LIBRARY

OP

USEFUL KNOWLEDGE.

NATURAL PHILOSOPHY.

POPULAR INTRODUCTIONS
TO NATURAL PHILOSOPHY.

NEWTON'S OPTICS.

DESCRIPTION OF OPTICAL
INSTRUMENTS.

THERMOMETER AND PYRO-
METER.
ELECTRICITY.
GALVANISM.
MAGNETISM.
ELECTRO-MAGNETISM.

"WITH

AN EXPLANATION OF SCIENTIFIC TERMS,

X.

PATERNOSTER-ROW.

LONDON I

PRINTED BY W.CIOWES,

Stamford-street.

CONTENTS.

Page

I. POPULAR INTRODUCTIONS TO NATURAL PHILOSOPHY . i— c

^~.J"

II. SIR ISAAC NEWTON'S OPTICS . , . 1—64

III. A DESCRIPTION OF OPTICAL INSTRUMENTS . . 1—60

.A>":'

IV. THE THERMOMETER AND PYROMETER . « 1—64
V. ELECTRICITY .».».. 1—64

VI. GALVANISM ...... 1—32

VII. MAGNETISM ...... 1 — 96

VIII. ELECTRO-MAGNETISM . . . . 1—100

IX. Glossary ....... 1—15

X. Index • ••••• 16—44

iy for the nail.

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INTRODU

[The present Treatise is intended to furnish introductions to the study of
different branches of Natural Philosophy, of the most elementary kind con-
sistent with accuracy. Many readers have now, for a considerable time, found
an excellent manual of this character in the well-known " Conversations on
Natural Philosophy." The Author and Proprietors of that work have, with
great liberality, authorized the Committee to use it freely for the purposes of
the present Treatise, which accordingly is entirely founded upon it, with hardly
any alterations except those which were necessary to adapt it to the form of the
publications of the Society.]

SECTION I. — On General Properties of Bodies.'

THIS Preface being intended as an elementary introduction to the Science
of Mechanics, we shall consider our readers as entirely ignorant of natural
philosophy, arid endeavour to adapt our explanations to the comprehension
of the most uninformed minds. No branch of Natural Philosophy can
be understood without some previous knowledge of the general properties
of bodies ; we shall therefore begin by taking a survey of these properties.

There are certain properties which appear to be common to all bodies,
and are hence called the essential properties of bodies : these are, Impene-
trability, Extension, Figure, Divisibility, Inertia, and Attraction.

By impenetrability is meant the property which bodies have of occu-
pying a certain space, so that, where one body is, another cannot be,
without displacing the former; for two bodies cannot exist in the same
place at the same time. A liquid may be more easily removed than a
solid body ; yet it is not the less substantial, since it is as impossible for
a liquid and a solid to occupy the same space at the same time, as for two
solid bodies to do so. For instance, if a spoon be put into a glass full of
water, the water will flow over to make room for the spoon.

Air is a fluid differing in its nature from liquids, but no less impene-
trable. If we endeavour to fill a phial by plunging it into a basin of
water, the air will rush out of the phial in bubbles, in order to make way
for the water ; for they cannot both exist in the same space, any more
than two hard bodies ; and if we reverse a goblet, and plunge it perpendi-
cularly into the water, so that the air will not be able to escape, the water
will not fill the goblet ; it rises, it is true, a considerable way into it,
because the water compresses or squeezes the air into a small space in the
upper part of the goblet ; but, as long as the air remains there, no other
body can occupy the same place.

If a nail be driven into a piece of wood, it will penetrate it, and both
the wood and the nail will occupy the same space that the wood alone did
before ; but it must be observed, that the nail penetrates between the
particles of the wood, by forcing them to make way for it ; for not a single
atom of wood remains in the space which the nail occupies ; and if the
wood is not increased in size by the addition of the nail, it is because
wood is a porous substance, like sponge, the particles of which may be
compressed or squeezed closer together ; and it is thus that they make
way for the nail.

ii INTRODUCTION TO MECHANICS.

We may now proceed to the next general property of bodies, extension.
A body which occupies a certain space must necessarily have extension ;
that is to say, length, breadth, and depth : these are called the dimensions
of extension, and they vary extremely in different bodies. The length,
breadth, and depth of a box, or of a thimble, are very different from those
of a walking-stick, or of a hair.

Height and depth are the same dimension, considered in different points
of view ; if you measure a body, or a space, from the top to the bottom,
it is called depth ; if from the bottom upwards, it is called height. Breadth
and width are also the same dimension.

The limits of extension constitute Jigure or shape : a body cannot be
without form, either symmetrical or irregular. Nature has assigned
regular forms to her productions in general. The most perfect natural
form of mineral substances is that of crystals, of which there is a great
variety. Many of them are very beautiful, and no less remarkable by
their transparency or colour, than by the perfect regularity of their forms,
as may be seen in the various museums and collections of natural history.
The vegetable and animal creations appear less symmetrical, and are still
more diversified in figure than the mineral kingdom. Manufactured sub-
stances assume the various arbitrary forms which the art of man designs
for them ; and an infinite number of irregular forms are produced by frac-
tures, such as broken china, or glass, or the fragments of mineral bodies,
which are broken in being dug out of the earth, or decayed by the effect
of torrents and other causes.

We may now proceed to divisibility ; that is to say, a susceptibility of
being divided into an indefinite number of parts. Take any small quan-
tity of matter, a grain of sand, for instance, and cut it into two parts ;
these two parts might be again divided, had we instruments sufficiently
fine for the purpose ; and if, by means of pounding, grinding, and other
similar methods, we carry this division to the greatest possible extent, and
reduce the body to its finest imaginable particles, yet not one of the
particles will be destroyed, and the body will continue to exist, though in
this altered state. A single pound of wool may be spun so fine as to
extend to nearly 100 miles in length.

The melting of a solid body in a liquid also affords a very striking
example of the extreme divisibility of matter ; when you sweeten a cup of
tea, for instance, with what minuteness the sugar must be divided to be
diffused throughout the whole of the liquid ! And if a few drops of red
wine be poured into a glass of water they will immediately tinge the
liquid throughout. The odour of lavender-water, or any other perfume,
will be almost as instantaneously diffused throughout the room if the bottle
be opened. The odour or smell of a body is part of the body itself, and
is produced by very minute particles or exhalations which escape from
odoriferous bodies, and come in actual contact with the nose ; and it
would be just as impossible to smell a flower, the odoriferous particles
of which did not touch the nose, as to taste a fruit, the flavoured particles
of which did not come in contact with the tongue. If a bottle of lavender-
water be left open a sufficient length of time, the whole of the liquid will
evaporate and disappear. But though so minutely subdivided as to be im-
perceptible to any of our senses, each particle would continue to exist ; for
it is not within the power of man to destroy a single particle of matter; nor
is there any reason to suppose, that in nature an atom is ever annihilated.

When a body is burnt to ashes, part of it, it is true, appears to be
destroyed ; the residue of ashes beneath the grate, for instance, is very

INTRODUCTION TO MECHANICS. iii

small compared to the coals which have been consumed within it. In
this case, that part of the coals, which one would suppose to be destroyed,
evaporates in the form of smoke and vapour, whilst the remainder is
reduced to ashes. A body in burning; undergoes, no doubt, very re-
markable changes ; it is generally subdivided ; its form and colour altered ;
its [extension increased : but the various parts, into which it has been
separated by combustion, continue in existence, and retain all the essential
properties of bodies. Smoke, indeed, when diffused in the air, becomes
invisible, but we must not imagine that what we no longer see no longer
exists. Were every particle of matter that becomes invisible annihilated,
the world itself would in the course of time be destroyed. The particles of
smoke continue still to be particles of matter, as well as when more closely
united in the form of coals : they are really as substantial in the one state
as in the other, and equally so when, by being diffused in the air, they
become invisible. No particle of matter is ever destroyed: this is a
principle which must constantly be remembered. Every thing in nature
decays and corrupts in the lapse of time. We die, and our bodies moulder
to dust : but not a single atom of them is lost ; they serve to nourish the
earth, whence, while living, they drew their support.

It should be observed, that when a body is divided, its surface or ex-
terior part is augmented. If an apple be cut in two, in addition to the
round surface, there will be two flat surfaces ; divide the halves of the
apple into quarters, and two more surfaces will be produced.

Inertia, the next essential property of matter, expresses the resistance
which inactive matter makes to a change of state. Bodies appear to be
not only incapable of changing their actual state, whether it be of motion
or of rest, but to be^endowed with a power of resisting such a change. It
requires force to put a body which is at rest in motion ; an exertion of
strength is also requisite to stop a body which is already in motion. The
resistance of a body to a change of state is, in either case, called its inertia.
In playing at cricket, for instance, considerable strength is required to
give a rapid motion to the ball ; and in catching it we feel the resistance
it makes to being stopped. Inert matter is as incapable of stopping of
itself, as it is of putting itself into motion. When the ball ceases to move,
therefore, it must be stopped by some other cause or power, which we
shall presently explain.

The last property which appears to be common to all bodies is
attraction, under which general name we may include all the properties
by which one atom of matter acts on another, so as to make the latter
approach or continue near the former. Bodies consist of infinitely small
particles of matter, each of which possesses the power of attracting or
drawing towards it, and uniting with any other particle sufficiently near to
be within the influence of its attraction. This power cannot be recog-
nized in minute particles, except when they are in contact, or at least
appear to be so : it then makes them stick or adhere together, and is
hence called the attraction of cohesion. Without this power, solid bodies
would fall in pieces, or rather crumble to atoms ; although we are so
much accustomed to see bodies firm and solid, that it seldom occurs to
us that any power is requisite to unite the particles of which they are
composed.

The attraction of cohesion exists also in liquids : it is this power which
holds a drop of water suspended at the end of the finger, and keeps
the minute watery particles of which it is composed united. But as this
power is stronger in proportion as the particles of bodies are more

iv INTRODUCTION TO MECHANICS.

closely united, the cohesive attraction of solid bodies is much greater
than that of fluids.

The thinner and lighter a fluid is, the less is the cohesive attraction of
its particles, because they are further apart ; and in elastic fluids, such as
air, there is no cohesive attraction whatever. Air, however, is of the
same nature as other bodies in all its essential properties ; nor is it
probable that the particles of air are destitute of the power of attraction,
but they are too far distant from each other to be influenced by it ; and
the utmost efforts of human art have proved hitherto ineffectual in the
attempt to compress them, so as to bring them within the sphere of each
other's attraction, and make them cohere.

It is owing to the different degrees of attraction of different substances,
that they are hard or soft ; and that liquids are thick or thin. This very
frequently, especially in bodies of the same nature, corresponds with what
we express by the term density, which denotes the degree of closeness
and compactness of the particles of a body : in these cases, whether in
solids or liquids, the stronger the cohesive attraction, the greater is the
density of the body. In philosophical language, however, density is said
to be^ that property of bodies by which they contain a certain quantity of
matter, under a certain bulk or magnitude. Rarity, though opposed to
density, as it denotes the thinness and subtlety of bodies, will admit
of the same definition ; for it implies merely a diminution of density :
thus we should say that mercury or quicksilver was a very dense fluid ;
ether, a very rare one, &c.

We judge by the weight of the quantity of matter contained in a
certain bulk, and bodies of the same bulk are said to be dense in pro-
portion as they are heavy. Thus we say that metals are dense bodies,
wood comparatively a rare one, &c. But it may be objected, that when the
particles of a body are so near as to attract each other, the effect of this
power must increase as they are brought by it closer together : so that
one would suppose' the body would gradually augment in density, till it
was impossible for its particles to be more closely united. Now, we
know that this is not the case ; for soft bodies, such as cork, sponge, or
butter, never become, in consequence of .the increasing attraction of their
particles, as hard as iron. The answer is, that in such ^bodies as cork
and sponge, the particles which come in contact are so few as to produce
but a slight degree of cohesion : they are porous bodies, which, owing to
their peculiar arrangement, abound with interstices which separate the
particles ; and these vacancies are filled with air, the spring or elasticity
of which prevents the closer union of the parts. But there is another
fluid much more subtle than air, which pervades all bodies ; this is heat
Heat insinuates itself p'more or less between the particles of bodies, and
forces them asunder; it may therefore be considered as constantly acting
in opposition to the attraction of cohesion, the one endeavouring to rend
a body to pieces, the other to keep its parts firmly united.

The more a body is heated, then, the more its particles will be sepa-
rated ; consequently bodies generally swell or dilate by heat : this effect
is very sensible in butter, for instance, which expands by the application
of heat, till at length the attraction of cohesion is so far diminished that
the particles separate, and the butter becomes liquid. A similar effect
is produced by heat on metals, and all bodies susceptible of being melted.
Liquids are made to boil by the application of heat ; the attraction of
cohesion, then, yields entirely to the expansive power ; the particles are
totally separated, and converted into steam or vapour. But the agency

INTRODUCTION TO MECHANICS; v

of heat is in no body more sensible than in air, which dilates and con-
tracts by its increase or diminution in a very remarkable degree.

To return to its antagonist, the attraction of cohesion; it is this power
which restores to vapour its liquid form, which unites it into drops when
it falls to the earth in a shower of rain, and which gathers the dew into
brilliant gems on the blades of grass : for rain does not descend from the
clouds at first in the form of drops, but in that of mist or vapour, which
is composed of very small watery particles; these, in their descent,
mutually attract each other, and those that are sufficiently near in con-
sequence unite and form a drop, and thus the mist is transformed into a
shower. The dew also was originally in a state of vapour, but is, by the
mutual attraction of the particles, formed into small globules on the blades
of grass : in a similar manner the rain upon the leaf collects into large
drops, which, when they become too heavy for the leaf to support, fall to
the ground.

Among the wonderful phenomena of nature, we must not omit to
point out a curious effect of the attraction of cohesion. It enables liquids
to rise above their level in capillary tubes : these are tubes the bores of
which are so extremely small that liquids ascend within them, from the
cohesive attraction between the particles of the liquid and the interior
surface of the tube. You may perceive the water rising in a small glass
tube immersed in a goblet of water. It creeps up the tube to a certain
height and there remains stationary, because the cohesive attraction
between the water and the internal surface of the tube is balanced by
the weight of the water within it. If the bore of the tube were narrower,
the water would rise higher ; and if you immerse several tubes of bores
of different sizes, you will see it rise to different heights in each of them.
In making this experiment, the water should be coloured with a little
red wine, in order to render the effect more obvious.

All porous substances, such as sponge, bread, linen, &c., may be con-
sidered as collections of capillary tubes : if you dip one end of a lump of
sugar into water, the water will rise in it, and wet it considerably above
the surface of that into which you dip it.

We shall now explain the attraction of gravitation. It is unnecessary
here to inquire whether it be only another modification of the same pro-
perties which produce the attraction of cohesion, which it certainly
resembles in this, that it really results from the attractive force of the
minute particles of matter of which bodies are composed. But, tracing it
only in its effects, we now speak of it as a force acting, unlike that of
cohesion, at considerable distances, and only perceptible in its effects
when many particles of matter are combined together in one mass. It
acts therefore on the largest bodies, and at immense distances as well as
small ones. Let us take, for example, one of the largest bodies in na-
ture, and observe whether it does not attract other bodies. What is it
that occasions the fall of a book when it is no longer supported? You
will say that all bodies have a natural tendency to fall. That is true ;
but that tendency is produced by the attraction of the earth. The earth,
being much larger than any body on its surface, forces every other, which
is not supported, to fall to it.

When you are accustomed to consider the fall of bodies as depending
on this cause, it will appear to you as natural, and surely much more
satisfactory, than if the cause of their tendency to fall were totally
unknown. Thus all matter is attractive, from the smallest particle to the
largest mass ; and bodies attract each other with a force proportioned to
the quantity of matter they contain.

vi . INTRODUCTION TO MECHANICS.

. When the attraction of cohesion and that of gravitation are opposed to
each other, the former, within the limits at which it acts, is generally
the stronger. Of this we have an instance in the attraction of capillary
tubes, in which liquids ascend by the attraction of cohesion, in opposition
to that of gravity. It is, however, necessary that the bore of the tube
should be extremely small ; for if the column of water within the tube is
not very minute, the attraction would not be able either to raise or sup-
port its weight, in opposition to that of gravity. It may be observed, also,
that all solid bodies are enabled, by the force of the cohesive attraction of
their particles, to resist that of gravity, which would otherwise disunite
them, and bring them to a level with the ground, as it does in the case of
liquids, the cohesive attraction of which is not sufficient to enable them
to resist the power of gravity. There is no attraction of cohesion between
the separate parts of pulverized bodies ; every grain of powder or sand is
composed of a great number of more minute particles, firmly united by
the attraction of cohesion ; but amongst the grains themselves there is no
sensible attraction, because they are not in sufficiently close contact.

The surface of bodies is so rough and uneven, that, when in actual
contact, they touch each other only by a few points. Thus, if a book,
the binding of which appears perfectly smooth, be laid on the table, so
few of the particles of its under surface come in contact with the table,
that no sensible degree of cohesive attraction takes place ; it does not
stick, or cohere to the table, and there is no difficulty in lifting it off. It is
only when surfaces perfectly flat and well polished are placed in contact,
that the particles approach in sufficient number, and closely enough to
produce a sensible degree of cohesive attraction. Take two hemispheres
of polished metal, press their flat surfaces together, having previously
interposed a few drops of oil, to fill up every little porous vacancy. It
now requires a weight of several pounds to separate them : but part of
this effect is due, as will be explained hereafter, to a pressure of the air
on their surface ; the residue is the effect of cohesion. The same cause
which occasions the fall of bodies produces also their weight; in other
words it is the attraction of gravity which makes bodies heavy. The power
which brings bodies that are unsupported to the ground causes those which
are supported to press upon the objects which prevent their fall, with a
weight equal to the force with which they gravitate towards the earth.

Attraction being mutual between two bodies, when a stone falls to the
earth, the earth should rise part of the way to meet it. But when, on
the other hand, you consider that attraction is proportioned to the mass of
the attracting and attracted bodies, you will no longer expect to see the
earth rising to meet the stone. You may possibly imagine that, according
to this theory, the hills should attract the houses and churches towards
them. The hills no doubt exert this influence, but they cannot move the
buildings, because they can neither overcome the attraction of cohesion
between the bricks and the mortar, nor that of gravity which fixes the wall
to the ground. There are, however, some instances in which the attrac-
tion of a large body has sensibly counteracted that of the earth. If a man,
standing on the declivity of an abrupt mountain, hold a plumb-line in his
hand, the weight will not fall perpendicularly to the earth, but incline a little
towards the mountain ; and this is owing to the lateral or sideway attraction
of the mountain interfering with the perpendicular attraction of the earth.

If no obstacle intervened to impede the fall of bodies, attraction would
make them all descend with an equal velocity, or quickness ; so that
those which fall from the same height would reach the earth in the same

INTRODUCTION TO MECHANICS. vi

space of time. It may be objected, that since attraction is proportioned to
the quantity of matter which a body contains, the earth must necessarily
attract a heavy body more strongly, and consequently bring it to the
ground more rapidly than a light one. In answer to this, it must be
observed that bodies have no natural tendency to fall any more than to
rise, or to move laterally, and that they will not fall unless impelled by
some force ; and this force must be proportioned to the quantity of matter
it has to move. A body consisting of 1000 particles of matter, forjnstance,
requires ten times the force of attraction to bring it to the ground, in the
same space of time, that a body consisting of only 100 particles does. If
you draw towards you two bodies, the one of 100, the other of lOOOlbs.
weight, will you not be obliged to exert ten times as much strength
to draw the heavier one to you in the same time that would be re-
quired for the lighter one ? Therefore if the earth draw a body of 1000 Ibs.
weight to it in the same space of time that it draws a body of 100 Ibs.,
it follows that it does actually, as we have stated it to do, attract the
heavier body ten times more than the lighter one. So that the more
matter there is in a body, the more forcibly it will gravitate ; the more
force there is, the more there is for the force to do. The consequence of
this should be, that all bodies, whether light or heavy, being at an equal
distance from the earth, should fall to it in the same time, or, in other
words, that their velocities should be equal. But experience seems to
contradict this, for we see bodies falling quickly or slowly in proportion
as they are heavy or light. We must inquire, therefore, what is the cause
which interferes with the regular action of gravity on bodies, and makes
them fall with such various degrees of velocity. This cause is the resist-
ance of the air through which bodies fall. They must force their way
through this medium ; and heavy bodies overcome this obstacle more
easily than lighter ones, for the resistance which the air opposes to the fall
of bodies is proportioned to their bulk, not to their weight ; the air, being
inert, cannot do more to support the weight of a cannon ball than that
of a ball of leather of the same size ; now as the cannon ball contains
perhaps 100 times more matter than the leather ball, it would require 100
times more resistance to impede its fall equally. The larger the surface
of a body the more air it covers and the greater is the resistance it meets
with from it. A sheet of paper expanded descends gently to the ground.
If rolled up in a ball it offers but a small surface to the uir, encounters but
little resistance, and falls with much greater velocity. The heaviest bodies
may be made to float awhile in the air by extending their surface so as to
counterbalance their weight ; gold is one of the most dense bodies we
know ; but when beaten into a very thin leaf it offers so great an extent of
surface in proportion to its weight that its fall is still more slow than that
of a sheet of paper. When bodies have but little bulk in proportion to their
weight, the resistance of the air has but a very trifling effect 5 and stones
of different sizes let fall from the top of a house will reach the ground
very nearly at the same time.

The air itself is also subjected to the law of gravity, the lower stratum is
actually in contact with the earth, and the superior strata are supported by
it, just as water at the bottom of a basin supports that which is at the
surface. But the air is an elastic fluid, the peculiar property of which is,
to resume, after compression, its original dimensions; and the air of the
atmosphere must be considered as constantly in a state of compression,
from the attraction of the earth ; it has therefore a constant tendency to
expand itself, and this is called the spring or elasticity of the air. This

viii INTRODUCTION TO MECHANICS.

compression is increased in the lower strata of air by the weight of the
upper strata, which rests upon them, and thus the air near the surface
of the earth is more dense than in the superior regions. The pressure
of the atmosphere has been compared to that of a pile of fleeces of wool,
in which the lower fleeces are pressed together by the weight of those
above ; these lie light and loose in proportion as they approach the up-
permost fleece, which receiving no external pressure is confined merely
by the power of its own gravity.

There are some bodies which do not appear to gravitate : smoke and
steam, for instance, rise instead of falling, but it is still gravity which
produces their ascent. The air near the earth being heavier than smoke,
steam, or other vapours, not only supports these light bodies, but, by its
own tendency to sink below them, forces them to rise. The principle
is just the same as that by which a cork, or a drop of oil, if forced to the
bottom of a vessel of water, rises to the top as soon as it is set at liberty :
the only difference being, that, in the case of the atmosphere, the weight
or density continually diminishes ; and the ascending body therefore does
not rise through the whole extent of the atmosphere, but only till it reaches
a stratum of which the weight is equal to its own ; and there, if no other
changes take place, it remains stationary. Smoke ascends but a very little
way ; it consists of minute particles of fuel carried up by a current of
heated air from the fire below. Heat expands all bodies ; it consequently
rarefies air, and renders it lighter than the colder air of the atmosphere ;
the heated air from the fire carries up with it vapour and small particles
of the combustible materials which are burning in the fire. When this
current of hot air is cooled by mixing with that of the atmosphere, the
minute particles of coal or other combustible fall, and it is this which
produces the small black flakes which render the air and everything in
contact with it, in London, so dirty.

Balloons ascend upon the same principle, the materials of which they
are made are heavier than the air ; but the air with which they are filled
is an elastic fluid of a different nature from the atmospheric air, and con-
siderably lighter ; so that, on the whole, the balloon is lighter than the air
which it displaces, and will consequently rise. Thus you see that it is
the resistance of the air alone which prevents bodies of different weight
from falling with equal velocities. Those which are lighter than the air
are forced by it to ascend ; those of an equal weight will remain stationary
in it, and those that are heavier will descend through it, and their descent
will be more or less retarded according to their weight. If you let fall a
crown piece and a piece of writing paper of exactly the same dimensions,
the crown piece will reach the ground much sooner than the paper, but if
you place the paper upon it so closely that no air shall intervene, the
paper will fall as rapidly as the crown piece. That bodies when not
supported by the atmosphere fall with equal velocities may be proved by
the air-pump, a machine by means of which the air may be expelled from
any close vessel placed upon it. Glasses of various shapes, called re-
ceivers, are employed for this purpose, and bodies of whatever size or
weight placed within them will fall from the top to the bottom in the same
space of time. The experiment is usually made with a guinea and^a
feather: they are placed on a brass plate in the upper end of the glass,
and as soon as the air is pumped out, by turning a screw the brass plate
is inclined, and the two bodies fall at the same moment, and reach the
ground at the same moment.

INTRODUCTION TO MECHANICS. ix

SECTION II. — On the Laws of Motion, and the Centre of Gravity.

THE science of mechanics is founded on the laws of motion ; it will
therefore be necessary to explain these laws before we examine the me-
chanical powers. Motion consists in a change of place. A body is in
motion whenever it is changing1 its situation with regard to a fixed point.
Now having observed that one of the general properties of bodies is
inertia, that is, an entire passiveness either with regard to motion or rest,
it follows that a body cannot move without being put into motion : the
power which puts a body into motion is called force ; thus the stroke of
the hammer is the force which drives the nail ; the pulling of the horse,
that which draws the carriage. Gravitation is the force which occasions
the fall of bodies, cohesion that which binds the particles of bodies
together, and heat a force which drives them asunder. The motion of
a body acted upon by a single force is always in a straight line, in the
direction in which it received the impulse.

The rate at which a body moves, or the length of time which it takes
to move from one place to another, is called its velocity ; and it is one
of the laws of motion that the velocity of the moving body is proportional
to the force by which it is put in motion. The velocity of a body is called
absolute, if we consider the motion of the body in space, without any
reference to that of other bodies. When, for instance, a horse goes fifty
miles in ten hours, his velocity is five miles an hour. It is termed
relative, when compared with that of another body which is itself in
motion. Thus a man sailing in a ship may remain at rest relatively to
the vessel, though he partakes of its absolute motion ; but if he walk the
deck in the same direction as that in which the ship is sailing, his
absolute motion will be increased by the rate at which he moves along it,
and his relative motion will be the difference between his own absolute
motion and that of the ship. So if two carriages go along the same road
in the same direction, their relative velocity will be the difference of their
absolute velocities ; if in opposite directions, the same. If they start
from the same point along two roads, making an angle with each other,
their relative motion will be measured by their distance, in a straight
line, from each other after a given time, and the direction of this relative
motion will be the direction of that line. The absolute velocity of a body
is measured by the space over which it moves, in some particular time,
selected as the standard ; the velocity per hour, for instance, would be
shewn by dividing the number of miles travelled over by the number of
hours occupied in the journey. Thus, if you travel one hundred miles
in twenty hours, and wish to know what is your velocity, you divide 100
by 20, and the answer will be 5 miles an hour. We say, also, that space
is equal to the velocity multiplied by the time ; if your velocity be three
miles an hour, and you travel six hours, you will have gone, in all, a
space of eighteen miles.

Uniform motion is that of a body which passes over equal spaces in
equal times. It is produced by a force having acted on a body once,
and having ceased to act, such as the stroke of a bat on a cricket-ball.
But it may be said, that the motion of a cricket-ball is not uniform, its
velocity gradually diminishing till it falls to the ground. In answer to
this objection, you must observe that the ball is inert, having no more
power to stop than to put itself in motion ; if it fall, therefore, it must be
stopped by some force superior to that by which it was projected ; and

x INTRODUCTION TO MECHANICS.

this force is gravity, which counteracts and finally overcomes that of
projection. If neither gravity nor any other force (such as the resistance
of the air or the friction of the ground) opposed its motion, the cricket-
ball, or even a stone thrown by the hand, would proceed onwards in a
right line, and with an uniform velocity for ever ! Yet we have no ex-
ample of perpetual motion on the surface of the earth ; because the
causes referred to ultimately destroy all motion, whether produced by
natural or artificial means.

When we study the celestial bodies, we find that nature abounds with
examples of perpetual motion, and that it conduces as much to the
harmony of the system of the universe, as the prevalence of it would to
the destruction of all stability on the surface of the globe. Providence
has therefore ordained insurmountable obstacles to perpetual motion here
below 5 and though these obstacles often compel us to contend with
great difficulties, yet the generalj result is that order, regularity 'and
repose, so essential to the preservation of the various beings of which this
world is composed.

Retarded motion is produced by some force acting on a body in a
direction opposed to that which first put it in motion, and thus gradually
diminishing its velocity.

Accelerated motion is produced when the force which put a body in
motion continues to act upon it during its motion, so that its velocity
is continually increased. Let us suppose, that the instant after a stone
is let fall from a high tower the force of gravity were annihilated : the
stone would nevertheless descend, for a body, having once received an
impulse, will not stop (unless some obstacle impede its course), but move
on with a uniform velocity. If, then, the force of gravity be not de-
stroyed after having given the first impulse to the stone, but continues to
act on it during the whole of its descent, it is easy to understand that its
motion will be thereby accelerated. Let us suppose that the impulse
given by gravity to the stone during the first instant of its descent be
equal to one ; the next instant we shall find that an additional impulse
gives the stone an additional velocity equal to one ; so that the accu-
mulated velocity is now equal to two ; the following instant another
impulse increases the velocity to three, and so on till the stone reaches
the ground. The spaces described in a given time follow a law slightly
different ; for it has been ascertained, both by experiment and calculations,
that heavy bodies descending from a height by the force of gravity, fall
sixteen feet in the first second of time, three times that distance in the
next, five times in the third second, seven times in the fourth, and so on,
regularly increasing both their velocities and the spaces described ac-
cording to the number of seconds during which the body has been falling.
Thus the height of a building or the depth of a well may be measured
by observing the length of time which a stone takes in falling from the
top to the bottom.

If a stone be thrown perpendicularly upwards, it is the same length
of time ascending that it is descending. In the first case the velocity
is diminished by the force of gravity, in descending it is accelerated by it.
The force of projection given to a body in throwing it upwards is equal
to the force with which it strikes the ground when it descends again, and
this latter force is the effect produced by gravity during the time of its fall.
If a stone be thrown upwards gently it will not rise high, and gravity will
soon make it descend ; if thrown with violence, it will rise higher, and
gravity will be longer in bringing it back to the ground. Suppose that

INTRODUCTION TO MECHANICS. xi

it be thrown with a force which will make it rise only sixteen feet, in
that case it will fall in one second of time. Now it is proved by expe-
riment, that an impulse requisite to project a body sixteen feet upwards,
will make it ascend that height in one second ; here then the times of
the ascent and descent are equal. But supposing it be required to throw
a stone twice that height, the force must be greater. Thus the impulse
of projection in throwing a body upwards, is equal to the accumulated
effect produced by gravity during its descent ; and it is the greater or
less distance to which the body rises, that makes these balance each
other, for it gives more time for the force of gravitation to act.

We must now explain to you what is meant by the momentum of bodies.
It is I he force, or power, with which a body in motion would strike against
another body, so as to set the latter in motion. The momentum of a
body is composed of its weight, multiplied by its velocity. The quicker
a body moves, the greater will be the force with which it will strike
against another body j so that a small light body may have a greater
momentum than a large heavy one, provided its velocity be sufficiently
great. For instance, the momentum of an arrow shot from a bow is
greater than that of a stone thrown by the hand. We know also by
experience, that the heavier a body is, the greater is its force, if it acts
in other respects under the same circumstances, therefore the whole
power or momentum of a body is composed of these two properties.
But why should not these be added together, instead of being multiplied
by one another ? It is found by experiment, that if the weight of
a body be represented by the number 3, and its velocity also by
3, its momentum will be as 9; not 6, as would be the case were
these figures added, instead of being multiplied together. The same
conclusion may very easily be deduced by reasoning. If two bodies,
one of one pound weight, the other of two, have the same velocity,
the moving force of the second, or its momentum, is double that of the
first. If a third body, also of two pounds, move with three times the
velocity of the second, its momentum, the weights being in this case
equal, is three times that of the second. But the momentum of the
second is twice that of the first; therefore the momentum of the third is
three times this quantity, or six times that of the first. By thus dividing
the process, and looking first to the effect of a change of the velocity,
and afterwards to that of the change of the weight, it becomes evident that
these effects are to be multiplied together.

The re-action of bodies is the next law of motion to be explained.
When a body in motion strikes against another body it meets with resist-
ance ; the resistance of the body at rest will be equal to the blow struck
by the body in motion ; or, in philosophical language, action and re-action
will be equal, and in opposite directions.

The most strikingexperiments on these subjects are made with elastic bo-
dies. Elasticity is a property, by means of which bodies that are compressed
return to their former state. If you bend a cane, as soon as it is at liberty
it recovers its former position ; if you press your finger upon your arm,
as soon as you remove it, the flesh, by virtue of its elasticity, rises and
destroys the impression. Of all bodies, those in the form of air or gas
are the most eminent for this property. Hard bodies are in the next
degree elastic: if two ivory or metallic balls be struck together, the
parts at which they touch will be flattened, but no mark is perceptible,
their elasticity instantly destroying all trace of it. If, however, a very
small spot of ink be placed on one of the balls at the point of contact,

Xll

INTRODUCTION TO MECHANICS.

it will be found after the contact to have spread, and will thus" shew that
there has been compression. Soft bodies, which easily retain impres-
sions, such as clay, wax, tallow, butter, &c. have very little elasticity.

The cause of elasticity is not well ascertained. Elasticity implies sus-
ceptibility of compression, and the susceptibility of compression depends
upon the porosity of bodies ; for were there no pores or spaces between
the particles of matter of which a body is composed, it could not be com-
pressed. But you must not hence infer, that bodies whose particles are
most distant from each other are most elastic. Elasticity implies not
only susceptibility of compression, but the power of resuming its former
state after compression. The pores of such bodies as ivory and metals
are invisible to the naked eye ; but it is well ascertained, that gold, one of
the most dense of all bodies, is extremely porous, and that its pores are
sufficiently large to admit water, under great pressure, to pass through
them. In cork, sponge, and bread, the pores form considerable cavities ;
in wood and many kinds of stone, when not polished, they are perceptible to
the naked eye ; whilst in ivory, metals, and most varnished and polished
bodies, they cannot be discerned. To give an idea of the extreme
porosity of bodies, Sir Isaac Newton conjectured that if the earth were so
compressed as to be absolutely without pores, its dimensions might pos-
sibly not be more than a cubic inch.

The elasticity of ivory is very perfect ; that is to say, it restores itself, after
compression, with a force very nearly equal to that exerted in compressing it.
If two ivory balls of equal weight be suspended by threads (fig. 1), and one
of them A be drawn a little on one side and then let go, it will strike against
the other ball B, and drive it off to a distance equal to that through which the

Fig. 1. Fig. 2. Fig. 3.

first ball fell ; but the motion of A will be stopped, because, when it strikes
B, it receives in return a blow equal to that it gave, and its motion is con-
sequently destroyed. Therefore, when one body strikes against another,
the quantity of motion communicated to the second body is lost by the
first, but this loss proceeds — not from the blow given by the striking
body, — but from the re- act ion of the body which it struck.

If six ivory balls of equal weight be hung in a row (Jig. 2), and the first
be drawn out of the perpendicular, and let fall against the second, none of
the balls will appear to move except the last, which will fly off as far as the
first ball fell. For when the first ball strikes the second, it receives a blow in
return, which destroys its motion. The second ball, although it does not
appear to move, strikes against the third ; the re-action of which sets it
at rest : the action of the third ball is destroyed by the re-action of the
fourth, and so on, till motion is communicated to the last ball, which, not
being re-acted upon, flies off. This effect takes place accurately only in
the case of perfectly elastic bodies.

If two balls of clay (fig. 3), which are not elastic, be suspended, and one
of them, D, be raised out of the perpendicular and let fall against the

INTRODUCTION TO MECHANICS. xiii

other E, only part of the motion of D, therefore, will be destroyed by it, and
the two balls will move on together to d and e, which are less distant from
the vertical line than the ball D was before it fell. Still, however, action
and re-action are equal ; for the action on E is only enough to make it
move through a smaller space, but so much of D's motion is now also
destroyed. If the elasticity of the balls be imperfect, the effect will be in-
termediate between the effects produced in the cases we have mentioned ;
that is to say, the ball struck will rise farther than in the case of non-elas-
tic bodies, and less far than in that of perfectly elastic bodies ; and the
striking ball will be retarded more than in the former case, but not stopped
completely, as in the latter. They will therefore move onwards both after
the blow, but not together, or to the same distance ; but in this, as in the
preceding cases, the whole quantity of motion destroyed in the striking
ball will be equal to that produced in the ball struck.

Birds, in flying, strike the air with their wings, and it is the re-action
of the air which enables them to rise or advance forwards. The force
with which their wings strike against the air must equal the weight of
their bodies, in order that the re-action of the air may be able to support
that weight ; the bird will then remain stationary. If the stroke of the
wings be greater than is required merely to support the bird, the re-action
of the air will make it rise; if it be less, it will descend: the lark some-
times remains with its wings extended, but motionless ; in this state it
drops rapidly into its nest. A bird expands his wings when he gives the
stroke, the re-action of which is to impel him onward, and contracts them
when in the opposite direction. The swimming of fishes is on the same
principle ; their fins are expanded and contracted in a like manner; and
a man in swimming strikes his hands out to produce the re-action which
impels him forward, and turns them edgewise to lessen the effect of the
contrary re-action. In rowing, the oars are lifted out of the water after
every stroke, so as completely to prevent any re-action in a backward
direction ; and even in moving them through the air they are turned edge-
wise, or feathered, as it is called, from its resemblance to the action of the
feathers of a bird in flying.

Let us now return to the subject of re-action, on which we have some
further observations to make. It is re-action being contrary to action
which produces reflected motion. If you throw a ball against a wall, it
rebounds ; this return of the ball is owing to the re-action of the wall
against which it struck, and is called reflected motion. A ball filled with
air rebounds better than one stuffed with bran or wool, for the elasticity of
the air re-acts after compression. If the ball be thrown perpendicularly
against a wall it returns straight towards the hand, though the action of
gravity draws it downwards before reaching it ; but if thrown obliquely
upwards, it rebounds still higher. We use the term perpendicular in pre-
ference to the more familiar word straight, because straight is a general
term for lines in all directions which are neither curved nor bent, and is,
therefore, equally applicable to oblique or perpendicular lines. A perpen-
dicular line has always a reference to something towards which it is
perpendicular; that is to say, that it inclines neither to the one side nor
the other, but makes an equal angle on either side.

Let the line A B (Jig. 4) represent the floor of the room, and the line
CD that in which you throw a ball against it: the line CD, you will
observe, forms two angles with the line A B, and those two angles are
equal. All circles are supposed to be divided into 360 equal parts, called
degrees ; the opening of an angle being, therefore, a portion of a circle,
must contain a certain number of degrees; the larger the angle, the

xir

INTRODUCTION TO MECHANICS.

greater the number of degrees, and two angles are said to be equal when
they contain an equal number of degrees : the two angles (ADC and
C D B, Jig. 4) are together just equal to half a circle ; they contain, there-
fore, 90 degrees each : 90 being a quarter of 360. An angle of 90 degrees
is called a right angle, and when one line is perpendicular to another, it
forms a right angle on either side. Angles containing more than 90
degrees are called obtuse angles (Jig. 5) ; and those containing less than
90 degrees are called acute angles (Jig. 6). Thus the angles of a square

Fig. 6.

Fig. 5.

table are right-angles, those of an octagon table obtuse angles, and
those of sharp pointed instruments, acute angles.

When a billiard player strikes the ball perpendicularly against the

Fig. 7.

cushion, it returns in the same direction
but when he sends it obliquely to the cushion
it rebounds obliquely on the opposite side;
the ball in this latter case describes an
angle, the point of which is at the cushion.
The more obliquely the ball be struck against
the cushion, the more obliquely it will re-
bound on the opposite side, so that a billiard
player can calculate with great accuracy in what
direction it will return. Fig. 7 represents a
billiard table : if a line AB be drawn perpen-
dicular to the cushion from the point where
the ball A strikes, it will divide the angle
which the ball describes into two parts, or two
angles ; the one will show the obliquity of the
direction of the ball in its passage towards the
cushion, the other its obliquity in its passage
back from the cushion. The first is called the

* v:\

INTRODUCTION TO MECHANICS.

angle of incidence, the other the angle of reflection, and theSe angles are,
if the bodies be perfectly elastic, equal.

We shall now explain the nature of compound motion. If a body be
struck by two equal forces, in opposite directions, it will not move. But
if the forces, instead of acting on the body in opposition, strike it in twe-
directions inclined to each other, at an angle of 90 degrees, if the ball
A (Jig. 8) be struck by equal forces at X and at Y, the force X
would send it towards B, and the force Y towards C ; and since these
forces are equal, the body cannot obey one impulse rather than the other.
Yet as they are not in direct opposition, they cannot entirely destroy
the effect of each other ; the body will therefore move, but, following the
direction of neither, it will move in a line between them, and reach D in the
same space of time that the force X would have sent it to B, and the force Y
would have sent it to C. Now, if two lines be drawn from D to join B and
C, a square will he produced, and the oblique line which the body describes
is called the diagonal of the square. Supposing the two forces to be
unequal, that X, for instance, be twice as great as Y ; then X will drive
the ball twice as far as Y, consequently the line A B (fig* 9) will be

Fig. 8. Fig. 9. Fig. 10.

twice as long as the line AC; the body will in this case move to D ;
and if lines be drawn from that point to B and C, you will find that
the ball will have moved in the diagonal of a rectangle. Let us now
suppose the two forces to be unequal, and not to act on the ball in
the direction of a right angle, but in that of an acute angle. The ball
will move from A to D (Jig. 10), in the diagonal of a parallelogram,
A B D C. Forces acting in the direction of lines forming an obtuse angle
will also produce motion in the diagonal of a parallelogram. For in-
stance, if the body set out from B instead of A, and was impelled by
the forces x and y, it would move in the dotted diagonal B C.

We shall now proceed to circular motion : this is the result of the action
of two forces on a body, by one of which it is projected forward in a right
line, whilst by the other it is continually directed towards a fixed point.
For instance, if I whirl a ball fastened to my hand with a string, the
ball will have a circular motion, because it is acted on by two forces,
that I give it, which represents the force of projection, and that of the
string, which confines it to my hand. If during its motion I were
suddenly to cut the string, the ball would fly otF in a straight line ;
being released from confinement to the fixed point, it would be acted on
but by one force, and motion produced by one force is always in a right
line. When a mop is trundled the threads fly from the centre ; but
being confined to it at one end, they cannot part from it; whilst the
water they contain is thrown off in straight lines. In the same way, the
flyers of a windmill, when put in motion by the wind, would be driven
straight forward in a right line, were they not confined to a fixed point,

XVi

INTRODUCTION TO MECHANICS.

round which they are compelled to move. The point to which the
motion of a small body, such as the ball with the string1, is confined,
becomes the centre of its motion ; for it may be considered as moving in
the same plane or flat surface. But when a body is not of a size or
shape to allow of our considering every part of it as moving in the
same plane, it revolves round a line, which is called the axis of motion.
In a top, for instance, when spinning- on its point, the axis is the line
which passes through the middle of it, perpendicularly to the floor.
The axle of the flyers of the windmill is the axis of its motion. The
centre of motion is not always in the middle of a body.

The middle point of a body is called its centre of magnitude, that is,
the centre of its mass or bulk. Bodies have also another centre, called
the centre of gravity, which shall be explained ; but, at present, we
must confine ourselves to the axis of motion. This line remains at rest,
whilst all the other parts of the body move around it ; when a top is
spun, the axis is stationary, whilst every other part is in motion round it.
A top, it is true, has also generally a motion forwards, besides its
spinning'motion ; and then no point within it can be at rest. But what is
said of the axis of motion relates only to circular motion ; that is, to motion
round a line, and not to that which a body may have at the same time
in any other direction.

There is one circumstance in circular motion, which must be carefully

Fig. 11.

attended to ; it is, that the further any
part of a body is from the axis of mo-
tion, the greater is its velocity : as you
approach that line, the velocity of the
parts gradually diminishes till you reach
the axis of motion, which is perfectly at
rest. The extremities of the vanes of a
windmill move over a much greater
space than the parts nearest the axis
of motion (Jig. 11). The three dotted
circles describe the paths in which three
different j>arts of the vanes move, and
though the circles are of different dimen-
sions, the vanes describe each of them
in the same space of time.
The force which confines a body to a centre, round which it moves,
is called the centripetal force ; and the force which impels a body
to fly from the centre, is called the centrifugal force: in circular
motion, these two forces balance each other ; otherwise the revolv-
ing body would either approach the centre or recede from it, ac-
cording as the one or the other prevailed. And should any cause
destroy the centripetal force, the centrifugal force would impel the body

to fly off from the centre. It would not,
however, fly off in a right line from the
centre ; but in a right line in the direction
in which it was moving at the instant of
its release : if a stone, whirled round in a
sling, gets loose at the point A (Jig. 12), it
flies off in the direction A B : this line is
called a tangent; it touches the circumfe-
rence of the circle, and forms a right angle
with a line drawn from that point of the

INTRODUCTION TO MECHANICS. xvii

circumference to the centre of the circle C. This force would, there-
fore, with more propriety be called the tangential than the centrifugal
force, or rather, the inertia of the body which inclines it to move in
the direction of the tangent is the tangential force. But motion in
the direction of the tangent would remove the body farther from the
centre ; a tendency, therefore, to such motion is a tendency to leave
the centre, and that part of its force which tends to produce motion thus
away from the centre is called the centrifugal force.

If a ball be thrown in an horizontal direction, it is acted upon by no
less than three forces: the force of projection first given to it; the
resistance of the air through which it passes; and the force of gravity,
which finally brings it to the ground. Gravity and the resistance of the
air act continually ; and as the whole effect produced by them is always
so great as to overpower any force of projection we can communicate
to a body, the latter is gradually overcome, and the body brought to the
ground ; but the stronger the projectile force, the longer will these powers
be in subduing it. A shot fired from a cannon, for instance, will go
much further than a ball thrown by the hand. Bodies thus projected
describe a curve line in their descent. If the forces of projection and of
gravity both produced uniform motion, the ball would move in the
diagonal of a parallelogram, but the motion produced by the force of pro~
jection alone is uniform, that produced by gravity is accelerated; and it is
this acceleration which brings the ball sooner to the ground, and makes it
fall in a curve instead of a straight line (see fig. 13). If a ball at
A be projected, in a horizontal direction, with a force capable of carrying
it to F (which we will suppose to be 100 feet) in a second, then, if it were
not acted upon by gravity, it would proceed from F to G, another 100
feet, in the next second, and the same distance G H in a third, and H I
in a fourth second. Now, if the ball, when at A, be allowed to fall, by
the force of gravity alone, from A towards E, it will fall 16 feet to B
during the first second* ; then three times as much, or 48 feet the next
second ; and five times as much, or 80
feet, in the third second ; and seven
times as much, or 1 12 feet, in the fourth
second. Then, in order to find the line
in which the ball will move, by the united
forces of projection and gravity, we must
draw a line B K parallel to the horizontal
line A F, and 16 feet below it ; then
another line C L, also parallel, at the dis-
tance of 48 feet more; then another line,
D M, 80 feet further; then another, EN, 112 feet further. Then, at the
end of the first second the ball will be at K, at the same distance from
B as F is from A ; at the end of the next second it will be at L, the
same distance from C that G is from A ; at the end of the third second
it will be at M ; and at the end of the fourth second at N ; and thus you
see the curve line A K L M N is described in its fall, instead of a straight
line, which would be the case if A B, B C, C D, D E, were all equal.

We have not taken notice of the resistance of the air, which much
complicates these results in practice. The principles of its operation
may easily be understood from the mode in which the other forces act ;
but the degree and manner in which it modifies their effects cannot be
shown without much difficulty and intricacy of explanation. It is, how-

* Sec page x.

C

Fig. 13.

C H Ct V ***

^-— — <,

ft

(3
i>

^^

K.

/

I*

/

M

xviii INTRODUCTION TO MECHANICS.

ever, sufficiently plain that this resistance increases with the velocity of
the ball, for the particles of air re-act on the ball in proportion to the
stroke they receive from it ; so that if the force of projection be doubled,
the resistance of the air is doubled also : nor is this all, for in doubling
the velocity of the ball, it passes through twice the quantity of air in the
same time, arid receives twice the resistance from each particle ; the whole
of the resistance must, therefore, be four times as great as in the first
instance. And if the velocity of the ball be tripled, it will pass through
three times the quantity of air ; will strike each particle with three times
the force, and receive three times the re- action ; which summed up will
make nine times the resistance.

The shortest mode of calculating the resistance is to multiply the
velocity by itself; thus, if the velocity be three, multiply it by three, and
the product will be nine. The product of a number multiplied by itself
is called its square.

The curve-line which a ball describes, if the resistance of the air be not
taken into consideration, is called in geometry a parabola. But when the
ball is thrown perpendicularly upwards, it will descend perpendicularly ;
because the force of projection, and that of gravity, are in the same line
of direction.

We have noticed the centres of magnitude and of motion, but we have
not yet explained what is meant by the centre of gravity. It is that point
about which all the parts of a body exactly balance each other, in every
position of the body ; if, therefore, that point is supported, the body will
not fall. Were any other point of the body alone supported, the surround-
ing parts no longer balancing each other, the body would fall on the side
at which the parts are heaviest; therefore, whenever the centre of gravity
is unsupported, the body must fall. This sometimes happens with an over-
loaded waggon winding up a steep hill, one side of the road being more
elevated than the other: let us suppose it to slope as described in jig. 14.
We will suppose that the centre of gravity of this loaded waggon is at the
point A. Now the eye will tell you, that a
waggon thus situated will overset ; and the reason Fig. 14.

is, that the centre of gravity, A, is not supported ;
for if a perpendicular line be drawn from it to the
ground at C, it does not fall under the waggon
within the wheels, and is, therefore, not supported
by them. A perpendicular line thus drawn from
the centre of gravity to the earth, is called the line
of direction. Let us in imagination take off the
upper part of the load ; the centre of gravity will
then change its situation, and descend to B, as
that will now be the point about which the parts of the less heavily laden
waggon will balance each other ; and the waggon will no longer upset,
for a perpendicular line from that point will fall within the wheels at D,
and be supported by them. You have heard that it is dangerous, when a
boat is in any risk of being upset, for the passengers to rise suddenly ;
this is owing to their raising the centre of gravity, and thus increasing the
chance of throwing it out of the line of direction. When a man stands
upright, the centre of gravity of his body is supported by the feet. If he
lean on one side, he will no longer stand firm. A rope-dancer performs
all his feats of agility, by dexterously supporting his centre of gravity;
whenever he finds himself in danger of losing his balance, he shifts the
heavy pole, which he holds in his hands, in order to throw the weight

INTRODUCTION TO MECHANICS.

xix

towards the side that is deficient ; and thus by changing the situation of
the centre of gravity, restores his equilibrium.

A stick is poised on the tip of the finger, by supporting its centre of
gravity, and it is for want of this support that spherical bodies roll down
a slope. A sphere being perfectly round, can touch the slope but by a
single point, and that point is not perpendicularly under the centre of
gravity, which therefore is not supported. The centre of gravity in this
case coincides with the centre of magnitude, but when one part of the
body is composed of heavier materials than another part, the centre of
gravity, being the centre of the weight of the body, will generally no longer
correspond with the centre of magnitude, though it may accidentally do so.
We defined the centre of gravity to be that point about which all
the parts of a body balance each other : you must consider it as
an abstract point, since there are cases in which it may be situated
at some distance from the body. Such, for instance, is the centre
of gravity of a ring, which is situated in the centre of the space which the
ring encircles ; and that point cannot be supported unless the ring be
held so that the line of direction will fall within the base of the support,
which will be the case, either if you poise the ring on the tip of your
Fi°: 15. finger, or if you suspend it by a string, as in Jig. 15.

If a body be suspended by that point in which the
centre of gravity is situated, it will remain at rest in
any position indifferently ; but if it be suspended by
any other point, it can rest only in the two following
positions : — Either when the centre of gravity is either
exactly above or below the point of suspension, so
that the point of suspension shall be in the line of
direction.

Bodies having a narrow base are easily upset, for if they are the least
inclined, their centre is no longer supported, as you may perceive \r\fig. 16.

\

Fig. 16.

A person carries a single pail of water with great
difficulty, owing to the centre of gravity being
thrown on one side, and the opposite arm is
stretched out to endeavour to bring it back to its
original situation. But two pails, one hanging
on each arm, are carried with much greater facility,
because they balance each other.
When two bodies are fastened together by a line, string, chain, or any
power whatever, they are to be considered as forming but one body. If
the two bodies be of equal weight, the centre of gravity will be in the
middle of the line which unites them (Jig. 17), but if one be heavier
than the other, the centre of gravity will be proportionably nearer the
heavy body than the light one (fig. 18). Were you to carry a rod or

Fig. 18.

Fig. 19.

c 2

xx INTRODUCTION TO MECHANICS.

pole with an equal weight fastened at each end of it, you would hold it in
the middle of the rod, in order that the weights should balance each other ;
whilst if it had unequal weights at each end, you would hold it nearest
the greater weight, in order to make them balance each other ; and if one
were very considerably larger than the other, the centre of gravity would
be thrown out of the rod into the heaviest weight (fig. 19).

SECTTON III. — On the Mechanical Powers.

WE will now proceed to examine the mechanical powers. They are six
in number, one or more of which enters into the composition of every
machine. The lever, the pulley, the wheel and axle, the inclined plane,
the wedge, and the screw.

In order to understand the power of a machine, there are four things to
be considered. 1st. The power that acts : this consists in the effort of
men or horses, of weights, springs, steam, &c.

2dly. The resistance which is to be overcome by the power; this is
generally a weight to be moved. The effect of the power, acting in
the manner in which in each particular case it is applied, must always
be superior to the resistance, otherwise the machine could not be put
in motion. For instance, were the resistance of a carriage equal to
the strength of the horses employed to draw it, they would not be able to
make it move.

3dly. We are to consider the centre of motion, or, as it is termed in
mechanics, the fulcrum, which means a prop ; this is the point about
which all the parts of the body move : and, lastly, the respective velocities
of the power, and of the resistance.

We shall first examine the power of the lever. The lever is an in-
flexible rod or beam, that is to say, one which will not bend in any direc-
tion. For instance, the steel rod to which a pair of scales is suspended is
a lever, and the point by which it is suspended, called the prop or fulcrum,
is also the centre of motion. The two parts of a lever divided by the
fulcrum are called its arms. Now, both scales being empty they are of
the same weight, and consequently balance each other (Jig. 20). We
have stated that when two bodies of equal weight were fastened together
the centre of gravity would be in the middle of the line that connected
them; the centre of gravity of the scales must, therefore, be in the middle
between them, as the fulcrum is, and, this being supported, the scales
balance each other.

You recollect, that if a body be suspended by that point in which the
centre of gravity is situated, it will remain at rest in any position indif-
ferently ; which is not the case with this pair of scales, for when we hold
them inclined, they instantly regain their equilibrium ; the reason of this
is, that the centre of suspension, instead of exactly coinciding with that
of gravity, is a little above it ; if, therefore, the equilibrium of the scales
be disturbed, the centre of gravity moves in a small circle round the
point of suspension, and is therefore forced to rise, and the instant it is
restored to liberty it descends and resumes its situation immediately below
the point of suspension, when the equilibrium is restored. It is this
property which renders the balance so accurate an instrument for weighing
goods. If the scales contain different weights, the centre of gravity will
be removed towards the scale which is heaviest, and being no longer
supported the heaviest scale will descend. The fulcrum of the balance
is moveable ; the lever may be taken off the prop and fastened on in

INTRODUCTION TO MECHANICS. xxi

Fig. 20. Fig. 21. Fig. 22.

another point which then becomes the fulcrum. In this case the equilibrium
is destroyed ; the longest arm of the lever is heaviest and descends. The
centre of gravity is not supported because it is no longer immediately below
the point of suspension ; but if we can bring the centre of gravity imme-
diately below that point, as it is now situated, the scales will again balance
each other. Now if a heavy weight be placed in the scale suspended to the
shortest arm of the lever, and a lighter one into that suspended to the longest
arm, the equilibrium will be restored. It is not, therefore, impracticable to
make a heavy body balance a light one ; and by this means an imposition
in the weight of goods might be effected, as a weight often or twelve
ounces might thus be made to balance a pound of goods. An ingenious
balance called a steelyard has been invented on the principle that a
weight increases in effect in proportion to its distance from the fulcrum.
In this machine a single pound weight, for instance, answers the purpose
of weighing any quantity of goods, simply by moving it along the lever ;
for, in proportion as it recedes from the fulcrum, it will balance five, ten,
twenty, or- perhaps even lOOlbs. weight. The hook by which the instru-
ment is suspended, forms the fulcrum ; it is, for instance, two inches distant
from the basin which is to contain the articles to be weighed, while the
opposite arm of the lever extends two feet ; a small weight is suspended
to it, and the graduations on the lever indicate the different powers of
this weight according- to the situation which it occupies on the long arm
of the lever ; when pushed to the extremity, a weight of 51bs. is equiva-
lent to 60 Ibs. placed in the basin. The same steelyard, when suspended
by a second hook, which divides the lever with less inequality, and corre-
sponds with another scale of graduation, is used for weighing smaller
quantities of goods, and the same weight, when hung at the extremity,
may be equal only to 20 Ibs. placed in the basin.

Let us now return to the balance (Jig- 22), and divesting it of the
basins, consider the lever simply. In this state the fulcrum is no longer
in the line of direction of the centre of gravity, but it is and must ever be
the centre of motion, as it is the only point which remains at rest while
the other parts move about it. When a lever is put in motion the longest
arm, or acting part of the lever must move with greater velocity than the
shortest arm, or resisting part of the lever, because it is furthest from the
centre of motion. When two boys ride on a plank drawn over a log of
wood, the plank becomes a lever, the log which supports it the fulcrum,
and the two boys the power and the resistance at each end of the lever.
When the boys are of equal weight the plank must be supported in the
middle to make the two arms equal ; — if they differ in weight, the plank
must be drawn over the prop so as to make the arms unequal, and the
lightest boy be placed at the extremity of the longest arm, in order that
the greater velocity of his motion may compensate for the superior gravity
of his companion, so as to render their momentums equal. But we know
that the action of the power must be greater than the resistance in

xxii INTRODUCTION TO MECHANICS.

order to put a machine in motion. For this purpose each boy at his
descent touches the ground with his feet, and the support he receives
from it diminishes his weight and enables his companion to raise him ;
thus each boy alternately represents the power and the weight, and the
two arms alternately perform the function of the acting and the resisting
part of the lever.

A lever, in moving, describes the arc of a circle, for it can move only
around the fulcrum or centre of motion. It would be impossible for one
child to rise perpendicularly to the point A (fig. 23), or for the other to

Fig. 23.

descend in a straight line to B, they each describe arcs of their re-
spective circles ; and you may judge from the different dimensions of the
circle how much greater the velocity of the little child must be than that of
the bigger one. Enormous weights may be raised by levers of this
description, for the longer the acting part of the lever in comparison to
the resisting part, the greater is the effect produced by it ; because the
greater is the velocity of the power compared to that of the weight.
You have seen a heavy snow-ball rolled over (fig. 24) by thrusting the
end of a strong stick beneath the ball, and resting it against a log of
Fig. 24. wood, or any object which can give it support,

near the end in contact with the snow-ball ? The
stick, in this case, is a lever ; the support, the prop
or fulcrum ; and the nearer the latter is to the re-
sistance, the more easily will the power be able to
move it.

There are three different kinds of levers : in the first, which compre-
hends the several levers we have described, the fulcrum is between the
power and .the weight. When the fulcrum is situated equally between the
power and the weight, as in the balance, the power must be greater than
the weight, in order to move it ; for nothing can in this case be gained
by velocity. The two arms of the lever being equal, the velocity of
their extremities must be so likewise. The balance is therefore of no
assistance as a mechanical power, but it is extremely useful to estimate
the respective weights of bodies. But when the fulcrum F of a lever
(fig. 25) is not equally distant from the power and the weight, and that
in amount, though greater in effect,the power P acts at the extremity of the
longest arm, it may be less than the weight W, its deficiency being com-

INTRODUCTION TO MECHANICS. xxiii

pensated by its superior velocity ; as we

^g* 25. observed in the see-saw. Therefore when

r ~~2\ a great weight is to be raised, it must

be fastened to the shortest arm of a lever,

^p and the power applied to the longest

"W arm; but if the case will admit of putting

the end of the lever under the weight,

no 'fastening will be required, as you may perceive by stirring the fire.
The poker is a lever of the first kind ; the point where it rests against the
bars of the grate, whilst stirring the fire, is the fulcrum ; the short arm, or
resisting part of the lever, is employed in lifting the weight, which is
the coals, and the hand is the power applied to the longest arm, or
acting part of the lever. A pair of scissars is an instrument composed
of two levers, united in one common fulcrum ; the point at which the two
levers are screwed together is the fulcrum; the handles, to, which the
power of the fingers is applied, are the extremities of the acting part of
the levers, and the cutting part of the scissars are the resisting parts of
the levers ; therefore the longer the handles, and the shorter the points of
the scissars, the more easily will they cut. Thus when pasteboard, or any
hard substance is to be cut, that part of the scissars nearest the screw or
rivet, is used. Snuffers, and most kinds of pincers, are levers of a similar
description, the great force of which consists in the resisting part of the
lever being very short in comparison of the acting part.

In levers of the second kind, the weight, instead of being at one end,
is situated between the power and the fulcrum (Jig. 26). In moving it,
the velocity of the power must necessarily be greater than that of the
weight, as it is more distant than the centre of motion. You may, perhaps,
have seen a snow-ball moved by means of a lever of the second
order, as well as by one of the first. The end of the stick (Jig. 27) that
is thrust under the ball rests on the ground, which becomes the

Fig. 26. Fig. 27.

fulcrum ; the ball is the weight to be moved, and the power the hands
applied to the other end of the lever. In this instance there is an
immense difference in the length of the arms of the lever, the weight being
almost close to the fulcrum, and the advantage gained is proportional.
Fishermen's boats are thus raised from the ground to be launched into
the sea, by means of slippery pieces of board, which are thrust under the
keel. The most common example that we have of levers of the second
kind is in the doors of our apartments : in these the hinges represent the
fulcrum, the hand, the power applied to the other end of the lever, and the
door, or rather its inertia, is the weight which occupies the whole of the
space between the power and the fulcrum. The whole weight and inertia
of the door may be regarded as collected into its centre of gravity; that
is to say, the resistance of the door is the same that would be offered by
a force equal to the inertia of the door, and passing through its centre
of gravity. Another very common instance is found in an oar : the blade

xxiv INTRODUCTION TO MECHANICS.

is kept in the same place by the resistance of the water, and becomes the
fulcrum ; the resistance is applied where the oar passes over the side of
the boat, and the hands at the handle are the power. Nut-crackers are
double levers of this kind : the hinge is the fulcrum, the nut the resist-
ance, and the hands the power.

In levers of the third kind (Jig. 28), the fulcrum is also at one of the
extremities, the weight or resistance at the other, and it is now the power

which is applied between the fulcrum
and the resistance. Thus the fulcrum,
the weight, and the power, each in it's
turn, occupies some part of the middle
3 of the lever between its extremities. But
in this third kind of lever, the weight being
|H further from the centre of motion than the
"W" power, the difficulty of raising it, instead of

being diminished, is increased. Levers of this description are used when
the object is to produce great velocity. The aim of mechanics, in general,
is to gain force by exchanging it for time ; but it is sometimes desirable
to produce great velocity by an expenditure of force. The treddle of a
common turning lathe affords an example of a lever of the third kind
employed in gaining time, or velocity, at the expense of force. A man,
in raising a long ladder perpendicularly against a wall, cannot place his
hands on the upper part of the ladder : the power, therefore, is neces-
sarily placed nearer the fulcrum than the weight, for the hands are the
power, the ground the fulcrum, and the ladder the weight, which, as in
the case of the door, may be considered as collected in the centre of
gravity of the ladder, about halfway up it, and consequently beyond the
point where the hands are applied. Nature employs this kind of lever
in the structure of the human frame. In lifting a weight with the hand,
the lower part of the arm becomes a lever of the third kind : the elbow
is the fulcrum ; the muscles which move the arm, the power ; and as
these are nearer to the elbow than the hand is, it is necessary that their
power should exceed the weight to be raised.

You may perhaps wonder that nature should have furnished us with
such levers, but the disadvantage is more than compensated by the con-
venience resulting from the structure of the arm. It is of more conse-
quence that we should be able to move our limbs nimbly, than that we
should be able to overcome great resistance ; for it is comparatively
seldom that we meet with great obstacles, and when we do, they can be
overcome by art. Besides, the Creator has endowed the muscular fibres
with prodigious strength, so that, upon the whole, this kind of lever is
best adapted to enable the arm to perform its various functions.

The pulley, which is the second mechanical power we are to examine, is
a circular flat piece of wood or metal, with a strino-
running in a groove round it, by means of which a
weight may be pulled up. Thus pulleys are used for
drawing up curtains, the sails of a ship, &c. When,
as in the examples alluded to, the pulley is fixed, it
does not increase the power to raise the weight. If P
represent the power, to raise the weight W (Jig. 29),
it is evident that the power must be greater than the
weight, in order to move it. A fixed pulley is useful,
therefore, only in altering the direction of the power,
and its most frequent practical application is to
make us to draw up a weight by drawing down the string con-

INTRODUCTION TO MECHANICS. xxv

nected with the pulley. But a moveable pulley affords mechanical as-
sistance (Jig. 30). The hand which sustains the cask by means of the
cord D E going; over the moveable pulley, does it more easily than it
it held the cask suspended to a cord without a pulley ; for the fixed
hook H, to which one end of the cord is fastened, bearing one half of
the weight of the cask, the hand has only the other half to sustain.
Now it is evident, that the hook affords the same assistance in
raising- as in sustaining the cask, so that the hand will have only one half
of the weight to raise. But observe that the velocity of the hand must be
'"- 30 double that of the cask ; for in order to raise the latter
one inch, the hand must draw the two strings (or rather
the two parts D and E into which the string is divided by
the pulley) one inch each ; the whole string being
shortened two inches, while the cask is raised only one.
Thus the advantage of a moveable pulley consists in
dividing the difficulty ; twice the length of string it is
true must be drawn, but only half the strengh is required
which would be necessary to raise the weight without such
assistance ; so that the difficulty is overcome in the same
manner as it would be by dividing the weight into two
equal parts, and raising them successively. The pulley,
therefore, acts on the same principle as the lever, the de-
ficiency of strength of the power being compensated by
its superior velocity ; and it is on this principle that all
mechanical power is founded. In the fixed pulley (Jig.
29) the line A C may be considered as a lever, and B the
fulcrum ; then the two arms A B and B C being equal, the lever will
afford no aid as a mechanical power ; since the power must be equal to
the weight in order to balance it, and superior to the weight in order to
raise it. In the moveable pulley (fig. 30) you must consider the point
A as the fulcrum ; A B or half the diameter of the pulley as the shortest
arm ; and A C or the whole diameter as the longest arm. It may,
perhaps, be objected to pulleys that a longer time is required to raise a
weight with their aid than without it : that is true, for it is a fundamental
law in mechanics, that what is gained in power is lost in time : this
Fie 31 applies not only to the pulley but to the lever and all the
other mechanical powers. It would be wrong, however, to
suppose that the loss was equivalent to the gain, and that
we derived no advantage from the mechanical powers ; for
since we are incapable of augmenting our strength, that
science is of wonderful utility which enables us to reduce the
resistance or weight of any body to the level of our strength.
This we accomplish, by dividing the resistance of a body
into parts which we can successively overcome ; and if it
require a sacrifice of time to attain this end, you must be
sensible how very advantageously it is exchanged for power.
The greater the number of pulleys connected by a string,
the more easily the weight is raised, as the difficulty is
divided amongst the number of strings, or rather of parts
into which the string is divided by the pulleys. Several
pulleys thus connected form what is called a system, or
tackle of pulleys (fig. 31). You may have seen them sus-
pended from cranes to raise goods into warehouses, and in
ships to draw up the sails. Here both the advantages of
an increase of power and a change of direction are united j

XXVI

INTRODUCTION TO MECHANICS.

for the sails are raised up the masts by the sailors on deck from the
change of direction which the pulley effects ; and the labour is faci-
litated by the mechanical power of a combination of pulleys. Pullies
are frequently connected, as described inj%. 32, both for nautical and
a variety of other purposes; but in whatever manner pulleys are con-
nected by a single string the mechanical power is the same in its
principle.

The third mechanical power is the wheel and axle. Let us suppose
(fig. 33) the weight W to be a bucket of water in a well, which is to be
raised by winding the rope, to which it is attached, round the axle : if this

Fig. 32.

Fig. 33.

be done without a wheel to turn the axle, no mechanical assistance is
received. The axle without a wheel is as impotent as a single fixed
pulley, or a lever, whose fulcrum is in the centre ; but add the wheel to
the axle, and you will immediately find the bucket is raised with much
less difficulty. The axle acts the part of the shorter arm of the lever, the
wheel that of the longer arm. The velocity of the circumference of the
wheel is as much greater than that of the axle, as it is further from the
centre of motion ; for the wheel describes a large circle in the same space

Fig. 34.

of time that the axle describes a small
one, therefore the power is increased
in the same proportion as the circum-
ference of the wheel is greater than
that of the axle. If the velocity of the
wheel were twelve times greater than
that of the axle, a power nearly twelve
times less than the weight of the bucket
would be able to raise it. Instead
of a wheel there is commonly attached
to the axle only a crooked handle,
which answers the same purpose
(Jig. 34). For the branch of the handle
A, which is united to the axle, repre-
sents the spoke of a wheel, and is as
effectuaFas an entire wheel ; the other branch, B, affords no mechanical
aid, merely serving as a handle ,to turn the wheel. Wheels are a very

INTRODUCTION TO MECHANICS. xxvii

essential part of most machines. They are employed in various ways ;
but, when fixed to the axle, their mechanical power is always the same ;
that is, as the circumference of the wheel exceeds that of the axle, so
much will the energy of the power be increased. In mills and
manufactures, you must have admired the immense wheel, the revo-
lution of which puts the whole of the machinery into motion ; and though
so great an effect is produced by it, a horse or two has sufficient power to
turn it ; but a steam-engine is both the most powerful and the most conve-
nient mode of turning the wheel. We have the advantage sometimes of a
gratuitous force, such as the stream of water to turn a watermill ; and the
wind which turns the vanes of a windmill. One of the great benefits
resulting from the use of machinery is, that it gives us a sort of empire
over the powers of nature, and enables us to make them perform that
labour which would otherwise fall to the lot of man. When a current of
wind, a stream of water, or the expansive force of steam performs our task,
we have only to superintend and regulate their operations.

The fourth mechanical power is the inclined plane. This is nothing
more than a slope, or declivity, frequently used to facilitate the drawing
up of weights. It is not difficult to understand, that a weight may with
much greater ease be drawn up a slope than it can be raised the same
height perpendicularly. But in this, as well as the other mechanical
powers, the facility is purchased by a loss of time (Jig. 35) ; for the weight,
instead of moving directly from A to C, must move from. B to C, and as
the height of the plane is to its length, so much is Ihe resistance of the
weight diminished. Thus, if a pulley be fixed at F, so that the string
from F to W may be parallel to B C, and a string fixed to the weight
W were connected with another weight P ; then if P bear the same pro-
portion to W that the line A C does to the line B C, the two weights
will balance each other, a considerable portion of the weight W being
supported by the plane B C, and only the residue by the power P.

The wedge, which is the next mechanical power, is composed of two
inclined planes (j%. 36) : you may have seen woodcutters use it to

Fig. 36.
Fig. 35.

cleave wood. The resistance consists in the cohesive attraction of the
wood, or any other body which the wedge is employed to separate ; and
the advantage gained by this power is in the proportion of half its
width to its length. The wedge, however, acts principally by being struck,
and not by mere pressure : the proportion stated is that which expresses
its power when acting by pressure only.

All cutting instruments are constructed upon the principle of the in-
clined plane, or the wedge. Those that have but one edge sloped, like
the chisel, may be referred to the inclined plane ; whilst the axe, the
hatchet, and the knife (when used to chop or split asunder), act on the
principle of the wedge. But a knife cuts best when drawn across the
substance it is to divide, as it is used in cutting meat, for the edge of a

xxviii INTRODUCTION TO MECHANICS.

knife is really a very fine saw, and therefore acts best when used like that
instrument.

The screw, which is the last mechanical power, is more complicated
than the others (fig. 37). It is composed of two parts, the screw and
the nut. The screw S is a cylinder, with a spiral protuberance coiled
round it, called the thread ; the nut N is perforated to contain the screw ;
and the inside of the nut has a spiral groove, made to fit the spiral thread
of the screw; just like the lid of a box which screws on. The handle
which projects from the nut is a lever, without which the screw is never
used as a mechanical power. The nut, with a lever L attached to it, is
commonly called a winch. The power of the screw, complicated as it
appears, is referable to one of the most simple of the mechanical powers,
the inclined plane. If a slip of paper be cut in the form of an inclined
plane, and wound round a pencil, which will represent the cylinder, it
will describe a spiral line corresponding to the spiral protuberance of the
screw (fig. 38). The nut then ascends an inclined plane, but ascends it
in a spiral instead of a straight line. The closer the thread of the screw

Fig. 38.

\\\\\\\\\

the more easy is the ascent, but the greater are the number of revolutions
the winch must make ; so that we return to the old principle, — what is
saved in power, is lost in time. The power of the screw may be increased,
also, by lengthening the lever attached to the nut; it is employed either
for compression or to raise heavy weights. It is used in cider and wine
presses, in coining, book-binding, and for a variety of other purposes.

All machines are composed of one or more of the six mechanical powers
we have examined. One more remark must be made relative to
them, which is, that friction in a considerable degree diminishes their
force. Friction is the resistance which bodies meet with in rubbing
against each other. There is no such thing as perfect smoothness or
evenness in nature. Polished metals, though they wear that appearance,
more than any other bodies, are far from really possessing it ; and their
inequalities may frequently be perceived through a good magnifying glass.
When, therefore, the surfaces of the two bodies come into contact, the pro-
minent parts of the one will often fall into the hollow parts of the other, and
occasion more or less resistance to motion. In proportion as the surfaces
of bodies are well polished, the friction is diminished; but it is always
considerable, and it is usually computed to destroy one-third of the power
of a machine. Oil or grease is used to lessen friction : it acts as a polish
by filling up the cavities of the rubbing surfaces, and thus making them
slide more easily over each other. It is for this reason that wheels are
greased, and the locks and hinges of doors oiled. In these instances the

INTRODUCTION TO MECHANICS. xxix

contact of the rubbing- surfaces is so close, and the rubbing- so continual,
that, notwithstanding their being polished and oiled, a considerable degree
of friction is produced. It is a remarkable circumstance, that there is
generally less friction between two bodies of different substances, than of
the same. It is on this account that the holes in which the spindles of
watches work, are frequently made of jewels ; and that when two cog-
wheels work in one another, the cogs of the one are generally made of
wood, and of the other of metal.

There are two kinds of friction ; the one occasioned by the sliding
of the flat surface of a body, the other by the rolling of a circular
body. The friction resulting from the first is much the most consi-
derable ; for great force is required to enable the sliding body to over-
come the resistance which the asperities of the surfaces in contact op-
pose to its motion, and it must be either lifted over, or break through
them ; whilst, in the other kind, the friction is transferred to a smaller
surface, and the rough parts roll over each other with comparative facility ;
hence it is, that wheels are often used for the sole purpose of diminishing
the. resistance of friction. When, in descending a steep hill, we fasten
one of the wheels, we decrease the velocity of the carriage by increasing
the friction, that is to say, by converting the rolling friction of one of
the wheels into the dragging friction ; and when casters are put to the
legs of a table the dragging is converted into the rolling friction.

The great fly-wheel which is frequently attached to steam-engines and
other large machines, acts in the first instance as a heavy weight to im-
pede their free uncontrolled motion. However paradoxical this mode of
improving machinery may appear, it is nevertheless of great advantage.
The motion of a machine is always more or less variable, owing to the
irregularity both of the power which works it, and of the resistance which
it has to overcome. Whether the power consists in wind, water, steam, or
the strength of animals, it cannot be made to act with perfect regularity,
nor can the work which the machine has to perform be always uniform.
Yet in manufactures, and most cases in which machinery is employed,
uniformity of action is essentially requisite, both in order to prevent injury
to the machine, and imperfection in the work performed. A fly-wheel,
which is a large heavy wheel attached to the axis of one of the principal
wheels of the machinery, answers this purpose, by regulating the action
of the machine : by its weight it diminishes the effect of increased action,
and by its inertia it carries on the machine with uniform velocity when the
power transiently slackens; thus by either checking or impelling the
action of the machine, it regulates its motion so as to render it tolerably
uniform.

There is another circumstance which we have already noticed as
diminishing the motion of bodies, and which greatly affects the power of
machines : this is the resistance of the medium in which a machine is
worked. All fluids, whether of the nature of air, or of water, are called
mediums ; and their resistance is generally proportioned to their density:
for the more matter a body contains, the greater the resistance it will
oppose to the motion of another body striking against it. It is therefore
more difficult to work a machine under water than in the air. Jf a machine
could be worked in vacua, and without friction, it would be perfect ; but
this is unattainable. A considerable reduction of power must, therefore,
be allowed for the resistance of the air.

INTRODUCTION TO ASTRONOMY.

SECTION I. — The Earth's Annual Motion.

JN attempting to give some general notions on astronomy, we shall not
begin by entering into an explanation of the system of the celestial bodies,
but select that portion which is most interesting to us, the earth ; and when
you have formed a distinct idea of the part which it performs in the
general | system, lead you from thence to form some conception of the
grandeur and immensity of the universe. Let us suppose the earth at its
p. , creation to have been projected

forwards into universal space.
We know that if no obstacle
impeded its course it would
proceed in the same direction,
and with a uniform velocity for
ever. In Jig. 1, A represents
the earth, and S the sun. We
shall suppose the earth to be
arrived at the point in which it
is represented in the figure,
having a velocity which would
carry it on to B in the space of
one month ; whilst the sun's at-
traction would bring it to C
in the same space of time. According to the laws of motion you would
conclude that the earth would move in the diagonal A D of the paral-
lelogram A B D C as a ball acted on by two forces will do. But it
must be observed that the force of attraction is continually acting upon
our terrestial ball, and producing an incessant deviation from its course
in a right line, which converts it into that of a curve line ; every point of
which may be considered as constituting the diagonal of an infinitely
small parallelogram.

Let us detain the earth a moment at the point D, and consider how it
will be affected by the combined action of the two forces in its new
situation. It still retains its tendency to fly off in a straight line ; but a
straight line would now carry it away to F, whilst the sun would attract it
in the direction D S. In order to know exactly what course the earth
will follow, another parallelogram must be drawn in the same manner as
the first ; the line D F describing the force of projection, and the line
D S that of attraction ; and you will find that the earth will proceed in the
curve line D G drawn in the parallelogram D F G E : ancl if we go on
throughout the whole of the circle drawing a line from the earth to the
sun to represent the force of attraction, and another at a right angle to
it, to describe that of projection, we shall find that the earth will proceed
in a curve line passing through similar parallelograms till it has com-
pleted the whole of the circle. It will then recommence a course, which it
has pursued ever since it first issued from the hand of its Creator, and
which there is every reason to suppose it will continue to follow as long
as it remains in existence. It affords an example, on a magnificent scale,
of the circular motion which was taught in mechanics. The attraction of

Fig. 2.

INTRODUCTION TO ASTRONOMY. xxxi

the sun is the centripetal force, which confines the earth to a centre ; and
the impulse of projection the force which impels the earth to quit the sun
and fly off in a tangent, and which, therefore, by the inertia of the body,
produces the centrifugal force.

A simple mode of illustrating the effect of these combined forces on the
earth is to cut a slip of card in the form of
a right angle (Jig. 2), to describe a small circle
at the angular point representing the earth,
arid to fasten the extremity of one of the legs of
the angle to a fixed point, which we shall con-
sider as the sun. Thus situated, the lines
forming the angle will represent both the forces
which act upon the earth ; and if you draw it
round the fixed point, you will see how the di-
rection of the force which opposes the centri-
petal force varies, constantly forming a tangent
to the circle in which the earth moves, as it
is constantly at a right angle with the centri-
petal force. You will naturally conclude that if the two forces which
produce this circular motion had not been so accurately adjusted,
one would ultimately have prevailed over the other, and we should
either have approached so near the sun as to have been burnt, or
have receded so far from it as to have been frozen. But we have de-
scribed the earth as moving in a circle, merely to render the explanation
more simple, for in reality these two forces are not so proportioned as to
produce circular motion ; and the earth's orbit, or path which it describes
round the sun, is not circular but elliptical or oval.

Let us suppose that when the earth is at A (fig. 3) its projectile force

Fig. 3.

does not give it a velocity sufficient
to counterbalance that of gravity, so
as to enable these powers conjointly
to carry it round the sun in a circle ;
the earth instead of describing the
line A C, as in the former figure,
will approach nearer the sun in the
line A B. Under these circum-
stances, it will be asked what is to
prevent our approaching nearer and
nearer the sun till we fall into it ; for
its attraction increases as we advance
towards it. There also seems to be
another danger. As the earth ap-
proaches the sun, the direction of its
motion is no longer perpendicular to
that of attraction, but inclines more
nearly to it. When the earth reaches that part of its orbit at B, the
force of projection would carry it to D, which brings it nearer the
sun instead of bearing it away from it; so that being driven, by
one power, and drawn by the other towards this centre of destruc-
tion, it would seem impossible for us to escape. But nature abounds in
resources. The earth continues approaching the sun with an accelerated
motion, till it reaches the point E : the projectile force now impels it
in the direction E F. Here then the two forces act perpendicularly
to each other, and the earth is situated as in the preceding figure, yet it

xxxii INTRODUCTION TO ASTRONOMY.

will not revolve round the sun in a circle for the following reason. The
centrifugal force increases with the velocity of the body, or in other words,
the quicker it moves the stronger is its tendency to fly off in a right line.
When the earth arrives at E, its accelerated motion will have so far
increased its velocity and consequently its centrifugal force, that the latter
will prevail over the force of attraction, and drag the earth away from
the sun till it reaches G. It is thus that we escape from the dangerous
vicinity of the sun ; and as we recede from it, both the force of its attrac-
tion, and the velocity of the earth's motion diminish. From G the direc-
tion of projection is towards H, that of attraction towards S, and the
earth proceeds between them with a retarded motion, till it has com-
pleted its revolution. Thus the earth travels round the sun, not in a
circle, but an ellipsis, of which the sun occupies one of the foci ; and in
its course the earth alternately approaches and recedes from it, so that
what at first appeared to be a dangerous irregularity, is the means by
which the most perfect order and harmony are produced. The earth
then travels on at a very unequal rate, its velocity being accelerated as it
approaches the sun, and retarded as it recedes from it.

Now it is mathematically demonstrable, that when a body moves round
Fig. 4. a Pomt towards which it is attracted, the areas

included between the line it describes and the
lines joining its place at different instants to
the attracting point, are equal in equal times.
The whole of the space contained within the
earth's orbit is (Jig. 4), divided into a number
of areas, or spaces, 1, 2, 3, 4, &c., all of which
are of equal dimensions, though of very differ-
ent forms ; some of them are long and nar-
row, others broad and short ; but they each
of them contain an equal quantity of space.
An imaginary line drawn from the centre of
the earth to that of the sun, and keeping pace
with the earth in its revolution, passes over
equal areas in equal times : that is to say, if it
is a month going from A to B, it will be a month going from B to C,
and another from C to E, and so on.

The inequality is not in fact so considerable as appears in figure 4 ; for
the earth's orbit is not so eccentric as it is there described ; and, in reality,
differs but little from a circle. That part of the earth's orbit nearest the
sun is called its perihelion, that part most distant from the sun its aphelion ;
and the earth is about three millions of miles nearer the sun at its peri-
helion than at its aphelion. You will learn with surprise that during the
height of our summer, the earth is in that part of its orbit which is most
distant from the sun, and it is during the severity of winter that it
approaches nearest to it. The difference, however, of the earth's distance
from the sun in summer and winter, when compared with its total distance
from the sun, is but inconsiderable, for three millions of miles sinks into
insignificance in comparison of 95 millions of miles, which is our mean
distance from the sun. The change of temperature, arising from this
difference, would in itself scarcely be sensible, and it is completely over-
powered by other causes which produce the variations of the seasons ; but
the explanation of these must be deferred till we have made some further
observations on the heavenly bodies. Since the earth moves with greatest
velocity in that part of its orbit nearest the sun, it must complete its

INTRODUCTION TO ASTRONOMY. xxxiil

journey through one half of its orbit in a shorter time than through the
other half; and in fact, it is about seven days longer performing our
summer half of its orbit than the winter half. The planets are celestial
bodies which revolve round the sun, on the same principle, and they are
supposed to resemble the earth also in many other respects ; and we are
led by analogy to consider them as inhabited worlds.

Some of the planets are proved to be larger than the earth : it is only
their immense distance from us which renders their apparent dimensions
so small. Now, if we consider them as enormous globes, instead of
small twinkling spots, we shall find it most consistent with our ideas of
the Divine wisdom and beneficence, to suppose that these celestial bodies
should be created for the habitation of beings, who are, like us, blessed
by His providence. Hence, in a moral, as well as a physical point of view,
it is most rational to consider the planets as worlds revolving round the
sun ; and the fixed stars as other suns, each of them probably attended
by its system of planets, to which they respectively impart their in-
fluence. Our telescopes are brought to such a degree of perfection,
that from the appearances which the moon exhibits when seen through
them, we have probable reason to conclude that it is a habitable globe ;
its mountains and valleys are very perceptible, and some astronomers
imagine that they have seen volcanoes in it.

The planets which are supposed to revolve round the fixed stars must
of course be much smaller than the suns which give them light ; and
the distance which makes these suns appear to us like stars must render
their planets quite invisible : besides the light of these planets would
be much more feeble than that of the fixed stars ; there would be exactly
the same sort of difference as between the light of the sun and that of
the moon, the first being a fixed star, the second a planet.

According to the laws of attraction, the planets belonging to our
system all gravitate towards the sun ; and this force, combined with
that of projection, occasions their revolution round the sun, in orbits
more or less elliptical, according to the proportion which these two
forces bear to each other. But the planets have also another motion :
they revolve upon their axis. The axis of a planet is an imaginary
line which passes through its centre, and on which it turns ; and it
is this motion which produces day and night. With that side of the
planet facing the sun, it is day ; and with the opposite side, which
remains in darkness, it is night. Our earth, which we consider as a
planet, is 24 hours in performing one revolution on its axis ; in that
period of time, therefore, we have a day and a night. Hence this revo-
lution is called the earth's diurnal or daily motion ; and it is this revo-
lution of the earth from west to east which produces an apparent motion
of the sun, moon, and stars in the contrary direction.

SECTION II. — On the Planets.

THE Planets are distinguished into primary and secondary. Those which
revolve immediately about the sun are called primary. Many of these are
attended in their course by smaller planets, which revolve round them :
these are called secondary planets, satellites, or moons. Such is our
moon, which accompanies the earth, and is carried with it round the sun.
The sun is the general centre of attraction to our system of planets ; but
the satellites revolve round the primary planets, on account of their
greater proximity. The force of attraction is not only proportional to

xxxiv INTRODUCTION TO ASTRONOMY.

the quantity of matter, but to the degree of proximity of the attracting
body. This power being weakened by diffusion, diminishes as the squares
of the distances increase. The square is the product of a number multi-
plied by itself; so that a planet situated at twice the distance at which
we are from the sun would gravitate four times less than we do, the pro-
duct of two multiplied by itself being four. The more distant planets,
therefore, move slower in their orbits, for their projectile force must be
proportioned to that of attraction. This diminution of attraction by the
increase of distance also accounts for the motion of the secondary round
the primary planets, in preference to the sun ; for the vicinity of the
primary planets renders their attraction stronger than that of the sun.
But since tjje attraction between bodies is mutual, the primary planets are
also attracted by their satellites. The moon attracts the earth, as well as
the earth the moon ; but as the latter is the smaller body, her attraction
is proportionally less. The result is, that neither the earth revolves
round the moon, nor the moon round the earth ; but they both revolve
round a point, which is their common centre of gravity, and which is
as much nearer the earth than the moon, as the weight of the former
exceeds that of the latter. It has been already stated (p. xix.) that if
two bodies were fastened together by a wire or bar, their common centre
of gravity would be in the middle of the bar, provided the bodies were
of equal weight ; and if they differed in weight, it would be nearer the
larger body. Attraction is the tie which unites the earth and moon ;
and if these bodies had no projectile force which prevented their mutual
attraction from bringing them together, they would meet at their common
centre of gravity.

The earth then has three different motions : it revolves round the sun,
upon its axis, and round the point towards which the moon attracts it;
and this is the case with every planet which is attended by satellites. The
complicated effect of this variety of motions produces certain irregularities,
which, however, it is not necessary to notice at present. The planets act
on the sun in the same manner as they are themselves acted on by their
satellites ; but the gravity of the planets (even when taken collectively) is
so trifling compared with that of the sun, that they do not cause the latter
to move so much as one-half of its diameter. The planets do not, there-
fore, revolve round the centre of the sun, but round a point at a small
distance from its centre, about which the sun also revolves. The sun
also revolves on his axis. This motion is ascertained by observing certain
spots which disappear and re-appear regularly at stated times.

The great distance of the planets renders their motion apparently so
slow, that the eye is not sensible of their progress in their orbit, unless
we watch them for some considerable length of time : in different seasons
they appear in different parts of the heavens. The most accurate idea
which can be 'given of the situation and motion of the planets will be
by the examination of the diagram (fig. 5), representing the solar system,
in which the principal planets, with their orbits, are delineated. The sun
is in the common centre of the whole, but, to avoid confusion in the figure,
he is not represented.

The orbits of the planets are so nearly circular, and the common centre
of gravity of the solar system so near the centre of the sun, that these
deviations are not noticed in the diagram. The dimensions of the pla-
nets, in their true proportions, will be found delineated in Jig. 6 : the
signs annexed to them are those used to represent the planets, which are
also used in fig. 1.

INTRODUCTION TO ASTRONOMY

XXXV

Fig. 6.

TT 9 9

Mercury is the planet nearest the sun : his orbit is consequently con-
tained within ours ; but his vicinity to the sun occasions his being nearly
lost in the brilliancy of his rays ; and when we see this planet, the sun is
so dazzling, that very accurate observations cannot be made upon him.
He performs his revolution round the sun in about 87 days, which is
consequently the length of his year ; the time of his rotation ou his axis
is not accurately known ; his distance from the sun is computed to be
37 millions of miles, and his diameter 3, "224 miles. The heat of this planet
is so great, that water cannot exist there but in a state of vapour, and
metals would be liquefied.

D 2

xxxvi INTRODUCTION TO ASTRONOMY.

Venus, the next in the order of planets, is 68 millions of miles from the
sun ; she revolves about her axis in 23 hours and 21 minutes, and goes
round the sun in 224 days 17 hours. The diameter of Venus is 7,687
miles. The orbit of Venus is within ours ; during1 nearly one-half of
her course in it we see her before sunrise, and she is called the morning-
star ; in the corresponding part of her orbit, on the other side, she rises
later than the sun. We then cannot see her rising, as she rises in the
daytime ; but she also sets later ; so that we perceive her approaching the
horizon after sunset : she is then called Hesperus, or the evening-star. i

The planet next to Venus is the Earth, of which we shall soon speak at
full length 5 at present we shall only observe, that we are 95 millions of
miles distant from the sun — that we perform our annual revolution in
365 days, 5 hours, and 49 minutes — and are attended in our course by a
single moon.

Then follows Mars. He can never come between us and the sun, like
Mercury and Venus j his motion is, however, very perceptible, as he may
be traced to different situations in the heavens ; his distance from the sun
is 144 millions of miles ; he turns on his axis in 24 hours and 39 minutes ;
and he performs his annual revolution in about 687 of our days : his
diameter is 4,189 miles. Then follow four very small planets — Juno,
Ceres, Pallas, and Vesta, which have been recently discovered, but whose
dimensions and distances from the sun have not been very accurately
ascertained.

Jupiter is next in order. This is the largest of all the planets ; he is
about 490 millions of miles distant from the sun, and completes his annual
period in nearly twelve of our years ; he revolves on his axis in about ten
hours ; he is above 1,400 times as large as our earth, his diameter being
89,170 miles. The respective proportions of the planets cannot therefore,
you see, be conveniently delineated in a diagram. He is attended by
four moons.

The next planet is Saturn, whose distance from the sun is about 900
millions of miles. His diurnal rotation is performed in ten hours and a
quarter ; his annual revolution in nearly thirty of our years ; his dia-
meter is 79,000 miles. This planet is surrounded by a luminous ring, the
nature of which astronomers are much at a loss to conjecture : he has
seven moons.

Lastly, we observe the Georgium Siclus, discovered by Dr. Herschel,
and which is attended by six moons. His numerous moons are, however,
far from making so splendid an appearance as ours ; for they can reflect
only the light which they receive from the sun ; and both light and heat
decrease in the same ratio or proportion to the distances as gravity ; —
consequently Saturn, which is at nearly ten times the distance at which
we are from the sun, has a hundred times less heat and light. To us
such a climate would not be habitable ; but this furnishes no argument
against the supposition that these planets are worlds, peopled with beings
whose bodies are adapted to the various temperatures and elements in
which they are situated. Whether we judge from the analogy of our own
earth, or from that of the great and universal beneficence of Providence,
we may reasonably conjecture this to be the case : and an inhabitant of
Mercury might with as much plausibility pity us for the intense coldness
of our situation, or those of Jupiter and Saturn for our intolerable heat,
as we can draw any inferences against their existence, from the circum-
stance that we, constituted as we are, could not live there.

Comets are supposed to be planets. The re-appearance of some of

INTRODUCTION TO ASTRONOMY. xxxvii

them at stated times proves that they revolve round the sun ; but in orbits
so extremely eccentric, and running- to such a distance from the sun, that
they disappear for a great number of years. If they are inhabited, it must
be by a species of beings very different, not only from the inhabitants of
this, but from those of any of the other planets, as they must experience
the greatest vicissitudes of heat and cold : their heat in that part of their
orbit nearest the sun is computed to be greater than that of red-hot iron.
In this part of its orbit the comet emits a luminous vapour, called the tail,
which it gradually loses as it recedes from the sun ; arid the comet itself
totally disappears from our sight in the more distant parts of its orbit,
which, in most cases, extends considerably beyond that of the furthest
planet.

The number of comets belonging to our system cannot be ascertained,
as some of them are several centuries before they make their re-appear-
ance. The number that are known by their regular re-appearance is very
small.

The ancients, in order to recognise the fixed stars, formed them into
groups, to which they gave the names of the figures delineated on the
celestial globe. In order to show their proper situations in the heavens,
they should be painted on the internal surface of a hollow sphere, from
the^centre of which you should view them : you would then behold them
as they appear to be situated in the heavens. The twelve constellations,
called the Signs of the Zodiac, are those which are so situated, that the
earth in its annual revolution passes directly between them and the sun.
Their names are — Aries, the Ram ; Taurus, the Bull ; Gemini, the
Twins ; Cancer, the Crab ; Leo, the Lion ; Virgo, the Virgin ; Libra,
the Balance; Scorpio, the Scorpion; Sagittarius, the Archer; Capri-
cornus, the Wild Goat ; Aquarius, the Water-carrier ; Pisces, the Fishes :
the whole occupying a complete circle, or broad belt, in the heavens,
called the Zodiac (Jig. 7). Hence, a right line drawn from the earth,
and passing through the sun, would reach one of these constellations, and
the sun is said to be in that constellation at which the line terminates.
Thus when the earth is at A, the sun would appear to be in the constella-
tion or sign Aries ; when the earth is at B, the sun would appear in
Cancer ; when the earth was at C, the sun would be in Libra ; and
when the earth was at D, the sun would be in Capricorn. This circle,
in which the sun thus appears to move, and which passes through the
middle of the Zodiac, is called the Ecliptic.

We have no means of ascertaining the distance of the fixed stars.
When therefore they are said to be in the Zodiac, it is merely implied that
they are situated in that direction, and that they shine upon us through
that portion of the heavens which we call the Zodiac.

Whether the apparent difference, of size and brilliancy of the stars
proceed from their various degrees of remoteness, or of dimension, is a
point which astronomers are not enabled to ascertain. Considering them
as suns, we know no reason why they should not vary in size, as well as
the planets belonging to them.

It may perhaps be objected to this system of the universe, that it is
directly in opposition to the evidence of our senses, to which it is plain,
and obvious that the earth is motionless, and that the sun and stars revolve
round it. But our senses, or at least the inferences we draw from them,
too often mislead us, for us to place implicit reliance on them. When
sailing on the water with a very steady breeze, the houses, trees, and every
object appear to move, whilst we are insensible of the motion of the

xxxv lii

INTRODUCTION TO ASTRONOMY.

Fig. 7.

* '* * * *
* * * 4

vessel in which we sail. It is only when some obstacle impedes our
motion that we are conscious of moving ; and were you to close your eyes
while sailing on calm water, with a steady wind, you would not perceive
that you moved, for you could not feel it, and you could see it only by
observing the change of place of the objects on shore. So it is with the
motion of the earth: every thing on its surface, and the air that surrounds
it, accompanies it in its revolution — it meets with no resistance, therefore
we are insensible of motion.

The apparent motion of the sun and stars affords us the same proof of
the earth's motion that the crew of a vessel has of their motion from the
apparent motion of the objects on shore. Imagine the earth to be sailing
round its axis, and successively passing by every star, which, like the
objects on land, we suppose to be moving, instead of ourselves. In
balloons, the earth appears to sink beneath the balloon, instead of the
balloon rising above the earth.

It is a law which we discover throughout Nature, and worthy of its
great Author, that all its purposes are accomplished by the most simple
means. We have no reason to suppose this law infringed, in order that
our earth may remain at rest, while the sun and stars move round us :
their regular motions, which are explained by the laws of attraction on
the first supposition, would be unintelligible on the last, and the order and
harmony of the universe be destroyed. What an immense circuit the sun
and stars would make daily, were their apparent motions real ! We know
many of them to be bodies more considerable than our earth ; for our eyes
vainly endeavour to persuade us, that they are little brilliants sparkling in
the heavens, while science teaches us that they are immense spheres,

INTRODUCTION TO ASTRONOMY. xxxix

whose apparent dimensions are diminished by distance." If the heavenly
bodies revolved round our earth in twenty-four hours, the centrifugal force
implied in so rapid a motion would be quite destructive, and no power
can be assigned which would be sufficient to balance it ; grindstones
driven by machinery in manufactories have been known to fly in pieces
from their great velocity. Why then should these enormous globes tra-
verse such an immensity of space, merely to prevent the necessity of our
earth revolving on its axis ? The motion produced by the revolution of
the earth on its axis is about thirteen miles and a half a minute to an
inhabitant of London. A person at the equator moves much quicker;
and one situated near the poles much slower, since they each perform
a revolution in twenty-four hours. But in performing its revolution
round the sun, every part of the earth moves with an equal velocity, and
this velocity is no less than a thousand miles a minute.

In ancient times, the earth was supposed to occupy the centre of the
system ; and the sun, moon, and stars to revolve round it. This was the
system of Ptolemy ; but so long ago as the beginning of the sixteenth
century it was discarded, and the solar system, such as we have shown,
was established by the celebrated astronomer Copernicus and his fol-
lowers, and is hence called the Copernican system. But the theory of
gravitation, the discovery of the source whence this beautiful and har-
monious arrangement flows, we owe to the powerful genius of Sir Isaac
Newton, who lived at a much later period.

It is far less difficult to trace by observation the motion of the planets,
than to divine by what power they are impelled and guided. The idea of
gravitation, it is said, was first suggested to Sir Isaac Newton by a cir-
cumstance from which one should little have expected so grand a theory
to have arisen. During the prevalence of the plague in the year 1665,
Newton retired into the country to avoid the contagion. When sitting
one day in his orchard, he observed an apple fall from a tree, which led to
a train of thought, whence his grand theory of universal gravitation
was ultimately developed. His first reflection was, whether the apple
would fall to the earth if removed to a great distance from it ; then, how
far it would be required to be removed from the earth, before it would
cease to be attracted ; would it retain its tendency to fall at the distance
of a thousand miles, or ten thousand, or to the distance of the moon ? —
and here the idea occurred to him that it was not impossible that the moon
herself might have a similar tendency, and gravitate to the earth in the
same manner as the bodies on or near its surface, and that this gravity
might possibly be the power which balanced the centrifugal force implied
in her motion in her orbit. It was then natural to extend this idea to the
other planets, and consider them as gravitating towards the sun, in the
same manner as the moon gravitates towards the earth. He followed up
this beautiful hypothesis by a series of calculations and demonstrations,
unparalleled for their originality, and the industry and judgment with
which they were conducted, until he established the stupendous doctrine
of universal gravitation ! Who would imagine that the simple circum-
stance of the full of an apple would have led to such magnificent results?
It is the mark of superior genius to find matter for observation and re-
search in circumstances which, to the ordinary mind, appear trivial,
because they are common, and with which they are satisfied because they
are natural, without reflecting that Nature is our grand field of observa-
tion— that within it is contained our whole store of knowledge : in a word,
that to study the works of Nature, is to learn to appreciate and admire.

xl INTRODUCTION TO ASTRONOMY.

the wisdom of God. Thus, it was the simple circumstance of the fall of
an apple which led to the discovery of the laws upon which the Coper-
uican system is founded; and whatever credit this system had obtained
before, it now rests upon a basis from which it cannot be shaken.

SECTION III.— On the Earth.

As the Earth is the planet in which we are the most particularly inte-
rested, we shall explain the effects resulting from its annual and diurnal
motions ; but for this purpose it will be necessary to make you acquainted
with the terrestrial globe. This globe, or sphere (fig. 8), represents the
earth. The line which passes through its centre, and on which it turns,

Fig. 8.
i\

K

is called its axis ; and the two extremities of the axis, A and B, are the
poles distinguished by the names of the north and the south pole. The
circle, C D, which divides the globe into two equal parts between the
poles, is called the equator, or equinoctial line; that part of the globe to
the north of the equator is the northern hemisphere ; that part to the south
of the equator, the southern hemisphere. The small circle, E F, which
surrounds the north pole, is called the arctic circle j that, G H, surround-
ing the south pole, the antarctic circle. There are two intermediate
circles between, the polar circles and the equator, — that to the north, I K,
called the tropic of Cancer ; that to the south, L M, called the tropic of
Capricorn. Lastly, this circle, L K, which divides the globe into two
equal parts, crossing the equator, and extending northward as far as the
tropic of Cancer, and southward as far as the tropic of Capricorn, is called
the ecliptic. The delineation of the ecliptic on the terrestrial globe is not
without danger of conveying false ideas ; for the ecliptic (as has before
been said) is an imaginary circle in the heavens, passing through the
middle of the zodiac, and situated in the plane of the earth's orbit.

In order to understand the meaning of the plane of the earth's orbit, let
us suppose a smooth, thin, solid plane cutting the sun through the centre,
extending out as far as the fixed stars, and terminating in a circle, which
passes through the middle of the zodiac. In this plane the earth moves

INTRODUCTION TO ASTRONOMY.

xli

in its revolution round the sun : it is therefore called the plane of the
earth's orbit; and the circle in which this plane cuts the signs of the
zodiac is the ecliptic. Let Jig. 9 represent such a plane, S the sun,
E the earth with its orbit, and A B C D the ecliptic passing through the

Fig. 9.

middle of the zodiac. Therefore the ecliptic relates'only to the heavens ;
but it is described upon the terrestrial globe to facilitate the demonstration
of a variety of problems in the use of the globes ; and besides, the ob-
liquity of this circle to the equator is rendered more conspicuous by its
being described on the same globe ; and the obliquity of the ecliptic
shows the inclination of the earth's axis to the plane of its orbit. But to
return to Jig. 8.

The spaces between the several parallel circles on the terrestrial globe
are called zones ; that which is comprehended between the tropics is dis-
tinguished by the name of the torrid zone; the spaces which extend from
the tropics to the polar circles, the north and south temperate zones ; and
the spaces contained within the polar circles, the frigid zones.

The several lines which, you observe, are drawn from one pole to the
other, cutting the equator at right angles, are called meridians. When
any one of these meridians is exactly opposite the sun, it is mid-day, or
twelve o'clock in the day, with all the places situated on that meridian ;
and, with the places situated on the opposite meridian, it is consequently
midnight. To places situated equally distant from these two meridians,
it is six o'clock. If they are to the east of the sun's meridian, it is six
o'clock in the afternoon, because the sun will have previously passed over

xlii INTRODUCTION TO ASTRONOMY.

them ; if to the west, it is six o'clock in the morning:, and the sun will be
proceeding towards that meridian.

Those circles which divide the globe into two equal parts, such as the
equator and the ecliptic, are called great circles — to distinguish them from
those which divide it into two unequal parts, as the tropics and polar
circles, which are called small circles. All circles are divided into 360
equal parts, called degrees; and these degrees into 60 equal parts, called
minutes. The diameter of a circle is a right line drawn across it, and
passing through the centre ; the diameter is equal to a little less than
one-third of the circumference, and consequently contains a length equal
to nearly 120 degrees, or more accurately, about 114J degrees, of the circle
itself. A meridian, reaching from one pole to the other, is half a circle,
and therefore contains 180 degrees ; and the distance from the equator to
the pole is half of a meridian, or a quarter of the circumference of a circle,
and contains 90 degrees.

Besides the usual division of circles into degrees, the ecliptic is divided
into twelve equal parts, called signs, which bear the name of the constella-
tions through which this circle passes in the heavens. The degrees mea-
sured on the meridians from north to south, or south to north, are called
degrees of latitude ; those measured from east to west on the equator, or
any of the lesser circles parallel to it, are called degrees of longitude ;—
these lesser circles are called parallels of latitude, because being every
where at the same distance from the equator, the latitude of every point
contained in any one of them is the same.

The degrees of longitude must necessarily vary in length according to
the dimensions of the circle on which they are reckoned : those, for
instance, at the polar circles, will be considerably smaller than those at
the equator. The degrees of latitude, on the contrary, never vary in
length, the meridians on which they are reckoned being all of the same
dimensions. The length of a degree of latitude is 60 geographical miles,
which is equal to 69 j English statute miles. The degrees of longitude
at the equator would be of the same dimensions were the earth a perfect
sphere ; but its form is not exactly spherical, being somewhat pro-
tuberant about the equator, and flattened towards the poles. This form
proceeds from the superior action of the centrifugal power at the equator.
The revolution of the earth on its axis gives every particle a tendency to
fly off from the centre. This tendency is stronger or weaker in proportion
to the velocity with which the particle moves. Now a particle situated
near one of the polar circles makes one rotation in the same space of
time as a particle at the equator; the latter, therefore, having a much
larger circle to describe, travels proportionally faster, consequently the
centrifugal force is much stronger at the equator than at the polar circles :
it gradually decreases as you leave the equator and approach the poles,
where, as there is no rotatory motion, it entirely ceases. Supposing,
therefore, the earth to have been originally in a fluid state, the particles
in the torrid zone would recede much farther from the centre than those in
the frigid zones : thus the polar regions would become flattened, and
those about the equator elevated. According to the same rule, our heads
move with greater velocity than our feet ; and on the summit of a moun-
tain, our velocity is greater than in a valley ; for the head is more distant
from the centre of motion than the feet — the mountain-top than the valley.
Even at the equator, however, the force of gravity preponderates very
considerably, being at the equator 288 times greater than the centrifugal
force.

INTRODUCTION TO ASTRONOMY. xliii

It would be natural to suppose that the prominence at the equator and
depression at the poles would render the attraction of gravity stronger at
the former, so that a body would weigh heavier at the equator than at the
poles. Tin's, however, is erroneous. The manner in which the force of
gravity varies at different spots on the surface of the earth depends on
considerations too complicated to be here explained ; but the general
result is that, although the difference in different situations is very small,
the nearer any part of the surface is to the centre of attraction, the more
strongly it is attracted. We refer, however, only to any situation on the
surface of the earth. Were you to penetrate into the interior, the attrac-
tion of the parts above you would counteract that of the parts beneath
you, and consequently diminish the power of gravity in proportion as you
approached the centre ; and if you reached that point, being equally
attracted by the parts all around you, gravity would cease, and you would
be without weight. Bodies therefore gravitate less, and consequently
weigh less, at the equator than at the poles, while their centrifugal force
is much greater ; and as this force tends to drive bodies from the centre,
it is necessarily opposed to, and must decrease the power of gravity.
There are then two causes which render bodies lighter in weight at the
equator than at the poles : the diminution of gravitation, and the increase
of the centrifugal force. Men of science have travelled both to the equator
and to Lapland, with a view of ascertaining this fact. The severity of the
climate and the obstruction of ice has hitherto rendered every attempt to
reach the pole abortive ; but the difference of weight of a body at the
equator and in Lapland is very considerable.

This difference cannot be discovered by simply weighing bodies; for if
the body under trial at the equator decreased in weight, the weight which
was opposed to it in the opposite scale must diminish in the same propor-
tion. For instance, if a pound of sugar did not weigh so heavy at the
equator as at the poles, the leaden pound which served to weigh it would
not be so heavy either ; therefore they would still balance each other. A
pendulum is the instrument used for the purpose of discovering the varia-
tions of gravity in different situations on the surface of the earth. A pen-
dulum consists of a line, or rod, to one end of which a weight is attached,
and it is suspended by the other to a fixed point, about which it is made
to vibrate. Without being put in motion, a pendulum, like a plumb-line,
hangs perpendicular to the general surface of the earth, by which it is
attracted ; but if you raise a pendulum on one side, gravity will bring it
back to its perpendicular position. It will, however, not remain sta-
tionary there, for the velocity it has received during its descent will impel
it onwards, and it will rise on the opposite side to an equal height: from
thence it is brought back by gravity, and again driven by the impulse of
its velocity. Were the motion of a pendulum not opposed by the resist-
ance of the air in which it vibrates, and by the friction of the part by which
it is suspended, it would be perpetual ; and were the force of gravity
which produces these vibrations always the same, they would be perfectly
regular, being of equal distances and performed in equal times. This is
the natural result of the uniformity of the power which produces these
vibrations: the force of gravity being always the same, the velocity of the
pendulum must consequently be uniform. But if the force of gravity is
less at the equator than at the poles, the vibrations of the pendulum will
be slower.

It was thus that the difference of gravity was discovered, and the true
figure of the earth ascertained ; for after having _made due allowance for

xliv

INTRODUCTION TO ASTRONOMY.

the effect of the centrifugal force, gravity was still found to be greater in
the polar than in the equatorial regions, owing to the spheroidal figure of
the earth. If then a pendulum vibrates faster at the poles and slower at
the equator, the inhabitants must regulate their clocks in a different
manner from ours, in which the pendulum vibrates once in a second
of time. The only alteration required is to lengthen the pendulum
in one case, and to shorten it in the other; for the velocity of the vibra-
tions of a pendulum depends on its length ; and when it is said that a
pendulum at the pole vibrates quicker than one at the equator, it is sup-
posing them both to be of the same length. A pendulum which vibrates
a second in this latitude is rather more than 39 inches in length. In
order to vibrate at the equator in the same space of time, it must be
shortened by a few lines ; and at the poles, it must be proportionally
lengthened.

We shall now explain the variation of the seasons, and the difference
of the length of the days and nights in those seasons — both effects
resulting from the same cause. In moving round the sun, the axis of the
earth is not perpendicular to the plane of its orbit ; in other words, its
axis does not move round the sun in an upright position, but slanting or
oblique. This you will understand more clearly if you carry a small globe
round a lamp or candle, whicli js to represent the sun Cfg- 10). You

Fig. 10.

must consider the ecliptic drawn on the small globe as representing the
plane of the earth's orbit ; and the equator, which crosses the ecliptic in
two places, shows the degree of obliquity of the axis of the earth in that
orbit, which is exactly 23£ degrees. The points in which the ecliptic in-
tersects the equator are called nodes. The globe at A is situated as it is
in the midst of summer, or what is called the summer solstice, which is on
the 21st of June. The north pole is then inclined towards the sun, and
the. northern hemisphere enjoys much more of his rays than the southern.
The sun now shines over the whole of the north frigid zone ; arid not-
withstanding the earth's diurnal revolution, which may be imitated by
twirling the ball on the wire, it will continue to shine upon it as long as
it remains in this situation, whilst the south frigid zone is at the same
time completely in obscurity.

INTRODUCTION TO ASTRONOMY. ' 1 fifi SIT"!

Let the earth now set off from its position in the summer solstice, ancl --•? ^ .
carry it round the sun : observe that the axis must be always inclined in
the same direction, and the north pole point to the same spot in the
heavens. There is a fixed star situated near that spot, which is hence
called the North Polar Star. The earth at B has gone through one
quarter of its orbit, and is arrived at that point at which the ecliptic cuts
or crosses the equator, and which is called the autumnal equinox. The
sun now shines from one pole to the other, as it would constantly do were
the axis of the earth perpendicular to its orbit, the inclination of the axis
being now neither towards the sun nor in the contrary direction. At this
period of the year, the days and nights are equal in every part of the
earth, excepting at the very poles ; but the next step she takes in her
orbit involves the north pole in darkness, whilst it illumines that of the
south. This change was gradually preparing as the earth moved from
summer to autumn ; the arctic circle begins to have short nights, which
increase as the earth approaches the autumnal equinox ; and the instant
it passes that point, the long night of the north pole commences, and the
south pole begins to enjoy the light of the sun. As the earth proceeds
in its orbit, the days shorten and the nights lengthen throughout the
northern hemisphere, until it arrives at the winter solstice, on the 21st of
December, when the north frigid zone is entirely in darkness, and the
southern enjoys uninterrupted daylight. Exactly half of the equator, it
will be observed, is enlightened in every position, and consequently the
day is there always equal to the night.

Observe that the inhabitants of the torrid zone have much more heat
than we have, as the sun's rays fall perpendicularly on them, while they
shine obliquely on the temperate, and almost horizontally on the frigid
zone ; for during their long day, the sun move* round at no great eleva-
tion above their horizon without either rising or setting ; the only ob-
servable difference is, that it is more elevated by a few degrees at mid-day
than at midnight ; but at the poles themselves, the sun travels round in
the course of four-and-twenty hours nearly at the same elevation from
the horizon, rising every day a very little higher from the vernal equinox
till midsummer, and declining after that period till the autumnal equinox,
when their long night begins.

To a person placed in the temperate zone, as we are hi England, the
sun's rays will shine neither so obliquely as at the poles, nor so vertically
as at the equator ; but will fall upon him more obliquely in autumn and
winter than in summer. Therefore, the inhabitants of the earth between
the polar circles and the equator will not have merely one day and one
night in the year, as happens at the pole ; nor will they have equal days
and equal nights, as at the equator ; but their days and nights will vary
in length at different times of the year, according as their respective poles
incline towards or from the sun, and the difference will be greater in pro-
portion to their distance from the equator. During the other half of her
orbit, the same effect takes place in the southern hemisphere as what we
have just remarked in the northern. When the earth arrives at the
vernal equinox, D, where the ecliptic again cuts the eqilator, on the 22nd
of March, she is situated, with respect to the sun, exactly in the same
position as in the autumnal equinox, excepting that it is now autumn in
the southern hemisphere, whilst it is spring with us ; for the half of the
globe which is enlightened extends exactly from one pole to the other :
the sun rises to the north pole, and sets to the south pole. On the two
days of the equinox the sun is visible at both poles ; but only half of it is
seen from either, the other half being concealed by the horizon.

xlvi INTRODUCTION TO ASTRONOMY.

The sun is nearly three of our clays in rising1 and set (inn- at the poles.
About thirty hours, or rather more, before he reaches the exact period of
the autumnal equinox, the upper edge or limb of the sun begins to be
visible at the south pole ; and it is there seen constantly travelling round
the horizon, and rising gradually higher and higher, till at the end of
about sixty hours, after revolving nearly 2J times round the horizon, the
whole of its orb is visible.

At the same moment that the edge of the sun becomes visible at the
south pole, the same edge which appears as the lower limb at the north
pole begins to dip below the horizon ; but the sun still continues visible,
travelling round the horizon, more and more of it. being hid, till, at the
end of sixty hours, it totally disappears, just at the same moment when it
is fully seen at the south pole. As the earth proceeds towards summer,
the days lengthen in the northern hemisphere, and shorten in the southern,
till the earth reaches our summer solstice, which brings it again to the
spot whence we first accompanied her.

The mind can find no object of contemplation more sublime than the
course of this magnificent globe, impelled by the combined powers of pro-
jection and attraction to roll in one invariable course around the source
of light and heat ; and what can be more delightful than the beneficent
effects of this vivifying power on its attendant planet? It is at once the
grand principle which animates and fecundates Nature.

The sun's rays afford less heat when in an oblique direction than when
perpendicular, because fewer of them fall upon an equal portion of the
earth. This will be understood better by referring to Jig. 1 1, which repre-
sents two equal portions of the sun's rays, shining upon different parts of

Fig. 11.

the earth. Here it is evident that the same number of rays fall on the
space AB as fall on the space B C ; and as A B is less than B C, the
heat and light will be much stronger in the former than in the latter.
A B, you see, represents the equatorial regions, where the sun shines per-
pendicularly; and B C the temperate and frozen climates, where his rays
fall more obliquely. This accounts also for the greater heat 'of summer,
as the sun shines less obliquely in summer than in winter.

In Jig. 12, the earth is represented as it is situated on the 21st of June,
when England receives less oblique, and consequently a greater number
of rays than at any other season; and Jig. 13 shows the situation of Eng-
land on the 21st of December, when the rays of the sun fall most obliquely
upon her. But there is also another reason why oblique rays give less
heat than those which are perpendicular; the former have a greater por-
tion of the atmosphere to traverse ; and though it is true that the atmos-
phere is itself a transparent body, it does not admit the passage of the

INTRODUCTION TO ASTRONOMY.
Fig. 12.

vy

xlvii

sun's rays quite freely ; and besides, it is always loaded more or less with
dense and foggy vapour, which the rays of the sun cannot easily penetrate ;
therefore the greater the quantity of atmosphere the sun's rays have to
pass through in their way to the earth, the fewer of them will reach it.
This will be better understood by referring to Jig. 14. The dotted line
round the earth describes the extent of the atmosphere, and the lines which
proceed from the sun to the earth, the passage of two equal portions of
the sun's rays to the equatorial and polar regions : the latter, from its
greater obliquity, passes through a greater extent of atmosphere.

The diminution of heat, morning and evening, is also owing to the
greater obliquity of the sun's rays ; and, as such, they are affected by
both the causes which have just been explained : the difficulty of passing
through a foggy atmosphere is more particularly applicable to them, as
mists and vapours are very prevalent about the time of sunrise and sunset.
But the diminished obliquity of the sun's rays is not the sole cause of the
heat of summer ; the length of the days greatly conduces to it ; for the
longer the sun is above the horizon, the more heat he will communicate

Fig. 14.

xlvlii INTRODUCTION TO ASTRONOMY.

to the earth ; and yet, though both the longest days and the most per-
pendicular rays are on the 21st of June, the greatest heat prevails in July
and August. To account for this, you must reflect that those parts of the
earth which are once heated retain the heat for a considerable length of
time ; and the additional quantity they receive occasions an elevation of
temperature, although the days begin to shorten, and the sun's rays to
fall more obliquely. For the same reason, we have generally more heat
at three o'clock in the afternoon than at twelve, when the sun is on the
meridian. As long as the sun continues to communicate more heat than
the earth parts with in a given time, so long the heat of the earth will
increase, even though the rate at which it receives new heat from the sun
is diminished.

The vicissitudes of seasons of the other planets vary according as their
axes deviate more or less from the perpendicular to the plane of their
orbits. The axis of Jupiter is nearly perpendicular to the plane of his
orbit; those of Mars and of Saturn are each inclined at angles of about
60 degrees; whilst the axis of Venus is believed to be elevated only 15 or
20 degrees above her orbit: the vicissitudes of her seasons must therefore
be considerably greater than ours.

There is one more observation to make relative to the earth's motion,
which is, that although we have but 365 days and nights in the year, she
performs 366 complete revolutions on her axis during that time. This is
owing to the progressive motion of the earth in its orbit whilst it revolves
on its axis : as it advances almost a degree westward in its orbit, in the
same time that it completes a revolution eastward on its axis, it must
revolve nearly one degree more in order to bring the same meridian back
to the sun. These small daily portions of rotation are each equal to the
three hundred and sixty-fifth part of a circle, which at the end of the year
amounts to one complete rotation. If the earth, then, had no other than
its diurnal motion, we should have 366 days in the year ; or rather, we
should have 366 days in the same period of time that we now have 365 ;
for if we did not revolve round the sun, we should have no natural means
of computing* years. If time be calculated by the stars instead of the sun,
the irregularity which we have just noticed does not occur, and that one
complete rotation of the earth on its axis brings the same meridian back
to any fixed star ; and yet the earth's advance in her orbit must change
her position with regard to the fixed stars, as well as with regard to the
sun ! This difficulty is explained by the distance of the fixed stars, which
is so immense, that our solar system is in comparison to it but a spot,
and the whole extent of the earth's orbit but a point ; therefore, whether
the earth remained stationary, or whether it revolved in its orbit during
its rotation on its axis, no sensible difference would be produced with
regard to the fixed stars. One complete revolution brings the same
meridian back to the same fixed star : hence the fixed stars appear to go
round the earlh in a shorter time than the sun by three minutes sixty-
six seconds of time, the time which the earth takes to perform the addi-
tional three hundred and sixty-fifth part of the circle, in order to bring
the same meridian back to the sun. Hence the stars gain every day three
minutes fifty-six seconds on the sun, which makes them rise that portion
of time earlier every day.

When time is calculated by the stars, it is called sidereal time ; when
by the sun, solar or apparent time ; and a sidereal day is three minutes
fifty-six seconds shorter than a solar day of twenty-four hours. The
difference of the solar and the sidereal year must also be explained : the

INTRODUCTION TO ASTRONOMY. xlix

common year, called the solar or tropical year, containing 365 days,
5 hours, 48 minutes, and 5:2 seconds, is measured from the time the sun
sets out from one of the equinoxes, or solstices, till it returns to the same
again ; but the year is completed before the earth has finished one entire
revolution in its orbit. This is owing to the spheroidal figure of the
earth, the elevation about the equator producing much the same effect as
if a similar mass of matter, collected in the form of a moon, revolved
round the equator. When this moon acted on the earth in conjunction
with, or in opposition to, the sun, variations in the earth's motion would
be occasioned, and these variations produce what is called the precession
of the equinoxes. The equinoctial points are therefore not quite fixed,
but have a retrograde motion : that is to say, instead of being every revo-
lution in the same place, they move backwards. Thus, if the vernal equinox
be at A (Jig. 15, next page), the autumnal one will be at B. instead of C,
and the following vernal equinox at D, instead of at A, as would be the
case if the equinoxes were stationary at opposite points of the earth's
orbit : so that though the earth takes half a year to move from one
equinox to the other, it has not then travelled through half its orbit ; and
consequently, when it returns again to the first equinox, it has not com-
pleted the whole of its orbit. In order to ascertain when the earth has
performed an entire revolution in its orbit, we must observe when the
sun retires in conjunction with any fixed star; and this is called a sidereal
year. Supposing a fixed star situated atE, the sun would not appear in
conjunction with it till the earth had returned to A, when it would have
completed its orbit. The sidereal is only about twenty minutes longer
than the solar year, so that the variation of the equinoctial points is very
inconsiderable.

In regard to time, we must further add, that the earth's diurnal motion,
on an inclined axis, together with its annual revolution in an elliptic orbit,
occasions so much complication in its motion as to produce many irre-
gularities : therefore true equal time cannot be measured by the sun. A
clock, which was always perfectly correct, would in some parts of the
year be before the sun, and in other parts after it. There are but four
periods in which the sun and a perfect clock would agree, which are — the
15th of April, the 16th of June, the 31st of August, and the 24th of
December. The greatest difference between solar time and true time
amounts to between fifteen and sixteen minutes. Tables of the equation
of time are constructed for the purpose of pointing out and correcting
these differences between solar time and equal or mean time, which is
the denomination given by astronomers to true time.

SECTION IV. — On the Moon.

LET us now turn our attention to the Moon. This satellite revolves
round the earth in the space of twenty-seven days eight hours, in an orbit
nearly coinciding with the plane of the earth's orbit, and accompanies us
in our revolution round the sun. Her motion, therefore, is of a compli-
cated nature ; for as the earth advances in her orbit whilst the moon goes
round her, the moon proceeds in a sort of progressive circle. There are
also other circumstances which interfere with the simplicity and regularity
of the moon's motion, but which are too intricate for us to notice at
present.

The moon always presents the same face to us, by which it is evident

£

1 INTRODUCTION TO ASTRONOMY.

that she turns but once upon her axis while she performs a revolution
round the earth ; so that the inhabitants of the moon have but one day
and one night in the course of a lunar month. Since we always see the
same hemisphere of the moon, the inhabitants of that hemisphere alone
can perceive the earth. One half of the moon, therefore, enjoys our light
every night, while the other half has constantly nights of darkness ; and
we appear to the inhabitants of the moon under all the changes or phases
which the moon exhibits to us.

Fig. 15.

Fig. 16.

These phases require some explanation. Injig. 16, let us say that S
represents the sun, E the earth, and ABCD, &c. the moon in different parts
of her orbit. When the moon is at A, her dark side being turned towards
the earth, we shall not see her ; but her disappearance is of very short
duration, and as she advances in her orbit we perceive her under the form
of a new moon : when she has gone through one-sixth of her orbit at B,
one quarter of her enlightened hemisphere will be turned towards the

INTRODUCTION TO ASTRONOMY. li

earth, and she will then appear horned, as in the figure 6 : when she has
performed one quarter of her orbit, she shows us one half of her
enlightened side, as at C, and appears as in the figure c; at D she is said
to be gibbous, and at E the whole of the enlightened side appears to us,
and the moon is at full. As she proceeds in her orbit she becomes again
gibbous, and her enlightened hemisphere turns gradually away from us
till she completes her orbit and disappears, and then again resumes her
form of a new moon. The small exterior figures, o, 6, c, d, &c., it will
be seen, represent the phases corresponding to the situations A, B, C, D,
&c. : the light part of the figures, «, 6, c, d, &c., alone being supposed
visible.

When the moon is at full, she is said to be in opposition ; when a new
moon, to be in conjunction with the sun. At each of these times, the
sun, the moon, and the earth are in "the same right line ; but in the first
case, the earth is between the sun and the moon ; in the second, the moon
is between the sun and the earth. An eclipse can take place only when
the sun, moon, and earth are in a right line. When the moon passes
between the sun and the earth, she intercepts his rays, or, in other words,
casts a shadow on the earth : then the sun is eclipsed, and the daylight
gives place to darkness, while the moon's shadow is passing over us.
When, on the contrary, the earth is between the sun and the moon, it is
we who intercept the sun's rays, and cast a shadow on the moon : she
then disappears from our view, and is eclipsed.

Why then have we not a solar and a lunar eclipse every month?

The planes of the orbits of the earth and moon do not exactly coincide,
but cross or intersect each other ; and the moon generally passes either on
one side or the other when she is in conjunction with, or in opposition to,
the sun, and therefore does not intercept the sun's rays, or produce an
eclipse ; for this can take place only when the earth and moon are in con-
junction near those parts of their orbits which cross each other (called the
nodes of their orbits), because it is then only that they are both in the
same plane, and in a right line with the sun. A partial eclipse takes
place when the moon, in passing by the earth, does not entirely escape
her shadow. When the eclipse happens precisely at the nodes, they are
not only total, but last for some length of time.

When the sun is eclipsed, the total darkness is confined to one parti-
cular part of the earth. In fig* 17 a solar eclipse is exhibited : S is the
sun, M the moon, and E the earth ; and the moon's shadow is not large
enough to cover the earth.

Fig. 17.

The lunar eclipses, on the contrary, are visible from every part of the
earth, where the moon is above the horizon. In Jig. 18, S represents the
sun, which pours forth rays of light in straight lines in every direction ;
E is the earth, and M the moon. Now a ray of light coming from one
extremity of the sun's disk in the direction AB will meet another coming

E 2

Hi INTRODUCTION TO ASTRONOMY.

from the opposite extremity in the direction CB ; the shadow of the earth
cannot, therefore, extend beyond B. As the sun is larger than the earth,
the shadow of the latter is conical : it gradually diminishes, and is much
smaller than the earth where the moon passes through it, and yet we find
the moon to be not only totally eclipsed, but some length of time in
darkness. The moon, shining only by reflected light, disappears abso-
lutely when the light of the sun is intercepted from her. She does not,
therefore, become invisible, like the sun, only from particular spots on the
earth's surface, but absolutely and universally, until the light of the sun
begins to shine upon her again. The length of an eclipse depends on the
respective distances and magnitudes of the sun, earth, and moon. The
diameter of the moon is about -^T of that of the earth, or the whole bulk
of the earth about 49 times that of the moon.

When the moon eclipses the sun to us, the earth is eclipsed to the moon ;
for if the moon intercepts the sun's rays, and casts a shadow on us, we
must necessarily disappear to the moon, but only partially — a black spot
will appear to pass over the earth, as in figure 17.

In the distant planets, few days elapse without an eclipse taking place ;
for among the number of satellites, one or other of them is continually
passing into the shadow of the planet, or between the planet and the sun.
Astronomers are so well acquainted with the motion of the planets and
their satellites, that they have calculated not only the eclipses of our
moon, but those of Jupiter, with such perfect accuracy, that it has afforded
a means of ascertaining the longitude. When, as on land, we know where
\ve are situated, there is no difficulty in ascertaining the latitude or longi«
tude of the place by referring to a map ; but the question is to find out
our situation when we do not know where we are : for instance, at sea,
interrupted in our course by storms, a map would afford no assistance in
discovering where we were. The latitude may be found by taking the
altitude of the pole : that is to say, observing the number of degrees that
it is elevated above the horizon, for the pole appears more elevated as we
approach it, and less as we recede from it. It is true that the pole is not
visible to us ; but the north pole points constantly towards one particular
part of the heavens, near which a star is situated, called the Polar Star.
The altitude of the polar star is therefore nearly the same number of degrees
as that of the pole; and, as this star is visible in clear nights from every part
of the northern hemisphere, it furnishes an easy mode of ascertaining the
latitude in all that half of the world. The latitude may be more accu-
rately determined by other observations, which may be made on the sun
or any of the fixed stars ; the situation, therefore, of a vessel at sea, with
regard to north and south, is easily ascertained. The difficulty is respecting
east and west — that is to say, its longitude. As there are no eastern poles
from which we can reckon our distance, some particular spot must be
fixed upon for that purpose. The English reckon from the meridian of
Greenwich, where the Royal Observatory is situated ; in French maps,
the longitude is reckoned from Paris.

INTRODUCTION TO ASTRONOMY. liii

The rotation of the earth on its axis in twenty-four hours, from west to
east, occasions, as we have already seen, an apparent motion of the sun
and stars in the contrary direction, and the sun appears to go round the
earth in the space of twenty-four hours, passing over fifteen degrees, or a
twenty-fourth part of the earth's circumference every hour : therefore, when
it is twelve o'clock in London, it is one o'clock in any place situated fifteen
degrees to the east of London, as the sun must have passed the meridian of
that place an hour before he reaches that of London. For the same reason
it is eleven o'clock to any place situated fifteen degrees to the west of
London, as the sun will not come to that meridian till an hour later. If,
then, the captain of a vessel at sea could know precisely what was the hour
at London, he could, by looking at his watch, and comparing it with the
hour of the spot in which he was, ascertain the longitude. For this pur-
pose he must be furnished with two watches — the one daily regulated by
the sun, and the other unaltered. The former would indicate the hour
of the place in which he was situated, and the latter the hour of London ;
and by comparing them together, he would be able to calculate his longi-
tude : this mode of finding the longitude is universally adopted. Watches
of a superior construction, called chronometers, or time-keepers, are used
for this purpose ; but the best watches are liable to imperfections, and
should the time-keeper go too fast or too slow, there would be no means
of ascertaining the error : implicit reliance cannot consequently be placed
upon them.

Recourse is therefore had to the eclipses of Jupiter's satellites. A table
is made of the precise time at which the several moons are eclipsed to a
spectator at London. When they appear eclipsed to a spectator in any
other spot, he may, by consulting the table, know what is the hour at
London; for the eclipse is visible at the same moment from whatever
place on the earth it is seen. He has then only to look at the watch
which points out the hour of the place in which he is, and by observing
the difference of time there, and at London, he may immediately determine
his longitude.

Let us suppose that a certain moon of Jupiter is always eclipsed at six
o'clock in the evening at London, and that a man at sea consults his watch,
and finds that it is ten o'clock at night where he is situated, at the moment
the eclipse takes place : he would be sixty degrees east of London; for the
sun, which travels (apparently) fifteen degrees an hour, must have passed
his meridian four hours before it reaches that of London ; for this reason,
the hour is always later than in London when the place is east longitude,
and earlier when it is west longitude. Thus the longitude can be ascer-
tained whenever the eclipses of Jupiter's moons are visible.

The latitude shows on what meridian you are situated, and the longi-
tude on what part of that meridian ; therefore, when you can ascertain
both these, you discover the very spot in which you are situated. But it
is not only the secondary planets which produce eclipses, for the primary
planets near the sun eclipse him to those at a greater distance, when they
come in conjunction in the nodes of their orbits; but as the primary
planets are much longer in performing their course round the sun than
the satellites in going round their primary planets, these eclipses very
seldom occur.

Mercury and Venus have however passed in a right line between the
earth and the sun, but being at so great a distance, their shadows did not
extend so far as the earth. No darkness was therefore produced on any
part of our globe ; but the planet appeared like a small black spot,

liv INTRODUCTION TO ASTRONOMY.

passing across the sun's disk : this is called a transit of the planet. It
•was by the last transit of Venus that astronomers were enabled to calcu-
late with some degree of accuracy the distance of the earth from the sun,
and the dimensions of the latter.

The tides are produced by the attraction of the moon. The cohesion of
fluids being much less than that of solid bodies, they more easily yield to
the power of gravity ; in consequence of which the waters immediately
below the moon are drawn up in a protuberance, producing a full tide, or
what is commonly called high-water, at the spot where it happens.
According to this theory, you would imagine that we should have full tide
only once in twenty-four hours — that is, every time that we were below
the moon — while we find that we have two tides in the course of twenty-
four hours, and that it is high-water with us and with our antipodes at
the same time.

This opposite tide is rather more difficult to explain than that which is
drawn up beneath the moon. In order to render the question more sim-
ple, let us suppose the earth to be everywhere covered by the ocean, as
injfig. IV. M is the moon, ABCD the earth. Now the waters on the
surface of the earth about A, being more strongly attracted than in any

Fig. 19.

other part, will be elevated, the attraction of the moon at B and C being
less ; but still it will be greater there than at D, which is the part most
distant from the moon. The body of the earth will therefore be drawn
away from the waters at D, leaving a protuberance similar to that at A :
so that the tide A is produced by the waters receding from the earth, and
the tide D by the earth receding from the waters.

The influence of the sun on the tides is less than that of the moon ; for
observe, that the tides rise in consequence of the moon attracting one
part of the waters more forcibly than another part : it is this inequality of
attraction which produces full and ebb tides. Now the distance of the
sun is so great, that the whole globe of the earth is comparatively but as
a point, and the difference of its attraction for that part of the waters
most under its influence, and that part least subject to it, is but trifling ;
no part of the waters will be much elevated above, or much depressed
below their general surface by its action. The sun has, however, a con-
siderable effect on the tides, and increases or diminishes them as it acts
in conjunction with, or in opposition to, the moon.

The moon is a month in going round the earth ; twice during that
time, therefore, at full and at change, she is in the same direction as
the sun. Both then act in conjunction on the earth, and produce very
great tides, called spring-tides, as represented inj%. 20, at A and B ; but
when the moon is at the intermediate parts of her orbit, the sun, instead
of affording assistance, weakens her power by acting in opposition to it ;
and. smaller tides are produced, called neap-tides, as represented in fig. 21.

INTRODUCTION TO ASTRONOMY.

Iv

Fig. 21.

Since attraction is mutual between the moon and the earth, we pro-
duce tides in the moon ; and these are more considerable, in proportion
as our planet is larger. Neither the moon nor the earth in reality assume
an oval form, for the land which intersects the water destroys the regu-
larity of the effect. The orbit of the moon being nearly parallel to that
of the earth, she is never vertical but to the inhabitants of the torrid zone :
in that climate, therefore, the tides are greatest, and they diminish as you
recede from it and approach the poles ; but in no part of the globe is the
moon immediately above the spot where it is high tide. All matter, by
its inertia, offers some resistance to a change of state ; the waters,
therefore, do not readily yield to the attraction of the moon, and the
effect of her influence is not complete until some time after she has
passed the meridian.

The earth revolves on its axis in about twenty-four hours : if the moon
were stationary, therefore, the same part of our globe would, every twenty-
four hours, return beneath the moon ; but as during our daily revolution
the moon advances in her orbit, the earth must make more than a com-
plete rotation in order to bring the same meridian opposite the moon : we
are three-quarters of an hour in overtaking her. The tides, therefore, are
retarded, for the same reason that the moon rises later, by three-quarters
of an hour every day. This, however, is only the average amount of the
retardation. The time of the highest tide is modified by the sun's
attraction, and is between those of the tides which would be produced by
the separate action of the two luminaries. The action of the sun, there-
fore, makes the interval different on different days, but leaves the
average amount unaffected.

INTRODUCTION TO HYDROSTATICS.

SECTION I. — On the Mechanical Properties of Fluids.

THE science of the mechanical properties of fluids is called Hydrostatics.
A fluid is a substance which yields to the slightest pressure. If you dip
your hand into a basin of water, you are scarcely sensible of meeting
with any resistance. Fluids, generally speaking, are bodies of less density
than solids. From the slight cohesion of their particles, and the facility
with which these slide over each other, it is conjectured, that they must be
small, smooth, and globular ; smooth, because there appears to be little
or no friction among them ; and globular, because touching each other
but by a point would account for the slightness of their cohesion.

Fluids are divided into two classes, distinguished by the names of
liquids and elastic fluids, or gases, which latter comprehends the air
of the atmosphere, and all the various kinds of air with which
chemistry makes us acquainted. We shall confine our attention at
present to the mechanical properties of liquids or non-elastic fluids.

Water, and liquids in general, are little susceptible of being compressed,
or squeezed into a smaller space than that which they naturally occupy.
This is supposed to be owing to the extreme minuteness of their particles,
which, rather than submit to compression, force their way through the
pores of the substance which confines them, as was shown by a cele-
brated experiment, made at Florence many years ago. A hollow globe
of grold was filled with water, and on its being submitted to great pressure,
the water was seen to exude through the pores of the gold, which it
covered with a fine dew. But more recent experiments, in which water
has been confined in strong iron tubes, prove that it is susceptible of
compression.

Liquids are porous, like solid bodies, but the pores are too minute to be
discovered by the most powerful microscope. The existence of pores in
liquids can be ascertained by dissolving solid bodies in them. If you
melt some salt in a glass full of water, the water will not overflow,
because the particles of salt will lodge themselves in the pores of the
liquid, so that the salt and water together will not occupy more space than
the water did alone. If you attempt to melt more salt than can find room
within these pores, the remainder will subside at the bottom, and occu-
pying a space which the water filled before, oblige the latter to overflow.
Spirit of wine may also be poured into water without adding to the bulk,
as the spirit will introduce itself into the pores of the water.

Fluids show the effects of gravitation in a more perfect manner than
solid bodies ; the strong cohesive attraction of the particles of the latter
in some measure counteracting the effect of gravity. In a table, for
instance, the strong cohesion of the particles of wood enables four slender
legs to support a considerable weight. Were the cohesion so far destroyed
as to convert the wood into a fluid, no support could be afforded by the
legs ; for the particles no longer cohering together, each would press
separately and independently, and would be brought to a level with the
surface of the earth,

INTRODUCTION TO HYDROSTATICS, Ivii

This deficiency of cohesion is the reason why fluids can never be formed
into figures or maintained in heaps ; for though it is true the wind raises
water into waves, they are immediately afterwards destroyed by gravity.
Thus liquids always find their level. The definition of the equilibrium of
a fluid is, that every part of the surface is equally distant from the point to
which gravity tends ; that is to say, from the centre of the earth. Hence
the surface of all fluids must partake of the spherical form of the globe and
be bulging. This is evident in large bodies of water, such as the ocean ;
but the sphericity of small bodies of water is so trifling as to render their
surfaces apparently flat.

The equilibrium of fluids is the natural result of their particles gravitating
independently of each other ; for when any particle of a fluid accidentally
finds itself elevated above the rest, it is attracted down to the level of the
surface of the fluid, and the readiness with which fluids yield to the slightest
pressure, will enable the particle by its weight to penetrate the surface of
the fluid and mix with it. But this is the case only with fluids of equal
density, for a light fluid will float on the surface of a heavy one, as oil on
water; and air will rise to the surface of any liquid whatever, being forced
up by the superior gravity of the liquid. Fig. 1. represents an instrument
Fig. 1. called a water-level, which is constructed

«A B. upon the principle of the equilibrium of

QJT" — — - '7 |Q fluids. It consists of a short tube, AB,

closed at both ends, and containing

water and a bubble of air ; when the tube is not perfectly horizontal the
water runs to the lower end, which makes the bubble of air rise to the
upper end, and it remains in the centre only when the tube does not
incline on either side. It is by this means that the level of any situation,
to which we apply the instrument, is ascertained.

Solid bodies, therefore, gravitate in masses, the strong cohesion of their
particles making "them weigh altogether, while every particle of a fluid
may be considered as a separate mass, gravitating independently. Hence
the resistance of a fluid is considerably less than that of a solid body.
The particles of fluids acting thus independently, press against each other
in every direction, not only downwards but upwards, and laterally or
sideways ; and in consequence of this equality of pressure, every particle
remains at rest in the fluid. If you agitate the fluid, you disturb this
equality, and the fluid will not rest till its equilibrium be restored.

Were there no lateral pressure, water would not flow from an opening
on the side of a vessel ; sand will not run out of such an opening, because
there is scarcely any lateral pressure among the particles. Were the
particles of fluids arranged in regular columns, as in Jig. 2,
there would be no lateral pressure, for when one particle is per-
pendicularly above the other, it can only press it downwards ;
but as it must continually happen that a particle presses between
two particles beneath (fig. 3), these last suffer a lateral pressure ;
just as a wedge driven into a piece of wood separates the parts
The lateral pressure is the result therefore of the pressure
downwards, or the weight of the liquid above ; and consequently
the lower the orifice is made in the vessel, the greater will be
ri the velocity of the water rushing out of it. Fig. 4 represents
OO the different degrees of velocity with which a liquid flows from
a vessel furnished with three stop-cocks at different heights. Since the
lateral pressure is entirely owing to the pressure downwards, it is not
affected by the horizontal dimensions of the vessel, which contains the
liquid, but merely by its depth ; for as every particle acts independently of

Iviii

INTRODUCTION TO HYDROSTATICS.
Fig. 4.

the rest, it is only the column of particles immediately above the orifice
that can weigh upon and press out the liquid.

In a cubical vessel, the pressure downwards will be double the lateral
pressure on one side, for every particle at the bottom of the vessel is
pressed upon by a column of the whole depth of the fluid, whilst the
lateral pressure diminishes equably from the bottom upwards to the sur-
face, where the particles have no pressure.

The pressure of fluids upwards, though it seems in direct opposition
to gravity, is also a consequence of their pressure downwards. When,
for example, water is poured into a tea-pot, the water rises in the spout
to a level with that in the pot. The particles of water at the bottom
of the pot are pressed upon by the particles above them ; to this
pressure they will yield, if there is any mode of making way for the su-
perior particles, and as they cannot descend, they will change their direc-
tion and rise in the spout.

Suppose the tea-pot to be filled with columns of particles of water
similar to that described in Jig. 5, the particle 1 at the bottom will be

Fig. 5.

Fig. 6.

pressed laterally by the particle 2,
and by this pressure be forced into
the spout, where, meeting with the
particle 3, it presses it upwards, and
this pressure will be continued from
3 to 4, from 4 to 5, and so on, till the
water in the spout has risen to a level
with that in the pot.

You may also reverse the experi-
ment by pouring water into the spout, and you will find
that the water will rise in the pot to a level with that in
the spout, for the pressure of the small quantity of water
in the spout will force up and support the larger quantity
in the pot. But this will be better exemplified byjtfg. 6,
in which a goblet is filled by means of a narrow tube. In
the pressure upwards, as well as that laterally, the force
results entirely from the height, and is quite independent
of the horizontal dimensions of the fluid. The tube, how-
ever, could never be filled by pouring water into the goblet,
because the water in the goblet cannot force that in the tube
above its own level, and as the end of the tube is considera-
bly highest, if water be poured into the goblet after it is full,
it will run over instead of rising in the tube above the level.

INTRODUCTION TO HYDROSTATICS. lix

The specific gravity of a body means simply its weight compared with
that of another body of the same size. When we say that substances,
such as lead and stones, are heavy, and that others, such as paper and
feathers, are light, we speak comparatively : that is to say, that the first are
heavy, and the latter light, in comparison with the generality of the sub-
stances in nature. Mahogany is a heavy body when compared to most other
kinds of wood, but light when compared to stone. Chalk is a heavy body
compared to coal, but light if compared to metal. You perceive therefore
that our notions of light and heavy are vague and undefined, and that
some standard of comparison is required, to which the weight of all other
bodies may be referred. The body which has been adopted as a standard
of reference is distilled water. It may perhaps appear surprising that a
fluid should have been chosen for this purpose, as it must necessarily be
contained in some vessel, and the weight of this vessel must be deducted.
This is true, when the specific gravity of fluids is to be estimated ; but
with regard to solids, it is necessary simply to weigh the body under trial
in water. If a piece of gold be weighed in a glass of water, the gold will
displace just as much water as is equal to its own bulk : a cubic inch of
water must make way for a cubic inch of gold. The bulk alone is to be
considered, the weight has nothing to do with the quantity of water dis-
placed ; for a cubic inch of gold does not occupy more space, and there-
fore will not displace more water, than a cubic inch of ivory, or any other
substance that will sink in water.

The gold will weigh less in water than it did out of it, on account of
the upward pressure of the particles of water, which in some measure
supports the gold, and by so doing, diminishes its weight. If the body
under trial be of the same weight as the water in which it is immersed, it
will be wholly supported by it, as was the water, the place of which it
occupies ; if it be heavier, the water will offer some resistance to its
descent ; and this resistance will in all cases be the same to bodies of
equal bulk, whatever be their weight. All bodies of the same size, there-
fore, lose the same quantity of their weight when completely immersed
in water. A body weighed in water loses as much of its weight as is
equal to that of the water it displaces ; so that were this water put into
the scale to which the body is suspended, it would restore the balance.

These observations, however, require some modification when applied
to the case of bodies lighter than an equal bulk of water, and which,
therefore, do not sink entirely in water. The method of ascertaining
their specific gravity will be presently pointed out. At present we may
observe, that the rule given above has an application even to them, if
forcibly immersed in water ; but the resistance, or upper pressure of the
water, being greater than the weight of the body, that weight is not
merely diminished, but the body has a tendency upwards equal to the
difference between the resistance and its weight.

When a body is weighed in water, in order to ascertain its specific
gravity, it may either be suspended to a hook at the bottom of the basin
of the balance, or, taking off the basin, suspended to the arm of the
balance (fig. 7). Now, supposing that a cubic inch of gold weighed
nineteen ounces out of water, and lost one ounce by being weighed in
water, the cubic inch of water it displaces must weigh that one ounce :
consequently gold would be nineteen times as heavy as water.

The specific gravity of a body lighter than water cannot be ascertained
in the same manner. If a body were absolutely light, it would float on
the surface without displacing a drop of water ; but bodies have all some

Ix

INTRODUCTION TO HYDROSTATICS,

Fig. 7.

L

weight, and will, therefore, displace some quantity of water. A body
lighter than water will not sink to a level with the surface of the water,
and therefore will not displace so much water as is equal to its bulk, but
a quantity equal to its weight. A ship sinks to some depth in water, and
the heavier it is laden the deeper it sinks, the quantity of water it dis-
places being always equal to its weight. This quantity cannot, however,
afford a convenient test of its specific gravity, from the difficulty of
collecting the whole quantity of water displaced, and of measuring the
exact bulk of the body immersed.

In order practically to obtain the specific gravity of a body which is
lighter than water, a heavy one, whose specific gravity is known, must be
attached to it, and they must be immersed together: the specific gravity
of the lighter body may then be easily calculated.

Bodies which have exactly the same specific gravity as water, will remain
at rest in whatever situation they are placed in water. If a piece of wood,
by being impregnated with a little sand, be rendered precisely of the
weight of an equal bulk of water, it will t*remain stationary in whatever
part of a vessel of water it be placed. If a few drops of water be poured
into the vessel (so gently as not to increase their momentum by giving
them velocity), they would mix with the water at the surface, and not sink
lower.

The specific gravity of fluids is found by" means of an instrument
called an hydrometer. It consists of a thin glass ball, A
(Jig. 8), with a graduated tube,B, and the specific gravity of
the liquid is estimated by the depth to which the instrument
sinks in it, for the less the specific gravity of the fluid, the
further will the instrument sink in it. There is a smaller
ball, C, attached to the instrument below, which contains a
little mercury ; but this is merely for the purpose of equi-
poising the instrument, that it may remain upright in the
liquid under trial.

The weight of a substance, when not compared to that of any other, is
perfectly arbitrary ; and when water is adopted as a standard, we may
denominate its weight by any number we please ; but then the weight of
all bodies tried by this standard must be signified by proportional num-
bers. If we call the weight of water, for example, 1, then that of gold
would be 19; or, if we call the weight of water 1000, that of gold
would be 19,000. In short, the specific gravity indicates how much
more or less a body weighs than an equal bulk of water.

INTRODUCTION TO HYDROSTATICS. Ixi

SECTION II. — On Springs, Fountains, fyc.

us iiow turn our attention to the various states in which the water
belonging to our globe exists. It is the same water which successively
forms seas, rivers, springs, clouds, rain, and sometimes hail, snow, and
ice. We will follow it through these various changes, and consider how
the clouds were originally formed. When the first rays of the sun warm
the surface of the earth, the heat, by separating the particles of water,
transforms them into vapour, which, being lighter than the air, ascends
into the atmosphere. The atmosphere diminishing in density as it is
more distant from the earth, the vapour which the sun causes to exhale,
not only from seas, rivers, and lakes, but likewise from the moisture on
the land, rises till it reaches a region of air of its own specific gravity,
and there it remains stationary. By the frequent accession of fresh
vapour it gradually accumulates, so as to form those large bodies
of vapour which we call clouds ; and these at length becoming too heavy
for the air to support, fall to the earth in the form of rain. If the
watery particles retained the state of vapour, they would descend only
till they reached a stratum of air of their own specific gravity ; but during
their fall several of the watery particles come within the sphere of each
other's attraction, and unite in the form of a drop of water. The vapour,
thus transformed into a shower, is heavier than any part of the atmos-
phere, and consequently descends to the earth. Observe, that if the waters
were never drawn out of the earth, vegetation would be destroyed by the
excess of moisture ; if, on the other hand, the plants were not nourished
and refreshed by occasional showers, the drought would be equally fatal
to them. Were the clouds constantly in a state of vapour, they could
never fall to the ground ; or were the power of attraction more than suffi-
cient to convert the vapour into drops, it would transform the cloud into
a mass of water, which, instead of nourishing, would destroy the produce
of the earth. We cannot consider any part of Nature attentively without
being struck with admiration at the wisdom it displays : we cannot con-
template these wonders without feeling our hearts glow with admiration
and gratitude towards their bounteous Author.

Water, then, ascends in tne form of vapour, and descends in that
of rain, snow, or hail, all of which ultimately become water. Some of
this falls into the various bodies of water on the surface of the globe, the
remainder upon the land. Of the latter, part re-ascends in the form of
vapour, part is absorbed by the roots of vegetables, and part descends
into the bowels of the earth, where it forms springs. The only difference
between rain and spring water consists in the foreign particles which the
latter meets with and dissolves in its passage through the various soils it
traverses. Spring water being more pleasant to the taste, and more
transparent, is commonly supposed to be more pure than rain water.
Excepting distilled water, however, rain water is really the most pure we
can obtain : it is this which renders it insipid, whilst the various salts and
different ingredients dissolved in spring water, give it a species of flavour,
without in any degree affecting its transparency ; and the filtration it
undergoes through gravel and sand in the bowels of the earth cleanses it
from all foreign matter which it has not the power of dissolving*.

When rain falls on the surface of the earth, it continues making its way
downwards through the pores and crevices in the ground. Several drops
meet in their subterraneous passage, unite, and form a little rivulet : this,

Ixii INTRODUCTION TO HYDROSTATICS.

in its progress, meets with other rivulets of a similar description, and they
pursue their course together in the interior of the earth, till they are
stopped by some substance which they cannot penetrate ; for though
we have said that water under strong compression penetrates the pores
of gold, when acted upon by no other force than gravity it cannot make
its way even through a stratum of clay. This species of earth, though not
remarkably dense, being of great tenacity, will not admit the passage of
water. When, therefore, it encounters any substance of this nature, its
progress is stopped, and the pressure of the accumulating waters forms a
bed, or reservoir.

Fig. 9 represents a section of the interior of a hill or mountain. A is a
body of water such as I have described, which, when filled up as high as
B (by the continual accession of waters it receives from the ducts or

Fig 9.

rivulets «, a, a, «), finds a passage out of the cavity ; and, impelled by
gravity, runs on, till it makes its way out of the ground at the side of the
hill, and there forms a spring, C. The spring, during its passage from
B to C, rises occasionally, upon the same principle that water rises in the
spout of a tea-pot, but it cannot mount above the level of the reservoir,
whence it issues ; it must therefore find a passage to some part of the
surface of the earth that is lower or nearer the centre than the reservoir.
Water may thus be conveyed to every part of a town, and even to the
upper stories of the houses, provided that it be originally brought from a
height superior to any to which it is conveyed.

Reservoirs of water are seldom formed near the summit of a hill, for in
such elevated situations there can scarcely be a sufficient number of rills
to supply one ; and without a reservoir there can be no spring. In such
situations, therefore, it is necessary to dig deep wells, in order to meet
with a spring ; and then it can rise in the well only as high as the
reservoir whence it flows.

When reservoirs of water are formed in very elevated situations, the
springs which feed it descend from higher hills in the vicinity. There is
a lake on the very summit of Mount Cenis, which is supplied by the springs
of the higher Alps surrounding it.

A syphon is an instrument commonly used to draw off liquids from
large casks or other vessels which cannot be easily moved. It consists
simply of a bended tube. If its two legs are of equal length, and filled
with liquid, if held perfectly level, though turned downwards, the liquid

INTRODUCTION TO HYDROSTATICS. Ixiii

will not flow out, but remain suspended in the tube (Jig. 10) ; for there
is no pressure of the atmosphere above the liquid, while there is a
V in pressure from below upwards upon the open ends of the
tube ; and so long as this pressure is equal on both ends,
the liquid cannot flow out; but if the smallest inclination be
given to the syphon, so as to destroy the equilibrium of the
water, it will immediately flow from the lowest leg. When
syphons are used to draw off liquids, the legs are made of
unequal lengths, in order to render the pressure of the liquid
unequal ; the shortest leg is immersed in the cask, and the
liquor flows out through the longest. To accomplish this, it
is however necessary to make the liquor rise in the shortest
leg, and pass over the bended part of the tube, which is
higher than the level of the liquor in the cask. There are two modes of
doing this : one is, after immersing the shortest leg in the liquor to be
drawn off, to suck out the air of the tube from the orifice of the longest
leg ; then the liquor in the cask, which is exposed to the pressure of the
atmosphere, will be forced by it into the tube which is relieved from
pressure. As long as the tube continues full, no air can gain admittance ;
the liquor will therefore flow on till the cask is emptied. The other mode
is to fill the syphon with the liquor, then stopping the two ends with the
fingers, immerse the shortest leg in the vessel, and the same effect will
follow. In either case, the water in the highest part of the syphon must
not be more than 32 feet above the reservoir ; for the pressure of the
atmosphere will not support a greater height of water.

The phenomena of springs which flow occasionally, and occasionally
cease, may often be explained by the principle of the syphon. The reservoir
of water which supplies a spring may be considered as the vessel of
liquor to be drawn off, and the duct the syphon, having its shortest leg
opening in the reservoir, and its longest at the surface of the earth whence
the spring flows ; but as the water cannot be made to rise in the syphon
by either of the artificial modes which we have mentioned, the spring will
not begin to flow till the water in the reservoir has risen above the level
of the highest part of the syphon : it will then commence flowing upon
the principle of the equilibrium of fluids ; but it will continue upon the
principle of the syphon; for, instead of ceasing as soon as the equilibrium
is restored, it will continue flowing as long as the opening of the duct is
in contact with the water in the reservoir. Springs which do not con-
stantly flow are called intermitting, and are occasioned by the reservoir
being imperfectly supplied.

Reservoirs of water which are formed in the bosom of mountains gene-
rally find a vent either on their declivity, or in the valley beneath ; while
subterraneous reservoirs formed in a plain can seldom find a passage
to the surface of the earth, but remain concealed, unless discovered by
digging a well. When a spring once issues at the surface of the earth, it
forms a rivulet, and continues its course externally, seeking always
a lower ground, for it can no longer rise : if therefore it flows into
a- situation which is surrounded by a higher ground, its course is stopped,
the water accumulates, and forms a pool, pond, or lake, according to the
dimensions of the body of water. Thus the Lake of Geneva is filled by
the Rhone, which passes through it. When the river enters the valley
which forms the bed of the Lake, it finds itself surrounded by higher
grounds : its waters, consequently, are pent up, and accumulate till they
rise to a level with that part of the valley where the Rhone continues its

Fig. 11.

Ikiv INTRODUCTION TO HYDROSTATICS.

course out of the Lake, and from whence it flows through valleys, occa-
sionally forming other small lakes, till it reaches the sea.

A Fountain is a spring conducted perpendicularly upwards by a spout

or adjutage, A (Jig. 11). It would rise as
high as the reservoir, B, were its motion
not impeded by the resistance of the air
and the friction against the sides of the
spout whence it issues ; besides, as all the
particles of water spout from the tube with
an equal velocity, and as the pressure of the
air upon the exterior particles diminish their
velocity, they will in some degree strike
against the under parts, and force them
sideways, spreading the column into a head,
and rendering it both wider and shorter
than it otherwise would be. Besides this,
the resistance of the air prevents even the first particles projected from
the tube from rising to the height of the water in the reservoir. Were
there no such resistance, it would rise to that height, and no higher : of
course, being resisted, the elevation to which it rises is diminished. On
both accounts, therefore, the height of such a fountain falls very consi-
derably short of the height of the water in the reservoir.

INTRODUCTION TO PNEUMATICS.

SECTION I. — On the Mechanical Properties of Air.

WE shall now examine the second class of fluids, distinguished by the
name of aeriform, or elastic fluids, the principal of which is the air we
breathe, which surrounds the earth, and is called the atmosphere. There
are a great variety of elastic fluids, but they differ only in their chemical,
not in their mechanical properties ; and it is the latter we are to examine.
There is no attraction of cohesion between the particles of elastic fluids,
so that the expansive power of heat has no adversary to contend with but
gravity; any increase of temperature, therefore, expands elastic fluids
prodigiously, and a diminution proportionally condenses them. The most
essential point in which air differs from other fluids is by its spring or
elasticity : that is to say, its power of increasing or diminishing in bulk,
according as it is less or more compressed — a power of which liquids are
almost wholly deprived.

The atmosphere is thought to extend to about the distance of 45 miles
from the earth ; and its gravity is such, that a man of middling stature is
computed, when the air is heaviest, to sustain the weight of about 14 tons.
Such a weight would crush him to atoms, were it not that air is also con-
tained within our bodies, the spring or elasticity of which counterbalances
the weight of the external air, and renders us insensible of its pressure.
Besides this, the equality of pressure on every part of the body enables
us more easily to support it : when thus diffused, we can bear even a
much greater weight, without any considerable inconvenience. In bathing
we support the weight and pressure of the water, in addition to that of
the atmosphere ; but this pressure being equally distributed over the body,
we are scarcely sensible of it : whilst if the shoulders, the head, or any
particular part of the frame were loaded with the additional weight of a
hundred pounds, we should feel severe fatigue. On the other hand, if
the air within a man met with no external pressure to restrain its elasticity,
it would distend his body, and at length bursting the parts which confine
it, put a period to his existence. The weight of the atmosphere, there-
fore, so far from being an evil, is essential to our existence. When a
person is cupped, the swelling of the part under the cup is produced by
taking away the pressure of the atmosphere; in consequence of which,
the internal air distends the part. The air-pump affords us the means
of making a great variety of interesting experiments on the weight and
pressure of the air. We have already seen, that in a vacuum produced
within the air-pump, substances of various weights fall to the bottom in
the same time.

We shall now point out some experiments which illustrate both the weight
and elasticity of air. If a piece of bladder be tied over a glass receiver,
open both at the top and bottom, when the air is taken away from the
under surface, so that there is no longer any re-action to counterbalance
the pressure of the atmosphere, the bladder is pressed inwards in propor-
tion as the receiver is exhausted ; and before a complete vacuum or void
is formed within the receiver, the bladder, unable to sustain the violence
of the pressure, bursts with a loud report.

F

Ixvi

INTRODUCTION TO PNEUMATICS.

A shrivelled apple placed within a receiver becomes plump from the
expansion of the air within it, as soon as the pressure of the external air is
taken away, and shrinks to its former dimensions when the air is again
let into the receiver. If two bodies be placed so close together that there
is absolutely no air between them, to counterbalance by its elasticity the
pressure of the air on their outer surfaces, that whole pressure resists
their separation. It is thus that a stone may be raised by a string fixed to
a piece of moistened leather pressed close upon it ; and thus also that in
the experiment detailed in page 6, the pressure of the atmosphere, as
well as the cohesion of the metallic hemispheres, prevents their separa-
tion.

A column of air reaching to the top of the atmosphere, and whose base
is a square inch, weighs 15 Ibs. when xthe air is heaviest. The rule that
fluids press equally in all directions applies to elastic fluids as well as to
liquids : therefore every square inch of our bodies sustains a pressure of
15Jbs., and the weight of the whole atmosphere may be computed by
calculating the number of the square inches on the surface of the earth,
and multiplying them by 15.

The weight of a small quantity of air may be ascertained by exhausting
the air from a bottle, and weighing the bottle thus emptied. Suppose
that a bottle, six cubic inches in dimension, weighs two ounces; if the air
be then introduced, and the bottle re-weighed, it will be found heavier by
two grains, shewing that six cubic inches of air (at a moderate tempera-
ture) weighs about two grains. In estimating the weight of air, the tem-
perature must always be considered, because heat, by rarefying air, ren-
ders it lighter. The same principle indeed applies, almost without
exception, to all bodies. In order to ascertain the specific gravity of air,
the same bottle may be filled with water, and the weight of six cubic
inches of water will be 1515 grains : so that the weight of water to that of
air is about 800 to 1.

A barometer is an instrument which indicates the state of the weather,
by shewing the weight of the atmosphere. It is extremely
simple in its construction, and consists of a glass tube,
A B (fig- 1), about three feet in length, and open only at
one end. This tube must first be filled with mercury, then
stopping the open end with the finger, it is immersed in a
cup, C, which contains a little mercury. Part of the mer-
cury which was in the tube now falls down into the cup,
leaving a vacant space in the upper part of the tube, to
which the air cannot gain access. This space is therefore
a perfect vacuum ; and consequently the mercury in the
tube is relieved from the pressure of the atmosphere,
whilst that in the cup remains exposed to it: therefore the
pressure of the air on the mercury in the cup supports that
in the tube, and prevents it from falling: thus the equili-
brium of the mercury is destroyed only to preserve the
general equilibrium of fluids. This simple apparatus is
all that is essential to a barometer. The tube and the cup
or vase are fixed on a board, for the convenience of sus-
pending it; the board is graduated for the purpose of
ascertaining the height at which the mercury stands in ihe
tube ; and the small moveable metal plate serves to show
that height with greater accuracy. The weight of the
atmosphere sustains the mercury at the height of about

Fig. 1.

INTRODUCTION TO PNEUMATICS. Ixvii

29J inches ; but the exact height depends upon the weight of the atmos-
phere, which varies much according to the state of the weather. The
greater the pressure of the air on the mercury in the cup, the higher it
will ascend in the tube. The air therefore generally is heaviest in dry
weather, for then the mercury rises in the tube, and consequently that
in the cup sustains the greatest pressure ; and thus we estimate the
dryness and fairness of the weather by the height of the mercury. We
are apt to think the air feels heavy in bad weather, because it is less
salubrious when impregnated with damp. The lungs, under these cir-
cumstances, do not play so freely, nor does the blood circulate so well :
thus obstructions are frequently occasioned in the smaller vessels, from
which arise colds, asthmas, agues, fevers, &c.

As the atmosphere diminishes in density in the upper regions, the air
must be more rare upon a hill than in a plain ; and this difference may
be ascertained by the barometer. This instrument is so exact in its
indications, that it is used for the purpose of measuring the height of
mountains, and of estimating the elevation of balloons. Considerable
inconvenience is often experienced from the thinness of the air in such
elevated situations. It is sometimes oppressive, from being insufficient
for respiration ; and the expansion which takes place in the more dense
air contained within the body is often painful : it occasions distension,
and sometimes causes the bursting of the smaller blood-vessels in the
nose and ears. Besides, in such situations, you are more exposed
both to heat and cold ; for though the atmosphere is itself transparent,
its lower regions abound with vapours and exhalations from the earth,
which float in it, and act in some degree as a covering, which preserves
us equally from the intensity of the sun's rays, and from the severity of
the cold.

Now since the weight of the atmosphere supports mercury in the tube
of a barometer, it will support a column of any other fluid in the same
manner ; but as mercury is the heaviest of all fluids, it will support a
higher column of any other fluid ; for two fluids are in equilibrium, when
their heights vary inversely as their densities : as, for instance, if a cubic
foot of one fluid weighs twice as much as a cubic foot of the other, a
column of the first ten feet in height will weigh as much as a column of
the other twenty feet in height. Thus the pressure of the atmosphere,
which will sustain a column of mercury of twenty-nine inches, is equal to
sustaininn- a column of water of no less than thirty-four feet above its level.
The weight of the atmosphere is therefore as great as that of a body of
water surrounding the globe of the depth of thirty-four feet ; for a column
of air of the height of the atmosphere is equal to a column of water of
thirty-four feet, or one of mercury of twenty-nine inches, each having the
same base.

The common pump is constructed on this principle. By the act of
pumping, the pressure of the atmosphere is taken off one part of the sur-
face of the water : this part therefore rises, being forced up by the pressure
communicated to it by that part of the water on the surface of which the
weight of the atmosphere continues to act. The body of a pump consists
of a large tube or pipe, whose lower end is immersed in the water which
it is designed to raise. A kind of stopper, called a piston, is fitted to
this tube, and is made to slide up and down it, by means of a metallic
rod fastened to the centre of the piston.

The various parts of a pump are delineated in jig. 2 (next page). A B is
the pipe or body of the pump ; P the piston ; V a valve,, or little door iu

F2

INTRODUCTION TO PNEUMATICS.

the piston, which, opening upwards, admits the water to rise through it,
but prevents its returning ; and y a similar valve in the body of the
pump. When the pump is in a state of inaction, the two valves are closed
by their own weight ; but when, by drawing down the handle of the
pump, the piston ascends, it raises a column of air which rested upon it,
and produces a vacuum between the piston and the lower valve, Y: the
air beneath this valve, which is immediately over the surface of the water,
consequently expands, and forces its way through it; the water then,
relieved from the pressure of the air, ascends into the pump. A few
strokes of the handle totally exclude the air from the body of the pump,
and fill it with water, which, having passed through both the valves,
flows out at the spout. Thus the air and the water successively rise in
the pump on the same principle that the mercury rises in the barometer.
Water is said to be drawn up into a pump by suction ; but the power of
suction is no other than that of producing a vacuum over one part of the
liquid, into which vacuum the liquid is forced by the pressure of the
atmosphere on another part. .vThe action of sucking through a straw con-
sists in drawing in and confining the breath, so as to produce a vacuum,
3>r at least to lessen materially the quantity of air, in the mouth : in con-
sequence of which, the air within the straw rushes into the mouth, and is
followed by the liquid, into which the lower end of the straw is immersed.
The principle is the same ; and the only difference consists in the mode of
producing a vacuum. In suction, the muscular powers answer the pur-

Fig. 2. Fig. 3.

pose of the piston and valves. The distance from the level of the water
in the well to the valve in the piston ought not to exceed thirty-two
feet, otherwise the water would not be sure to rise through that valve,
for the weight of the air is sometimes not sufficient to raise a column of
mercury more than twenty-eight inches, or a column of water much more
than thirty-two feet ; but when once it has passed that opening, it is no
longer the pressure of air on the reservoir which makes it ascend — it is
raised by lifting it up, as you would raise it in a bucket, of which the

INTRODUCTION TO PNEUMATICS. Ixix

piston formed the bottom. This common pump is, therefore, called the
sucking and lifting pump, as it is constructed on both these principles.

The forcing1 pump consists of a forcing power added to the sucking
part of the pump. This additional power is exactly on the principle of
the syringe : by raising the piston, the water is drawn up into the pump ;
and by making it descend, it is forced out. The large pipe, A B (Jig. 3),
represents the sucking part of the pump, which differs from the lifting
pump only in its piston, P, being unfurnished with a valve, in consequence
of which the water cannot rise above it. When, therefore, the piston
descends, it shuts the valve, y, and forces the water (which has no other
vent) into the pipe, D : this is likewise furnished with a valve, V, which,
opening outwards, admits the water, but prevents its return. The water
is thus first raised in the pump, and then forced into the pipe, by the
alternate ascending and descending motion of the piston, after a few
strokes of the handle to fill the pipe, from whence the water issues°at the
spout.

SECTION II. — On Wind and Sound.

WE are now to give some account of the nature of Wind and Sound.
Wind is the motion of a stream or current of air, which may be produced
by a variety of causes ; but the most common one is a partial change of
temperature in the atmosphere : for when any one part is more heated
than the rest, that part is rarefied ; and thus, becoming lighter than the
air around, it rises, and the surrounding air presses in towards that part, in
order to restore the equilibrum : this spot, therefore, receives wind from
every quarter. Those who live to the north of it experience a north wind ;
those to the south, a south wind; and those who live on the spot where these
winds meet and interfere have turbulent and boisterous weather — whirl-
winds, hurricanes, rain, lightning, thunder, &c. This stormy weather occurs
most frequently in the torrid zone, where the heat is greatest: the air
being more rarefied there than in any other part of the globe, is lighter,
and consequently ascends; whilst the air about the polar .regions is
continually flowing from the poles, to restore the equilibrium. This
motion of the air, did no obstacles interfere, would produce a regular
and constant north wind to the inhabitants of the northern hemi-
sphere, and a south wind to those of the southern hemisphere ; but
these winds do not meet without previously changing their direction.
The atmosphere accompanies the earth in its diurnal motion : it travels,
therefore, with greater or less velocity as it is nearer the equator,
or more distant from it. When therefore the air flows from the north
or south to restore the atmospherical equilibrium at the equator, this
air, not having acquired the velocity of the equatorial regions, cannot
keep pace with the earth, which, travelling faster, passes through it ; and
as the earth moves from west to east, its motion through the air produces
a regular east wind at the equator. The winds from the north and south
combine with this easterly wind, and form what are called the trade-
winds. The composition of the two winds north and east produces a
constant north-east wind ; and that of the two winds south and east pro-
duces a regular south-east wind. These winds extend to about thirty

Ixx INTRODUCTION TO PNEUMATICS.

degrees on each side of the equator, the regions further distant from it
experiencing- only their respective north and south winds.

The light air about the equator, which expands and rises into the upper
regions of the atmosphere, ultimately flows from thence back to the poles,
to restore the equilibrium. If it were not for this resource, the polar
atmospheric regions would soon be exhausted by the stream of air, which,
in the lower strata of the atmosphere, they are constantly sending towards
the equator. There is therefore a sort of circulation in the atmosphere:
the air in the lower strata flowing from the poles towards the equator,
and in the upper strata flowing back from the equator towards the poles.
An example of this circulation on a small scale may be seen in the air of
a room, which being more rarefied than the external air, a current is
pouring in from the crevices of the windows and doors, to restore the equi-
librium ; but the light air with which the room is filled must find some
vent, in order to make way for the heavy air which enters. If the door
be set a-jar, and a candle held near the upper part of it, the flame will be
blown outwards, showing that there is a current of air flowing out from
the upper part of the room; and if the candle be placed on the floor
close by the door, the flame will bend inwards, shewing that there is also
a current of air setting into the lower part of the room. The upper
current is the warm, light air, which is driven out to make way for the
stream of cold, dense air which enters below.

There are also periodical trade-winds, commonly called monsoons,
which change their course every half-year. This variation is produced
by the earth's annual course round the sun, when the north pole is
inclined towards that luminary one-half of the year, and the south pole
the other half. During the summer of the northern hemisphere, the
countries of Arabia, Persia, India, and China, are much heated, and
reflect great quantities of the sun's rays into the atmosphere, by which it
becomes extremely rarefied, and the equilibrium consequently destroyed.
In order to restore it, the air from the equatorial southern regions, where
it is colder (as well as from the colder northern parts) must necessarily
have a motion towards those parts. The current of air from the equa-
torial regions produces the trade-winds for the first six months in all the
seas between the heated continent of Asia and the equator. During the
other six months, when it is summer in the southern hemisphere, the ocean
and countries towards the southern tropic are most heated, and the air
over those parts most rarefied ; then the air about the equator alters its
course, and flows in an opposite direction.

The breaking-up of the monsoons is the name given by sailors to the
shifting of the periodical winds; they do not change their course sud-
denly, but by degrees, as the sun moves from one hemisphere to the other.
This change is usually attended by storms and hurricanes, so that those
seas are seldom navigated at the season of the equinox.

It is less easy to account for the great variety of winds which prevail in
the temperate zones ; but when we consider that so large a portion of the
atmosphere is in continual agitation in the torrid zone, these agitations in
an elastic fluid, which yields to the slightest impression, must extend
every way to a great distance. The air in all climates will suffer more or
less perturbation, according to the situation of the country, the position
of mountains, valleys, and a variety of other causes : hence it is easy to
conceive that almost every climate must be liable to variable winds. On
the sea-shore a gentle sea-breeze generally sets in on the land in the

INTRODUCTION TO PNEUMATICS. Ixxi

afternoon, to restore the equilibrium which had been disturbed by reflec-
tions from the heated surface of the shore during the earlier part of the
day ; and about midnight, when the earth is cooled and the air condensed,
it flows back towards the sea.

The air, being a gravitating fluid, is affected by the attraction of the
moon and the sun, in the same manner as the water, and must there-
fore have tides. These tides, however, are of no practical interest or
importance.

We have considered the effects produced by the wide and extended
agitation of the air ; but there is another kind of agitation of which the
air isvsusceptible — a sort of vibratory tremulous motion, which, striking
on the drum of the 'ear, produces Sound. Sonorous bodies, such as bells,
musical instruments, &c., are merely the agents by means of which that
peculiar species of motion is communicated to the air. A bell runs; in
vacuo under the air-pump gives no sound.

Air, though by far the most common, is not the only vehicle of sound.
Liquids are capable of conveying the vibratory motion of a sonorous body
to the organ of hearing; for sound can be heard under water. Solid bodies
also convey sound, as you may be convinced by a very simple experiment.
If a string be fastened, round a poker, by the middle; the poker raised
from the ground by the two ends of the string, and one end being held to
each ear, if the poker be then struck with a key, the sound will be con-
veyed to the ear by means of the strings in a much more perfect manner
than if it had no other vehicle than the air.

Bodies are called sonorous which produce clear, distinct, regular, and
durable sounds — such as a bell, a drum, musical strings, wind instru-
ments, &c. They owe this property to their elasticity; for an elastic
body, after having been struck, not only returns to its former situation,
but, having acquired momentum by its velocity, like the pendulum, it
springs out on the^ opposite side. If the string A B (Jig. 4), which is

[Fig. 4.

made fast at both ends, be drawn on one side to C, it will not only
return to its original position, but proceed onwards to D. This is the
first vibration; at its t termination, the string, being stretched into the
position A D B, will again tend to return to its natural state A B, and
will, therefore, return to that line and pass on beyond it to E, and thence
back again to F : then in the same manner to G and H ; the resistance
of the air continually destroying some of the motion, so that the extreme
points, EFG H, are continually nearer to the line AB, until the whole
motion is destroyed, and the string comes to rest in the position A B.

The tremulous motion given to the air by the vibration of a sonorous
body is very similar to the motion communicated to smooth water when a
stone is thrown into it. This first produces a small circular wave round
the spot in which the stone falls : the wave spreads, and gradually com-
municates its motion to the adjacent waters, producing^similar waves to

Ixxii INTRODUCTION TO PNEUMATICS.

a considerable extent. The same kind of waves are produced in the air
by the motion of a sonorous body ; but with this difference, that air, being
an elastic fluid, the motion does not consist of regularly extending
waves, but of vibrations composed of a motion forwards and backwards,
similar to those of a sonorous body. The aerial undulations also take
place in all directions, and are spherical. The first sphere of undulations
which are produced immediately around the sonorous body, by pressing
against the contiguous air, condenses it. The condensed air, though
impelled forward by the pressure, re-acts on the first set of undulations,
driving them back again. The second set of undulations which have
been put in motion, in their turn communicate their motion, and are
themselves driven back by re-action. Thus there is a succession of waves
in the air. The air is immediately put in motion by the firing of a
cannon ; but it requires time for the vibrations to extend to any distant
spot. The velocity of sound in air is computed to be at the rate of 1142
feet in a second.

The direction of the wind makes less difference in the velocity of sound
than might be imagined. If the wind sets from us, it bears most of the
aerial waves away, and renders the sound fainter; but it is not very
considerably longer in reaching the ear than if the wind blew towards us.
In fact, the wind cannot possibly retard the progress of the sound, by
more than its own rate of motion : and as the velocity of sound is about
780 miles in an hour, the velocity of even a high wind bears too small a
proportion to it to affect very materially the rate at which sound travels.
The nearly uniform velocity of sound enables us to determine the distance
of the object whence it proceeds : as that of a vessel at sea firing a cannon,
or that of a thunder-cloud. If we do not hear the thunder till half a
minute after we see the lightning, we conclude the cloud to be at the dis-
tance of six miles and a half.

An echo is produced when the aerial vibrations meet with an obstacle
having a hard and regular surface, such as a wall, or rock: they may thus
be reflected back to the ear, and produce the same sound a second time ;
but the sound will then appear to proceed from the object by which it
is reflected. If the vibrations fall perpendicularly on the obstacle, they
are reflected back in the same line j if obliquely, the sound returns
obliquely on the other side of the perpendicular, the angle of reflection
being equal to the angle of incidence.

Speaking-trumpets are constructed on the principle of the reflection of
sound. The voice, instead of being diffused in the open air, is confined
within the trumpet; and the vibrations which spread and fall against the
sides of the instrument, are reflected according to the angle of incidence,
and the form of the instrument is so regulated, that the whole of the
vibrations are collected into a focus ; and if the ear be situated in or
near that spot, the sound is prodigiously increased. Fig. 5 represents the

Fig. 5.

speaking-trumpet; the rays, as they issue from its mouth, are distin-
guished by being dotted ; and they are brought to a focus at F. The

INTRODUCTION TO PNEUMATICS. Ixxiii

trumpet used by deaf persons acts on the same principle ; the rays, in
this case, being collected into a focus near the smaller end of the trumpet
which is applied to the ear. The trumpets used as musical instruments
are also constructed on this principle, so far as their form tends to
increase the sound ; but, as a musical instrument, the trumpet becomes
itself the sonorous body, which is made to vibrate by blowing into it, and
communicates its vibrations to the air.

If a sonorous body be struck in such a manner that its vibrations are all
performed in equal times, the vibrations of the air will correspond with
them, and be equal also; and thus, striking uniformly on the drum of the
ear, they produce an uniform sensation on the auditory nerve, and excite
the same uniform idea in the mind ; or, in other words, we shall hear one
musical tone. But if the vibrations of the sonorous body be irregular,
there will necessarily follow a confusion of aerial vibrations ; for a second
vibration may commence before the first is finished, meet it half way on its
return, and interrupt it in its course. The quicker a sonorous body
vibrates, the more acute or sharp is the sound produced. The duration
of the vibrations of strings or chords depends upon their length, the
thickness or weight, and their degree of tension : thus the low, bass notes
of a harp or piano are produced by long, thick, loose strings ; and the
high, treble notes, by those which are short, small, and tightly strung : so
that the different length and size of the strings serves to vary the duration
of the vibrations, and, consequently, the acuteness or gravity of the notes.

Among the variety of tones, there are some which, sounded together,
please the ear, producing what we call harmony, or concord. This is
thought to arise from the agreement of the vibrations of the two sonorous
bodies ; so that some of the vibrations of each strike upon the ear at the
same time. Thus, if the vibrations of two strings are performed in equal
times, the same tone is produced by both, and they are said to be in
unison. If a violin is to be tuned in unison with another, the strings
must be drawn tighter if too low, or loosened if at too high a pitch, in
order to bring them to vibrate in equal times with the strings of the
other instrument.

But concord is not confined to unison, for two different tones har-
monize in a variety of cases. If one string (or any sonorous body
whatever) vibrate in double the time of another, the second vibration of
the latter will strike upon the ear at the same instant as the first vibration
of the former ; and this is the concord of an octave. If the vibrations of
two strings are as two to three, the third vibration of the first corresponds
with the fourth vibration of the latter, producing the harmony called a
fifth : so that when the key-note is struck with its fifth, you hear every
third vibration of one, and every fourth of the other at the same time.
The key-note struck with the fourth is likewise a concord, and the vibra-
tions are as three to four. The vibrations of a major third with the key-
note are as four to five ; and those of a minor third, as five to six.

There are other tones which, though they cannot be struck together
without producing discord, if struck successively, give us the pleasure
which is called melody.

INTRODUCTION TO OPTICS,

SECTION I. — On Optics.

OPTICS is one of the most interesting branches of Natural Philosophy:
it is the science of vision, and teaches us how we see objects. In
this science, bodies are divided into luminous, opaque, and transparent.
A luminous body is one that shines by its own light — as the sun, the fire,
a candle, &c. But all bodies that shine are not luminous: polished
metal, for instance, when it shines with so much brilliancy, is not a lumi-
nous body, for it would be dark if it did not receive light from a luminous
body : it belongs, therefore, to the class of opaque, or dark bodies, which
comprehend all such as are neither luminous nor will admit the light
to pass through them ; and transparent bodies are those which admit
the light to pass through them,

such as glass and water. Trans- Fig. 1.

parent or pellucid bodies are fre-
quently called mediums ; and the
rays of light which pass through
them are said to be transmitted by
them. Light, when emitted from
the sun, or any other luminous
body, is projected forwards in
straight lines in every possible di-
rection, or at least appears to move
as it would on that supposition :
so that the luminous body not only
seems the general centre whence
all the rays proceed, but every
point of it may be considered as a
centre which radiates light in every
direction (Jig. 1). A ray of light
is a single line of light projected
from a luminous body ; and a pen-
cil of rays is a collection of rays
proceeding from any one point of
a luminous body, as^g-. 2.

Philosophers are not agreed as
to the nature of light. Some main-
tain the opinion that it is a body consisting of detached particles, which are
emitted by luminous bodies, in which case the particles of light must be
inconceivably minute, since, though they must cross each other in every
direction, they are never known to interfere with each other; others sup-
pose it to be produced like sound, by the undulations of a subtle fluid
diffused throughout all known space. In some respects, light is obedient
to the laws which govern bodies ; in others, it appears to be independent

Fig. 2.

INTRODUCTION TO OPTICS.

Ixxv

of them. Thus, though its course corresponds with the laws of motion,
it does not seem to be influenced by those of gravity ; for it has never
been discovered to have weight, though a variety of experiments have
been made with a view of ascertaining that point. We are, however, so
ignorant of the .intimate nature of light, that an attempt to investigate it
would lead us into a labyrinth of perplexity, if not of error. We shall
therefore confine our attention to such of its properties as are well ascer-
tained.

To return then to the examination of the effects of the radiation of light
from a luminous body ; since the rays are projected in straight lines,
when they meet with an opaque body through which they are unable to
pass, they are stopped short in their course, for they cannot move in a
curve line round the body. The interruption of the rays of light by the
opaque body produces therefore darkness on the opposite side of it ; and
if this darkness fall upon a wall, a sheet of paper, or any object what-
ever, it forms a shadow, for shadow is nothing more than darkness
produced by the intervention of an opaque body, which prevents the
rays of light from reaching an object behind it. You might suppose
from this definition of a shadow, that it would be perfectly black ; but it
frequently happens that light from another body reaches the space where
the shadow is formed, in which case the shadow is proportionally fainter.
This happens if the opaque body be lighted by two candles : if you
extinguish one of them, the shadow will be both deeper and more distinct.
Yet it will not be perfectly dark, because it is still slightly illuminated by
light reflected from the walls of the room, and other surrounding objects.

There are several things to be

observed in regard to the form and l&' '

extent of shadows. If the lumi-
nous body A (Jig. 3) be larger than
the opaque body, B, the shadow
will gradually diminish in size till it
terminate in a point j if smaller,
the shadow will continually increase
in size, as it is more distant from
the object which projects it. The shadow of a figure, A, (Jig. 4) varies
in size according to the distance of the several surfaces, B, C, D, E, on

Fig. 4.

which if is described. Two lights produce two shadows from the same
object. The number of lights (in different directions), while it decreases
the intensity of the shadows, increases their number which always corre-

Ixxvi

INTRODUCTION TO OPTICS.

spends with that of the lights ; for each light makes the opaque body cast
a different shadow, as illustrated by Jig. 5. It represents a ball, A,
lighted by three candles, BCD: the light B produces the shadow 6, the
light C the shadow c, and the light D the shadow d. Now what becomes
of the rays of light which opaque bodies arrest in their course, and the
interruption of which is the occasion of shadows ? Thfs leads to a very
important property of light, Reflection.

Fig. 5.

When rays of light encounter an opaque body, which they cannot tra-
verse, part of them are absorbed by it, and part are reflected, and rebound
as an elastic ball which is struck against a wall. Light in its reflection
is governed by the same laws as solid perfectly elastic bodies. If a
ray of light fall perpendicularly on an opaque body, it is reflected back in
the same line towards the point whence it proceeded ; if it fall obliquely,
it is reflected obliquely, but in the opposite direction, the angle of inci-
dence being equal to the angle of reflection.* If the shutters be closed,
and a ray of the sun's light admitted through a very small aperture, and
reflected by a mirror, on which the ray falls perpendicularly, but one ray
is seen, for the ray of incidence and that of reflection are both in the
same line, though in opposite directions, and thus are confounded together.
The ray, therefore, which appears single, is in fact double, being composed
of the incident ray proceeding to the mirror and the reflected ray returning
from the mirror. These may be sepa- Fig. 6.

rated by holding the mirror, M (fig. 6),
in such a manner that the incident ray,
A B, shall fall obliquely upon it ; then
the reflected ray, B C, will go off in
another direction. If a line be drawn
from the point of incidence, B, perpendi-
cularly to the mirror, it will divide the
angle of incidence from the angle of re-
flection, and these angles will be equal.

It is by reflected rays only that we see opaque objects, Luminous

* See Mechanics.

INTRODUCTION TO OPTICS. Ixxvii

bodies send rays of Jig-lit immediately to our eyes ; but the rays which
they send to other bodies are invisible to us, and are seen only when
reflected or transmitted by those bodies to our eyes. Yet it may be
observed that the ray of light on its passage from the sun to the mirror,
arid its reflection, have been spoken of as visible, though in neither case
were those rays in a direction to enter our eyes. The fact is, that what is
seen is the light reflected to the eye by small particles of dust floating in
the air, and on which the ray shone in its passage to and from the
mirror. So when, in common phrase, we speak of seeing the sun shining
on an opposite house, it is impossible to see a single ray which passes
from the sun to the house : no rays are visible but those which enter our
eyes ; therefore it is the rays which are reflected by the house, and not
those which proceed from the sun to the house, that are seen. Why,
then, does one side of the house appear to be in sunshine, and the
other in the shade ; for if we cannot see the sun shine upon it, the
whole of the house should appear in the shade ? That side of the house
on which the sun shines reflects more vivid and luminous rays than the
side which is in shadow, the latter being illumined only by rays reflected
upon it by other objects : these rays are therefore twice reflected before
they reach our sight ; and as light is more or less absorbed by the bodies
it strikes upon, every time a ray is reflected its intensity is diminished.
Thus, on a large sheet of water the sun appears to shine on one part
only, though the whole of it is equally exposed to its rays. This partial
brilliancy of water is more remarkable by moonlight, on account of the
deep obscurity of the surrounding parts. To account for this, it must be
remembered that the direction of a reflected ray depends on that of the
incident ray; the sun's rays, therefore, which fall with various degrees of
obliquity upon the water, are reflected in directions equally various : some
of these will meet the eye, and it will see them, but those which fall
elsewhere are invisible to it*.

Let us now examine by what means the rays of light produce vision.
They enter at the pupil of the eye, and proceeding1 to the retina or optic
nerve, which is situated at the back of the eye-ball, there describe the
figure, colour, and (with the exception of size) form a complete repre-
sentation of the object from which they proceed. If the shutters be
closed, and a ray of light admitted through the small aperture, a picture
may be seen on the opposite wall similar to that which is delineated on
the retina of the eye: it exhibits a picture in miniature of the garden, and
the landscape would be perfect were it not reversed. This picture is
produced by the rays of light reflected from the various objects in the
garden, and which are admitted through the hole in the window-shutter*
It is called a camera obscura {dark chamber}, from the necessity of
darkening the room in order to exhibit it.

The rays from the glittering weathercock at the top of the alcove, A
(fig. 7), represent it at a; for the weathercock being much higher than
the aperture in the shutter, only a few of the rays, which are reflected by
it in an obliquely descending direction, can find entrance there. The
rays of light moving always in straight lines, those which enter the room
in a descending direction will continue their course in the same direction,
and will, consequently, fall upon the lower part of the wall opposite the
aperture, and represent the weathercock reversed in that spot, instead of
erect in the uppermost part of the landscape ; and the rays of light from
the steps, B, of the alcove, in entering thejaperture, ascend, and describe
* See Mechanics.

Ixxviii

INTRODUCTION TO OPTICS,

them in the highest instead of the lowest part of the landscape ; whilst
the rays proceeding from the alcove, which is to the left, describe it on
the wall to the right. Those which are reflected by the walnut-tree, C D,
to the right, delineate its figure in the picture to the left, c d. Thus the
rays, coming in different directions, and proceeding always in straight
lines, cross each other at their entrance through the apertures : those
from above proceed below, those from the right go to the left, those from
the left towards the right; thus every object is represented in the picture
as occupying a situation the very reverse of that which it does in nature,
excepting the flower-pot, E F, which, though its position is reversed,
does not change its situation in the landscape ; for being immediately in
front of the aperture, its rays fall perpendicularly upon it, and conse-
quently proceed perpendicularly to the wall, where they delineate the
object. It is thus that the picture of objects is painted on the retina of
the eye. The pupil of the eye, through which the rays of light enter,
represents the aperture in the window-shutter ; and the image delineated
on the retina is exactly similar to the picture on the wall.

The retina of the eye exhibits a much more perfect image than any
mirror : the extensive landscape beheld from the window is there repre-
sented with the greatest accuracy. Art would in vain attempt to paint
so small and distinct a miniature ; but Nature works with a surer hand
and a more delicate pencil. That Power which forms the feathers of the
butterfly and the flowerets of the daisy can alone pourtray so admir-
able and perfect a miniature. As the rays intersect each other on entering
the pupil, in the same manner as they do on entering the camera obscura,
the image is reversed. The scene, however, does not excite the idea
of being inverted, because we always see an object in the direction of the
rays which it sends to us. How it is that we do so is a point rather diffi-
cult to explain clearly. The following, however, seems to be the best
explanation : — A ray which comes from the upper part of an obje«t de-
scribes the image on the lower part of the retina; but experience having
taught us that a ray which strikes the retina there comes from above, we
consider that part of the object it represents as uppermost. The rays

INTRODUCTION TO OPTICS.

Ixxix

proceeding from the lower part of an object fall upon the upper part of
the retina ; but as we know their direction to be from below, we see that
part of the object they describe as the lowest. When you wish to see
an object above you, you look upwards ; when an object below, you look
downwards. You look up to see an elevated object, for it is only thus
that the rays which proceed from it fall upon the retina of your eyes,
and they must do so if you are to see the object ; but the very circumstance
of directing your eyes upwards convinces you that the object is elevated,
and teaches you to consider as uppermost the image it forms on the retina,
though it is in fact represented in the lowest part of it. When yon look
down upon an object, you draw your conclusion from a similar reason-
ing. It is thus that we see all objects in the direction of the rays which
reach our eyes.

The different apparent dimensions of objects at different distances pro-
ceed from our seeing, not the objects themselves, but merely their image
on the retina. Fig. 8 represents a row of trees, as viewed in the camera

Fig. 8.

obscura ; the direction of the rays from the objects to the image is ex-
pressed by lines. Observe that the ray which comes from the top of.the
nearest tree, and that which comes from the foot of the same tree, meet
at the aperture, forming an angle of about twenty-five degrees : this is
called the angle of vision, being that under which we see the tree. These
rays cross each other at the aperture, and represent the tree inverted in
the camera obscura. The dimensions of the image are considerably
smaller than those of the object, but the proportions are perfectly pre-
served. The upper and lower ray, from the most distant tree, form an
angle of not more than twelve or fifteen degrees, and an image of propor-
tional dimensions. Thus two objects of the same size, as the two trees
of the avenue, form figures of different sizes in the camera obscura,
according to their distance, or, in other words, according to the angle of
vision under which they are seen.

The experience we acquire by the sense of touch corrects the errors
of our sight with regard to objects within our reach ; we are so perfectly
convinced of the real size of objects which we can handle, that we do not
attend to their apparent difference. The opposite house does not appear
to you much smaller than if you were close to it ; and yet you see the
whole of it through one of the windows of the room you sit in, and the
image of the house on your retina must be very considerably smaller than
that of the window through which you see it. Those accustomed to draw
from nature are well aware of this difference. When we look up an
avenue, the trees not only appear smaller as they are more distant, but
seem gradually to approach each other till they meet in a point, for the

Ixxx

INTRODUCTION TO OPTICS.

road which separates the two rows forms a smaller visual angle, in pro-
portion as it is more distant from us ; therefore the width of the road
seems gradually to diminish as well as the size of the trees, till at length
the road apparently terminates in a point, at which the trees seem to
meet.

In sculpture we copy Nature as she really exists ; in painting we
represent her as she appears to us — that is to say, we do not copy the
objects, but the image they form on the retina of the eye.

If an object, with an ordinary degree of illumination, does not sub-
tend an angle of more than half a minute of a degree, it is invisible.
There are, consequently, two cases in which objects may be invisible,
either if they are too small, or so distant as to form an angle less than one
second of a degree. The fixed stars subtend much smaller angles, and
yet are visible ; but they are bodies luminous in themselves, and possess
much more than an ordinary degree of illumination. In like manner, if
the velocity of a body be so small that the arc which it describes in an
hour does not subtend an angle of more than twenty degrees, its motion is
imperceptible : consequently a very rapid motion may then be imper-
ceptible, provided the distance of the moving body be sufficiently great ;
for the greater its distance, the smaller will be the angle under which its
motion will appear to the eye. It is for this reason that the motion of
the celestial bodies is invisible, notwithstanding their immense velocity ;
for the greatest apparent motion of any celestial body does not exceed
fifteen degrees in an hour, being that seemingly produced in a body at
the equator by the revolution of the earth. The greatest of the real
motions is that of the moon, and even that does not exceed about thirteen
degrees in a day. The real velocity depends altogether on the space
comprehended in each degree ; and this space depends on the distance
of the object and the obliquity of its path. Now we cannot judge of the
velocity of a body in motion unless we know its distance ; for, supposing
two men to set off at the same moment from A and B (Jig. 9), to walk
each to the end of their respective lines C and D, if they perform their

walk in the same space of time, they
must have proceeded at a very different
rate ; and yet to an eye situated at E,
they will appear to have moved with
equal velocity, because they will both
have gone through an equal number of
degrees, though over a very unequal
length of ground. Sight cannot be
implicitly relied on : it deceives us,
both in regard to the size and the dis-
tance of objects — indeed our senses
would be very liable to lead us into
error, if experience did not set us right.
Nothing more convincingly shows how
requisite experience is to correct the
errors of sight, than the case of a young
man who was blind from his infancy,
and who recovered his sight at the
age of fourteen, by the operation of
couching. At first he had no idea
either of the size or distance of ob-
jects, but imagined that every thing he saw touched his eyes j and it was

Fig. 9.

A

INTRODUCTION TO OPTICS.

Ixxxi

not till after having repeatedly felt them, and walked from one object to
another, that he acquired an idea of their respective dimensions, their
relative situations, and their distances.

Since an image is formed on the retina of each of our eyes, it would
seem that we ought to see objects double. In fact, however, we do not;
and perhaps the best solution which has been offered of the difficulty is
this, that the action of the rays on the optic nerve of each eye is so perfectly
similar, that they produce but a single sensation: the mind, therefore,
receives the same idea from the retina of both eyes, and conceives the
object to be single. It is, however, safer to treat the fact as one estab-
lished by experience, but not admitting of any satisfactory explanation ;
for the manner in which external objects act upon the mind admits of
no direct observation, and all theories respecting it can therefore rest on
no sound foundation. Persons afflicted with a disease in one eye, which
prevents the rays of light from affecting it in the same manner as the
other, frequently see double.

The image of an object in a looking-glass is not inverted, because
the rays do not enter the mirror by a small aperture, and cross each other,
as they do at the orifice of a camera obscura, or the pupil of the eye.

When a man views himself in a mirror, the rays from his eyes fait
perpendicularly upon it, and are reflected in the same line ; they proceed,
therefore, as if they had come from a point behind the glass, and the
.same effect is produced, as if they proceeded from an image of the object
described behind the glass, and situated there in the same manner as the
object before it. This is not the case only with respect to rays falling
perpendicularly on the glass, but with all others. Thus in Jig. 10, a
ray proceeding from the point C to D is reflected to A, and arrives there
in the same manner as if it had proceeded from E, a point behind the
glass, at the same distance from it as C is in front of it.

A man may see himself at full length in a mirror which is not more
than half his height (Jig. 10). The ray of light, A B, from his eye which
falls perpendicularly on the mirror, B D, will be reflected back in the
same line ; but a ray, C D, from his feet, which falls obliquely on the
mirror, will be reflected in the line D A ; and since we view objects in
the direction of the reflected rays, which reach the eye, and the image
appears at the same distance behind the mirror as the object is before it,
we must continue the line A D to E, and the line A B to F, at the termi-

Fig. 10.

Fig. 11.

xxxii INTRODUCTION TO OPTICS.

nation of which the image will be represented. The line D E is equal
to D C, or to D A ; and the line A B D therefore, which represents the
necessary length of the mirror, is half the line E F, which represents the
height of the person. The man could not see the whole of his person in
a7 much smaller mirror; for a ray of light from his feet would fall so
obliquely on it, that it would be reflected above his head, so that he could
not see it. This is shown by the dotted line {fig- 10). A man cannot
see himself in a mirror if he stand to the right or the left of it, because
the incident rays falling obliquely on the mirror will be reflected obliquely
in the opposite direction, the angles of incidence and of reflection being
equal-

Fig. 11 represents an eye looking at the image of a vase, reflected by a
mirror : it must see it in the direction of the ray A B, as that is the ray
which brings the image to the eye ; prolong the ray to C, and in that spot
will the image appear. You must observe, that in a glass mirror it is not
the glass that reflects the rays which form the image, but the mercury
behind it. The glass acts chiefly as a transparent case, through which
the rays find an easy passage. Could mirrors be made of mercury, they
would reflect more perfectly ; but mercury is a fluid. By amalgamating it
with tin-foil, it becomes of the consistence of paste, attaches itself to the
glass, and forms, in fact, a mercurial mirror, which would be much more
perfect without its glass cover, for the purest glass is never completely
transparent : some of the rays, therefore, are lost during their passage
through it, by being either absorbed, or irregularly reflected. This im-
perfection of glass mirrors has introduced the use of metallic mirrors, for
optical purposes. All opaque bodies would be mirrors, were their surfaces
sufficiently smooth; but the surface of bodies in general is so rough and
uneven, that their reflection is extremely irregular, which prevents the
rays from forming an image on the retina. You may easily conceive the
variety of directions in which rays would be reflected by a nutmeg-grater,
on account of the inequality of its surface, and the number of holes with
which it is pierced. Now all solid bodies resemble the nutmeg-grater in
these respects, more or less ; and it is only those which are susceptible of
receiving a polish, that can be made to reflect the rays with regularity.
As hard bodies are. of the closest texture, the least porous, and capable of
taking the highest polish, they make the best mirrors: none, therefore,
are so well calculated for this purpose as metals.

There are three kinds of mirrors used in optics : the plane or flat,
which are the common mirrors we have just mentioned, convex mirrors,
and concave mirrors. The reflection of the two latter is very different
from that of the former.

The plane mirror, we have seen, does not alter the direction of the
reflected rays, and forms an image behind the glass exactly similar to the
object before it: for it forms an image of each point of the object at the
same distance behind the mirror, that the point is before it ; and these
images of the different points together make up one image of the whole
object. A convex mirror has the peculiar property of making the reflected
rays diverge, by which means it diminishes the image ; and a concave mirror
makes the rays converge, and, under certain circumstances, magnifies the
image. Let us begin by examining the reflection of a convex mirror. This
is formed of a portion of the exterior surface of a sphere. When several
parallel rays fall upon it, that ray only which, if prolonged, would pass
through the centre, or axis of the mirror, is perpendicular to it. In order
to avoid confusion, we have, in fig. 12, drawn only three parallel lines,

INTRODUCTION TO OPTICS. Ixxxiii

AB, CD, EF, to represent rays falling; on the convex mirror MN: the

Fig. 12.

middle ray, you will observe, is perpendicular to the mirror, the others
fall on it obliquely. The three rays being- parallel would all be perpendi-
cular to a flat mirror ; but no ray can fall perpendicularly on a spherical
mirror, which is not directed towards the centre of the sphere, just as a
weight falls perpendicularly to the earth when gravity attracts it towards
the centre. In order, therefore, that rays may fall perpendicularly to the
mirror at B and F, the rays must be in the direction of the dotted lines,
which meet at the centre, O, of the sphere, of which. the mirror forms
a portion.

Now let us observe in what direction the three rays AB, CD, EF,
will be reflected. The middle ray falling perpendicularly on the mirror,
will be reflected in the same line ; the two others falling obliquely, will
be reflected obliquely to G and H, for the dotted lines are perpendiculars,
which divide their angles of incidence and reflection, or they will proceed
as if they came from the point L ; and since we see objects in the direction
of the reflected ray, we shall see an image, answering to that which would
be produced by a body placed at L, which is the point at which the
reflected rays, if continued through the mirror, would unite and form
an image. This point is equally distant from the surface and centre
of the sphere, and is called the imaginary focus of the mirror. A focus
is a point at which converging rays unite; in this case called an imaginary
focus, because the rays only appear to unite there, or rather proceed
after reflection in the same direction as if they came from behind the mirror,
from that point : for they do not pass through the mirror, since they are
reflected by it.

If the rays diverge before they fall on the mirror, they will diverge still
more after reflection j but in this case also they will diverge as if they
proceeded from a point within the mirror, which is the focus of those rays.
The rays, therefore, which really proceed from a point in front of the
mirror, will appear to proceed from a point within it, at which they would
unite, and form an image. This point within the mirror, like the
imaginary focus of parallel rays, is always a point in the line joining the
centre of the sphere, with the point without the mirror, from which the
rays really proceed.

If, instead of supposing a single luminous point, we imagine a body of
some magnitude placed before the mirror, the rays of light which proceed

G

Ixxxiv

INTRODUCTION TO OPTICS.

Fig. 13.

A

O

from each point of it will be reflected "exactly in the same manner as if
that was a single luminous point ; and an image of that point therefore
will be formed as before, in the line joining that point to the centre of the
sphere. An image being thus formed of each point in the object, there
•will be an image of the whole object, formed by the collection of these
images of its different parts.

This Image will necessarily' be
smaller than the object itself. If
AB be an object placed before
the convex mirror, X Y, and lines
be drawn from its extreme points,
A B to O, the centre of the sphere
of which the mirror forms part,
the image of the point A will be at
«, a point in the line A O ; that of
B at 6, a point in the line B O ;

and of course the image of every intermediate point somewhere between
a and b. Or, in other words, the rays which really proceed from A are
seen after reflection as if they proceeded from a ; those from B as if they
proceeded from b ; and all others as if from some point between them.
The lines A O, B O, converge to a point at O ; and the points a, b, which
are nearer to O than A, B are, are necessarily nearer together than A, B.
The space, therefore, from which the rays after reflection appear to pro-
ceed is less than that occupied by the body itself, as the image is smaller
than the object.

A concave mirror is formed of a
portion of the internal surface of a
hollow sphere, and its peculiar pro-
perty is to make the rays of light con-
verge. If three parallel rays, A B,
C D, E F, fall on the concave

Fig. 14.

mirror, M N (Jig. 14), the middle
ray will be reflected in the same
line, being in the direction of the
axis of the mirror, and the two
others will be reflected obliquely,
as they fall obliquely on the mirror.

The two dotted perpendiculars divide their angles of incidence and reflection;
and in order that these angles may be equal, the two oblique rays must be
reflected to L, where they will unite with the middle ray. Thus, when any
number of parallel rays fall on a concave mirror, they are all reflected to a
focus : for in proportion as the rays are more distant from the axis of the
mirror, they fall more obliquely upon it, and are more obliquely reflected :
in consequence of which they come to a focus in the direction of the axis of
the mirror j and this point is not an imaginary focus (as with the convex
mirror), but the true focus at which the rays unite. If rays fall convergent
on a concave mirror (fig. 15), they are sooner brought to a focus, L, than
parallel rays : their focus is therefore nearer to the mirror MN. Divergent
rays are brought to a more distant focus than parallel rays, as in fig. 16,
where the focus is at L ; but the true focus of mirrors, either convex or
concave, is that formed by parallel rays, which is equally distant from the
centre and the surface of the sphere, as in fig. 12 and fig. 14. If a
metallic concave mirror of polished tin be exposed to the sun, the rays
will be collected into a very brilliant focus ; and a piece of paper held

INTRODUCTION TO OPTI
Fig. 15.

Fig. 17.

in this focus will take fire; for rays of light cannot be concentrated

without accumulating a proportional quantity of heat : hence con»

cave mirrors have obtained the name of

burning1 mirrors. If a burning taper

be placed in the focus (fig. 17), the

ray which falls in the direction of the

axis of the mirror will be reflected back

in the same line ; but two other rays,

drawn from the focus, and falling on the

mirror at B and F, will be reflected to

A and E. Therefore the rays which

proceed from a light placed in the focus

of a concave mirror fall divergent upon

it, and are reflected parallel : it is

exactly the reverse of the former figure, in which the sun's rays fell

parallel on the mirror, and were reflected to a focus. In .other words,

when the incident rays are parallel, the reflected rays converge to a focus ;

when the incident rays proceed from the focus, they are reflected parallel :

this is a very important law of optics. We have said that the image was

formed in the focus of a concave mirror, yet glass concave mirrors

are often seen, where the object is represented within the mirror, in the

same manner as in those which are convex. This is the case only

when the object is placed between the mirror and its focus ; the image

then appears magnified behind, or within the mirror.

SECTION II. — On Refraction and Colours.

REFRACTION is the effect which transparent mediums produce on light in
its passage through them. Opaque bodies reflect the rays, and transpa-
rent bodies transmit them ; but it is found that if a ray, in passing from
one medium into another of different density, fall obliquely, it is turned
out of its course. The power which causes the deviation of the ray is not
fully understood, nor completely ascertained ; but the appearances are
the same as if the ray (supposing it to be a succession of moving particles,
which is for this purpose the most convenient way of considering it) were
attracted by the denser medium more strongly than by the rarer. Let us
suppose the two mediums to be air and water: when a ray of light passes
from air into water, it appears to be more strongly attracted by the
latter. If then a ray, A B (Jig. IS), fall perpendicularly on water, the

Ixxxvi

INTRODUCTION TO OPTICS.

attraction of the water acts in the same direction as the course of the
ray ; it will not therefore cause a deviation, and the ray will proceed
straight on to E ; but if it fall obliquely, as the ray C B, the water will
attract it out of its course. L<et us suppose the ray to have reached the
surface of a denser medium, and that it is there affected by its attraction.
If not counteracted by some other power, this attraction would draw it
perpendicularly to the water at B, towards E ; but it is also impelled by
its projectile force, which the attraction of the denser medium cannot
overcome : the ray, therefore, acted on by both these powers, moves in a
direction between them, and instead of pursuing its original course to D,
or being implicitly guided by the water to E, proceeds towards F, so that
the ray appears bent or broken. C "B^Jig. 19) represents a ray passing

Fig. 18.

Fig. 19.

A

E

Fig. 20.

obliquely from glass into water; glass being the denser medium, the ray
will be more strongly attracted by that which it leaves, than by that
which it enters. The attraction of the glass would act in the direction
AB, while the impulse of projection would carry the ray to F: it moves,
therefore, between these directions towards D; so that when a ray
passes from a dense into a rare medium, a refraction takes place in
the opposite direction to that observed when the ray passes from a rare
into a dense medium. The distance at which the denser medium produces
its effect upon a ray is so small as to be insensible : the ray appears,
therefore, to be refracted only at the point at which it passes from one
medium to the other, and passes on in a straight course through each.

If a shilling (Jig. 20) be placed
at the bottom of an empty tea-
cup, and the tea- cup at such a
distance from the eye that the rim
shall hide the shilling, it will
become visible by filling the cup
with water. In the first instance,
the rays reflected by the shilling-
are directed higher than the eye,
but when the cup is filled with
water, they are refracted by its
attraction, and bent downwards at
quitting it, so as to enter the eye.
When the shilling becomes visible
by the refraction of the ray, you do not see it in the situation which it really
occupies, but an image of "it higher iu the cup; for as objects ,al ways

INTRODUCTION TO OPTICS.

Ixxxvii

appear to be situated in the direction of the rays which enter the eye, the
shilling1 will be seen in the direction of the refracted ray at B.

The manner in which an oar appears bent in water is a similar effect
of refraction. The line drawn from the point A to the shilling in the
last figure may now be conceived to represent an oar plunged in water;
the lowest point being* represented in the figure by the little circle, will,
as before, have an image of itself formed apparently above it, as at B.
In like manner every point of the oar below the surface of the water will
have an image of itself formed above it at some point in the dotted line
in the figure, and the whole dotted line will represent the whole image
of that part of the oar which is immersed. The part of the oar above the
water, extending to A, is seen in its natural position ; that below the
water is seen as if reaching along the dotted line to B ; the oar, there-
fore, appears bent or broken at the surface of the water. The fact of the
formation of an image above the true place of the body does not depend
on the situation of the eye. The representation of the eye in the figure
therefore, which was introduced in the former paragraph, to explain the
cause of a single ray, is not necessary or particularly applicable to the
present subject. 1

When we see the bottom of a clear stream, the rays which it reflects,
being refracted in their passage from the water into the air, will make the
bottom appear more elevated than it really is, and the water will conse-
quently appear more shallow. Accidents have frequently been occasioned
by this circumstance ; and boys who are in the habit of bathing should be
cautioned not to trust to the apparent shallowness of water, as it will
always prove deeper than it appears.

The refraction of light prevents our seeing the heavenly bodies in
their real situation. The light they send to us being refracted in
passing into the atmosphere, we see the sun and stars in the direction

Fig. 21.

E

of the refracted ray, as described in Jig. 21, where the dotted line
represents the extent of the atmosphere, above a portion of the earth,
E B E. A ray of light coming from the sun, S, falls obliquely on it at
A, and is refracted to B ; then, since we see the object in the direction
of the refracted ray, a spectator at B will see an image of the sun at C,
instead of the real object at S. If the sun were immediately over our
heads, its rays falling perpendicularly on the atmosphere would not be
refracted, and we should then see it in its true situation. To the inha-
bitants of the torrid zone, where the sun is sometimes vertical, its rays
are then not refracted. There is, however, another obstacle to seeing the
heavenly bodies in their true situation, which affects them in the torrid
zone as well as elsewhere. Light is about eight minutes and a half in
its passage from the sun to the earth ; therefore, when the rays reach

Ixxxviii

INTRODUCTION TO OPTICS.

us, the sun has quitted the spot he occupied on their departure ; yet we
see him in the direction of those rays, and consequently in a situation
which he had abandoned eight minutes and a half before. In speaking
of the sun's motion, we mean his apparent motion, produced by the diurnal
rotation of the earth, for the effect being the same whether it be our
earth or the heavenly bodies which move, it is more easy to represent
things as they appear to be, than as they really are. The refraction of
the sun's rays by the atmosphere prolongs our days, as it occasions our
seeing an image of the sun, both before he rises and after he sets ; for
below the horizon he still shines upon the atmosphere, and his rays are
thence refracted to the earth. So likewise we see an image of the sun
before he rises, the rays that previously fall upon the atmosphere being
reflected to the earth.

In passing through a pane of glass the rays suffer two refractions,
which being in contrary directions, produce nearly the same effect as if no
refraction had taken place.

Fig. 22, A A represents a thick pane of glass seen edgeways. When
the ray B approaches the glass at C, it is refracted by it ; and, instead of
continuing its course in the same direction, as the dotted line describes,

Fig. 22.

Fig. 23.

.A

it passes through the pane to D ; at that point returning into the air, it is
again refracted by the glass, but in a contrary direction, and in conse-
quence proceeds to E. Now the ray B C and the ray D E being parallel,
the light does not appear to have suffered any refraction ; for if a ray of
light passes from one medium into another, and through that into the first
again, the two refractions being equal and in opposite directions, no
sensible effect is produced ; for the direction is the same, and the little
space by which the ray is thrown to one side, as represented in fig. 22, is
necessarily less than the thickness of the medium, and the thickness of a
pane of glass is too little to be worth considering. But this is the case
only when the two surfaces of the refracting medium are parallel to each
other; if they are not, the two refractions may be made in the same
direction. Thus, when parallel rays (fig. 23) fall on a piece of glass hav-
ing a double convex surface, which is called a lens, that only which falls in
the direction of the axis of the lens is perpendicular to the surface ; the
other rays falling obliquely are refracted towards the axis, and will meet
•at a point beyond the lens, called its focus. Of the three rays A, B,C,
•which fall on the lens D E, the rays A and C are refracted in their
passage through it, to a and c, and on quitting the lens they undergo a
second refraction in the same direction, which unites them with the ray B,

INTRODUCTION TO OPTICS.

Ixxxix

at the focus F*. The focal distance, or distance of the focus from the
surface of the lens, depends both upon the form of the lens and of the
refractive power of the substance of which it is made ; in a glass "lens,
both sides of which are equally convex, the focus is- situated nearly at the
centre of the sphere of which the surface of the lens forms a portion : it
is at the distance, therefore, of the radius of the sphere.

There are lenses of various forms, which are represented in fig. 24.
The property of those which have a convex surface is to collect the rays

Fig. 24.

of light to a focus ; and of those which have a concave surface, to dis-
perse them : for the rays A, C, falling on the concave lens X Y (Jig. 25),
instead of converging towards the ray B, which falls on the axis of the
lens, will each be attracted towards the lens, both on entering and quitting
it, and will, therefore, by the first refraction, be made to diverge to a, c,
and by the second to d, e. Lenses which have one side flat, and the
other convex or concave, as~A and B,Ji$. 24, are called plano-convex and
plano-concave lenses. The focus of the former is at the distance of the

Fig. 25. Fig. 26.

diameter of a sphere, of which the convex surface of the lens forms a
portion, as represented in Jig. 26. The three parallel rays, A, 13, C, are
brought to a focus by the plano-convex lens X Y at F.

Thus far we have only spoken of the refraction of parallel rays; if the

* The refractions will at once appear to be in the same direction if perpendiculars to
the surface of the lens be drawn at (he points where the ray enters and quits it: bearing
in mind that the lens being denser than the air, the ray is drawn towards the perpen-
dicular ou entering, and/m/i it on quitting the lens.

xc INTRODUCTION TO OPTICS.

rays diverge originally, they will be less convergent or more divergent
after refraction, but they would in general still finally meet at a point, or
appear to diverge from one. Taking the case of a convex lens, the
point at which they would meet would be farther from the lens than that
in which parallel rays meet, and continually farther and farther, as the
rays were more divergent, or as the body from which they proceeded was
brought nearer to the lens. An image* would therefore be formed, but
continually farther and farther from the lens, as the body approached it ;
and the image is smaller or larger than the body, as it is nearer to or
farther from the lens than the body itself is. If the body is brought as
near to the lens as the distance of the focus for parallel rays, no image
would be formed, for the rays would be refracted parallel to each other ;
and if the body were brought still nearer, the rays would diverge after
refraction. The case of a convex lens is one of the most simple and the
most important ; but the same principle may easily be extended to other
cases. Fi 27>

We shall next explain the re-
fraction of a triangular piece of
glass, called a prism (jig- 27).
The sides are flat; it cannot
therefore bring the rays to a focus,
nor can its refraction be similar to
that of a flat pane of glass, because c , (i^ —]£

it has not two sides parallel. The

refractions of the light, on entering and on quitting the prism, are both in
the same direction"!". On entering the prism P, the ray is refracted from
B to C, and on quitting it from C to D. If the window-shutters be
closed, and a ray of light, admitted through a small aperture, fall upon a
prism, it will be refracted, and a spectrum, A B (Jig. 28), representing
airthe colours of the rainbow, will be formed on the opposite wall. It is
difficult to conceive how a piece of white glass can produce such a variety
of brilliant colours ; but the fact is, that the colours are not formed by the
prism, but existed in the ray previous to its refraction ; for the white rays
of the sun are composed of coloured rays, which, when blended together,
appear colourless or white.

Sir Isaac Newton, to whom we are indebted for the most important
discoveries respecting light and colours, was the first who divided a white
ray of light, and found it to consist of an assemblage of coloured rays,
which formed an image upon the wall, such as is exhibited (Jig. 28), in
which are displayed the following series of colours — red, orange, yellow,
green, blue, indigo, and violet. Now a prism separates these coloured
rays by refraction. It appears that the coloured rays have different

* Wo speak of the formation of an image at a point wherever the rays after reflection
or refraction proceed, as if they diverged from that point. The object is then seen as
an image of it placed there would lie. This image, however, has not, under common
circumstances, any real existence. The rays pass through the point in question ; Init as
they are only seen by an eye in the direction of their motion, a spectator any where else
will not see them at all, and the spectator who does see them will only know the direction
in which they move. If, however, a screen be placed at the focns, so as to intercept and
reflect the rays, the existence of the image will be proved by the actual formation of a
distinct picture of the object upon the screen ; if the screen be placed nearer or farther
off, so that the rays have not yet accurately converged to, or have begun to diverge from,
the focus, there will be a contused spot of light, but no distinct representation.

f This will at once appear, as in the case of the lens, by drawing perpendiculars to the
surface of the prism where the ray enters and quits it.

INTRODUCTION TO OPTICS.
Fig. 28.

xci

V

degrees of refrangibility ; in passing through the prism, therefore, they
take different directions, according to their susceptibility of refraction.
The violet rays deviate most from their original course : they appear at
one end of the spectrum A B. Contiguous to the violet are the indigo
rays, being those which have somewhat less refrangibility : then follow, in
succession, the blue, green, yellow, orange, and, lastly, the red, which are
the least refrangible of the coloured rays. The union of these colours, in
the proportions in which they appear in the spectrum, produces in us the idea
of whiteness. If a card be painted in compartments with these seven
colours, and whirled rapidly on a pin, it will appear white. But a more
decisive proof of the composition of a white ray is afforded by re-uniting
these coloured rays, and forming with them a ray of white light. This
can be done by letting the coloured rays, which have been separated by a
prism, fall upon a lens, which will make them converge to a focus ; and when
thus re-united, they will appear white as they did before refraction. The
prism P (Jig. 29) separates a ray of white light into seven coloured rays ;
and the lens L L brings them to a focus at F, where they again appear
white : thus, by means of a prism and a lens, we can take a ray of white
light to pieces, and put it together again.

Fig. 29.

This division of a ray of white light into different colours, being caused
by the unequal refrangibility of the different coloured rays, must take
place, more or less, whenever the ray suffers refraction. Thus the rainbow,
which exhibits a series of colours so analogous to those of the spectrum,
is formed by the refraction of the sun's rays in their passage through a

xeii INTRODUCTION TO OPTICS.

shower of rain, every drop of which acts as a prism, in separating the
coloured rays as they pass through it.

1 The sun's rays may be collected to a focus by a lens in the same
manner as they are by a concave mirror: in the first, the rays pass
through the glass, and converge to a focus behind it; in the latter they
are reflected from the mirror, and brought to a focus before it. A lens,
when used for this purpose, is called a burning glass ; and if, when
the sun shines bright, a piece of paper be held in the focus of the rays,
it will take fire. This experiment succeeds best with brown or any
dark-coloured paper ; for though it is true that the lens collects an equal
number of rays to a focus, whether the paper held there be white or
coloured, the white paper appears more luminous in the focus, because
most of the rays, instead of entering into the paper, are reflected
by it ; and this is the reason that the paper is not burnt ; whilst, on the
contrary, the coloured paper, which absorbs more light than it reflects,
soon becomes heated and takes fire.

It is supposed that the tendency to absorb or reflect rays depends on
the arrangement of the minute particles of the body, and that the diversity
of arrangement renders some bodies susceptible of reflecting one coloured
ray, and absorbing the others ; whilst other bodies have a tendency to
reflect all the colours, and others again to absorb them all. A body
appears to be of the colour which it reflects ; as we see it only by reflected
rays, it can appear but of the colour of those rays. Thus grass is green,
because it absorbs all except the green rays : it is, therefore, these only
which the grass and trees reflect to our eyes, and which make them
appear green. The sky and flowers, in the same manner, reflect the
various colours of which they appear to us : the rose, the red rays ; the
violet, the blue ; the jonquil, the yellow, &c. If you imagine that these
are the permanent colours of the grass and flowers, you are mistaken.
Whenever you see those colours, the objects must be illuminated; and
light, from whatever source it proceeds, is of the same nature, composed
of the various coloured rays, which paint the grass, the flowers, and every
coloured object in nature. Objects in the dark have no colour, or are
black, which is the same thing. You can never see objects without light.
Light is composed of colours, therefore there can be no light without
colours ; and though every object is black, or without colour in the dark,
it becomes coloured as soon as it becomes visible.

An object placed in a coloured ray of light which has been refracted
by a prism, will appear of the colour of the ray in which it is placed. A
sheet of white paper will take all the colours indifferently, but a coloured
body will appear most brilliant when placed in the ray which it naturally
reflects. But though bodies, from the arrangement of their particles,
have a tendency to absorb some rays and reflect others, yet they are not
so perfectly uniform in their arrangement as to reflect only pure rays of
one colour, and perfectly absorb the others. A body reflects, in great
abundance, the rays which determine its colour, and the others in a
greater or less degree, in proportion as they are nearer or farther from
its own colour, in the order of refrangibility.

Bodies which reflect all the rays are white ; those which absorb them
all are black. Between these extremes they appear lighter or darker, in
proportion to the quantity of rays they reflect or absorb. A rose is of a
pale red : it approaches nearer to white than black, it therefore reflects
rays more abundantly than it absorbs them. Pale-coloured bodies reflect
all the coloured rays to a certain degree, which produces their paleness,

INTRODUCTION TO OPTICS. xciii

approaching to whiteness ; but one colour they reflect more than the rest:
this predominates over the white, and determines the colour of the body.
Since, then, bodies of a pale colour in some degree reflect all the rays of
light, in passing through the various colours of the spectrum, they will
reflect them all with tolerable brilliancy, but will appear most vivid in the
ray of their natural colour. The green leaves, on the contrary, are of a
dark colour, bearing a stronger resemblance to black than to white : they
have, therefore, a greater tendency to absorb than to reflect rays. Blue
often appears green by candle-light, because this light is less pure than
that of the sun; and when refracted by a prism, the yellow rays predomi-
nate ; and as the admixture of blue and yellow forms green, the super-
abundance of yellow gives to blue bodies a greenish hue.

The sun appears red through a fog, owing to the red rays having
a greater momentum, which gives them power to traverse so dense an
atmosphere. For the same reason the sun generally appears red at
rising and setting : as the increased quantity of atmosphere which the
oblique rays must traverse, loaded with the mists and vapours which are
usually formed at those times, prevents a large proportion of the other
rays from reaching us. The colour of the atmosphere, commonly called
the sky, is blue ; — now since all the rays traverse it in their passage
to the earth, it would be natural to infer that it should be white ; but we
must not forget that we see none of the rays which pass from the sun
to the earth, excepting those which meet our eyes ; and this happens
only if we look at the sun, and thus intercept the rays, in which case,
you know, it appears white. The atmosphere is a transparent medium,
through which the sun's rays pass freely to the earth ; but when reflected
back into the atmosphere, their momentum is considerably diminished,
and they have not all of them power to traverse it a second time. The
momentum of the blue rays is least; these, therefore, are the most im-
peded in their return, and are chiefly reflected by the atmosphere ; or it
may be that, without any question of momentum, the colour which the
particles of air most readily t reflect is blue — just as grass reflects the
green, or a rose the red rays. This reflection is performed in every pos-
sible direction ; so that wherever we look at the atmosphere, some of
these rays fall upon our eyes : hence we see the air of a blue colour. If
the atmosphere did not reflect any rays, though the objects on the sur-
face of the earth would be illumined, the skies would appear perfectly
black. This would not only be very melancholy, but it would be per-
nicious to the sight, to be constantly viewing bright objects against a
black sky.

When bodies change their colour, as leaves which wither in autumn,
or a spot of ink which produces an iron-mould on linen, it arises from
some chemical change, which takes place in the internal arrangement of
the parts, by which they lose their tendency to reflect certain colours,
and acquire the power of reflecting others. A withered leaf thus no
longer reflects the blue rays : it appears, therefore, yellow, or has a slight
tendency to reflect several rays which produce a dingy brown colour.
An ink-spot on linen at first absorbs all the rays ; but, exposed to the air,
it undergoes a chemical change, and the spot partially regains its ten-
dency to reflect colours, but with a preference to reflect the yellow
rays ; and such is the colour of the iron-mould.

Fig. 31.

SECTION III. — On the Structure of the Eye and Optical Instruments.

THE body of the eye is of a spheri- Fig. 30.

cal form (fig. 30). It has two mem-
braneous coverings ; — the external

one, a a «, is called the sclerotica :

this has a projection in that part of

the eye which is exposed to view,

b b, which is called the cornea,

because, when dried, it has nearly

the consistence of very fine horn,

and is sufficiently transparent for

the light to obtain free passage

through it. The second membrane

which lines the cornea, and enve-
lopes the eye, is called the choroid,

c c c : this has an opening in front

just beneath the cornea, which forms

the pupil, d d, through which the

rays of light pass into the eye. The

pupil is surrounded by a coloured

border of fibres, called the iris, e e,

which, by its motion, always pre-
serves the pupil of a circular form,

whether it be expanded in the dark

or contracted by a strong light.
(Fig. 31.)

The construction of the eye is so admirable, that it is capable of adapt-
ing itself, more or less, to the circumstances in which it is placed. In a
faint light the pupil dilates so as to receive an additional quantity of rays ;
and in a strong light it contracts, in order to prevent the intensity of the
light from injuring the optic nerve. The eyes suffer pain, when from
darkness they suddenly come into a strong light ; for the pupil being
dilated, a quantity of rays rush in before it has time to contract ; and
when we go from a strong light into obscurity, \ve at first imagine
ourselves in total darkness ; for a sufficient number of rays cannot gain
admittance into the contracted pupil to enable us to distinguish objects ;
but in a few minutes it dilates, and we clearly perceive what was before
invisible. The choroid, c c, is imbued with a black liquor, which serves
to absorb all the rays that are irregularly reflected, and to convert the
body of the eye into a more perfect camera obscura. When the pupil is
expanded to its utmost extent, it is capable of admitting ten times the
quantity of light that it does when most contracted. In cats, and animals
which are said to see in the dark, the power of dilatation and contraction
of the pupil is still greater : it is computed that their pupils may receive
one hundred times more light at one time than at another. Within these
coverings of the eye-ball are contained three transparent substances, called
humours. The first occupies the space immediately behind the cornea,
and is called the aqueous humour, ff, from its liquidity and its resem-
blance to water. Beyond this is situated the crystalline humour, g g,

INTRODUCTION TO OPTICS. xcv

which derives its name from its clearness and transparency : it has the
form of a lens, and refracts the rays of light in a greater degree of per-
fection than any that have been constructed by art : it is attached by fibres,
mm, to each side of the choroid. The back part of the eye, between the
crystalline humour and the retina, is filled by the vitreous humour, h h,
which derives its name from a resemblance it is supposed to bear to glass
or vitrified substances. The membranous coverings of the eye are
intended chiefly for the preservation of the retina, i i, which is by far the
most important part of the eye, as it is that which receives the impression
of the objects of sight. The retina consists of an expansion of the optic
nerve, of perfect whiteness : it proceeds from the brain, enters the eye
at n on the side next the nose, and is finely spread over the interior
surface of the choroid. The rays of light which enter the eye by the
pupil, are refracted by the several humours in their passage through
them, and unite in a focus on the retina.

Rays proceed from bodies in all possible directions; we must, therefore,
consider every part of an object which sends rays to our eyes as points
from which the rays diverge, as from a centre. Divergent rays, on entering
the pupil, do not cross each other ; the pupil, however, is sufficiently
large to admit a small pencil of them ; and these, if not refracted to a
focus by the humours, would continue diverging after they had passed
the pupil, would fall dispersed upon the retina, and thus the image of a
single point would be expanded over a large portion of the retina. The
divergent rays from every other point of the object would be spread over
a similar extent of space, and would interfere and be confounded with the
first, so that no distinct image could be formed on the retina.

Fig. 32 represents two pencils of rays issuing from two points of the
tree A, B, and entering the pupil, refracted by the crystalline humour D,
and forming distinct images of the spot they proceed from on the retina,
at «, b. Fig. 33 differs from the preceding, merely from not being sup-
plied with a lens : in consequence of which the pencils of rays are not
refracted to a focus, and no distinct image is formed on the retina. The
rays issuing from two points of an object are alone delineated, and
the two pencils in Jig. 33 distinguished by describing one of them
with dotted lines. The interference of these two pencils of rays will

Fig. 32.

enable you to form an idea of the confusion which would arise from
thousands and millions of points at the same instant pouring their
divergent rays upon the retina. The refraction of the several humours
unites the whole of a pencil of rays, proceeding from any one point of an
object, in a corresponding point on the retina, and the image is thus ren-

XCVI

INTRODUCTION TO OPTICS.
Fig. 33,

dered distinct and strong. If you conceive (mjig. 32) every point of the
tree to send forth a pencil of rays similar to those, A, B, every part of the
tree will be as accurately represented on the retina as the points a, b. You
may perhaps inquire why, since the eye requires refracting humours in
order to form a distinct representation on the retina, the same refractions
are not necessary for the image formed in the camera obscura ? It is
because the aperture through which we receive the rays into the camera
obscura is so extremely small, that but very few of the rays diverging from
a point gain admittance ; but if the aperture [be enlarged, and furnished
with a lens, the landscape will be more perfectly represented.

That imperfection of sight which arises from the eyes being too promi-
nent, is owing to the crystalline humour, D (fig. 34) being too convex ;
in consequence of which it refracts the rays too much, and collects a
pencil, proceeding from the object A B, into a focus, F, before they reach
the retina. From this focus, the rays proceed diverging, and conse-

Fig. 34.

quently form a very confused image on the retina, at a b. This is the
defect of short-sighted people ; and it is remedied by bringing the object
nearer to the eye ; for the nearer an object is brought to the eye the more

Fig.35.

divergent the rays fall upon the
crystalline humour, and conse-
quently do not so soon converge
to a focus : this focus, therefore,
either falls upon the retina, or at
least approaches nearer to it, and
the object is proportionally dis-
tinct, as in Jig. 35. The nearer,
therefore, an object is brought to
the crystalline or to a lens, the
further the image recedes behind it.

another resource for objects which they cannot approach to their eyes:
this is Jo place a concave lens, C Da(^. 36), before the eye, in order to

But short-sighted persons have

INTRODUCTION TO OPTICS.
Fig. 36.

XCVll

T>

increase the divergence of the rays, the effect of a concave lens being
exactly the reverse of a convex one. By the assistance of such glasses,
therefore, the rays from a distant object fall on the pupil as divergent aa
those from a less distant object ; and, with short-sighted people, they
throw the image of a distant object back as far as the retina. Those who
suffer from the crystalline humour being too flat, apply an opposite
remedy : that is to say, a convex, lens LM {fig, 37), to make up for the
deficiency of convexity of the crystalline humour, O P. Thus elderly
people, the humours of whose eyes rre decayed by age, are under the

Fig: 37.

M

necessity of using convex spectacles ; and when deprived of that resource,
they hold the object at a distance from their eyes, for the more distant the
object is from the crystalline, the nearer the image will be to it. These
two opposite defects are easily comprehended ; but it is difficult to con-
ceive how any sight can be perfect, for if the crystalline humour be of a
proper degree of convexity to bring the image of distant objects to a
focus on the retina, it will not represent near objects distinctly ; and if, on
the contrary, it be adapted to give a clear image of near objects, it will
produce a very imperfect one of distant objects. It is true, that every
person would be subject to one of these two defects, were it not in em-
power to increase or diminish, in some degree, the convexity of the crys-
talline humour, and to project it towards, or draw it back from the object,
as circumstances require. In a young, well-constructed eye, the fibres to
which the crystalline humour is attached have so perfect a command over
it, that the focus of the rays constantly falls on the retina, and an image
is formed equally distinct both of distant objects and of those which are
near. We cannot, however, see an object distinctly, if we bring it
very near to the eye, because the rays fall on the crystalline humour too
divergent to be refracted to a focus on the retina. The confusion, there-
fore, arising from viewing an object too near the eye, is similar to that
which proceeds from a flattened crystalline humour ; the rays reach the
retina before they are collected to v focus {fig. 38). If it were not for
this imperfection, we should be aMe to see and distinguish the parts of

H

xcviii INTRODUCTION TO OPTICS.

Fig. 38. Fig.

objects which are now invisible to us from their minuteness ; for could
we bring them close to the eye, their image on the retina would be so
much magnified as to render them visible. The microscope is con-
structed on. this principle. The single microscope (Jig. 39) consists
simply of a convex lens, in the focus of which the object is placed, and
through which it is viewed. By this means, you are enabled to bring
your eye very near the object, for the lens A B, by diminishing the
divergency of the rays before they enter the pupil C, makes them fall
parallel on the crystalline humour D, by which they are refracted to a
focus -m the retina, at R R. The lens magnifies the object merely by
allowing us to bring it nearer to the eye ; those lenses, therefore, which
have the shortest focus will magnify the object most, because they enable
us to bring the object nearest to the eye. On the other hand, a lens that
has the shortest focus is most convex ; and its protuberance will prevent
the eye from approaching very near to the object. This inconvenience
is remedied by making the lens extremely small : it may then be spherical
without occupying much space, and thus unite the advantages of a short
focus, and of allowing the eye to approach the object.

A double microscope is a more complicated instrument (Jig. 40), in
which you look not directly at the object AB, but at a magnified
image of it, ab. In this microscope two lenses are employed: the one,
Jj M, is placed so near the object, that the image which it forms is farther
from the lens than the object itself is ; the image therefore is larger than the
object itself, and it is further magnified by being viewed through another lens,

Fig. 40.

N O, which acts on the principle of the single microscope, and is called the
eye-glass. The solar microscope is the most wonderful, from its great
magnifying power : in this we also view an image formed by a lens, not
the object itself. A ray of light is admitted into a darkened room through
a small aperture in the window-shutter, and the object A B (fig. 41), which
is a small insect, placed before the lens C D, and nearly at its focus :

INTRODUCTION TO OPTICS.
Fig. 41.

xcix

the lens itself being placed at such a distance from the opposite wall
that an image may be accurately formed upon it ; the image E F there-
fore will be represented on the opposite wall in the same manner as
the landscape was in the camera obscura — with this difference, that it
will be magnified, instead of being diminished, because it is farther from
the lens than the object A B ; while the representation of the landscape
was diminished, because it was nearer the lens than the landscape was : a
lens, therefore, answers the purpose equally well, either for magnifying or
diminishing objects. In this state, the image produced by the solar
microscope is faint and indistinct, a very small ray of light being diffused
over a prodigiously magnified image ; but if the aperture be enlarged, so
as to admit a more considerable pencil of rays, and a lens X Y (Jig. 42)

Fig. 42.

placed in it to bring it to a focus on the object AB, the image will be
much more distinct. There is but one thing more required to complete
the solar microscope, which is a small mirror, PQ (placed on the outside
of the window-shutter), which receives the incident rays, S S, and reflects
them on the lens X Y. This microscope can be used only when the sun
shines, and is adapted to transparent objects. Very minute objects, such
as are viewed in a microscope, are generally transparent; but when
opaque bodies are to be exhibited, a, second mirror M N (Jig. 43) is used
to reflect the light on the side of the object next the wall : the image is
then formed by light reflected from the object, instead of being formed by
rays transmitted by it. A magic lantern is constructed on the same
principle — with tlys difference, that the light is supplied by a lamp instead

INTRODUCTION TO OPTICS.
fig. 43.

of the sun. The microscope thus enables us to see and distinguish objects
which are too small to be visible to the naked eye. But there are objects
which, though not really small, appear so to us, from their distance. To
these we cannot apply the same remedy, for when a house is so far off as
to be seen under the same angle as a mite which is close to us, the effect
produced on the retina is the same : the angle it subtends is not large
enough for it to form a distinct image on the retina. It is impossible in
this case to bring the object to the eyes, but by means of a lens we
may bring an image of it nearer to us ; but then, the object being very
distant from the focus of the lens, the image would be exceedingly
smaller than the object itself, and in most cases it would even be so small
as to be invisible to the naked eye. To obviate this difficulty, we must look
at, the image through another lens, which, acting as a microscope, enables
us to bring the image close to the eye, and thus renders it visible. This
instrument is a telescope. • In Jig. 44, the lens C D forms an image, E F.

Fig. 44.

A

of the object A B ; and the lens X Y serves the purpose of magnifying that
image : and this is all that is required in a common refracting telescope.
Observe that the image is not inverted on the retina, as it usually is : the
object therefore appears to us inverted. When it is necessary to represent
the image erect, two other lenses are required ; by which means a
second ima^e is formed, the reverse of the first, and consequently upright.
These additional glasses are used to view terrestrial objects, for no incon-
venience arises from seeing the celestial bodies inverted.

When a very great magnifying power is required, telescopes are con-
structed with concave mirrors instead of lenses. Concave mirrors pro-
duce by reflection an effect similar to that of convex lenses by refraction.
In reflecting telescopes, therefore, mirrors are used in order to bring the
image nearer the eye; and a lens or eye-glass, as in the refracting tele-
scope, to magnify the image. The advantage of the reflecting telescope
Js, that mirrors whose focus is six feet will magnify as much as lenses of
a hundred feet.

A POPULAR ACCOUNT

N-EW

CHAPTER I.

Of the State of Optical Science before
the time of Newton.

(1.) THE splendid phenomena of optics
must have been among the first natural
appearances to attract the attention of
mankind. Of all the objects in nature,
light is perhaps the most pleasurable.
Vision, at once the most perfect and
useful of the senses, wholly depends on
it. By its agency the sphere of our ob-
servation and experience is indefinitely
enlarged. It brings us sure and im-
mediate intelligence of existences and
events, whose places are remote, and
thus gives us a certain degree of omni-
presence. Setting aside all the beau-
tiful variety of form and figure, and the
gorgeous pheaomena of colours, which
it is the means of disclosing, light it-
self is a delightful perception. Nature
supplies it so continually and so abun-
dantly, that we are apt to forget its
value; but, in cases where habit has
not blunted the sense of pleasure, it
seems to produce singular enjoyment.
The infant eagerly directs its gaze to the
window or the lamp, and stretches forth
its hand as if to grasp an object so
agreeable. Persons blind from infancy,
but whose organs are not absolutely
opaque, derive exquisite pleasure from
the perception of the cloudy light which
the imperfectly transparent humors al-
low them.

The property of light soonest noticed
was most probably its rectilinear pro-
pagation, by far the most important of
its qualities, and one with which all the
others are intimately connected. It was
impossible to observe the effects of
opaque bodies on light, the confines of
their shadows and their effects on the
sense of sight, without at once discover-
ing this important law. An opaque
body B (flg. 1.), placed in a right line
A C joining another opaque body A and

aluminous point C, deprived that other
opaque body A of the light it received

Fig. 1.

from the point C ; but if the same
opaque body B was placed in any
curved or crooked line ABC, joining the
luminous point G and illuminated body
A, no such obscuration was produced.
Again, if a straight line be drawn from
the eye to a luminous point, and also
any curved or crooked line drawn from
the eye to the same point, an opaque
body placed any where in the straight
line will deprive the eye of the percep-
tion of light ; but if the same opaque
body be placed any where in the curved
or crooked line, the perception of light
continues. Facts like these must have
occurred so constantly at all times, and in
all places, the inference from them is so
evident and immediate, that it is impos-
sible to suppose that the rectilinear pro-
pagation of light was not known even
in the most rude and savage state.

(2.) Some of the phenomena of re-
flection must also have attracted atten-
tion, and excited inquiry at a very early
period. The inverted picture of a land-
scape in the placid water of the lake or
river must have been viewed with as-
tonishment. Polished surfaces, natural
and artificial, presented themselves in
sufficient abundance to furnish nume-
rous experiments on reflection, and thus
from the rectilinear propagation of light
the step to the law of reflection was not
very difficult. The equality of the angles
of incidence and reflection was taught
in the Platonic school, and probably was
known considerably prior to that date.
B

2

A POPULAR ACCOUNT

Aristotle devoted considerable atten-
tion to the phenomena of rainbows, par-
helia, halos, &c. He observed these
with accuracy, and ascribed the rain-
•bow to the imperfect reflection of the
sun's rays by drops of rain. Some of
the properties of concave and convex
spherical mirrors appear to have been
taught in the Alexandrine school, and
are explained in a treatise on optics
contained in the works of Euclid.

(3.) The phenomena of refraction not
being of so striking a kind, or of such
frequent occurrence as those of reflec-
tion, do not seem to have been noticed
until a later period. The power of re-
fractors to collect the sun's rays to a
focus, so as to burn any substance ex-
posed to their influence, was, however,
long known. Burning refractors are
very distinctly described in Aristophanes
comedy of The Clouds, and Aristotle
observed the broken appearance of a
stick held obliquely in water, and at-
tempted to account for it. In the first
century of the Christian era several
optical phenomena were investigated by
Seneca, and among others he observed
that writing viewed through a glass
bottle filled with water was magnified.
This was, probably, the first discovery
of the magnifying power of a refracting
medium, bounded by convex surfaces.
Seneca also noticed the colours pro-
duced by a prism or angular piece of
glass, and observed that they were si-
milar to those of the rainbow.

(4.) The most distinguished among
the ancients for discovery in optical
science was Claudius Ptolemy, the
celebrated astronomer of Alexandria,
who flourished in the second century of
the C hristian era. This philosopher was
the first who observed, with any degree
of scientific precision, the phenomena
of refraction. He appears to have sys-
tematized, improved, and imparted a
greater degree of accuracy to all that
was previously known concerning the
reflection of light at plane and curved
surfaces, but he must justly be con-
sidered as the first and exclusive dis-
coverer of the principal phenomena of
refraction or dioptrics.

Ptolemy observed that if a visible
object, a piece of money for example, be
laid on the bottom of a vessel, and the
eye so placed that the edge of the vessel
just intercepts the view of the object,
upon filling the vessel with water, the
object will seem to be gradually raised,
until at length it comes distinctly into

view. This fact led him directly to the
true nature of refraction, and shewed
that the visual ray in passing out of the
water at the surface, was bent towards
the eye of the spectator, so as to make
the object on the bottom of the vessel
appear higher in the water than its
real position. He invented an instru-
ment to measure the deflection of the
ray in passing from the water into air.
This instrument consisted of a circle,
carrying two sights on its graduated
rim, and a third sight at the centre.
The circle was immersed in the water,
with its plane perpendicular to the sur-
face, and so that the surface of the
water coincided with one of its diameters,
and that one of the sights on the rim was
above and the other below the surface.
The eye being placed at the sight above
the surface, the sight below the surface
was moved upon the rim, until the three
sights appeared to lie in the same
straight line. The distances of the sights
on the rim, from the highest and lowest
points of the circle, then shewed the
angles of refraction and incidence.

With this instrument Ptolemy ob-
served and calculated the refractions
corresponding to every ten degrees of
incidence in the quadrant, the refraction
being made between air and water. He
also measured and calculated the re-
fractions between air and glass and
water and glass, by cutting the glass
into a semi-cylinder of the same dia-
meter as the graduated circle, and ap-
plying the semicircle to the end of the
semi-cylinder.

The results of these experiments
showed that the deflection of the ray
between glass and water was less than
in either of the cases between water or
glass and air. From this he was led to
conclude that the difference of the den-
sities of the media was the cause of re-
fraction, since water and glass differed
less in density than air and glass, or air
and water. This fortunate observation
suggested the probability that light from
celestial objects, in passing into our at-
mosphere from the surrounding me-
dium, whatever it be, might suffer a
deflection, the consequence of which
would be that our view of the whole
face of the heavens must be distorted ;
objects appearing variously removed
from their true places, according to their
various positions with respect to the
highest point or the zenith. The only
point which would not be removed from
its true place under such circumstances,

OF NEWTON'S OPTICS.

would be the zenith itself ; for the ray
passing from that point to the eye, must
enter the atmosphere perpendicularly ;
and he observed that in his experiments,
whenever the ray met the surface of the
water or glass perpendicularly, there was
no deflection.

Thus he perceived that the zenith was
a fixed point, with reference to which
he should be enabled to ascertain this
interesting fact. The test to which he
submitted this is a remarkable instance
of philosophical acuteness. In his ex-
periments he had observed that the
more obliquely the ray met the refract-
ing surface, the greater was the deflec-
tion of the refracted from the incident
ray. Hence he supposed that those ob-
jects which were more remote from the
zenith, and the light of which met the
atmosphere more obliquely, were more
removed from their proper places. In
other words, that the distortion of the
firmament by the refraction of the at-
mosphere was greater near the horizon
than near the zenith. Accordingly, he
observed the positions of the same star
in different parts of its diurnal path in
the heavens, and found that it appeared
not to move in a lesser circle parallel to
the celestial equator, but that it con-
tinually deviated in a slight degree from
such a circle ; that this deviation was
greater, the more distant the star was
from the zenith or highest point, and
that the deviation always brought the
star nearer to the zenith.

Such were thephenomenahe observed,
and they were precisely what his experi-
ments suggested. The last mentioned
circumstance, of the deviation being
always towards the zenith, showed that
the refracted ray was bent towards the
perpendicular, which proved that the
density of the atmosphere must be greater
than that of the fluid, if such there be,
which pervades the region beyond it.

(5.) A long interval elapsed after the
age of Ptolemy, before the science of
optics made any advance. About the
beginning of the twelfth century, some
steps were made towards improvement
in the theory of vision. In the thirteenth
century Roger Bacon devoted consider-
able attention to the study of optics ;
and although he cannot be said to have
extended the bounds of the science by
positive discoveries, yet he has so plainly
described the effects on vision produced
by lenses and their combinations, that
we cannot, with any regard to justice,
deny him a share in the honour of the

invention of spectacles, telescopes, and
microscopes. In describing the effects
of convex lenses of glass, he states
that "they are useful to old men, and to
those that have weak eyes, for they may
see the smallest letters sufficiently mag-
nified." Respecting the effects of com-
binations of lenses, he says — " We shall
see the object near at hand, or at a dis-
tance, and under any angle we please.
And thus from an incredible distance we
may read the smallest letters, and may
number the smallest particles of dust
and sand, by reason of the greatness of
the angle under which we see them ; and
on the contrary, we may not be able to
see the greatest bodies just by us, by
reason of the smallness of the angles
under which they appear; thus a boy
may be as big as a giant, and a man as
big as a mountain, forasmuch as we
may see the man under as great an angle
as the mountain, and as near as we
please. Thus, also, the sun, moon, and
stars, may be made to descend hither in
appearance, and to appear over the heads
of our enemies, and many things of
a like sort, which would astonish un-
skilful persons."*

It will be perceived that Bacon not only
describes the effects of telescopes, but
also distinctly alludes to their causes. It
is very difficult to conceive that he could
have written thus, without having actu-
ally constructed the instruments, and
witnessed the effects which he describes.

(6.) In the sixteenth century, Mau-
rolycus explained, with great exactness,
the structure and functions of the eye,
more especially of the crystalline humor.
He showed that those defects which are
called long-sightedness, and short-
sightedness, proceeded from too small or
too great a refracting power in the eye,
and showed how and why these defects
were removed by the use of convex and
concave lenses. Maurolycus failed to
discover the formation of the picture on the
retina, and the functions of that coat, from
the difficulty of reconciling an inverted
image with our perception of erect objects.

About this time Baptista Porta," a
Neapolitan philosopher, invented the
camera obscura. He observed that if a
small hole be made in the window-shutter
of a darkened chamber, the images of
external objects will appear depicted in
their proper colours on the opposite wall.
He then tried the effect of a convex lens
fixed in the aperture, and found that the

* See the Edinburgh Encyclopaedia — OPTICS.
B 2

A POPULAR ACCOUNT

images became much more perfect and
distinct. In this way the features of
persons outside could be discerned, and
known from the image. The pictures,
however, thus produced, were inverted.
To remedy this, he proposed that, instead
of being projected on the wall, the rays
from the lens should be received upon a
convex mirror properly placed and
adapted to the lens.

Observing that the images thus formed
by the convex lens were magnified, and
seen with more distinctness than with the
naked eye, it occurred to him that if a
convex lens were presented to an object
so as to form an image at its focus, an
eye placed a short distance behind that
image would see the object magnified.
Such an arrangement was, in fact, a
telescope without an eye-glass.

(7.) The phenomenon of the rainbow
could not have failed to attract attention
at a very early period. A work has
been brought to light by Venturi, writ-
ten, in 1311, by Theodoric of Saxony, a
Dominican friar, in which a rational
explanation of the double bow is given.
Extensive extracts from this work, with
the figures of the single and double re-
flection within the drops, may be seen
in the sixth volume of the Annales de
Physique et de Chimie.

The art of printing not having been
then invented, it is probable that the
work of Theodoric did not gain publi-
city, and was not generally known.
Accordingly, we find several eminent
philosophers, at the end of the sixteenth
and commencement of the seventeenth
centuries, engaged in attempts to solve
the problem of the rainbow. Some
conceived the exterior bow to be a
reflected image of the interior one,
and thus accounted for the inversion
of the colours. Fleschier, of Breslau,
attributed the production of the colours
of the rainbow to two refractions by
the drop, but conceived that a reflec-
tion took place at another drop before
the light reached the eye. It is curious
to observe how slowly and gradually the
laws of nature are discovered.

Soon after the year 1600, Antonio de
Dominis, archbishop of Spalatro, re-
duced the phenomenon of the rainbow
to actual experiment, and proved that
one reflection only, with two refractions,
is sufficient to produce the effect. He
filled a hollow globe of glass with water,
and having placed it in a proper position
with respect to a beam of solar light,
found that when viewed in the same

direction, and under the same circum-
stances, as the drops of rain which form
the bow, the same colours were pro-
duced, and were disposed in the same
order. He conceived that a solar ray
entering at the upper part of the drop,
was retracted to the back of it, where it
suffered a reflection by the inner sur-
face ; passing out again at the lower
surface, that it reached the eye of a spec-
tator properly placed, and produced a
perception of the colours of the bow.
He accounted for the colours in the fol-
lowing manner. The red rays issued
from the nearest part of the inner sur-
face of the drop, and having traversed a
less quantity of water, preserved the
greater degree of intensity ; for the red
colour was always considered to be pro-
duced by the most intense and active
portion of the light. The green and blue
rings, on the contrary, were those which
were reflected from that part of the
posterior surface of the drop which was
most distant from the point of final
emergence, and having, therefore, tra-
versed a greater quantity of water, were
more faint. The other colours of the
bow were conceived to be formed by these
three mixed in various proportions.

If a straight line be drawn from the
sun to the centre of the drop, and con-
tinued through the centre, it will meet
the posterior surface, at a certain point.
De Dominis conceived that the rays
which produced the same colour were
similarly situate with respect to this
point, and therefore that such rays ought
to form with the line drawn from the
sun to the eye of the spectator equal
angles. Hence he inferred that each
colour should appear in a circular band
or in the surface of a cone of which the
eye is the vertex, and the line from the
eye to the sun the axis. Upon these
principles he accounted for the shape of
the bow and the order of the colours,
and confirmed his theory by correspond-
ing experiments with the glass globe.

It has been considered extraordinary
that it should be reserved for De Dominis
to make the first important step towards
an explication of the most singular and
beautiful phenomenon in nature. Mon-
tucla declares his work to be obscure
and confused, and to betray an unusual'
ignorance even of so much of optical
science as was generally known in that
day. Some difference of opinion exists,
however, as to the extent of his claims
to a share in the merit of the explana-
tion of this phenomenon. Montucla

OF NEWTON'S OPTICS.

states, that he cannot concede to him
greater merit than that of having had a
glimmering of the true explanation of
the interior bow. From the manner in
which he explains the course of the rays,
it seems doubtful whether he was even
aware of the second refraction which the
rays suffered on their emergence from
the drop. Montucla, however, denies
him any participation in the discovery of
the cause of the exterior bow, and states
that he had not the most remote idea of
the double reflection which constitutes
the character of this phenomenon. Dr.
Brewster, on the other hand, an high
authority on this subject, says, that
although his attempt at explaining the
interior bow is sufficiently absurd, yet
that it is impossible for any philosopher,
free from the influence of national par-
tiality, to deny that the Italian prelate
has given such an explanation of the
general phenomena of the exterior bow,
that any other philosopher of more
optical knowledge, and of inferior acute-
ness, could not fail, without any stretch
of intellect, to give precision and perfec-
tion to the explanation. Newton's own
opinion appears to have been similar to
that last quoted.

About the period at which De Dominis
instituted his experiments, Kepler pro-
posed an explanation of the rainbow, in
a letter to Harriot. He conceives that
the solar ray which touches a drop of
rain is refracted, and penetrating the
drop meets the posterior surface, from
which it undergoes a partial reflection ;
that again penetrating the drop, it
emerges, and at its emergence undergoes
a second refraction, after which it reaches
the eye of the spectator. Kepler here
approximated very closely to the true
solution of the problem. If, however,
the ray which finally reached the eye
were a tangent to the drop when inci-
dent on it, the bow would be much
smaller than it is known to be. Har-
riot did not, on this occasion, give to the
subject that attention to which its im-
portance entitled it, but excused himself
to Kepler by business and indisposition,
promising that he would one day develop
the mystery. It is evident that he agreed
with Kepler in the necessity of a reflection
intermediate between the two refrac-
tions, which, indeed, considering the re-
lative positions of the sun, the drops,
and the spectator, was sufficiently ob-
vious. But he said nothing which can
guide us to a knowledge of how far he
was acquainted with the true theory of
the phenomenon.

(8.) The accumulated facts and experi-
ments furnished by various philosophers,
and the numerous suggestions of optical
writers, on the use and applications of
lenses, and their combinations, among
which those of Roger Bacon and Bap-
tista Porta are more especially entitled
to notice, had now prepared the way for
the construction, we will not say inven-
tion, of telescopes and microscopes. The
approach to the construction of the
telescope, like the passage from dark-
ness through twilight to broad day, is so
gradual, that it is almost impossible, if
we deny the invention to Bacon, to as-
sign it to any other. Descartes assigns
the discovery to James Metius, a Dutch-
man, and a citizen of Alkmaer. He
commences his Dioptrics with this hu-
miliating confession : — " To the shame
of science, this admirable invention was
the fortuitous result of experience. About
thirty years ago, a person named James
Metius, who had never studied, although
his father and brother had devoted them-
selves to the profession of mathematics,
but who took great pleasure in making
mirrors and burning glasses, having
occasion for glasses of different forms,
happened to look through two of them,
of which one was convex and the other
concave. He applied them to the ends
of a tube, and thus happily formed the
first telescope."

Not satisfied with this origin of the
telescope, some authors have sought for
one still more humiliating to science,
and to the pride of human intellect. It
is said that the children of a spectacle-
maker at Middleburg, happening to play
in their father's shop, were amusing
themselves with looking at a weather-
cock with two glasses, the one convex
and the other concave ; and happening
to place them in a fit position, they be-
held the object magnified, and brought
close to them. They communicated their
astonishment to their father, who, to
make the experiment more convenient,
fixed the glasses in a proper manner up-
on a board. Presently another person ad-
justed the lenses at the ends of a tube, so
as to exclude the lateral light, which dis-
turbed the vision, and thus made the ob-
jects appear more brilliant and distinct.
The next improvement, which trod upon
the heels of the last, was to use tubes
which moved one within another, so as
to admit of any adjustment of the lenses
which might be found necessary.

Without insisting further on this ac-
count of the invention, which, besides
being unsupported by evidence, is at-

A POPULAR ACCOUNT

tended with circumstances which give it
small probability, we shall notice the
better substantiated claim of Zacharias
Jansen, urged by Borelli, and attested
by legal witnesses, who were regularly
sworn by the consular magistrates of
Middleburg in the year 1655. Zacha-
rias Jansen appears to have been a
spectacle-maker at Middleburg, and
the witnesses were his children, a son
and daughter. The son assigns the in-
vention to the year 1590, and the daugh-
ter to the year 1610. Both, however,
agree in the fact of the invention ; and
the difference in the dates assigned to it,
instead of invalidating their testimony,
ought to be considered favourable to its
truth, since it shows that no conspiracy
existed between them. Other witnesses
give the honour of the invention to Jean
Lapprey, a spectacle-maker in the same
place.

All these circumstances, considered
with reference to the general state of
optical knowledge at the time, render
it probable that telescopes were con-
structed by several persons nearly at
the same period, each being ignorant
of what had been done by the others.
Effects so singular and so brilliant as
those produced by the telescope could
not be long confined to one country. It
will easily be believed, that such an in-
strument could not continue a mere toy
of amusement, or matter of curious ob-
servation to philosophers. Among those
who applied it to the great ends of
science, the name of Galileo stands fore-
most. If we could credit his own ac-
count, it would not be more than justice
even to assign to this philosopher a
share in the honour of the invention,
although we cannot concede to him a
priority. He states that he was at Venice
when a report of the wonderful effects of
this discovery was spread abroad. Doubt-
ful at first to what degree of faith state-
ments apparently so incredible were en-
titled, he awaited a confirmation of the
intelligence, which he received in letters
from Paris. Being credibly assured of
the reality of the powers ascribed to the
new instrument, but uninformed of the
particulars of its construction, he ap-
plied himself to the investigation of those
particulars, by the aid of the established
theory of refraction, and completely suc-
ceeded in discovering them. He forth-
with applied a convex object-glass, and
concave eye-glass, to the extremities of
a tube, and directing it to distant ob-
jects, found that it rendered them three
times as large as they appeared to unas-

sisted vision. The success of this first
attempt stimulated him to further exer-
tion, and he constructed a second tele-
scope which magnified about eight times.
Finally, sparing neither labour nor ex-
pense on a subject which seemed to pro-
mise results so important, he produced
a telescope which magnified thirty times,
and with this he discovered the satellites
of Jupiter, the solar spots, and other
phenomena.

This account of Galileo's proceedings
is given upon his own authority. It does
not seem, however, likely that Galileo
could have remained in total ignorance
of the means which produced the won-
derful effects which had excited such
general attention. Besides, he acknow-
ledges that he did not trust to mere pub-
lic report, but received a letter expressly
on the subject from " the noble James
Badovere at Paris." Of this letter, which
was written to inform Galileo on the
subject, he does not give the particulars.
Is it likely that in such a communica-
tion, made to such a man, no allusion
would be made to the means of produc-
ing the effects described, nor that lenses
were not distinctly mentioned ? Add to
this, that the general problem of magni-
fying distant objects by the modification
of the rays of light by refraction or re-
flection, or both, which we must sup-
pose to be that which Galileo asserts
that he solved, is very indeterminate,
and such as would be extremely unlikely
to be deduced from the general theory of
optics as then known. Still less proba-
ble is it that of the infinite variety of so-
lutions, which so indeterminate a pro-
blem admits, he would have chanced to
be led to that particular one which had
been practised in the north of Europe.

(9.) The honour of the invention of the
astronomical telescope belongs indispu-
tably and exclusively to Kepler. In his
work on Dioptrics, he distinctly suggests
the substitution of a convex eye-glass
for the concave one previously used. He
shows that in this case the image will
necessarily be inverted ; but the advan-
tage of an enlarged field of view, in
which this instrument excels the Galilean
telescope, seems to have escaped his no-
tice. To this we may, perhaps, impute
the circumstance of Kepler's never hav-
ing actually constructed telescopes upon
the principle which he suggested, con-
sidering, probably, that as they presented
inverted images, their use for terrestrial
objects would be awkward and inconve-
nient, and that they possessed no ad-
vantage in astronomical observations

OF NEWTON'S OPTICS.

over those used by Galileo and others.
Kepler was, however, aware of the me-
thod of correcting the position of the
image, by the use of additional eye-
glasses ; but probably considered that
the complexity of the instrument, and
the loss of light, would render it inferior
to the Galilean telescope.

The optical discoveries of Kepler were
not confined to the telescope. He made
experiments on refraction by lenses, and
succeeded in establishing some of the
properties of their foci. He also ex-
plained the formation of the inverted
image on the retina. He attributed erect
vision from an inverted image to an
operation of the mind, by which it refers
the lower part of the image to the upper
side of the eye, but considered it beyond
our power to determine the manner in
which the mind perceives the images of
objects upon the retina. He investigated
the power of the eye to accommodate
itself to different distances, and attri-
buted it to the contracting power of the
ciliary processes.

(10.) At the period to which we now
refer, the beginning of the seventeenth
century, the most important discovery
in the theory of refraction since the time
of Ptolemy, was made by Willebrord
Snellius, professor of mathematics at
Leyden. This philosopher, by a careful
comparison of numerous refractions at
different incidences, found that if a
sphere were described round the point of
incidence, and a cylinder circumscribed
this sphere, having its axis perpendicular
to the refracting surface, the parts of the
incident and refracted rays, between the
centre of the sphere and the cylinder,
were in a constant ratio, so long as the
refracting medium remained the same.

To explain this important law more
fully, let I, Jig. 2, be the point of inci-
Fig. 2.
A

dence, and S I S the refracting surface,
S B S being a vertical section of the re-
fracting medium, or rather a section
supposed_to pass through the incident

ray, and perpendicular to the surface.
With I as centre, and any distance I S,
describe a circle in the plane of the sec-
tion, and draw tangents at S S, perpendi-
cular to the refracting surface. Let the
incident rays meet the upper tangent at
P', P", P'", &c. ; and let the correspond-
ing refracted ray meet the lower tangent
at p', p", p"'y &c. It was found by Snel-

P'T P"I P''I
lius, that the ratios J_,_..__,g».

were equal so long as the media S A S
and S B S remained the same. Taking
the radius I A or I B as the unit, the
lines P'l, P"I, P'"!, Sec. are the cosecants
of the angles of incidence ; and the lines
p'l, p"l, p'"l, &c. are the cosecants of
the angles of refraction. These quanti-
ties are, therefore, in a fixed proportion.
The cosecants of angles being the reci-
procals of their sines, it follows that
when the media on each side of the re-
fracting surface are given, the sine of the
angle of incidence bears to the sine of the
angle of refraction an invariable ratio.

(11.) The discovery of the law of refrac-
tion has been sometimes erroneously as-
cribed to Descartes. With a degree of
disingenuousness and want of candour,
or rather of common justice, which not
unfrequently characterised the conduct of
that great philosopher, he has announced
in his Dioptrics, published eleven years
after the death of Snellius, the law of
refraction as the result of his own in-
quiries, without taking the slightest no-
tice of the previous discovery of Snel-
lius, although there is no doubt that he
was acquainted with it ; and there is even
strong reason to believe that Descartes
had the manuscripts of Snellius in his
hands, and availed himself of the full
and unrestricted use of them.

About the time of the death of Snel-
lius, which happened in 1626, at the
early age of thirty, Descartes applied
himself to optical investigations ; and, in
1637, published his Treatise on Dioptrics.
Guided by the law of refraction, with the
recent discovery of which he was made
acquainted, he made some important ad-
ditions to, and improvements in, the
science. The fact that spherical lenses
were incapable of collecting rays of light
into an exact focus, had been known by
experience ; and the cause of this sphe-
rical aberration, as well as its quantity
and laws, were easily deduced from the
Snellian law. To comprehend the nature
of this defect of spherical lenses, let the
light incident upon them be conceived to
be divided into a number of concentrical

B

A POPULAR ACCOUNT

rings ; the exterior ring, or that which
is most distant from the centre of the
lens, will be collected by refraction into
a focus, at a point on the axis of the lens
at a certain distance from the surface.
The next ring of light within the last
will be also collected into a point, but at
a greater distance from the surface ; and
so on, each ring of light is collected into
a focus, the distance of which from the
surface increases, as the distance of the
ring from the centre of the lens dimi-
nishes. To illustrate this, let L L be a
section of the lens at right angles to its
axis; and suppose its surface divided
into rings as repre- Fig. 3.

sented in fig. 3, and
let the order of the
rings be reckoned
from the edge of
the lens towards the
centre, calling the
external ring the
first ring, the next
within that the se-
cond ring, and so
on. Let L L,fig.4, be a section of
the lens by a plane through its axis;

Fig. 4.

and suppose that the whole lens, ex-
cept the first ring, be covered by an
opaque circular cover, and light be inci-
dent on its surface, this light will be re-
fracted to a certain point ft in the axis ;
and this point will, therefore, be the
focus of ihe first ring. Again, removing
the cover from the lens, let another be
substituted, which will leave the second
ring alone exposed to the light. The
rays will now be collected in the point
fs. The same process being continued,
and the third, fourth, and other rings
being successively exposed to the light,
their foci will be found at the points /3,
,/j, &c. the rings nearest to the centre of
the lens having their foci most distant
from its surface. It was easily deduced
from the law of refraction, that, the foci
of the rings near the centre of the lens
were much closer together than those
near its surface.

Since the images of objects were

known to be formed by collecting the
rays emerging from them into the focus
of the lens, it followed from these con-
siderations that each of the rings, into
which we have supposed the surface of
the lens to be divided, must produce a
separate image ; and thus an indefinite
number of images of the same object
would be found at different distances
from the lens, and arranged in regular
succession along the axis. This effect,
called spherical aberration, caused a con-
fusion in the appearance of the image,
which confusion was increased with the
magnitude and curvature of the lens.

Seeing that this defect was essential
to the very nature of spherical lenses,
Descartes proposed to investigate the
figure of a lens which should be free
from this defect; and such that each
ring, into which its surface would be
divided, might collect the rays to a focus
at the same distance from the lens.
The high analytical acquirements of this
mathematician, united with the know-
ledge of Snellius's law, rendered the so-
lution of this problem a matter of no
great difficulty. He accordingly found
a class of curves, since called the Car-
tesian ovals, which possessed the re-
quired property. When the densities of
the medium of incidence and the me-
dium of refraction are given, the figure
of the surface, which will collect into an
exact focus rays emerging from any
given point, will always.be determined
by one of these ovals. When the incident
rays are parallel, the oval becomes a
conic section *.

Ignorant of the cause of the principal
defect of lenses, which was subsequently
discovered by Newton, Descartes ex-
pected much greater results from this
discovery than it was capable of pro-
ducing. He invented machines, and en-
gaged skilful artists to grind spheroidal
lenses, according to the figures sug-
gested by his theory. After the expen-
diture of much labour and ingenuity, no
adequate advantage was obtained, and
at this day, when practical science has
attained such an extraordinary degree of
perfection, the spherical lenses are still
universally used.

(12.) In unfolding the theory of the
rainbow Descartes has been singularly
happy, and certainly has brought the
explanation of this phenomenon as near
to perfection as could be done by one
who was ignorant of the different re-

* For an account of these curves, see Lardner's
Algebraic Geometry, p. 452-

OF NEWTON'S OPTICS.

frangibility of light. Here, as in the
case of the law of refraction, this geo-
meter has fixed an indelible stain upon
his fame, by a similar act of unprinci-
pled selfishness and injustice towards
Antonio de Dominis, to whom he never
even alludes in his treatise. Descartes
explained why the interior bow has a
diameter of 42° ; this was an easy in-
ference from the theory of De Dominis
and the law of Snellius. But the genius
of Descartes was very conspicuous in
his solution of the problem of the ex-
terior bow. He shewed that the sun's
rays entering the inferior part of the
drop, emerged with a second refraction
from the superior, after undergoing two
reflections within. The double reflec-
tion accounted for the faintness of the
exterior bow, and the inversion of its
colours. Thus we see that ignorance of
the different refrangibility of light alone
prevented this philosopher from bringing
the theory of the rainbow to absolute
perfection.

(13.) In the middle of the seventeenth
century a discovery was made by Eras-
mus Bartolinus, a Danish mathemati-
cian, which formed the first of the most
brilliant train of experiments, and may
be considered the basis of the most
splendid speculations which ever adorn-
ed the annals of science ; speculations
which regard not merely the phenomena
of light and vision, but which seem to
furnish man with new sensibilities, which
are to touch what the microscope is to
sight, and disclose to his view wonders
of nature which would refuse to un-
veil themselves to any other power. In
this extensive field, more than in any
other, has the philosophic genius of
our own times shone forth.

Bartolinus received from some Danish
merchants, who frequented Iceland, spe-
cimens of the crystal, of extraordinary
transparency and dimensions, since
known by the name of Iceland spar,
which is carbonate of lime, in a crystal-
lized form. While making optical ex-
periments with pieces of this crystal, he
discovered that it exhibited a double
image of objects seen through it, and
consequently inferred that each ray of
light in passing through it was cloven
into two rays, which were refracted at
different angles, while the angle of in-
cidence of the whole ray of which they
were the component parts remained the
same. Slight observation was sufficient
to prove that both of these refracted
rays did not obey the Snellian law, for

if that were the case, both images of an
object seen through the crystal would
maintain the same position with respect
to a perpendicular to the refracting sur-
face, while the crystal was turned round
that perpendicular as an axis. Such,
however, he found not to be the fact.
By the revolution of the crystal, one of
the images only was observed to obey
the above-mentioned law. Hence he
inferred that one of the two refractions
was performed according to the com-
mon law, but that the other obeyed an
extraordinary law, not before noticed by
opticians. The results of these experi-
ments were published by Bartolinus, in
a work, entitled Experimenta crystalli
Islandici dis-diaclastici quibus rnira et
insolita ref radio detegitur. Copen-
hagen, 1699.

The publication of this work soon
drew the attention of the celebrated Huy-
gens to the subject of double refraction.
Previously to this he had published a
theory of refraction and reflection, found-
ed on the hypothesis, that light, like
sound, was propagated by the undula-
tions of a subtle and elastic medium,
which he supposed to pervade all
space permeable to light. Others held
that light was corporeal, and com-
posed of infinitely small and subtle
corpuscles, which were emitted from
every luminous body, and entering
the eye impinged upon the retina, and
produced sensation. This corpuscular
theory, which is as old as Pythagoras,
was adopted by Newton for the expli-
cation of the phenomena of optics ; but
it is proper to add, that he is careful not
to intermingle with the reasoning from
his experiments any assumptions from
this theory, which could at all affect the
validity of his results, all of which are
entirely independent of any hypothesis,
and such that any theory must account
for before it can be admitted as a true
one.

Huygens took up the subject of double
refraction, in order to obviate any ob-
jections to his undulatory theory, which
might arise from it. He accordingly
explains distinctly the law which regu-
lates the refraction of the extraordinary
ray, and attempts to shew how the phe-
nomenon may be accounted for by sphe-
roidal undulations, while ordinary re-
fraction is ascribed to spherical undu-
lations. As, however, this part of the
subject is not intimately connected with
the discoveries of Newton, we shall not
here enter into further detail upon it,

10

A POPULAR ACCOUNT

but refer the reader to our history of
optics, in which this and many other
matters entirely omitted here will be
treated at large.

While examining the two rays into
which the incident ray is cleft by the
Iceland spar, Huygens discovered one
of the facts from which Newton subse-
quently deduced the polarisation of light.
He found that if these two rays entered
a second prism of the same crystal si-
milarly placed with respect to the ray,
each of the two rays were again re-
fracted, according to the same law which
regulated their refraction in the first
crystal ; but he found that if the second
prism were placed at right angles to the
first, the laws by which the rays were
refracted were interchanged. In every
intermediate position of the second crys-
tal, each of the two rays from the first
crystal were divided into two, so that
the whole primitive ray was resolved
into four. These complicated pheno-
mena Huygens confessed himself unable
to solve, and it remained for the genius
of Newton to explain the curious and
beautiful property of polarisation.

(14.) Newton was born in 1642, and
in 1663 James Gregory published his
invention of the reflecting telescope,
consisting of two concave mirrors, the
larger of which, receiving rays from the
object, was perforated at its axis, and
opposite to the perforation and facing
the larger, the smaller was placed so as
to receive the rays reflected from the
larger, and reflecting them to bring them
to a focus in a tube placed in the per-
foration. The image thus formed was
viewed with an eye-glass as in the as-
tronomical telescope.

Three years after this Newton in-
vented his reflecting telescope, in which
he dispensed with the perforation in the
principal speculum, thereby preserving
the central rays which were most es-
sential to the formation of a perfect
image. The rays reflected from the
great speculum in Newton's telescope
were again reflected to an aperture in
the side of the tube by a plane reflector
placed at an angle of 45° with the axis
of the tube. At this aperture they were
viewed in the usual way through an eye-
glass.

(15.) In 1665 the work of Francis
Maria Grimaldi, an Italian Jesuit, was
published, containing his discovery of
the inflexion or diffraction of light. He
admitted a ray of light through a small
hole into a darkened chamber, and ob-

served that it was diffused into the form
of a cone, and that the shadows of
bodies placed in this light were larger
than they would have been had the
light passed without interruption or de-
flexion at their edges. Upon more mi-
nute and careful inspection, he observed
that these shadows were skirted with
three coloured fringes, which grew nar-
rower as they receded from the body.
He also observed, that when the light
was strong there were similar streaks of
colour within the shadow, which in-
creased in number in the same shadow
when it was received at a greater dis-
tance from the body. From these phe-
nomena he concluded that light, in
passing the boundaries of an opaque
body, was turned from its rectilinear
course, an effect which has been called
inflexion or diffraction.

Another experiment instituted by this
philosopher indicated a still more ex-
traordinary property of light. Through
two small apertures he admitted two
cones of light into a darkened chamber,
the apertures being so placed that the
cones did not penetrate one another
until they reached a considerable dis-
tance from them. Beyond this distance
the light was received upon a screen,
and it was observed that the part of the
screen illuminated by both cones was
the least bright, and that its degree of
illumination was increased by depriving
it of the light of one of the cones, by
stopping one of the apertures. Thus
it appeared that a body may actually
become more obscure by increasing
the quantity of light which shines
upon it.

We have in this chapter attempted to
give a short sketch of the principal dis-
coveries in optics before the time of
Newton. In doing this we shall per-
haps be accused of having given an un-
equal or disproportionate attention to
particular topics, and of having touched
too slightly upon others. It must how-
ever be remembered that our design was
not^ to write a history of optics, but to
shew the reader how far the way had
been paved for Newton, and what ad-
vances had been already made in some
of those theories which he has the merit
of completing ; and on the other hand
to shew that on other subjects, such
as the unequal refrangibility of light,
absolutely nothing was known, nor any
advance made, even in the shape of
conjecture.

We shall proceed in the next chapter

OF NEWTON'S OPTICS.

II

to give the reader, in a popular form, an
account of Newton's Optics, a work
which communicated to the world a
series of discoveries of transcendent
beauty, and which would themselves
have been sufficient to have conferred
immortality on the name of Newton,
even had he never written his PRIN-

C1PIA.

CHAPTER II.

The Compound Nature of Solar Light
established by Experiments.

(16.) BEFORE he enters upon the in-
teresting detail of the experiments which
led to his great optical discoveries, New-
ton explains in eight axioms the leading
principles and established results of the
science before his time. We have, in
the preceding chapter, enlarged more
fully on the subject than he has done in
the axioms, that the reader may be the
better enabled to appreciate the dis-
coveries we have to explain. The
propositions respecting the properties
of light which Newton gives under the
title of axioms, are not to be understood
as partaking of the nature of mathe-
matical axioms. On the contrary, many
of them are results, not only of careful
experiment, but of very subtle and com-
plex reasoning. " I have," says New-
ton, " now given, in axioms and their
explications, the sum of what hath been
hitherto treated of in optics. For what
hath been generally agreed on, I con-
tent myself to assume, under the notion
of principles, in order to what I have
further to write. And this may suffice
for an introduction to readers of quick
wit and good understanding, not yet
versed in optics ; although those who
are already acquainted with this science,
and have handled glasses, will more
readily apprehend what followeth."

The following are the propositions
assumed by Newton as axioms : —

Axiom I.

The angles of reflection and refraction
lie in one and the same plane with the
angle of incidence.

Axiom II.

The angle of reflection is equal to the
angle of incidence.

Axiom III.

If the refracted ray be turned directly
back to the point of incidence, it shall be
refracted into the line before .described
by the incident ray.

Axiom IV.

Refraction out of the rarer medium
into the denser, is made towards the
perpendicular ; that is, so that the
angle of refraction be less than the
angle of incidence.

Axiom V.

The sine of incidence is either accu-
rately or very nearly in a given ratio to
the sine of refraction.

Axiom VI.

Homogeneal rays which flow from
several points of any object, and fall
perpendicularly, or almost perpendicu-
larly, on any reflecting or refracting
plane or spherical surface, shall after-
wards diverge from so many other
points, or ,be parallel to so many other
lines, or converge to so many other
points, either accurately or without any
sensible error ; and the same thing will
happen, if the rays be reflected or re-
fracted successively by two or three or
more plane or spherical surfaces.

Axiom VII.

Wherever the rays, which come from
all the points of any object, meet again
in so many points, after they have been
made to converge by reflection or re-
fraction, there they will make a picture
of the object upon any white body on
which they fall.

Axiom VIII. 1

An object seen by reflection or re-
fraction, appears in that place from
whence the rays, after their last reflec-
tion or refraction, diverge in falling on
the spectator's eye.

The work opens with the announce-
ment of the discovery that LIGHTS

WHICH DIFFER IN COLOUR, DIFFER
ALSO IN REFRANGIBILITY.

For those readers who are not familiar
with the science, we shall first explain
the meaning of this property, and next
describe the nature of the experiments
by which Newton established it.

Let S &ss,jig. 5, be a section of any
transparent medium, as water or glass.
Let R be a ray of red light incident at
I, and supposed to proceed from some
red object in the direction I R, or to
have been transmitted through red
glass. In like manner, let B be a ray
of blue light incident at I', in a direction
B I', parallel to R I, and proceeding as
before from a blue object in that direc-
tion or transmitted through blue glass.
The angles of incidence . formed by the

12

A POPULAR ACCOUNT

rays R and B, with the perpendiculars

P to the refracting surface will be equal,

Fig. 5.

because the rays R and B are parallel.
On entering the medium they will be
both deflected by the law of refraction
towards the perpendiculars p. Let r be
the course after refraction of the red
ray,? and b the course after refraction of
the blue one. If these rays were equally
refrangible they would be equally de-
flected towards the perpendicular. Such,
however, is not the case. The red ray
r will be less deflected from its original
direction than the blue ray b, and there-
fore the angle under the lines r and p
will be greater than the angle under b
and p. Hence, red light is said to be
less refrangible than blue light.

We might expect that this effect
would be easily reduced to experiment
by colouring two sticks, one red, and
the other blue, and placing them paral-
lel to each other, obliquely in water. In
this case, the broken appearance which
the sticks should exhibit would be dif-
ferent, the angle formed by the parts in
the water with those in the air being
more obtuse in the red than in the blue
stick. It happens, however, that the
difference between their deflections is
so small, that, in this way, it would be
very difficult to render it perceptible.

To obviate this inconvenience, and
render the difference of refractions so
great as to be easily perceptible, Newton
contrived that the different lights should
each be twice refracted, so that being
originally parallel, they should thus ac-
quire the sum of the divergencies which
such refraction alone would give them.
This he accomplished by using a glass
prism, in the manner which we shall
now describe.*

* We advise every reader who has not seen the
prismatic spectrum, before he proceeds further, to
procure a prism, or a common angular piece of
glass with plane sides, and to transmit through it a
beam of the sun's light admitted through an aperture
in.™e window.shntter of a dark room. The facility

th which he will comprehend Newton's interesting
experiments will thus be much increased.

( 1 7. ) Let ABC, Jig. 6, represent a sec-
tion of a triangular glass prism at right

Fig. 6.

B

angles to its axis (an end view of it.) A
ray of red light entering at I in the direc-
tion RI will, by the refraction of the
glass, be deflected in the direction r, to-
wards the perpendicular to the surface
B C, on which it is incident. On meet-
ing the second surface B A, it will emerge,
suffering a second refraction on passing
into the air. But here, as it passes from
glass into air, it will be deflected from
the perpendicular to the surface B A in
the direction /, which increases still
more its deviation from its original course
R. In like manner, a blue ray incident
in the same direction at I will suffer two
deflections at the surfaces of the prism ;
but in each case will be more deflected
from its original direction R, than the
red ray. The deviation of the blue from
the red ray, on emerging from the sur-
face B A, will evidently be found, by
adding together the two deviations at
the surfaces B C and B A. Thus, al-
though either deviation alone might be
too small to be perceptible, yet the sum
of both will produce a sensible effect.

( 1 8.) To reduce this to absolute experi-
ment, Newton states that he took a black
oblong stiff paper, terminated by parallel
sides, with a line drawn from side to
side, dividing it into two equal parts.
One of these was coloured with an in-
tense red, and the other with an intense
blue. He then covered the wall and
shutters, surrounding the window of the
room with black cloth, to prevent the
light reflected from them from interfer-
ing with his experiment. On a table
before the window, also covered with
black, he placed the coloured paper, the
line dividing the colours being perpendi-
cular to the plane of the window. Things
being thus arranged, he held the prism,
with its angle B,y?g-. 7, upwards, so that
the light coming from the paper RB
should be twice refracted before it reach-
ed the eye placed behind the prism, as
represented in the cut. Upon viewing
the appearance through the prism, he
found that the blue half of the paper
appeared a little more elevated than the

OF NEWTON'S OPTICS.

red half, which plainly proved that the
rays from the blue half were more de-
flected from their original direction than
those of the red half. Had the rays been
equally refrangible, the parts of the
paper would have maintained the same
relative position, although the place of
the entire paper would appear to be
elevated.

Fig. 7.

In order to confirm the conclusion
deduced from this experiment, Newton
viewed the same paper with the refract-
ing angle B, Jig. 8, of the prism pre-
sented downwards. He found, as he
expected, that the blue half of the paper
was lower than the red, being more de-
pressed, owing to the greater refraction
of the rays.

Fig. 8.

upon the light. The greater that refract-
ing power, other things being the same,
the nearer to the lens will be the focus
into which the rays are collected. It was
also well known that the place of the
focus might always be detected by the
presence of an image of an object placed
before the lens. We have already stated
(p. 8), that a certain degree of indis-
tinctness will attend this image, when
the lens is spherical ; but still a certain
point of greatest distinctness will always
be found, which may be regarded as the
place of the focus, were all spherical
aberration removed. Now, if it be true
that light of different colours is differ-
ently refrangible, and that according to
the result of the experiments already
described, red light is less refrangible
than blue light, it would follow that the
images of red and blue objects, formed
by the same lens, ought to be found at
different distances from it.

To apply this test to the coloured
paper already mentioned, Newton wrap,
ped round it an extremely fine thread
of black silk, so as to form fine black
lines upon the red and blue ground, as
represented \nfig. 9 ; where, for distinc-
tion, the black lines are parallel on the
red, and cross each other on the blue.
He covered the wall of a dark chamber
with black cloth, and attached to it the

Fig. 9.

( 1 9.) He now determined to submit the
question to a different test. It was pre-
viously known that the position of the
focus, in which rays from any luminous
object are collected by a lens, depended
on the refracting power of the glass

coloured paper, with the silk
thread wound round it, as
in the figure. Immediately
under this he placed a light,
so as to illuminate the paper
thoroughly, the flame not
rising above its lower ter-
mination ; and, therefore,
not intercepting any of the
light reflected from it. He
then placed a double convex lens M N,

\X

Fig. 10.

Jig. 10, of about four inches and a quar-
ter diameter, at about six feet two inches

from the paper. The image of the paper
was received upon a white screen, at the

14

A POPULAR ACCOUNT

same distance behind the lens. This
screen was slowly moved to and from
the lens, until that position rr was ob-
tained, in which the black lines upon red
were seen with the greatest distinctness
depicted in the image on the screen.
This was evidently the focus of the red
rays, because the black lines became
distinctly visible only when the red boun-
dary was precisely defined. In this po-
sition the black lines on the blue part of
the image were faint, confused, and
scarcely distinguishable. The screen
was then moved slowly towards the lens.
As it moved the lines on the red part
became faint and confused, while those
on the blue part became clear and dis-
tinct, being found to be as distinct when
the screen was removed an inch and a
half nearer to the lens, as the lines on
the red had been in the first position ;
while, on the other hand, the lines upon
the red part were scarcely observable.
The conclusion was irresistible; the
same lens placed at the same distance
from red and blue surfaces illuminated
by the same candle, brought the rays
from the one surface to a focus nearer
than those from the other ; and, there-
fore, had a greater refracting power on
the blue rays than on the red ; which
conclusion harmonized exactly with the
results of the prismatic experiments pre-
viously instituted.

(20.) Notwithstanding the very con-
clusive nature of these experiments,
Newton reasons from them with a de-
gree of caution and circumspection truly
philosophical. " From these experi-
ments," he says, " it follows not that
all the light of the blue is more refran-
gible than all the light of the red ; for
both lights are mixed with rays differ-
ently refrangible, so that in the red there
are some rays not less refrangible than
in the blue, and in the blue there are
some rays not more refrangible than in
the red ; but these rays, in proportion to
the whole light, are but few, and serve
to diminish the event of the experiment,
but are not able to destroy it : for if the
red and the blue colours were more
dilute and weak, the distance of the
images would be less than an inch and
an half ; and if they were more intense
and full, that image would be greater, as
will appear hereafter."

These experiments were conclusive
respecting light which proceeded from
the colours of natural bodies. But it
still remained to analyze the direct solar
light, and to determine the nature of the

beams of white light, which are the
means of rendering all coloured objects
visible, and of causing them to transmit
the coloured light which emerges from
them.

(21.) If a ray of light, direct from the
sun, be admitted through a small aper-
ture A, fig. 11, in the window-shutter of

Fig. 11.

a dark chamber, a circular image of the
sun will be formed by the rays admitted
through the hole, and may be received
upon a paper screen, as at S S'. We
shall, for the present, suppose the hole
to be so small, that its diameter may be
neglected, and it may be regarded as a
physical point. The rays which proceed
from the several points of the sun's
disc entering the aperture A, cross
each other, and that from the highest
point proceeds to S' the lowest point
of the image, while that from the
lowest proceeds to S the highest point
of the image. In like manner the ray
from the right-hand side of the sun
proceeds to the left-hand side of the
image, and that from the left-hand side
to the right-hand side of the image. In
the same way every point of the sun's
disc is referred to that point of the image
diametrically opposite in position. The
image is therefore inverted in whatever
way it be considered with reference to
the sun.

The magnitude of the image, or illu-
minated spot, on the screen, evidently
depends on the distance of the screen
from the aperture A, increasing as that
distance increases ; and the diameter of
this image subtends at the hole A the
same angle as the sun subtends at it, or
as the apparent diameter of the sun.

We have here supposed that the hole
has no sensible magnitude, or is a phy-
sical point. If this be not the case, and,
on the other hand, the aperture have a
sensible diameter, all that we have above
stated will be true of every separate
point in it : so that there will be innu-
merable images of the sun ; the centres
of which will be diffused over a space

OF NEWTON'S OPTICS.

Fig. 12.

upon the screen equal to the size and
shape of the aperture. At present we
will consider the aperture circular. To
find, then, the magnitude of the illumi-
nated spot upon the screen, first describe
a circle A B, jig. 12, whose diameter is
equal to that of the
hole. Then, adding to
the radius of this cir-
cle another line, A C,
whose length is such
as would subtend at
the hole an angle
equal to the sun's se-
midiameter, with the
whole distance O C
describe a circle. This circle will be the
magnitude of the illuminated spot on
the screen.

(22.) Instead of allowing the beam of
light to pass directly from the aperture
to the screen, let it be intercepted by a
prism ABC near the hole, with its re-
tracting angle B presented downwards.
The refraction by the surfaces of this
prism might be expected to deflect the
beam from its original course, and raise
it to an higher position, as represented
in Jig. 13. The part of the screen on

Fig. 13.

15

Fig.14.

•3

•r

which the refracted ray falls, is, accord-
ingly, elevated by the effect of the prism,
but this is not the only effect produced.
The breadth of this spectrum, as it is
called, is exactly equal to the diameter
of the illuminated spot which would be
projected on the screen, if the prism
were removed. But, instead of a cir-
cular image being projected on the
screen, the illuminated part assumes
an oblong form, such as RV, fig. 14.
The sides are straight, and the ends
semicircular — the length being perpendi-
cular to the axis of the prism.

It appears, therefore, that of the rays
which pass through the prism, some are
refracted to a higher part of the screen
Hum others, those toward the end V
being more elevated by the refraction

than those towards the end R.
From this Newton inferred,
that of the light which passed
through the prism, some was
more, and some less refracted,
those rays which passed to-
wards the highest points, V,
being more refracted than
those which passed towards
the lowest points R.

The oblong form given to the
illuminated part of the screen,
was not the most curious
or surprising effect produced
by the prism. This image or K
spectrum exhibited the most beautiful se-
ries of colours, each depicted on the screen
with a degree of splendour and inten-
sity far exceeding those of the colours of
any natural object. Beside these the most
brilliant colour which nature presents,
or the most refined efforts of art could
exhibit, would seem faded and dim. The
lower extremity, R, exhibited the most
dazzling red, and above this, in regular
succession, were ranged the colours,
orange, yellow, green, blue, and indigo,
the upper termination, V, being violet.
These colours were not separated by
distinct limits, but the tints seem to melt
imperceptibly one into another, it being
impossible to determine exactly where
any one ended and the next began. Thus
the red was tinted off insensibly into the
orange, the orange into the yellow, and
so on.

From these observations, it is appa-
rent that the red light, and all that por-
tion of light which partook of this cha-
racter, being deflected to the lower part
of the spectrum, is less refracted than
the blue light, and those colours of the
same class which are refracted to the
upper part. It would therefore seem to
be an obvious inference, that the solar
beam incident on the prism, was a mix-
ture of different kinds of light ; that the
prism acting on the component parts re-
fracted some of them in a greater, some
in a less degree ; those partaking of the
blue character being more refracted than
those of the red kind. This inference
seems also to harmonize with the former
experiments made upon coloured bodies
(18, 19), whereby it was proved that
red light is less refrangible than blue.

(23.) Newton repeated the preceding
experiment in another way. The prism
being placed as before, at the aperture
in the window shutter, he placed his eye
behind it, so as to receive the rays emerg-
ing after the second retraction from the

1G

A POPULAR ACCOUNT

prism. In this case the eye supplied the
place of the screen in the former experi-
ment, the rays which formed the spec-
trum entering the pupil as they emerged
from the prism. He found the same
effects to ensue. He beheld the oblong
spectrum as before, the order of the
colours being in all respects the same.

(24.) It appears by the experiment
which we have described that the action
of the prism was such as to dilate the
ray in a direction at right angles to its
length, and thereby to give the spectrum
the oblong form. Now if this dilatation
was the consequence of the action of the
material of the prism, and not of the
various refrangibility of the component
parts of solar light, it must inevitably
follow that a second prism placed with
its length vertical, and consequently at
right angles to the first, so as to refract
the light sideways, would dilate the ray
as much in the horizontal direction as
the first did in the vertical. The infer-
ence from this would necessarily be, that
the combined action of two prisms would
be to give a square spectrum, as much
length being obtained by the action of
the one, as breadth by the action of the
other.

Accordingly Newton tried this expe-
riment. Let RV,y2g-. 15, be the spec-
trum produced by the
first prism. He placed
another prism with its
length vertical, and
consequently at right
angles to the first, and
when the rays were in-
tercepted by it, the ef-
fect was, that instead
of the spectrum, V R,
being spread over a square surface, it
retained its breadth, but was transferred
to the position V'R/.

This result furnished a most convinc-
ing and beautiful confirmation of the
theory of Newton. The rays at the

j 5
"

upper end of the spectrum were deflected
by the second prism, through the dis-
tances A B, from their former position,
while the rays at the lower end were
only deflected through the smaller spaces
a b. The breadth of the spectrum re-
mained unaltered, plainly shewing that
the second prism had no power to dilate
the rays which formed it. The rays
which were most refracted by the first
prism, were those of the blueish charac-
ter, which occupied the upper part of
the spectrum. These same rays were
also most refracted by the second prism,
being most removed from their first po-
sition (through the spaces A B). Also
the rays which were least refracted by
the first prism, were those of the reddish
character which occupied the lower part
of the spectrum ; and these also are least
refracted by the second prism, being
those which are least removed from their
places through the distances a b.

To put this question even more beyond
dispute, Newton received the rays from
the second prism on a third, placed with
its length parallel to the length of the
spectrum, and found the same effect re-
peated, the third position of the spec-
trum being inclined to the second in the
same manner as the second was inclined
to the first, but no dilatation taking
place, and the breadth of the spectrum
remaining the same. He states that he
used a fourth prism with the same re-
sult.

It is important to remember that in all
these experiments the light is all inci-
dent on each prism at the same angle.
For if its parts fell upon the surface at
different angles, different quantities of
refraction would be the natural and ne-
cessary result.

(25.) Newton contrived a very elegant
experiment to shew the regularity with
which the prisms determined the magni-
tude, figure, and position of the spectrum.
Before two small apertures F,/, fig. 16,

in the window shutter he placed two
similar prisms A B C, a b c, the one im-

mediately beneath the other, with their
lengths parallel and horizontal. These

OF NEWTON'S OPTICS.

17

cast two spectrums R' V', r' v't on the op-
posite wall so as to lie in the same right
line, and having the lengths at right an-
gles to the floor, the lowest point R', or
red end of one, being contiguous to the
highest point v', or violet end of I he other.
A third prism, DH, was now placed
with its length vertical, and of course at
right angles to the other two, and so as
to receive the rays emerging from them.
The two spectrums were immediately

translated from their former positions to
other positions RV, rv, no longer in the
same line, but similarly inclined to the for-
mer, and therefore parallel to each other.
(26.) The next test to which Newton
submitted the problem was even more
conclusive and convincing than any of
the preceding. Through an aperture, O,
fig. 17, in the window-shutter he admitted
a beam of the sun's light which he re-
ceived upon a prism ABC, placed before

Fig. 17.

the aperture. The spectrum produced
by the refraction of this prism was re-
ceived upon a screen perforated by a
small hole O'. The several coloured
lights of the spectrum being diffused
over a considerable space upon the
screen, and the aperture O' being small,
the light of but one colour passed
through it, while the prism A B C re-
mained stationary ; but when this prism
was slowly turned round its axis, the
spectrum moved upwards and down-
wards on the screen, so as to transmit
in succession the lights of the several
colours through the aperture O'. At a
distance of about twelve feet from this
screen another was placed, having, in like
manner, a small aperture O". The beam
of coloured light transmitted through the
aperture O' was received upon the se-
cond screen, diffusing itself over a space
of some magnitude. A small ray of
this light passing through the aperture
O" was received upon a prism A' B' C'
placed immediately behind it, and by this
prism was refracted to a certain point
upon the opposite wall.

The prism A'B'C' remaining fixed,
the prism ABC was turned until the red
end of the spectrum fell upon the hole
O'. A ray of red light now passed from
O' to O", and was refracted by the
prism A' B' C' to the point R on the wall.
This point was marked. By a slight
motion of the prism ABC, the orange
light was next brought upon O', and a

ray of it passing in the direction 0' O"
fell upon the prism at the same angle as
the red light had before been incident.
This orange ray was refracted by the
prism A' B' C' to a point O on the wall
a little above the point R to which the
red had been brought. The yellow,
green, blue, indigo, and violet rays were
in succession transmitted in the same
way to the prism A'B' C1, all being in-
cident upon it at the same angle, and
they were severally found to be refracted
to the points Y, G, B, I, and V. Thus
it appeared that the several coloured
lights into which the sun-beam was re-
solved by the prism ABC were, under
the same circumstances, differently re-
fracted by the prism A' B 'C', each light
being refracted the more, the nearer its
situation to the violet end of the spectrum.
The conclusiveness of this result
would have satisfied an ordinary in-
quirer, and it would have immediately
been made the basis of a theory. The
ardour of discovery wras, however, in
Newton tempered by philosophical cir-
cumspection", and in the unwearied
patience of his research, he left untried
nothing which could put his hypothesis
to the proof, and overturn it if false. In
the records of scientific discovery there
is not a more splendid instance of an
investigation in which theory and ex-
periment mutually guide and support
each other. In the first experiments,
Newton found that the coloured lights
C

18

A POPULAR ACCOUNT

reflected from the surfaces of natural
bodies were differently refrangible ; and
subsequently he shewed that the colours
produced by the refraction of a sunbeam
by a prism were also differently re-
frangible, and that the colours which,
reflected from natural bodies, were most
refrangible were also the colours most
refrangible in the refracted solar light.
But in order, as it were, to identify the
two experiments on natural colours and
coloured light, he instituted the following
experiment.

(27.) By means of two prisms, as de-
scribed in (25), he projected two spec-
trums on the wall, so as to be placed
end to end in the same direction, and
so that the violet end of the one joined
the red end of the other. At some dis-
tance from the wall he placed a slender
piece of white paper, with straight and
parallel edges, and so arranged that the
red light R,Jrg. 18, of the one spectrum

Fig. 18.

ILOTGBIV

should illumine one half of 1he paper,
•while the violet light, V, of the other
spectrum illuminated the other half,
The paper thus appeared of two colours,
red and violet, similar to the painted
paper used in the experiment described
in (18). The remaining lights of each
spectrum passing beyond the paper fell
upon the wall, which was hung with
black, in order that light reflected from
it might not disturb the experiment. It
is evident that this arrangement was
such as to place the coloured light pro-
duced by the refraction of the prism
exactly under the same circumstances
as the light reflected by the colours of
natural bodies in the experiment already
alluded to. Accordingly Newton viewed
the illuminated paper through a prism
held parallel to it, as in that experiment,
and found exactly the same result, vis:.
that the violet half was separated from
the red by a greater refraction, so that
the parts of the paper, instead of forming
one straight band, were now separated
from one another, but placed in parallel
directions. Instead of using a band of
paper, he sometimes used a white thread,
one half of which he placed in the violet,
and the other in the red light, and ob-
served the same effect, the thread
appearing to be broken, and one half of
it moved out of its place, but parallel to
its former position.

In this experiment, by turning one of
the prisms upon its axis, he was enabled
to illuminate one half of the thread
successively with the violet, indigo,
blue, green, and the other prismatic co-
lours, while the other prism, main-
taining its position constantly illuminated
the other half of the thread with red
light. Upon viewing these phenomena
successively with a third prism, he found
that in each case the parts of the thread
illuminated with lights of different co-
lours were separated, but the separation
was greatest between the red and violet,
less between the red and indigo, still less
between the red and blue, and so on,
being very small between the red and
orange. But when both parts of the
thread were illuminated with the reds of
the two spectra, the thread appeared no
longer broken. It is scarcely necessary
to observe, that all these phenomena
were such as must have been easily
foreseen from the supposed unequal re-
frangibility of differently coloured lights.
Newton might have carried this expe-
riment further, and probably he did so,
although he has not particularly men-
tioned it. He might have thrown on
the thread every possible distinct pair
of the prismatic colours, by moving both
prisms on their axis, and the result
would be that the apparent separation
of the parts of the thread, when viewed
through the prism, would have been
great in proportion to the distance be-
tween the two colours in the spectrum.

(28.) All those experiments instituted
by Newton, in which the refracted light
was received upon a screen, were re-
peated with the same success, the light
being admitted immediately to the eye
from the prism, without the use of
the screen. Thus the experiment (25)
in which the two spectra, lying in the
same line, were refracted by the prisms
to parallel lines, was repeated thus. The
spectra were viewed through a prism
without disturbing the screen, and their
apparent position, as seen through the
prism, was found to be the same as when
refracted by the third prism, and receive4
upon the screen.

Two prisms ABC, A'B'C',/g-. 1 9, were
also placed at apertures in the window-
shutter, the refracting angle of one being
directed upwards, and that of the other
downwards. The spectra produced by
these prisms were both in an upright
position ; but had the colours arranged
in an opposite order, the red end being
the higher in the one, and the lower in

OF NEWTON'S OPTICS.

19

the other. One of the prisms was slowly spectrum produced by the other. The

turned on its axis, until the spectrum two spectra thus appeared to form one,

produced by it was thrown upon the which, in its colours, differed from either

Fig. 19.

of them, the red of the one being mixed
with the violet of the other, the orange
with the indigo, and so on. This mixed
spectrum was now viewed through a
third prism, held as represented in the
figure. The effect which ensued was, the
separation of the spectra, which assumed
the cross position shewn in the figure.
This experiment is another variety of
that which we last described ; the in-
clined positions assumed by the spectra,
with respect to their first position, was
explained in (24) to arise from the
different refrangibilities of the rays.
In the present experiment, this inclined
position is given at the same time to two
spectra. The inclinations are in opposite
directions with respect to the first po-
sition, because the lights which form the
spectra are disposed in an opposite order.
(29.) The following experiment, men-
tioned by Newton, is another beautiful
example of the analysis of mixed lights.
A circular piece of white paper A, about
one inch in diameter, was placed before
a black wall, and using the two prisms
mentioned in the last experiment, the
paper A, Jig. 20, was illuminated at
the same time with the red light p. 2o
from the one, and a deep violet
light from the other. By this
mixture the paper assumed a
rich purple colour. The circle
A was then viewed through
a prism at some distance, and
the appearance exhibited was
two circles, R and V, the cir-
cle R, nearer to the paper
being red, and the more re-
mote one, V, violet. The prism
in this case refracted the red
and violet light, mingled in
the circle A, through different
angles ; the red being least re-
frangible was removed to R,
and the more refrangible vio-
let light carried so far as V.

To confirm this, the apertures before
the prisms, which cast the red and violet
lights on A, were in turns covered, so as
alternately to deprive the circle A of
these lights. It was accordingly found
that the circles R and V alternately va-
nished, plainly proving that all the light
of R came from the prism which cast
the red light on A, and that all the light
of V came from the prism which cast
the violet light on A.

By turning one of the prisms at the
window upon its axis, the circle A was
successively illuminated with all the pris-
matic lights, while the other prism, being
stationary, constantly projected on it an
intense red light. The effect produced
to an eye viewing these changes through
the third prism was, that the circle V
changed its colour according to the
change in the light used with the red
upon A. But the change of colour was
not the only alteration observed in V.
Its position was also changed. When
the blue light was mixed with the red,
it appeared nearer to R. Still nearer
when green was mixed with red on A.
In a word, the circles R and V always
exhibited the colours mixed upon A,
and their separation from A and from
each other always corresponded to the
separation of the prismatic spectrum
from the direct course of the light, and
to the separation of the two lights in the
spectrum from each other.

From all these experiments no doubt
could remain that lights which differed
in colour differed also in refrangibility.
One test more however remained, ana-
logous to that which was applied to
the colours of natural objects in (19).
If the difference of refrangibility be ad-
mitted, it will necessarily follow that the
same double convex lens will have dif-
ferent foci for differently coloured lights,
the focus of the more refrangible light
being nearer to the lens than that of the
p. Q

20

A POPULAR ACCOUNT

less refrangible. Thus if the lens be
exposed to a beam of violet light pro-
ceeding from a given object, and col-
lects that light to a focus at a certain
point, it should collect red light to a
focus at a more distant point, and the
lights of intermediate colour to points
between these extreme limits.

(30.) In order to reduce the doctrine
to this test, Newton cast a strong red
light, by means of a prism, upon the
page of an open book in a dark room.
At a certain distance from the book
thus illuminated, he placed a double
convex lens, so as to give an image of
the book at its focus ; this image was
received upon a sheet of white paper
properly placed behind the lens. The
book and the lens being fixed in their
respective positions, the paper was
moved until that situation was found in
which the image of the page and its
letters were most distinctly depicted on
the paper. This position was of course
the focus to which the red light re-
flected from the book was collected by
the lens. The prism in this experiment
was so placed, that as the sun moved
in the heavens, the several coloured
lights of the spectrum were successively
cast upon the book, without disturbing
either its place or those of the prism
or the lens. The position of the book
was ascertained in which the letters ap-
peared most distinct, and it was found
that as the successive prismatic lights,
in regular order from red to violet, passed
over the book, the place of greatest
distinctness gradually approached the
lens, so that the full violet light required
for distinctness that the book should be
two inches and an half nearer to the
lens than for the red light.

In this experiment it was necessary to
render the chamber extremely dark, in
order to prevent the pure prismatic light
cast upon the book from being diluted
by the white light which might be scat-
tered about the room. In proportion as
this adventitious light was admitted, it
was found that the distance between the
extreme foci became less. And this is
plainly the reason why the distance be-
tween the extreme foci of prismatic light
was found to be so much as two inches
and an half, while the distance for light
reflected from natural bodies was only
one inch and an half (19). For the
colours of natural bodies never have the
extreme vividness, purity, 'and splendour
which are obtained by the prism.

(31.) The doctrine of the different re-
frangibility of light led to an obvious

consequence respecting its inflexibility,
which Newton easily foresaw, and from
which he derived another beautiful test
to establish the truth of his theory. To
render this intelligible to those who are
not familiar with optical investigations,
we must here be permitted a short di-
gression.

It will be recollected that long before
the time of Newton, it was known that
the deflection of a ray of light, in pass-
ing from a denser into a rarer trans-
parent medium, was/romthe perpendi-
cular. Thus, let A, Jig. 21, be air, and
Fig. 21.

M ' P'

P JV1

G glass, and let P I be a ray incident on
the surface S S, which separates the air
from the glass. Let M M' be the per-
pendicular at the point of incidence I.
Since air is less dense or heavy than
glass, the ray P I, instead of persevering
in the direction P I, will take a direction
further from the perpendicular I M',
such as I P'. This fact was long known.
The law of this deflection, as discovered
by Snellius, has been already explained ;
but it may be explained under another
point of view, as follows : —

Round the point of incidence I, fig.
22, describe a circle in a perpendicular
Fig. 22.

plane. Let P I be the incident ray in
the denser medium, and P' I the re-
fracted ray in the rarer medium. Draw
P M, P' M7 perpendicular to the surface
S S'. It was found, that at whatever
angle P I might be incident, the propor-
tion of M I to M' I would be the same,
so long as the media on each side of
the surface remained unaltered. Thus,
suppose MI were two-thirds of M'l,
r 1 being incident at I, its refraction may
be thus found : take a distance I m!
from I, of which I m is two-thirds, or
such that I m' is equal to I m and the

OF NEWTON'S OPTICS.

21

half of I m, and from m' draw m! p'
perpendicular to lm' ; the line I p' will
be the direction which the ray p I will
take in passing through the denser me-
dium.

Now suppose that a ray PI,/#. 23,
Fig. 23.

were to meet I in such a direction that
M I is two-thirds of I S. If in this case,
in conformity with the rule just ex-
plained, we take a length from I equal
toIM, together with the half of I M,
that length will be I S', and S' would be
the point from which the perpendicular
(m'p1, see last fig.*) should be drawn to
meet the circumference. But this point
S' being itself on the circumference, the
perpendicular (m'p') altogether disap-
pears, its length being reduced to abso-
lutely nothing ; and the point (p') of
the circumference to which the ray is
deflected, would be the point S' itself.
Thus the Snellian law would shew that,
in this case, the ray incident at I would
not pass into the rarer medium. Ex-
perience, however, proved that, in this
case, the ray PI was reflected according
to the common law of reflection, in the
direction I P', making the angle P' I S
equal to I PS.

If the incident ray made any angle
p I m less than P I M, the law of Snel-
lius likewise became inapplicable. For,
in this case, m I being more than two-
thirds of I S, the distance from I taken
upon I S would be beyond the point S,
and therefore outside the circle, so that
the perpendicular could never meet the
circle ; and accordingly the refracted
ray could have no direction conformable
to this law. In all such cases experi-
ment shewed that the ray was reflected.
In this illustration we have supposed
the fixed proportion to be two-thirds,
but the conclusion may be drawn if any
other proportion be adopted.

It appears, therefore, that in passing
from a denser medium into a rarer there
is a certain degree of obliquity beyond
which the ray cannot be refracted ; and,
on the contrary, will be reflected back
into the denser medium, according to

the common law of reflection. It fur-
ther appears, by what has just been ex-
plained, that this degree of obliquity,
which limits the possibility of refraction,
depends on the degree of refraction
which the ray suffers in passing from
the one medium into the other, and that
the limiting obliquity is greater where
this refraction is greater.

Aware of this property of refracting
media, Newton perceived, that if the
doctrine of unequal refrangibility were
granted, it would follow that the limiting
obliquity, in passing from a denser me-
dium to a rarer, would vary with the
refrangibility of the light, being greater
for the violet and more refrangible lights
than for the red and less refrangible
lights. Thus it would follow that those
lights which had a greater aptitude for
refraction, were also more susceptible
of reflexion, and vice versa. He ac-
cordingly submitted the doctrine to this
test by the following ingenious experi-
ment.

(32.) He took two prisms, B A V,fig.
24, and C DB, of the same quality of

Fig. 24.

vibgyor

glass, having the angles A and D right,
and the angles at B and C, in each 45°,
and placing their broadest faces B C to-
gether, tied them in this position, so as
to form a square prism. This compound
prism was placed before an aperture F,
which admitted a beam F M of the sun's
light, so as to fall perpendicularly on the
face AB of the prism. This ray was
incident upon the thin plate of air B C
between the prisms, which were not
brought into absolute contact. Passing
through this and the second prism, it

22

A POPULAR ACCOUNT

emerged from the surface C D parallel
to ifslncidence ; for since the refracting
angles of the two prisms, A C B and
C B D, are turned in opposite directions,
and are equal, they neutralise each
other's effects, and the light emerges as
it entered. The ray M O, emerging from
the compound prism, was received upon
another prism I H K, and was dilated in
the same manner as if it had been re-
ceived directly from the aperture F, and
the spectrum was received in the usual
way upon the screen at P.

Another prism XV Y was placed
above the compound one, in such a posi-
tion as to receive a ray reflected from
the surface B C ; and above it a paper
screen, placed to receive the light re-
fracted by it.

Let us suppose that in the position
thus given to all the prisms, a vivid
spectrum appears on P, and no appear-
ance of light is exhibited on p. The
compound prism is now slowly turned
round its axis in the direction A C D B,
so as gradually to increase the obliquity
of the ray F M to the surface B C. At
a certain position, a strong violet light
will appear upon the screen p at v.
Maintaining the prisms, for the present,
in this position, let the circumstances of
the experiment be examined. The violet
part of the spectrum on P will be found
to have disappeared. If a screen be in-
terposed between the prism I H K, and
the compound prism, it will be found
that the light which falls upon it will be
of a colour which would result from the
mixture of all the colours of the spec-
trum, except the violet. If a screen be
interposed between the prism X V Y and
the compound prism, it will be found
that the light which falls upon it is the
violet. The inference is most obvious.
The incident ray F M has obtained the
limiting obliquity, corresponding to the
most refrangible or violet light. This
light is accordingly reflected by the sur-
face B C, and is transmitted through the
prism X V Y to the screen p, where it
appears. The violet light not passing
with the other parts of the sun-beam
through the prism BCD, has disap-
peared from the spectrum on P, which,
therefore, now terminates with the in-
digo light. The beam M O, before it is
dilated by the prism I H K, is in fact the
sun-beam FM deprived of the violet
light, which has been reflected at M,
and, therefore, it is composed of all the
colours of the spectrum except violet, as
appears by the spectrum, which is now

exhibited on P. Thus it follows that
violet light is totally reflexible at an ob-
liquity, which is insufficient to prevent
the refraction of lights of other colours.
The prism A C D B was now slowly
turned round a little more in the same
direction, and the effects observed. The
screen JP, in addition to the violet lights,
was now illuminated by the indigo i% of
which the spectrum on P was observed
to be deprived. The place of the indigo
light on p was also next to v, but so as
to be less refracted. On interposing the
screen between the prism I H K and the
compound prism, it was found to be
illuminated with a colour, which would
result from the mixture of all the colours
of the spectrum, except violet and in-
digo ; and on the other hand, on inter-
?osing it between the prism X V Y and
H K, it was illuminated with a colour
which would be produced^ by the mix-
ture of violet and indigo.

The inference from these effects is
consistent with the former one. The
prism A C D B had now attained that
position which gave the incident ray
F M the limiting obliquity of the indigo
light. It was accordingly reflected, to-
gether with the more reflexible violet
light. The lights of other colours were
transmitted, and produced the ett'ects
observed on P, and between I H K and
ACDB.

This process was continued, the prism
ACDB being slowly and gradually
turned on its axis in the same direction.
The lights, blue, green, yellow, orange,
and red, successively disappeared from
the spectrum on P, and at the same
times appeared in succession in their
proper places on p. In each case the
colour of the light in the beam M O was
such as would result from the mixture
of the colours on P, and the colour of
the light M N was such as would result
from the mixture of the colours on p.

All these effects are obvious and
beautiful consequences, and therefore
confirmations of Newton's doctrine. As
the obliquity of the incident ray F M to
the surface B C is gradually increased,
it becomes successively equal to the
limiting obliquities of the several co-
loured lights, and in the same succes-
sion reflects them to the screen p ; the
other screen P being in the same order,
and at the same instants of time, de-
prived of them. Thus it appears, not
only that lights of different colours
are differently refrangible, but also that
they are differently reflexible ; and that

OF NEWTON'S OPTICS.

23

which are more refrangible are
so more reflexible, and vice versa.

CHAPTER III.

the Methods of obtaining Homoge-
neous Light.

(33.) BY the results of the experimental
investigations described in the last chap-
ter, Newton was convinced that solar
light was not simple and homogeneous,
but was a mixture or composition of
many lights, differing from each other
in certain respects, but more particularly
in refrantribility. This quality he adopt-
ed as a test for pure unmixed or homo-
geneous light. A beam, every ray of
which was equally refrangible, and
which, therefore, did not admit of being
dilated by a prism, he considered to be
pure homogeneous light. Although the
parts into which the solar beam was de-
composed were shewn, by the experi-
ment described in p. 17, to be inca-
pable of further dilatation; yet this
method did not give all the purity to the
light which philosophical exactitude de-
manded, for the reasons which we shall
now explain.

It will be recollected that the aperture
in the window-shutter (p. 14) casts a
circular illuminated spot on the opposite
wall, the diameter of which is equal to
the diameter of the hole, together with a
line, which being drawn upon the wall
would subtend at the hole an angle
equal to the apparent diameter of the
sun. But this circular spot is not uni-
formly bright. It is more faintly illu-
minated at its edges than at its centre,
the cause of which will be easily under-
stood. Let O O',fg. 25, be the hole in

Fig. 25.

the window shutter ; S S' the sun's dia-
meter. A ray of light from S', passing
the upper boundary O of the hole, falls
upon a screen at P ; and a ray from S,
passing the lower edge O' of the hole,
falls upon the screen at P'. Now, it is
apparent that no part of the sun's disc,
except the lowest point S', can shine

upon P, the upper part of the window
shutter intercepting the light from all the
other points ; and that no part of his
disc, except the point S, can shine upon
P', the lower part of the window shutter
intercepting the light. Hence it appears
that the points P P', and the entire of
the edge of Ihe illuminated circle on the
screen, will be more faintly illuminated
than any of the parts nearer to its centre.

The ray from S' passing above the
lower edge of the hole at O' will illumi-
nate L, and thus the point L will be
exposed to light from the entire disc
of the sun. The same may be said
of L/, and of all intermediate points.
The several points from L to P'
will be exposed to light from only a
part of the sun's disc, that part being
smaller the more distant the point is
from L, so that the light becomes gra-
dually more faint from L to P'. The
same may be said of the light from L'
to P. From this it appears that the
circular illuminated spot on the screen
is composed of a small circle whose
diameter is L L', uniformly illuminated,
surrounded to the distance L P' by a
ring of light of gradually decreasing
brightness, and fading away until it be-
comes insensible. This ring is called
the penumbra *.

Now since the effect of the prism, as
has been already proved, is to stretch
out this luminous circle into an oblong
form, the breadth being the same, and
the sides and ends being illuminated
by the rays which form the penumbral
ring surrounding the unrefracted circle,
it follows that the sides and ends of the
spectrum will be bounded by a penum-
bral skirt of the breadth of P L' ; and
such, in fact, was the result of the ex-
periments.

In the experiments which Newton
now desired to institute, it was necessary
that the light should be obtained of as
uniform an intensity as possible, and
therefore it was necessary to remove or
very much diminish the penumbral fringe
which we have just described. But
it was still more necessary that light
should be obtained which was perfectly
pure and homogeneous, and it so hap-
pened that the same cause which pro-
duced the fringe and varied the intensity
of the light, also impaired its purity.

* The hole may be so small, that its apparent di-
ameter at the screen will be less than that of the sun.
In this case the part of the figure L I/ will disappear,
and the whole spot on the screen will have the cha-
racter of penumbra, the centre being the most lumi-
nous point.

24

A POPULAR ACCOUNT

This \vill be understood by attending to
the effects of the prism on the circular
spot.

Let S, fig> 26, be the circular illumi-
nated spot cast upon the screen
by the light proceeding directly Fig. 26.
from the aperture without be-
ing intercepted by the prism.
Let S A be the distance on the
screen to which the least re-
frangible rays are deflected.
These rays, which before fell
upon the circle S, will now
fall upon the circle A. Let
S Z be the distance on the
screen to which the most re-
frangible of the rays incident
on S are deflected by the
prism. These rays, which be-
fore fell upon the circle S,
now fall upon the circle Z.
The rays of all the interme-
diate degrees of refrangibility
are deflected by the prism,
so as to fall upon circles whose /" \
centres occupy the entire space ( s J
from A to Z. From A take \^^s
a distance A C, equal to
twice the radius of the circle A, or,
what is the same, to the breadth of the
spectrum. A circle described round the
centre C, with a diameter equal to A C,
will evidently touch the circle A, but
neither circle will be within or upon the
other. It is evident, however, that every
circle of the same diameter, whose cen-
tre lies between A and C, must lie partly
upon the circle A, and partly upon the
circle C. Hence it is evident, that the
rays which, deflected from S, illuminate
the circle A, and those which illuminate
the circle C, are not intermixed. But
between the points C and A is a consi-
derable space, and between the degrees

of refrangibility which would cause rays
to be deflected to these points, are many
intermediate degrees, in virtue of which
rays would be deflected upon innume-
rable circles, whose centres lie between
C and A. It follows, therefore, that all
these lights of intermediate refrangibi-
lities are intermixed with lights upon
the circles A and C, neither of which,
therefore, shine with pure homogeneous
light. Since the lights of different re-
frangibilities which are thus intermingled
are diffused over circles whose centres
occupy the space A C, the number of
such circles must be proportional to
the space A C, or to the breadth of the
illuminated circle S, increasing and di-
minishing as that breadth is increased
or diminished. In this proportion, then,
lights of different refrangibilities are
mixed in the spectrum ; and, therefore,
every means which can diminish the
breadth of the spectrum will propor-
tionally increase the purity of the lights.
The first method which occurred to
Newton to accomplish this, and, at the
same time, to remove the penumbral
fringe already mentioned, was to per-
forate the screen in the space L L' (fig.
25), and to receive the light trans-
mitted through the perforation on a
second screen behind the first. By this
means the penumbral ring would be
received upon the first screen at L' P,
L P', and the uniform light of L L' would
be admitted to the second screen through
the perforation. In this case, the breadth
of the illuminated circle on the second
screen, and therefore that of the spec-
trum, would be nearly equal to that of
the perforation. In proportion as the
breadth of the spectrum would be thus
reduced, the intermixture of heteroge-
neous lights would be diminished, and, as
27.

we have just explained, the penumbral
light would be altogether intercepted.

He, however, accomplished what he
aimed at more effectually by the follow-

ing method. At a distance of ten or
twelve feet from the hole O, Jig. 27, he
placed a lens L, which formed an image
of the hole at O'. If the lens were so

OF NEWTON'S OPTICS.

placed that O L and 0' L should be
equal, which was always possible, the
magnitude of the image O' would be
exactly equal to that of the hole. This
image was therefore considerably less
than the illuminated circle cast on
the wall or screen without the inter-
position of the lens. A prism A B C,
placed behind the lens, deflected the
rays emerging from it, and formed
the oblong spectrum RV, the breadth
of which was equal to O', and which
was free from any penumbra. The
length of the spectrum in these expe-
riments is never changed. In the pre-
sent experiment he succeeded in reduc-
ing the breadth to about one- sixtieth of
the length ; and therefore diminished
the mixture of heterogeneous lights in
any part of it proportionally. Although
it was impossible by this method, or
perhaps by any other, to obtain a beam
of absolutely homogeneous light, yet
what was thus obtained was sufficiently
simple and homogeneous for all the
purposes of experiment.

Some other ingenious expedients were
resorted to for the simplification of light.
The diminished breadth of the spectrum,
while it gave a purified homogeneous
light, gave a very small quantity of it.
To obtain it in greater abundance, it
occurred to Newton to form a great
number of small holes in the shutter, in
the same horizontal row, so as to obtain
several of these spectra placed parallel
to each other, and thus form one broad
one, in which the lights should be as
homogeneous as in a single one of small
breadth. Or what was equivalent, and
still more effectual, instead of a row of
holes, he formed one narrow slit in the
shutter extending in an horizontal direc-
tion, so as to admit a thin sheet of light.
By this means a spectrum of any required
breadth may be formed, in which the
light is as pure and homogeneous as in
a spectrum formed by light admitted at
a round hole, whose diameter is equal to
the breadth of the slit.

Newton suggests another very ingeni-
ous means of obtaining a spectrum, in
which lights would be supplied of dif-
ferent degrees of purity and intensity.
Let a narrow triangular hole be cut in
the shutter in an horizontal direction, as
fig. 28, O o. The sunbeam admitted
through this hole will have an edge like
a knife. The broadest part O of the ray
will form a spectrum RV, in which the
intermixture will be in proportion to the
base O of the triangle ; but as the ray

25

diminishes in breadth towards o, the cor-

respondingparts of the spectrum, towards

Fig. 28.

R V, are increased in the purity of the
light, heterogeneous rays being less in-
termixed as the breadth of the triangle
is diminished, as is evident from the
figure. Having a spectrum of this kind,
experiments may be tried either in its
stronger, though less simple light, on the
side RV, or in the weaker and more
simple light on the side r v, as may seem
more suitable to the objects of the in-
vestigation.

In all experiments on homogeneous
light, Newton states that he found it
necessary to use great precaution in order
to be secure of success. The chamber
should be carefully darkened to avoid the
disturbance arising from rays of white
light scattered casually about. The glass,
both of the prisms and lens, should be
free from veins, striae, air bubbles, and
other inequalities. He found it so dif-
ficult to procure good prisms, that he
frequently used transparent liquids in-
closed in hollow prisms, formed of pieces
of plate glass fixed together at proper
angles.

With light simplified by the methods
we have now explained, he tried the
experiment explained in (26), and
found that a pure homogeneous ray ad-
mitted of no dilatation by the prism, and
therefore concluded that the light of
which it was composed was all equally
refrangible. With a view of establish-
ing the same principle, he took two small
circular pieces of while paper, and illumi-
nated one with light direct from the sun,
and the other with homogeneous light
obtained by one of the methods which

26

A POPULAR ACCOUNT

we have just explained. Viewing these
circles thus illuminated through a prism,
he found that the white circle was dilated
into an oblong spectrum ; but that the
other still appeared as perfectly circular
as when viewed without the intervention
of the prism. He now placed minute
objects in the homogeneous light, and
viewing them through the prism, found
them distinct ; but when the same objects
in the sun's direct unrefracted beam
were similarly viewed, they became con-
fused and indistinct.

law. The experiment by which this was
determined was as follows.

CHAPTER IV.

Law

iw of refraction of homogeneous light
— imperfection of refracting telescopes
— Newton's reflecting telescope.

(34.) NEWTON having now succeeded in
establishing the unequal refrangibility of
the rays which compose solar light, his
next step was to determine the law by
which each species of light was refracted.
It was about the year 1665, being then
in his 25th year, that he appears to have
commenced his investigations respecting
the composition of light ; and on the 8th
of February, 1672, he communicated his
discovery to the Royal Society, of which
he had just been elected a Fellow. About
fifty years prior to this period, Snellius
discovered the law of refraction, which
being afterwards adopted by Descartes,
was well known, and generally received
at the period to which we now allude.
The philosophers who observed this law
were not aware that all the rays of light
were not equally refrangible ; and New-
ton concluded that they adapted their
measures to the mean rays, or those
which lie in the middle of the spectrum,
and which therefore have an intermediate
refrangibility between those of the red
and violet, the least and most refrangible
rays. Hence he concludes from the
experiments of his predecessors, that
the green light of the prismatic spec-
trum is refracted according to the Snel-
lian law.

Newton showed, by the experiment
which we shall now describe, that when
the sun's rays were all incident on the
prism at the same angle, the sines of the
refractions of the several component rays
always have to each other the same pro-
portion. It follows, therefore, that they
must always have the same proportions
to the sine of the angle of incidence, for
since the green ray obeys this law, and
they all have fixed proportions with re-
spect to it, they must all obey a similar

Let RV (Jig. 29) be the spectrum
produced by a prism in the usual way.
Let another prism be placed at right
angles to this, as described in (24), and
let R' V be the new position which the
spectrum assumes. The distances RR',
V V measure upon the screen the de-
flections of the red and violet rays, and
lines parallel to these, as GG', measure
the deflection of the intermediate rays.
From the proportion of these lines, that
of the sines of the refractions of the
several kinds of rays was determined by
mathematical reasoning. The prism
placed at right angles to the first was
now removed, and another with a dif-
ferent refracting angle was substituted
for it, which removed the spectrum to
the position Rr/V". The proportion of
the sines of the refractions was now
deduced from measurement and com-
putation as before, and was found to be
unaltered. A third prism, with a dif-
ferent refracting angle, was used with
the same result. Thus it appeared that
the sine of refraction of each light bears
a fixed proportion to that of the green
light, while the sine of refraction of the
green light bears a fixed proportion to the
sine of incidence, by the law of Snellius ;
from whence it follows that in each kind
of light the sine of the angle of refraction
bears a fixed proportion to that of the
angle of incidence, but that this propor-
tion is different in light of different kinds.

Newton gives an investigation, by
which he shows that this law of refrac-
tion may be deduced independently of
experiment, by mathematical reasoning,
from the supposition that bodies refract
light by acting upon its rays in directions
at right angles to their surfaces. We
shall not give here the details of this in-
vestigation, which also furnishes an ex-
planation of the fact we have already
stated, namely, the total reflection of the
light at the limiting obliquities. It is,
however, in the Principia that this latter
consequence is deduced from it.

(35.) Having determined the proper-

OF NEWTON'S OPTICS.

27

tion of the sine of incidence and refrac-
tion for the red, green, and violet, sup-
posed to pass from air into glass, to be
as follows : —

Red Sin. inc. Sin.ref. : : 77 50
Green Sin . inc. Sin . ref. : : 77| 50
Violet Sin . inc. Sin . ref. : : 78 50
he proceeded to investigate the effect
which this difference in the proportion
produces on the images of objects formed
by the object-glasses of refracting tele-
scopes. When the curvature of the
object-glass is not great, compared with
its diameter, the angles of incidence of
rays proceeding from a point at any

Fig.

OAUV^J VI T^l Y OHIO,!!, Ctll^lVO GUW

ne proportion as the angles themselves:
1, therefore, in the case to which we

considerable distance are very small, the
rays being evidently very nearly perpen-
dicular to the surface of the glass. By
a well known principle of mathematics,
the sines of very small angles are in the
samei
and,

now allude, the angles of incidence will
be to those of refraction as the above
numbers, and the deviations of the rays
above mentioned will be as the differences
of those numbers. That is to say, the
deviations of the red, green, and violet,
will be as 27, 2 7i, and 28.

Let L be a lens presented to a dis-

30.

tant object from which the rays may
be considered parallel. Let V be the
focus to which the violet, or most refran-
gible rays are collected ; and R the point
to which the red, or least refrangible rays
are collected ; and let G be the focus of
the medium or green rays. The places
of the points, V, G, R, may be determined
by geometrical reasoning, if the propor-
tions of the sines of incidence and refrac-
tion as above given, and the curvature of
the lens, be known. The result is, that if
twice the distance, LG, be divided into
55 equal parts, the space VRis equal to
one of these parts.

It may also be proved that if O be a
ucid point, and V, G, R, the foci of the
violet, green and red light from it, and
as before twice the distance L G be di-
vided into 55 equal parts, V R will have
the same proportion to one of those
parts as O G has to O L.

It will be observed that the violet rays
diverging from V meet the red rays con-
verging towards R at a certain point be-
tween V and R, and that at this distance
all the rays which are refracted by the
lens are collected into the smallest pos-
sible circle. The diameter of this circle
being computed when the incident rays
are parallel, was found to be about the
55th part of the diameter of the lens.

(36.) To verify these inferences, Newton
repeated again the experiment described
in (19), but adopted the method men-
tioned in p. 24, of rendering the pris-
matic light homogeneous. In the course
of these experiments he encountered
many practical difficullies arising from
imperfections, such as veins, air bubbles,
&c. in the glass of which his prisms

were formed, of which, as well as of his
efforts to avoid or remove them, he gives
a very detailed and interesting account.
He also found considerable difficulty in
determining the exact foci of the lights
of blueish character at the upper end of
the spectrum, owing to their extreme
faintness. On the whole, however, he
succeeded in satisfying himself that the
foci of the rays of different colours were
at those points to which the computation
made on their supposed unequal re-
frangibility assigned them. Thus it ap-
peared that the object glass of a refract-
ing telescope formed as many distinct
images of an object placed before it, as
there are lights of different degrees of
refrangibility : that these images dif-
fered in colour, the blueish ones being
nearest to the object glass, and the red-
dish most remote from it, and that be-
tween these were included images of a
greenish and yellowish hue ; that these
images extended over a space along the
axis of the telescope, equal to about
2-55ths of the focal length of that glass ;
and that the smallest space into which
the innumerable images of the same
point in the object can be collected on a
plane at right angles to the axis of the
telescope, is a circle, whose diameter
amounts to about a 55th part of the
diameter of the object glass. " So that
it is a wonder," says Newton, " that
lelescopes represent objects so distinct
as they do. But were all the rays of
light equally refrangible, the error arising
only from the sphericalness of the figures
of glasses would be many hundred times
less."
The effect called spherical aberration,

28 A POPULAR ACCOUNT

and its cause, have been already indi-
cated in (11.) Let LL',/£'. 31, be the
section of a plano-convex object-glass
made by a plane passing through its
axis, and let parallel rays of pure ho-
mogeneous light be supposed to fall on

Fig. 31.

L
A'

Square of the radius of the
spherical surface of the
lens, (radius being 1200
inches) 1440000

Square of the sine (2) of re-
fraction 4

Their product

5760000

the plane side, perpendicular to the sur-
face. If the surface of the lens be con-
ceived to be divided into a number of
concentrical rings, as described in (11),
the foci of each ring will be more distant
from the lens, the nearer the ring is to
the edge of the lens. Let/ be the focus
of the marginal ring, and F that of the
central rays. The foci of all the inter-
mediate rings will lie between F and /.
The rays diverging from all the foci
between / and F are collected in a
circle having the line A A' for its dia-
meter, and this is evidently the smallest
space within which all these rays are
collected. The diameter of this circle,
therefore, measures the lateral aberra-
tion which parallel rays would sustain
from the sphericity of the lens ; and
Newton calculated this, in order to
compare the imperfection of telescopes,
arising from this cause, with that im-
perfection which arises from the un-
equal refrangibility of light.

By geometrical reasoning, the details
of which we cannot properly introduce
here, it is proved that the square of the
radius of the spherical surface of the
lens, multiplied by the square of the
sine of refraction, has to the square of
half the breadth of the lens L L/ mul-
tiplied by the square of the sine of the
angle of incidence the same proportion
as half that breadth bears to the aberra-
tion A A'.

Newton then proceeds to show that if
the object-glass were a plano-convex
lens, having its plane side turned to-
wards the object, having the radius of
its convex surface 100 feetor 1200 inches,
and the diameter of the lens four inches,
the diameter of the smallest circle into
^vhich equally refrangible rays would be
collected, would be about goooo^h of an
inch. The calculation is as follows, the
proportion of the sine of incidence to
that of refraction being supposed to be
3 to 2.

Square of half the breadth of the
lens, (the breadth being 4) . . 4

Square of the sine (3) of the angle
of incidence 9

Their product .... 36

The proportion of these products is
that of 160, 000 to 1 ; and such is the
proportion of half the breadth of the
lens (i. e. two inches) to the aberration,
which is, therefore, the 160,000th part
of two inches, or the 80,000th part of
an inch.

The diameter of the lateral aberration
arising from unequal refrangibility of
light, would, in the case of the lens just
described, be the fifty-fifth part of four
inches, or four fifty-fiths of an inch.
The lateral aberration produced by the
spherical form of the lens has, there-
fore, to that produced by the unequal
refrangibilily of light, so small a pro-
portion as 1 to 5800.*

It follows, therefore, that the im-
perfection of telescopes, which arises
from the spherical form of lenses, bears
an exceedingly small proportion to that
which is caused by the unequal refran-
gibility of light. But even the small er-
ror arising from the spherical form may
be almost removed, as Newton suggests,
by a compound object-glass, formed by
two glass lenses with water between them.
So that thus all the labours of Descartes,
and others who devoted themselves to
the formation of spheroidal lenses were
fruitless, since even had they succeeded
in producing lenses absolutely free from
spherical aberration, the effect would not
have been perceptible.

(37.) Reasoning thus, Newton did not
hesitate to pronounce the improvement
of refracting telescopes desperate, a con-
clusion which forms a striking exception
to the almost superhuman sagacity
which characterised all the philosophical
researches of this extraordinary man.
What renders this error the more won-
derful, is that the property of light,

* This proportion is calculated with reference to
the green or mean rays. If, however, it be taken
with reference to those rays which produce the
strongest effect in vi*ion, it will only be as 1 to 1200,

OF NEWTON'S OPTICS.

29

on which the perfection of refracting
telescopes has since been found to de-
pend, seems to have pressed itself for-
ward, and even to have courted his at-
tention in the very experiments from
which he deduced his erroneous conclu-
sion. (Fig. 32.) Let ABC, A'B'C'
Fig. 32.

be two prisms, formed of different trans-
parent substances. Let S B, S' B' be
rays of the sun falling on them in pa-
rallel directions. Let B V, B'V be the
most refracted or violet light in each,
and B R, B'R' the least refracted or red
light. The deviation of the rays from
their original direction, produced by the
refraction of the prisms, will be different
for each component part of the incident
light. Newton supposed that the de-
viations of the different coloured lights
from the common direction when inci-
dent, have to each other a certain fixed
proportion ; so that with the same aver-
age refraction or deviation from their
common original direction, they would
be dilated or separated from each other
in the same degree. This may perhaps
be more easily comprehended if thus
explained. Let ABC, A' B' C' be two
prisms of different materials, receiving
parallel rays S B, S' B', of solar light.
Let the lines B M, B' M' divide the an-
gles V B R, V B' R', formed by the ex-
treme red and violet rays into equal parts,
or so that V B M shall be equal to
R B M, and also V B' M' to R' B' M'.
Also, suppose the prisms to have such
refracting angles, that the rays B M,
B' M' shall be parallel. The deviations
of these rays, s B M, s' B' M', from their
original directions B s, B'*' must be
equal. Under these circumstances New-
ton concluded that the angles V B R
and V'B'R' would be equal, and that
the deviation of every ray in the spec-

trum VR, from its original direction B s,
would be equal to the deviation of the
similar ray in the spectrum V R' from
its original direction B'*'. Such is not
the fact, and it is almost inconceivable
how Newton, who had avowedly ex-
amined the spectra produced, not only
by prisms of different kinds of glass, but
also by liquids contained in hollow glass
prisms, could have escaped noticing a
fact that would at once have led him to
the discovery of achromatic telescopes.

In fact the prisms being circumstanced
as we have just described, so as to pro-
duce equal deflections of the sunbeam
from its original direction, the dilatation
or dispersion of the rays from each other,
and which may be measured by the di-
vergence V B R, V B' R' of the extreme
rays, will be different according to the
material of which the prism is composed.
Newton, on the other hand, concluded
that when the deflection of the sun's
beam by different prisms was the same,
the dispersion would also be the same.
Had he thought of measuring the lengths
of spectra produced by different prisms,
equally deflecting the light, he could not
have tailed to have found them different,
and would have naturally been led to the
discovery of achromatic telescopes, as
we shall now explain.

Since prisms of different materials,
with an equal deflection of the beam,
produce spectra of different lengths, and
since also the length of a spectrum va-
ries with the position of the prism, or,
what is the same, with the deflection of
the light, it follows that if two prisms of
different materials be exposed to beams
of the sun's light, one of them may be
turned until such a position be given to
it, that the length of the spectrum pro-
duced by it shall be equal to the length
of the spectrum produced by the other
prism.

In this case the deflection of the beam
by the two prisms producing equal
spectra must be different, for if not, as
we have before stated, the spectra would
have different lengths. If one of the
prisms be inverted with respect to the
other, all other things remaining the
same, the spectra will still keep the same
length, but the colours will be reversed.
Now suppose that instead of transmit-
ting different beams of light through the
two prisms, the same beam be succes-
sively transmitted through them, the one
being placed behind the other, but the
former arrangement being in all other
respects preserved, it is quite evident

30

A POPULAR ACCOUNT

that the dispersion of the one prism
will have a tendency to neutralise the
dispersion of the other, and that in the
beam emerging from the second prism, the
prismatic lights will be so mingled as to
render the emergent beam nearly colour-
less. This will appear from considering
that the tendency of the one prism to
disperse the rays in one way, bringing the
violet ray highest and the red lowest, is
exactly equal to the tendency of the
other to disperse the light in the oppo-
site way, bringing the violet lowest and
the red highest. But this mutual com-
pensation will not obtain in the deflection
of the light, since the power of the se-
cond prism to deflect downwards is not,
in its actual position, equal to the power
of the first to deflect upwards ; so that
the prism which has the less deflecting
power will destroy so much of the de-
flecting effect of the other as is equal to
its own, but an effective deflection will
remain, by which the beam will be
turned from its original direction.

Thus we arrive at the important fact,
that a beam of light may have its direc-
tion changed by refraction, so that the
directions of all its component rays shall
be equally changed, or nearly so, al-
though they be differently refrangible.
What may be done by prisms may
also be effected by lenses ; and therefore
an object-glass of a telescope may be
so constructed as to collect all the
rays of different refrangibilities nearly
to the same focus,* and thus an achro-
matic telescope may be formed. Such
was the discovery that Newton left to
adorn a future age, a discovery pre-
sented to him by his own experiments, a
fact rendered not improbable by his
own reasoning, consistent with his own
theory, and soliciting investigation and
inquiry at almost every step of his own
researches, yet which investigation and
inquiry he seems, by an unaccountable
pertinacity, to have stepped out of his
way to avoid.

Newton seems not to have maintained
an uniform opinion at all times on this
point. The first edition of his Optics was
published in 1704, and the second in
1717. In both of these he pronounces
the improvement of refracting telescopes

* In strictness, two prisms or lenses will only bring
two colours accurately together, the law of disper-
sion being different throughout the whole spectrum:
the rest, however, will be very nearly coincident, and
consequently colour very nearly got rid of. By the
combination of three prisms or lenses, three colours
may be accurately combined, and the rest still more
nearly than before ; and so in succession.

to be desperate. And yet, in a letter to
Mr. Oldenburg, dated July, 1672, three
years before his " discourse about light
was written at the desire of some gen-
tlemen of the Royal Society," he vindi-
cates himself from a charge of Dr.
Hooke, " who reprehended him for
laying aside the thoughts of improving
optics by refractions," in the following
words — " What I said was in respect of
telescopes of the ordinary construction,
signifying that their improvement is not
to be expected from the well figuring of
glasses, as opticians have imagined. But
I despaired not of their improvement by
other constructions, which made me cau-
tious to insert nothing that might inti-
mate the contrary. For although suc-
cessive refractions which are made all
in the same way do necessarily more and
more augment the errors of the first re-
fraction; -yet it seemed not impossible
for contrary refractions so to correct
each other's inequalities, as to make their
difference regular, and if that could be
conveniently effected, there would be no
further difficulty. Now to this end I
examined what may be done not only by
glasses alone, but by a complication of
divers successive mediums ; as by two
or more glasses or crystals, with water
or some other fluid between them ; all
which may together perform the office of
one glass, especially of the object-glass,
on whose construction the perfection of
the instrument chiefly depends. But
what the results in theory or by trials
have been, I may possibly find a more
proper occasion to declare."

Jn this passage he hints at the prin-
ciple on which achromatic telescopes
depend, and even the manner of ap-
plying that principle in their construc-
tion, and yet fifty years of his life
after this employed in perfecting his
theory, seem only to have confirmed his
error.

(38.) Abandoning all further inquiry
into the methods of improving refracting
telescopes, Newton, at the part of his
Optics to which we have now arrived,
proceeds to explain his contrivances for
the construction of a reflecting telescope.
In the end of a tube he placed a con-
cave spherical reflector, which he con-
structed of metal, and polished with his
own hands. The image from this was
deflected by another plane reflector
placed in the axis of the tube, so as to be
received by an eye-glass in the side of
the tube at which it was viewed by the
observer. He suggests the possibility of

OF NEWTON'S OPTICS.

31

constructing a concave reflector of glass,
as being in some respects preferable to
metal, but does not seem to have car-
ried this into effect. Newton was fully
aware of the defects of reflecting tele-
scopes compared with refractors, owing
to the much greater loss of light in
reflection, and the greater aberration
proceeding from their spherical form.
These, however, he thought inconsider-
able when compared with those defects
of the refracting telescope, which pro-
ceeded from the unequal refrangibility
of light.

CHAPTER V.
The Theory of Colours.

(39.) THE colours exhibited by refracted
and reflected light were phenomena with
which philosophers had been familiar
before the time of Newton. These ef-
fects were generally ascribed to the ac-
tion of the reflecting or refracting body,
and to the edges of opaque bodies which
marked the limits of shadow, in impart-
ing to the light qualities which it did not
possess before encountering these bodies.
Thus it was thought, that in passing
through glass or other transparent sub-
stances formed into a prism, the solar
beam is endued with a virtue by the ac-
tion of the medium upon it, by which it
reddens, or otherwise colours any body
which it afterwards illuminates. In like
manner, it was supposed, that in passing
the edge of an opaque body a similar
effect might be produced.

Before he proceeds to explain and
establish his theory of colours, Newton
shows that this hypothesis of his prede-
cessors is untenable and inconsistent
with facts. The coloured spectrum be-
ing produced by the prism in the usual
way, the lights of the several colours
may be successively intercepted by the
interposition of an opaque body, so that
any one of the colours may bound its
shadow. These colours will remain un-
altered by thus passing the edge of the
opaque obstacle ; and therefore he con-
cludes that the light in passing the body
receives no modification which affects
its colour.

He further shows, that the same light,
refracted in the same manner, passing
the same opaque edges, will throw upon
the paper which it illuminates different
colours, according to the direction in
which the paper is placed with respect
to the rays. He argues, that if the co-

lorific property were a virtue imparted
to the ray by the edges of the aperture
through which the light is admitted, or
by the refracting medium through which
it has passed, this could not happen,
inasmuch as the colouring quality would
then be independent of the position of
the paper.

But perhaps the most conclusive ar-
gument against this theory is derived
from the experiment explained in (32).
It appears in that experiment that the
confines of shadow produce no effect
whatever ; for the colour of the whole
of the light emerging from the com-
pound prism is always the same, that
in the middle of the beam being in
nowise different from that at the bor-
ders. Neither can the colour proceed
in this case merely from the action
of the glass, because it changes from
white to yellow, orange, red, &c., that
action remaining the same. Besides
this, the refractions being equal, and
in contrary directions, would mutually
destroy each other's effects. It may
further be argued, that if the light owed
its colour to the action of the glass, it
would not have the colour before its
passage through the prism K I H ; yet
it was found, in that experiment, that
when all the colours in the spectrum P
were made to vanish, except the red,
the light producing that colour on the
screen P was found to produce the same
colour on a screen which received it be-
tween the compound prism and K I H,
before it was refracted by the latter.
Thus the light which reddens the screen
P would also redden it if unrefracted by
the prism K I H, and the same may be
said of the lights of other colours.

From these and, indeed, all other ex-
periments which have been described, it
abundantly appears, that " all homo-
geneous light has its proper colour an-
swering to its degree of refrangibility, and
that this colour is unalterable, either by
refraction or by reflection." When pure
homogeneous light of any colour illu-
minates a body, whatever the natural
colour of that body may be, it will ap-
pear, when so illuminated, to have the
colour of that light only which shines
upon it. The apparent colour of the
body is that of the light which it re-
flects ; and it can reflect no light but
that which shines upon it. Thus if a
body whose natural colour is blue be
placed in a dark chamber, and illumi-
nated by the red light of the prismatic
spectrum, it will appear red ; and, on

32

A POPULAR ACCOUNT

the other hand, a body whose natural
colour is red, illuminated in the same
way with blue light, will appear blue.

(40.) In the theory derived by Newton
from the experiments which have been
explained, the white light of the sun is
supposed to be compounded of several
component lights which have qualities
different each from the others. They
are all refrangible according to the same
law discovered by Snellius (10); but,
as we have already shown, they possess
this quality in different degrees. This
property is accompanied by another in-

timately connected with it. Any two of
the component parts of solar light which
differ in refrangibility, differ also in
colour ; and therefore the light of the
sun is composed of various species of
light of different colours, the mixture of
which produces whiteness.

Newton next proceeds to determine
the degrees of refrangibility correspond-
ing to the rays of different colours. To
determine this by experiment, he de-
lineated, on a paper, the outline of the
spectrum, fig. 33, F A P G M T, and
refracting the sun's light by a prism, as

Fig. 33.

.''F

•"<$ *

-J-

h

k

1 7/1

/•''A

T"!

1

;

£.

=_

.-''!

jNw,

described in p. 15, he held the paper
so that the spectrum might exactly fall
upon the space marked out upon it.
He employed an assistant, whose per-
ception of colours he considered to be
better than his own, who drew lines
across the paper, marking the confines
of the several colours. Thus ab divided
the red from the orange; c d, the orange
from the yellow; ef, the yellow from the
green ; g h, the green from the blue ; i k,
the blue from the indigo ; Im, the indigo
from the violet. This experiment was
frequently repeated on the same, as
well as on different papers, and the
results were found to be generally ac-
cordant. Let G M be drawn to X, so
that MX shall be equal toGM, the
spaces measured from G to the several
boundaries of the colours were found to
have the following proportion :
XG, XI, X?', Xg, Xe, Xc, Xa, XM,

1, I, f. i> §> f> TV *•*
The spaces measured along the spec-
trum occupied by the lights of the several
colours may be considered to measure
the differences of the sines of refraction
of those rays having one common sine
of incidence. But the proportion of
the sine of incidence to that of refraction
from glass into air has been already as-
certained to be 50 to 77 for the least,
and 50 to 78 for the most refrangible
rays ; it follows, therefore, that if 50 be
the common sine of incidence, the sines
of refraction for the rays at the bounda-
ries of the several colours, beginning

* The analogy observed by Newton between the
proportion of these intervals and the musical inter-
vals must be regarded as merely fanciful,

from the red, will be 77$, 77 J, 77^, 77^,
77f, 77JJ, 78, which may be familiarly
explained thus. Let A B, fig. 34, be

Fig. 34.

s

the surface of glass from which the ray
S I passes at the point I into air. Let
the ray S I be solar light. Round the
point I as centre describe a circle, and
through I draw a diameter E F perpen-
dicular to the refracting surface A B.
From the point C, where the ray meets
this circle, draw C D : this is the sine
of incidence. Let it be divided into 50
equal parts. Upon I B from 1 take a
length lr' equal to 77 such parts, and
draw r'r perpendicular to I B ; again,
take I o', equal to 77$ of those parts, and
draw o'o perpendicular to I B. In the
same manner, take I yf, I g', I b', I i',
Iv', equal to 77$, 77£, 77$, 77|, 77|, 78
parts respectively, and draw, as before,

ry, g'g, b'b, i'i, o'o. From I draw I rt
o, ly, Ig, Ib, It, Iv. These lines
will determine the directions of the red,
orange, and the other rays corresponding

OF NEWTON'S OPTICS.

33

to the different degrees of refrangibility ;

the ray I B being after refraction re-
solved into I r, I o, &c.

If a ray of light be successively trans-
mitted through several transparent media
having different refracting powers, it
may so happen that, on its emergence
from the last of these media, it shall take
a direction parallel to that which it had
when incident upon the first of them. In
this case the several refractions which
the ray suffers in passing through the
media, compensate and neutralise each
other, so as to produce, on the whole, no
deflection of the ray from its original
course. Newton observed that, under
these circumstances, whenever the inci-
dent ray was white, the refracted ray
was also white. But he found, on the
other hand, that if the refractive powers
of the media were not thus related,
and that a deflection of the incident ray
from its original direction finally took
place, a separation of the white ray into
its component colours was produced.
From these results he inferred that the
same succession of media, which mutu-
ally neutralised the refractions of any
one species of homogeneous light, also
neutralised them on all the others, so
that if one component part of the solar
beam emerged parallel to its incident
direction, all the others would emerge
with it in the same directions, thus form-
ing an emergent white beam. But, on
the other hand, that if on the whole any
deflection of the incident beam were
finally produced, such deflection would
be different for the different component
lights ; and, therefore, a decomposition
or dispersion would ensue.

(4 1 .) From these facts experimentally
exhibited, Newton inferred, by mathema-
tical reasoning, the following theorems :

I. The differences between the sines
of incidence and refraction, when the
ray passes from several different media
into the same medium, are to one another
in a given proportion.

II. The proportion of the sines of in-
cidence and refraction for any one species
of homogeneous light from one medium
into another, is composed of the propor-
tions of these sines from the first medium
into any third medium, and from that
third medium into the second medium.

By the first of these theorems, the re-
fractions of all sorts of rays from any
medium into air may be found, if the
refraction of any one sort be known. By
the latter, the refraction out of one me-
dium into another may be found, if the

refractions of both of them into a third
medium be known.

" These theorems," says Newton,
" being admitted into optics, there would
be scope enough of handling that science
voluminously after a new manner ; not
only by teaching those things which tend
to the perfection of vision, but also by
determining mathematically all kinds of
phenomena of colours which could be
produced by refractions. For to do this
there is nothing else requisite than to
find out the separations of heterogeneous
rays, and their various mixtures and
their proportions in every mixture. By
this way of arguing, I invented almost all
the phenomena described in these books,
besides some others less necessary to the
argument ; and by the successes I met
with in the trials, I dare promise, that
to him who shall argue truly, and then
try all things with good glasses and suf-
ficient circumspection, the expected event
will not be wanting."

(42.) Although colour is one of the qua-
lities of homogeneous light, it is not, like
the degree of refrangibility, a test of its
purity or homogeneity. For compound
lights may be produced, the tints of
which will not be distinguishable from
those of homogeneous light. If the red
and yellow lights produced by a prism
be projected on the same white paper,
they will give it an orange tint, precisely
the same as the pure homogeneous
orange light, which lies between the red
and yellow lights in the spectrum. If
another white paper be illuminated by
this pure orange light, it will have ex-
actly the same appearance as to colour
as the paper which receives the com-
pound light. But if these two papers
thus illuminated be viewed from a dis-
tance through a prism, it will be found
that no change will take place in the ap-
pearance of the paper illuminated by the
pure orange light, while that which re-
ceives the compound light will be divided
into two images of its component colours,
red and yellow. In the same manner
any two alternate colours in the spec-
trum will, by their mixture, produce the
intermediate tint. Thus blue and yellow
will produce green, and so on.

(43.) " Whiteness, and all grey colours
between white and black, are formed by
mixtures of all the colours; and the
whiteness of the sun's light is com-
pounded of all the prismatic colours
mixed in a due proportion." The expe-
riments by which Newton verified and
established this important proposition,
D

34

A POPULAR ACCOUNT

are characterised with such singular ele-
gance and ingenuity, that we shall not
apologise for giving the particulars of
them at some length.

The prismatic spectrum being project-
ed on a screen, a white paper was 'held
before it, in such a manner as not to in-
tercept the rays from the prism, and so
that the paper should be as nearly as
possible equally distant from all the
colours. Under these circumstances, the
paper appeared white. The colours which
produced this white were evidently the
several colours of the spectrum reflected
from the screen upon the paper, and

consequently reflected in the same pro-
portions as they hold in the spectrum it-
self; from whence we may infer that the
mixture of these colours produces white.
If any of the colours of the spectrum be
intercepted, the paper will appear to be
illuminated with that colour which would
be produced by the mixture of those
which remain; a circumstance which
further confirms the inference, that the
white produced, when no light is inter-
cepted, is the consequence of the mix-
ture of all the colours.

Let the spectrum fig. 35, be pro-
jected upon a lens M N, which will

cause the coloured light to converge to
its focus G, and there to fall on white
paper. If the paper thus illuminated be
moved to and from the lens, it will be
found that when near the lens the paper
will be intensely coloured. As its dis-
tance from the lens is increased, the co-
lours will seem to approach each other,
and be collected into a smaller space,
until at last, at the focus G, they will be
collected and perfectly mixed together:
here the illuminated spot on the paper
will be white. By removing the paper to
a greater distance from the lens, the
rays which before converged, having
crossed each other at the focus G, will
now diverge. The colours also will be
inverted, those rays which were above in
the former case being now below, and
vice versa.

Lot the paper be now placed at the
focus, so as to be illuminated with white
light free from colour. We are to prove
that this whiteness arises from the ad-
mixture of all the coloured lights of the
spectrum in their due proportions. Let
all the colours except the red be inter-
cepted by an opaque screen, placed be-
tween the prism and the lens. The spot
on the paper will now appear red. By
raising the screen let the orange be ad-
mitted with the red through the lens.
The spot on the paper will now take a
tint which would be produced by a mix-
ture of red and orange, Again, let the

yeiiow light be admitted, and a similar
result will be obtained, the colour being
one which would be produced by the
mixture of red, orange, and yellow. In
a word, let any number of the prismatic
colours be intercepted between the lens
and the prism, and the colour on the
paper will be that due to the mixture of
those colours which are not intercepted.
From which we infer that if no colour
be intercepted, the white light on the
paper must arise from the mixture of all
the colours.

Let XY, jig. 36, be an instrument
formed like a comb, with teeth about an
inch and a half broad, at intervals of
about two inches asunder. By inter-
posing successively the teeth of this in-
strument between the prism and the
lens, a part of the colours was inter-
cepted, while the rest went through be-
tween the teeth. The teeth of this in-
strument being passed before the lens,
all the colours are successively thrown
upon the paper. Now, when this mo-
tion is rapid, so that the colours on the
paper succeed each other in very quick
succession, the eye loses all sense of co-
lour, and the paper appears white. Yet
it is certain that the paper is not at any
instant white. In this case the percep-
tion of whiteness is produced by the
continuance of the impression which
each colour makes upon the sense of
sight, until all the other colours have

OF NEWTON'S OPTICS.
"X

likewise affected the organ. The effect
is thus compounded of the influences
of the several colours upon the eye, as
much as if they all affected it at the
same moment.

(44.) In this explication of the pheno-
menon just described, we assume the fact,
that when a visible object affects the eye,
it continues to be perceived after it has
ceased to be present. Thus, if a light
be suddenly extinguished, the light itself
and all the objects which it rendered
visible continue to be seen for a certain
short space of time after the extinction.
This curious fact admits of very simple
proof. If a burning coal or lighted stick

be moved rapidly in a circle, it will be
seen in every part of the circle at once,
so as to have the appearance of a ring
of fire ; which proves that the impres-
sion which the light in one part of the
circle makes upon the eye, continues
until it returns again to the same part of
the circle, to make another impression.

The colours of the spectrum may be
recomposed, so as to form white light, by
a second prism, instead of the lens men-
tioned in the last experiment. Let R V,
fig. 37, be the spectrum formed by the
prism ABC, and let this be viewed
through another prism a b c, placed in
such a manner that the rays which con-

verge from RVwill be received as if
they emerged from a circular image of
the sun at s. In this case the rays enter
the eye exactly as they would if it were
placed before the aperture F, and pre-
sented towards it. The colours pro-
ceeding from R V are thus mixed on
entering the eye, and appear white.

If any of the colours of the spectrum
RV be removed by intercepting a part
of the light between R V and the prism

ABC, the colour which will be per-
ceived through the prism a be, will be
that which would be formed by the mix-
ture of the remaining colours. But if
the comb mentioned in the last experi-
ment be quickly moved between RV
and ABC, so as to throw the several
colours on the screen in rapid suc-
cession, the eye will again perceive
white, for the reason already explained.
The same result was obtained by va-
D2

36

A POPULAR ACCOUNT

rious other means, such as projecting
several spectra produced by different
prisms, on the same part of the same
paper, by moving several spectra ra-
pidly up and down, &c. &c. In all
these cases the colours submitted to ex-
periment were, however, prismatic. To
establish his theory more completely,
Newton now proceeds to inquire whether
the colours of natural bodies were en-
dued with qualities similar in all respects
to those of prismatic light. To accom-
plish this, having procured powders of
colours similar to those of the spectrum,
he mixed them together as nearly as
possible in the proportion which they
were found to hold in the spectrum.
He found that the mixture was not a
pure white, such as that produced by
the composition of the prismatic colours,
but was a dim, greyish white ; such, in
fact, as would be produced by mixing a
small quantity of black with a pure
brilliant white.

(45.) It was not difficult, to account
for this circumstance, which Newton
appears even to have foreseen. The co-
lours of natural bodies arise from a qua-
lity, in virtue of which they reflect one
component part of the solar beam more
copiously than the others, and therefore
affect the sense of sight with the colour
so reflected. Thus a body which we
call red, is one which reflects a "very
large portion of the red light of the solar
beam, and absorbs nearly the whole of
the other six colours. But it is found
that no body reflects "the light of its
proper colour so copiously as a white
body would reflect the same light. If a
white and a red object be placed beside
each other in a dark room, and both be
illuminated with red homogeneous light,
by means of a prism, the white object
will be more intensely red than the
red one.

(46.) Since then coloured bodies do not
any of them reflect all the light of their
proper colour, we are not to expect by
their mixture to obtain a clear white,
but rather such an obscure white as
would result from imperfect illumina-
tion. That the colour produced by mix-
ing powders in the manner already men-
tioned is exactly of this kind, Newton
proved by the following ingenious ex-
periment.

He placed the mixture of powders on
the floor of the chamber, and beside it a
piece of white paper. The room being
darkened, a beam of light was admitted,
so as to illuminate intensely the powder,

the white paper remaining near it, but in
the shade. Viewing them from a dis-
tance, he could perceive no difference,
both appearing to have exactly the same
whiteness. Another person happened to
enter the room during the experiment,
and Newton, without informing him of
the previous arrangement, asked him,
"Which of the two whites were the bet-
ter, and in what they differed ?" After he
had deliberately viewed them, he an-
swered, " That both were good whites,
that he could not say which was better,
nor wherein they differed." Thus it was
evident that the colour produced by the
mixture of the powders was a true white,
but only deficient in the degree of white-
ness ; just as twilight is as true alight as
broad sunshine, differing from it only in
quantity.

(47.) Having established the important
fact, that white must result from the mix-
ture of all the colours of the spectrum in
the proper proportions, Newton proceeds
to the consideration of the more general
question a§ to the colour which would
result from the composition of any given
colours in any assigned proportion. For
this problem he gives the following very
ingenious solution.

With the centre O, fig. 38, and a
radius O D, describe a circle AD F, and
let the circumference of this circle be
divided into 447 equal parts. Take A B,

Fig. 38.

consisting of 80 parts, BC of 45, CD
of 72, DEofSO, EFof45, FGof45,
and the remaining part, G A, will conse-
quently consist of 80 parts. Let the first
part AB represent a red colour; the
second B C an orange; the third C D a
yellow, and so on in the order of the
spectrum. Let it be conceived that
these are all the colours of uncom-
pounded light gradually passing one into

OF NEWTON'S OPTICS.

37

another as they appear in the spectrum,
so that, in effect, the circumference of
the circle will exhibit, as it were, a round
prismatic spectrum. By the principles of
mechanics, let the centres of gravity of
the arcs AB, B C, &c. be respectively
found, and let these points be r, o, y, g,
b, i, and v. Now, suppose that it be re-
quired to determine the tint which would
result from the mixture of red, green,
and blue, in certain given proportions.
Let circles be described round the points
r, g, and b, the magnitudes of which are
to be made proportional to the quanti-
ties of the three colours in the proposed
mixture. Let the common centre of
gravity of these circles be found, and let
it be m; and from the centre O through
m draw Om, to meet the circle at t. The
colour at the point t will be the tint
sought, and the line Om will represent
" its fullness, or intensity, that is, its
distance from whiteness." Thus, if t
should fall exactly in the middle of any
of the arcs, AB, B C, &c. the tint will
be the purest of the corresponding co-
lour ; but if it be distant from the middle
point, it will partake of the colour which
occupies the next arc, towards which it
lies. Again, if m fall on the centre O,
the colour will be, as it were, infinitely
diluted, and will be a perfect white ; but,
on the other hand, the nearer m is to the
circumference, the more intense and
florid the tint will be.

Newton conceived this method to be
sufficiently accurate for practice, al-
though not mathematically true. This
is a subject, however, in which much im-
provement has been introduced in later
times. It would not be to our purpose
here to enter upon it, our design being
merely to present to the reader in a po-
pular form a sketch of the labours of
Newton in the science of light. Those
who desire a short account of the mo-
dern discoveries, will find one in the ad-
mirable article on LIGHT, by Mr. Her-
schel, in the Encyclopaedia Metropoli-
tana.

In applying his theory of light to ex-
plain the phenomena of the colours of
natural bodies, Newton assumes, " that
every body reflects the rays of its own
colour more copiously than the rest, and
derives its colour from their excess, or
predominance, in the reflected light."
When a beam of solar light falls upon a
violet, a decomposition immediately en-
sues. The red rays, and those of the less
refrangible character, are either trans-
mitted through the body, or absorbed

and stifled ; those of the bluish, or violet
hue, and of the more refrangible species,
are copiously reflected, and produce in
the spectator the effect which in ordinary
language is denominated the violet colour
of the object.

Several ingenious experiments support
this reasoning. A natural object, what-
ever be its colour, will, if placed in ho-
mogeneous light, take for the time the
colour of that light, proving thereby its
capability of reflecting, in some degree,
lights of all colours. But when it is
placed in homogeneous light of its own
colour, it will appear much more re-
splendent than in light of any other
colour. Hence we infer that it possesses
a capability of reflecting light of its own
colour more abundantly than light of
any other colour. Thus cinnabar, a red
substance, placed in homogeneous red
light, exhibits a splendid red; let it,
however, be illuminated with green or
blue light, and it will assume these
colours, but with great faintness.

The colours of transparent liquors
vary with their thickness. If a red
liquor be poured into a glass of conical
or tapering shape, and held between the
light and the eye, it will appear of a pale
dilute yellow at the narrowest part of the
glass ; a little higher, where the glass is
wider, it becomes orange ; higher still it
becomes red ; and, finally, in the widest
part, exhibits a deep dark red. We
must, therefore, infer that a small quan-
tity, or thickness of the liquor, intercepts
a portion of the violet and indigo rays,
so that the remaining rays which it trans-
mits form a pale yellow. A greater
quantity of the liquor, besides stopping
the violet and indigo, also arrests the blue
rays, and a part of the green, transmit-
ting the other component parts of light,
the mixture of which produces an orange.
A still greater quantity of the fluid will
intercept all the green, and a great part
of the yellow, so that the transmitted
light approaches to a red, becoming a
deep dark red, when the quantity of the
fluid is so great as to absorb the whole
of the orange light.

W^e have in this description assumed
several distinct effects, but the changes
of colour are not sudden, but take place
by an imperceptible gradation, an ob-
vious consequence of the tapering form
of the glass. If the glass were formed
of a number of cylinders rising one above
another, the diameter of each exceeding
that below it by a certain magnitude, the
changes of colour would be sudden and

38

A POPULAR ACCOUNT

distinct ; and the liquid in each cylinder
would present, in the vertical direction,
an uniform colour.

(48.) Connected with the power of trans-
parent liquids to reflect and transmit the
different component parts of solar light,
Newton mentions two very remarkable
facts noticed by Halley and Hooke, but
which these eminent philosophers were
unable to explain. Halley, having de-
scended in a diving bell to the depth of
several fathoms in the sea, observed,
upon holding his hand in the sun's light,
which penetrated the water, and shone
into the bell through a small glass win-
dow in the top, that the light upon his
hand was red. Whereupon he examined
the lower part of his hand illuminated by
light reflected from the water below, and
found it green. This circumstance is
thus accounted for by Newton. The
sea-water reflects back the violet and
blue rays most easily, and transmits most
copiously the red. In the sun's light
transmitted to considerable depths, the
red rays therefore predominating, objects
illuminated by them assume a red hue.
At depths to which the violet rays cannot
penetrate, the reflection of the blue,
green, and yellow light separated from
the red, which is transmitted, must com-
pound a green.

Two liquids may be obtained, one of
which transmits the rays of the red
character, and the other those of the
blue, the former intercepting the bluish
light, and the latter the red. If both
liquids be placed between a spectator and
the light, they will be found perfectly
opaque, although either alone is trans-
parent. This is evident, since all the
rays which can be transmitted by either
are intercepted by the other. Hooke
casually, and without anticipating or ex-
pecting the result, actually tried this ex-
periment. He filled two hollow glass
wedges, one with a red, and the other
with a blue liquor. On placing the
wedges together, and looking through
them at the light, he found them abso-
lutely opaque.

(49.) We have explained, according to
the Newtonian theory, the most striking
phenomena of coloured lights produced
by prisms. The explication of others
will be found in every elementary treatise

on optics. One very singular prismatic
phenomenon, however, still remains to be

noticed, and is entitled to attention, as

well for the strong confirmation of New-
ton's theory which it furnishes, as from

the ingenious manner in which that
theory is shown to account for it.

Let HKG, fig. 39, be a prism placed be-
fore an open window, with its base H E I G

Fig. 39.

horizontal, the face F K G I presented
to the light of the clouds, and let the
base be viewed through the face F K H E
by an eye at S. The" base H E I G will
now be observed to be separated into two
parts by a beautiful iridescent arch,
formed of colours of violet and bluish
tints. This arch is concave towards the
eye, and that part of the base which is
towards the edge I G, or above the arch,
exhibits a most vivid reflection of the
firmament, not yielding in splendour to
the direct view- of the heavens. On the
other hand, the lower division of the
base next the edge E H, appears nearly
dark, reflecting but a very small portion
of the light incident upon it. The arch
next this sombre space is fringed with a
violet colour, which is gradually tinted
off into a vivid blue towards the convex
edge, which bounds the bright part of the
base.

To account for this curious phenomenon,
it must be remembered that the different
parts of solar light are differently re-
flexible ; also, that when rays of light are
incident on the base of a prism, having
previously passed through its side, there
are certain angles of obliquity at which
it will be impossible for the rays to pass
through the base, and they will then be
reflected. The limit of obliquity at which
they will cease to penetrate the base, and
will be reflected, depends on their degree
of refrangibility. The most refrangible,
and consequently the most reflexible rays,
are the violet, next to these the indigo,
then the blue, and so on through the
other colours of the spectrum, the red
being least reflexible. Let H E I G be

OF NEWTON'S OPTICS.

the base of the prism. From the eye, let
lines he supposed to be drawn to the base,
nclined to it at that angle which limits
the reflexion of the violet li^ht. These
lines being all equally inclined to the base,
must meet it at points which lie in the
arc of a circle. Let this arch be V V,
fig. 40. Again, let lines be drawn at the

FigAQ .

1, r.

limiting angle of the indigo rays. This
angle being less than that for the violet,
the corresponding arc 1 1' will be beyond
V V. In the same manner the limiting
arcs B B', GG', Y Y', O O', RR', cor-
responding to the other prismatic lights,
blue, green, &c. may be drawn.

It follows then, that all the violet rays
in the solar light will be reflected from
the part of the base of the prism whose
boundary is VV'GI; all the indigo
from 1 1' G I ; all the blue from B B' G I ;
all the green from GG'GI; all the
yellow from YY'GI; all the orange
from O O' G I, and all the red from
R R' G I. Hence it appears that the
space between the arcs, V V and 1 1', is
illuminated with a pure violet light only ;
that between 1 1' and B B' is illuminated
by both violet and indigo mixed; be-
tween B B' and G G' there is a mixture
'of violet, indigo, and blue; between
G G' and Y Y' is a mixture of the former
colours, with the addition of green ; from
Y Y' to O O', yellow is added to the com-
pound ; the next arched band introduces
orange, and the last the red. Now the
last mixture constitutes a pure white.
The former also a white, but one which,
being deprived of the pure red rays, takes
a faint tint approaching a bluish colour,
but which is not distinguishable from a
perfect white. In the next space the red
and orange being removed, the mixture
produces a greenish blue, which rapidly
deepens, and becomes a strong blue,
when the yellow rays are removed. The
arc towards its inner termination is a
pure violet.

Newton next applies his theory to ex-
plain the phenomena of rainbows. As

this subject has been already fully dis-
cussed in our treatises on Optics accord-
ing to the same principles, and in exactly
the same manner as it is treated by
Newton, it is not necessary to repeat it
here.

CHAPTER VI.

On the phenomena exhibited by thin
transparent plates — the theory of the
Jits of easy reflexion and transmission
deduced from these phenomena.

(50.) THE first book of Newton's Optics
contains the discussions which have
been detailed in the last four chapters.
In these investigations a ray of light
upon its impact on the surface of any
medium is considered to undergo one
of two effects, viz. either to pass into the
medium on which it impinges in a deter-
minate direction, in which direction it is
supposed to persevere through the entire
medium ; or to be reflected back from
the surface into the medium from whence
it came, following also and persevering
in a rectilinear course. We are now
about to accompany this great scrutineer
of nature through a more subtle analysis
of the process to which- a beam of light
is submitted when it encounters the sur-
face which separates two media of dif-
ferent densities.

If it were possible to divide the me-
dium which a ray of light penetrates
into a series of plates, the thickness of
which should be minute to an extreme
degree, and to examine the state of the
ray during its transmission through
each of them, we should attain the end
which we desire. Although it would
perhaps be difficult to effect this very
minute subdivision by direct mechanical
means, yet numerous expedients present
themselves, and those too of a character
sufficiently familiar, by which the phe-
nomena in question may be brought
under examination. Indeed, these phe-
nomena were long the subjects of daily
observation, and may almost be said to
have been the sport and toy of children ;
but, like many other natural effects, not
less wonderful, which are continually
passing under our eyes, they had failed
to excite the attention or stimulate the
curiosity of those who, by faculties and
acquirements, were qualified to behold
in them manifestations of the laws and
principles on which the works of nature
are constructed.

(51.) If a small quantity of soap be
mixed with water, the latter acquires a

40

A POPULAR ACCOUNT

tenacious or glutinous quality, in virtue
of which it may be blown into bubbles,
or it may be thrown into that state by
mere agitation. Every one is familiar
with the various colours which these
bubbles reflect. Similar appearances
are exhibited by glass when blown into
bubbles of sufficient tenuity. Since these
effects are not produced when the bound-
ing surfaces of the medium are more
distant from one another, we are com-
pelled to suppose that when the light
first enters the transparent medium, it is
put into some state in which it does not
continue during its entire course through
the medium. This inference is as sin-
gular and important as it is inevitable.
Suppose that the two surfaces of water
impregnated with soap were at a dis-
tance of one inch asunder — a ray of
light entering the first surface perpen-
dicularly, would penetrate the water, and
passing through the second surface,
would issue from the water at the other
side, preserving its original direction.
Now, suppose that the second surface
of the water, instead of intercepting the
course of the ray at the distance of an
inch from the first surface, meets it at a
distance from that surface, equal to the
thickness of a certain part of the soap-
bubble, to which we have alluded— the
ray will no longer be allowed to pass
out in its original state at the second
surface. On the contrary, if it be white
solar light, that part of it which has a
certain colour, say red, will be reflected
back in the direction from which it
came, while the remainder only of the ray
which, combined with red, would pro-
duce white, will be transmitted. It there-
fore follows that, in this instance, after
the ray has penetrated the water through
a space equal to the supposed thickness
of the bubble, that portion of it which is
red is put into such a state, that were it
to encounter the second surface, it could
not penetrate it, and would be reflected.
This state, however, does not continue ;
for when the white ray is allowed to
proceed further into the water before it
is intercepted by the second surface, it
will be brought into a state in which it
will penetrate that surface, and be trans-
mitted into the ambient medium. Such
is an example of the class of facts which
form the basis of the experiments and
investigations which we are now about
to explain.

(52.) The property which we have in-
stanced in water and glass is common to
all transparent media. The evanescent

and fluctuating nature of a water-bubble
renders it an inconvenient object of ex-
perimental inquiry. Glass is better, but
still is difficult to procure, and to retain
in the highly attenuated state which is
necessary to manifest the desired effects.
By the following contrivance, Newton
rendered air, though at the first view
an unpromising agent, available for the
purposes of deliberate and close experi-
mental observation.

He procured a double convex lens,
the object-glass of a fifty foot telescope,
and consequently having a degree of
convexity so small as to be scarcely per-
ceptible. On this he placed the plane
surface of another lens, so that the two
surfaces were in absolute contact at the
centre, the distance between them in-
creasing with the increased distance from
that centre. A lens of air was thus in-
closed between two glass lenses. This
air lens was plane on one side and con-
cave on the other, losing all thickness
at the centre at which may be conceived
an infinitely small space, filled by the
point of contact of the glass lenses.
Taking this point of contact as a centre,
let us suppose a number of concentric
circles to be traced on the lenses as
represented in fig.
41. Let the'smallest
circle be called 1 , the
next 2, the next 3,
and so on. It is
plain that the thick-
ness of the air under
the circle 1, is less
than under the circle

2. In like manner
the thickness under

the circle 2 is less than under the circle

3, and so on. A section of the lenses
exhibiting the thickness of the air be-
tween the lenses under the several circles
1, 2, 3, &c.is represented \r\fig. 42.

Fig. 42.

Upon exposing these lenses to a beam
of light, a very minute black spot was
observed at the centre. Immediately
around this circular spot was a ring of blue
colour, which gradually emerged from the
black, so as to assume the perfect blue
tint at some distance from the black
spot. This blue ring was surrounded
with a white one, into which it in like

OF NEWTON'S OPTICS

41

manner gradually melted, assuming tints
more and more dilute, until it became
absolutely white. This white ring was
again bordered by a yellow one, which
in its turn was tinted off in a red ring.

In this series of coloured circles the
blue of the first circle was very faint ;
the white of the second was brilliant.
The gradual tints assumed by the yellow
of the third ring in passing into the red
of the fourth, produced between them
an orange ring.

A second series of rings succeeded,
the first of which, surrounding the red
ring of the last series, was violet, after
which appeared in regular succession
four other rings of blue, green, yellow
and red. In this series the green was
yellowish, the yellow brilliant, and the
red partaking of a crimson hue.

After this came a third series. The
first ring in this series surrounding the
red of the last was purple, which was
regularly succeeded by four rings of
blue, green, yellow and red. Of these
the green was brisk and copious, being
a rich grass green, and the yellow was
particularly splendid ; but the red had
a more faded appearance, and partook
more of the pink and crimson, than the
vermilion.

After these succeeded a fourth series,
consisting of two colours, green and red.
The green, in passing into the red, ex-
hibited a yellowish pink hue. Then suc-
ceeded three other series, each consisting
of two rings ; the inner ones being various
shades of green, and the outer ones
various shades of red ; each of the colours
become more and more dilute, as the
diameters of the rings increased.

(53.) Due consideration of these phe-
nomena suggested some very important
conclusions. Of the light which pene-
trated the glass within the central black
circle none was reflected, for in that case
the circle would take the colour of the
reflected light. The incident light was,
therefore, in this case, either stifled and
absorbed by the glass, or was transmit-
ted. To ascertain this, the eye was
placed behind the lenses, so as to receive
the transmitted light. The central spot
now appeared white, proving that all the
light incident on the glass was here
transmitted. Again, of the light incident
upon the first ring of the first series, the
rays composing a bluish colour alone
were reflected. The remaining rays were
transmitted, as appeared by viewing the
lenses on the other side ; the colour of
the first ring being that which was com-

plemental to its reflected colour, or that
which combined with the reflected colour
would produce white. In the same way,
each ring of each series was found to
transmit the colour complemental to that
which it reflected, which was proved by
viewing the light through the lenses.

These phenomena were attended with
many circumstances, which rendered it
probable that some connexion subsisted
between the colours of the reflected and
transmitted light, and the thickness of
the air-lens, at the place where these
colours were produced. The same colour
was observed to be arranged in a circle
round the centre of the lens. It was
evident, that in all parts of such a circle
the thickness of the air-lens was the
same. Again, in passing from one con-
centric circle to another, the tint was ob-
served to undergo a change. In dif-
ferent concentric circles the thickness of
the air-lens was different. Here, then,
were two important steps towards the
discovery of a connexion between the
colour of the light, and the thickness of
the air, which reflected or transmitted it.
By pressing the glass lenses together,
so as to force them into closer contact,
the diameter of each circle, at which the
air-lens had a given thickness, would ob-
viously be increased. If it were true,
that the colour of the reflected light de-
pended on the thickness of the air at the
points of reflexion, it would follow, that
upon pressing the glass lenses together,
each coloured circle would be enlarged.
It was accordingly found, that upon ap-
plying such pressure, the central spot
was increased, and each coloured circle
expanded its dimensions, and retreated
from the centre. These indications were
further confirmed, by pressing the lenses
more closely at one side of the centre
than at the other, the colours still re-
treating from the points of closest con-
tact.

(54.) Aware of theheterogeneous nature
of solar light by previous investigations,
Newton considered it probable that these
coloured rings were not the effects of
one simple cause, or of a single action of
the transparent medium on the solar ray,
but conjectured that it might rather be
the result of the combined actions on all
or several of the component parts of
light. To simplify the phenomena, and
thereby facilitate the analysis, he deter-
mined to expose the lenses successively
to the different species of homogeneous
light, and to observe and carefully note
the separate effects of each. He con-

42

A POPULAR ACCOUNT

eluded, that these effects being severally
known, there could be no difficulty in
combining them, so as to account for the
phenomena produced by compound solar
light.

With this view, he decomposed a sun-
beam, by means of a prism, and casting
successively on the lenses the several co-
loured lights in the spectrum, he ob-
served and carefully noted the pheno-
mena. In each case the rings appeared,
and even in greater numbers, than in the
case of the compound solar light. They,
however, no longer exhibited any variety
of colour, the central spot being now
surrounded by rings of the same colour
as the light cast upon the lenses, sepa-
rated by dark rings, in which, like the
central spot, all light seemed to be trans-
mitted, and none reflected. Upon look-
ing through the lenses towards the light,
the intermediate rings just mentioned, as
well as the central spot, appeared of the
colour of the prismatic light, to which
the lenses were exposed ; and, on the
other hand, those rings which by reflect-
ed light appeared coloured, were now
dark, no light being transmitted.

Let S S, fig. 43, be a section of the
air-lens, and suppose a beam of homo-

Fig. 43.

geneous red light projected on it from
the direction A, and perpendicular to its
surface. The centre of the lens being o,
let 1 be the place of the first ring of red
light, as viewed from A. At 2 will be a
dark ring, at 3 a second ring of red light,
at 4 a dark ring, and so on, the central
spot o being dark. Now, let the lens be
viewed from B, so as to receive the rays
transmitted through it. The central spot
o will appear red, the first ring 1, which
before was red, will be dark, the ring 2
will be red, 3 dark, and so on ; all the
rings which were dark, when viewed
from A, being red when viewed from B,
and vice versa.

Upon exposing the lenses to orange,
yellow, and the other species of homoge-
neous light, similar effects were observ-
able ; the bright rings always taking the
colour of the light incident on the lenses,
and being separated by dark rings, which
being viewed from B appeared bright,

the bright ones, as in the case of the red
light, appearing dark. One difference,
however, was remarkable, viz. that the
rings formed by the less refrangible rays
were larger than those formed by the
more refrangible. Thus the first red
ring was larger than the first orange one
and this larger than the first yellow ring,
and so on, the first violet ring being least.

(55.) The existence of a connexion be-
tween the colour of the reflected and
transmitted light, and the thickness of
the air-lens, being now manifest, Newton
applied his attention to measure this
thickness at the places of the several
dark and bright rings. The extreme
minuteness of the magnitude to be as-
certained rendered the application of
direct measures impracticable. The first
principles of elementary geometry, how-
ever, furnished a method of effecting the
measurement with the greatest accuracy.

* Let O T,/g-. 44, be the plane side of

Fig. 44.

the air-lens, and C D the concave side,
and let the circle, of which C D is an
arc, be completed. Let O B be its dia-
meter. By the principles of optics, the
length of O B may be deduced, from ob-
serving the focus of the convex glass
lens used in the experiments, provided the
refracting power of the glass be known.
Let it be required to ascertain the thick-
ness T P of the air-lens at T. Draw the
line P O, and from P draw PA parallel
to T O. By actually measuring the dia-
meter of the ring at the distance T, the
line T O, or P A, will be determined ; and
from the extreme minuteness of T P, the
line PA may be considered as practically
equal to P O. The right angled triangles
OPB, and OAP, are similar; and,

* This investigation may be omitted by those who
are not familiar with the eiements of geometry.

OF NEKTON'S OPTICS.

43

therefore, O P bears the same proportion
to OB, as OA, or TP, bears to OP.
Now, since the magnitudes of O P, and
OB, are known, we know how many
times OB is greater than OP. Then

0 P will be the same number of times
greater than O A, or T P. Thus, if O B
were 182 inches, and O P 8-79ths of an
inch, the thickness T P is found by com-
mon arithmetic to be about — -th

1774784

part of an inch.

Calculating in this manner, Newton
found a very singular analogy to subsist
between the thicknesses at which the
bright and dark rings of each colour
were produced. Let the thickness at
which the first bright ring of any ho-
mogeneous colour is produced, be called

1 ; the thickness at the next bright ring
will be 3 ; the next 5 ; the next 7 ; and
so on ; the thicknesses of the successive
bright rings being represented by the odd
integers. Again, the thickness of the
air at the dark ring, which immediately
succeeded the first bright ring, was found
to be twice its thickness at the first bright
ring ; and therefore the central spot being
considered as the first dark ring, the
second dark ring will be at the thick-
ness 2. The third dark ring was found
to be at the thickness 4 ; the fourth at 6,
and so on : the thicknesses of the air at
the several dark rings being represented
by the even integers.

The proportion which we have now
explained was found to prevail among
the rings, whatever might be the colour
of the light projected • on the lens ; but
the absolute magnitudes of the rings
was, as we have stated, different in each
kind of light. This will, perhaps, be
better understood by example. Suppose
an inch divided into 180,000 equal parts,
and let one of those parts be the thick-
ness at which the first ring of a certain
colour, say green, appears, homogeneous
green light being projected on the lens.
At a thickness equal to two of these
parts will be a dark ring. At a thick-
ness equal to three of these parts will
appear the second bright green ring, and
so on alternately.

The rings of the other colours will
succeed each other in a similar manner,
with this difference, that the thickness at
which the first ring appears will be less
for the more refrangible rays, i. e. those
of a bluish tint, and greater for the less
refrangible rays, which take the yellow
or red hues, and that the intervals be-
tween the rings will also be less for the

former rays than for the latter. Hence
we may easily perceive how the suc-
cession of coloured rins:s is produced
when compound solar light is projected
on the lens. In this case each compo-
nent part of the light forms its own set
of rings, and the rings of one colour in-
termixing with those^of another, form the
several series of coloured rings already
described.

(56.) All that has been observed re-
specting the rings produced by the light
reflected from the lens will apply, with the
requisite modifications, to the rings pro-
duced by the light transmitted through
it. These latter, however, are much less
vivid than the reflected rings.

Before we proceed further in our ac-
count of these phenomena, it may be
useful, in impressing them on the me-
mory of the reader, to give some ac-
count of the manner in which Newton
explained them. He considers that every
ray of light, in its passage through the
surface which separates two media of
different densities, is put into a certain
transient constitution or state, which, in
the progress of the ray, returns at, equal
intervals, and disposes the ray at every
return to be easily transmitted through
the next refracting surface, and between
the returns, to be easily reflected by it.
Let A B (fig. 45) be a ray of pure
homogeneous light falling perpendicu-
larly on a refracting surface, S S, Let us
suppose that the medium through which

Fig. 45.
A

7

9

3

4 •--

6.

7-

A B has passed is glass, and that the
medium included between the surfaces
S S and S' S' is air. Take B 1 in the di-
rection of AB, and equal to the thickness
of the air between the lenses at which
the first ring of the homogeneous light,
now supposed to fall on S S, appeared.
From 1 take the intervals 2, 3, 4, &c.
equal to B 1. The action of the surface,
SS, upon the ray, is supposed to

44

A POPULAR ACCOUNT

be such as to put it in a state in
which it would be easily transmitted by
another similar refracting surface, such
as S' S', if that surface received the ray
immediately after its passage through
S S. But this state of easy transmission
does not continue. When the light has
arrived at 1 it is in a state of easy re-
flection, so that if it were intercepted at
1 by such a surface as S' S', it would be
reflected back in the direction 1 B A.
After passing the division 1, the state of
the ray is again changed, and when it
has arrived at the division 2, it is again
in a state of easy transmission, as at B.
If the surface, S' S', therefore, met the
ray at 2, the ray would pass freely
through it in the direction 3, 4, &c. In
passing from 2 to 3 the ray again
changes its state, and is found at 3 to
be in the same disposition to be reflected
as it was at 1 ; and such reflection
would, in fact, take place if the surface,
S' S', intercepted the ray at 3. In this
manner the ray passes alternately into
states of easy transmission and reflec-
tion, at the successive points, 4, 5, 6, &c.

(57.) These alternate states of the ray
Newton calls fits, the light being in jits
of easy reflection at the points 1, 3, 5,
&c. ; and in fits of easy transmission
at the points 2, 4, 6, &c. The spaces
B2, 24, &c., or 13, 35, Sec., he calls
the interval of the fits. Although these
phrases imply a theory or hypothesis,
yet Newton intends them merely as
names for effects which are known to
exist, and distinctly disclaims the adop-
tion of any hypothesis, endeavouring in
every case to render his inferences inde-
pendent of everything except the result
of experiment or observation.

In describing the phenomena of phy-
sical science it is extremely difficult, if not
impossible, to avoid expressions and
terms which imply a theory or a sup-
posed cause for effects. Every writer,
but more especially he who promulgates
new facts, should be cautious to remind
the student that the language of cau-
sation, the use of which in physics is
inevitable, is nothing more than a me-
thod of expressing the classification of
effects ; and that, when we are said to
"discover the cause" of any appear-
ance, nothing more is to be understood
than that we have found a class of phe-
nomena to which it belongs and must
be referred. There is no philosopher
who seems more conscious of the neces-
sity of this than Newton ; and, accord-
ingly, in the introduction of the phraseo-

logy to' which we have just alluded, he
warns his reader that he does not pre-
tend to affirm " what kind of action this
(the fits) is ; whether it consists in a cir-
culating or a vibrating motion of the
ray, or of the medium, or of something
else. Those that are averse to assenting
to any new discoveries but such as they
can explain by an hypothesis, may for
the present suppose that, as stones, by
falling upon water, put the water into
an undulating motion, and all bodies, by
percussion, excite vibrations in the air ;
so the rays of light, by impinging on any
refracting or reflecting surface, excite
vibrations in the refracting or reflecting
medium or substance, and by exciting
them agitate the solid parts of the re-
fracting or reflecting body, and by agi-
tating them cause them to grow warm
or hot ; that the vibrations thus excited
are propagated in the refracting or re-
flecting medium or substance, much
after the manner that vibrations are pro-
pagated iri the air for causing sound,
and move faster than the rays, so as to
overtake them ; and that when any ray
is in that part of the vibration which
conspires with its motion, it easily breaks
through a refracting surface ; but when
it is in the contrary part of the vibration,
which impedes its motion, it is easily
reflected ; and by consequence, that
every ray is successively disposed to be
easily reflected or easily transmitted, by
every vibration which overtakes it.
But, whether this hypothesis be true
or false, I do not here consider. I con-
tent myself with the bare discovery that
rays of light are, by some cause or
other, alternately disposed to be reflected
or refracted for many vicissitudes."

(58.) By the observation of air between
glass lenses, Newton ascertained the
fact that, after passing through a certain
thickness of air, a ray would be reflected
or transmitted, according to the degree
of thickness of the air and the species of
the light. By actual admeasurement he
ascertained the least thickness at which
each species of homogeneous light
would be reflected, a magnitude which
will easily be perceived to be equal to
half the interval of the fits. We have
already observed that this interval is
different in different kinds of light, being
greater for the less refrangible rays than
for the more refrangible. The following
Table exhibits the interval for lights of
the different degrees of refrangibility.
If an inch be supposed to be divided
into ten millions of equal parts, the

OF NEWTON'S OPTICS.

45

Difference.
0
5
5
3

1*

4
4
4

number of these parts, in the interval
for each ray, is expressed in the second
column.

Ten mlllionths
of an inch.

Extreme rays 133

Red rays 128

Intermediate 123

Orange rays 120

Intermediate 117^

Yellow rays 113i_

Intermediate 109^

Green rays 105-!:

Intermediate 101^

Blue rays 98

Intermediate 94A

Indigo rays

Intermediate

Violet 87

Extreme rays

That the magnitudes, which appear
in the third column of this Table, should
be subjects of accurate computation,
founded on measurements performed by
the hand of man, must, we conceive, be
matter of the greatest wonder and admi-
ration. An inch being divided into five
million of equal parts, a distance equal
to one of these parts is ascertained by
positive measurement, in the estimation
of the intervals of the fits of indigo rays
and those intermediate between them
and the violet! Magnitudes of such
minuteness far exceed even the powers
of imagination. We have, perhaps, a
distinct idea of the hundredth part of an
inch, by imagining ,the tenth of an
inch divided into ten equal parts. But
when we are required to conceive one
of these ten parts divided into fifty thou-
sand equal parts, imagination altogether
fails, and we cease to attach to the name
of such a magnitude any positive con-
ception. Nevertheless, this magnitude
is, as we have seen, capable of measure-
ment as accurately as any other, how-
ever gross and perceptible, so far does
the power of reason exceed that of the
imagination.

(59.) The principles which we have
now explained will be found sufficient to
account for all the phenomena of co-
lours reflected, or transmitted by trans-
parent media. Let us suppose that two
glass surfaces are placed parallel at
the distance of 120 ten-millionth parts
of an inch, inclosing between them
a plate of air. If pure orange light fall
perpendicularly on the glass, it will pass
through, being transmitted freely by the
plate of air between the plates of glass.
For on passing from the first glass plate
into the air it is in a fit of easy trans-

mission ; and since the distance between
the plates is equal to the interval between
the fits (see Table), it will be again in
a fit of easy transmission when it en-
counters the surface of the second plate
and will consequently pass through. If,
in this case, the eye be placed behind
the second plate, the coloured light will
be perceived to be transmitted.

Now suppose the two plates to be
placed parallel as before, but only at
half the former distance asunder. The
orange light, on meeting the surface of
the second plate, will be in a fit of easy
reflection, and will consequently return
through the plate of air in the direction
from which it came. Having the same
space to move through, it will be in a
fit of easy transmission when it has
again reached the surface of the first
plate, and will consequently be trans-
mitted. If the eye be placed before the
first plate, it will be coloured over with
the orange light.

Let us now suppose that the plates
are placed at the distance of 240 ten-
millionths of an inch asunder. This is
equal to two intervals of the fits of orange
light, and therefore that light being in a
fit of easy transmission when it encoun-
ters it, will be transmitted at the sur-
face of the second plate. An eye placed
behind the second plate will receive the
transmitted light, and the plate will take
an orange hue.

In the middle of the interval between
120 and 240, that is, at the distance of
180 ten-millionths of an' inch from the
first plate, the ray will be in the middle
of the second interval of its fits, and will
therefore be in a fit of easy reflection.
If it encounter the second surface at this
distance, it will consequently be reflected.
When it has returned to the distance
120, it will again be in a fit of easy trans-
mission, which fit will return again after
passing through the distance 120, at
which point it will again meet the surface
of the first plate, and will be transmitted
through it. Thus an eye placed before
the first plate will perceive the orange
light reflected.

Thus the space followed by the ray ]f
passing through the air being divide^
into parts equal to 60 ten-millionths u
an inch, at each alternate point of divi
sion the orange ray will be in a fit o
easy transmission, and at the interme
diate points it will be in a fit of easj
reflection. In the one case, the orange
light transmitted will appear to an eye.
placed behind the second plate, while to

46

A POPULAR ACCOUNT

an eye placed before the first no colour
is apparent. In the other case the re-
verse will happen, the orange light being
perceptible to an eye placed before, the
first plate, while no colour appears to an
eye placed behind the second.

All that has been here observed of the
orange light will be equally applicable
to the red, yellow, and all the other co-
lours, the interval of the fits only being
different. It should, however, be ob-
served, that the ray does not pass sud-
denly into its fits of reflection and trans-
mission on arriving at the several points
of division which we have mentioned,
but passes gradually from its complete
fit of easy reflection to its complete fit
of easy transmission, and vice versa,
being, in the intermediate space, in a
state to be partially reflected and trans-
mitted.

(60.) When'a beam of white solar light
falls perpendicularly on the plates, each
component part is put into fits separately,
and in the same manner as would hap-
pen if that part alone had been incident
on the plate. The thickness of the plate
of air may be such, that several compo-
nent rays may meet the second surface
in fits of easy reflection, while the other

Fig

parts meet it in fits of easy transmission.
In this case, the tint exhibited to an eye
placed before the first plate will be one
which is compounded of the colours of
those rays which meet the second plate,
in fits of easy reflection, while the tint
exhibited to an eye behind the second
plate will be compounded of the colours
of those rays which meet the second
surface in fits of easy transmission. It
will happen frequently that the second
surface will encounter a ray in such a
manner as to divide the interval of the
fits unequally, so that the light will be
partly reflected and partly transmitted.
In this case, the tint seen on each side of
the plates is determined as before, by
the composition of the colours reflected
and transmitted, due regard being had
to their quantities.

(61.) Newton has given an ingenious
scale for determining the colours reflected
by the second surface, at any proposed
distance from the first. We shall here,
however, adopt another method of illus-
tration, not differing in principle from
that of Newton, but better adapted for
popular illustration.

Draw two lines* AX (fig. 46) and
AY at right angles, and taking any

,46.

/

t S

ln_/

o'

,A

4 B

7 8 C

thven points R, O, Y, G, B, I, V on the
efie A Y, draw through them lines pa-
'Mlelto AX; Let RR' represent the
aiterval of the fits of red light, and let
this space be repeated, so that R'R",
ri/'R"', &c. shall be equal to R R'.
Draw a waving curve line touching the
parallel through R in the points R, R',
il", &c., and let the points, at which the
iistance of the curve from the parallel
:s greatest, be situated exactly in the

0 TO D P 3?

middle between every two successive

Eoints of contact, that is, perpendicu-
irly above the points r, r', r", &c.
When the thickness of the air between
the plates is equal to RR', or R R", or
R R"', &c. no red light is reflected, the
ray being at those points in a fit of easy
transmission. On Ihe other hand, in the
middle of each interval, or at the thick-

* This scale is used by Mr. Herschel in his able,
" Treatise on Light."

OF NEWTON'S OPTICS.

nesses Rr, Rr', R?1", Sax. the reflection
of red light is most intense, the ray being
at these points in a fit of easy reflection.
From the first entrance of the red ray at
the plate A Y until it reaches rt its dis-
position to be reflected is increasing, and
consequently, the curve from R to r
may be so shaped, that its increasing
perpendicular distance from R r may be
proportionate to the quantity of red light
reflected at each increasing thickness.
If this be done, the perpendicular dis-
tance of the curve from r will represent
the quantity of red light reflected when
the ray attains its first fit of easy reflec-
tion. As the ray passes from r to R' it
gradually changes its phase, and the
reflected light constantly decreases like
the distance of the curve from r R', until
at length, like that distance, it dwindles
into nothing at R', the entire light being
here transmitted, the ray having attained
its fit of easy transmission. The same
process is repeated as the ray passes
from R' to R", from R" to R'", and
so on.

It thus appears, that the quantity of
red light reflected by the plate of air
intervening between the two plates of
glass may be exhibited. Take R p
equal to the distance between the plates,
or the thickness of the plate of air.
Draw p m perpendicular to Rp and
meeting the curve at m. Then p m will
bear the same proportion to ra as the
quantity of red light reflected at the
thickness R;; bears to the quantity re-
reflected at the thickness R r when the
ray is in a fit of easy reflection. It is
evident that the quantity of red light
reflected at any other thickness may be
similarly found.

Let O O' be taken to represent the
interval of the fits of orange light, and
let this interval be repeated O' O",
O" O'", See., as in the former case. Let
a curve be drawn as before, touching
the parallel at the points which mark
the fits of easy transmission, and such
that its distance from the parallel will
always be proportionate to the quantity
of orange light reflected at each thick-
ness of the plate of air. The other
curves are to be drawn in the same
manner, the distances YY', GG', BB',
1 1', V V' representing the intervals of
the fits of the yellow, green, blue, in-
digo, and violet lights respectively. It
is evident that, by this scale, the quan-
tity of light of each colour which is re-
flected at any given thickness may
always be exhibited, and the colours

which compose the tint, perceived by
reflection, may thus be determined, both
in quantity and quality*

It will be observed, that the intervals
R R', O O', Y Y', &c., continually dimi-
nish in passing from the red to the
violet light. This is conformable to
what has been explained ; the interval
of the fits being shorter for the more
refrangible the lights. It is to this
circumstance that the coloured rings
between the lenses is owing ; for if the
fits of all the component parts of light
were equal, the rings would be alter-
nately white and black.

To explain more fully the manner of
determining a tint corresponding to a
given thickness, let the line AX be
divided at 1,2, 3, &c. Let A 1 be a
thickness much less than R r, half the
interval of the fits of red light. Through
1 draw a line parallel to AY, and
crossing all the curves. The parts of
this line intercepted between each curve
and the corresponding parallel to A X,
express the quantities of the respective
colours reflected by the air at this thick-
ness. It thus appears, by inspection,
that the quantities of red and orange
are small ; of yellow not much more,
but that the proportion rapidly increases
as we approach the blue and violet.
The excess of light of a bluish tint,
therefore, which enters the reflected
light, will give that light a corresponding
character. If the thickness of the plate
of air be much less than A 1, the lines
representing the reflected light will gra-
dually disappear, and no light will be
reflected. Accordingly, it was found in
the experiment with the glass lenses
described in (54), that at the centre,
where the glasses were in contact, and
for a small distance round it, a black
spot was perceivable, arising from the
absence of reflected light. In this case,
the air immediately around the centre
was too thin to reflect the light in any
sensible quantity, as appears by the
scale which we are now describing.

Let a parallel to AY be drawn
through 2. The lines which now repre-
sent the reflected lights are nearly equal.
The red tints have not reached their
maxima, and the blue tints have passed
theirs. The intermixture of these pro-
duce a brilliant white.

Referring again to the experiments
with the glass, we found that the blw
ring which immediately succeeded tht
central black spot was followed by i
ring of splendid white. Here, then, the

48

A POPULAR ACCOUNT

thickness of the air between the lenses
had so far increased as to be represented
by A 2.

As the parallel to AY is moved to-
wards 3, the lines which are intercepted
between the upper curves and their
respective bases rapidly diminish and
disappear, while those which correspond
to the bases Y Y', and O O' attain their
maxima. Hence it appears, that by
this increase of thickness the reflection
of the bluish lights is subdued, and the
yellow and orange tints appear. When
the parallel arrives at 3, the line r a re-
presenting the red light attains its max-
imum, the blues and violets altogether
disappearing. Here the reflected tint
will be red.

The first white ring which appeared
between the lenses was succeeded by a
yellow which passed through an orange
into a red one. The increased thickness
of the air between the lenses, in receding
from the centre, accounts for this suc-
cession of colours as explained above.

As the parallel to A Y moves towards
4, 5, the lines representing the quantities
of the red lights reflected gradually
diminish and disappear, the indigo being
on the increase, and the violet at its
maximum. Hence the light reflected
at this thickness will have a violet hue.
Such is, in fact, the first ring of the
second series between the lenses (54).
As the parallel arrives at 5, the violet is
on the decrease, the blue attains its
maximum, and the reddish tints vanish.
The colour reflected will, therefore, be
blue, and corresponds to the blue ring
in the second series between the lenses.

When the parallel to AY arrives at 6,
the lines representing the reds and violets
disappear, the blue and green are only
partially reflected, and the yellow is at its
maximum. The partial reflexion of the
blue and green mixed with the intense
yellow, produces a yellowish green. This
thickness of the air is that at which the
green ring is reflected in the second
series of rings between the lenses. After
the parallel passes 6, the yellow predo-
minates, the other lights being but
faintly reflected, as appears by inspect-
ing the curves. The yellowish green
last mentioned, therefore, gradually
changes into a bright yellow. This
corresponds to the yellow ring of the
*;econd series.

As the parallel approaches 8, the red
Ijnd orange increase ; the greens, yel-
-, and blues nearly vanish ; the vio-

ts are copiously reflected, but the

stronger influence of the red and orange
gives the reflected light a glowing crim-
son hue. This corresponds to the last
ring of the second series between [the
lenses.

At C, the red and orange are copi-
ously reflected ; the yellow, green, and
violet are not reflected in any perceptible
quantity, but the indigo is abundant and
the blue considerable. The result of the
composition of these lights is a rich
purple of a ruddy character, which cor-
responds to the first ring of the second
series between the lenses. By following
in this manner the parallel as it moves
from A Y, we shall be able to trace dis-
tinctly the lights reflected by plates of
air of every degree of thickness, and we
shall perfectly account for the succes-
sion of coloured rings, as far as they
are observable in the experiment with
the lenses.

(62.) We have here constructed curves
to represent the quantities of reflected
light of seven distinct degrees of refrangi-
bility. But it must be remembered that
solar light consists of rays of every de-
gree of refrangibility between certain
extreme limits. To describe the phe-
nomena perfectly, there should there-
fore be an infinite number of other
curves between each pair of those
already exhibited, and with bases of
intermediate magnitude. Two such
curves having bases nearly equal, cor-
respond to lights which differ but little
in refrangibility, and which have no
perceptible difference in colour. When
the thickness of the plate of air becomes
considerable, the number of repetitions
of the bases of two curves, whose bases
have a very small difference, will be so
great, that the points of contact will be
separated by a considerable interval.
Accordingly, it must at length happen,
that when one ray is most copiously re-
flected, another of very nearly the same
colour will not be reflected at all. As
this is necessary to explain why a thick
plate of air will only reflect white light,
and that but faintly, we shall explain it
more fully.

Let Y Y', instead of representing the
interval of the fits of the yellow light,
represent that of the fits of green light
differing a little in refrangibility and
not perceptibly in colour from the light
whose fits are at the interval G G'.
Suppose that the difference between
Y Y' and G G' is the fortieth part of
G G' ; after passing through a thickness
equal to twenty repetitions of G G', the

OF NEWTON'S OPTICS.

49

highest point of the curve Y Y' will
correspond to the lowest of G G', so
that although no green light of the re-
fransribility of G be reflected, yet light
of the same colour, but of the refrangi-
bility of Y, will be reflected in abun-
dance ; so that at considerable thick-
nesses, green light, of some degrees of
refrangibility, must always be reflected.
Now, what we have to observe of green
light is equally applicable to light of all
other colours : so that it follows, that
light of all colours will be both reflected
and transmitted at considerable thick-
nesses.

(63.) Since a mixture of lights of all
colours constitutes white light, it follows
that the light reflected and transmitted by
a thick plate of air will always be white ;
but since some is transmitted, and some
reflected, neither the reflected nor trans-
mitted light will be so intense as the
lights which are reflected and trans-
mitted by very thin plates.

The recurrence of the fits of lights of
nearly equal refrangibility do not sud-
denly attain the state which we have
described. They approach it gradually.
Hence we may account for the dilute
appearances of .the colours reflected as
the thickness of the air increases, as is
perceived by observing the rings of
colour at a considerable distance from
the centre of the lenses. They become
faint by degrees, and finally disap-
pear.

(64.) Another circumstance attending
the phenomena of the rings is accounted
for on the grounds to which we havejust
adverted. When homogeneous light
was projected on the lens, the rings
appeared in much greater numbers, and
more distinctly defined than when com-

pound solar light was used. In fact, in
this case their number seemed quite
interminable. The colours were here
not diluted and neutralized by the ad-
mixture of rays of all hues, as we have
shown to be the case when white light
was projected on the lenses.

(65.) Seeing that the interval of the
fits changed with every change of refran-
gibility of the light, the problem to de-
termine the relation between the interval
of the fits and the degree of refrangibility
obviously presented itself. This, how-
ever, Newton failed to solve. He fan-
cied that the intervals of the fits of the
seven coloured lights bore an analogy
to the lengths of a string which sound
the seven musical notes.

(66.) Hitherto we have conceived the
light to fall perpendicularly on the plate
of air, and therefore to be reflected per-
pendicularly by it. If the light enter
the air obliquely to its surface, it will be
reflected at the same obliquity. In tbis
case the light is affected by the oblique
incidence in a singular manner. The
intervaj. of its fits is lengthened, and
bears a certain proportion to the length
of the interval when perpendicular.
Newton succeeded in detecting this pro-
portion, and in showing how the length
of the fit depended on the obliquity of
the light.

Although the phraseology of mathe-
matics may be considered in some de-
gree necessary to express this curious
relation, yet we hope to render it intel-
ligible to the general reader, if a mode-
rate portion of attention be given to the
following explanation. Let the line
PD (fig. 46.) represent the interval of
the fits of any species of homogeneous
light incident perpendicularly on a thin

Fig. 4 7 . Cmo « P

50

A POPULAR ACCOUNT

plate of air, having previously passed
through a plate of glass. We are re-
quired to find the interval of the fits^when
the same ray enters the air obliquely to
its surface, as in the direction A P.

With P as a centre, and P D as a
radius, let a semicircle CDC' be de-
scribed, and let C C' be considered as
the surface of the plate of air. Let P F
be the direction which the ray AP would
take, if it were refracted by passing from
air into glass. From F, draw F n per-
pendicular to C P. Continue the line
A P to the circle at E, and from E draw
E m perpendicular to C P. Divide the
interval mninio 107 equal parts, and
let o be the point of division nearest to
m. From o draw o p perpendicular to
C P, and meeting the circle in p. From
D draw D G, touching the circle at D,
and from P draw Pp, and continue it to
meet D G at T. Continue PD below
D, and take P H equal to P T. Through
H draw H K perpendicular to H P, and
continue P E until it meet it at I. Then
P I will be the interval of the fits of the
oblique ray, and P H will be the thick-
ness of a plate of air through which
the ray passes between two successive
fits.

It appears from hence that the inter-
val of the fits of an oblique ray depends
on three things ; first, the angle of obli-
quity ; second, the refrangibility of the
light ; and third, on the refracting power
of the media which bound the thin trans-
parent plate through which the light
passes. The relation by which the in-
terval depends on these elements, ap-
pears complex in the preceding explana-
tion. This complexity is, however, only
apparent, and arises from the necessity
of throwing our account of it into a
popular form. Expressed in the lan-
guage of Trigonometry and Algebra, it
is sufficiently simple.*

(67.) The manner in which Newton de-
duced this relation from observation of
the rings may easily be conceived, by
recurring to the method by which he

* It is very doubtful, as Newton does not give the
details of his observations at great obliquities, and
as the making the observations with accuracy at
obliquities beyond 75° must be a matter of great
difficulty, whether the construction is to be depended
on; particularly as the observations, as far as from 0°
to 75°, agree sensibly with the simple formula, secant
< obliquity (or, if we use the figure in the text, P H,
instead of being equal to PT, will be equal to the
distance from P to the intersection of P E produced
•with D T), This formula is consistent with the
theory of undulations. Biot does not say that he has
repeated these observations at great obliquities, and
found them to be correct. He Is evidently at a loss
to know how they were made.

discovered the interval of the fits, when
the incident light was perpendicular.
In that case, the thickness of the plate
of air, which reflected the colour in each
ring, was found by measuring the dia-
meter of the ring. When he viewed
the rings obliquely, he found them en-
larged, and, consequently, the thickness
of the air at which the light was reflected
was increased. Its increased thickness
was computed in the same manner as
before, by measuring the diameter of
the ring. By a careful comparison of
the thickness thus deduced with the
thickness which reflected the same
colour perpendicularly, he discovered
the method already explained of finding
the magnitude PH by knowing PD and
the obliquity. The thickness at which
a given tint will be reflected at a given
obliquity being found, the interval of
the fits was easily discovered. In the
case where the ray penetrated the me-
dium perpendicularly, twice the thick-
ness of the air was equal to the length
of the course of the ray in passing
through it and returning, and this, there-
fore, was the interval of the fits. But
when the ray, as in the present case,
penetrates the air obliquely, the course
of the ray in passing through it and re-
turning is more than twice the thickness
of the air. Let PH (Jig. 48) be the

Fig. 48.
P

A.

rr.

thickness of the air, and let A H be the
ray entering, and H B emerging. The
lines AH and H B taken together,
form the course of the ray within the
plate of air. These lines, therefore,
taken together, or twice A H, is the in-
terval of the fits.

In explaining the laws which govern
these phenomena, we have referred con-
stantly to a thin plate of air inclosed
between two surfaces of glass. The
presence of air, or any other material
agent, is not necessary for the produc-
tion of the effects. The lenses which
we have described (54) would exhibit
the same rings of colour if placed under
an exhausted receiver. All that is ne-
cessary to produce the phenomena is,
that two refracting surfaces should be
placed close together, so as to include
between a thin transparent space. In
passing the first surface the rays will be

OF NEWTON'S OPTICS.

51

put into fits of easy reflection and trans-
mission, and those which meet the se-
cond surface in a fit of easy transmission
will penetrate it, while those which meet
it in a fit of easy reflection will be re-
flected from it.

(68.) The interval of the fits changes
with the refracting power of the thin
medium traversed by the light. The
relation of this interval to the refracting
power was detected by Newton, and
may be exhibited as follows :

With a centre C (Jig. 49) and a radius
C A, describe a semicircle, and from any

Fig. 49.

point G, through the centre C, draw the
right line G C H. From H draw H m
parallel to B A, and meeting at m a line
E D drawn through the centre perpendi-
cular to B A. Let H m represent the
length of the interval of the fits when
the ray traverses a vacuum. To find
the length of a fit when the medium tra-
versed by the ray is water, draw C H in
that direction which a ray of light com-
ing in the direction GC would take in
passing from a vacuum, through the
surface B A, into water. Let this direc-
tion be C H', and from H' draw H'm'
parallel to B A. This line H' m1 will
be the interval of the fits when the ray
passes through water. Again, if the
medium be glass, draw C H" in the
direction which the ray G C would take
in passing from a vacuum into glass,
and H" m" is the interval of the fits.

Thus it appears that the greater the
refracting power of the medium tra-
versed by the light, the shorter will be
the interval of its fits.

Newton discovered this law by intro-
ducing transparent liquids between the

lenses, whose refracting powers were
greater than that of air. " Upon wetting
the object glasses at their edges the
water crept in slowly between them, and
the circles thereby became less and the
colours more faint, insomuch that, as
the water crept along, one-half of them,
at which it first arrived, would appear
broken off from the other half and con-
tracted into a less room. By measur-
ing them I found the proportion of their
diameters to the diameters of the like
circles made by the air, to be about seven,
to eight, and consequently, the intervals
of the glasses at like circles, caused by
the water and air, to be as three to
four*. Perhaps it may be a general
rule, that if any other medium more or
less dense than water be compressed be-
tween the glasses, their intervals, at the
rings caused thereby, will be to their
intervals caused by interjacent air, as the
sines are which measure the refraction
out of that medium into air." These
sines are the lines H m, H' m', H" m",
&c. in our explanation.

(69. ) It is remarkable, that whatever be
the matter of which the then transparent
medium traversed by the light is com-
posed, the same colours will be reflected
in the same order. This arises from the
circumstance of the fits being regulated
by the same law in all substances, one
differing from another only in the length
of the interval of the fits, and whenever
the length of the fit for any one species
of homogeneous light undergoes any
change arising from a change in the re-
fracting power of the medium, the in-
tervals for all the other species of homo-
geneous light undergo a proportional
change.

(70.) The scale for determining the
tints corresponding to different thick-
nesses of the transparent medium would,
accurately constructed, be a very exact
method, not only of ascertaining those
tints, but also the primary colours of
which they are composed. Newton has
given the following Table of the thick-
ness of air, water, and glass, which re-
flect tints corresponding to the several
series of rings described in (54). The
numbers express millionths of an inch.

* The intervals of the glasses, being in the proper,
tion of the squares of the diameters of the rings, are
more accurately as 49 to 64 ; very nearly the same
ratio.

E2

A POPULAR ACCOUNT

THE THICKNESS OF COLOURED PLATES AND PARTICLES OF AIR,
WATER, AND GLASS.

Air.

Water.

Glass.

Very Black

i

3

1 0

Black

1*

8
3

20

Beginning of Black .
Blue

2
22

I

If
I'l

Their Colours of the first Order'

White

•^s
51

3

3|

71

54

4i

Orange

3

^

51

Red

g

63

54

Violet

8i

71

Indigo

12s

9!

R>

Blue

141

10i

q11

Of the second Order t . >

15i

}U

9f

Yellow
Orange • .

16f
172

12*
13

10§
II1

Bright Red

184

13a

111

Scarlet

19*

143

12'

••'i

21

15s

13ii

22 -1

164

14i°

Blue

932

Ijli

15Tfc

25 !

18 9*

161

Yellow

27i

20'

174

Red

29

213

184

32

24

20§

Blueish Green
Green

24
35f

25£
26i

22

22|

Yellowish Green ....
Red

367
4(H

27
301

23*
26

Of the fifth Order !

jreenish Blue

46

341

29§

J

52^

39|

o4

Of the sixth Order

58|
ff

44

AQ 3

38

bo

48 f

Of the seventh Order . . .. . .<

71

531

45-J

Ruddy White

77

57|

49§

A comparison of^ this Table with the
scale in fig. 46 will give a tolerably ac-
curate notion of the reflected tints and
their composition. But" it also answers
the further purpose of measuring the
thickness of a medium too minute to be
estimated in any other way. Thus the
size of the minute parts of natural bodies
may be determined from their colours.
" If two or more thin plates be laid upon
one another, so as to form one plate
equalling them all in thickness, the re-
sulting colour may be determined. For
instance : Hook observed that a faint
yellow plate of Muscovy glass laid upon
a blue one constituted a very deep purple.
The yellow of the first order is a faint
one, and the thickness of the plate ex-
hibiting it is 4$, to which add 9, the

thickness exhibiting blue of the second
order, and the sum will be 13|, which
is the thickness exhibiting the purple of
the third order."

CHAPTER VII.
The Theory of Colours, continued.

(71.) IN explaining the theory of co-
lours, it was shown that the colours
of natural bodies arose from an apti-
tude in them to reflect the rays of
some colours rather than those of
others. The experiments and investi-
gations noticed in the preceding Chap-
ter, now enabled Newton to advance a
step further in the inquiry into the
causes of the colours of natural bodies.
He accordingly proceeds to discuss the

OF NEWTON'S OPTICS,

53

principles to which bodies owe this apti-
tude to reflect some lights rather than
others, and to trace this aptitude to the
internal constitution and essential nature
of the bodies.

A very close analogy may be ob-
served between the phenomena of re-
flection and refraction. It has been
proved that those species of light which
were most refrangible were also most
reflexible. But it further appears that
those svirfaces at which light is most
powerfully refracted have a propor-
tionately intense power of reflection.

A surface which separates glass from
air has a considerable refractive power.
That its reflective power is also great,
will be seen by looking into the side of
a prism at its base, the eye being placed
so as to view the base obliquely and the
side perpendicularly. The reflection of
light from the base will be so intense,
that every object seen in it has all the
vivid splendour of reality.

The surface which separates glass
from water has a much less refracting
power, and, accordingly, we find the
reflective power proportionately dimi-
nished. If the prism just mentioned be
laid with its base upon water, the splen-
dour of the reflection will be considerably
impaired, and the images seen in the
base will no longer have the intense
brilliancy which was observable when
the medium below the base was air. To
this cause the diamond owes its lustre.
Its refracting power is greater than that
of most other substances. In confor-
mity with the principle here laid down,
its reflecting power is proportionately
great, and we find the reflection of light
from its inner surfaces of a correspond-
ing intensity. On the other hand, if a
surface separates two media of equal
refracting powers, no refraction takes
place in passing from the one medium to
the other, and so neither do we find any
reflection. Thus, at the point at which
the lenses touched in the experiment
described in (54), no light was reflected
at whatever obliquity it was viewed.

(72.) Newton, therefore, concludes that
the reason why uniform pellucid sub-
stances, such as water, glass, or crystal,
reflect no light except from their surfaces,
is, because every part within those sur-
faces has an uniform refracting power, or
has in every place the same density. He
infers that reflection of light may always
betaken as an indication of a change
of density in the medium at that point
where the reflection is made, and that it

is a test by which a change of density
may be as certainly ascertained as by
refraction.

A ray of 'light falling on the surface
of a body in a fit of easy transmission,
enters it. 'if, after penetrating to a small
depth, it meet with a point at which
there is a change of density, and be at
the same time in a fit of easy reflection,
it will retrace its course. According to
this view, every body, how opaque so
ever it be, is transparent to a certain
depth within its surface. Experiments
sufficiently evince the truth of this pro-
position. If a very thin lamina of the
most opaque substance be suspended
before a hole through which a beam of
light is admitted into a dark room, it
will be manifestly transparent, and light
will be perceptible through it, provided
it be sufficiently attenuated.

From all this it seems not impossible
that the intimate particles or molecules
of opaque or coloured bodies, are sepa-
rated by minute pores or spaces, which
are either entirely void or filled with
some subtle material of a different den-
sity from the molecules of the body, in
the same manner as a liquid pervades the
particles of a solid substance which it
holds in solution. If we admit that light
penetrates the external surface of opaque
bodies, a fact which experience proves,we
must also admit the existence of spaces
within that body, filled by some medium
differing in density from the molecules
of the body itself ; for, without this dif-
ference of density, there could be no re-
flection, and, consequently, no opacity.

(73.) There are many experiments by
which phenomena are elicited, which
seem to support the hypothesis that the
discontinuity of parts is the cause of opa-
city. If the pores of an opaque body be
filled with any substance of nearly the
same density or refracting power, it loses
its opacity, and becomes diaphanous.
Paper and oil have nearly equal refract-
ing powers. Dry paper is opaque ; but,
when steeped in oil until its pores are in
a great degree filled with that fluid, it
acquires a proportionate degree of trans-
parency. In the same manner, linen
cloth, and many other substances of
very imperfect transparency, will have
that quality perceptibly increased by
being steeped in liquids whose refracting
power is nearly equal to their own.

On the other hand, the most trans-
lucent bodies may be rendered opaque
by extricating from their pores the
matter which pervades them, which may

54

be done by separating their parts by
mechanical division ; or the subtle per-
vading; agent may be banished by heat
or chemical action. Thus, pulverised
glass and the shavings of horn are no
longer transparent. Wet paper becomes
opaque upon evaporating the moisture
by heat ; and salts and other substances,
held in solution, lose their transparency
by precipitation.

On such grounds Newton assumes
that the parts of opaque bodies are se-
parated by interstices ; but he even goes
further. He affirms that these inter-
stices must have certain determinate
magnitudes, and there are limits within
which they cannot come. In the
experiment with the lenses (54), the
black central spot, within which the
transmission of the light was total, was
much larger than the point of contact
of the glass, and no light was reflected
until the air attained a certain thickness.
In like manner, if a soap-bubble be
examined, it will be found that as the
liquid which forms it subsides, the top
becomes so thin as to reflect no light,
and, consequently, appears black. In
order, therefore, to reflect light, the dis-
tances between the particles of a body
must be such as to allow the pervading
medium, whatever it be, to exist in suffi-
ciently thick parts ; otherwise the dis-
tances between the particles would be
less than half the interval of the fits, and
the ray, throughout the whole thickness
of the medium, would be in a fit of easy
transmission.

This Newton considers to be the case
with water, glass, translucid stones,
and other transparent substances. That
there are spaces between their particles
is beyond doubt. But those spaces, re-
latively to the pervading medium, are
circumstanced in the same way as the
air immediately around the point of con-
tact of the lenses ; they are too small to
reflect light, and the ray never has space
enough in any of them to pass from a
fit of easy transmission into one of easy
reflection, and consequently it is wholly
transmitted.

(74.) This theory derives some further
support from the effects of the solution
of solid substances in liquid. The par-
ticles, by a subdivision, inconceivably
minute, are brought into closer con-
tact; and, although the solid after
solution, fills a greater space, and the
aggregate of the interstices of its parts
must therefore be increased, yet the
subdivision is so minute that every sin*

A POPULAR ACCOUNT

gle interstitial space is very much dimi-
nished. This diminution is carried so
far, that it is less than the interval be-
tween a fit of easy transmission and one
of easy reflection, and consequently the
light is transmitted, and the solution is
diaphanous.

(75.) There are many analogies which
support the opinion, that the colours of
natural bodies are produced upon the
same principle, governed by the same
laws, and attended with effects in all
respects the same as those produced by
their transparent plates, as explained
in the last chapter. One of these thin
transparent plates, provided it be of uni-
form thickness, will, in every part, re-
flect the same colour. Let it be slit into
threads, or broken into fragments ; every
piece will separately reflect the same
colour as the entire plate. A heap of
these fragments will still appear of the
same colour ; and if a natural body be
considered as amass of such fragments,
it must, on the same grounds, exhibit
the same colours.

It will be recollected, that thin plates,
viewed at different obliquities, shifted
their tints, the length of the fits, and,
therefore, the species of the light re-
flected, changing with every change in
the obliquity of the light to the plate.
In conformity with this, many natural
bodies vary their hues with the point
from which they are viewed. The plu-
mage of birds, and more especially of
the peacock, presents a splendid in-
stance of this. Changeable silks, and
almost all dyed clothes, are attended
with a similar effect. A spider's web,
when finely spun, exhibits colours. All
these phenomena bear a close and ob-
vious analogy to those which occupied
our attention in the last chapter.

(76.) But, further, some substances
reflect one colour and transmit another,
like the air between the lenses in (54).
Examples of this will be found in leaf
gold, some species of stained glass, and
the infusion of lignum nephriticum.
The coloured powders used by painters
change their tints by mere grinding.
The parts being thus reduced in size,
reflect various colours, in the same
manner as a transparent plate would by
reducing its thickness.

No example more strongly supports
Newton's reasoning than those which
are so common in chemical experiments,
in which various changes of colour are
produced by the admixture of different
species of liquids. Two liquids, each of

OF NEWTON'S OPTICS.

55

which is perfectly transparent, will fre-
quently, when mixed, be strongly co-
loured. This may be conceived to arise
from the mutual action of the corpus-
cles of the two liquids : they either create
or destroy the connexion of particles,
and cause the molecules to swell or
shrink, whereby not only their bulk but
their density may be changed.

This change of the magnitudes of the
particles and their interstices is an ade-
quate cause for the change of colour,
according to Newton's theory.

It is observable in experiments such
as those described in the last chapter,
that when the thin transparent medium
which reflects the colours is more dense
than the surrounding medium, the co-
lours are much more vivid and brisk,
and less liable to shift their tints with a
change in the direction in which they
are viewed. This will be evident, by
comparing the effect of thin films of
glass or mica, the ambient medium
being air, with that of a thin plate of air,
inclosed between two pieces of plate
glass. From this circumstance, Newton
supposes that the parts of bodies on
which their colours depend must be
denser than those which fill the remain-
ing spaces within the surface of the
body. For the colour of a body being
generally produced by light reflected at
all angles, if the particles reflecting
colour were rarer than the surrounding
medium, all tints would be reflected at
different obliquities, and so the body
would appear white or grey. But if the
parts reflecting colour be more dense
than the others, the lights reflected nearly
perpendicular will, by predominating
over the oblique ones, give their own
colour to the body.

(77.) All the preceding reasoning led
Newton directly to the inference, that the
colour of a body furnishes a means of
determining the magnitude of the ulti-
mate transparent corpuscles of which it
is composed. Many circumstances render
it probable, as Newton conceived, that
the parts of bodies have, for the most
part, the same refractive density as those
of water or glass. This being assumed,
it follows that the diameter of the cor-
puscle of a body, which has any pro-
posed colour, is equal to the thickness
of a plate of water or glass, which would
reflect the same colour, and which may
always be determined by the Table, p. 52,
and by the Scale, fig. 46. Thus, if it be
desired to know the diameter of a cor-
puscle, which, being of equal density

with glass, will reflect green of the third
order; 16J expresses the number of
millionth parts of an inch in it.

(78.) The received opinion respecting
the cause of reflection of light was, that
its particles impinged upon the hard sur-
face of the solid parts of the reflecting
body, and were reflected by the reaction
of that surface, in the same manner as
an elastic sphere is reflected when it
strikes a hard plane. This opinion
seemed to be countenanced by the law
of reflection of light, which is the same
as that which regulates the reflection of
all elastic spheres impinging upon hard
surfaces. Newton, however, held that
light does not impinge on the solid and
impervious parts of bodies, but is re-
flected without having encountered these
surfaces. He shows many difficulties
which attend the hypothesis, that light
impinges on the solid parts of bodies in
reflection, among which the following
may be noticed.

If light, after passing through glass,
be reflected by the surface, the ambient
medium being air, the reflection will be
stronger than it would be if the light
had passed through the air and been
reflected by the surface of the glass,
and much stronger than if the ambient
medium had been water. This effect
would lead to the conclusion, that the
particles of air repelled the light with
greater force than those of either water
or glass. But it is still more unac-
countable on this theory, that upon
withdrawing the air from about the
glass by an air-pump, the reflection of
the light from the surface which sepa-
rates the glass from the vacuum is still
stronger than in either of the former
cases.

If the light, after passing through the
glass, be incident on the surface more
obliquely than at an angle of 41°, it will
be wholly reflected ; but, at all obliqui-
ties less than this, it will be transmitted.
Hence, if we admit that the impact on
solid parts is the cause of reflection,
and therefore the penetration of in-
terstices the cause of transmission, we
are compelled to suppose, that at obli-
quities greater than 41°, all the parts of
the light encounter solid parts of air,
but at less obliquities they all pass into
pores or interstices ; and, on the other
hand, that in passing through air, and
impinging upon glass, it never fails to
meet pores enough to transmit nearly
the whole of it, even at the most oblique
incidences, Some may suppose that

A POPULAR ACCOUNT

the light, after passing through the glass,
is reflected— not by the parts of air
contiguous to the glass, but by the last
particles of the glass itself. This suppo-
sition, however, is scarcely intelligible,
and, besides, is contradicted by wetting
the surface of the glass, which immedi-
ately affects the reflection, which could
not be the case, if the reflection were
made by the parts of the glass.

It has been shown that two kinds
of homogeneous light — say blue and
red — might be incident at the base
of a prism, with the same obliquity,
and so that the red should be wholly
transmitted, and the blue wholly re-
flected. In this case it seems incon-
ceivable, that the red rays should every-
where fall on pores, and the blue rays
on solid parts. A similar difficulty at-
tends the explication of all the pheno-
mena of thin plates explained in the
last chapter, for we must there suppose,
that the transmitted rays, being all of
certain species, meet only with pores
while the rays of the complemental co-
lours encounter nothing but solid parts.

Still greater difficulties attend the
explanation of the phenomena of reflec-
tion of light at polished surfaces, ac-
cording to this theory. The substances
used in polishing glass and other sur-
faces, only bring the roughness of the
surface to a very fine grain, so that the
inequalities become too small to be
perceptible either to sight or touch.
Hence, since innumerable inequalities
exist, irregularly distributed over the
most polished surface, it follows, that if
the light were reflected by impinging on
the surfaces of the solid parts, it would
be as irregularly scattered about by such
a surface as by the roughest.

(79.) Newton, therefore, considers that
light is reflected — " not by a single point
of the reflecting body, but by some
power of the body which is evenly dif-
fused over its surface, and by which it acts
upon the ray without immediate contact."

By a comparison of the refracting
powers of substances with their densi-
ties, Newton traced an evident con-
nexion between them. In all bodies,
whether solid, liquid, or aeriform, ex-
cept such as be of an unctuous or sul-
phureous nature, the refracting powers
are very nearly in the proportion of
their densities. A body of an unctuous
or sulphureous nature, compared with
one which is not so, has a greater rela-
tive refracting power than the propor-
tion of their densities indicates. But

even in this case, bodies of this nature,
when compared with one another, have
refracting powers in a ratio not very re-
mote from that of their densities.

The sulphureous principle, therefore,
seems to increase the action of bodies
on light ; and Newton conjectures that
this principle, existing more or less in
all bodies, may be the cause of all the
phenomena of reflection arid refraction.
" And as light congregated by a burn-
ing glass acts most upon sulphureous
bodies to turn them into fire and flame,
so, since all action is mutual, sulphurs
ought to act most upon light. For that
the action between light and bodies is
mutual, may appear from this consi-
deration ; that the densest bodies which
refract and reflect light most strongly,
grow hottest in the summer sun, by the
action of the refracted and reflected
light."

This last observation of Newton has
been since proved incorrect. The most
dense bodies, if transparent and colour-
less, do not grow hot. However, the
theory which has formed the subject
of this chapter, and which has been
given substantially as Newton himself
has left it, must be received as matter
of hypothesis. To discuss its merits,
and compare it with subsequent expe-
riments and theories, would greatly
exceed the limits of the present treatise.
Our object here is to give a succinct
popular account of " Newton's Optics,"
and not to discuss the relative merits of
the different theories which have been
advanced to represent the phenomena.

CHAPTER VIII.
Thick transparent Plates.

(80.) THE colours produced by the
reflection and transmission of light by
thin transparent plates are not pecu-
liar to these, but may also be exhibited,
under certain circumstances, by plates
of considerable thickness. By a series
of experiments with concave glass re-
flectors, silvered on the convex side,
Newton made these phenomena mani-
fest, and applied his hypothesis of the
fits of easy reflection and transmission
successfully, in accounting for them,
and in reducing them to the laws which
were explained in the sixth chapter.

Before we proceed to describe these
experiments, it is necessary to remind
the reader of the several effects which
a beam of light undergoes when it en-
counters the surface of a transparent

OF NEWTON'S OPTICS.

57

medium. A part is reflected, according
to the regular law of reflection, and a part
entering the medium is regularly re-
fracted. But besides these, which may be
considered as the principal parts, there is
still another portion which is scattered
about in all directions around each point
of incidence, both within and without
the refracting medium. It is this portion
of the incident light which serves to
render the surface visible to an eye
placed in any situation with respect to
it. For if no light proceeded in any
other directions than those of the regu-
larly reflected and refracted rays, the
surface would be invisible to every eye,
except to one placed in these directions.
It is to the portion of light scattered
thus irregularly that the phenomena,
which we are now about to describe,
are to be traced.

Newton procured a concave glass
speculum or mirror, silvered on the con-
vex side, and in every part exactly a
quarter of an inch thick. This he placed
in a darkened chamber opposite the
window, through which a beam of light
was admitted. The radius of the sphere on
which the mirror was ground was about
six feet, so that the centre of the mirror
was at that distance from its surface.
At the centre of the mirror he placed a
white opaque paper screen, having a
small hole in it, and so adjusted the
mirror and the screen, that the beam of
light, passing through the hole in the
screen, should fall upon the mirror, and
be reflected back from the mirror to the
same hole.

Let AB (fig. 50) be the mirror, C D

Fig. 50.

Ihe screen placed before it, so that the
hole O shall be at the centre of the mirror.
The rays of light, passing from O to the
mirror, will fall perpendicularly on it,
and consequently all the regularly re-
flected rays will return to O ; and as far
as these rays are concerned, the side of
the screen presented to A B will be as
dark as if no light passed through 0 to

the mirror. Nevertheless, upon observ-
ing the screen presented towards A B,
Newton found this not to be the case.
He observed upon the paper " four or
five concentric irises or rings of colours
like rainbows, encompassing the hole.
These rings, as they grew larger, became
fainter and diluter, so that the fifth was
scarce visible. Yet, sometimes, when
the sun shone very clear, there appeared
to be faint lineaments of a sixth and a
seventh."

There was a very obvious analogy be-
tween these rings, and those exhibited
between the lenses in 54. The co-
lours did not succeed each other in
the order of the reflected rings, but in
that of those which were in that case
transmitted. In the centre, at and
around O, was a white round spot.
This was skirted or fringed with a dark
grey, which insensibly brightened into a
violet ring. This was followed by a
circle of indigo, one of pale blue, a
greenish yellow, a vivid pure yellow, and,
finally, a red, which deepened into a
purple on Ihe outer edge. Such was the
first iris which surrounded the white
central spot.

This was encompassed by a second
series of coloured rings, of which the
first was a dark purple, the outer boun-
dary of the red in the former series.
This was followed in succession by cir-
cles of blue, green, yellow, and red.

The colours of the third and fourth
series were green and red. Those of the
fifth were so faint as to be scarcely dis-
tinguishable.

In order to trace their connexion with
rings exhibited by air inclosed between
the lenses, Newton now measured their
diameters, and found exactly the same
proportion subsist among them as pre-
vailed among the rings seen by trans-
mission in that case. In order to make
the relation of these phenomena still
more manifest, he now transmitted
through the hole O, not a beam of com-
pound solar light, but of pure homoge-
neous light, obtained in the usual way
by a prism. Transmitting through O in
succession each of the colours, he ob-
served the effects on the screen. Rings
now only appeared of that colour which
fell upon the speculum. If the specu-
lum were illuminated with red, the rings
were totally red with dark intervals ; if
with blue, they were totally blue, and so
of the other colours. With whatever
colour they were illuminated, the same
proportion subsisted among their dia-
meters as was observed with the lenses.

58

A POPULAR ACCOUNT

But if the colour was varied, they va-
ried their magnitude. In the red they
were largest ; in the indigo and violet
least ; and in the intermediate colours,
yellow, green, and blue, they were of
several intermediate sizes, answering to
that colour.

Hence it was plain, as in the case of
the lenses, that when the speculum was
illuminated with white light, the several
colours of the rings were produced by
the superposition of the rings which
were separately formed by projecting
successively on the speculum the several
component elements of white light. The
several tints of the rings produced by
the white light admitted, in this case, of
being found by such a scale as was ex-
plained in 61.

The diameters of the corresponding
rings of different colours varied with the
refrangibility of the light, and varied
exactly according to the same propor-
tion, as in the case of the rings seen
between the lenses.

In order to ascertain whether the co-
lours seen upon the screen were the
mere effects of light and shade, and not
to be attributed to the lights proceeding
from the mirror, Newton placed his eye
where the rings appeared plainest, and
directing his view towards the mirror, he
beheld the speculum all tinged over with
waves of colours, like those seen be-
tween the lenses ; and, like these, the
rings swelled and contracted as they
were viewed more or less obliquely by
moving the eye from or towards the
centre of the speculum. A bystander,
during this experiment, observed upon
the eye of the observer the same co-
loured light as he perceived in the spe-
culum.

On comparing the manner in which
these rings were produced with that in
which the like phenomena were caused,
as described in Chapter VI., the only
differences which are observable are the
thickness of the glass, and the posterior
surface of it being silvered. The thin
transparent medium, forming a soap-
bubble, has the same figure as the spe-
culum used in the present case, with
this difference, that the colours are
seen on the convex side, whereas, in the
present, they are seen in the concave
side. This circumstance, however, so
far from impairing the analogy, renders
it more perfect when the order of co-
lours is considered, for the rings are not
those seen by reflexion in thin trans-
parent ^mediums, but by transmission.
Hence, if the soap-bubble were viewed

upon the concave side by an eye placed
within it, the colours would be identical
with those exhibited by the concave spe-
culum in the present case.

To ascertain whether the silvering
upon the back of the speculum had any
part in producing the phenomena of the
rings, Newton tried the effect of a simi-
lar concave glass, without silvering.
The result was the production of the
same rings, but with more faint colours,
owing to the reflecting power of the
second surface being diminished by the
want of silvering. It therefore appeared
that the circumstance of the back of the
speculum being silvered, had no other
effect than that of increasing the inten-
sity of the colours.

Notwithstanding the identity of the
two phenomena, as well as the complete
similitude which existed between the
manner in which they were produced,
Newton did not feel himself warranted
in applying the theory of the fits of
easy reflection and transmission in ac-
counting for them, until he established
the fact that the two surfaces of the glass
were indispensable for their production.
Were it possible for a single surface, i. e.
the concave surface alone, to produce
the phenomena, this theory would have
been quite inapplicable, and altogether
inadequate to explain them. To reduce
this question to the test of experiment,
Newton procured a concave speculum
of polished metal, which reflected only
from one surface. On presenting this
to the light, in the same manner as the
glass speculum, no rings were produced.
This result was decisive of the point, and
proved that the two surfaces were neces-
sary to the production of the rings.

It was plain that the light, regularly
reflected from the speculum, had no part
in these phenomena; for, as all the in-
cident light radiated from the centre of
the sphere, it fell perpendicular on the
speculum, and was therefore reflected
back perpendicularly to the same centre.
The rings, therefore, must owe their ex-
istence to the light irregularly scattered
by the surfaces of the glass. By the
experiment already mentioned to have
been made with the metallic speculum,
it appeared that they could not be pro-
duced by the light scattered by the irre-
gular reflection of the first surface alone ;
for, if that were possible, the metallic
speculum, by the surface of which the
light was thus scattered, would have ex-
hibited them. The cause of the pheno-
mena was, therefore, to be looked for in
the light irregularly refracted by the

OF NEWTON'S OPTICS.

59

glass, and reflected by the posterior sur-
face. The rays thus irregularly re-
fracted by the first surface fell upon the
second surface at various obliquities,
and were reflected by it regularly and ir-
regularly, so as to return through the
first surface at various obliquities.

We must here call the recollection of
the reader to a property of thin plates
already mentioned. A plate of a cer-
tain thickness reflects or transmits a
certain colour, the rays of light being
perpendicular to it. The rays being still
perpendicular, every change of thickness
will produce a corresponding change of
hue in the reflected and transmitted lights.
The very same effects, the same shifting
of tints, which is thus produced by a
change of thickness, the rays remaining
perpendicular, may also be produced by
changing the direction of the rays, the
thickness of the plate remaining unva-
ried. Thus a plate of a given thickness,
at a certain obliquity, will transmit red
light, at another yellow, at another green,
and so on. Also, if a ray of a particu-
lar colour gradually increases its obli-
quity from the perpendicular direction,
it will be alternately reflected and trans-
mitted for many successions.

The glass mirror used in the experi-
ment to which we now refer was, in
every 'part, of exactly the same thick-
ness ; therefore the rings of colour could
not, as in the case of the lenses, be
ascribed to a regular variation of thick-
ness. The light, however, reflected irre-
gularly by the silvered surface of this
speculum, encountered the first surface
after reflection at various obliquities.
Those rays which met the first surface
at obliquities proper for transmission,
were transmitted to the screen, where
they depicted their proper colour, and
those which fell upon it at other angles
were intercepted. Thus, suppose a ray
of homogeneous red light radiated from
the centre to the speculum, allj the re-
gularly reflected rays returned to the
centre. Those which being irregularly
reflected by the second surface met the
first surface at obliquities proper for
transmission, were propagated in cor-
responding directions to the screen,
and they produced a red tint. These
rays being regularly disposed in circles
round the axis of the speculum, de-
picted circles of red light on the screen,
and the rays returning to the first sur-
face at intermediate obliquities not
being in fits of transmission, were in-
tercepted, and thus caused the dark

circles between the luminous red ones
on the paper. What we have here said
of red light may be equally applied to
homogeneous lights of other colours ;
and, hence, will easily be collected the
cause of the various coloured rings
produced when the light which emanates
from the centre of the speculum is com-
pound solar light, this effect being
nothing more than the result of the si-
multaneous exhibition of the rings of
each colour.

(81.) Newton points out one diiference
between the effect of the speculum on
the light and that of the thin plates, de-
scribed in Chapter VII. In the latter
case the colours are produced by alter-
nate reflections and transmissions of
the light at the second surface of the
plate, after one passage through it ;
but here they pass fronf the first surface
to the second, and then return from the
second to the first, there being either
transmitted to the screen or reflected to
the silvered surface, according as they are
in fits of easy transmission or reflection.

By measuring the diameters of the
rings of the different lights, and com-
paring this with the distance of the
screen from the speculum, Newton as-
certained the obliquities at which the
lights of different colours emerged from
the speculum, and thence derived the
angles at which they were incident on
the first surface after reflection at the
second. On comparing these with the
obliquities corresponding to the lights
of different colours deduced from the
theory established in Chapter VII., he
found a perfect accordance.

(82.) In order, however, to put the mat-
ter to a more decisive test, he calculated
the effect which the thickness of the
glass which formed the speculum would
produce upon the diameters of the seve-
ral rings, and found that, according to
the theory of fits of easy reflection and
transmission, the squares of the diame-
ters of corresponding rings produced
by different mirrors should be in the in-
verse proportion of the thickness of the
mirrors. He, therefore, procured ano-
ther speculum, ground on both sides, to
the same sphere with the former. Its
thickness was •$% parts of an inch, and
the diameters of the first three bright
rings, measured between the brightest
parts of their orbits, at the distance of
six feet from the glass, were 3, 4J, 5£
inches. Now, the thickness of the other
glass being £ of an inch, was to the
thickness of this as i to ft, or as 62 to

GO

A POPULAR ACCOUNT

20, or 31 to 10, or as 310,000,000,
100,000,000, which numbers are very
nearly as the squares of 17,607 and
10,000, and in the proportion of the first
of these to the second, are the diameters
of the bright rings made by the thinner
glass, 3, 4i, 5§, to the diameters of the
corresponding rings made by the thicker
glass, l{i, 2f, 2i|. This perfect^accord-
ance of the phenomena with the results
of theory was considered by N ewton to be
conclusive as to the validity of his hypo-
thesis.

CHAPTER IX.

Experiments on the Inflexion of Light.
(83.) IN the account of the state
of optical science before the time of
Newton, which was given in the first
chapter, the discovery of the diffraction
or inflexion of light by Grimaldi was
noticed. This phenomenon was too
striking to escape the attention of
Newton. He accordingly instituted some
experiments with a view to the investi-

gation of this property ; but he seems not
to have had leisure or inclination to pur-
sue the subject to as great an extent, or
to obtain results so satisfactory as those
which have been discussed in the pre-
ceding chapters. In the commencement
of his third book, however, be describes
some experiments which he made, and
hazards some conjectures on the pro-
bable cause of these phenomena, which,
as well as some other matters connected
with physical science in general, and
with the phenomena of heat and light in
particular, he proposes in the form of
queries. He states that he was inter-
rupted in his investigations; that he could
not afterwards think of taking these
things into further consideration ; and as
he had not finished his design, he leaves
these queries, in order to a further search
to be made by others.

The experiments which led Grimaldi
to the discovery of inflexion were the
following. Through a small aperture
A B (fig. 51) he admitted a beam of

M I G K L

light into a dark room. He observed
that the light thus admitted, spread itself
in the form of a cone, and illuminated a
large portion of a screen C D, held per-
pendicular to its direction, and at some
distance from the hole, the illuminated
part of this screen increasing with the in-
crease of distance. When an opaque
body E F was held in the light so as to
cast a shadow on the screen C D, this
shadow was found to be much larger
than it would have been if the light had
passed the edges of E F in right lines.
From B draw the straight line B F G
and from A the line A E H. Also from
A draw the right line A F I, and from B
the right line BEL. Now, if the light
passes the edges of E F in right lines,
it. is plain that the space G H will be
altogether deprived of light, and that
from the limits G H to I and L respec-
tively there will be a partial orpenumbral
illumination ; but that beyond the limits
I L,the illumination of the screen will be
as complete as if the opaque object E F
were not present. This was, however,

00, IV

])

found not to be the fact, for the shadow

extended beyond I and L to wider limits,

as M and N.

Beyond this shadow. Fig. 52.

and skirting it, Grimaldi

observed three coloured

fringes, the broadest and

brightest of which was

next the shadow. If X

(Jig. 52) represent the

edge of the shadow, the

space N N was blue, M

was colourless, and O O

red. In the second fringe, " JN" oq, RT V
Q Q was faint blue, P colourless, and
11 R red. This fringe was narrower than
the first and more faint. The third fringe
T T, S and V V, was similar to the other
two, but still narrower and fainter.

"When the opaque body has a salient
angle as D (fig. 53), the fringes were
arched round it, and at a re-entrant
angle they intersected each other as at C.
Newton repeated and varied these ex-
periments of Grimaldi. He pierced a
plate of lead with a pin, making a hole

OF NEWf ON'S OPTICS.

61

in it, the diameter of which was the
42nd part of an inch. Through this hole
he admitted a beam of the sun's light,
and found that the shadows of hairs,
pins, straws, and such slender sub-
stances, appeared, when received upon
a screen, to be much larger than
they would be if the light passed their
edges in unbroken straight lines. A
human hair, the breadth of which was
the 230th of an inch, held at 12 feet from
the hole, cast a shadow upon a screen
held at 4 inches from it, which was the

T.

GOth part of an inch broad, that is, above
four times broader than the hair. Like
effects were observable with other opaque
bodies, and at all distances.

These effects were found to be entirely

independent of the refractive power of

the air, or of the nature of the body whose

shadow was formed. When the hair was

surrounded with moisture, or inclosed

with a liquid between glass planes, the

effect was the same. The scratches

made on glass, and the veins in plates

of that material, produced shadows of

the same kind. Thus whatever was the

origin of this effect, it was evident, that

it did not proceed from the refractive

powers of bodies, nor on any quality

connected with the nature or properties

of bodies of particular species.

The way in which Newton conceived
the rays to be affected in passing the
body may be thus explained. Let X
(fig. 54) be a section of the hair at right

angles to its length. Let AD, B E, C F,
KN, LO, MP, be rays of light ap-
proaching the hair in parallel directions.
The ray A D is deflected at D in the di-
rection D G, and falls upon the screen at
G. In like manner the ray K N, at the
same distance below the hair, is deflected
in the direction N Q, meeting the screen
at Q. Newton supposes that the force
which deflects the light diminishes as the
distance from the hair increases. Con-
sequently, the rays B E and L O will be
less deflected than AD and K N, and
their deflected directions E H and O R
will cross those of the nearer rays, if the
screen be sufficiently distant from the hair.
In like manner, the more distant rays
C F and M P will be still less deflected,
and will also cross both the last rays.
This continues until the distance of the
ray from the hair becomes so great, that
the deflecting power is lost, and the rays
proceed in straight lines to the paper as
T I and V S. "These rays at I and S
bound the shadow, and at the hair in-
clude within them the deflected rays, all

of which they must cross between the
hair and the screen.

The continued intersections of the de-
flected rays with each other between the
hair and the screen form curves, which
are concave towards the shadow, and the
distance between which increases as
the distance from the hair increases. It
is evident also that the illuminated space
on the screen, immediately beyond the
boundaries L S of the shadow is exposed
to the light of the deflected rays between
T I or V S and the hair, as well as to the
direct rays which proceed without T I
and V S and parallel to them.

The shadows of bodies, such as
metals, stones, glass, wood, horn, ice,
&c., exposed to light in this way, were
skirted with three coloured fringes. The
colours observed by Newton in these
fringes were as follows: the first or
innermost fringe was violet, deep blue,
light blue, green, yellow, and red. The
second and third fringes which imme-
diately succeeded this were blue, yel-
low, red ; but their colours were faint.

63

A POPULAR ACCOUNT

Upon measuring the breadths of
these fringes at their brightest parts, and
also the breadths of their intervals,
Newton found them nearly in the same
proportion, and so that the fringes and
intervals taken in regular succession
were as the following quantities :

i, L-±-±-L

V 2 V3 V4 V 5

His next experiments were on the
shadows produced by the edges of sharp
knives placed across the aperture through
which the light was transmitted. The
light admitted between the edges being
received upon a screen at some distance,
the edges were moved towards one ano-
ther, until their distance did not exceed
the four hundredth part of an inch. The
light upon the screen now parted in the
middle, and formed two parallel lines.
The intermediate shadow was so black
and dark that all the light which passed
between the knives seemed to be bent,
and turned to the one side or the other.
As the edges approached, the shadow
grew broader, and the lines of light nar-
rower, until at length, when the edges
were in actual contact, the light wholly
vanished. This experiment Newton
considered conclusive of the greater de-
flection of the rays which were nearer
the body.

From these experiments, he concluded
that the light which formed the first
fringe passed the edge of the knife at a
distance not less than the eight hun-
dredth part of an inch ; that the light
of the second fringe passed the edge of
the knife at a greater distance than that
of the first, and the light of the third
fringe at a still greater distance.

From these and other circumstances,
Newton concluded that the distances at
which the light forming the fringes
passed the knives are not altered by the
approach of the knives, but that the
angles at which the light is inflected
are increased by their approach ; the
knife which is nearer each ray deter-
mining the direction in which it is bent,
and the other knife increasing the deflec-
tion.

When the rays fell very obliquely
on the ruler, at the distance of the third
of an inch from the knives, the dark lines
between the first and second fringes
bounding each shadow, intersected each
other at the distance of the fifth of an
inch from the termination of the light.
Hence Newton computed the distance
between the edges, at the concourse of

these lines, to be the hundred and six-
tieth part of an inch. Since this con-
course is in the middle of the light which
passes between the edges, it follows that
one-half of the light passes each edge
at a distance not greater than the three
hundred and twentieth part of an inch.

Upon increasing the distance of the
ruler from the knives, he found that the
distance of the concourse of the dark
lines before mentioned, from the termi-
nation of the light, was more than the
fifth part of an inch. Hence it appeared
that the light, which in this case passed
between the knives at the concourse of
the dark lines, passed at a greater dis-
tance from the edges than the hundred
and sixtieth part of an inch.

From these experiments Newton
concluded that " the light which makes
the fringes, is not the same light at all
distances from the knives; but when
the ruler is held near the knives, the
fringes are made by light which passes
the edges at a less distance, and is more
bent than that which forms the same
fringes at a greater distance." It will
be perceived that this is consistent with
the explanation of the phenomena of in-
flection already given.

When the shadows were received upon
paper at a great distance from the
knives, the fringes assumed the form
of hyperbolic curves, being nearly
straight where the distance between the
edges was considerable, but bending
into arches after intersecting. Let C A,
C B,y7j»\ 55, represent the projections of
the edges of the knives upon the paper.
The entire light would fall within the
angle A C B, were there no inflection.
Through C draw D E equally inclined
to CA and C B. The curve eis re-
presents the boundary of the shadow of
the blade A C ; fk t represents the dark
line which separates the first and se-
cond fringe, and glo the dark line
which separates the second and third
fringe. The lines xip, y kg, zlr, are
similarly related to the shadow of the
other edge.

The two systems of curves, which are
perfectly similar, intersect at the points
i, k, I ; so that the shadows of the edges
are marked by the lines et>and#t/?,
until the intersection of the fringes,
and then each of those lines crosses the
fringe corresponding to the other edge.
Then those lines cross the fringes, dis-
tinguishing them from another light
which begins to appear at *, and illu-
minates the triangular space ip DEs.

OF NEWTON'S OPTICS.

63

Fig. 55.

Newton now repeated the first ex-
periment described in page 219; but, in-
stead of using compound solar light,
he used homogeneous light, obtained by
a prism placed behind the hole through
which the light was transmitted. He
found the shadows of all bodies held in
this light bordered with fringes, not as
before of different colours, but of the
colour of the transmitted light. Those
made by the red light were largest, and
those made by the violet, least; the in-
termediate colours having intermediate
breadths.

From these experiments he inferred
that the rays which formed the fringes
in the red light passed by the body at a
greater distance than those which formed
the violet or any of the intermediate
colours. So that the action of the body
on the less refrangible rays at a given
distance was equal to its action on the
more refrangible rays at a less distance,
and thus occasioned the tints of the
fringes without changing the colour or
properties of any component part of the
solar light.

Such was the state to which Newton's
experimental investigations had arrived,
when they were unfortunately inter-
rupted. In the queries, however, an-
nexed to this book, and which have
been already alluded to, he throws out
some suggestions for the consideration of
future inquirers. The following queries
relate to the subject with which we have
just been engaged.

1 . Do not bodies act on light at a dis-
tance ; and by their action bend its rays ?
and is not this action strongest at the
least distance?

2. Do not the rays which differ in re-
frangibility differ also in flexibility? and
are they not, by their different inflexions,
separated from one another, so as, after
separation, to make the colours in the
three fringes above described ? and after
•what manner are they inflected to make
those fringes ?

3. Are not the rays of light in passing
by the edges and sides of bodies bent
several times backwards and forwards
with a motion like an eel ? and do not
the three fringes of coloured light above-
mentioned arise from three such bend-
ings?

4. Do not the rays of light which fall
upon bodies, and are reflected or re-
fracted, begin to bend before they arrive
at the bodies ; and are they not reflected,
refracted, and inflected by one and the
same principle, acting variously in va-
rious circumstances ?

The theory hinted at in these queries
is sufficient to represent the principal phe-
nomena of inflexion ; but it must be con-
fessed that the undulatory theory affords
rather a more satisfactory generalization
of those complicated effects, and has been
more generally adopted in the investiga-
tions of modern philosophers. Neither the
material nor undulatory theory can be
said to be satisfactorily established ; but
it would seem that every phenomenon
which can be brought under the former,
can also, with equal facility, be explained
by the latter ; while there are some known
effects in strict accordance with the lat-
ter, which cannot, without great difficulty
and the introduction of gratuitous hy-
pothesis, be accounted for by the former.
For the most part, however, the lan-
guage of either may be translated into
the other.

To discuss the question respecting the
experiments which have been just de-
scribed, and the inferences drawn from
them, it would be necessary to refer to
the original experiments and reasoning
of Grimaldi, and the later investigations
of Dr. Young and Fresnel. Such de-
tails are, however, foreign to the objects
of the present treatise.

The remaining queries which termi-
nate this book relate to the probable
connexion and causes of heat and light,
the sense of vision, the cause of gra-
\itation, and other subjects in phy-

(54

A POPULAR ACCOUNT

sics, which cannot properly find a place
here. To this, however, we must make
exception of that (the 26th) in which
he points out the polarity of light, to
the merit of having first suggested which
he has an undoubted right. " If the
planes of perpendicular refraction of one
piece of Iceland spar be at right angles
with those of another, the rays which
are refracted after the usual manner by
the first crystal will be all refracted
after the unusual manner in passing
through the second crystal ; and the rays
which are refracted after the unusual
manner in passing through the first
crystal will be all of them refracted
after the usual manner in passing
through the second." Hence Newton
concluded, that the rays have sides, pos-
sessing different properties, in virtue of
which they will be differently refracted,
according to the direction in which the
crystal is presented to them. If the
ray be successively transmitted through
two crystals, and is placed in the same
manner with respect to the planes of
both, it will be refracted in the same
manner by both ; but if that side of
the ray which looks towards the coast
of unusual refraction in the first crys-
tal be at right angles with that side
of the same ray which looks towards
the coast of unusual refraction in the
second crystal, the ray shall be re-
fracted after several manners in seve-
ral crystals. There is nothing more
required to determine whether the rays
of light, which fall upon the second crys-
tal, shall be refracted after the usual or
after the unusual manner, but to turn
about this crystal, so that the coast of
its unusual refraction may be on this
or on that side of the ray. And there-
fore every ray may be considered as
having four sides or quarters, two of
which opposite to one another incline the
ray to be refracted after the unusual man-
ner, as often as either of them are turned
towards the coast of unusual refraction,
and the other two, whenever either of
them are turned towards the coast of
unusual refraction, do not incline it to
be otherwise refracted than after the
usual manner. The first two may
therefore be called the sides of unusual
refraction.

We cannot better conclude the rapid
sketch which we have attempted to give of
this extraordinary production of human
genius, than by quoting some precepts
which have been most admirably illus-
trated in the works of this illustrious
philosopher. "The main business of

natural philosophy is to argue from phe-
nomena without feigning hypotheses, and
to deduce causes from effects until we
come to the very First Cause, which cer-
tainly is not mechanical ; and not only
to unfold the mechanism of the world,
but chiefly to resolve these and such
like questions : What is there in places
almost empty of matter, and whence is
it that the sun and planets gravitate to-
wards one another without dense mat-
ter between them ? Whence is it that
nature doth nothing in vain ? and whence
arise all that order and beauty which
we see in the world ? To what end are
comets, and whence is it that planets
move all in one and the same way
in orbits concentric, while comets
move in all manner of ways, in orbits
excentric ; and what hinders the fixed
stars from falling on one another? How
came the bodies of animals to be con-
trived with so much art, and for what
ends were their several parts ? Was the
eye contrived without skill in optics, and
the ear without knowledge of sounds ?
How do the motions of the body fol-
low from the will, and whence is the in-
stinct of animals ? Is not the sensory of
animals that place to which the sensitive
substance is present, and into which the
sensitive species of things are carried
through the nerves and the brain, that
there they may be perceived by their
immediate presence to that substance ?
And these things being rightly dis-
patched, does it not appear from phe-
nomena.that there is a Being incorporeal,
living, intelligent, omnipresent, who, in
infinite space, as it were in his sensory,
sees the things themselves, intimately
and thoroughly perceives them, and
comprehends them wholly by their im-
mediate presence to himself; of which
things the images only, carried through
the organs of sense into our little sen-
soriums, are there seen and beheld by
that which in us perceives and thinks.
And though every step in this philoso-
phy brings us not immediately to the
knowledge of the First Cause, yet it
brings us nearer to it, and on that ac-
count is highly to be valued. * * * *
And if natural philosophy in all its parts
shall at length be perfected, the bounds
of moral philosophy shall also be en-
larged. For so far as we can know by
natural philosophy what is the First
Cause, what power he has over us and
what benefits we receive from him, so
far our duty towards him as well as that
towards one another, will appear to us
by the light of nature."

UUIVEHSITY

OPTICAL INSTRUMENTS.

INTRODUCTION.

THE construction of optical instru-
ments has, in almost every instance,
originated with eminent philosophers
and mathematicians. Their gradual
perfection has been a natural result of
the difficulties which were presented to
the progress of discovery, by the in-
efficient and inaccurate means which
science possessed ; and thus, the same
great minds that have struck out and
pursued vast and splendid ideas in their
investigations of nature, have only been
enabled to follow up their own concep-
tions by applying themselves to the
practical improvement of the instru-
ments with which they had commenced
their discoveries. For instance, we are
indebted to Newton for the construction
of the first reflecting telescope that was
ever made, although the idea had been
previously suggested by Dr. Gregory.
Leuwenhoek, one of the most assiduous
naturalists of his day, earned on his
curious researches in the animal and
vegetable economy, with microscopes
made by his own hands. The late Dr.
Herschel, whose astronomical disco-
veries were the result of the profoundest
mathematical knowledge, constructed
the most powerful telescopes ever
known, which, like the Gregorian, bear
the name of their inventor. Indeed, the
ordinary makers of optical instruments
have been often men of considerable sci-
entific attainments ; — and it is from this
union of a theoretic and practical know-
ledge, that these instruments, as is the
case with almost every other important
invention and improvement, have been
conducted to their present very high
state of perfection, in an almost un-
limited adaptation to all the purposes of
science, and all the wants and luxuries
of common life.

In the limited extent of this treatise
on Optical Instruments, the chief object
will be to point out the principle of
their construction, freed from technical

mathematics, and to describe the im-
proved state in which the more im-
portant are adapted to their several
purposes. The various modes in which
many instruments, not differing in their
principle and end, are mounted, depend
upon the varying taste of the artist, the
caprice of fashion, or the demands of
luxury. Any minute details of the ex-
ternal parts of the instruments would
be therefore perfectly useless. Indeed,
the external parts of the ordinary in-
struments can be much better under-
stood by an inspection at the opticians*
shops, than in volumes of description.
So indifferent is the outward form of
the common instruments, that makers
living even in the same neighbourhood
vary as much in their mountings as the
artists of London or Paris. We shall,
therefore, confine our observations to
the essential parts in the construction,
of particular instruments, passing over
their accidental varieties of mounting ;
— in the same way that the anato-
mist speaks of the various parts of the
human body, without enquiring whether
the subject upon which he has made his
observations was dark or fair, tall or
short.

The ancients seem to have been but
little acquainted with dioptrical instru-
ments, or those by which the light is
refracted and transmitted; from their
earliest history, however, they appear
to have been conversant with the laws
of the reflection of objects from the sur-
face of water and polished metals, or
that department of optical science called
catoptrics. The first application of their
knowledge of this branch of science, with
which we are acquainted, is that of the
burning mirrors employed by Archi-
medes, a philosopher of Syracuse, about
200 years before the Christian era, who,
at the siege of that city, by Marcellus,
the Roman Consul, employed them to
destroy the besieging navy. The me-
thod by which this was probably accom-
plished is thus described by the histo-

OPTICAL INSTRUMENTS.

rian Tzetzes :— " When the fleet of Mar-
cellus was within bowshot, the old man
Archimedes brought an hexagonal mir-
ror, which he had previously prepared,
at a proper distance from which he also
placed other smaller mirrors of the same
kind that moved in all directions on
hinges, which when placed in the sun's
rays directed them upon the Roman
fleet, whereby it was reduced to ashes."
We are also informed that Proclus in
the same way destroyed the fleet of
Vitellius at the siege of Byzantium.

CHAPTER I. — Mirrors — Plane Looking-
Glasses — Concave Mirrors — Burning
Mirrors — Convex Mirrors.

(1 .) Mirrors are surfaces of polished me-
tal, or glass silvered on its posterior side,
capable of reflecting the rays of light
from objects placed before them, and
exhibiting to us their image. There are
three classes of mirrors, distinguishable
by the figure of their reflecting surface.
These are plane, concave, and convex. —
The reflection of light by either of these
mirrors observes this constant law, that
the angle which the incident ray makes
with the reflecting surface, is equal to
the angle of reflection. We may explain
this law by a figure. Let c d, (fig* 1 .)
be a section of the reflecting surface :

Fig. \.

c7

0 an object before the mirror ; and 0 a
rays proceeding to the surface in the
point a. The angle O ac which the rays
make with the surface of the mirror is
called the angle of incidence ; and the
direction in which an observer will see
the object O in the mirror at a, must be

1 a, the angle of reflection I a d being
equal to the angle of incidence O a c.
If we suppose O I to be two persons
viewing each other in a plane mirror or
looking-glass c d, the direction in which
each observeth the other at the point a is
a o or a i, but the apparent place of their
images will be behind the glass at the
point i or o, the distance behind corres-
ponding with that of their situation before
the mirror. This deception proceeds from

our common experience, which leads us
to expect the object to be in the direction
in which the rays come to our eyes, in-
stead of in the real place of the object.
The illusion is so complete, that domes-
tic animals, when viewing themselves in
a looking-glass for the first time, often
have their passions strongly excited.
When a person is viewing himself in
a Booking- glass, if he measure the size
which he appears on the glass, the
image will be one half his real magni-
tude, let his distance from the glass be
in any manner varied. For, as it was
stated above that the image appears
behind the glass exactly at the same dis-
tance as the object is before it, it must
be evident that as the mirror is half way
between him and his apparent image, it
will cut in half the cone of rays which
proceed from his image to his eye. This
is shown mfig. 2, where c c? is a section

Fig. 2.

of a plane mirror or looking-glass, and
a b the face of a person viewing himself
in it ; now rays from every part of his
face fall upon the reflector, from which
they are sent to the eye, forming a cone
of rays whose apex enters the pupil ;
consequently, as this cone is cut by the
glass halfway between the apex and its
base, which base is himself, the mea-
sure of the line ef will be half of a b.

(2.) Concave Mirrors are those whose
polished surfaces are spherically hollow.
The properties of these mirrors may be
easily understood, when we consider

Fig. 3.

OPTICAL INSTRUMENTS.

their surface as composed of an indefinite
number of small planes all of which make
a determinate angle with each other, so
as to throw all the rays to a point. (See
fig. 3.) Let a b c be a concave mirror,
and let d be the centre of curvature,
and ooo rays of light from a distant
object (the cross in the wood-cut must
be supposed at a considerable distance)
falling on the mirror at a b c, making
different angles with its surface ; these
rays when reflected at equal angles to
their incidence, as may be seen by the
dotted lines which are perpendicular,
will meet in a point /, called the focus
of the mirror, where* an image of the
object will be formed in an inverted po-
sition. The distance of this focal point
from the surface of the mirror, when the
curvature is moderate, will be equal to
half its radius d b. The importance of
concave mirrors in the construction of
reflecting telescopes, in which construc-
tion they are commonly called specula,
will be shown hereafter under that head.
The employment of concave mirrors
in collecting the heat of the sun's rays
from the whole of its' surface to a single
point, thus accumulating a very great
degree of heat, for the combustion and
fusion of various natural substances
that are infusible in the greatest heat
capable of being produced from ordi-
nary fire, may be exemplified, amongst
those of modern date, by the burning
mirror of M.deVillette. The diameter
of this metal speculum was 3 feet 1 1
inches, and its focal distance or pointy
from the surface was 3 feet 2 inches. The
composition of this metal was of tin
and copper, which reflects the light very
powerfully, and is capable of a high de-
gree of polish. When exposed to the
rays of the sun, by Drs. Hams and De-
saguliers, a silver sixpence was melted
in 7| seconds when placed in its focus ;
a copper halfpenny melted in 16 seconds
and liquefied in 34 seconds ; tin was
melted in 3 seconds ; and a diamond,
weighing 4 grains, lost iths of its weight*.
The intensity of heat, obtained by burn-
ing mirrors or lenses, will always be as
the area of the reflecting surface ex-
posed to the sun is to the area of the
small circle of light collected in its
focus ; thus, the diameter of the spot

* The burning mirror constructed by Count Buffon,
was a polyhedron, 6 feet broad and as many high, con-
sisting of 168 small mirrors, or flat pieces of looking-
glass, each 6 inches square. By means of this instru-
ment, with the faint rays of the sun in the month of
March, he set on fire boards of beech wood at 150

at the focus of
was 0.358 of an inch, and the diameter
of the mirror 47 inches ; hence the area
of these circles was as 0.3582 to 47a,
that is, the intensity of the sun's rays
was increased 17257 times at the focal
point. The loss of light occasioned in
passing through the medium of which
the lens is composed, together with that
lost by reflection from the surface of
mirrors, must, however, be deducted
from this theoretical calculation. See
Photometers.

Concave mirrors afford many curious
and pleasing illustrations of their pecu-
liar properties. For example :— when a
person stands in front of a concave mir-
ror, a little further from its surface
than its focus (or half the radius of its
concavity), he will observe his own
image pendant in the air before him,
and in an inverted position : this
image will advance and recede with
him ; and, if he stretch out his hand,
the image will do the like. Exhibitions
have been brought before the public
in which a singular deception was ob-
tained by a large concave mirror. A
man being placed with his head down-
wards, in its focus an erect image of
him was exhibited, while his real per-
son was concealed, and the place of
the mirror darkened; the spectators
were then directed to take a plate of
fruit from his hand, which in an instant
was dexterously changed for a dagger,
or some other dangerous weapon.

(3.) Convex mirrors are chiefly em-
ployed as ornaments in apartments.
The objects viewed in these are dimi-
nished, but seen in an erect position ;
the images appear to emanate from a
point behind the mirror; this point,
which is its focus, will be half the radius
of convexity behind their surface, and
is called the negative, or imaginary
focus, because the rays are not actually
collected as by a concave mirror, whose
focus is called virtual.*

feet distance. This machine had the conrenience of
burning downwards, or horizontally at pleasure, each
speculum being moveable, so as, by the means of
three screws, to be set to a proper inclination lor
directing the rays towards any given point. It thns
turned either in its greater focus or in any nearer
interval, possessing this great superiority over com-
mon burning glasses whose foci are fixed and deter-
mined. Buifon at another time burnt wood at the
distance of more than 120 feet, and silver was fused
at 50 feet. See the article Burning Apparatus in
the Edinburgh Encyclopaedia.

• The reflecting surfaces of cylinders have been
occasionally used in optical amusements, for render-
ing anamorphoses (distorted or deformed pictures) of
their proper shape when reflected from its surface.
B2

OPTICAL INSTRUMENTS.

CHAPTER II.— Lenses—Burning Lenses
— Polyzonal Lenses.

Lenses. — (4.) A lens is any" transparent
substance, as glass, crystal, water, or
diamond, having one or both of its sur-
faces curved to collect or disperse the
light transmitted by it. The lenses in
general use are made of glass, and are
usually called magnifying glasses. Glass,
however, does not possess a greater
share of the magnifying property than
other transparent substances. In fig. 4,

there are six differently shaped lenses,
shown in section. A is called a plano-
convex, from having one side flat, and
the other spherically rounded. B is a
double convex, and has both sides sphe-
rically rounded. When these sides are
unequally curved, as at C, it is termed
a crossed lens. D is a plano-concave,
having one side spherically hollow. E
is a double concave with both sides
hollow. F is a Meniscus (so called
from its moon shape), and has one side
convex and the other concave.

Fig. 4.

(5.) The passage of light, when trans-
mitted by a plano-convex lens, is shown
mfigs. 5 and 6. Let A (fig. 5.) represent

Fig. 5.

a section of the lens, and B an object at
an infinite distance, as a star (in the
figure a cross is placed to assist the
conception). Now, the lines from B to
A will represent the rays proceeding
from every part of the object B, to every
portion of the surface of the lens A, and
from the distance of the object they
will be parallel to each other. Only five
rays are drawn in the figure, to prevent
confusion ; but it should be constantly
remembered that the light strikes every
part of the lens. The first surface of
the lens next the object B is flat ; and
as all the rays fall perpendicularly on
it, and the attraction on each side of
the rays is equal, they will pass on in
their right-lined direction, till they meet
the curved surface of the lens, when
all the rays will be bent or refracted, so
as to meet in a point F, called the focus
of the lens ; but the central ray being
perpendicular to the curved surface is

not bent. This constant law of refrac-
tion, or bending, is always observed by
rays of light in their passage from one
medium to another of diiferent density,
whose surface is oblique to their direc-
tion ; and the rays of light in passing
from a dense medium, as glass, &c.
into "a less refractive medium, as air,
will be bent so as to form a greater
angle with a perpendiular to that sur-
face, than it had at first. Let the dotted
line c d be a perpendicular to the sur-
face of the lens at the point e. Now,
the angle a e c, called the angle of inci-
dence, is less than the angle F e d, called
the angle of refraction, as is shown by
continuing the ray a to i.

(6.) But whenever light strikes an ob-
lique surface in its passage from a rare
medium, as air, to a denser, as glass, it
will be refracted nearer the perpendi-
cular, as shown by fig. 6, in which
the lens is reversed. The ray of light
a eis there bent nearer the perpendicu-
lar c d, as shown by the dotted line
e r, and when this ray is transmitted by
the plane side, it will be refracted to F,

Fig. 6.

OPTICAL INSTRUMENTS.

making a greater angle with the perpen-
dicular to the flat side, as it now enters
a rarer medium.

(7.) If, instead of supposing the object
at an infinite distance, and consequently
a point, we imagine it removed to some
finite distance, a like action in a pro-
portional degree will be observed, only
the object will not be reduced to a point,
but its image will be formed at F,'whose
size is equal to the angle under which
the object would be seen without the
lens. Let a c (fig. 7.) be the object,
and b the lens: now, this object will
subtend the angle a b c from every
portion of the lens, and an inverted
image d e will be formed under the
equal angle d b e ; for whenever right
lines intersect each other, the opposite

angles are always equal. This experi-
ment may be proved by a common
convex glass lens. Suppose the distance
of the focal point is six inches, and the
lens two inches in diameter, the image
of a distant object may be seen on a wall
when the lens is held a little more than
its focal length from it ; then let the size
of the image be measured : now remove
the lens, and measure the apparent size
of the object, while the eye of the ob-
server is in its place, taking the distance
of the focus of the lens for the point of
measurement, and it will be found of
the same size in both cases. If a wafer
is made to adhere to the surface of the
lens, so as to stop a portion of the light,
the size is not altered, but the image
will be formed less bright.

Fig. 7.

(8.) When an object is placed in the
focus of a lens, the rays diverging from it
will, by the action of the lens, be rendered
parallel ; this case, however, is only the
reverse of the former, the place of the
object and image being changed. But
this astonishing circumstance will take
place : when observed on the side for
parallel rays, the objects will appear
magnified or increased in size, should
the distance of the object from the lens
be less than the eye can see it without.
Let a b (fig. 8.) be the nearest distance
at which an object can be seen distinctly

without the assistance of a lens, and b d,
the distance of the object when seen
through the lens, equal to half the
distance a b ; now the angle c bg, which
the object subtended without the lens,
is only half the angle e bf- and, there-
fore, the object will be magnified twice,
when seen by the lens ; and if the dis-
tance had been only £, j, ^o, the object
would have been magnified 3, 4, or 10
times in length and breadth, and, conse-
quently, its surface increased 9, 16, or
100 times.

(9.) Concave lenses obey the samelaws
of refraction as convex, but as the cur-
vature is reversed, the rays are bent out-
wards ; hence a concave lens will ren-
der parallel rays diverging, as may be

seen in fig. 9, where a is the object, a b
rays proceeding parallel to each other
from the object. These rays, when
transmitted by the concave lens, are
made to diverge, as if they came from

OPTICAL INSTRUMENTS.

the point/, the imaginary focus of the
lens; the perpendicular cd shews that
the rays obey the same law of refraction
as in a convex lens. It should be here
observed, that when the lens is concave,

Fig. 9.

the focal point will be on the same side
as the object, and it is termed negative,
as objects are diminished by concave
lenses.

When converging rays from a convex
lens are transmitted by a concave, they
are rendered parallel, as shewn in fig.
10, where the converging rays at c

Fig. 10.

(from" a convex lens) are brought pa-
rallel L at p, after passing through the
lens.

(1 0.) The manner in which the foci
of lenses of different curves are calcu-
lated, and how the foci of combined
lenses may be obtained, are as follows.
When the lenses are made of plate
glass, the focal distance is nearly the
diameter of the sphere from which we
may suppose a plano-convex lens to be
cut, or it is equal to twice the radius of
the circle that forms the convex surface
Of the lens. For example, if the globe of
glass is one inch in diameter, and a por-
tion is cut off to form a plano-convex
lens, the focus will be one inch, or twice
the radius of the circle. If the lens is
double convex, the focus will be equal to
the radius, or half the diameter. When
the lens is crossed or unequally convex,
the focal length will be twice the product
of the two radii, divided by the sum of
the radii. For example, let the radius on
one side be 2 inches, and on the other

side 6 inches ; the focus of this will be
2 x 2 x 6 = 24, divided by 2 + 6 = 8
or 3 inches. The focus of the miniscus
lens is found by dividing twice the pro-
duct of the two radii by their difference.
Example ; let the radius on the convex
side be 2 inches, and on the concave
side 4, the focus is 2 x 2 x 4 = 16 di-
vided by 4 —2 = 2, or 8 inches, the focus
of the lens*.

If two lenses are placed in contact,
the compound focus, when each lens
has the same power, will be half the
focus of the single lens. When two
convex lenses are in contact, having dif-
ferent focal lengths, then, as the sum of
the two foci is to one of them, so is the
other to the compound focus required.
For example, let the foci of the lenses
be 2 and 6 ; then, as 2 + 6 = 8 : 2.'. 6 :
1^, the compound focus. Lastly, if two
lenses are not in contact, the compound
focus is found by dividing the product
of the two lenses by the sum lessened
by their distance. Example: let the
foci of the lenses be 2 and 4, their
distance 2 ; then 2x4 = 8 divided by
(2 + 4) — 2=4 gives 2 as the compound
focus.

(11.) If lenses be made of different
substances, although the curves may be
the same, the focal lengths will vary ;
while, in like mediums, the action will
always be equal. Let a b (Jig. 11.) be
a ray of light, and let it enter the me-
dium c c? at the point b ; instead of con-
tinuing in a right line to e it will pass on
in the direction b f, should the medium
cdbe denser than the first a b ; now if
on the point b a circle be drawn, and a
line sit parallel to the surface of the
medium, touching the incident ray a b
be produced to e, this line will be the sine
of incidence ; and if another line pr be
drawn in the same manner to the re-
fracted ray, it will be the sine of refrac-
tion. Now if the angle a b c be varied
to any degree, the sine s i will always
be in the same proportion to the sine of
refraction, p r. If the dense medium is
water, the sine p r will be f of 5 i. When

* In many cases.it is found advisable to ascer-
tain the radii of the two surfaces of a convex lens,
as well as its focus, by a more accurate manner.
This may be effected by forming a reflected image
by the posterior surface, which distance will be half
of the radius of curvature (or one quarter the focus of
a plano-convex lens) ; then, by exposing the other
side, we obtain the radii of the opposite surface.
This method was adopted by Professor Robinson, to
measure the different radii of double and triple
achromatic object-glasses.

OPTICAL INSTRUMENTS. 7

glass is used, the sines are as 2 to 3 focus, so as to decrease the diameter of

nearly, and in diamond as 2 to 5.
Fig. 11.

When a ray is passing out of a dense
medium into a rarer, the direction will
be changed, and the ray bf will now be
bent further from the perpendicular, so
as to make the sines the reverse of the
former case. Out of water they will be
as 4 to 3 : from glass as 3 to 2, and
from diamond as 5 to 2. The theorems
just described for finding the foci of
lenses are called geometrical, and will
be nearly the same as the refracted,
when the lens is made of plate glass.
The refracted focus is only oiist.part
less than the geometrical, when ascer-
tained by accurate experiment. The
refracted focus of lenses of other media
may be obtained by dividing the geo-
metrical focus by the quotient obtained
when the sine of incidence (z), minus
the sine of refraction (r), is divided by

half the sine of refraction. ( l —• \
\ l*r )

(12.) Convex Lenses, in their simple
state, have been applied to collect the
heat of the sun's rays for purposes si-
milar to that of burning mirrors. One
of the largest lenses that have been
mounted for these purposes, was that
made of flint glass by Mr. Parker. This
lens was 3 feet in diameter, and when
mounted, exposed a surface of 330
square inches to the sun's light; its
focal distance was 3 feet 9 inches, and
the diameter of the circular spot of
light was one inch. But in order that
the light might be condensed as much
as possible, he employed another lens,
13 inches diameter, and of 29 inches

the focal point to 3-8ths of an inch. The
apparatus on which it was mounted is
shown in Jig. 12 : a is the large convex
lens mounted in a ring, and connected
to the smaller lens b by wooden ribs c c ;
the lower rib has a piece e attached to it,
capable of adjustment to or from the
smaller lens : to this bar is fixed the
holder d, having an universal joint. On
this holder, the substance to be experi-
mented on is placed. The following
are some of its effects on bodies placed
in its focus ; 20 grains of pure gold was
fused in 4 seconds ; 1 0 grains of pla-
tina fused in 3 seconds, and a diamond,
weighing 10 grains, exposed for 30 mi-
nutes, lost 4 grains. This lens, which
is now in the posession of the Emperor
of China, cost 7001.

In large burning lenses the weight of
the glass employed becomes of consider-
able importance ; and to effect as great
a saving as possible, Count Buffon has
proposed to construct them of circular
rings, as shown in/ig-. 13, where the lens

Fig. 13.

is composed of three pieces, two rings, a
and b, and a lens c. When, however,
the size is very great, the rings may be

8

OPTICAL INSTRUMENTS.

composed of several pieces, as shown
by the front view E, where the 'lens is
built of ten pieces. These instruments
have been denominated by Dr. Brewster,
who suggested this division, Polyzonal
lenses.

The following advantages of these
lenses have tbeen laid down by. Dr.
Brewster.

1 . The difficulty of procuring a mass
of flint-glass proper for a solid lens of
great dimensions, is in this construction
completely removed.

2. If impurities exist in the glass of
any of the spherical segments, or if an
accident happen to any of them, it^can
be easily replaced at a very trifling ex-
pense. Hence, the spherical segments
may be made of glass much more pure
and free from flaws and veins than the
corresponding portions of a solid lens.

3. From the spherical aberration of
a convex lens, the focus of the outer
portion is nearer the lens than the focus
of the central parts, and, therefore, the
solar light is not concentrated in the
same point of the axis. This evil may,
in a great measure, be removed in the
present construction, by placing the
different zones in such a manner that
their foci may coincide.

4. A lens of this construction may be
formed by degrees, according to the con-
venience and means of the artist. One
zone, or even one segment may be
added after another, and at every step
the instrument may be used as if it were
complete, without the rest of the zone
to which it belongs ; and it will contri-
bute, in the proportion of its area, to
increase the general effect.

5. If it should be thought advisable
to grind the segments separately, or
two by two, a much smaller tool will be
necessary than if they formed one con-
tinuous lens. But, if it should be reck-
oned more accurate to grind each zone
by itself, then the various segments may
be easily held together by a firm cement.

6. Each zone may have a different
focal length, and may, therefore, be
placed at different distances from the
focal point, if it is thought proper.

CHAPTER III. — Spectacles — Periscopic
Spectacles.

(13.) Spectacles* — When two lenses
are mounted in a frame to fix before the

* These instruments are said to have been in-
vented about the year J290.

eyes, they are denominated spectacles :
the lenses are employed to render the
objects before the wearer more distinct.
The eye, which consists of a convex
lens,^ called the crystalline lens, re-
fracts the light proceeding from the
object placed before it in the same
manner as a convex glass : the image of
the object is formed at the focus of
the lens, where it is received on a
screen at the back of the eye; this
screen, called the retina, is an ex-
pansion of the optic nerve, which con-
veys the sensation of vision to the mind.
As the crystalline lens of the eye will
only produce distinct vision when the
focus is thrown on the retina, it is ob-
vious that should any defect occur with
respect to that organ, indistinct and im-
perfect vision will arise. Thus, if the lens
of the eye is not of a proper convexity
to bring the image on the screen, an in-
distinctness must ensue. This is the case
when the lens through age has become
flattened ; the image will then be thrown
beyond the retina, and thus convey an
imperfect representation of the object
to the mind. To obviate this defect, we
must make the rays pass through a glass
of sufficient convexity to assist the eye,
and enable it to form the image at the
required place, which is in this in-
stance done by shortening the focal
distance of the crystalline lens of the
eye. If, on the contrary, the eye should
be too convex, or short-sighted, as
is often the case with young persons,
then the image will not be formed at a
sufficient distance from the lens of the
eye to reach the retina, and thus imper-
fect vision of distant objects is produced.
To remedy this defect concave lenses
must be resorted to, in order to diverge
the rays before they enter the eye, and
thus lengthen the focus of the crystal-
line lens to form an image on the retina.
When the eyes are not directed near
the centre of the spectacle-glasses, the
obliquity of their surface to the rays will
be increased, so as to occasion a con-
fused appearance of the object. A great
portion of this confusion is removed
in the spectacles now usually made,
when compared with those formerly em-
ployed, whose size, being very large,
augmented the imperfection ; for it
may be observed that when objects are
seen through spectacle-glasses, no
more of the glass is employed at one
view than a portion equal to the size of
the pupil of the eye ; this on an average
may be reckoned at the eighth of an

OPTICAL INSTRUMENTS.

inch in diameter. Thus, 'we see how
small a portion is used for the purposes
of vision ; but as it would be tedious to
require the eye always to look through
a small aperture, the glasses are left of
a sufficient size to admit of a moderate
degree of motion ; and, as we require a
greater latitude horizontally than verti-
cally, their figure is made of an oval
form.

In the selection of spectacle-glasses
great care should be used in examining
them, and the first point of importance
is the goodness of the material of which
they are formed ; this should be free
from all veins or small bubbles, for if
one of these occur in the portion
through which we look, it will greatly
impair the eyes. The next circumstance
is the colour of the glasses ; the best
adapted for general purposes is a pale
blue. The figure of their surfaces should
be perfectly spherical, for if they are
curved more in one direction than in
another, they will injure the sight, unless
they are cylindrically formed, as for some
particular disease. The polish should be
clean, and free from flare, which too
often arises from the manner in which
they are usually polished on heteroge-
neous surfaces, producing what is
technically termed a curdled glass.
See the method of grinding and polish-
ing lenses, described in (48).

(14.) Dr.Wollaston, in order to allow
the eyes a considerable latitude without
fatigue, invented a peculiar form of
glasses, called by him periscopic, from
two Greek words signifying seeing about;
their form is that of a meniscus with
the concave side always turned towards
the eye. When they are intended
for long-sighted persons, or old age,
the anterior surface, or that next the
object, is formed spherically convex,
with a curve deeper than the concave,
so as both to gain the required power,
and compensate for the divergency oc-
casioned by the concave side ; this form
is shown at A, (fig. 14.). The peri-
Fig. 14.

scopic form employed for correcting
the defect of a short or near sight is
shown in section at B, having its an-
terior surface convex, as in the former
case ; but here the concavity on its pos-
terior side is increased to procure the
required divergency, and compensate for
the convex side.

CHAPTER IV. — Telescopes — Common
Astronomical Telescope — Day Tele-
scope— Dynameters.

(15.) A Telescope is an optical instru-
ment employed for viewing distant ob-
jects, by increasing the apparent angle
under which they are seen without its
assistance ; and hence the effect on the
mind of an increase in size, or, as com-
monly termed, magnified representation.
The construction of the Telescope is,
perhaps, one of the most important ac-
quisitions that the sciences ever attained,
as it unfolds to our view the wonders of
the heavens, and enables us to obtain
data for astronomical and nautical pur-
poses.

The invention of this instrument is
somewhat uncertain, and is ascribed to
different individuals, as John Baptista
Porta, Jansen of Middleburg, and
Galileo. The time of its first construc-
tion was about the year 15 90.

The simplest construction of this in-
strument consists of two convex lenses,
so combined as to increase the apparent
angle under which distant objects are
seen. If we take a convex lens, and
place it in a similar position to the object,
as that inyTg-. 7, and another of shorter
focus in the position Jig. 8, with a dis-
tance between them equal to the sum of
their foci, a telescope will be formed,
and the magnifying power will be in
proportion to the focus of the two lenses.
Let O (/o-. 15) be the object lens, and
suppose it 8 inches focus, and e the
eye lens, of 2 inches focus, the distance
between these two lenses must be ten
inches, if the object be at an infinite
distance, as a star ; but when the object
is terrestrial, the distance between the
two lenses must be increased to adjust
for distinct vision : on this account the
eye lens is mounted in a tube, sliding
within another tube in which the object-
glass is fixed, and, therefore, can be
drawn out for near objects. As the
size of objects is dependent on the angle
under which they are seen, the image
F, formed by the object-glass, in the
focus of the eye-glass e, will subtend

10

OPTICAL INSTRUMENTS,
Fig. 15.

the angle c e d, which is four times the
angle cod that the object subtends, for
the distance Fo is four times Fe : hence
the magnifying power may be found, by
dividing the focal length of the object-
glass by the focus of the eye-glass, when
the quotient will be the power. Objects
seen through this telescope are in-
verted, and on that account it is inap-
plicable to land observation ; but at sea
it is occasionally used at night, and in
hazy weather when there is little light ;
it is hence called a night telescope.

(16.) The common astronomical tele-
scope is of the same principle of con-
struction as the preceding. The inver-
sion of the object is immaterial in its
application to celestial observations ;
but the disadvantage of this instru-
ment is felt when very high powers are
required, for then the objects are ren-
dered dark and obscure, and if the
aperture of the object-glass is increased
to admit more light, the formation of
the object is confused. M. Huygens,
however, made a telescope of this con-
struction, in which he was enabled to
use an aperture of 6 inches, by making
the focus of the object-glass 123 feet in
length, and, by changing the eye-lenses,
any required power was produced. From
experiments on different combinations,
he found that to obtain the greatest
distinctness and light, the focus of the
object-glass, its aperture, and the power
of the instrument, should be according
to the following table :

Focus of

Aperture of

the Object
Glass.

the Object
Glass.

Feet.

Inches.

1

0.545

2

0.76

3

0.94

4

1.08

5

1.21

10

1.71

20

2.43

30

3.00

40

3.48

50

3.84

100

5.40

120

5.9

Focus of
the Kye
Glass.
Inches.

0.605

0.84

1.04

1.18

1.33

1.88

2.68

3.28

3.76

4.20

5.95

6.52

Magnifying1
Power.

20

27.6

33.5

39.5

44

62

88
108
125
140
197
216

(17.) The common day -telescope is an
instrument of this class, with the addi-
tion of two other lenses of the same
power as the eye-lens e ; these lenses
will produce an erect image of the ob-
ject when placed at a fixed distance
from each other, equal to the sum of
their two focal lengths. Let o (fig. 1 6.) be

Fig. 16.

OPTICAL INSTRUMENTS.

11

the object-lens, which may be the same
focus as that in fig. 16, eee three lenses
of equal power. Now, if the focus of
each eye-lens e is two inches, as in the
former case, then each eye-glass must
be placed at a fixed distance of 4 inches
from each other ; and the distance be-
tween the object-lens o, and the nearest
eye-lens, will be 1 0 inches, this distance
increasing as the objects to be viewed
approach the instrument. The power
of day-telescopes may be calculated in
the same manner as the astronomical ;
for the two additional lenses produce
no effect in the amplification of the
objects.

(18.) The magnifying power of tele-
scopes may be ascertained without a
knowledge of the foci of the glasses, by
means of a dynameter ; this apparatus
simply consists of a strip of mother-
of-pearl, marked with equal divisions,
from the T£o to T^O of an inch apart,
according to the accuracy required ;
this measure is attached to a magnifying
lens in its focus, in order to make the
small divisions more apparent. When
the power of a telescope is required, the
person must measure the clear aperture
of the object-glass ; then holding the
pearl dynameter next the eye-glass, let
him observe how many divisions the
small circle of light occupies when the
instrument is directed to a bright ob-
ject. Then by dividing the diameter of
the object-glass by the diameter of this
circle of light, the power will be ob-
tained.

CHAPTER V. — Aberration of Reflectors
and of Lenses — Glass and Diamond
compared — Huy gens' Eye Piece —
Ramsderis Eye Piece — Newton" s
Parabolic Lenses — Chromatic Dis-
persion.

(19.) The field of vision, or number
of objects seen by the telescopes^g-s..^
and 16, is very limited, the eye-lenses
not being sufficiently large, as is shown
by the dotted lines i i mfig. 1 5, which do
not enter the eye lens e, and are not
received by the eye. Now, if the dia-
meters of these lenses were increased,
the objects would be rendered indistinct,
arising from the rays, spread over the
surface of the lens from any point in
the object, not being collected again in
another point after refraction. This
error is occasioned by the figure of the
lens, and is called the spherical aberra-
tion by figure.

As a lens is formed with two sur-
faces, and, consequently, has two re-
fractions, we shall first investigate the
aberration of a spherical reflecting sur-
face.

In (2) the focus of a concave sphe-
rical reflector was stated to be half
the radius distant from its surface ; this,
however, is only the case with parallel
rays near the centre. When we are
desirous of employing specula for
telescopes, they require to be made of
the parabolic or hyperbolic form, to
unite all the rays to one point : the rays
that fall on the extreme parts of a sphe-
rical reflector, forming an image nearer
the speculum than those that fall on its
centre. In fig. 17, F is the focus of cen-

Fig. 17.

tral rays, and the point/ the focus of the
extreme rays A C, while, along the axis
from / to F, images from the different
parts of the reflector will be formed of
the same object ; these, not coinciding,
will confuse one another. The quantity
F/is called the longitudinal aberration,
and will be equal to half the aperture of
the speculum squared (a b) 2, divided by
4 times the radius of curvature (d b), or
a b 2 .

—^ nearly, in specula whose sur-
face is spherical.*

This spherical aberration produces an
indistinctness of vision, by spreading
out every mathematical point of the
object into a small spot in its picture ;
which spots, by mixing with each other,
confuse the whole. The diameter of
this circle of confusion, at the focus of
central rays F, over which every point is
spread, will be LK (fig. 17.) ; and when
the aperture of the reflector is mode-
rate it equals the cube of the aperture,
divided by the square of the radius

• The focus of rays reflected by any curve will be
equal to half "the distance of the tangent from the
centre or half d D for A a.

12

OPTICAL INSTRUMENTS.

I C3\: this circle is called the aberra-

\bd2 )

tion of latitude.

(20.) The aberration produced by a
lens with a spherical surface is shown

in fig. 18, where aB c is a section of a
plano-convex lens. Let the plane side
be exposed to parallel rays, and let a A
be an extreme pencil of rays ; D the
centre of curvature ; D / the axis of the

Fig 18.

lens ; and F the focus of a slender pen-
cil of incident rays, at an infinitely
smaller distance from the centre. Now,
as the extreme ray a A is perpen-
dicular to the plane surface, it will
pass directly through to the convex
side, where it will be refracted to /,
crossing the axis in that point, for D A
is perpendicular to the curve at a, and
a D the sine of incidence, n D the sine
of refraction ; hence, an image of the
object will be formed at F by the cen-
tral rays, and another image of the same
object will be formed at / by the ex-
treme rays ; while, from F toy, images
of the same object will be formed by the
intermediate portion of the lens. The
longitudinal aberration F/ bears a cer-
tain ratio to the thickness or versed sine
B P ; and when the lens is placed in
the position shown in the figure, it is
equal to | or 4& times B P ; this quan-
tity will be decreased, when the curved
surface of the lens is exposed to paral-
lel rays, that is, when the refraction of
the first surface is made nearer the per-
pendicular, or when the ray is bent in
passing from a rare into a dense me-
dium, and this difference out of air into
glass, will be in the proportion of 27 to
7 ; so that when the convex side is
placed next the radiant, the longitudinal
aberration will be only I of the thick-
ness BP, or 1.166.

When a crossed convex lens is used,
the proportions of the radii of whose
surfaces are as 1 to 6, and the most
convex side is exposed to the distant
radiant, the longitudinal aberration will
be the least possible quantity ; viz. if.
or 1.0714 of the thickness of the lens,
When the radii of a double-convex

lens are equal, the aberration is | of its
thickness ; therefore, this lens is not so
good as a plano-convex of the same
thickness, in its best position. The
longitudinal aberration F / increases as
the square of the aperture, when the
curvature of the lens is not altered;
and is inversely as the focal distance,
when the aperture is constant.

The lateral aberration, which is the
actual confusion of the image at the
focus of central rays, is equal to the
longitudinal aberration F/, multiplied

by~— , or the aperture of the lens di-
13 r

vided by the focal distance, which is
equal to K H. Now, if rays are drawn
from the different parts of the lens, it
will be found that they will be refracted
through a small circular space I R,
whose diameter will be \ of K H ; hence,
this point must be considered as the
focus of the lens. The lateral aberra-
tion of lenses increases as the cube of
the aperture, if the radius remain the
same, or inversely as the square of the
radius when the apertures are the same.
These laws may be considered as de-
termining the relative aberration of all
lenses ; yet it is found that if we employ
media of different refractive powers,
and form each into lenses of like curva-
ture, the separation or spreading out of
the rays at their focal point will be dif-
ferent ; that possessing the highest
refractive power producing the least
aberration, though its amplifying power
will be greatest : thus,' if three lenses
were ground in the same tool, one of
plate glass, and the others of sapphire
and diamond, they would possess very

OPTICAL INSTRUMENTS.

13

different magnifying powers, aberra-
tions, and separatio?i of colour, or
chromatic dispersion, (this latter error
is explained in 23 :) their respective va-
lues are shown in the following Table :

Piano convex lenses
with convex side ex-
posed to parallel rays.

Glass

Sapphire

Diamond

Magnifying Longitudinal
.power*. aberration.

150
250
400

1.167
1.005
0.950

Chromatio
dispersion .

48
26
38*

But this difference in the longitudinal
aberration would be much greater if the
lenses were so formed as to give the
same magnifying powers ; for this error
always decreases as the squares of their
respective radii, while the lateral aber-
ration or area of the circle of confusion
will be as their cubes.

Hence, in sapphire and diamond lenses
of high magnifying powers, the indis-
tinctness arising from their figure would
barely be discernible in practice, thus
producing a kind of natural aplanatic
magnifier.

The valuable properties possessed by
these stones were ' known to Sir Isaac
Newton, and Martin, and have been
more particularly pointed out by Dr.
Brewster, in his Treatise on New Optical
Instruments. But their hardness and
crystalline form probably occasioning
difficulties in the formation of spherical
polished surfaces almost insurmount-
able, has retarded their adoption as
lenses : however, we have lately learned
that Mr. Pritchard has succeeded in
forming these substances into lenses,
and their \ application to the microscope
is so favourable, that, if any new disco-
veries are to be made in the minutife of
nature, they seem most likely to develope
them.

The process by which these lenses
are worked, and their application to the
microscope, are detailed in the Journal
of Science of the Royal Institution,
vol. ii. page 15 (New Series). This
paper was communicated by Dr. Go-
ring, who suggested to Mr. Pritchard
the advantages which diamond lenses
would most probably possess.

The adaptation of these lenses to
telescopes in place of the ordinary eye-
glasses would, in all probability, be at-
tended with equal success, where every
circumstance calculated to produce a
perfect representation of the object is of
the utmost importance.

* When it is considered, that the refraction of
diamond is nearly three times that of glass, it
follows, that in equal refractions its dispersion will
be only one-third of the latter.— See Optks, p. 24.

21. The great advantage of duly con-
sidering the aberration of lenses will be
evident, if we combine two lenses, of
twice the focal distance, instead of one,
to produce any given power, as the
aberration will, be decreased to one
quarter of that of a single lens of equi-
valent power, and, therefore, the aper-
ture of the compound lens may be in-
creased, while the error will be less than
in a single « lens. In the common tele-
scopes (figs. 15 and 16), if two lenses
were used, instead of the single object
and eye-glass, as there shown, the aper-
tures of each might be increased, and,
consequently, the instruments would be
improved in light and field.

(22.) M. Huygens has demonstrated,
that when the greatest possible distinct-
ness is required for the eye-piece of a te-
lescope, it may be obtained by two plano-
convex lenses, placed as in/g-. 19, with

Fig. 19.

their plane sides outward, and the focus
of the eye-lens E must be § of that of
the field-lens F, with a distance between
them equal to the difference of their
focal lengths. This combination, from
the purpose it has been adapted to, is
called the astronomical positive eye-
piece ; and the telescope, by this addi-
tion, will have four times the distinct-
ness of a single lens D, of equivalent
power, while the distortion of the object
will only be \ of that produced by a sin-
gle lens ; for the refraction of the object-
lens brings the image of the marginal
rays nearer to itself than the central,
therefore the image will be formed
convex next the lens F, as shown by
the aiTOw; and as the radius of
curvature of the lens F is twice
that of the single lens D, the distor-
tion will be decreased in the square
of this ratio, or 4 times. On this ac-
count, a similar combination is used for
the eye-pieces of telescopes for astrono-
mical quadrants, and other graduated
instruments, when the convex side of
the field-lens is turned towards the eye-
glass E, because equal divisions on a
micrometer correspond with equal an-
gles, subtended by objects measured by
this instrument, This combination, which

14

OPTICAL INSTRUMENTS.

is called Ramsden's Micrometical Eye-
piece, has one great disadvantage, viz.
that it requires the eye to be placed ex-
ceedingly near to the eye-lens E.

(23.) Being now in possession of
a combination that will dimmish the
aberration produced by the eye-piece of
a telescope, our limit of magnifying
power and light will arise from the
errors occasioned by the object-glass ;
and this, we have seen, may be dimi-
nished by having the curves of the two
surfaces as 1 to 6, with the most con-
vex side outermost; for this lens has
been shown to have less aberration than
any other*. Secondly, by using two
lenses of twice the focus in contact, to
produce the required refraction, and
thus diminish the error four times.
But, although this error may, by the
means here pointed out, be rendered
very small and almost imperceptible, yet
it is magnified in the same proportion
as the objects ; and when high powers
are used, the indistinctness will become
sensible.

Sir Isaac Newton conceived that the
surfaces of the lens might be formed of
some mathematical curve which would
entirely obviate this error; and, by inves-
tigation he found that, if the surface were
described bythe revolution of a parabola,
and the radiant or object be at an infinite
distance, the rays would be collected to
a point, and be free from all aberration.
He afterwards formed tools to grind and
polish lenses of this figure, but when
made, although the error by figure was
perfectly corrected, it was discovered that
the white heterogeneous pencils of light
(before that time considered as homoge-
neous) in their passage through the lens,
were divided into their several consti-
tuents of red, orange, green, blue, and
violet, in the same manner as by a prism,
and hence lenses of this figure became
useless.

* Dr. Brewster, in his Edition of Ferguson's Lec-
tures (vol. ii. p. 299), states, that in order to render
the common refracting telescope as perfect as possible
without making it achromatic, the exterior surface
of the object-glass should be ground to a radius equal
to 5-9thsof its focal length; and the radius of the
interior surface, or that next the eye, should be 5
times its focal length. In eye-glasses, the radius of
the surface next the object should be 9 times its focal
distance, and that of the surface next the eye 3-5ths
of the same distance. By this means, the aberration
arising from the spherical figure of the lenses will be
nothing for objects placed in the direction of their
axis, and the least possible for objects removed from
the axis. According to Huygens, the spherical aber-
ration was the least possible, when the radii of the
surfaces are as 6 to 1. But though this be true for
objects placed in the axes of the lenses, yet a conside-
rable aberration remains when the objects are placed
on one side of the axis.

(24.) To illustrate the chromatic disper-
sion produced by a lens, let a A, Jig. 20,
be a white compounded pencil of light
proceeding from any luminous body,
and falling on the lens B ; parallel to its
axis, at the point a, this pencil of light
will not be refracted colourless, but the
red rays will cross the axis at r, and

Fig. 20.

the violet, which will be attracted by the
lens more than the other colour, crosses
the axis at v ; and along the interme-
diate space from r to v, will be formed
a coloured spectrum of orange, yellow,
green, and blue ; the proportional quan-
tity of these colours, and the total length,
will vary according to the substance
of which the lens is formed.* Sir Isaac
Newton, by most accurate observations,
found that in common glass, when the
sine of the angle of the incident rays
A a was 50°, the sines of refraction of
the red and violet rays were 77° and 78°,
the mean refraction of the pencil being
77£°. Now, if we call the sine of in-
cidence «, the sine of refraction for red
rays r, and of the violet v, it is found
that the diameter of the circle of disper-
sion d s, through which all the colours
pass, will be as (v— r) is to (v-\-r— 2t?)
or as 1 to 55, so that the diameter is
5ith part of the aperture of the lens,
which is equal to half the diameter of
the circle of dispersion at the focus of
central rays r. The circle of dispersion
that will comprehend any particular
colour, or set of colours, may be easily
calculated. Thus, all the orange and
yellow will pass through a circle, whose
diameter is 5£ffth of the aperture of the
lens. When it is considered that an
object-glass 5J inches in diameter has a

* Sir Isaac Newton imagined, that the different
colours divided the spectrum formed by all sub-
stances in the proportions of a musical canon. This
is found to be a mistake : for when the spectrum is
formed by a prism of crown glass, and another of
precisely the same length is formed by the side of
it with a prism of flint glass, the confines between
the green and blue will be found precisely in the
middle of the first spectrum ; but in the second, it
will be considerably nearer to the red extremity :
indeed, there are hardly two substances that dis-
perse the colours in the same proportions. Oil of
cassia exerts the strongest action on green light,
and sulphuric acid the least.

OPTICAL INSTRUMENTS.

15

circle of dispersion ^th of an inch in
diameter, it may be surprising that any
picture of an object can be distinguished ;
but the superior vivacity of the orange
and yellow light in comparison with the
rest, make the effect produced by the
confusion of the colours much less sen-
sible, and will allow this aperture to be
used when the focal length of the lens
is considerable.

(25.) We may now compare the diame-
ters of the circles of the chromatic and
spherical aberration together. If we take
a standard telescope of approved good-
ness, it has not been found possible to
give more than 4 inches aperture to an
object-glass of 100 feet focal distance,
so as to preserve sufficient distinctness ;
and if the diameter of the circle of sphe-
rical aberration is computed for this lens,
it will not exceed iso'oooth part of an
inch, while the chromatic, if restricted
to 3ioth of the aperture, which is hardly
a fifth of the whole dispersion, (or dia-
meter of the circle of orange and yellow

light,) is ggi of an inch, and is there-
fore about 1900 times greater than the
other. But when the aperture of a lens
is increased to 30°, the spherical aberra-
tion will be found equal to the chroma-
tic in a glass lens ; but this aperture can
only be used for eye lenses or micro-
scopes.

CHAPTER VI. — Reflecting Telescopes —
the Newtonian — the Gregorian — the
Cassegrainian — Sir W. Herschell's —
Mr. Ramage's.

(26.) With these disadvantages to con-
tend against in refracting substances,
Sir Isaac Newton, in the year 1666,
turned his attention to reflected light,

in which the angle of all the coloured
rays are equal. By pursuing this idea he
entirely obviated the chromatic error. In
the first telescope he made by reflection,
the distinctness with which objects were
seen through it was surprising, when
compared with the refracting telescopes
of those times ; for though the focal dis-
tance of the metal was only 6£ inches,
it would carry a power of 38 with equal
distinctness to a 4 feet refractor. The
form of the metal was spherically con-
cave ; but by investigation he ascer-
tained that if the form had been that of
a parabola, there would not have been
any spherical aberration produced ; and
if we examine the spherical aberration
by figure of a spherically concave metal,
and compare it with that of a plano-
convex lens ground in the same tool,
the former will be 4, while the latter
is 9. But when it is considered that the
focus of the glass lens is 4 times that
of the metal, (for the focal distance of a
plano-convex lens is twice the radius,
and that of a concave reflector half the
radius,) to make their foci equal, the
curvature of the lens must be 4 times
that of the speculum ; and it has been
shown that the error by figure increases
inversely as the square of the radius :
hence the aberration of the lens will be
to that of the reflector as 4a x 9 to 4,
or as 36 to 1, and the distinctness will
be inversely as the areas of these circles,
which are as the squares of their respec-
tive diameters ; so that the distinctness
of a reflector will be 1296 times greater
than that of a lens of the same focus and
aperture.

(27.) The Newtonian telescope (fig.
21.) consists of a concave parabolic
metal A, fixed at the end of the tube
ddd; the plane speculum c is fixed to

(Fig. 18.)

a wire, having its other end attached to
a dove-tailed sliding-piece i i, and the
face of the plane metal is inclined to the
axis of the tube and the large speculum
at an angle of 45°. In the sliding piece
i i, opposite the small metal, is inserted
a short tube to hold an eye-piece, which

is a single lens with its flat side outer-
most, or the astronomical eye-piece
(fig. 19.) ; but as the colour produced
by these eye-lenses is not corrected,
another combination, called the nega-
tive achromatic eye-piece, should be
used, which will be described when

1G

OPTICAL INSTRUMENTS.

treating on the chromatic correction of
lenses. The adjustment of this instru-
ment to distinct vision is made by a
rack and pinion attached to the sliding
piece and great tube of the telescope, by
\vhich the eye-piece and small speculum
is brought nearer or farther from the
large metal. Let r r be the rays of
light coming from a distant object, and
falling on the large speculum A, these
rays would be reflected to the focus e;
but meeting with the oblique flat metal
c, are reflected to/, where an image of
the distant object will be formed, and is
received by the eye-lens g, by which the
rays are rendered parallel. The power
of a Newtonian reflector is proportional
to the relative focal distances of the
concave metal and the eye-lens. For
example, let the focal distance A e be
40 inches, and the focus of the eye lens
g, half an inch, the power will be 80. It
should here be observed, that the same
instrument which is free from aberra-
tion for astronomical observation will
not be so for terrestrial uses ; for the
rays in the former case are parallel,
while they are divergent in the other.
The curve, therefore, of the large spe-
culum when required for the latter pur-
poses, should be elliptical, having the

object in one focus, and the focus of
the eye-lens in the other.

This telescope, which is more simple
than other reflectors, may be greatly im-
proved according to the method of Dr.
Brewster, who has proposed, (for tele-
scopes of moderate size, where a front
view cannot be used,) to employ two
glass prisms in place of the small
plane. By the experiments of Major
Kater, it appears that one-third of
the rays of light is lost when re-
flected by a speculum at a vertical in-
cidence, and probably not more than 68
out of 100 are reflected at an angle of
45°, as in the Newtonian small metal ;
in addition to this, the imperfection of
surface and figure in metals, which
makes the rays stray 5 or 6 times more
than the same imperfection in a refract-
ing surface,* as well as the difficulty of
working metals as perfect as glass,
induced him to suggest this improve-
ment. Let a b, (Jig. 22,) be the great
speculum, and r a, r b parallel rays
from a distant object reflected to a
focus at F ; the cone of rays, however,
is intercepted by the achromatic
prism c d, and refracted to /, where a
distinct image is formed in the anterior
focus of the eye-glass e^by which it is

Fig. 22.

magnified. The double prism c d, being
composed of a prism of crown glass c,
and another of flint d, united by a ce-
ment of mean refractive power, the loss
of light by transmission through the two
prisms, says Dr. Brewster, will not ex-
ceed 600 rays out of ] 0,000, as the light
transmitted through a lens of glass, ac-
cording to Dr. W. Herschell, is 9,485
out of 10,000 incident rays. Hence, the
light lost by the prism is only £ of that
lost by reflection.

The Newtonian telescopes made by
Hadley had, in place of a plane metal
a right angular prism P substituted,
haying its sides perpendicular to the
incident and emergent rays. In this, as
is accomplished by the two prisms of

Dr. Brewster, the image will be erect,
and a less quantity of light lost than
by a mirror of the common kind.

(28.) Another class of reflecting tele-
scopes was invented by Dr. Gregory, in
1660, but they were not made till some
years after the Newtonian, from the dif-
ficulty of forming the metals. The Gre-
gorian reflector is, however, preferred
to the Newtonian, and is most com-
monly used, because the observer is
stationed in a line with the object,
whereas, in the Newtonian he is at
right angles to it. Fig. 23 is a sec-
tion of the Gregorian reflector. B D
is a concave metal, whose surface

t * jSee Newton's Optics,

OPTICAL INSTRUMENTS;

17

Fig. 23.

should be formed by the revolution of
the hyperbolic curve : this speculum has
a small hole in its centre. E is another
concave elliptical small metal placed in
the axis of the larger one, at a distance
from it a little more than the sum of their
focal distances. H are the eye-lenses
sliding in a tube fixed behind the large
speculum : the adjustment is made by
the screw s s, which moves the small
metal to or from the great speculum.
Let r B and r D be two parallel rays
from a distant object, these will be re-
flected to the focus F of the large metal,
where an image will be formed, and the
rays, crossing each other, fall upon the
small speculum E ; and if the focus of
this metal had coincided with the focus F,
the rays would have been reflected pa-
rallel, but now they form a direct image
at I, and this image is viewed by the
eye-piece or a single lens at H. The
magnifying power of this instrument
may be computed thus : — suppose the
focus of the large speculum B F is 9
inches, and the focus of the small metal
1 4 inch : then will the angle be in-
creased six times ; but this must be
multiplied by the ratio of the distances
I H, the focus of the lens, and the dis-
tance I F ; and if these are as 1 to 8,
the amplification of the object will be
6 x 8 = 48 times.

(29.) The Cassegrainian reflector is
constructed in the same way as the
Gregorian, with the exception of a small
convex spherical speculum, instead of
one a little concave ; and as the focus of
this metal is negative, it is placed at a
distance from the larger metal, equal to
the difference of their foci, and only one
image is formed, viz. that in the focus
of the eye-glass ; on this account,
the distinctness is considerably greater
than in the Gregorian. Mr. Ramsden,
in the G9th volume of the Philosophical
Transactions, states, that this construc-
tion is preferable to either of the former
reflectors, because the aberrations of the
two metals have a tendency to correct
each other ; whereas in the Gregorian,
both the metals being concave, any
error in the specula will be doubled.

By assuming such proportions of the
foci of the specula, as are generally
employed in these instruments,which are
about as 1 to 4, he asserts that the
aberration or indistinctness occasioned
by the figures of the reflectors (sup-
posing each worked equally true) in the
Cassegrainian construction, is to that in
the Gregorian as 3 to 5.

(30.) In sidereal observations of ne-
bulae and small stars, abundance of
light is necessarily required, and by
whatever means a loss of light by re-
flection or refraction can be prevented,
the adoption of such a construction
would be advisable. Sir W. Herschel,
from an investigation of the loss of light
occasioned by the small speculum in
reflectors, constructed an instrument
which entirely obviated the use of the
second metal, by what he called the
front view telescope. The diameter of
the polished surface of the speculum of
his large instrument was 4 8 inches, and
its focal distance 40 feet. This metal,
which weighed when taken from the
casting, 2118 lb., was placed at the end
of an iron tube 4 feet 1 0 inches in dia-
meter ; the other end is elevated towards
the object, and has attached to it a single
eye-lens in the focus of the metal ; the.
observer is mounted in a gallery move-
able with the instrument, having his
back to the object. The light obtained
from so large a surface by this instru-
ment was truly surprising, and enabled
objects otherwise invisible to become
extremely interesting. This telescope,
erected at Slough, near Windsor, was
completed on the 28th of August,
1789, and on the same day the sixth
satellite of Saturn was discovered*.
The frame of this instrument having
greatly decayed, it has been taken down ;
and another, of 20 feet focus and 18
inches diameter, erected in its place,
by his son Mr. J. Herschell, in 1822.

* A full description of this instrument will be
found in the Transactions of the Royal Society for
1793, explained by means of 18 plates and 63 pages
of letter-press ; and an ample detail is given of every
circumstance relating to the mechanical construc-
tion of this instrument.

13

OPTICAL INSTRUMENTS.

Fig. 24.

MH, RAMAGE'S REFLECTING TELESCOPE, ERECTED AT THE ROYAL
OBSERVATORY, GREENWICH, IN THE YEAR 1820,

OPTICAL INSTRUMENTS.

10

(3 1 .) The largest front view reflecting
telescope at present in this country, is
that erected at the Royal Observatory,
at Greenwich, by Mr. Ramage, in 1820.
The diameter of the concave reflector is
15 inches, and its focus 25 feet ; the
mechanical arrangement of the stand is
greatly simplified. A perspective view
of the whole instrument is shown at
fig. 24. The tube is composed of a
twelve-sided prism of deal | inch thick.
At the mouth c is a double cylinder of
different diameters on the same axis ;
around this a cord is wound by a winch,
and passes up from the small cylinder
over a pulley a, and down through the
pulley b, on to the larger cylinder at c.
Now, when the winch is turned to raise
the telescope, the endless cord is un-
wound from the smaller cylinder, and
wound on to the larger : the difference of
the size of the two cylinders will be
double the quantity raised, and a me-
chanical force to any extent may thus
be obtained by duly proportioning the
diameters of the two cylinders ; by
this contrivance the necessity for an
assistant is superseded. The instru-
ment, when not in use, is let down into
the box d d, and covered with canvas,
to prevent dust or moisture from tar-
nishing the speculum.

CHAPTER VII. — Theory of Achromatic
Telescopes— Double Object- Glass.

(32.) Having noticed all the valuable
modifications of the reflecting telescope,
we must now return to the refracting
one. The most obvious and important
improvement m this instrument consists
in the formation of object-glasses free
from the errors of chromatic and sphe-
rical aberration, whence they were deno-
minated achromatic telescopes. But
as this word merely signifies freedom
from colour, which in common tele-
scopes is sometimes effected without a
correction of the figure or spherical
aberration, Sir W. Herschel has, there-
fore, veiy properly denominated a per-
fect telescope aplanatic, from two Greek
words « from xxavos error, that is, free
from all errors.

In (Note to 24) it was stated, that the
length of the spectrum produced by
lenses varied, when formed of different
substances ; thus, if two lenses are
made of the same focal length, the one
of flint glass and the other of crown, the

length or diameter of the coloured image
in the flint will be to that produced by
the crown lens as 3 to 2 nearly. Now,
if we make the focal lengths of the lenses
in this proportion, that is, as 3 to 2, the
coloured spectrum produced by each
will be equal ; but if the flint lens be
concave and the crown convex, when
placed in contact they will mutually
correct each other, and a pencil of white
light refracted by the compound lens
would remain colourless. Unfortunately
in the formation of such a lens, the dis-
persion of the flint glass is so variable,
that trials on each specimen require to
be made, before the absolute propor-
tional dispersion of the substances can
be ascertained*. As the achromatic
object-glass is the most delicate test of
the dispersion of the medium, it is best
found by forming a piece of the flint
glass into a concave lens, and combining
it with a convex of crown glass whose
focal length is known, and varying the
curvature of the flint till the dispersions
are corrected, i. e. till the purple or lilac
fringe surrounding a white object on a
black ground is observed on one side of
the focus, and a green on the other,
when converted into the object-glass of
a telescope and using a powerful eye-
glass. Now, if the compound focus be
accurately measured, and the focus of
the convex lens known, the propor
tionate foci of each may be ascertained,
and, consequently, the proportion of their
dispersive powers is found. This cor-
rection of the spectrum will not correct
the error of each colour, for the propor-
tional lengths of the blue, green, or red
light are variable in different sub-
stances* : thus flint glass is found to re-
fract green light considerably less than
crown glass, in the proportion of the
whole refraction of the red and violet
light : so that when the divergency of

* 1. Sulphuric acid exceeds all other transparent
bodies in its action on the green rays, while oil of
cassia exerts the least action upon them of all known
substances.

2. It is ascertained that in all minerals in which
a metal is the principal ingredient, those which
have the greatest density have also the greatest
faculty of producing colour, while in all the precious
stones a high refractive power is attended with a
low dispersive power.

3. The dispersive powers of resins, gums, oils; and
balsams greatly exceed water, and correspond in
some measure with their refraction.

4. The muriatic and nitric acids exceed water in
dispersion, while the phosphoric, citric, sulphuric, and
tartaric acids, surpassing them in refraction, possess
very low dispersive powers. (See the Tables of Re-
fractive and Dispersive Powers of different Sub-
stances in the Treatise on Optics.)

c 2

20

OPTICAL INSTRUMENTS.

the red and violet light, caused by the
refraction of the two mediums, is equal,
the divergency of the red and green
light is always greater in the crown than
in the flint, and the divergency of the
violet is always less in the crown glass*.
Hence it must be observed, that in
order to have a complete correction of
all the colours, more than two media
must be used, and, therefore, the best
telescopes have their object -lens com-
posed of three kinds of glass. When
the dispersive ratio is known, and the
refractive focus of the compound lens
is given, the refractive focus of each
must be calculated, and to obtain the
radius of the tools for working the lenses,
their refractive foci must be converted
into the geometrical shown as at (11),
when the object-glass would be com-
pleted, and the task would not be dif-
ficult to perform; but the spherical aber-
ration, although much less in quantity,
is more troublesome to correct, and in
making this correction, the proportion of
the radii of the two surfaces of the con-
vex lens must be assumed. When a suit-
able selection is made, the aperture of the
lens being given, the spherical aberra-
tion must be calculated, when its thick-
ness is ascertained^. And lastly, the
curvatures and thickness of a concave
flint glass must be found that will ex-
actly balance the spherical aberrations
produced by the convex glass ; always
keeping the foci of the two lenses in the
ratio of their dispersive powers.

(33.) The radii of curvature of the
different surfaces of the lenses necessary
to form a double achromatic object-
glass, when the sine of incidence is to
the sine of refraction in the crown glass
as 1.528 to 1, and in the flint as 1.5735
to 1, the ratio of their dispersive powers
being as 1 to 1.524, and assuming the
curvatures of the concave as 1 to 2, are
shown in the following TABLE. The first
column F is the compound focus of the
object-glass in inches ; r the radius of
the anterior surface of the crown ; R
its posterior side ; r' the radius of the
anterior side of the concave lens of
flint glass ; and R' its posterior surface.

* Dr. Blair.

t The longitudinal aberration of a lens of glass
may be found by the following general theorem,
where r is the radius of the first surface, R the se-
cond surface, and T the thickness of the lens.
/ 2? f* + 6 r R + 7 R2 „ N
\ 6 + r x~R3 +

F r R r' R'

12 3. 4.652 4.171 8.342

21 6. 9.304 8.342 16.684

30 7.5 11.63 10.428 20856

36 9. 13.956 12.552 25.027

43 12. 18.603 16.684 33.869

60 15. 23.260 20.856 41.712

120 30. 46.520 41.712 83.42 1

In these computations it may be re-
marked that the radius of the anterior
surface of the concave being less than
the posterior side of the convex, admits
of its approach without touching in the
centre, which should always be a neces-
sary practical condition.

CHAPTER VIII. — Aplanatic Telescopes
of Clairaulfs Construction — Mr. J.
F. HerscheWs Object- Glass— Triple
Object- Glasses — Fraunhofer 's and
Tulley's Telescopes — Galilean Tele-
scope and Opera Glass — Achromatic
Opera- Glass — Dr. Brewstcfs Fluid
Opera- Glass.

(34.) The problem for the choice of
the proportional curvature of the as-
sumed convex lens is of the kind called
indeterminate, /or admitting of an infi-
nite variety of solutions. In conse-
quence of this, it allows an endless
number of combinations of lenses, and
each may be free from spherical aber-
ration. It becomes therefore a matter of
considerable delicacy to fix our choice
among them, and numerous construc-
tions have been calculated by different
authors. Thus Clairault, a French
mathematician, has given a construc-
tion in which the two internal surfaces
are worked of equal radii, the one con-
vex and the other concave, so as to ad-
mit of being cemented together, and thus
avoiding the loss of light by reflections
at the two surfaces. But having em-
ployed indices of dispersion in his com-
putations, higher than what are usually
met with in practice, and when those
most likely to be obtained are used, the
radii change so rapidly as to render this
construction difficult to interpolate,
where the artist is no algebraist ; and
hence it must lose much of its value to
the practical optician.

(35.) Another construction has lately
been proposed by Mr. J. F. Herschel,
in which he states, that the destruction
of the spherical aberration is ensured,
not only for parallel rays from celestial
objects, but also for those that diverge
from objects situated at a moderate

OPTICAL INSTRUMENTS.

21

finite distance ; and on these conditions
he has rendered the problem deter-
minate, \vhile the radii resulting from
the construction are such as will satisfy
the following more important condi-
tions : — 1st. The curvatures assigned to
each surface are more moderate than
in any other theoretical combination.
2nd. The exterior surfaces of the com-
pound lens vary within narrow limits by
any variation in either the refractive
or dispersive powers that generally occur
in practice. 3rd. That the two interior
surfaces approach in all cases so near
to coincidence, that no sensible error
can arise from neglecting their dif-
ference ; finally, he states as a theorem,
which will be found sufficiently exact in
practice, that a double object-glass
will be free from aberration, provided
the radius of the exterior surface
of the crown lens be 6.720, and of
the flint 14.20, the focal length of the
combination being 10.00, and the radii
of the interior surfaces being computed
from these data so as to make the focal
lengths of the two glasses in the direct
ratio of their dispersive powers. Fig.
25, is a section of this object-glass, the

anterior glass A, or that which receives
the incident ray, is an unequally convex

Fig. 25.

lens of crown glass, the flatter side being
placed outermost; the posterior glass
B is a meniscus of flint glass.

The rule here stated is given only as an
approximation, and will no doubt be suf-
ficiently exact for ordinary practical pur-
poses ; but when object-glasses of great
aperture and value are to be constructed,
their radii must be computed more
strictly, and for this purpose we shall
subjoin; Mr. HerschelTs TABLE, calcu-
lated upon the rigorous formulae as
given in the Philosophical Transactions
for 1821 :—

Dimensions of an Aplanatic Double Object- Glass.

Refractive index of the Crown Lens

Ditto ditto of the Flint Lens. . ,

1.524

1.585

> Compound Focal Length, 10,000.

1st Surface Convex.

SdSurface

SdSurface

4th Surface Convex.

Jto

a

Convex-

Concave.

I

<u

Bo«g

22 o

.

SS*o

go -g

i-!

0

ii

!*J

'i0- g

i|

111

I;!

o

S -

1

-I

° ° «

o <3~

Illl

ri

»

tn

a
I

.1

° ° D •

.2 s?'u;5

I0t0 4)
|||l

M
J

5

I

1

IS

i««i

l*«.s

}i

1*^0

Is^l

1

o

ft

K

£.SU

•s.sE

^'"°

t£'r:*

0.50

6.7185

+0.0500

— 0.0036

4.2827

4.1575

14.3C97

+0.9921

— 0.3962

5.0

10.0000

0.55

6.7184

+0.0740

-0.0011

3.6332

3.6006

14.5353

+ 1.0080

—0.5033

4.5

8.1818

0.60

6.7069

-1 0.0676

4- 0.0037

3.0488

3.0640

14.2937

+ 1.1049

—0.5659

4.0

6.6667

O.G5

6.7316

+0.0563

+0.0125

2.5208

2.5566

13.5709

+ 1.1614

—0.6323

3.5

5.384G

0.70

6.8273

+0.0335

+ 0.0312

2.0422

2.0831

12.3154

+ 1.1613—0.7570

3.0

4.2858

j

0.7517. 0816

-0.0174

+0.0568

1.6073

1.6450

10.5186

+ 1.3847

-0.7207

2.5

3.3333

OPTICAL INSTRUMENTS.

The dimensions in the table are com-
puted on the supposition of the focal
length of the object-glass being 1 0 ; and
to adjust them to any other assigned
focal length, all that is required is to
increase or diminish the radii here set
down on the proportion of the assigned
focal length (in inches, feet, or parts of
any given scale) to ten parts of the
same scale.

When the refractive powers of the
two media are exactly 1.524 and 1.585
(which are nearly their average values)
respectively, and the dispersive ratio is
any one of the numbers in the first
column, this table gives at once the
exact values of the radii required ; but
when this is not the case, we must
proceed as follows :' — Suppose (for ex-
ample's sake) we would find the proper
radii for the surface of an object-glass
of 30 inches focal length : the refractive
index of the crown lens being 1.519,
and that of the flint 1.589, the disper-
sive power of the former being to that

of the latter 'as 0.567.1 or 0.567 being
the dispersive ratio.

The computation must first be made
as for an object-glass of 1 0 inches focus,
and first, we must determine the focal
length of the separate lenses, to this end.

1. Subtract the decimal (0.567) re-
presenting the dispersive ratio from
1.000, and the remainder multiplied by
1 0, is the focal length of the crown lens
(in this case 10 x 0,433, or 4,330.)

2. Divide unity by the decimal above
mentioned (0.567,) subtract 1,000 from
the quotient and multiply the remainder
by 10, and we get the focal length of
the flint lens. In this case before us

g-^- = 1.7635 and 0.7635x10= 7.635

is the focal length required. We must
next determine by the tables the radii
of the 1 st and 4th surfaces for the dis-
persive ratios their set down, (0.55 and
0.60), next less and next greater than
the given one. For this purpose we
have

Refractive powers given 1.519
Ditto ditto in the table 1.524
Differences

The given refraction of the crown
being less and the flint greater, than
their average value on which the table
is founded. Looking out now opposite

- 0.005 and + 0.005

to 0,55 in the first column for the varia-
tions in the two radii corresponding to
a charge of + 0,010 in each of the two
refractions, we find as follows : —

For a charge = +
Ditto ditto = +

1st surface 4th surface.

0.010 in crown -f 0.0740 + 1.0080
0.010 in flint — 0.0011 — 0.5033

But the actual variation in the crown
instead of + 0.010, being + 0.004, we
must take the proportional parts of

these, changing the sign in the case of
the crown : Thus we find the variations
of the first and last radii to be : —

For — 0,005 variation in crown —
For + 0.004 ditto in flint -

1st surface.
0.0370
0.3004

4th surface.
0.5040
0.2010

Total variation from both causes — 0.3374 and — 0.7053
But the radii given in the table are + 6.7184 and + 14.5353
Hence the radii interpolated are, 6.6810 and 13.8300

If we interpolate (by a process exactly similar) the same two radii for a "dis-
persive ratio 0.60, we shall find respectively: —

1st surface. 4th sxirface.

For — 0.005 variation in crown — 0.0338 and — 0.5524

For + 0.004 ditto in flint + 0.0015 — 3.2264

Total variation — 0.0323 and - 0.7788

Radii in the table 6.7069 14.2937

Interpolated radii 6.6746 and 13.5149

OPTICAL INSTRUMENTS.

23

Having thus got the radii correspond-
ing to the actual refractions, for the two
dispersive ratios 0.55 and 0.60, it only

For 0.600
For 0.550

Diff. + 0.050

We then say as 0.050 : 0.567—0.550
= 0.017:; -0.0064 : —0.0022 and 0.50

: .017:: -0.3151 : -0.1071, so that

6.6810-0.0022 and 13.8300 -0.1071 ;
or 6.6788 and 13.7229 are the true radii
corresponding to the given data :

Thus we have in the crown lens : —

Focal length =4.3300)

Radius of first surface = 6.6788 >
Index of Refraction = 1.519o)
From which data it is easy to com-
pute, by rules familiar to every optician,
the radius of the other surface, which

remains to determine their values for
the intermediate ratio 0.567 by propor-
tional parts thus : —

1st radius.

6.6746

6.6810

4th radius.

13.5149

JL3.8300

-0.3151

will come out, 3.3868. Again in the
flint lens we have for the

Focal length = 7.635

Radius of first surface = 13.7229
Index of refraction = 1.589

Whence we find 3.3871 for the radius
of the other surface.

The four radii are thus obtained
for a focal length of 1 0 inches ; and
to obtain them "for 30 inches we have
only to multiply them by 3, and we
obtain finally. In the case proposed,
the

Radius of 1st surface, of 2nd, 3rd. 4th.

20.0364 inches, 10.1604 inches, 10.1613 and 41.1687.

So that here the radii of the two ad-
jacent surfaces scarcely differ more
than T^ooth of an inch, and may of
course be cemented together, should it
be thought desirable.

(36.) The triple object-glass is con-
structed in the same manner as the
double, but as we have two convex
lenses to produce the required refrac-
tion, the total spherical aberration will
be less in a triple lens than in a double
one, and, therefore, requires less correc-
tion by the flint ; while the secondary
spectrum may be greatly diminished by
making one convex lens of crown, and
the other of Bohemia, or Dutch plate
glass. The theorems for finding the
proportionate foci of the two lenses are
indeterminate ; for theoretically, it is im-
material whether their foci be equal, or
in any other proportion, provided the
compound lens be in the ratio of the
dispersion of the flint. Again, the radii
of the concave may be varied to any
convenient curvature, as there are two
convex lenses, and, therefore, the aber-
ration cannot be greater than the con-
vex lenses will correct*. When the
radius of each surface is equal, and the
ratio of dispersion and refraction is the

* In a double achromatic object glass, the convex
lens should be assumed tirst, as the flint might be
formed with more aberration than the convex could
correct.

same as in the double achromatic object-
glass, and the compound focus of the
lens is 30 inches, the radii of the first
convex

21.35.
.93.

will be

fr = 21.
1R = 15.

of the concave

second convex

fr"
-j R,,

= 21.35.
15 g3

(37.) The largest triple achromatic
telescope ever constructed, has lately
been erected in the observatory of the
imperial university at Dorpat, on the
I Oth of November, 1824, and was made
by Fraunhofer, the late director of the
Optical Institute, at Benedictbauern,
near Munich.

The concave is formed from a piece
of dense flint glass made by Guinand,
and has a greater dispersive power than
any obtained before. It is perfectly
free from veins ; and the diameter of
the object-glass exceeds that of any
other telescope, having a clear aperture
of 9T6o inches, and a focal distance of
25 feet. This instrument is mounted on
a metal stand, and although of the im-
mense weight of 5000 Russian pounds,
is moveable in every direction with the
slightest exertion, all the moveable parts
being balanced by counter- weights. It
has 4 eye-glasses, the lowest magnifying

24

OPTICAL INSTRUMENTS.

175 times, and the highest 700 times:
the cost of this instrument was 1300/.
sterling. Another telescope has lately
been made of similar materials in Eng-
land by Mr. Tulley ; the aperture of the
object-glass is 6T8?, and its focal length
is 12 feet. This instrument is now in
the possession of Dr. Pearson.*

(38.) The Galilean telescope was in-
vented in 1590, by the illustrious person
from whom it derived its name ; but as it
is susceptible of little improvement from
the nature of its construction, it is sel-
dom used except for opera glasses, in
which the shortness of the construction
renders it available. It consists of a sin-
gle convex or achromatic object-glass,
whose focal length is usually from 4 to
8 inches, which it rarely exceeds. The
eye-glass is a double or plano-concave
lens from £ an inch to 2 inches focus ;
the distance between the two glasses
is equal to the difference of their focal
lengths, and the power is in the ratio of
their foci, as in the astronomical tele-
scope. Pig. 26 is a section of the Gali-

lean construction for an opera-glass.
Let O be the object-glass of 6 inches
focus, E the concave eye-glass 2 inches
focus ; the distance O E will be 4
inches, and the power will be expressed
by 6 inches divided by 2 inches, equal
to 3 times. The distinctness of the Ga-
lilean construction exceeds that of any
other, and arises from the rays of light
proceeding from the object directly
through the lenses without crossing or
intersecting each other ; whereas, in
the combination of convex glasses, they
intersect one another to form an image
in the focus of the object-^lass ; and
this image is magnified by the eye-lens
with its imperfections and distortions.
With a power of 8 and a 12 or 16 -inch
object-glass, the satellites of Jupiter
have been distinctly observed ; while a
common astronomical telescope of 4 or
5 feet focal length has scarcely rendered
them visible. The area or field of view
in this instrument is very limited, and,
on that account, it cannot be used for
high powers, as the objects seen at one

«r. 26.

view are always as the area of the pupil
of the eye, and not as the area of the
eye-glass, as in convex lenses. Thus, if
ab (fig. 20.) is larger than the pupil of
the eye they will not be seen, although
refracted by the lens E ; but it should
be remembered, that as the rays, when
they have passed through the eye-glass,
are not converged to a focus, the nearer
the eye is placed to the" lens E, the more
numerous will be the objects seen at one
view.

(39.) The construction of opera-
glasses might be achromatic, when
made with only two lenses, provided the
focal length of the object and eye-lens
are in the ratio of the dispersive and
refractive powers of the media from
which they are formed. Thus, if an
object lens be made of rock-crystal,
whose focal distance is 5, and the eye-

* See Astronomical Transactions, vol, ii.

lens be formed of oil of cassia," fixed
between two parallel pieces of glass, or
other convenient substance so curved
as to give a focus of 1.02, the combina-
tion will be achromatic, with an amply-
fying power of 4T9o times. If the con-
cave lens had been formed of flint-glass,
with the same object lens, a magnifying
power of two might be obtained, and
the combination would be free from all
colour. If oil of aniseed were used for
the concave, the power obtained to make
the instrument achromatic would be
2.82 times*.

The following TABLE exhibits the
refractive and dispersive powers of diffe-
rent substances capable of producing
achromatic combinations ; and it should
be remarked, that the medium used for
the eye-lens must have the greatest dis-
persive power : —

* Dr, firewater's treatise on new optical Instru-
cts.

OPTICAL INSTRUMENTS.

25

Substance! to be employed for Refr.ictiv-

Kye-Lenses. Power.

1 . Oil of Cassia 1 . 64 1

2. Aniseed 1.601

8. Cummin 1.508

4. Cloves 1 . 535

5. Sassafras 1.532

6. Sweet Fennel Seed 1 . 506

7. Spearmint .... 1.481

8. Pimento 1.507

9. Flint Glass ., .1.616

CHAPTER IX. — Dr. Brewster 's Telescope
for measuring Distances - — Double
Image Telescope — Graphic Telescope
— Achromatic Eye-Pieces.

(40.) A telescope for measuring the
distances of objects was invented by Dr.
Brewster, for which he obtained a pa-
tent in 1810. When describing the tele-
scope/g-*. 15 and 16, it was stated, that
as the object approached the instru-
ment, the eye-tube required to be drawn
out to adjust the instrument for distinct
vision : now the measure of the quantity
required to be drawn out, if registered
on the sliding tube by divisions, and by a
corresponding mark on the outer tube,
would determine the distances of the ob-
jects. The increase of the focal length
of the object-glass may be found by
dividing the square of the focal length
of the object-glass by the distance of
the object, minus the focus of the ob-
ject-lens : thus, if the focus of the ob-
ject-lens is 2 feet, and the distance of
the object 50 feet, the tube must be

persire

ower.

.139

Substances to be employed fof
Object-Lenses.

Befraetive
Power.

.534

Dispersive
Power.

036

.077

Plate Glass

.527

032

.065

Water

. 336

035

.062

874

029

.060

Sulphuric Acid

440

031

.055 1

. 368

032

.054

562

026

.052 ;

Topaz ..... ...

638

024

.048

2 470

038

.052 |
.048

drawn out

or 1 inch from the

oO — 2

solar focus, to adjust for distinct vi-
sion of the object ; but the length of
these divisions would be in a decreasing
ratio, and the quantity is almost imper-
ceptible for great distances : thus, if two
objects be 100 feet apart, and the near-
est 200 feet from the observer, the dif-
ference of adjustment for these two ob-
jects would be only Tfj(j of an inch. In the
patent construction this quantity may be
increased to almost any required length,
by means of two object-glasses; the
inner one is about £ of the focal length
of the principal lens, and being fixed to
the eye-tube slides along with the eye-
piece. This instrument has been found
useful at sea in determining the distance
of head-lands, and in war, when an
enemy's ship is gaining sail ; this is de-
termined by taking two observations,

and noting the difference of the quan-
tity required to be drawn out or pushed
in to obtain a distinct vision ; and when
the time between each observation is
known, the rate of the vessels sailing
may be ascertained.

(41.) Another instrument, nearly for
the same purpose as the one just de-
scribed, but possessing more accuracy
for measuring angles and distances, was
constructed by the Abbe Rochon. This
instrument is the ordinary telescope,
with a double refracting prism of rock-
crystal, placed before the eye-glass ; this
prism produces two images of an object.
These images may be made to approach
to or recede from one another by alter-
ing the distance of the prism from the
eye-glass; but the object is better
effected by a method proposed by Dr.
Brewster, in which the first glass of
the telescope, together with the prism,
which may be cemented to it, is made
to slide to or from the other glasses of
the eye- piece, and the quantity moved is
registered by divisions on the tube.
This variable eye-piece, which changes
the power of the telescope, is so ad-
justed, that the two images of the object
shall be in accurate contact ; when this
is done, if an observation is made to find
the angle, the index, by previous expe-
riment, will give it ; but when it is desi-
red to know the rate of motion of the
object, another observation must be
made at a determinate interval, and the
difference of adjustment required in the
two observations to bring the images in
contact will determine its distance, the
scale being previously ascertained by ex-
periment or calculation. The imperfect
crystallization of quartz, and the difficulty
of working it, led Dr. Brewster to adopt
the colourless topaz of New Holland to
form his double-refracting prisms, it
being much freer from veins and imper-
fections, besides possessing the advan-
tage of a lower dispersive power. In

26

OPTICAL INSTRUMENTS.

certain sections of the crystal, when we
require only a very small separation of
the images, we may preserve, on both
sides, the natural surface of the cleav-
age. The other means of procuring a
double image proposed by Dr. Brewster,
we shall subjoin, as being well worthy
consideration for micrometical purposes.
1 . The double image may be produced
by a small bisected plane speculum,
placed between the eye-lens and the eye,
and one of the halves may be made to
move by a screw, not for the purpose of
bringing the images in contact, but in
order to vary the constant angle, ac-
cording as it is wanted, for large or
small discs. 2. The duplicature of the
image may be effected by bisecting the
eye-lens, or by placing a bisected lens
between the eye-lens and the eye. 3.
The two images may be formed by a
slightly-inclined face, ground upon a
highly-polished and parallel plate of

fluor-spar, one" image being seen by half
of the pupil through the parallel plate,
and the other through the inclined face.
Fluor-spar is recommended because of
its producing a less dispersion under a
given angle than any other substance ;
and even this might be removed by the
ordinary means.

(42.) The graphic telescope is em-
ployed for the delineation of objects
situated at any distance from the in-
strument, which may be represented of
any required size ; and is used for draw-
ing portraits, landscapes, and architec-
tural subjects. It was invented in 1811,
by Mr. C. Varley,who obtained a patent
for it. This instrument consists of an
astronomical telescope of low power,
placed between two plane reflecting
specula, with a particular construction
of the eye-piece to correct the distortion
of the image produced by the eye-glass.
Fig. 27, is a section of the instrument.

Fig. 27.

O is the object-glass, E the eye-glass,
and F F two meniscus field-glasses,
whose form and distance from the eye-
glass are so adjusted as to produce a flat
and enlarged field of view. The rays of
light proceeding from an object enter
the side of the tube, and impinge upon
the flat speculum s, placed at an angle
of 45° with the axis of the telescope,
as is shown in section at A ; the rays
will be reflected by this speculum along
the axis and through the object-glass O,
and are converged to a focus near the field
lens F, where an inverted image of the
object will be formed and may be received
by the eye-glass E ; lastly, they will
strike the other inclined flat reflector, and
be reflected up to the eye placed above
at a. When a piece of paper is placed
below the speculum r, the observer being
supposed to look down on the specu-
lum, keeping both eyes open, he will be
able to see an image of the object on the
paper when the instrument is adjusted
to its proper focus. This representation

maybe traced with one or both eyes, and
the size of the image may be varied by
altering the distance of the paper from
the speculum, or by changing the mag-
nifying power of the instrument. If the
first speculum s be taken away, the
sides of the objects are reversed (as on
an engraved copper-plate,) the specu-
lum r forms the image erect, which was
inverted by the eye-glass E, and pre-
vents the telescope from intercepting a
view of the paper.*

* The inventor of this instrument has employed it
very extensively in sketching from nature ; and in the
mountainous districts of Wales the views become of
great value on account of their accuracy ; he has also
made with this instrument some very correct views of
the seat of the late Lord Byron, at Newstead Abbey,
Nottinghamshire. In drawing shipping and boats it
is extremely valuable, as the numerous lines in the
details of these objects are not of a geometrical
figure. We have been informed that this instrument
was the one employed in making the panoramic
view of the metropolis from the top of St. Paul's,
for the exhibition of which a building in the Regent's
Park, called the Coliseum, has recently been erected.

OPTICAL INSTRUMENTS.

27

(43.) The achromatic or negative
eye-piece generally adapted to astrono-
mical telescopes, is a combination of
lenses intended to correct the dispersion
produced by the eye-glass, independent
of the object lens. Let O, Fig. 28, be

the object-glass of a telescope free from
chromatic dispersion, and E the eye-
glass ; let F be the focus of the object-
glass O, where an inverted image is
formed. Now the pencil of white light
A a b, when transmitted by the lens E,

Fig. 28.

will be divided into its component co-
lours, so that b R will be the direction
of the red rays, and b V the path of the
violet, and the angle V b R in crown-
glass will be oV °f a b R. The rays
B a d passing through a part of the lens
whose surfaces are less inclined to each
other, will be less refracted and less
dispersed in the same proportion nearly,
and d I will be the path of the red rays,
and dv of the violet ; therefore, the two
violet rays will be very nearly parallel

when the two red rays are rendered
parallel. Hence it must happen, as
coloured rays do not unite at the bottom
of the eye, that the object will appear
bordered with coloured fringes, and a
black line seen near the margin on a
white ground will have a ruddy and
orange border on the outside, and a
blue border within, and this confusion
will increase nearly in the same pro-
portion as the visual angle b I c.

(44.) Fig. 29, is a section of the
achromatic eye-piece. Let A B be a
compounded pencil of white light pro-
ceeding from the object glass, B F a
plano-convex field glass, with the plane
side next the eye-glass E. Now the
red rays of the pencil A B after refrac-
tion would cross the axis in R, and the
violet rays in V, but meeting the eye-
glass E, the red rays will be refracted
to c r, and the violet to c v, when they
will cross one another, in the axis at
the point c and unite ; for the violet ray
being nearer the axis of the lens E,
will suifer less refraction than the red,
and when the eye is placed in the axis
ate, the object will appear colourless.
The distance of the two lenses F E, to
produce this correction, when made of

crown glass, whose dispersion is as 77 to
78, when the incidence is 50, must be
equal to half the sum of their focal dis-
tances nearly ; or, more exactly, the
distance between the two lenses must
be equal to half the sum of the focal
distance of the eye-glass, and the dis-
tance at which the field-glass would
form an image of the object-glass ; for
the point R is the focus to which a ray
coming from the centre of the object-
glass is refracted by the field-glass, con-
sequently, this distance must be varied
according to the distance of the objects,
and also, as the length of the object-
glass ; for the same combination will
not be correct for a long and a short
object-glass, nor for celestial and ter-
restrial observations ; for, when it is

28

OPTICAL INSTRUMENTS.

correct for one case, and the distance
of the image or focal point is varied,
the divergency of the rays will alter,
and the dispersion will be either too
much or too little corrected. The pro-
portion of the foci of these two lenses
should be as 2 to 3, to produce the
largest field and the least distortion.

(45.) The eye-pieces employed to
produce an erect image for terrestrial
telescopes admit of numerous arrange-
ments : the simplest was shown in fig. 1 6,
composed of three similar lenses placed
at equal distances ; but as there is no
field-lens in this construction, the view

is limited, though the chromatic dis-
persion is in part corrected by the
middle lens, and may be totally de-
stroyed by using an eye-glass of shorter
focal length than the other two, and at
a less distance from the middle glass.
Another combination may be produced
by two glasses so placed that the lens
which receives the inverted image
formed by the object-glass shall form
an image on the other side, inverted
with respect to the first, but erect with
the object, and this image may be viewed
with an eye-glass. This construction
is shown in jig. 30. Let i be the in-

Fig. 30.

verted image formed by the object-glass,
o a a pencil of light from the centre of
the object-glass falling on the lens A ;
this pencil will be dispersed after re-
fraction, and a r will be the path of the
red rays, and a v of the violet .; at the
focal point e an erect image will be
formed, and may be viewed by the lens
E. Now the red rays at r falling on
this lens will be less refracted than the
violet at v, and will cross the axis in R,
while the violet v crosses the axis
at V ; hence the dispersion will be
greatly increased both by reason of the
spherical aberration, and their greater
refrangibility, so that objects seen
through this eye-piece will be more

fringed with colours than in any other
construction.

(4G.) The last construction might
be improved by using two lenses dis-
posed in a similar manner to the nega-
tive eye-piece (44,) instead of the lens
E. These would enlarge the field and
correct the dispersion ; but as the sphe-
rical aberration is very great, two lenses
should be used in place of the lens
A, in order to make the combination
complete ; thus a four-glass eye-piece
would be formed that is perfectly cor-
rected, if duly proportioned distances
and foci are used. This is accomplished
in the following combination (fig. 31.)
where the lenses are in the order of the

Fig. 31.

letters A B C D beginning with the
lens next the object-glass, and the dis-
tances, &c. of each lens are as follows :

1.18, B C = 1.83, C D = 1.105. Stops
should be placed between the lenses
A and B, and a larger one between C
focal length of A=l| = B = 2J, C = 2 and D ; these stops are to prevent any
an dD = l£, and their distances AB = 2£, false light from passing through the

lenses to the eye. The more black stops
there are introduced into a telescope,
provided they do not hinder the pencils
of light proceeding from the object, the
better will the instrument perform.

BC = 3|,CD = 2f. This eye-piece will
be nearly free from chromatic dispersion
and aberration. In a very good eye-
piece of Ramsden's, the focal lengths
were found to be of A.= . 0775, B= 1.025,
C = i.oi, D = .79 ; the distances A B =

OPTICAL INSTRUMENTS.

29

(47.) Tho brightness of any object
seen through a telescope, in comparison
with its brightness when seen by the
naked eye, may, in all cases be easily
found by the following formula. Let n
represent the natural distance of a visi-
ble object at which it can be distinctly
seen, and let d represent its distance
from the object-glass of the instrument.
Let m be the magnifying power of the
instrument, that is, let the visual angle
subtended at the eye by the object when
at the distance n, and viewed without
the instrument, be to the visual angle
produced by the instrument, as 1 to m.
Let a be the diameter of the object-
glass, and p be that of the pupil. Let
the instrument be so constructed that
no parts of the pencils are intercepted
for want of sufficient apertures of the
intermediate glasses. Lastly, let the
light lost in reflection or refraction be
neglected.

The brightness of vision through the
instrument will be expressed by the

fraction ( an \ the brightness of na-

\mp d)

tural vision being 1. But although this
fraction may exceed unity, the vision
through the instrument will not be
brighter than natural vision. For when
this is the case, the pupil does not re-
ceive all the light transmitted through
the instrument.

In microscopes, n is the nearest limits
of distinct vision nearly eight inches ;
but a difference in this circumstance,
arising from a difference in the eye,
makes no change in the formula, be-
cause m changes in the same proportion
with n.

In telescopes n and d may.be ac-
counted equal, and the formula be-

comes *

CHAPTER X.— Stands for Telescopes —
Method of making Grinding and Po-
lishing Specula and Lenses — Method

' of centering and adjusting Lenses.

(48.) To describe the numerous va-
rieties of stands or supports for tele-
scopes would be both prolix and tri-
fling, as different artists generally adopt
such contrivances as they think most
likely to please the fancy of the pur-
chaser ; but the chief consideration in
a scientific point of view is, to obtain
a steady and immoveable stand free

* Barlow.

from vibration. To effect this, ' the
instrument should be supported at
both ends, to give steadiness, and to
prevent its being affected by the wind ;
for every vibration will be increased in
the sanis ratio as the amplification of
the instrument, and produce a tremu-
lous or dancing motion in the objects.
Thus a superior telescope badly sup-
ported may be rendered inferior to a
common one fixed on an immoveable
stand. The materials of which stands
are composed should be capable of
transmitting as little vibration as pos-
sible ; thus, the vibration of a frame of
cast iron in one piece, although per-
fectly steady, would be sufficient to de-
stroy distinct vision. Wooden stands
are preferable, provided firm diagonal
braces are used, so as to form immove-
able triangular frames. Where iron is
required for durability, plates of lead
should be screwed between each piece to
stop the vibrations. In reflecting tele-
scopes the difficulty of preventing vibra-
tions greatly impairs their value ; for the
metals, particularly the small one, are
easily set in vibration, and unless the arm
of the small metal is damped, the vision
is frequently indistinct. When a Grego-
rian telescope has been taken fronTits
stand, and placed on a lump of soft
clay, the distinctness has been such as
to enable a person to read a bill placed
at 900 feet ; while on the stand it could
be read only at the distance of 650 feet,
although no apparent tremor could be
discerned when on the stand.

(49.) A good composition for the
specula of reflectors is one of the most
important desiderata in the making of
telescopes. The qualities most in re-
quest are, a sound uniform metal, free
from all microscopic pores ; not liable
to tarnish by absorption of moisture
from the atmosphere ; not so hard as to
be incapable of taking a good figure
and exquisite polish, or so soft as to
be easily scratched ; and possessing a
high reflective power. The various com-
positions employed for specula differ
more in the admixture of minor ingre-
dients than in their essential materials.
Copper and tin (bronze metal) are the
metals mostly employed, with small
quantities of arsenic, silver, and brass.
The proportions generally employed are,
copper 32 parts, grain tin 15, with the
addition of two parts of arsenic to ren-
der it more white and compact. The
Rev. Mr. Edwards, in a treatise annexed

30

OPTICAL INSTRUMENTS.

to the Nautical Almanac for 1 787*, says
that if 1 of brass, and 1 of silver, be
used with only one of arsenic, a most
excellent metal will be obtained, which is
whiter, harder, and more reflective than
any other he ever met with. With re-
spect to the practical value of this com-
position we cannot speak, but having
made specula for reflecting instru-
ments ourselves, we can vouch for the
goodness of the following, both with
respect to the exquisite figure and polish
it is capable of assuming, and its free-
dom from pores. To make this compo-
sition, take two parts of copper, as pure
as it is possible to be procured ; (for the
goodness of the speculum will depend
on the purity of the materials employed)
this must be melted in a crucible by
itself ; then put in another crucible, 1
part of pure grain tint. When they
are both melted, mix and stir them with
a wooden spatula, keeping a good flux
on the melted surface to prevent oxida-
tion : this metal must be quickly poured
into the moulds, which may be made of
founders' loom ; the intended face al-
ways being downwards. Where the spe-
culum is required particularly good, the
best mode of casting is to have an iron
mould made with a vertical tube at-
tached on one side, and the bottom of the
tube to end in a bulb ; the melted metal
is then to be poured down the tube, and
will fill the bulb and mould, leaving a
sufficiency in the tube to give pressure.
The bulb being lower than the mould
will retain any dense impurities, and
the tube the lighter ones, while the spe-
culum will be uniform and dense.

Having thus procured the speculum,
the next thing will be to grind it to the
required figure ; this is effected on a con-
vex brass or hard metal circular tool,
carefully turned to a gauge of the required
curve. This tool is fixed on a post or ug-
right, and the speculum is held in the
hand by means of a convenient holder
cemented on its back. The grinding is
then commenced with coarse emery-
powder and water, when the roughness
is taken off by moving the speculum
across the tool in different directions
walking round the post: finer emery

* This treatise which is now very scarce, is re-
publishing in the Technological Repository.

t It is most probable that the best proportions
would be as their respective atomic weight, that is,
thrice 32 of copper, and 58 of tin, as the metal would
then be more intimately combined.

is used in the same way, till the surface
of the speculum has become uniform.
The next step will be to smooth it by
means of fine washed flour emery, gra-
dually passing from one degree to the
next finer, and washing the tool and
speculum between each application of
emery, to prevent any gritty particles
from scratching the metal. When
the speculum is completed, and of
the required figure, it is next to be
polished. This is done either by taking
a convex tool similar to the grinder,
or the grinder itself, and covering it
with pure pitch evenly spread over
its surface ; while warm a concave tool
of the same figure as the speculum is
then worked over its surface wet.
When the proper figure is obtained,
washed putty (i. <?. combined oxide of tin
and lead) is poured on the pitch, and the
speculum polished thereon by moving
it as before. During the process of
grinding and polishing, the tools must
be carefully examined by the gauge, and
if they happen to get out of the true
figure, the speculum must be worked
more on the edge, or middle, as the
case may require. Instead of the ver-
tical post above mentioned, a lap is
sometimes employed, which produces a
much better figure and more expedi-
tiously. A lap consists of a common
lathe communicating a slow and regu-
lar motion to a vertical mandral, on
which the grinding or polishing tool is
fixed ; in using the lap, the artist is en-
abled to stand in the same place, and
has more command over the work.

Lenses are ground precisely in the
same manner as specula, but the po-
lishing is different. Here the concave
or convex polishing tool is made of
brass, and when turned of a proper
curve, a smooth thick piece of felt
(cloth) is stretched over the tool and
cemented to it ; the outer surface is then
imbedded with washed putty powder.
After this is done, the lens, or block of
lenses, is worked on it with cross mo-
tions ; if the powder be employed too wet
the fibres of the cloth will rise up, and
polish not only the surface, but also the
small hollow s left in the grinding. This
effect, from the nature of the polishing
surface being heterogeneous, generally
takes place to a greater or less extent
when viewed by a microscope ; these
cavities being polished admit the light
and disperse it, instead of it being col-
lected as with a uniform surface. When

OPTICAL INSTRUMENTS.

31

these faults are visible to the eye,tthe lens
is called curdled. If we are desirous of
procuring; an uniform and perfect sur-
face, the polishing tool must be homo-
geneous, and the best material for its
formation is good clean bees wax, hard-
ened by the addition of red sulphate of
iron, dry and finely washed. This com-
position, when of the proper temper, is
melted over the brass tool ; and when
cold can be turned to the required
curve. The advantage of this improve-
ment, besides its uniformity, is that
should any hard scratching particles
insinuate themselves between the tool
and glasses, they sink and are imbedded
in the wax, and thus their injurious
effects are prevented. The polish of
lenses made in this manner is clear and
defined when examined by a micro-
scope ; when the shadow of a bar is
brought across them. This method is
now employed by one of the first opti-
cians in the metropolis.*

Centering of Lenses. — The centering
of lenses for accurate instruments is of
great importance, more especially for
the object-glasses of achromatic tele-
scopes. Different opticians employ their
own methods, but one of the best is
done by reflection : let the lens to be
centered be cemented on to a brass
chuck, having the middle turned away
so as not to touch the lens, but near the
edge, which will be hid when mounted ;
this rim is very accurately turned flat
where it is to touch the glass. When the
chuck and cement is warm it is made to
revolve rapidly: while in motion a lighted
candle is brought before it and its re-
flected image attentively watched. If this
image has any motion, the lens is not
flat or central : a piece of soft wood
must therefore be applied to it in the
manner of a turning tool, till such time
as the light becomes stationary. When
the whole has cooled, the edges of the
lens must be turned by a diamond, or
ground with emery. This method of
centering and adjusting object-glasses
by their reflected images, was laid before
the public by Dr. Wollaston, and has
been used by our first opticians for a
considerable time.

' * The method of grinding and polishing lenses
from diamonds for microscopes, by Mr. Pritchard,
will be found in the Quarterly Journal of the Royal
Institution, vol. ii, page 14, new series.

Concluding Remarks on Telescopes.

(48.) The applications of the telescope to
the purposes of man are so numerous,
that their details would far exceed the
boundaries of our treatise. Amongst
its principal uses, however, besides
those accompanying the descriptions of
the various modifications of that in-
strument, may be enumerated the fol-
lowing : — The accurate determination of
the longitude of the various places on
the earth's surface, is ascertained by the
telescope, by observing the immersions
and emersions of the four satellites of
the planet Jupiter ; from thence, by the
aid of a good chronometer, with the time
of any known place, the situation of the
unknown spot is determined. Before
the invention of the telescope, naviga-
tors were compelled to keep within sight
of the coast in sailing from one country
to another, and thus were often endan-
gered while passing an hostile or rocky
shore ; by the assistance of this instru-
ment, the voyage is made direct to the
intended place without fear or danger.

To the astronomer the telescope is his
principal and most important guide. It
enables him to determine, with precision,
the transits of the planets and stars
across the meridian. The computation
of astronomical and nautical tables, to
determine the revolution of the planets on
their axes, and their relative polar, and
equatorial diameters, is derived from ob-
servations by the telescope. We are by
this instrument enabled to discover the
analogy between the laws which govern
the motions of the planets and those of our
earth ; their parallax, and from thence
their distances. The aberration of light
and the motions of the siderial systems
in space, unfold wonders which must
excite the imagination of the most pro-
found philosophers in the highest pos-
sible degree. The harmony and sim-
plicity displayed in such immense
worlds prove the design and wisdom by
which they were created ; and the won-
derful facts thus ascertained raise the
most ordinary mind up to a sublime
contemplation of the great Creator.

In surveying of land, the telescope is
highly useful, and for this purpose is
mounted on a stand, with an horizontal
and vertical motion registering by divi-
sions the degrees and minutes of inclina-
tion or position of the instrument. For
the more accurate reading o^these divi-

32

OPTICAL INSTRUMENTS.

sions, the two limbs are furnished with
a Vernier's scale. Spirit levels and a
magnetic needle are usually attached to
the^instrument ; and from the purposes
to which it is applied, a telescope with
this mounting is called a Theodolite,
derived from" two Greek words ^a<
to see, and «V- the way or distance, —
that is, an instrument for seeing or de-
termining distances. The method by
which the distances and heights of re-
mote objects are ascertained is by mea-
suring the angles subtended by the ob-
ject, and computing trigonometrically
therefrom.

The continental writers have much
exaggerated the powers and penetration
of the telescope ; indeed the time is
not far distant, when it was gravely as-
serted, that works of art had been re-
cognised in our satellite the moon. The
fallacy of this circumstance may be
easily shown to our readers by the fol-
lowing simple considerations.— Let a
person direct the tubes of a telescope
(without the glasses) to any celestial
object, and there fix them ; he will soon
find that in a short space of time, the
object will have removed from before
the mouth of the tube. Now this mo-
tion of the celestial bodies, which is only
apparent, arises from the revolution of
our earth on her axis ; and the quan-
tity of this motion may be determined
with facility, thus : — the earth is known
to revolve once about her axis in 24
hours, and as eveiy circle is supposed
to be divided into 360 equal parts or
degrees, the apparent time any celestial
body takes to describe one degree, will
be found by dividing the 24 hours by
360, which gives us 4 minutes as the
time an object would pass the mouth of
the tube if it only takes in one degree
of the heavens.

Now, if we suppose the glasses to be
placed in the tubes, the magnfying power
of the instrument being 60, and we
direct it (as before) to an object, as the
moon, whose diameter is about half a
degree, the time of her passing or tran-
sit will be one minute, if the field of
view be, as in the ordinary telescopes,
about 30°, which the moon would ex-
actly occupy. If the power of the tele-
scope be increased 1 0 times, the eye-
piece having the same angle of vision,
onlv iis part of the moon would be seen
at once, 100 being the square of 10, the
increased power of the instrument ; and
the time this portion of the moon would

pass the telescope is 6 seconds. Again,
if we increase the power 10 times, so
that its linear amplification of an object
is 6000 times, only a Toioo part of the
moon's surface could be seen in the
field of view ; or the planet Saturn, whose
apparent diameter is 10 seconds, would
just fill it, and the time of their passing
the instrument would be only T6o of a
second.*

Having thus shown the amazing velo-
city a planet passes the mouth of a tele-
scope with these high powers, we shall
next proceed to point out the apertures
and amplification necessary for observ-
ing some given measure on the surface
of the moon. First, we must determine
the angle every object must subtend to
the eye, in order to render it visible :
this is found on an average for different
sights to be one minute, that is, when
an object is removed from the eye about
3000 times its own diameter it will only
be just distinguishable. From this we can
now determine the extent of the smallest
part of the moon's surface discoverable
by the unassisted eye. Its real diameter
being 2100 miles, which divided by the
number of minutes which its apparent
diameter subtends, (viz. 30,) gives us 70
miles as the measure of the least distinct
spot seen by the naked eye ; therefore,
we know that, if a telescope magnifies
70 times, we can just discern a spot
one mile in diameter on the moon's
surface ; and to recognise any object 1 0
feet in diameter, we shall find by this
rule the magnifying power of the tele-
scope must be 37100 times, and the
diameter of an object-glass or metal
for such an instrument may be found
by the method described in (18) ;
which if we suppose a pencil of rays
?V of an inch in diameter will admit
sufficient light to the eye, the diameter
of the speculum must be 62 feet, and
its focal distance 309 feet, when an eye-
glass of yUh of an inch is employed.
These calculations must convince the
reader of our inability for making such
observations ; for if the impossibility of
procuring such enormous instruments
were overcome, they would be so un-
wieldy as entirely to prevent our using
them.

* It is necessary for us to state, that, for the ob-
servation of small stars and nebulae with reflecting
instruments this power is occasionally employed ; as
even under these circumstances they possess no sensi-
ble diameter, and a regular motion is communicated
to the telescope in the plane of the equator by means
of a clock or other mechanism.

THE THERMOMETER AND PYROMETER.

THE ancients were unacquainted with
any more certain mode of marking
the variations of temperature, than
the indications of the senses, and the
limited knowledge derived from ob-
serving the melting or combustion of
different substances. In modern times,
.instruments have been invented for
noting variable degrees of heat and
cold, which, under the designation of
thermometers, or thermoscopcs, py-
rometers, or pyroscopes, are now in
general use in every part of the civi-
lized world. Their names are derived
from the Greek terms ttyus, M?, sig-
nifying heat, fire, and pir^ov, trxoveos, a
measure, an investigator.

The principle on which all such in-
struments are constructed, is the change
of bulk which every body undergoes by
alteration of its temperature.

All homogeneous bodies, except wa-
ter, within a few degrees of its freezing
point, expand by heat and contract by
cold.* Their expansion, then, may
afford a relative, measure of the in-
crease of temperature ; and their
contraction, of its diminution. This
law holds good in gases, liquids, and
solids ; and, accordingly, matter in those
three states of existence has been em-
ployed in the construction of instru-
ments for measuring the intensity of
heat and cold.

The changes of volume which gases
or aeriform bodies undergo, were first
employed for this purpose; liquids,
such as spirit of wine, oils, or mercury
were next used ; and lastly, the
changes in the bulk of solids were ap-
plied to measure the variations of
higher temperatures, which would have
too much expanded gaseous and liquid
bodies.

The designation of thermoscope or
pyroscope might be, with most pro-
priety, applied to such instruments ;
but, in conformity to common usage, it
is proposed in this treatise to apply the

* Clay, a seeming exception, is not a homogeneous
substance, of which afterwards.

general term thermometer to the in-
struments depending on the expan-
sions of aeriform and liquid bodies,
and pyrometer to those in which the
expansion of solids is the measure of
the elevation of temperature; and the
subject will be treated under the fol-
lowing heads.

I. Of the common Thermometer.

1 . Its history and construction.

2. The precautions necessary in its
construction and graduation.

II. Of the Pyrometer.

III. Of Register Thermometers.

IV. Of the Differential Thermometer,
and its modifications.

V. Of some peculiar applications of
the Thermometer.

VI. Of the imperfections common to
all instruments for the indication of
heat.

CHAPTER I.
Of the Common Thermometer.

§ 1. History and Construction of the
Thermometer.

THE invention of the thermometer, like
almost every other discovery of great
utility, has been claimed for different
philosophers ; and national vanity has
occasionally been enlisted in sup-
port of the pretensions of rival claim-
ants. There seem, however, but two
whose titles are worthy of notice.

The Italian writers generally give the
honour to their countryman Santorio
Saniorio, long a physician at Venice,
and afterwards a professor at Padua,
who flourished about the beginning of
the seventeenth century ; and who had
obtained just celebrity by his discovery
of the insensible perspiration of the
animal frame : the Dutch philosophers
as unhesitatingly ascribe it to Cornelius
Drebbel, a physician of Alkmaar, who
appears to have enjoyed a high reputa-
tion as a chemist, a mathematician,
and an inventive mechanical genius.

THERMOMETER AND PYROMETER.

Santorio expressly claims the inven-
tion as his own,* and he is supported
by Borelli t and Malpighi ; $ the title
ofDrebbel is considered as undoubted
by Boerhaave § and Musschenbroek.||
It would now be difficult, perhaps, to
decide the controversy ; but it is worthy
of remark, that Santorio, who was born
in 1561, and died in 1636,5[ did not
publish his claim to the invention till
1626;** and, although thermometers
are alluded to by Robert Find, within
the first quarter of that century, yet
as he travelled both in Germany and
Italy for six years, we can draw no
inference from that circumstance. Cer-
tain it is, that thermometers were con-
structed about the same time, both in
Italy, and in Holland, on the same prin-
ciple ; and though the instruments of
Drebbel were well known in Holland
and England, before the fame of San-
torio appears to have reached the
North-West of Europe, the most re-
cent writers have generally considered
the latter as the real inventor of the
thermometer. It is, however, by no
means improbable that each may be
justly entitled to the merit of a dis-
coverer.

Be this as it may, the instrument
was, from its imperfect construction,
of little use in the hands of either, and
required the successive labours of dif-
ferent philosophers to render it a tole-
rably accurate indicator of the varia-
tions of temperature.

The thermometer ascribed to San-
torio and to Drebbel, is pre- p> ,
cisely the same in form and °'
principle. It consists of a glass
tube, with a ball blown on one
of its extremities A, (fig. 1,)
and having the other end
open. A portion of the air in
the ball is expelled by heat,
and then the open end of the
tube is immersed in any liquid
contained in the cup c. As the
ball cools, the included air
diminishes in volume, and the
liquid is forced into the stem,
as at b, by the pressure of cl
the atmosphere, until it re-
places the volume of air which was

* Comment, in Galen, et in Avicen.
t De Motu Animalium. Prop, clxxv.
J Opuscula Posth. p. 30.
§ Klementa Chemise, torn. i. p. 152.
IIElem. Phil. Nat. § 780.— Tentam Exp. Acad.
Cim.

II Tiraboschi Storia, torn. viii. P. 1,323.
** Commentaria in Avicennam.

expelled by the heat. When a
heated body is applied to the ball A,
the air will again be expanded, and
depress the liquid in the stem ; and, if
this stem be a cylinder, a scale of equal
parts applied to it will enable the ob-
server to form some idea of the dif-
ference between the relative tempera-
ture of bodies applied to the ball. On
the removal of the heated body, the
volume of the included air again di-
minishes, and the liquid again rises in
the stem by atmospheric pressure, un-
til the elasticity of the air within the
instrument is in equilibria with that of
the surrounding atmosphere. Instru-
ments constructed on this principle are
termed air thermometers ; because their
action depends on the elasticity of air ;
and from their having been originally
employed to mark the changes of atmo-
spheric temperature, they are described
by the older writers under the name of
weather-glasses; a denomination also
given to barometers.

Drebbel appears to have devised a
variety of the instrument more delicate
in its indications. The globular form
of the common bulb, and its small size,
rendered it less susceptible of slight
changes than a flattened bulb of larger
diameter ; and Boerhaave describes the
bulb of Drebbel's thermometer, as com-
posed of two shallow segments of large
spheres, as in'jtfg-. 2. A, united at their
edges, and in/g-. 2. B, where it is seen
in profile.

Fig. 2. A. Fig. 2. B.

In the obscure, and often almost un-
intelligible, writings of our countryman,
Dr. Robert Flud, published about the
beginning of the seventeenth century,
frequent mention is made of the ther-
mometer, or, as he calls it, speculum

THERMOMETER AND PYROMETER.

Calendarium ; and the common air
thermometer is repeatedly figured in
his singular work, De Philosophia Moy-
siaca* with its stem equally divided
into an ascending and descending series,
each of 7 degrees, respectively appro-
priated to winter and to summer. It is
obvious, that the size of an air thermo-
meter, on such principles, is only limited
by convenience, and the length of the co-
lumn of liquid which the pressure of
the atmosphere can sustain in the tube.
As originally made, they were unwieldy,
they could not be applied to high tem-
peratures, and were, besides, liable to
two very important objections, as indi-
cators of the atmospheric changes of
temperature, — they were liable to be
affected not only by heat and cold, but
by the varying pressure of the atmo-
sphere ; and the scales adapted to
them were arbitrary, and without, fixed
points for the comparison of observa-
tions made with different instruments.

The first objection was foreseen and
obviated by the scientific members of
the Florentine academy del Cimento,
assembled under the auspices and pa-
tronage of Fernando II., Grand Duke
of Tuscany. In the first article in the
published transactions of that learned
body.f we find a full description and
delineation of a thermometer from
which the influence of atmospheric
pressure is excluded. The expansion
of spirit of wine is employed to ascer-
tain the temperature, instead of the
dilatation of air ; and the instrument is
sealed hermetically, as it is termed, or
has its orifice closed by melting the
glass, after the introduction of as much
spirit as fills the bulb and a portion of
the stem. The method employed by
the Florentine academicians is nearly
that still used by the makers of the
instrument ; namely, by heating the
bulb in the flame of a lamp, to expel
the air, and then immersing the open
end of the tube in the liquid destined to
fill the thermometer. As the ball cools,
the atmospheric pressure forces the
liquid into the stem and ball, to supply
the vacuum ; and the orifice is closed
by melting with the blowpipe the end
of the tube, from which any excess of
the liquid may be previously expelled
by again heating the ball. (Fig. 3.)

The Florentine academicians appear
also to have been aware of the neces-

* Folio, Goydae, 1633.

f Saggi di Natural! Esperienze.

sity of adapting some fixed Jftgv, 3
scale to the tube ; but their
attempts were not very suc-
cessful. They described the
thermometer as consisting of
a ball and tube of such rela-
tive size, " that on filling it
to a certain mark of its neck
with spirit, the cold of snow
and ice will not cause it to fall
below 20 degrees measured on
the stem; nor, on the other
hand, the greatest heat of
summer expand it more than
80 degrees."* This method
is undoubtedly erroneous, in-
asmuch as the last point could
be of no determinate tem-
perature; and their method
of graduation is in itself
rather rude. The tube is
directed to be divided by
compasses into ten equal parts, these
divisions are to be marked " by a little
button of white enamel ; and these may
be further subdivided by the eye, and
the intermediate degrees marked by
buttons of glass, or of black enamel."

This instrument was variously modi-
fied by them to suit different purposes.
The ball was occasionally enlarged, and
the tube reduced in thickness to render
the instrument more sensible ; and in
the work already quoted, we find a
figure of a thermometer of this sort,
with the stem spirally twisted to render
it more portable, and less liable to
accident.

Another invention of those philoso-
phers to indicate changes of tempera-
ture may be here noticed. It consisted
of hermetically sealed spherules of glass,
of different specific gravities, introduced
into a wide tube filled with pure spirit.
The degree of the Florentine thermome-
ter at which each sank was noted, and by
hanging this instrument in an apart-
ment, it somewhat slowly showed the
variations of the temperature of the
surrounding air.f Imperfect as these
attempts were, they paved the way to
very important improvements in ther-
mometers.

The indefatigable Boyle appears early
to have turned his attention to the im-
provement of the thermometer, and his
first attempts were on the air thermo-
meter, or the weather-glass, as it was
then styled. He rendered the instru-

* Saggi di Natural! Esperienze, p. 4,
f Saggi, p. 10.

B2

THERMOMETER AND PYROMETER.

ment more convenient, by making one
reservoir for the liquid and for the air
at the bottom of the tube ; and thus the
thermometer might be conveniently
dipt in a fluid, or applied to any body
for ascertaining its temperature. " The
thermometer," he says, " being made
by the insertion of a cylindrical pipe of
glass (open at both ends) into a phial
or bottle, and by exactly stopping with
sealing wax, or very close cement, the
mouth of the phial, that the included
air may have no communication with
the external, but by the newly men-
tioned pipe."* If a portion of any liquid
sufficient to cover the lower extremity
of the pipe, be contained in the bottle,
it is obvious, that the expansion of the
enclosed air will elevate the included
liquid in the cylindrical pipe ; and this
liquid will again descend on the contrac-
tion of the enclosed air \Jig. 4,5. Mr.

Fig. 4. Fig. 5.

Boyle likewise showed that no depend-
ence could be placed on the indications
of open air thermometers, under different
degrees of atmospheric pressure; and
he states, that on plunging the bulbs of
different thermometers in liquids of very
different specific gravities, as mercury
and water, thej liquor in the stem stood
at unequal heights, though both had
been long exposed to the same tempera-
ture.

The Florentine thermometer was
about that time introduced into Eng-
land, and duly appreciated by both
Boyle and Hooke. The specimen seen
by these philosophers was filled with
colourless spirit, but they made use of
spirit of wine, tinged by cochineal,
( Of a lovely red ;" and, says Boyle,

us pleasant to see how many inches
a mild degree of heat will make the

• Works of Hon. Rob«rt Boyle, folio, vol. ii, p.

tincture ascend in the cylindrical stem of
one of these useful instruments."* Boyle
was fully aware of the imperfection of
the scales hitherto applied to the ther-
mometer, and sought to discover a re-
medy. He proposed to obtain a fixed
point in the scale, by marking the height
of the liquid in the stem of the instru-
ment, when the ball was placed in
thawing oil of aniseeds ; a point
which he preferred to that of thawing
ice, because the former could be readily
obtained at any time of the year. His
method of making two or more com-
parable thermometers, however, would
be found extremely difficult, if not im-
possible, in practice; it is best ex-
plained in his own words. " For if you
put such rectified spirit of wine into a
glass, the cavity of whose spherical,
and that of its cylindrical part, are as
near, as may be, equal to corresponding
cavities in the former glass, you may
by some heedful trials, made with
thawed and recongealed oil of aniseeds,
bring the second weather-glass to be
somewhat like the first ; ^and if you
know the quantity of your spirit of
wine, you may easily enough make an
estimate, by the place it reaches to in
the neck of the instrument, whose capa-
city you also know, whether it expands
or contracts itself to the 40th, the 30th,
or the 20th part, Sec. of the bulk it was
of, when the weather-glass was made."t
Boyle mentions that an " ingenious
man""% had proposed the freezing of
distilled water, as a fixed point in the
scale of thermometers ; but he himself
evidently gives the preference to the
congealing point of aniseed oil. Dr.
Halley proposed to regulate the scale by
the uniform temperature of such a ca-
vern as that under the Observatory of
Paris, or the point at which spirit
boils ; and he also suggests the fixing
of the scale from the boiling of water.
This point he considered as an invari-
ably fixed one, not liable to alteration
from external circumstances ; and the
same idea was entertained by Amon-
tons. With a single point so fixed, the
method attempted by Boyle, Halley,
and Hooke was to calculate the pro-
portion of the stem to the ball, and thus
to determine the increase in bulk of the
whole liquid, by a certain temperature.
Dr. Hooke describes a method of ob-
taining this by comparing the expansions

* Works, vol ii. p. 249.
f Works, vol. ii. p. 247.
| He undoubtedly alludei to Hooke,

THERMOMETER AND PYROMETER.

of the thermometer to be graduated,
with those of the liquid in an accurately
formed cylinder of metal, two inches in
diameter and depth, and having cement-
ed to its top a glass pipe, just ^ of the
diameter of the cylinder:* measure off
two inches of the stem, above the cylin-
der of metal, and divide the space be-
tween them into 10 equal parts, so that
each division of the stem will = Tg'5^ of
the capacity of the cylinder. The ther-
mometer to be graduated has the com-
mencement of its scale, or 0°, fixed by
marking the point at which the included
liquid stands in the stem, when the bulb
is plunged in distilled water just begin-
ing to freeze ; and the rest of the pro-
cess he details in these words. " Fill
this cylindrical vessel with the same
liquid wherewith the thermometers are
filled, then place both it and the ther-
mometer you are to graduate in water
that is ready to be frozen, and bring the
surface of the liquor in the thermo-
meter to the first mark, or 0° ; then so
proportion the liquor in the cylindrical
vessel, that the surface of it may just be
at the lower end of the small glass
cylinder ; then very gently and gradu-
ally warm the water, in which both the
thermometer and the cylindrical vessel
stand, and as you perceive the tinged
liquor to rise in both stems, with the
point of a diamond give several marks
on the stem of the thermometer, at
those places which, by comparing the
expansion in both stems, are found to
correspond to the divisions of the cylin-
drical vessel ; and having by this means
marked some few of the divisions on
the stem, it will be very easy by these
to mark all the rest of the stem, and
accordingly to assign to every division
a proper character."-!* This ingenious
method is, however, more difficult in
execution than any one, unacquainted
with such operations, will readily sup-
pose ; and it presupposes, what is not
easy to accomplish, a very perfect ad-
justment of the metallic cylinder and
the glass stem in the standard instrument.

Dr. Hooke appears invariably to have
used in his thermometers spirit of wine
" highly tinged with the lovely colour of
cochineal, which he deepened by pouring
in it some drops of common spirit of
urine.1'

The sagacity of our illustrious New-
ton saw the importance of improving
thermometers. He appears to have been

* Alicrographia.

| Micrographia, p. 3;).

early aware of the inconvenience of
spirit as a thermometric fluid, and em-
ployed linseed oil to fill his thermo-
meter. It has the advantage of being
able to endure a very considerable tem-
perature, without endangering the burst-
ing of the tube, and therefore can be
applied to a higher range of tempera-
ture than a spirit thermometer. It has
the disadvantage, however, to be more
sluggish in its movements, and to ad-
here much to the inside of the tube,
while it differs greatly in its fluidity at
different temperatures. Newton per-
ceived the convenience of having two
fixed points in the construction of the
scale ; and he used the freezing and boil-
ing points of water as the most suitable
for this purpose.* His method of gra-
duating his oil thermometer is given in
the Principia. The oil, at the tempe-
rature of melting snow, was supposed to
consist of 10,000 equal parts, which,
when heated to the temperature of the
human body, expanded to 10,256 ; at
the temperature of water strongly boil-
ing to 10,725; and at that of tin be-
ginning to congeal, to 11,516 parts. In
the first instance the ratio of expansion
is as 40 to 39 ; in the second as 15 to
14 ; and in the third as 15 to 13 nearly.
Hence, by taking the temperature of the
oil in the ratio of the rarefaction and
assuming 12 as the heat of the human
body, the temperature of water briskly
boiling will be 34 degrees, and of con-
gealing tin 72 degrees.!

Newton continued his scale of tem-
perature farther by observing the rate
of cooling of heated bodies, until he
could apply his thermometer to them,
on the principle that equal decrements
of temperature take place in equal
times. It was thus he estimated the
temperature of iron heated to the utmost
intensity of a small kitchen fire equal to
194 degrees, ar.d in a fire of wood'about
200 or 210 degrees of the same scale.

It is perhaps unfortunate for the phi-
losophy of heat that more sublime
and dazzling objects drew Newton to
other pursuits. Though he led the way
to just views of the subject, neither he,
nor any of his predecessors, appear to
have been aware of the influence of the
varying atmospheric pressure on the
boiling points of liquids ; nor do any of

* Phil. Trans.

f " Ponendo caloris olei ipsius, rarefactione pro-
portionnlis, et pro calore corporis humani scribendo
12, prodest calor aqua? ubi vehementer ebullit par-
tium34, et calor stanni ubi liquescit prodest
72." Priucip.

6

THERMOMETER AND PYROMETER.

them seem to have considered that the
varying expansions of the thermome-
tric liquids at different temperatures,
and the expansions of the glass of the
instrument, must have materially af-
fected every attempt to subdivide the
stem of the thermometer into fractional
parts of the whole bulk of the con-
tained liquid.

One of these questions, however,
seems to have about that time engaged
the attention of philosophers, viz. whe-
ther equal increments of temperature
caused equal expansions of the thermo-
metric fluid. Dr. Brooke Taylor tried
the experiment with an oil thermometer,
by mixing definite portions of hot and
cold water, and measuring the tempe-
rature of the mixture. His conclusion
was in the affirmative, but the delicacy
of his instruments was unequal to the
solution of this nice problem, although
he has the merit of pointing out how the
problem is to be solved.

The construction and uses of ther-
mometers early engaged the attention
of the French Acadtfmie des Sciences ;
and several -were constructed by Mr.
Hubin for that learned body ; but nei-
ther these, nor the thermometers placed
in the observatory of Paris by De La
Hire, appear to have been graduated
on any fixed principle. The Memoirs
of the Academy contain several descrip-
tions of thermometers, and an account
of many interesting observations, with
these instruments ; but the first altera-
tion in their construction deserving of
notice is the air thermometer of Geof-
froy, which from the short description
appears to be an improvement on that
of Boyle, inasmuch as it is not affected
by atmospheric pressure. He describes
the tube as without any opening, except
one, which descends almost Fig. 6.
to the bottom of the ball,
and there dips into a small
portion of coloured liquid.*
There is no figure given
in the original, and but a
very rude one in our Phi-
losophical Transactions^
seemingly from the de-
scription. It is not stated
how the ball was joined to
the tube, but it was most
probably by cement, as re-
presented mfig. 6.

M. Amontons clearly saw

M* M™Acad> tom' xiii< P- 12°- Jt was read in
iviay, i /Uv.

t Phil, Trans, vol. xxiii. p. 962.

the importance of fixed points in the
thermometric scale, and proposed to ob-
tain them from the boiling J^Q. 7
point of water.* His
thermometer consisted of
a tube four feet in length,
ending below in a ball
bent upwards, as in fig. 7,
and open at the other ex-
tremity. The measure of
the temperature was the
elasticity of a given por-
tion of air included in the
ball, and subjected to a
pressure equal to two at-
mospheres, by adding to
the usual atmospheric
pressure that of a column of mercury of
28 French inches. Each half- inch of
his tube is therefore equal to one inch
under the usual pressure ; and hence at
a mean pressure of 28 French inches,
the volume of the compressed air is
really equal to 56 inches under the
usual pressure.

In passing from the mean tempera-
ture of a Parisian spring to the heat of
boiling water, Mr. Amontons found that
these 56 inches were increased by one-
third, or 1 8 inches 8 lines, and therefore
he fixed the boiling point of his scale at
56 + 18,8 = 74 inches 8 lines. To mea-
sure this on Amontons's principle a tube
of 47 inches is quite sufficient ; for 74
inches 8 lines minus 28 inches, the at-
mospheric pressure which need not be
considered in the length of the tube, is
equal to 46 inches 8 lines ; and, indeed,
as in Amontons's process, the compres-
sion at high temperatures is rather more
than in the duplicate ratio of the air we
breathe, the mercury in boiling water
will not rise above 45 of his scale.f
There is a slight discrepancy between
the original account of Amontons's ther-
mometer and that given by Martine,
who states its boiling point at 73 inches,
and its freezing point at 51 1 inches;
but, according to the Academicians, the
latter will be at 52 inches and about 8
lines. The ingenious contrivance of
the double pressure enabled him to
apply the instrument to measure the
temperature of boiling water, by a tube
less than four feet in length.:}:

Although the idea of Amontons was
a fine approximation to an universal
standard for a thermometric scale, the
instrument is liable to such objections

* Mdmoires de 1'Acad. for 1702.

f Me"moires de 1'Acad. des Sciences, tom. xv.

% M£m. Acad. des Sciences, tom. xv. for 1702

THERMOMETER AND PYROMETER.

that its principle seems scarcely ever to
have been put in practice, except by its
inventor and the Marchese Poleni.* It
is difficult to construct two instru-
ments which shall correspond, from the
varying expansibility of air according
to its moisture or dryness ; the indica-
tions are liable to be affected by the
fluctuations of atmospheric pressure ; it
is liable to be deranged by the escape of
a portion of the included air, when the
instrument is moved about ; it is, more-
over, too unwieldy, and very liable to be
broken.

Much about the period when those
attempts to perfect the thermometer
were made in France, important im-
provements on it were effected in the
north of Germany and in Holland, by
the introduction of quicksilver as the
thermometric fluid.

The objections we have stated to the
use of the spirit thermometers, and to
the oil thermometer of Newton, led the
way to the employment of quicksilver
in the construction of the instrument.
Dr. Halley alludes to several advantages
of quicksilver as a thermometric fluid, but
seems to have rejected it on the ground
of its slight expansion by heat,t although
this objection might have so easily been
obviated by increasing the disproportion
between the bulb and the diameter of
the tube. On this account the claim set
up for his title to priority of invention
may justly be denied. It is most pro-
bable that science is indebted for this
great improvement to Roemer, the cele-
brated astronomer of Dantzic, to whom
the invention is ascribed by Boerhaave,
as well as the first idea of the scale now
known as that of Fahrenheit. Boer-
haave further adds, that as early as
1709, Roemer observed with that in-
strument a natural cold so intense as to
sink the mercury to the beginning of
the scale.J Thermometers of this con-
struction began to be made by Daniel
Gabriel Fahrenheit, a native of Dantzic,
who afterwards lived at Amsterdam, in
so admirable a manner, that he has
generally been considered the original
inventor ; they were speedily spread over
the north of Europe under his name,
and still maintain their ground in se-
veral countries, especially in Britain.

It has commonly been alleged, that
at the time when Roemer' s or Fahren-

heit's scale was proposed, its zero was
derived from the artificial cold pro-
duced by a mixture of salt and snow,
then supposed to be the lowest possi-
ble reduction of temperature. This,
however, seems to be inaccurate : Boer-
haave * gives a different account of the
matter, which is repeated in the Philo-
sophical Transactions, f The zero
was fixed from '' the lowest cold ob-
served in Ysland" (Iceland); which
was supposed to be as low a tempera-
ture as was likely to become the object
of philosophic investigation : but when
artificial methods of reducing the tem-
perature of bodies much lower, and
occasional natural colds brought the
mercury below that point, a scale of
equal parts was extended below the 0° ;
the ascending series of degrees being
distinguished by sign + or plus, and
the descending series by the sign — or
minus.

The principle which dictated the pe-
culiar division of the scale is as follows.
When the instrument stood at the
greatest cold of Iceland, or 0 degree,
it was computed to contain 11,124
equal parts of quicksilver ; which,
when plunged in melting snow, ex-
panded to 11,156 parts; hence the in-
termediate space was divided into 32
equal portions, and 32° was taken
as the freezing point of water : when
the thermometer was plunged in
boiling water, the quicksilver was ex-
panded to 11,336 parts ; and therefore
212° was marked as the boiling point
of that fluid.;}; In practice, Fahren-
heit determined the divisions of his
scale from two fixed points, the freez-
ing and boiling of water : the theory of
the division, if we may so speak, was
derived from the lowest cold observed
in Iceland, and the expansions of a given
portion of mercury.

The mercurial thermometer was used
by the Italian philosopher Renaldini
before the end of the seventeenth cen-
tury : and he proposed, in 1694, an in-
genious method of graduating it between
the freezing and boiling points of water,
by successive mixtures of determinate
weights of boiling and ice cold water.

The great advantages of Fahrenheit's
thermometer over every other pre-
vious invention, consisted in its appli-
cability to a greater range of tempera-

* Phil. Trans. No. 421.

f Phil. Trans, vol. xvii. p. 652.

I Boerhaavii Chemise, tool. i. p. 720,

• Chemiae, torn. i. p. 720.

f Vol. xliv. p. 680.

f For 11156 — 11124 = 32, and 11336— 1124=«2 12.

THERMOMETER AND PYROMETER.

ture, from the freezing to the boiling
point of quicksilver, in its not soiling
the containing tube, and in its receiv-
ing the impressions of heat and cold
more readily, while its density rendered
capillary tubes filled with it perfectly
visible; and thus the instrument be-
came more portable and delicate. We
may also remark, that at the period of
its invention, there was no other scale
in use that could pretend to vie with it
in accuracy ; and it still possesses the
peculiar advantages, that from the low-
ness of its 0°, the observer is seldom
troubled with negative degrees, and
from the number of its divisions i^has
rarely, in ordinary operations, to use
fractions of a degree.

We are indebted also to Fahrenheit for
ihe knowledge of the fluctuation of the
boiling point of water, according to the
difference of atmospheric pressure.*
Le Monnier, in 1739, confirmed this fact,
by noting the temperature of boiling
water on the top of Mount Canigou,
one of the Pyrennees; and in 1744
it was fully established by Martin
Folkes, who found that water boiled on
the summit of Pic du Midi 15° of
Fahrenheit's scale lower than at Bag-
neres ; and at the latter place 3|° lower
than at Bordeaux ; while he proved
that elevation in the atmosphere had no
sensible influence on the stability of the
freezing point.t These facts led to
an important correction in fixing the
boiling point of water or other liquids.

It would now be a waste of time to
describe minutely the various thermo-
meters which were in use in France and
England before the time of Fahrenheit.
They were all without fixed points in
the scale; and though they were vaunted
as constructed after the models in the
Royal Observatory at Paris, or in the
apartments of the Royal Society of Lon-
don, they gave most discordant results.
An analysis of the most noted of them
has been elaborately and ingeniously
attempted by Dr. Martinein his valuable
Essays, and the results presented in the
very convenient form of a tabular view.
We shall therefore pass at once to notice
some of the other more accurate ther-
mometers that have been employed in
different parts of Europe, although the
principle in them all is similar to what
has been already described.

The thermometer with which the

'* Phil. Trans, xxxiii. No. 381.
t Phil. Trans, vol. xliii. p. 32.

Dutch philosopher Cruquius made the
observations published in the Philoso-
phical Transactions, (vol. xxxiii. No.
381,) \vas an air thermometer, on which
he states the freezing point of water to
be indicated by 1070°, and boiling
water by 1 5 1 0° : the lowest known cold,
which seems to have been the begin-
ning of his scale, he gives = 1000°.

The objections to the thermometer of
Amontons are clearly stated by Reau-
mur, * who proposed to adopt the
freezing and boiling points of water as
fixed points in the scale, but employed
spirit as the thermometric fluid. He
unquestionably fell into error when he
stated that 1000 parts of strong spirit
dilated to 1087.5 parts in passing from
the freezing to the boiling point of
water; for how could strong spirit
sustain so high a temperature without
being partially converted into vapour ?
His proposal was to use spirit of just
such strength, that between these two
temperatures it should expand from
1000 to 1080; and, commencing his
scale or 0° at the freezing point of
water, he made the boiling point 80°.
The principle of this construction was
good ; but Dr. Martine has shown that
from the large size of the bulbs of his
thermometers, which were from 3 to 4
inches in diameter, and the short time
they were immersed in the freezing
mixture, they could not have acquired
an uniform temperature ; and accord-
ingly Martine found their freezing point
too high,t and the error in the boiling
point from the cause already alluded to,
must have been still greater.

These errors might have been obviat-
ed by the use of quicksilver instead of
spirit. This was accordingly soon done ;
by whom first is uncertain, although
there is strong reason to believe by De
Luc ; and the mercurial thermometer,
with the 0° at the freezing point of
water, and 80° as its boiling point, soon
became general in France, and well
known over Europe under the name of
Reaumur's Thermometer. The only
material objections to such a scale,
when the instrument is accurately
made, arise from the largeness of the
divisions rendering fractional parts of a
degree of frequent occurrence, and the
elevation of 0° often introducing + and
— degrees in a series of observations-,
even at common natural temperatures.

The mercurial thermometer of Mons.

* M£moires de 1'Acad. des Sciences,
jr Martina's Essays, Edin. 1792, p. 23.

THERMOMETER AND PYROMETER.

0

J. De Lisle of St. Petersburg, differs
little in principle from the instruments
just mentioned ; but its graduation is
inverted. His 0° is at the'boiling point
of water, and he continues the gradua-
tion dwcnirards: and conceiving the
mercury, at that temperature, to be di-
vided into 100,000 parts, he determined
the degrees by the contractions of the
whole mercury as it cooled, expressed
in such parts.* The distance between
the freezing and boiling points of water
on this scale is 150°, as ascertained by
Dr. Martine, who examined one of Dr.
Lisle's original thermometers : but this
thermometer seems to possess no advan-
tage over those just described, and never
came into general use except in Russia,
where it is still employed.

Our countryman, Dr. Stephen Hales,
employed another thermometer in his
experiments on vegetable physiology.
The 0° was at freezing water, and the
highest point was ascertained by placing
the instrument in hot water, on ^ which
wax was just beginning to congeal ;
the intervening space was divided into
100°.t This near approach to a true
centesimal scale was defeated by the un-
certainty of the upper point, arising
from his using spirit instead of mercury
in the tube, and the difficulty of ascer-
taining the exact moment of the conge-
lation'of the wax.

In the year 1742, the Swedish philo-
sopher Celsius, professor at Upsal,
divided centesimalty the thermometer
known in the north by his name, and
which has, since its tacit adoption by
the French chemists, obtained additional
celebrity as the ThermomHre Centi-
grade. Celsius commences his scale
at the freezing point of water, and di-
vides the space between that point and
the height of the mercurial column in
boiling water into 100°. This appears
a more natural and simple division than
any that had been previously proposed,
and it possesses several advantages ;
but it has two inconveniences of some
importance in many practical opera-
tions. Thus, from the high position of
the 0°, natural colds are frequently to
be noted by a descending series of fi-
gures, and one column of observations
may be hence embarrassed by + and —
degrees ; while from the large space
intercepted between the degrees, the
observer is frequently obliged to com-
pute fractional parts of a degree.

* Phil. Trans, vol. xxxix. p. 221, for 1736.
t Vegetable ritatics, vol. i. p. 5^.

M. de la Lande, in 1804, proposed a
new thermometric scale, the 0° or mean
point of which he would fix at the mean
temperature of the earth ; which he
gives as = to 9°. 5 of Reaumur's scale ;
and his degrees were to be the ten mil-
lionth part of the volume of the mer-
cury in the instrument. Among the
advantages of such a division, he con-
siders the simplification of expression in
meteorological observations — thus, 30°
would express the heat of summer and
cold of winter ; 40° a hot summer and
severe winter; while the smallness of
the degrees would obviate the use of
fractions of a degree. The boiling
point of water would be at + 133,
and the congelation of mercury at —
74° ; ice would melt at — 1 8°, and the
zero of Fahrenheit would be at — 44.*

This proposition has never been
adopted ; and its advantages seem over-
rated by the inventor. It only obviates
one of the objections urged against the
scale of Celsius, and is inferior in sim-
plicity either to a millesimal division of
the interval between the freezing and
boiling point of water, or to the ther-
mometric scale proposed by the late
Dr. Murray of Edinburgh. That acute
philosopher proposed to employ the
freezing and boiling points of mercury
itself as the extremes of his scale, and
to divide the intervening space into
1000°. It is a more natural division
than any hitherto proposed, inasmuch
as it is taken from relations of the best
thermometric fluid itself to heat : and if
we suppose these two points to have
been accurately fixed at — 40° and -r-
655° of Fahrenheit, the freezing point of
water would be 99°, and its boiling
point 347° on Murray's scale.

The advantages of this scale, are that
it will very seldom, in natural tempera-
tures, render the introduction of — de-
grees necessary, and the smallness of
the divisions supersede the employment
of fractional parts of a degree, in ordi-
nary cases ; two circumstances of con-
siderable importance in a long series of
thermometric observations.

Magellan informs us, f that M.
Achard of Berlin invented a thermome-
ter for ascertaining high temperature,
which is a true Pyrometer, and might
have been introduced in the next sec-
tion. It consists of a ball and tube of
semitranslucent porcelain, highly baked,

•Journal de Physique, 1804. Nicholson's Jour-
nal, 8vo. vol. ii. p. 61.
t Snr la Tbeorie du Feu Ele"mentaire, 1780,

10

THERMOMETER AND PYROMETER.

containing" a fusible alloy of two parts
of bismuth, one of lead, and one of tin.
In the temperature of the air, it remains
solid in the tube; it becomes fluid
about the boiling point of water ; then,
as a fluid, expands by increase of tem-
perature ; and its expansion being seen
through the semitranslucent tube,
which is divided into equal parts or de-
grees, becomes an indication of the
temperature applied to the ball.

This invention promises to be of con-
siderable utility, and is capable of ex-
tension by the employment of less
fusible metals. From the simplicity of
its construction, it is rather surprising
that it has not been more generally
known, and employed in potteries,
where the instrument could be easily
made. An instrument on this con-
struction would be a better method of
uniting the scales of the common ther-
mometer and pyrometer than any here-
tofore employed.

Of these various thermometric scales
there are but three in very general use,
viz. that of Fahrenheit, Celsius, and
Reaumur. Fahrenheit's is chiefly used
in Britain, North America, and Hol-
land : the scale of Celsius was adopted
by the French, and is now employed in
most parts of the north and middle
of Europe: Reaumur's was the only
one used in France before the Revolu-
tion, and is still that best known in
Spain and in some other continental
states ; but it is further important, as
affording the terms in which numerous
very valuable observations are recorded.

For these reasons it is useful to have
formulae for readily converting one
scale into the equivalent degrees of the
other two. The freezing point of water
on Fahrenheit's scale is at 32°, and on
those of Celsius and Reaumur at 0°,
while it boils on each respectively at
180°, 100°, and 80°, above that point.
Hence the degrees of Fahrenheit are to
those of Celsius as 180 : 100 = 18 : 10
= 9:5, and to those of Reaumur as
180: 80 = 18:8 = 9:4— , or 9° of Fah-
renheit are equal to 5° of Celsius and
to 4° of Reaumur. Therefore, when we
wish to convert the degrees of Celsius
into those of Fahrenheit, we have to
multiply the number of the former by 9,
divide by 5, and add 32; to reduce
the degrees of Fahrenheit into those of
Celsius, the converse of the proposition
will give the required result; that is,
from the degree of Fahrenheit subtract
32, then multiply by 5, and divide by 9.

When we wish to convert the degrees of
Reaumur into those of Fahrenheit, we
have to multiply by 9, divide by 4, and
add 32 ; and subtracting 32 from the
given degree of Fahrenheit, multiplying
the remainder by 4, and dividing by 9,
will give the equivalent degree of Reau-
mur's scale.

The following short formulae will
apply to each case :

1.

3.

4.

(F - 32) X 5

(F- 32) x 4

These formulae apply to all degrees
above the freezing point of water ; but
when negative degrees of Celsius are to
be converted into the equivalents on
Fahrenheit's scale, multiply the degree
of Celsius by 9, divide by 5, and the
difference between the quotient and 32
is the required degree of Fahrenheit :
or when negative degrees of Fahrenheit
are to be reduced to their equivalents
on the scale of Celsius, add 32 to the
given degree of Fahrenheit, then multi-
ply by 5, and divide by 9. By substi-
tuting 4 for 5, the same formulae will
apply to Fahrenheit and Reaumur, all
which may be thus expressed :

1.

2.

3.

4.

— C =

-F

— R =

(F + 32) X 5

9R

4

32.

(F + 32) X 4

The formulae are convenient for re-
ducing a few examples from one scale
to another ; but when they perpetually
occur in reading or writing it is very
useful to have comparative tables, fron
which, by one glance, the desired infor-
mation may be obtained.

§ 2. Precautions necessary to be ob-
served in constructing accurate Ther-
mometers.

A general idea has been already
given of the mode of constructing a

THERMOMETER AND PYROMETER.

11

thermometer, but where much accuracy
is required there are many niceties that
demand attention.

1. The tube should be of equal
diameter throughout the whole stem.
As obtained from the glass-house, the
tubes are in reality frusta of very elon-
gated hollow cones, which, by extension,
become more or less nearly cylindrical ;
and as the divisions of the scale are
usually equal, it is very important that
the tube should not perceptibly differ
from a true cylinder.

For these purposes, after a tube has
been chosen by the eye as equal in
calibre as possible, the best makers
blow a bulb on it, and introduce a
short column of mercury into the stem,
perhaps an inch in length, which is accu-
rately measured on a fine scale of equal
parts, in different portions of the tube,
as the column is, by the heat of the
hand, moved from the bulb to the open
extremity of the tube. Should the mer-
curial column subtend the same
number of divisions on the scale in
every part of the tube, it may be consi-
dered as a perfect tube for a thermome-
ter.

The late Mr. Wilson, of Glasgow, in-
troduced thermometric tubes of an
elliptical bore. The advantage of this
form is, that a very small column of
mercuiy is much more visible when it is
expanded at right angles to the line of
vision. If due precaution be taken to
ensure the equality of the tube this
form answers well, especially for ordi-
nary purposes ; but where great nicety
is required, we would recommend the
cylindrical tube.

2. The form and proportion of the
bulb may vary according to the purpose
for which the instrument is to be ap-
plied. The larger the bulb in propor-
tion to the stem, so much more deli-
cately susceptible of changes of tempe-
rature will be the thermometer. The
spherical bulb is to be preferred, for
this shape is least likely to be affected
by the varying pressure of the air ; but
when the bulb is very large this form

"venders the thermometer less susceptible
of minute changes of temperature, and
pyriform or cylindrical bulbs are usually
adopted. All large bulbs are more or less
sensibly affected even by slight pres-
sure. An examination of more than fifty
common thermometers, with large sphe-
rical bulbs, in the work- shop of an
excellent artist, afforded the writer of
this article an opportunity of observing

that by slightly compressing their bulbs
between the finger and thumb, the mer-
cury in the stem rose and fell alter-
nately several degrees, as the pressure
was increased or diminished. The bulb
and stem are usually in the same
straight line, but for various purposes
the bulb is occasionally placed at
various angles to the stem.

In forming the bulb the mouth must
not be employed to blow it, otherwise
moisture will condense in the tube,
which is expelled with much difficulty,
and if suffered to remain, will greatly
impair the value of the thermometer.
Good instrument-makers use a small
bottle of caoutchouc, or elastic gum,
fastened by a thread on one end of the
tube, while the other extremity is soft-
ened by the flame of a tallow lamp,
urged by a blowpipe. By compressing
the bottle, after the orifice of the soft-
ened end of the tube is closed by the
aid of another rod of glass, a bulb is
formed of any required size ; but a neat
workman will rarely consider the first
blown bulb sufficiently well formed for
his purpose. It is generally dilated till
it bursts ; the glass, while still soft, is
compressed into a rounded mass, and a
fresh bulb formed of a regular shape,
and size proportioned to the calibre of
the tube. Should the artist not intend
to fill the tube immediately, he usually
hermetically seals the other end of the
tube to prevent the entrance of damp
air or dust.

3. The precautions necessary in fill-
ing thermometers with mercury are
exceedingly well given in Nicholson's
Chemistry.*

The mercury should be clean, dry
and recently boiled, to expel air as
much as possible. Mercury is often
cleaned by thermometer - makers by
agitating it in a phial, for some time,
with sand, and then straining it through
leather ; for nice instruments it should
be distilled from iron filings, or reduced
from its sulphurets, in clean iron
vessels, at a moderate heat.

The bulb to be filled is heated in the
flame of a lamp, and the open extremity
of the tube is immersed in the mercury •
as the bulb cools, the pressure of the
atmosphere forces the fluid into the
tube and ball. Mr. Nicholson recom-
mends, that the bulb should be but
moderately heated at first; so as, on
cooling, to become only half filled. He
advises the open end ot the tube to be

* Edition 3rd, p. 24.

12

THERMOMETER AND PYROMETER.

kept under the surface of the mercury,
and the instrument to be retained as
nearly in the horizontal position as pos-
sible, while the flame of a newly snuffed
candle is applied to the bulb, so as to
boil the included mercury. Thus the
remaining air will be expelled ; and on
removing the candle, the mercury will
suddenly fill the ball and part of the
tube.

4. To ensure a delicate thermometer,
the mercury is next to be boiled in the
thermometer. For this purpose a slip
of clean writing paper is to be rolled
tightly around the upper part of the
tube, so as to form, beyond the orifice,
a cup or cylinder capable of containing
as much mercury as the bulb : secure
this round the tube with a thread, put
a drop of mercury into the paper cavity,
and again apply heat to the bulb, hold-
ing the tube by the part covered by the
paper. The mercury will soon boil, and
about one -half of the contents of the
ball will rush up into the paper cup.
On removing the bulb from the candle,
the mercury will suddenly return. Re-
peat this operation again and again,
until the speedy boiling of the mercury,
and the diminished noise and agitation,
show that the whole has been well
heated, and air and moisture expelled
from it.

Should there be the least moisture in
the tube before this part of the opera-
tion, it is very likely to burst the bulb ;
and the same accident is likely to hap-
pen, if the mercury be too strongly
boiled the first or second time.

An experienced eye will readily judge
what range of scale the thermometer
will have ; but this point can easily be
ascertained, before the tube is closed,
by heating the bulb in the mouth, and
then immersing it in cold water or
melting ice. When the latter is used,
the operator can at pleasure fix how
far from the bulb he will have the
freezing point ; for, by keeping the tube
more or less filled, he can adjust that
point to any desired height.

5. The tube is now to be hermetically
sealed, that is, closed by the fusion of
the glass at the upper extremity, which
for this purpose is previously drawn to
a capillary orifice. When it is intended
to free the tube entirely from air, which
is the best method with mercurial ther-
mometers, heat is again to be gently
applied to the bulb, which at the same
moment is to be softened by another
flame, and closed in the usual way, as

soon as the mercury reaches the extre-
mity of the tube. When the ball has
cooled a little the sealing is rendered
more secure by fusing the glass more
fully around the top, so as completely
to obliterate the orifice. If the vacuum
be perfect, the mercury will fall to the
extremity of the tube on inverting the
thermometer, unless the calibre be ab-
solutely capillary ; in which case capil-
lary attraction will overcome the force
of gravity, and the mercury will retain
its position in the tube, in every situa-
tion of the instrument,

Where there is a complete vacuum
in the tube, the mercury must be well
boiled before the sealing, as above di-
rected ; and when we choose a thermo-
meter, the ready falling of the mercury,
on inversion of the tube, is the best test
we can have that the mercury has been
well freed from air and moisture. This
vacuum is not, however, so essential to
the true action of the thermometer as
was once supposed. A thermometer
with a small dilatation of the tube
when sealed, containing some common
air, has lately been recommended as
preferable to the instrument with a va-
cuum on the surface of the mercury.

M. Flaugergues* first called attention
to the fact, that when old thermometers
are placed in melting ice, they seldom
fall quite so low as the mark of freezing
on their stems, especially when the
whole air has been expelled from them.
This difference he found to amount
sometimes to 0.9 of a degree. The
same fact has been confirmed by MM.
De la Rive and F. Marcety}- and also by
Bellani $ and Arago. § The writer of
this article possesses three thermome-
ters ; one very delicate, made by Rams-
den, and two well made instruments by
Lovi of Edinburgh, all which have been
in his possession upwards of a quarter
of a century. On lately placing them
in a vessel filled with pounded ice, in a
warm apartment, they all showed a
slight elevation of the freezing point.
That made by Kamsden has a capillary
tube and small spherical ball; the
other two have small pyriform bulbs,
and the mercury readily falls to the ex-
tremity of the tube on inverting them :
yet Ramsden's stood about 0.6 of a
degree above the freezing point, and the
other were just perceptibly above it.

* Bibliothique Universelle, torn. xx. 1823.

f Ib. torn. xxii.

| Giornale di Fisica, torn. v.

§ Aunales de Chimie, torn, xxxii.

THERMOMETER AND PYROMETER.

1.3

M. Flaugergues aif ributes this change
to the effect of long continued atmo-
spheric pressure on the bulbs of ther-
mometers, in which there is no air to
counteract it. De la Rive and Marcet
give the same explanation, and remark
how this circumstance must affect the
result of all experiments on the cold
produced in vacuo.

Arago is not inclined to attribute this
elevation of the zero to atmospheric
pressure on the bulb ; since he found it
equally affecting thermometers with
very thick and very thin bulbs. He in-
clines to ascribe it to the disengagement
of air, which either adhered to the glass
or the mercury, and its accumulation in
the upper pail of the bulb, so as to
affect the column in the stem.

The most complete observations on
this point are those of Bellani,* who
acknowledges two sources of variation
in the zero of thermometers. That ele-
vation of the zero, first noticed by Flau-
gergues, according to him, goes on
gradually increasing for a limited pe-
riod, but ceases after a year or two. He
ascribes it to the extreme slowness with
which glass once softened has the equi-
librium among its particles restored.
He found, that some months after gra-
duation, a thermometer did not sink
quite to the freezing point when im-
mersed in melting ice; if laid by for
some months, and again tried, its zero
will be still higher ; but after some time
this irregularity ceases. He found that
this effect was not diminished by leav-
ing the thermometer open at the top,
and it was sensible even in spirit ther-
mometers.

The other irregularity noticed by
Bellani is detected in the following
manner.— Let a thermometer, having
such a range that ^ of a degree is
appreciable, after lying by for some
months, be plunged into melting ice, and
its height accurately noted, then into
boiling water, and again into ice, it will
now stand lower by about T15 of a de-
gree than at its first immersion in the
liquefying ice. This effect he ascribes
to the extreme slowness with which the
expanded glass can regain its former
state of contraction, compared to the
mercury.

These deductions appear to be per-
fectly just ; and we are further indebted
to Bellani for an ingenious method of
showing that the air, if not wholly, is

* Bellani, Giornale di Fisica, torn, v.

chiefly retained in thermometers and
barometers by the glass, not by the
mercury. He introduced a portion of
unboiled mercury into a bulb, contain-
ing mercury which had ceased to give
out any air, and found that this intro-
duction did not renew the agitations
which the first application of heat to
the bulb had occasioned.

The difficulty of freeing thermometers
from air is admitted by Arago, while he
recommends boiling the mercury in the
bulb as the best method of effecting the
expulsion of the air; and he quotes
some unpublished experiments of Du-
long, to show the tedious manipulations
which are necessary for this purpose.

We would recommend the boiling to
be performed in the manner stated, un-
til the agitation of the fluid caused by
the air ceases ; and after the tube is
closed, the observations of Bellani
would incline us to recommend, for
delicate instruments, that the attempt
to fix the freezing point should be de-
ferred, until the glass might be supposed
to have contracted to its state of equi-
librium ; after which, there would pro-
bably be little change in the dimensions
of the bulb.

6. We come now to the last and most
delicate step of the process, the adapta-
tion of the scale to the instrument.

In the manufacture of thermometers
this is conveniently done by plunging
the new instrument, along with a stand-
ard thermometer, into two liquids at
different temperatures : but the gradua-
tion of this standard instrument is a
work of such nicety and importance,
that a committee of seven members of
the Royal Society was formed to in-
vestigate the subject, and their elabo-
rate report is given in vol. Ixvii. part ii.,
where all the requisite circumstances
are distinctly noticed, and the best ma-
nipulations minutely described.

Two fixed points are sought ; and the
freezing and boiling points of water are
most convenient for that purpose. To
find the first, nothing more is necessary
than to place the thermometer to be gra-
duated, after it is filled, in melting snow
or ice, in such quantity around the ball
and tube, as to bring it to the desired
temperature. When the mercury has
become stationary in the tube, a mark
is to be made on the tube with a file,
just opposite to the top of the mercurial
column ; and that mark fixes the freez-
ing point of the scale of the instrument.
The determination of the boiling point

14

THERMOMETER AND PYROMETER.

is much more difficult, because it is
affected by atmospherical pressure, and
even by the form of the vessel in which
the water is heated.

The Committee of the Royal Society*
recommend that the boiling point
ought to be fixed under a barome-
trical pressure of 29.80 inches. For
the graduation of the thermometer they
recommend that the bulb should not be
immersed in the water; because they
found, that according to the depth of
this immersion the mercury rose to a
greater height in the tube. They re-
commend a vessel of tin plate, pro-
vided with a cover which fits easily on,
and rendered steam-tight by a ring of
woollen cloth between it and the vessel.
This cover has two apertures — a chim-
ney, with an area not less than half a
square inch, and two or three inches
high, to carry off the steam of the boil-
ing water ; and a hole for a cork,
through which the thermometer tube
is inserted in such a manner, that the
ball does not touch the surface of the
water, but may be surrounded with an
atmosphere of steam ; while no more of
the tube should be above the cork than
is sufficient to show the height to which
the mercury rises when the water is
briskly boiling. When all things are
thus adjusted, a thin plate of metal is to
be laid over the chimney, to prevent the
escape of the steam as it is formed ; heat
is to be applied to the bottom of the
vessel ; and when the mercury has re-
mained a few minutes stationary in the
atmosphere of steam, its height is
carefully to be marked with a file on
the tube.

The water may be distilled, or any soft
water, such as clear rain water, be used ;
for, if there be much saline ingredient
in the water, this will affect the boiling
point, and may lead to error.

Various mechanical contrivances have
been proposed for more conveniently
fixing the tube in the cover, but they are
of little comparative importance. Some
prefer plunging the ball into the water
to the depth of two or three inches : in
this case there is no necessity for a plate
of metal on the chimney, nor for the
tightness of the cover; but the adjust-
ment of the boiling point is to be made
for the barometer at 29.50 inches. To
those unprovided with such a vessel
the following method is recommended.
Wrap several folds of linen, or flannel,

* Phil. Trans, vol. Ixvii. part ii.

round the tube, nearly as high as the
supposed boiling point, which may be
guessed at by previous immersion of
the bulb in boiling water : hold the ther-
mometer in an ascending current of
boiling rain water about two or three
inches below the surface ; pour boiling
water three or four times on the~ cohering
of the tube, at intervals of some seconds ;
and waiting a few seconds, after the last
affusion, to allow the water to be in
brisk ebullition, mark the height of the
mercury in the tube, which will be the
boiling point of the instrument.

Having thus obtained two fixed
points, the freezing and boiling points
of water, it is easy to mark off corre-
sponding divisions on the scale which
is to be graduated. If the tube be truly
cylindrical, nothing more is necessary
than to divide the intervening space into
as many equal parts as it is^ intended to
have degrees between those points.
Should the tube not be of uniform bore,
the size of the divisions ought to be ac-
commodated to the inequalities of the
tube. This may be done by taking in-
termediate points in mixtures of water
at different temperatures ; and after
marking them on the tube, proportion-
ing the size of the degrees, at short in-
tervals, to the varying diameter of the
tube. This method of graduating from
intermediate points ought, in nice in-
struments, to be adopted, however true
the tube may appear ; but a tube with
sensible inequalities is in general to be
avoided.

Although it would be advisable to fix
the boiling point when the barometer
is at the height above recommended,
this may be attended with serious in-
convenience to artists ; and philosophers
have therefore investigated the correc-
tion to be made for every ordinary varia-
tion of atmospheric pressure.

The first considerable series of ex-
periments on this subject are those of
De Luc, in 1762, published in his in-
teresting Recherches sur les Modifica-
tions de I" Atmosphere,* which were
extended and verified by Sir George
Shuckburgf in 1775 and 1778. Em-
ploying Reaumur's scale, De Luc as-
certained, that if y represent the
height of the barometer, T the height of
the thermometer above the freezing
point, expressed in hundredths of a
degree of this scale, when immersed in
boiling water ; and a the constant number

Vol. i. 382 ; vol. ii. 338.
t Phil. Trans, vol. Ixix. partii.

THERMOMETER AND PYROMETER.

10387, the following formula will ex-
press the height of such thermometer
when plunged in boiling water under
every variation of barometric pressure.
99

5o

or, as expressed in the more usual way
of considering all the figures after the
index as decimals, De Luc's formula
would stand thus :

99 x 100 . „,
2 log y - a = T.

De Luc's researches and his formula
are reduced to English measures, and
adapted to Fahrenheit's thermometer by
Horsley, in a valuable paper in the
Philosophical Transactions;* where a
table is computed for the direction of
artists in adjusting the boiling point. It
is unnecessary to give his equation of
the boiling point, because the later ex-
periments of Shuckburg, and of the
Committee of the Royal Society, enable
us to present a more complete table for
the direction of British artists in cor-
recting the height of the boiling point in
every ordinary fluctuation of the baro-
meter.

Barometer when the boiling
point is found by immersion

Correction in
lOOOths of the in-
terval between

Steam.

Water.

freezing and
boiling of water.

30'60

10 1

•50

9

30'71

•41

8

•50

•29

7

•48

•18

6

i

•37

•07

5

>j

•25

•95

4

•14

•84

3

•03

•73

2

29-91

•61

1

•80

•50

0

•69

29-39

1

•58

•28

2

•47

•17

3

•36

•06

4

•25

28-95

5

|

•14

•84

G

•03

•73

7

28-92

•62

8

•81

•51

9

•70

10 J

•59

The use of this table requires no fur-
ther explanation : but it is necessary to
remark, that it presupposes the thermo-
metric tube to be cylindrical, or of equal

* Vol. liiv. parti.

dimensions throughout, before the indi-
cations of the table can be received as
quite correct ; yet, unless the irregularity
of the tube be considerable, a small
correction will scarcely produce any
sensible error in the instrument.

In proportioning the bulb to the
tube, the eye and experience of the artist
are usually judged sufficient for the pur-
pose ; or they are copied as nearly as
possible from standard instruments.
M. Durand has, however, thought it
necessary to propose an algebraic for-
mula for determining the proportions
they ought to bear to each other ; but
there are practical difficulties in the way
of its application, which render his for-
mula an exercise rather of his own
ingenuity than of utility to the artist.

During the various improvements of
the common thermometer, the air ther-
mometer was almost wholly neglected
until of late years ; but the attention of
philosophers was directed to the changes
of bulk which solids undergo by altera-
tions of temperature, as a measure of the
relative degrees of heat.

CHAPTER II.

History and Construction of Pyro-
meters.

1. THE impracticability of applying
the known modifications of the thermo-
meter to bodies much heated, induced
the celebrated Musschenbroek, before
the middle of the last century, to employ
the expansions of solid rods of metal to
indicate the temperature of such bodies ;
and he gave the name of pyrometer to
his invention.

As the expansions of solids are ex-
tremely minute, it was necessary to
devise some method of rendering them
perceptible ; and the mechanism repre-
sented in/g-. 8 was the Dutch philoso-
pher's arrangement for this purpose,
a is a metallic prism 5.8 inches in length
and 0.3 in thickness, resting in a notch
in the upright i, where it is secured by a
screw, and heated by the lamp b with
five wicks. The prism is pinned to the
end of a bar c, which has twenty-five
teeth in one inch of its length, and forms
a rack sliding smoothly on the table of
the instrument through the two holdfasts
seen in the figure, and playing in the
six-leaved pinion d on the same axis as
the wheel /, which is furnished with
sixty teeth. This wheel plays in another
pinion <?, of six leaves also, which is on
the axis carrying the index g, which

THERMOMETER AND PYROMETER.

moves round the circle h, divided into
300°. The consequence of this arrange-
ment is, that if the expansion of the
metal were to push the rack e one inch
forward, it would turn the pinion d 4|
times round; and the wheel/, moving
at the same rate, will carry the pinion e,
and consequently the index (10 x 44)
= 41 1 round. Hence the index would
have moved over (4 1 § x 300), or 12,500
divisions of the scale ; or each degree
of the instrument is equivalent to Ts|o5
of an inch of the expansion of the prism
a. Similar prisms of different metals
applied in like manner to the instru-
ment.enabled Musschenbroek to measure
the different expansibility of steel, iron,
copper, brass, and lead, with considera-
ble accuracy :* but there is always some
uncertainty in the movements of so
many loosely connected teeth and pini-
ons ; and this pyrometer was improved

by

2. Desaguliers,t who instead of prisms
substituted cylinders, as wires are
more easily procured than prisms of

equal dimensions. For the first pinion
he employed steel slightly roughened
by the file in the same direction as the
teeth. Thus a more equable motion
was given to the instrument. The
toothed wheel and second pinion were
supplied by a wheel and roller, having
grooves in their circumference for re-
ceiving a watch-chain, by which motion
was communicated to the index. The
dial plate was square and movable, in
order to stretch the watch-chain as
there might be occasion. A thin plate
of rough steel ^ inch wide, slightly
convex towards the first roller, was
substituted for the rack ; and this last,
which in Musschenbroek's pyrometer
was made to travel lightly over a small
bit of fine watch-spring, moved in
Desaguliers over a well constructed fric-
tion wheel, or roller.

These changes improved the delicacy
of the instrument very considerably ;
but it soon underwent other modifica-
tions.

3. The pyrometer of Mr, John Ellicot,

Fig. 9.

m

of London, is seen in fig. 9, a a is a
flat plate of brass screwed 1o

*Tentanu Acad. del Cimento.
t Desagulier's Experimental Philosophy,!, 421,

mahogany S.>IQ, A: which the thiee brass
a thick uprights bbb are firmly attached.

The pyrometric pieces consist of two
metallic bars : the flat on« c c is of

THERMOMETER AND PYROMETER.

17

steel, and is that by which the expan-
sions of all the other metals are to be
compared together. Its extremity to
the risrht passes through a hole in the
upright, and is fixed to a spring which
may be tightened by the screw ra. Its
other extremity is free, and presses
against a snail on the axis of the lever
/. The other bar e e is a prism of any
metal, the right end of which rests on
the end of the screw /, while its other
bears on a snail on the axis of the lever
h. When the bars are expanded by the
heat of the spirit lamp g, they move
the levers, to each of which is attached
a slender watch-chain : the chain from
the lever / passes round a pulley \
inch in diameter, fixed on the axis
round which the inner graduated circle
« of the dial moves ; the chain from the
lever h passes round a similar pulley
on the axis of the index, as seen in the
figure ; and the expansions of this bar
are marked by the index on the fixed
outer circle. Both pulleys have a
thread wrapped round them in a con-
trary direction on each, and then pass-
ing over the pulleys at o to the weight
k, which acts as a counterbalance to
bring back the index and movable
circle as the bars cool. The index and
circle are both adjusted to the be-
ginning of their scales by means of the
screws /, m, at the commencement of
each experiment; and when the tem-
perature applied expands the standard
bar to a given degree, as indicated on
the inner circle, the index will show on
the outer circle the relative expansi-
bility of whatever metal is applied to
the instrument at e e.*-

This instrument was chiefly intended
by its ingenious inventor, a chrono-
meter-maker by profession, for ascer-
taining the relative expansion of the
metals usually employed in the con-
struction of pendulums ; an important
object, for which many of the best py-
rometers have been devised.

In this instrument the dial is about
three inches in diameter ; the levers
two inches and a half in length, and
the proportions of the several parts
such that the expansion of ,"& inch in
the bar will move the index wholly
round the circle ; or each degree will
mark the 75Vjj of an inch in the length-
ening of the bar. From the mean of
numerous experiments, Ellicot ascer-

• This description is taken from an original in-
strument now before the author.

tained the following to be the relative
expansions of seven metals . —

Steel. Iron. Gold. Copper. Brass. Silver. Lead.

56 60 73 89 95 103 149,*

which is more nearly in the ratio of the
conducting power of the different me-
tals, than ot any other of their physical
properties.

4. n the 44th volume of the Philo-
sophical Transactions is a description
of another pyrometer by Dr. Cromwell
Mortimer, which, though less accurate
and convenient than Ellicot's, is worthy
of notice, especially as it may be em-
ployed to show the alterations of
atmospheric temperature.

a, b, fg. 1 0, is a round rod of brass
Fig. 10. A.

or steel, % inch in diameter and three

* Phil. Trans, vol. xxxix. p. 297.
C

18

THERMOMETER AND PYROMETER.

feet long, its upper extremity termi-
nating in a hardened steel point one
inch more in length, and entering a
hole in a steel plate on the under
side of the lever e, while its lower
end rests on a point attached to the
metallic plate at d. c, d are plates of
iron joined at d, and at different other
points, as in the figure : at x, x they
are turned half round, to allow the
application of heated bodies, as sand
or water, to the bar, which is immersed
in the heated bodies to a certain mark
as at b. In the original instrument
this mark was at 1^ inch from the
bottom: e,f is a lever moving round
an axis in g. A string from the end of
its longest arm passes twice round the
pulley h, and is kept tight by a weight
i of £lb., while there is another weight
I, at the short arm of the lever, suffi-
cient to counterbalance the weight of
the longer arm, and to keep the point a
in close contact with the lever, m, n, o,
a dial, of which the face is seen at B,
graduated to correspond to Fahren-
heit's and Reaumur's degrees, which
are indicated by an index fixed on the
axis of the pulley h. The frame of the
instrument is of oak. The lever from
p to a = 4 inches ; from a to g — 1.5
inch; from g to/= 12 inches; the
pulley = 0.5 inch ; the dial = 11 inches
in diameter. In the original the melt-
ing point of different substances is in-
dicdted by their chemical signs in the
outer circle of the dial.

This instrument appears to have been
of considerable delicacy, and to have
marked minute changes of atmospheric
temperature very readily : but the size
is inconvenient ; and it must now be
regarded rather as an instrument of
curiosity than utility.

5. The pyrometer, figured 11, the
invention of Mr. Froteringham, a Lin-
coln grazier, combines simplicity with
considerable delicacy. It was also in-
tended to indicate the changes of at-
mospheric temperature, a, a is a bar
of iron four feet long and 1 1 inch wide,
having a polished brass surface screwed
to it with steel screws, which are fitted
to short slips in the brass that allow the
expansion of the iron bar, without that
of the brass ornamental surface, to
affect the hardened steel apex b. This
apex moves the lever c, which raises
the lever d; both turning on well made
central disks. A chain from the ex-
tremity of the lever d is lapped twice
round the pulley/ on the axis of the

index, which moves round a graduated
circle g. The counterpoise i brings
back tne index as the levers fall. The
screw h is for adjusting the index to
the beginning of the scale. It is very
obvious that such an instrument would
be capable of showing the expansions
of the bar in proportion to the differ-
ence between the arms of the levers ;
and, it is said, that the original in-
strument, in the library of a philoso-
phical society at Spalding, indicated
the changes of the heat of the weather
with great precision.*

6. All these instruments, however,
yield in accuracy to the invention of the
celebrated Smeaton, which is described
in the Philosophical Transactions .t

In this instrument the expansions of
the metallic bars, heated by water, are
measured by means of a micrometer
screw; a principle which had been
before employed by the great chrono-
meter-maker Graham, for the adjust-
ment of the rods of a rendulum.

From the principle of its construc-
tion, this instrument is called the Mi-
crometer-Pyrometer, /g-. 12.

The basis of this instrument a, b, c, d
is of solid brass, which was chosen as

* Phil. Trans, vol. xlv. p. 125.
t Phil. Trans, vol. xlviii. p. 487.

THERMOMETER AND PYROMETER.
Fig. 12.

19

of a mean expansibility among the me-
tals, ef is the bar to be measured,
resting on two notches, one attached to
the fixed upright a b, and the other to
the principal lever h i. A is a strong
arbor fixed to the basis, and intended
to receive the ends of two screws h, I,
upon which the principal lever h, i
turns ; o is a slender steel spring in-
tended to press the lever against the
extremity of the bar; and p is a check-
rod to support the lever, when the bar
is removed, t is called the feeler ; it is
in the form of the letter T, and is sus-
pended freely, but without shake, be-
tween the points of the screws m, n. q
is the handle of the feeler, which is
movable on a loose joint, so that the
feeler may be moved by the handle
without being irregularly affected by
the pressure of the hand. The princi-
pal part of the instrument is s, the
micrometer screw, and w the graduated
circle or index-plate fixed on the screw,
which indicates the revolutions of the
screw on the index v. The micrometer
screw passes through two solid heads
perforated by a corresponding screw ;
the piece y z is made somewhat springy,
and tends to draw the micrometer screw
backward from d; by which its threads
press uniformly against the correspond-
ing threads in the holes, and keep the
motion equable and easy.

When the instrument is used, its
basis and the bar are immersed in a tin
vessel containing water, as marked by
the dotted line, which is heated by se-
ven lamps applied below. The vessel
is provided with a cover ; and a deli-
cate mercurial thermometer is sus-
pended in the water, for regulating and
ascertaining the temperature employed,

which is not intended to exceed that of
boiling water.

The expansion of the bar presses the
lever and feeler towards the end of the
micrometer screw, which, as well as the
extremity of the feeler, is tint with har-
dened steel. The handle q is laid hold
of, and by it the feeler is moved up and
down, while the screw is turned, until
its steel point comes in contact with the
end of the screw. Mr. Smeaton found
that he could judge of that contact more
accurately by the ear, than by the eye
or the touch.

The turns of the index-plate counted
by its edge and the divisions of the
index, show the expansion of the bar ;
and its length when cool may be found
in the same manner, either before or
after the experiment above described.
In this instrument the bar acts against
the centre of a lever of the second order t
the fulcrum of which is in the basis ;
and when both are expanded, the free
extremity of the lever moves through
a space double of the difference between
the expansion of the bar and of the
basis : hence, when we know the length
of the lever from its axis to the point of
suspension of the feeler, the distance
from that axis to the point of contact of
the bar, the number of threads of the
micrometer screw in an inch, and the
number of degrees on the circumference
of the index-plate, we can compute the
value of these degrees in fractions of an
inch. In the original pyrometer the
following were the proportions :
From axis of lever to point of inches.

suspension 5.875

fulcrum to point of contact 2.895

Length of 70 threads of the screw 2.455
Division of index-plate 100°.
c 2

20

THERMOMETER AND PYROMETER.

Hence the value of each division of
the index-plate will = 7rr^^s °f an
inch ; and as, when the instrument was
well adjusted, the difference of contact
was very perceptible when the screw
was moved through £ of a division, the
53T43th of an inch of expansion was de-
terminable by this pyrometer, with which
Mr. Smeaton ascertained the expansi-
bility of many solids.

The following table is the result of
his experiments, showing in 10,000dths
of an inch the expansion of rods of
different kinds of matter, in passing
from the freezing to the boiling point of
water.

White glass barometer tube . .100
Martial regulus of antimony . .130

Bistered steel 138

Hard steel 147

Iron 151

Bismuth 167

Copper hammered 204

Alloy, 8 copper, and 1 tin . . .218

Cast brass 225

Alloy, brass 16, tin 1 229

Brass wire 232

Telescope speculum metal . . . 232
Alloy, 2 brass, 1 zinc .... 247

Fine pewter 274

Grain tin 298

Soft solder, 2 lead, 1 tin . . . .301
Alloy, 8 zinc, 1 tin, slightly ham-
mered 323

Lead 344

Zinc 353

Zinc hammered out 1 inch per foot 373
These experiments correspond as
nearly with the results obtained by
Ellicot, as the difference of the instru-
ments admit. They introduced a pre-
cision hitherto unknown in the law of
expansion of solid bodies ; and are still
quoted with approbation in those nice
disquisitions which have paved the way
to the perfection of horology, and the
modern refinements in geodesical ope-
rations, while they have extended our
knowledge of the effects of heat.

7. The metalline thermometer of Mr.
Keane Fitzgerald comes next in order
of time ; but it is chiefly applicable to
mark the alterations of atmospheric tem-
perature. • Its general construction will
be readily learnt from fig. 13.*

The basis of the instrument is a piece
of well seasoned deal, on which a system
of levers is fixed ; a a is the pyrometric
bar, 2 feet long, the upper extremity of

* Phil. Trans, vol. li. p. 523.

which bears against the fulcrum z
Fig. 13.

while its other end rests on a small
hemisphere of metal on the short arm of
the lever b. The long arm of this lever
is 2| times as long as the other ; b is
joined by a pivot to the rod c, 2 feet

2 inches in length, which bears against
the short arm of d, and the long arm of
d is 2| times as long as the former.
The rod e is 2 feet 4 inches long, and
is jointed to /, as in the figure. The
long arm of fis 4 times the length of
its short arm, and terminates in a
slender arch-head, which is attached to
the lower end of the rod g by a watch-
chain, as in the figure. This last rod is

3 feet long, and is kept perpendicular
by sliding between two friction rollers
p, v, its connection with the arch-head, its
suspension from the lever y, and its
friction on the pulley h. The weight of
the levers, &c. is counterbalanced by
the springs m and o, and the spring of
y is nearly neutralized by the pressure
of x. The pulley h is fixed at 2 feet
6 inches from the lower end of g, and
is 3 inches in diameter. Two cords
fixed to the spring q, pass twice round

THERMOMETER AND PYROMETER,

21

the pulley h in different ways, and thence
go over the pullies at t, respectively, 1
inch and ^ inch in diameter. These last
are put on the common axes of the
indices k, I, in the same manner as the
hands of a clock. The face of the dial
is 12 inches in diameter, and from the
construction, the index / ranges 48
times, and the index k 12 times as much
as the bar g. The dial has on it three
circular scales ; the inner is divided into
240°, corresponding to those of Fahren-
heit's thermometer ; the middle is di-
vided into 360°; and the outer into
1080 parts, marking 18 for each degree
of the thermometer, and 12 for each
degree of the circle.

This instrument may be used as a
pyrometer in low temperatures ; for the
bar a is removable ; and from the con-
struction, each division of the outer
circle is equivalent to an expansion of
755T<>th of the bar.

Used in this way, Mr. Fitzgerald in-
forms us that the dilatations of metallic
bars 2 feet long, at the same tempera-
ture, were as follows :

Divisions.

Spelter or zinc . . . . = 1570
Zinc 18, copper 2 parts . . = 1550

Brass =1120

Iron = 785

Steel = 695

which agrees pretty well with the ex-
periments of Smeaton and Ellicot.

When used as a thermometer the in-
dex k marks 74 divisions in passing from
the usual extremes of temperature in
our climate, and 212 divisions from
freezing to boiling water.

Mr. Fitzgerald experienced some
difficulty in proportioning the strength
of the springs to the weight sustained
by the levers, and he improved the in-
strument by the adoption of pulleys and
counterpoise weights, as in fig. 14,
which he ingeniously converted into a
register thermometer, by adapting two
light index hands a, a, fixed to two
brass circles moving between friction
wheels, attached to a fixed circle d.
They were so nicely fitted as to move
readily by a weight of 8 grains hung on
them. These hands are moved in op-
posite directions, by a small stud in the
under surface of the index f, which re-
ceives its motion from a cord passing
from the pulley h round a small wheel
on its axis.

This alteration of the instrument was
intended only to note the changes of the
atmosphere, which it seems to have done

with much delicacy ; for it had a range
Fig. 14.

of 72 inches from the common changes
of the heat of the weather in London ;
and it would show an alteration amount-
ing to 50 or 60 degrees of its scale,
when the pyrometric bar of the instru-
ment was five or six times breathed
upon*

8. In Ferguson's Lectures two pyro-
meters, the invention of that great self-
taught mechanician, are described.

Fig. 1 5 was merely intended to exhibit
to his audience the expansions 'of bodies
by heat, yet is worthy of notice.

a a, a mahogany board, on which are
fixed four brass studs ; of these b sup-
ports a screw for adjusting the 'pyro-
metric bar/, which rests in notches in
the studs cd. The extremity of the bar
presses against the crooked lever g-, which
acts on the index i i ; the stud e holds
the spring h, which brings back the in-
dex when the bar cools. The lever g
(of the second order) has the portion
between the point of contact of the

* Phil. Trans, vol. lii. p. 146.

THERMOMETER AND PYROMETER,

bar, and where it touches the index 20
times as long as the space between the
point of the bar and its fulcrum ; and
the space between the end of the lever
and the free end of the index is just 20
times the length of that between the
point of the lever and the axis of the
index; hence, when the bar expands
5^5th of an inch, the point of the index
will have moved over (20 x 20) = 400
times as much space, or one inch ; or
if the bar expand 3oWtn of an inch, the
index will move j\jth.

The scale is divided into inches and

Fig. 16

tenths ; and the mere friction of the
bar /, which is removable at pleasure,
with a piece of flannel till it becomes
sensibly warm, will be sufficient to show
variations of the index. Ferguson states
that it gave the following results: —
with bars of iron and steel, 3 ; copper,
4£ ; brass, 5 ; tin, 6 ; lead, 7.

9. In the supplement to his lectures
there is however a much more delicate
pyrometer described, (fg. 16,) which
will show the expansion of a bar of me-
tal to the sskotfth of an inch, or even to
the 9 0,0 00th.

The frame a b is of mahogany, sup-
ported on short pillars, so as to admit a
lamp under it for heating the bar/"; one
end of which lies in a cavity in the piece
of metal g, and the other, after passing
on a friction wheel over the cross-bar
h h, presses against the short lever e e.

The manner in which this short lever
acts on the index is seen in the adjoining
diagram A, where k is the short lever
that moves under the dial d between
friction wheels. On the side of k are
1 5 teeth in the space of one inch, which
play in the twelve leaves of the pinion

THERMOMETER AND PYROMETER.

/ on the axis of the wheel m m. This
wheel has round its circumference 100
teeth, which work in the ten leaves of
the pinion n, on the axis of the wheel o
of 100 teeth, that gives motion to the
pinion p of ten leaves, on the axis of
which the index is fixed.

As the wheels m and n have each
100 teeth, and the pinions n and p ten
leaves, it is obvious that when the wheel
m has made one revolution the pinion
p, and of course the index, will have
made 100 revolutions ; as the pinion /
has twelve leaves, and the bar i k has
fifteen teeth to one inch (equivalent to
12^1 it is obvious that while ik moves
one inch the pinion/? will have moved
100 + i or 125 times round, and the in-
dex would at the same time be carried
125 times round the circle dd. This
circle is graduated into 360 degrees, and
beina: eleven inches in diameter, it is
subdivided into half degrees. Hence
each degree of that circle will be equi-
valent to an expansion of 125 x 360
= _5£_5th of an inch of expansion in
the bar/; and as the half degrees can
readily be distinguished on the dial, the
instrument will show expansions only
amounting to ?5&3oth part of an inch.
A silk thread is several times wound
round the axis of n and passes to the
slender spring s, which keeps the teeth
of the pinions and wheels in close con-
tact, and pulls back the train of wheels
vrhen the cooling of the bar / allows
the short bar i k to recede.

The inner circle of the dial is divided
into eight parts, corresponding to so
many thousandths of an inch in the ex-
pansion of the bar/, or t^th of an inch
for each degree of the outer circle over
which the index has moved. Bars of
different metals laid in g for a given
time, and exposed to the same lamp,
afford an indication of their relative ex-
pansibility ; and to ensure equality in
the bars it is recommended to have
them wire drawn through the same
hole. There is, however, in this instru-
ment no accurate measure of the tem-
perature applied to each bar ; and, not-
withstanding the delicacy of the move-
ment, it seems inferior to Ellicot's
pyrometer, as it wants a constant and
uniiorm standard by which to compare
the expansions in each separate experi-
ment.

10. A new method of ascertaining the
expansibility of different substances was
suggested by the late Mr. Jesse Rams-
den, and on his hint it was attempted

by the ingenious and indefatigable D
Luc, whose researches on the barometer
gave this subject an increased interest
to his mind. The object in view was to
determine the relative expansion of so-
lids by observation with a microscope
furnished with a micrometer. The
microscopic pyrometer of De Luc is
seen in Jig. 17, where a b represents a

Fig. 17.

strong board of even-grained deal ; to
which the frame c c c c is firmly joined,
that when a b is suspended vertically
from a strong post, the front of the in-
strument bearing the microscope d d is
towards the operator.

The microscope is securely united to
the frame by the braces e e and the
cross-bar /; and the whole of this part
of the apparatus can be moved up or
down by the slides g g, which fit so
tightly on their centres as to require
slight blows with a hammer on the
frame to cause them to move down,
and may be further tightened by the
screws Hi. The microscope is kept
horizontal by the cross-pieces, and by an
inner sliding frame not seen in the
figure. The microscope is so adjusted
that an object is distinctly seen when a
full inch from its lens ; and it is furnished
with a micrometer, movable by k, for
ascertaining the expansions of the rods
subjected to experiment. A piece of
thick deal I is seen at the top of the
frame lying horizontally from a groove

24

•THERMOMETER AND PYROMETER.

cut in the board a b to the front of the
frame ; this is movable by means of
the screw A, and is perforated by a
piece of cork m firmly driven into it
until level with its lower surface. The
cork is then pierced vertically to receive
the glass rod o o o, which is thus sus-
pended in a thin cylindrical glass jar
p, 21 inches high, and 4 inches in
diameter, filled with water. The glass
rod is the standard of comparison, and
to it is attached a rod s s, of the metal
to be tried, by two connected rings r r,
which are tightened on the rods by two
screws. Another set of rings v is ap-
plied higher up; but through this the
rod s s freely slides, while it firmly clips
the glass rod by means of a screw. A
delicate thermometer hangs in Ihe centre
of the jar p to note the temperature of
the water, which is occasionally agitated
to secure uniformity of temperature by
the rod q q. A syphon z to draw off
the water completes the apparatus.*

In using this pyrgmeter, warm water
is poured into the jar, in order to heat
the rods ; the rods are adjusted to the
focus of the microscope by the screw
n; the thermometer gives the degree
of heat employed ; and, by means of
marks on the bars, their relative ex-
pansion is given in divisions of the
micrometer, the value of which is
known by previous experiments. The
connection of the rods is more dis-
tinctly seen at A ; but it is unnecessary
to give a more minute description of an
instrument which has been superseded
by the more accurate and more elegant
contrivance of Ramsden, so elaborately
detailed by General Roy, to which we
shall presently advert.

11. From experiments with this instru-
ment, De Luc ingeniously applied a cor-
rection to the scale of barometers for
temperature, by what, in the same
paper, he calls " metallic thermome-
ters" The scale of the barometer
was fixed on a bar of metal of known
expansibility, so as to raise the scale in
exact proportion to the expansion of
the mercury ; and thus the mere in-
spection of the barometric scale will
give the true height, without the trouble
of applying the equation or formula of
correction for temperature, as in ordi-
nary observations.

12. We come now to certainly the most
complex, but the most perfect of all
contrivances for determining the rela-

* Phi). '1 rans. vol. Ixviii. part i. p. 43?.

tive expansions of solids, the micro-
scopic pyrometer of Ramsden, contrived
by that eminent artist, for determining
with the utmost possible precision, the
expansibility of the rods employed by
General Roy, in the geodesical opera-
tions that are the foundation of the
great trigonometrical survey of Britain.
Fig. 18 contains plans and sections of
this beautiful contrivance, and although
we do not propose to enter into a mi-
nute detail of the different parts of the
instrument, a general description will
show, to those who have not consi-
dered such subjects, the nice precau-
tions which are necessary to accuracy
in like operations, while it explains its
construction. Ramsden's pyrometer is
attached to a strong and well joined
deal table, or frame 5 feet long, 28
inches broad, and 42 inches high ;
of which an end elevation is seen fig.
18, B ; the plan of its top will be best
understood from an inspection of A.
a b and c d are troughs of deal,
(firmly screwed to the table) 3 inches
in diameter, and a little longer than the
frame ; a b projects a little over the
table, but c d is in a line with the
frame, as may be seen at B. Each
trough contains a cast iron prism, 1|
inch on each side, firmly fixed in the
troughs, at the ends a and c, by means
of brass collars embracing the prisms,
and tightened by screws as at G, while
the ends b and d pass freely through
loose collars, without any shake, when
their dimensions are altered by tempe-
rature. The prism a b is called the
eye prism ; because it carries at each
end the eye-pieces of the microscopes
Imn, and o p r; which are figured on
a larger scale at F and E. The other
prism c d is called the mark prism ;
because it carries at one end the mark I,
and at the other cross wires H; efis a cop-
per boiler 2| inches wide, and 3| deep,
rather shorter than the wooden troughs.
The centre of the boiler, or rather of
the object lens standing perpendicular
to it, is 5.81 inches from the cross
wires of the mark in c d, and 20.33
inches from the wire of the micrometer
attached to the corresponding eye-piece.
The boiler rests on five small rollers,
seen in the enlarged section D. The
boiler, like the troughs, has acock to the
right hand ; and in the plan A, it is
represented with a bar in it, to show
the position of the rods to be tried.
The water in the boiler is heated by the
12 spirit lamps gggg, standing on four

THERMOMETER AND PYROMETER.

25

26

THERMOMETER AND PYROMETER.

movable shelves, and showing only
their handles h h h h, when under the
boiler.

The boiler contains another essential
part of the apparatus, viz. two brass
slides, composed of two cheeks, kept
at equal distances by cross bars as in
C, where a prism or bar is represented
as resting in the centre of the slides.
The long slide reaches from microscope
to microscope, and has its cheeks If
inch deep. It is attached to the boiler
only at the point w, and it rolls on the
small roller x, near the left hand of D.
The right hand end of the long slide is
shut up by a piece of strong brass y, y,
supporting two rings, for the part n of
the fixed microscope. The short slide
v, v, v, v, is only 14 1 inches long ; its
cheeks are H inch deep, kept parallel by
braces, as seen in D. It moves within
the long slide ; and its outer end rests
on the cylindrical surface of the last
brace of the long slide, fitted to receive
it, while a narrow longitudinal bar z
moves freely in the notch of a bridge
B, framed for it in the long slide. The
outer end of the short slide is shut up
by a similar piece of brass to that clos-
ing the opposite extremity of the long
one. The bar or rod to be examined
abuts against the piece of brass y,
it rests on the three rollers s s s, I inch
in diameter, and is kept in the centre of
the slides by three milled nuts eee> that
screw up so as not to press too much
on the sides of the bar. At / is a tube
and wire moving through a collar of
oiled leather, that by means of a helical
spring presses a flat piece of metal
attached to the wire, against the shut
end of the short slide and rod to be
measured, so as to keep the other ex-
tremity of the rod in contact with y.
On the application of heat, the rod ex-
pands, and overcoming the slight resis-
tance of the spring, carries before it the
short slide, and with it the tube con-
taining the object lens of the micro-
meter microscope o, p, r, a space pro-
portional to the temperature applied;
and it is this space, measured by the
micrometer, that determines the nume-
rical value of the expansion of the rod.
The microscope tubes are divided
into three pieces, for the convenience of
applying the instrument to measure
rods shorter than five feet. For this
purpose the centr.il screening tube of
the fixed microscope, supported on the
mahogany prism i k by a collar, may
be moved and damped at any part of

that prism ; the eye-piece, in like man-
ner, may be moved along the eye prism ;
but the object lens tube was left in the
rings of the slide, and another lens of
the same focus was clamped to the
cheeks of the slide at suitable distances.
The standard prisms, during each
experiment, were kept at the freez-
ing temperature, by being sur-
rounded with pounded ice. The mi-
croscopes were then accurately ad-
justed to the marks, by bringing the
cross wires to bisect them, and until this
was accomplished, the rod to be mea-
sured was also surrounded with ice.
The lamps were then applied to the boiler,
and the elongation of the rod, at the
boiling heat, was ascertained by the
micrometer attached to the microscope
o, p, r. In these delicate investiga-
tions there were two observers, who si-
multaneously used both microscopes,
lest any alteration had taken place in
the fixed end of the rod ; and to ensure
accuracy, the experiments were twice at
least repeated.

The value of the indications of the
micrometer, on which so much depends,
was previously thus ascertained :

The head of the micrometer screw
= 0.9 inch in diameter, and was divided
into fifty equal parts, each of which was
reckoned two ; and they were therefore
numbered to 100. Fifty-five revo-
lutions of the head were found equal
to 0.77175 of an inch; it follows that
there are 71.27 threads of the screw in
one inch ; and seven revolutions and
nearly T~ths move the micrometer wire
•jyh of an inch ; consequently T5Vh of
part of a revolution, or half a division of
the head, will answer to a motion of
something more than 0.00014th of an
inch. Having found 7.13 revolutions
equal to 0.1 inch at the wires, it is ob-
vious that the number answering to 0.1
inch at the mark being aiso found and
added to the former, their sum will give
the measure of 0.1 inch at the object
lens of the microscope o, p, r, or the
space through which the free end of the
rod has moved by the change of tempe-
rature. This last point was ascertained
by experiment to be = 24.93 revolutions
of the micrometer head ; which being
added to 7.13 = 32.06, " for the number
of revolutions measuring a motion of
0.1 at the object lens, or an expansion
of T\jth of an inch," or half a division of
the micrometer head is equivalent to an
expansion of the rod under examination
of an inch ; and \ of a division,

THERMOMETER AND PYROMETER.

27

•which may readily be seen by the eye,

= 640-00-

Such is the microscopic pyrometer of
Kamsden ; an instrument not indeed
suitable for ordinary purposes, but ad-
mirably adapted for obtaining an accu-
rate estimate of the comparative expan-
sibility of different solids ; an object of
the highest importance, not only in
bringing to perfection the delicate in-

struments required by the refinements
of modern philosophical investigations,
but essential to the perfection of diffe-
rent kinds of machinery in daily use,
and even to a successful investigation
of the laws and nature of heat itself.
With this instrument Roy determined
the expansion of the seven solids in the
annexed table.

Expansion of

By 180°.
Revolutions. Parts.

By F.

Parts.

Dutch brass 35.69 = 19^5

English plate brass, a rod . 36.41 = 20 T2o3o
Ditto, in the form of a trough 36.45 = 20 JL%

Steel rod 22.02=12T^_

Cast iron prism . . . . 21 34 = 11 T*/°

Glass rod 15.54= 8T<y_

Ditto tube 14.93 =

13. The instruments hitherto noticed
are inapplicable to very high tempera-
tures, or to ascertain the heat of closed
fire-places ; an object, in many processes
in the arts, of the utmost importance. To
supply this deficiency, our celebrated
Wedgwood took advantage of the pro-
perty which clay has of contracting
by heat, and remaining afterwards in
that state of contraction. This property
is not, strictly speaking, an exception to
the general law of expansion by increase
of temperature : clay is not a homoge-

Inch.
on 5 feet.

0.111323
0.113568
0.113693
0.(J68684
0.066563
0.048472
0.046569

Inch.
on 1 foot.

0.0222646
0.0227136
0.0227386
0.0137368
0.0133126
0.0096944
0.0046569

neous body, but a mechanical mixture
of argil and silex, which by the influence
of heat are brought into more intimate
union, and therefore diminish in bulk ;
until a temperature sufficiently high to
melt them, that is, to convert them into
a homogeneous mass, is applied : after
which the product obeys the general law
of expansion by heat. Availing himself
of this property, Mr. Wedgwood em-
ployed as pyrornetric pieces cylinders of
dain clay, slightly flattened on
as seen in A B, Jig. \ 9,*

formed by pressing the clay into an
iron tube, and baked in a potter's fur-
nace. It was found, after repeated
trials, that the pieces of clay contracted
more and more in an uniform ratio
to the degree of heat communicated to
them, and permanently retained this
contraction ; so that by applying them
when cold to a scale, an indication of
the degree of heat was obtained.

The scale employed by Wedgwood
consisted of two brass rods \ inch

square, and two feet in length, fixed on
a brass plate converging ly, so that they
were distant at one end just 0.5, and at
the other 0.3 inch. For convenience
the rods are usually divided and fixed as
in the figure on the plate, forming two
nearly parallel grooves- •»• With the
above -stated convergence the whole

• Phil. Trans, vol. Ixxii. Ixxiv. Ixxvi.
f The degree of convergence bi'injr only one-tenth
of an inch in a foot, is not perceptible in the tigure.

THERMOMETER AND PYROMETER.

groove is divided into inches and tenths,
making 240 degrees in the whole scale;
and the higher the temperature to which
the pyrometric piece has been exposed,
the further will it slide up the scale.

In order to compare his scale with
Fahrenheit's mercurial thermometer,
which cannot measure a temperature
much beyond 600°, Mr. Wedgwood was
compelled to make use of the expansions
of a pyrometric piece of fine silver, ap-
plied to a gage on the same principle as
that above described. By this, the ex-
pansions of the silver for 50° and 212°
Fahrenheit were first noted ; and then
the silver and clay pyrometric pieces
were compared at the same temperature.
By such means Wedgwood estimated
the value of each degree of his scale at
130° of Fahrenheit ; and he reckoned
that the 0° of his scale corresponded
with the 1077°.5 of the common scale.
On this principle comparative tables of
the two thermometers have been con-
structed ; but their accuracy depends on
two circumstances which have not been
determined to the satisfaction of the
philosophic world. Clay being a hete-
rogeneous mixture, it by no means fol-
lows that its contractions are equable at
different temperatures ; and even were
this ascertained, there is great doubt
how far the means employed by Wedg-
wood did accurately estimate the degree
of Fahrenheit at which his scale com-
mences.

There is still another serious objection
to the general use of such an instrument.
It occurred to the ingenious inventor,
that different portions of clay would pos-
sess different degrees of contractibility ;
and he endeavoured to secure unifor-
mity, to a certain extent, by laying in a
large stock of Cornish clay, which he
hoped would supply innumerable pyro-
metric pieces of the same quality. It
was found, however, that spontaneous
changes take place in such clay, which
render its indications liable to variation
at distant intervals : or pieces, now
formed of the same clay, will not give
the same indication with pieces baked
several years ago. Attempts were
made to remedy this inconvenience by
forming a clay of uniform quality of
fixed proportions of silex and alumine.
Fine Cornish clay yielded, on analysis,
two parts of silex and three of alumine ;
and such a mixture made into a paste
with fths their weight of water, has been
recommended for the fabrication of
pyrometric pieces. The method detailed

by Wedgwood should then be followed
in moulding them. The paste is first
to be rammed into a metallic mould
0.6 inch wide, 0.4 deep, and 1 inch
long : they should be dried in the air,
and when quite desiccated, Wedgwood
gaged them in another mould exactly
0.5 of an inch wide, and of the form
given in the figure. Before they are
baked they will, of course, just enter the
widest end of the scale, resting at 0°.
When contracted by baking to £th of their
bulk, they will pass to the 120°; and
when reduced to fths, they would pass to
the 24 (j°, or the extremity of the scale ;
but Mr. Wedgwood never did obtain a
higher temperature than 160°. From
these proportions each degree of Wedg-
wood's scale is equivalent to «, contrac-
tion of' o£oth part of the pyrometric
piece.

The difficulty of obtaining clay of an
uniform quality, and not liable to spon-
taneous change, has lately given rise to
a suggestion of employing pyrometric
pieces" formed of Chinese agalmatolite ;
a suggestion of Mr. Sivright of Meg-
getland, well worthy of attention.*

A more formidable objection was
started by some foreign chemists to
Wedgwood's scale ; one, indeed, that
would have overturned the theory of the
instrument. It was alleged, that the
effect of a long continued, or often re-
peated, exposure to even inferior degrees
of heat, would cause contraction of the
clay, after it had undergone the action
of a higher temperature. This point
has been examined with much care by
Guyton de Morveau, who has shown,
in his valuable essay ,•}• the inaccuracy of
this opinion ; although he contends that
Wedgwood has greatly erred in the at-
tempts to convert his scale into degrees
of Fahrenheit's thermometer, as we shall
immediately notice.

On the whole, the pyrometer of
Wedgwood is an instrument well adapt-
ed to the purposes of the potter, or to
convey some idea of the relative heat of
furnaces ; but we cannot regard the de-
termination of the celebrated inventor
as giving even a tolerable approximation
to relative degrees of high temperatures
by other scales. As, however, Mr.
Wedgwood's tables of temperature are
often quoted, we shall here subjoin
them, with the corresponding degrees
of Fahrenheit, according to his calcu-
lation.

« hclinb. Hiil. Jouinal, vol. vi. p. 179.

| Annales de Chiinie, vol Ixxiv, Ixxviii. xc.

THERMOMETER AND PYROMETER.

29

w.

Red heat in full day-light ............ ou

Enamel heat ..... "• . ............... 6

Brass melts. ......................... 21

Swedish copper melts ................ 27

Fine silver melts .......... .. ........ 2tt

Settling heat of flint glass- • .......... 29

Fine gold melts ...................... 32

Delft ware baked .................... 41

Working heat of plate glass .......... 57

Flint glass furnace, low heat ........ 70

Cream coloured ware baked .......... 86

Welding heat of iron, least ........ 90

Ditto ditto greatest ........ 95

Stone ware, baked ................. U>'J

Derby China vitrefies ................ 112

Flint glass furnace, high heat ........ 114

Inferior Chinese porcelain softened ---- 120

Bow porcelain vitrified .............. 121

Plate glass furnace, greatest heat ____ 124

Smith's forge, greatest heat .......... 125

Cast iron begins to melt .............. 130

Bristol porcelain vitrifies ............ 135

Hessian crucible melted .............. 150

Cast iron thoroughly melted .......... 150

Chinese porcelain, best sort softened . . 156
Greatest heat of an air furnace eight
inches in diameter; deduct soften
Nankeen porcelain at all .......... 160

Extremity of Wedgwood's scale ...... 240

F.

K>77°
1K57
3!!n/
4~>:\7
471?
4847
5237
6407
84H7
10,177
12,257
12,7/7
13,427
14,3:17
15,637
1. 1,897
16,677
16,807
17,197
17,327
17,9/7
18.627
20,577
20,577
21,557

21 ,877
32,277

These results are rendered doubt-
ful by the causes already noticed;
and the experiments of Morveau and
Daniell with pyrometers of platina lead
to very different results.

14. The metallic thermometer of
Regnier is described in a report of the
French Institute for 1 798.* The inven-
tor had remarked, that when a thin
metallic rule, resting on a table, is raised
by the middle, it forms a segmental arc
of which the versed sine, that is a line
perpendicular to the chord, drawn to the
centre of the arc, is twelve times longer
than the space through which the extre-
mity of the bar has moved ; and, on this
principle, he proposed to construct an
instrument for noting variations of atmo-
spheric temperature. The small models
which he exhibited answered perfectly ;
but his intention was, to apply his
invention to instruments on a larger
scale for public use.

The instrument consists of two plates
of yellow copper, two metres long, fixed
in an iron frame, in a bent position,
with their concave surfaces toward
each other, as in the sketch, fig. 20.
On one is fixed a pinion of eight leaves,
on an axis, the end of which supports
an index to mark the temperature. To
the centre of the other plate is attached
a toothed rack, in the position of the
versed sine of the curve, playing in the
leaves of the pinion. When the plates
are cooled they approach each other,
when heated their centres recede; and the
only circumstances of consequence in
the position of the bars or plates are,

* Mtfmoiresde 1'Institut. Nationale, torn. ii. an. ?.

that they should be at some little distance
Fig. 20.

from each other, and so bent that they
cannot become parallel by any reduction
of temperature to which they may be
exposed. Regnier found, that two such
bars, of two metres in length, by a
change of temperature equal to 60 cen-
tesimal degrees, changed the relative
position of their centres, or had a play
equal to 65 millimetres ; but the cor-
rection for the expansion of the iron
frame reduces this by -?5 ; so that there
remains about 26 millimetres for the
real play of the centres of the bars ;
and if the frames, in public instruments
of this sort, are made of stone, that
change, by diminishing the expansi-
bility" of the frame, will increase that of
the bars. Regnier gave a radius of 659
millimetres to his index ; so that it
will traverse over a circle of 1.299 me-
tres in diameter. The pinion has 8
leaves in a diameter of 27 millimetres ;
and these proportions are such, that a
temperature of 60 degrees centesimal
will nearly cause a whole revolution of
the index round a dial 4.085 metres in
circumference. Hence each degree
would be about 68 millimetres in size,
or rather more than 2£ inches ; and
consequently might be distinctly seen at
some distance.

15. The platina pyrometer of Guyton
de Morveau, /£. 21, was laid before the
French Institute in 1804, and was de-
signed to measure the heat of open fire-
places and of furnaces.

Its basis is a small, yet solid plate
«, b of highly baked porcelain, in which
is a grove capable of containing a flat
bar of platina c, 1.75 inch in length,
0.2 of an inch broad, and about 0.1 of
an inch in thickness. One end of this

30

THERMOMETER AND PYROMETER.

bar rests or abuts against the bottom of
Fig. 21.

the groove ; the other presses against
the short arm of a bended lever, Ihe
long arm of which, moving on a pivot,
becomes the index of the instrument.
The short arm of this lever is just one
twentieth ot the length of the long
arm, which in the original instrument
was equal to 1.8 inch; consequently
the space moved over by the long arm
will be twenty times as great as the mo-*
tion caused in the short arm by the ex-
pansion of the bar.

A finely graduated arc of a circle, of
which the index is a radius, is fixed on
the porcelain ; and each degree of this
arc is subdivided into ten parts by a
vernier on the extremity of the index
itself, and thus the instrument is capable
of indicating an expansion of s^oth
part of the radius. All these parts are
of platina.

With this instrument Guyton made
many experiments, the general result of
which proves, that Wedgwood has
greatly erred in assigning too high a
temperature for the degrees of his scale,
a result confirmed by the later experi-
ments of Daniell. Guyton ascribes
Wedgwood's error to his estimating the
fusing point of silver much too high.
It was by means of a pyrometric piece
of fine silver that Wedgwood connected
his scale with that of Fahrenheit ; and
an error with respect to that metal must
viciate all the results. According to
Morveau, the fusing point of silver
ought to have been at 22° W. instead of
28° : and each degree, instead of being
equivalent to 130° of F., ought to have
been no more than 62°.5 ; while the
commencement of his scale should have
been at, 517° F., instead of at 1077°.5.

There is some reason, however, to
believe, that Morveau has stated a red
heat in day rather too low ; for thermo-

meters of mercury and of oil can sustain
a temperature of 517° F. without any
luminousness even in the dark.

Morveau appears to have taken great
pains to connect the scale of his pyro-
meter with the common thermometer ;
and he is probably nearer the truth
than Wedgwood.*

His corrected table of Wedgwood's
temperatures is as follows : —

AVedg.

Mercury boils 2° =

Zinc melts 3 =

Antimony melts 7 =

Silver mrlts 22 =

Copper melts 27 =

Gold melts , . 32 =

Iron welds 95 =

Cast iron melts ISO =

Porcelain melts ,. 155 =

Manganese melts 160 —

Malleable iron melts 175 =

Nickel melts 175 =

Platina melts ..175 =

Fah.
642.75
705.26
95523

1822.67
2 03. 18
2517.63
6508.88
8696.24
9633.68
1051712
1 1454.56
1145456
11454.56

16. In 1803, Mr. James Crighton, of
Glasgow, published a new "metallic
thermometer," in which the unequal
expansion of zinc and iron is the moving
power. A bar is formed by uniting a
plateofzinc,/g-.22, c, d, 8 inches long, 1

Fig. 22.

inch broad, and | inch thick, to a plate of
iron a, b of the same length. The
lower extremity of the compound bar
is firmly attached to a mahogany board
at e, e ; a pin / fixed to its upper end
plays in the forked opening in the short
arm of the index gt g. When the tem-
perature is raised, the superior expan-

* Annales de Cbimie, torn. Ixxiv. Ixxviii, xc.

THERMOMETER AND PYROMETER.

31

sion of the zinc c d will bend the whole
bar, as in the figure ; and the index g
will move along the graduated arc,
from right to left, in proportion to the
temperature. In order to convert it
into a register thermometer, Crighton
applied two slender hands h, h on the
axis of the index: these lie below the
index, and are pushed in opposite direc-
tions by the stud /, a contrivance seem-
ingly borrowed from the instrument of
Fitzgerald.

On the whole, the principle of this
pyrometer is just ; but it does not seem
to possess any considerable advantages
over several of those already noticed.

17. We have some doubts of the
propriety of noticing a sort of air py-
rometer,fig. 23, proposed by M. Schmidt

of Jasy in Moldavia.* It is so evi-
dently a mere theoretic proposal, and is,
besides, an expensive, clumsy, and pro-
bably not very accurate mode of ascer-
taining high temperatures. It consists

of a bottle a, and narrow tube b of pla-
tina, the former to receive the impres-
sion of the heat, and the latter to convey
the expanded air into c c, an air-tight
cistern partially filled with water. The
cover of the cistern is perforated by
three holes ; in one of which the end of
the platina tube is cemented ; in the
second is fixed a glass tube d, contain-
ing a common thermometer ; and in
the third, a slender graduated tube e,
which dips into the water in the cistern.
The thermometer is for ascertaining
the temperature of the included air of
the cistern before the experiment ; the
graduated tube for ascertaining the
temperature communicated to the pla-
tina bottle by the ascent of this water
raised by the pressure of the expanded
air on the surface of the fluid in the
cistern. Any further description would
be superfluous.

The pyrometer of Mr. Daniell (Jig.
24) was first described in Brande's
Quarterly Journal* The moving power
is a rod or wire of platina 1 0.2 inches
in length, and 0.14 inch in diameter,
fixed in a tube of blac;klead ware a, b, c,
by a flanch within and a nut and
screw without the tube at a. This tube
has a shoulder moulded on it at b, for
the convenience of always inserting it
into the furnace, or muffle, to the same
depth. From the extremity of the pla-

Fig. 24.

.A

tina rod at b proceeds a fine wire of
the same metal, ^ inch in diameter,
which comes out of a brass ferrule d,
and passes two or three times round
the axis of the wheel t, B, Jig. 24. It
then bends back, and is attached to a
slender spring m n, which is fixed by
one end to the pin at ?i, on the outside
of the ferrule.
The substitution of a silk string for

• Nicholson's Journal, 8vo, series, vol. ii. 141.

that part of the platina wire lapped
round the wheel, and connecting it with
the spring, has rendered the motions of
the index more sensible. The axis of
i is = 0.062 inch, and the diameter of
the wheel one inch : its teeth play in
the teeth of another wheel just one-
third of its diameter, by which the
wheel k has three times the movement
of i ; and the index on the axis of k

•JVol. xi.p.309.

32

THERMOMETER AND PYROMETER.

moves therefore three times round for
every revolution of i. The action of
the spiral spring m draws round the
wheel * and the index, when the ex-
pansion of the platina rod permits it to
act. The dial is divided into 360 de-
grees. By experiment, Daniell ascer-
tained that each degree of his scale
= 7 degrees of Fahrenheit's : and he has
published an account of some well con-
ducted experiments on the fusing points
of some of the metals with this instru-
ment, which very widely differ from the
results obtained by Wedgwood, but
nearly agree with those of Morveau,
Mr. Daniell found, that after being
exposed to high temperatures, the py-
rometer did not fall to the point from
which it set out ; a circumstance which
he attributes, with justice, to changes
in the form of the tube induced by a
high temperature. This is certainly an
imperfection in the principle of the in-
strument ; but if the degrees of heat
be marked by the ascending series, its
indications seem tolerably correct, and,
although perhaps little to be depended
on in nice investigations, it may become
an useful instrument to manufacturers
who make use of high temperatures.
The tube should not be exposed to
a naked fire, except it be of wood char-
coal; because the foreign ingredients
of fossil coal will adhere or incorporate
with the blacklead ware of the tube.
On these grounds we should feel more
inclined to recommend the pyrometer
of Morveau, which, besides, is extremely
portable ; and, being wholly exposed to
the heat, is less liable to be affected by
extrinsic circumstances in its indications.
The following table exhibits some of
Daniell's comparative results.

50° Fahrenheit = 7°. 2' Daniell.

100 =140

150 = 22.5

200 = 30.5

250 = 38.5

300 = 45.4

350 = 51.5

400 = 58.5

450 = 66.9

50J = 73 5

550 =770

580 = 840
600° by calculation he estimates at 86°.4.

I>. F.

Melting point of tin 63 441

bismuth 66 462

lead 87 609

Boiling point of mercury 92 644

Melting point of zinc 94 658

Red heat in full daylight 140 980

Heat of a parlour fire 163 1141

Melting point of brass 267 1869

^ silver 319 2233

copper 364 2543

gold .. 370 2590

cast iron... ,. 497 3479

18. Messrs. Breguet, the celebrated
chronometer-makers, have lately con-
structed a most elegant and delicate
pyrometer, or metalline thermometer,
of which we give a figure. (See fig.
25.)

Fig. 25.

It consists of a helix formed of three
metals of unequal expansibility. The
exterior plate of this delicate helix
is of silver, the interior of platina, and
between them is one of gold. Two only
are necessary to the perfect action of
the instrument ; but from the difference
of expansibility between silver and pla-
tina, they would be liable to separate by
sudden changes of temperature ; and a
thin plate of gold, which is of interme-
diate expansibility, is interposed. The
whole form a single flat plate or wire
about isoth of an inch in thickness.
The upper extremity of the helix is fixed
to the brass support b, which by its
form insulates the helix, and permits its
coiling and uncoiling freely. To its
lower extremity is attached a gold needle
e, kept horizontal by a small counter-
poise. This needle moves round a gra-
duated circle representing degrees of
the centigrade scale. When the am-
bient air is heated, the expansion of the
metals carries round the needle in the
direction of the coils of the helix, and a
diminution of temperature moves it in
the opposite direction by relaxing the
coils. Experiment has proved that
equal increments of temperature move
the needle over equal spaces of the
scale, so that it is comparable with other
thermometers.

THERMOMETER AND PYROMETER.

33

The sensibility of the instrument is re- The height of the instrument, from
presented as very great, when compared which this description is drawn, is 3

to a mercurial thermometer ; and it is
applicable to such purposes as ascer-
taining the temperature of a vacuum,
which the mercurial thermometer is able
to do less accurately, because of the di-
latability of its bulb by the removal of
pressure.

inches, including the feet, which are half
an inch ; the diameter of the helix is
rather less than T3o ; and its length is
1 \ ; the diameter of the graduated circle
is 2 inches inside, and its breadth \.

1 9. The instrument delineated in fig.
26, from one now before us, is a

Fig. 26.

beautiful instrument of the same kind,
the work of the Parisian artist, Fre-
derick Houriet, which appears to be
little known in this country. It is of
the size of a thin ordinary watch, with a
dial A divided according to the centi-
grade scale. The mechanism is covered
by a thin plate of metal, which opens
like a hunting watch ; and the instru-
ment is so delicate as to move, in less
than a minute, after it is laid on the
hand, at an ordinary temperature of
GOT.

The pyrometric piece is the bent
compound bar a, b, a, b, composed of
a plate of steel on the side a, and of
another of brass on the side b, united
together into one bar. The steel plate
is Jo inch in thickness, and the brass
twice as much, forming a bar 9.5
inches in length, and about } inch in
depth. One extremity is firmly secured
to the frame at c: the rest of it is
free, bent up for the convenience of
size, and secured against any accidental
injury from rude handling by passing
between two steel studs h, h. Its free
extremity is terminated by a plate of
steel d screwed to it, and projecting 0 . 3
inch beyond it, to press against the
short arm e of the lever//. The long
arm of the lever ends in an arch- head
with thirty teeth, that play in the teeth
of a small wheel g with twenty-two
teeth, which is fixed on the axis of the
slender hand. The lengthening of the
bar pushes the short arm of the lever,

and the arch-head moves over a space
proportional to the difference in length
of the arms of the lever. How this
motion is communicated in an increased
ratio to the index is obvious from the
construction. Under the cock i, which
supports the common axis of the index,
and g, is a spiral spring of flattened
gold wire, intended to bring back the
index when the contraction of the bar
allows it, and to retain the piece d in
contact with the short arm of the le-
ver e. The instrument is adjusted by
means of a steel screw k working in a
small tube, which perforates the end
piece d.

The whole instrument is most deli-
cately made, and it accords in its indi-
cations with a mercurial centigrade
thermometer, with which it has been
carefully compared ; forming one of the
most elegant metalline thermometers
hitherto described.

CHAPTER III.

History and Construction of Register
Thermometers.

THE original suggestion of a thermo-
meter which could register its own indi-
cations in the absence of the observer, is
due to the celebrated John Bernoulli,
who describes such an instrument in a
letter to Leibnitz ; * and an instrument

* Leibnitzii et Bernoulli Commercium Philosoph.
et Mathcmat.

THERMOMETER AND PYROMETER.

on nearly the same principle was con-
structed by Kraft : * but their con-
trivances are inferior to several others
of a later period, and do not require a
detailed notice. We shall therefore pro-
ceed to describe the most approved re-
gister thermometers.

1. Lord Charles Cavendish commu-
nicated to the Royal Society different
forms of thermometers, intended to
register the maximum and minimum
temperature in the absence of the ob-
server, t His lordship's thermometer for
showing the maximum is represented in
%. 27. It consists of a cylindrical bulb,

Fig. 29.

Fig. 28. . Fig. 27-

and a stem terminating in an open
capillary orifice, covered by a glass cap
or ball e completely closing the ther-
mometer. The bulb and part of the
stem are filled with mercury, the rise
and fall of which indicate the tempera-
ture in the usual way ; above the mer-
cury a portion of alcohol is intro-
duced, sufficient to fill the rest of the

* Van Swinden, Comparaisondes Thermomfctres.
t Phil. Trans, vol. 1. for 175?.

tube and a small part of the cap. When
the mercury rises it drives the spirit
before it into the cap e, from which it
cannot return while the instrument re-
mains erect ; and the deficiency of spirit
in the tube, on the subsiding of the
mercury, measured by a proper scale, will
show how much the maximum rise of
the thermometer exceeded its height at
the time of the observation.

To prepare it for a fresh observation,
the thermometer is to be heated by the
hand until the spirit fills the whole
tube, which is then to be inclined so
that the spirit in the cap may cover the
capillary orifice : as the ball cools, the
spirit will drain into the tube thus in-
clined, and fill it as completely as before.

Fig. 28 is a construction of the same
instrument, intended to obviate the in-
convenience of so weighty a bulb as that
filled with mercury must be.

Lord C. adds a correction which
should be made on account of the
difference of expansion of mercury and
spirit, in computing the deficiency, if
this be measured by the same scale as
the ascent of the mercury : the degrees
computed by the column of spirit will
exceed those of the mercurial column
by i of a degree for every 10° of Fah-
renheit of difference between them.

Fig. 29 is his lordship's minimum
thermometer. Its bulb, f of the ball d,
and part of the leg b, are to be filled
with spirit of wine ; from b to c is
occupied by a column of mercury, and
about i of d contains a portion of this
fluid ; a little alcohol is likewise intro-
duced above the mercury before the
orifice of the tube at e is closed in the
usual manner. The mercury at c will,
when furnished with a proper scale,
indicate the present temperature in the
ordinary way ; but, when the spirit in
the bulb contracts by cold, the mercury
will rise in the short leg of the siphon
from b into the ball d, from which it
cannot get back into the tube b. This
will therefore occasion a deficiency of
mercury in that leg, which, measured by
a proper scale attached to the short leg
of the siphon, and subtracted from the
present height of the mercury in the
long leg, will show the lowest point to
which the thermometer had fallen
during the absence of the observer.
To prevent the mercury falling in too
large drops into the ball d, by which the
delicacy of the instrument would be
impaired, a solid but fine thread of glass
passes through the short leg to the

THERMOMETER AND PYROMETER.

35

narrow neck / of the ball d, by which
the passage is still further contracted, so
that the mercury trickles through in
most minute division. The instrument
is prepared for a new observation by
being inclined so as to bring the mer-
cury in d to cover the orifice at /; the
bulb is then heated, and the mercury
is expelled from the ball into the short
leg of the siphon, until it be filled with
that fluid.
Fig. 30 is another form of the last

Fig. 30.

instrument, which has the advantage of
being more easily adjusted, and is less
liable, from slight motion, to have the
mercury which has passed into a
brought back into the tube.

These instruments are extremelyjjin-
genious contrivances ; but there are
some practical difficulties in their con-
struction ; and the minimum thermo-
meter is rather liable to be broken from
the size of the bulb, and the several
bendings of the tube. Hence Lord C.
Cavendish's thermometers never appear
to have come into general use, although
very well adapted for certain purposes,
as for ascertaining the temperature of
the ocean at great depths. It is there-
fore unnecessary here to notice the cor-
rections pointed out by Mr. Cavendish
in its applications to various purposes.

2. Next in point of time is the con-
trivance of Fitzgerald ; which has been
already noticed, as well as that of
Crighton ; for rendering their metallic
thermometers indicators uf the maxima
and minima of temperature during the
absence of the observer.

3. The Register Thermometer in-
vented by Mr. James Six, of Colchester,
was h'rst described in the Philosophical
Transactions,* and is represented in
fig. 31. It is, in fact, a spirit of wine

Fig. 31.

thermometer, with a long cylindrical
bulb, and a tube bent in the form of a
siphon with parallel legs, and termi-
nating in a small cavity. A portion
of the two legs of the siphon from a
to b is filled with mercury ; the bulb,
and the remainder of both legs of the
siphon, as well as a small portion of
the cavity, are filled with highly recti-
fied alcohol. The double column of
mercury is intended to give motion to
the two indices c, d; the form of which
is better seen at A. Each index con-
sists of a bit of iron wire inclosed
in a glass tube, which is capped at each
extremity by a button of enamel. Their
dimensions are such, that they would
move freely in the tube, were it not for
a thread of glass drawn from the upper
cap of each, and inclined so as to press
against one side of the tube, forming a
delicate spring of sufficient power to

« Vol. Ixxii.
D 2

36

THERMOMETER AND PYROMETER.

retain the attached index at any part of
the tube, to which it is raised by the
column of mercury. The action of the
instrument will now be readily under-
stood. When an increase of tempera-
ture expands the spirit in the bulb, it
depresses the mercury in the limb a,
and proportionally raises it in the limb
b of the siphon : the mercurial column
in the latter raises the index d before it ;
and when the mercury sinks in that leg,
the bottom of the index d, retained at
that height by the glass spring, will in-
dicate how high the mercury had risen.
When the spirit in the bulb contracts
by cold, the mercury in the limb b de-
scends, and the consequence is a pro-
portional ascent of the column in the
side a ; which likewise carrying the in-
dex c before it, leaves its lower extre-
mity at the point to which the column
of that side had risen. In this manner
the maximum and minimum tempera-
tures are seen at any desired interval of
time ; and all that is necessary to pre-
pare the instrument for a fresh observa-
tion is to bring down both indices to
the surface of their respective columns
by means of a magnet, which will
act on the bit of iron wire included in
the body of each index. From the
above description, it is obvious, that
there must be an ascending scale to
measure the degrees of expansion in 6,
and a descending scale applied to a to
mark the contraction of the spirit. Mr.
Six graduated his thermometers by
placing them in water at different tem-
peratures, and marking on his scales
the heights corresponding to every 5° of
a standard mercurial thermometer im-
mersed in the same liquid. This elegant
invention has become a common instru-
ment ; and on account of the ease with
which the glass spring of the index may
be broken off, many instrument makers
substitute a slender bristle, tied to the
upper part of the index, and lapped
round its body, as at B. This renders
the spring less easily spoiled by the
careless shifting of the index ; but the
hair, by being long steeped in spirit, is
liable to have its elasticity destroyed ;
and a slender silver or platina wire
•would be preferable. The usual dimen-
sions of the instrument are, a bulb
from 6 to 1 6 inches in length, and from
0.2 to 0.3 inch in internal diameter;
the siphon from the ~ to the ^ of an
inch in width, and of a length propor-
tioned to the size of the bulb ; the in-
dices about 1 inch long ; the terminal

expansion of the tube is, in most of the
instruments now made, rather too small ;
in Six's original instrument, this part was
a cylinder of 2 inches in length, by half
an inch in diameter, to a bulb of 16
inches in length, and Tf inch in internal
diameter.

The chief defect of Six's thermome-
ter arises, as in most other contrivances
of this sort, from the unequal expan-
sion of the spirit, and the introduction
of two liquids of very different expansi-
bility in the instrument ; while, from the
construction, it would be difficult to ap-
ply any general correction to its indica-
tions. It does not indicate the expan-
sion of the spirit only, but also that of the
mercurial column ; which, where nice
observation is required, would be of some
moment ; and the necessary friction of
the indices will also tend to diminish
the effect of expansion. Yet this in-
strument is a valuable addition to me-
teorology ; and is probably the most
convenient for ascertaining the tempe-
rature of the ocean, at great depths, of
any hitherto given to the public.

4. The day and night thermometers
of Dr. John Rutherford, from the sim-
plicity of their construction, and low
price, have in some measure super-
seded the register thermometer of Six.
This ingenious and elegant device was
first published in the Transactions of
the Royal Society of Edinburgh,* and is
represented in fig. 32 ; where A re-
presents a spirit, and B a mercurial
thermometer, each provided with its
own scale, placed 'horizontally on
the same piece of box wood or ivory.
B contains, as an index, a bit of steel
wire, which is pushed before the mer-
cury, and is left in that situation to
mark how high the temperature had
been. A contains a glass index half an
inch long, with a small knob at each
end ; it lies in the spirit, which can
freely pass beyond it when expanded by
heat ; when contracted by cold, from
the attraction between spirit and glass,
the last film of the column of spirit is
enabled to overcome the slight friction
of the index on the inside of the tube,
and to carry it back towards the bulb.
This attraction is so considerable, that
although the index will move freely up
and down in the spirit, on inclining the
instrument, it will rest on the last film,
and require several smart concussions
given to the thermometer, to make it

* Edin. Phil. Trans, vol. iii.

THERMOMETER AND PYROMETER.
Fig. 32.

escape into the empty part of th
From the position of both thermoi
it is obvious, that to bring both indices
to the surface of the respective fluids, it
is only necessary to incline the instru-
ment toward C ; and it is thus prepared
for a fresh observation.

The accuracy of Rutherford's thermo-
meters depends on the ease with which
they are constructed, and the applica-
tion of a due correction for the inequa-
lities of expansion between them ; this
can be more readily accomplished
than with Six's thermometer ; because
the indications of each fluid are inde-
pendent of each other. The discre-
Eancies between both thermometers
ave been carefully examined by De
Luc,* and more lately by De Wildt, of
Hanover ;f the results will be given in
the sixth chapter, from which the cor-
rection can be applied. Such a correc-
tion will render them applicable to the
nicest meteorological observations of
maxima and minima. For more ordi-
nary purposes they are very convenient,
as not being easily deranged, and being
adjusted, for each observation, with the
utmost facility.

5. In the Transactions of the same
society,^ we find another register ther-
mometer, by Mr. Alexander Keith, a
gentleman of great mechanical inven-
tion, and long an active member of that
society. It is represented in our Jig. 33 ;
where a b is a glass tube, 14 inches
long, and £ inch in calibre, sealed at the
top, and below communicating with a
bent tube b, d, 7 inches long, and 0.4
inches in diameter, open at the top,
where it is cemented to a metallic plate
e, which supports the ivory scale e, e,
6f inches long. From a to b, the tube
is filled with highly rectified alcohol,
and from b to c with mercury. At c is
a conical float of ivory or glass, resting
on the surface of the quicksilver, and

» Recherches sur les Mod. de 1'Atmosphfcre.

f Jameson's Edin. Phil. Journal for October, 1826.

^ Edin. Phil. Trans, vol. iv.

' I

supporting a kneed wire h, intended for
moving two indices of black silk i, kt
that slide along the fine gold wire g, f,
as will be readily seen from the figure.

To prepare the instrument for obser-
vation, the indices are drawn, by means
of a crooked wire prepared for the pur-
pose, till they touch each side of the
knee of the float wire. It is obvious
that, as the heat alters the dimensions
of the column of spirit in a, b, the mer-
cury will rise or fall in the small tube,
and the float swimming on the surface
of the mercury will raise or depress the
knee h, which will move the indices ac-
cordingly on the wire g,f. The instru-
ment is defended from wind or rain by
the glass case I, I, which, by means of
its metal collar, fits tight on e, and is
only removed to adjust the indices.

This instrument, it is true, is influ-

THERMOMETER AND PYROMETER.

38

enced by atmospheric pressure, when
its cover is removed ; but the effects of
barometrical variations are scarcely
appreciable except in an air thermo-
meter ; and when made on a large
scale, is applicable, as Mr. Keith
has shown, to the important purpose of
marking the periods of the atmospheric
changes of temperature. This he effect-
ed by making the large tube 40 inches
long, but retaining the original width;
"Awhile the small tube is increased in
diameter, but not in length. The float c
is enlarged, and the float wire carries,
instead of the knee, a soft pencil, which
is made to press lightly against a hollow
vertical cylinder, 7 inches long and 5
in diameter, moved by clock-work, once
round in 31 days. This cylinder is
covered with smooth paper, ruled lon-
gitudinally into 31 columns, to corre-
spond to the days of the month ; and
every column is subdivided into 6 equal
parts, each corresponding to 4 hours.
The cylinder is ruled across into 100
divisions, intended to correspond to the
1 00° of Fahrenheit marked on the or-
dinary scale of the instrument, which is
unnecessary when the cylinder is applied.
Thus, as the cylinder revolves, the
point of the pencil will trace a line on
the paper more or less deviating from a
horizontal line, as the mercury rises and
falls ; and thus the paper will present a
chart of the variations of the thermo-
meter for a whole month, the value of
which, in degrees of Fahrenheit, will be
indicated by the numbers on the margin
of the paper. Keith recommends the
observer to have a copper plate for
giving ruled impressions on smooth
paper, to be applied monthly to the
cylinder ; and these, bound up together,
will present tabular views of the fluc-
tuations of the thermometer for everv

month. It is hardly necessary to state
that a similar contrivance is applicable
to the indications of the barometer.

6. We are indebted to Mr. Henry Home
Black adder, for some very ingenious
methods of ascertaining the tempera-
ture of the air, at any given hour, by a
subsequent inspection of a thermo-
meter. His first invention resembles
one of Rutherford's thermometers, sus-
pended on a pivot. If a spirit ther-
mometer be preferred, it is to be hung
vertically and inverted, so that the index
may rest on the last film of the liquid.
Suppose that we desire to know the
temperature at 5 o'clock, A. M., a lever
connected with a clock is applied, so as
to bring the thermometer to a horizontal
position at that hour ; and, at the same
time, the motion causes the bulb of the
thermometer to approach some 'source
of heat a little higher than that of the
air ; as, for instance, a small lamp, by
which the spirit would rise beyond the
now horizontal index, leaving it at the
point to which the spirit had contracted
before the reclination of the instru-
ment. When a mercurial thermometer
is employed, the instrument is also hung
vertically, but with its bulb lowermost ;
and the index, therefore, resting on the
mercury. It is brought to a horizontal
position by the same means as the other
thermometer ; and then its bulb, com-
ing into contact with a camel-hair pen-
cil, kept continually moist with water,
is cooled so as to cause the mercury to
shrink, and leave the index at the height
of the column while the instrument was
in the upright position.

By a subsequent improvement he
has contrived to dispense with the index
altogether. This modification is seen in
Jig. 34, where two thermometers are
placed parallel on the same piece of box

Fig. 34.

-*—

MM?,, nh.ua

wood or ivory : a 6 is a common mer-
curial thermometer ; c c? is of the same
size, but is not hermetically sealed. The
end of its stem is ground flat, and is in-
troduced into the neck of a small ball at
e ; which, as well as the stem, contains

some mercury. The stem is pushed up
until it just reaches the ball, to which it
is cemented by colourless varnish.

When these thermometers are in the
upright position, the globule of mercury
in e covers the orifice of the tube ; and

THERMOMETER AND PYROMETER.

on applying the heat of the hand to d,
the mercury in the stem joins that in
the ball ; and the stem will' remain filled
with the mercury while the instrument
is vertical : but when it is brought into
a horizontal position by the machinery
above mentioned, and the bulbs ap-
proximated to the pencils ef, suspended
over them for that purpose, and supplied
with liquid through the channels in
them, the mercury in the ball will leave
the orifice of the stem, and that in the
latter will descend, as represented in the
figure ; and its subsequent contraction
is marked by an inverted scale ; which,
with the indications of the other ther-
mometer, will enable us to ascertain
the temperature at the moment of the
reclination of the instrument. Thus, as
both instruments are equal, if" the same

diminution of temperature has sunk a b
to 50°, and has produced a contraction
of 10° in ed; it is evident the sum of
both numbers will show the degree at
which the common thermometer stood at
the moment of the change of position ;
which in this case has been 60°.

This idea is most ingenious, and is
said in practice to work exceedingly
well. It promises to be useful in meteo-
rological investigations, although not so
complete as the register thermometer of
Keith, which continues to note its own
indications for a whole month; while
that requires to be readjusted for each
observation.

7. We shall conclude this chapter by
a notice of another register thermometer
invented by Dr. Traill, and seen in fig.
35. It is a single spirit thermometer,

Fig. 35.

in which a column of mercury 5 of an
inch in length is introduced: at each
end of this column lies an index of fine
steel wire, gilded by means of a galvanic
circuit, to prevent oxidation in the
spirit. An inspection of the figure will
show how the variations of bulk of the
spirit in the bulb will move the column
of mercury ; and by this the indices are
pushed in opposite directions, but will
remain at the lowest and highest points
to which they are driven by the mer-
cury. The difference between the two
scales will be the length of the mercu-
rial column. The indices are brought
in contact with the mercury by a
magnet.

This thermometer has the advantage
of giving the maxima and minima by
the changes in volume of a single fluid ;
for the expansion of so short a column
of mercury is quite inappreciable. The
defect of this construction is the liability
of the mercury to separate by sudden
motions of the instrument. This is
least likely to happen when the mercu-
rial column is short and the calibre
of the tube is minute ; and it is to
admit of a fine tube that gilded steel
wire is preferred to an index coated
with glass.

CHAPTER IV.

Differential Thermometers, and their
Modifications.

THERMOMETERS of this kind are not
affected by general changes of tem-
perature in the surrounding medium ;
but are delicate indicators of partial
changes affecting one of their balls.
Some of the forms of the air ther-
mometer described by Van Helmont,
bear a general resemblance to the in-
strument known by the name of differ-
ential thermometer: but they were
rudely constructed, without a fixed
scale, and unsusceptible of accuracy,
or of application to the delicate inves-
tigations required by modern experi-
mental philosophy. We are indebted
to the ingenuity of Professor Leslie of
Edinburgh for a perfect differential
thermometer, and its application to
some very important purposes.

In January 1800, he published a de-
scription of a new hygroineter and
photometer,* of which the principle de-
pends on the difference in the volume
of air contained in two equal balls of
glass, connected by a tube bent in the

* NicholbOn'ti Jouraal, 4to, vol. iii. p. 461.

40

THERMOMETER AND PYROMETER.

form of a letter U, when the balls are
unequally heated. This difference is
measured by the motion of a coloured
liquid contained in the bent tube. " In
ordinary cases," says Leslie, " the in-
termediate liquor would continue sta-
tionary ; for the air in both balls having
the same temperature, and, consequently,
the same elasticity, the opposite pres-
sures would exactly counteract each
other ;" but if one ball becomes colder
than the other, " it is manifest that the
liquor would be pushed towards it by
the superior elasticity of the air included
in the other." This is the principle
which suggested to him the

1. Differential Thermometer, used
with so much skill and ingenuity in those
delicate investigations on heat, with
which he was occupied from the above
period, until the publication of his work
early in 1804. The differential thermo-
meter in its most usual form is repre-
sented in (fig, 36,) where a, b, are two

Fig 36.

equal glass spherules, connected by the
tube c, d, e,f, slightly dilated just below
the ball a, and at e, and partially filled
with a coloured liquid, as represented in
the figure. The dilatation below a, is
intended as a reservoir of liquid ; and
that at e, for the more easy adjustment
of the liquor to the commencement of
the scale, by passing bubbles of air from
one ball to the other. The liquid re-
commended by Leslie,* after many
trials, is strong sulphuric acid, tinged

k* Experimental Inquiry, p. 4J7, 1804.

by carmine. The scale he adopts, is
millesimal, from the freezing to the
boiling point of water ; or 10 degrees of
it are equal to one of the scale of Cel-
sius. The instrument is cemented to a
wooden foot, either immediately, or is
furnished with a sliding stem to adapt
it to different heights. Each leg of the
instrument is usually from 3 to 6 inches
in length, and the balls are from 2 to 4
inches apart. The calibre of the stem
e,f, is from the 1-5 Oth to 1-6 Oth of an
inch ; that of the rest of the syphon a
little larger.

When exposed in a room, or in the
open air, the differential thermometer
remains stationary at 0°, whatever may
be the temperature of the ambient air ;
but if one of its balls be more heated
than the other, the unequal expansion
of the included air puts the coloured
fluid in motion. In employing this in-
strument in experiments on the radia-
tion of caloric, its principal use, the ball
a is that to which the heat is applied ;
or is, as Leslie calls it, the sentient ball ;
and the mounting of the liquid in the
other stem indicates the difference, of
elasticity of the air in both balls, and
hence the name of the instrument. It
owes to its insensibility to general
changes of temperature, its peculiar fit-
ness for measuring the influence of ra-
diation.

The theory of the instrument sup-
poses that gases expand uniformly with
equal increments of temperature : this
is, perhaps, not strictly true; yet, as
Leslie remarks, it is - so nearly correct,
that, in the limited range of the instru-
ment, the irregularity from that cause is
quite inappreciable.

It is worthy of remark, that, a few
weeks after the publication of Mr. Les-
lie's " Inquiry," a part of the " Philo-
sophical Transactions" of London ap-
peared, which contained a set of expe-
riments almost similar to many of his,
and a description of an instrument, in
principle precisely similar to his differ-
ential thermometer. This was

2. Rumfords Thermoscope.

It consisted also of two horizontal balls,
united by a syphon ; and the only differ-
ence from Leslie's thermometer is, that
the scale is attached to the horizontal
part of the tube (which is the longest
portion) ; and the coloured liquid is a
bubble moving to and fro, when the
balls are unequally heated, in the hori-
zontal part of the tube, llumford pro-

THERMOMETER AND PYROMETER.

41

fesses to have borrowed the idea of the
thermoscope from Leslie's hygrometer ;
but the latter has roundly charged him
with a more direct plagiarism from the
differential thermometer ; to which ac-
cusation we do not recollect that any
satisfactory answer has been published.
The differential thermometer has un-
dergone several alterations of form to
adapt it to particular purposes as an air
thermometer. One of the most com-
mon is seen at Jig. 37, where the
ball b is cemented to the tube, after
the introduction of the liquid, as in the
old air thermometer, but this form has
been rendered more elegant and con-
venient by the modification of Dr. De
Butts of Baltimore, (See Jig. 38,) in
Fig. 38.

Fig. 37.

which no cement is necessary ; for
the stem and both balls are united by
the blow-pipe. These instruments are
to be either fixed perpendicularly on a
stand, or suspended. The liquid is con-
tained in the lower ball, and the heat is
applied to the upper one ; so that the
stem is provided with a descending scale.

3. Leslies, or the Thermometric
Hygrometer.

When the ball of the differential ther-
mometer containing the supply of co-
loured liquid is coated with several folds
of tissue paper, and kept moist with
distilled water, the instrument becomes
an Hygrometer : for the descent of the
coloured liquid in the other stem will
mark the diminution of temperature
caused by the evaporation of the water
from the humid surface ; and as this
effect is proportional to the relative dry-
ness of the ambient air, it will give an
indication of the comparative quantity
of water suspended in the atmosphere,
at the different times of observation. In
most cases, two minutes are sufficient to
produce the full effect on the instru-
ment ; and the included liquid then be-
comes stationary, until the whole mois-
ture is exhaled from the ball. The
drier the ambient air is, the more
rapidly will the evaporation go on;
and the cold produced will be greater.
When the air is nearly saturated with
moisture, the evaporation goes on slow-
ly ; the cold produced is moderate, be-
cause the ball regains a large portion
of its lost heat from surrounding bodies ;
and the degree of refrigeration of the
ball is an index of the dryness of the air.
Could we ascertain with precision the
capacity of air for moisture, at different
temperatures, this hygrometer would
likewise afford a measure of the abso-
lute quantity of water suspended in the
air. The most approved form of the
instrument, according to Leslie, is seen
in fig. 39. The balls are parallel, and
bent from each other ; a is covered
smoothly with several folds of tissue
paper, which is to be kept continually
moistened with pure water, drawn from
the vase d, by the capillary attraction
of a few fibres of silk. In order to ob-
viate any inequality from the disturbing
effect of light, the ball b is formed of pale
blue glass ; and the papered ball is co-
vered with thin Persian silk of the same
hue.

Should the water become frozen on
the ball, this hygrometer will still act ;
for evaporation goes on from the sur-

42

THERMOMETER AND PYROMETER.

Fig. 39.

face of ice, in proportion to the dry-
ness of the air. Mr. ^Leslie estimates,
that when the ball is moist, air, at the
temperature of the ball, will take up
moisture equal to the sixteen thou-
sandth part of its weight, for each de-
gree of his hygrometer ; and as ice in
melting, requires l-7th of the caloric
consumed, in converting water into va-
pour, when the papered ball is frozen,
the hygrometer will sink more than
when wet by 1° in 7° ; and hence in the
frozen state, we must increase the value
of the degrees l-7th: so that each of
them will correspond to an absorption
of moisture, equal to one-fourteen thou-
sandth part of the weight of the air.

When this hygrometer stands at 15°,
the air feels damp ; from 30° to 40°, we
reckon it dry ; from 5 0° to 60°, very dry ;
and from 70° upwards, we should call
it intensely dry. A room will feel un-
comfortable, and would probably be
unwholesome, if the instrument in it did
not reach 30<>.* In thick fogs it keeps
almost at the beginning of the scale.
In winter, in our climate, it ranges from
5° to 15°; in summer often from 1 5° to
55° ; and sometimes attains to 80° or 90°.
The greatest degree of dryness ever
noticed by Leslie, was at Paris in the
month of September, when the hygro-
meter indicated 120°.

* Leslie " On the Relations of Air, Heat, and
Moisture," p. 70,

The thermometric hygrometer is of
two forms ; the stationary, (Jig. 39,)
and importable, which resembles the in-
strument delineated in (figAl), without
its glass shade. This last form is de-
fended by a wooden case which screws
over it, to fit it for the pocket.

4. Leslie's Photometer.

This elegant instrument is the differen-
tial thermometer, covered by a case of
transparent glass, and having one of its
balls either painted black, or, what is
better, formed of black glass enamel.
The Stationary Photometer (Jig. 40)

Fig. 40

has both its balls at the same height,
and covered by a spherical shell of the
most transparent glass ; which, with the
annexed glass tube, defend the balls
from the disturbing influence of cur-
rents of air.

The Portable Photometer (fig. 41)
has the balls in the same vertical line,
in order to admit a turned tube of
wood A, of the same form as its cover
a, a, to screw on the brass collar d, as
a defence to the instrument when in the
pocket; and for further convenience

THERMOMETER AND PYROMETER.

43

Fig. 41.

the socket is made to unscrew from the
sole of the instrument. The ball, b, is
of black, or deep reddish-brown enamel,
while c is as diaphanous as possible.
The graduation, and other parts of the
photometer, are on the same scale and
construction as in the differential ther-
mometer.

Dr. Franklin, and others, had re-
marked the superior power which dark
colours possessed of absorbing the ca-
lorific influence of the sun's rays ; and
Dr. Watson, afterwards Bishop of Llan-
daff, had, in 1773, observed, that when
a thermometer, having its ball black-
ened, was exposed to the sun's light, it
rose 1 0° higher than it had previously
done in a similar situation. The re-
searches of Mr. Leslie put this fact in a
more striking point of view, and led to
the invention of this instrument.

The theory of the photometer hangs
on the supposition, that the intensity of
lii;ht emitted from any body, is always

Eroportional to the temperature excited
y its incidence on the blackened ball.
This is probably true with regard to the
undecomposed rays of the sun, in which
the caloric and the light, if different

kinds of matter, are intimately blended ;
but there is strong reason to suspect, that
light emitted by terrestrial bodies is not
always proportional to the concomitant
temperature. Thus the intense splen-
dour of phosphorus burning in oxy-
gen gas, gives out far less heat than the
comparatively dull combustion of hy-
drogen in the same gas ; and we have
found this photometer often more af-
fected by the emanations from a fire so
duD, that not a single letter could be
discerned in a well-printed page, than
by the degree of daylight, by which we
could read the same print with plea-
sure and facility. It is differently af-
fected too by light of different colours,
where their illuminating property ap-
pears the same ; and the experiments of
Herschel, Englefield, and others, show
that the maximum of heat in the solar
beam, decomposed by the prism, by no
means corresponds with the illumina-
tion, but is even altogether beyond the
margin of the spectrum.*

As a measure, however, of the inten-
sity of undecomposed solar light, it ap-
pears to support the character it re-
ceives from the inventor. — " The pho-
tometer," says he, " exhibits distinctly
the progress of illumination from the
morning's dawn to the full vigour of
noon, and thence its gradual decline
till evening spreads her sober mantle.
It marks the growth of light from the
winter solstice to the height of sum-
mer, and its subsequent decay through
the dusky shades of autumn ; and
also enables us to compare, with nu-
merical accuracy, the brightness of
different countries — the brilliant sky
of Italy, for instance, with the murky
air of Holland."

The direct impression of the sun's
rays at noon, about the summer sol-
stice, in this country, equals from 90°
to 100° of this instrument ; and at mid-
winter, the force of the solar beams is
from 25° to 28°. The indirect light,
from a summer's sky, at noon, is from
30° to 40°; in winter, it is from 10° to
1 5°. In the most gloomy weather,, in
summer, the photometer rarely indicates
less than 10° at noon ; but in winter it
sometimes barely exceeds a single de-
gree.

The observations on the light of day
with this instrument should always be
made in the open air ; and the direct

* Herschel, Phil. Trans. 1800; Englefield, Journ.
Roy. Institution, vol. i.

THERMOMETER AND PYROMETER.

44

effect of the sun's rays noticed, as well
as the indirect reflection from the sky.

5. Pyroscope.

When one ball of the differential ther-
mometer is smoothly covered with thick
silver leaf, or inclosed in a polished sphere
of silver, and the other ball is naked,
it forms the pyroscope ; an instrument
intended by its inventor, Mr. Leslie, to
measure the intensity of heat radiating
from a fire into a room, or the frigorific
influence from a cold body. A figure
is unnecessary, as the instrument is
usually made either like the differential
thermometer, like that represented in
(fig. 37,) or the hygrometer, (fig. 39.)
The theory of its construction and
application is, that all the rays inci-
dent on the metallic surface, are re-
turned from it ; while those that reach
the transparent ball expand the air
within it, and depress the coloured li-
quid in the stem. In this way the com-
parative radiation from various bodies
may be ascertained ; and it is so delicate
an instrument, that in a warm room it
will be visibly affected by a pitcher of
cold water, at the distance of a few
inches.

6. The JEthrioscope of Leslie is ano-
ther modification of the differential ther-
mometer which we shall here notice.
One of its most usual forms is given in
Jig. 42 ; and is what the inventor calls the
Pendant JEthrioscope. The ball a of the
thermometer is inclosed within a brass
sphere, d, d, without touching it ; and
for the convenience of adjustment, this
sphere may be unscrewed in the middle.
The other ball, b, which is about one half
the diameter of the first, is in the centre
of an oblong spheroidal cup, c, c, which
may be covered by a top that fits on at
/,/. The coloured liquid in the stem
is supported by capillary attraction in
the dilated extremity of the tube, where
it joins the ball a. The brass work is
highly polished, and the inside of the
spheroidal cup is well gilt.*

This very elegant instrument is in-
tended, in the language of Mr. Leslie,
" to indicate the cold pulses emanating
from the sky ;" or, in other words, to
give a comparative idea of the radia-
tion proceeding from the surface of the
earth toward the region of perpetual
congelation in the atmosphere. The
brass coverings defend both balls from
the influence of the sun's rays, or

* Edin. Phil. Trans., vol. viii.

other adventitious sources of heat ; and
when the ball b is cooled by radiation
toward the heavens, the air within it
contracts, and the elasticity of that
within a, forces up the liquid in the
stem, the height of which marks the in-
tensity of the radiation.

When the cover is on, the liquid re-
mains at 0° ; but when it is removed,
and the instrument presented to a clear
sky, either by night or by day, it in-
stantly begins to rise, and continues
to mount until the ball b has sustained
the greatest diminution of temperature,
which radiation at that time can pro-
duce.t

The circumstances which favour ra-
diation from the surface of the earth to-
ward the sky, namely, a clear and calm
atmosphere, are admirably pointed out
by Dr. Wells, in his excellent Essay on
Dew : and this instrument becomes a

f Leslie appears to have been led to this invention
by some of his own experiments on radiant caloric ;
but it is proper to state, that Dr. Wollaston had
shown, that when a delicate thermometer, in the focus
of a concave metallic mirror, is presented to the sky,
cold is indicated.

THERMOMETER AND PYROMETER,

45

valuable indicator of the state of the
air favourable to the deposition of that
interesting meteor. Such is its extreme
delicacy, that, w hen rising, its progress
is checked by the smallest cloud sailing
over it ; and it may be kept in a state of
oscillation, by being alternately stretched
beyond the edge of a parasol, and drawn
within its shade, when the sky is clear
and serene.

Besides the forms given above, Leslie
describes two others, the Standard and
the Sectorial. The first has the stem
bent up as in the hygrometer, and one
of the balls covered with silver leaf or
gilt, near the side of the cup c, c, with
the naked ball in the centre, as the
balls a, b, in A. The second has the
cup, c, c, formed with a notch in its
bottom, for admitting a partial vertical
motion round the ball b. The motion
is given by means of a toothed sector
and pinion ; from which the name is
derived. This form is applicable to
ascertain the radiation from the earth,
when we ascend a mountain, or rise in
a balloon.

7. The differential hygrometer and
photometer would have come into
more general use had they indicated the
maximum and minimum between any
two times of observation. In their pre-
sent construction they only show the
state of the atmosphere at the moment
of observation, and, therefore, require
an attention which few have leisure or
inclination to bestow on meteorolo-
gical observations. To render them
more extensively useful the following
alteration is suggested, by which they
are brought nearly to the thermoscope
of Rumford in form. The tubes con-
necting the balls have the upright part
of their stems shortened, and the hori-
zontal part extended : instead of a co-
loured fluid filling the stems, there is a
short column of mercury introduced
into the horizontal part of the stem ;
the motion of which, towards either ball,
carries before it a piece of steel w ire,
which constitutes the index of maximum
and of minimum change during the
absence of the observer. This con-
struction will be readily understood from
the figure 43, which represents the re-
Fig. 43.

gister hygrometer. A double scale lies
along the horizontal part of the instru-
ment. When the indices are adjusted
for a fresh observation, they are brought
by a magnet to each extremity of the
little column of mercury.*

CHAPTER V.

Of some peculiar Applications of the
Thermometer.

THERE are a few applications of the
thermometer to certain useful pur-
poses, which ought to find a place in
the history of the instrument ; I allude
particularly to the Statical Thermometer
of Dr. Gumming, the Balance Thermo-
meter of Mr. Kewley, the "Hygrometri-
cal Instrument of Mr. Daniell, and the
Barometrical Thermometer of Mr.
Wollaston.

1. The Statical Thermometer of Dr.
Gumming, now of Chester, was con-
trived by that gentleman, in 1808, and
intended by him as a mode of opening
and closing windows and ventilators in
apartments, by the variations in tempe-
rature of the included air. This inge-
nious application of statical principles
was shown to numerous friends at dif-
ferent times, in his residence at Den-
bigh, and was afterwards, for a consi-
derable time, exhibited in the Denbigh
Dispensary. The general form of the
instrument is represented in Jig. 44,
where a b is a glass matrass, or a ball
and tube of iron ; the globular termi-
nation of which is capable of containing
four or five pints of air, and the tube
is about twenty-five inches in length,
and from one to two inches in diame-
ter. A portion of the tube is filled
with mercury ; and in this state it is
inverted, and its extremity plunged in
a cylindrical jar for containing the same
fluid. The ball is covered by a net of
strong cord, or of wire, which forms a
ring at the top, for the suspension of the
ball and tube. From this ring passes a
cord over the pulley, d; and it may
either pass upward under the pulley, e,
to be attached to the frame of a swing
window, as shown at g ; or downwards
over the pulley/, to be fixed to the ven-
tilator h. "When the heat of the apart-
ment expands the air in the ball, it de-
presses the mercurial column in the
tube, b ; by which the whole instrument

* This instrument acts rather slowly, and the
motion of the indices is not quite smooth; but it ap-
pears capable of supplying a desideratum in me-
teorological observations— a register hygrometer,

46

THERMOMETER AND PYROMETER.

becomes as much lighter as the weight
of the mercury expelled from the tube,

Fig. 44.

and the weight at the top of the win-
dow, or of the ventilator, opens these
apertures which were kept shut by the
weight of the statical tube and ball.
On the other hand, when the cooling
of the air in the chamber causes the
contraction of the air included in the
ball, the pressure of the atmosphere
forces the mercury into the tube, which
thus becomes so much heavier ; and as
it descends, it drags with it the window
irame, or ventilator, attached to it.

This simple and very ingenious con-
trivance is applicable to hot-houses,
ooms, and apartments of every de-

1D' that are liable to consider-
changes of temperature; and it

possesses considerable powers : for in a
tube two inches in diameter, every inch
in the rise or fall of the mercury is
equivalent to a moving power of about
one pound. It is liable to be slightly
altered also by changes in atmosphe-
ric pressure.

Dr. Cummings's attention was drawn
to the importance of regulated tem-
perature in the treatment of disease,
whether in public institutions or in
private practice, and the contrivance
above noticed was the method by which
he endeavoured to obtain this important
object : but he soon perceived that the
principle was applicable to various mete-
orological purposes ; and the instrument
has, in his hands, undergone successive
modifications and improvements, until
it has become the basis of a thermo-
meter, hygrometer, and photometer,
capable of registering their own indi-
cations, by the aid of clock-work, at any
given time. Details of these different
contrivances would lead us into too
Fig. 45.

THERMOMETER AND PYROMETER.

47

wide a field ; but it may be proper to
state, that finished drawings of them all
have been in the possession of a dis-
tinguished member of the Meteorolo-
gical Society of London for upwards of
four years. The principle of them will
be readily understood from the pre-
ceding figure, (fig. 4 5,) in which a repre •
sents an air thermometer ; b a barometer,
suspended from the opposite side of the
wheel c c, to compensate the influence of
variations in atmospheric pressure on the
instrument; dd is a siphon-cistern, in
both sides of which the mercury will
always remain on the same level ; / is
an index, to which a pencil may be
fixed, for tracing the variations of the
instrument on a plate revolving by
means of clock-work.

The portions of the tubes which dip
into the mercury should be of equal
substance; and the tube of the air-
thermometer should be a cylinder ca-
pable of containing twice as much mer-
cury as the corresponding portion of the
barometer which counterpoises it. A
small correction may be required for the
varying immersion of the tubes, pro-
duced by the oscillations of the instru-
ment. This must be determined by ex-
periment, and allowed for in the gra-
duation of the scale.

2. The Balance Thermometer of Mr.
Kewley is a contrivance for a similar
purpose, and is represented in fig.
46, A.

This instrument is the subject of a
patent, the date of which is 1816. It

Fig. 46.

consists of a tube of glass, a, a, closed
at c, and terminating at e in a ball,
which communicates with another tube
of smaller diameter, which also termi-
nates in a ball at d, having a communi-
cation with the external air at /. The
tube «, a, and one half of the ball, e,
are filled with spirit, or any light easily
expansible fluid. The other tube, from
e to d, is filled with mercury. The
whole is suspended in the iron frame,
h, i, /?, m, B, by means of two clamping

Eieces, which are adjusted to the tubes
y the screws, o, o. The centre of gra-
vity is suitably adjusted by means of
the milled nut, sunk in the transverse
part of the frame, and receiving the
screw, i • in order that the whole may
librate on the knife edges, m, n, des-
tined to rest on surfaces resembling the
suspension frame of a common balance.
A brass scale, p, is moved by the nut, k,
on the arbor of which is a pinion playing
in the teeth of the plate, p.
It is obvious, that by adjusting the

n

mercury in each arm of this balance, it
will be in cequilibrio ; but when the
spirit in a is expanded by heat, it will
force some more of the mercury into
the ball, d, and that arm of the instru-
ment will preponderate ; when it again
contracts, the atmospheric pressure will
cause the mercury to resume its original
situation.

The instrument may be used as a
thermometer, by ascertaining at what
temperature it is in equilibrio, and
when either end preponderates, finding
how much is necessary to restore the
balance by the motion of the brass
plate, p ; but its chief value arises from
its applicability to shut and open doors
or windows, according to the tempera-
ture of the apartment ; in which case,
a lever, or tooth-wheel, is fixed on
one of its centres of oscillation. It is
almost needless to remark, that the
whole may be constructed of iron, and
of any convenient size. In point of
simplicity, and cheapness, however, it is

48

THERMOMETER AND PYROMETER.

inferior to that proposed by Dr. Gum-
ming, which preceded it in date ; al-
though until now no detailed account of
that invention has been published.
f 3. The Barometrical Thermometer of
the Rev. F. Wollaston is seen in fig.
47, A. It is a thermometer with a

Fig. 47.

large bulb, devised for the measurement
of altitudes, by observing the tempera-
ture at which liquids boil- on different
elevations ; * on the principle first
pointed out by Fahrenheit,-!' that the
boiling of a fluid varies with the pres-
sure of the atmosphere. Cavallo first
applied this principle to the measure-
ment of heights, $ and the instrument
proposed by Mr. Wollaston is intended
to facilitate this method.

The bulb of the mercurial thermome-
ter he proposed to use, is one inch in dia-
meter, with a dilatation k, as seen in B,
and ending in a capillary tube, five inches
long, which is not closed in the usual

* Phil. Trans, for 181?.
t Phil. Trans, vol. xxxii.
% Phil. Trans, vol. Ixvi..

manner, but, after being broken off
smoothly, is sealed by a little cap of
glass, as at i, i. ^The scale is 4.15
inches long, divided into 100 parts, and
may be subdivided by a vernier into 1000
parts ; giving 241 parts to each inch of
the scale ; and to facilitate observation,
these are read off by a small lens jointed
to the index, but not represented in our
figure. The index is moved by a mi-
crometer screw, d. The thermometer is
supported by means of stuffing between
two circular plates of metal, c, c, through
which it passes, and which are tightened
by screwing them together. They form
a metallic collar, that may be screwed
by either end into the top of the copper
boiler, f, g, which becomes the case of
the thermometer on inverting it; and
then the bulb is protected by a copper
cap C, which also serves as a measure
of the due quantity of water to be used
in the experiment. The portion of the
copper tube below the bottom, g', of the
boiler, is capable of holding the lamp, e,
which is attached to it by two sliding
wires. Thus the instrument becomes
very portable. The boiler is 5.5 inches
deep and 1.2 in diameter, with an aper-
ture at the top to permit the escape of
the steam, by which the heat is applied
to the bulb.

When we have to determine an alti-
tude, the boiling point is noted at the
bottom of the eminence, and again when
we have ascended ; and the value of the
difference between those points on the
scale having been ascertained by expe-
riment, we can estimate the height
ascended, provided no change has, in the
mean time, taken place in the barome-
trical pressure ; or these points may be
simultaneously found by two observers.
The only correction required is, for the
specific gravily of air at different tem-
peratures, which may be found by Ge-
neral Roy's tables. The use of the di-
latation, h, is to receive the expanded
mercury, before it arrives at the boiling
point : and the small cap, i, i, is intended
to receive a globule of mercury, to be
detached occasionally from the "column
in the stem, when it is wished to alter
the range of the scale to suit various
altitudes. The method of separating
this globule is, to elevate the mercury,
by heating the bulb until the thread of
metal may be shaken over the flat end
of the capillary tube ; and when we wish
to join the globule again to the thread,
the two portions of mercury are brought
into contact by heat, and then as the in-

THERMOMETER AND PYROMETER.

strument, held in the vertical position,
cools, the globule, will follow the thread
into the stem.

This instrument is of great delicacy ;
being capable of showing a difference
of altitude of not more than three feet ;
but, unfortunately, although not very
bulky, it is not very portable, from the
liability of the stem to be broken by the
weight of the bulb, even from the usual
jolting of a carriage ; an accident which
happened thrice to the writer of this,
within one month. From this cir-
cumstance, and its price, it is not likely
to supersede the barometer in geological
surveys.

4. M. Le Roi was the first who sug-
gested the temperature at which dew
begins to be deposited as a method of
ascertaining the moisture of the air.
De Luc has the merit of having proved
that the quantity and force of vapour in
a vacuum of any given dimensions, are
equal to its force and quantity, in an
equal volume of air, at the same tem-
perature ; or that the force and quantity
of vapour in the air are dependant on
its temperature.* This was confirmed
by Mr. J. Dalton, t who investigated
the force of vapour, at every tempera-
ture, from 0° to above 212° Fahren-
heit, and expressed this force by the
height of the mercurial column, which
it could support in a Torricellian
tube. These results are given in a ta-
bular form, and are thus easily applied
to hygrometric purposes. Dalton finds
the dew point, like Le Roi, by pouring
cold water into a glass, and marking the
temperature at which it just ceases to
cause the deposition of dew on the sides
of the glass, in the open air. This is
the point at which, in an air of that
temperature, dew would just begin to be
formed. From this fact he is able to
infer, not only the force exerted by the
vapour, but its quantity in a perpendi-
cular column of the whole atmosphere,
and the force of evaporation at the time
of observation.

Thus, if the dew point be 45°, the
force of vapour in Dalton's table = 0.3 16
of an inch of the mercurial column, or
the one-ninety fifth of the whole atmos-
pheric pressure ; or, if the specific gra-
vity of steam be 0.70, the weight of
the steam or vapour in a given volume
of air will be the one hundred and thirty-
sixth part of the whole. Now, as the force
of a whole atmosphere of steam, at the

* Recherches sur ]es Modifications de 1'Atmosphere.
t Manchester Memoirs, vol. v. 535. — vol. i. new
series, p. 252.

49

surface of the earth," would be the weight
of a perpendicular column of it, and as
in a mixed atmosphere of steam and air,
the force exerted by each is as their re-
lative weights, it follows, that when the
dew point is 4 5°, the whole superincum-
bent column of vapour in the atmosphere,
being equal to the one-ninety-fifth of
the whole atmospheric pressure, will be
equivalent to a pressure of 4.30 inches
of water ; or the vapour, if condensed,
would afford that depth of water. From
these data, Dalton has shown how we
can find the force of evaporation at a
given time : for the quantity of water
evaporated from a given surface is
proportional to the maximum force of
vapour at the temperature of that sur-
face ; it being understood that the va-
pour is still in contact with a surface of
water. Hence, if we have the dew point
45°, while the temperature of the air
is 50°, by subtracting the force of va-
pour at 45° from that of 50°, we shall
obtain the force of the evaporation at
that time— thus, .375 — .316=.059, the
force of evaporation.

5. It is on this principle that DanielZ's
Hygrometer is constructed ; the inven-
tion of a gentleman distinguished for
his meritorious labours in meteorology.
It was published in 1820, along with a
meteorological table, and seems to have
been suggested by the cryophorus of
Wollaston. J The form of the instru-
ment is seen, as last improved by Mr.
Daniell, in (fig. 48.) The ball a is of

Fig. 48.

t Quarterly Journal of Science, vol. viii. 299 — see
also vol. ix. &c,

THERMOMETER AND PYROMETER/

50

black glass, about one and a quarter
inch in diameter, and is connected with
a ball d of the same size, by a bent tube
one-eighth of an inch in diameter. A
portion of sulphuric aether, sufficient to
fill three-fourths of the ball a, is intro-
duced ; a small mercurial thermometer,
with a pvriform bulb, is fixed in the
limb a o, the atmospheric air is ex-
pelled as completely as possible ; and
the whole is sealed at e. The ball d is
covered with muslin ; the whole is sup-
ported on a brass stand /, g, on which
is another delicate mercurial thermo-
meter. The tube can be removed from
the spring tube h ; and the whole, to-
gether with a phial of aether, packed
neatly in a box, that goes easily into the
pocket. The method in which the dew
point is indicated by this instrument, is
as follows : — The aether is all brought
into the ball a, by inclining the tube ;
the balls are placed perpendicularly ;
the temperature of the ambient air is
now noted; aether is poured from a
dropping tube that fits the mouth of
the small phial, on the muslin cover of
d ; and, the cold produced by its eva-
poration, causing a condensation of the
elastic aethereal vapour within the ball,
produces a rapid evaporation from a,
by which the temperature of the ther-
mometer in it sinks ; and when the
black ball is thus cooled to the dew
point, a film of condensed vapour, like
a ring, surrounds the ball. If the ther-
mometer be at that instant noticed, we
obtain the true dew point of air at the
temperature indicated by the other ther-
mometer.

The observation is made in a very
short period ; and much of the labour
required in the method of Le Roi is
saved. There seems but one objection
to this very ingenious instrument, and
it is one, which, even with much prac-
tice, is not easily obviated. The surface
on which the dew condenses, is small,
and requires a peculiar light to be well
seen; while the attention of the ob-
server, distracted between the close in-
spection of the surface of the black ball,
and the included thermometer, is not
always able to fix with absolute preci-
sion the dew point.

6. This instrument has been modified
in

Mr. Thos. Jones's Hygrometer ;
an instrument on exactly the same
principle as the original invention of
Daniell, but simpler in construction,
more compact, and less expensive. It
is seen in fg. 49, consisting of a deli-

Fig. 49.

cate mercurial thermometer, with its
tube at a, b, bent so as to bring its cy-
lindrical ball c, parallel with, and at a
little distance from, its stem. The bulb
is one inch long, and is terminated by
a flattened surface d, of black glass,
which projects a little beyond the sides
of the bulb. The bulb below the
flattened surface is covered with black
silk. The instrument is supported on
the wire ef, which is attached to the
scale by a pivot, that allows the black
surface to be inclined to the light, and
the whole, with a phial of aether, are
contained in a small case. *

When used, the temperature of the
air is first noted ; then aether is poured
on the silk cover of the bulb ; and the
condensation of the dew is seen on the
black extremity of the bulb.

The difficulty of marking the incipient
condensation on these instruments is
the same ; and it has produced various
modifications of the instrument.

7. Dr. Cumming, of Chester, finds that
the dew point is most conspicuously
shown, by inclosing the bulb of a deli-
cate thermometer, covered by a sponge,
in a tube of planished tinned iron, silver,
or platina. When the sponge is moist-

* Phil, Trans, for 1826, part ii. p. 23,

THERMOMETER AND PYROMETER.

ened with any very evaporable fluid,
such as aether or alcohol, and a stream
of air blown through the tube, a more
rapid and more conspicuous deposition
of dew takes place on the surface of the
metallic tube, than we ever recollect to
have observed in similar experiments.

Fig. 50 is Dr. Gumming' s hygrometer
fitted to a portable air-syringe, by which
a current of air is produced through
the tube B B. The bulb of the delicate
thermometer within it is surrounded
with fine sponge, to retain the evapo-
rable fluid ; the tube B B is of highly
polished metal, with an aperture in its
upper part covered with a glass tube,
for the inspection of the thermometer,
as represented in the figure.

Fig. 50.

CHAPTER VI.

On the Imperfections common to all
Instruments for the Indication of
Heat.

1. THE terms thermometer and pyro-
meter might lead to the supposition,
that the instruments so designated were
actual indicators of the quantity of ca-

loric contained in those bodies to which
they are applied ; but a single experi-
ment is sufficient to show that this view
is erroneous. If we place equal quan-
tities of water and of snow, both at
temperature 32°, in a room at 60°, the
temperature of the water will, as indi-
cated by the thermometer, after some
time, rise considerably ; but the effect
of the heat on the ice will only be to
melt it partially, while its temperature
remains steadily at 32°. Here we have
caloric received by the ice which does
not affect the thermometer.

The principle upon which the ther-
mometer and the pyrometer act is, the
tendency which heat or caloric has to
diffuse itself among contiguous bodies.
When applied to a hot body, they ac-
quire a portion of the heat from that
body ; and when applied to a cold one,
they communicate to that body a por-
tion of their own caloric. These changes
in the quantity of its own caloric are in-
dicated by changes in the bulk of the
thermometric fluid, orpyrometric piece;
and such instruments, therefore, do
no more than show a certain excess
of heat given out by the hottest to the
coldest body. On this ground the
names of thermoscope and pyroscope
are more suitable for such instruments
than their more common designations.

That different bodies, in equal quan-
tities, whether measured by weight or
volume, contain unequal quantities of
caloric, has been established by the
investigations of Boerhaave, Black,
Wilcke, Irvine, Crawford, Lavoisier, &c.
It does not belong to this place to enter
into this subject, but it is sufficient to
mention the grounds for this important
conclusion.

If we mix one pound of water at
212°, and as much water at 32°, when
due precautions are employed to mix
them without loss of heat or the addi-
tion of extraneous temperature, the ther-
mometer plunged in the mixture will
indicate very nearly 122°, or the arith-
metical mean between the extremes ;
which proves that equal quantities of
the same body contain quantities of calo-
ric proportional to their temperature. If,
however, we mix a pound of mercury
with a pound of water, at different tem-
peratures, when the mercuiy is the hot-
test, the temperature of the mixture will
be greatly below the mean ; and when
the water is the hottest body, the mix-
ture will be greatly above the mean tem-
perature. A series of such experi-
E 2

5-2

THERMOMETER AND PYROMETER.

ments have shown that the same quan-
tity of caloric which can raise the tem-
perature of water only 4°, will raise that
of mercury 112°. If this be the case at
every temperature, which is most pro-
bable, the quantity of caloric in water is
to that in an equal weight of mercury, at
the same temperature, as 1 1 2 : 4 =28:1.
Besides this method of finding out
the comparative quantity of caloric in
bodies, there is another founded on the
fact, that ice in melting absorbs an uni-
form quantity of caloric. Professor

a, is the innermost ; and is destined to
receive the heated body, the subject of
experiment. The space between it and
the second vessel, b, is to be filled with
pounded ice, or snow, as well as the
perforated cover, /, of the cage a. It
is the melting of this snow which af-
fords the indication of the comparative
quantity of caloric in the bodies sub-
mitted to experiment ; it rests on a wire
sieve, at the bottom of the cylinder b, and
is received at the orifice of the pipe d.
To guard against the effects of external
temperature, the cavity between the
vessel b, and the exterior one, is filled
also with pounded ice or snow ; the ge-
nerallid h, of the whole being also covered
with snow, and its edges resting in a

Wilcke, of Copenhagen, first conceived
the idea of employ ing the melting of ice or
snow, for the purpose of ascertaining the
comparative quantity of caloric in dif-
ferent bodies ; and this method was im-
proved in the hands of Lavoisier and La-
place, by the invention of the Calori-
meter.

It consists of two vessels of tinned
iron, and a wire cage, which are fitted
so that one may be inserted within the
other, leaving a cavity between the sides
of each. (See Jig. 51.) The wire- cage,

groove e, lined with the same material,
the interior of the instrument is defended
from all direct access of external tem-
perature. The two tubes, g, in the lid,
are for the introduction of thermome-
ters; but during experiments those
tubes are shut up, to prevent the ac-
cess of currents of air through the ca-
lorimeter. The water collected between
the outer and second vessel may be
drawn off by the pipe i. Experiments
of this kind should be made in a room
at a temperature of 32°. Before com-
mencing the experiment, the snow is sa-
turated -with moisture by its melting, to
obviate as much as possible the error from
not collecting the whole of the water.
The indications of this instrument,

THERMOMETER AND PYROMETER.

53

from the cause just alluded to, and the
impossibility of altogether obviating the
effects of currents of air through it,
during experiments, are not susceptible
of such accuracy as the more simple
method of mixture ; although this also
is liable to lead to erroneous conclusions,
from the difficulty of obtaining the true re-
sult of the effect of the different mixtures.

2. There is an obvious source of
error in the indications of all instru-
ments employed to measure tempera-
ture, in as much as the apparent changes
in the volume of the thermometric sub-
stance are not the real augmentation or
diminution of bulk it undergoes. The
glass of the thermometer, and the frame
of the pyrometer, are also expanded
and contracted by changes of tempera-
ture ; so that our instruments only show
the excess of the expansions of the ther-
mometric fluid, or pyrometric bar over
those of the glass and frame ; by which
the true indications are diminished.

From the extreme nicety of some of
the investigations in which the mercu-
rial thermometer is employed, a compa-
rative ratio of the expansions of mer-
cury and glass has been most diligently
sought after by De Luc, Ramsden, Roy,
and others. From their investigations
it has been ascertained that all solids
and liquids vary in their rate of expan-
sion ; but that the expansibility of glass
depends so much on the manufacture
of that article, and varies so much in
the different kinds, that no general
equation as a correction for this source
of eiTor can be of practical utility. Even
the form of the glass rod is material.
Roy gives the expansion of a glass
tube = 0. 0046569th ; of a solid glass
rod = 0.0096944th, in passing from the
freezing to the boiling point of water.*

An Important series of experiments
by Lavoisier and Laplace have been
published by Biot,-i- from which it ap-
pears that of twenty-three solids tried,
glass was the least expansible of them all.

The length of different glass rods,
which at 32° Fahrenheit= 1.00000000, at
2 12° Fahrenheit is augmented as follows:

Glass of St. Gobain 1 .00089089 —T/25

Glass tube, without tead 1 .00087 572= TT^5

Ditto 1.00089760r=TT'T5

Ditto 1.00091751=Tsy5'

French glass, with lead ...1 . 00087 199r=TT»-y
English flint glass 1 .00081166 ~

This will show the impossibility of
any general correction being applied ;
and the adoption of any formulae for
this purpose would be an affectation of
accuracy, of which, unfortunately, the
subject is incapable. If, however, such
formula is considered desirable, it may
easily be constructed from the experi-
ments of De Luc on glass tubes $ at
different temperatures, which, reduced
to the scale of Fahrenheit, are,

Temp.

32°

50

70
100
120

Bulk.
100000
100006
100014
100023
100032

Temp.

150°

167

190

212

Bulk.
100044
100056
100069
100083

* Phil. Trans., vol. Ixxv.

f Traite de Physique, t. ii. 153.

3. Another error of some magnitude
is produced by the inequalities of the
expansions of the same substances by
equal increments of temperature.

If we could consider expansion simply
as the effect of the application of heat,
equal increments of temperature should
produce equal rates of increase of vo-
lume ; but the expansion is the re-
sultant, in solids and in liquids, of two
opposite forces — of the repulsive energy
of caloric opposed by the cohesion of
the particles of matter; and, accord-
ingly, it not only differs in the different
kinds of solid and liquid matter, but
in the same body at different tempera-
tures. As might be expected from this
view, it must be in an increasing ratio
with the temperature ; because the force
of cohesion must diminish with the dis-
tance of the particles of matter.

In aeriform bodies, the force of cohe-
sion does not exist ; and we might in-
fer that equal increments of heat
would produce equal expansions, in all
gases, at all temperatures. In gases,
the ratio might even be expected to de-
crease, in a minute but inappreciable
degree, with the temperature ; because
the increased distance of the particles
will tend to diminish the repulsive
energy.

Experiment in these particulars ac-
cords with theory. The ratio of expan-
sion in solids and liquids is found to be
an increasing one, as the temperature
is augmented, and is very different in
each substance ; while in the gases, it is
not only equable in the same gas, but
equal in all.

The manner in which an increasing

t Recherches, t, i.

54

THERMOMETER AND PYROMETER.

ratio of expansion must affect thermo-
meters and pyrometers, graduated on
the principle of equal degrees between
two fixed points, is obvious ; and it
early became an object of solicitude.
Drs. Halley and Brooke Taylor devised
the method of investigation — namely,
by mixing together equal weights of
water at different ascertained tempera-
tures, and finding how much the ther-
mometer, plunged in that mixture, dif-
fered from the mean temperature.
From not sufficiently attending to the
various requisites to obviate error,
they did not arrive at the true conclu-
sion. We owe to De Luc the more
successful investigation of this problem;
by a train of nice experiments, in which
he endeavoured to guard against the
sources of error arising from the cool-
ing effect of the vessel containing the
mixture, and of the escape of vapour,
he proved that the different therm o-
metric fluids do not expand in a uni-
form ratio to the quantities of caloric
applied; but follow an increasing rate
as thdr temperature is raised. Mer-
cury he found to be the most regular
in its expansions ; yet it also showed
very sensible deviations. "When equal
weights of water at 32° and 212° were
mixed, the mercurial thermometer did
not indicate the mean temperature 122°,
but only 119°; an oil thermometer,
in the same experiment, stood no higher
than 117°, and one of spirit, of wine at
108°; while, with a thermometer filled
with water, the temperature of the mix-
ture appeared only to be 75°. His ex-
periments showed the great superiority
of the mercurial over the alcoholic ther-
mometer ; but this superiority, it pro-
bably owes, as Mr. John Dalton has
remarked, in a great measure to the dis-
tance of the ordinary range of tempera-
ture, from the freezing and boiling point
of mercury; for the experiments of
De Luc show that the irregularities of
all fluids are much augmented about
the points of their consolidation and
passing into vapour.

From the usual method of graduating
the mercurial thermometer from two
fixed points only, the error from inequa-
lity of expansion will be greatest at the
mean between the two points; when, ac-
cording to De Luc, it amounts to about 3°
Fahrenheit below the real temperature.
In the following table are given the
result of De Luc's researches, on the
two fluids chiefly used for thermometers
— mercury and alcohol. In the first

column are the indications of the mer-
curial thermometer, according to De
Luc's or Reaumur's scale; in the
second, the indications of the alcoholic
thermometer at the corresponding tem-
peratures ; and in the third, are, what
ought to be, the real temperatures as
discovered by experiment.

Mercurial Ther. Alcohol Ther. Real Tern.

80
75.
70

80.0..
73.8..
67 8

80.0
75.28
. 7056

65.

61.9..

65.77

60.

56.2..

60.96

55.

50.7..

56.15

50.

47.3..

.,.*.. 51.26

45.

40.2..

46.37

40.

35.1..

41.40

35.

30.3..

36.40

30.

25.6..

31.32

25.

21.0..

26.22

20.

16.5..

21.12

15.

12.2..

.. .,..15.94

10.

7.9..

10.74

5,

3.9..

5.43

0.

0.0. .

0.0

There is reason to believe, however,
that De Luc states the irregularity of
the mercurial thermometer too high.
Dr. Crawford investigated this point
with great care, and concluded, that
when the difference of temperature of
the two portions of fluid did not exceed
100° F., the average deviations of the
mercurial thermometer were not above
0.25 of a degree. De Luc himself
allows, that the result of the mixtures
must be inaccurate, if the capacity of
the water operated on is changed during
the experiment. In mixing together
hot and cold water, the probability is,
that the diminished volume of the mix-
ture causes a diminution of capacity ;
and, consequently, an increase of tem-
perature, beyond what is due to the
heat of the two portions mixed together.
By exposing a mercurial thermometer
in a vessel, in which the included air
was exposed to the frigorific influence
of melting snow, and the heat of watery
vapour at 212°, he found that it indi-
cated 121°, or only a single degree less
than the arithmetical mean.* From
comparison with air thermometers, Gay
Lussac inferred that the mercurial ther-
mometer was equable in its expansions,

* Experiments on Animal Heat.

THERMOMETER AND PYROMETER,

55

between the freezing and the boiling
point of water.* Petit and Dulong
have investigated the subject, by a com-
parison with the air thermometer, and
with the expansions of a pyrometer of
very infusible metals (platina and cop-
per) which appear, by the experiments
of Lavoisier and Laplace, to be very
equable in their expansions, below the
boiling point of water. They found
that the irregularity in mercury in-
creases with the temperature ; and
would even appear to be greater than
the rise of the mercurial thermometer
indicates, were not the increasing ex-
pansion of the mercury diminished by
the increasing ratio of the dilatation of
the glass itself, f Hence the source of
error in thermometers arising from the
expansion of the glass, is rather advan-
tageous than detrimental to their accu-
racy.

The difference between the indica-
tions of the alcoholic and mercurial
thermometers, as lately ascertained by
Dr. De Wildt, do not materially differ
from the determinations of De Luc ;
and having been obtained for every 5°
of Reaumur's scale, apparently with
much care, we give the result as a table
of correction for Rutherford's thermome-
ters ; for which purpose they were in-
tended by the author.

Mercury. Spirit. Mercury. Spirit

—45° = _-28°.50 -}-20°...+160.48

40 ....25 .92 25 20 .97

35 23 .19 30 25 .60

30 20 .32 35 30 .38

25 17 .30 40 35 .31

20 14 .13 45 40 .38

15 10 .82 50 45 .60

10.... 7 .36 55 50 .97

5 3 .75 60 56 .48

0 0 .00 65 62 .14

+5 + 3 .90 70 67 .95

10 7 .95 75 73 .90

15 12 .14 80 80J.OO

The indications of air thermometers
were at one time supposed liable to un-
certainty, from the inequalities of their
expansion. Guyton and Prieur ima-
gined that they progressively expanded
in a greater ratio than the temperature, §
but this has been proved to be errone-
ous ; and the mistake probably arose

* Annales de Chimie, t. xliii.

-j- Annales de Chimie et Physique, t. ii. p. 240.

J Jameson's Edin. Phil. Journal, Oct, 1826, and
Kastner Archiv fur die Gesammte Natural, Decem
1825.

$ Journal de 1'Ecole Polytechnique.

from their neglecting the effect of hy-
grometric water in the gases. General
Roy* found that their expansion fol-
lowed a ratio, decreasing with the ele-
vation of temperature ; and the same
result was obtained by Mr. Dalton.-f- Dr.
Murray, $ Gay Lussac, § and Petit and
Dulong ; || but there is reason to con-
clude that this apparently decreasing
ratio in the expansion of gases is owing
to the error caused by the unequable
expansions of the mercury in the ther-
mometer, and the dilatation of the bulb of
that instrument ; and philosophers now
agree to consider the expansions of
gases equable and equal, as before
stated; especially as the decreasing
ratio disappears, if we apply De Luc's
correction of the real mean between
32° and 212°.

From the foregoing observations, we
may conclude, that the air thermometer
requires no correction of its indications ;
that the accuracy of the mercurial
thermometer is not materially affected
by the inequalities of the expansions
of the mercury, in ordinary ranges of
temperature ; that the expansion of al-
cohol is pretty uniform, until about 30°
R. or 100° F. : above that point its ex-
pansions become more irregular ; but it
has the advantage over every other liquid,
of marking the lowest degrees of natural
or artificial cold hitherto observed.

The irregularities affecting pyrome-
ters, except from alteration in the size of
the substances supporting the bars, are
extremely minute. The experiments of
De Luc and Roy would lead to the con-
clusion, that the expansibility of solids
is not quite equable. Roy thinks that
this slight irregularity may be apparent
rather than real; but Lavoisier and
Laplace state, that the expansions of
solids keep pace with those of the mer-
curial thermometer, from the freezing
to the boiling point of water ; and Petit
and Dulong assert, that the expansion
of metals is progressive above 212° F.

These irregularities, if they exist, are
so minute, as not, in any ordinary prac-
tical purpose, to affect the indications
of pyrometers; and at high tempera,
tures extreme accuracy is seldom of
much consequence.

The differences arising from two py-
rometers of different materials may be
corrected by Table IV. in the appendix

» Phil. Trans, vol. Ixvii.

t Manchester Memoirs, vol. v. p. 599.

J Edin. Phil. Trans. § Ann. de Chim. t. xliii.

|| Annales de Chim, et Phys, t. ii.

THERMOMETER AND PYROMETER.

of the results obtained in 1782, by La-
voisier and Laplace, lately recovered
by Biot. Each substance at 32° F.=
1.00000000.

There is one precaution in graduating
thermometers, which will render any
irregularities of little consequence ; that
is, in forming the scale, not to rest satis-
fied with only two fixed points, and

equally dividing the intervening space ;
but to obtain intermediate points by
means of mixtures, or by comparison
with a standard instrument so formed ;
and by the shortness of the intervals
adapting the scale to inequalities in the
bore of the tube, or to the less impor-
tant irregularities just now considered.

57

APPENDIX,

No. I.— TABLES OF CORRESPONDENCE OF THE DIFFERENT
THERMOMETRICAL SCALES.

(From Da. MURRAY'S System of Chemistry).

TABLE FOR THE CENTIGRADE THERMOMETER.

Centigrade.

Reaumur's.

ahrenheit's.

enti grade.

Reaumur's.

Fahrenheit's.

Centigrade.

Reaumur's.

Fahrenheit's.

100

80.

212.

53

42.4

127.4

6

4.8

42.8

99

79.2

210.2

52

41.6

125.6

5

4.

41.

98

78.4'

208.4

51

40.8

123.8

4

3.2

39.2

97

77.6.

206.6

50

40.

122.

3

2.4

37.4

96

76.8

204.8

49

39.2

120.2

2

1.6

35.6

95

76.

203.

48

38.4

118.4

1

0.8

33.8

94

75.2

201.2

47

37.6

116.6

0

0.

32.

93

74.4

199.4

46

36.8

114.8

-1

-0.8

30.2

92

73.6

197.6

45

36.

113.

-2

-1.6

28.4

91

72.8

195.8

44

35.2

111.2

-3

-2.4

26.6

90

72.

194.

43

34.4

109.4

-I

-3.2

24.8

89

71.2

192.2

42

33.6

107.6

-5

-1.

23.

88

70.4

190.4

41

32.8

105.8

-6

-1.8

21.2

87

69.6

188.6

40

32.

104.

-7

-5.6

19.4

86

68.8

186.8

39

31.2

102.2

-8

-6.4

17.6

85

68.

185.

38

30.4

100.4

-9

-7.2

15.8

84

67.2

183.2

37

29.6

98.6

-10

-8.

14.

83

66.4

181.4

36

28.8

96.8

-11

-8.8

12.2

82

65.6

179.6

35

28.

95.

-12

9.6

10.4

81

64.8

177.8

34

27.2

93.2

-13

-10.4

8.6

80

64.

176.

S3

26.4

91.4

-14

-11.2

6.8

79

63.2

174.2

32

25.6

89.6

-15

-12.

5.

78

62.4

172.4

31

24.8

87.8

-16

-12.8

3.2

77

61.6

170.6

30

24.

86.

-17

-13.6

1.4

76

60.8

168.8

29

23.2

84.2

-18

-14.4

-0.4

75

60.

167.

28

22.4

82.4

-19

-15.2

-2.2

74

59.2

165.2

27

21.6

80.6

-20

-16.

-4.

73

58.4

163.4

26

20.8

78.8

-21

-16.8

-5.8

72

57.6

161.6

25

20.

77.

-22

-17.6

-7.6

71

56.8

159.8

24

19.2

75.2

-23

-18.4

-9.4

70

56.

158.

23

18.4

73.4

-24

-19.2

-11.2

69

55.2

156.2

22

17.6

71.6

-25

-20.

-13.

68

54.4

154.4

21

16.8

69.8

-26

-20.8

-14.8

67

53.6

152.6

20

16.

68.

-27

-21.6

-16.6

66

52.8

150.8

19

15.2

66.2

-28

-22.4

-18.4

65

52.

149.

18

14.4

64.4

-29

-23.2

-20.2

64

51.2

147.2

17

13.6

62.6

-30

-24.

-22.

63

50.4

145.4

16

12.8

60.8

-31

-24.8

-23.8

62

49.6

143.6

15

12.

59.

-32

-25.6

-25.6

61

48.8

141.8

14

11.2

57.2

-33

-26.4

-27.4

60

48.

140.

13

10.4

55.4

-31

-27.2

-29.2

59

47.2

138.2

12

9.6

53.6

-35

-28.

-31.

58

46.4

136.4

11

8.8

51.8

-36

-28.8

-32.8

57

45.6

134.6

10

8.

50.

-37

-29.6

-34.6

56

44.8

132.8

9

7.2

48.2

-38

-30.4

-36.4

55

44.

131.

8

6.4

46.4

-39

-31.2

-18.2

54

43.2

129.2

7

5.6

44.6

-10

-32.

-10.

58

THERMOMETER AND PYROMETER.

TABLE FOR REAUMUR'S THERMOMETER.

Reaumur

Centigrade

Fahrenheit

Reaumur

Centigrade

Fahrenheit

Reaumur's

Centigrade

Fahrenheit's.

80

100.

212.

42

52.5

126.5

4

5.

41.

79

98.75

209.75

41

51.25

124.25

3

3.75

38.75

78

97.5

207.5

40

50.

122.

2

2.5

36.5

77

96.25

205.25

39

48.75

119.75

1

1.25

34.25

76

95.

203.

38

47.5

117.5

-0

0

32.

75

93.75

200.75

37

46.25

115.25

-1

-1.25

29.75

74

92.5

198.5

36

45.

113.

-2

-2.5

27.5

73

91.25

196.25

35

43.75

110.75

-3

-3.75

25.25

72

90.

194.

34

42.5

108.5

-4

-5.

23.

71

88.75

191.75

33

41.25

106.25

-5

-6.25

20.75

70

87.5

189.5

32

40.

104.

-6

-7.5

18.5

69

86.25

187.25

31

38.75

101.75

-7

-8.75

16.25

68

85.

185.

30

37.5

99.5

-8

-10.

14.

67

83.75

182.75

29

36.25

97.25

-9

-11.25

11.75

66

82.5

180.5

28

35.

95.

-10

-12.5

9.5

65

81.25

178.25

27

33.75

92.75

-11

-13.75

7.25

64

80.

176.

26

32.5

90.5

-12

-15.

5.

63

78.75

173.75

25

31.25

88.25

-13

-16.25

2.75

62

77.5

171.5

24

30.

86.

-14

-17.5

0.5

61

76.25

169.25

23

28.75

83.75

-15

-18.75

-1.75

60

75.

167.

22

27.5

81.5

-16

-20.

-4.

59

73.75

164.75

21

26.25

79.25

-17

-21.25

-6.25

58

72.5

162.5

20

25.

77.

-18

-22.5

-8.5

57

71.25

160.25

19

23.75

74.75

-19

-23.75

-10.75

56

70.

158.

18

22.5

72.5

-20

-25.

-13.

55

68.75

155.75

17

21.25

70.25

-21

-26.25

-15.25

54

67.5

153.5

16

20.

68.

-22

-27.5

-17.5

53

66.25

151.25

15

18.75

65.75

-23

-28.75

-19.75

52

65.

149.

14

17.5

63.5

-24

-30.

-22.

51

63.75

146.75

13

16.25

61.25

-25

-31.25

-24.25

50

62.5

144.5

12

15.

59.

-26

-32.5

-26.5

49

61.25

142.25

11

13.75

56.75

-27

-33.75

-28.75

48

60.

140.

10

12.5

54.5

-28

-35.

-31.

47

58.75

137.75

9

11.25

52.25

-29

-36.25

-33.25

46

57.5

135.5

8

10.

50.

-30

-37.5

-35.5

45

56.25

133.25

7

8.75

47.75

-31

-38.75

-37.75

44

55.

131.

6

7.5

45.5

-32

-40.

-40.

43

53.75

128.75

5

6.25

43.25

-33

-41.25

-42.25

THERMOMETER AND PYROMETER.

TABLE FOR FAHRENHEIT'S THERMOMETER*

Fahrenheit's.

Reaumur's.

Centigrade.

Fahrenheit'*.

Reaumur's.

Centigrade.

Fahrenheit's.

Reaumur's.

Centigrade.

212

80.00

100.00

170

61.33

76.66

128

42.66

53.33

211

79.55

99.44

169

60.88

76.11

127

42.22

52.77

210

79.11

98.88

1G8

60.44

75.55

126

41.77

52.22

209

78.66

98.33

167

60.00

75.00

125

41.83

51.66

208

78.22

97.77

166

59.55

74.44

124

40.88

51.11

20T

77.77

97.22

165

59.11

73.88

123

40.44

50.55

206

77.33

96.66

161

58.66

73.33

122

40.00

50.00

205

76.88

96.11

163

58.22

72.22

121

39.55

49.44

204

76.41

95.55

162

57.77

72.77

120

39.11

48.88

203

76.00

95.00

161

57.33

71.66

119

38.66

48.33

202

75.55

94.44

160

56.88

71.11

118

38.22

47.77

201

75.11

93.88

159

56.44

70.55

117

S7.77

47.22

200

71.66

93.33

158

56.00

70.00

116

37.33

46.66

' 199

74.22

92.77

157

55.55

69.44

115

36.88

46.11

198

73.77

92.22

156

55.11

68.88

114

36.44

45.55

197

73.33

91.66

155

54.66

68.33

113

36.00

45.00

196

72.88

91.11

154

54.22

67.77

112

35.55

44.44

195

72.44

90.55

153

53.77

67.22

111

35.11

43.88

194

72.00

90.00

152

53.33

66.66

110

84.66

43.33

193

71.55

89.44

151

52.88

66.11

109

34.22

42.77

192

71.11

88.88

150

52.44

65.55

108

33.77

42.22

191

70.66

88.33

149

52.00

65.00

107

33 . 33

41.66

190

70.22

87.77

148

51.55

64.44

106

32 . 88

41.11

189

69.77

87.22

147

51.11

63.88

105

32.44

40.55

188

69.33

86.66

146

50 . 66

63.33

104

32.00

40.00

187

68.88

86.11

145

50.22

62.77

103

31.55

39.44

186

68.44

85.55

144

49.77

62.22

102

31.11

38.88

185

68.00

85.00

143

49.33

61.66

101

80.66

38.33

184

67.55

84.44

142

48.88

61.11

100

80.22

37.77

183

67.11

83.88

141

48.44

60.55

99

29.77

37.22

182

66.66

83.33

140

48.00

60.00

98

29.33

36.66

181

66.22

82.77

139

47.55

59.44

97

28.88

36.11

180

65.77

82.22

138

47.11

58.88

96

28.44

35.55

179

65.33

81.66

137

46.66

58.33

95

28.00

35.00

178

64.88

81.11

136

46.22

57.77

91

27.55

34.44

177

61.41

80.55

135

45.77

57.22

93

27.11

33.88

176

64.00

80.00

134

45.33

56.66

92

26.66

33 . 33

175

63 . 55

79.44

133

44.44

56.11

91

26.22

32.77

174

62.11

78.88

132

44.55

55.55

90

25.77

32.22

173

62.66

78.33

131

44.00

55.00

89

25 . 33

31.66

172

62.22

77.77

130

43.55

54.44

88

24.88

31.11

171

61.77

77.22

129

43.11

58 . 88

87

24.44

30.55

* All the decimals in this Table are circulating decimals.

60

THERMOMETER AND PYROMETER.

TABLE FOR FAHRENHEIT'S THERMOMETER CONTINUED.

Fahrenheit's. Reaumur's.

Centigrade.

Fahrenheit's

Reaumur's. Centigrade.

Fahrenheit's.

Reaumur's.

Centigrade.

86

21.00

30.00

43

4.88

6.11

0

-14.22

-17.77

85

2S.55

29.44

42

4.44

5.55

-1

-14.66

-18.33

84

23.11

28.88

41

4.00

5.00

-2

-15,11

-18.88

83

22.66

28.33

40

3.55

4.44

-3

-15.55

-19.44

82

22.22

27.77

39

3.11

3.88

-4

-16.00

-20.00

81

21.77

27.22

38

2.66

3.33

-5

-16.44

-20.55

80

21.33

26.66

37

2.22

2.77

-6

-16.88

-21.11

79

20.88

26.11

36

1.77

2.22

-7

-17.33

-21.66

78

20.44

25.55

35

1.33

1.66

-S

-17.77

-22.22

77

20.00

25.00

34

0.88

1.11

-9

-18.22

-22 . 77

76

19.55

24.44

33

0.44

0.55

-10

-18.66

-23.33

75

19.11

23.88

32

0.

0.

-11

-19.11

-23.88

74

18.66

23.33

31

-0.44

-0.55

-12

-19.55

-24.44

73

18.22

22.77

30

-0.88

-1.11

-13

-20.00

-25.00

72

17.77

22.22

29

-1.33

-1.66

-14

-20.44

-25.55

71

17.33

21.66

28

-1.77

-2.22

-15

-20.88

-26.11

70

16.88

21.11

27

-2.22

-2.77

-16

-21.33

-26.66

69

16.44

20.55

26

-2.66

-3.33

-17

-21.77

-27.22

68

16.00

20.00

25

-3.11

-3.88

-18

-22.22

-27.77

67

15.65

19.44

24

-3.55

-4.44

-19

-22 . 66

-28.33

66

15.11

18.88

23

-4.00

-5.00

-20

-23.11

-28.88

65

14.66

18.33

22

-4.44

-5.55

-21

-28.55

-29.44

64

14.22

17.77

21

-4.88

-6.11

-22

-24.00

-30.00

63

13.77

17.22

20

-5.33

-6.66

-23

-24.44

30.55

62

13.33

16.66

19

-5.77

-7.22

-24

-24.88

-31.11

61

12.88

16.11

18

-6.22

-7.77

-25

-25.33

-31.66

60

12.44

15.55

17

-6.66

-8.33

-26

-25.77

-32.22

59

12.00

15.00

16

-7.11

-8.88

-27

-26.22

32.77

58

11.55

14.44

15

-7.55

-9.44

-28

-26.66

33.33

57

11.11

13.88

14

-8.00

-10.00

-29

-27.11

-33.88

56

10.66

13.33

13

-8.44

-10.55

-30

-27.55

-34.44

55

10.22

12.77

12

-8.88

-11.11

-31

-28.00

-35.00

54

9.77

12.22

11

-9.33

-11.66

-32

-28.44

-35.55

53

9.33

11.66

10

-9.77

-12.22

-33

-28.88

-36.11

52

8.88

11.11

9

-10.22

-12.77

-34

-29 . 33

-36.66

51

8.44

10.55

8

-10.66

-13.33

-35

-29.77

-37,22

50

8.00

10.00

7

-11.11

-13.88

-36

-30 . 22

-37.77

49

7.55

9.44

6

-11.55

-14.44

-37

-30 . 66

-38.33

48

7.11

8.88

5

-12.00

-15.00

-38

-31.11

-38.88

47

6.66

8.33

4

-12.44

-15.55

-39

-31.55

39.44

46

6.22

7.77

3

-12.88

-16.11

-40

-32.00

-40.00

45

5.77

7.22

2

-13.33

-16.66

44

5.33

6.66

1

-13.77

-17.22

THERMOMETER AND PYROMETER,

61

No. II.— TABLES OF EXPANSIONS OF BODIES BY HEAT.
I. TABLE OF EXPANSIONS ACCORDING TO SMEATON.

SUBSTANCES.

DILATATION FOR A LENGTH
EQUAL TO UNITY.

In decimal
fractions.

In vulgar
fractions.

Blistered steel ....

0.00115000

F*o

Tempered steel

0.00122500

sic

Bismuth . ....

0.00139167

Copper hammered . .

0.00170000

Jjg

Copper 8 parts with 1 of tin . .

0.00181667

Cast brass

0.00187500

5^3

Brass, 16 parts with 1 of tin

0.00190883

5'_

Fine pewter

0.00228333

4^8

Grain tin ....

0.00248333

4*3

Iron . . .

0.00125833

yI5

Brass wire . .

0.00193333

517

Speculum metal

0.00193333

5T7

Lead ....

0.00286667

liy

Antimony . . . .

0.00108333

1*4

Lead2 parts with 1 of tin

0.00250533

Copper 2 parts with 1 of zinc

0.00205833

*b

White glass (barometer tube,) .

0.00083333

Zinc . . . .

0.00294167

3575-

Zinc hammered

0.00310833

322

Zinc 8 parts with 1 of tin .

0.00269167

fH

II. TABLE OF EXPANSIONS ACCORDING TO ROY.

SUBSTANCES.

FOR A LENGTH EQUAL. TO
UNITY.

In decimal
fractions.

In vulgar
fractions.

Steel rod .....

0.00114450

«*4

Brass scale, Hamburgh .

0.00185550

sl-J :

Brass plate rod, English

0.00189296

1558

Brass plate trough, English .

0.00189450

sis

Cast Iron prism .

0.00111000

S*T

Glass tube .....

0.00077550

Tins 9

Glass rod ....

0.00080833

T,b

III. TABLE OF EXPANSIONS ACCORDING TO TROUGHTON.

SUBSTANCES.

LINEAR DILATATIONS FHOM THE
TEMPERATURE OF FREEZING TO
BOILING WATER.

In decimal
fractions.

In vulgar .
fractions.

Steel . .....

0.0011899

555

Silver ... . .

0.0020826

-1-

Copper . ...

0.0019188

S£T

Iron Wire . *

0.0014401

oh

Platina. .

0.00099218

To'oS

Palladium (according to W7ollaston) .

0.0010000

Toulo

62

THERMOMETER AND PYROMETER.

IV. TABLE OF THE LINEAR DILATATION OF DIFFERENT SUBSTANCES FROM THE TEM-
PERATURE OF FREEZING TO THAT OF BOILING WATER, ACCORDING TO THE
EXPERIMENTS OF LAVOISIER AND LAPLACE.

SUBSTANCES.

DILATATION FOR A LENGTH
EQUAL TO UNITY.

In decimal
fractions.

In vulgar
fractions.

At 212°

Glass of St. Gobain ,

0.00089089

ifa

Glass tube without lead

0.00087572

IT1* 2

Ditto. ,

0.00089760

IT*!?

Ditto ....

0.00091751

ToSTT

English Flint Glass

0.00081166

rfa

French glass with lead

0.00087199

IT'??

Copper «

0.00172244

iti

Ditto

0.00171222

tk

Brass ....

0.00186671

ill

Ditto ....

0.00188971

Hammered Iron • , .

0.00122045

III

Iron Wire • . . .

0.00123504

sis

Hard Steel ....

0.00107875

ilv

Soft Steel ....

0.00107956

Tempered Steel ....

0.00123956

gly

Lead

0.00284836

•H

Malacca Tin ....

0.00193765

sis

Cornish Tin

0.00217298

Cupelled Silver

0.00192974

Ih

Parisian Standard Silver . «

0.00190868

Pure Gold . .

0.00146606

9*1

Parisian Standard Ditto not softened .

0.00155155

6*5

Ditto, softened ....

0.00151361

eil

V. TABLE OF THE EXPANSIONS OF LIQUIDS.

The expansions in this table were determined by Mr. D alt on. They are equal
to what would be produced by an elevation of temperature from the freezing to
the boiling point of water ; the volume at the former being 1.

Mercury it.. .0200 — -^

Water , .0466 =r ft.,

Water saturated with salt 0500 = 5V

Sulphuric acid 0600 = ^

Muriatic acid 0600 =: ^\

Oil of turpentine 0700 " T\

^Ether 0700 =: TV

Fixed oils 0800 = ^.5

Alcohol .., .0110= |

Nitric acid ,,....,, ,,., .0110= |;

THERMOMETER AND PYROMETER. 63

No. III.

TABLE OF REMARKABLE TEMPERATURES ACCORDING TO FAHRENHEIT'S

SCALE.

Iron red hot in the twilight . . . . . . 884

Heat of a common fire (Irvine) . . . . ' . 790

Iron bright red in the dark . . . . , .752

Zinc melts . . . . . . . 700

Quicksilver boils (Irvine) ...... 672

(Dalton) . . . , . 660

(Crichton) . . . . . .655

Linseed oil boils . . . . ... 600

Lead melts (Guyton, Irvine) ...... 594

Sulphuric Acid boils (Dalton) . . . . 590

The surface of polished Steel acquires a deep blue colour . . . 580

Oil of Turpentine boils . . . . , 560

Phosphorus boils ....... 554

Bismuth melts (Irvine) . . . . . . 476

The surface of polished steel acquires a pale straw colour . . . 460

Tin melts (Crichton, Irvine) . . . . . 442

A Compound of equal parts of Tin and Bismuth melts . .. . 283

Nitric Acid boils . . . . . . 242

Sulphur melts •»#.... 226

A saturated Solution of Salt boils . . . . . . 21

Water boils, (the barometer being at 30 inches) ; also a Compound of 5 of Bismuth,

3 of Tin, and 2 of Lead, melts . . . . .212

A Compound of 3 of Tin, 5 of Lead, and 8 of Bismuth, melts . . 21 0

Alcohol boils . . . . . . .174

Bees' Wax melts . . . . . . 142

Spermaceti melts . . . . . . . 1 33

Phosphorus melts . . . . . . 100

vEther boils . . . . . . .98

Medium Temperature of the Globe . . . . ... 50

Ice melts . . . . . . .32

Milk freezes . . . . . . . 30

Vinegar freezes at about . . . . . .28

Strong Wine freezes at about . . . . . . 20

A Mixture of 1 part of Alcohol, and 8 parts of Water, freezes . . 7

A Mixture of Alcohol and Water in equal parts, freezes . . . T

A Mixture of 2 parts of Alcohol and 1 of Water, freezes . . .11

Melting point of Quicksilver (Cavendish) . . . 39

Liquid Ammonia crystallizes (Vauquelin) . . . .42

Nitric Acid, spec. gr. about 1.42, freezes (Cavendish) . . 45

Sulphuric ./Ether congeals (Vauquelin) • . • .47

Natural Temperature observed at Hudson's Bay . . . 50

Ammoniacal Gas condenses into a liquid (Guyton^ . . .54

Nitrous Acid freezes (Vauquelin) . . . . . 56

Cold produced from diluted Sulphuric Acid and Snow, the materials being at the

temperature of 57 . . , . . 78|

Greatest Artificial Cold yet measured (Walker) . . . .91

ADDENDA ET CORRIGENDA,

Page 6. The description of
7, in the text, is taken from the Memoirs
of the Academy of Sciences. It is pro-
bable that in the construction of the
instrument, Amontons employed a tube
of narrow calibre, to enable the included
air to support the mercurial column.
The proportion between the tube and
ball maybe inferred from what is stated
of the expansion of the whole, by the
heat of boiling water.

P. 10. In the beginning of the para-
graph, just after the first set pf formulae,
there is an error which requires correc-
tion. Instead o f these formula? applying
to all degrees above the freezing point,
read, "these formulae apply to all de-
grees above the zero of each scale ;"

P. 14. The notation employed by
De Luc requires some explanation.
y denotes the height of the barometer
in sixteenths of a Parisian line ; T the
height of a thermometer, plunged in
boiling water, above the melting point
of ice, in hundredths of a degree of
De Luc's scale ; and a the constant quan-
tity 10387, which Horsley thinks, from
some of De Luc's experiments, should
have been 10369 ; but in his investiga-
tions he retains the first number, as
probably adopted on good grounds.

The logarithms used by De Luc are
the tables of Briggs, in which the seven
figures of the tables, as well as the
indices, are reckoned integers ; or he
considers the eighth figure in the place

of units : but it is most convenient to
reckon all the figures after the index
as decimals, and the formulae will be

99xlOOr „,

-- Log. y-a=T. ,

P. 15. In the second column of the
table, 29. should be opposite to .95,
instead of .39.

The terms lower and higher, in the
third column of the same table, are so
an the original paper ; but the directions
will be more plain, if the reader will re-
collect, that where lower is indicated,
the correction is " to be subtracted ;"
and when higher is used, the correction
is "to be added."

P. 23, Col. 1. The action of the pi-
nions in Ferguson's Pyrometer is ill ex-
pressed in the text ; and the reader is
requested to substitute for the member
of the sentence commencing with " and
the bar, tyc." in the 14th line, the fol-
lowing — -"and as the bar k has IrBeen
teeth to one inch, it is obvious that
when k moves one inch, the pinion I
will have made one revolution and a
quarter, and the pinion p will have
moved 100 x 1|, or 125 times round."

P. 29. In Wedgwood's table, Col. 1,
line 27, for deduct, read, " did not."

In the same page, Col, 2, the radius
of Regnier's instrument should have
been -649 millimetres, its diameter 1.298
metres, and its circumference 4.079
metres.

ELECTRICITY.

CHAPTER I.
General Facts and Principles.

(1 .) THE science of ELECTRICITY, which
now ranks as one of the most important
branches of Natural Philosophy, and
which embraces so many subjects of
inquiry, exceedingly curious in them-
selves, and highly interesting from their
relations with" every department of na-
ture, is wholly of modern creation. The
ancients were, indeed, acquainted with
a few detached facts, depending on the
agency of electricity; such as the at-
tractive power which amber acquires by
being rubbed, the benumbing shocks
which are experienced on touching the
torpedo (or electrical eel), and the ap-
pearance of those sparks or streams of
light which, on some occasions, are seen
to issue from the human body. But no
suspicion was entertained that these
phenomena had any connexion with one
another ; and far less was it imagined
that they were the effects of a power
pervading all material bodies, and ex-
tensively concerned in all the operations
of nature.

(2.) It was only by slow degrees that
this knowledge was acquired. The first
step towards a generalization of the
phenomena was made by Dr. Gilbert,
an English physician, who, in the year
1GOO, published a very original and va-
luable treatise on the magnet. He re-
marked th -it several other bodies besides
amber can, by friction, be made to at-
tract light bodies ; and he was thus led
to the discovery of a property common
to all of them. The Greek name for
amber being «X£xrSay (Electron), the
bodies possessed of this property were
denominated Electrics; and the "power
they manifested was termed ELECTRI-
CITY. The observations of Boyle, Otto
Guericke, Newton, and a lew other phi-
losophers of the same period, contributed
somewhat to the extension of our know-
ledge on this curious subject ; but even
the information collected during the
whole of that century amounted to
nothing that could be- entitled to the

name of science. The real science of
Electricity can, properly speaking, be
considered as taking its rise only in a
later age ; and it was the first fruit of
that active spirit of investigation, which
at the commencement of the eighteenth
century was rapidly diffusing itself over
Europe. The establishment of the Royal
Society of London appears to have had
considerable influence in promoting the
cultivation of electricity: for we find
that almost every discovery of import-
ance in this science was made by the
members, and is recorded in the Trans-
actions of that Society. But it was not
until the present century that the ex-
tensive relations which connect electri-
city with so many other branches of
physical science, were discovered, and
their importance appreciated. Already
have we seen, in this short era, the rise
of a new science, founded on that pe-
culiar modification of Electricity, which
is known by the name of GALVANISM.
Hence, have we derived new instru-
ments of analysis, new paths of research,
and new powers of extending the- do-
minions of science; hence, have we
been able to trace alliances between
several of the great agents concerned
in the phenomena of the material uni-
verse. ELECTRO-CHEMISTRY has thus
arisen as one of the connecting branches
between remote divisions of the Philo-
sophy of Nature. Still more recently
there has been opened to us, in the sub-
ject of ELECTRO-MAGNETISM, another
new province of science, which esta-
blishes a natural connexion between
two powers hitherto regarded as dis-
tinct.

So rapid has been the march of scien-
tific improvement, that it is difficult for
those whose attention has not been
steadily and exclusively devoted to these
particular objects, to keep pace with
the progress of discovery. The mate-
rials collected by the numerous labourers
in these wide fields of inquiry have
poured in upon us so fast, that there
has scarcely yet been time for marshal-
ling them in their proper places, and for

ELECTRICITY.

disposing them in the order best fitted
for instruction. It is to be lamented
that there exists as yet no general and
comprehensive treatise embracing the
whole of these extensive and compli-
cated subjects of modern research ; and
that the student has still to gather the
information he seeks from a multitude
of journals and other miscellaneous
sources, where they lie irregularly scat-
tered, and are not to be arranged, or
even found, without a great expenditure
of time and labour. It is the aim of
these treatises to supply, in some degree,
this deficiency, in as far, at least, as
relates to the instruction of those who
have no previous acquaintance with the
subject, and are desirous of being initi-
ated in the principles of the science.

(3.) In order to convey the clearest
and most philosophical views of the sub-
ject we are about to treat, we shall begin
by stating, independently of all theory,
the most general facts relating to Elec-
tricity ; presenting them at first in their
simplest form. We shall, in the se-
cond place, review the theories which
have been framed for the purpose of
connecting these facts in the mind. We
shall thus be enabled, lastly, to study
their combinations, to unravel their com-
plicated results, and to follow them in
their practical applications.

(4.) The general facts relating to
Electricity may be reduced to the six
following heads : —

1. EXCITATION.

2. ATTRACTION.

3. REPULSION.

4. DISTRIBUTION.

5. INDUCTION.

6. TRANSFERENCE.

§ 1. Of Excitation, Attraction, and

Repulsion.

(5.) If a piece of amber, or sealing-
wax, or a smooth surface of glass, per-
fectly clean and dry, be briskly rubbed
with a dry woollen cloth, and imme-
diately afterwards held over small and
light bodies, such as pieces of paper,
thread, cork, straw, feathers, or frag-
ments of gold leaf, strewed upon a table,
these bodies will be seen to fly towards
the surface that has been rubbed, and
adhere to it for a certain time. The
surfaces which have acquired by fric-
tion this attractive power are said to be
excited ; and the substances thus sus-
ceptible of excitation are termed elec-
trics, in contradistinction to such as
are not excitable by a similar process

and which are, therefore, termed non-
electrics.

(6.) The principal electric substances
in nature are the following : viz. amber,
gum-lac, resin, sulphur, glass, talc, the
precious stones, silk, the fur of most
quadrupeds, and almost all vegetable
substances (excepting charcoal), which
have been thoroughly deprived of mois-
ture, as, for example, baked wood, and
very dry paper.

(7.) After the bodies which had been
attracted by the excited electric have re-
mained in contact with it a certain time,
the force which held them together
ceases to operate : the bodies then re-
cede from the electric, and if the latter
be again presented to them, they will,
provided they have touched no other
body, be repelled, or driven off, instead
of attracted. This change from attrac-
tion to repulsion takes place more
slowly with some substances than with
others : some bodies will adhere to the
electric a considerable time before they
recede ; while others, and especially me-
tallic bodies, are repelled the instant
after contact :— the reason of this will
afterwards be seen.

(8.) It is also to be noticed that two
bodies which have both of them been
in contact with the same electric, mutu-
ally repel each other.

(9.) The phenomena of electrical at-
traction and repulsion are best observed
when electrics of considerable size are
employed. For the experiments we are
about to describe, it is convenient to
have them of a cylindrical shape, which
admits of their being more easily carried
in the hand, and more readily trans-
ferred to wherever we may wish to place
them. We may employ as our electric
a thick cylinder of sealing wax, or one
of sulphur. If glass be chosen, it should
be in the form of a tube of considerable
diameter, and should, previously to the
experiment, be gently warmed before
the fire, in order to expel all moisture
from its surface. As a rubber we may
use a silk handkerchief, a piece of clean
flannel, or the fur of a quadruped ; but
the material which produces the greatest
effect when rubbed with glass is an
amalgam (or mixture) of mercury with
tin or zinc. Whatever be the substance
employed, it should be perfectly dry ;
to ensure which condition it should, pre-
viously to being used, be held for some
time before the fire.

(10.) When, by attending to these pre-
cautions, a sufficiently powerful excite-

ELECTRICITY.

ment of the cylinder has been obtained,
we may observe several other remark-
able phenomena, besides those of attrac-
tion. It' the experiment be performed
in a dark room, flashes of light, of a
bluish colour, will be perceived during
the friction, extending over every part of
the surface rubbed ; and sparks, attended
with a sharp snapping sound, will be
seen to dart around it in various direc-
tions. If a round body, as a metallic
ball, be presented to it, and moved from
one end to the other, a succession of
sparks will be obtained as the ball passes
along the surface ; and if the knuckle
be presented instead of the metallic
ball, each spark will be accompanied by
a pricking sensation. When the ex-
cited cylinder is brought near to the
face, an unpleasant sensation of tickling
is felt in the skin, as if it had been co-
vered with a cobweb.

(11.) If a globe of metal be sus-
pended in the air by silk threads, and if,
while in this situation, it be rubbed by
an electric, such as silk, fur, or the
outside of the skin of a cat, it will also
become electrical, and exhibit the same
properties of attraction and of repulsion
as if it had been itself an electric. The
circumstance of its being thus insulated
or cut off from the contact of any sub-
stance, except the air and the electric
which sustains it, is essential to the suc-
cess of this experiment.

(12.) Various modes have been de-
vised for exhibiting distinctly the attrac-
tive and repulsive agencies of electricity ;
and for obtaining indications of its pre-
sence, when it exists only in a feeble
degree. Instruments for this purpose
are termed Electroscopes. One of the
simplest of these is the Electroscope of
Haiiy, which is very similar to that
formerly proposed by Dr. Gilbert. It
consists of a light metallic needle, ter-
minated at each end by a light pith
ball, which is covered with gold leaf,

Fig. 1.

(13.) In some cases it is more con-
venient to employ a pair of similar
balls, suspended from a brass ball fixed
to the end of a glass handle, by very fine
silver wires, or by hempen threads, pre-
viously steeped in a solution of salt, and
afterwards dried. See fig. 2.

Fig 2.

(14.) C avallo has contrived an elec-
troscope of the same kind, which has
the advantage of being more portable
than that of Haiiy, while it is, perhaps,
equally sensible. It is formed by two
fine silver wires, each carrying at one
of their ends a little ball made of cork,
or of the pith of the elder tree ; the
other ends of the wires being suspended
from a cork, which is rather long, and
tapering at both ends, so as to fit either
way into the mouth of a varnished glass
tube, serving both as a handle to the
instrument when in use, and as a case
for it when carried in the pocket. When
it is to be employed as an electroscope,
the wires with pith balls are placed so
as to hang out from the end of the
tube, and will indicate by their diver-
gence any electricity which may be
communicated to them. (Fig. 4.) When

Fig. 3.

Fig. 4.

and supported horizontally by a cap at
its centre, on a fine point. The attractive
or repulsive power of any electrified
body presented to one of the balls, will
be indicated by the movements of the
needle.

the instrument is not in use, the wires
are put into the tube, by inverting it,
and closing it with the other end of the
cork. (Fig. 3.)

(15.) For studying the circumstances
B 2

ELECTRICITY.

attending electrical attraction, we should
be provided with stands, from the ends
of which are suspended by their respec-

Fig5.

tive threads one or two pith balls, about
the size of a small pea, as shown in

§ 2. Distribution and Transference.

(16.) If an excited electric be brought
near a pith ball suspended by silk, the
ball will, in the first place, approach the
electric (fig. 6.), indicating an attrac-
tion towards it, and if the position of
the electric will allow, the ball will come
into contact with it and adhere to it for
a short time ; but it will presently after-
wards recede from the electric, showing
that it is now repelled, (fig. 7.) If we

Fig. 6.

Fig. 7.

now remove the electric, and present to
the ball which has thus touched it, a
second ball which has had no previous
communication with any electric, we
find that these two balls attract one
another, and come into contact. The
same actions are repeated between this
second ball and a third, which may be
presented to it ; and so on in succession,
but with a continued diminution of in-
tensity. This diminution plainly indi-
cates a diminished power, in conse-
quence, as it would seem, of its being
distributed among a number of bodies.
(17.) In the prosecution of these
experiments, therefore, the effects will
be more distinct, if, instead of small
pith balls, we employ a globe of metal
of larger size, which will allow of the
reception of a considerable quantity of

this electric influence by contact with
the excited electric. A globe, suspended
by silk threads, as the pith balls are,
and which has extensively touched the
electric, will act upon these balls pre-
cisely in the same way as the original
electric would have acted upon them,
and may accordingly be substituted for
it in all these experiments. It is, indeed,
exactly in the same condition as the
globe rubbed by an electric, already
mentioned. ($ 11.)

(18.) From the whole of these facts
we necessarily infer that the electric has
imparted to the ball or globe which
came in contact with it, properties ex-
actly similar to those which had been
excited in itself by friction. By repeated
contact with a number of bodies, an
excited electric is found to lose its elec-
trical powers in the same degree as
these powers have been acquired by the
bodies themselves ; and fresh excitation
alone can renew them. It is evident,
then, ,that the unknown agent, which
we have termed Electricity, is capable
of transference, in the same sense in
which we speak of heat being communi-
cated or transferred from one body to
another, and that, like heat, it is weak-
ened by diffusion among a number of
bodies.

(19.) If the electrified ball be touched
with the finger, it will be deprived of
the whole of its electricity, which will
pass into the body of the person who
touches it. It is now reduced to its
original or natural state, and is again
susceptible of being attracted, either by
an excited electric, or by another body
to which electricity has previously been
communicated.

If the electrified body, instead of
being touched with the finger, had been
touched by a rod of metal held in the
hand, the effect would have been the
same in both cases : hence we may
infer that the metallic rod is capable
of conveying away from the body the
whole of its electricity. But if a glass
rod be substituted, the result is very
different ; the body touched is found to
retain the whole of its electricity, not-
withstanding the contact of the glass
rod. We are thus led to the conclusion
that some substances, such as glass,
are incapable of conducting electricity ;
while others, such as the metals and
the human body, readily convey that
influence.

(20.) It is invariably found that all
electrics are, at the same time, non-con-

ELECTRICITY.

ductors. Conductors, on the other
hand, are non-electrics. The two qua-
lities of a capability of excitation, and a
power of conducting electricity, appear
to be incompatible with each other—
for the one is always found to diminish
in proportion as the other increases.
The permanence of electricity in metallic
bodies, which are suspended in the
air by silk threads, shows that the air
as well as the silk is a non-conductor.
Bodies which are in this way surrounded
on all sides by non-conductors are said
to be insulated. When this condition
is not observed, that is, when the body
is in contact with conducting bodies
which communicate with the earth, its
electricity will escape by the channel
which is thus opened for it, and will be
lost by diffusion in the mass of the
earth, which is formed of conducting
materials, and which may be regarded as
the great reservoir both for the absorp-
tion and supply of electricity. Hence
we see why it is not possible to accu-
mulate electricity in a conducting body
while it is held in the hand, and why
electrics alone are capable of permanent
excitation.

(21.) The insulating power of atmos-
pheric air depends principally upon
two circumstances, its density and its
diyness. Air with the density which it
has under the ordinary pressure of the
atmosphere, if perfectly dry, is a remark-
ably good insulator, even although it
be rapidly renewed on the surface of
the electrified body. This is shown by
an experiment of Franklin's, in which
he whirled an electrified ball round his
head, by means of a silk line, with great
rapidity, so as to make it perform many
hundred revolutions, without being
able to perceive that it had thereby lost
any sensible portion of its electricity.
Neither an increase, nor a diminution
of temperature, appears to lessen its in-
sulating power. But in proportion as
the air is rarefied by the removal of the
superincumbent pressure, its power of
confining electricity diminishes, till, at
last, when the rarefaction is very great,
it opposes scarcely any resistance to the
passage even of very feeble electricity ;
and it may be then classed among con-
ductors. 'This is the case with the im-
perfect vacuum produced by the air-
pump, from which it is almost impos-
sible to exclude minute quantities of
air. Even in the space left, in the upper
part of the tube of a barometer by the
descent of the mercury, or the Torri-

cellian vacuum, as it is called, there is
in general present a minute portion of
air, as well as of mercurial vapour,
which are sufficient to conduct electri-
city. Small globules of air usually
adhere to the mercury, and to the sides
of the tube ; and these, upon the re-
moval of pressure, expand into an
atmosphere. It has been asserted by
Mr. Morgan that if great care be taken
to remove every source of error, by the
employment of very pure mercury, and
by boiling it for a long time in the tube,
a perfect vacuum may be obtained,
which does not conduct electricity. His
experiment, however, was made long
ago, and requires careful repetition,
before the result can be confided in.

(22.) The circumstance which chiefly
determines the conducting power of
common air is that of its containing a
greater or less quantity of moisture.
Water is a very good conductor of elec-
tricity ; and that portion which is sus-
pended in the air tends powerfully to
carry off electricity from the bodies
which are charged with it, and which
are surrounded by air. Moisture also
easily attaches itself to glass and other
electrics, and deprives them of the
power of insulation. Hence, the same
experiments which succeed in a clear
dry day, will often fail when tried in
damp weather : and hence we see the
utility of previously drying every part
of the apparatus, in order to exclude as
much as possible the interference of
moisture.

(23.) The conducting powers of most
bodies are influenced by changes of tem-
perature, and also of form. Thus, al-
though water in its liquid state is a good
conductor, yet, when congealed, in the
form of ice, its conducting powers are
much impaired ; and at a very low tem-
perature, namely, at — 13°of Fahrenheit's
scale, ceases altogether. Mr. Achard,
who observed this fact, formed ice of
this temperature into a spheroid, and
mounting it upon an axis, was able to
excite it by friction as any other electric.
On the other hand, by raising the tem-
perature of water, its conducting powers
are increased. Charcoal is also found
to transmit electricity with more facility
when hot, than when cold. Glass, which
is a non-conductor when cold, becomes
a tolerably good conductor when heated
to redness ; and a similar change takes
place in sulphur and in resinous bodies
when melted ; and also in baked wood
when heated. Reducing substances to

ELECTRICITY.

powder has often an effect upon their
powers of conducting electricity. Snow
conducts less readily than ice of the
same temperature. The same is the
case with powdered charcoal, when
compared with the same substance in
its entire state. But glass, on the con-
trary, acquires some conducting power
by being pulverized, as was ascertained
by Van Swinden, who extended the same
observation to sulphur.

Many bodies, which, in their usual
state, are good conductors of electricity,
lose this power when they are made
very dry. This is the case with recent
vegetable and animal substances, their
conducting power appearing to be de-
rived solely from the fluids they con-
tain.

(24.) Strictly speaking, there is no
substance hitherto known that is per-
fectly impervious to electricity ; for the
intensity of that agent may be so in-
creased as to force it, for a certain small
distance, through all bodies : neither is
there any body in which the conducting
power is infinitely great ; that is, which
opposes no resistance to the transmission
of electricity. If the degree of conduct-
ing power which bodies possess could
be ascertained with sufficient precision,
they might be arranged in progressive
order ; but the present state of our
knowledge affords only an approxima-
tion to such a series. As a table of
this kind, however, with all its imper-
fections, may be of great use, we sub-
join the following, in which the different
bodies are arranged in one series, begin-
ning with those which have the greatest
conducting power, and terminating with
those that have the least. The order in
which they possess the power of insu-
lating is, of course, the reverse of this.

Catalogue of Bodies in the Order of their

conducting Power.

The perfect, or least oxidable metals.
The more oxidable metals.
Charcoal prepared from the harder

woods, and well burned.
Plumbago.

The concentrated mineral acids.
Powdered charcoal.
Dilute acids.

Solutions of metallic and neutral salts.
Metallic ores.
Animal fluids.
Pure water.

Ice above —13° Fahrenheit,
Snow.
Living vegetables.

Living animals.

Flame.

Smoke.

Steam.

Metallic salts.

Salts with alkaline or earthy bases.

Rarefied air.

Vapour of alcohol.

Vapour of ether.

Earths and stones in their ordinary

state.

Pulverized glass.
.Flowers of sulphur.

Dry metallic oxides.

Oils.

Vegetable ashes.

Animal ashes.

Diy transparent crystals.

Ice below — 13° Fahrenheit.

Phosphorus.

Lime.

Dry chalk.

Native carbonate of barytes.

Lycopodium.

Caoutchouc, or Indian rubber.

Camphor.

Siliceous and argillaceous stones in
proportion to their hardness.

Dry marble.

Porcelain.

Baked wood.

Dry atmospheric air, and other gases.

White sugar, and sugar crystallized.

Leather.

Dry parchment.

Dry paper.

Cotton.

Feathers.

Hair, especially that of a living cat.

Wool. "

Dyed silk.

Bleached silk.

Raw silk.

Transparent gems.

Diamond.

Talc.

Metallic vitrifications.

Glass, and other vitrifications.

Fat.

Wax.

Sulphur.

Resins, and bituminous substances.

Amber.

Gum-lac.

Although the precise point in the
scale which forms the separation be-
tween conducting and insulating bodies
must, of course, be somewhat indefinite,
we have endeavoured to mark it by the
division in the above table.

(25.) It appears, from the experiments

.ELECTRICITY.

of Mr. Coulomb, that a thread of gum-
lac is the most perfect of all insulators,
and is ten times more effec-tual than a
silk thread as dry as it can he made ;
"for the former, when only one inch and
a halt' in length, insulated as well as a
fine silk thread of fifteen inches. When
the thread of silk was dipped in fine
sealing-wax, it was equal in power to a
thread of pure lac of six inches, that is,
of four times its length. Professor Ro-
bison found that the conducting power
of silk thread depends greatly on its
colour ; or, in other words, on the nature
of the drug with which it is dyed. When
of a brilliant white, or a black, its con-
ducting power is the greatest ; and a high
golden yellow, or a nut brown, renders
it the best insulator. Glass, even in its
dryest state, and in situations where it
was impossible that moisture could have
access to it, is stated by the same author
to insulate considerably better than silk ;
and when drawn into a slender thread,
and coated with gum-lac, it acted as
well as a thread of lac of one-third of
the length. It was found, howrever, at
the same time, that extreme fineness
was requisite ; for it dissipated in pro-
portion to the square of its diameter.
The insulating power of glass is remark-
ably injured by having a bore, however
fine, unless that bore admits of being
also coated with lac. Human hair,
when completely freed from every thing
that water could wash out of it, and
then dried by lime, and coated with lac,
was equal to silk. Fir, cedar, larch,
and the rose-tree, when split into fila-
ments, and first dried by lime, and after-
wards baked in an oven, which just
made paper become faintly brown,
seemed scarcely inferior to gum-lac.
The white woods, as they are called, and
mahogany, were much inferior. Fir,
baked and coated with melted lac, seems,
therefore, the best support when strength
is required. The lac may be rendered
less brittle by a minute portion of pure
turpentine, which has been cleared of
water by a little boiling, without sensi-
bly increasing its conducting power.
Lac, or sealing-wax, dissolved in spirits,
is far interior, for these purposes, to
what it is when melted by heat.

(26.) The laws which regulate the
gradual dissipation of electricity from
bodies in a state of imperfect insulation
have been investigated with great ability
by Coulomb. Three causes chiefiy
operate in depriving a body under these
circumstances of its electricity :— First,

the imperfection of the insulating pro-
perty in the solids by which it is sup-
ported. Secondly, the contact of suc-
cessive portions of air, every particle of
which carries off a certain quantity of
electricity. Thirdly, the deposition of
moisture upon the surface of.the insu-
lating body, which establishes commu-
nications with its remote ends, and may-
be considered as virtually increasing its
conducting power.

(27.) With regard to the first cause,
Mr. Coulomb has completely ascertained
that for all fine cylindrical fibres, such
as hair, silk, filaments of gum-lac, &c.
if the nature of the substance, the dia-
meter of the fibre, and the dispersive
state of the air are supposed constant,
the length of the fibre requisite for the
complete insulation of a given intensity
of electricity, varies as the square of
that intensity. Theory, therefore, leads
to the conclusion that, however great
may be the intensity, there is always a
certain length beyond which a filament
of any of these bodies becomes a perfect
insulator ; and we find, in practice, that
by diminishing the intensity of the elec-
tricity, or increasing the length of the
substance it has to traverse, a sufficiently
accurate degree of insulation may be
obtained. With respect to the second
source of dissipation, it was found that
in a given state of the atmosphere, as
far as it could be determined by the
indications of the barometer, thermo-
meter, and hygrometer, the dissipation
at each instant of time, varied directly
as the intensity of the electricity.

(28.) There is one very material cir-
cumstance relating to the dissipation of
electricity that should here be men-
tioned, although its explanation must
be deferred till the principles on which
it depends have been developed ; and
it is, that the power of retaining elec-
tricity in any body is much influenced
by its shape. The form most favourable
to its retention is that of a sphere ; next
to which is a spheroid, and a cylinder
terminated at both ends by a hemi-
sphere. On the other hand, electricity
escapes most readily from bodies of a
pointed figure, especially if the point
projects to a distance from the surface.
In such bodies it is scarcely possible,
indeed, to accumulate any sensible de-
gree of electricity, on account of its
rapid dissipation from the point. In
like manner pointed bodies receive elec-
tricity more readily than those of any
other form.

ELECTRICITY.

§ 3. Of the two species of Electricity.

(29.) We have hitherto viewed elec-
trical phenomena as arising from the
operation of a single agent, which could
be called into action, and transferred
from one body to another. We have
seen that bodies, which have received
their electricity from excited glass, repel
one another, and are likewise repelled
by the excited glass. The same thing
happens with respect to those bodies
\vhich have received their electricity
from excited sealing-wax. But upon
examining the action of any of the bo-
dies belonging to the one set, upon any
of those belonging to the other, we find,
that instead of repelling, they attract
each other. Thus, the ball which has
received its electricity from the glass,
attracts that which has been electrified
by the sealing-wax, and is attracted by
it ; but, what is still more remarkable,
the moment these balls have come into
contact, provided they have both been
electrified in the same degree, they
cease at once to exhibit any signs of
electricity, as if the electricities of both
were suddenly annihilated by their
mutual communication. Thus there
appears to be two different, and, in
some respects, opposite kinds of elec-
tricities ; the one obtained from glass,
the other from sealing-wax. Du Fay,
by whom this distinction was first no-
ticed, denominated the former the vi-
treous, and the latter the resinous elec-
tricity.

(30.) The mode of action which these
two electricities exert on matter, may
be expressed by the following law :
namely, that bodies charged with either
species of electricity, repel bodies charged
with the same species, but attract bodies
charged with the other species; and that,
at equal distances, the attractive power
in the one case is exactly equal to the
repulsive power in the other.

Accordingly, if we wish to ascer-
tain what is the species of electricity
with which a given body is charged,
we have only to approach it to a
small insulated pith ball, which has
previously been touched either with
excited glass or with excited sealing-
wax. If the body in question repel it
in the former case, or attract it in the
latter, its electricity is vitreous ; if the
contrary happens, it is resinous.

(31.) Although each of these two elec-
tricities, when taken separately, acts in
a manner precisely similar to the other,

they nevertheless exhibit in all their rela-
tions to each other a marked contrariety
of nature. Hence they are naturally
viewed as agents having opposite quali-
ties, which completely neutralize one
another by combination.

(32.) Another remarkable circum-
stance which characterizes these agents,
is, that the excitation of one species of
electricity is always accompanied by the
excitation of the other ; and both are
produced in equal degrees. Thus, when
glass is rubbed by silk, or flannel, just
as much resinous electricity is produced
in the silk, or flannel, as there is vitreous
electricity produced in the glass; and
whatever electrified bodies are repelled
by the one are attracted in the same
degree by the other. If one of the sub-
stances happen to be a conductor, and
be held in the hand, the whole of the
electricity which the friction excites in
it will disappear as soon as it is pro-
duced, from its escaping through the
body of the person holding it, and being
lost in the earth. But if the precaution
be taken of insulating the rubber, its
electricity will become manifest, and is
always found to be of the opposite spe-
cies to that which is excited in the body
which is rubbed.

(33.) Since the two surfaces rubbed
acquire opposite electricities, it follows
as a consequence of the law above
stated, that they must attract one ano-
ther ; and this is found invariably to be
the case. If a white and a black ribbon
of two or three feet long, and perfectly
dry, be applied to each other by their
flat surfaces, and are then drawn re-
peatedly between the finger and thumb,
so as to rub against each other, they
will be found to adhere together, and if
pulled asunder at one end, will rush
together with great quickness. While
united they exhibit no sign of electricity,
because the operation of the one is just
the reverse of that of the other, and
their power is neutralized and inopera-
tive. If completely separated, how-
ever, each will manifest a strong elec-
trical power, the one attracting those
bodies which the other repels.

(34.) The very act of separation is
accompanied by appearances which in-
dicate that considerable portions of the
electricities excited on each of the sur-
faces fly back to the opposite surface,
and by their union become as it were
extinguished or inoperative ; and it is
only the remaining quantities which
have adhered more tenaciously to the

ELECTRICITY.

surfaces, that retain their activity. When
the experiment is made in the dark,
flashes of light attend these sudden ex-
changes of electricity, passing between
the two surfaces, and accompanied with
a rustling noise.

(35.) Numberless experiments have
been made with a view of ascertaining
the conditions that determine the species
of electricity excited in the respective
bodies of which the surfaces are made
to rub against each other, but they have
led to no satisfactory conclusion. The
mechanical configuration of the surface
appears to have a greater influence in
the result than the peculiar nature of
the substance itself. If a plate of glass
with a polished surface be rubbed
against one which is roughened, the
former always acquires the vitreous,
and the latter the resinous electricity.
No approach to an explanation of this
peculiarity has ever been made. Smooth
glass acquires vitreous electricity by
friction with almost every substance,
except the back of a cat, which gives it
the resinous electricity ; but roughened
glass, if rubbed with the same sub-
stances, becomes charged with resinous
electricity, while the rubbing bodies ac-
quire the vitreous. Sealing-wax, rubbed
with an iron chain, acquires, if polished,
the resinous electricity ; but if its sur-
face is previously rough with scratches,
the vitreous. Silk, rubbed by resin,
takes the resinous, but with polished
glass, the vitreous electricity. The fol-
lowing is a list of several substances
which acquire vitreous electricity when
rubbed with any of those which follow
it in the order in which they are set
down ; and resinous electricity if rubbed
with any of those which precede : —

The back of a cat.

Polished glass.

Woollen cloth.

Feathers.

Wood.

Paper.

Silk.

Gum-lac.

Roughened glass.

In the experiment just mentioned, in
which a black and a white ribbon are
rubbed together, the former is found to
be resinously and the latter vitreously
electrified. But if two pieces of the
same ribbon of the same length be
rubbed, the one being drawn lengthwise
and at right angles over a part of the

other, the one which has suffered fric-
tion in its whole length acquires vitreous,
and the other resinous electricity. In
like manner, when the whole length of
the bow of a violin is drawn over a
limited part of the string, the hairs of
the bow exhibit a vitreous, and the
string a resinous electricity, the body
whose excited portion is of the least
extent being generally found to be re-
sinously electrified. But in truth, the
slightest difference in the conditions of
these and similar experiments on the
species of electricity arising from fric-
tion, will be often sufficient to produce
opposite results.

(36.) Electrical excitation may also
be produced by the friction of liquids
or of gases against solid bodies. This
is the case when mercury is made to
fall, in a fine shower, under the ex-
hausted receiver of an air-pump, against
the glass. If a current of atmospheric
air be directed against a pane of glass,
by means of a pair of bellows, the glass
becomes vitreously electrified.

§ 4. Induction.

(37.) Another class of electrical phe-
nomena must here be noticed. When-
ever a body is charged with electricity,
although it be perfectly insulated, and
of course all escape of that electricity
prevented, it tends to produce an elec-
trical state of the opposite kind in all the
bodies in its vicinity. Thus the vitreous
electricity tends to "induce the resinous
electricity in a body that is situated
near it ; and this with greater energy,
as the distance is smaller. This effect
is termed the induction of electricity,
and may be ranked among the general
facts, or laws of the science. The fur-
ther development of the consequences
it leads to, must, for the present, be post-
poned, as we shall hereafter be better
prepared to understand them. But there
is one of its results which we shall now
point out, as it refers immediately to
the phenomena that have already occu-
pied our attention.

(38.) If an electrified body, charged
with either species of electricity, be pre-
sented to an unelectrified or neutral
body, its tendency, in consequence of
the law of induction, is to disturb the
electrical condition of the different parts
of the neutral body. The electrified body
induces a state of electricity contrary
to its own in that part of the neutral
body which is nearest to it ; and conse-
quently a state of electricity similar to its

.ELECTRICITY.

own in the remote part. Hence the neu-
trality of the second body is destroyed by
the action of the first ; and the adjacent
parts of the two bodies, having now
opposite electricities, will attract each
other. It thus appears, that the attrac-
tion which is observed to take place
between electrified bodies and those
that are unelectrified, is merely a con-
sequence of the altered state of those
bodies, resulting directly from the law
of induction ; and that it is by no
means itself an original law, or primary
fact in the science.

(39.) The effects of induction will
be in proportion to the facility with
which changes in the distribution of
electricity among the different parts of
a body can be effected, a facility which
corresponds with the conducting power
of the body. Hence the attraction ex-
erted by an electrified body upon ano-
ther body previously neutral, will be
much more energetic if the latter be a
conductor, than if it be an electric, in
which these changes can take place
only to a very small extent. This is
confirmed by the following experi-
ment : suspend by fine silk threads of
equal length, two small balls of equal
dimensions, both made of gum-lac,
but one having its surface covered
with gold leaf. Place these two pen-
dulums, as they may be called, at a
little distance from one another, so as
to admit of a comparison of their mo-
tions ; and then present to them an
excited electric, which may be either a
tube of glass, or a cylinder of sealing-
wax. It will at once be seen that the
ball, with a metallic covering, which
readily admits of the transfer of elec-
tricity from one side to the other, will
be much more readily and powerfully
attracted, than the other ball which
allows of no motion in its electricity.
The latter ball will, by slow degrees,
however, assume electrical states of the
same kind as the gilt ball, and will be
feebly attracted. As this change is very
slowly effected, so it is more permanent
when once produced ; and the plain
ball adheres for a considerable time to
the electric which has attracted it. The
gilt ball, on the contrary, is sooner re-
pelled, by its readily receiving the
charge of electricity imparted to it by
the electric. A degree of permanent
electricity, however, is also induced on
this ball, in consequence of its gradual
penetration into the substance of the
gum-lac.

CHAPTER II.

Theories of Electricity.

(40.) It is impossible to arrive at the
full comprehension of the multifarious
facts relating to any of the physical
sciences without the aid of some lead-
ing principles, or modes of viewing
them, by which their connexions can
be represented to the mind, so as to
combine them into an intelligible system.
We begin by classing the different
agents in nature, designating them by
specific names ; we next endeavour to
conceive these agents as possessed of
certain powrers or qualities adapted to
the production of the observed effects.
In the case of light, for example, we
may conceive the phenomena to result
from the action of material particles,
emanating in all directions from the lu-
minous body, and obeying certain laws
in their course ; or we may adopt ano-
ther hypothesis, namely, that they pro-
ceed from the undulations of an elastic
medium pervading space. By employ-
ing either the one^or the other of these
hypotheses, we acquire great facility in
tracing the connexions of the pheno-
mena of optics, and retaining them in
our minds. This advantage is not im-
mediately dependent on the truth of the
particular hypothesis we employ for
that purpose : for, in the example before
us, it is evident they cannot both be
true, and yet they both answer this
end. But, of course, the utility of an
hypothesis will be proportionate to the
degree of exactness with which it ac-
cords with the phenomena. No incon-
venience can arise from its adoption, as
long as we bear in mind that our rea-
sonings are founded on a mere hypo-
thesis, and as long as we hold our-
selves in readiness'to abandon it, the
moment we meet with facts with which
it is decidedly inconsistent.

(41.) The hypothesis .which naturally
suggests itself for the explanation of
electrical phenomena is that of a very
subtile and highly elastic fluid, pervad-
ing the earth and all other material
bodies, but itself devoid of any sensible
gravity. We must suppose this fluid
to be capable of moving, with various
degrees of facility, through the pores, or
actual substance of different kinds of
matter. In some, as in those we call
conductors, or non- electrics, such as
the metals, it moves without any per-
ceivable obstruction : but, in glass,
resin, and, in general, in all bodies

ELECTRICITY.

n

called electrics, or non-conductors, it
moves with great difficulty. Moreover,
as the phenomena appear to point out
the existence of two distinct kinds of
agencies, we may further assume that
there are two distinct species of electric
fluid, which we shall, for the present,
name the vitreous and the resinous
('li'i'tricith's. They must each have,
when separate, the same general pro-
perties as have already heen enume-
rated ; but, in relation to each other,
there must be a complete contrariety in
their natures, so that when combined
together, their actions on the bodies in
their vicinity, or on the particles of
electric fluid contained in those bodies,
are exactly balanced ; and all visible
action ceases. It is in this state of
union, in which they perfectly neu-
tralize one another, that they exist in
bodies which may be said to be in their
natural state with regard to electricity.

(42). Thus, then, may the problem
be solved, in which it is required
to conceive an agent, analogous, in
many respects, to other known agents,
and to assign to it such properties as
will, in their results, correspond to all
the observed phenomena. In order to
apply to it this latter test, we must
trace all the consequences which flow
from the suppositions wre have made,
and strictly compare them with the facts
both as presented to us by nature, and
as resulting from experiment. These
facts, it will be recollected, are redu-
cible to those of excitation, attraction,
and repulsion, distribution, induction,
and transference.

(43.) Excitation. From various
causes, of which the friction of sur-
faces is one, the state of union in
which the two electricities naturally
exist in bodies, is disturbed; their
latent powers are called forth by their
separation; the vitreous electricity is
impelled in one direction, while the resin-
ous is transferred to the opposite side ;
and each can now manifest its pecu-
liar energies. When accumulated in any
body, or part of a body, each fluid acts
in proportion to its relative quantity,
that is, to the quantity which is in excess
above that which is still retained in a
state of inactivity by its union with
electricity of the opposite kind. Thus
when glass is rubbed with a metallic
amalgam, a portion only of the elec-
tricities at the two surfaces is decom-
posed ; the vitreous electricity resulting
from this decomposition attaches itself

to the glass ; the resinous, to the amal-
gam. What remains in each surface
undecomposed continues to be quite
inert, and has no other influence on the
phenomena, than being ready, on the
continuance of the decomposing action,
to furnish a fresh supply of both fluids
to the bodies in the vicinity.

(44.) Distribution. Each of these
fluids, being highly elastic, their par-
ticles repel one another with a force
which increases in proportion as their
distance is less : and this force acts at
all distances, and is not impeded by the
interposition of bodies of any kind, pro-
vided they are not themselves in an
active electrical state. From the most
careful analysis of the phenomena, it
has been deduced that the exact law of
this force is the same as that of gravi-
tation, namely, that its intensity is in-
versely as the square of the distance.

The mode in which the electricity
imparted to a conducting body, or to a
system of conductors, is distributed
among its different parts, is in exact
conformity to the results of this law, as
deduced by mathematical investigation.
But we reserve the examination of this
subject for a future chapter.

While the particles of each fluid repel
those of the same kind, they exert an
equally strong attraction for the particles
of the other species of electric fluid. This
attraction, in like manner, increases with
a diminution of distance, and follows
the same law as to its intensity, namely,
that of the inverse ratio of the square
of the distance. This force, also, is not
affected by the presence of any interven-
ing body.

(45.) Transference. Since the two
electricities have this powerful attrac-
tion for each other, they would always
flow towards each other and coalesce,
were it not for the obstacles that are
opposed to their motion by the non-con-
ducting properties of electrics. When
these obstacles are overcome, and a free
channel is open for the passage of the
electricities, they rush into union
with great force and velocity, producing,
in their transit and confluence, seve-
ral remarkable effects. After their
coalescence, their power seems to be
at once annihilated, or, more properly
speaking, it remains dormant, until call-
ed into play by the renewed separation
of the fluids.

(46.) Attraction and Repulsion. The
repulsion which is observed to take
place between bodies that are insulated

12

ELECTRICITY.

and charged with any one species of
electricity, for other bodies similarly
charged, is derived from the repulsive
power which the particles of this fluid
exert towards those of their own species.
Let us suppose a body charged with
electricity to be suspended in the air, or
otherwise surrounded by a non-con-
ducting medium, which allows it to
move freely. As long as this body re-
mains alone, the outward pressure
which the electric fluid exerts against
the insulating medium that confines it,
will, by the laws of hydrostatics, be
equal on all sides ; and the body, thus
balanced by equal and opposite pres-
sures, will nave no tendency to move.
But if another body, similarly circum-
stanced, be brought near it, the repul-
sive action between the similar electri-
cities contained in these bodies, will
diminish the outward pressures of each
fluid against the sides of the bodies,
(6, c,fig. 8.) which are adjacent to each

other ; and it will, at the same time,
increase the outward pressure on the
opposite or remoter sides (a, d.) Both
these causes conspire to destroy the
equilibrium ; each body is impelled in
the direction of the preponderating
force, that is, in a direction from the
other body ; and an effect, which may
be called repulsion, takes place. The
very same explanation, it is evident,
applies to both kinds of electricity, their
properties being in this respect exactly
alike.

If, on the other hand, a body charged
with vitreous electricity be presented to
one that is charged with resinous electri-
city, the attraction of these two fluids will
diminish the outward pressure on the
remote sides oi the bodies, and increase
it on the adjacent sides ; hence, the
bodies will be urged towards each other,
and motions indicative of attraction will
result. Thus, in all cases, do the move-
ments of the bodies represent the forces
themselves which actuate the particles
of the developed electricities they con-
tain.

(47.) Induction. The law of induc-
tion is a direct consequence of the hypo-
thesis we are considering. Wherever
pne of the electricities exists in an active

state, it must repel the particles of the
same electricity in all surrounding bo-
dies, and attract those of the opposite
species : or, in other words, it tends to
decompose their united electricities, ac-
cumulating the electricity of the oppo-
site species towards the nearest side,
and impelling that of the same species
towards the remote side. The body
thus acted upon is no longer neutral,
although it contains, on the whole, its
natural quantities of both electricities ;
but, in consequence of their partial dis-
tribution, electrical appearances will be
exhibited in its different parts. The
further prosecution of this branch of
the subject must also be postponed to a
subsequent chapter, our present object
being merely to point out, in a general
way, the coincidence of the fundamental
facts with the proposed theory.

(48.) Thus far we have proceeded
upon the hypothesis of there being two
distinct electric fluids, having certain
properties in common, but each being
characterized by a certain modification
of these properties. It is, however,
equally possible to account for ah1 the
phenomena with the same exactness, on
the supposition of their resulting from
the agency of a single electric fluid.
This simplification of the theory may be
considered as the discovery of the im-
mortal Franklin, although it had oc-
curred at the same period to Dr. Wat-
son ; for it was Franklin who first
pointed out the mode in which it might
be successfully applied to explain some
of the most remarkable phenomena of
the science. Several particular points
in his theory, as he originally proposed
it, were defective, and were found on
strict examination to be at variance with
ascertained facts. It is to ^Epinus and
to Cavendish that we owe the rectifica-
tion of these errors : and the theory of
Franklin, as thus amended, whatever
alterations the future progress of disco-
very may oblige us to make in it, will
ever remain one of the most beautiful
specimens of this kind of reasoning
which philosophy has produced. Of
this hypothesis we shall now present a
brief outline ; and point out the mode
in which it explains the phenomena.

(49.) We set out, then, with sup-
posing that there exists in all bodies a
subtile fluid, which we shall call the
electric fluid ; — that its particles repel
one another with a force varying in-
versely as the square of the distance ; —
that they attract the particles of all

ELECTRICITY.

13

other matter, or some specific ingredient
in that matter, with a force following the
same law of the inverse square of the
distance ; — that this fluid is dispersed
through the pores of bodies, and from
some unknown peculiarity, can move
through them with various degrees of
facility, according as they are conduc-
tors or non-conductors. Bodies are
said to be in their natural state with
regard to electricity, when the repulsion
of the fluid they contain for a particle
of fluid at a distance is exactly balanced
by the attraction of the matter in the
body for the same particle. In this
state they may be considered as satu-
rated with the electric fluid. Whenever
they contain a quantity of fluid greater
than this, they are said to be positively
electrified, or to have positive electricity.
When, on the other hand, there is a
quantity less than that required for satu-
ration, the body is said to be negatively
electrified, or to' have negative electricity.
In the former case, it is the fluid that is
redundant, or in excess ; in the latter,
it is the matter which is left unsaturated
that should be considered as the redun-
dant principle. The state of positive
electricity, then, consists in a redun-
dance of fluid, or in matter that is over-
saturated, as it has been termed ; that
of negative electricity, in a deficiency of
fluid, or in matter under-saturated, or,
what is an equivalent expression, in
redundant matter. In mathematical
language, the former condition may be
expressed by the sign plus ; the latter
by that of minus. In considering the
mutual electrical actions of bodies, the
portions in which the matter and the
fluid mutually saturate each other, need
not be taken into account, since their
actions, as we have seen, are perfectly
neutralized : and we need only attend to
those of the redundant fluid and the
redundant matter.

(50.) When a body contains more
than its natural proportion of electric
fluid, the surplus will, by the repulsive
tendency of its particles, overflow and
escape, if such escape be allowed, until
the body is reduced to its neutral state.
When under- saturated, the redundant
matter will attract fluid from all quar-
ters from which it can receive it, until it
is again brought to its neutral state.
This efflux, or influx, is prevented either
when the body is surrounded on all
sides by substances, through the pores
of which the fluid cannot pass, or when
the body itselt' is of that nature.

(51.) The mutual recession of two
positively electrified bodies is a direct
consequence of the repulsion of the re-
dundant fluids contained in each, which,
being attached to the matter by their
attraction for it, impel it in the direction
of their own repulsion. In the same
way the mutual approximation of two
bodies in opposite electrical states is the
immediate effect of the attraction of the
redundant fluid in the one, for the redun-
dant matter in the other ; and vice
versa, for this attraction is mutual.

(52.) A difficulty does, indeed, occur
when we attempt to apply the theory to
the case of two bodies which are both in
a state of negative electricity, that is, in
which there exists in boih certain quan-
tities of matter unsaturated with electric
fluid. What action does the theory, as
hitherto stated, point out as the result in
this particular case ? Plainly none. All
those portions of the matter of each
body which are still saturated, together
with the fluid which saturates them,
can have, as we have already seen, no
effect either of attraction or repulsion.
The only active element is the unsatu-
rated matter ; but the hypothesis does
not assign any action of this matter
upon other matter at a distance. Yet
we learn from experience that the bodies,
under these circumstances, actually
repel one another. In order, therefore,
to render the hypothesis conformable to
fact, we are obliged to annex to it ano-
ther condition ; namely, that the parti-
cles of simple matter, that is, of matter
uncombined with the electric fluid, exert
a repulsive action on one another. It
is singular that so acute a mind as that
of Franklin should not have discerned
this defect in his own theory, or perceived
that this further condition was abso-
lutely requisite for the explanation of
the phenomena. Without it, indeed, we
should be unable to explain the want of
action between two neutral bodies ; for
the repulsion of the fluids in both bo-
dies being balanced by the attraction of
the fluid in the one for the matter in the
other, the remaining attraction of the
fluid in the second body for the matter
in the first would be uncompensated by
any repulsion, and the forces would not
be held in equilibrium, as we find they
really are.

(53.) The law of electrical induction
is an immediate consequence of the
Franklinian theory. When a body
charged with electricity is presented to
a neutral body, the redundant fluid of

14

ELECTRICITY.

the former exerts a repulsive action on
the fluid in the latter body ; and if this
happens to be a conductor, it impels a
.certain portion of that fluid to the remote
end of this body, which becomes at that
part positively electrified ; while its
nearer end, which the same fluid has
quitted, is consequently in the state of
negative electricity. If the first body
'had been negatively electrified, its unsa-
turated matter would have exerted an
'attractive force on the fluid in the
second body, and would have drawn it
nearer to itself, producing an accumu-
lation or redundance of fluid at the
adjacent end, and a corresponding defi-
ciency at the remote end: that is, the
former would have been rendered posi-
tive, and the latter negative. All this
is exactly conformable to observation.

(54.) The phenomena of transference
are easily explicable on this hypothesis ;
and they arise from the destruction of
the equilibrium of forces, which con-
fined the fluid to a particular situation
or mode of distribution.

(55.) There is, indeed, no fact ex-
plicable by the hypothesis of a double
fluid, which is not explained with
equal facility by that of a single
fluid, with the condition already stated.
The explanation by the first is easily
converted into an explanation by the
second, by substituting the expressions
of positive and negative for those of
vitreous and resinous electricities ; and
considering the action of the latter as
arising from the influence of redundant
or unsaturated matter, to which is
ascribed in the Franklinian hypothesis
a similar operation to that of the resi-
nous electricity in the hypothesis of Du
Fay. The hypothesis of a single fluid
has, it must be allowed, the advantage
of greater simplicity : but, on the other
hand, it lies open to the objection of its
involving a condition which appears, at
first view, to be at variance with our
preconceived notions of the primary
laws of matter, and more especially with
that of gravitation ; namely, that which
implies the mutual repulsion of its par-
ticles when void of electricity.

When viewed as a mere hypothesis
calculated to facilitate our comprehen-
sion of the phenomena and of their con-
nexions, it is a matter of indifference
which we employ, for they will either of
them answer the purpose. In our fu-
ture explanations we shall, in general,
adhere to the language of the Frank-
linian theory, as being the simplest, and

generally the most convenient ; and be-
cause a conversion of terms the reverse of
that just now pointed out will in all cases
enable us to supply the explanation of
the same phenomenon according to the
theory of Du Fay. As to the question
which of these two hypotheses ap-
proaches the nearest to the real state of
things, we are not yet prepared to dis-
cuss the arguments that could enable us
to decide it ; and we must, therefore, wait
till we can resume the subject in the
sequel.

The further development of these
theories, and of the law of induction,
in particular, must, for the present, be
postponed, since they require us to be
acquainted with many practical details
relating to the accumulation of elec-
tricity, and its management when ap-
plied to various objects of experimental
research.

CHAPTER III.
Electrical Machines.

(5G.) THE essential parts of an instru-
ment for procuring large supplies of
electricity for the purposes of experi-
ment, or an electrical machine, as it is
called, are the electric, the rubber, the
prime conductor, the insulator, and the
machinery for setting the electric in
motion.

(57.) The electric, by the excitation
of which the electricity is to be de-
veloped, may be made of various mate-
rials. Globes of sulphur were employed
by the earlier electricians for that pur-
pose ; but polished glass is found, on
the whole, to be the most convenient
substance. The original form given to
it by Hauksbee, who was the inventor
of the electrical machine, was that of a
globe, which he caused to revolve upon
a vertical axis. The most convenient
forms, however, are those of a hollow
cylinder, or of a flat circular plate, revolv-
ing upon a horizontal axis. When used
in the form of a globe or cylinder, it has
sometimes been found advantageous to
line the inside of it with a thin layer of
a resinous composition, consisting of
four parts of Venice turpentine, one of
resin, and one of bees' wax. This must
be introduced in sufficient quantity into
the inside of the globe or cylinder, and,
when the glass is brought gradually to
an equal degree of heat throughout the
melted substances, is allowed to spread
itself over the interior surface, by turn-

ELECTRICITY.

t5

ins; the globe or cylinder about its axis.
The principal use of such a coating is
to improve bad machines, for it is not
required in good ones.

The earlier electricians contented
themselves with using; the hand as a
rubber, till a cushion was introduced
for that purpose by Professor Winkler,
of Leipsic. The cushion is usually
made of soft leather, generally basil
skin, stuffed with hair or wool, so as to
be as hard as the bottom of a chair,
but yet sufficiently yielding to accom-
modate itself, without much pressure,
to the surface of the glass to which it
is applied.

(58.) Of cylindric machines, the sim-
plest and most perfect construction is
that invented by Nairne, and which is
represented in fig. 9. The glass cylinder
C is from 8 to i 6 inches in diameter, and

Ft'g.9.

from one to two feet long, supported, for
the purpose of insulation, on two up-
right pillars of glass, which are fixed to
a firm wooden stand. Two hollow me-
tallic conductors P, N , equal in length to
the cylinders, are placed parallel to it,
one on each side, upon two insulating
pillars of glass, which are cemented into
two separate pieces of wood that slide
across the base so as to allow of their
being brought within different distances
from the cylinder. To one of these
conductors N, the cushion is attached,

being fastened to it by the intervention
of a bent spring, the purpose of which
is to keep it equally pressed against the
cylinder in every part of its revolution.
The pressure of the cushion is also fur-
ther regulated by an adjusting screw
adapted to the wooden base, on which
the glass pillar that supports the con-
ductor is fixed. From the upper edge
of the cushion there proceeds a flap P
of thin oiled silk, which is sewed on the
face of the cushion about a quarter of
an inch from its upper edge. It extends
over the upper surface of the glass
cylinder to within an inch of a row of
metallic points, proceeding like the teeth
of a rake from a horizontal rod, which
is fixed to the adjacent side of the oppo-
site conductor P. The motion of the
cylinder must- always be given in the
direction of the silk flap ; and it may
be communicated either by a single
handle, or by a multiplying wheel W,
as in the figure: the latter produces
more electricity in the same time, but
the labour of turning is increased nearly
in the same proportion. On some ac-
counts it is more convenient to place the
conductor to which the rubber is not
attached, at right angles to the cylinder ;
and this is the plan adopted in the com-
mon electrical machines.

(59.) The conductor P, to which the
rubber is not attached, is generally
called the prime conductor, or the posi-
tive conductor, as the electricity with
which it becomes charged is positive.
It is a cylindrical tube, each end ter-
minating in a hemisphere. There is no
advantage in its being made of solid
materials, for the electricity is contained
only at the surfaces. It may be made
of thin sheet brass, or copper, or tin, or
of pasteboard, covered with gold leaf or
tin foil. Care must be taken that its
surface be free from all points and aspe-
i ities ; and the perforations which are
made in it, and which should be about
the size of a quill, for the purpose of
attaching wires, and other kinds of ap-
paratus, should have their edges well
rounded and smoothed off. For 'the
more perfect insulation of the conduc-
tor, it is advisable to apply upon the
glass pillar which supports it, a varnish
of gum-lac, or of sealing wax.

(60.) The degree of excitation pro-
duced in the glass depends much upon
the substance employed as a rubber.
Mr. Singer observes that dry silk is
very efficacious, but that the most
powerful effects are obtained by the use

16

ELECTRICITY.

of an amalgam of tin, zinc, and mer-
cury, applied by means of hog's lard, to
the surface of leather or oiled silk. That
part of the cushion which comes in con-
tact with the glass cylinder, should be
coated with an amalgam of this kind,
spread evenly over its surface, until
level with the line formed by the seam
which joins the silk flap to the face of
the cushion. No amalgam should be
placed over this seam, nor on the silk
flap ; which last should be wiped clean
whenever the continued motion of the
machine shall have soiled it, by deposit-
ing dust or amalgam on its surface. The
same attention is requisite to the sur-
face of the glass, which often becomes
covered with black spots and lines,
more particularly when the amalgam
has been recently applied. It is essen-
tial to remove these as often as they
are formed in any quantity, since they
tend to lessen the power of the machine.
The surface of the amalgamated cushion
is also soon soiled; for the excited
glass constantly attracts dust from sur-
rounding bodies, and this dust is col-
lected by the rubber as the glass passes
it. If the dust is removed after every
course of experiments, by separating
the cushion from the negative conductor,
and gently rubbing its surface, and the
surface of the silk flap, with a dry
linen cloth, the machine may be kept in
good order without a frequent renewal
of the amalgam ; such renewal being
only necessary when that which has
been applied becomes irregularly dis-
tributed over the cushion, or impreg-
nated with dust.

(61.) The amalgam recommended by
Mr. Singer, is made by melting together
one ounce of tin and two ounces of
zinc, which are to be mixed, while
fluid, with six ounces of mercury, and
agitated in an iron, or thick wooden
box, till cold. It is then to be reduced
to very flne powder in a mortar, and
mixed with a sufficient quantity of hog's
lard to form it into a paste. When amal-
gams have a large proportion of mer-
cury, their action is variable and tran-
sient. The best cement for attaching
the cylinder to its pivots, is made by
mixing five pounds of resin, one pound
of bees' wax, one pound of red ochre,
and two table- spoonfuls of plaster of
Paris. The ochre and plaster of Paris
should be well dried, and then added to,
and alternately mixed with the other
ingredients, when they are in a state of
fusion.

The plate machine, fig. 1 0, was ori-
ginally proposed by Dr. Ingenhouz, and
has been since much improved by Cuth-
bertson. This machine, in its most per-

Fig. 1 0.

feet form, consists of a circular plate of
glass, turning on an axis that passes at
right angles through its centre : it is
rubbed by two pair ^of cushions, fixed at
opposite parts of the circumference by
elastic frames of thin mahogany, which
are constructed so as to press the glass
plate between them with the requisite
force, by means of regulating screws.
A brass conductor P, supported by
glass, is fixed to the frame of the ma-
chine, with its branched extremities
opposite to each other, and near the
extreme diameter of the plate, in a direc-
tion at right angles to the vertical line
of the opposite cushions. The branched
extremities of the conductor are fur-
nished with pointed wires, that serve to
collect the electricity from the surface
of the excited plate.

(62.) It is not quite determined which
of these two arrangements affords the
greatest quantity of electricity from the
same surface ; but the cylinder is less
expensive, and less liable to accidents
than the plate, and it appears to possess
nearly equal power.

(63.) From what has already been
explained of the general laws of elec-
tricity, the mode in which these ma-
chines act will readily be understood.
The friction of the cushion against the
glass cylinder produces a transfer of
electric fluid from the former to the
latter; that is, the cushion becomes
negatively, and the glass positively, elec-
trified. The fluid which thus adheres
to the glass, is carried round by the re-
volution of the cylinder ; and its escape
is at first prevented by the silk flap

ELECTRICITY.

17

which covers the cylinder, until it comes
to the immediate vicinity of the metallic
points, which being placed at a small
distance from the cylinder, absorb nearly
the whole of the electricity as it passes
near them, and transfer it to the prime
conductor. Positive electricity is thus
accumulated in the prime conductor,
while the conductor connected with the
cushion, being deprived of this electri-
city, is negatively electrified.

But if both these conductors are
insulated, this action will soon have
reached its limit : for when the cushion
and its conductor have been exhausted
of their fluid to a certain degree, they
cannot by the same force of excitation
supply any further quantity to the glass.
In order to enable it to do so, we must
replenish it, as it were, that is, restore
to it a quantity equal to what it has
lost. This purpose will be answered by
placing it in communication with a con-
ducting body of large dimensions ; or,
what is still more effectual, by making
it communicate with the earth, which is
an inexhaustible source of electric fluid.
In order, therefore, to supply the prime
conductor with a constant stream of
electricity, we must destroy the insula-
tion of the cushion, by placing on the
conductor to which it is fixed, a metal-
lic chain, or wire, extending to the
ground. If, on the other hand, we wish
to obtain negative electricity, by means
of the same machine, we must keep the
negative conductor insulated, and con-
nect the prime conductor with the
ground, in order to allow the fluid to
escape from it as soon as it is collected
from the cylinder. The fluid will thus
continue to be drawn without interrup-
tion from the negative conductor, as it
now meets with no impediment to its
discharge on the opposite side of the
machine.

That the quantity of positive electri-
city produced in one conductor is exactly
equal to that of the negative electricity
in the other, is proved by the fact that,
if the two conductors are connected by a
wire, no signs of electricity are obtained
in any of the conductors on turning the
machine : but if the wire be not con-
tinuous, but interrupted by short inter-
vals, a succession of sparks appear at
each interval, indicating the passage of
a stream of fluid from the one side to
the other ot the apparatus.

(64.) A person standing on a stool
with glass legs is thereby insulated;
and if, in this situation, he touch the

prime conductor, either with his hand,
or through the intermedium of a metal-
lic rod, or chain, he may be considered
as forming part of the same system of
conductors. When the machine is
worked, therefore, he will partake with
the conductor of its charge of electri-
city, and sparks may be drawn from
any part of his body by the knuckle of
any other person who is in communica-
tion with the ground.

CHAPTER IV.

Effects of Electrical Attraction and,
Repulsion.

(65.) HAVING obtained, by the elec-
trical machine, the means of accumu-
lating considerable quantities of elec-
tricity, we are enabled to multiply
and extend our observations of the
phenomena, and to examine with
more precision their correspondence
with the results of theory. The effects
of electrical attractions and repul-
sions may be exhibited much more
distinctly, and on a larger scale than
with the simpler instruments we had
previously employed. The experiments
formerly mentioned on the alternate
approach and recession of light bodies,
may be repeated with either conductor
of the machine, when charged with
electricity, and we may note with more
accuracy the differences which occur in
the rapidity with which the changes
from one electrical state to another
take place according as the bodies are
more or less good conductors of elec-
tricity. A pith ball, or a fragment of
gold leaf, is very strongly and immedi-
ately attracted by the electrified con-
ductor, and the instant after it has come
into contact with it, is repelled ; but
it is now attracted by the other bodies
in its neighbourhood, to which it com-
municates its own electricity, and then
is again in a state to be influenced by
the conductor, and to be again attracted :
and this alternation of effects will con-
tinue as long as the conductor remains
charged.

(66.) These alternate and rapid
movements are best seen by placing
these small bodies between two metallic
plates, placed as in fig. 11, the one over
the other, at a certain distance ; the
upper one communicating with the
prime conductor, the lower one with the
ground. If figures of men and women
are cut out of paper and placed between

18

ELECTRICITY.

the two plates, they will exhibit a rapid
dance, while they fetch and carry the
electricity from the upper to the lower
plate ; or contrariwise, if the conductor
be in the negative state.

A* II.

. (67.) This alternation of attractions
and repulsions accompanying the trans-
ferring electricity by moveable conduc-
tors, is also illustrated by the motions of
a ball (fig. 12.), suspended by a silk

Fig. 12.

thread, and placed between two bells, of
which the one is electrified, and the other
communicates with the ground. The
alternate motion of the ball between
the two bells will produce a continued
ringing. As thus described, it is a
mere toy, but the same arrangement
has been applied to the philosophical
purpose of giving notice of changes
taking place in the electrical state of the
atmosphere.

(68.) The mutual repulsion of bodies
that are similarly electrified gives rise
to many amusing appearances. The
filaments of a feather will separate from
each other and diverge, when electrified,
presenting a singular and unnatural
appearance. A small figure in the

shape of a human head, covered with
hair, when placed upon the conductor
and electrified, will exhibit the appear-
ance of terror from the general bristling
up and divergence of the hair. A lock
of wool highly charged with electricity
will, in like manner, swell out to a large
size, in consequence of the mutual re-
pulsion of the filaments which compose
it. On approaching a needle to it, held
in the hand, whereby its electricity is
quickly drawn off, the cotton will sud-
denly shrink into its original dimen-
sions.

(69.) We 'have already adverted to
the effects of fusion in rendering some
bodies conductors, which in their solid
state had the contrary property. This
is the case with sealing-wax ; and ac-
cordingly, if melted sealing-wax be
electrified, its particles will tend to
separate by their mutual repulsion, and
to draw out into filaments. Let a piece
of sealing-wax be fixed on the end of a
wire, and be set fire to, but the flame
immediately afterwards blown out.
While the surface of the wax is still
melted, present it, at the distance of
some inches, to the electrified conductor,
a number of extremely fine filaments
will immediately dart out from the
sealing-wax to the conductor, on which
they will be condensed into a kind of
net- work resembling wool. If the wire
with the sealing-wax be stuck into one
of the holes of the conductor, and a
piece of paper be presented at a mode-
rate distance to the wax, just after it
has been ignited, on setting the machine
in motion, a net-work of wax will be
formed on the paper. The same effect,
but in a slighter degree, will be pro-
duced, if the paper be briskly rubbed
with a piece of Indian rubber, and the
melting sealing-wax be held pretty near
the paper immediately after it has been
rubbed. If the paper, thus covered
with filaments of sealing-wax, be gently
warmed before the fire, the wax will
adhere to it, and exhibit permanently
the result of the experiment. Still more
beautiful are the appearances produced
by camphor subjected to a similar pro-
cess. For the purpose of obtaining them,
a spoon, holding a piece of lighted cam-
phor, must be kept electrified by work-
ing the machine, while it communicates
with the conductor ; the camphor will
then throw t ut curious ramifications*
which appear to shoot like those of a
vegetable.
(70.) It is on the same principle that

ELECTRICITY.

the escape of a conducting fluid, such
as water, through a narrow aperture, is
promoted by electrifying it. If a small
metallic vessel filled with water be sus-
pended from the prime conductor, and
there be placed in the water one end of
a glass syphon, with a capillary bore of
such a diameter as that the water will
scarcely drop from it ; upon turning the
cylinder of the machine so as to convey
electricity to the vessel and its con-
tents, the water immediately flows in a
stream, and, if the electrical charge
be veiy powerful, the descending current
will be seen to separate into several
branches.

(71.) If a sponge, saturated with
water, be suspended from the prime
conductor, the water will at first only
drop gradually from the sponge ; but
when the conductor has become strongly
electrified, the drops will fall plenti-
fully, and, in the dark, will produce
the appearance of a luminous shower
of rain.

(72.) Advantage is taken of the re-
pulsive property of electrified bodies
for the construction of an Electrometer,
or instrument adapted to measure the
intensity of the electricity they may con-
tain. Henley's electrometer (fig. 13.)

Fig. 13.

consists of a slender rod of very light
wood, r, serving as an index, terminated
by a small pith ball, and suspended
from the upper part of a stem of w^ood,
,9, which is fitted to a hole in the upper
surface of the conductor. An ivory
semicircle, or quadrant, q, is affixed to
the stem, having its centre coinciding
with the axis of motion of the rod, for
the purpose of measuring the angle of
deviation from the perpendicular, which
the repulsion of the ball from the stem
produces in the moveable rod. The
number of degrees which is described
by the index, affords some evidence of
the quantity of electricity with which
the apparatus is charged ; though the

instrument has obviously no pretensions
to being an exact measure of its inten^
sity.

(73.) One of the most delicate in-
struments for detecting the presence of
electricity is that which was invented
by Mr. Bennet, and is usually called
the gold-leaf electrometer ; although it
is, properly speaking, only an electro-
scope. It consists (fig. 14.) of two

Fig. 14. ,'

narrow slips of gold leaf, g, suspended
parallel to each other, in a glass cylin-
der, which secures it from disturbance
by accidental currents of air, and at-
tached to the end of a small metallic
tube, which terminates above either in
a flat surface, S, of metal, or in a me-
tallic ball. Two slips of tin- foil 1 1, are
pasted to the inside of the cylinder, on
opposite sides, in a vertical position,
and so placed as that the gold leaves
may come in contact with them, when
their mutual repulsion is sufficiently
powerful to make them diverge to that
extent. These slips of tin-foil terminate
in the foot of the instrument, and thus
are in communication with the earth.
A very minute charge of electricity
communicated to the upper end of the
tube, is immediately transmitted to the
gold leaves, which are thus made to
repel each other ; but if the repulsion
is such as to make them strike against
the tin-foil, their insulation ceases, and
their electricity is carried off; and
being now rendered neutral, they cease
to repel one another, and, collapsing,
resume their original position.

(74.) The most perfect electrometer
for measuring very small quantities of
electricity, is the apparatus contrived
by Coulomb, and to which he has given
the name of the torsion balance. It is
represented in its simplest form in fig.
15, and consists of a cylindrical glass
jar, covered at the top by a circular
glass plate, with a hole in its centre.
Through this hole a single fibre of
c 2

20

ELECTRICITY.

the web of the silken worm descends
nearly to the bottom of the jar, and

Fig. 1 5.

carries at its lower extremity a trans-
verse needle. This needle consists of
either a filament of gum-lac, or a silk
thread or piece of straw coated with
sealing-wax. At one end it is termi-
nated by a small pith-ball, and at the
other by a disc of varnished paper, act-
ing merely as a counterpoise to the
ball. The upper end of the silk fibre is
affixed to a kind of button having a
small index, and capable of being turned
round upon a circular plate divided into
degrees. One side of the jar is perfo-
rated to allow of the insertion of a short
horizontal bar, having a small metallic
sphere at each of its ends, the one being
in the inside and the other on the out-
side of the jar; and the former being
so situated as just to allow the ball of
the suspended needle to come in con-
tact with it in the course of its revolu-
tion. By turning the button, or the
index, the needle may be brought into
this, or any other required position with
regard to the ball. It is found by ex-
periment that the angle of torsion of
the silk fibre is, within a certain range
of distance, very nearly in the direct
ratio of the force which acts in pro-
ducing the torsion ; and therefore, it the
two balls be placed in contact by turning
the button, and then similarly electri-
fied, the distance to which they are
repelled by the angular motion of the
suspended ball, attbrds a measure of
the repulsive force exerted. In like
manner, the distance which the sus-
pended ball is made to move when it
is attracted by the fixed ball, when the
two have opposite electricities, gives
accurate measures of the attractive
forces. It was by the employment of this
apparatus, in a very elaborate series of
experiments, that Coulomb was enabled

to establish very satisfactorily the exact
law of variation, both of the attractive
and repulsive forces, arising from elec-
tricity, with relation to the distance,
which we have already stated.

CHAPTER V.
Distribution of Electricity.

(75.) IT had long been observed, that
the quantity of electricity which bodies
are capable of receiving, does not follow
the proportion of their bulk, but depends
principally upon the extent of their sur-
face. It was found, for instance, that a
metallic conductor in the form of a globe,
or cylinder, contains just as much elec-
tricity when hollow, as it does when solid.
Hence it was evident that the electricity
resides altogether at the surface, or at
least does not extend equally through-
out the whole mass of the body. But
it was only by applying to the theory
all the refinements of mathematical in-
vestigation, that precise notions could
be formed of the exact distribution of
the electric fluid in bodies of different
shapes. The labours of Cavendish,
Coulomb, Poisson, and Ivory, have fur-
nished the means of determining this
problem in every case, however com-
plicated; and whenever a comparison
has been instituted between the results
of experiment and of theory, the most
perfect agreement has been found be-
tween them. Thus all the phenomena
of electricity are found to be in exact
conformity with the mechanical conse-
quences of the theory : they can be an-
ticipated with rigorous precision, and
can even be reduced to numerical cal-
culation in their minutest details, as
well as in their most intricate combina-
tions.

(76.) For the purpose of measuring
the proportional quantities of electri-
city with which different parts of the
same; or of different bodies are charged,
no instrument is so well fitted as the
balance of Coulomb, of which an ac-
count has just been given. What pecu-
liarly adapts it for these experiments, is
its extreme sensibility, by which the
slightest variation in the intensity of the
attractive or repulsive force produces a
very considerable effect in the movement
of the horizontal needle. In some of
the experiments related by Coulomb, a
force only equal to the 279ih of a grain
was sufficient to make the needle per-
form an entire revolution round the

ELECTRICITY.

21

circle : the 360th part of this force,
therefore, or less than the 100,000th of
a grain, might be estimated by each
degree of its angular motion.

In order to apply to the instrument
only such forces as it is capable of
measuring, and of collecting at the
same time from the different parts of
bodies such minute quantities of elec-
tricity as are exactly proportional to
those with which they are themselves
charged, Coulomb employed what he
calls a proof plane, which is simply a
small circular disc of gilt paper, d, (fig.
15,) fixed to the extremity of a very
slender cylinder of gum-lac, and thus
completely insulated. If we wish, then,
to ascertain the proportions in which
electricity is distributed on the surfaces
or interior of any particular body, we
first insulate that body as completely
as possible, and impart to it a small
quantity of electricity by a spark from
the prime conductor. We next touch
any of the points on its surface, the
electricity of which we may wish to
measure, with the little gilt disc, holding
it by the other end of its insulating
handle ; then carrying the plane to the
torsion balance, of which the moveable
ball has been previously charged with
an electricity of the same kind, we bring
it for an instant in contact with the
fixed ball. We then withdraw it, and
the fixed ball being now electrified in
the same manner as the moveable one,
repels the latter with a force measured
by the angle of torsion, at which the
moveable ball stops. While the little
plane and the balls of the balance remain
the same, the division of the electricity
between the little plane and the move-
able ball preserves the same uniform
proportion ; and thus the repulsive force
which results, and which drives off the
moveable ball, is proportional to the
quantity of electricity with which the
little plane is charged. It has been
proved, by a series of well-contrived
experiments, that this quantity is ex-
actly proportional to the quantity of
electricity which really exists at the
point of the body with which it has
been placed in contact. By applying
the same test and method of admeasure-
ment to various other points of the body
we are studying, we may determine the
manner in which the electricity is dis-
tributed in all its parts ; for the method
is applicable even to the interior of the
body, if we pierce it with a small hole
terminating at the part whose electricity

we wish to examine, and pass the proof
plane into it till it is applied to the
bottom of the aperture. Care must be
taken, however, in conducting these last
experiments, that the proof plane be not
suffered to touch any other part of the
body except that of which the electricity
is to be determined, and not even the
sides of the aperture through which it
is introduced, as such contact would
entirely falsify the result.

The following are among the princi-
pal results of these investigations : —

(77.) In a solid body having the form
of a perfect sphere, and charged with
positive electricity, the whole of the
fluid is, in consequence of the repulsion
of its own particles, which is every-
where directed from the centre out-
wards, accumulated in a thin stratum
at the very surface of the sphere. If the
body be charged with negative elec-
tricity, the deficiency of fluid will take
place only in the superficial stratum of
matter.

(78.) If, instead of being spherical,
the body have any other form, the elec-
tricity will still be chiefly confined to
the surface ; and if it have an elongated
form, there will be a greater charge in
the remoter parts than in those nearer
to the middle.

(79.) This result of theory, respecting
the limitation of electricity to the mere
surface, is confirmed in the most deci-
sive manner by the experiments of
Coulomb. A conducting body of the
form represented by the section, /g. 16,

Fig. 16.

/ ' . I3||\

^ tll^

had small pits made in various parts of
its surface. They were half an inch in
diameter, and some of the most shallow
were not depressed more than one- tenth
of an inch below the surface. When
the body was electrified, and the small
proof plane applied in accurate contact
to the bottom of these pits and depres-
sions, care being taken that it should
not touch their margin, and then applied
to the electrometer, no indication of
its having received any electricity could
be perceived ; whereas the contact of
the same proof plane with any part of
the even surface showed the latter to be
strongly electrified. .

ELECTRICITY.

(80.) The following experiment of
Biot's contains also a striking practical
illustration of the same truth. Let a
(fig. 17.) represent a section of any
spheroid of conducting matter, suspend-
ed by a thread which perfectly insulates
it. Let c c be two caps formed of gilt-

Fig. 17.

paper, tin-foil, or any conductor, and
such that, when united, they accurately
iit the surface of the spheroids ; and let
them be also furnished with insulating
handles of gum-lac. Let there be com-
municated to the ball, a, any degree of
electricity ; and then let the two caps,
held by their insulating handles, be care-
fully applied to its surface. Upon the
removal of these caps, it will be found
that the whole of the electricity has been
Abstracted from the spheroid, so that it
will no longer affect the most delicate
electrometer ; whilst the two caps will
be found, upon accurate trial, to have
acquired precisely the same quantity of
electricity which had at first resided in
the body a.

We may conclude, both from theory
and experiment, therefore, that al-
though, strictly speaking, the electricity
must reside within the substance of
conducting bodies, it extends, in fact,
to a depth so small as to be inappreci-
able by any known methods of observa-
tion,

(81.) The effect of an expansion of
surface in lessening the intensity of
electricity, while its absolute quantity
remains the same, is well illustrated by
the following experiment mentioned by
Biot. Fig. 18 represents an insulated
cylinder, a b, moveable round a hori-
zontal axis, and capable of being turned
by an insulating handle h. Around the
-cylinder is coiled a thin lamina of any
metal, c, the end of which is semicir-
cular, and has attached to it a silk
thread /. The whole apparatus com-
municates with an electroscope e, formed
of two_ linen threads, each terminating
m a pith ball. On" communicating a

charge of electricity to the cylinder, the
threads and balls of the electroscope
diverge. Upon taking hold of the silk

Fig. 18.

thread, and unrolling the metallic lamina
from the cylinder, the balls gradually
collapse ; thus indicating a diminution
in the intensity of electrical repulsion.
If the lamina be sufficiently long, the
electrical charge may be spread over so
great an extent of surface, as to allow
the balls to hang perpendicularly and
come in contact. But on winding up
the lamina, the intensity of the electricity
is restored, and the balls diverge to the
same extent as before, allowance being
made for the small dissipation of elec-
tricity which may have occurred from
the contact of the air during the experi-
ment.

(82.) In the case of a long and slender
lamina of conducting matter, charged
with electricity, Coulomb found that its
intensity continued nearly uniform from
the middle of the lamina to within a
short distance from the ends ; at that
part it rapidly increased ; and at the
very extremity it became twice as much
as at the middle part. In a circular
plate, the electricity is accumulated in
much greater quantities at the circum-
ference than about the centre ; the in-
tensities being in the proportion of 2.9
to 1 : that is, the intensity at the centre
is nearly one- third of that at the circum-
ference.

(83.) If the body be an oblong sphe-
roid, arising from the revolution of an
ellipse on its smaller axis, the thickness
of the strata of electricity, or, in other
words, its intensity, at the extremities
of the two axes, is exactly in the pro-
portion of the respective axes them-
selves. It thus appears, that if the
ellipsoid be much elongated, the inten-
sity must be very feeble at the equator,
but very great at the poles. A still
more rapid augmentation of the relative

ELECTRICITY.

intensity at the extremities takes place
in bodies of a cylindric or prismatic
form ; and the more so as their length
bears a greater proportion to their
breadth. Coulomb found by experi-
ment that, in a cylinder thirty inches
long and two inches in diameter, the
intensity of the electricity at the ends
was to its intensity at the middle, or at
any part more than two inches from the
extremity, as 2.3 to 1. Pursuing this
train of reasoning, it will lead us to a
conclusion of some importance, namely,
that, if the conducting substance be
drawn out into a point, the intensity of
the electricity at that point will be ex-
ceedingly great ; and that the point will
accordingly absorb and draw into] itself
nearly the whole of the electricity that is
contained in the body. This vast con-
centration of electricity is found actually
to take place in all points that project
beyond the general surface.

CHAPTER VI.
Transference of Electricity*

(84.) WE are next to consider the
condition of bodies during the preva-
lence of those forces which tend to over-
set the electric equilibrium, over those
which tend to preserve it. The pressure
exerted by the electric fluid against the
non-conducting medium, such as the
air, which opposes an obstacle to its
escape, is in a ratio compounded of the
repulsive force of its own particles at
the surface of the stratum of fluid, and
of the thickness of that stratum ; but as
one of these elements is always propor-
tional to the other, the total pressure
must, in every point, be proportional to
the square of the thickness. If this
pressure be less than the resistance, or
coercive force, as it has been called, of
the air, the electricity is retained ; but
the moment it exceeds that force, in
any one point, the electricity suddenly
escapes, just as a fluid confined in a
vessel would rush out if it were to burst
open a hole in the side of the vessel.

(85.) It is only a certain proportion
of the whole quantity of electricity in
the conducting body that thus suddenly
escapes ; but the irruption of it is
marked by many very striking pheno-
mena, all indicative of the abruptness
and violence with which the change is
effected. A sharp snap is heard, ac-
companied by a vivid spark, and there
are evidences of an intense heat being

evolved in the line which the electricity
takes.

(86.) The passage of the electric fluid
through a perfect conductor is unat-
tended with light. Light appears only
where there are obstacles in its path
by the interposition of imperfect con-
ductors ; and such is the velocity with
which it is transmitted, that the sparks
appear to take place at the very same
instant along the whole line of its course.
Thus, if a row of small fragments of
tin-foil be pasted on a piece of glass,
fig. 1 9, and electricity be sent through

Fig. 19.

them by connecting one of its ends with
the conductor of an electrical machine,
while the other end communicates with
the ground, it will not be possible to
detect any difference of time in the oc-
currence of the light in the different
parts, so that the whole series of lumi-
nous points, if sufficiently near, appear,
in the dark, like a vivid and continuous
line of light. By varying the arrange-
ment of the tin-foil, we may distribute
the light in any manner we please, so as
to exhibit a brilliant delineation of the
figure they represent. Even when con-
ducting bodies appear to be in contact,
if the experiment be made in the dark, a
spark is generally seen to pass between
them, unless the bodies be pressed toge-
ther with considerable force. Hence,
a chain appears luminous at each link,
while conveying a charge of electricity.

(87.) The longest and most vivid
sparks are obtained between two con-
ductors having a rounded form, and the
more so in proportion as they are both
portions of spheres of large diameter.
This may be exemplified in a common
electrical machine, by presenting a me-
tallic ball of large size to that side of
the prime conductor which is furthest
from the cylinder of the machine. In
such cases, however, the electricity
being of weaker intensity, the distance
between the conducting bodies requisite
for the transfer of electricity through
the air, or what is termed the striking
distance, is necessarily small. If the
ball is of smaller diameter, or the con-
ductor of a more elongated shape, the
electricity at its surface is of higher

24

ELECTRICITY.

intensity, and will therefore pass through
a greater extent of air ; the spark is in
this case of considerable length, appear-
ing as a long streak of fire extending
from the conductor to the ball, and
instead of being directed towards one
point, being distributed to various points
throughout a certain extent of the sur-
face of the ball.

Often, when very long, the spark is
seen to have an angular or zig-zag
course, (see fit?. 20.) exactly like that of
a flash of lightning.— This irregularity

Fig. 20.

is probably occasioned by the fluid dart-
ing obliquely in its course to minute
conducting particles that are floating in
the air, a little removed from the direct
line of passage. Even particles of
moisture suspended in the air would be
sufficient to occasion these deviations.
The presence of such particles will ac-
count also for the appearance of lateral
scintillations, which frequently seem to
diverge from the principal stream of
electricity. The greater the number of
such intermediate conductors, or step-
ping-stones, as it were, for the electricity,
the more readily will the balance be-
tween the forces be overset, and the
irruption of electric fluid determined.

When the air is either sufficiently
moistened, or sufficiently rarefied, the
electric fluid passes through it with
comparative facility, and its track is
indicated by streams of light, probably
occasioned by many parallel series of
minute sparks passing from particle to
particle.

(88.) Electrical light differs in no
respect from the light obtained from
other sources. Dr. Wollaston found
that, when observed through a prism,
the ordinary colours arising from the
decomposition of light are obtained ;
but the prevailing tint of colour will
vary according to the different sub-
stances through which the sparks pass,
or to the nature of the surface from
which they emanate, or by which they
are received. Dr. Brewster found that
it is capable of undergoing polarization,
either by transmission through a doubly

refracting crystal, by reflection at the
proper polarizing angle from a polished
plane surface, or by oblique refraction
through a series of glass plates.

(89.) The brilliancy of the electrical
spark is proportional to the conducting
power of the bodies between which it
passes. When an imperfect conductor,
such as wood, is employed, the electric
light appears in the form of faint red
streams ; but metals afford them of
great brilliancy. Its colour is subject
to variation, from a great number of
different circumstances. Sparks pass-
ing through balls of wood or ivory, are
of a crimson colour ; but this depends
also upon their position with regard to
the surface. If two pointed wires be
inserted obliquely and in opposite di-
rections into a piece of soft deal, having
their points an inch and a half distant,
but penetrating to different depths below
the surface, and so that the line joining
them is in the direction of the fibres,
the sparks passing from the one to the
other, will exhibit different colours at
different depths ; and if one of the points
be inserted deeper than the other, all
these colours will appear at once, ac-
cording as the electric light is trans-
mitted at various depths. Electric
sparks passing from one polished me-
tallic surface to another are white ; but
if the finger be presented to an electri-
fied conductor, the sparks obtained are
violet. They are green when taken from
the surface of silvered leather ; yellow
when taken from finely powdered char-
coal ; and of a purple colour when taken
from the greater number of imperfect
conductors. If one of the bodies be-
tween which the spark takes place is a
green plant, the light is red ; and the
same is the case with water or ice. In
the vapour of ether green sparks are
seen when the eye is placed close to the
tube : but they appear reddish when
viewed at a considerable distance. Even
between the same two metallic con-
ductors the colour may vary from the
most brilliant white to the most delicate
violet, according to the distance through
which the electricity is transmitted, and
according to the resistance of the me-
dium which it is compelled to tra-
verse. In exceedingly rarefied air, the
colour of the spark is green ; in denser
air, it acquires a blue tint, and passes
to a violet and purple, in proportion as
the condensation of the air is increased.
Transmitted through other gases, the
colour varies according to their density.

ELECTRICITY.

25

In carbonic acid gas, the spark is white
and vivid ; in hydrogen gas, it is faint
and red.

(90.) It should be recollected, in
making these experiments, that in pro-
portion as the medium is more rare, its
conducting power increases, and a
smaller intensity of electricity is re-
quired for the production of light. In
the ordinary vacuum produced by the
air-pump, the passage of electricity is
rendered sensible by streams or co-
lumns of diffused liii'nt occasionally
varying in their breadth and intensity,
and exhibiting movements which give
them a marked resemblance to the co-
ruscations of the Aurora Borealis. After
rarefying the air contained in a glass
jar, about one foot long and eight inches
in diameter, to the 500th part, Mr.
Smeatofi placed the jar upon a lathe,
and caused it to revolve rapidly, whilst
at the same time he rubbed it with his
hand. A considerable quantity of lam-
bent flame appeared under his hand,
variegated with all the colours of the
rainbow. The light was steady ; but
every part of it was constantly changing
colours. When a very perfect vacuum
is made in a glass cylinder covered with
a brass plate, the electric stream will
pass between it and the plate of the re-
ceiver of the air-pump, in a continued
stream of the same size throughout its
whole length. If a Torricellian vacuum
be formed in the upper portion of a long
bent glass, tube filled with mercury, and
inverted, by placing the legs of the bent
tube in separate "basins of mercury,
when electricity is transmitted through
the tube, light is seen to pervade the
vacuum in a continued arch of lam-
bent flame, without the least diver-
gency.

(91.) It was natural to suppose, be-
fore sufficient consideration had been
bestowed upon the subject, that the
light which appears during the passage
of electricity, was actually the electric
fluid itself, which, at some certain
degree of accumulation, was in itself
luminous ; and such was the notion en-
tertained by the early electricians. But
since we know that common atmos-
pheric air becomes luminous by violent
compression, and we must also pre-
sume that electricity exerts a very sud-
den and powerful pressure upon the
air by its passage through that resisting
medium, we are certainly justified in
drawing the inference that the same
phenomena proceed in both cases from

the same cause. Biot has adopted this
opinion, which appears to be more con-
sonant with philosophical views of the
subject than any other : for it is certain
that the whole of the electrical light
that appears is not more than what may
proceed from the mechanical compres-
sion of the air, the vapours, and other
constituents of the medium through
which the passage of the electricity is
effected.

(92.) The sound which accompanies
these various modes of transference is
subject to corresponding modifications,
dependent likewise, no doubt, upon the
degree and the suddenness of the im-
pulses given to the air. The full, short,
and undivided spark is attended with a
loud explosion; the more lengthened
spark, with a sharper snap, which be-
comes more broken and rattling in pro-
portion to the distance it has to traverse.
The luminous streams produced by a
succession of minute sparks are scarcely
productive of noise, but are accompa-
nied only by a faint rustling sound, like
that of a stream of wind through a nar-
row chink.

(93.) A peculiar odour has some-
times been perceived in the neighbour-
hood of an electrical machine which has
been briskly worked, so as to emit for
some time a great number of sparks ;
and it has been thought to resemble
that of phosphorus. This is also pro-
bably owing to some unknown chemical
decomposition effected by the electricity
during its passage through the air.

(94.) We have already had occasion
to remark the great increase of inten-
sity which the electric fluid acquires at
the extremity of all elongated parts of
conducting bodies ; and the indefinite
augmentation of this intensity which
takes place at the apex of all projecting
points. This high intensity will neces-
sarily be accompanied with a powerful
tendency in the fluid to escape ; a cir-
cumstance which furnishes a natural
and exact explanation of the rapid dis-
sipation of electricity which takes plac.e
from all bodies of a slender and pointed
form.

The following experiments illustrate
these positions. Let the insulated con-
ductor of a machine be furnished with
a pair of pith-balls, suspended by a fine
wire, and charged with either species of
electricity ; the divergence of the balls
will indicate the presence and degree of
this electricity. If a metallic rod with
a ball at one end be held in the hand,

ELECTRICITY.

and the ball presented to the conductor,
taking care not to bring it sufficiently
near to draw a spark, the balls will be
but little affected, and their divergence
will continue for a considerable time.
But if the rod terminate in a sharp point,
instead of a ball, and the point be pre-
sented to the conductor at the same dis-
tance as the ball was in the former case,
the electroscope will immediately col-
lapse, showing that the electrical charge
has entirely disappeared : it has, in
fact, been rapidly drawn off by the
pointed rod. It is quite immaterial to
the success of the experiment whether
we affix a point to the conductor itself,
or whether we present to it a point held
in the hand ; the escape and dispersion
of the electricity being equally pro-
moted by the presence of a point, whe-
ther the fluid be given out or absorbed ;
for it is scarcely necessary to remark
that the very same kind of reasoning
applies equally to both the positive and
negative conditions of electricity.

(95.) Currents of air always accom-
pany the discharge of electricity, whe-
ther positive or negative, from pointed
bodies ; for each particle of air, as soon
as it has Deceived its electricity from
the point, is" immediately repelled by the
body. These currents tend powerfully
to increase the dissipation of the elec-
tricity, by bringing in contact with the
point a continued succession of particles
of air, that are not yet electrified, and
are, therefore, ready to receive a charge.
Many amusing experiments are founded
on this principle. Let two cross wires,
(/?g-. 21.) the ends of which terminate in

Fig. 21.

in a direction opposite to that of the
stream ; and this taking place at all the
four points, the whole system will
revolve backwards with considerable
rapidity.

The following is another form in
which this experiment may be made.
Two wires, (Jig. 22.) are stretched in

Fig. 22.

points, bent in a similar direction with
respect to the axis, be supported by
means of a cap upon a fine point, and
electrified by being placed upon the
prime conductor of a machine. Each
of the points will give off a stream of
electricity : this will remove a part of
the pressure which the fluid would have
exerted on that side if no efflux had
taken place ; but as the pressure of the
fluid on the opposite side of the wire,
in the opposite direction, still operates
in full force, the wire will be impelled
in the direction of that] force, that is,

the direction of a plane, slightly in-
clined to the horizon, between four in-
sulating pillars. Across these wires,
another wire is made to rest, termi-
nating by small balls at each end, and
having a cross wire fixed to it at right
angles, with two bent points, as in the
former experiment. When this system
is electrified, the dispersion of the elec-
tricity from the points produces a re-
volution of the bars, which makes the
transverse bar roll up the inclined plane.

An apparatus consisting of wires ter-
minating in points, and having balls
annexed to them to represent the planets,
may be constructed so as to revolve
when electrified ; and thus to imitate the
planetary motions. Such an apparatus
has been called an electrical orrery.

(96.) It should be observed, how-
ever, that a point loses its power of
concentrating and dispersing electricity
when it is surrounded by other parts
of the conducting body which are
equally prominent ; as when it is placed
between two balls, or inclosed in a tube,
or when it does not rise above the ge-
neral surface of the body. The effect
of one point is much diminished even
by the vicinity of another point ; so that
if several points placed near each other
be presented to the conductor, the elec-
tricity is drawn off much less rapidly,
and will be transferred by sparks in-
stead of forming a continued stream.

(97.) When the transfer of electri-
city takes place between smooth surfaces
of a certain extent, no difference can be
perceived in the nature and appearance
of the spark, whichever be the position
of the negative surface. But in the
passage of electricity through points,

ELECTRICITY.

27

the effect is considerably modified by
the species of electricity with which the
bodies are charged ; or, in other words,
by the direction in which the fluid moves.
When the electric fluid is escaping out
of a pointed conductor, the luminous
appearance is that of diverging streams,
as represented in fig. 23 ; forming
what is termed a pencil of light, and

Fig. 23.

resembling the filaments of a brush.
When, on the contrary, the electric
fluid is entering into the pointed body,
the light is much more concentrated at
the point itself, having a resemblance
to a star, in which, if any streams
appear, they are disposed like radii, and
equally^o in all directions. An approach
to these different modifications may be
remarked when sparks pass between
balls of small diameter, especially if
the charge is high. Thus the direction
of the lateral ramifications sent out
from the principal line, in the branched
spark, fig. 20, is from the positive to
the negative surface.

(98.) In describing the above ap-
pearances, we have, as usual, referred
to the hypothesis of Franklin: but if
we adopt that of the two electricities,
we have only to consider the appearance
of the pencil of light as arising from the
double current of the vitreous electricity
issuing from the point, and of the resi-
nous electricity passing into it : while
the star will be the effect of the irruption
of the resinous, and the absorption of
the vitreous electricities. But this re-
markable difference in the phenomena
produced, according to the particular
species of electricity with which the
point is charged, has always been urged
as a convincing argument in favour of
the Franklinian theory. They appear
very strongly to indicate the emanation
of some material fluid from the positive,
and its reception by the negative point.
The diverging lines on the one side, and
their inflections on the other, represent
exactly the paths of particles flowing
out as from a pipe, and urged forwards
by a force which gives them sueh. a

projectile velocity as to prevent their
spreading out beyond a certain distance
from the direct line of projection. But
this very velocity will carry the par-
ticles that happen to have deviated
most, somewhat beyond the point to
which they are attracted : while the
attraction to this latter point will tend
to deflect them from the line of their
path, and gradually turn them back, so
that they will arrive at the point of
attraction by very different paths, and
some even by a retrograde motion.
Hence, while in the first case they form
a diverging cone of rays, in the latter
they must be distributed on all sides of
the point like the rays of a star. The
annexed diagram, fig. 24, will suffi-
ciently illustrate this explanation by

Fig. 24.

representing the supposed course of the
particles of electric fluid, passing through
the air from the positive to the nega-
tive point. What weight the argu-
ment derived from this phenomenon
may be allowed in deciding the question,
will be discussed in the sequel.

(99.) The difference which we have
now described in these two appearances,
may be employed, on many occasions,
as a useful criterion of the species of
electricity, at least, which is passing
from one conductor to another, if not of
the absolute direction of its motion. For,
if a needle be presented to an electrified
body, the appearance of a star on the
needle will show that the electricity of
that body is positive ; while, on the con-
trary, a luminous brush on the needle
will indicate that the body is negative.

(100.) The influence of a point pro-
jecting a short distance from the surface
of a body, is greater when that body is
negative than when it is positive. Hence,
a spark is more readily obtained in the
latter case than in the former.

On this principle an instrument has
been invented by Mr. Nicholson for
distinguishing the negative from the
positive electricity. It consists simply
of two metallic balls fixed at the ends of
two curved rods of glass, and moveable
like branches on a joint, so as to admit
of the balls being placed at different

28

ELECTRICITY.

distances from each other, when held by
a handle proceeding from the joint. A
short point projects from one of the balls
on the side adjacent to the other ball ;
and this point affords a spark at a
shorter distance when positively, than
when negatively electrified.

CHAPTER VII.
Development of the Law of Induction.

(101.) WE have next to trace the
consequences of that important law of
electricity which has been called the
Law of Induction.

Active electricity existing in any sub-
stance tends always to induce the op-
posite electrical state in the bodies that
are near it. Now it is impossible, as
we have already seen, to induce one
electrical state in any body without at
the same time producing the opposite
state in the same body, or in the one
which is immediately contiguous. Ac-
cording to the simpler theory, the accu-
mulation of electricity in any one part
can be effected in no other way than by
withdrawing it from another part, nor
can it be abstracted from the one with-
out being received by another ; so that
there is always an equal degree of nega-
tive as of positive electricity, and vice
versa, in every case. According to the
more complex theory, if we decompose
the natural electricities residing in any
body, we must at the same moment ob-
tain equal quantities of both the vitreous
and resinous electricities. It follows,
therefore, that if the bodies subjected to
the inductive influence are non-conduc-
tors, although the tendency to produce
the opposite electricity still exists, yet
in consequence of the immobility of the
fluid, it can produce no visible change.
In proportion as the body opposes less
resistance to the passage of electricity,
the operation of the disturbing force
becomes sensible ; and in order to fix
our ideas, let us first take the case of
a positively charged electric, acting by
induction on an insulated conducting
body. The redundant fluid in the for-
mer will tend to repel all the fluid con-
tained in the latter: a portion of this
fluid will, therefore, be driven from the
side adjacent to the first body, towards
the remoter side. The adjacent side
will thus be rendered negative ; the
remote side, positive. But this will take
place to a certain extent only : for there
is a limit at which the repulsion of the

fluid accumulated at the remote end,
will just balance the repulsion of the
fluid in the electric, added to the attrac-
tion of the under- saturated matter, in the
near end ; and when this limit has been
attained, the flow of electric fluid from
the near to the remote end of the body
will cease, and an equilibrium will be
established.

(102.) Experiment shows the perfect
coincidence of theory with the actual
fact. Let a cylinder of metal, NP, (see
ftg. 25,) of some length, with rounded
ends, and furnished in different parts

Fig. 25.

with pairs of suspended pith-balls, to
serve as electroscopes, being previously
insulated, be placed in the vicinity of an
electrified globe of glass, E, taking care
that it be not sufficiently near to receive
any quantity of electricity by transfer-
ence.

We shall find that eveiy pair of balls,
except those situated in a particular
plane Mm, about the middle of the
cylinder, will immediately diverge, indi-
cating the electrical states of the parts
from which they are suspended. Those
at either extremity of the body, n,p,
diverge the most ; and the divergence
diminishes as we approach the middle
plane before mentioned, at which the
body is in the natural or neutral state.
The position of this plane of neutrality,
Mm, varies according to the distance of
the electric, and the relation which that
distance bears to the length of the body
itself. If we further examine the species
of electricity residing in the different
parts, we shall find it to be negative in
all the parts nearer to the electric than
the neutral plane, and positive in all
those more remote. We may ascertain
with much greater accuracy these elec-
trical states by the employment of the
proof plane and electrometer of Cou-
lomb, than by the pith-balls ; and the
results are then found to correspond,
with the most rigorous precision, with
the deductions from the theory of elec-
trical action.

(103.) These effects, it should be
remarked, are simply the result of the
action of electricity at a distance ; for
they depend upon no other circumstance.

ELECTRICITY.

29

They take place in an equal degree
whatever substance be interposed be-
tween the bodies which are exerting this
action on one another, provided the
interposed substance undergoes no
change in its own electrical state; a
condition which is fulfilled in electrics
only. Thus, induction will take place
just as effectually through a plate of
glass, as if no such substance had inter-
vened.

(104.) Let us now suppose that the
acting body, E, is, instead of an electric,
a conducting body, a globe of metal, for
example, charged with positive electri-
city. The primary effects of this globe
on the cylinder will be the same as in
the former case ; but the electrical state
which the globe has induced on the
cylinder will re- act upon its own electri-
city. The negative electricity, that is,
the under-saturated matter at the nearer
end of the cylinder N, exerts a tendency
to induce positive electricity in the
globe, and more especially upon the
adjacent side, F : that is, it will tend, by
its attraction for the fluid, to draw it io
that side, and thus render it still more
highly positive than it was before. This
can only be done at the expense of the
other side, O, from which the fluid must
betaken, and which is, therefore.rendered
less charged with fluid, that is, less po-
sitive than before. But this new distri-
butiorj of the electric fluid in the globe,
by increasing the positive state of the
side, F, next to the cylinder, tends to
augment its inductive influence on the
fluid in the cylinder ; that is, to drive
an additional quantity of fluid from the
negative to the positive end. This is
followed, in its turn, by a corresponding
reaction on the globe, and so on, con-
stituting a series of smaller adjustments,
until a perfect equilibrium is established
in every part. When this has been
attained, the electrical states will, it is
evident, be of the same kind as those
consequent upon the immediate actions,
though somewhat increased in intensity
by the series of reactions.

The following experiment is a practi-
cal illustration of the preceding reason-
ing. Furnish the metallic globe with
electroscopes on its opposite surfaces ;
•when the globe is insulated and alone,
any electricity communicated to it will
diffuse itself equally over the surface,
and both the electroscopes will diverge
equally. But no sooner do we bring
near to it a conducting body, than the
balls of the electroscope at the side most

distant from that body begin to col-
lapse, while those at the nearer side
diverge to a greater degree than before ;
thus showing the nature of the reflex
operation of the induced electricity of
the conductor upon the body from
which the induction originated.

(105.) It should be recollected that in
all the changes we have thus traced as
the effects of induction, there has been
no transfer of electricity from either of
the bodies to the other ; as was suffi-
ciently proved, indeed, by their taking
place equally if a plate of glass be inter-
posed. Another proof is afforded by
the circumstance that the mere removal
of the bodies to a distance from one
another, is sufficient to restore each of
them to their original state. The globe
remains as positively electrified as be-
fore ; the cylinder returns to its condi-
tion of perfect neutrality ; nothing has
been lost, and nothing gained on either
side. The experiment may be repeated
as often as we please, without any varia-
tion in the phenomena. But this would
not be the case if the cylinder were
divided in the middle, and one or both
of the parts were removed separately,
while they still remained under the in-
fluence of the globe. The return of the
electric fluid from the positive to the
negative end being thus prevented, each
part will retain, after its separation, the
electricity which had been induced upon
it. The nearer portion will remain ne-

fative ; the remoter portion, positive,
f the division had been in three parts,
the middle part only would have been
neutral. The experiment may be made
by joining two or more conductors end-
wise, as shown in Jig. 2G, so that they

Fig. 26.

-NT:

may act as a single conductor when
placed near to the electrified globe, and
after induction has thus been produced,
removing them separately, and examin-
ing their electrical states. If E be po-
sitive, N will be found negative, P po-
sitive, and M neutral.

(106.) Another modification of effect
will take place when an insulated con-
ductor, rendered electrical at both ends
by induction, is made to communicate
with another conductor. Let us first
suppose that a long metallic conductor

30

ELECTRICITY.

is brought irito contact "with the remote
end of the first cylinder P,(fig. 25),
which has been rendered positive by in-
duction. The fluid accumulated at this
end will now pass into the conductor,
and will remove to the most distant part
of the conductor. The transit will now
take place before actual contact, and
will be manifested by the appearance
of a spark when the bodies are brought
within the striking distance. The re-
moval of this fluid to a greater distance
will occasion a disturbance in the equi-
librium that had before been esta-
blished. The repulsion which that fluid
had excited, and which had contributed
to prevent any more fluid from being
propelled from the negative end N, is
now considerably weakened by the
greater distance at which it acts ; and
more fluid will leave the negative end,
which end will consequently become
more highly negative. This change of
distribution will again occasion a further
effect, by its reaction on the fluid in the
globe whence the action originally pro-
ceeded ; and another series of changes
and adjustments will follow, until a new
condition of equilibrium takes place,
and then the fluid will be at rest.

(107.) Thus we learn that the effects
of induction on a conductor are aug-
mented by increasing its length; they
would, therefore, be greatest of all, if
we could give it infinite length : but the
same condition is attainable by placing
the conductor in communication with
the earth, which will accordingly carry
off all the fluid which the electrified
body is capable of expelling from the
nearest end. Accordingly, if we touch
with the finger, or with a metallic rod
held in the hand, the remote end of an
insulated conductor under the influence
of induction, we obtain a spark, more
or less vivid according to the intensity
of the electricity so induced ; and the
conductor so touched has now only one
kind of electricity, namely, the one op-
posite to that of the electrified body
which is acting upon it. The part
touched is brought into a state, in which
it appears to be neutral as long as it
remains in the vicinity of the electrified
body; because the actions of the re-
dundant fluid, and unsaturated matter
in the two bodies, exactly balance one
another. But it all the while really con-
tains less fluid than its natural share, in
consequence of the repulsive tendency
of the fluid in the body which produces
the induction ; and this negative state

will readily become ' active, if the con-
ductor that has been touched be again
insulated, and then removed from the
influence of the former. This peculiar
condition of a body, in which its parts
are really undercharged or overcharged
with fluid, although, from the action of
electrical forces derived from bodies in
its vicinity, a state of equilibrium is
established, and no visible effect results,
has been denominated by Biot, dis-
guised electricity.

(108.) It is also worthy of remark,
that if the communication between the
insulated conductor and another longer
conductor, or the earth itself, be made
at either end of the former, the same
effect will result, and the electric fluid
accumulated at its remote end will be
carried off by the longer conductor,
although, it will have, in one case, to
pass round through the end nearest to
the body which repels it. The opera-
tion which here takes place may be il-
lustrated by the motion of a fluid in a
syphon. A repulsive force is acting
upon the fluid, both in the shorter and
the longer column ; but with regard to
the motion of the fluid in the bent chan-
nel, the one force is in opposition to the
other, and the tendency of the fluid in
the longer column prevailing over that
in the shorter, will draw off the latter,
round the bend of the supposed syphon.
Thus in the bent conductor A N JP (fig.
27,), the repulsion exerted by the fluid
in E for that in the longer column N P,

Fig. 27.

being greater than its repulsion for that
in the shorter column N A, the fluid in
A will be carried over the bend N, not-
withstanding its tendency to move from
N towards A.

(109.) We have hitherto supposed
the acting body to be positively electri-
fied; but precisely the same effects
would happen with regard to degree,
although opposite as to the species of
electricity, if it had been negatively elec-
trified : and the same explanations will
in every respect apply, with the requisite
substitution of the terms negative for
positive, and of attraction for repulsion,
and vice versa. A little reflection will
also easily show the application of the

ELECTRICITY.

theory of the double electricities to ex-
plain the same phenomena.

(110.) Another consequence of the
induction of electricity must not be over-
looked,, namely, that the bodies between
which it takes place, necessarily attract
one another : for the action of the adja-
cent sides F and N ( fig, 25), which are
brought into opposite electrical states,
is greater than the action of those sides
which are in the same electrical states,
F and P, and which are more distant :
hence the attractive force always exceeds
the repulsive. We have already seen
that this circumstance sufficiently ex-
plains the fact that conducting bodies,
previously neutral, are attracted by
electrified bodies. Another fact, which
appears more singular, and which can-
not be accounted for on any other prin-
ciple, is also a direct consequence of
the law of induction. If a small body
weakly electrified, be placed at a dis-
tance from another and a larger body,
more highly charged with the same
species of electricity, it will, as usual,
be repelled ; but there is a certain
distance within which if it be brought,
attraction will take place, instead of re-
pulsion. This happens in consequence
of the inductive influence producing so
great a change in the distribution of
electricity, as to give a preponderance
to the attractive forces of the adjacent
parts of the two bodies, over the repul-
sive forces that take place in the other
parts, and which would have alone
acted if the fluid had been immoveable.

(111.) From the principles now laid
down, it will be easy to understand how
induction may operate through a suc-
cession of conductors, which are all of
them insulated, except the last ; and
which are separated from each other by
distances greater than that at which a
transfer of electricity would take place.
If, under such circumstances, the first be
electrified, alternate states of opposite
electricities will be produced in the two
ends of each conductor in succession.
In all the ends nearest to the first body,
the electricity will be of the opposite
kind to that with which the first has
been charged ; in the other ends it will
be of the same kind as that of the first
body. The vicinity of these opposite
electricities will tend powerfully to re-
tain them in that condition, and will
diminish their electric action on sur-
rounding bodies. A large portion of
the electricities so arranged and re-
tained, is, therefore, in the condition

designated by the term disguised elec-
tricity.

(112.) In proportion as the interrup-
tions to the continuity of the line of
conductors are more numerous, the
more nearly will such a system ap-
proach to the condition of an imper-
fectly conducting body. The same prin-
ciple admits of being extended, with
some modifications indeed, to the con-
stitution of electrics themselves, as we
shah1 have occasion to notice in the
sequel.

CHAPTERVIII.

Accumulation of Electricity by
Induction.

(113.) THE most important application
of the principle of induction is that by
which a vast accumulation of electricity
is obtained in a small space, while its
intensity, or tendency to escape, is at
the same time rendered exceedingly
small. This condition exactly corre-
sponds to that which has been termed
disguised electmcity.

(114.) Let two circular metallic plates
P and N (fig. 28), be placed the one
immediately over the other, but sepa-
rated by a non-conducting medium, such
as the air, or, what is still better, a plate
of glass. Let the upper one P, commu-

nicate, by a wire M, with the prime con-
ductor of the electrical machine ; and
let the lower one N, be insulated by
resting upon three glass supporters.
Let P be charged with a certain quan-
tity of electric fluid. The fluid natu-
rally contained in N, will be repelled by
the"fluid in P, and will quit the upper
surface of N in order to occupy its lower
surface. When this change has taken
place, let N be touched by a wire W,
establishing a communication between
it and the ground. All the fluid which
was accumulated in the lower surface
of N, will be earned off by the wire,
and the whole plate will thus be nega-
tive, or undercharged with fluid. The

32

ELECTRICITY.

redundant matter in N will, by its at-
traction for the fluid, draw more of it
into the upper plate, which will be sup-
plied from the conductor of the ma-
chine through the wire M ; and such
an additional quantity will be accumu-
lated in P, as will balance the increased
attraction of the matter inN, and main-
tain it at the same intensity as the fluid
in the prime conductor. That this is
what really happens will be rendered
evident by placing an electroscope upon
the prime conductor ; for the moment
the plate N communicates with the
ground, the balls of the electroscope
collapse, showing that the intensity of
the fluid in the prime conductor is sud-
denly reduced by the great quantity
that has been absorbed by the plate P.
The machine must now again be set in
motion, in order to supply the electri-
city which has been thus abstracted
from the conductor. The operation of
each plate on the other may be con-
sidered as that of increasing its electri-
cal capacity, or of rendering a large
proportion of its electricity latent or
disguised.

(115.) It is evident that the quantity
of electric fluid driven out of the lower
plate by the action of the fluid in the
upper one, can never be quite equal to
that of the fluid with which the upper
one is itself charged, and the difference
will be greater in proportion to the dis-
tance of the plates. When they are
very close to each other, these two quan-
tities approach very near to an equality ;
and this circumstance it was that mis-
led Franklin into the belief that they
were actually equal.

(116.) The capacity for accumula-
ting electricity corresponding to a given
intensity in the upper plate depends
upon the distance between the plates,
provided always that the intervening
electric opposes a sufficient obstacle to
the direct transfer of the electricity
from the one to the other; and is in
some inverse ratio to that distance.
The lower plate, N, which communi-
cates with the ground by the wire W,
although strongly negative, is rendered,
by the vicinity of the fluid in the upper
plate P, neutral with respect to fluid in
the wire W : that is, the attraction of
its unsaturated matter, although nearer,
is exactly balanced by the repulsion of
the redundant fluid in the upper plate,
which, although really stronger, is, from
the greater distance at which it acts,
only equal to the former. With refer-

ence to fluid in the wire M, however,
the action of the redundant fluid in P,
is not balanced by that of the unsatu-
rated matter in N, which latter is both
weaker in itself and more distant. Thus,
while N is neutral with respect to the
conductors which touch it, P is in a
slight degree active, in consequence of
this small preponderance of force, and
a portion of its fluid tends to escape.
Hence, if N be again insulated, by
removing the wire W, and the wire M
be now made to communicate with the
ground, this portion of the fluid in P
will pass off by it ; but not any larger
quantity, for the remaining portion is
retained by the attraction of the unsa-
turated matter in N. P is, by this loss,
rendered neutral, as N had before been,
and it now no longer acts on the fluid
beyond it in M. The influence of P on
that fluid is greater than that of N in
respect to its greater vicinity, but less
in as far as regards the intensity of
action, and the compensation is exact,
feut under these circumstances, N,
which was before neutral, becomes in
its turn active, and now that the repul-
sion of the fluid in P is diminished, will
absorb a certain quantity of the fluid as
soon as it is touched by W, after P has
been again insulated. By this contact,
N is again restored to the neutral state,
a fresh portion of fluid in P is released
from the attraction of N, and P is again
active. By repeating these alternate
contacts a sufficient number of times,
we gradually deprive the plates of their
whole charge of electricity ; alternately
imparting small portions to the negative
plate, and taking away the like portions
from the positive one, until they are
both brought to their natural unelectri-
fied state. The quantities of fluid wh.ch
are thus successively added and ab-
stracted were found, by the calculations
of Laplace, to be in geometrical pro-
gression.

(117.) The most convenient mode of
obtaining the accumulated electricity
arising from induction is by the employ-
ment of coated glass, that is, of a plate of
glass, on each side of which is pasted a
sheet or coating of tin- toil. Care must
be taken to leave a sufficient margin of
glass uncovered by the metal, for pre-
venting the transfer of electricity from
the one coating to the other round the
edge of the glass ; and all sharp angles,
or ragged edges in the coatings, should
be avoided, as they have a great tendency
to dissipate the charge.

(118.) The following experiment of
Professor Richman, (the philosopher
who fell a sacrifice to his zeal for elec-
trical science by a stroke of lightning
from his apparatus,) is very instructive.
Let a pane of glass placed vertically,
and seen edgewise in fig. 29, be coated
on both sides, and furnished with two

Fig. 29.

igs alterna

small electroscopes, p, n, consisting of
two pith-balls, one attached to each of
the coatings. Let the coating P be

charged positively, while the coating N
is made to communicate with the ground.
The electroscope p will stand out from
the plate, and n will hang down close
to its coating, as long as N communi-
cates with the ground. But in propor-
tion as P loses electricity by gradual
dissipation in the air, the ball p will
gradually, but very slowly descend. If
we now insulate N, p will fall down at
first very speedily, and then more
slowly, till it reaches q, about half its
first elevation. The ball n will at the same
time rise to nearly the same height ;
the angle between the two electroscopes
continuing nearly the same as at first.
AVhen n has ceased to rise, both balls
will very slowly descend, till the charge
is lost by dissipation. If we touch N
during this descent, n will immediately
fall down, and p will as suddenly rise
nearly as much ; the angle between
the electroscopes continuing nearly the
same. Remove the finger from N, and
p will fall, and n rise, to nearly their
former places ; and the slow descent of
both will again recommence. The same
thing will happen if we touch P, p will
fall down close to the plate, and n will
rise to m, and so on ; and this alternate
touching of the coatings may be repeated
some hundreds of times before the
plate is entirely discharged. If we sus-
pend a crooked wire, bent, as shewn atW,
(Jig. 29,) having two pith-balls, from an
insulated point, s, above the plates, it
will vibrate with great rapidity, the

ELECTRICITY.

balls striking the coatin
and thus restoring the equilibrium by
steps ; each contact being attended by
a spark.

(119.) If, instead of this gradual
discharge, a direct communication is
made between the two coatings by a
metallic wire extending from the one to
the other, the whole of the electric fluid
which was accumulated in the. positive
coating rushes with a sudden and violent
impetus along the conductor, and passes
into the negative coating, thus at once
restoring an almost complete equili-
brium, and rendering every part very
nearly, though not absolutely, neutral ;
for as there must always be some slight
difference in the quantity of electrical
charge in the two coatings, where one
of them is in communication with the
ground, there must always be a certain
excess, however minute, of electricity,
after the balance has been struck.

(120.) This sudden transfer of a large
quantity of accumulated electricity is a
real explosion ; it gives rise to a vivid
flash of light, corresponding in intensity
to the magnitude of the charge. The
effect of its transmission is much greater
tli an that of the simple charge of the
prime conductor of the machine; for
while the latter gives a spark only, the
former imparts what is called an electric
shock, and the sensation it produces
when passing through any part of the
body is of a peculiar kind. We shall
describe their effects in a future chap-
ter ; at present we must confine our
attention to the purely electrical condi-
tions of the phenomenon.

(121.) The presence of the coating is
not absolutely essential to the charge
and discharge for the twro surfaces of the
glass plate ; for if the glass be fur-
nished with moveable coatings, and
charged in the usual manner, upon re-
moving the coatings (taking care that
they be touched only by electrics,) the
greater part of the electricity will be
found to have attached itself to the sur-
faces of the glass plate, where they are
retained by their mutual inductive in-
fluence. In this state the charged plate
of glass may be gradually discharged
by making a communication between
its several parts in succession. It can-
not be discharged at once, for want of a
common intermedium for the simulta-
neous transference of the electricity of
the different parts of the surface. But
if this be supplied by replacing the
former coatings, or adding new ones,

34

ELECTRICITY.

the complete discharge may be effected
as before.

(122.) By peculiar management a
charge may be given to a plate of glass
independently of any coating whatever.
For this purpose, it must 'be held by
one corner, and passed before a ball,
connected with the prime conductor of
a machine, so that it may successively
come in contact with every part of the
middle of the plate of glass, while the
finger, or any conducting body commu-
nicating with the ground, is held oppo-
site to it on the other side. Thus the
glass will be charged, and will be in the
same state as the glass from which the
coatings had been removed.

(123.) We often find, a short time
after the discharge of coated glass, that
t has acquired spontaneously a small
charge, producing a faint spark when a
second communication is made between
the coatings by the discharging wire.
This, which is called the residual charge,
arises from two causes : first, a portion
of the electricity adheres to the un-
coated surface of the glass: and se-
condly, another part has penetrated
from the coating for some little depth
below its surface. Both these portions
slowly return to the coatings after they
have been deprived of their original
charge, and give it a fresh charge.
"When a very large extent of coated
glass is employed, this residual charge
may even amount to a considerable
quantity, and the experimenter should
be cautious not to expose himself to
the shock which he might thus receive,
if he inadvertently touched the appa-
ratus before he had properly discharged
it. That charges are capable of pene-
trating even through the entire thick-
ness of the glass is proved by the
curious fact, that a coated, cylindrical
jar may be discharged merely by keep-
ing up for a sufficient time a continu-
ance of the minute vibrations excited
by rubbing it with the finger, or by
making it ring. A discharge may also
be effected by heating the glass, which
renders it a conductor of electricity.

(124.) The most convenient form for
coated glass for experimental purposes,
is that of a cylinder or jar. In the ear-
lier periods of electrical research, jars
were filled with water, mercury, or iron
filings, which furnished the interior
coating, while the exterior coating was
supplied either by water, in which the
jar was immersed, or by the hand of
the operator, who for that purpose

grasped the outside of the jar : a rod 'of
metal was employed to communicate
the charge from the prime conductor of
the machine to the inner coating. On
making a communication between the
exterior and interior coatings, by means
of a circuit of conducting substances,
the discharge took place, and the shock
made to pass through the circuit thus
formed. This instrument having been
made known principally through the
experiments of Kleist, Cuneus, and
Muschenbroeck, at Leyden, the name
of the Leyden phial, or jar, was gene-
rally applied to it. It is at present con-
structed as shewn iny?#. 30, by apply-

Fig. 30.

ing coatings of tin-foil on both sides of
the jar or bottle, leaving a sufficient
space uncovered at its upper part to
secure it from the risk of a spontaneous
discharge, which might take place if
the coatings were not separated by a
sufficient interval. A metallic rod, rising
two or three inches above the jar, and
terminating at the top in a brass ball,
which is often called the knob of the jar,
is made to descend through the cover,
till it touches the interior coating. It
is through this rod that the charge of
electricity is conveyed to the inner coat-
ing, while the outer coating is made to
communicate with the ground. We
have already seen, that if this last con-
dition be not observed, the inner coating
can receive no charge, and only a feeble
spark will pass from the conductor to
the knob.

(125,.) The outer coating may be
made to communicate with the ground
by holding it in the hand ; and on pre-
senting the knob of the jar to the prime
conductor when the machine is in mo-
tion, a succession of sparks will pass
between them, while at the same time
nearly an equal quantity of electricity
will be passing out from the exterior
coating, through the body of the person

ELECTFICITY.

who holds it, to the ground. If, instead
of this, the jar be placed on an insulat-
ing stand, and a ball of metal, or the
knuckle of the finger, be held near the
outside of the jar, \ve have evidence of
the escape of the electricity from the
latter by a succession of sparks simul-
taneous with those that occur between
the prime conductor and the knob of
the jar.

(126.) If, instead of touching the
outer coating of a jar supported on an
insulating stand, we bring into contact
with it the knob of a second jar, of
which the outer coating communicates
with the ground, as shewn in jig. 31,

Fig. 31.

the electricity which is expelled from
the outer coating of the first jar passes
into the inner coating of the second jar,
and thus both jars are charged. Thus
may charges be given to a succession
of jars, so placed as that the inner
coating of each shall communicate with
the outer coating of the one that pre-
cedes it in the series ; taking care that
the outer coating of the last jar com-
municates with the ground. All the
jars will be found to be charged in a
similar manner. It is evident, however,
that the charge must diminish in in-
tensity as it is conveyed from each jar
to the next, because the quantity of
electricity which is expelled from the
exterior is never quite equal to that
which passes into the interior.

(127.) For the sake of greater dis-
tinctness we have ah1 along supposed the
interior of the jar to be charged with
positive electricity, but the very same
effect would take place if the knob of
the jar were charged negatively by com-
munication with the negative conductor.
A similar change in the electrical state
of the coatings would result from placing
the jar on an insulating stand, and then

forming a "communication between the
outer coating and the prime conductor,
while the knob is made to communicate
with the ground. The only (iiiiercnce
is, that the outer coating would then
be active and the inner one neutral;
but these conditions would again be
reversed as soon as the knob was dis-
connected with the ground, and the
outer coating touched with the hand.

(128.) If two jars, the one charged
positively, the other negatively, be
placed on two separate insulating
stands, and their knobs then connected
by a conduct or, vrhich is itself insulated,
no explosion will take place, although
the two coatings, which are thus brought
into communication, are in opposite
electrical states. But if the two outer
coatings be at the same time connected,
an explosion will take place, and both
jars will be discharged.

(129.) Since the susceptibility of re-
ceiving a charge depends upon the
proximity of the metallic surfaces, while
the passage of the electricity from the
one to the other is interrupted by the
interposition of a non-conducting sub-
stance, it is evident that, in the con-
struction of the Leyden j ar, the thick-
ness of the glass is an important con-
sideration. The thinner the glass, the
greater.. will be the power of taking a
charge ; but the power of retaining the
charge will be less, on account of the
diminished resistance which the glass
will afford to the passage of the elec-
tricity through it. If the charge be
higher than what the jar will bear, the
glass will be broken by the violence with
which the electricity forces a passage
through its substance. Muscovy talc,
even in very thin laminae, resists much
better than glass, and is, therefore, ca-
pable of receiving and of retaining a
much higher charge. Another limit
to the charge which a jar is capable of
retaining, arises from the liability of the
electricity to pass from one coating to
the other, round the edges of the glass.
(130.) These spontaneous discharges,
as they are called, are facilitated by the
deposition of moisture on the glass,
forming a chain of conducting particles
in the very line which the electricity
has a strong tendency to take. Hence,
it is a requisite precaution to keep the
apparatus in as dry a state as possible ;
and the deposition of moisture may be
guarded against most effectually by
covering the uncoated part of the glass
with a layer of sealing-wax, or other
D 2

ELECTRICITY.

resinous varnish. The liquid should be
applied with a flat, camel-hair pencil,
the glass being previously warmed.

On the other hand, it is a curious
circumstance, that there is a degree of
humidity in the inside of the jar, not
only compatible with a high charge, but
which even contributes to retain it.
This effect was accidentally observed
by Mr. Brooke, and afterwards by Mr.
Cuthbertson, who states that a jar will
take a much greater charge, namely,
one- third more, if its inside be consider-
ably damped by blowing into it with the
mouth through a tube reaching to the
bottom. The explanation of this re-
markable fact has been given by Pro-
fessor Robison on the principles for-
merly explained, namely, that there is
no electric intensity so great, but that it
may be insulated by the least imperfect
conductor, provided the latter be long
enough, and so constituted as that the
intensity of the electricity it contains
shall diminish by sufficiently gentle gra-
dations. An uniform dampness, in-
deed, will not do this ; but it will dimi-
nish the abruptness of the variations of
intensity, and thus give security against
a spontaneous discharge. A similar
protection against the breaking of the
glass is afforded by placing a layer of
paper between the glass and the tin-foil,
and making it extend also an inch be-
yond the coating.

• (131.) Glass balloons of a spherical
shape, being of more uniform thickness
than jars, would be much preferable for
the construction of an apparatus of this
kind, were it possible to apply an uni-
form coating to the inside. Professor
Robison recommends the following con-
struction for a portable jar, which he
found to answer exceedingly well. A
long-necked phial was made of sheet
tin, and then coated entirely on the out-
side with line sealing-wax, one thirtieth
of an inch thick. The sealing-wax was
then coated with tin- foil, all but the
neck. It is evident, that the wax here
acts the part of the glass in the common
*ar, the tin plate corresponding to the
inner coating and wire, and the tin-foil
to the outer coating. The dissipation
is almost nothing if the neck be very
small ; and it only requires a little cau-
tion to avoid bursting by too high a
charge. Even this may be prevented
by coating the sealing-wax so near to
the end of the neck, that a spontaneous
discharge must happen before the accu-
mulation is too great, Alternate layers

of tin- foil and hard varnish form also a
very compendious battery. It admits
of a surprising accumulation, without
shewing any vivid electricity ; but it
must be used with more caution, lest it
should be spoiled by a spontaneous dis-
charge, in which case we cannot disco-
ver where the flaw has happened, and
the whole is rendered useless.

(132.) By combining together a suffi-
cient number of jars we are able to
accumulate an enormous quantity of
electricity : for this purpose all the in-
terior coatings of the jars must be made
to communicate by metallic rods, and a
similar union must be established among
the exterior coatings. When thus ar-
ranged, the whole series may be charg-
ed, as if they formed but one jar ; and
the whole of the accumulated electricity
may be transferred from one system of
coatings to the other, by a general and si-
multaneous discharge. Such a combina-
tion of jars is called an Electrical Battery.

(133.) It is evident, that an apparatus
of this kind, consisting of a great number
of parts, must be more liable to derange-
ment than a single jar: for if any one
of the jars should happen to break by a
spontaneous explosion, the whole battery
would be rendered useless, until the
broken jar be removed. It is prudent,
therefore, to secure the adjacent jars
from actual contact, by fixing them in a
box having thin partitions ; the coated
bottoms of the jars resting on a trellis
of wire, or on a sheet of tin-foil, which
may establish a general communication
between them ; while the rods from the
interior coatings are connected above by
cross wires, having balls at their ex-
tremities in order to obviate the dis-
sipation of the electricity. On the other
hand, by limiting the communications to
a certain number of jars, we have it in
our power to charge only a part of the
battery, without employing the whole.

CHAPTER IX.

Management of Electrical Jars and

Batteries.

(134.) FOR the purpose of making the
direct communication between the inner
and outer coating of ajar or battery, by
which a discharge is effected, the in-
strument shown in fig. 32, and which is
Fig. 32.

ELECTRICITY.

37

called 'the Discharging Rod or Jointed
Discharger, may be conveniently em-
ployed. It consists of two bent metallic
rods, terminated at one end by brass
balls, and connected at the other by a
joint, which is fixed to the end of a glass
handle, and which, acting like a pair of
compasses, allows of the balls being
separated at different distances. When
opened to the proper degree, one of the
balls is made to touch the exterior
coating, and the other ball is then quickly
brought into contact with the knob of
the jar, as represented in Jig. 33, or with
Fig. 33.

any part of the system of the interior
coatings, and thus a discharge is effected ;
while the glass handle secures the person
holding it from the effects of the shock.

035.) If we wish to send the whole
charge of electricity through any par-
ticular substance which may be the
subject of experiment, we must so ar-
range the connecting conductors, as
that the substance shall form a neces-
sary part of the circuit of the electricity,
as it is termed. With this view, we
must place it between two good con-
ductors, one of which is in communica-
tion with the outer coating; and the
circuit may then be completed by con-
necting the other conductor with the
inner coating by means of a discharging
rod, to one branch of which, if neces-
sary, a flexible chain may be added.

(136.) In order to direct the charge
with more certainty and precision, an
apparatus, called the Universal Dis-
charger, was contrived by Mr. Henley,
and is represented in jig. 34. It con-
Fig. 34.

sists of a wooden stand with a socket
fixed in its centre, to which may be
occasionally adapted a small table T,
having a piece of ivory (which is a non-
conductor) inlaid on its surface. This
table may be raised and kept at the
proper height by means of a screw S.
Two glass pillars P, P are cemented
into the wooden stand. On the top of
each of these pillars is fitted a brass
cap, having a ring R attached to it, and
containing a joint, moving both verti-
cally and horizontally, and carrying on
its upper part a spring tube, admitting
a brass rod to slide through it. Each
of these rods is terminated, at one end,
either by a ball, a point, or a pair of
forceps, and is furnished at the other
extremity with a handle of solid glass.
The body through which the charge is
intended to be sent, is placed on the
table, and the sliding rods, which are
moveable in every direction, are then, by
means of their insulating handles,
brought in contact with the opposite
sides, and one of the brass caps being
first connected with the outside of the
jar or battery, the other may be brought
in communication with the inner coat-
ings, by means of the discharging rod
above described. For some experiments
it is more convenient to fix the sub-
stance, on which the experiment is to
be made, in a mahogany frame, con-
sisting of two boards, which can be
pressed together by screws, and which
may then be substituted for the table
T. In either of these ways the charge
can be directed through any part of the
substance with the greatest accuracy.

(137.) The quantities of electricity
which can be accumulated in any given
extent of coated glass, are in the inverse
proportion to the thickness of the glass.
Different jars or batteries, therefore,
will, according to the thinness of their
sides, and the quantity of coated surface
they contain, have different capacities
of holding charges of electricity. But
in any given instrument of this kind,
the quantity of the charge communicated
to it by a machine may be measured by
the intensity of the electricity in the
prime conductor, which communicates
with the interior coating. Some esti-
mate of the intensity may be obtained
by the employment of Henley's quadrant
electrometer already described, ($ 72,)
the index of which rises very slowly
while the battery is charging, till it
reaches a certain elevation, correspond-
ing to the capacity of the battery. If

38

ELECTRICITY.

the electricity be accumulated beyond
this limit, a spontaneous discharge takes
place, and the process must then be re-
newed in order to obtain a full charge.
It is more prudent, however, to stop
before this degree of accumulation is
attained: and one great advantage of
Henley's electrometer is, that it shows
us the progress of the charge, and how
far we may proceed with safety.

(138.) But the most effectual security
against fracture from a spontaneous
discharge, is to form an interrupted
circuit, of which the parts, where the
interruption occurs, terminate by me-
tallic balls, placed at a certain distance
from each other. By varying the in-
terval between them, we may regulate
the quantity of electricity which we
shall allow to accumulate in the battery ;
for the moment it exceeds the quantity
of which that interval is the striking
distance (§ 87,) an explosion happens,
by the electricity forcing its wray through
the air from one ball to the other. If the
balls be brought very near each other,
a discharge will take place with a com-
paratively small accumulation: when
farther separated, a greater charge will
be retained, because a higher intensity
of electricity is required in order to pass
through the larger intervening space.
It is on this principle that the instru-
ment, called Lane's Discharging Elec-
trometer, is constructed. It consists of
a brass ball, ~B,fig. 35, placed at the end

Fig. 35.

quired : the other end of the moveable
rod must be connected, by means of a
chain or wire, with the outer coating.

The chief use of this instrument is to
allow a jar to discharge itself sponta-
neously through any previously arranged
circuit, without employing a discharging
rod, or moving any part of the apparatus ;
and also to produce successive explo-
sions nearly of the same strength. The
magnitude of the charge is measured by
the distance at which the balls are
placed ; and the power of the machine
may be estimated by the number of
explosions, which, at any given dis-
tance, take place in equal times. In
Mr. Lane's experiment the shocks were
twice as frequent when the interval be-
tween the balls was l-24th of an
inch, as when twice as much : hence he
concluded that the quantity of electri-
city required for a discharge is in exact
proportion to the distance between the
surfaces of the balls. But the indica-
tions of this instrument are in reality
subject to great fallacy, on account of
the variable state of the atmosphere,
which affects its conducting power;
the quantity of dust which, even during
the course of an experiment, is liable
to be attracted, and to collect upon the
balls ; and also from the roughening
and tarnishing of the metallic surfaces
produced by frequent electric explo-
sions. This last imperfection is one to
which brass balls are particularly ex-
posed ; and might, if it were worth
while, be remedied by having the balls
made of fine silver.

(139.) Another contrivance for re-
gulating the amount of the charge
which we may wish to send through any
substance, is that invented by Cuth-
bertson, and termed the Balance Elec-
trometer. It consists of a metallic rod,
R, fig. 36, terminated by two equal
balls A, B, and balanced, like a scale-

Ftg. 36.

of a short metallic rod R, which moves
through a tubular piece, supported by
a bent glass stand S. This stand is
made so as to be capable of being fixed,
by its other extremity, to the rod pass-
ing up from the interior coating, and
adjusted so that the ball B is immedi-
ately opposite to the knob' of the jar,
and may be brought to the exact strik-
ing distance from it which may be re-

ELECTRICITY.

39

beam, upon knife-edged centres. One
of the arms of this beam is graduated,
and carries a slider, which, when set
at different distances from the centre of
motion, acts on the lever with a pro-
portionate weight from one grain to
sixty. The ball A, at the extremity of
this loaded arm, rests on a similar ball
D, below it, which is supported by a
bent metallic tube T, proceeding from
the same stand as that which supports
the rods ; the whole being insulated by
a glass pillar P. At a little distance
below the ball B, at the other extre-
mity of the beam, another ball C, insu-
lated by the glass pillar Q, is placed;
this last ball is to be connected by a
chain with the outer coatings of the
battery, while the metallic support of
the balance is connected with the inner
coatings. When a charge is commu-
nicated to the battery, the two balls A
and D, which are in contact, become
repulsive of each other ; and when the
force of this repulsion is sufficient to
raise the weight on the loaded arm of
the beam, the other arm will be forced
down, and the ball B coming in con-
tact with the ball C, the circuit will be
completed and a discharge take place.
As the force of the repulsion depends
upon the intensity of the charge, the
weight it has to overcome affords a
measure of this intensity, and enables us
to regulate its amount.

The practical application of accumu-
lated electricity to various purposes of
experiment, involves considerations
which relate to the laws observed by
electricity in its movements, and which
more properly belong to the subject of
the ensuing chapter.

CHAPTER X.

Of the Motion of accumulated Elec-
tricity.

(140.) IN forming arrangements for
directing the passage of accumulated
electricity, it should be borne in mind
that the electric fluid will, on these oc-
casions, always pass through the best
conductors, although they may be more
circuitous, in preference to those which
are more direct, but have inferior con-
ducting power: and it must also be
recollected, that when different paths
are open for its passage, along con-
ductors of equal power, the electricity
will always take that which is the
shortest. Thus if a person, holding
a .wire between his hands, discharges a

jar by means of it, the whole of the
fluid will pass though the wire, without
affecting him : but if a piece of dry
wood be substituted for the wire, he
will feel a shock ; for the wood, being
a worse conductor than his own body,
the charge will pass" through the latter,
as being the easiest, although the
longest circuit. During its transit
through the human body, in like man-
ner, the shock is felt only in the parts
situated in the direct line of communi-
cation ; and if the charge be made to
pass through a number of persons who
take one another by the hand, and form
part of the circuit between the inner
and outer coatings of the jar, each will
feel the electric shock in the same
manner and at the same instant ; the
sensation reaching from hand to hand,
directly across the breast. By varying
the points of contact, however, the
shock may be made to pass in other
directions, and may either be confined
to a small part of a limb, or be made
to traverse the whole length of the
body from head to foot.

(141.) By accurate experiments it ap-
pears that the force of the electric shock
is weakened, that is, its effects are di-
minished, by employing a conductor of
great length for making the discharge.
But it is difficult to assign a limit to the
number of persons through which even
a small charge of electricity may be
sent, so that all shall experience the
shock ; or to the distance along which
it may be conveyed by good conductors.
At an early period of electrical inquiries,
much interest was attached to the de-
termination of these points. The Abbe
Nollet passed an electrical shock from
a small phial through a hundred and
eighty of the French guards in the pre-
sence of the king ; and at the Carthusian
convent in Paris, the monks were formed
into a line of above a mile in length, by
means of iron wires held between them :
on the discharge of the phial, the sensa-
tion was felt at the same moment by all
the persons composing this extensive
circuit. Many experiments were made
both by the English and French 'elec-
tricians with a view to ascertain the
space which a discharge can be made
to traverse, and the velocity with which
it is transmitted. Of these the most
ingenious and satisfactory were the ex-
periments planned and executed by Dr.
Watson, with the assistance of the lead-
ing members of the Royal Society. A
circuit was formed by a wire which ex-

40

ELECTRICITY.

tended the whole length of Westminster
bridge, at a considerable height above
the river: one end of this wire com-
municated with the outer coating of a
charged phial, the other being held by a
person on the opposite side of the river,
who formed a communication with the
water by dipping into it an iron rod
held by the other hand. The circuit
was completed by another person, who
stood near the phial, and who likewise
dipped an iron rod into the river with
one hand, and was enabled, by means
of a wire held in the other, to effect a
contact with the knob of the phial.
Whenever the discharges took place, the
shocks were felt by both persons ; thus
proving that the electric fluid must have
been in motion along the whole line of
the circuit, including both the wire
above and the river below,

In another experiment, made on Shoot-
ers'-hill, at a time when the ground was
remarkably dry, the electricity was made
to perform a circuit of four miles ; being
conducted for two miles along wires
supported upon baked sticks, and for
the remaining distance, also of two miles,
through the dry ground. As far as
could be ascertained, by the most careful
observation, the time in which the dis-
charge was transmitted along that im-
mense circuit was perfectly instantane-
ous : nor has any other trial that has
yet been made afforded the least ap-
proach to a measurement of the velocity
with which electricity moves.

(142.) On this subject, however, an
important distinction should be made
between the actual movement of each
individual particle of electric fluid, and
the transmission of an impulse along a
series of such particles, for the one may
bear hardly any proportion to the other :
just as we find that sound proceeds with
a velocity incomparably greater than
that of the particles of air which are
concerned in its propagation. In like
manner the portion of blood, which raises
the artery at the wrist, where the pulse
is felt, is not the identical portion of
blood which is thrown out from the heart
by the contraction of that organ pro-
ducing that pulsation : the impulse, in
all these cases, being propagated like a
wave, from one particle to another.
There is, therefore, no reason to sup-
pose that the same particles of electric
fluid, which enter at one part, have tra-
versed from one end to the other the
whole line of conducting substances
which form the circuit.

(143.) If we conceive the conducting
bodies which compose the circuit to be
divided into an indefinite number of
filaments, every one of which is capable,
in an equal degree, of conveying the
electric fluid, it is evident that the united
power of these filaments, or what is
the same thing, the capability of the
body itself to convey a charge of elec-
tricity, is in proportion to the number
of these elementary filaments which it
contains, that is, to the magnitude of its
transverse section, without any relation
to its form. Thus, the same metallic
rod will conduct a charge equally well,
whether it be flattened, or divided into
several smaller wires, or whether it con-
sist of a single cylinder of the same
area.

(144.) If the size of the conductor be
sufficiently great, the whole charge may
be conveyed without any sensible ob-
struction or retardation, and therefore
without any tendency to deviate from
the direct line of its course. But it is
otherwise when the conductor is too
slender to afford a ready passage to the
fluid which is pressing onwards : and it
is important to inquire into the conse-
quences to which these obstructions may
give rise.

(145.) The first effect of an impedi-
ment to the free passage of accumulated
electricity must be a retardation of its
motion-. It is reasonable, therefore, to
expect that with a circuit composed
either of bad conductors, or of con-
ductors of inadequate size, although
good, the discharge will not be effected
so instantaneously, nor so completely ;
and that the shock which accompanies it
will be diminished in its violence. This
principle may find its application on oc-
casions where it is desirable to soften
the intensity of 'the shock, as in the
medical employment of electricity, where
imperfect conductors are on this ac-
count sometimes preferable, both for
taking sparks and shocks.

(146.) A second eftect resulting from
an obstruction to the flow of electricity,
is a tendency in the fluid to diverge
from the direct line of its course, and to
fly off to different objecis in the vicinity.
This is frequently exemplified in the
case of lightning, which, on striking a
building, is apt to take a very irregular
and seemingly capricious route, darting
towards conducting bodies which may
happen to attract it, although at some
distance from the immediate direction it
was pursuing. The position of such

ELECTRICITY

conducting bodies would appear to have
a material influence in determining the
striking distance. It was remarked by
Dr. Priestley, that the explosion from a
large battery extends to a greater dis-
tance over the surface of water than in
air alone.

(147.) An effect which seems to de-
pond upon this tendency in the fluid to
divergence in consequence of obstruc-
tion, all hough it has by some been re-
ferred to a different principle, is that
which has been termed the lateral ex-
plosion. When a large jar or battery
is discharged '"by a metallic wire which
is held in the hand without the protec-
tion of any glass or other insulating
handle, it often happens that a slight
shock is felt in the hand that grasps the
wire, especially if the charge of elec-
tricity be very considerable. This ap-
parent divergence or overflow of electric
fluid, when rushing in large quantities
through a narrow space barely sufficient
to contain it, may also be rendered
visible in other ways. If one end of a
chain be connected with the outer coat-
ing of a charged jar, while the remainder
of the chain is lying loosely upon a
table, on discharging the jar in a dark-
ened room, by a discharging rod, in
the usual way* it will be found that the
chain, although it makes no part of the
circuit, is rendered luminous by the
passage of sparks from one link to an-
other. The following experiment, made
by Dr. Priestley, may also be regarded
as a case of lateral explosion. Let a
thick metallic rod R, ^. 37, be sup-
Fig. 37.

ported on an insulating stand, and
placed with one of its ends in contact
with the outer coating of a Ley den jar ;
and at a distance of half an inch from
its other extremity place a long con-
ducting body B, of at least six or seven
feet in length, and only a few inches in
breadth. Let a chain C, be now placed
upon the table, so that one of its ends
may be about an inch and a half dis-

tant from the outer coating of the jar,
and apply one end of the discharging
rod D, to the other extremity of the
chain. As soon as the other ball of the
discharging rod is made to touch the
knob of the jar, so as to effect a dis-
charge, a brilliant spark is seen to ex-
tend between the insulated rod R, and
the adjacent conductors. This lateral
spark has the same length and brilliancy
whether it be received on flat or smooth
surfaces, or on sharp points.

It is stated by Dr. Priestley, that
the effect we have been describing takes
place without any apparent change in
the electrical state of the conductor B ;
and hence Cavallo conceived that the
lateral spark was sent out from the jar,
and returned to it almost at the same
instant, allowing of no perceptible time
for an electrometer to be affected. Dr.
Robison, however, always observed,
on repeating the experiment, that a
very delicate electrometer was affected
under these circumstances : and the
same observation is confirmed by Biot.
The phenomena of the lateral ex-
plosion have been attempted to be
explained by the electricity exerting,
during its passage, an inductive in-
fluence, of which the effects may be
expected to cease the moment the cause
is removed. But this explanation ap-
pears to* be less satisfactory than the
one which attributes the phenomena to
an expansive propulsion, followed by
an immediate recession of electric fluid,
produced by obstructions to its free
passage in the circuit of conductors.

CHAPTER XL
Effects of Electricity upon Bodies.

(148.) HAVING considered the cir-
cumstances attending the motion of
electricity with reference chiefly to the
fluid itself, we next proceed to give an
account of the effects which it produces
upon bodies by its passage through
them.

(149.) Independently of electrical at-
traction and repulsion, it does not appear
that the simple accumulation of electricity
in any quantity in bodies, as long as it
remains quiescent, produces the least
sensible change in their properties. A
person standing upon an insulating stool
may be charged with any quantity of
electricity from a machine, without being
perceptibly affected, until the equilibrium
of the fluid is disturbed, by drawing
sparks from his body, or from the prime

42

ELECTRICITY.

conductor with which he may be in com-
munication.

We have already seen, indeed, (§ 78,
79, 80,) that it is only a very small part
of an electrified body, namely, the mere
surface, that is in an active state, either
of positive or negative electricity, and
that the rest of the substance of the body
is in a state of perfect neutrality.

(150.) It also appears that the unin-
terrupted passage of any quantity of
electricity through a perfect conductor,
such as a rod of metal which is of suffi-
cient thickness to convey it, occasions
no perceptible alteration in the mecha-
nical properties of the conducting body.

(151.) On the contrary, very consi-
derable effects are produced when a
powerful charge is sent through a wire,
which from the smallness of its size will
not admit of the whole quantity to pass
with perfect freedom ; or through a sub-
stance which, although large, is deficient
in conducting power ; or, in other words,
which opposes a degree of resistance to
the passage of electricity. Thus, an iron
conductor will carry off the whole elec-
tricity of a thunder-cloud in safety and
in silence, while a beam of wood, or a
tree, struck by lightning, is shivered into
a thousand fragments.

(152.) When electricity thus changes
the physical properties of bodies, its
operation may, in general, be referred to
that of separating their particles in the
line of its course. This separation is
effected with more or less violence, ac-
cording to the intensityand quantity of the
charge, and is frequently attended by the
evolution of heat and light. The mecha-
nical effects of electricity resemble those
which would be produced by a material
agent driven with great velocity and force
through the substance of the body. Some
of these effects, on the other hand, seem
to be the consequences of the expansion
produced by heat; but many of the
changes induced by electricity are of a
chemical nature, and such as mechani-
cal agencies alone are insufficient to ex-
plain. We proceed to describe these
several effects more particularly.

§ 1. Mechanical Effects of Electricity.

(153.) The cohesion of the particles of
solid bodies may be conceived to oppose
some resistance to the tendency of elec-
tricity to separate these particles from
one another ; for we find that fluids are
more violently acted upon than solids, by
the passage of the electric discharge. If
the stem of a capillary tube, such as is

employed for making thermometers, be
filled with mercury, and placed so that
the filament of this metal forms part of
the circuit ; on the discharge being
made, the glass tube will be burst, and
its fragments, together with the mer-
cury, will be completely dispersed. If a
fluid of inferior conducting power, such
as water, be contained in a tube of
larger diameter than in the preceding
experiment, the passage, even of a mo-
derate charge, will be sufficient to break
the tube, and scatter its contents. Oil,
alcohol, and ether, oppose still greater
resistance than water to the ^.passage of
electricity, and they are expanded and
scattered with still greater violence by a
discharge being made to pass through
them.

(154.) Beccaria introduced two wires
through holes in the opposite sides of a
perforated ball of solid glass of two in-
ches diameter, the ends of the wires
being separated by a drop of water,
which occupied the centre of the perfo-
foration. On passing a shock through
the wires and intervening drop, the ball
was shattered with great violence. By
a similar arrangement, Mr. Morgan
succeeded in breaking green glass bot-
tles filled with water, when the distance
of the wires between which the explosion
passed exceeded two inches. In this
way, also, glass tubes, half an inch thick,
with a bore of the same diameter, were
burst with a very moderate charge, in
Mr. Singer's experiments. If a cup-like
cavity be turned in a piece of ivory, ca-
pable of receiving the half of a" light
wooden ball, with a small conical cell at
the bottom of the cavity, and two wires
be inserted into it through the sides of
the ivory ; on putting a drop of water,
alcohol, or ether between the wires, and
placing the ball over ^them in its cavity,
and sending a charge through the drop
of fluid, part of it will be suddenly con-
verted into vapour, and the ball will be
propelled with great violence/ Even a
common drinking glass, filled with water,
may be broken by the explosive force
with which vapour is formed at the
point where the electricity passes. Bec-
caria constructed a small mortar with a
ball, behind which a drop of water was
placed, so as to be between the two
wires that passed through the sides of
the mortar. The charge being sent
through the two wires, the drop of water
was expanded with such force, as to-
drive out the ball with great velocity
Mr. Lullin, of Geneva, found that, by

ELECTRICITY.

43

using oil instead of water in this ex-
periment, the ball was projected with
still greater force.

( 1 55 .) If two wires be introduced into
a soft piece of tobacco-pipe clay, so that
their ends be near each other, and
a shock passed through them, the clay
will be curiously expanded in the inter-
val between the wires. The experiment
will not succeed if the clay be either too
dry or too moist. If the clay be too dry,
or' the shock too powerful, the mass will
be shivered into innumerable fragments.
If the clay be placed in the tube of a to-
bacco-pipe, or in a glass tube, the ex-
pansion of the clay will be so consider-
able as to shatter "the tube which con
tains it.

(156.) The expansion of air by the
passage of the electrical fluid, either in
the form of sparks or shocks, is shown
in the following experiment of Kinners-
ley, the apparatus for which has been
called the Electrical Air Thermometer.
It consists of a glass tube closed at both
ends by air-tight brass caps, through
which two wires slide in the direction of
the axis of the tube. These wires are
terminated by brass balls, which are
made to approach within the striking
distance. To an aperture in the bot-
tom of the lower cap is fitted a bent
tube of glass which turns upwards, and
is open at both ends; the bent partis
filled with mercury, or with a coloured
fluid, which may indicate by its rising
or falling in the tube any dilatation or
contraction that may take place in the air
within the vessel. It is found that every
time a spark passes between the brass
balls, the fluid suddenly rises, but de-
scends again to its former level imme-
diately after each explosion ; thus show-
ins: that the dilatation of the air, produced
by the abrupt passage of electricity, is but
of momentary duration.

(157.) When a strong electrical charge
is sent through a very confined portion
of air, the explosive effects produced by
it are as considerable as those we have
seen exhibited by denser fluids. Thus if
a piece of plate glass of the size of a
square inch, and half an inch in thick-
ness, be laid flat upon the small table of
Henley's universal discharger, ($ 136,)
and pressed down by a weight, and the
points of the sliding wires be set oppo-
site to each other and against the under
edge of the glass, so that the electricity
may pass beneath it, the charge of a
large jar transmitted in this wray will
break the glass into innumerable frag-

ments, and even reduce a portion into an
impalpable powder. If the mouth of a
small mortar made of ivory, with a ca-
vity of half an inch diameter and an inch
deep, be stopped by a cork, fitted so as
to close the aperture accurately, yet
without much friction, and if two wires
be inserted through the sides of the mor-
tar so that their points within the cavity
be separated by an interval of about a
quarter of an inch, a strong charge
being sent through the wires will expand
the air within the cavity so suddenly as
to project the cork to some distance.

(158.) Solid bodies of a porous tex-
ture, such as wood, are easily torn
asunder by an electric charge. If two
holes be drilled in the opposite ends of
a piece of wood, about half an inch
long, and a quarter of an inch thick,
and the ends of two wTires inserted in
the holes, so that their points may be at
the distance of a quarter of an inch ; on
passing a strong charge through them,
the wood will be split in pieces. Stones,
loaf-sugar, and other brittle and im-
perfectly conducting substances, may be
broken in a similar way.

Place a piece of dry writing paper
upon the table of the universal dis-
charger, and having removed the balls
from the ends of the sliding wires, press
the points of the wires against the paper
at the distance of two inches from each
other ; if a powerful shock be now sent
through the wires, the paper will be
torn in pieces. If a number of wafers
be placed on the table, instead of paper,
they will be dispersed in a curious man-
ner, and many of them broken into
small fragments.

(159.) A singular result is obtained
by the following variation in the circum-
stances of the last experiment, which
was made by Mr. Lullin. Suspend a
varnished card by silk threads, (see fig.
38,) in such a manner that two blunt

Fig. 38.

wires proceeding from the two sides of
a jar or battery, may be in contact with

44

ELECTRICITY.

the opposite sides of the card, but at the
same time half an inch distant from
each other; when the discharge is
made between the wires, and along
the surface of the card, the latter is
found to be perforated, but always at
the point where the wire communicating
with the negative side of the battery had
touched it. The same perforation takes
place at this point, even when a hole
has been previously made at the point,
where it is touched by the positive wire.

The course of the electric fluid may
be traced with more precision, by hav-
ing both sides of the card coloured,
previously to the experiment, with ver-
milion, for it will then leave on the
card a well defined black line extending
from the point of the positive wire to
the perforation ; and a diffused black
mark on the opposite side of the card,
around the perforation, and next to the
negative wire.

(160.) When the electrical discharge
is made to pass in a perpendicular di-
rection through the thickness of a card,
which may be effected by placing it
against the outer coating of a Leyden
jar, and setting the lower ball of the
discharging rod against the other side
of the card, so that its thickness may be
interposed between it and the tin-foil,
and making the explosion in the usual
way, as represented in Jig. 33, (§ 134,)
the card will be perforated. At the edge
of the perforation, on each side of the
card, there will be a small bur or pro-
trusion, which is always larger on the
side next to the jar, than on that next to
the discharging rod ; the former being
the negative, and the latter the positive
side. By passing the shock through a
quire of paper, instead of a single card,
the progress of this effect at different
depths from the surface may be accu-
rately analysed. Mr. Symmer, who de-
vised this experiment, observed that the
ragged edges were for the most part
directed outwards from the body of the
quire. Upon examining the leaves se-
parately, however, he found that the
edges of the holes were bent regularly
two different ways, and more remarka-
bly so about the middle of the quire ;
one edge of each hole being throughout
its course forced one way, and the other
edge in the contrary direction, as if the
hole had been made in the paper by
drawing two threads through it in oppo-
site directions.

(161.) ri he following variation of the
experiment illustrates the nature of the

mechanical impressions made by elec-
tricity. Let a sheet of tin-foil be placed
in the middle of a quire of paper ; ori
making the discharge through it, the
tin-foil is found to have received two
indentations in opposite directions, and
the leaves of paper are rent in such a man-
ner, that on both sides of the tin-foil the
burs point towards the outsides of the
quire ; but the indentations upon the
tin -foil, and the burs on the paper, are in
opposite directions. If another quire of
paper be taken, and two sheets of tin-
foil be placed within it, so that they are
separated by the two middle leaves of
the quire, the result will be that all the
leaves will be perforated, excepting the
two within the tin-foil, and in these two
leaves there will be two impressions or
indentations in opposite directions.

(162.) The mechanical effects we have
just described have been often adduced,
not only as proofs of the materiality of
the electric fluid, but also as positive
indications of the direction of its motions,
according as either the one or the other
of the two theories of electricity is
adopted. But this is a subject which we
reserve for future discussion.

(163.) The fracture of glass by the
electrical explosion has already been
adverted to, ($ 129 ;) but there are still
a few circumstances attending it which
deserve to be noticed. The edges of the
fractured portion appear well defined on
the positive side ; while on the negative
side they are splintered, as might be
expected from the passage of a material
agent from the former to the latter. It
is remarkable also, that a perforation
may be made in glass by a very mode-
rate discharge, when the glass is in con-
tact with oil or sealing-wax. Thus if
a small phial, or glass tube, closed at
one end, be filled with olive oil, and a
pointed wire, bent at right angles, and
passing through a cork fitted to the
mouth of the phial or tube, be intro-
duced into it, so that the point may touch
any part of its inside beneath the sur-
face of the oil ; on suspending the ves-
sel by its wire to the prime conductor of
an electrical machine, and applying to
the outside, either the knuckle, or a
brass ball, exactly opposite to the point
of the wire within, so that a spark may
pass between them, it will be found to
have made a small perforation through
the glass ; by bringing the wire in con-
tact with different parts of the glass, a
great number of holes may thus be
made in it. The effect of the oil appears

ELECTRICITY.

45

to be that of controlling the tendency of
the electric fluid to diverge, and of con-
centrating the whole power of the charge
into a single point.

( 1 64.) This repulsive tendency is also
well illustrated by the following experi-
ments made by Dr. Priestley. If a clean
brass chain, previously dipped in melted
resin, be laid upon paper, and the
charge of a battery of at least 32 square
feet "be sent through it, the resinous
coating will be thrown off from every
part of the chain, which will be left per-
fectly clean, and free from resin. If a
brass chain be laid upon a piece of
glass, and a similar charge passed
through :it, the glass will be marked in
a beautiful manner on every part of its
surface, where it had been touched with
the chain, every spot having the width
and colour of the link. The metal may
be scraped off the glass at the outside of
the marks, but in the middle part it is
forced within the pores of the glass.
Dr. Priestley communicated a similar
tinge to glass, by means of a silver
chain, and small pieces of other metals ;
but he could not succeed with large
pieces.

(165.) The effects of accumulated
electricity upon metallic bodies, are re-
ferable, for the most part, to the agency
of the heat produced by its passage
through them ; yet the phenomena, in
many cases, indicate also the operation
of other forces. By the transmission,
through a piece of metal, of repeated
shocks, which are not powerful enough
to effect its fusion, or even ignition, a
permanent alteration may be produced
in its form, such as would not have re-
sulted from heat alone. Dr. Priestley
and Mr. Nairne found by experiment,
that a chain through which an electrical
charge had passed, undergoes a diminu-
tion in its length. A piece of hard
drawn iron wire, ten inches long and
one hundredth of an inch in diameter,
was found, after fifteen discharges, to
have lost one inch and one tenth of its
length ; and the increase of thickness
seemed to be in proportion to this lon-
gitudinal contraction, for the wire had
not perceptibly lost any of its weight
during the experiment. A copper wire
plated with silver, of the same dimen-
sions as the former, underwent, by the
same treatment, a diminution of length
two thirds as great as that of the iron
wire.

On the other hand, if the shocks
be transmitted through a wire which

has a weight suspended by it, so as
to irive it considerable tension, the length
of the wire becomes increased instead
of diminished, as in the above experi-
ment. This is evidently owing to the
influence of the heat which accompa-
nies the passage of the electricity, and
which diminishes the cohesion of the
particles of the metal, and disposes
them to yield to the extending force
which the weight supplies.

$ 2. Evolution of Heat by Electricity.

(166.) The ignition and fusion of metals
by the electric discharge, are phenomena
which have been long observed. Thus
by passing a strong charge through
slender iron wires, they are ignited, and
partly melted into globules. It was
formerly believed that very large bat-
teries were necessary for obtaining this
effect; but if the wire be sufficiently
fine, the electricity accumulated in a
single jar of moderate size will suffice
for its production. The best material
for exhibiting this effect, is the finest
flatted steel sold at the watchmakers'
tool shops, under the name of watch
pendulum wire. Van Marum has given
a statement of the lengths of wires of dif-
ferent diameters, and of different metals,
which his powerful machine enabled
him to melt; when they were drawn
to the thirty-second part of an inch in
diameter, he found that he could fuse
120 inches of lead wire, and the same
quantity of tin wire ; five inches of iron
wire; three inches and a half of gold
wire ; and only one quarter of an inch
of wires of silver, copper, or brass.

(167.) From the experiments of
Brooke and of Cuthbertson, it has been
inferred that the length of wire which is
thus melted by the electric discharge,
varies as the square of the quantity of
accumulated electricity which is sent
through it ; thus a combination of two
jars, charged to an equal degree, will
melt four times the length of wire which
one jar will melt.

(168.) While the electric battery thus
effects the fusion, and even in some
cases the volatilization of metals, the
phenomena appear also to indicate the
action of propelling or dispersive forces,
as if the agent concerned in their pro-
duction was endowed with great mecha-
nical momentum. Thus the densest
metals are rent and dispersed with vio-
lence by the passage of accumulated
electricity If a slip of gold or silver
leaf be placed on white paper, and a.

46

ELECTRICITY.

strong shock passed through it, the
metal will disappear with a bright flash,
and the impulse with which its particles
are driven against the paper will produce
a permanent stain of a purple or grey
colour. Franklin found that if the
metallic leaf be placed between two
panes of glass firmly tied together, the
explosion, provided the glass withstands
the concussion, will leave on each of
its surfaces an indelible stain, in conse-
quence of some of the metallic particles
being actually forced into the substance
of the glass, and being then inaccessible
to the action of chemical solvents applied
to the surface of the glass. Sometimes
it is found that these metallic stains
extend to a greater distance than the
breadth of the piece of metal. It often
happens, however, that the pieces of
glass themselves are shattered to pieces
by the discharge.

(169.) The colours produced by the
electric explosion of metals have been
applied to impress letters or ornamental
devices on silk and on paper. For this
purpose Mr. Singer directs that the out-
line of the required figure should be
first traced on thick drawing paper, and
afterwards cut out in the manner of
stencil plates. The drawing paper is
then placed on the silk or paper intended
to be marked ; a leaf of gold is laid upon
it, and a card over that ; the whole is
then placed in a press or under a weight,
and a charge from a battery sent through
the gold leaf. The stain is confined by
the interposition of the drawing paper to
the limit of the design, and in this way a
profile, a flower, or any other outline
figure may be very neatly impressed.

(170.) The heat evolved by electricity,
like most other of its effects, is in pro-
portion to the resistances opposed to its
passage. The less the conducting power
of a metal, the greater is the portion of
it which the same shock can ignite or
destroy. A rod of wood of considerable
thickness being made part of the circuit,
has its temperature sensibly raised by a
very few discharges. Most combustible
bodies are capable of being inflamed by
electricity, but more especially if it be
made to strike against them in the form
of a spark or shock obtained by an in-
terrupted circuit, as by the interposition
of a stratum of air. In this way may
alcohol, ether, camphor, powdered resin,
phosphorus, or gunpowder be set fire
to. The inflammation of oil of turpen-
tine will be promoted by strewing upon
it fine particles of brass filings, If the

spirit of wine be not highly rectified, it
will generally be necessary previously to
warm it, and the same precaution must
be taken with other fluids, as oil and
pitch ; but it is not required with ether,
which usually inflames very readily.
But, on the other hand, it is to be re-
marked that the temperature of the body
which communicates the spark appears
to have no sensible influence on the heat
produced by it. Thus the sparks taken
from a piece of ice are as capable of in-
flaming bodies as those from a piece of
red-hot iron. Nor is the heating power
of electricity in the smallest degree dimi-
nished by its being conducted through
any number of freezing mixtures which
are rapidly absorbing heat from sur-
rounding bodies.

(171.) Light, as well as heat, is emit-
ted during the electric discharge at every
point where the circuit is either inter-
rupted, or is occupied by bodies of infe-
rior conducting powers. A moderate
charge will produce a bright spark when
made to pass through water, and the
spark is still more luminous in oil, alco-
hol, or ether, which are worse conductors
than water : on the contrary, in fluids of
greater conducting power there is greater
difficulty of eliciting electric light. Thus
a much higher charge is required to
produce a spark in hot water than in
cold ; a still higher in saline solutions ;
and in concentrated acids, light can be
obtained only when their volume is very
small ; so that it is necessary for that
purpose, to draw a line of the acid upon
a plate of glass with a camel's hair
pencil. This is illustrated by the follow-
ing experiment mentioned by Singer.
Draw a line with a pen dipped in water
on the surface of a slip of glass ; place
one extremity of the line in contact with
the coating of a Leyden jar, and at six
inches distance upon the line place one
knob of the discharging rod ; when the
jar is fully charged, bring the other ball
of the discharger to the knob of the jar,
and the discharge will take place lumi-
nously over the six inches of water. Next,
trace a line with a pen dipped in sul-
phuric acid on a slip of glass, as in the
former experiment, and place one extre-
mity of it in contact with the outside of
the jar; the ball of the discharger may
then be placed on the glass at twelve
inches distance, and the electric fluid
will pass as brilliantly over that interval
as over the six inches of water. In
either of these experiments, if the line of
fluid be wider in any particular part, the

ELECTRICITY.

'47

light of the discharge will appear less
brilliant in passing that portion ; this
must arise from the greater division
of the fluid when passing over an ex-
tended conductor than over one that is
narrow.

§ 3. Chemical Effects of Electricity.

(172.) Electricity exerts a most ex-
tensive and important influence in effect-
ing: changes in the chemical composition
of bodies; but as this influence is most
conspicuously exerted in that particular
mode of agency, which is known by the
name of Galvanism, this subject will
more properly be considered in the treatise
on that branch of electrical science. For
the present, we must content ourselves
with adducing a few instances, illustra-
tive of the chemical effects of electricity
in the forms under which it has now
been presented to our notice.

(1 73.) Some of the chemical changes
consequent on powerful electrical ex-
plosions, appear to be merely the effects
of the heat which is evolved in that pro-
cess. The surfaces of metallic bodies
through which accumulated electricity is
made to pass are frequently oxidated ;
this is seen more especially in the case
of wires that have been fused or vola-
tilized by the electric discharge. It is
known that metals intensely heated are
disposed to combine with the oxygen of
the atmosphere, and, consequently, to
assume the state of oxides ; it is simpler,
therefore, to ascribe this effect in the
present case to a cause which is known
to be in operation at the same moment,
than to any peculiar or determining
agency of electricity. A multitude of
experiments are on record in which the
partial oxidation of metals has been ef-
fected by electric explosions. This subject
was prosecuted with minute and labori-
ous attention by Van Marum, by Cuth-
bertson, and more lately by Singer. It
is remarked by this last experimentalist,
that the oxides of metals produced in
this way appear to consist of several
distinct portions of different degrees of
fineness ; when a wire is exploded in a
receiver, part of the oxide immediately
falls to the bottom, but another portion
remains suspended in the air for a con-
siderable time, and is at length gradually
deposited. It is probable that this cir-
cumstance may in part account for the
different colours of oxides produced in
close receivers and in the open air, for in
the latter case a portion of the oxide is
always lost.

(174.) Under other circumstances,
electricity is found to exert a power the
reverse of the former ; for it decomposes
metallic oxides, extricating their oxygen,
and restoring them to the metallic state.
This deoxidating power was known to
several of the earlier electricians. Bec-
caria reduced the oxides of tin and of
mercury to their metallic state by elec-
tricity. In order to effect this change, a
quantity of the oxide may be introduced
into a glass tube, and pointed conducting
wires inserted through corks at the op-
posite ends of the tube, so that a portion
of the oxide may lie between them. This
apparatus is then to be placed on the
table of the universal discharger, (§136,)
and repeated shocks are to be sent
through the oxide until its partial or
total reduction is accomplished. Ver-
milion, which consists of sulphur and
mercury, is very easily decomposed by
this process, and by a very moderate
charge.

(175.) When a succession of electric
discharges from a powerful electric ma-
chine are sent through water, a decom-
position of that fluid takes place, and it
is resolved into its two elements of oxy-
gen and hydrogen, which immediately
assume the gaseous form. This fact wras
discovered in 1789, by Messrs. Dieman,
Paetz, and Van Troostwyck, who had
formed themselves into a society for ex-
perimental research in Holland ; and it
completed the chain of evidence by which
the great discovery of the composition of
water, made five years before by Caven-
dish, is established. The abovemen-
tioned Dutch chemists being occupied,
in conjunction with Mr. Cuthbertson, in
investigating the effects of electricity
when passed through different bodies,
were desirous of ascertaining its effect
on pure water. They employed for this
purpose an apparatus consisting of a
glass tube, twelve inches long and one-
eighth of an inch in diameter, through
one end of which a gold wire was in-
serted, projecting about an inch and a
half within the tube ; that end was then
hermetically sealed. Another wire was
introduced at the other end of the tube,
which was left open, and passed up-
wards, so that its extremity came to a
distance of five-eighths of an inch from
the end of the first wire. The tube was
then filled with distilled water, which
had been freed from air by an excellent
air-pump, and inverted in a vessel con-
taining mercury. A little common air
-was let into the top of the tube, in

48

ELECTRICITY.

order to prevent its being broken by the
discharge. Electrical shocks were then
passed between the two ends of the
wires through the water in the tube by
means of a Leyden jar, which had a
square foot of coated surface. This jar
was charged by a very powerful double
plate machine, which caused it to dis-
charge twenty-five times in fifteen revo-
lutions. At each explosion bubbles of
air were formed, and rose to the top of
the tube. As soon as a sufficient quan-
tity had collected to leave the upper end
of the wire uncovered by the water, so
that the shock had now to pass through
a portion of the mixed gases, they were
instantly kindled ; a reunion of the ele-
ments took place ; water was again form-
ed, and the space they had occupied was
immediately filled with fluid from below,
so as to restore every thing precisely as
at the outset of the experiment. It was
ascertained by the most decisive che-
mical tests, that the gases thus obtained
consisted of a mixture of oxygen and
hydrogen gases.

(176.) It may appear somewhat para-
doxical that the same agent should, in
the course of the same experiment, pro-
duce at one time decomposition, and
at another combination of the same ele-
ments. The simplest way of reconciling
this apparent discordance, is to suppose
that the combination of the gases is the
effect of the heat evolved during its
forcible transit through an aeriform
fluid that opposes' considerable resistance
to its passage ; while the decomposition
of the liquid is the direct consequence of
the agency of electricity when not inter-
fered with by heat.

(177.) Until lately, it was thought ne-
cessary to employ powerful machines
and large jars in order to effect the de-
composition of water by electricity, and
that mere sparks from acommon.machine
were inadequate to accomplish this pur-
pose. That there is in this respect, how-
ever, no essential distinction in the ope-
ration of these two forms of electricity
has been satisfactorily shown by Dr.
"Wollaston. This distinguished philos<5-
pher, perceiving, with his accustomed
sagacity and penetration, that the de-
composition would depend on duly pro-
portioning the strength of the charge to
the quantity of water, and that the quan-
tity exposed to its action at the surface
of communication depends on the extent
of that surface, inferred that by reducing
the surface of communication the de-
composition of water might be effected

by smaller machines, and with less pow-
erful excitation than had hitherto been
applied to this object. Having procured
a small wire of fine gold, and given to it
as fine a point as possible, he inserted it
into a capillary glass tube; and after
heating the tube, so as to make it adhere
to the point, and cover it in every part,
he gradually ground it down, till, with
a pocket lens, he could discern that the
point of the gold was exposed. When
sparks from a prime conductor of an
electrical machine were made to pass
through water by means of a point so
guarded, a spark, extending to the dis-
tance of one-eighth of an inch, would
decompose water when the point ex-
posed did not exceed one 700th of an
inch in diameter. With another point,
estimated at one 1500th, a succession of
sparks one-twentieth of an inch in length
afforded a current of small bubbles of
air. With a still finer filament of gold,
the mere current "of electricity, without
any perceptible sparks, evolved gas from
water.

(178.) When a solution of sulphate of
copper was subjected to the action of
electricity by means of these slender
conducting wires, the metal was revived
around the negative wire; but upon
reversing the direction of the current of
electricity, so that the same wire now
became positively electrified, the copper
which had collected around it was re-
dissolved, and a similar precipitate was
deposited on the opposite wire, which
was now the negative one. Similar ex-
periments made with other metallic
solutions were attended with analogous
results ; the negative wire always sepa-
rating oxygen from its combinations,
the positive wire always attracting it,
and effecting its union with the bases
presented to it. With solutions of neu-
tral salts, the alkaline or earthy bases
were attracted by the negative, while the
acids were attracted by the positive wire.
The experiments of Sir Humphry Davy
have confirmed these results as far as
concerns the chemical action of common
electricity ; Jbut as this is a subject which
bears more immediate relation to che-
mistry and to galvanism, it would not
be right to enlarge upon it in the present
treatise.

(179.) The magnetic effects of elec-
tricity will likewise form the subject of a
distinct treatise, as they now constitute
a new branch of science, under the title
of ELECTRO-MAGNETISM,

ELECTRICITY.

49

$ 4. Effects of Electricity upon Animals.

(180.) Having seen that the effects
of electricity on inanimate matter are of
various kinds, we should he led to ex-
pect that its operation on living bodies
would he still more complicated ; for in
addition to its mechanical and chemical
agencies, it can hardly fail of exerting
considerable influence on the living
powers, and more especially on the
functions of the nervous system. It is
unnecessary to describe the sensations
excited in the body by receiving electric
sparks or shocks, since most persons
in the present day are familiar with
them. It is curious, however, to take
a retrospective view of the mode in
which the effects of the Leyden phial
were announced to the world, on their
first discovery. The philosophers whp
first experienced, in their own person,
the shock attendant on the transmission
of an electric discharge, were so im-
pressed with wonder and with terror by
this novel sensation, that they wrote the
most ridiculous and exaggerated ac-
count of their feelings on the occasion.
Muschenbroek states, that he received
so dreadful a concussion in his arms,
shoulder, and heart, that he lost his
breath, and that it was two days before
he could recover from its effects; he
declared also, that he should not be in-
duced to take another shock for the
whole kingdom of France. Mr. Alle-
mand reports, that the shock deprived
him of breath for some minutes, and
afterwards produced so acute a pain
along his right arm, that he was appre-
hensive it might be attended with seri-
ous consequences. Mr. "Whikler informs
us, that it threw his whole body into
convulsions, and excited such a ferment
in his blood, as would have thrown him
into a fever, but for the timely employ-
ment of febrifuge remedies. He states,
that at another time it produced copious
Weeding at the nose; the same effect
was produced also upon his lady, who
was almost rendered incapable of walk-
ing. 1 hese strange accounts naturally
excited the attention and wonder of all
classes of people ; the learned and the
vulgar were equally desirous of expe-
riencing so singular a sensation, and
great numbers of half-taught electri-
cians wandered through every pail of
Europe to gratify this universal cu-
riosity.

(181.) As it is probable that the elec-
tric fluid meets with greater impedi-

ment in passing from the surface of one
bone to another, at the parts where
the continuity of substance is inter-
rupted by the joints, this circumstance
explains why the shock is often more
especially felt at the joints than in any
other pail of a limb. But if the shock
be directed more particularly through
muscles, its effects are chiefly shown by
exciting a convulsive and involuntary
action of those muscles. This is often
observed to take place in a paralysed
limb, when electric shocks are sent
through it, although the nerves of the
limb are at the time incapable of con-
veying the impressions which produce
sensation. Mr. Morgan states, that if
the diaphragm be included in the circuit
of a coated surface of two feet in extent,
fully charged, the sudden contraction of
the muscles of respiration will act so
violently upon the air in the lungs, as
to occasion a loud and involuntary
shout ; but if the charge be small, a fit
of convulsive laughter is induced, pre-
senting a most ludicrous exhibition to
the by-standers.

(182.) It is on the nervous system,
however, that the most considerable ac-
tion of electricity is exerted. A strong
charge passed through the head, gave
to Mr. Singer th sensation of a violent
but universal blow, and was followed
by a transient loss of memory and in-
distinctness of vision. If a charge be
sent through the head of a bird, its
optic nerve is usually injured or de-
stroyed, and permanent blindness in-
duced: and a similar shock given to
larger animals, produces a tremulous
state of the muscles, with general pros-
tration of strength. If a person who is
standing receive a charge through the
spine, he loses his power over the mus-
cles to such a degree, that he either
drops on his knees, or falls pros-
trate on the ground ; if the charge be
sufficiently powerful, it will produce im-
mediate death, in consequence, proba-
bly, of the sudden exhaustion of the
whole energy of the nervous system.
Small animals, such as mice and spar-
rows, are instantly killed by a shock
from thirty square inches of glass. Van
Marum found that eels are irrecovera-
bly deprived of life when a shock is sent
through their whole body; but when
only a part of the body is included in
the' circuit, the destruction of irritability
is confined to that individual part, while
the rest retains the power of motion.
Different persons are affected in very

50

ELECTRICITY.

different degrees by electricity, accord-
ing to their peculiar constitutional sus-
ceptibility. Dr. Young remarks, that
a very minute tremor, communicated to
the most elastic parts of the body, in
particular to the chest, produces an agi-
tation of the nerves, which is not -wholly
unlike the effect of a weak electricity.
^ (183.) The bodies of animals killed
by electricity, rapidly undergo putrefac-
tion, and the action of electricity upon
the flesh of animals is also found to
accelerate this process in a remarkable
degree. The same effect has been ob-
served in the bodies of persons de-
stroyed by lightning. It is also a well-
established fact, that the blood does not
coagulate after death from this cause.

(184.) It has not been determined
with any degree of certainty, whether
electricity, in its ordinary mode of ap-
plication, exerts any sensible influence
on the functions of the animal system.
Ihe Abbe Nollet persuaded himself,
from the experiments he made on man
and animals, that the perspiration was
increased during the time they were
electrified ; and De Bozes had noticed
that the pulse was quickened under the
same circumstance. But Van Marum,
on repeating these experiments in a
variety of ways, met with such variable
and contradictory results, that he could
deduce from them no satisfactory con-
clusion respecting the real operation of
electricity ; and, indeed, if we take into
account the powerful influence which
the imagination exerts on most persons
who are the subjects of such experi-
ments, as well as on those who witness
them, there appears but little chance,
amidst such multiplied sources of fal-
lacy, of arriving at the truth. The
only general fact, perhaps, which ap-
pears to be established, is that elec-
tricity acts as a stimulant both to the
muscular and the nervous systems.

(185.) When the energetic effects of
the shock from the Leyden phial were
first made known, the most sanguine
expectations were immediately raised,
that electricity would prove an agent
of considerable power in the cure of
diseases. It was supposed that as a
stimulant, it would have many advan-
tages over other remedies ; for it can
be administered in various degrees of
intensity, which may be regulated with
great exactness ; and its application
can be directed especially to the organ
vve wish to affect, and can be limited to
that organ, so as not to interfere with

the functions of the general system.
Accordingly we find, that at one period
electricity was in great repute as an
efficacious remedy in a number of dis-
eases ; but at present it is seldom
employed except in a very few. It is
not unfrequently had recourse to in
palsy, contractions of the limbs, rheu-
matism, St. Vitus's dance, and some
kinds of deafness, and impaired vision ;
it has also been applied to discuss tu-
mours, to remove obstructions, and to
relieve pain.

(186.) Electricity may be adminis-
tered medicinally in four different ways.
The first and most gentle is under the
form of a continued stream, or aura as it
is termed, derived from a wire or pointed
piece of wood connected with the prime
conductor of the machine, held by an
insulated handle, at the distance of one
or two inches from that part to which it
is to be directed ; an impression is felt,
similar to a current of air ; and in this
way it may be borne by parts of great
sensibility, such as the eye. The se-
cond mode is by directing sparks of
various sizes to the affected part, by
means of a metallic ball at the extremity
of abrass rod, which is within a moderate
distance from the part ; or else by plac-
ing the patient on an insulating stool,
and while he is in communication with
the prime conductor of the machine,
taking sparks from him by another per-
son with a metallic ball at the end of a
rod which he holds in his hand. The
size and intensity of the spark will, of
course, be regulated by the distance at
which the ball is placed from the
body, provided the machine be steadily
worked. The third mode is that by
shocks from the discharge of a Leyden
phial, which is, of course, the most se-
vere and painful method of applying
electricity. Great caution is required
against the indiscriminate application
of this last method, which is not wholly
free from danger. The fourth mode is
by Galvanism, hereafter to be noticed.

§ 5. Effects of Electricity upon Vege-
tables.

(187.) It has also been imagined that
electricity acts as a stimulus to vegetable
life : and many fanciful projects of im-
provements in horticulture by the aid
of artificial electricity have been enter-
tained. It is needless, however, to en-
large upon these visionary speculations,
the fallacy of which has been sufficiently
shown by the late Dr. Ingenhouz, who,

ELECTRICITY.

upon the most accurate inquiry, found
that the vegetation of plants was in no
sensible decree either promoted or re-
tarded by common electricity. The
experiments of Van Marum, however,
in which be found that electricity in-
creased the evaporation of plants, ap-
pear to be entitled to some confidence ;
but still the effect observed, may, as he
himself remarks, have been occasioned
by the increased current of air from the
parts of the electrified leaves. His
observations on the influence of elec-
tricity on the sensitive plant, (Mimosa
pudica,) deserve also to be noticed.
The mere approach of an electrified
conductor, whether charged with posi-
tive or negative electricity, produced no
effect upon the plant ; but when sparks
were taken from it, the leaves collapsed,
just as they would have done by con-
cussions of a mechanical nature, and in
other respects the plant underwent no
change. In the Hedysarwn gyrans, a
plant remarkable for the continual ro-
tatory motions of its leaves, electricity
appeared to have no sensible influence
either in accelerating or retarding these
movements.

(188.) The passage of shocks through
living plants immediately destroys the
vitality in the parts through which the
shock has been sent. It is, indeed, very
easy to kill plants by means of elec-
tricity. A very small shock, according
to Cavallo, sent through the stem of a
balsam, is sufficient to destroy it. A
few minutes after the passage of the
shock, the plant droops, the leaves and
branches become flaccid, and its life
ceases. A small Leyden phial, contain-
ing six or eight square inches of coated
surface, is generally sufficient for this
purpose, which may even be effected by
means of strong sparks from the prime
conductor of a large electrical machine.
The charge by which these destructive
effects are produced, is probably too
inconsiderable to burst the vessels of
the plant, or to occasion any material
derangement of, its organization ; and,
accordingly, it is not found, on minute
examination of a plant thus killed by
electricity, that either the internal ves-
sels or any other parts have sustained
perceptible injury.

(189.) It appears from the experi-
ments of Mr. Achard, that the fermen-
tation of vegetable matter is accelerated
by electricity.

(190.) The general conclusion de-
ducible from these inquiries' is, that

feeble electricity exerts no perceptible
influence on cither animal or vegetable
life : but when transmitted in powerful
shocks, its destructive effects are simi-
lar to those which are produced by
lightning.

CHAPTER XII.
Instruments adapted to collect weak

Electricity.

(191.) BEFORE we proceed to consider
the developement of electricity under
various circumstances, it will be proper
to give a description of several instru-
ments which have been contrived for
the purpose of collecting and exhibiting
weak degrees of electricity, that would
otherwise escape detection. All these
instruments derive their efficacy from
the principle of electric induction ; and
their mode of operation will be best
understood by previously directing our
attention to the electrophorus.

(192.) The instrument termed the
Electrophorus was invented about the
year 1774 by Professor Volta, a name
which is associated with many im-
portant discoveries in the science of
electricity. It consists of three parts:
the essential part, which supplies the
electricity, being a cake of some electric
substance, (E,Jtg. 39,) such as sulphur,

Fig, 39.

gum lac, sealing-wax, pitch, or other
resinous composition ; this is melted
on a conducting plate S, called the
sole, wrhich is formed with a rim to
contain it, and the fluid then allowed
to congeal. The third part of the ap-
paratus consists of a circular metallic
plate C, provided with an insulating
handle fixed upon its upper surface.
This is called the cover ; and is some-
times made of wood, covered on all
sides with tin-foil well rounded at the
edges to prevent the dispersion of elec-
tricity. In order to bring the apparatus
into a state of activity, the surface of
the cake is excited by friction with fur
or flannel, and is thus rendered nega-
tively electrical. The cover, held by its
insulating handle, must now be placed
E 2

ELECTRICITY.

52

on the cake : in this situation it does
not come sufficiently in contact with
the cake to receive its electricity, but
acquires by induction an opposite state
at its lower surface, and a similar state
at its upper; that is, the cake being
negative, the under side of the cover
will be positive, and the upper side
negative. If, while in this state, the
upper negative surface be touched with
the finger, or with any other conductor
communicating with the earth, a spark
will pass from the latter to the cover,
so as to restore the electric equilibrium ;
the quantity of electricity thus super-
added being retained in the cover by
the inductive influence of the cake.
But when the plate is raised, provided
it be held by its insulating handle, the
action of the cake being withdrawn, the
cover is found to be charged with posi-
tive electricity, which may be imparted
to an insulated conductor, or to a
Leyden jar. This operation maybe re-
peated an indefinite number of times,
since the electricity of the cake con-
tinues unimpaired during the process,
and thus may a charge be communi-
cated to the jar of an intensity equal to
that of the cover of the electrophorus
when raised. The instrument has been
known, indeed, to retain its power un-
diminished for months, and may there-
fore be regarded as a sort of magazine
of electricity. It is obvious, that if the
cover were simply placed on the cake,
and again raised without previously
touching it, it would then exhibit no
sign of electricity. If the sole of the
electrophorus be insulated, a spark may
be obtained from it, when the cake
has been excited ; and if while placed
on the cake the cover be touched with
the finger, and at the same time the
sole be touched with the thumb, a sen-
sible shock will be felt in that part of
the hand.

(193.) Volta is also the inventor of
an instrument acting on the same prin-
ciple as the electrophorus, and which he
termed the condenser, of which the
purpose is to collect a weak electricity,
spread over a large surface, into a body
of small dimensions, in which its in-
tensity will be proportionably increased,
and therefore become capable of being
examined. A small metallic plate,
connected with the substance of which
the electricity is to be determined, is
brought within a very small distance of
another plate communicating with the
earth. The small portion of electricity

received from the substance to be tried
by the first plate, acts by induction on
the second plate, and occasions it to
acquire the opposite electrical state:
this latter state reacts upon the first
plate, increasing its capacity for the
electricity which it had first received, and
tends to accumulate a larger quantity
in it, which quantity it must derive from
the substance with which it communi-
cates. This mutual action and reaction
continues till an equilibrium is attained.
If the communication between the sub-
stance tried and the first plate be
broken off, and the plate thus insulated
be removed from the contiguity of the
second plate, the accumulated electricity
with which it is charged will become
evident upon its application to an or-
dinary electroscope, such as those de-
scribed in § 13 and 14.

(194.) Various have been the forms
given to the condenser, according to
the fancy of electricians, without any
change in the principle on which it
acts. In general, the two plates are
merely separated by a thin stratum of
air. Sometimes their surfaces are
covered with a non-conducting varnish,
which prevents any communication of
electricity from the one plate to the
other, while it allows of a very near ap-
proach of the plates to each other ; but
this method is liable to objection, from
the permanent electricity which the
varnish sometimes contracts by friction,
and which may interfere with the re-
gular operation of the instrument. One
of the most convenient forms is that of
the condensing electrometer, (fig. 40,)
Fig. 40.

in which the first plate of the condenser
A, is fixed to the cap of the gold-leaf
electroscope; the second plate B, which
communicates by a chain with the
ground, being moveable round a joint
C, and thus capable of being turned
back and removed from the first plate,
so as to allow its electricity to be ma-
nifested by the divergence of the gold
leaves.

ELECTRICITY.

53

(195.) The instruments called Dou-
blers are so contrived, that by executing
certain movements, very small quantities
of electricity communicated to a part
of the apparatus may be continually
doubled, until it becomes perceptible
by an electroscope. The first invention
of this kind was that of Mr. Bennet,
which consists of three brass plates,
which we shall call A, B, and C. The
plate A has an insulating handle fixed
in its centre, while the piate B has a
similar handle fixed in its circumference.
The under side of A, and both sides of
B are covered with varnish. The third
plate C is also of brass, and is only
varnished on its upper side, the lower
side communicating with the gold-leaf
electroscope. The body whose electri-
city is to be tried, is made to communi-
cate with the under side of the plate
C, which touches the electroscope,
while B is placed upon C, and then
touched with the finger : the communi-
cation with the electrified body is then
removed, and B is lifted up by its insu-
lating handle. A is then placed, by
means of its handle, upon B thus ele-
vated. A is then touched, and, after
withdrawing the finger, is separated
from B. In this process B acquires an
electricity contrary to that of C ; and A
an electricity contrary to that of B, that
is, the same as that of C. If the plate
A, thus electrified, be next applied to
the under surface of C, and B be again
applied over C, and touched with the
finger as before, it will be acted upon
by the electricities contained both in C
and A, and thus acquire, by induction,
nearly double the quantity which it had
done in the first operation. The con-
sequence of this will be that nearly all
the free electricities of A and C will be
concentrated in C. A may now be re-
moved, and after withdrawing the finger
from B, B may also be removed, and
C will be left with double the quantity
of electricity which it had received from
the body with which it was originally
,made to communicate.

If after this duplication the electricity
of the plate C be still too feeble to be
indicated by the electroscope, the same
series of operations must be repeated
ten or even twenty times ; when by
doubling it every time, the smallest
conceivable quantity of electricity must
at last be rendered sensible ; since, at
the end of the twentieth operation, it
will be augmented more than 500,000
times. Although the frequent repeti-
tion, of the operations may p-ppear tedi-

ous, yet, by a little practice, the art is
readily acquired, and the whole process
need not occupy a minute. Great care
must be taken in conducting these ex-
periments, not to excite any electricity
by the friction of the finger, or by any
other means, in the varnished sides of
the plates. In order to obviate this
source of error, Cavallo contrived a
form of the instrument, that enabled
the plates to be brought within a veiy
small distance of one another, yet
without actual contact, so as to enable
him to dispense altogether with the em-
ployment of varnish. But notwith-
standing every precaution of this kind,
it is always found that the instrument ex-
hibits electricity of itself, although none
has been previously communicated to
it : so that its indications cannot be at
all depended upon for the detection of
very minute quantities of electricity. It
is unnecessary, therefore, to describe the
particular mechanisms invented by Dr.
Darwin, and improved by Nicholson,
for bringing the plates into the requisite
positions, and effecting in succession
the necessary contacts, by the simple
rotation of a winch, aided by wheel-
work : instruments which have gone by
the names of the moveable, or revolving
double?', and the multiplier of electri-
city, and which are now superseded in
practice by instruments more sensible
and certain in their operation.

CHAPTER XIII.

Developement of Electricity by Changes
of Temperature and of Form.

(196.) THERE are certain mineral bo-
dies, which, from being in a neutral
state at ordinary temjperatures, acquire
electricity simply by being heated or
cooled. This property is possessed only
by regularly crystallized minerals ; and
of these the most remarkable is the
tourmalin, which is a stone of consi-
derable hardness, found in many parts
of the world, and particularly in the
island of Ceylon. The Dutch, who
first became acquainted with it in that
island, gave it the appellation of As-
chentrikker, from its property of attract-
ing ashes when it is thrown into the
fire. It appears from the researches of
Dr. Watson, that its attractive proper-
ties were known to Theophrastus, who
describes it under the name of Lyncu-
rium. Linnaeus has termed it the Za-
pis Electricus, (Electric stone.) The
form of its crystals is generally that of
a nine- sided prism, terminated by a

51

ELECTRICITY.

three -sided pyramid at one end, and by
a six-sided pyramid at the other. Le-
mery noticed its electric properties in
the year 1717; but the first scientific
examination of them was made by
.^Epinus in 1756, and published in the
Memoirs of the Berlin Academy. He
found that when a crystal of tourmalin
has its temperature raised to between
100° and 212° of Fahrenheit, one ex-
tremity, which is that terminated by the
six-sided pyramid, becomes charged
•with positive electricity, while the other
extremity is negative ; so as to be capa-
ble of affecting a delicate electroscope.
When the stone is of considerable size,
flashes of light may be seen along its
surface. Mr. Wilson, who made many
experiments on this subject, observed
that a flat tourmalin retained its electri-
city without diminution, after exposure
to intense heat for half an hour ; but
Canton, upon repeating these experi-
.ments, did not obtain the same result.
Hauy states, that very high degrees
of heat destroy the electricity of the
tourmalin. After this has been effected,
it recovers its electricity as it gradually
cools : but in that case the electric states
are generally reversed ; that extremity,
or pole, as it has been called, which
was before positive, is now negative,
and vice versa. It is only at the sum-
mits of the pyramids, by which the
crystal is terminated, that the electricity
is manifested ; the intermediate por-
tions exhibiting no sign of electrical ex-
citation, unless the stone be broken in
pieces ; and then each fragment is
found to possess a positive and a nega-
tive pole, like the entire crystal. This
fact bears a striking analogy to a cor-
responding property in magnets. At
the ordinary temperature of the atmo-
sphere, the tourmalin may be rendered
electrical by friction.

(197.) There are several other gems
and crystallized minerals which possess
the same property as the tourmalin.
The luminous appearance of some dia-
monds, when heated, is ascribed by
Sir Humphry Davy to their electrical
excitation. The substance called the
Boracite, composed of borate of mag-
nesia, which crystallizes in cubes, having
its edges and angles defective, becomes
electrical by heat, and in one variety
presents no less than eight sides, alter-
nately in different states ; that is, four
positive and four negative ; the oppo-
site poles being in the direction of the
axes of the crystal. In those varieties
in which only four of the angles of the

crystal are truncated, that is, cut off by
planes, while the rest are either entire,
or are replaced by more than one plane,
it is always the former of these angles
that become positive, and the latter
negative.

(198.) Similar properties are possess-
ed by the Topaz, which consists of
siliceous fluate of alumina ; its electric
poles are situated upon the two opposite
summits of the secondary crystal. In
some varieties, Hauy found a series of
consecutive poles alternately positive
and negative. Axinite, Mesotype, and
Prehnite, become electrical by the ap-
plication of heat : as also the two fol-
lowing metallic oxides, namely Gala-
mine, which is an oxide of zinc, and
Sphene, or calcareo- siliceous oxide of
titanium. Mr. Dessaignes has lately
shown that all metallic bodies are capa-
ble of a feeble electric excitation by
changes of temperature. It results
from the researches of Hauy, that this
electrical property in mineral bodies is
intimately related to the laws of their
crystallization, and also to the direction
in which the light is most readily trans-
mitted through them.

(199.) There are a great many sub-
stances which become electrified on
passing from the liquid to the solid
form. This happens to sulphur, gum
lac, bee's wax, and in general all resi-
nous bodies. Unless proper precautions
be taken, however, we frequently obtain
no indications of this electricity, because
it is usually disguised, that is, rendered
inactive by the opposite electricity of
the contiguous substances. Thus, if
sulphur be melted over the fire in an
iron ladle, and then set by to cool and
harden, it exhibits no sign of electricity ;
because the negative electricity of the
sulphur is exactly counterbalanced by
the positive electricity accumulated in
the iron vessel which contains it. But
if the sulphur be removed from the
vessel, which may be done by again
heating it for a short time, so as just
to melt the surface in contact with the
iron, and allow of its being detached
when the ladle is inverted, on suffering
the sulphur to cool in this situation, its
electricity becomes very apparent. If
sulphur be melted in a wine glass, the
conical shape of which admits of its
being taken out when cold, the opposite
electricities of the two surfaces will then
manifest themselves, that of the sulphur
being negative, and that of the glass
positive ; but when the sulphur is re-
place4 iu the glass, all indications of

ELECTRICITY.

electricity disappear. The electricity
developed by the process of cooling was
called by Wilke, who first observed it,
spontaneous electricity, in contradistinc-
tion to that which, originating from
friction, he called excited electricity.
Van Marum, however, attributes the
electricity developed by the separation
of the two substances, to a species of
friction ; for he remarks, that the electri-
city does not manifest itself till the sul-
phur begins to contract in thfc act of con-
gelation, and that it attains its maximum
at the point of the greatest contraction.

(200.) It is reasonable to suppose
that whatever change was produced in
the electrical state by congelation, the
reverse would be produced by liquefac-
tion. We are not aware of any ex-
periments which bear directly upon this
question.

(201.) The conversion of bodies into
the state of vapour, as well as the con-
densation of vapour, is generally attend-
ed by some alteration of their electrical
condition ; and the bodies in contact
with fhe vapour are thereby rendered
electrical. Thus, if a plate of metal
strongly heated be placed upon a gold-
leaf electroscope, and water be dropped
upon the plate, at the moment the va-
pour rises the leaves of the electroscope
diverge with negative electricity. The
general fact was noticed by Laplace,
Lavoisier, and Volta, in the year 1781 ;
and was found to extend both to solids
and to liquids passing into a gaseous
form. De Saussiire made an extensive
series of experiments on the ebullition
of water and other fluids, with a view
to ascertain the degree and kind of elec-
tricity developed during this process.
But investigations of this kind are at-
tended with great difficulty, from the
multitude of minute circumstances which
are liable to affect the results ; and we
accordingly find, that different experi-
ments of the same kind often afford the
most opposite conclusions.

(202.) In general it is found, that the
vaporization of water by simple ebulli-
tion produces negative electricity in the
remaining fluid, or vessel which con-
tains it : the vapour itself being positive.
On the contrary, when aqueous vapour
is condensed into water, it becomes ne-
gative, leaving the bodies with which it
was last in contact in a state of positive
electricity. Yet in some of De Saus-
sures's experiments, when the heat was
communicated to a quantity of water
contained in an insulated metallic ves-
sel, by throwing into it a mass of red-

hot iron, the electricity was very strongly
positive. This difference in the result
was probably owing to the chemical de-
composition of the water in the latter
experiment, a circumstance which, as
we shall presently see, is itself a source
of electricity. It is principally on ac-
count of the interference of chemical
actions with the regular operations of
temperature, and of the complications
introduced by electric induction, that
experiments on this subject have hither-
to presented such anomalous, and, appa-
rently, discordant results.

CHAPTER XIV.

Developement of Electricity by Contact,
Compression, and other mechanical
Changes in Bodies, and also by their
Chemical Action.

(203.) IT had long been suspected,
rather than proved, that a feeble degree
of electricity is evolved by the contact
or collision of different metals : but this
important fact was established in the
clearest manner by Volta, about the year
1801. The apparatus he employed in
his investigations on this subject con-
sisted of two discs, the one of zinc, the
other of copper, (fig. 41,) rather more

Fig. 41.

than two inches in diameter, ground
perfectly plane, and having in their cen-
tres insulating handles perpendicular to
their surfaces, by means of which the
plates could be brought into contact,
without being actually touched with the
hand. With this precaution the discs
were made to approach till they touched
one another ; they were then separated,
by keeping them parallel as they were
drawn back. The electricity they pos-
sessed after this separation was then
examined by means of the condenser ;
and, that the effects might be rendered
more distinct, : the electricity produced
by a number of successive contacts,
(taking care to restore the discs to the
neutral state after each contact,) was
accumulate:! in the same condenser. It
was constantly found that the copper
disc charged the condenser with nega-
tive, and the zinc disc with positive elec-

56

ELECTRICITY.

tricity. Thus it was established as a
general fact, that these two metals, in-
sulated, and in their natural state, are
brought, by mutual contact, into oppo-
site electrical states ; the zinc acquiring
positive electricity, and the copper be-
coming, in an equal degree, negative.

(204.) No explanation has yet been
given of this curious fact, which seems
to be at variance with all the previously
ascertained laws of electric equilibrium.
The transfer of electricity from one me-
tal to the other during their contact, im-
plies the operation of some new force
which no theory has yet embraced.
While the contact is preserved, neither
of the metals gives any indication of its
electrical state, the electricity being dis-
guised ; as would be the case of that of
the coatings of a Leyden jar, if we could
suppose them both in actual contact,
but yet incapable of allowing any trans-
fer of the electricity from the one to the
other, so as to restore both to the state
of neutrality.

We shall have occasion to resume the
consideration of this curious subject in
the treatise on Galvanism, with the
theory of which it appears to have an
intimate relation.

(205.) There are some bodies which
are rendered electrical by pressure. This
property is possessed in the most re-
markable degree by that transparent
variety of carbonate of lime which is
known by the name of Iceland spar.
According to Hauy, if a crystal of this
spar, which has the form of a rhomboid,
be held in one hand by two of its oppo-
site edges, and if at the same time two
of its parallel planes be lightly touched
by two fingers of the other hand, and
then brought near to the small needle of
the electroscope, (§ 12) a decided attrac-
tion will be perceptible. By applying a
more powerful pressure, the electrical
effects will be still more considerable ;
the electricity being in all cases positive.
Hauy observes that this property resides
principally in those crystalline minerals
that are capable of being reduced by
mechanical division to plane and smooth
laminae : such as the Topaz, especially
the colourless variety ; Euclase, Arra-
gonite, Fluate of lime, and Carbonate of
lead. Among those substances in which
friction excites negative electricity, there
are some which require only to be
pressed, for the production of the same
effect. An instance occurs in elastic
bitumen, when it has been cut into a
proper shape for the experiment. Mr.
Becquerel has lately discovered that

many other substances, such as cork,
bark, hairs, paper, and wood, possess
the property of producing electricity by
compression.

(206.) Many substances, when re-
duced to powder, exhibit electricity, if
they are made to fall upon an insulated
metallic plate. This fact was first no-
ticed by Mr. Bennet, after he had in-
vented his gold-leaf electroscope. He
found that powdered chalk, put into a
pair of bellows, and blo\vn upon the cap
of the electroscope, communicates to the
instrument positive electricity, when the
pipe of the bellows is about six inches
from the cap ; but the same stream of
powdered chalk electrifies it negatively
at the distance of three feet. On being
blown in a more copious stream from a
pair of bellows without the pipe, the
electricity is always negative; and the
same effect takes place when the powder
is let fall from another plate upon the
cap of the instrument. This subject was
pursued by Cavallo ; but the most com-
plete set of experiments relating to it is
that of Singer, who employed in his re-
searches the two following methods :
first, that of sifting the powders on the
cap of a delicate electrometer through a
fine sieve, which was thoroughly cleaned
after each operation ; and secondly, that
of bringing an insulated copper plate
repeatedly in contact with extensive sur-
faces of the powders spread on a dry
sheet of paper ; the copper plate being
brought in contact with the condenser
after every repetition of the contact,
until a sufficient charge was communi-
cated.

(207.) The following substances, ac-
cording to Singer, produce negative
electricity when sifted on the cap of the
electrometer: viz. copper, iron, zinc,
tin, bismuth, antimony, nickel, black
lead, lime, magnesia, barytes, strontites,
alumine, silex, brown oxide of copper,
white oxide of arsenic, red oxide of lead,
litharge, white lead, red oxide of iron,
Acetate of copper, sulphate of copper,
sulphate of soda, phosphate of soda,
carbonate of soda, carbonate of am-
monia, carbonate of potash, carbonate
of lime, muriate of ammonia, common
pearl-ashes, boracic acid, tartaric acid,
cream of tartar, oxy muriate of potash,
pure potash, pure soda, resin sulphur,
sulphuret of lime, starch, orpiment.

(208.) The following substances pro-
duce positive electricity under the same
circumstance : viz. wheat flour, oat-
meal, lycopodium, quassia, powdered
cardamom, charcoal, sulphate of potash,

ELECTRICITY.

57

nitrate of potash, acetate of lead, oxide
of tin.

(209.) The following catalogue ex-
hibits the results of the experiments of
contact with a copper plate ; the dif-
ferent substances being arranged under
the head of the electricity they really
acquire, which is contrary to that of the
copper plate. Positive : lime, barytes,
strontites, magnesia, pure soda, pure
potash, common pearl-ashes, carbonate
of potash, carbonate of soda, tartaric
acid. Negative : benzoic acid, boracic
acid, oxalic acid, citric acid, silex,
alumina, carbonate of ammonia, sul-
phur, resin. These experiments were
several times repeated with uniform
results.

(210.) The above mode of electrical
excitation is probably merely a species
of friction, differing only from the more
ordinary instances by the mode of its
application. But in other cases the
electrical effects of contact are more
distinctly exhibited, as when zinc filings
are poured through holes in a plate of
copper, upon the cap of an electro-
meter.

(211.) The following experiment,
founded on one devised by Professor
Lichtenberg of Gottingen, is an elegant
illustration of the opposite electrical
states of different powders. With the
knob of a charged jar, trace on the
surface of a smooth plate of glass, or of
any resinous substance, various lines at
pleasure ; and then repeat the same
operation in other parts with the knob
of a jar charged with the opposite
electricity. Let the surface thus pre-
pared be gently dusted, by means of a
powder-puff, with a mixture of pow-
dered sulphur and red lead, previously
triturated together in a mortar. By the
contact and friction thus produced, the
sulphur has been rendered negative,
and the red lead positive ; and each of
the powders, when projected on the
plate, will attach itself to the oppositely
electrified lines, forming a series of red
and yellow outlines. It is also observ-
able, that the configurations assumed
by these and other powders differ ac-
cording to the species of electricity im-
pressed upon the plate ; positive elec-
tricity producing an appearance resem-
bling feathers, and negative electricity
an arrangement more like stars.

(212.) The most important circum-
stance in this inquiry, is the connection
between electricity and the chemical
properties of matter. It is observed by
Sir H. Davy, that most of the sub-

stances that act distinctly upon each
other electrically, are likewise such as
act chemically, when their particles
have freedom of motion : this is the
case with the different metals, with sul-
phur and the metals, with acid and
alkaline substances. Of two metals in
contact, the one which has the greatest
chemical attraction for oxygen acquires
positive electricity, and the other the
negative : so that if arranged in the
order of their oxidability, as follows,
zinc, iron, tin, lead, copper, silver, gold,
platina, each will become positive when
brought into contact with any that fol-
low it in the series, and negative with
any of those which precede it. In con-
tacts of acids with bases, as of crystals
of oxalic acid with dry quicklime, the
former is negative, the latter positive.
All acid crystals when touched by a
plate of metal render it positive, the
crystals themselves becoming negative.

(213.) Bodies that exhibit electrical
effects by mutual contact, previous to
their chemical action on each other,
lose this power during combination.
Thus if a polished plate of zinc be made
to touch a surface of dry mercury, and
quickly separated, it is found positively
electrical, and the effect is increased by
heat ; but if it be so heated as to amal-
gamate, that is, unite chemically with
the mercury, it no longer exhibits any
signs of electricity. The case is analo-
gous with copper and sulphur ; and
iron, when applied to mercury, produces
more electricity than zinc, apparently
from its being incapable, under ordinary
circumstances, of forming a chemical
combination with mercury.

(214.) On the other hand, there can
be no question that electricity is oc-
casionally, if not universally, elicited
during chemical action. We have just
seen that a dry acid becomes negative
by contact with a metal, which is con-
sequently thereby rendered positive. In
this case no chemical combination had
taken place. But Becquerel has shown
that if the acid, instead of being in a
dry crystalline form, be in a liquid state,
and capable of acting chemically on the
metal, the acid will become positive and
the metal negative. The same conclu-
sion may also be deduced from the
experiments of Lavoisier and Laplace,
on the action of dilute sulphuric acid
on iron filings. That the oxidation of
metals gives rise to electricity has been
also shown by the experiments of Dr.
Wollaston, from which it would appear
that the electricity obtained in the .com-

58

ELECTRICITY.

mon electrical machine is derived prin-
cipally from this source. When he
employed as the rubbing substance an
amalgam of silver or of platina, which
are metals very little subject to oxida-
tion, he could obtain no electricity. An
amalgam of tin, on the other hand, sup-
plied" a large quantity of electricity.
Zinc acts still better than tin ; but the
best amalgam for this purpose is made
•with both tin and zinc, a mixture which
oxidates more readily than either metal
separately. As a further trial whether
oxidation assists in the production of
electricity, a small cylinder with its
cushion and conductor was arranged in
a vessel so contrived that the contained
air could be changed at pleasure. After
ascertaining the degree of excitement
produced in atmospheric air, carbonic
acid was substituted, but the excitement
could not be renewed ; while it was im-
mediately reproduced on the readmisston
of common air. It must be acknow-
ledged, however, that Sir H. Davy, in
repeating these experiments, arrived at
opposite results ; for he states, that the
machine acted equally well in hydrogen
gas as in atmospheric air, and was even
more active in carbonic acid gas, a cir-
cumstance which he attributes to the
greater density of this gas.

(215.) Electricity is often developed
by processes quite independent of che-
mical changes. This is evident from its
production by the friction of two bodies
of the same kind upon one another,
as has been already noticed, (§ 35 ;) and
also by the strong electricity which is
manifested on the separation of the
parts of the same body. rlhus, if a
piece of dry and warm wood be suddenly
-rent asunder, the two surfaces which
have separated are found to be elec-
trified, the one positively, the other
negatively ; and a flash of light is per-
ceived if the experiment be made in the
dark. The same phenomenon is ob-
served when the plates of mica (Mus-
covy glass) are suddenly torn asunder ;
and even when a stick of sealing-wax is
broken across ; the two surfaces of
fracture being in each case positive and
negative respectively. Dr. Brewster
discovered that the fracture of the un-
annealed glass tears, called Prince Ru-
pert's drops, was attended with the evo-
lution of electrical light, which pervaded
the whole drop, so that its form was
distinctly visible in the dark. The light
appears even when the expeiiment is
made under water.

(216.) There is every reason to pre-

sume that electricity is essentially con-
cerned in the processes that are carried
on in the living system both of animals
and vegetables. In the animal economy
more particularly, the operation of this
agent is indicated in the processes of
secretion, in the actions of the muscles
and nerves, and probably, indeed, in all
the vital functions. There are several
kinds of fish, which are endowed with
the power of accumulating large quan-
tities of electricity, which they can dis-
charge at pleasure through conducting
bodies that come in contact with them,
and thus communicate powerful shocks.
This power is possessed in an eminent
degree by the torpedo, which is a species
of ray ; but it is also met with in the
Gymnotus electricus, the Silurus electri-
cus, the Trichiurus indicus, and the
Tetraodon electricus. But as this, as
well as other subjects relating to animal
electricity, involve considerations which
properly belong to Galvanism, we must
defer treating of them until this branch
of electrical science is before us.

CHAPTER XV.
Electricity of the Atmosphere.

(217.) As the subject of atmospheric
electricity is more especially a branch of
the science of Meteorology, we shall con-
tent ourselves, in this place, with a very
brief outline of the principal facts relat-
ing to it.

(218.) The atmosphere is very gene-
rally in an electrical state. This may be
ascertained by employing a metallic rod
elevated to some height above the ground,
and communicating at its lower end,
which should be insulated, with an elec-
troscope. In order to collect the elec-
tricity of the higher regions cf the air, a
kite may be raised, in the string of which
a slender metallic wire should le inter-
woven, so as to conduct the electricity.
If the electroscope be sufficiently sensi-
ble it will usually indicate the prevalence
of positive electricity in the atmosphere,
the intensity of which increases accord-
ing as the stratum examined is more
elevated. In the ordinary slate of the
atmosphere its electricity is invariably
found to be positive : and is stronger in
winter than in summer; and dining the
day than the night. From the time of
sunrise it increases for two or three
hours, and then decreases towards the
middle of the day, being generally weak-
est between noon and four o'clock. As
the sun declines its intensity is again
augmented, till about the time of sun-

ELECTRICITY.

59

set, after which it diminishes, and con-
tinues feeble during the night. In cloudy
weather the electrical state is much more
uncertain ; and when there are several
strata of clouds, moving in diiferent
directions, it is subject to great and
rapid variations, changing sometimes
from positive to negative, and back
again, in the course of a few minutes.
On the first appearance of fog, rain,
snow, hail, or sleet, the electricity of the
air is generally negative, and often
highly so ; but it afterwards undergoes
frequent transitions to opposite states.
On the approach of a thunder-storm
these alternations of the electric condition
of the air succeed one another with re-
markable rapidity. Strong sparks are
sent out, in great abundance, from the
conductor ; and it becomes dangerous to
prosecute experiments with it in its in-
sulated state.

(219.) The analogy between the elec-
tric spark, and more especially of the
explosive discharge of the Leyden jar,
with atmospheric lightning and thunder,
is too obvious to have escaped notice,
even in the early periods of electrical
research. It had been observed by Dr.
Wall and by Gray, and still more point*
edly remarked by the Abbe Nollet. Dr.
Franklin was so impressed with the
many points of resemblance between
lightning and electricity, that he was
convinced of their identity, and deter-
mined to ascertain by direct experiment
the truth of his bold conjecture. A
spire which was erecting at Philadel-
phia he conceived might assist him in
this inquiry ; but, while waiting for its
completion, the sight of a boy's kite,
which had been raised for amusement,
immediately suggested to him a more
ready method of attaining his object.
Having constructed a kite by stretching
a large silk handkerchief over two sticks
in the form of a cross, on the first ap-

Searance of an approaching storm, in
ime 17.V2, he went out into a field,
accompanied by his son, to whom alone
lie had imparted his design. Having
raised his kite, and attached a key to the
lower end of the hempen string, he in-
sulated it by fastening it to a post, by
means of silk, and waited with intense
anxiety for the result. A considerable
time elapsed without the apparatus
•giving any sign of electricity, even al-
though adenss cloud, apparently charg-
ed with lightning, had passed over the
spot on which they stood. Franklin was
just beginning to despair of success,
when his attention was caught by the

bristling up of some loose fibres on the
hempen cord ; he immediately presented
his knuckle to the key, and received an
electric spark. Overcome with the emo-
tion inspired by this decisive evidence of
the great discovery he had achieved, he
heaved a deep sigh, and conscious of an
immortal name, felt that he could have
been content if that moment had been
his last. The rain now fell in torrents,
and wetting the string, rendered it con-
ducting in its whole length ; so that
electric sparks were now collected from
it in great abundance.

It should be noticed, however, that
about a month before Franklin had made
these successful trials, some philoso-
phers, in particular Dalibard and De
Lors, had obtained similar results in
France, by following the plan recom-
mended by Franklin. But the glory of
the discovery is universally given to
Franklin, as it was from his suggestions
that the methods of attaining it were
originally derived.

(220.) This important discovery was
prosecuted with great ardour by philo-
sophers in every part of Europe. The
first experimenters incurred consider-
able risk in their attempts to draw down
electricity from the clouds, as was soon
proved by the fatal catastrophe, which,
on the 6th of August, 1753, befel
Professor Richman, of Petersburg,
whose name has already been before us,
($ 123.) He had constructed an appa-
ratus for observations on atmospherical
electricity, and was attending a meeting
of the Academy of Sciences, when the
sound of distant thunder caught his ear.
He immediately hastened home, taking
with him his engraver, Sokolow, in
order that he might delineate the ap-
pearances that should present them-
selves. While intent upon examining
the electrometer, a large globe of fire
flashed from the conducting rod, which
was insulated, to the head of Richman,
and passing through his body, instantly
deprived him of life. A red spot was
found on his forehead, where the elec-
tricity had entered, his shoe was burst
open, and part of his clothes singed.
His companion was struck down, and
remained senseless for some time ;' the
door-case of the room was split, and the
door itself torn off its hinges.

(221.) The protection of buildings
from the effects of lightning, is the most
important practical application of the
theory of electricity. We have only
room for a few observations on the
principles on which conductors for this

60

ELECTRICITY.

purpose should be constructed. They
should be formed of metallic rods,
pointed at the upper extremity, and
placed so as to project a few feet above
the highest part of the building they are
intended to secure ; they should be con-
tinued without interruption till they de-
scend into the ground, below the foun-
dation of the house. Copper is prefer-
able to iron as the material for their
construction, being less liable to destruc-
tion by rust, or by fusion, and possessing
also a greater conducting power. The
size of the rods should be from half an
inch to an inch in diameter, and the
point should be gilt, or made of platina,
that it may be more effectually preserv-
ed from corrosion. An important con-
dition in the protecting conductor is, that
no interruption should exist in its con-
tinuity from top to bottom : and advan-
tage will result from connecting together
by strips of metal all the leaden water
pipes, or other considerable masses of
metal in or about the building, so as to
form one continuous system of conduc-
tors, for carrying the electricity by dif-
ferent channels to the ground. The
lower end of the conductors should be
carried down into the earth till it reaches
either water, or at least a moist stratum.

For the protection of ships, chains
made of a series of iron rods linked
together, are, by their flexibility, most
conveniently adapted. They should ex-
tend from the highest point of the mast
some way into the sea, and the lower
part should be removed to some dis-
tance from the side of the ship, by a
wooden spar or outrigger.

The air of close rooms, vitiated by
respiration, is found to be negatively
electrified.

CHAPTER XVI.
Theoretical Views of the Nature of

Electricity.

(222.) THE preceding history of the
phenomena relating to electricity, may
prepare us for the discussion of some in-
teresting inquiries concerning the real
nature of this powerful and mysterious
agent, and the theory of its operation.

The first question that presents itself
is with respect to its materiality. Besides
the well-known mechanical forces which
belong to ordinary ponderable matter,
the phenomena of nature exhibit to our
view another class of powers, the pre-
sence of which, although sufficiently
characterised by certain "effects, is not
attended with any appreciable change in
the weight of the bodies with which they

are connected. To this class belong
heat, light, electricity, and magnetism :
each of which, respectively, produces
certain changes on material bodies, either
of a mechanical or chemical nature,
which it is natural to regard as the
effects of motion communicated by the
impulse of material agents, of so subtile
and attenuated a kind, as to elude all
detection when we apply to them the
tests of gravity or inertia. If we admit
heat and light to be material, analogy
will lead us to ascribe the same charac-
ter to electricity and to magnetism, not-
withstanding their being imponderable.

(223.) But the materiality of electri-
city has also been maintained on other
grounds. The pungent sensation of the
electric spark, the smart blow which
accompanies the shock, the vivid line of
light which marks its course, the varied
sounds which attend its passage through
the air, and the irresistible fury with
which it bursts asunder the densest
textures, all seem to denote the mecha-
nical effects of sudden and powerful im-
pulse ; all seem to imply the rushing of
a stream of fluid possessed of momentum
adequate to produce these energetic mo-
tions. Can we refuse to ascribe the cha-
racter of materiality to that which we
not only see and hear, but feel also ?

(224.) This argument has been en-
deavoured to be strengthened by a va-
riety of experiments, from which the
communication of impulse in a particu-
lar direction with respect to the species
of electricity has been inferred. The
stream of air, which proceeds from a
pointed conductor when electricity is
issuing from it, appears as if the air
were carried forward along with the
electric fluid. The direction of its mo-
tion is still more decidedly indicated by
the different luminous appearances which
accompany the escape of the fluid from,
or its reception by, a pointed conductor.
(See/g-. 23.) We have already had oc-
casion to notice the manner in which
this curious fact appears to support
the hypothesis of Franklin, implying the
singleness of the electric fluid, (§ 98.)

(225.) The following experiment has
also been adduced by Cavallo and by
Singer, in support of the same opinion.
Place on the table of the universal dis-
charger a card bent lengthwise over a
round ruler, so as to form a hollow cy-
lindrical groove ; or, what is still better,
two straight sticks of sealing-wax, laid
parallel to each other, so that the junc-
tion of their rounded edges may form a
groove. In this groove place a pith- ball

ELECTRICITY.

of about half an inch in diameter, and
arrange the wires of the discharger with
their points in ths direction of the groove,
and at four inches from each other, the
ball being equally distant from each. On
passing a small charge from one wire to
the other, the ball will be driven from
the positive to the negative wire, and this
effect will be constant if the wires ter-
minate in points ; but if they are knob -
bed, the ball frequently vibrates between
them, because the influence of the at-
tracting surfaces upon the ball interferes
with the regularity of the effect, and often
renders the result equivocal.

(•226.) The nature and place of the
perforation effected in a card by the
passage of an electric charge, of which
we have already given an account (§ 159),
appear to favour the same view of the
subject. The following experiment, also,
shews that the impulse is communicated
most forcibly in the direction from the
positive towards the negative conductor.
A light float-wheel, the vanes of which
are mads of card paper, inserted in the
circumference of a cork turning freely
on a pin passed through its centre as an
axle, will be put in motion by presenting
to it an electrified point, apparently in
consequence of the impulse of the stream
of air which issues from the point.
Whether the point be positively or nega-
tively electrified, the direction of the
motion, as well as of the stream of air, is
always the same. But if the wheel be
placed on an insulating stem, as in fig.
42, and introduced between the pointed
Fig. 42.

wires of the universal discharger, which
are to be placed as accurately as possible
opposite to each other, and at the distance
of an inch or more from the upper vanes ;
on connecting one of the wires with the
positive, and the other with the negative
conductor of an electrical machine, and
exciting it, the wheel will move as if im-
pelled by a stream from the positive to
the negative wire. On reversing the
connections, so that the electricity of
each wire is changed, the motion of the
wheel will likewise be reversed.

(227.) If a card be placed vertically,
by inserting it in a small piece of cork
that may form a base of about a quarter

of an inch wide for it to stand upon, but
so lhat it may be overthrown by the
smallest impulse ; and the pointed wires
of the universal discharger be brought
opposite to each other, and about a
quarter of an inch below the upper edge
of the card, which stands at an equal
distance between them ; on connecting
the wires with a machine, or with an in-
sulated jar, so as to effect an electric
discharge between them, the card will
be thrown down, and will constantly
fall from the positive, and towards the
negative wire.

(228.) The determination of a stream
of electrified air in this direction is also
rendered very sensible by the motions of
smoke or vapour placed in the circuit
of the electricity. Thus the flame of a
taper placed between two oppositely
electrified balls will constantly be blown
from the positive to the negative side.
Fig. 43 represents two hollow metallic

Fig. 43.

balls, about three quarters of an inch in
diameter, insulated on separate glass
pillars, by which they are supported at
a distance of two inches from each other :
the upper part of each ball is hollowed
into a cup, into which a small piece of
phosphorus is to be put. A small candle
has its flame situated mid-way between
the balls, one of which is connected with
the positive, and the other with the nega-
tive conductor of the machine. When
the balls are electrified, the flame is
agitated, and inclining towards the one
which is negative, soon heats it suffi-
ciently to set fire to the phosphorus it
contains, whilst the positive ball remains
perfectly cold, and its phosphorus un-
melted. On reversing the connections
of the balls with the machine, the phos-
phorus in the other ball will now be
heated and will inflame.

(229.) However plausibly it may have
been inferred, from a superficial view
of these facts and experiments, that the
electric fluid actually possesses momen-
tum, and that it moves in a particular
direction, a more rigid analysis of the
phenomena will show that they in no

62

ELECTRICITY.

decree warrant such an inference. All
the mechanical effects that attend the
transfer of electricity are ultimately re-
solvable into the sudden action of a
repulsive power exerted among the par-
ticles of matter which are situated in the
line of its course. They are only parti-
ticular instances of the fundamental law
of electric action, that bodies charged
with the same kind of electricity repel
one another. Thus the particles of
air electrified by a pointed conductor
are repelled by that conductor, and
repel it also ; and, moreover, repel one
another: and the same, effect takes
place whether their electric state be of
the positive or negative kind. Hence
the stream of air which proceeds from
any electrified point is very naturally
accounted for. If the quantity of elec-
tricity which is transferred is consider-
able, it excites a more violent commo-
tion among the particles which it in-
fluences in its passage. The intense
energy of its repulsive action produces
the most sudden and forcible expansion
of that portion of the air which occupies
this line ; this air thus expanding must
be impelled laterally against the sur-
rounding particles, and must occasion
their sudden compression. The evolution
of heat and light is the necessary conse-
quence of this violent compression ; and
the vibratory impulse being propagated
in all directions is the source of the sound
which attends the electric explosion. The
sensation to which the passage of the elec-
tric shock through our bodies gives rise, is
also, evidently, referable to an impression
made on the nerves by the same repul-
sive action. In all this we can discern
no positive proof of the operation of a
material agent extraneous to the body
itself and acting by mechanical impulse.
The materiality of electricity, therefore,
must still rest upon a similar foundation
with that of heat, or of light.

(230.) If the electric power, or fluid,
if we choose to consider it as such, does
not act by its mechanical momentum, the
arguments in favour of the motion of a
single fluid from the positive to the nega-
tive body, derived from the appearances
of the streams of electric light, (§ 97, 98,)
the impulsion of a pith-ball, (§ 225,) the
perforation of a card, ($ 159,) the rota- j
tion of a windmill, (§ 226,) and the de-
termination of the flame of ataper (§ 228)
in one constant direction, must fall to the
ground, and can evidently be of no avail
in deciding the great question, whether
there be two electric fluids or only one.
But, still, it is incumbent upon us to in-

quire upon what principle these remark-
able differences in the phenomena of
positive and of negative electricity can
be accounted for, consistently with either
hypothesis.

(231.) On an attentive examination of
the phenomena they appear to be expli-
cable on the supposition that the air, or
medium through which the electricity
passes, is, in the language of one theory,
more disposed to admit of the passage
of the vitreous than of the resinous elec-
tricity; or, to speak consistently with
the Franklinean theory, that it is ^ jre
disposed to receive the electric
from a conductor which is chare*,
it, than to part with it to an underch
conductor which absorbs it. Th:
sequences of this hypothesis are, tin
vitreous electricity meets with h
sistance in passing out from a body u- ^
the air, and is therefore carried forward
more readily and more directly than the
resinous electricity. The latter, in con-
sequence of meeting with greater resist-
ance to its exit, is more diffused in the
surrounding space.

On the Franklinean theory the same
effects will follow with reference to the
propulsion of the electric fluid from the
positive, and its absorption by the nega-
tive body.

(232.) That the peculiarity of the me-
chanical effects of the different species
of electricity depends upon the proper-
ties of the air, which is the vehicle of
its agency, and not upon any specific
power in the agent itself, is shown by a
modification of the experiment described
in § 159, in which a varnished card,
suspended between two conductors,
was perforated at the point where it was
touched by the negative wire. On re-
peating the same experiment under the
receiver of an air-pump, Mr. Tremery
found, that in proportion as the air is
exhausted, the place where the card is
perforated by the electric shock ap-
proaches nearer to the positive wire.
When the pressure of the air is reduced
to one-half, the hole is at the middle
point between the two wires. At every
discharge, a flash is seen to pass from
each conductor to the place of perfora-
tion. The curious appearances pre-
sented by the edges of the perforations
made in the leaves of a quire of paper,
already detailed ($ 160, 161,) are not
reconcileable with the supposition of
a mechanical impulse acting only in one
direction, but indicate the equal repul-
sive action of both kinds of electricity,
when the disturbing influence of the

ELECTRICITY.

63

air is withdrawn. It is a confirmation
of this hypothesis, respecting the pecu-
liar kind of obstruction which air op-
poses to the passage of electricity, that
other substances have been discovered
in which a similar property exists. Mr.
Ermann, of Berlin, has found that the
flame of alcohol is possessed of a greater
conduct ilia; power with regard to posi-
tive, than to negative electricity. Alka-
line soap, on the contrary, conducts ne-
gative electricity better than positive ;
and will, therefore, serve to insulate a
IV degree of the latter, at the same
ti> hat it permits the passage of the

:.) It has always been urged as a
objection to the theory of a single

c fluid, that it necessarily involves
* Condition of a mutual "repulsion
K ^ng the particles of ordinary matter.
8ee$ 52. Before attempting to combat
this objection, it will be proper to enter
into a somewhat fuller illustration of the
position than we have already done ;
and for this purpose we shall avail our-
selves of the assistance of a few dia-
grams, calculated to aid our conceptions
of the forces concerned in the mutual
Actions of electrified or neutral bodies.
For the sake of greater distinctness, we
shall suppose the whole of the matter
in the body, of which we are studying
the actions, to be concentrated in a
small space, and we shall represent this
matter by a black square. In like man-
ner, we shall suppose that the whole of
the electric fluid contained in the same
body is condensed into a small space,
denoted by a white circle. The mutual
actions of the matter or electric fluid in
two adjacent bodies, are expressed by
lines passing from the one to the other
respectively ; the attractions being dis-
tinguished by unbroken lines, and the
repulsions by dotted lines.

(234.) Fig. 44 represents a body B in

Fig. 44.

a neutral state of electricity, by which
is to be understood, that the quantity
of fluid it contains exists in a propor-
tion so exactly adjusted to the quantity
of matter, as that its repulsion for a
particle F of electricity at any distance.
precisely balances the attraction of the
matter for that same particle. "While
this equilibrium is preserved among the
forces which would impel any electric
fluid external to the body both from it

and towards it, it is evident that the
body will neither acquire nor lose elec-
tricity, but remain quiescent, whether
it be insulated or not.

(235.) The state of the forces ope-
rating between two similar neutral
bodies is shown in Jig. 45. Here it

Fig. 45.

follows from the condition of neutrality,
as above defined, that the two attractive
forces, denoted by the two whole lines,
are each of them equal to the repulsive
force between the two fluids, denoted
by the upper dotted line. Actuated by
these forces only, therefore, the two
bodies would attract each other. The
addition of a second repulsive force be-
tween the two portions of matter, as
represented by the lower dotted line, is
therefore necessary to account for the
state of equilibrium which we find,
under these circumstances, really ob-
tains. Some persons have conceived,
that by assuming the repulsive force of
the electric particles to be double the
attractive forces of the same particles
for matter, the equilibrium might be
explained without having recourse to
the mutual repulsion of the particles of
matter : forgetting, that such an as-
sumption is incompatible with that of
the neutral state of the bodies, which
is the condition under which we are now
examining them.

(236.) The repulsion of two bodies,
each containing twice the quantity of
electric fluid requisite for the saturation
of their respective matter, is illustrated
byy?o-. 46. All the forces represented

Fig. 46.

64 ELECTRICITY.

by the lines must, from the hypothesis,
be regarded as equal in point of in-
tensity : but the number of repulsive
forces is as -five, while that of the at-
tractive forces is only as four: the
former therefore will prevail.

(237.) Precisely the same result will
obtain in the case of two negatively
electrified bodies, in which, as repre-
sented in fig. 47, the quantity of matter

Fig. 47.

is twice as much as the fluid can satu-
rate. In the former case it was the
repulsion between the two portions of
fluid which were in excess that de-
stroyed the equilibrium ; while in this
case the same effect is produced by the
mutual repulsion of the unsaturated
portions of matter.

(238.) Lastly, we may collect from
an examination of fig. 48, where a body

Fig. 48.

.positively electrified is supposed to be
placed near one that is negatively elec-
trified, that the ultimate effect will be
determined by the attraction between
the fluid in excess in the former, and
the unsaturated matter in the latter :

all the other attractions and repulsions
exactly compensating each other.

(239.) It is a great, though a com-
mon error to imagine, that the condition
assumed by ^Epinus, namely, that the
particles of matter, when devoid of
electricity, repel one another, is in op-
position to the law of universal gravi-
tation established by the researches of
Newton ; for this law applies, in every
instance to which inquiry has extended,
to matter in its ordinary state, that is,
combined with a certain proportion of
electric fluid. By supposing, indeed,
that the mutual repulsive action be-
tween the particles of matter is, by a
very small quantity, less than that
between the particles of the electric
fluid, a small balance would be left in
favour of the attraction of neutral
bodies for one another, which might
constitute the very force which operates
under the name of gravitation: and
thus both classes of phenomena may
be included in the same law.

(240.) An objection has been urged
by Biot against the hypothesis of a
single, fluid, on the ground that it im-
plies an equal degree of attraction be-
tween the fluid and every species of
matter, whereas in the case of other
agents, such as heat and magnetism,
the degree of their attraction is very
different towards different kinds of
matter. This objection does not apply
to the hypothesis of the two fluids, for
they are assumed as acting independ-
ently of any specific attractions for the
bodies which contain them : hence their
distribution in those bodies follows the
same law, whatever be the specific
nature of the materials of which the
latter are composed.

(241 .) We arrive, then, at the conclu-
sion that there is no fact in electricity
which cannot be explained on either of
the two hypotheses : but to which side
the balance of probabilities may incline,
when the respective merits and demerits
of each are taken into account, remains,
perhaps, to be decided more by the
taste than the judgment of the inquirer.

ERRATA IN PART

Page. Col. Line.

6, 1,

7, 1,
9, 1,

ib.

3, from bottom, omit the line — "Ice

above— 13° Fahrenheit."
10. omit—*" of six inches, that is."
36, for " resinous," read " vitreous."

1. 37, for " vitreous," read " resinous.

Page. Col. Line.

20, 1, I, for " silken," read " silk."
2'2, 1, 10, for " spheroid^," read " spheroid."
ib. 2, 10, from bottom, fur "smaller," read
" greater."

GALVANISM.

CHAPTER I.
Origin of Galvanism.

(1.) THE term GALVANISM " is em-
ployed to designate a peculiar form of
electric agency, elicited under particular
circumstances, and capable of produc-
ing certain effects on bodies, not usually
resulting from the ordinary modes of
excitation. The first notice that we
find of any phenomenon referable to this
branch of electricity, occurs in a meta-
physical work, published in 1767, and
entitled, The General Theory of Plea-
swes, by a German writer of the name
of Sulzer, who observed, that by apply-
ing two metals, one above, and the other
below the tongue, and then bringing
them into contact, a peculiar taste was
perceived. He ascribed this sensation
to some vibratory motion, excited by
the contact of the metals, and commu-
nicated to the nerves of the tongue.
Content with this loose and fanciful
explanation, Sulzer appears to have
pursued the inquiry no farther ; and the
curious fact he had announced remained
for many years unnoticed, until the at-
tention of the philosophic world was
drawn to the subject, by the discovery
of Galvani. Important discoveries in
science seem often to arise from ac-
cident ; but, on closer examination, it is
found that they always imply the exer*
cise of profound thought. As the fer-
tility of the soil is essential to the germi-
nation and growth of the seed which
the wind may have scattered on its sur-
face, so it is principally from the quali-
ties of mind in the observer that an
observation derives its value, and may
be made eventually to expand into an
important branch of science. This has
been remarkably exemplified in the ori-
gin of galvanism. Its founder, Gal-
vani, was professor of anatomy at
Bologna, and had early distinguished
himself by his attainments and his zeal
in his profession, and especially by the
ardour with which he cultivated com-
parative anatomy. It happened, in the

year 1790, that his wife, being con-
sumptive, was advised to take, as a
nutritive article of diet, some soup made
of the flesh of frogs. Several of these
animals, recently skinned for that pur-
pose, were lying on a table in the labo-
ratory, close to an electrical machine,
with which a pupil of the professor was
amusing himself in trying experiments.
While the machine was in action, he
chanced to touch the bare nerve of the
leg of one of the frogs with the blade of
the knife that he held in his hand ; when
suddenly the whole limb was thrown
into violent convulsions. Galvani was
not present when this occurred, but re-
ceived the account from his lady, who
had witnessed, and had been struck
with the singularity of the appearance.
He lost no time in repeating the expe-
riment, in examining minutely all the
circumstances connected with it, and in
determining those on which its success
depended. He ascertained that the con-
vulsions took place only at the moment
when a spark was drawn from the prime
conductor, and the knife was at the same
time in contact with the nerve of the
frog. He next found that other metallic
bodies might be substituted for the
knife ; and very justly inferred that they
owed this property of exciting mus-
cular contractions to their being good
conductors of electricity.

(2.) Far from being satisfied with
having arrived at this conclusion, it only
served to stimulate him to the further
investigation of this curious subject;
and his perseverance was at length re-
warded by the discovery, that similar
convulsions might be produced in a frog,
independently of the electrical machine,
by forming a chain of conducting sub-
stances between the outside of the mus-
cles of the leg, and the crural nerve.
Galvani had previously entertained the
idea that the contractions of the muscles
of animals were in some way dependent
on electricity ; and as these new expe-
riments appeared strongly to favour
this hypothesis, he with great ingenuity
applied it to explain them. He com-

GALVANISM.

pared the muscle of a living animal to a
Leyden phial, charged by the accumula-
tion of electricity on its surface ; while
he conceived that the nerve belonging
to it performed the function of the wire
communicating with the interior of the
phial, which would, of course, be charged
negatively. In this state, whenever a
communication was made, by means of
a substance of high conducting power,
between the surface of the muscle and
the nerve, the equilibrium would be in-
stantly restored, and a sudden contrac-
tion of the fibres would be the conse-
quence.

(3.) The discoveries of Galvani were
no sooner made known to the scientific
world, than they excited very general
interest ; and philosophers in every
country in Europe vied with each other
in repeating his experiments, in varying
them in all possible ways, and in invent-
ing all kinds of hypotheses to account
for the phenomena. Some regarded
them as the effects of a new and un-
known agent, differing altogether from
electricity; while others, adopting the
views of Galvani, recognised them to be
electrical, but attributed them to a pe-
culiar modification of that power, re-
siding in the animal system only, and
which they accordingly distinguished by
the name of Animal Electricity. But
the discovery of new facts contributed
more and more to multiply and strengthen
the analogies between galvanism and
electricity: till at length all doubt of
the identity of the agent concerned in
all these phenomena was removed by
the discovery of the Galvanic, or Vol-
taic Pile. Whatever share accident
may have had in the original discovery
of Galvani, it is certain that the inven-
tion of the pile, an instrument which
has most materially contributed to the
extension of our knowledge in this
branch of physical science, was purely
the result of reasoning. Professor Volta,
of Pavia, a name already familiar to
electricians,* was led to the discovery of
its properties by deep meditation on the
developement of electricity at the sur-
face of contact of different metals. f
We may justly regard this discovery as
forming an important epoch in the his-
tory of galvanism : and indeed, since
that period, the terms, Voltaism, or
Voltaic Electricity, have often, in ho-
nour of this illustrious philosopher, been

e on Electricity, § 192, 193.

used to designate that particular form
of electrical agency, which is the subject
of the present treatise.

Previously to our entering into a de-
tailed exposition of the facts relating to
this science, and of the theories which
have been proposed for their explana-
tion, it will be necessary to direct our
attention to the nature -of those arrange-
ments of bodies, which are the sources
of galvanic power.

CHAPTER II.
Simple Galvanic Circles.

(4.) THE process usually adopted for !
obtaining galvanic electricity is to inter-
pose between two plates of different
kinds of metal a fluid capable of exert-
ing some chemical action on one of the
plates, while it has no action, or at least
a different one, on the other plate : and
then to establish a communication be-
tween the plates at some other part,
either by their direct contact with one
another, or by the intervention of con-
ducting substances. Let us take, for
example, a plate of zinc, Z, and another
of copper, C, (fig. 1.) and immerse
Fig.l.

them, to a certain depth only, in diluted
sulphuric acid, A, contained in a glass
vessel, keeping their lower edges at a
little distance from one another: then,
inclining them towards each other, let
us bring their upper edges, which are
out of the fluid, into contact, as repre-
sented in the figure. The arrangement
we have thus termed constitutes what
is called a galvanic circle, in its simplest
form, of which the three parts, or ele-
ments, are zinc, acid, and copper ; each
of these bodies being^ in contact with the
two others. Under these circumstances
it is found that a quantity of electricity
is set in motion ; a continued current of
electric fluid passing from the zinc to the

GALVANISM.

3

acid— from the acid to the copper—
from the copper back again to the zinc
— and so on, in a perpetual circuit.
Such at least must be the explanation
of the phenomena on the hypothesis of
Franklin, implying the singleness of the
electric fluid. But if the theory of Du
Fay, which recognises two different
fluids, be adopted, what has just been
stated must be understood to refer ex-
clusively to the current of vitreous elec-
tricity. Now, according to that theory,
every such transfer of electricity con-
sists of an interchange of the two fluids :
the current of vitreous electricity just
mentioned, must, therefore, necessarily
be accompanied by an opposite current
of resinous electricity ; that is, of one
flowing from the zinc to the copper;
from the copper to the acid ; and from
the acid to the zinc. Hence in our fu-
ture explanations of the phenomena of
galvanism, it will be sufficient to ex-
press the former of these currents only ;
provided we bear in mind that the trans-
fer of any quantity of vitreous electricity
in a given direction, implies the transfer
of an equal quantity of resinous electri-
city in the opposite direction.

(5.) The same effects will take place,
if, instead of allowing the metallic plates
to come in direct contact, the communi-
cation betwen them be effected by wires,
(as shewn in fig. 2.) extending from the

Fig 2.

one to the other. The circuit of elec-
tricity will thus be lengthened, but the
currents will move in the same direction
as before ; that of the positive electri-
city being denoted in the figure by the
position of the arrows ; namely, in the
fluid, from the zinc towards the copper ;
and along the wires, from the copper to
the zinc. The completion of the circuit

by means of wires, enables us to 'direct
the electric current through such bodies
as we may wish to subject to its opera-
tion, and at the same time gives us the
power of interrupting or renewing at
pleasure the communication between the
two metallic plates, by merely separat-
ing or joining together their remote ex-
tremities at Y. When united, the wire
W, which proceeds from the copper-
plate C, is imparting electricity to the
wire X, which touches the zinc plate Z ;
hence, the former is considered as being
in a positive, and the latter in a nega-
tive state.

(6.) The electrical effects of the sim-
ple apparatus just described are, in
general, too feeble to be perceived, un-
less by very delicate tests. The fact
mentioned by Sulzer, and the experi-
ments of Galvani on the muscles of
frogs, in their original form, afford,
however, examples of the operation of
simple galvanic circles. When the
tongue is interposed between zinc and
copper, the saliva in contact with the
metals performs the part of the acid in
the experiment above mentioned, and
the stream of electricity in its passage
from the zinc to the copper, through
the substance of the tongue, affects the
nerves of that organ, so as to give rise
to sensations of taste. In Galvani's
experiment, muscular contractions were
produced by forming a connection be-
tween two different metals, one of which
was applied to the nerve, and the other
to the muscles of a frog's leg. It is
evident that such an arrangement com-
poses a galvanic circle, deriving its
activity from the chemical properties of
the fluids in those parts of the frog that
are in contact with the metals. Al-
though the quantity of electricity set in
motion by this slight action, must be
supposed to be exceedingly minute, it is
yet sufficient, when passing over the
exquisitely sensible nerves of the tongue,
or through the highly irritable fibres of
a frog, to produce a very considerable
impression.

(7.) It has even been found possible,
by means of a very small galvanic circle
of the same simple kind as that which,
we have described, to produce some of
the more energetic effects of galvanism,
such as raising the temperature of the
wire which conducts it to a red heat. •
We are indebted to the ingenuity of Dr.
Wollaston for the contrivance of an ap-
paratus, which he calls an elementary
galvanic battery, capable of exhibiting
u 2

GALVANISM.

this effect.* He found that a single
plate of zinc, of the size of a square
inch, when properly mounted, and sus-
pend'ed in dilute sulphuric acid, between
two copper plates of similar dimensions,
was more than sufficient to ignite a wire
of platina, one three-thousandth of an
inch in diameter, which formed part of
the connection between the two metals.
(8.) It will readily be conceived, that
by enlarging the size of the plates, their
power will be proportionally increased.
The first battery of this kind, on a very
large scale, was that constructed by Dr.
Hare, professor of chemistry in the uni-
versity of Philadelphia, and called by
him a Calorimotor, from its remarkable
power of producing heat.t It consisted
of sheets of zinc, and of copper, formed
into coils, so as to encircle each other,
separated only by interstices of a quarter
of an inch in width. This construction
is shown iufig. 3, which exhibits a hori-
Fig. 3.

zontal section of the plates as they are
coiled together : the thick line Z, repre-
senting the zinc, and the thinner line C,
the copper plate. The zinc sheets were
nine inches by six ; the copper fourteen
by six ; more of the latter metal being
required ; as in every coil it was made
to commence within the zinc, and com-
pletely to surround it on the outside.
Each coil was about two inches and a
half in diameter ; their number amounted
to 80 ; and by means of a lever they
could all be let down at the same mo-
ment into as many glass jars, two inches
and three quarters diameter inside, and
eight inches high, placed so as to receive
them, and containing the acid liquor in-
tended to act upon the zinc.

(9.) To the class of simple galvanic
circles must also be referred the magni-

* See Thomson's Annals of Philosophy, vol. vi.
p. 209. While these pages were in the press we have
sustained an irreparable loss in the death of Dr. Wol-
laston, a philosopher whose unrivalled acuteness of
observation, soundness of judgment, and integrity of
mind, directed to the highest objects of science, place
his name among the mosteminent of its benefactors.
\ Silliman's Journal, iii, 105, and Annals of Phi-
New Series, i. 330,' •

ficent battery belonging to the London
Institution, and which was constructed
under the direction of Mr. Pepys. * It
consists of two plates only, the one of
zinc, and the other of copper, coiled
round a cylinder of wood, and prevented
from coming into contact by ropes of
horse hair, which is a non-conducting
substance, interposed in various places
between them . The length of each plate
is 60 feet, and its breadth two feet ; the
total surface being 400 square feet. In
order to charge this battery, the whole
coil is immersed in a tub containing
acid of the proper strength.

CHAPTER III.
Compound Galvanic Circles.

(10.) MANY of the effects of galvanism
require for their production the combined
influence of a number of plates, arranged
so as to form what is termed a com-
pound galvanic circle. To this class
belongs the galvanic pile, discovered by
Volta, and announced by him in a paper
which he transmitted, in the year 1800,
to the Royal Society. He had been led
by theory to conceive that the effect of
a single pair of metallic plates might be
increased indefinitely by multiplying
their ^number, and disposing them in
pairs, with a less perfect conducting
substance interposed between each pair.
For this purpose he provided an equal
number of silver coins, and of pieces of
zinc, of the same form and dimensions ;
and also circular discs of card, soaked
in salt water, and of somewhat less
diameter than the metallic plates, Of
these he formed a pile or column, as
shown in fig. 4 : in which the three sub-
Fig. 4.

stances, silver,f zinc, and wet card, de-
noted by the letters S. Z, andW, were

* Philosophical Transactions for 1823, p. 187.

t Copper might have been used instead of silver,
as in the single galvanic circle already described,
(§ 4, 5.)§with the same effect.

GALVANISM.

made to succeed one another in the same
regular order throughout the series.
The efficacy of this combination realized
the most sanguine anticipations of the
discoverer : it far exceeded in power the
single circle already described. If the
uppermost disc of metal in the column
be touched with the finger of one hand,
previously wetted, while a finger of the
other hand is applied to the lowermost
disc, a distinct shock is felt in the arms,
similar to that from a Leyden phial, or
still more nearly resembling that from
an electrical battery weakly charged.
A repetition of shocks is obtained for an
indefinite period, whenever the circuit is
completed by touching the two ends of
the pile with the moistened fingers. The
strength of the shock is, as might be
expected, greater in proportion to the
number of plates of which the pile is
composed.

If the pile were raised to any consi-
5 derable height, it would

obviously be in danger
of oversetting : this
may be prevented by
placing the discs be-
tween three vertical
glass rods, properly var-
nished, and cemented
into two thick pieces of
wood, one of which
serves as a base, and
the other as a cover to
the pile. See fig. 5.

(11.) Any number
of these piles may be
combined so as to form a battery,
by making a metallic communication
between the last plate of the one and
the first of the next, and so on ; taking
care that the order of succession of the
plates in the circuit be preserved invio-
late, as is shown in fig. 6, where the dark

Fig. 6.

may be arranged in a form somewhat
different from the preceding, and corre-
sponding more nearly to the elementary
galvanic circle in its simplest state al-
ready described ($ 4, 5). In this new ar-
rangement the metallic plates, instead of
being piled one above the other, are
placed side by side in a vertical position,
and combined together in pairs, consist-
ing each of one zinc and one copper (or
silver) plate, connected at their upper
edges by slips of metal, passing from
the one to the other. A sufficient num-
ber of glasses being provided, and filled
with water, or some acid or saline solu-
tion, they are to be placed side by side,
so as to form a circle. The two plates
belonging to each pair are then to be
immersed in the fluids contained in two
different, but adjoining, glasses ; the
zinc plate, for instance, in the first glass,
and the copper in the second. The
plates of the second pair must be im-
mersed, in a similar way, in the second
and third glasses ; and so on successively
throughout the series, taking care to
preserve the same order of alternation in
the metals. It is evident that by this
arrangement, (of which an horizontal
section is shown infg. 7, where the dark

Fig. 7.

lines represent the copper, and the light
lines the zinc plates.
(12.) The component parts of the pile

lines indicate the copper, and the lighter
lines the zinc plates in each pair,) each
vessel will contain one plate of zinc and
one of copper, which, as they belong to
different pairs, are not connected toge-
ther, except through the medium of the
intervening fluid in that particular
vessel.

(13.) The first apparatus of this kind
was constructed by Volta, who employed
for that purpose a circular series of
cups, and hence gave it the name of

GALVANISM.

Couronne de tosses. If the circuit be
interrupted at any one point, by remov-
ing one or more of the vessels, the in-
strument is rendered similar in its opera-
tion to the pile, and the metallic plates
at each end of the series which are not
immersed in fluid, may be connected to-
gether by means of wires in order to
complete the circuit. Such an arrange-
ment is shown in fig. 8, where the zinc
Fig. 8.

and copper plates are marked respec-
tively with the letters Z and C : and the
course of the electric fluid denoted by
the arrows.

(14.) It is also to be observed, that in
every compound galvanic circle, such
as is exemplified in this apparatus, the
direction of the electric current is pre-
cisely the same as in a simple galvanic
circle composed of the same elements.
In the present case, where zinc and cop-
per are the metals employed, and the
fluid acts upon the former so as to oxi-
date it, a stream of positive electricity is
continually circulating from the zinc to
the copper plate contained in the same
vessel, through the oxidating fluid which
separates them ; and is transferred from
the copper to the zinc plate contained in
the next vessel, along the slip of metal
which connects them. Following its
course in this manner to the end of the
series, we find the electric current pass-
ing on from the last copper plate, con-
tained in the last vessel, to the zinc
plate connected with it, and thence con-
veyed along the wires of communica-
tion, round to the copper plate at the
other end of the series. The direction of
this current is shown in the figure by
the arrows above and below. It is evi-
dent, therefore, that that end of the bat-
tery which is terminated by a zinc plate
is that from which electricity is given
out to the wire, and is, consequently, the
positive end, or pole, as it is called, of
the battery. For the same reason, the
opposite end, or that terminated by the
copper plate, and which receives the

electricity from the wire, is the negative
pole. The same observations apply to
the galvanic pile ; the zinc end being the
positive, and the copper (or silver) end
the negative pole.

(15.) It will be perceived that the de-
nominations of the zinc and copper ends
of the pile or compound battery, as being
positive and negative, are exactly the
reverse of what obtains in the single
galvanic circle, where, as we have seen,
it is the copper plate which is positive,
and the zinc negative, with relation to
the communicating wires. But as the
direction of the electrical currents is the
same in the compound as in the simple
circle, this contrariety in the qualities of
the poles appears, at first sight, para-
doxical. But the difficulty vanishes
when we advert to the circumstance,
that in the simple galvanic circle the
conducting wire communicates directly
with that plate which is in contact with
the fluid part of the apparatus ; while in
the compound circle it proceeds, not
from the plate immersed in the fluid, but
from that which is associated with it,
and, therefore, of a different kind. The
compound circle reduced to its condition
of greatest simplicity would be repre-
sented by the following series, consisting
of five parts, namely,

copper — zinc— fluid — copper — zinc.
In this arrangement the copper end is
negative, and the zinc end positive. By
merely removing the two terminal plates,*
which, in fact, are no ways concerned
in the effect, we bring it to the state of
the single circle, consisting simply of

zinc— fluid — copper :
here we find the zinc end negative, and
the copper end positive. It is highly ne-
cessary to possess clear ideas of this dif-
ference, since much ambiguity has arisen
from inattention to it in describing ex-
periments, and • reasoning upon their
results, more especially in the study of
electro-magnetism, hereafter to be con-
sidered.

(16.) A much more compendious
form may be given to a battery con-
structed on the., principle of the Cou-
ronne de tasses, by employing a trough
divided into numerous compartments by
partitions, the whole being made of non-
conducting materials. This will admit
of the plates being brought nearer to each
other, and of a much greater number

* Volta, in conformity with tlie theory he had
adopted, considered these terminal plates as adding
to the galvanic power. Bat we shall afterwards point
out the incorrectness of that theory.

GALVANISM.

being contained in a given space. The
zinc and copper plates are united in
pairs, as before, by a slip of metal pass-
ing from the one and soldered to the
other : each pair being placed so as to
enclose a partition between them, and
each cell containing a plate of zinc con-
nected with the copper plate of the suc-
ceeding cell, and a copper plate joined
with the zinc plate in the preceding cell.
Such an apparatus is called a trough
battery, and is represented in Jig. 9.

Fig. 9.

The trough, T, may be made of baked
mahogany, with partitions of glass :
but it is found more convenient to con-
struct the whole of one material, and
Wedgwood ware answers best for this
purpose. Each trough is usually fitted
up with ten or twelve cells. The plates,
P, adapted to them, are connected to-
gether by a slip of baked wood, so as to
allow of their bein«; let down into the
cells, or lifted out, together. A further
advantage arises from this construction,
that the plates and the fluid being inde-
pendent of each other, the former may
be readily cleaned or replaced, when
worn or injured, without disturbing the
fluid : and the latter may, in like man-
ner, be removed and changed with the
utmost facility. A number of these
troughs may be combined with great
ease, by connecting together the termi-
nal plates of the adjoining troughs, by
slips of copper; taking care, as in the
case of the pile, ($ 11.) to preserve
throughout the whole series the same
order of alternation in the plates, by
connecting the zinc end of one battery
with the copper end of the next.

The voltaic battery belonging to the
Royal Institution, which is of immense
power, is constructed on the plan above
described, and consists of 200 separate
parts, each part composed of ten double

plates, and each plate containing thirty-
two square inches. The whole number
of double plates is 2000, and the whole
surface 128,000 square inches.

(17.) A trough battery on another

construction was invented by Mr.

Cruickshanks, and is represented in

fig. 10. Plates of zinc and of copper,

Fig. 10.

united by their flat surfaces by solder-
ing, are employed to form the partitions
themselves, and are fixed into grooves in
the sides of a trough of baked wood,
which is a bad conductor of electricity,
so as to leave sufficient intervals to hold
small quantities of fluid. They must, of
course, be arranged so that all the zinc
surfaces shall be on one side, and all
the copper surfaces on the other. The
battery is charged by filling the cells
with a saline solution, or with dilute
acid, and the galvanic circuit completed
by bringing the two wires proceeding
from the ends of the battery in contact
with one another. The section, /g-. 11,
Fig. 11.

will tend to elucidate the principles of
its action. Troughs of this construc-
tion, however, are exceedingly liable to
get out of order, from the action of the
liquid on the wood, which it tends to
warp. The plates require to be fixed
into the grooves by cement, in order to
render them water tight ; but this ce-
ment is apt to crack from the warping
of the wood, and other causes, and the
liquid insinuating itself into the fissures,
impairs the power of the instrument by
destroying the insulation of the cells.

(18.) The power of a battery is con-
siderably increased when both surfaces
of each plate of zinc, in contact with the
oxidating fluid, are opposed to a surface
of copper. In order to accomplish this,

GALVANISM.

it will be necessary to add a second
copper plate to each pair, so that every
cell may contain one zinc and two cop-
per plates, the former being placed be-
tween the latter. This plan, which was
suggested by Dr. Wollaston, was adopt-
ed by Mr. Children in the construction
of a very large battery, in which each
plate was six feet long, by two feet eight
inches broad, so that it presented thirty-
two square feet of surface.*

(19.) An ingenious application of this
principle was made by Mr. Hart of
Glasgow, in the construction of a gal-
vanic battery, requiring no other ma-
terial for confining the fluid, than the
metals themselves which form the
circles. This he accomplished by con-
verting the double copper plates into
cells, by adding sides and bottoms, so as
to enable them to hold the acidulous
fluid into which the zinc plates are im-
mersed. The cells are formed by cut-
ting a sheet of copper into the form
shown \nfig. 12.t They are then folded
Fig 12.

up as seen in fig. 13, and the seams
Fig. 13.

* Philosophical Transactions for 1815, p. 363.

t The engraving- fig. 12, is here reduced in its
dimensions from the original drawing. It should
have been of the size required to form, when folded,
.the cell represented in fig. 13.

grooved. A drop of tin is run into each
lower corner to render the cells per-
fectly tight. Fig. 14 represents the zinc

plate, having a piece of screwed brass-
wire cast into the top of it for the pur-
pose of suspension. Fig. 15 is a section

Fig. 15.

of the battery, showing how the copper
tail of the first cell is connected with the
zinc plate of the second, and so on.
The connexion is rendered perfect by
joining them with a drop of solder.
Each zinc plate is kept firmly in its
place by three small pieces of wood.
The whole series is then fixed, by means
of screw-nuts fitted on to the brass wires,
to a bar of baked wood, previously well
varnished. When the battery is to be
used, it must be lifted off the frame,
and dipped into a wooden trough,
lined with lead, containing the acid. It
is then placed on the frame and is ready
for action. Such a battery, with an
equal number of zinc plates, is found to
possess considerably greater power than
the best batteries of the ordinary con-
struction.*

(20.) Various contrivances have been
employed for converting a compound
voltaic battery, consisting of a certain
number of alternations of plates, into a

* Edinburgh Journal of Science, iv. 19,

GALVANISM.

battery having a smaller number, or
even into one corresponding in principle
to the simple battery with a single pair
of plates, such as the calorimotor.
These changes may be effected by alter-
ing the connexions of the plates, and
uniting several plates of the same metal
together, so that they may act as only
one plate ; or if the effect of a calorimotor
be desired, connecting all the zinc plates
together, and also all the copper plates,
so that the whole may act only as a
single pair.

CHAPTER IV.

Effects of Galvanism.

(21.) THERE are three principal cir-
cumstances in which the electricity
produced by the voltaic battery differs
from that obtained from the ordinary
electrical machine; first, the very low
degree of intensity in which it exists in
the former, when compared with the
latter ; secondly, the very large quantity
of electricity which is set in motion by
the voltaic battery ; and thirdly, the
continuity of the current of voltaic elec-
tricity, and its perpetual reproduction,
even while this current is tending to
restore the equilibrium. The effects of
the voltaic pile have been compared by
the inventor of that instrument to those
of an electric battery of large dimen-
sions, but charged only to a low degree ;
in which case,'as appears from what has
already been said on this subject in the
Treatise on Electricity, a large quantity
of electricity may be contained, with a
very small tendency to escape, or, in
other words, with a very feeble in-
tensity. The comparison is, in many
respects, just ; but it fails in regard to
the third property we have noticed as
belonging to the voltaic apparatus ;
namely, the continuity of the current
arising from its perpetual reproduction
and circulation.

However considerable may be the
power collected in a highly charged
electric battery, the whole of that power
is at once expended as soon as the cir-
cuit is completed. Its action may, while
it lasts, be sufficiently energetic ; but it
is exerted only for an instant ; and, like
the destructive operation of lightning,
can effect, during its momentary pas-
sage, only sudden and violent changes,
which it is beyond the power of the
experimentalist to regulate or control.

On the contrary, the voltaic battery con-
tinues, for an indefinite time, to develop
and supply vast quantities of electricity,
which, far from being lost by returning
to their source, circulate in a perpetual
stream, and with undimished force.
The effects of this continued current on
the bodies subjected to its action, will,
therefore, be more definite, and will be
constantly accumulating; and their
amount will, in process of time, be in-
comparably greater than even those of
the ordinary electrical explosion. We
shall accordingly find that changes in
the composition of bodies are effected
by galvanism which can be accom-
plished by no other means. Hence
may be conceived the advantages which
have accrued to science from the ac-
quisition of an instrument of such vast
power, and admitting of such extensive
application in the wide field of chemical
research.

It will be convenient to study the
effects of galvanism in their relation to
the three circumstances which have
been noticed as characterizing its ope-
ration when contrasted with those of
ordinary electricity.

§ 1. Ordinary Electrical effects re-
sulting from Galvanism.

(22.) The degree of intensity in which
the electricity developed by a single
galvanic circle exists, is so extremely
low, that its action produces none of
the usual phenomena exhibited by the
common electrical machine. Even from
the largest calorimotor that has yet
been constructed, it is not possible to
obtain indications of electrical attraction
and repulsion, such as are given by the
feeblest degree of excitation to a piece
of sealing-wax. With a few alternations
of plates and interposed fluid, as in the
pile or trough battery, electrical indica-
tions may be obtained, by means of .an
ordinary condenser. It is necessary in
these experiments to advert to the dis-
tinction already pointed out (§ 15.) be-
tween single and compound circles as
to the denomination of the extremities
or poles of the battery. In the com-
pound circles the zinc side is found, to
be positive and the copper negative.
When fifty pairs of plates are employed,
a delicate gold-leaf electrometer will be
affected, without the aid of the condenser,
and with a series of one thousand
groups, even pith balls are made to
diverge. In order to exhibit these ef.

10

GALVANISM.

fects, the wire proceeding from one ex-
tremity of the battery should be con-
nected with the foot of the electrometer ;
while the wire proceeding from the op-
posite extremity is made to touch the
cap. It was by means of the revolving
doubler (see Electricity, § 195.) that
the electrical states of the two ends of
the voltaic pile were first ascertained by
Messrs. Nicholson and Carlisle.*

(23.) Since the ends of the two wires,
which proceed from the two poles [of
the voltaic battery, are in opposite states
of electricity, we might naturally expect
that they would attract one another.
Such an attraction actually does take
place, as Biot found by experiment ;f
but it does not become sensible, unless
a battery composed of a great number
of plates is employed,

(24.) The general conclusion dedti-
cible from the facts that have now been
stated, is that the intensity of the elec-
tricity developed by galvanic combina-
tions is increased, according as the
number of alternations in the elements
which compose them is greater, and
that it bears no proportion to the mag-
nitude of their surfaces.

(25.) If the voltaic battery be of suf-
ficient size, its electricity may be trans-
ferred to a common electrical battery,
which will then become charged to the
same degree of intensity. Nothing more
is necessary for this purpose than to
connect the outer and inner coatings of
the electrical battery, respectively, with
the two poles of the voltaic battery ;
when the charge will be instantly com-
municated to the former. If on re-
moving it from the voltaic battery this
electricity be discharged, and the same
communications be renewed, a similar
charge will again be received ; and the
same process may be repeated an in-
definite number of times. If, instead
of removing the electrical battery, we
allow it to remain connected with the
voltaic battery, a rapid succession of
sparks may be obtained from it by con-
necting a wire with the outer coating,
and repeatedly striking the knob of the
phial with the other end of the wire. If
the series of plates in the voltaic battery
consist of three or four hundred alter-
nations, these rapid explosions are so
powerful as to ignite the end of the wire,
if it be of iron, and to cause it to throw
off an abundance of sparks, consisting

* Nicholson's Journal, 4(o. iv. 174.
t Biot, Traite de Physique, ii. 511.

of small particles of iron in a state of
intense combustion. With a series of
one thousand, each discharge is at-
tended with a sharp sound, and will
burn thin metallic leaves. This is the
more remarkable as the same voltaic
battery may not have sufficient power
to produce these effects by itself, or un-
connected with an electrical battery.
The shortest possible contact with the
voltaic battery is sufficient for giving
the whole of the charge which it is ca-
pable of communicating. This was ap-
parent in some experiments made by
Van Marum and Pfaff with a battery
having 137£ square feet of coated sur-
face, and which was charged to the
same degree of intensity as the pile with
which it was made to communicate, by
a contact which did not last for the
twentieth part of a second.*

$ 2. Luminous effects of Galvanism.

(26.) It is only when the electricity of
a voltaic battery possesses a sufficient
intensity, that it becomes capable of
passing through air. With the calo-
rimotor the intensity is too feeble to
enable it to traverse the smallest per-
ceptible interval between metallic con-
ductors, so that they must be brought
into actual, or at least apparent, con-
tact, before any sensible effect is pro-
duced. In a pile or trough battery, on
the other hand, composed of a consider-
able number of alternations of plates,
on bringing together the wires from the
opposite poles, the transfer of electricity
begins while they are yet at a sensible
distance from one another : and as in
the case of ordinary electricity, this tran-
sit through the air is accompanied by
vivid light. The sparks occur every
time the contact between the wires is
broken, as well as when it is renewed.
This phenomenon, which does not take
place with the electricity furnished by
the ordinary means, is characteristic of
voltaic electricity, and is a consequence
of its continuous supply. The stream
continues to flow, notwithstanding the
interruption to the line of circuit, and
as long as the conductors remain within
the striking distance ; and although this
happens only for an instant, there is
still sufficient time for the appearance
of a spark.

(27.) The most splendid exhibition of
electric light is that obtained by placing
pieces of charcoal, shaped like a pencil,

• Annales de C/iimie, xl. 289.

GALVANISM.

11

at the ends of the two wires in the inter-
rupted circuit, and bringing their points
into contact. When the experiment
was tried with the powerful battery of
the Royal Institution, already noticed
($ 16.), a bright spark passed between
the two points of charcoal, when they
came within the distance of the thirtieth
or fortieth of an inch ; and immediately
afterwards more than halfcof each pen-
cil of charcoal, the length of which was
one inch, and the diameter one- sixth of
an inch, became ignited to whiteness.
By withdrawing the points from each
other, a constant discharge took place
through the heated air, in a space at
least equal to four inches, forming an
arch of light in the form of a double
cone, of considerable breadth, and of
the most dazzling brilliancy. This phe-
nomenon is represented in fg. 16 ; in

Fig. 16.

which W, X, are the conducting wires
communicating with the ends of the
battery ; C, C, the pieces of charcoal,
and A the luminous arch of electrical
light, making the passage of electricity
through the air. When any substance
was introduced into this arch, it in-
stantly became ignited : platina melted
in it, as wax in the flame of a candle :
some of the more refractory substances,
as quartz, the sapphire, magnesia, and
lime, all entered into fusion : fragments
of diamond, and points of charcoal and
of plumbago quickly disappeared, and
seemed converted into vapour, even
when the connection was made in highly
rarefied air, and apparentlywithout hav-
ing undergone previous fusion. When
the pieces of charcoal were placed in
the receiver of an air-pump, in propor-
tion as the air was abstracted, the dis-
tance at which the discharge took place
increased : and when the Height of the
mercury in the barometrical gage was
only one quarter of an inch, the sparks
were nearly half an inch in length ; and
by then withdrawing the points from
each other, the discharge passed through
a space of six or seven inches, pro-
ducing a most brilliant coruscation of
purple light. The whole of the charcoal

became intensely ignited, and some pla-
tina wire attached to it melted with
bright scintillations, and fell down in
large globules.* A battery of a hundred
pair of plates of six inches square will
suffice to exhibit these phenomena on a
smaller scale. Charcoal, carefully pre-
pared from some of the harder woods,
such as beech, lignum vitae, or box
wood, answers best for these experi-
ments. The arched form of the stream
of light passing between the two char-
coal points is perceptible even when the
points are within half an inch of each
other.

The light obtained by voltaic elec-
tricity in the manner now described
exceeds in intensity any other that art
can produce. It often exhibits in suc-
cession a variety of the 'prismatic co-
lours ; and supplies some of the rays
which are deficient in the solar beams.
It is so dazzling as to fatigue the eye
even by a momentary impression ; and
it effaces, by its superior lustre, the
light of lamps in an apartment other-
wise brilliantly illuminated, and which,
on the sudden cessation of the galvanic
light, appears for a short time as if left
in darkness. It is a light which so
nearly emulates the brightness of the
sun's rays, as to be applicable for the
purpose of illuminating objects in a
solar microscope; and even with the
magic lantern it has been found capable
of exhibiting on a large scale, as was
done by Mr. W. Allen in his lectures,
all the pleasing and endless variations
of the kaleidoscope.

(28.) The employment of charcoal in
these experiments might lead to a sus-
picion that the light mighf, in part at
least, arise from combustion ; but many
circumstances concur to prove that it is
quite independent of this cause. During
the continuance of the light, although
the charcoal be in a state of ignition, yet
it suffers but little loss of weight. The
light is evolved with equal splendour
when the experiment is made in gases
that contain no oxygen, such as azote
or chlorine, and in which therefore com-
bustion could not be maintained : and it
is moreover found that during the igni-
tion, neither the gas nor the charcoal
has undergone any chemical change.f
Light from voltaic electricity may also
be obtained, though with diminished in-

_* Davy's Elements of Chemical' Philosophy, p.

{'Children, Philosophical Transactions for 1815,
p: 369, j

12

GALVANISM,

tensity, under water, alcohol, ether, oils,
and other fluids of inferior conducting
power.
§ 3. Evolution of Heat by Galvanism.

(29.) The evolution of heat is one of
the effects which accompany the action
of the voltaic, as well as of the electric
battery ; but there is a remarkable dif-
ference in the circumstances which favour
its production in the two cases. In the
common electrical apparatus, heat is not
sensibly evolved where the electricity
moves with perfect freedom, but only
when some resistance is opposed to its
passage, and when there is a sudden
restoration of its equilibrium, accom-
panied with light and sound. But in
the voltaic battery, an elevation of tem-
perature is observed to take place when
the circuit remains complete, when no
light is evolved, and when the stream
of electricity is conducted in the most
silent manner. That the mere passage
of voltaic electricity through bodies
raises their temperature, is proved by
making a wire, forming part of the cir-
cuit, pass through a known quantity of
water, contained in a vessel, with a
thermometer immersed in the fluid. The
heat acquired by the water soon be-
comes sensible by the rise in the ther-
mometer, which even attains the boiling
point ; and the water continues in ebul-
lition as long as the experiment is con-
tinued.

(30.) The circulation of voltaic elec-
tricity produces an elevation of tempe-
rature, not only in that part of the
circuit which connects together the poles
of the battery, but also in the battery
itself, every part of which, both the
plates of metal and the fluid in the cells,
become heated when the apparatus is
in an active state. But the elevation of
temperature is found not to be equal
throughout the series ; and the ditfer-
ence is dependant on causes which have
not yet been accurately determined.
Mr. John Murray found a gradual in-
crease of temperature in the successive
cells from the negative to the positive
pole ; and when a number of different
troughs were joined together, the cells
at the extremities of each were less
heated than those towards the middle ;
the maximum of heat was at a part
situated nearer to the positive pole ; and
the temperature gradually diminished
in the direction of the negative pole.*
(31.) Ignition, in various degrees, is

* Edinburgh Philosophical Journal, xiv. 57.

produced by the" passage of voltaic elec-
tricity through metallic wires, when their
size and length are properly propor-
tioned to the kind of apparatus, and to
the quantity of electric fluid they have
to convey. Iron wire is in general easy
to ignite, and is often fused into glo-
bules ; and steel wire is made to burn
with a rapid and brilliant combustion.
A wire of platina, a metal not suscepti-
ble of being acted on by the air, may be
kept at a red, or even white heat, for an
indefinite length of time, by voltaic elec-
tricity. As long, indeed, asj the battery
retains its power, there appears to be no
limit to the continual evolution of heat.

(32.) The order in which the different
metals are raised to a red heat by the
action of galvanism, was ascertained by
Mr. Children, with the aid of a very
powerful apparatus of his own con-
struction, to be as follows, namely, pla-
tina, iron, copper, gold, zinc, silver.
Between copper and gold the difference
is inconsiderable; and with regard to
platina and iron, their relative places in
the scale seem to depend upon the tem-
perature acquired. The relations of tin
and lead to the other metals could not
be ascertained in these experiments, on
account of their melting before they
could be raised to a red heat. A beau-
tiful illustration of the difference exist-
ing in metals as to their capacity of
ignition, is obtained by placing in the
circuit a wire or chain composed of
alternate portions, or links of platina
and silver soldered together ; it will then
be found that the silver links are not
sensibly heated, while all those of pla-
tina become equally and intensely ig-
nited.

(33.) It would appear that the heat
produced by the voltaic battery is more
intense than can be excited by any other
process. In the experiments detailed
by Mr. Children,* the action of his
powerful apparatus raised to a red heat,
visible in full daylight, the whole of a
wire of platina, one tenth of an inch in
diameter, and five feet and a half in
length. It also effected the fusion of a
variety of substances on which the heat
of the best wind- furnaces makes no im-
pression.

(34.) When very thin metallic leaves
are placed in the electric current of a
powerful voltaic battery they take fire,
and by continuing the action, may be
made to burn with great brilliancy. In

* In thtj Philosophical Transactions for 1815, p,
368-370,

GALVANISM.

13

order to exhibit these effects, the me-
tallic leaves .should be suspended to a
bent wire proceeding from one extre-
mity of the battery, and then a broad
metal plate connected with the opposite
extremity should be gradually brought
near to them till contact is produced.
The brilliancy of the effect is heightened
by covering the plate with gilt foil.
Gold leaf, thus treated, burns with a
vivid white light tinged with blue, and
produces a dark purple or brown oxide.
Silver leaf gives out a brillant emerald
green light, and leaves an oxide of a
dark grey colour. Copper produces a
bluish white light, accompanied with
red sparks ; its oxide is dark brown. Tin
exhibits nearly the same phenomena,
excepting that its oxide is of a lighter
hue. Lead burns with a beautiful pur-
ple light ; and zinc with a vivid white
lighCinclining to blue, and fringed with
red. For the distinct appearance of
these colours, it is necessary that the
contacts should be made with a metal,
and not with charcoal ; for the intense
white light emitted by the latter, would
overpower the peculiar colours arising
from the combustion of the metal.*

(35.) A beautiful effect, noticed by
Van Marum, is produced by connecting
a slender iron wire with one of the poles
of a powerful voltaic battery, and bring-
ing its end in contact with the surface
of some mercury connected with the
other pole. Vivid combustion takes
place both in the mercury and in the
wire ; giving rise to an abundant emis-
sion of sparks, and appearing like a star
or sun dispersing thousands of rays on
every side. TmV splendid spectacle may
be prolonged at pleasure, by taking care
to continue the depression of the iron
wire, in proportion as the metallic par-
ticles are dispersed by the combustion.

(36.) Inflammable bodies, such as
oils, alcohol, ether, and naphtha, are
easily inflamed by means of galvanism,
when charcoal points in the circuit of
the battery are brought near each other
on the surface of these fluids ; and gun-
powder may readily be made to explode
under the same circumstances.

(37.) The difference in the operation
of voltaic and ordinary electricity is
very manifest in their mechanical effects.
The forcible separation of the particles
of bodies, and destruction of their cohe-
sion, characterize more especially the
electrical explosion, in which the fluid

* Singer's Elements of Electricity, p. 408.

appears to force for itself a passage
through every obstacle ; while the heat
which occasionally manifests itself dur-
ing this sudden effect, seems as if it were
merely the effect of the compression and
collision of the particles which are thus
forcibly impelled. But the elevation of
temperature which accompanies the
passage of voltaic electricity, on the
contrary, appears to be its immediate
and direct effect; for the mechanical
texture of the substance which conveys
the electricity remains unaltered. If
electricity in its common form possess
any power of igniting bodies, its opera-
tion is too transient and momentary to
produce any extensive effect ; and its
tendency is rather to separate and dis-
perse the body into minute fragments,
than to unite the particles into globules
by fusion. We have seen that charcoal
is very readily ignited by galvanism, but
it will sustain a strong discharge from
an electric battery without any percepti-
ble rise in its temperature; nor is it
possible to ignite it by this means.
Whether reduced to fine powder, or cut
into thin plates, or made to taper to a
point, it resists all attempts to raise it to
a red heat, or even to impart to it any
sensible warmth, though subjected to
the action of the most powerful battery
that has yet been tried. Even when an
apparently continuous stream of electri-
city, obtained from a large electrical
machine, was made to pass through
pointed wires coated with spermaceti, no
part of the spermaceti was melted.

§ 4. Electro-Magnetic Effects of Gal-
vanism.

(38.) We must rank among the more
remarkable of the physical effects pro-
duced by the transit of voltaic electricity
through conducting bodies, the induc-
tion ot magnetism in iron, and the influ-
ence exerted on bodies which possess
magnetic properties. But as the study
of the connections which subsist between
these phenomena implies a previous
knowledge of magnetism, and consti-
tutes, indeed, a distinct branch of
science, it will be proper to reserve their
consideration for a future treatise. It
may be as well, however, to remark
in this place, that the discovery of the
electro-magnetic effects of galvanism
have furnished us with the most delicate
tests for detecting very minute portions
of voltaic electricity, so that many of the
results of simple galvanic arrangements,

14

GALVANISM.

to be hereafter mentioned, have been ob-
tained by magnetic galvanometers.

§ 5. Chemical changes effected by
Galvanism.

(3 a.) In the Treatise on Electricity,
some of the chemical changes which re-
sult from the operation of this agent in
its ordinary form were noticed ; and ex-
periments were described in which water,
and a few'.saline bodies were decomposed
by a succession of electric discharges
from a powerful machine. But the
power of galvanism to effect changes in
the composition of bodies subjected to
its action is incomparably greater ; and
its application has led to a series of dis-
coveries which constitute a new era in
chemistry, and rank among the most
brilliant in the annals of physical
science.

(40.) The chemical agency of galvanism,
unlike its power of eliciting heat, is
manifested, not while it is traversing
substances of great conducting powers,
but, on the contrary, when it meets with
impediments to its passage; and it is
exerted chiefly on substances, generally
fluids, which convey electricity only par-
tially and imperfectly. That we may
acquire clear ideas of the connection of
the chemical phenomena relating to gal-
vanism, it will be necessary to trace
them from their origin, and attend to
what takes place in the simplest gal-
vanic circle composed of two dissimilar
metals and an interposed fluid.

(41.) If a plate of zinc, and another
of copper, be immersed in very dilute
sulphuric acid, - without touching or
communicating with each other, the
zinc will be acted upon by the acid ;
part of the water will be decomposed,
its oxygen combining with the zinc and
forming oxide of zinc ; and its hydrogen
will be disengaged in the form of gas
from the surface of the zinc plate. The
oxide of zinc, in proportion as it is pro-
duced, will be dissolved by the acid,
thereby forming sulphate of zinc. The
plate of copper, which has been immersed
in the same fluid, will, during all this
time, have undergone no change ; the
acid, in its diluted state, being incapable
of acting upon it. But if, while the
above process is going on, the metals be
brought into contact, either directly, or
by the intervention of some metallic in-
termedium, the following changes will
ensue. In the first place, the oxidation
and solution of the zinc will proceed
with much greater rapidity and energy

than it did before ; and in the second
place, it will not be accompanied by the
evolution of the same quantity of hydro-
gen gas from the oxidating surface.
There will, indeed, be a disengagement
of hydrogen from the whole fluid, in
quantity exactly corresponding to that
of the oxygen derived from the water ;
but the greater part of this hydrogen
will now make its appearance on the
surface of the copper plate, whence it
will arise in a copious stream of bubbles.
But still the copper will itself remain
apparently unaffected by this change in
the circumstances of the experiment.
In process of time, indeed, when a con-
siderable proportion of sulphate of zinc
has been dissolved in the fluid, the
quantity of disengaged hydrogen is
found gradually to diminish, and a thin
film, composed partly of metallic zinc
and partly of filaments of oxide of zinc,
is deposited on the surface of the cop-
per ; as soon as this happens the gal-
vanic action ceases.

(42.) If an acid, such as the nitric
acid, capable of acting upon the copper,
as well as upon the zinc, be employed
instead of the sulphuric acid, similar
phenomena will take place, with this
additional circumstance, that the action
of the acid upon the copper will cease
the instant the galvanic circuit is com-
pleted ; and instead of nitrous gas being
formed on the surface of the copper,
which happens before the circuit is
formed, only bubbles of pure hydrogen
will make their appearance ; and the
copper is protected from all further
action, the zinc being, as in the former
case, oxidated and dissolved with addi-
tional energy. It is on this principle
that Sir H. Davy has effected the pro-
tection of the copper sheeting of ships
from the corrosion of sea water, by
placing in contact with it pieces of zinc
or iron, on which sea water exerts a

freater chemical action than on copper,
ee Phil. Trans, for 1824, p. 151, and
242; and for 1825, p. 328.

(43.) In compound voltaic batteries,
the same chemical changes which have
been just described as occurring in the
simple galvanic circle, take place in
each of the portions of fluid intervening
in the compartments between the plates.

(44.) The chemical agency of galva-
nism is exerted in a no less remarkable
manner on fluid conductors placed in the
circuit between the poles of the battery.
Among the simplest of its effects is the
resolution of water into its two gaseous

GALVANISM.

elements, oxygen and hydrogen. The
discovery of this fact is due to the united
researches of the late Mr. Nicholson and
of Mr. (now Sir Anthony) Carlisle, and
was one of the immediate consequences
of the invention of the pile by Volta.

(45.) The most convenient mode of

exhibiting the decomposition of water by

galvanism is to fill with water a glass

tube, (see/0-. 17.) to each end of which

Fig. 17.

a cork has been fitted so as to confine
the water, and to introduce into the tube
two metallic wires, by passing one at
each end through the gork which closes
it, allowing the extremities of the wires,
that are in the water, to come so near
each other as to be separated by an in-
terval of only a quarter of an inch. The
wires being then respectively made to
communicate with each of the two poles
of a voltaic batteiy, the following phe-
nomena will ensue. If the wire con-
nected with the positive pole of the bat-
tery consists of an oxidable metal, it is
rapidly oxidated by the water surround-
ing it — while at the same time a stream
of minute bubbles of hydrogen gas arises
from the surface of the other wire, which
is in connection with the negative pole.
But if we employ wires made of a metal
which is not susceptible of oxidation by
water, such as gold or platina, gas will
be extricated from both the wires, and
by means of a proper apparatus may be
collected separately. This may be ac-
complished by taking two glass tubes, or
receivers, closed at one end, and filled
with water; this fluid is retained by
inverting them over a sufficient quantity
of water contained in a glass vessel, as
js shown in Jig. 1 8. Each tube is to be

furnished with a platina wire, P andN,

passed through the closed extremity,
and descending within it through its
whole length. The open ends are then
to be placed as near to each other as
their position in the water will allow ;
and the wires are to be connected re-
spectively with the opposite poles of a
voltaic battery. Gas will immediately
be seen to rise from each of the wires,
but in different quantities. The tube
containing the negative wire, N, will be
soon filled with hydrogen gas, while the
other, which is traversed by the positive
wire, P, will, in an equal time, be only
half filled with oxygen gas. This arises
from the circumstance that the volumes
of the two gases, which form water when
combined, or which are the products of
the decomposition of water, are in the
above proportion ; that is, the volume
of the hydrogen is to that of the oxygen
gas as two to one. That the water is in
this experiment perfectly resolved into
its two elements is satisfactorily proved
by mixing together the gases thus ob-
tained, and firing the mixture by the
electric spark ; when the whole instantly
loses its gaseous form, and is reconverted
into water.

(46.) If the water employed in the
preceding experiment be not perfectly
pure, other substances besides oxygen
and hydrogen will also make their ap-
pearance at the two wires, and the
apparent formation of such substances
from water was the occasion of great
perplexity to the earlier experimentalists.
But Sir H. Davy succeeded in proving,
by a most masterly train of investigation,
that when every precaution is taken to
ensure the purity of the water subjected
to the operation of galvanism", the only
products obtained are the two gaseous
elements of water, oxygen, and hy-
drogen.

(47.) In these experiments it became
manifest, that under the influence of vol-
taic electricity neutral salts, existing in
any solution, were decomposed, the acid
portion being accumulated around the po-
sitive wire, on the same points where the
extrication of oxygen took place ; while
the bases, whether earthy, alkaline, or
metallic, were, at the same moment,
transferred along with the hydrogen to
the negative wire. The best mode of
exhibiting these decompositions, is to
employ two cups, made either of glass,
or, where great precision is requisite, of
agate, or of gold ; the liquids contained
in these cups being connected together
by a few fibres of moistened asbestos,

1C

GALVANISM.

and subjected to the action of the vol-
taic battery. If the liquid contain any
soluble saline compound, such as sul-
phate of soda, or common Glauber's
salt, and the operation be continued a
sufficient time, the whole of the acid
contained in the salt will be found col-
lected in the positive cup, and the whole
of the alkali in the negative cup. Nor
is any considerable solubility in the body
placed in the circuit necessary for its
decomposition by galvanism. Two cups
made of compact sulphate of lime, con-
taining pure water, were connected to-
gether by fibrous sulphate of lime, moist-
ened by pure water, and the voltaic
current transmitted through them, Af-
ter an hour the fluids were accurately
examined, when it was found that the
negative cup contained a pure and satu-
rated solution of lime, partially covered
with a calcareous crast ; while the po-
sitive cup was filled with a moderately
strong solution of sulphuric acid. Sul-
phate of strontites, and fluate of lime,
subjected to the same process, yielded
similar results : sulphate of barytes,
from its greater insolubility, proved
more difficult of decomposition ; but the
difficulty was at length overcome. The
analysis of many mineralogical speci-
mens, 'of which the composition was
much more complicated, was greatly
elucidated by the application of voltaic
electricity, which effected the extraction
of all the acid and alkaline matters they
contained.

(48.) For the production of these
effects it is immaterial in what part of
the fluid line of circuit the decomposable
body happens to be situated. This will
appear by placing three cups, side by
side, in a line (fig. 19.), and connecting

Fig. 19.

them together by moistened asbestos.
Let a solution of sulphate of potash, or
any other neutral salt, be put into the
middle cup, and blue infusion of cab-
bage into the other cups. When these
fluids are placed in the circuit of the
voltaic battery, by immersing the wires
into the fluid in the outer cups, the sul-
phuric, or other acid, will collect in the

positive cup, and render its blue infu*
sion red, while the alkali will pass into
the opposite cup, and tinge its blue con-
tents green.

(49.) When metallic solutions are
subjected to the decomposing action of
galvanism, a deposition of the metal,
generally in the form of minute crystals,
takes place on the negative wire, and
oxide is also deposited around it ; while
the acid passes over, as before, into the
positive cup. This effect takes place
with solutions of iron, zinc, and tin, as
well as with the more oxidable metals.

(50.) When a solution of nitrate of silver
has been placed on the positive side, and
distilled water on the negative, the whole
of the connecting asbestos becomes co-
vered with a thin metallic film of silver.
We have been the more particular in
noticing these effects, because, as was
before observed ($ 41.), they occur to a
greater or less extent in the fluids which
occupy the cells of the battery, and have
a considerable influence in modifying,
and ultimately destroying the power of
the instrument.

(51.) Phenomena of a still more ex-
traordinary nature, presented themselves
to Sir H. Davy in the further prosecu-
tion of these inquiries. It was disco-
vered that the elements of compound
bodies were actually conveyed by the
influence of the electric current through
solutions of substances, on which, under
other circumstances, they would have
exerted an immediate and powerful che-
mical action, without any such effect
being produced. Acids, for example,
may be transmitted from one cup, con-
nected with the negative pole, to ano-
ther cup on the opposite or positive
side, through a portion of fluid in an
intermediate cup tinged with any of the
vegetable coloured infusions, which are
instantly reddened by the presence of
an acid, without occasioning the slight-
est change of colour. The same hap-
pens also with alkalies. If three cups
be arranged as before, (see Jig. 19.) and
connected with each other in a series by
moistened cotton, the middle cup, and
also the one next to the positive side of
the battery, being filled with blue in-
fusion of cabbage, or of litmus ; and the
cup next to the negative side containing
a solution of sulphate of soda ; on the
series being placed in the voltaic circuit,
a red tinge will soon be perceived in
the water of the positive cup, which
will become strongly acid. It is evi-
dent that the sulphuric acid so trans-

GALVANISM.

17

ferred must have passed through the
fluid in the middle vessel, but without
affecting the coloured solution in its
passage. By reversing the connections
with the poles of the battery, a similar
transfer of the alkali will be made ; it
will be collected in the tinged water of
the negative cup, which it will render
green ; but the intermediate portion of
fluid will not, either in this or in the
former case, exhibit any trace of the
substance which is carried through it by
the influence of electricity.

(52.) No union, under similar cir-
cumstances, is found to take place, be-
tween acids and alkalies, when either of
these active chemical principles is trans-
mitted by voltaic electricity through the
other, provided the compound which
they would form by their union remains
soluble ; for should the compound be in-
soluble, the union takes place, and the
product, on falling to the bottom of the
fluid by its superior gravity, is removed
from the line of the electric action. When,
for example, sulphuric acid is attempted
to be passed through a solution of bary-
tes, or vice versa, barytes through a solu-
tion of sulphuric"" acid, sulphate of ba-
rytes is formed, which being insoluble in
the fluid, falls down as a precipitate,
and being removed from the action of
the electric current, proceeds no further
in its course. If some basis of me-
chanical support be provided, whereby
this removal from the voltaic influence
can be prevented, the transfer may some-
times be continued, notwithstanding the
body has assumed a solid form ; thus
magnesia or lime will pass along moist
asbestos, from the positive to the nega-
tive sides ; but if a vessel of pure water
be interposed, they do not reach the
negative vessel, but sink to the bottom.
In like manner when nitrate of silver
was on the positive side, and distilled
water on the negative, the silver, as we
have already seen, passed along the
transmitting fibres of the asbestos, so as
to cover it with a thin metallic film.

(.)3.) When the fluids placed in the
same voltaic circuit are connected, not
by fluids, but by pieces of metal, such
as wires, the changes above described
take place in each separate portion of
fluid, each alternate metallic surface
performing the functions of a positive
and negative polarity, according to its
place in the circuit of, the electric current.
Those parts into which the electricity is
entering possess properties corresponding
to those of the negative wires or poles of

the battery ; and those which are giving
exit to the electricity, act as positive
wires. The former will collect around
them the several bases of neutral and me-
tallic salts, and the hydrogen of the de-
composed water ; the latter will collect
oxygen, and the compounds in which
oxygen predominates, such as the acids.
(54.) The decomposition of the alka-
lies and of the earths, which crowned
this brilliant career of discovery, is, in
point of theory, only a particular in-
stance of the general fact above stated,
namely, that combustible substances are
carried to the negative wire, and oxygen
evolved at the positive wire. Various
other applications have been made of the
voltaic battery to the purposes of che-
mical decomposition. Sulphuric acid
is resolved by its means into oxygen gas
and sulphur. Phosphoric acid, in like
manner, yields oxygen gas and phos-
phorus. Ammonia separates into hy-
drogen and azote, with a small propor-
tion of oxygen. Oils, alcohol, and ether,
when acted on by a powerful battery,
deposit charcoal, and give off hydrogen,
or carburetted hydrogen. But it would
be encroaching too far on the province
of chemistry to extend our illustrations
of this subject to any greater length.

§ 6. Physiological effects of Galvanism.

(55.) The action of voltaic, as well as
of common electricity, on a living ani-
mal is chiefly exerted on the functions of
the nervous system. It is shown in the
production of sensation, in the excita-
tion of muscular contraction, and in al-
tering the products of secretion.

(56.) If any considerable part of the
human body form part of the circuit of
a voltaic pile or battery, a separate
shock is experienced every time a con-
nection is made with the poles of the
apparatus ; provided the skin through
which the electric current is to pass be
sufficiently moist to allow of its being
transmitted : for in its usual dry state
the cuticle, or outer skin, is scarcely
pervious to electricity of such low inten-
sity as that afforded by galvanism. The
most effectual method of receiving the
whole force of the battery is to wet both
hands with water, or what answers still
better, with a solution of common salt,
and to grasp a silver spoon in each ; the
circuit is then to be completed by touch-
ing one pole of the battery with one
spoon, and the opposite pole with the
other spoon. Another mode is to plunge

c

18

0ALVANISM.

a finger of each hand into two separate
vessels filled with water, into which the
extremities of the two wires from the
battery have been immersed. The shock
received from the voltaic pile is similar
to that resulting from a large electrical
batfery very weakly charged : and its
intensity is greater in proportion to the
number of series of elements composing
the pile. Twenty pair of plates are ge-
nerally sufficient to give a shock, which
is sometimes felt in the arms : with a
hundred pair it extends to the shoul-
ders.

(57.) Independently of the shock felt
on the first impression of voltaic elec-
tricity communicated from the battery,
the continued flow of the current
through the body, as long as it forms
part of the circuit, is generally accom-
panied by a continued aching pain. If
it pass through any external part de-
prived of cuticle, it produces a severe
smarting or burning sensation, which,
if the exposed surface be large, continues
to increase till it is scarcely support-
able. This painful feeling is experienced
if the slightest cut, burn, or excoriation of
any kind, happen to be in the path of
the electrical current : and it will be ex-
cited in these parts, even by a single
pair of plates, forming a galvanic com-
bination. It has been remarked by
Volta that the pain is of a sharper kind
on those sensible parts of the body, in-
cluded in the circuit, which are on the
negative side of the pile ; that is, where
the electricity flows out from the body,
than where it enters : a fact which has
also been noticed with regard to the
pungency of the common electrical
spark.*

(58.) The impression made by voltaic
electricity on some of the nerves of the
face, when they form part of the circuit,
is accompanied by the sensation of a
vivid flash of light. The simple appli-
cation of a piece of zinc and one of silver
to the tongue or lips, frequently gives
rise, at the moment of the contact 01 the
metals, to this perception of a luminous
flash : but the most certain way of ob-
taining this result is to press a piece of
silver as high as possible between the
upper lip and the gums, or to insert a
silver probe into the nostrils ; while, at
the same time, a piece of zinc is laid
"upon the tongue ; and then to bring the
two metals into contact. Another mode
is to introduce some tinfoil within the

Nicholson's Journal, 4to. iv. 180.

eyelid, so as to cover part of the globe
of the eye, and place a silver spoon in
the mouth, which must then be made to
communicate with the tinfoil by a wire
of sufficient length ; or conversely, the
tinfoil may be placed upon the tongue,
and the rounded end of a silver probe
applied to the inner corner of the eye ;
and the contact established as before.
The flash which results from the action
of a pile, applied in this way, is very
powerful ; and if the plates were nume-
rous, the experiment might occasion
permanent injury to the sight. This
phenomenon is evidently produced by an
impression communicated to the retina,
or optic nerve, and is analogous to the
effect of a blow on the eye, which is
well known to occasion the sensation of
a -bright luminous coruscation, totally
independent of the actual presence of
light. In like manner the flash from
galvanism is felt whether the eyes be
open or closed, or whether the experi-
ment be made in day-light or in the
dark. If the pupil of the eye be watched
by another person when this effect is
produced, it will be seen to contract at
the moment when the metals are brought
into contact. A flash is also perceived
at the moment the metals are separated
from each other.

(59.) The peculiar taste which is per-
ceived when different metals are applied
to different parts of the tongue, and
made to touch each other, has already
been noticed. It is essential to the
success of the experiment, that the sur-
face of the tongue should be moist;
for when the tongue is previously wiped
very dry, the effect is considerably di-
minished, and it is not at all perceptible,
if the surface is absolutely dry. The
quality of the metal laid upon the tongue
influences the kind of taste which is
communicated ; the more oxidable metal
giving rise to an acid, and the less oxi-
dable metal to an austere or alkaline
taste. Similar differences have been
observed by Berzelius, with regard to
the sensations excited in the tongue by
common electricity, directed in a stream
upon that organ, from a pointed con-
ductor; the taste of positive electricity
being acid, and that of negative electri-
city caustic and alkaline. This circum-
stance would tend to prove that the
taste perceived in the galvanic experi-
ment is owing to the actual presence of
acids and alkalies, derived from the
chemical decomposition of the salts
contained in the saliva, by the galvanic

GALVANISM.

19

action ; and that it is not merely the effect
of a direct impression of the electric
current on the nerves of the tongue.

(60.) When the current of voltaic
electricity is made to pass along a nerve
distributed to any of the muscles of
voluntary motion, these muscles are
thrown into violent contractions of a
convulsive kind. It was an observation
of this kind that led the way, as we have
already seen, to the discovery of the gal-
vanic influence. The muscles of a frog
are, indeed, peculiarly sensible to this
influence, and are therefore the fittest
for the exhibition of this phenomenon,
with very weak galvanic powers. The
susceptibility of some of the animals
belonging to the class of vermes, is also
very great. If a crown piece be laid
upon a plate of zinc of larger size, and
a living leech be placed upon the silver
coin, it will suffer no inconvenience as
long as it remains in contact with the sil-
ver only ; but the moment it has stretched
out its head so as to touch the zinc, it
suddenly recoils, as if it had experienced
a painful shock. An earthworm will
also exhibit the same kind of sensitive-
ness ; and the same effect is still more
strikingly exhibited by the nais, which
is an aquatic worm. Humboldt found
that the lerncca, or water-serpent, and
even the tcenia, ascaris, and other spe-
cies of intestinal worms, had their move-
ments accelerated by the influence of
galvanism, which also speedily de-
stroyed their life. Powerful shocks from
a voltaic battery are no less immediately
fatal to animals, than discharges from
an ordinary electric battery.* Small
animals are easily killed by discharges
which would only produce a temporary
stunning effect on larger animals.

(61.) Convulsive movements may be
excited by galvanism in the muscles of
an animal, after its death, as long as
they retain their contractility. These ef-
fects become exceedingly striking, when
large animals are made the subjects of
experiment, and when powerful batte-
ries are employed. Thus if two wires,
connected with the poles of a battery of
a hundred plates, be inserted into the
ears of an ox, or sheep, when the head
is removed from the body of the animal
recently killed, very strong actions will
be excited in the muscles of the face
every time the circuit is completed.
The convulsions are so general, as often
to impress the spectator with a belief

* See Treatise ou Electricity. 5 182.

that the animal has been restored to the
power of sensation, and that he is en-
during the most cruel sufferings. The
eyes are seen to open and shut sponta-
neously, they roll in the sockets as if
again endued with vision ; the pupils are
at the same time widely dilated. The
nostrils vibrate as in the act of smell-
ing ; and the movements of mastication
are imitated by the jaws. The strug-
gles of the limbs of a horse galvanised,
soon after it has been killed, are so
powerful as to require the strength of
several persons to restrain them.

(62.) It is needless to enter into the
details of experiments of a similar kind
performed in hospitals on limbs re-
moved by amputation ; or on the bodies
of criminals soon after their execution. A
great number of these are stated to have
been made at Turin, on the victims of
the guillotine ; and in this country, Al-
dini, by operating with a considerable
number of plates on the body of a cri-
minal executed at Newgate, produced
effects very similar to those already de-
scribed in the sheep and ox ; but which
were necessarily of a more impressive
character, from their conveying the
more terrific expressions of human pas-
sion and of human agony.

(63.) Muscles whose actions, like
those of the heart, are not under the
guidance of the will, are less easily af-
fected by galvanism than the muscles of
voluntary motion. But Fowler, Vassali,
Humboldt, Nysten, and others, have
sufficiently proved that even these mus-
cles may, by the proper application of
this power, be made to contract.

(64.) The most curious and hitherto
unexplained of the physiological effects
of galvanism, are those on the functions
of secretion, especially on that of the
gastric juice, a fluid which is essentially
subservient to the process of digestion.
But these topics appertain more to phy-
siology than to the subject of the pre-
sent treatise.

CHAPTER V.
Theory of Galvanism.

(65.) THE various attempts which have
at different times been made to explain
the phenomena of galvanism, by the
application of the laws which are known
to govern those of ordinary electricity,
have on the whole been attended with
very indifferent success ; and the theory
c 2

GALVANISM.

of this science remains, even at the pre-
sent day, involved in considerable un-
certainty and obscurity. No very dis-
tinct or satisfactory account has yet
been given of the nature of that force,
which originally disturbs the electrical
condition of the different parts of the
voltaic apparatus, and constitutes the
primary source of galvanic power. It
was long the prevailing hypothesis, that
this force was the same with that which
gives rise to the developement of elec-
tricity during the contact of dissimilar
metals; a fact, the principal circum-
stances attending which have been
stated in the treatise on Electricity.
(§ 203.) But in proportion as a more
extensive acquaintance with the pheno-
mena afforded the means of a more ac-
curate analysis, the insufficiency of this,
which was termed the Electrical Theory,
became more apparent ; and it is now
fully established, that the primary agent
in the evolution of electricity, is the force
of chemical attraction. This latter view
of the subject, has led to what may be
called the Chemical Theory of Gal-
vanism.

(66.) Every scientific theory must
have for its basis some general fact,
comprehending a multitude of subordi-
nate phenomena, which are its more or
less direct consequences. The chemical
theory of galvanism assumes the fol-
lowing as the most general fact in that
science : namely, that chemical action,
occurring between a fluid and a solid
body, is always accompanied by the dis-
turbance of electric equilibrium ; in con-
sequence of which a certain quantity of
electricity is developed, or, in other
words, converted from a latent into an
active state. So intimate, indeed, is the
connection between the electrical and
the chemical changes, that the chemical
action can proceed only to a certain ex-
tent, unless the electrical equilibrium
which has been disturbed be again re-
stored. The oxidation of metallic bo-
dies (that is, their combination with
oxygen) is more especially accompanied
by the developement of large quantities
of electricity. rihus it has been ascer-
tained, that when a plate of zinc is
chemically acted upon by dilute sul-
phuric acid, which produces first oxide,
and then sulphate of zinc, the metal
becomes negatively electrified, while the
liquid is in the same degree positively
electrified. This fact, when stated con-
formably to the hypothesis of Franklin,
implies the abstraction of the electric

fluid from the zinc, and its transference
to the liquid product of the combina-
tion : but, when translated into the lan-
guage of the hypothesis of a double
fluid, must be understood as the separa-
tion of the two electricities by the che-
mical action, and the determination of
the resinous or negative electricity in
the direction of the zinc, and of the
vitreous or positive electricity in the
direction of the oxidating liquid. In
order to avoid perplexity, however, we
shall continue to adhere to the simpler
of these hypotheses ; and advert only to
the conditions and movements of posi-
tive electricity. ($ 4.)

(67.) That two conducting bodies,
such as zinc and acid, thus remain, the
one in a negative, and the other in a
positive electrical state, notwithstanding
their being in contact, is known to us
as a matter of fact ; but it is a fact
which is not explicable by any of the
laws of ordinary electrical phenomena,
or, in other words, it is not reducible to
any other more general fact. We must
for the present, therefore, be content to
leave it as a subject of future inquiry,
to determine to what peculiarity in the
circumstances attending the changes of
chemical composition it is owing, that
the electric equilibrium is permanently
disturbed, and what is the unknown ob-
stacle that prevents its restoration. A
similar difficulty occurring in the case
of the electricity produced by contact,
has been noticed in the treatise on .Elec-
tricity. ($204.)

(68.) As long as the chemical action
proceeds, the transfer of electricity from
the metal to the fluid continues ; but the
rapidity of the process is checked by the
circumstance, that as soon as the quan-
tity transferred has accumulated so as
to reach a certain degree of intensity,
which is generally exceedingly low, all
action ceases, the chemical affinities be-
ing balanced by an opposing electrical
force. But in consequence of the gra-
dual absorption of electricity by the
metal from surrounding bodies, and the
gradual dissipation of the superabun-
dant electricity of the fluid, this state is
never reached ; or, if attained, does not
long subsist : and the chemical affini-
ties continue to produce their effects,
though more slowly than if their opera-
tion were uncontrolled by the electrical
force. But if, on the other hand, by
the interposition of good conductors, a
ready passage be afforded for the elec-
tricity from the fluid, where it is accu-

GALVANISM.

21

mulated, to the metal where it is defi-
cient, then the obstacle to the further
exertion of Ihe chemical affinities be-
tween these two bodies will be removed,
and the action will now proceed with
much greater energy. This is precisely
what is accomplished by galvanic com-
binations. Some metal, such as cop-
per, silver, e;old, or platina, not suscep-
tible of oxidation by the fluid employed,
is applied to this fluid, collects from it
the redundant electricity, and then being
brought into contact with the zinc, or
metal acted upon by the fluid, commu-
nicates to it this electricity, and thus
continually restores the electric equi-
librium, the very instant after it has
been disturbed. We find, accordingly,
that under these circumstances, that is,
whenever the galvanic circuit is com-
pleted, the oxidation of the zinc proceeds
with renewed activity ; but ceases, or at
least takes place more slowly, whenever
this circuit is interrupted.

(69.) In order to take a more com-
prehensive view of the subject, we may
state the following as the conditions that
are essential to galvanic action. First,
the presence of three elements is re-
quired, which we shall designate by the
letters A, Z, and C. Between the two
first of these, A and Z, some chemical
affinity must exist, adequate to produce
combination and developement of elec-
tricity ; while the same action, or at
least the same degree of that action, is
not exerted between the third element C,
and either of the former. Secondly, it
is necessary that one of the two first
bodies, which we shall suppose to be Z,
be a solid ,* and that it possess a high
degree of conducting power with regard
to electricity. As it is a general law in
chemistry that no chemical action can
take place between two bodies, unless
one of these bodies be in a fluid state, it
follows that as Z is a solid, so A must
be a fluid body ; on the other hand, the
body C may be either solid or fluid.
Thirdly, it is requisite that all the three
bodies be in mut ual contact, so as to com-
pose a kind of circular arrangement, as
is represented in fig 20. If all these
conditions be fulfilled, it is found that a
continued stream or current of electri-
city will circulate in a determinate di-
rection through the bodies thus placed,

* Sir H. Davy has shown that chemical action
taking place between two fluids, although intense, is
not attended with the disturbance of the electric
equilibrium. Philosophical, Transactions for 182G,
p. 399, 400.

as long as the chemical action continues.
If the bodies Z, A, and C, be respec-

Fig. 20.

tively zinc, acid, and copper, the surface
of contact between Z and A will be that
at which the chemical action and con-
sequent developement of electricity takes
place ; for C may be considered as act-
ing merely the part of a conductor of
that electricity between A and Z ; and
the current will circulate in the direc-
tion denoted in the figure by the arrows,
that is, from A to C, and thence to Z.

(70.) The absolute quantity of elec-
tricity which is thus developed, and made
to circulate, will depend upon a variety
of circumstances, such as the extent of
the surfaces in chemical action, the faci-
lities afforded to its transmission, &c.,
— causes the operation of which we shall
afterwards have occasion to examine..
But its degree of intensity, or tension
as it is often termed, will be regulated by
other causes, and more especially by the
energy of the chemical action. In a
single galvanic circle, however, it is ne-
cessarily very low, being limited by the
nature of the process to which it owes
its origin, and to which it is in some re-
spects opposed. It may be much in-
creased, however, by combining toge-
ther the power of a number of circles,
as is done in the pile and voltaic battery.
Taking the common trough battery as
an example, and tracing the several steps
of the process, we shall find that the
electricity which the liquid in the first
cell has acquired from the first plate of
zinc exposed to its action, is taken up
by the copper plate belonging to the
second pair, and transferred to the second
zinc plate, with which it is connected.
This second plate of zinc, having thus
acquired a larger portion of electricity
than its natural share, is capable of
supporting a more intense chemical ac-
tion than it would otherwise have done;
and hence it communicates a larger
quantity of electricity to the fluid in the
second cell. This increased quantity is

GALVANISM.

22

again transmitted to the next pair of
plates, and renders the third zinc plate
capable of maintaining a still more pow-
erful chemical action than the preceding
plate ; and thus every succeeding alter-
nation is productive of a further increase,
both in the quantity and intensity of the
electricity developed.

(71.) The simplest cases are those in
which no chemical action whatever is
exerted either between the fluid A and
the body C, or between C and Z ; and
the force of the electric current will then
be proportional simply to the energy of
the chemical action taking place between
A and Z. But either A and C, or C
and Z, may also have some chemical
action on one another ; and it will de-
pend on the nature of that action whe-
ther the electric force to which it gives
rise opposes or concurs with the force
resulting from the action between A and
Z. If the two actions be of the same
kind, as, for example, if they should
both be oxidating actions, the electric
forces resulting from them will be in op-
position to each other ; for while the one
is impelling the current from Z to A, the
others will tend to impel it from C to A,
or from Z to C, that is, in a contrary
direction. The effective electromotive
force will, in all these cases, ^be equal to
the difference between the two that are
thus opposed to each other. On the
other hand, if the chemical actions be-
tween A and C, or between Z and C,
should happen to be of an opposite kind,
with regard to their electrical tendencies.
to that between Z and A, they will com-
municate to the developed electricity an
impulse in the same direction, and the
resulting electromotive force will be equal
to the sum of the conspiring forces.

(72.) We have seen that the third ele-
ment C may be either a solid or a fluid
body, and we may therefore distinguish
galvanic circles into two kinds, according
as C has the one or the other of these
two forms. In the first, the circle is
composed of two solids and one fluid ;
in the second, of one solid and two fluids.
Of the solid elements capable of forming
galvanic combinations, the most effica-
cious are the metals, and charcoal. Of
fluid elements, those which exert a pow-
erful chemical action upon the former,
such as the mineral acids, alkaline solu-
tions, sulphurets, solutions of neutral
salts, and Water containing oxygen gas,
or atmospheric air. The energy of the
galvanic power will depend altogether
upon that of the chemical action, and

can never be excited when the latter
condition is wanting. Thus silver and
gold evolve no galvanic influence when
in contact with pure water, which is in-
capable of acting chemically upon either
of these metals ; but the addition of
nitric acid, or any other fluid decom-
posable by silver, to the water, imme-
diately renders this combination of ele-
ments an active galvanic circle.

(73.) With regard to the direction
given to the electrical current by the
chemical action of two bodies, we may
lay it down as a general rule, to which
there are but few exceptions, that the
electricity is determined from the solid
to the fluid which acts upon it chemi-
cally. This we have already seen ex-
emplified in the instance so frequently
referred to of the ternary arrangement of
zinc, acid, and copper. Another, and
very common mode of expressing the
same fact is, to say that the zinc is ren-
dered positive with regard to the copper,
and, vice versa, the copper negative with
reference to the zinc. In this sense,
that is with relation to the action of acids
and other oxidating fluids, every oxi-
dable metal is positive with regard to a
metal which is oxidable in a less degree.
In order to determine beforehand the
effect of any combination of two metals
in a galvanic circle with any of the acids,
it will be convenient, therefore, to ar-
range the metals in the order of their
oxidability. With this view the follow-
ing catalogue has been given by Sir
Humphry Davy : * viz.

Potassium and its amalgams.

Barium and its amalgams.

Amalgam of zinc.

Zinc.

Cadmium.

Tin.

Iron.

Bismuth.

Antimony (?).

Lead.

Copper.

Silver.

Palladium.

Tellurium.

Gold.

Charcoal.

Platina.

Iridium.

Rhodium.

(74.) In a ternary galvanic arrange-
ment with acids, then, each metal in the
above list is positive to all those which

# Philosophical Transactions for 1826, p. 408.

GALVANISM.

23

follow it ; and the more so in proportion
as the two metals are more distant from
each other in the scale. Thus zinc and
iron will compose a weaker circle than
zinc and silver ; and zinc and platina
will form one of still greater power. It
may be observed, however, that the pre-
cise order in which the metals stand in
such a scale as the above, must be un-
derstood as only strictly true with rela-
tion to the particular acid employed, and
even to the particular decree of dilution
that has been given to the acid. For
we find slight variations in the order of
relation of the metals with different acids,
or even with the same acid in different
states of concentration.

(73.) When alkaline solutions are em-
ployed as the fluid agent, instead of
ado's, the same general order is observed
in the metals, with regard to their mutual
electrical relations. The principal ex-
ception is with regard to iron, which
is here found to occupy a place interme-
diate between copper and silver. Thus
a combination of iron and copper will,
by immersion in an acid, form a circle
in which the electricity will be determined
from the iron to the acid, thence to the
copper, and thence to the iron ; that is
to say, the iron will be positive with re-
gard'to the copper. But if the same
combination of iron and copper be acted
upon by an alkaline solution,* and more
especially by ammonia, the iron is nega-
tive with regard to the copper ; for here
the chemical action of the fluid upon the
copper is stronger than upon the iron,
and the electricity is therefore determined
to the fluid from the copper, and not
from the iron as in the former case.
The same results are obtained when tin
is employed in conjunction with copper,
and with ammonia, f

(76.) With solutions of hydro-sulphu-
rets, the several metals stand also nearly
in the same order, as to their electrical
relations, as with acids — with a few ex-
ceptions, however, as will appear from
the following catalogue, given by Sir
H. Davy : —

Zinc.

Tin.

Copper.

Iron.

Bismuth.

Silver.

Platina.

Palladium.

* Davy, Elements of Chemical Philosophy, p. 148.
f I>e la Rive, Annales do CMmie, xxxvii.'m

Gold.
Charcoal.

We may observe, that here also cop-
per is positive with regard to iron ; so
that when these two metals form a circle
with a solution of hydro-sulphuret, the
electrical current is in an opposite di-
rection to what it is when the same
combination of metals is plunged in
acids.

(77.) It need hardly be observed, that
every thing that has been stated with
regard to single galvanic circles applies
also to compound circles, whether in
the form of the pile, or the trough bat-
tery, composed of the same elementary
parts.

(78.) We have next to consider the
second class of galvanic circles ; those,
namely, which are composed of a single
solid and two fluid elements.

The arrangement assumed in this
case by the three elements of the circle,
may be represented by the same dia-
gram as before, /£•. 20. Z will then de-
note the solid ; A the acting fluid, and
C the conducting fluid. As there is a
necessity for separating the two fluids,
they may be contained in separate ves-
sels, and be made to communicate by
means of a bent tube, inverted like a
syphon, full of some conducting liquid,
and passing over from the one to the
other of the two fluids. Sir H. Davy
uses, in many of his experiments, fibres
of moistened asbestos in place of the
tube, for establishing a communication
between the fluids. "Two plates of the
same metal are then to be immersed in
the fluids, and made to communicate by
wires, or slips of the same metal.

(79.) Sir H. Davy has distinguished
three different kinds of circles of the
second class.*

The first and most feeble is composed
of single metallic plates, arranged in
such a manner, that two of their sur-
faces are in contact with different fluids,
one capable, and the other incapable,
of oxidating the metal. Zinc, acid, and
water, occupying the situations of Z, A,
and C, in the diagram, may be taken as
an example ; and it will be seen that
the only difference between this ar-
rangement and those of the former
class, consists in the substitution in the
circle of water for copper ; but the func-
tion of each of these parts is essentially
the same, namely, that of simply con-
ducting electricity between the other

* Philosophical Transactions for 1801, p. 398.

<24

GALVANISM.

two elements. As the conducting power
of fluids, however, is much inferior to
that of metals, the electrical indications
will be more feeble than in circles of
the first class ; and, indeed, will scarcely
be sensible unless we employ the more
easily oxidable metals, such as tin and
zinc. But powerful effects may be ob-
tained by combining a number of such
circles in a pile or battery. For con-
structing an instrument of the for-
mer kind, Sir H. Davy directs pieces of
polished tin, about an inch square and
one-twentieth of an inch thick, to be
piled up with woollen cloths of the
same size, moistened some in water,
and some in dilute nitric acid, in the
following order, — tin, acid, water, and so
on. It is proper to observe the pre-
caution of placing the cloth moistened
with acid underneath the one which is
moistened with water ; for, as the acid
is specifically heavier than the water,
little or no mixture of fluid will then
take place. Twenty such alternations
will produce a battery capable of acting
weakly on the organs of sense, and of
slowly decomposing water. When zinc
is the metal used, it is necessary, on
account of its rapid oxidation in water
containing atmospheric air, to use three
cloths ; the first moistened with a weak
solution of hydro-sulphuret of potash,
which has no power of acting upon
zinc, and which prevents it from being
acted upon by the water; the second
moistened with a solution of sulphate
of potash, of greater specific gravity
than the solution of hydro-sulphuret ;
and the third wetted with an oxidating
fluid, such as an acid, specifically hea-
vier than either of the solutions. In
this case, if, proceeding upwards, the
order be as follows — zinc, — oxidat-
ing solution, — solution of sulphate of
potash, — solution of hydro-sulphuret of
potash, very little mixture of the fluids,
or chemical action between them will
take place ; and an alternation of twelve
series of this kind, forms a battery ca-
pable of producing sensible galvanic
effects. The direction of the electrical
current is, as usual, from the zinc to
the oxidating fluid.

(80.) It has often been remarked that
porter drank out of a pewter pot has a
brisker taste than when taken out of a
glass. Professor Robison ascribed this
to the influence of galvanism, arising
from the circle formed by the metal and
two different .'fluids. He considered
that in the act of drinking, one side of

the pewter pot is exposed to the action
of the saliva which moistens the lip,
while the other side of the metal is
touched by the porter ; the circle being
completed when the latter fluid comes
in contact with the tongue.

(81.) The second kind of galvanic
combinations with a single metal, con-
sists of a series of plates composed of a
metal capable of being acted upon by
sulphuretted hydrogen, in contact with
solutions of hydrosulphurets on the one
side, and water on the other, placed in
a regular order of alternation. Under
these circumstances, a current of elec-
tricity is produced, the direction of
which is the reverse of what it is in the
former case, the surface of the metallic
plate in contact with the solution of sul-
phur being positive, while that in con-
tact with acid is negative. Eight series
will produce sensible effects. Copper,
silver, and lead are each capable of
forming this combination; their com-
parative activity being in the order in
which they are here enumerated, that
is, copper the most, and lead the least.*

(82.) A familiar instance of the ope-
ration of galvanism in promoting the
combination of sulphur with silver,
occurs in the employment of a silver
spoon in eating the yolk of an egg ; a
galvanic circle of the second kind being
formed by the yolk, which contains
sulphur, the silver spoon, and the saliva
of the tongue.

(83.) The third kind of combinations
unite the power of the two former, and
consist of a single metal, acted upon on
one side by an acid, and on the other
side by the hydro- sulphurets. Copper,
silver, or lead may here be employed,
and the order of their powers is the
same as in the preceding instance. The
pile may be constructed in the same
manner as the pile with zinc in the first
kind of combination ; the cloths moist-
ened with acid being separated from
those moistened with solution of hydro-
sulphuret by an intermediate cloth
soaked in solution of sulphate of pot-
ash. Three plates of copper, or silver,
arranged in this manner, in proper
order, produce sensible effects ; and a
pile composed of twelve or thirteen
series is capable of giving weak shocks
and of rapidly decomposing water. The
current of electricity is determined as in
the two former cases.

(84.) Greater permanency may be

* Philosophical Transactions for 1801, p. 400.

GALVANISM.

given to the effects of these combina-
tions of a single metal with two fluids,
by a disposition of the plates similar to
the trough of Cruickshanks, with par-
titions alternately of metal and of horn
or glass; and with the cells filled al-
ternately with the different solutions,
according to the kind of combination
employed ; these fluids being connected
in pairs with each other, by slips of
moistened cloth, carried over the non-
conducting plates.

(85.) Efficient galvanic circles may
also be formed with a single metal and
with the same fluid solvent, (an acid,
for example,) provided the action of
the latter is different on the two sides
of the metal, by being of different de-
grees of strength. Thus, if one of the
branches of a tube, bent in the form of
a V, contain concentrated sulphuric
acid, while the same acid in a diluted
state occupies the other branch, in which
case the two fluids will, on account
of the difference in their specific gra-
vities, remain without mixing with each
other; and two portions of the same
metal, zinc for instance, be then im-
mersed in these fluids, and made to
communicate with each other, galvanic
electricity will be evolved, and deter-
mined from the metal to the diluted
acid, in consequence of the action of
this portion of acid upon the zinc being
greater than that of the concentrated
acid. But with those metals, which are
more acted upon by the latter than by
the former, the influence of the concen-
trated acid will preponderate, and the
current will be determined in an op-
posite direction. In like manner it has
been observed, that two solutions of
common salt, the one concentrated, the
other diluted, form a galvanic circle
with copper; that metal being more
acted upon by the latter than by the
former, became negative to the one
and positive to the other.*"

(86.) The application of these princi-
ples will explain a variety of apparently
anomalous facts, which are continually
presenting themselves in the course of
experimental researches. Sir H. Davy
observed, for instance, that when two
pieces of the same polished copper were
introduced at the same moment into the
same solution of hydro-sulphuret of pot-
ash, there was, as might be expected, no
action ; but if they were introduced in
succession, there was a distinct, and

* Becquerd, Annalcs de Chimie et de Physique,
xxxv. 120.

often, if the interval of time was consi-
derable, a violent electrical effect; the
piece of metal first plunged in being ne-
gative with relation to the other. This
is owing to the rapid formation at the
surface of contact of sulphuret of cop-
per, which, by its presence, prevents, or
at least diminishes, the further action of
the fluid ; the clean surface of the plate
last introduced is therefore attacked
comparatively with greater force, and
determines a galvanic effect.* Many
singular and apparently capricious
changes of electric states occur in these
and other experiments of the same kind,
whenever new substances are produced
by the chemical action, which at first
adhere to the metal, but are liable to
be detached in smaller or larger por-
tions, and thus occasion sudden altera-
tions in the conditions of the galvanic
elements.

(87.) Having thus seen how, under
certain circumstances, it is possible to
form various galvanic combinations
with a single metal and a single fluid, it
remains for us to notice the attempts
that have been made to produce the
same effect without the aid of any me-
tallic substance, or even of charcoal.
Lagrave announced that by placing upon
each other alternate layers of muscle
and of brain, from a human body, with
pieces of moistened cloth or leather in-
terposed, he formed a pile which pro-
duced galvanic effects/!- Dr. Baconio,
of Milan, composed a galvanic pile en-
tirely of vegetable substances : namely,
discs of red beet-root, two inches in
diameter ; and discs of walnut-tree, of
the same size, divested of their resin by
digestion in a solution of cream of tar-
tar in vinegar. With a pile so con-
structed, and with a leaf of scurvy-
grass as a conductor, he is said to have
excited galvanic convulsions in a frog.$
Aldini also succeeded in producing the
same effect without the intervention of
any metallic substance ; sometimes by
bringing into contact the nerve of one
animal with the muscle of another, and
at other times by employing the nerves
and muscles of the same animal. In
some of his experiments the most pow-
erful contractions were excited, .by
bringing the parts of a warm-blooded
animal into contact with those of a cold-
blooded animal. On introducing, for

* Philosophical Transactions for 1826, p. 393.
t Journal de Physique, Ivi. 235 ; and Nicholson's
Journal, v. 62.
$ NicJwlsvu's /0Kr/»a/,.xviii. 159.

GALVANISM.

example, into one of the ears of an ox
recently killed a finger of one hand,
moistened with a solution of salt, and
holding in the other hand a prepared
frog, when the spine of the frog was
made to touch the tongue of the ox,
convulsions took place in the limb of the
frog. In like manner, when he held a
prepared frog by one hand, moistened
with solution of salt, and applied the
crural nerves of the animal to the tip
of his own tongue, convulsions were
produced.* Many of these experiments
were made in presence of the members
of a commission of inquiry appointed by
the French Institute: and they have
since been repeated with success in
London, at the Anatomical Theatre in
Great Windmill Street.

(88.) It is well known that several
fishes, such as the torpedo, which is a
species of ray ; the gymnotus electricus,
or the electric eel ; the silurus electri-
cus, a species peculiar to some of the
rivers in Africa ; and also the trichiurus
indicus, and tetraodon electricus, which
are fishes found in the Indian ocean,
possess the power of giving electrical
shocks to animals that touch them, or
communicate with them by electrical
conductors. Anatomical investigation
has shown that this power resides in
organs of a very peculiar construction.
In the torpedo they are composed of a
great multitude of vertical and parallel
membranous plates, arranged in longi-
tudinal columns of quadrangular, pen-
tagonal, or hexagonal forms, with a loose
net-work of tendinous fibres passing
transversely and obliquely between the co-
lumns, and uniting them firmly together.
Each column is, moreover, divided by a
•great number of thin horizontal parti-
tions, placed over each other at very small
distances, and forming numerous in-
terstices, which appear to contain a
fluid. All these parts are supplied by a
great abundance of blood-vessels, and
by a still more extraordinary proportion
of nerves.

(89.) In the regular arrangement of
their plates these organs have a marked
resemblance to a voltaic battery; we
know nothing, however, of the imme-
'diate source "from which they derive
electrical properties. Mr. Cavendish
compared the action of the torpedo to
that of a large electricaljar very weakly
charged : and Volta considered it as still
more analogous to that of the galvanic
pile. Sir Humphry Davy, with a view

* Nicholson's Journal, iii, 298,

to ascertain the justness of Volta' s com-
parison, passed the shocks given by
living torpedos through the interrupted
circuit made by silver wire through
water, but could not perceive that it pro-
duced the slighest decomposition 'of that
fluid. The same shocks made to pass
through a fine silver wire less than one
thousandth of an inch in diameter did
not produce ignition. Volta, to whom
Sir H. Davy communicated the results
of these experiments, considered the
conditions of the organs of the torpedo
to be best represented by a pile, of which
the fluid substance is "a very imperfect
conductor, such as honey ; and which,
though it communicated weak shocks,
yet did not decompose water. Sir H.
Davy also ascertained that the electrical
shocks given by the torpedo, even when
powerful, produced no sensible effect on
an extremely delicate magnetic electro-
meter. In a paper recently read at the
Royal Society, he explains these nega-
tive results by supposing that the motion
of the electricity in the organ of the tor-
pedo is in no measurable time, and wants
that continuity of current requisite for
the production of magnetic effects.

(90.) Mr. Geoffrey St. Hilaire has
found an organic structure very similar
to that of the torpedo in other animals
of the ray genus, which, nevertheless, do
not possess any electrical powers.

(91.) Electrical effects are obtained
from a pile composed of thin plates of
different metals in the usual order, with
discs of writing paper interposed between
them. This species of pile was the in-
vention of Mr. De Luc, who gave it the
name of the electrical column. It may
be constructed of pieces of paper, silver-
ed on one side, by means of silver leaf,
and alternated with thin leaves of zinc ;
taking care that the silvered surfaces of
the paper discs are always in the same
direction. A very large number of these
may be contained in a glass tube of mo-
derate length, previously well dried,
having its ends covered with sealing-
wax, and capped with brass. The most
extensive instrument of this kind was
made by Mr. Singer, and consisted of
twenty thousand series. Each of the
two ends or poles of the column affect
the electrometer, and exhibit electrical
attractions and repulsions ; the appara-
tus will even give sparks, and commu-
nicate shocks of considerable force : but
it possesses no sensible power of chemi-
cal decomposition when applied to fluids
in the interrupted circuit. If two

GALVANISM.

27

upright electrical columns be placed side
by side, with their poles in opposite di-
rections, and connected at their upper
ends, while a small bell is attached to
the lower end of each ; the whole will
act as one column, and each bell will,
in consequence of the electrical actions,
be alternately struck by a brass ball
suspended between them; and thus a
continual ringing will be produced as
long as the machine remains in action,
which is generally for a considerable
time. This action is, however, kept up
solely by the presence of moisture in the
paper, for it does not take place atx all
when the paper is perfectly dry; and
although the process of oxidation is very
slow, the more oxidable metal is in pro-
cess of time found to be tarnished.

(92.) An apparatus somewhat analo-
gous to that of De Luc was constructed
by Hachette and Desormes with pairs
of metallic plates, separated by layers of
farinaceous paste, mixed with common
salt. To this instrument, although it
evidently owed its efficacy to the mois-
ture of the paste, they gave the very in-
appropriate name of dry pile. It has
the same properties as the electric
column, except that it is unable to give
a shock. A pile having nearly similar
powers was also constructed by Profes-
sor Zamboni, of Verona, with discs of
paper, gilt or silvered on one of their
sides, while the other side was covered
with a layer of pulverized black oxide of
manganese, mixed with honey. Both
this and the former instrument retained
their power fora great length of time.

(93.) Piles formed simply of discs of
copper and moistened card, placed al-
ternately, were found by Ritter to have
no power of developing electricity by
their own action, but to be capable of
receiving a charge by being placed in the
circuit of a powerful voltaic battery,
and of thus acquiring, though in an
inferior degree, all the properties of the
battery itself from which it derived its
activity. The properties of these se-
condary piles, as they have been called,
are obviously the effect of a series of
electrical inductions, extending from end
to end ; and the apparatus is found to
retain its charge for a very considerable
time, provided it be kept insulated, and
the communications between the two
poles are not renewed too frequently.

(94.) Having thus traced the various
ways m which galvanic power may be
excited, we have next to examine the
influence of different circumstances, by

which its quantity, intensity, and mode
of action are regulated. We have al-
ready seen that the intensity of the
electricity developed by a single galvanic
circle, bears no relation to the extent of
surface of the elements which compose
that circle. It follows, therefore, that
however much we may increase the
quantity of electricity by employing very
large plates, as in the calorimotor, we
cannot obtain from such an instrument
any of those effects which require for
their production a certain intensity, as
well as quantity of electricity. In order
to obtain these latter effects, we must
employ the compound battery, consist-
ing of a considerable number of alter-
nations of the same elements. The
former of these instruments, accord-
ingly, will be capable of producing such
effects as depend upon mere quantity,""
without regard to intensity ; such as
the evolution of heat, the ignition and
deflagration of the metals, and electro-
magnetic phenomena. The compound
apparatus, on the other hand, will afford
the more ordinary electric appearances,
(such as the spark, and the phenomena
of attraction and repulsion,) will affect
the electrometer, or condenser, and will
communicate a charge to a Leyden jar;
for in all such operations, intensity of
electricity is the most essential requisite,
and the power of the battery to produce
them is found to be augmented by
every increase in the number of the
alternations. But there is also a third
class of effects, more peculiarly apper-
taining to galvanism, which take place
by the transmission of the electric cur-
rent through bodies of inferior conduct-
ing power ; such as liquids of various
kinds, and living organized structures,
both animal and vegetable : producing
in the former chemical decomposition,
and in the latter various physiological
effects, such as nervous excitation,
muscular contraction, and affections of
secretion. For the production of these
effects it is necessary, not only that the
electricity be sufficiently powerful,
both in respect to intensity and to
quantity, but also that it should flow in
a continuous current. It is from the
difficulty of supplying this latter re-
quisite, that the electricity derived from
the common electrical machine, is,
under ordinary circumstances, incapable
of decomposing water in the way that
is so readily accomplished by voltaic
electricity. It is from deficiency of
intensity, on the other hand, that we are

28

GALVANISM.

unable to obtain the same effects from
the calorimeter, which amply fulfils the
conditions of quantity and continuity.
The electricity which it furnishes, how-
ever abundant in quantity, does not
possess sufficient intensity to overcome
the obstacle presented by the smallest
thickness of water, or other liquid of
low conducting power ; and is, for the
same reason, incapable of penetrating
through the skin, or traversing through
any other part of an animal body.
Hence we can obtain from it neither
chemical nor physiological effects. The
electricity furnished by the electric
column of De Luc, again, though of
sufficient intensity to produce the shock
and other effects of a sudden influx, is
too deficient in quantity to produce
chemical action ; and the same general
observations apply to the electricity of
the torpedo.

(95.) Every circumstance that faci-
litates the passage of the electric current
in all parts of the circuit, will tend to
increase the quantity that circulates.
The degree of conducting power pos-
sessed by the fluid parts of the circle,
will, therefore, have an important in-
fluence on the power of the apparatus.
Hence the addition of various saline
bodies to the fluid is found to increase
the efficacy of the voltaic battery, pro-
bably, in part at least, by increasing the
conducting power of the fluid ; but as
such substances generally also promote
chemical action, it is always in some
degree doubtful what part of the effect
is to be ascribed to the one or the
other of these causes.

(96.) As the fluid element of the
circle is the part having the smallest
conducting power, the electric current
will be retarded by having to pass
through any considerable extent of fluid.
"With a view to augment the activity of
the battery, it is an object to bring the
two metallic surfaces of Z and C very
near each other, so that the distance the
electricity has to pass from the one to
the other, through the fluid, shall be as
small as possible ; and for the same rea-
son the surface of C, which collects the
electricity from the fluid, should be suf-
ficiently extensive to effect this purpose
completely. We hence perceive the reason
of the advantage derived from employ-
ing in the common trough battery, ac-
cording to the suggestion of Dr. Wol-
laston, a double plate of copper to each
plate of zinc, so that each surface of the
latter metal acted upon by the fluid,

may have a surface of copper opposite
to it, (§ 18.); and also of enveloping
each coil of zinc plate, in the calori-
motor, by a coil of sheet copper, (§8.)
Mr. Marianini has extended this prin-
ciple still further, and has found that
the maximum of effect takes place when
the surface of the copper is no less
than eight times greater than that of the
zinc.

(97.) There is yet another cause of
impediment to the motion of the electric
current of a singular kind, and which
produces very considerable effect. It
appears from the experiments of Mr.
Augustus De la Rive, that voltaic elec-
tricity, in passing out of one conducting
body into another of a different kind,
always sustains some loss of its intensity.*
The amount of this loss varies much in
different cases, according to the nature
of the two conductors ; and it is differ-
ent with different degrees of intensity.
In the case of the passage of the elec-
tricity from a fluid to a metal, or vice
versa, it is very great, and it is sensible
even when it has to pass from one li-
quid to another, or along a mixed con-
ductor composed of two different kinds
of solids. The impediment arising from
the mere change, of conductor is quite
independent of the peculiar conducting
powers of the one or the other of the
substances through which the electri-
city passes. Mr. De la Rive found, for
example, that a much greater obstacle
existed to the transmission of the elec- '
tricity between sulphuric acid, especially
when concentrated, and platina, than be-
tween nitric acid and the same metal ;
and accordingly, on sending the electric
current from a voltaic battery through a
number of portions of sulphuric acid,
contained in separate glasses, and con-
nected by arcs of platina wire, it proved
to be a much worse conductor than when
nitric acid was employed in a similar
arrangement. But the conducting powers
of each system of compound conductors
were immediately rendered equal by
dipping the ends of the platina wires in
nitric acid, before immersing them in
the sulphuric acid.t

(98.) In general the more readily a
metal is acted upon by liquid conductors,
the less is the diminution of intensity
which is sustained by the passage of
electric currents through them. Mr.
De la Rive states it to be a general

* Annales de Chimie et de fhytique, xxx*ii. 267.
f Ibid. p. 2/3.;

GALVANISM.

29

law, that, independently of the effects of
chemical action, the influence of the
obstacle opposed to the passage of elec-
tricity from a fluid to a solid conductor,
is such, that when two metallic surfaces,
either of the same or of different metals,
are immersed in a fluid, so as to form a
galvanic circle, that metal which trans-
mits the electricity with the least loss of
intensity is positive with respect to the
other metal.*

- (99.) The influence of this retarding
cause varies also with the intensity of
the current itself. The loss of electricity,
from its passage through a number of
metallic plates^ is scarcely sensible when
the current is very energetic, as, for in-
stance, when it proceeds from a battery
composed of a great nnmber of plates ;
but it becomes more and more percepti-
ble, according as the original intensity
of the current is less considerable. It
is also remarkable that the current is
disposed to pass more readily through
imperfect conductors, which present a
great degree of resistance, when it has
previously been made to traverse a great
number of metallic plates. This was
illustrated in two comparative experi-
ments, in the first of which a current,
originally of high intensity, was reduced,
by passing through a considerable num-
ber of plates, till it was equal in inten-
sity to one originally weaker, that had,
in the second experiment, passed through
a smaller number ; of the two currents,
thus apparently rendered equal in every
respect, it was nevertheless found that
the one which had previously passed
through the greater number of plates,
was thereby rendered capable of passing
through any succeeding plate with less
loss ot intensity than the other current.
The phenomena, he states, correspond
to those which would take place, if we
could imagine that there were two dis-
tinct kinds of electric current — the one
capable of passing indiscriminately
through all sorts of conductors, good or
bad ; the other capable of passingthrough
good conductors alone. The passage of
the currents through successive plates
gradually effect the separation of these
two portions, the plates arresting the
one which cannot pass so readily through
bad conductors, and giving free passage
to the other portion.

(100.) M. Dela Rive has applied this
theory to the explanation of the different
effects resulting from the increase of the
number of the plates. If the pile, he

* Annales dc Chimie et de Physique, xxxvii. iW4.

says, consist only of a small number of
plates, the electricity produced by it, not
having undergone the above process of
filtration, as it may be called, only one
part of it will be capable of passing
through an imperfect conductor, which
is presented to it, and the other part will
be arrested ; but if a good conductor be
presented, the whole of the electricity
finds a ready passage, and will produce
corresponding effects. Electricity of the
former kind only will be capable of pro-
ducing chemical decompositions, and of
passing through organized bodies ; but,
in the latter case, it will be adequate to
the production of all the calorific and
magnetic effects. These modifications
of electricity would, if this theoiy were
established, have a remarkable analogy
with those of light and of haat, under
circumstances somewhat parallel.

(101.) It must be observed, however,
that one source of the diminution of
effect consequent on the multiplication
of surfaces, exists in the transfer of ele-
ments which takes place in the fluid from
galvanic action. This transfer, as is re-
marked by Sir H. Davy, in as far as it
has actually occasioned the deposit of a
positive element on the negative surface,
and vice versa, has an immediate in-
fluence in checking the further progress
of the galvanic action ; and arrests it
completely when it has proceeded to a
certain extent. Hence the powers of
batteries are found to diminish by the
continuance of their action, and ulti-
mately to cease. This change we have
already noticed in treating of the che-
mical actions of the simple galvanic cir-
cle. ($ 41.)

(102.) It is obvious that the several
causes of retardation now stated render
it exceedingly difficult to determine, pre-
vious to actual experiment, the relative
powers of different batteries, composed
of different materials, and consisting of
different numbers of alternations of its
parts.

(103.) It is not easy to understand
the manner in which the chemical ele-
ments of a body decomposed by gal-
vanism, are carried to their respective
stations in the voltaic circuit. Thus if
the influence of a powerful battery> be
transmitted through water, it will ope-
rate in decomposing that fluid, although
the wires which form the communica-
tion with the poles be at a consider-
able distance from each other. They
may even be placed in separate vessels,
provided the portions of water in which

30

GALVANISM.

they terminate are made to communi-
cate with one another by means of a
syphon full of water, or even by moist-
ened threads. We find, under these
circumstances, the whole of the oxygen
of the decomposed water transferred to
the positive, while the hydrogen is col-
lected at the negative wire. Two ques-
tions may here be asked : first, in what
part of the circuit does the decomposi-
tion take place ? secondly, in what mode
are the elements of the decomposed par-
ticles transferred to such distant points,
without any indication being afforded of
their movements, which must be exceed-
ingly rapid, in order to traverse through
so long a space ? The velocity of this
transfer would appear to be very consi-
derable from the following experiment
made by Dr. Roget, in the year 1807.
The ends of two platina wires, commu-
nicating with the poles of a powerful
battery, were introduced into two sepa-
rate vessels of water, communicating by
means of a long tube, bent into the form
of a syphon, and filled with a solution
of common salt. The whole length of
the fluid part of the circuit between the
two wires was 46 inches. Microscopes
were applied to the ends of the wires,
for the purpose of enabling the observer
and an assistant, (who was the late Mr.
Sylvester,) to ascertain the precise mo-
ment when the gases made their appear-
ance at the respective wires. No sen-
sible interval of time could be perceived
between the appearance of the oxygen
gas at the positive, and of the hydrogen
gas at the negative wire, when the com-
munications with the battery were made.

(104.) The transfer of material and
ponderable substances, such as those
which constitute the elements of water,
might be expected, even with a moderate
velocity, to occasion visible currents in
the fluid through which they pass ; for
their motion, by whatever force pro-
duced, must be accompanied by a certain
momentum, sufficient to displace the
particles of the fluid through which they
pass. Dr. Roget could, however, detect
no appearance of current or displace-
ment of fluid ; such as would be indi-
cated by movements among the minute
globules of dust, or other extraneous
matters suspended in the water, even
with the assistance of the microscope.
Mr. Wilkinson and Professor de la Rive,
have also arrived at the same conclu-
sion, by employing microscopes of high
magnifying power.

(105.) These phenomena of transfer

have appeared to some so inexplicable, '
upon the commonly received doctrine of
the composition of water, that they have
had recourse to a new hypothesis in order
to solve the difficulty. Professor Ritter
was led to consider water as a simple
substance, forming oxygen by its com-
bination with positive electricity, and
hydrogen by its union with negative
electricity ; and this theory was adopted
by several other philosophers. Monge
endeavoured to account for the pheno-
mena, by supposing that water formed
compounds with excess of oxygen on
the one hand, or excess of hydrogen on
the other ; which compounds passed in
opposite directions between the two
wires, each depositing on their arrival
the superabundant ingredient. Dr. Bos-
tock conceived that the water was de-
composed at the positive wire only,
where its oxygen was disengaged ; and
its hydrogen, uniting with electricity,
was carried invisibly along with it to
the negative wire, where this union
being [dissolved, the electricity passed
on through the wire, and the hydrogen
appeared in its gaseous form.

(106.) The following mode of ex-
plaining these phenomena was sug-
gested by Dr. Roget, in a paper which
was read to the Philosophical Society
of Manchester, in 1807.

"We may conceive the agency of
electricity to extend throughout the
whole of the fluid line connecting the
two wires. The hydrogen existing in
every particle of water in this line, will,
if it possess a positive electrical polarity,
according to the hypothesis of Mr.
Davy, be repelled by the positive, and
attracted by the negative wire. We
may consider the row of particles of
hydrogen abstractedly from those of
oxygen. While the former are moving
together, by the agency of thejelectricity,
in a direction towards the negative wire,
all those particles which have not yet
reached that wire, will merely have to
pass over in succession from one par-
ticle of oxygen to the next, among those
of the other row. They will not appear
in the form of gas, because the instant
each has quitted the particle of oxygen
with which it was associated, it meets
with another to combine with ; and this
process will be continually repeated,
until it has arrived at the end of the
line, when, finding no oxygen to unite
itself with, it will make its appearance
in the form of gas. In like manner, the
first particle of hydrogen, in the series,

GALVANISM.

by its abandoning the first particle of
oxygen, which finds no other particle of
hydrogen to replace it, causes the oxygen
to appear at that point in the form of
gas. We have thus the two gases formed
at each end, not from the same indivi-
dual particle of water, but from the two
which happen at that moment to be in
contact with the wires. The production
of the two gases will take place at the
same instant in both places, each par-
ticle having only to move ore step, that
is, from one particle to the adjoining
one, instead of having to traverse the
whole extent of the line, and no current
will be perceptible in the fluid. If this
theory be correct, the operation of gra-
vity in favouring the descending current
of ihe heavier element, namely oxygen,
might be rendered sensible ; and that
this is actually the case appears by an
observation of Mr. Sylvester, that when
the wire giving out oxygen is placed at
a much lower level than that which
gives out hydrogen, the effect is sensibly
greater than when the positions are
reversed."

(107.) Similar explanations of the
mode of transfer have been given by
Dr. Henry, and by Grotthus ; and from
the following passage in Sir H. Davy's
last paper on the subject,* it would
seem jthat he entertained views some-
what similar. " If it be supposed that
the fluid is divided into two zones, xli-
rectly opposite in their powers to the
poles of the battery, the virtual change
may be regarded as taking place in the
two extremities of these zones nearest
the neutral point ; so that by a series of
decompositions and recompositions, the
alkaline matters and hydrogen separate
at one side, and oxygen, pure, or in union,
at the other. In this way the electricity
may be regarded as the transporter of
the ponderable matters, which assume
their own peculiar characters at the mo-
ment when they arrive at the point of
rest." That visible motions are sometimes
produced in fluid conductors when
transmitting the electric current, has
been shown by Sir H. Davy, who no-
ticed the very singular convulsive agi-
tations into which mercury is thrown,
when placed within the circuit of a pow-
erful voltaic battery discharged through
water.f These motions, which are fre-
quently of a violent and capricious kind,
have also attracted the attention of Mr.

» Philosophical Transactions for 1826, p. 416, 417
t Elements of Chemical Philosophy, p. 172,

Herschel, and he has made them the
subject of an interesting research, of
which an account is contained in the
Philosophical Transactions.*

(108.) The following singular fact
has been noticed by Mr. Porrett : — If a
vessel be divided by a membranous par-
tition into two compartments, of which
the one is filled with water, and the other
contains but a very small quantity, and if
the positive wire from a voltaic battery be
inserted into the former, and the nega-
tive wire into the latter, the water will
be impelled from the first compartment
into the second, through the partition,
and will at length rise to a higher level
in the latter than in the former.f Mr.
A. De la Rive, upon repeating these
experiments, arrived at the same result,
when he employed distilled or river
water, which has but a small conduct-
ing power; when, however, a saline
solution of sufficient strength was used,
no such effect of impulsion was per-
ceptible. But the reality of such an
effect under the above circumstances, is
sufficient to establish the existence of
a mechanical force derived from the
current of voltaic electricity.

(109.) We have already had occasion
to observe that a theory, founded upon
totally different views of the sources of
galvanic power from those which have
now been stated, has been applied to the
explanation of the phenomena. As this,
which has been termed the electric
theory of galvanism, has been adopted
by several eminent philosophers, it
ought not to be considered as undeserv-
ing of notice in this place.

(110.) It was conceived by Volta, the
original author of this theory, that the
primary source of the electricity liberated
during the action of a galvanic appa-
ratus, might be traced to the contact of
the dissimilar metals. He assumed as
a fundamental fact, that during the
whole time that these metals are in
contact, a certain force is in constant
operation, tending to effect a transfer of
electricity from the one metal to the
other. To this force he gave the name >
of electromotive force. When, for ex-
ample, zinc and copper are in contact,
the alleged operation of this force i's to
impel the electricity from the copper to
the zinc, so as to maintain in the latter
a positive state, when compared with
the former, which will, consequently,

* For 1824, p. 163.

t Thomson's Annals of Philosophy, viii. 74,

32

GALVANISM.

itself be in a negative state with relation
to the zinc. If either of these states be
reduced to a more neutral condition by
communication with other bodies ; that
is, if the redundant electricity of the
zinc be carried off, and the deficiency
of electricity in the copper be supplied
from other sources, the electromotive
force will, he conceived, immediately
renew this difference 6f condition, and
thus maintain a continual and rapid
current of electric fluid, flowing always
in the same direction.

(Ill .) It was further assumed in this
theory that liquids have no electromo-
tive power when in contact with metals :
and that this negative property enabled
them to transmit the electricity evolved
by the contact of the zinc and copper,
and which is accumulated in the zinc,
back again to the copper ; whence it is
again transferred to the zinc ; and so on
in a perpetual circle. In compound
galvanic circles, the electromotive force
residing in the surfaces of contact be-
tween the two metals in each pair of
plates, are all tending in the same direc-
tion ; and the several impulses they
give to the electricity conspire together
to increase the effect, which will there-
fore be the sum of all the forces taken
separately. Thus will a continued and
powerful stream of electricity be deter-
mined from the negative to the positive
pole of the battery, ready to circulate
through any conducting line of com-
munication extending between the two
poles. The office of the fluid is consi-
dered, in this theory, as simply that of
conducting the electricity from the one
metal to the other : its chemical action
on either of these being regarded as a
mere accidental circumstance, not in
any way concerned in the production of
galvanic or electrical effects. The effec-
tive quantity of electricity which actually
circulates in the voltaic battery is sup-
posed to be determined altogether by
the degree of conducting power possessed
by the liquid : for it is assumed that
the quantity which the electromotive
force existing at even the smallest sur-
face of contact between dissimilar me-
tals could set in motion, if the move-
ments of that electricity were not
impeded by the difficulty of its trans-
mission through fluids, would be incom-
parably greater than that which any
conducting fluid can discharge.

(112.) Such is the general outline of
the electric theory, which it is scarcely
necessary to pursue in its various appli-

cations, because there are several
facts which appear so totally at variance
with the immediate consequences of its
fundamental hypothesis, as to warrant
us in rejecting it. Chemical action be-
tween some of the elements of a galvanic
combination is so invariably connected
with the production of electrical effects,
that it would be a violation of all just
rules of philosophy not to consider these
two classes of phenomena as standing
to each other in the relation of cause
and effect. The quantity of galvanic
effect is always in proportion to the
energy of the chemical action. The
extent of contact between the two me-
tals, on the other hand, appears to
have no relation to the quantity of elec-
tricity which is developed. Combina-
tions producing galvanic effects may be
formed, as we have seen, with a single
metal only, when two fluids are present ;
and indeed, on other occasions, without
the presence of any metallic substance
whatever. We have also seen that the
same metals do not in all cases stand in
the same invariable electrical relation
to each other ; but that this relation is
determined by the chemical properties
of the; fluid with which they are placed
in contact. (§ 75.) All these facts are
irreconcilable with the electric theory.

(113.) Were any further reasoning
necessary to overthrow it, a forcible
argument might be drawn from the
following consideration. If there could
exist a power having the property
ascribed to it by the hypothesis,
namely, that of giving continual im-
pulse to a fluid in one constant direction,
without being exhausted by its own
action, it would differ essentially from
all the other known powers in nature.
All the powers and sources of motion,
with the operation of which we are
acquainted, when producing their pecu-
liar effects, are expended in the same
proportion as those effects ate pro-
duced; and hence arises the impossi-
bility of obtaining by their agency a
perpetual effect ; or, in other words, a
perpetual motion. But the electro-mo-
tive force ascribed by Volta to the me-
tals when in contact, is a force which
as long as a free course is allowed to
the electricity it sets in motion, is never
expended, and continues to be exerted
with undiminished power, in the produc-
tion of a never-ceasing effect. Against
the truth of such a supposition, the pro- ,
babilities are all but infinite.

MAGNETISM.

CHAPTER I.
General Facts and Principles.

(1.) THE attractive power of the load-
stone for iron was known in times of
very remote antiquity, and has been, in
all ages, a subject of curiosity and of
wonder. It is a property which seems,
at first sight, so unconnected with every
other, as to form of itself a separate
class among- natural phenomena: and,
although an immense mass of knowledge
relating to Magnetism has been accu-
mulated by the labours of successive
generations, and embodied into a science
of high rank and importance, yet the
field it comprises is of comparatively
limited extent. This arises from the
great simplicity which characterises both
the phenomena and the laws that govern
them ; a quality, however, which pecu-
liarly invites a philosophic mind to
undertake their investigation. A still
more powerful motive to this inquiry
will present itself when we reflect on
the signal benefits mankind has derived
from magnetism as applied to the pur-
poses of navigation. The discovery of
the compass, by the aid of which, the
manner, however distant from land,
amidst cloudy skies, or in the darkest
nights, is enabled, at all times, to steer
his course with certainty, and traverse
in all directions the wide expanses of
ocean which separate the countries and
continents of our globe, must unques-
tionably rank among the great discove-
ries that have essentially contributed to
advance the civilization of the human
race.

(2.) The term MAGNETISM expresses
the peculiar property occasionally pos-
sessed by certain bodies, more especially
by iron and some of its compounds,
whereby, under certain circumstances,
they mutually attract or repel one
another, according to determinate laws.

(3.) This property was first noticed
in a mineral substance called the native
magnet, or the loadstone, which is an
ore of iron, consisting chiefly of the

two oxides of that metal, together with
a small proportion of quartz and alu-
mina. Its colour varies in different spe-
cimens, according to minute differences
in the proportion of the oxides, and the
nature of the other substances with
which they may be found united : but it
is usually of a dark grey hue, and has
a dull metallic lustre. It is found in
considerable masses in the iron mines
of Sweden and Norway, and also in
different parts -of Arabia, China, Siam,
and the Philippine Islands. Small load-
stones have occasionally been met with
among the iron ores of England.

(4.) There are several modes in which
a piece of iron may be rendered mag-
netic, or converted into what is called
an artificial magnet; and for all pur-
poses of accurate experiment such a
magnet is much to be preferred to a
loadstone. The following is a simple
and ready method of obtaining artificial
magnets with a view to the investigation
of the magnetic properties.

Let a straight bar of hard tempered
steel, devoid of all perceptible magnetism,
be held in a vertical position (or still
better, in a position slightly inclined to
the perpendicular, the lower end deviat-
ing to the north,) and struck several
smart blows with a hammer ; it will be
found to have acquired, by this process,
all the properties of a magnet.

(5.) These properties are the four fol-
lowing:— viz. 1. Polarity. 2. Attrac-
tion of unmagnetic iron. 3. Attraction
and repulsion of magnetic iron. 4. The
power of inducing magnetism in other
iron. These we shall now explain and
illustrate.

§ 1. Polarity.

(6.) If a bar, which has been ren-
dered magnetic, be supported in such a
manner as to have entire freedom of
motion in a horizontal plane, and be re-
moved from the neighbourhood of all
ferruginous bodies which might influ-
ence it, it will spontaneously turn round,
and, after a few oscillations, will finally
B y

MAGNETISM.

settle in a position directed nearly north
and south. If it be disturbed from this
situation and placed in any other direc-
tion, it will, as soon as it is again at li-
berty to move, resume its former posi-
tion. The end of the bar which points
to the north, is that which was lower-
most at the time it acquired its magnet-
ism by hammering: the end which,
during that operation, had been the
upper one, is consequently that which,
when the magnet is free to move, directs
itself to the south. The two ends of a
magnet of this form are called its poles :
the one which spontaneously turns to the
north, being distinguished as the north,
and the other as the south pole: and
the tendency of the magnet to assume
the above described position is called its
Polarity. The straight line joining the
two poles of a magnet is called its axis.
(7.) There are several ways of sup-
porting a magnet so as to enable it to
manifest its polarity. The readiest mode
is to suspend it by a thread, fastened

Fig. 1.

Fig. 2.

round it at the middle, so that it
may be sufficiently balanced to pre-
serve its horizontal position as it turns
freely round its centre. It cannot, in-
deed, turn thus, without, at the same time,
either twisting or untwisting the thread
by which it hangs ; and the reaction of the
thread, the fibres of which tend to resume
their original situation, or the force of
torsion, as it is called, may prevent the
magnet from assuming the precise po-
sition to which its polarity would have
brought it. But by employing a very
slender thread, and taking it of sufficient
length, the force of torsion may be so
much reduced as to be quite insensible
in the experiments about to be described.
(8.) Another convenient mode of ex-
amining the horizontal movements of
the magnetized bar is to poise it on its
centre, hollowed into a cap, which is
made to rest on a fine point fixed in a stand.

When thus fitted up, it acts like the
needle of a mariner's compass ; and in
its principle is identical with that in-
strument;

(9.) We may sometimes find it more
expedient to fix the magnet on a piece
of cork, and thus make it float on water,

Fig. 3.

in a basin. In this case, we must take
care, however, that it be kept at a suffi-
cient distance from the sides of the
vessel to prevent its being affected by
the capillary attractions of the water.

The same precaution must be used if
the magnet be made to float on the sur-
face of mercury, which is an excellent
mode of giving it complete liberty of
motion. But the vessel containing the
mercury should be at least six inches in
diameter, in order to guard against the
effects of the curvature of the surface
of the mercury near the sides. When
the surface of the mercury is very clean
and bright, which happens only when
the metal is very pure, it allows of the
ready motion of pieces of iron floating
upon it. But it soon tarnishes, and the
film of oxide which forms on the surface,
becomes a great impediment to the
freedom of motion of the floating body.
The best way of rendering it clean, is to
strain it through a funnel of paper,
rolled up into a cone, having a small
aperture at the point, of about the for-
tieth of an inch in diameter.

§ 2. Attraction of Iron.

(10.) If either pole of a magnet be
brought near any small piece 'of soft
unmagnetic iron, it will be found to at-
tract it. Iron filings, for instance, are
immediately collected together when a
magnet is placed among them ; and
they adhere more especially to the poles
(as shewn kin fig. 4, A), from which,

MAGNETISM.

Fig. 4.

when the magnet is*- lifted up, they re-
main suspended in thick clusters. (Fig.
4, B.) A small number of filings are
also found adherent to the intermediate
parts of the bar, but they are evidently
attracted much more feebly than those
at the ends ; it may also be remarked
that there is a part of the magnet, gene-
rally mid-way between the two ends, to
which the filings have no tendency to
adhere at all, and which appears there-
fore to have no power of attraction.
Thus it appears that the attractive
forces, whatever be their nature, reside
chiefly at the poles.

(11.) It is an established law of na-
ture, the knowledge of which we have
derived by induction from a vast variety
of phenomena occurring in every part
of the material universe, that all action
is attended by a corresponding reaction,
equal in degree, but opposite in its kind,
to the action itself. Mechanical philo-
sophy, in all its departments, abounds
with exemplifications of this funda-
mental principle ; many of these, indeed,
are matters of familiar observation.
The stretched rope pulls back with equal
force at both its ends ; the compressed
spring resists equally in two opposite
directions ; the exploding powder, at
the same moment that it propels the
ball, gives to the gun its recoil. In all
the effects resulting from cohesion, from
elasticity, from caloric, from animal
force, from gravitation, whether actuat-
ing the minutest particles of matter, or
the largest masses ; whether exerted on
the rolling waters of the ocean, or dis-
played on the grander scale of the pla-
netary movements, the same universal
law is rigidly observed. To every phy-
sical force there is opposed another and
a similar force. No material agent can
produce an effect upon another, without
being at the same time subjected to an
equal reaction from that other agent.
An attracting body must of necessity be
itself attracted, and a repelling body re-
pelled. This perfect reciprocity of ac-
tion takes place in all the agencies of
electricity; — it exists also in those of
magnetism.

(12.) If the two bodies exerting a
mutual action upon one another, be
very different in their size, the smaller
of the two will necessarily exhibit the
effects of this action more strongly than
the larger, because, its mass being less,
the same force will communicate to it a
greater velocity of motion. We find,
accordingly, that the small fragments of
iron in the experiment just described,
appear to fly towards the magnet, while
their reciprocal action on the magnet
itself is imperceptible. But this latter
action may be rendered sensible by try-
ing its effect on a magnet poised or sus-
pended in any of the ways above men-
tioned ; and we shall find that on pre-
senting a piece of soft iron to either of
the poles of the magnet, the latter is
slowly attracted by the iron. The at-
traction, therefore, between the magnet
and the iron is reciprocal.

Let us next see what influence mag-
nets have upon one another.

§ 3. Attraction and Repulsion of Mag-
netic Iron.

(13.) For the purpose of examining
the mutual action of two magnets, we
may either present to the poised magnet
another magnet held in the hand, or we
may place two poised magnets in dif-
ferent positions with respect to each
other. We shall find by sufficiently
varying these positions, that when the
poles of different magnets are brought
near one another, they in some cases
appear to be attracted towards each
other, while in others they manifest a
mutual repulsion. This, however, does
not happen capriciously ; for if we mark
the poles according to the distinction
already pointed out, we shall find that
two north poles always repel each other :
— that two south poles also repel each
other : — but that the north pole of one
magnet invariably attracts, and is of
course attracted by the south pole of
another magnet.

(14.) It thus appears that there are
two species of magnetic powers, the
northern and the southern, which in
their mode of action are perfectly simi-
lar, but in their effects are directly op-
posite.

(15.) Such of our readers as have
studied our Treatise on Electricity must
here be struck with the pointed analogy
which subsists between the phenomena
of magnetic attraction and repulsion,
and those of electricity. In both there
exists the same character of double
B 2

MAGNETISM.

agencies of opposite kinds, capable,
when separate, of acting with great
energy, but being, when combined toge-
ther, perfectly neutralized and exhibit-
ing no sign of activity. As there were
two electrical powers, the positive and
the negative, or, as some prefer denomi-
nating them, the vitreous and the re-
sinous, so there are two magnetic
powers, distinguished as the northern
and the southern polarities ; or, as some
choose to designate them, the austral
and the boreal. The parallel is most
exact. Both sets of phenomena are go-
verned by the same characteristic law,
which may be expressed by the follow-
ing concise and general formula, namely,
— between like powers there is repul-
sion ; between unlike, attraction.

§ 4. Induction.

(16.) The communication of mag-
netic properties to iron or steel by the
mere approach of the poles of a magnet,
is also analogous in its principal cir-
cumstances to electric induction.

Fig. 5.

If the north pole, N, of a magnet A
(fig. 5), be brought near to the end s of
an unmagnetized bar of iron, B, that
end will immediately acquire the pro-
perties of a south pole, while the oppo-
site, or distant end, n, will at the same
time be converted into a north pole. If,
instead of the north pole, N, the south
pole, S, had been presented to the bar,
the changes effected in B would have
been just the reverse ; the adjacent end
would have acquired the northern, and
the distant end the southern polarity.

(17.) Thus we may observe that each
pole of a magnet induces the opposite
kind of polarity in that end of the iron
which is nearest to it, and the same kind
on the remotest end ; just as happens in
the induction of electricity, in which the
positive state induces the negative, and
the negative the positive state, in those
parts of an insulated conductor which
are nearest to the electrified body ; and
a similar state of electricity in the dis-
tant end.

(18.) That the iron, while it remains
in the vicinity of the magnet, possesses
the magnetic properties, may be shown
by a variety of experiments.

First, it attracts other iron. If we
take, for instance, a key (fig. 6), and

Fig. 6.

hold it horizontally near one of the poles
of a strong magnet, also lying in a hori-
zontal position, but not touching the
key ; and if we then apply another light
piece of iron, such as a small nail, to the
other end of the key, the nail will hang
from the key, and will continue to do so
while we slowly withdraw the magnet
horizontally from the key. When the
magnet has been moved beyond a cer-
tain distance, the nail will drop from the
key, because the magnetism induced on
the key becomes, at that distance, too
weak to support the weight of the nail.
That this is the real cause of its falling
off may be proved by taking a still
lighter fragment of iron, such as a piece
of very slender wire, and applying it to
the key. The magnetism of the key
will still be sufficiently strong to support
the wire, though it could not support
the nail : and it will continue to support
the wire, even when the magnet is yet
further removed; at length, however,
when the distance is still greater, the
wire, in its turn, drops off.

The same effects may be observed if
the nail be placed in contact with the
near end of the key ; but they are gene-
rally less distinct, on account of the di-
rect influence which the magnet exerts
on the nail, and which interferes in some
degree with the action of the key, so
that the results become complicated.

(19.) The same series of phenomena
take place when the key is held above
or below the pole of the magnet, or on
either side of it. The key will hold the
nail or wire suspended from either end,
as long as the magnet is near enough to
exert sufficient influence on the key.

(20.) If the key be laid upon a piece
of paper on a table, and several small
bits of wire, or iron filings, be strewed
round one end of the key, which we sup-
pose to be devoid of all magnetic pro-
perties, no adhesion will be perceptible
between the fragments of iron and the
key. Let us now approach the pole of a
magnet to the other end of the key ; we
immediately observe the filings and
lighter pieces of iron spontaneously, and
of one accord, move towards the key, and

MAGNETISM.

adhere to it just as if the key had itself
become a magnet. They also col-
lect and cohere together, as if animated
by a common sympathy. When this
has taken place, let us suddenly remove
the magnet : that moment all these ef-
fects cease at once, the key returns to
its natural or unmagnetic state ; the
bits of iron which had attached them-
selves to it immediately fall off, and
show no tendency either to cohere among
themselves, or to adhere to the key.

(21.) Secondly, the vicinity of a mag-
net to a piece of iron gives it the pro-
perty of attracting and repelling the re-
spective poles of another magnet, in the
same way as a magnet would have done.
The truth of this proposition may easily
be proved by placing a small compass
needle poised as in fig. 2, in various si-
tuations relative to the ends of the key or
any other piece of iron of a lengthened
shape, while in the vicinity of the magnet.
It will be seen by this examination that
the piece of iron has acquired by induction
two poles, the qualities of which will be
discovered by their attractions or repul-
sions of the poles of the compass needle,
as they are respectively presented to
each ; and it will be found that these
two poles are disposed in the manner
specified above.

(22.) Thirdly, the iron, which has be-
come magnetic by induction, has at the
same time acquired the power of in-
ducing a similar state of magnetism on
the iron in its neighbourhood. Thus,
while the bar B,y?g-. 5, is rendered mag-
netic by the influence of the magnet A,
it exerts itself a similar power on an-
other bar C, rendering it also magnetic.
The bar C, in its turn, will act in like
manner upon another bar, D, and so on.
In this way the influence of the magnet A
may be made to extend along a series of
iron bars or pieces of any other shape,
each acquiring magnetism by the induc-
tive power of the preceding piece ; and in
its turn inducing magnetism on the next.

(23.) But this is not all. The piece
of iron which has been rendered magne-
tic by the vicinity of a magnet, not only
acts upon the other iron that is near it,
but also reacts upon the magnet from
which its power is derived, and increases
the intensity of its magnetism. The
power of a magnet is, in fact, augmented
by the exertion of its inductive influence
on a piece of iron in its neighbourhood.
A simple experiment is sufficient to
prove this fact.

Let a piece of iron be suspended from
one of the poles of a straight magnet ;

and let the weight which this magnet
will carry be ascertained by attaching to
the iron a scale, capable of holding the
weights necessary for this trial, and
which may be gradually increased till
the piece of iron drops off from the
magnet. Repeat this experiment, hav-
ing previously placed a bar of iron in
contact with the other pole of the mag-
net, and it will be found that the mag-
net will now support a much greater
weight ; showing the increase of power
it has derived from the presence of the
bar of iron which has been applied to
the other pole, and the induced magnet-
ism of which, although solely derived
from the magnet, reacts, by a kind of
secondary induction, upon that magnet.
We have already had occasion, in the
Treatise on Electricity, to notice the
same kind of reaction in the case of elec-
tric induction. The increased intensity
which a magnet acquires by induction
often leads to the permanent acquisition
of power by the magnet. Hence we may
understand the reason why a magnet that
is employed for magnetizing a neutral bar
of steel, by means of its inductive power,
becomes itself stronger by the operation.

(24.) It is a necessary consequence of
the law of magnetic induction that it is
accompanied by attraction : for the po-
larity of the adjacent end of the piece of
iron on which the magnetism is induced,
is always of the opposite kind to that
of the pole of the piece which induces it :
according to the fundamental law of
magnetism, therefore, a mutual attrac-
tion must take place between them. The
remote end of the piece on which the
magnetism has been induced is indeed
repelled, because its polarity is similar
to that of the inducing pole : but it is
evident that the attractive action of the
adjacent and dissimilar poles will always
be stronger than the repulsive action of
the more distant poles ; and will there-
fore always prevail.

(25.) This remark leads us" to a very
important step in the generalization of
the magnetic phenomena. We have
hitherto spoken of the attractive power
of magnets for iron as one of the
primary facts in the science : but we now
see that it is merely a necessary result of
a more general law, namely, that of in-
duction, together with the law of action
of the two polarities upon each other : —
or, in other words, that it is itself com-
prehended in these more general facts.
A magnet attracts a piece of unmag-
netic iron, not from any inherent dispo-
sition to attract it in that state, but in

MAGNETISM.

consequence of its inductive influence,
which converts it, for the time, into a
second magnet, having its poles so dis-
posed with relation to the first magnet,
that the adjacent parts have always op-
posite polarities ; and attraction, there-
fore, takes place hetween them. Thus
the pieces A and B, jig. 5, attract each
other, simply hecause the induced mag-
netism of the end s is of the opposite
kind to that of the pole N. In like man-
ner B and C attract each other, because
the polarities of the adjacent poles n and
s' being of a dissimilar kind, their mu-
tual action is attraction. With respect
to this ultimate effect, the inductive in-
fluence of either pole is exactly alike,
and leads to the same result.

(26.) We may now understand the
reason why, when a magnet is placed in
a heap of iron filings, and then lifted
up, the filings attach themselves in clus-
ters to the poles, arranging themselves
in lines, and adhering together by a force
of attraction which extends from each
individual particle to those which precede
and follow it. They form, indeed, by
their mere juxtaposition, under the in-
fluence of the large magnet, a series of
minute magnets, of which the poles are
similarly situated in each, and being
alternately north and south, the adjacent
ends attract one another.

(27.) This disposition of the poles
may be verified by making an experi-
ment of the same kind on a larger scale,
suspending from the end of a strong
magnet a piece of iron, such as a key (fig.
7), from the lower end of which a smaller
key maybe made to hang
in consequence of its in-
duced magnetism. To this
may be appended a still
smaller piece of iron, such
as a nail; and we may
thus proceed, adding piece
after piece, till the lower
one will exert only suffi-
cient attraction to sus-
tain a very small weight
of iron, such as a small
needle. The polarities of
the lower ends of each
piece, if examined previ-
ously to each additional
piece being appended to
it, will be found to be
constantly of the same
kind as that of the lower
end of the magnet from
which the whole is sus-
pended. This may be
ascertained by its at-

tracting or repelling the poles of a small
magnetic needle balanced on a point,
and supported on a stand, as shown at

(28.) The knowledge of the general
fact, that magnetic induction always
tends to produce attraction between the
adjacent parts of the bodies which act
upon each other, enables us to explain
many phenomena, which might other-
wise appear to be at variance with the
simple laws of attraction and repulsion
already stated. Thus we find that the
dissimilar poles of two magnets attract
each other with a force which is greater
than the repulsion exerted between the
similar poles of the same magnets ; and
this happens because, in the former
case, the tendency of induction is to
strengthen the magnetic power of the
adjacent dissimilar poles; but, in the
latter case, where the poles are similar,
each pole tends, by its inductive influ-
ence, to weaken the magnetism of the
other. This is exemplified in a still more
striking manner, when a weak magnet
is brought near to a much more power-
ful one : in which case we find that,
although when they are at a moderate
distance from each other, either pole of
the weaker magnet is repelled by the
similar pole of the strong one ; yet, if it
be brought very near, and especially if it
be made to touch the latter, it is at-
tracted, and will even adhere with some
force to the strong magnet. This effect
evidently results from the powerful in-
ductive influence of the strong magnet,
which, for the time, destroys the feeble
polarity of the weak magnet at the part
immediately adjacent, and impresses
upon it a polarity of an opposite kind :
whence attraction follows as a neces-
sary consequence of contrary polarities.

(29.) The intensity of magnetic power
developed by induction in an iron bar,
was found by Mr. W. S. Harris to be
inversely as the distance of the inducing
pole from the adjacent end of the bar on
which it acts. It would also appear,
from the experiments of the same gen-
tleman *, that the intensity of the mag-
netism induced on the remote end of the
bar, is, with the same inductive power
acting on the nearer end, inversely as
the length of the bar.

Magnetic induction is not confined in
its operation to any particular direction ;
thus a bar of iron will be rendered mag-
netic if placed at right angles, or at any
other inclination to the axis of the mag-

* Transactions of the Royal Society of Edinburgh
for 1829.

MAGNETISM.

net, or line joining its two poles, as in
the situations represented in Jig. 8. The

'Fig. 8.

ends, s, s, of the bars B, C, adjoin-
ing the north pole N of the magnet A,
will still become south poles by induc-
tion, and the ends ra, ra, north poles.
Under these circumstances, if the incli-
nation be less than a right angle, as in
the case of C, the opposite pole of the
magnet s begins to exert an inductive
influence on the other end of the bar n,
which concurs with that of the pole N
in rendering n a north and s a south
pole. The most favourable position for
the bars receiving the full inductive in-
fluence of both poles is that of parallelism
with the magnet A, as shown mjig. 9.
Fig. 9.

(30.) Complicated effects result from
bringing the magnet in contact with
other parts than the ends of a piece of
iron. Thus, if the north Fig. 10.
pole of a magnet be placed ||||[||S

in the middle of an iron
bar, as in Jig. 10, both
extremities of the bar are
rendered north poles, while
the middle is a south
pole.

(31.) If the north pole of a magnet be

placed on the centre of a round iron

plate (Jig. 11), so that its axis may be

Fig. 11. Fig.

perpendicular to it, the plate will have a
south pole in its centre, and every part
of its circumference will have the pro-
perties of a weak north pole. If the plate
have the form of a star (Jig. 12), each

of the points will have a stronger
northern polarity than in the last case.
Analogous effects may be observed in
pieces of iron of an irregular shape
when acted upon by a magnet ; the part
immediately adjoining to the north pole
of the magnet acquires the properties of
a south pole, and all the remote protu-
berances have a feeble northern polarity.

$ 5. Complex Induction.

(32.) When two magnets are placed
so as to exert an inductive influence on
the same bar, it will depend on their re-
lative position, whether they shall con-
spire to produce the same polarities in
the ends of the bar, or whether they
shall oppose each other, and produce
contrary polarities. If the bar B, fig.
13, lie between the two magnets A and

Fig. 13.
A T* c

S tf " '" S

C, and be in the same line with them,
and if N, the north pole of A, be adja-
cent to S, the south pole of C, the inter-
mediate bar B will receive a magnetism
of the same kind from the inductive
power of both the magnets, its south
pole s being adjacent to N, and its
north pole n to S. Its magnetic power
will, therefore, be considerably greater
from the united influence of the two
magnets, than it would have been from
the influence of one only.

(33.) It may here be observed that the
order in which the poles of these pieces
A, B, and C, succeed one another, is
exactly the same as that which obtains
in the successive induction of magnetism
along a series of iron bars, as in fig. 5.
Now the same consequences follow from
this arrangement in the one case as in
the other. We have just seen that when
C is a bar already rendered magnetic,
the magnetism of the iron bar B, which
the magnet A had induced upon it, is
increased by the presence of C. In like
manner, we find that the magnetism of
the bar B, in the case above referred to,
fig. 5, is increased by the presence of ano-
ther piece of iron C, placed at its end, al-
though that piece, previously to its being
so placed, was entirely free from magnet-
ism. This increase is owing to the
piece C having become magnetic by its
position with respect to B, which had
itself been rendered magnetic by the
vicinity of the magnet A. C having
thus become a temporary magnet by

MAGNETISM.

the action of B, reacts upon B> and, ex-
erting a new inductive influence, tends
to increase its magnetism in the same
manner as if it had been a permanent
magnet. Thus it is that in a series of
iron bars held together by induced mag-
netism, each piece tends to increase the
strength of the preceding piece, and
the whole coheres together -with greater
force than if no such reaction took
place. This circumstance affords a fur-
ther explanation of the strong cohesion
we observe among the particles of iron
filings, which hang in long threads from
the poles of a magnet.

(34.) A closer attention to the conse-
quences which flow from magnetic in-
duction, will also enable us to explain
another remarkable fact, which the ad-
hesion of the strings of iron filings pre-
sents. It is that each separate filament,
although composed of parts that attract
each other in the direction of their
length, yet shew a tendency to keep dis-
tinct from the neighbouring filaments,
and even appear to repel one another.
In order to understand this, let us con-
sider the condition of several slender
iron bars placed side by side, and ad-
hering to the north pole of a magnet, as
shewn in fig. 14. The in- •£•„ 14
ductive power of the mag-
net, as we have seen, will
render each of the ends in
contact with that pole, a
south pole, while all the re-
mote ends will be north
poles. Hence, the bars will
all have their similar poles
near each other, and this
will happen at both their
extremities, and they will
accordingly repel one ano-
ther. As long as they ad-
here to the magnets by one end, this
repulsion will be prevented by that ad-
hesion from shewing itself; but at the
other ends, which are at liberty to move,
it will be strongly manifested, and the
bars will be observed to separate or
diverge from one another. Now this is
very nearly the condition of the fila-
ments composed of particles of iron.
The polarities of those parts of each
which are in contact, are neutralized and
become scarcely sensible ; but those of
the extremities, being uncompensated,
exert their full power, and produce the
observed repulsion of the filaments.

(35.) This effect of induction is exceed-
ingly well illustrated by the following
experiment of Cavallo: let two short

pieces of iron wire, fig. 15, be each fas-
tened to a thread, the threads being
joined at their other ends and formed
into a loop, by which they are to be sus-
pended from a hook or pin, so as to have
full liberty to move. On bringing the
pole of a magnet, the south pole for
instance, at a certain distance below the
wires, it will occasion them to recede
from each other, as shewn in fig. 16, in-

Fig.15. Fig.]6. Fig. 17.

9

dicating the repulsion which takes place
between the adjacent ends of the wires,
in consequence of their being similarly
affected by the inductive power of the
magnet ; the lower ends of both being
rendered north poles, and the upper ends
south poles. This divergency of the
wires will continue to increase until the
magnet has approached to a certain
limit. But if the magnet is brought
nearer than this limit, its own attractive
force becomes so strong as to overpower
the repulsion that exists between the
lower ends of the wire ; and therefore
brings them nearer to each other, as
shewn in fig. 1 7 ; while the repulsion
of the upper ends s, s, still conti-
nues to manifest itself, by keeping them
remote from one another. On removing
the magnet entirely, the wires imme-
diately collapse, their magnetism being
only of a transitory nature. But if the
same experiment be made with sewing
needles, instead of soft iron wires, the
needles will often continue to repel each
other after the removal of the magnet,
having acquired some degree of perma-
nent magnetism by the circumstances in
which they have been placed.

(36.) If four wires be suspended in a
manner similar to those in the last ex-

periment, each by its separate thread,
the induction of a simil ar magnetism upon
all of them will produce a mutual re-
pulsion among them, and they will of
course all diverge from one another.
But if the wires be made of steel, so as
to retain whatever magnetism may be
communicated to them, and a northern
polarity be given to the lower ends of
two of the wires, but a southern pola-
rity to the lower ends of the other two
wires, when each of these pairs is
kept apart, the wires will repel each
other ; but if both pairs are brought to-
gether, all the four wires will unite and
adhere together. The reason is that
those wires which have opposite polari-
ties attracting each other, unite to form
a pair in which the polarities are ba-
lanced, and the repulsion each had be-
fore exerted towards those which were
similar in the other pair, is now entirely
neutralized. The same thing will hap-
pen, however numerous are the pairs of
wires that are dissimilarly magnetized.

(37.) In order that a bar of iron may
receive the combined inductive influence
of two magnets, it is not necessary that
they should all be situated in the same
line as in the example already given.
The same effect will result if the bar be
at right angles to the two magnets, as in
the following figure (18), provided the

Fig. 18.

MAGNETISM. ^ « ^*?*^ V

evident that an effect of an opposite kind
must result when the dissimilar poles of

Fig. 19. Fig. 20. ^SlTTA *

** * A.

two ends be immediately acted upon re-
spectively by the poles of opposite deno-
minations of the magnets. The attrac-
tion of a bar in this situation is much
increased by the conspiring inductive in-
fluence of the two magnets ; and the
force exerted is more than double of that
by which it would have adhered to either
of the magnets when singly employed.
This may be verified by attaching the
scale of a balance to the bar (fg. 19,)
and adding weights till its adhesion to
the magnets is overcome : these weights
will be found to exceed the sum of the
weights which the two magnets would
have supported, by means of the adhe-
sion of the same iron bar, if they had
been applied separately.

(38.) While such is the effect of the ap-
plication of the dissimilar poles of two
magnets to the ends of a bar of iron,
namely, that of conspiring to induce the
same kind of magnetism, it is likewise

the magnets are both applied to the same
end of a bar. These poles, being of dif-
ferent kinds, will produce contrary ef-
fects ; their inductive influence will op-
pose, instead of assisting, each other,
and the magnetism induced on the bar
will be only that resulting from the dif-
ference, instead of the sum of their in-
tensities. If the bar be of some length,
and if the magnets be of equal strength,
and applied close to each other, their
actions upon the remoter parts of the
bar will be so nearly equal, that they
will almost entirely neutralize each
other, and no sensible degree of mag-
netism will be excited. Thus if while a
key is supported by a magnet as in fig.
20, we gradually bring down upon it a
second magnet, with its lower pole of the
opposite kind to the lower pole of the
first magnet, it will tend to induce in the
key a polarity of an opposite kind to
that which it has received from the first
magnet. In as far as it exerts that in-
fluence it diminishes this magnetism,
and consequently weakens the attrac-
tion. Another cause also operates in
diminishing the attraction. The polarity
induced upon the adhering end of the
key is of the contrary kind to that of the
pole of the first magnet; it is therefore
of the same kind with that of the second
magnet which is brought near it, and
which, therefore, as far as the key re-
tains its induced magnetism, must repel
it. Accordingly, it happens that when
the second magnet, if sufficiently power-
ful, is brought within a certain distance
from the upper end of the key, it de-
stroys its power of adhering to the mag-
net, and the key drops off.

A similar counteraction of magnetic
induction will take place, when the other
pole of the second magnet, that is the

10

MAGNETISM.

pole of the same denomination as that
of the first magnet to which the key ad-
heres, is applied to the lower end of the
key. The first action of the lower mag-
net, as it approaches the key under
these circumstances, is to repel it ; but
on being brought still nearer, its induc-
tive influence becomes so great as to re-
verse the poles of the key, which is now
attracted by the pole which before re-
pelled it : it then generally drops off and
adheres to the lower magnet.

(39.) The effect of applying similar
poles to the two ends of a bar of iron is
generally that of inducing the opposite
polarity on both ends of the bar, and the
same polarity at the middle. Thus,^*
21, the north poles N, N, ofthetwomag-

Fig. 21.

A

nets A and C being applied lengthwise to
the ends of an intermediate bar B, will
render it a magnet with three poles,
those at the end being south poles, and
the middle being a north pole. In this
case the bar will be attracted by both
the magnets, though less powerfully than
when the acting poles of the latter are
of opposite kinds, as in the situation
shewn by/g-. 13. In more complicated
cases, and more especially when the
form of the piece of iron is irregular, it
is difficult to predict the exact mode in
which the poles will arrange themselves
when magnetism is induced upon it by a
single magnet, and still more when the
operation of two or more magnets, es-
pecially if they be of unequal strength,
is to be estimated. The following, how-
ever, is one of those cases in which the
process that takes place is more obvious,
and which furnishes an amusing illus-
tration of the general principle.

(40.) Take a piece of iron, C (Jig.
22), formed into the shape pjff 22
of a fork, or of the letter — '
Y, and suspend it by one
of the branches of the fork
to the north pole of a mag-
net A ; its lower end will
immediately acquire a nor-
thern polarity, and will at-
tract another small piece of
iron, such as a key, which
may therefore easily be
supported by it. ^While the
key is thus hanging from
its lower end, apply to the
other branch of the fork the

south pole of another magnet B, the
key will instantly drop off. The reason
is that the magnet B tends to induce
upon the remote or lower end of the fork,
a contrary polarity to that which is in-
duced upon it by A, and thus destroys
its power of attracting. The fork will
have a south pole at a, a north pole at b,
while its lower end will be neutral. If, on
the contrary, the northpole of the magnet
B had also been applied to the branch b
of the fork, its influence would have
conspired with that of A in inducing a
northern polarity at C, and the key
would have been more strongly attracted.

§ 6. Different Qualities of Iron and
Steel with regard to Magnetic Sus-
ceptibility and Retentiveness.
(41.) All the effects we have hitherto
described, as attendant on the induction
of magnetism on iron, are of a temporary
nature, depending altogether on the in-
fluence excited by the neighbouring
poles of a magnet ; for we find that, the
moment the magnet is removed, all these
effects cease, and the iron returns to its
original state of neutrality, and loses all
its magnetic properties. But the case
is different when steel is made the sub-
ject of experiment. Magnetism, it is
true, may be induced on steel ; but the
induction proceeds very slowly, and is,
at first, much more feeble than it is with
iron. On the other hand, steel does not,
like iron, lose what it has acquired ; for,
on the removal of the magnet which
gave it the magnetic properties, it re-
tains these properties permanently : it
has, in fact, become itself a real magnet.
(42.) This remarkable difference ex-
isting between iron and steel in their re-
spective susceptibility to receive, and
capacity to retain magnetism, must, no
doubt, arise from some peculiar ar-
rangement of their particles, the exact
nature of which is at present entirely
unknown.

It is, however, exceedingly analogous
to the difference in the qualities of elec-
trics and non- electrics with regard to
the power of conducting electricity ; and
the magnetic phenomena depending
upon it admit of being explained on an
hypothesis very similar to that by which
we are enabled to account for the elec-
trical phenomena which correspond to
them. The two magnetic polarities
may be conceived to reside constantly
in all iron or steel, and in the natural
or neutral condition of these bodies, may
be regarded as in a state of equilibrium,

MAGNETISM.

11

and as equally 'distributed throughout
the whole mass. But this state of equi-
librium in a bar is disturbed by the in-
fluence of a magnetic pole in the vici-
nity, which exerts an inductive influ-
ence. This new force which comes into
play, tends to transfer one kind of pola-
rity to one end of the bar, and the op-
posite polarity to the other end. In iron,
these changes are readily effected, on
account of the facility which its peculiar
texture affords for the transmission of
these agencies in both directions. No
sooner is the cause which has produced
these changes, and maintains the sepa-
ration of the polarities, removed, than
all the effects cease ; for there exists no
obstacle to the return of the two polari-
ties to their original situations: they
revert, therefore, to their former state of
equal distribution, and the condition of
neutrality is restored in every part. But
it is not so with steel ; the constitution of
which is such as to interpose impedi-
ments to the transfer of the polarities
from one part to another. It requires a
certain time before the obstruction, what-
ever be its nature, can be overcome ; and
before the new state of distribution,
which induction tends to establish, can
be completed ; nor can the changes
themselves ever be accomplished to the
same extent as they are in bodies which
present no such obstacles. It is also a
necessary consequence of the resistance
which the texture of steel presents to
any changes taking place in its mag-
netic condition, that these conditions,
when once induced, tend to remain in
a great degree fixed. Hence we see
the reason why steel bars admit of being
rendered permanently magnetic, while
the magnetism induced upon iron is
only temporary. A steel bar, which has
as great a degree of magnetic power as
it is capable of retaining, is said to be
saturated with magnetism.

(43.) In order to obtain an exact
knowledge of the progress of magnetic
induction in steel, we should place a bar
of this material very near to the pole of a
strong magnet, and in the same line with

Fig. 23.

it, and provide ourselves with a very small
and delicate compass needle, poised
on its centre, as in fig. 2, by which the
polarity of each part of the bar may be
examined in succession. It will be

evident that an inductive 'action com-
mences immediately on the magnet's
being presented to the bar ; for the latter
is attracted and adheres very strongly
to the magnet from the very first.

The end next the magnet has, there-
fore, acquired a polarity of a contrary
nature to that of the magnet. For the
sake of greater clearness of illustration,
we shall suppose the actual pole of the
latter to be a north pole. The near end
of the steel bar is at once converted into
a south pole ; but if we examine the re-
mote end we do not find it so immedi-
ately converted into a north pole. A
sensible time is required for effecting
the change in the latter ; and it will be
found, upon a more careful investiga-
tion, that the different parts of the bar
from south to north, acquire, in suc-
cession, this northern polarity, which
at last settles in the extremity. If
the bar be of considerable length, it

Fig. 24.

often happens that the northern pola-
rity never reaches thus far, but stops at
a nearer point ; and in that case, we
generally find a weaker south pole ap-
pearing at some greater distance ; and
this pole also travels slowly onwards
till it attains its furthest limit. This
is often succeeded by another north
pole, and even a greater number of
alternations will sometimes take place ;
each successive pole, however, be-
coming weaker and more diffused in
proportion as they are more nume-
rous and more distant. The points
where the polarities thus change from
the one kind to the other have been
called consecutive points. It is evident
that alternations of this kind must very
much disturb the regularity of the mag-
netic actions of bars in which they
exist, and complicate the resulting
phenomena.

It would appear, from a variety of
facts hereafter to be detailed, that a
certain time is in all cases required for
the complete operation of magnetic in-
duction.

(44.) There are certain circum-
stances and modes of treatment which
tend to quicken the progress of this in-
duction. The first of these is concus-
sion. Whatever excites a tremulous or
vibratory motion among the particles of
the steel, promotes thejtrahsmission of

12

MAGNETISM.

the magnetic polarities, and favours the
induction of magnetism. Striking on
the bar with a hammer is found to pro-
duce this effect in a remarkable degree ;
and the more so if it occasion a ring-
ing sound in the steel, which is an in-
dication that its particles are very gene-
rally thrown into vibratory motion. But
any other cause producing agitation
among the particles assists in the in-
duction of magnetism.

(45.) The transmission of an electric
discharge through a steel bar under the
influence of a magnet, is sufficient to
produce permanent magnetism. That
the electricity acts here only by its me-
chanical operation, is proved by the
effect being the same, whatever be the
direction in which it is transmitted ;
that is, whether the positive stream of
electricity be made to pass from right
to left, or from left to right, along any
part of the steel bar ; or whether it be
passed longitudinally or transversely
through it. This mechanical operation
of electricity is, however, to be carefully
distinguished from an influence of a to-
tally different description, which it is
capable of exerting in producing mag-
netism, and the operation of which will
be the subject of a distinct treatise here-
after to be published.

(46.) Heat also appears to act by re-
moving the obstructions to the trans-
mission of magnetism which exist in
steel, in its ordinary state, and by thus
reducing it nearly to the condition of
soft iron. Accordingly, if a steel bar
be heated, and placed in circumstances
favourable to magnetic induction, if it
be placed, for example, in the immedi-
ate vicinity of a magnet, and then sud-
denly cooled, it will be found, on its
removal from the magnet, to have be-
come strongly and permanently mag-
netic. The greatest degree of magnetism
is produced by heating the steel to red-
ness, and, while it is under the influence
of a strong magnet, quenching it sud-
denly with cold water.

(47.) It will readily be understood
that since the magnetism of a steel bar
remains permanent, only because the
peculiar texture of the steel presents
an insuperable obstacle to its resuming
its natural state of uniform distribution,
all the causes which diminish this ob-
structing force, will give occasion to the
escape of portions of this imprisoned
magnetism, and will make the bar ap-
proach nearer to a neutral condition.
In other words, its magnetism will be

impaired and weakened, by the very
same causes which favoured its acqui-
sition when under the inductive process.
It is accordingly found that any mecha-
nical concussion, or any rough usage,
has a tendency to destroy the power of
a steel magnet. Dr. Gilbert, who was
one of the earliest discoverers in this
science, found that a magnet which he
had impregnated very strongly was very
much impaired by a single fall on the
floor: and it has been observed since
his time, that a magnet is more injured
by falling on a stone pavement, or re-
ceiving blows which cause it to sound
or ring, than by being struck with any
soft or yielding substance.

(48.) In like manner the application
of heat to a magnet is invariably at-
tended by a dissipation of its magnetic
power. It is even sensibly affected by
the heat of boiling water ; and a red heat
totally destroys its magnetism. It has
been observed by Mr. Canton, that if
the temperature of the magnet has been
raised only to that of boiling water, al-
though it loses much of its power during
the operation, yet that a great part of it
is again recovered on its becoming cool.
But after it has been heated to redness,
no part of its magnetism is recovered on
cooling.

(49.) The precise nature of the in-
fluence which heat has upon magnetism
is far from being clearly understood;
and there appears to be much discord-
ance in the accounts given by different
authors on this subject. This appears
to have arisen in a great measure from
a want of attention to the circumstance
that the operation of heat is of two
kinds : for while, on the one hand, it
facilitates the induction of magnetism,
on the other it weakens magnetic action.
In those cases where the effects depend
upon the readiness with which a piece
of iron receives magnetism by induction,
heat will favour this process ; thus, soft
iron is more disposed to be attracted by
a magnet when hot than when cold,
provided the heat be not excessive. But
in as far as relates to permanent mag-
netism, the action of heat is to impair
or destroy it ; so that steel, when heated,
is less capable of retaining its power
than it is when cold. This happens
in consequence of its being brought
nearer to the condition of soft iron, by
the separation of its particles. By rais-
ing the temperature sufficiently high, to
a red heat for instance, the whole of its
permanent magnetism is at once de-

MAGNETISM.

13

stroyed, though it will still be suscep-
tible, while in that state, of receiving tem-
porary magnetism by induction, and
therefore of being attracted by another
magnet

(50.) The direct tendency of heat to
diminish magnetic power must also be
taken into account in our estimate of
the preceding phenomena. It not only
promotes the destruction of permanent
magnetism, but diminishes likewise the
effects of that which is of a tempo-
rary nature. The degree in which heat
possesses this direct influence, can be
estimated only under circumstances in
which no permanent change has been
produced in the magnetism of a bar
subjected to the change of tempera-
ture ; that is, provided we find on the
return of the bar to its former tempera-
ture, that it has retained all the power it
had before the experiment. The limit
beyond which no proper distinction can
be accurately drawn between the effects
of this twofold operation of heat, ap-
pears, according to the experiments of
Mr. Christie, to be below 100° of Fah-
renheit. From this temperature down-
wards the power of a magnet increases
as it becomes colder ; and this augmen-
tation proceeds as far as the lowest
temperature that has been tried.

(51.) The following are the results of
an extensive series of experiments upon
this subject made by Mr. Christie*.
Commencing with a temperature of
-3° of Fahrenheit, up to one of 127°,
the intensity of magnetic power de-
creased as the temperature of the mag-
nets increased. From an experiment
he made at the Royal Institution, in
conjunction with Mr. Faraday, in which
a small magnet, enveloped in lint, well
moistened with sulphuret of carbon,
was placed on the edges of a basin con-
taining sulphuric acid, under the re-
ceiver of an air-pump, he found that the
intensity of the magnet increased to the
lowest point to which the temperature
could be reduced, and that the intensity
decreased on the admission of air into
the receiver, and consequent increase
of temperature. This, he observes, is
in direct contradiction to the notion
which has been entertained of intense
cold destroying the magnetism of the
needle. Captain Middleton had an-
nounced •!• his having frequently observed
that a compass appeared to be deprived

* Philosophical Transactions for 1825, p. 62.
t PhilosophicalTrans. for 1733, vol. xl. p. 310.

of all magnetic power from cold, while
he was navigating among the 'ice in
Hudson's Bay ; but recovered its power
when brought into the cabin and warmed
by the fire : and that this repeatedly oc-
curred. There can be no doubt that
this must have been owing to some other
cause than the one he assigned.

With a certain increment of tem-
perature, the decrement of intensity is
not constant at all temperatures, but
increases as the temperature increases.
From a temperature of about 80° the
intensity decreases very rapidly as the
temperature increases, and beyond the
temperature of 100°, a portion of ihe
power of the magnet is permanently de^
stroyed.

(52.) The effects produced on unmag--
netized iron, by changes of temperature*
were observed by Mr. Christie to be
directly the reverse of those produced on
a magnet ; an increase of temperature
causing an increase in the magnetic,
power of the iron, the limits between,
which, he observed to be 50° and 100°.
This is in perfect conformity with the
views we have above explained, of the
nature of the operation of heat with re-
gard to magnetism.

(53.) Although the direct tendency of
heat to diminish magnetic power may, in
a red hot bar, be not sufficient to prevent
its receiving induced magnetism, yet
when the temperature is still further
raised, even this capability is destroyed •
and accordingly we find that, at a white
heat, iron appears to be totally insuscep-
tible of any magnetic action. There are
still, however, some curious anomalies
occurring in the magnetic action of iron
at these very high temperatures, of which
further investigation alone can furnish
the explanation.

(54.) In .the account we have now
given of trie properties of iron and of
steel, with regard to their capabilities of
acquiring and of retaining magnetism,
we have all along referred to those of
pure metallic iron in its softest and most
ductile state, and to those of steel which
has been brought to its greatest degree
of hardness by immersion in cold water
after being heated ; for it is in these two>
states that they exhibit the strongest
contrast in these respects. We often,
however, meet with this metal in states
possessing intermediate degrees of the
above qualities ; that is, acquiring mag-
netism with less facility than soft iron,
and retaining less of it than hard steel.
It may be laid down as a general propo-

14

MAGNETISM*

sition, liable however to some excep-
tions, that the power of retaining mag-
netism in any specimens of iron or steel,
is in proportion to its hardness.

(55.) But some of the combinations
of iron with other substances affect its
capacity for magnetism, independently of
the hardness of the compound. A slight
degree of oxidation pervading the mass
of iron appears to increase its power of
retaining magnetism ; but a greater de-
gree renders it totally insusceptible of
being affected by the magnet, or of
possessing any magnetic properties
whatsoever. Combinations with phos-
phorus, with arsenic, or with tin, were
found by Mr. Gay Lussac to produce
compounds somewhat resembling those
of carburet of iron or steel in their ca-
pability of retaining magnetism. Every
thing depends, however, upon the pro-
portions in which these several sub-
stances are united with the iron ; for if
they exceed a certain quantity, they
totally incapacitate the compound from
acquiring any magnetic properties.

(56.) It is only the finest and purest
soft iron, free from all knots and veins,
that returns to the state of perfect neu-
trality after it is removed from all extra-
neous magnetic influence. Iron is sel-
dom found in this perfectly pure state ;
but even the purest iron may be rendered
capable of permanently retaining mag-
netism, if it has been twisted or ham-
mered violently. The slight superficial
oxidation it undergoes by the action of
the atmosphere, will also make it suscep-
tible of some degree of fixed magnetism.
But in its common state, iron may, on
the whole, be regarded as incapable of
any long retention of the magnetism
which it may have received by induc-
tion.

(57.) It would appear from the ex-
periments of Mr. Scoresby*, that the tex-
ture of all iron, even the most malleable,
presents a certain degree of resistance to
the transmission ot magnetic power;
for if a bar of iron be placed in circum-
stances favourable to its acquiring mag-
netism by induction, it does not acquire
it in the degree of intensity of which it
would be capable, were there no such
internal obstructions to the transmission.
If, under these circumstances, it be sub-
jected to percussion, which, as we have
seen, favours the transfer of magnetism
in obedience to the attractive and re-

* Transactions of the Royal Society of Edin-
burgh, vol. ix. p. 252.

pulsive forces that act upon it, it is
found to acquire a much higher inten-
sity of magnetic power than it would
have received without such percussion.
Nor is the whole of this power lost on
the removal of the inducting cause ; a
part is retained by the iron, the inter-
nal structure of which appears to have
undergone some alteration by the per-
cussion.

As connected with this subject, we
may notice the following curious obser-
vation of the same experimentalist*.
Bars which had been strongly magne-
tized, and had their magnetisms de-
stroyed or neutralized, either by ham-
mering, heating, or by the simultaneous
contact of the two poles of another mag-
net placed transversely, were always
found by him to have a much greater
facility for receiving polarity in the same
direction as before, than in the contrary
direction. Hence, it generally happened
in his experiments, that one blow with
the original north end downward, pro-
duced as much effect as two or three
blows did with the original south end
downwards. He also observed, that the
polarity of pokers, generally supposed to
be permanent, and considerable in inten-
sity, was rather transient and weak ; for
in no instance did he meet with a poker
the magnetism of which he could not
destroy by a blow or two with a hammer
on the point ; and in general, two blows,
even when the poker was held in the
hand, and not rested upon any thing,
were sufficient to invert the poles.

(58.) Soft steel is not much more reten-
tive of magnetism than iron in its ordinary
state. It is only when hardened that its
magnetic powers become in any degree
sensible. Dr.Robison states, that when
steel is tempered to that degree which
fits it for watch springs, it may acquire
a strong magnetism, which it exhibits
immediately on the removal of the mag-
net. But it dissipates very rapidly ; and
in a very few minutes it is reduced to
less than one half of the intensity it mani-
fested while in contact with the magnet,
and to less than two-thirds of what it was
immediately on removal from it. It con-
tinues to dissipate for some days, though
the bar be kept with care ; but the dissi-
pation diminishes fast, and it retains at
least one-third of its greatest power for
any length of time, unless carelessly kept
or injudiciously treated.

(59.) Steel tempered for cutting-tools

* Philosophical Transactions for 1822, p. 251.

MAGNETISM.

15

(the same author observes), such as
chisels, punches, and drills for metal,
acquires magnetism still more slowly
by induction, and receives less of it while
in contact with the magnet ; but it is
less disposed to lose it, and finally re-
tains a larger portion of what it had ac-
quired. Steel made as hard as possible
is still longer in acquiring all the mag-
netism which simple juxtaposition can
give to it. It acquires Jess than the
former ; but ultimately retains a much
greater proportion. The loadstone, or
native ore of iron, resembles very hard
steel in these respects ; that is, in the
time necessary for its greatest impreg-
nation, and in the dm ability of the ac-
quired magnetism.

(60.) We have seen that iron, or any
of its compounds, when free from mag-
netism, is attracted by a magnet only in
consequence of the induction of mag-
netism upon it by the magnet which
attracts it. It follows, therefore, that
the degree of susceptibility to induc-
tion may be accurately measured by
the attraction which results from this
property. With this view Mr. Barlow
made a series of experiments to ascer-
tain the relative attraction which dif-
ferent species of iron and steel had for
the magnet ; and obtained the following
specific results, the relative magnetic
power of each substance being expressed
by numbers*.

Malleable iron 100

Soft cast steel 74

Soft blistered steel . 67

Soft shear steel

Hard blistered steel.

Soft shear steel ....

Hard cast steel. . . .

66
53
53

49
Cast-iron.. 48

$ 7- Fracture-

(61.) We have hitherto been able to
trace a very close analogy between the
phenomena of magnetism and electri-
city, us far at least'as relates to the law
of action, and the influence of induc-
tion ; but in pursuing it beyond this point
it fails us entirely. Electricity, whether
positive or negative, is not only capable
of being excited by induction, but may
be actually transferred from one body
to another ; but the transference of the
magnetic polarities is a phenomenon
which was never, in a single instance,
known to take place. A body may,

* Philosophical Transactions for 1822, p, 117.

without difficulty, be rendered positively
or negatively electrified ; that is, it may
be charged with a redundance of one or
other of the two kinds of electricity ; and
the influence or agency, call it by what
name we please, that has been gained
by one body, is the same as that which
has been lost by the other. It is not so
with magnetism. There is never any
transfer of properties, but only the ex-
citation of those which were already in-
herent in the body operated upon. We
always find in the same magnet, that the
intensities of the two polarities, although
each may occupy different portions of
it, or be concentrated in some points,
and diffused over others, yet still on the
whole exactly compensate each other.
We never can obtain a portion of iron
or steel endowed wholly with either the
northern or the southern polarity. Each
appears to be strictly confined within the
boundary of the surface of the body
which contains it.

(62.) When a conductor of electricity,
of an oblong shape, is placed near an
electrified body, but not sufficiently near
to receive any part of its electricity, it
becomes electric by induction, the two
ends of the body having opposite electri-
cities. If, under these circumstances,
the conductor be divided across the mid-
dle, and the two portions removed to a
distance from one another, we obtain the
two electricities separate; each por-
tion retaining the electricity that had
been induced upon it. The condition of
a magnet appears to be exactly analo-
gous to this in reference to the distribu-
tion of magnetic power; for the northern
polarity appears 'to be collected in one
half of its length, and the southern po-
larity in the other; and each of these
agencies seems, indeed, to be almost
entirely concentrated in the very extre-
mities of the bar. What, then, ought
to happen conformably with this analogy,
were we to break a magnet (A., fig. 25,)

Fig. 25.

across its middle ? Might we not expect
by this means to obtain the two polari-
ties separate, each still contained in the
same portions where they had before re-
sided ?

(63.) The result of this experiment is
exceedingly curious, and what certainly

16

MAGNETISM.

no previous reasoning could have led us
to anticipate. Each portion, B C, of the
fractured magnet is at once converted
into a magnet, perfect in itself; that is,
each respectively has a north pole at one
end, and a south pole at the other. That
end of the magnet which, previously to
the fracture, was a north pole N, con-
tinues to be a north pole, while the
other end of that fragment s, that is, the
broken end, becomes a south pole. The
converse is true of that fragment B
which originally contained the south pole
of the magnet. It thus appears that the
two fractured surfaces n and s, are now
converted, the one into a north, and the
other into a south pole, although that
part had, in the original magnet, been
apparently in a neutral state.

(64.) Similar consequences ensue
from the subdivision of one of these
fragments into any number of portions,
however great; each lesser fragment
constituting in itself a complete magnet
furnished with its two poles.

(65.) It is observed by ^pinus, who
made many experiments on the effects
of the fracture of magnets, and the ob-
servation has been confirmed by others,
that the neutral point in each fragment
of the broken magnet is at first much
nearer to the place of their former union
than to their other ends. He states that
in the space of a quarter of an hour
after the separation, the neutral points
advance nearer to the middle of each,
and continue to do so, by small steps,
for some hours, and sometimes days,
and finally become stationary at the
centre.

When a magnet is split according to
its length, the two portions will have
sometimes contrary, and sometimes the
same poles as they had when they formed
one piece. When one portion is much
thinner than the other, the slender frag-
ment generally has its poles reversed*.

CHAPTER II.

Laws of Magnetic Forces.

§ 1 . Relation of Intensity to Distance.

(66.) It would be inconsistent with the
elementary views to which we are at
present confining ourselves, to engage in
the investigation of the mathematical
law which regulates the variations of in-
tensity of the magnetic forces, both at-

> Philos°Phieal Transactions, vol. xxiv.

tractive and repulsive, at different dis-
tances. We shall here only observe that
this law has been made the subject of
diligent and careful inquiry by some of
the most eminent philosophers of modern
times ; and shall content ourselves with
merely stating the final result of their
labours. It has been ascertained most
satisfactorily that the same law of varia-
tion obtains in magnetic attractions and
repulsions with relation to proximity, as
in the electrical : namely, that the in-
tensity of the force by which magnetic
polarities act upon each other is inverse-
ly as the square of their distance. In
this respect, therefore, they agree, not
only with the electrical forces, but also
with that of gravitation ; and it would
appear, indeed, to be a property com-
mon to all forces which emanate in every
direction from a central agent.

(67.) The variations of the intensities
of magnetic attractions and repulsions
exerted between any two poles depend
solely upon the distances at which they
are placed ; and are in no degree affected
or interfered with by the interposition of
other bodies which are not themselves
magnetic. Numerous experiments have
been made with a view of discovering
whether there exists any substance
which can modify or intercept the action
of magnets when placed between them
and the body acted upon ; but the result
has been uniformly the same : namely,
that the intervening bodies, of whatever
kind they were, provided they were not
susceptible of magnetism, occasioned no
difference in the observed effects.

This subject, however, involves a ques-
tion, hereafter to be discussed, as to the
magnetic susceptibilities of substances
which are not of a ferruginous nature.

§ 2. Mutual Action of Two Magnets.

(68.) The general law of magnetic
force with relation to distance being once
established, it becomes interesting to
follow its consequences and applications
under a variety of circumstances. These
consequences are always, even in the
simplest cases, more complicated than
electrical arrangements ; becatfse in
magnets the two polarities are "'ways
conjoined, and their influence is never
perfectly isolated. In studying the mu-
tual actions between two magnets, or
even between one magnet and the small-
est conceivable piece of iron, we have
always four polarities in activity, the two
residing in one body, and the two re-
siding in the other ; these polarities are

MAGNETISM.

17

net strictly confined to particular points
in the magnet : for although much con-
centrated at the two ends, they exist
with less intensity in other parts of the
magnet.

(69.) Let us, however, suppose, for
the sake of simplification, that the mag-
netic forces emanate solely from the
two poles at the extremities of the mag-
net M,Jlg. 26, with its axis placed hori-

Fig. 26.

zontally, while a smaller magnet B sus-
pended on a point, or in other words, the
needle of a mariner's compass, and
which we shall therefore designate as
the needle, is presented to it in the vi-
cinity of its north pole, N, and with its
centre in a line with the axis of the mag-
net. The north pole of the magnet at-
tracts the south pole of the needle, and
tends to turn it in the direction indicated
by the arrow at s. It also repels the
north pole of the needle, turning it in the
direction indicated by the arrow at n.
These two actions, it will be seen, both
conspire to give the needle a rotatory
motion in the same direction with regard
to its centre, and to bring it into the po-
sition represented in the next figure, (27,)

in which the south pole of the needle is
turned directly towards the north pole of
the magnet.

The influence of the south pole, S, of
the magnet operates in a manner exactly
contrary to that of its north pole ; but
Leing at a greater distance, its intensity
is less ; and all that it can effect is to
subtra. ^ somewhat from the forces \vith
which ,the needle would have been im-
pellt. .f the north pole of the magnet
had acted alone. The general result as
to the rotatory motion, is therefore de-
termined by the predominance of the
actions of the north pole of the magnet,
and remains as before stated.

(70.) The tendency in one magnet to
assume a particular position with rela-
tion to another magnet, is termed its

directive force *. It results, as we have
seen, from the conjoined influence of two
forces, the one acting on the north, and
the other on the south pole, and is there-
fore equal to the sum of these forces.

(71.) If we now consider what ten-
dency the needle has to approach to, or
recede from the magnet, we shall find
the same forces, which in the former in-
stance conspired together, now op-
posing each other. It is, in the first
place, evident that while the needle is in
the position shewn iny?g\ 26, that is, at
right angles to the magnet, the attrac-
tion of the adjoining north pole of the
magnet for the south pole of the needle,
is balanced by its repulsion for its north
pole ; and the needle, although strongly
urged by these forces to turn round its
centre, has no tendency, on the whole, to
recede from, or approach the magnet.
When, however, it arrives at the posi-
tion shewn in/g-. 27, its south pole s
being nearer to N than its north pole n,
the attractive action is more powerful
than the repulsion, and the needle is,
consequently, now impelled towards the
magnet. But the force which thus im-
pels it results from the difference only of
two contrary forces, the one attractive,
the other repulsive.

(72.) Hence we may conclude that the
directive force, which consists of the
sum of two forces, is in all cases con-
siderably greater than the attractive force
exerted upon the whole needle ; this
latter force being only equal to the diffe-
rence between the same forces. The
ratio between the directive and attractive
forces will be increased, either by di-
minishing the length of the needle, or
increasing that of the magnet. Hence
the polarity of a small needle may be
considerable, while its attraction is quite
insensible.

(73.) Let us next transfer the needle
to the situation shown in Jigs. 28 and 29,
in which its centre is in a line drawn
from the centre of the magnet, and at
right angles to its axis ; the needle being
supposed, as in the former case, to be so
balanced as to turn freely in a plans
which passes through the centre of the
needle, and both poles of the magnet.
Let it be placed in the position indicated
in fig. 28, with one of its poles di-
rected towards the middle of the mag-
net. The directive force is here com-
pounded of four forces: the attrac-
tions of N for s and of S for n ; and

* Dr. Gilbert expressed it by the term verticity,

c

18

MAGNETISM.

Fig. 28,

the repulsive ; the former will therefore
prevail, and the needle will have a ten-
dency to move towards the magnet in
the direction of the line connecting their
centres.

(75.) A similar process of reasoning,
derived from the same principles, will
enable us to determine the resultants
of the forces which act upon the needle
when its centre is situated in different
directions relatively to the axis of the
magnet : and consequently what will be
its. movements, and what its final posi-
tion of equilibrium. In oblique posi-
the repulsions of N for n, and of S for s. tions, indeed, the process of investiga-
They respectively impel the poles n s of tion becomes more complicated, for it is
the needle in the directions denoted by necessary to take into consideration the
the small arrows parallel to the lines in
which these forces act. Those which

act.

act upon the remote pole of the needle s,
compose a resultant having the direction
of the upper horizontal arrow R, at
right angles to the length of the needle,
which is also the radius of its revolution.
Those forces which act upon the oppo-
site pole n, compose another resultant
force in the opposite direction, expressed
by the lower horizontal arrow r. Now
these two resultant forces having oppo-
site directions, and acting at the opposite
ends of the needle which turns upon its
centre, conspire in producing a rotation in
the same direction with relation to that
centre ; and will tend to bring the needle
into the position shown in fig. 29, in

Fig. 29.

which its direction is parallel to that of

the magnet, but in which its poles are

reversed when compared with those of

the magnet ; that is, the north pole of to the magnetic position that the case

the needle being on the side of the south will admit of. It will therefore be situ-

different intensities of each of the four
forces concerned, with reference not only
to the respective distances of the poles of
the needle from those of the magnet, but
also to their respective directions in the
plane of rotation.

(76.) If the plane of rotation, to which
the movements of the needle is limited,
be one which does not pass through the
poles of the magnet, the complication of
the problem becomes still greater. There
are, however, three general results at
which we may arrive, which tend very
much to simplify the resolution of ques-
tions relating to this subject.

(77.) The first is, that if we suppose
the needle to be at perfect liberty to
move on its centre in all directions, the
position of equilibrium at which it will
arrive by the conjoint action of all the
forces which impel it, will always be si-
tuated in the plane which includes the
poles of the magnet and the centre of
the needle. This plane may be called,
for the sake of distinctness, the magnetic
plane ; and the position assumed by the
needle in this plane may be called its
magnetic position.

(78.) Secondly, when the movements
of the needle are limited to any particu-
cular plane, its position of equilibrium is
that which makes the nearest approach

pole of the magnet, and its south pole on
the side of the north pole of the magnet.
This relative situation has been called
by some authors the subcontrary posi-
tion.

(74.) Here also it may be remarked,
that in consequence of the greater prox-
imity of the poles of the different deno-
minations, compared with that of the
poles of the same name, the sum of
the attractive forces exceeds that of

ated in a plane passing through the mag-
netic position, and at right angles to the
plane of revolution.

(79.) Thirdly, if the plane of revolu-
tion be perpendicular to the magnetic
position, the needle will be in a state of
equilibrium with regard to the forces
exerted upon it by the magnet, in all
positions. Such a plane may be called
the plane of neutrality. An example of
this is shown in/^.30, where the needle,

Fig. 30.

MAGNETISM. 19

given the position of the magnet M,
(/#. 31,) and of its two poles N and S,

Fig. 31.

turning on a horizontal axis in a line
with the magnet, is limited in its motion
to a vertical plane perpendicular to its
position of equilibrium. In this case it
will have no tendency to assume any one
position in preference to another, in
this plane.

§ 3. Magnetic Curves.

(80.) In order still further to genera-
lize our views, let us conceive the needle to
be exceedingly short when compared with
the length and distance of the magnet ;
and we shall then arrive at still more
simple conclusions with regard to its po-
sitions of equilibrium in the magnetic
plane. The two poles of the needle may,
with regard to the action of the magnet,
be considered as coincident ; the inten-
sities of the actions of any one of the
poles of the magnet upon them are so
nearly equal that their differences may
be regarded as infinitely small. The at-
traction of the magnet for this minute
needle, an attraction which, as we have
seen, depends upon their difference,
must accordingly be inappreciable. But
the directive force, on the contrary, de-
pending on the sum of these actions,
must be very effective ; and it is to the
operations of the latter of these forces
only that we need direct our inquiries.

(8 1 .) The problem to be solved is this :

and also the place of the centre C of the
needle, which is supposed to be at liberty to
revolve only in the magnetic plane, to find
the direction C T, at which the rotatory
force resulting from the action of the
north pole N of the magnet on the two
poles of the needle in the direction C N,
exactly balances that resulting from the
action of the south pole S of the magnet
on these poles, in the direction C S, each
force having an intensity reciprocally
proportional to the squares of these re-
spective lines.

It may be mathematically demon-
strated * that if such be the law of the
magnetic forces, the direction of the
needle is that of the tangent of a pecu-
liar curve of an oval shape, which
has been denominated the magnetic
curve. Every magnet having two poles
N and S (fg. 3'2) has a system

* Demonstrations of this and of the other fun-
damental properties of the magnetic curves are
given in the Journal of the Royal Institution, for
Feb. 1831, by the author of this Treatise.

Fig. 32.

C2

20

MAGNETISM.

netic curves related to the line joining
these poles, and which may be called its
axis. The general form and disposition of
these curves, according to their different
distances from the magnet, is shown in
the figure.

(82.) The magnetic curves have the
following remarkable property ; namely,
that the difference of the cosines of the
angles, which lines, drawn from any
point in the curve to the two poles,
make with the axis, taken on the same
side, is constant. Thus, in the curve
S C C' C" N, fig. 33, the sum of the

Fig. 33.

cosines of the angles C N X and C S X,
is equal to the sum of the cosines of the
angles C' N X and C' S X. When, how-
ever, the angle C" S X exceeds a right
angle, its cosine being negative, it will
be the sum (instead of the difference)
of the cosines of the polar angles
C" N S, C" S N, that is constant. When
the angle C'" N X is also obtuse, both
the cosines being negative, it is again
their difference that is constant.

(83.) If two radii of equal length, N n,
S s,fig.34, be made to revolve in the same

Fig. 34.

—

J\:

N and S, while their other extremities,
n and s, are kept continually in such a
relative position as that a line drawn
through them shall always be perpen-
dicular to the axis N X, then the line,
constituted by the successive points of
intersection C, C' of the radii, will be
a magnetic curve*.

(84.) The most expeditious method
of delineating a great number of mag-
netic curves related to the same base,
in order to obtain a general view of the
entire system of these curves, is to de-
scribe from each pole, N, S (fig. 35), as
a centre, the equal circles or semi-circles,
A A, B B, with as large a radius as the
paper will conveniently admit of; and,
dividing the axis, produced till it meets
both circles, into any number of equal
parts, to mark off, on the circumferences
of both the circles, the points where they
are cut by perpendiculars from these
points of division ; then, drawing radii
from the centre of each circle to the
divisions of its respective circumference,
the mutual intersections of these radii
will form different series of points indi-
cating the course of the magnetic curves
which pass through them. In the pre-
sent case these curves are composed of
a succession of diagonals of the lozenge-
shaped interstices formed by the inter-
secting radii, as is shown in the upper
half of fig. 35.

(85.) The forms and disposition of
these curves are elegantly illustrated by
the lines in which iron-filings arrange
themselves when acted upon by a power-
ful magnet. In order to exhibit them,
we need only place a sheet of paper or
pasteboard immediately over a straight
magnetic bar laid flat upon a table, and
scatter lightly some very fine iron-filings
over the pasteboard ; which is best done
by shaking them through a gauze bag.
If we then tap gently upon the paper,
so as to throw them into a slight agita-
tion, they will arrange themselves with
great regularity in lines, which exactly
follow the course of the magnetic curves,
extending from one pole of the magnet
to the other. These minute fragments
of iron, being rendered magnetic by in-
duction, have their dissimilar poles
fronting each other, and therefore at-
tract one another, and adhere together

* The author of this Treatise has constructed a
system of rulers by which magnetic curves may be
mechanically delineated, founded on the principle
stated in the text. The description of this instru-

j- j- , ., . ment is contained in the paper above referred to in

direction rounfl their respective centres the journal of the Boyai institution.

MAGNETISM.
Fig. 35.

21

in the direction of their polarities, which
is that of the tangent to the magnetic
curve : thus affording a beautiful ocular
exemplification of the mathematical pro-
perties of these curves.

(86.) By continuing to tap upon the
paper, the filings arrange themselves still
more visibly into separate lines ; but
here a curious, and perhaps unlocked for
phenomenon presents itself. The lines
gradually move and recede from the
magnet, appearing as if they were re-
pelled, instead of attracted, as theory
would lead us to expect. This arises
from the circumstance that each particle
of iron, or cluster of particles, is thrown
up into the air by the shaking of the
paper, and, while unsupported, imme-
diately turns on its centre, and acquires
a position more or less oblique to the
plane of the paper. This is shown in
Jig. 36, in which M represents a section

Fig. 36.

of the magnet, P P a section of the
paper, and // the position of the fila-
ments of iron thrown up into the air.
The end of each filament nearest to the
magnet is thus turned a little down-
wards, and the filament falls upon the
paper at a point a little more distant
than that which it before occupied ; and
thus, step by step, it moves further and
further from the magnet, till it reaches
the edge of the paper and falls off.

(87.) When the magnet, instead of
being beneath the paper, is held above
it, the effect is just the reverse. In
this latter case, the lower ends of the
filaments having a tendency to turn
towards the magnet, the filings gradually
collect under it, when made to dance by
the vibrations of the paper, instead of
falling outwards as they did before.
This will be rendered apparent by fig.
37, where the letters indicate the same
objects as in the preceding figure.

Fig. 37.

\ \ \

22

MAGNETISM.

(88.) Magnetic curves of a different
kind are constituted by the balanced
actions of two poles of the same de-
nomination placed near to each other.
When, for instance, a second north pole
N' (fig. 38) is substituted, instead of the
south pole S, both poles will act in a
similar manner, and in directions not
very different.

Fig. 38,

lar. For this purpose they must be
taken in a different order of arrange-
ment, and followed in the lines of the other
diagonals of the lozenge-shaped intervals
between the intersecting radii ; that is,
of the diagonals which cross those con-
stituting the curves in the former case,
as is shown in the lower half of fig. 34.
These divergent curves, as they have
been called in contradistinction to the
former or convergent ones, are deli-
neated in. fig. 39 ; and may, in like man-

Fig. 39.

In order to render the conditions of
this case as simple as those of the last,
\ve must suppose that the action of the
south poles belonging to the two north
poles N, N', whose action we are ex-
amining, is, from their remoteness, too
feeble to influence the results. In the
former case, where the actions of the
two poles were of a contrary kind, the
resultant of their joint action, or the line
C T (fig. 31) passed in a direction in-
termediate between N C prolonged and
C S, and therefore cut the axis N X
at some point in the prolongation of
N S. But in the present case, the two
magnetic poles being of the same kind,
their action is similar, and their resultant
is a force of which the direction is inter-
mediate to the lines C N and C S ; and
this line produced must cut the axis
somewhere between N and N'. In con-
sequence of this change of position,
which produces a change in the sign of
the cosine of the angle GST, which is
now C N'T, the relation of the cosines of
the polar angles is as follows ; namely,
that the sum (and not, as before, the
difference) of the cosines of the angles
which lines, drawn from any point in
the curve to the two poles, make with the
axis, taken on the same side, is constant.
This applies to the case in which the
angle formed by C N' with the pro-
duced axis is acute, and its cosine posi-
tive. When it is obtuse (or CN'N
acute), the cosine becoming negative, it
is their difference which is constant.

(89.) The intersections of the radii,
drawn according to the method above
described, § 82, will also point out the
course of those curves which belong to
the case where the acting poles are simi-

ner, be exhibited by the arrangements
of iron-filings round two similar poles.

(90.) When the actions of the four
poles of two magnets are taken into ac-
count, the magnetic curves expressive of
the direction of a needle influenced by
them, become, of course, much more
complicated.

CHAPTER III.

Terrestrial Magnetism.

$ 1 . Variation of the Compass.

(91.) IT has been already stated ($ 6),
that if a magnetic bar be poised on its
centre so as to move freely in a horizon-
tal plane, and if no ferruginous body be
sufficiently near to affect it sensibly, it
will assume, when left at liberty, a di-
rection nearly north and south. When
disturbed from this situation, it returns,
after several oscillations, to the same
position. On this property is founded
the mariner's compass, which is of such
essential use in navigation. In moving
horizontally towards the position which
it thus tends to assume, the needle of the
compass is said to traverse.

(92.) It is found that in this country,
as well as throughout Europe, the north
pole of the compass deviates a certain
number of degrees to the westward of
the exact northern direction. This de-
viation from the true geographical me-
ridian haa been called the magnetic

MAGNETISM.

23

declination; but it is more usually known
by the term Variation of the Compass.
The vertical plane which passes through
the direction of the horizontal needle at
any particular place is termed the mag-
netic mpri'iian of that place, in contra-
distinction to the geographical or true
meridian, which is a vertical plane pass-
ing through the poles of the earth.

(93.) There are but few places on the
earth where the compass points directly
to the poles ; that is, where it exhibits
no variation. As far as observation
has extended, these places are situated
in a line which encompasses the globe,
and is called the line of no variation.
In many of its portions it appears to
form part of a great circle of the sphere,
but in others it deviates much from re-
gularity, presenting many flexures in its
course. It may be considered as com-
mencing from a point which may be
designated as the principal arctic mag-
netic pole of the earth, and the exact
situation of which is not yet perfectly
ascertained, although the late voyages
of discovery in these regions have en-
abled us to form a tolerable approxima-
tion to the precise spot, which appears
to be a point somewhere to the west-
ward of Baffin's Bay. After crossing
the United States of North America it
passes along a tract of the Atlantic, a
little to the eastward of the windward
West India Islands, till it touches the
north-eastern point of the South Ame-
rican continent. Thence it stretches
across the Southern Atlantic towards
the south pole, where navigators are
unable to follow it. It re-appears in the
eastern hemisphere to the south of Van
Dieman's Land, and passing across the
western part of the Australian continent,
is again found in the Indian Archipe-
lago. Here, according to Biot, it di-
vides into two branches, one of which
crosses the Indian Sea and enters Asia
at Cape Comorin ; it then traverses Hin-
dostan and Persia, and passing through
the western part of Siberia stretches over
to Lapland and the Northern Sea. The
second branch pursuing a more directly
northern course, traverses China and
Chinese Tartary, and makes its exit from
Asia in the eastern division of Siberia,
where we again lose it in the Arctic seas.
Between these there must exist an inter-
mediate line of no variation in some
part of the continent of Asia ; but the
observations we possess regarding it are,
as yet, too imperfect to admit of any
attempt to trace it correctly.

(94.) If we consider these Asiatic

lines of no variation as composing a
single band, we may then consider the
globe as divided by this and the corre-
sponding American line into two hemi-
spheres. In that hemisphere which
comprehends Europe, Africa, and the
western parts of Asia, together with the
greater portion of the Atlantic, the va-
riation is to the west. In the oppo-
site hemisphere, which comprises nearly
the whole of the American continents,
both North and South, and the entire
Pacific Ocean, together with a certain,
portion of Eastern Asia, the variation is
to the east.

§ 2. Dip of the Magnetic Needle.

(95.) But in order to arrive at a
knowledge of the real influence which
the earth exerts on a magnetic needle, it
is not sufficient to ascertain the position
it assumes when its movements are con-
fined to a horizontal plane, as it is in the
mariner's compass of the ordinary con-
struction : we must place it in such cir-
cumstances as will allow it to move
freely in a vertical plane also. But to
effect this in an unexceptionable man-
ner is extremely difficult. The great ob-
stacle with which we have to contend is
the force of gravity, which by acting in
one direction, interferes with the opera-
tion of the force of terrestrial magnetism,
which acts in a different and in an ob-
lique direction.

(96.) The readiest mode of removing the
influence of gravity, is to affix a steel nee-
dle to a cork, or other buoyant substance,
and to immerse it in water, adjusting the
specific gravity of the two bodies, so that
they may remain suspended in the middle
of the fluid without any tendency either
to float or to sink ; taking care at the
same time that the centre of gravity of
the whole coincides with the centre of
its figure, so that, when the needle is
unmagnetic, and united to the cork, the
two together, placed in any position in
the fluid, shall have no tendency to take
any other position. If the needle be
now rendered magnetic, and replaced as
before, it is found to assume a position
nearly vertical, that is, making an angle
with the plumb line of about 20 degrees,
the north pole of the needle being turned
about 25 degrees to the westward of the
true north. Its deviation from the plane
of the meridian is equal to the variation
of the horizontal needle. Its inclination
to the horizontal plane, or 70°, is called
the dip. But this method, though well
fitted for illustrating the general fact, and
the principle on which it depends, is not

24

MAGNETISM.

adapted for accurate measurement.
For this purpose we must have recourse
to other contrivances.

(97.) The magnetic force may, by the
ordinary dynamic method of the resolu-
tion of forces, be resolved into two
forces, the one acting vertically, the other
horizontally. The latter of these forces,
namely, the horizontal force, is the only
one with the action of which gravitation
does not interfere ; and accordingly, the
mariner's compass indicates by its mo-
tions, the effects of this part of the ter-
restrial magnetic force, and this only.
In order to ascertain the vertical force,
we must proceed in a different manner.
The needle must be furnished with an
axis, at right angles to its length, and
adjusted very carefully, so that it may
pass as exactly as possible through its
centre of gravity. This, of course, can
only be done when the needle is wholly
free from magnetism, and secured, in the
manner hereafter to be pointed out, from
all magnetic influence which the earth
might exert upon it. The axes should be
supported horizontally in such a man-
ner as to allow the needle complete free-
dom of motion in a vertical plane. The
needle being thus balanced, will have no
tendency to incline to one side rather
than to another, and will remain at rest
in any position in which it may happen
to be left, as long as no extraneous force
is applied to it. When this has been
accomplished, the needle is to be mag-
netized, by the methods hereafter to be
described, as strongly as possible, and it
is then to be replaced on its supports,
which are to be turned so that the plane
in which the needle is allowed to move,
may coincide with that of the magnetic
meridian. It will be found that, in this
situation, the end of the needle to which
a northern polarity has been imparted,
will preponderate, or dip, as it is called,
and after a certain number of oscilla-
tions, will settle at a determinate point.
The line which its axis assumes under
these circumstances, is termed the mag-
netical direction, or position. The dip of
the needle was first observed by Norman.

(98.) The inclination of the needle, or
dip, like the variation, differs in differ-
ent parts of the globe. The latest accu-
rate observation of the dip in London,
of which we have any record, is that of
Captain Sabine, who ascertained it, in
August, 1828, to be 69° 47'.* Asa

* Philosophical Transactions for 1829. Since the
ove Was written, \ve are informed that the dip

38' ate\v a?c"riained by CaPt- Segelcke to be 69*
PS at Woolwich, in Nov. 1830

general rule, to which, however, there
are many exceptions, the dip diminishes
as we approach the equator, and in-
creases as we recede from it on either
side. Towards the polar regions it is
very great, and as we come near to the
poles, it approaches to a right angle. At
the magnetic poles themselves, the
dipping needle would, of course, be ex-
actly perpendicular to the horizon.
Those places on the earth where the
needle is perfectly horizontal, that is,
where there is no dip, are in a line that
encircles the globe, and is termed the
magnetic equator.

(99.) As the magnetic poles are not
situated exactly at the poles of the earth's
relation, but at some little distance from
them; so, the magnetic equator does
not coincide with that of the earth ;
though it does not in any part deviate
widely from it. In a general way we
may consider it as a great circle of the
globe inclined to the terrestrial equator
at an angle of about 12 degrees ; its in-
tersections with it being situated at the
longitudes 113° 14' west, and 66° 46'
east from the meridian of Greenwich.
Such, at least, is the result given by all
the observations made for an extent of
more than one half of its circuit, in the
Atlantic and Indian Oceans, and that
part of the Pacific which is nearest to
the South American continent, as ap-
pears from a table of these observations
given by Biot*. But a remarkable ano-
maly is met with when we trace the
course of the magnetic equator across
the Pacific Ocean. This line is found
in the southern hemisphere in the
American continent, and joins the equa-
tor as before-mentioned, at a longitude
of about 113°; but still further to the
westward, at longitude 156° 30' it is
again met with at a distance from the
equator and to the south of it. In the
Sea of China at 116° east longitude, it is
found to the north of the equator,
which it must therefore have crossed at
some intermediate point ; and it is again
inflected towards the south, so as to tra-
verse the equator at the eastern node
already mentioned.

It appears, therefore, from these ob-
'servations, that there are at least three
points in the terrestrial equator where the
magnetic equator coincides with it ; and
the probability is, that there are four :
because, if the latter curve passes to the
northern side of the equator at its western
coincidence, it must again cross it before

* Traite de Physique, tome III. p. 130.

MAGNETISM.

25

it can arrive at the southern situation in
which it has been met with in longitude
156.}°. These inflexions will, therefore,
assume a figure, with relation to the
terrestrial equator, somewhat like that
represented in fig. 40, where the dotted
line m m m, is the magnetic, and the
continuous line, e e, the terrestrial
equator.

Fig. 40.

§ 3. Variations in the Intensity of
Terrestrial Magnetism .

(100.) Besides the variation and the
dip, which together constitute the mag-
netic position, and which differ much in
different situations, there is also a third
circumstance highly deserving our atten-
tion in connexion with this subject,
namely, the intensity of the force which
directs the needle towards this position.
Extensive observations of the relative in-
tensities of the magnetic force of the earth
in different parts of its surface, are of
greater value in enabling us to under-
stand the general system of terrestrial
magnetism than those in the dip or va-
riation. We know that this force varies
greatly in different latitudes ; but our
information with regard to the exact
amount of this variation is exceedingly
scanty, both from its importance not
having been felt, and the consequent
omission of the proper observations
with regard to it, and also from the
greater difficulty there is in conducting
the experiments which are required to
ascertain it. -

(101.) The best mode of estimating
the comparative intensities of the mag-
netic action in the same needle in two
different places, is to count the number
of oscillations which it makes in a given
time, a minute for example, on its being
disturbed from its position of equili-
brium, while it is resuming that position.
The movements of the needle being re-
gulated by the same dynamical la\vs
which govern the oscillations of the pen-
dulum, it is a necessary consequence of
those laws, that the intensity of the force
producing the oscillations, is propor-
tional to the square of the number of
oscillations performed in a given time.
Mr. Graham appears to have been the
first who devised this method of mea-
suring the magnetic intensities.

(1 02.) The first accurate observations
of this kind were those made by Hum-
boldt, and by De Rossel : who have
completely established the general fact,
that the intensity of the force of terres-
trial magnetism increases as we recede
from the equator, where it is weakest,
till we approach the poles : at the mag-
netic poles themselves, it is probably
greater than at any other spot. We
have every reason to expect that great
light will be thrown on this department
of the science from the labours of Pro-
fessor Hansteen of Christiana, who is
now travelling at the expense of the
King of Sweden, and with the permis-
sion of the Emperor of Russia, for the
purpose of observing the magnetic dip,
variation, and intensity, over the whole
of the North of Europe and of Asia. He
has especially directed his attention to
trace the course of the lines of equal in-
tensity, or isodynamic lines as they have
been called: that is, the lines connecting
those places where a needle freely sus-
pended in the magnetic direction, and
drawn a certain number of degrees from
this position, makes the same number of
vibrations round the point of rest in an
equal time.

$ 4. Hypothesis of the Magnetism of
the Earth.

(103.) From a consideration of the
general facts that have now been stated
with respect to the influence of terres-
trial magnetism, it will be sufficiently
evident that the earth acts upon mag-
netised bodies in the same way as if
it | were itself a magnet ; or rather as
if it contained within itself a powerful
magnet lying in a position nearly coin-
ciding with its axis of rotation. This
hypothesis was originally proposed by
Dr. Gilbert in his work entitled
" Physiologia nova de Magnete, et de
Tellure magno magnete," published in
the year 1 6 00; and Kepler ranks this
hypothesis among the greatest disco-
veries in the annals of science.

(104.) In order to make this hypo-
thesis agree with facts, we must assume
that that pole of the terrestrial magnet
which is situated in the northern regions
of the earth, attracts the north pole of
the compass needle, and consequently
that it has the same properties as the
south pole of an ordinary magnet. The
opposite pole of the earth, or that situ-
ated in the antarctic regions, has the con-
trary properties, for it attracts the south
pole of the compass ; and therefore cor-

26

MAGNETISM.

responds in its properties to the north
pole of a common magnet.

(105.) It may be necessary to remark
that this circumstance of the south pole
of the terrestrial magnet being situated
near the north pole of the earth, and vice
versa, has occasionally created a confu-
sion of terms. Some authors have taken
a fancy to reverse the names we have
hitherto given to the magnetic polari-
ties : assuming that it is more correct to
set out by calling that property which
distinguishes the pole of the terrestrial
magnet situated in the northern regions,
the northern polarity : and consequently
to give the name of the south pole to that
pole of the compass, or ordinary magnet,
which is attracted towards it, and which
of course has the opposite polarity. For
the same reason they would call the ant-
arctic pole of the magnet of the earth, the
south pole, and that end of the needle
which is turned towards it, the north
pole. Mr. Savery * endeavoured to avoid
this confusion of terms by using the
word end, in contradistinction to that of
pole ; and this phraseology is adopted
by Mr. Christie in his papers in the Phi-
losophical Transactions, as appears from
the following passage : " To prevent
any ambiguity, I must here state, that
by the south pole of a magnet, I under-
stand always the end which, when the
magnet is freely suspended, points to-
wards the north pole of the earth ; so
that the north end is the south pole, and
the south end the north pole of a mag-
netic needle ." It matters little which
set of terms are used, provided they are
clearly denned, and all persons agree to
abide, by these definitions. But where-
ever a diversity of practice exists, it is
then best to adhere to that which most
generally prevails : in the present case
the authorities in favour of the nomen-
clature we have adopted are much the
most numerous.

(106.) Some have attempted to avoid
the confusion which the changes just
mentioned would lead to, by the intro-
duction of the terms Boreal and Austral
instead of north and south : the former
set of terms having reference to the na-
tural magnetism of the earth, the latter
to that of the needle, or artificial mag-
net : that is, they would express what
we have all along called the northern
polarity, by the term Austral polarity ;
and the southern polarity, they would
translate by the expression Boreal po-

* Phil. Trans, for 1730, p. 295.
t Ibid, for 1823, p. 344.

larity. We should not have dwelt upon
this comparatively unimportant topic,
were it not that this change of language
is sanctioned and adopted by Biot and
most of the Continental writers on mag-
netism.

(107.) Assuming it, then, as an hypo-
thesis, that the earth contains in its axis,
or near it, a powerful magnet, let us
note the consequences which follow from
it, and compare them with the facts.
We shall begin with those that relate to
the inductive power of the earth's mag-
net : a power which will be exerted in
the direction which a magnetised needle,
at perfect liberty to move, would assume
in consequence of the action of terrestrial
magnetism; that is, in the direction of the
magnetic position. In this part of the
globe this position is, as we have seen,
not very far from the perpendicular to the
horizon. A bar of unmagnetised iron
placed in the vertical position, or near it,
ought therefore to become magnetic
from the influence of the earth, and
merely in consequence of its position.
Its lower end should exhibit the proper-
ties of a north pole, and its upper end
those of a south pole. All this agrees
perfectly with experience. An iron bar
held nearly upright will be found, at its
upper end, to attract the north pole of
a compass needle, and repel the south

?ole : it is, therefore, itself a south pole,
ts lower end, on the contrary, will at-
tract the south, and repel the north pole
of the compass : and has therefore a
northern polarity. That these properties
of the ends of the bar depend altogether
on the position of the bar itself, is proved
by reversing its position ; when the two
ends will be found to have exchanged
polarities merely by their change of
situation : the upper end being always a
south, and the lower end a north pole.
On the other hand, if the bar be placed
in a position at right angles to the mag-
netic position, (for example, horizontally,
and with the ends directed to the east
and west,) it will not exhibit any cha-
racteristic magnetism.

(108.) The magnetism which a soft
bar of iron derives from its position with
relation to the earth, is, as we have just
seen, of a transitory kind ; immediately
lost on turning the bar so that it makes
a right angle with the magnetic position ;
and again acquired, but with contrary
poles, when its position is reversed. But
this is not the case with harder bars, for,
by remaining for a considerable time in
a vertical position, they are found to ac-

MAGNETISM.

quire a sensible and permanent magne-
tism. This is generally the case with
the stationary iron bars belonging to a
building, and even with pokers and other
fire-irons which have long been kept in
an upright position. This circumstance
will also readily account for the perma-
nent magnetism of that class of iron ores
to which the loadstone belongs. Indeed
it is perhaps not going too far to assert
with Professor ^Robison that all the
magnetism which we observe, whether
in nature or art, is either the immediate
or the remote effect of the magnetism of
the earth.

(109.) All the phenomena which we
have already described as the conse-
quences of induced magnetism proceed-
ing from ordinary magnets, are exem-
plified also in the case of that derived
from the magnetism of the earth. It is
most readily induced, but soonest lost,
in the softest kinds of iron and steel ; it
is slowly acquired, but more permanently
retained in hard-tempered steel. Per-
cussion promotes the change, of what-
ever kind it may be, which the position
of the bar relatively to the earth has a
tendency to produce. Hence we see the
reason why the steel bar described in § 6
became permanently magnetical by
being struck, while in a vertical position,
with a hammer. Mr. Scoresby found
that even a bar of soft iron, held in any
position, except in the plane of the mag-
netic equator, may be rendered magne-
tical by a blow with a hammer or other
hard substance ; and both ends seem to
acquire, by this treatment, an equal de-
gree of magnetism.

On the other hand, an iron bar, pos-
sessing permanent polarity, when placed
any where in a direction at right angles
to the magnetic position, and struck
several times, has its magnetism always
much weakened, and may even be de-
prived of the whole of its magnetism by
a single blow. This affords, indeed, an
excellent method of depriving iron of its
magnetism. Rough treatment of any
kind, such as filing or scouring the
surface of iron, and more especially
bending or twisting it, when in the mag-
netical position, tends to impart to it the
magnetism corresponding with that po-
sition ; or to destroy its previous mag-
netism, if it be subjected to the same
treatment in a position at right angles
to this.

Iron heated to redness, and quenched
in water, in a vertical position, was found
by Mr. Scoresby to become magnetic j

the upper end acquiring the southern, and
the lower end the northern polarity. Hot
iron, according to the same experimen-
talist, receives more magnetism of posi-
tion than the same when cold. An iron
bar is rendered magnetical by passing
an electrical discharge through its axis,
provided it be in a position favourable to
induction by the earth : and the polarity
it acquires corresponds with the effects
of this induction. Electricity appears
to act, in this instance, merely by its
mechanical agency, and independently
of a peculiar influence of another kind
which it possesses, and which will be
the subject of future inquiry.

(110.) Let us now examine how far
Dr. Gilbert's hypothesis corresponds
with the actual phenomena of the varia-
tions of magnetic position in different
parts of the globe. For this purpose, it
will be necessary to revert to what was
explained in a former chapter regarding
the positions which a small needle as-
sumes when under the influence of a
strong magnet in its vicinity, and va-
riously situated with respect to it. These
positions, we have seen, are tangents
to a magnetic curve passing through the
two poles of the great magnet, arid
through the centre of the needle. The
direction of the tangent, which is the
same as that of the dipping-needle,
together with that of a vertical line,
or one perpendicular to the horizon,
will determine the plane of the magnetic
meridian, for it is the plane which in-
cludes both these lines. The compass-
needle, which turns in an horizontal plane
only, will arrive at its position of equi-
librium when it is situated in the plane
of the magnetic meridian, because it
then makes the nearest approach of
which it is susceptible, to the position of
the dipping-needle, which is that towards
which the magnetic influence of the
earth tends constantly to bring it.

(111.) In those parts of the globe
where the dip is very small, the hori-
zontal needle is capable of taking a posi-
tion very nearly approaching to that of
the dipping-needle : hence the terrestrial
magnetism is exerted in bringing it to
this position with very little loss of its
force. This happens in the equatorial
regions of the earth. In high latitudes,
on the contrary, where the dip is great,
the forces which actuate the horizontal
needle, act more obliquely, and there-
fore to great disadvantage : hence the
compass-needle is more feebly impelled ;
the point of rest is less decidedly marked,

28

MAGNETISM.

and the compass traverses slowly. The
absolute intensify of the terrestrial force
is, indeed, greater in the latter case than
in the former ; but the increase is not
sufficient to compensate for the greater
obliquity of its action. If we could
place ourselves exactly over the north
or south magnetic pole of the earth,
the dipping-needle would take a vertical
position, and the horizontal compass
would no longer be sensible to the in-
fluence of terrestrial magnetism, but
would remain at rest in any position in
which it might happen to be placed.

(112.) All these consequences of the
hypothesis which ascribes terrestrial
magnetism to the influence of a magnetic
power in the central regions of the earth,
and of which the direction nearly coin-
cides with its axis of rotation, may be
experimentally illustrated by placing a
strong magnet in the centre of an arti-
ficial globe. The points on the surface
which are opposite to the poles of the
magnet, are to be marked as the terres-
trial magnetic poles. A great circle
being traced equidistant from these
poles, will be the magnetic equator,
dividing the globe into the northern and
southern magnetic hemispheres. Great
circles passing through the poles, and
crossing the equator at right angles,
will be magnetic meridians ; of which
the one which also passes through the
poles of the earth's rotation will t>e the
lines of no variation. Smaller circles
parallel to the magnetic equator, will
indicate situations when the dip is the
same in all. The lines of equal varia-
tion will be curves of particular forms
less easily determinable. The accord-
ance of fact with theory may now be
verified by placing in different situations,
on the surface of a globe so prepared, a
small needle suspended as freely as pos-
sible by a fine thread, which holds it
balanced as nearly as possible at its
centre of gravity, and observing the
positions it assumes in each situation.

(1 13.) But when we come to compare
the regular lines thus traced from theory,
on the supposition of a single central
magnet, with the lines which obser-
vation points out as those indicating
the actual variations of the magnetism
of the earth, we meet with very remark-
able discordances. Many have been the
attempts made to explain the irregula-
rities and anomalies in the course of the
magnetic lines by suppositions of various
kinds. There is reason for believing
that the northern and the southern

magnetic poles do not occupy points on
the globe diametrically opposite to each
other, which would be the case if the
magnetic influence emanated from the
centre of the earth. It has been sup-
posed, in consequence, that the terres-
trial magnet, or centre of magnetic
force, was eccentric. But this suppo-
sition alone will not suffice ; for there
are various indications of the influence
of more than one pole in each hemi-
sphere of the earth ; and the probability
is that these poles are of very unequal
intensities. Other irregularities exist
which appear to owe their existence to
the influence of causes entirely local and
of limited extent, such as might be sup-
posed to be derived from large masses
of iron situated at different depths be-
neath the surface of the earth.

(114.) The observations best calcu-
lated to decide the important question
of the existence of secondary magnetic
poles, appear to be those of the varia-
tions of magnetic intensity, from which
we derive the knowledge of the isodyna-
mic lines already adverted to (§ 102) ; for
these lines will necessarily arrange them-
selves in regular order around the point
or points in each hemisphere when the in-
tensity is greatest, that is around each
respective pole. If these poles were
single, and placed opposite to each other
in the globe, one in the northern and the
other in the southern hemisphere, the
lines of equal intensity would form pa-
rallel circles, analogous to those of geo-
graphic latitude. Captain Sabine re-
marks * that the observations on this
subject, made previously to those of Pro-
fessor Hansteen, appeared to corrobo-
rate such an hypothesis ; for, although
they extended widely over the magnetic
parallels in the northern hemisphere,
namely, from the least almost to the
greatest intensity, yet they were confined,
in respect to longitude, to a space little
more than a quarter of a hemisphere ;
and to that quarter which is immedi-
ately opposite to the countries visited by
Professor Hansteen. Within the space
that had been thus examined, the isody-
namic curves appeared to arrange them-
selves with comparative insignificant
deviations, in parallel circles around a
point situated in the north-eastern part
of Hudson's Bay, and, as nearly as could
be judged, about the intersection of the
sixtieth degree of geographical latitude

t * Quarterly Journal of Science, Sept. 1829, p. 3.

MAGNETISM.

29

with the meridian of 80° west of Green-
wich.

But M. Hansteen was led by a more
careful consideration of the slight ap-
parent deviations which had been no-
ticed, and of the general disposition on
the globe of the lines of dip and varia-
tion, to infer the existence of a second
point of principal magnetic action in
the northern hemisphere. This fact
may now, indeed, be regarded as fully
established by his recent observations ;
the isodynamic curves being found to
arrange themselves systematically round
two poles, the one in Hudson's Bay and
the other in Siberia ; and to be governed
in the courses which they follow, partly
by their distances respectively from
those points, and partly by a disparity
in the absolute attractive force at the
points themselves : the maximum in-
tensity in Siberia appearing to be weaker
than that in Hudson's Bay, and existing
at a point situated in longitude 102° east
of Greenwich, which is as nearly as can
be judged, 180° from the present posi-
tion of the corresponding point in Hud-
son's Bay, and in a latitude somewhat
to the north of 60°, but which, it is to
be hoped, will soon be more particularly
determined.

§ 5. Progi'essiue Changes of Varia-
tion and of Dip.

(115.) The most singular and unac-
countable circumstance relative to ter-
restrial magnetism remains yet to be
noticed ; namely, that it does not remain
constantly the same in the same place,
but undergoes a slow and progressive
change. The variation of the compass
is itself variable, not merely in different
regions of the globe, but at different
periods of time. Thus, the needle in
London, in the beginning of the seven-
teenth century, was inclined a few de-
grees to the eastward of the true north.
In 1659 or 1660, it pointed exactly
north ; or in other words, the variation
was reduced to zero; and, of course,
London was at that time one of the
points of the line of no variation. After
this, the variation became westerly, and
has continued so to the present time.
The line of no variation, therefore, has
been progressively, but slowly moving
in a westerly direction, and has now
passed over to North America.

Similar changes have taken place at
Paris ; but the line of no variation ap-
pears to have passed over that city
rather later than it did over London:

for it was not till the year 1G64 that the
magnetic coincided with the true meri-
dian. In 1814, it was 22° 34' west. In
October, 1829, the variation at Paris
was ascertained, by M. Arago, to be
22° 12' 5" west*.

At London, the westerly variation
continued to increase till the year 1818,
when it amounted to 24° 30.' This ap-
pears to have been its maximum ; for
since that time it has somewhat dimi-
nished, and is at present about 24°.

It appears, from the table given by
Mr. Gilpin f, that the annual change in
the variation has diminished, in each
successive period, since the beginning of
the last century. In the preceding cen-
tury, that is from 1622 to 1692, the an-
nual change was about 1 0' ; from 1 723
to 1773, it was about 8'; from 1787 to
1795, about 5'; from that time to 1802,
only l'.2: in 1818 it was reduced to
zero.

(116.) The dip has also undergone
corresponding changes, though less con-
siderable ones than the variation. In
1680, the dip in London was 73° 30';
in 1723 it was 74° 42': since which time
it has been observed to diminish pro-
gressively, though, as it would seem,
not quite regularly.

Authorities and Localities.

In 1773 it was 72° 19' Dr. Heberden.
1786 „ 72 8} Gilpin, Royal
1805 „ 70 21 J Society'sRooms
Phil. Trans, for
1806, p. 419.

1818 „ 70 34 Capt. Kater, Re-
gent's Park.
1821 „ 70 3 Captain Sabine,

i C his wick.

1828 „ 69 47 " Ditto.
1830 „ 69 38 Capt. Segelcke,

Woolwich.^

On the continent of Europe the dip
has undergone a similar diminution of
late years. The dip at the observatory
at Paris, in the year 1814, was 68° 36',
according to the determination of
M. Bouvard. In June, 1829, it was as-
certained by M. Arago to be 67° 41'. 3.

(1 1 7.) Captain Sabine, by comparing
the present dip with that observed for
the last fifty years, concludes that the

» Annuaire pour 1'An 1830.

i Phil. Trans, for 1806, p. 395.

t For the information relative to the last of these
determinations, we are indebted to the kindness
of Professor Barlow, of Woolwich, who states that
Captain Segelcke, of the Norwegian Navy, and
a friend of Professor Hansteen, employed in tiiis
determination of the dip, the same needle which
the latter had with him in hia recent tour in
Siberia.

30

MAGNETISM.

mean annual diminution is about 3'.
Mr. Barlow finds that these observa-
tions accord much more nearly with
what would take place on the supposi-
tion of a uniform motion of revolution in
the magnetic pole round the pole of the
earth. From the most authentic ob-
servations on the dip and variation
of the needle in London, he calculates
that the longitude of the northern ex-
tremity of the magnetic polar axis
which it obeys was, in 1818, 67° 41'
west, and its latitude 75° 2' north. If
we suppose that the motion of this pole
has been uniform since the year 1660,
when, from the disappearance of varia-
tion, its longitude must have been zero,
and 1 hat it has preserved the same dis-
tance from the terrestrial pole, its an-
nual motion of revolution must have
been about 25'. 4. It would, therefore,
require eight hundred and fifty years
to make an entire revolution of 360°.
Computing from these data, it would
follow that the variation ought to reach
its maximum when the longitude of the
magnetic pole is 70° 23' west. It would
have arrived at this situation about the
year 1 823 ; about which time, as it would
appear, the variation was stationary,
having attained its real maximum, and
having, since that period, actually re-
troceded.

(118.) On calculating what the dip
should be in 1823, according to Mr.
Barlow's hypothesis, a very near agree-
ment with actual observation is found
to take place. It follows, however, from
this hypothesis, that the dip has not an
uniform decrease, but that it is changing
much more rapidly at this present time
than it has ever done since magnetical
observations have been made. Its de-
crease, during the five years preceding
1824, has been nearly half a degree, and
it ought to have diminished to an equal
extent during the following five years.
Mr. Barlow* has computed that in

The Variation should be and the Dip

1828 . . 24° 29' . , 69° 43'
1833 . . 24 26 . . 69 21
The near accordance of these results
with what has actually been observed,
is considered by him as a strong confir-
mation of the truth of his hypothesis.

(119.) It would appear, then, both
from observation and from theory, that
the dip is at present changing more
rapidly than the variation; and the

-P * ?ee his Essay on Magnetic Attractions, 2nd
Edition, p, 218,

theory leads to the expectation that it
will continue to decrease together with
the dip, for about two hundred and
fifty-five years, at the end of which
period, that is in 2085, the longitude of
the magnetic pole will be 180°; the
variation will then be nothing, and the
dip only 56°, which will be its minimum ;
they will then both increase together for
the next two hundred and sixty years,
when the needle will have its greatest
easterly variation, and will then again
return towards the north, the variation
decreasing, but the dip still increasing,
for one hundred and sixty-five years
longer, namely, till about the year 2510,
when the magnetic pole will be again in
the meridian of London ; the variation
will then be zero, and the dip will amount
to 77° 43'. It is to be observed, how-
ever, that Mr. Barlow advances this
merely as an hypothesis, the truth of
which remains to be determined by
future experience.

(120.) A curious hypothesis was ad-
vanced by Dr. H alley, and supported
with some ingenuity, in order to explain
the progressive changes that take place
in the variation of the compass. He
supposes the globe we inhabit to be a
mere external shell, enclosing, towards
its centre, a detached magnetic nucleus,
of a spherical shape, which revolves with
the external shell on a similar axis, with
nearly the same velocity. He supposes
both these spheres to be magnets, having
each two poles ; but the poles of
the one not exactly corresponding in
situation with the poles of the other.
The difference of the periods of rotation
of the two spheres, he conceives to be
exceedingly small, yet sufficient to be-
come sensible after the lapse of years,
and to occasion a change in the relative
situation of the two sets of magnetic
poles ; and hence would arise changes in
the direction of their resulting actions,
and corresponding changes in the varia-
tion of the magnetic needle. However
ingeniously this hypothesis may have
been framed, it was too bold and fanciful
to have been ever generally adopted. Its
author, indeed, has the candour to ac-
knowledge that it is beset with nume-
rous difficulties, which further experience,
extended over a long period of time, can
alone enable us to remove. He con-
cludes his paper in the Philosophical
Transactions in which he has developed
his theory, with the following sentence :
* But whether these magnetical poles
move altogether with one motion, or

MAGNETISM.

31

with several — whether equally or un-
equally — whether circular or libratory ;
if circular, about what centre ; if libra-
tory, after what manner ; are secrets as
yet utterly unknown to mankind, and
are reserved for the industry of future
ages*.'

§ 6. Diurnal Changes of Variation
and Intensity.

(121.) Independently of the changes
already noticed, the position of the mag-
netic needle is liable to certain slight
variations, according to the time of the
day, and also according to the season of
the year. The daily change in the va-
riation was discovered in 1724 by Mr.
George Graham, and has been confirmed
by many subsequent observers. This
change, however, is exceedingly minute,
and requires the most careful observa-
tion, and the most delicate instruments
to render it sensible, even in the hori-
zontal needle ; and it is still more diffi-
cult of detection in the dipping needle,
which does not admit of the same de-
gree of delicacy of suspension.

(122.) Professor Barlow, to whom the
science of magnetism is so much in-
debted for its more recent improvements,
has devised a mode of rendering these
diurnal oscillations much more percep-
tible, by diminishing the ordinary direc-
tive power of the needle, through the
influence of one or two magnets, placed
in such positions with respect to the
needle as to counteract, and thereby
neutralize, as it were, the terrestrial ac-
tion. The effect of the ordinary action
being thus removed, he was led to ex-
pect that the extraordinary cause, what-
ever it might be, which produced the
daily variation, would exhibit its effects
much more perceptibly ; and thus not
only the amount of the changes it pro-
duces, but also the period of their taking
place, and of the maximum of their ope-
ration, might be ascertained with great
precision. These expectations have been
amply realized by the success of his own
experimental researches, and also by
those of Mr. Christie, which are de-
tailed in several papers in the ' Philoso-
phical Transactions t.'

(123.) The general result of the ex-
periments of the latter of these obser-
vers was, that the deviation of the hori-
zontal needle from the mean position was
easterly during the forenoon, and was

* Phil. Trans, for 1683, p. 220.
f For 1823, 1825, and 1827.

of greatest amount at about eight o'clock,
thence returning quickly to its mean po-
sition, which it attained between nine
and ten o'clock, after which it became
westerly ; at first increasing rapidly, so
as to reach its maximum at about one
o'clock in the afternoon, and then slowly
receding during the rest of the day, and
arriving at its mean position by about
ten o'clock at night. The state of the
weather, and more particularly that of the
temperature, had considerable influence
on the nature and extent of the changes.

(124.) Mr. Christie remarks that the
changes which are observed to take place
cannot be explained by a change in the
directions alone of the terrestrial forces,
but that their characters agree, as nearly
as we can possibly expect, with the effects
that would take place from an increase
of intensity at the time that the direction
deviates towards the west.

(125.) The occurrence of diurnal
changes of intensity at Christiana in
Norway have been ascertained by
M. Hansteen ; the same conclusion being
deducible from his observations of the
vibrations of a needle very delicately
suspended ; and also from those of Mr.
Christie, made with a different appara-
tus, and by a totally different method.

(126.) M. Hansteen found that the
minimum intensity occurs about half
past ten o'clock in the morning, that is,
about two hours after the westerly devi-
ation has commenced, and the maximum
intensity at half-past seven in the even-
ing, that is, about the same time after
the return towards the east. Mr. Chris-
tie* found that the terrestrial magnetic
intensity is the least between ten and
eleven o'clock in the morning ; the time,
nearly, he observes, when the sun is on
the magnetic meridian ; that it increases
from this time until nine and ten o'clock
in the evening ; after which it decreases,
and continues decreasing, during the
morning, until it attains its minimum
already stated.

(127.) The general dependence of
these variations of magnetic position on
diurnal changes of temperature is suffici-
ently apparent from the results hitherto
obtained. But the prosecution of the in-
quiry involves considerations of another
kind, connected with a subject we have not
yet touched upon, namely, electro-mag-
netic and thermo-magnetic phenomena.

(128.) The mean diurnal changes of
variation were found by Mr. Canton to

* Philosophical Transactions for 1825, p, 51,

32

MAGNETISM.

differ at different seasons of the year,
being greatest in June and least in De-
cember *, and he has given the results of
his observations in a tabular form. Mr.
Gilpin investigated this subject at a later
period, and gives also tables of the diur-

nal changes of variations in each month,
which he found to be very different in
different years. The following table
contains the results of these different ob-
servations *.

Mean Diurnal Changes of Variation.

Months.

Canton In
1769.

January' 7' 8"

February 8 58

March 11 17

April 12 26

May 13 0

June 13 21

July 13 14

August 12 19

September 11 43

October 10 36

November 8 9

December . . .658

Gilpin

in

1787.

1793.

10'. 2 ..

4'. 3

10 .4 ..

4 .6

15 . ..

8 .5

17 A ..

11 .7

18 .9 ..

10 .4

19 .6

12 .6

19 .6 ..

12.5

19 .4 ..

12.1

15.5 ..

9 .8

14 .3 ..

7 .0

11.1

3 .8

8.3 ..

3 .8

CHAPTER IV.
Theories of Magnetism.

§ 1. Mechanical Theories.

(129.) IN the general view we have
now given of the present state of our
knowledge with regard to magnetism, we
have strictly confined ourselves to the
statement of facts, unmixed with hypo-
thetical speculations as to the nature of
the powers from which they proceed.
We have solely endeavoured to genera-
lize the facts, as far as their nature and
extent would warrant. The result has
been their reduction to a small number,
such as the mutual attractions and re-
pulsions of magnetic iron according to
certain laws, — the induction of these
properties on other iron, — the differences
in the capacities of receiving and of re-
taining these properties, existing in dif-
ferent kinds of ferruginous bodies, — and
the magnetic influence of the globe of
the earth.

(ISO.) But the human mind is so
constituted as to refuse being restrained
within the boundaries of a rigid induc-
tive philosophy. Incited by an irresisti-
ble desire of exploring the secrets of
Nature, it scruples not as to the means
of forcing her to disclose them; and
borne on the wings of imagination and
conjecture, presses forwards with an
eagerness which often betrays it into
courses widely deviating from the truth.
Yet good is often found to result from

* Phil, Trans, for 1759, p. 39S.

these erratic excursions of our faculties :
they infuse fresh interest into the pursuit
of knowledge ; they inspire with the hope
of success ; they invigorate those powers
which must be exerted to attain it. The
spark which kindles a train of light is
sometimes struck out in the conflict of
discordant speculation; and amidst a
multitude of attempts, some effort, more
happy than the rest, elicits an important
discovery. No great or comprehensive
fact in science was ever established,
without being preceded by a bold though
sagacious conjecture. Hypothesis of
some kind or other is invariably the pre-
cursor of truth.

(131.) Magnetism, ever since it occu-
pied the attention of philosophers, has
been a fertile soil for hypothesis. That
a shapeless and unorganized lump of
metal should have the power of drawing
towards itself another equally rude and
unfashioned piece lying at a distance,
or of forcing it to move away, as if both
were alive, and animated by some prin-
ciple of active sympathy ; and that all
this should take place, whatever may be
the number or kind of the intervening
bodies, nay even in the apparent absence
of any connecting medium, are pheno-
mena of too remarkable a nature not to
excite in us a lively curiosity to learn
their cause. No wonder that the an-
cients, who had but imperfect notions of
the real objects of philosophy, were im-
pressed with a vague notion of their
connexion with immaterial agency. Mag-

* Phil. Trans, for 1806, pp. 416, 417.

MAGNETISM.

33

netic attraction was ascribed by Thales
to the secret influence of a species of
mind, or soul, residing in magnets.
This was also the doctrine taught by
Anaxagoras, who extended it to many
other phenomena in nature. Others en-
deavoured to account for the attraction
between the loadstone and iron by the
vague notion of a kind of sympathy ex-
isting between these two bodies. In
later times Cornelius Gemma imagined
that the connexion between them" was
established by what he cads invisible
rays. Cardan asserted that the iron
was attracted because it was of a cold
nature, and Costeo de Lodi, because
iron was the natural food of the magnet.

(132.) But, consigning these wild va-
garies to the oblivion they merit, let us
consider whether the phenomena of
magnetism are capable of being re-
duced to any class of physical actions
with which we are more "familiar. In
accounting for a motion which we see
take place, we have a natural repugnance
to admit of the existence of a power
of action at a distance ; or, in other words,
to conceive that a body can act where
itself is not: and we always incline to
that supposition which implies the mo-
tion to be the effect of impulse. We
naturally ask, agreeably to this prepos-
session, whether the movements of mag-
netic bodies may not be occasioned by
the impulse of some subtile ethereal fluid
impinging on their surfaces ; emanating,
for instance, from one end, and passing
into the other, or circulating in invisible
currents around the magnets from pole
to pole. This fluid might, for instance,
be conceived to emanate from one pole,
to enter at the other, and permeating
the substance of the magnet, again to
issue from its former outlet.

Such was the train of thought that
obviously occurred to those who first
witnessed the arrangement which iron
filings, loosely scattered around a mag-
net, assume in consequence of its
influence. The filings have the ap-
pearance which would be given by a
stream of fluid brushing by them,
and turning each individual filament
in the direction of its course ; which
course might accordingly be easily
traced in the regular and symmetric
curves that are exhibited to the eye.
In the infancy of the science, and in the
absence of any other hypothesis, many
were the speculations advanced as to
the mode in which these supposed
streams of magnetic fluid produced the

observed effects. Descartes was the
foremost among those philosophers who
laboured to account for all the unex-
plained movements in nature by the im-
pulsion of fluids circulating in vortices ;
and he naturally viewed the phenomena
of magnetic action as strongly corrobo-
rating his system. Euler also, who
sought to explain various natural ap-
pearances by the intervention of an
ethereal fluid, did not fail to apply his
favourite hypothesis to the elucidation
of magnetism. He even went so far as
to imagine the possibility of there exist-
ing in the substance of iron numerous
canals, through which the ether circu-
lated, furnished with valves which regu-
lated the direction in which it moved.
It was not until the phenomena had
been examined with greater care, and
were rigorously subjected to the in-
ductive process, that juster notions of
the nature of the magnetic forces came
to be entertained. With the knowledge
we now possess of the actual law of
magnetic attraction and repulsion, it
must be at once perceived that all hypo-
theses founded on the impulse of a fluid
in motion, are irreconcileable with that
law, and must therefore be totally dis-
carded.

§ 2. Theory of Mpinus.

(133.) The obvious analogy which pre-
sents itself between the phenomena of
magnetism and those of electricity, na-
turally suggested the probability that
the same mode of explanation might
apply to both, and laid the foundation
of the first rational theory of magnet-
ism. While ^Epinus was intent upon
improving the beautiful electrical theory
of Franklin, he was struck with the re-
markable similarity in the attractions
and repulsions exhibited by the tourma-
line, when it is heated, to those of mag-
netic bodies ; and it occurred to him
that the phenomena of magnetism might
be derived from the agency of a peculiar
fluid, having properties very similar to
those of the electric fluid, but which
acted exclusively upon iron. The prin-
cipal difference between the two sets of
phenomena was, that, in the case 'of
electricity, the agent, whatever be its
nature, is actually transferred from one
body to another; but in magnetism
there is merely induction, but never any
transference. In as far, however, as
respects mere attraction, repulsion, and
induction, electricity and magnetism pre-
sent phenomena that are perfectly pa-
D

34

MAGNETISM.

rallel to one another. Franklin's inge-
nious theory of the two modes of elec-
tric agency, the one consisting in an
excess, the other in a deficiency of fluid,
and the happy explanation it afforded
of the opposite electrical states resulting
from induced electricity, and its accu-
mulation in the Leyden phial, were ap-
plied with great ingenuity by JEpinus to
the contrariety of magnetic polarities in
the opposite ends of a magnet, and the
induction of similar magnetic states in
an unmagnetized bar of iron. His sys-
tem of magnetism, when digested into a
series of propositions, may be stated as
follows : —

(134.) 1. There exists in all bodies
capable of acquiring magnetic proper-
ties, a subtile fluid, which may be called
the magnetic fluid.

2. The particles of this fluid repel one
another with a force which decreases as
the distance increases.

3. The particles of the magnetic fluid
attract, and are attracted by the parti-
cles of iron, with a force varying accord-
ing to the same law.

4. The particles of iron repel one
another according to the same law.

5. The magnetic fluid is ijicapable of
quitting the body in which it is contained,
but it is capable of moving within the
substance of pure iron and of soft steel
without any considerable obstruction.
It is more and more impeded in its mo-
tion as the steel is tempered harder;
and in very hard tempered steel, and in
some of the ores of iron, it moves with
the greatest difficulty.

(135.) In order to judge of the degree
in which the theory is qualified to re-
present the facts, we must study the se-
veral consequences which flow from the
above suppositions, and then compare
them with the actual phenomena which
are presented to our observation.

(136.) Each particle of iron, by the
hypothesis, attracts a particle of mag-
netic fluid, placed at any particular dis-
tance, with a certain force. We may
conceive that magnetic fluid is gradually
added to the particle, until the quantity
thus added is such that the force of re-
pulsion which the fluid exerts upon any
distant particle of magnetic fluid, ex-
actly balances the attractive force of
the iron for that same particle. This
quantity may be regarded as the natural
quantity of fluid belonging to that par-
ticle of iron. According to this definition,
therefore, a mass of iron, all the par-
ticles of which contain their natural

quantity of fluid, must be neutral with
regard to its action on any other par-
ticle of fluid, and also on any other par-
ticle of iron. Such is the condition of
unmagnetical iron or steel. Its mag-
netism is neutral, or in a state of equi-
librium.

(137.) But should, from any cause,
this state of equilibrium be destroyed,
and magnetic fluid be either accumu-
lated beyond its natural quantity as re-
lates to the iron, or reduced below that
proportion, the part where this excess
or this deficiency exists becomes active
— that is, acquires the properties of
either a north or a south pole. As the
fluid can never pass beyond the surface
of the mass of iron in which it is con-
tained, the total quantity residing in that
mass must remain precisely the same,
whatever be its mode of distribution;
and therefore the excess of fluid in those
parts where it is accumulated or re-
dundant, must be exactly compensated
by the redundant iron, if we may so ex-
press it, in those parts where the fluid
is deficient. In all cases it will be only
the redundant fluid or the redundant
iron that constitutes the active parts of
the magnet.

(138.) It follows as a direct conse-
quence of the second condition of the
hypothesis, that the pole of one magnet
in which the fluid is redundant will repel
the pole of another magnet in which it
is also redundant ; because the fluid in
each is mutually repulsive of the other.

(139.) From the third condition of
the same hypothesis, it likewise follows
that the pole having an excess of fluid,
or the overcharged pole, as we may
call it, of one magnet, will attract and
be attracted by the pole in which the
fluid is deficient, or the undercharged
pole of the other ; and this action must
be reciprocal.

(140.) It is also a necessary conse-
quence of the fourth condition that the
redundant iron in the undercharged pole
of one magnet repels every similarly con-
stituted pole in other magnets ; because,
by the hypothesis, iron repels iron.

(141.) Hence we deduce the general
law that similar poles repel, and dissi-
milar poles attract one another, — a law
identical with that we have already de-
duced from experiment.

(142.) Let us next see what account
the theory gives us of the induction of
magnetism. If the overcharged pole of
a magnet be brought near the end of a
bar of iron in its natural or unmagneti-

MAGNETISM.

35

cal state, the redundant fluid in the
former, exerting a repulsive influence on
the fluid at the nearest end of the latter,
will give it a tendency to move towards
the remote end. It will obey this ten-
dency, provided the texture of the iron
offers no obstruction to its transmis-
sion ; and a certain portion of the fluid
will accordingly be transferred from the
near to the remote end. The bar will
now exhibit magnetic properties; its
near end being undercharged, will pos-
sess a polarity of an opposite nature to
that of the magnetic pole presented to
it ; the remote end, being overcharged,
will have a polarity of the same kind as
the pole of the magnet.

(143.) A series of changes exactly the
converse of these will take place when,
instead of the overcharged pole, we
present the undercharged pole of the
magnet to the bar. The redundant iron
now attracts the magnetic fluid of the
bar, and draws it towards the adjacent
end, converting it into an overcharged
pole, while the other end, from which
the fluid has been drawn, becomes the
undercharged pole.

(144.) The effects, however, do not
end here. The bar, thus rendered mag-
netic, reacts upon^ the magnet from
which it had derived its power, and tends
to increase the magnetism it originally
possessed. This increased magnetism,
in its turn, tends to produce an augmen-
tation of the induced magnetism of the
bar ; and these alternate actions and re-
actions proceed till all action is balanced
and every thing remains quiescent. In
soft iron this is accomplished almost in
a moment : but in steel the process is
somewhat different ; for its texture
presenting great impediment to the mo-
tion of the magnetic fluid, the changes
of distribution take place much more
slowiy, and to a less extent than they do
in iron. The adjacent end of a steel bar
soon acquires a degree of polarity oppo-
site to that of the end of the magnet
presented to it ; but the polarity of the
same kind travels slowly onwards, and
does not reach the other extremity of
the bar till after a considerable time ;
and if the bar be very long, may possibly
never reach it.

In this last case, we have a curious
phenomenon produced from the influ-
ence of a secondary induction ; namely,
the appearance of a second set of poles,
at a certain distance from the first. Thus,
if a north pole has been presented, the
adjacent end of the bar will be a south

pole ; at a little distance from this we
shall have a north pole; beyond this
again will appear another south pole,
and perhaps at the furthest end a se-
cond north pole. Sometimes, indeed,
there will be only three poles, the middle
one being of an opposite character to
those at the two ends, which are similar
to one another.

(145.) Let us now remove the mag-
net; what will happen to the iron bar?
The cause which maintained the mag-
netic fluid in the forced state of excess
at one end, and of deficiency at the
other, no longer operating, the fluid will
now tend to resume its original state of
uniform distribution over the whole mass
of iron ; and if no obstacle exist in the
structure of the iron to impede its mo-
tion, it will immediately revert to that
state. But if the bar be of steel, which
presents obstacles to the passage of the
fluid, which the force derived from its
tendency to equable diffusion is insuffi-
cient to overcome, the fluid which had
passed will remain stationary, and the
induced magnetism will continue as at
first — that is to say, the bar will have
been converted into a permanent magnet.

(146.) On the same principle may be
explained the effect of hammering, or
any other kind of mechanical concus
sion, in impairing the magnetism of a
steel bar; for the tremulous motions
excited among the particles will open a
passage for the fluid, which will thus
escape from the situations where it is
condensed, and return to those where it
is rarefied.

(147.) Heat, as we have seen, weak-
ens and finally destroys magnetic
power ; its operation may in like man-
ner be understood, by its occasioning
the separation of the particles of iron to
a greater distance than before. Hence
the interstices will be enlarged, and the
obstacles to the motion of the fluid will
be diminished, or even entirely removed.
The magnetic fluid will thus be enabled
to regain its natural state of uniform
diffusion among the particles. But in-
dependently of its mechanical operation,
there are yet many other ways in which
heat may be conceived to contribute to
the destruction of magnetism. It may
change the action of the particles of
iron on those of the fluid, or of the fluid
on each other, and by altering the dis-
tribution of the fluid with respect to the
particles of iron, may greatly affect the
law of action between one magnet and
another.

D 2

36

MAGNETISM.

§ 3. Correction of jfipinus's Theory.

(148.) Thus far do the facts accord with
the hypothesis of ^Epinus, and thus far
may we admit that hypothesis to be a sa-
tisfactory explanation of these facts. But
in one important application it entirely
fails; it does not explain the consequences
that are observed to follow on the division
of a magnet at the neutral point. Theory
would lead us to expect that we should,
in this case, obtain the different polari-
ties separate, one in one piece, and the
other in the other. The fact we know to
be totally different: each part becomes
a regular magnet with two poles, one of
which retains the character it had before
the separation. '

,/Epinus attempted to remove this dif-
ficulty by supposing that, in the act of
fracture, a portion of the fluid actu-
ally escaped from the overcharged pole,
while another portion entered into that
which was undercharged, — effects which
he conceived might result from the
sudden change in the balance of mag-
netic forces consequent upon the frac-
ture. But this explanation, as Professor
Robison remarks, is far from satisfac-
tory.

(149.) The only rational mode of re-
conciling this fact with the system of
yEpinus, is to consider a magnet as an
aggregate of small particles of iron, each
of which individually has the properties
of a separate magnet ; that is, has two
poles of its own: the arrangement of
these particles being such that all the
poles are disposed in a regular order of
alternation ; so that in every part of the
mass of iron, each pole of one particle is
in contact with the contrary pole of the
next in the series. These adjacent poles
of course neutralize one another, with
regard to their magnetic action, and it
is only those which are situated at the
extremity of the line, and which are not
associated with any other, that consti-
tute the active poles of the entire mag-
net. Hence it is at the surface, and
more particularly at the extremities,
that polarity is manifested; and hence
when a magnet is broken across, the
fractured ends at once exhibit the oppo-
site polarities they had before possessed,
but which had been masked by their
cohesion.

(150.) A practical illustration of this
view of the subject may be afforded by
placing a number of small magnets of
equal strength in a line, with their oppo-
site poles in contact, as exhibited in fig.

41. It will be found that almost the
only polarity that^is sensible, appears

Fig. 41.

Stl, _ t ^^_

at the two extremities N and S ; the
intermediate portions formed by the
junction of the opposite poles n and s
being, to all appearance, neutral. If the
series be broken at any one point as at
F, the two portions G and H will imme-
diately present the properties of separate
magnets, and the new poles N1 and S1
being now separated, exhibit their na-
tural activity.

(151.) According to this view of the
subject, the induction of magnetism will
consist, not in the actual transference of
the magnetic fluid from one extremity
to the other of the iron bar which has
been rendered magnetic ; but in a
change of this nature taking place in
every particle individually, and by which
each particle is converted into a separate
magnet.

§ 4. Theory of two Magnetic Fluids.

(152.) The theory with respect to
magnetism which has of late more gene-
rally prevailed, is founded on the sup-
position, that its phenomena are occa-
sioned by the agency of two magnetic
fluids, residing in the particles of iron,
and incapable of quitting them ; one of
which fluids imparts the northern and
the other the southern polarity. They
have been denominated respectively
the Austral and Boreal fluids. The
particles of each of these two kinds of
fluids attract those of the other, but
repel those of the same kind. When in
combination with each other, these
fluids are neutral and inert; each be-
coming active only when separate. The
decomposition of the united fluids is
effected by the inductive influence of
either the one or the other when acting
independently. It is obvious that, as
far as regards the distribution and ac-
tion of the two magnetic fluids in each
individual particle, "this theory is pre-
cisely similar to that of the two electric
fluids, of which an account has been
already given in our Treatise on Elec-
tricity; it is therefore unnecessary to
pursue its development in those particu-
lars, for the reader need only refer to

MAGNETISM.

37

that treatise, and substituting the terms
austral and boreal fluids for those of vi-
treous and resinous electricities, will
find that all the- details of the electrical
theory will apply to that of magnetism.
(153.) But however the laws of the
theory of a double magnetic fluid may
be analogous or identical with those of
the theory of a double electrical fluid,
their application is somewhat different
in the two cases, in consequence of the
difference of circumstances under which
they act. The electrical fluids, when
decomposed or separated from each
other, are capable of being extensively
transferred along the particles of bodies,
and of being collected and accumulated
at the surfaces, where they have a
tendency to escape ; and where, if that
tendency exceed a certain limit, they ac-
tually do escape, either passing into the
bodies in immediate contact, or flying
off through the air to distant bodies.
In this manner each kind of fluid may
be separately, and in any quantity, trans-
ferred from one body to another. No-
thing of this kind takes place with
regard to magnetism ; the magnetic
fluids are never found to quit the bodies
to which they are attached, however small
those bodies, however intimate the con-
tact with other iron, however long the
contact may be continued, and however
powerful the forces by which the fluids
are impelled. The phenomena conse-
quent on the division or fracture of a
magnet lead us also to the conclusion
that no sensible quantity of either austral
or boreal fluid is ever transported from
one part to another of the same piece of
iron or steel. Hence, in order to accom-
modate the theory to these facts, we
must introduce as a new condition of
the hypothesis on which it is founded,
that within the substance of a mag-
netized body, the two magnetic fluids,
when they are decomposed by the in-
fluence of magnetizing forces, undergo
displacements to an insensible distance
only.

(154.) It is not necessary to deter-
mine whether the extremely small
spaces, within which these displacements
and motions of the magnetic fluids are
restricted, be actually the same as the
spaces occupied by the constituent mole-
cules of the iron ; it is sufficient for the
purposes both of theory and of the cal-
culations founded upon that theory, that
they be extremely small in comparison
with the whole volume of the body, or
even with the smallest dimensions that

ever come under the cognizance of our
senses. Poisson, who has given us a
beautiful developement of this theory*,
designates these very minute spaces or
portions of a magnetic body by the name
of the magnetic elements of that body.
There is also no necessity for making
any particular supposition with regard
to" the form or respective disposition of
these elements, provided we simply con-
sider them as insulated from each other
by intervals impermeable to either of
the magnetic fluids.

(155.) The quantities of each kind of
fluid contained in every magnetic ele-
ment must be considered, with reference
to all our experiments, and to all the
powers we can apply, as without limit ;
that is to say, the forces we can com-
mand, in any magnetizing process, are
never sufficient to exhaust or separate
the whole of the fluids. For, when a
body is magnetized by the inductive in-
fluence of a neighbouring magnet, the
intensity of its magnetic state, as shown
by its effects, increases without limit, in
proportion as we employ a magnet of
greater force ; which, of course, implies
that we have not yet effected the decom-
position or separation of the whole quan-
tity of the neutral or combined fluid
which that body contains. In like man-
ner, we find it impossible to separate
completely the two electric fluids con-
tained in any particular body.

(156.) Besides the obstacles, which
appear to be insuperable, to the trans-
mission of the magnetic fluids from
one magnetic element to another, there
must exist, in the substance of certain
bodies, some impediment of another
kind, which obstructs the motion of the
fluids from one part to another of the
same magnetic element. The effect of
this power, which is somewhat analogous
to the force of friction, is to arrest the
particles of both fluids in the situations
which they occupy; and thus to op-
pose, in the first place, the separa-
tion of these fluids, and, in the next,
their return to the situations from
which they had been displaced, and
where they would unite to recompose a
neutral, fluid. This force is termed, by
Poisson, the coercive force. In soft iron
the coercive force is either wanting, or
is extremely feeble ; in steel and in the
loadstone it is very energetic; and it
exists in various degrees of intensity in

* Memoires de 1'Iustitut de France, tome v., p.
247. The introduction to this memoir is given in
the Annales de Chimie, tome xxv., p. 113.

38

MAGNETISM.

different kinds of steel. This coercive
force, which exists in iron with regard
to the. magnetic fluids, is analogous in
all respects to the resistance which
glass, resin, and other non-conducting
bodies, present to the passage of the
electric fluids through their substance.
Having thus established the founda-
tions of the hypothesis of a double mag-
netic fluid adopted to the particular cir-
cumstances of the case, we must next en-
deavour to ascertain precisely the distri-
bution of the austral and boreal fluids in
magnetized bodies, conformably with
these principles ; and afterwards exa-
mine the nature of their combined ac-
tions upon bodies at a distance.

(157.) For this purpose, we may first
take the simpler case of a cylindrical
needle of soft iron, of very small dia-
meter, and of any given length, as re-
presenting an elementary longitudinal
filament of that metal. In the natural
state of the needle, the two fluids it
contains are united in equal proportions
throughout its substance, so that their
actions, being equal and opposite at
all distances, totally destroy each
other, and no sign of magnetism is ex-
hibited. If we next suppose these fluids
to be subjected to the action of mag-
netizing forces, proceeding from one or
more centres, situated in the line of the
axis of the needle produced, these
forces will now cause the fluids to sepa-
rate from each other ; but each particle
of austral or boreal fluid can, by the hy-
pothesis, move only a very short distance
from its primitive situation; and the
two fluids, in their new arrangement,
will succeed each other alternately,
throughout the length of the needle,
which will, accordingly, be divided into
very small portions, composing a series,
each part of which will contain, as it did
in the neutral state, the two fluids in
equal quantities. The united actions of
every particle of decomposed fluid in
this series upon a particle of magnetic
fluid in any particular situation, compose
a resultant force, the intensity and di-
rection of which remain to be deter-
mined by the application of mathemati-
cal analysis.

(158.) We may now proceed to con-
sider the more complicated case of a
magnetized body of indeterminate form
and dimensions. Attention must here be
paid to the lines or directions in which
the separation of the two fluids takes
place throughout its substance, and
in which they are arranged alternately,

as we have just seen exemplified in the
case of the simple filament. These
lines or filaments will, in general, be
curved ; the nature of the curvatures
depending on the form of the body, and
on the external forces which act on the
two fluids. They are termed by Poisson
lines of magnetization, and may be con-
sidered as constituted by series of mag-
netic elements, following one another in
the same regular consecutive order of po-
lar arrangement. We have to determine,
then, for each point of the body which
is the subject of investigation, the direc-
tion of the line of magnetization, which
is also the line of polarity ; and the ac-
tion of the magnetic element on any
other point given in position, either
within or without the body. This action
is the difference of the forces exerted by
the two fluids contained in the element,
arising from the slight separation of
the austral and boreal particles, which
constitutes the state of polarity. It may
excite surprise that forces depending on
such small differential distances as
those of the two centres of austral and
boreal forces in each magnetical element,
should, nevertheless, be capable of pro-
ducing mechanical effects so conside-
rable as those exhibited by the magnetic
attractions and repulsions of bodies. By
applying to the subject the methods of
analytical investigation, Poisson arrived
at the conclusion, that the result of the
action of all the magnetical elements of
a magnetized body is a force equivalent
to the action of a very thin stratum, co-
vering the whole surface of the body,
and formed of the two fluids, the austral
and the boreal, occupying different parts
of it. We have a similar instance in the
case of electrical attractions and repul-
sions of mechanical effects, sometimes
very powerful, being produced by strata
of fluids collected at the surfaces of con-
ductors, and having a thickness so ex-
ceedingly minute as to be inappreciable
by any of our senses. As these observed
effects of the two magnetic agents result
only from the differences of two con-
trary powers, we can form no estimate
of the real magnitude of the forces be-
longing to each separate power, that is,
to each of the two portions of austral or
boreal fluid belonging to the same mag-
netic element ; but can only infer that
they are incomparably greater than the
resulting forces which are actually in
operation, and of which we witness the
effects.

(159.) In the memoir on the Theory

MAGNETISM.

39

of Magnetism, already referred to, M.
Poisson deduces, from the theory above
stated, the analytical equations which
express, for all possible cases, the laws
of the distribution of magnetism within
bodies that are rendered magnet ical by
induction, and those of the actions, whe-
ther attractive or repulsive, which they
exert on points given in position. The
first problem to be resolved is to reduce
the resultants of all the attractions and
repulsions of the magnetic elements of
a magnetized body, of any imaginable
form, "on such points, situated either
within or without the surface, to three
directions at right angles to one another.
By adding to the resultants which relate
to any interior point those of the ex-
ternal magnetic forces which act upon
the body, he obtains the whole forces
which tend to separate the two fluids
that are united at that particular point.
Were the matter of the body to oppose
no sensible degree of resistance to the
displacement of the fluids in each mag-
netic element, or, in other words, if there
were no coercive force, it would be ne-
cessary, in order that there might be an
equilibrium, that the attractions and re-
pulsions should destroy one another ; or,
speaking algebraically, that their sum
should be equal to zero ; since, if any of
them were uncompensated, they would
effect a new decomposition of the neutral
fluid, which may be regarded as inex-
haustible, and the magnetic state of
the body would be altered. The sum of
the resultants must, therefore, be made
equal to zero, with respect to each of the
three rectangular directions to which
they are referred. The equations of
equilibrium, thus formed, will always be
possible, and they will serve to deter-
mine, for each point of a magnetized
body, the three unknown quantities
which they comprehend; namely, the
intensity of the action of a magnetic ele-
ment on a given point, and the two an-
gles which determine the corresponding
direction of the line of polarity. At the
extremities of each element, these joint
resultants will not vanish ; they will
give rise to pressures from within each
element, tending outwards, and counter-
balanced by the obstacle, of which the
nature is unknown, but which opposes
the passage of the fluid from one ele-
ment to another, and also its escape from
the surface.

(160.) When the coercive force of
the magnetized body is also taken into

account, it will then be sufficient for the
magnetic equilibrium that the resultant
of all the exterior and interior forces,
acting upon any point of the body, no-
where exceeds the given magnitude of
the coercive force : so that, in this case,
the equilibrium may take place in an
infinitude of different ways, and the
problem is, in this respect, wholly inde-
terminate. This indeterminateness is a
source of considerable difficulty in the
resolution of questions of this nature.
The following general consequence,
however, may be deduced from the
equations of magnetic equilibrium
formed in the manner above described ;
namely, that although in a solid body,
magnetized by induction, the austral
and boreal fluids are distributed in an
active state throughout the whole mass
of that body, yet the attractions and re-
pulsions which it exerts externally are
precisely the same as if they proceeded
from a very thin stratum of each fluid,
occupying the surface only, both fluids
being in equal quantities, and distributed
in such a manner as that their total ac-
tion upon all the points in the interior
of the body is equal to nothing. If the
body be hollow, or contain an empty
space within it, and if the centres from
which magnetic forces proceed be situ-
ated within this space, the body must
be considered as terminated by two thin
strata of fluid, situated, the one on the
external, and the other on the internal
surface; and the action of these two
strata on any point, within the substance
of the body, joined to that of all the
given centres of magnetic action, must
produce a perfect equilibrium; and, in
this case, the two fluids may be in diffe-
rent quantities in each of the thin strata,
provided that they be always in equal
quantities in the two surfaces taken
together.

(161.) Thus it appears, that the
theory of magnetic attractions and re-
pulsions is reduced to the same princi-
ples, and leads to the same formulae, as
the theory of electric forces in conduct-
ing bodies ; and the perfect correspond-
ence between the two may be illustrated
in the following manner. We may sup-
pose an aggregate mass composed of
minute grains of metal, or other con-
ductor of electricity, each grain being of
so small a size that its dimensions may
be neglected in comparison with the
whole mass, and each being surrounded
by a substance impermeable to^electri-

40

MAGNETISM.

city, but not sensibly adding to its bulk.
On bringing a body thus constituted near
an electrified body, every one of the
grains would immediately become elec-
trical by induction ; and, in this condition
of the body, it has been mathematically
proved that the attractions and repulsions
which the body would exert externally,
would be the same with those of a ho-
mogeneous conducting body of the same
form and size, subjected to the same ex-
ternal forces : although, in the latter
case, the two electric fluids would be
transferred to the opposite extremities
of the body, while, in the former, they
would be obliged to remain in the con-
stituent masses to which they originally
belonged. An electrical body, consti-
tuted in the manner here supposed, pre-
sents us with a disposition exactly analo-
gous to that of a magnetical body ; and
is therefore calculated to give us a very
distinct idea of the distribution of the
magnetic fluids when that body is mag-
netized. The electricity inherent in the
tourmaline appears to be disposed in the
manner above described ; and this stone
accordingly affords an excellent illus-
tration of the hypothesis under our con-
sideration. See Electricity, $ 197.

(162.) Another general consequence
of the theory is, that a magnetic needle,
placed in the interior of a hollow sphere
of soft iron, and so small as not to exert
any sensible influence on the sphere, will
not be subject to any magnetic action
from a magnetic force proceeding from
a point external to the sphere ; or, in
other words, all magnetic action, whe-
ther of the earth or of any number of
magnets placed without the hollow
sphere, will be completely intercepted by
the sphere with reference to all magnetic
bodies contained in its interior. And
conversely, such hollow sphere will
totally prevent the action of a magnet
placed within it from being exerted on
any body placed without the sphere.

(163.) The formulae derived from this
theory have been applied by Poisson to
another case, which, as we shall after-
wards find, is one of considerable prac-
tical importance in navigation, namely,
that of a hollow sphere of iron, magnet-
ized by the influence of the earth, that
is, by the action of a force of which the
origin is very remote, and which may,
therefore, be considered as uniform in
magnitude, and acting in parallel direc-
tions on all the points of the body in
question. From the resolution of the
equations of magnetic equilibrium ob-

tained in this case, it appears that al-
though the magnetism is by no means
confined to the superficial strata of
the sphere, and although its intensity
may be determined for any particular
point of the solid mass of the shell, yet the
magnitude of the three component forces
produced by it is wholly independent of
the thickness of the shell, and is deter-
mined only by the radius of the external
surface, and by the co-ordinates belong-
ing to the position of the point on which
the forces act. When the distance of
this point from the centre of the sphere
is very great, compared with the radius,
each of the three forces is very nearly as
the cube of the radius directly, and as
the cube of the distance inversely. We
shall have occasion, in a future chapter,
to notice the remarkable coincidence of
the results of observation with the de-
ductions from this theory, affording a
very important confirmation of the ac-
curacy both of the analysis itself, and of
the theory from which it is derived.

(164.) In a subsequent memoir*, M.
Poisson extends his researches so as to
obtain a more diversified comparison of
the theory with the phenomena; and
with this view resolves the general equa-
tions he had before established, in the
case of bodies having forms less simple
than that of the sphere. Such a resolu-
tion, however, is attainable only in a very
limited number of cases, of which the
elliptic spheroid is an example. After
giving the formulae relating to a spheroid
of which the axes have any imaginable
relations to each other, he particularly
considers the two opposite cases of
spheroids extremely flattened and ex-
tremely elongated. The former may re-
present a plate, of which the thickness
varies very slowly near the centre, and
decreases from the point to the circum-
ference ; for its action on points near its
centre must be sensibly the same as that
of any other plate of uniform thickness
and of very great extent. The latter, or
the extremely elongated spheroid, ap-
proaches very nearly to the form of a
needle or bar, of which the diameter
decreases from the middle to the extre-
mities, varying at first very slowly ; and
its action on points near its middle can
differ but little from that of a bar of
which the diameter is constant, and very
small in proportion to its length. The
consideration of these three cases, which
readily admit of the application of the
analytical formulae, is of considerable

* Memoires de 1'Jnstitut, tome v., p. 481.

MAGNETISM.

41

importance, as they allow of a strict
comparison of the results of experiment
with the deductions from theory ; and
the accurate accordance which has been
obtained between them, in every instance
in which such a comparison has been
instituted, affords the strongest evidence
of the correctness of the views on which
that theory is founded.

(165.) It has already been observed
that we have no data for determining
the question as to the size of the mag-
netic elements, compared with that of
the constituent molecules : we know not
whether they are coincident with these
molecules, or whether they occupy only
the interstices between the molecules :
neither can we determine whether they
do not actually comprehend certain de-
finite aggregates of molecules, or whe-
ther they are constituted in the intervals
of these aggregates. All that we can
be certain of is, that the sum of all the
magpetic elements, added to the sum of
all the unmagnetic elements (that is, the
spaces, whether occupied by matter or
not, which are devoid of magnetic fluid),
must together make up the total appa-
rent volume of the body under conside-
ration. Now the ratio between these
two sums may vary, not only in different
kinds of bodies, but also in the same
body, in different circumstances. It
may, for instance, be very materially af-
fected by changes of temperature : and
this consideration will probably furnish
a key to the explanation of many of the
anomalous appearances we have already
had occasion to notice in $ 49.

(166.) The hypothesis of two mag-
netic fluids was first propounded by
Wilke and Brugmann ; but the first real
foundations of the theory were laid by
Coulomb, who, by the exercise of sin-
gular perseverance and sagacity, pre-
pared and established all the physical
principles upon which it rests. It has
recently occupied the attention of Pois-
son, and appears to have received its
last finish from his masterly hand ; for
by applying to it the refinements of mo-
dern analysis, this distinguished mathe-
matician has succeeded in discovering
formulae which represent, numerically,
all the principal phenomena of the sci-
ence, even in their minutest details, and
which furnish us with a ready and con-
sistent explanation of the physical mode
by which they are produced.*

* A popular view of this theory is given by Biot,
in a note to his French translation of Fischer's
Physique Mechanique, 4th edition, page 342.

(167.) Professor Prevost, of Geneva*,
has laboured to frame a theory of mag-
netism which shall dispense with all at-
tractive or repulsive agencies, and in
which all the phenomena shall be re-
solvable into the effects of impulsion.
For this purpose he admits two magnetic
fluids, each giving its respective polarity
to the two ends of a magnet, and neu-
tralizing each other by combination ; but
adopting the hypothesis of Le Sage, as
to the existence of another infinitely
more subtile fluid, pervading all space,
and giving rise by its inconceivably
rapid movements to all the phenomena
of gravitation, cohesion, and chemical
attraction, he supposes the magnetic
fluids themselves to be set in motion by
this primary and universal agent. But
it would be impossible in this place to
engage in the development of so abstruse
and complicated a system as this.

CHAPTER V.

Methods of making Artificial Magnets.
$ 1. General Principles.

(168.) THE art of communicating mag-
netic power to bodies capable of retaining
it, is founded on the proper application of
the principles already explained ; and
the practical results of experience in this
art have, as might be expected, furnished
some of the most interesting illustra-
tions of the theory of magnetism. We
have seen that acquired magnetism of
every kind, whether temporary or per-
manent, of which the origin can be
traced, has been derived, by induction,
from a similar power already existing in
some other body. In this respect, then,
it differs from electricity, which may be
elicited from bodies all of which were
previously in a neutral state, by a variety
of processes either of a mechanical or
chemical nature. But the body which
is the cause of magnetism in another
body must itself be in an active state of
magnetism, and may be either a magnet,
whether natural or artificial, or else it
must be the globe of the earth itself: it
is therefore highly probable, that the
magnetism of the earth is the original
source of all other magnetism. This
view of the subject excludes, of course,
all consideration of electro-magnetic in-
fluence,which belongs to another division
of the science, hereafter to be treated of.
It will then be shewn that electricity in

* Surl'Origine des Forces Magnetiques. 8vo.
Geneve, 1788.

42

MAGNETISM.

motion is a source of magnetism, and
that strong grounds exist for the belief,
that even the magnetism of the earth has
its origin in electric currents circulating
round the equatorial regions.

In giving an account of the methods
of procuring artificial magnets, we shall
begin with those which depend solely on
this source of magnetic power, and which
would enable us to obtain them if we
were unprovided with any instrument
previously magnetized.

(169.) The success of every plan that
can be put in practice for obtaining
magnets must depend on two circum-
stances : first, the efficacy of the induc-
tion ; and, secondly, the fixation of the
magnetism that has been induced. That
quality and temperament of steel which
is most favourable to the former, is least
favourable to the latter of these objects ;
but various methods may be devised
which shall answer both these intentions.
The particular purposes intended to be
answered by the magnetic instrument we
are constructing, will frequently deter-
mine our preference of one or other of
these methods, as well as guide us in the
choice of the material to be used, and of
the form and dimensions to be given
to it.

(170.) Magnetism is most readily com-
municated to an unmagnetic bar of iron
or steel by means of certain combina-
tions of steel bars already magnetized
to saturation, which combinations may
be regarded as highly-charged magnetic
batteries ready for action, and capable
of exerting a powerful influence, in in-
ducing magnetism, on all the iron in
their vicinity. But an apparatus of this
kind cannot at all times be commanded,
nor can it at once be constructed : it
must be the result of a long preliminary
process, of which the object is to impart
to each single bar additional quantities
of magnetism, until it has acquired, by
gradual steps, the full measure it is
capable of retaining. We shall first,
then, point out the methods of mag-
netizing those bars which are to com-
pose the apparatus or battery just men-
tioned.

§ 2. Method by Percussion.

(171.) The most Advantageous form
for the steel bars thai are to be employed
for this purpose, is taat of a rectangular
prism, of which the length is about ten
times the breadth, and about twenty
times the thickness. Six or eight bars
of this kind, and tof equal size in every

respect, should be provided. We have
already seen that a certain degree of
magnetism may be given to each of these
bars by a few blows with a hammer,
while they are held in a vertical position
($ 6). This effect results, as we have also
seen (§ 107), from the direct inductive
power of the earth. But the efficacy of
this power will be very considerably in-
creased, if it be combined with the in-
ductive influence of other masses of iron
placed near it, or in contact with it: not-
withstanding the iron itself, which thus
adds to the effect, derives its own power
from the same source, namely, the mag-
netism of the earth. Thus, Mr. Scoresby
found that a steel bar which acquired a
feeble magnetism by being hammered
vertically when resting upon stone or
pewter, received a considerable accession
of power when subjected to the same
degree of hammering while it was placed
upon a parlour poker, also kept in a ver-
tical position. The poker, under these
circumstances, became strongly mag-
netic, and in this state exerted upon the
bar a much more powerful inductive in-
fluence than the earth alone could have
done. Hence the magnetism of an iron
bar, although temporary and dependent
on position alone, may serve as a very
important auxiliary in the development
of the magnetism in steel bars, which is
capable of being permanently retained.
This is, in fact, the great principle on
which the art of making artificial mag;
nets of high power is founded.

(172.) The effect of the auxiliary iron
bar, or of the poker used in the above
experiment, is greater in proportion as
it is longer ; but as it would not be con-
venient to employ a bar of iron beyond
a certain length, the magnetizing process
may be continued by the aid of a still
more powerful auxiliary, namely, very
long bars of soft steel. Mr. Scoresby
provided himself with two bars of this
description, thirty inches long, and one
inch broad ; and also with a large bar of
soft iron. This iron bar was first ham-
mered in a vertical position. It was
then laid on the ground, with its acquired
south pole towards the south, and upon
that end of it the large steel bars were
made to rest while they were hammered;
they were also hammered upon each
other. On the summit of one of the
large steel bars, each of the small bars
(which were eight inches long, and half
an inch broad), held also vertically, was
hammered in succession. In a few mi-
nutes they had all received considerable

MAGNETISM.

43

magnetic power. He then had recourse
to other methods, of which we are pre-
sently to speak, for still further increas-
ing: their power, till they were saturated
with magnetism.

(173.) It may here be remarked, that
in this, as well as in every other method
for procuring artificial magnets, we ad-
vance only by successive steps, gaining
a little additional power by each suc-
cessful process, and employing that
which has been acquired by one bar
in contributing to the increase of
the power of another; while this in
its turn gives us the means of re-
acting upon the first. This we are
enabled to do in consequence of the re-
markable circumstance'attending the in-
duction of magnetism, and to which we
have already adverted ($23), namely,
that the power of the magnet which ex-
cites magnetism in another body, is it-
self increased, instead of being diminish-
ed, by such excitation. Hence, by pro-
per management, the power of the;seve-
ral parts of the apparatus is capable of
continual increase, limited only by
their capacity for receiving and retain-
ing magnetism.

(174.) After having in this way mag-
netized, by the help of terrestrial mag-
netism, a certain number of steel bars,
we may proceed with them in imparting
magnetism to others ; and being thus
provided with a stronger power, may, in
our subsequent operations, dispense
with that which we had at first em-
ployed.

§ 3. Method by simple Juxtaposition.

(175.) Simple induction by juxtapo-
sition with one or more powerful mag-
nets may suffice for the impregnation of
very small magnets. But, for this pur-
pose, it is not sufficient to place the latter
in contact with one of the poles of a
magnet, in the manner represented in
the following figure (fig. 42) ; be-

cause, although the small bar or needle
becomes magnetic by remaining a suffi-
cient time subjected to the influence of
the large magnet, yet we find that its
two ends do not exhibit a magnetism of
equal strength. That which has been in
contact with the magnet appears to be
the most powerful, in consequence of its
magnetism being more concentrated at
the very extremity ; while in the re-

moter end it is more diffused and there-
fore less energetic. This inequality is
in a great measure remedied by em-
ploying two magnets of as nearly equal
power as possible, with their dissimilar
poles fronting each other, and 'placing
the needle to be magnetized in a lint-
bet ween them; as shown in fig. 43.

Fig. 43.

The effect of such a combination is
always more than twice as great as
that of each of the magnets when em-
ployed singly. It is difficult, however,
even with every precaution, always to
avoid the superinduction of consecutive
poles in the intermediate parts of the
needle.

(176.) The principle we have already
referred to, of the increase of power
which a magnet acquires by inducing
magnetism on other bodies, may here
again be applied in augmenting the in-
fluence of the magnets we employ in the
preceding case. " If a long bar of soft
iron be applied to each of the poles of
these magnets which are most distant
from the needle to be subjected to their
action, the power of the magnets will be
greatly augmented. A more' convenient,
and perhaps equally efficacious method,
is to place the magnets A and B, parallel
to each other (y?o-.44), while the small

Fig. 44.

bar C, to be magnetized, is in contact
with the two dissimilar poles at one
end ; and to unite those at the other
end by a bar of soft iron, R. This bar
becomes strongly magnetical by the
joint induction of both the magnets, and
the magnetism it thus acquires reacts
powerfully in strengthening the magnets
themselves ; and the needle which con-
nects their other poles participates in
this augmentation of effect. These
auxiliary pieces of soft iron, which serve
to retain and conc-entrate the magnetism
of steel bars, are;-1 ailed armatures.

(177.) Were th«i theory of ^Epinus, in
its original form, perfectly correct, no-
thing more would be required for im-
pregnating steel bars with all the mag-
netism they are capable of receiving, than
following the methods we have now

44

MAGNETISM.

pointed out. But we have seen reason
to conclude that this theory can be ap-
plicable only to the minutest individual
particles of which magnetic bodies are
composed, and that a magnet should
really be viewed as an aggregate of an
indefinite number of minute magnets.
This consideration cannot but have an
important influence on the art of impart-
ing magnetism to such an aggregate;
as it will lead us to apply our means to
effect, as far as it is in our power, a
change in the magnetic state of each
portion of the aggregate. The greater
the proximity of the pole of the magnet
which is to effect this change, to the
part in which the change is to be pro-
duced, the greater will be the effect
produced. Agreeably to this view of
the subject, we shall succeed in con-
verting a steel bar into a magnet of
greatest power, by subjecting every part
of the surface of the bar successively to
the contact of the magnetizing pole.
Let us trace the consequences of the
practical application of this principle.

§ 4. Method by the Single Touch.

(178.) One of the earliest methods
which was employed for giving magnet-
ism to a bar C (fig. 45), was to lay it flat

A C B

on a table, and placing an artificial mag-
net, M, on one of its ends, A, and at right
angles to it, to slide it along the surface
of the bar till it arrived at the other end
B ; and then, lifting it cautiously to a suffi-
cient height to render its inductive influ-
ence insensible, to bring it down again
to its former situation, and renew the
operation. This was repeated several
times on each of the surfaces of the bar,
the pole of the magnet being always
passed in the same direction, and the
same pole employed.

It is evident that when the mag-
net is first applied to the end of the
bar, it will induce in that end a po-
larity of the opposite kind to that of
the pole N of the magnet which is in
contact with it. Let us suppose, for the
convenience of explanation, that this is
a north pole ; the end A of the bar to

which it is first applied will first become
a south pole, and the portions at a little
distance from that end will acquire an
equal degree of northern polarity. But
as the magnet advances along the bar, a
similar change will be induced in each
successive particle of the surface which
it approaches and touches ; that is,
each particle will now be converted into
a south pole, although it had before been
rendered a north pole. In as far as this
takes place, therefore, the advance of the
magnet reverses the effect it had at first
produced. In like manner, the mag-
net has no sooner quitted the end to
which it was first applied, than it tends to
induce in it the northern polarity, at the
same- time that it renders the part which
it then touches, a south pole. The same
succession of changes, and reversal of the
magnetism of each part, takes place
during the whole of the progress of the
magnet along the bar, with the excep-
tion of the end which it touches last. It
leaves this end of the bar in the state of
a south pole, while the other end re-
mains a north pole. The intermediate
parts may be considered as constituting
a series of small magnets, with all their
north poles turned towards A, and their
south poles towards B.

(179.) However conformable to the-
ory this method of magnetizing may
appear to be, experience shows that it is
very little superior to that by simple con-
tact. It has also, like 1hat method, the
disadvantage of frequently producing
consecutive poles ; and these more es-
pecially occur when the bar to be mag-
netized is of some length, or consists of
very hard steel. They are also very
readily produced if care be not taken to
prevent the magnet from resting for a
longer time on some portions of the bar
than on the rest ; for in this case the
poles are multiplied very much in the
manner stated in a former chapter
($ 3 0, 3 1 ) ; a pole of one kind being formed
at the point where the contact has been
too long protracted, and two others of
the contrary denomination in the imme-
diate vicinity.

(180.) A singular circumstance cha-
racterizes this method of magnetizing by
touching, as it is called. If, after a bar
has been impregnated with as much mag-
netism as it is capable of receiving by
this method from a strong magnet, an
attempt be made to increase the effect
by renewing the same operation upon
the bar with a weaker magnet than
the one that was first employed, the

MAGNETISM.

45

immediate result is a loss instead of an
augmentation of power ; " and the bar
remains only magnetized to a degree
corresponding with the lesser power of
the last magnet by which it has been
touched. The reason of this will easily
be understood, when we consider that
the first effect produced by the second
magnet is, to reverse the poles -which
already exist in the bar, and afterwards
to induce the same kind of polarity ;
but this last effect it can produce only
in a degree corresponding to its own
strength. It therefore destroys, during
the former part of the operation, more
than it can supply in the latter.

The only exception which might be
conceived to exist to this destructive ac-
tion of a weaker magnet, would be in the
case where the latter was composed of a
very soft material, so that the magnetism
of the bar was capable of affecting its
polarity, so as to destroy it when of a
similar kind to the part of the bar with
which it came in contact, and convert it
into an opposite polarity.

(181.) An attentive 'consideration of
the stages of the process we have detailed,
will show us that the destructive opera-
tion of the second magnet is produced
chiefly in the first half of the bar ; for
if the'weaker magnet were first applied
to the middle of the bar, and then made
to slide on to the end in the same direc-
tion as before, over the latter half, its
effect on the first half would only tend
to strengthen the polarity already im-
pressed upon it. Nor would there, in
that case, be any injurious effect pro-
duced if the second magnet were suffi-
ciently soft in its texture to admit of
having its polarity changed by the mag-
netism of the bar. This consideration
leads us to another important stage in
the progress of improvement in the art
we are studying.

§ 5. Dr. Knight's Method.
(182.) This improvement consists in
employing two magnets in the same
operation, applying two dissimilar poles
of these magnets each to a different half
of the bar to be impregnated, and con-
fining its action to that portion of the
bar, which of course should be much
smaller than the magnets. For this pur-
pose the two magnets are to be joined
lengthwise, with their dissimilar poles in
contact, and laid on the bar to be magne-
tized, in the manner represented in/g-.
46, where A and B are the magnets, and
C the bar to be magnetized ; so that the

Fig. 46.

point of junction of the magnets shall be
immediately over the middle of the bar.
Then separating the magnets, by draw-
ing them opposite ways in the direction
of their length as far as the extremities
of the small bar, they are next to be re-
moved to a considerable distance, and
again joined; and afterwards laid a se-
cond time on the middle of the bar, in
the same manner as at first. This ope-
ration is to be repeated several times on
each of the sides of the bar. By this
method, which was first practised by Dr.
Gowan Knight about the middle of the
last century, steel bars could be rendered
much more powerfully magnetic than by
any of the means before in use.

(183.) The great superiority of Dr.
Knight's method is owing, not merely
to the circumstance before noticed — that
each pole of the magnets acts only upon
that half of the bar which is intended to
receive a magnetism of an opposite kind,
and that its inductive effect on the other
half has never to be destroyed, — but
also to the inductive influence of the two
poles being combined together during
the whole of the operation. In every por-
tion of the bar which lies between the
two poles of the magnets that are thus
applied, their influence conspires to in-
duce the kind of magnetism that it is
desired to produce. In those portions
of the bar, indeed, which lie on the other
sides of the poles of the magnets, they
oppose each other: but it will be per-
ceived that their effect is here only that
resulting from the difference of their re-
spective influences ; while, in the former
case, when they act upon the interme-
diate portions, it is as the sum of that
influence. The superiority of the com-
bined influence is even greater than the
united powers of the single magnets, as
we have already had occasion to point
out.

§ 6. Duhamel's Method.

(184.) If the magnets employed be
large and powerful, and the bars very
short and slender, it is easy, by the' pre-
ceding method, to magnetize them to
saturation. Soon after the publication
of Dr. Knight's method, small bars thus
magnetized were distributed over Europe,
and were eagerly sought after by the cul-
tivators of natural philosophy. It was

46

MAGNETISM.

soon found, however, that the attempt to
magnetize bars of a greater length by
this process was generally less successful,
or at least failed in giving to them all the
power of which they were susceptible.
Philosophers therefore renewed their ef-
forts to devise methods of greater and
more universal efficiency. M. Duhamel,
of the French Academy of Sciences, in
conjunction with M. Authcaume, at
length devised the following plan, which
was found to succeed even with bars of
considerable dimensions.

(185.) He first laid the two bars of
steel intended to be magnetized, and
which were made of equal length, paral-
lel to each other, C D (see Jig. 47), and
Fig. 47.

connected their extremities by two
shorter bars of soft iron, R r, so as to
form altogether a right-angled parallelo-
gram. Then taking two parcels of bars
already magnetized, M ra, the separate
bars of each parcel being placed with
their respective poles in the same direc-
tions, and firmly tied together, he brought
the poles of opposite kinds, N, S, into
contact over the middle of one of the
steel bars forming the parallelogram,
giving them a certain inclination to the
bars as seen in the figure. The angle
they formed with each bar was generally
about forty-five degrees, so that they
formed with each other a right angle.
Then separating them from each other, he
made them slide gently, and with an equa-
ble motion, towards the extremities of the
bar. This operation was repeated on the
same bar as often as appeared requisite.
The inclined parcels of magnets were
then taken to the opposite bar of the pa-
rallelogram, and applied to them in the
same manner ; taking care, however, to
reverse the disposition of the poles of the
magnets, so that the side on which the
north pole was placed in the one case,
was occupied by the south pole in the
other. After the bars had been rubbed
sufficiently on the one side, they were
turned on the other side, and the same
operations repeated on them in that situ-
ation.

(186.) It is evident, that in as far as
the magnets exert their conjoined influ-
ence on the portions of the bars that lie
between them, and act only upon their
respective halves of the bars, the method
of Duhamel possesses all the advantages
of that of Dr. Knight. The combination
of many separate magnets in each bun-
dle, however, gives them greater power
in operating the requisite inductions — a
power, indeed, which appears to be con-
siderably greater than that which a sin-
gle magnet of the same size as that of
the combined magnets would possess.
But the principal improvement in Du-
hamel's plan consists in the disposition
of the bars in a parallelogram in con-
junction with connecting pieces of soft
iron, which, acting as armatures, afford
an advantage of a similar kind to that
already explained in § 176. In propor-
tion as the steel bars acquire magnetism,
these connecting pieces participate in the
acquisition of a similar power, and serve
to retain it in the bars themselves ; just as
the electricity which is imparted to the
inner coating of a Leyden jar is retained
by the reciprocal influence of the in-
duced and contrary electricity of the
outer coating. The magnetism of the
bars is retained by a similar influence,
and greater facility is thus afforded to
increase its amount by the subsequent
additions it is receiving from the action
of the magnets as they pass along the
surface.

§ 7, Method by Double Touch:
Process of Mitchell.

(187.) While Duhamel was endea-
vouring to perfect his method in France,
the same object was occupying the at-
tention of experimental philosophers in
England ; and much about the same pe-
riod new processes for magnetizing bars
were invented by Mitchell and by Canton.

(188.) Mr. Mitchell, of Cambridge,
published his improved method in 1750.
He employed two parcels of strongly
magnetized bars (M m, Jig. 48), joined

Fig. 48.

in a manner similar to those above de-
scribed, and placed them parallel to each
other, but with the poles of each parcel
reversed, leaving between the two par-
cels an interval of about a quarter or a

MAGNETISM.

47

of an inch. He then arranged a
number of equal steel bars (A, B, C, D,
E) in a straight line, and made one extre-
mity of the conjoined magnets slide at
right angles over the line of steel bars.
He did not, however, limit himself to one
direction, but moved them backwards and
forwards the whole length of the united
surfaces of the bars ; repeating the ope-
ration on each side until he had obtained
as great an effect as possible.

In order to equalize as much as pos-
sible the magnetic power of the two ends
of each bar, it is expedient to commence
each operation by laying the conjoined
magnets on the middle of the line of bars,
and to pass the magnets over each half
of the line an equal number of times ;
at the conclusion of which, the magnets
being brought again to the middle,
they should be raised perpendicularly,
so as not to disturb the lateral effects
which had been produced. Mr. Mitchell
found that the steel bars B, C, D, which
were intermediate in the series, acquired
by this process a very great degree of
magnetic power. Those which formed
the extreme bars of the series A, D, were
much less impregnated ; but by remov-
ing them from this situation, and trans-
ferring them to the middle of the series,
and then repeating the same operations,
they quickly acquired the same degree
of magnetism as the rest.

(189.) The process above described,
which soon acquired much celebrity,
was called the method by double touch ;
and it is asserted by its inventor, that
two magnets will impart more magnetic
power to a bar of their own size, when
employed in this peculiar mode, than a
single magnet of five times the strength
of the former, when applied after the
manner of the single touch. The opera-
tion of the two poles of the conjoined
parcels of magnets on those portions of
the bars over which they pass will readily
be understood from what has been said
with respect to the methods of Knight
and Duhamel. They act by the sum of
their inductive powers on those parts
of the bar that are situated between
them, but with the difference of those
powers on all those parts which lie be-
yond them ; and the former is therefore
always greatly more efficient than the
latter. The superiority is the more con-
siderable in the present case, inas-
much as the magnets are nearer to each
other, and therefore act with much
greater power when they co-operate, but
are nearly inefficient when _they oppose

one another. The latter of these forces,
therefore, will never have sufficient
energy to destroy, or even much di-
minish, the effect which had been pro-
duced by the former; and thus the
magnetism of each portion receives con-
tinual accessions of strength every time
the magnets are made to pass over it.
The long line of bars operates in a man-
ner similar to the pieces of soft iron at
the extremities of those in the paral-
lelogram of Duhamel — that is, the ex-
ternal bars act as armatures to those
which lie between them ; and hence may
be understood why these intermediate
bars receive the strongest impregnation.

(190.) The different processes for
communicating magnetism which we
have now described, comprise all those
methods that are essentially different in
their principle ; all others which have
been proposed may be regarded as va-
rieties merely in the combinations of
which these principles are susceptible.
We shall only, therefore, notice those
which have been most in repute.

(191.) Mr. Canton published, in 1751,
a method which he considered as su-
perior to any of those previously em-
ployed. He placed the bars intended to
be magnetized so as to form a parallelo-
gram with connecting bars or arma-
tures of soft iron, as in the method of
Duhamel. He then had recourse to the
method of double touch as prescribed
by Mitchell; after which he separated
the two bundles of magnets, and in-
clining them to the bars in contrary di-
rections, as Duhamel had done, he com-
pleted the operation by making them
slide from the middle towards the extre-
mities. The combination of these two
processes was considered by Canton as
an improvement upon the method of
Mitchell. There is, however, great rea-
son to think, as Coulomb and Biot
have remarked, that these successive
operations are quite superfluous, and
that the bars are left at the end of them
precisely in the same state as if only the
last had been employed.

$ 8. JEpinuiiS Method.

(192.) j^Epinus introduced modifica-
tions into the process of the double
touch, of greater importance and much
more judiciously conceived. He first
formed the parallelogram of steel bars
in the manner of Duhamel ; but in place
of the auxiliary cross bars of soft iron,
he connected the ends of the steel bars by
means of other steel bars which hadpre-

MAGNETISM.

viously been rendered powerfully mag-
netic. He next placed the two compound
magnets end to end, with their dissimilar
poles adjoining each other, but separated
by a small piece of wood (fig. 49),

Fig. 49.

which kept them asunder for a short
space; and then, inclining them so that
they formed a very obtuse angle with each
other, he placed them on the middle of
one of the steel bars, and, without sepa-
rating them, made them slide backwards
and forwards along the surface of the
bar ; repeating the operation, with the
usual precautions as to the direction of
the poles, on the other bar, and on both
sides of each.

With regard to the position of the
magnets, it is evident that this process
is analogous to that of Duhamel; but
as the magnets, during the whole time
they are rubbed upon the bars, are kept
in the same relative situation with re-
spect to each other, their operation de-
pends upon the principle of the double
touch peculiar to Mitchell's process.
JEpinus tried different angles of inclina-
tion for the magnets, with a view to
discover that which gave the greatest
effect ; and concluded that the maximum
of effect was obtained when the mag-
nets made angles of fifteen or twenty
degrees on each side with the steel bar
on which they were to act.

(193.) When an inquiry was insti-
tuted as to the comparative efficacy of the
methods of Duhamel and of yEpinus, the
latter was found to possess this advan-
tage— that it enabled the experimenter
to magnetize bars of considerable length
and thickness by means of bars which
themselves possess no great power,
which was not the case with the process
of Duhamel. At the same time the
method of ./Epinus is liable to many in-
conveniences; in the first place, we
scarcely ever obtain, by its means, an
•equal degree of magnetic power in the
two ends of the bars to which it is ap-
plied. This will appear by placing any
one of these bars on a table, and laying
on it a sheet of paper, on which are
strewed some very fine iron filings;

when it will be seen, by the manner in
which the filings arrange themselves,
that the neutral point of the bar does not
occupy the exact middle of the bar, but
is sensibly nearer to that end to which
the magnets used in the operation of
touching it had been last applied.

In the second place, magnets formed
by the process of .^Epinus are much more
liable to have consecutive poles than
those obtained by Duhamel's process ;
and this is especially the case if the
magnets are of some length. These
consecutive poles, which are irregularly
formed in various parts of the magnet,
are, it is true, in general extremely
feeble ; but still they must always im-
pair very considerably the directive
force, which becomes a very serious
objection when the magnets are intended
for compass-needles. The inequality of
strength or of diffusion of the two
principal poles is also disadvantageous
with a view to the same object. Hence
the process of Duhamel will always be
found preferable for the construction of
compass-needles; while that of-^pinus
is more serviceable when it is wished to
obtain a very considerable magnetic
power in large bars, for the purpose of
batteries, or other magnetic combina-
tions, where it imports little whether
the neutral points be exactly coincident
with the centres of each individual piece.

§ 9. Coulomb's Process.

(194.) The attention of M. Coulomb,
already distinguished by his researches
in electricity, was engaged for a con-
siderable period in perfecting the art of
making magnets; and his numerous
communications to the French Academy
and Institute contain a great mass of
valuable observations on this subject.
Some of the results of his experiments
are given by Biot, in his "Traite tie
Physique."*

(195.) The magnetic apparatus for
impregnating a steel bar consists, as we
have seen, of two parts ; the first is that
which is fixed, and applied to the bars
in such a manner as to act by its con-
tinued inductive operation ; this includes
the armatures of soft iron, as well as the
fixed magnets that may be substituted
for them : and the second is the moveable
magnetic bars, or combinations of bars,
which are made to slide and rub over
the bar to be magnetized. For the
construction of the fixed part of the
apparatus, Coulomb employed bars of

MAGNETISM.

49

steel tempered at a cherry-red heat, and
from twenty to twenty-four inches in
length, of rather more than half an inch
in breadth, and one-fifth of an inch in
thickness. These he first magnetized to

to each other, and uniting them by their
poles of the same denomination, he ar-
ranged them into two assemblages, com-
posed of five bars in each, separated by
small parallelepipeds of soft iron, which

saturation by means of other magnets projected a little beyond their extremities,
procured by any of the methods already and performed the <

described; then, placing them parallel common to

Fig. 50.

office of an armature,
the whole set (see fig. 50).

The moveable part of the apparatus he
usually formed of four bars of steel,
tempered at the same heat as the pre-
ceding, and of the same dimensions as
to breadth and thickness, but only six-
teen inches long. After magnetizing
them as strongly as possible, he united
two of them by their widths, and two
others by their thicknesses, forming a
packet consisting of four magnets placed
as close as possible.

(196.) In proceeding to operate with
this apparatus, the large assemblages of
magnets, with their armatures, are
placed opposite to each other ; so that
the magnets which compose them shall
lie in the same line, with their north and
south poles opposite to each other, but
separated by a space nearly equal to
the length of the bar to be magnetized,
which latter bar is to be laid between
the two sets of magnets, resting on the
cross bars or armatures, for the space
of about the fifth of an inch. Then the
moveable magnets are laid upon the
centre of the bar, and inclined on each
side in opposite directions, so as to farm
with each half of the bar an angle of
twenty or thirty degrees.

Every thing being thus prepared, it is
at the option of the experimenter to
proceed according to the manner of
Duhamel, by drawing each packet of the
moveable magnets away from the middle
of the bar, along that half of it which
lies on its own side, as far as the ex-
tremity ; or, following the directions of
^Epinus, to retain the magnets in their
relative situation, by placing between
them a piece of wood or of copper, so as
to keep their poles at an invariable dis-
tance of one quarter or one fifth of an
inch from each other, and preserving
their inclinations, to slide them back-
wards and forwards from the centre to
each extremity of the bar, until each
half of Jt shall have been subjected to

an equal number of frictions. After the
last movement has been completed,
when the magnets will have been brought
back to the centre, where the movement
had commenced, they are to be raised
perpendicularly to a height sufficient to
obviate all sensible disturbance of the
magnetic state of the bar, and the ope-
ration repeated on its other side.

(197.) If the pieces which compose
the moveable magnets have not previ-
ously received all the magnetic power of
which they are susceptible, as will gene-
rally happen if we have not previously
the command of a sufficient apparatus
for that purpose, their united power,
when assembled in the manner already
described, will still produce in the bars
subjected to their action in the above
process, a degree of magnetism greater
than that which they themselves pos-
sess. We may therefore avail ourselves
of the latter of these bars for composing
a new set of magnets, which will accord-
ingly be more powerful than the former :
and then, obtaining by their means still
more highly impregnated magnets, we
may again disunite them, and subject
them to the action of still more energetic
combinations, formed by the bars last
impregnated. By the continued repeti-
tion of these processes, employing one
set of magnetic bars alternately in raising
the intensity of those of another set, it is
evident that we shall finally succeed in
effecting their complete saturation.

(198.) "When it is required to magne-
tize bars of very considerable size, Cou-
lomb recommends that the moveable
apparatus of magnets should be com-
posed of a much greater number of
pieces than in the instance above given ;
and that these pieces should be disposed
in rows, each successive row projecting
beyond the last, as shown in Jig. 51 : thus
the pole of each, which generally resides
at the very extremity of the bar, will
E

50

MAGNETISM.

come immediately in contact with the
bar to be magnetized, when the com-
pound magnet is applied to it with the
proper inclination, and the whole will
powerfully conspire in producing the
same effect.

(199.) The parallelograms of steel bars
and soft iron should be kept firm by
wedges, in the manner of printers' types,
and the extremities of the magnetic bars
should be perfectly cleaned. In rubbing
the bars, it is recommended by some
authors to apply considerable pressure ;
but Captain Kater found increased pres-
sure rather injurious than beneficial.
Dr. Robison conceived that by wetting
their extremities he obtained a greater
effect ; but he found that the least drop of
oil between the bars greatly obstructed
the operation of the magnets, as was
also the case when the smallest piece of
the thinnest gold leaf intervened. He
found that bars which were rough re-
ceived a more powerful magnetism
than those which were moderately po-
lished ; but that, if moderately rough,
they acquired the first degrees of mag-
netism more expeditiously than smooth
bars, but did not ultimately receive so
strong an impregnation as the latter.

§ 10. Comparative Advantages of the
different Processes.

(200.) An account of Coulomb's ex-
periments on the comparative advan-
tages of the different, methods of mag-
netizing bars of different thicknesses,
lengths, and forms, is given by Biot in
his " Traite de Physique." The strength
of each magnet was estimated by the
number of oscillations which it per-
formed in a given time on each side of
the magnetic meridian by the influence
of terrestrial magnetism. The following
are some of the results of his inquiry.

He found that steel wires of small dia-
meter may be rendered magnetic to an
equal degree, whether touched by the
method of Duhamel or of yEpinus, or
even when simply rubbed with the single
pole of a strong magnet, for in all these
cases they became magnetized to satura-
tion. AVhen a plate of unannealed steel
of greater width than the wires, but

of equal length and thickness, was sub-
jected to experiment, a slight difference
was perceptible in the degree of magne-
tism resulting from the different methods,
those of Duhamel and ^Epinus being the
most efficacious. The difference was
more perceptible when the steel was
made of a harder temper, and increased
still more when thicker plates of steel w ere
tried. The processes of Duhamel and
of .yEpinus, when applied to plates of
which the thickness is less than one
twelfth of an inch were nearly of equal
power ; but when they exceed this thick-
ness, that of ^Epinus was decidedly the
most efficacious. In the case of bars
sixteen inches long, one inch broad, and
about one third of an inch thick, the
comparative intensities of the magne-
tism produced by the methods of JEpi-
nus and Duhamel were nearly in the
proportion of nine to eight.

(201.) Captain Kater also made a
series of valuable experiments on the
effects of different methods of magnet-
izing, and on the influence of extent of
surface, independently of the mass, on
the directive force.* The directive force
was estimated by means of the balance
of torsion of Coulomb. This instrument
consists of a fine wire terminated above
by an index at right angles to it, which
is moveable round a circle divided into
degrees. To the lower end of the wire a
cradle is attached for receiving the needle
which is the subject of experiment. The
instrument being adjusted so that the
needle was in the magnetic meridian
when the wire had no torsion, the index
was turned, and the wire consequently
twisted, until the needle was made to
deviate 60° from its original position.
The number of degrees passed over by
the index would then be the measure of
the directive force of the needle.

The needles to be magnetized were
right-angled parallelograms, five inches
long ; the one seven-tenths of an inch
broad, and the other of half this
breadth. The broadest was reduced in
thickness till it was of the same weight
as the other, namely, one hundred and
forty-two grains. The magnets em-
ployed were first placed perpendicularly
on the centre of the needle, their oppo-
site poles being joined; their lower ex-
tremities were then separated and kept
asunder by placing between them apiece
of wood a quarter of an inch thick, Iheir
upper extremities remaining in contact.

* Philosophical Transactions for 1821, p. 104,

MAGNETISM.

51

The magnets were then slid along the
needle backwards and forwards from
end to end, and this was repeated on
both sides, till it was conceived that the
full effect had been produced ; and the
directive force of the magnet thus ob-
tained was noted. The process was
repeated with the same apparatus, ex-
cepting that the magnets were separated
at the top by a piece of wood of the
same thickness as that at the bottom.
The effect was considerably diminished
by this change. When the lower ex-
tremities of the needle were separated
by a piece of wood to the distance of half
the length of tho needle, the upper
extremities remaining in contact, the
effect was greater than in the first expe-
riment. A further augmentation of
power was obtained when the magnets
were joined and placed perpendicularly,
as before, on the centre of the needle,
and then moved in opposite directions
from the centre to the extremities, keep-
ing each magnet perpendicular to the
needle ; afterwards rejoining them at a
distance from the needle, replacing them
on its centre, and thus continuing the
operation. The needles were then mag-
netized according to the method of
Duhamel, the magnets being inclined at
an angle of about forty-five degrees, and
carried as before from the centre to the
ends of the needle. This was attended
with a still greater increase of effect ;
but it was increased still further when
the magnets formed with the needle an
angle of about twenty degrees. The
maximum of effect took place when this
angle was reduced to about two or three
degrees ; for when the magnets were
laid flat on the surface of the needles,
and drawn from the centre to the ends,
the effect was not so great as in the last
case.

(202.) On the whole, Captain Kater
concludes that the best mode of commu-
nicating magnetism to a needle, is. by
placing it in the magnetic meridian, join-
ing the opposite poles of a pair of bar
magnets (the magnets being in the same
line), and laying the magnets so joined,
flat upon the needle with their poles
upon its centre; then having elevated
the distant extremities of the magnets,
so that they may form an angle of about
two or three degrees with the needle,
they are to be drawn from the centre of
the needle to the extremities, carefully
preserving the same inclination ; and
having joined the poles of the magnets
at a distance from the needle, the ope-

ration is to be repeated ten or twelve
times on each surface.

(203.) We have seen (§ 171), that
terrestrial magnetism may be made
available for procuring artificial magnets
by the help of percussion ; and it now
remains that we point out the methods
of employing it, in the absence of all
other magnetized bodies, in exciting
magnetism by friction, either by the
single touch or by a method analogous
to that of Dr. Knight.

(204.) Let the needle to be magnetized
be placed horizontally in the magnetic
meridian and fixed in that situation, and
apply to the middle of it the lower end
of a long iron bar, a poker, for instance,

Fig. 52. Fig. 53.

held vertically ; and immediately oppo-
site, at the lower side of the needle, ap-
ply the upper end of a second bar of a
similar description to the first, as seen
in fig. 52. Then draw each bar, still
kept in a vertical position, towards the
opposite ends of the needle (Jig. 53), tak-
ing care that the upper bar be drawn
towards the side of the needle intended
to be its south pole, and the lower
bar towards the intended north pole ;
then, separating the bars, remove them
to a distance, and bring them again
perpendicularly to the middle of the
needle; and repeat this operation a
sufficient number of times on each side.
This simple process will often, if the
needle be small, be sufficient to magne-
tize it to saturation. The principle on
which it is founded is sufficiently obvious.

§11. Cantons Process.

(205.) The process of Canton, already
alluded to, proceeds upon the same
principle, and does not require the pre-
E 2

52

MAGNETISM.

vious possession of any magnet. As the
knowledge of it, therefore, may be of
use to those who are not provided with
any magnetic apparatus, we shall pre-
sent the following outline of that part
of it which appears to be the really effi-
cient process.

(206.) Six bars of soft steel are to be
provided, each three inches long, one
quarter of an inch broad, and one twen-
tieth of an inch thick, together with two
pieces of iron, each half the length of
one of the bars, but of the same breadth
and thickness ; and also six bars of hard
steel, each five inches and a half long,
half an inch broad, and three twentieths
of an inch thick, together with two
pieces of iron of half the length, but the
whole breadth and thickness of one of
the hard bars. All these bars are to be
marked at one end by a line quite round
them, in order to distinguish the poles.

(207.) Two bars of iron, or an iron
poker and tongs (Jig. 54), are to be taken :

Fig. 54.

the larger they are, and the longer they
have been used, so much the better.
Let the poker be fixed upright, and held
by the knees, and let one of the soft steel
bars, having its marked end downwards,
be tightly fastened to it by a piece of
sewing silk, and held with the left hand ;
then grasping the tongs with the right
hand a little below the middle, and hold-
ing them in a vertical position, let the
bar be rubbed with the lower end, from
the bottom to the top, about ten times
on each side. This will give it sufficient
magnetic power to lift a small key from
the marked end, which will, of course,
be a north pole.

(208.) Having magnetized four of the
soft bars in this manner, the other two
(fig. 55) are to be laid parallel to each
other, at the distance of about one
fourth of an inch between the two
pieces of iron belonging to them, a north
and a south pole against each piece of
iron. Two of the four bars that have
been already made magnetical are then
to be united, so as to make a double bar
in thickness, the north pole of one even
with the south pole of the other ; and the
remaining two being placed next to
these, one on each side, so as to have
two north and two south poles together,
the north and south poles are to be se-
parated at one end by a large pin put
between them; they are then to be

placed perpendicularly with that end
downwards, on the middle of one of the
parallel bars — the two north poles to-
wards that end intended to be made the
south pole, and the two south poles to-
wards the intended north poles. Next
slide them backwards and forwards
three or four times the whole length of
the bar, and removing them from the
middle of this bar, place them on the
middle of the other bar as before di-
rected, and go over that in the same
manner. Then turn both the bars the
other side upwards, and repeat the ope-
ration.

Having done this, remove the two
bars from between the pieces of iron,
and placing the two outermost of the
touching bars in their stead, let the
other two be the outermost of the four
to touch these with. This process being
repeated till each pair of bars have been
touched three or four times over, they
will thus acquire a considerable magnetic
power.

Next put together the six bars, as
was done with the four, and touch

MAGNETISM.

53

with them two pair of "the hard bars,
placed between their iron armatures, at
the distance of about half an inch from
one another. The soft bars may now
be laid aside, and two of the hard bars,
placed between their iron armatures,
may be magnetized by means of the
other four, which should be held apart
at the lower end, at an interval of about
one fifth of an inch ; to which distance
they are to be separated after they are
set on the parallel bar, and brought to-
gether again after they are taken off.
The same process as that above described
is now to be continued until each pair
has been touched two or three times
over.

The whole of this process may be
gone through in less than half an
hour; and each of the larger bars, if they
had been previously well hardened, may
be made to lift twenty-eight troy ounces,
or even more. Bars thus impregnated
will give to a hard bar of the same size
its full virtue in less than two minutes ;
and may, therefore, answer almost every
purpose in natural philosophy much
better than the natural loadstone, which
has seldom sufficient power to impreg-
nate hard bars.

§ 12. Horse-shoe Magnets.

(209.) Magnets in the form of a
straight bar are less convenient when
the action of both the poles is wanted,
as happens in various experiments, es-
pecially such as concern the raising of
weights by the force of magnetic attrac-
tion. In order to bring the two poles
near each other, artificial magnets are
often made in the shape of a horse-shoe,
(fig. 56,) or sometimes a more semicircu-

Fig. 56.

Fig. 57.

lar form is given to them (fig. 57). These
horse-shoe magnets, as they are called,
may be rendered magnetic by the same
process as a straight bar ; the magnets
by which they are rubbed being, of

course, made to follow the curvature of
the bar. The method of^Epinus is best
suited for the communication of mag-
netism to bars of this shape.

(210.) Horse-shoe magnets, that have
their poles brought very near to each
other, are exceedingly convenient as
substitutes for the compound magnets
employed in the process of magnetizing
by the double touch. They fulfil, in-
deed, all the purposes of compound
magnets in this operation ; and if placed
at once on the middle of the needle to
be magnetized, with the poles turned in.
a direction the reverse of that of the
poles intended to be given to the needle,
and then moved backwards and forwards
along the surface of the needle, taking
care to pass over each half of it an equal
number of times, and repeating the same
operation on the other side, the needle is
speedily and effectually rendered mag-
netic. The readiness with which this
may be put in practice, and the absence
of all previous preparation, are strong
recommendations in favour of this form
of magnet.

(211.) Powerful magnetic batteries
are sometimes constructed by uniting a
number of horse-shoe magnets, laying
them one over the other with all their
poles similarly disposed, and fastening
them firmly together in a leathern or
copper case.

' $ 13. Preservation of Magnets.

(212.) From what has been already
said respecting those circumstances
which tend to produce or to destroy
magnetism, we may easily devise rules
for the preservation of magnets, for,
unless kept with care, and with the ob-
servance of certain precautions, they
soon lose their power.

(21 3.) If a single magnet be kept in
an improper position,that is, one differing
much from that which it would assume in
consequence of the action of terrestrial
magnetism, in process of time it becomes
gradually weaker ; and this deterioration
is most accelerated when its poles have
a position the reverse of the natural one.
Under these circumstances, indeed, un-
less the magnet be made of the'hardest
steel, it will in no long time lose the
whole of its magnetic power. Two
magnets may also very much weaken
each other if they be kept, even for a
short time, with their similar poles
fronting each other. The polarity of the
weaker magnet, especially, is rapidly
impaired, and sometimes is found to be

54

MAGNETISM.

actually reversed. More frequently, how-
ever, there arises, from this opposition
of powers, considerable irregularity and
confusion in the poles of both magnets.
Heat, as we have seen, impairs mag-
netism : care should therefore be taken
to avoid exposing magnets to a high
temperature. We should likewise be
very cautious to avoid all rough and
violent treatment of a magnet ; for we
have seen how quickly its virtue is lost
by any concussion or vibration among
its particles. A fall on the floor, espe-
cially if it strike against any hard sub-
stance, will materially weaken it : rub-
bing with coarse powders, for the purpose
of polishing it, and grinding, in order to
bring it to any required form, are equally
injurious. A natural loadstone will, in
like manner, suffer by such an operation ;
hence we should attempt to alter its natu-
ral form as little as possible ; and when
it is necessary to do so, it should be
effected very rapidly by cutting it briskly
in the thin discs of a lapidary's wheel.

(214.) Although the loadstone retains
its magnetic virtue more tenaciously
than any artificial magnet that can
be constructed, yet even this body re-
quires a certain management for the
permanent preservation of its power.
For this purpose it should be armed, as
it is called; that is, an armature of iron
should be applied to both its poles. In
order to do this most effectually, we
must first ascertain the situation of the
poles of the loadstone ; and cutting off
all the superfluous parts, give it the
shape of a parallelepiped, having the
poles in the middle of two opposite sur-
faces, and at the same time taking care to
preserve the axis, which passes through
the poles, of as great a length as can be
obtained : for it has been observed, that
any curtailment of the magnet in the
direction of this line deprives it of force
in a greater degree than when shortened
in any other direction.

(215.) Two plates of very soft iron
must next be provided, equal in breadth
to the surfaces containing the poles, and
a little longer than those surfaces ; so
that, when applied to them, a portion of
each plate shall project beyond the load-
stone to a small extent. Infix. 58» ^r
represents the sections of these iron
plates affixed to the opposite sides of the
loadstone L ; and P p the projecting
pieces. These projecting pieces should
be much narrower than the other portion
of the plates. For loadstones weighing
less than an ounce, the lower surfaces

of the projections need not exceed the
tenth of an inch ; and so in proportion
for larger loadstones. The thickness of
the plates, also, must be regulated by
the strength of the loadstone, and can
scarcely be determined without previous
trial in each particular case. The best
way, therefore, is to make them tolera-
bly thick at first ; and then file off suc-
cessive layers, until we find, by actual
experiment on the power of the loadstone
after each reduction, that we cease to
obtain any advantage ; for the power
increases gradually to a certain limit,
at which the filing ought to be discon-
tinued. The armature of a loadstone
should be fixed on it very firmly, by
wires, or by an external case, which
should be made of any metal which is
not susceptible of magnetism. Load-
stones are sometimes cut into a spherical
shape, in imitation of the earth, and are
then called terrella. Their armatures
should, in that case, be adapted to the
curvature of the surface, and should
each cover about a quarter of that
surface.

(216.) The addition of armatures to a
loadstone is found to have a very favour-
able effect in augmenting its strength,
and this increase of strength goes on for
a considerable time after they have been
applied. But there is another, and a
still more important advantage resulting
from them, in enabling us to direct the
power of the loadstone, and to concen-
trate it into a small space. The polarities
of loadstones are often diffused over a
considerable part of their surface ; and
these scattered forces could never be
made to bear upon any point on which
they are required to act, unless by the
intermedium of some substance which
might collect and unite them. The iron
armatures supply this intermedium.
They receive at their expanded part the
inductive influence of all the scattered
poles residing in the surfaces to which
they are applied ; and this influence
being transferred to the narrow extre-
mity, is there concentrated, and acts
with full effect. By this expedient also,
the resultant forces, derived from each

MAGNETISM.

55

single pole, are brought near to each
other, and their directions rendered pa-
rallel: they are, therefore, made to
conspire in various actions which
require the joint operation of both
poles, such as that of eliciting magne-
tism by the double touch. The same
advantages, indeed, are procured by this
construction as \ve have already seen
obtain in the case of horse-shoe mag-
nets when compared with straight mag-
netic bars. Thus we find that a load-
stone which, in the natural state, would
appear to be exceedingly feeble, will pos-
sess, when properly armed, very consi-
derable magnetic powers.

(217.) The armature of a loadstone
not only contributes to exalt its magnetic
virtue, but also furnishes us with the
means of preserving it uninjured. Its
two poles, being now transferred to the
extremities of the armatures, or to each
foot of the armature, as it has been
called, on connecting these poles, by ap-
plying to them a bar of soft iron, A (Jig.
58), we may effectually prevent the dissi-
pation of their magnetism. This cross-
bar performs a similar function with
relation to these poles that the iron plates
do to the loadstone itself — it acts as a
secondary armature ; and we find, after
applying this bar, that the apparatus
gradually acquires greater power up to
a certain limit. Such therefore is the
mode in which loadstones, when not in
use, should always be kept, with a view
to the preservation of their powers.

(218.) Directions of a similar kind,
and derived from the same principles,
apply also to the preservation of artificial
magnets. Horse-shoe magnets should
have a short bar of soft iron (A,y?#. 59)
Fig. 59. adapted to connect the two
poles, and should never be
laid by without having such
a piece of iron adhering to
them. Bar magnets should
be kept in pairs, lying pa-
rallel to each other, with
their poles turned in con-
trary directions, and the dis-
similar poles on each side
connected by a bar of soft
iron ; so that the whole may
form a parallelogram as in
fig. 60. They should fit into
A a box when thus arranged,

so as to guard against accidental con-
cussions, and to preserve them from the
dampness of the atmosphere. Magnets
should be polished, not indeed with a
view to the increase of their magnetism,

Fig. 60.

but because they are then less liable to
contract rust. It is convenient that those
ends which have the northern polarity
should be marked with a line all round,
in order to distinguish the respective
poles in each magnet.

CHAPTER VI.
Magnetic Instruments.

(219.) IN every branch of science,
the value of a correct theory is best
estimated by the extent and importance
of its practical applications. The nearer
its approach to perfection, the greater
the assistance we derive from it in con-
structing instruments for the accurate
measurement of spaces, of times, or of
forces, and for the accomplishment of
the objects which relate to that particu-
lar science. It is thus that in mag-
netism, if the theory, developed in the
preceding part of this treatise, be cor-
rect, it ought to furnish principles for
the construction and management of
magnetic instruments, such as the com-
pass, dipping-needle, &c. Our limits
permit us only to point out the leading
principles which particularly deserve
attention in the case of each kind of
instrument.

§ 1. Of the Compass.

(220.) The term compass is a general
name for all instruments calculated to
indicate the position of the magnetic
meridian, or of objects with reference to
that meridian, whether adapted for being
used on land, and in the bottoms of mines,
where we have a stable support at com-
mand, or for observation at sea, where
the perpetual agitation of the surface
deprives us of that advantage.

The first class, which merely show
us the direction of the magnetic meri-
dian, includes the Land Compass, the
Mariner's Compass, and the Variation
Compass; the second, or those which
mark the angular distances of objects
from this meridian, are called Azimuth
Compasses.

(221.) Whatever modifications may
be rendered necessary by the particular
purpose of the compass, the essential
parts of which it consists are the same

56

MAGNETISM.

in all— namely, a magnetised bar of
steel, generally termed the needle, hav-
ing at its centre a cap fitted to it, which
is supported on a sharp-pointed pivot
fixed in the base of the instrument. In
the mariner's compass, the needle is
also affixed to a circular plate, or card,
the circumference of which is divided
into degrees, while an inner circle de-
scribed upon it is marked with the
thirty-two points of the compass, or
rhumbs, as they are called. The pivot of
support rises from the bottom of a circu-
lar box, which contains the needle and
its card, and is covered with a piece of
glass,* in order to protect them from
dust, and prevent their being disturbed
by the agitations of the external air.
The compass box is suspended within a
larger box, by means of two concentric
brass circles, or gimbals, as they are
called; the outer one being fixed by
horizontal pivots, both to the inner
circle which carries the compass box,
and also to the outer box ; and the two
sets of axes being in directions at right
angles to one another. By the combi-
nations of movements determined by
these axes, the inner circle, with the
compass box and its contents, always
retains a horizontal position, during the
rolling of the ship.

(222.) The qualities required in the
needle of the compass, for the perfect
performance of its office, are these : —
first, its directive force compared with
its weight, or with the mass which that
power has to set in motion, should be
as great as possible ; while, secondly,
the impediments to the exertion of that
force, and which consist principally in
the friction between the cap and pivot,
should be as small as possible. Hence
it becomes important to consider the
relation subsisting between these op-
posing forces, and to ascertain those
conditions which give the greatest pre-
ponderance to the directive force.

(223.) The friction that takes place
between the pivot and the cap which
rests on it, will, in different compasses,
bear a certain proportion to the pressure
on the points of support, provided these
parts are constructed precisely in the

* An electrical state of the glass cover, acci-
dentally excited by friction, has been known to
occasion a sensible disturbance of the needle, by
attracting its ends. This attraction, when it ex-
ists, may be at once destroyed by moistening the
surface of the glass. See Phil. Trans, for 1746,
p. 242. See also the observations on the local and

ectrical influences on compasses by Lieutenant
Johnson, in the 21st volume of the Quarterly Jour-
nal of Science, p. 274.

same manner in each case. This pres-
sure is proportional to the weight of
the needle and the parts which turn
with it. Coulomb concluded, from a
set of experiments he made with a view
to ascertain this particular point, that
when the pivots terminate in a sharp
point, and the caps are made of very
hard materials, the friction is very nearly
proportional to the square root of the
cube of the weights. But after long
use, the point of the pivot becomes
blunted, and the surface of contact with
the bottom of the cap is considerably
enlarged. In this state the friction is
found to be simply proportional to the
pressure.

(224.) Assuming this, then, to be the
law of relation between them, let us
take a magnetized needle of any given
size and shape, and support it upon a
pivot in the usual manner. Let us next
place upon it another needle, precisely
similar in all its dimensions, and mag-
netized to the same degree. The pres-
sure on the pivot will now be double
what it was before ; and therefore the
friction, which is proportional to that
pressure, will be double also. But the
directive force, though increased, will
not be twice as great as with the single
needle ; because, as was formerly shown,
the reaction of the similar poles of the
two magnets tends to diminish the
power of each. Hence the ratio be-
tween the directive force and the resist-
ance is diminished, and the compound
needle is less sensible to the magnetic
influence of the earth, and less fitted for
indicating the magnetic points of the
compass. The same mode of reasoning
applies to any increase of thickness that
may be given to the needle. Hence it
appears, that when all other conditions
are the same, needles of very small
thickness possess the greatest sensibi-
lity to terrestrial magnetism. To this
general proposition there is, however, a
limit ; inasmuch as excessive thinness
in the needle would endanger its bend-
ing by its own weight, which would be
attended with a considerable loss of
power.

(225.) With regard to the most ad-
vantageous length for a compass needle,
it appears that when we have passed a
certain limit, which is about five inches,
an increase of length is accompanied by
an increase in the directive force in the
same proportion ; but when the thick-
ness remains the same, the weight, and
consequently the friction, increases in

MAGNETISM.

57

the very same ratio ; no advantage,
therefore, as to directive power can be
obtained by any increase of length.
Beyond the limit just mentioned, there-
fore, all needles having the same trans-
verse dimensions should, according to
theory, be equally sensible, whatever be
their lengths. But it is found in prac-
tice, that needles which exceed a very
moderate length are liable to have seve-
ral consecutive poles, attended, as we
have seen, with a great diminution of
directive force. On this account, short
needles, made exceedingly hard, are
generally preferable.

(226.) The next object of atten-
tion in the construction of a compass
needle is the shape which is most fa-
vourable to the acquisition of the
greatest directive power. Various
have been the forms given to com-
pass needles ; the choice having been
regulated more by the whim and fancy
of the maker, than by any reference to
scientific principles. The forms most
frequently met with are the cylindric,
the prismatic, that of a rhombus or
parallelogram, and that of the flat Iv*
tapering like an arrow at the extremi-
ties. Coulomb, who made many expe-
riments on the subject, gave a decided
preference to the last mentioned of these,
as being that which, with a given weight
of needle, retains the strongest directive
force. On the other hand, he found,
that any expansion of the needle at its
extremities, a form which has sometimes
been recommended, is attended with a
sensible diminution of power. From the
whole of his experiments, he was led to
the general conclusion, that in needles
of the same form, their directive forces
are to each other as their masses.

(227.) This inquiry has been still fur-
ther pursued by Captain Kater, whose
paper in the Philosophical Transactions,
already alluded to ($ 201), contains an
account of a series of experiments for
determining the best kind of steel for
a compass needle, and the best form
that can be given to it. He found,
on comparative trial, that the directive
force is little, if at all, influenced by
extent of surface, but depends almost
entirely on the mass of the needle,
when magnetized to saturation. Two
needles were prepared of that kind
of steel which is called blistered steel,
and two of spur steel, each weighing
66 grains. They were of the form of a
long ellipse, five inches in length and
half an inch in width. One of each

kind was pierced, as shown in fig. 61 ;
the weight so lost being made up by

Fig. 61.

additional thickness. It is evident that
these pierced needles had, though of
equal mass, much less extent of surface
than those which remained solid. Hav-
ing formerly had in his possession a
compass of extraordinary power, the
needle of which was composed of pieces
of steel wire put together in the shape
of a rhombus, he procured two needles
of this form (Jig. 62), made from a
Fig. 62.

piece of clock spring, which J.s of that
kind of steel called-sheav steel. In oner
thf .os "piece was of brass; in the
urner, formed of part of a clock spring.
They weighed only 45 grains.

(228.) The results of the inquiry
were, that shear steel is capable of re-
ceiving" the greater magnetic force ; and
that the pierced rhombus is the best
form for a compass needle. Needles of
cast steel were also tried, but were
found so very inferior, as at once to be
rejected. In the same plate of steel, of
the size of a few square inches only,
portions are found, varying considera-
bly in their capability of receiving mag-
netism, though not apparently differing
in any other respect.

(229.) Captain Kater next endea-
voured to determine the effects of va-
rious modes of hardening and tem-
pering the needles. He found that
hardening a needle throughout consi-
derably diminishes its capacity for mag-
netism. The greatest directive force
was obtained by a needle which was
soft in the middle, and its extremi-
ties hardened at a red heat. He at
first thought that the most effectual
means of increasing its retentive power,
would be first to soften it throughout,
and then harden it at the extremities,
instead of first entirely hardening it, and
afterwards softening it in the middle.
But subsequent experience induced him
to attribute the difference of effect to a
difference in the degree of heat to which
the needle is exposed in softening it in

58

MAGNETISM.

the middle. Repeated exposure to heat
was found considerably to impair the
susceptibility of the needle to retain the
magnetic power communicated to it ;
an effect which does not appear to be
owing to any decarbonization of the
steel. Captain Kater suggests that this
deterioration may arise from a perma-
nent expansion produced in the texture
of the steel 'by the repeated application
of heat; for the springs of clocks,
which was the material used in his ex-
periments, being made by passing the
steel through rollers, when it undergoes
great compression, it is probable that
the state of condensation thus induced
is exceedingly favourable to the reten-
tion of magnetism.

(230.) The process which, on the
whole, he recommends as the most ef-
fectual for giving to a needle the great-
est susceptibility of directive power, is
first to harden it throughout at a red
heat, and then to soften it from the
middle to within an inch of each extre-
liiity, by expcsi"^ it to a heat sufficient
to cause the blue colour which arises
again to disappear.

(231.) The effect of previously po-
lishing the needle to be magnetized was
not found by Captain Kater to have any
sensible influence on its capacity for re-
ceiving directive power. Neither did
any advantage result from the employ-
ment of increased pressure in applying
the magnets over the surfaces of the
needle during the process for magnet-
izing them ; but, on the contrary, in
one instance it seemed to be attended
with a diminution of effect.

(232.) It is an important requisite in
a compass needle that its polarities
should be concentrated as much as
possible in its two extremities, and un-
disturbed by the action of any conse-
cutive poles existing at intermediate
points. We have already had occasion
to remark (§ 193) how much the direc-
tive force of a needle is impaired by
irregularities in the distribution of its
magnetism, attended either by a multi-
plicity of poles, or by an inequality in
the strength of the two principal poles.
It is on this account that Duhamel's
process of magnetizing is so much pre-
ferable to that of /Epinus for imparting
magnetism to compass needles ; being
more conducive to uniformity of effect
in every portion of the needle. But
even with all the care that can be
bestowed, we cannot always be certain
of obtaining perfect regularity in the

disposition of the magnetic power of a
steel bar, whatever shape we may give
to it, or whatever process we may em-
ploy for its magnetization.

(233.) The consequence of the un-
equal distribution of magnetism on the
two sides of the needle, is evidently to
produce a deviation of its axis from the
true magnetic meridian ; and the instru-
ment will therefore fail to point out the
real direction of this meridian. There
is only one way of discovering the ex-
istence and the amount of the deviation
proceeding from this cause; it is to
reverse the needle, that is, to turn up-
wards that surface which was before the
under surface ; and when thus reversed
to balance it as nearly as possible in the
same point in its axis as that on which
it was before supported. If the needle,
in this new state of suspension, finally
settles in a position somewhat different
from that it before assumed, we may
conclude that the axis indicated by its
figure is not its true magnetic axis ;
and that the latter, which alone tends to
arrange itself in the magnetic meridian,
lies, in a situation exactly bisecting the
i,,o positions assumed by the needle
in these two different modes of suspen-
sion.

(234.) ;When compasses are con-
structed of two separate pieces of steel
bars, slightly bent at an obtuse angle in
the middle, so as to allow a space for
the placing of the brass cap on which
it is to be suspended at the centre, and
the two pieces joined by their extremi-
ties so as to compose a lozenge-shaped
combination, they are exceedingly liable
to the imperfection just noticed. For,
unless the ends of the separate pieces
which compose such a needle have been
brought, by tempering, to an exactly
equal degree of hardness, that side
which is the hardest will retain more
magnetic power than the other side ;
and will, consequently, have a stronger
tendency to place itself in the magnetic
meridian. The needle will, accordingly,
incline on the side which favours
this tendency, and the line joining its
extremities, and which must be regarded
as the axis of its figure, will deviate
from the magnetic meridian. This evil
will have a tendency to increase by
time: for the stronger magnetism of one
side, will tend first to impair, and at
length destroy, or even finally to re-
verse, the polarities of the parts on the
other side to which they are adjacent.

(235.) The mode in which compass

MAGNETISM.

needles are to be suspended, is'well de-
serving of attention. In order to pro-
vide a concave surface, by which the
needle may rest on the pivot which is to
support it, such, that the point of sus-
pension may be just above the centre of
gravity, it is generally necessary, in the
(Straight needle, to make a perforation in
its centre, and to rivet into the hole a
piece of hammered brass, the lower side
of which has been hollowed into a coni-
cal cavity, while its upper convex sur-
face is allowed to project a little above
the level of the upper surface of the
needle. It is found, however, that brass is
not capable of being rendered sufficiently
hard to resist the continued action of the
point against which it rubs in every mo-
tion of the compass. In process of time
it is worn into an irregular hole, giving
rise to great friction, and loss of mobi-
lity in the compass. This defect is
usually remedied by inserting in the
upper part of the brass, a piece of po-
lished agate, ground concave, with a de-
cided centre. The best compasses, made
for nautical use, are thus furnished with
agate caps.

(236.) Some have considered the per-
foration of the needle at the centre, for
the purpose of suspension, as prejudicial
to the regularity of the magnetic power,
and as tending to the creation of an ad-
ditional number of poles. But, in re-
ality, the derangement occasioned by the
perforation of a magnetic bar at the
point of neutrality, is not found to be at-
tended with any sensible inconveniences
in practice*. If the shape given to the
needle be that of the pierced rhombus,
as recommended by Captain Kater, no
such objection will arise, since the cross-
bar which connected the obtuse angles
of the rhombus has nothing to do with
the magnetism of the steel bars forming
the sides of the parallelogram. It would,
no doubt, be easy to balance a straight
bar, without removing any part of its
substance, by the addition of a rim of
copper, or other non-magnetic substance,
to the circumference of the card, so that
the centre of gravity of the moveable
part of the instrument may be brought
sufficiently low to be under the point of
suspension. But a little reflection will
show, that more would be lost than
gained by this expedient ; for every ad-
dition that is made to the weight of the
parts which have to move along with
the needle, lessens the efficacy of the

* Coulomb, Mcmoires de Mathematique et de
Physique pres«ntcs a 1' Academic. Tom. ix., 1780.

magnetic force which gives them mo-
tion, and the friction also, being aug-
mented in the same proportion, con-
spires to diminish the freedom of the
motion of the needle, and to impair its
sensibility.

(237.) The best precaution to be taken
for ensuring the steadiness of the move-
ments of the compass, under all circum-
stances, is to balance the needle accu-
rately upon its centre, before the card is
applied. Care should be taken that the
card is uniform throughout in its thick-
ness and texture, and be perforated with
a circular hole in its centre, so that
when united to the needle, the equili-
brium of the whole may be perfectly
preserved. In order to fix it to the
needle, the latter is tapped with two
small screw-holes, at the distance of
about half an inch from each end ; and
the card being placed so that the meri-
dian line marked on it is in the same
vertical plane with the axis of the needle,
and holes being made in it opposite to
those in the needle, small screws are in-
troduced, so as firmly to fasten them
together. In order to secure the steadi-
ness of the compass during the violent
and irregular movements to which the
ship is liable, the suspension of the box
by the gimbals should be made with
great care; the several axes of motion
being so adjusted as that the point of
suspension on which the needle, with its
card, is supported, be exactly in the
same line with both these axes.

(238.) Complaints are frequently
made by seamen, that, in a rough sea,
the ordinary compasses are so unsteady
as to prevent their being easily ob-
served ; an inconvenience, which they
are apt to ascribe to the needle's being
too strongly magnetic, and therefore too
easily disturbed by the irregularities in
the motion of the vessel. This supposed
defect they endeavour to remedy by
adding a weight to the card : and this is
often done, very injudiciously, by load-
ing it with sealing-wax. Sometimes
they stick a few pieces of paper on the
under side of the card, to serve as vanes
which, acting upon the air, may create
a resistance to the oscillations of the
needle. It has even been proposed,
with a similar design, tQ make the
needle move in oil, or other liquid,
keeping it still suspended, as usual, on
its pivot — the fluid serving to check the
vibrations. But all these expedients,
calculated to diminish the mobility of
the needle, by counteracting the opera-

GO

MAGNETISM.

tion of its directive force, are productive
of an evil of a much more serious kind,
than any that can arise from mere un-
steadiness : for it is evident that the
very same cause which makes the com-
pass partake of the irregular motions of
the ship, forces it, in the same degree,
to deviate from its proper position in
the magnetic meridian. While the card
remains apparently steady, the steers-
man will pursue his course, unsuspicious
of danger, until the first warning of his
error may, perhaps, he the sudden ap-
pearance of a shore, from which he had
imagined himself at a considerable dis-
tance. The real remedy for the incon-
venient vacillations of the compass is
that we have already pointed out,
namely, the accurate adjustment of the
point of suspension in the line of the axis
of rotation of the gimbals, which, as we
have before observed, ought to intersect
one another at right angles, in that
same point. In addition to this, it may
be advantageous to increase the weight
of the magnet, provided its directive
force be at the same time augmented.
This may be effected by employing, as
the compass needle, a magnet of greater
thickness, or by combining several
needles together, laying them parallel to
one another; for if both the magnetic
power and the weight, (and conse-
quently the friction,) increase in the
same proportion, the directive power
will remain the same as before ; and the
compass, thus constructed, being hea-
vier, will be deranged to a less extent by
the same disturbing force : and when
deranged, will be brought back by the
directive force to its proper bearing, with
the same facility as in an instrument of
the ordinary construction.

(239.) It is to be recollected that if
a needle, in its unrnagnetic state, be so
constructed as that it shall be accurately
balanced when resting on a point at its
centre, and shall maintain itself in a
horizontal position, and if it be after-
wards magnetized, the influence of ter-
restial magnetism will cause it to as-
sume an inclined position, one of its
ends preponderating, as if it had ac-
quired additional weight. In order,
therefore, to restore the equilibrium,
and bring it back to the horizontal
plane, it will be necessary to add a cor-
responding weight to the other end of
the needle.

The degree of inclination in the un-
balanced needle, depends upon the
amount of the dip, which, as we have

seen, varies in different parts of the
world, according to the situation of the
place with regard to the magnetic poles
of the earth. Hence, when the compass
is transported to a distant part of the
globe, a different adjustment must be
made of the weight applied to correct
the tendency to dip. These adjustments
are best effected by means of a sliding
piece of brass placed under the needle,
and the position of which may change,
according to circumstances, on the one
side or the other, to any distance that
may be necessary In long voyages,
during which the changes of latitude are
considerable, the position of this regu-
lating weight requires to be frequently
shifted, in order to accommodate the
needle to the varying changes of incli-
nation incident to the variations of lati-
tude.

(240.) The Azimuth Compass differs
from the ordinary Mariner's Compass
only in the circumference of its inner
box being provided with sights, through
which any object, "either in the horizon,
or above it, may be seen, and its bear-
ings from the magnetic points of the
compass determined, by reference to the
position of the card, with respect to the
sights. For this purpose the whole box
is hung in detached gimbals, which turn
on a strong vertical pin, fixed below the
box, which is thus capable of being
moved round horizontally, and of the
sights being directed to whatever object
is to be viewed through them. On one
side of the box there is usually inserted
a nut or stop ; which, when pushed in,
presses against the card and stops it ;
this is done to enable the observer to
read off the number of degrees of the
card, which correspond with an index,
or perpendicular line, drawn in the in-
side of the box. They are also some-
times read off by means of a wire stretch-
ing from one sight to the other across
the centre of the card.

(241.) Analogous to this instrument is
the land or surveying-compass, which is
also furnished with sights, and means for
reading off the degrees on the card. This
latter object is effected in a very inge-
nious manner, by a contrivance of Mr.
Schmalcalder, for which he procured a
patent. The card is balanced in the usual
manner, and contained in a round brass
box, with two sights, the one to which
the eye is applied being furnished with
a triangular prismatic lens, and the
other being an open sight, with a vertical
horse-hair lineextending along its middle.

MAGNETISM.

01

The pupil of the eye being bisected
by the upper edge of the prism, as in
the Camera Lucidu, the object and that
part of the circumference of the card
on which the degrees are marked, are
seen at the same time ; the former by
direct vision, the latter by reflexion from
the internal surface of the inclined face
of the prism: and thus the coincidence
of the two may be accurately noted.
A prism of the same kind is also ap-
plied in Gilbert's patent Azimuth Com-
pass.

(-242.) The Variation Compass, de-
signed to exhibit the diurnal changes of
variation in the horizontal magnetic
needle, has generally a needle of much
greater length, than those of other kinds
of compasses ; and as it is not required
to move round the whole circumference,
the box, instead of being circular, is
oblong, so as to admit of a deviation of
only 20 or 25 degrees from the middle
line. A vernier scale, with a magnifier,
is usually applied in order to estimate
the changes of position of the needle
with greater precision.

$ 2. Of the Local Attraction of Vessels.

(243.) The indications of the mari-
ner's compass at sea are liable to error
from a cause which, till lately, had
never been supposed capable of affecting
the needle. It consists in the attraction
which the large quantity of iron, con-
tained in various parts of the ship, exerts
upon the magnetic needle : for although
the action of each individual piece of iron
may, at the distance at which it is placed,
be quite insensible ; yet the united action
of the whole quantity dispersed in every
part of the vessel, may amount to a
considerable sum, and occasion a very
perceptible deviation of the compass
from its true position in the magnetic
meridian. This will happen more espe-
cially in ships of war, which contain a
large number of guns, of iron-shot, and
of water-tanks, and various parts of the
frame-work of the ship which are now
made of iron.

(244.) If we suppose each particle
of iron to exert a certain attractive
force upon the magnetic poles of the
compass needle, according to a certain
law, hereafter to be determined, it
is easy to understand how the com-
bined effect of all these forces may be
considered as equivalent to one simple
resultant force acting in a certain direc-
tion. If the quantity of iron be con-
sidered as equally distributed on both

sides of the ship, and the compass be
placed, as is usual, in the binnacle, in
the after part of the ship, this resultant
force, which represents the combined
action of the iron, will be situated in a
vertical plane passing through the com-
pass, and through the axis of the ship,
and will, moreover, have a certain in-
clination to the horizon. In the northern
regions of the globe, the inductive in-
fluence of the earth on unmagnetic iron
consists in carrying the southern pola-
rity upwards, and the northern polarity
downwards, ($ 107,) in a ^direction pa-
rallel to that of the dipping'needle. The
action on the compass of a piece of
iron thus brought into a state of in-
duction will, therefore, be precisely
similar to that of a magnet having the
position of the dipping needle, and
placed at a considerable distance from
the compass. If it be placed, with re-
lation to the compass, exactly in the
magnetic meridian, (that is, to the mag-
netic north or south of the compass,) it
can have no effect in disturbing its po-
sition. This will generally be the case
when the course of the ship coincides
with the magnetic meridian, and the
needle of the compass is in the direction
of the axis of the ship. But if the ship's
head be turned to the eastward, and the
resultant force of the iron in the ship be
directed in a line downwards from the
compass, that force will be represented
by a magnet placed in the same oblique
line ; and the south pole of that magnet,
being uppermost, will act with most
power, and will attract the north pole of
the compass needle, causing it to deviate
towards the east. The same magnet,
placed to the westward of the compass,
which would correspond with the ship's
head being turned to the west, would oc-
casion a westerly deviation of the com-
pass needle. In the southern hemi-
sphere, when the inductive influence of
the earth has a contrary direction, the
opposite effects would result from the
action of the iron in the ship ; for the
action would then be represented by a
magnet, having a position with respect
to its poles, the reverse of what it had in
the former case.

(245.) The earliest record of any ob-
servation of the effect of this local at-
traction of vessels, occurs in the voyages
of Captain Cook ; but the reason of the
deviation of the needle does not appear
to have been suspected. The first dis-
tinct statement of the real cause of this
anomaly is contained in a report from

MAGNETISM.

Mr. Downie, Master of H. M. S. Glory,
in which there is the following passage :

« I am convinced that the quantity

and vicinity of iron in most ships have
an effect in attracting the needle ; for
it is found by experience that the needle
will not always point, in the same direc-
tion when placed in different parts of
the ship. Also, it is rarely found that
two ships steering in the same course,
by their respective compasses, will go
exactly parallel to each other ; yet these
compasses, when compared on board
the same ship, will agree exactly*/

(246.) The next observations on this
subject were those of Captain Flinders f,
who, whilst surveying the south coast
of New Holland, in H. M. S. Investi-
gator, in 1801 and 1802, remarked con-
siderable differences in the direction of
the magnetic needle, when there was no
other apparent cause for them, than the
differences in the direction of the ship's
head. This occasioned much perplexity
in laying down the bearings, as it was
very difficult to find the proper allow-
ances to be made for this deviation of
the compass in estimating them. With
a view of trying how far an alteration
in the disposition of the iron might
tend to remedy this source of error,
Captain Flinders first removed two guns,
which had stood near the compass,
into the hold, and afterwards fixed the
surveying compass exactly a-midships
upon the binnacle ; for at first it was oc-
casionally shifted to the weather-side as
the ships went about; but neither of
these two arrangements produced any
material effect in preventing the devia-
tions of the compass. When the ship's
head was to the east, the deviation was
westward ; and the contrary, when the
ship's head was to the west : when it
was nearly north or south, no deviation
was perceptible. These differences, aris-
ing from a change in the direction of
the ship with regard to the points of the
compass, were less considerable as he
proceeded to lower latitudes ; and on ap-
proaching the line of no variation, upon
the south coast of New Holland, the
deviations of the compass were smaller
than either before or afterwards. In
reasoning on the cause of these devia-
tions, he supposes * the attractive power
of the different bodies in the ship, which
are capable of effecting the compass,

* Walker's Treatise on Magnetism, published in
1794 ; quoted by Mr. Barlow in his Essay on Mag-
netic Attractions.

i Philosophical Transactions for 1805, p. 186,

to be collected into something like a
focal point, or centre of gravity ; and
that this point is nearly in the centre of
the ship, where the shot are deposited,
for here the greatest quantity of iron is
collected together/ He further sup-
poses that this point is endowed with
the same kind of attraction as the pole
of the hemisphere where the ship is ;
consequently, in New Holland, the south
end of the needle would be attracted by
it, .and the north end repelled. On this
hypothesis, which appears to be the true
one, he explains the phenomena he had
observed, and also deduces from it as
a necessary consequence, that the devi-
ations of the compass, arising from the
attraction of the iron in the ship, must,
when the ship is on the north side of the
magnetic equator, be directly the reverse
of those he had observed in the southern
hemisphere; that is, the north end of
the needle would be attracted, and the
south end repelled. This theory was
confirmed by other observations, made in
the same ship, in the British Channel.

(247.) The observations of Captain
Flinders excited considerable attention
at the time they were published ; and a
course of experiments was, in conse-
quence, made, by order of the Admiralty,
in various ships in the Nore. It was
found that, in every ship a compass
would vary considerably in its position
on being removed from one part of the
ship to another. Although the general
fact was completely established by these
experiments, they did not then lead to
any further investigation, until the sub-
ject was again brought into notice by
Mr. Bain, who, in a useful treatise which
he published on the Variation of the
Compass, placed in a striking point of
view the fatal consequences which might
attend this source of error. The atten-
tion of the public was also particularly
drawn to the subject at this time, in con-
sequence of the proposed expeditions to
the Arctic regions, from which it was ex-
pected that much important information
would result with regard to terrestrial
magnetism. The local attraction of the
vessels sent out on these expeditions was
made a particular object of inquiry ; and
the results of the numerous experiments
made for that purpose, are detailed by
Captains Ross and Parry in their ac-
counts of their respective voyages ; and
also by Captain Sabine, in a paper in
the Philosophical Transactions *. It is

* For 1819, p. 112,

MAGNETISM.

63

stated by the last of these observers that,
in the Isabella and Alexander, the bin-
nacle compasses of the two ships were
soon found to diiter very materially from
one another, in indicating the course
steered. The difference was frequently
one point, or eleven degrees and a quar-
ter. No dependence whatever could be
placed on the agreement of compasses
in different parts of the ships, or, of the
same compass with itself, if removed
but a few inches. Even in the neigh-
bourhood of the binnacles, the varia-
tion, as observed amidships, was from
8° to 10° greater than the result of
azimuths taken by a compass placed
between two and three feet on the lar-
board side, and an equal difference, in a
contrary direction, took place, on remov-
ing the compass to the starboard side ;
all of \vhich introduced great difficulties
in the ship's reckoning.

(2-iS.) An extensive investigation of
the subject was now instituted by Mr.
Barlow, with a view of discovering some
principle of computation, or other
method, for correcting this source of
error, in all parts of the world. The re-
sults of the first experiments he made
for this purpose, were published by him
in 1820* ; and, in 1824, there appeared
a second and greatly extended edition of
the same work, developing the mathe-
matical principles which regulate the
action of unmagnetic iron upon a mag-
netized needle. His situation, as Pro-
fessor of the Royal Military Academy of
Woolwich, gave him the means of pur-
suing his experiments upon a very ex-
tended scale ; as he could procure, with
facility, considerable masses of iron,
such as balls and shells of every deno-
mination, and having that regularity of
figure which was most favourable to
the application of mathematical formulae.
As the inquiry is important, not merely
from its application to the subject of the
local attraction of vessels, but also in its
bearings on the whole theory of magnet-
ism, we shall briefly state the principal
results which he obtained.

(249.) Mr. Barlow ascertained, that a
ball of iron produces no disturbance of
the compass needle, when the latter is
situated in any part of a plane passing
through the centre of the ball, and at
right angles to the direction of the dip-
ping needle, in the place where the ex-
periment is made. The angle of the

* Under the title of ' An Essay on Magnetic At-
tractions,'

inclination of this plane to the horizon
is, therefore, the complement to the
angle of the dip. In London, where the
latter may be taken at 70°, this angle is
consequently 20°. The section of this
plane of neutrality, as it may be called,
by a horizontal plane, passing through
the centre of the ball, will be a line
directed to the magnetic east and wesf.
If a hollow sphere, of considerable dia-
meter, be supposed to extend around
the ball, and to be concentric with it,
the plane above defined will, by its in-
tersection with the sphere, form a great
circle, which may be regarded as the
magnetic equator of that sphere, with
relation to the magnetic action of the
ball.

(250.) Another plane of neutrality is
constituted by a vertical plane, also
passing through the common centre of
the ball and sphere, and including the
magnetic direction, that is, the line of
the^dip : this plane is evidently that of
the magnetic meridian ; and it also inter-
sects a great circle on the imaginary
sphere.

We have termed these two planes the
planes of neutrality, in preference to
adopting the name of planes of no at-
traction, by which Mr. Barlow has de-
signated them, because, as Poisson has
remarked, it is not the whole of the
attractive force exerted by the iron ball
that vanishes in these planes ; but only
that part of this force which occasions
deviations in the natural position of the
needle, which, indeed, is the only force
of which we are now studying the effects.
Strictly speaking, however, there re-
mains another force, acting in a direc-
tion parallel to the dipping-needle, but
of an opposite nature to the action of
the earth, and tending, therefore, to re-
tard the oscillations of the needle.
There is, indeed, no plane in which the
attraction of a sphere, or, in general, of
any body magnetized by the earth's in-
fluence, becomes evanescent.

(251.) In like manner, other meri-
dional great circles may be conceived
on the sphere cutting the equator at right
angles, and meeting at the two poles of
that equator; and the situation of any
point at the surface of the sphere, may
be designated by its distance from the
equator, measured on the meridional
circle which passes through the point,
and which may be defined its magnetic
latitude; together with its distance from
any one meridian, fixed upon as the first
meridian, measured on a smaller circle

MAGNETISM.

parallel to the equator, and passing
through the point in question, which
distance might be termed its magnetic
longitude. Mr. Barlow assumes as his
first meridian, the circle which passes
from the pole to the magnetic east and
west points of the horizontal plane in-
stead of the vertical meridional plane. —
We cannot help thinking, however, that
the multiplication of these planes would
have been better avoided, by assuming
the latter, necessarily referred to on so
many occasions, as the first meridian.

(252.) Having settled these definitions,
the law of action deducible from the ex-
perimental investigation of Mr. Barlow,
may be very simply expressed. The
amount of the angular deviation of a
compass needle, the motion of which is
limited to a horizontal plane, from the
true magnetic meridian, at any point on
the surface of the sphere, is such, that
the tangent of the angle of deviation is
directly proportional to the rectangle of
the sine and cosine of the latitude of that
point multiplied into the cosine of its
longitude. As it is extremely convenient
to express propositions of this kind in
the concise and perspicuous language of
algebra, we shall present the above pro-
position in that form ; denoting the angle
of deviation by the symbol A ; the lati-
tude by A ; the longitude by I. The
formula will then be as follows,

tan. A = sin. x cos. x sin. /.
But since the product of the sine and
cosine of an angle is equivalent to the
sine of twice that angle, the formula
admits of this simplification, and it will
then be,

tan. A = sin. 2 A cos./.

(253.) The results of a numerous
series of experiments made by Mr. Bar-
low, when the centre of the compass was
placed in every variety of position, with
respect to an iron ball, approximated so
closely to those which were given by
computation from the above formula,
that no doubt can remain of the accu-
racy of the law from which it is deduced.
They have been further verified by Mr.
Christie, by a somewhat different method
of procedure, of which he has given an
account in the Transactions of the Cam-
bridge Philosophical Society *.

(254.) The next object of inquiry was
the law of attraction, with relation to
distance ; and the result at which Mr.
Barlow arrived was, that, when the
position, with regard to latitude and lon-

* Vol. i. P. H7.

gitude, remains the same, the tangents
of the angles of deviation are reciprocally
proportional to the cubes of the dis-
tances. Now as it has been estab-
lished, that the magnetic force varies
inversely as the square of the distance,
it will follow, that the square of the tan-
gent of deviation is directly as the cube
of the force ; or, that the tangent of de-
viation is directly proportional to the
| power of the force. In order to con-
vert this proportionality into an equa-
tion, it is necessary to introduce a certain
constant co-efficient for the number ex-
pressing the distance. This co-efficient,
when the distance is estimated in inches,
Mr. Barlow finds to be .00080382. If
this be called A, and the distance de-
noted by d, the formula, comprising all
the variable quantities in one equation,
becomes,

tan. A =

Ac/3

(255.) The influence of the mass, and
also of the surface with relation to the
mass, of the iron sphere in modifying its
action, were next made the subjects of
investigation. Having at first employed
solid balls, weighing, respectively, 288
and 128 pounds, the results appeared to
lead to the conclusion, that the tangents
of the deviations were proportional to the
cubes of the diameter, that is, directly
as the masses. But when similar ex-
periments were made with hollow shells,
of the same diameter as the former balls,
Mr. Barlow was not a little surprised to
find that no difference was perceptible
between the results of these and of the
former trials. Hence he concluded,
that the power of attraction was inde-
pendent of the mass, and resided wholly
in the surface of the metal ; and all sub-
sequent- experiments confirmed the ac-
curacy of this conclusion. The in-
ference he drew was expressed in
the following proposition, namely, that
the tangents of the deviation are pro-
portional to the cubes of the diameters,
or to the square root of the cube of the
surfaces, whatever may be the weight or
thickness of the sphere. Subsequent
experience, however, taught him that
this law is subject to a limitation in re-
spect to the thickness of the metal in
which the magnetic power resides ; for
if that thickness, be less than the thir-
tieth of an inch, the power is not fully
developed, and its action is diminished.

This conclusion has been since veri-
fied by Captain Kater, who found, on

MAGNETISM.

65

employing three cylinders of iron, the
one being solid, and the other two hol-
low, but all equal in surface, that the
deviation of the compass needle, occa-
sioned by the attraction of soft iron,
depends on the extent of surface of the
iron, and is wholly independent of the
mass ; excepting a certain thickness,
Amounting to about two-tenths of an
inch, which is requisite for the full de-
velopment of its attractive energy *. It
may be remarked, by the way, that the
circumstance of the effective power be-
ing limited to the surfaces of bodies, or
nearly so, is another striking instance
of the analogy which subsists between
the magnetic and the electric agencies.
These inductive results of observation
are all in strict conformity with the theo-
retical deductions of Poisson already
adverted to, § 163.

Introducing into the general forr
mula this new variable quantity,
namely, the diameter, or radius, of the
sphere of iron, which we shall express
by r : it becomes

tan. ^ ""- 2* cos-*-
Ac?3

(256.) These rules and formulae are
capable of being applied in another
manner ; for, instead of conceiving the
imaginary sphere to surround the iron
ball, we may imagine a similar sphere
concentric with the point of suspension
of the needle ; and it will then be obvious
that the centre of the ball will have the
same relative position in the latter sphere,
as the pivot of the compass has with re-
spect to the former ; so that the refer-
ence may be made indifferently to either:
and when the mass of iron is irregular,
which is the more usual case, it will be
more convenient to refer the common
centre of attraction of the iron to an
imaginary sphere circumscribing the
compass.

(257.) It must be observed, however,
that in every instrument a limit exists
within which the above law ceases to
obtain. This limit arises from the influ-
ence which the inductive power of the
needle may exert upon the iron pre-
sented to it ; lor we have already seen
that the consequence of this induction
is attraction of the adjacent pole of the
magnet, whichever pole that may hap-
pen to be. Hence it follows that when
the compass is brought so near to the

iron, as to act upon it by induction, the
laws above determined are superseded
by those dependent on this latter cause,
and are therefore no longer applicable.
In all the experiments made by Mr.
Barlow, care was taken that the dis-
tances should be such as to be entirely
exempt from this disturbing cause.

(258.) Having established the law of
action on the compass, as far as regards
masses of iron of regular geometric
forms, the next object was to determine
whether the same law obtains with
masses of irregular shapes. This would
evidently not be the case, if the popular
notion were true, that the poles of apiece
of iron, under the influence of terres-
trial induction, reside exclusively at the
opposite extremities of the mass ; where-
as if the entire action admits of being
referred to one common centre of at-
traction, in the same manner as the com-
bined effect of the gravitation of all the
particles of a body of irregular figure
may be considered as directed on a sin-
gle point, known by the name of the
centre of gravity, it is reasonable to
expect that the same laws are common
to both. Experiments tried, with this
view, upon a twenty-four pounder,
showed the existence of a plane of
neutrality in the most irregularly- shaped
masses of iron, and completely esta-
blished the identity of the operation of
the attractive and repulsive forces in all
cases, whether the iron was presented
in isolated masses, or dispersed in every
variety of situation throughout the ship.

(259.) The actual amount of deviation
produced in the ship's compass by its
local attraction, will, of course, be dif-
ferent in different vessels. With an
easterly or westerly course, it has been
observed in these latitudes to vary from
five to twelve or fourteen degrees : it is
of greater amount as the ship is in
higher latitudes ; and diminishes, with-
out however vanishing, at the equator ;
and again increases as we approach the
south pole. Mr. Barlow, iu a paper
lately published in the ' Philosophical
Transactions*,' gives the following
table of the deviation observed in dif-
ferent ships, on the best authorities,
from which a general idea may be
formed of the extent of error that may
thus arise, and also of its average
amount.

• Philosophical Transactions for 1819, p, 129.

« For 1831, p. 217.

66

MAGNETISM.

loc.,,1

Attraction.
33'
7
30
27
22
36
43
30

Ship. Commander. Pit

Conway Capt. Basil Hall Portsmouth

Leven . Capt. Owen , . Northfleet

Barracou a Capt. Cutfield . Do.

Hecla . Capt. Sir E. Parry Do.

Fury . Capt. Hopner . . Do.

Griper . Capt. Clavering . Nore . .

Adventure Capt. King: • • • Plymouth

Gloucester Capt. Stuart . . Channel .

giving a mean of 8° 44' at the east and
west points in these latitudes.

(260.) The last of these ships, the
Gloucester, was reported as being ' in-
variably drawn, in consequence of this
deviation, to the southward of her in-
tended place, notwithstanding the great-
est care being taken in steering her/
Had it not been ascertained, by taking
an observation, that this error was al-
together the effect of local attraction, it
would probably have been ascribed to
the influence of an unknown current.
The real deviation, estimated in dis-
tance, would occasion the vessel, after
running ten miles, to be more than a
mile and a half to the southward of her
reckoning ; and so on in proportion as
the distance increased. An error of
this magnitude, occurring in a narrow
channel and in a dark night, were it
unknown or disregarded, might lead to
the most disastrous consequences. The
wreck of his Majesty's ship Thetis,
which lately happened on the coast of
Brazil, has been ascribed, with some
probability, to a mistake of this kind.
The following is the account given of
this accident in the * United Service
Journal : ' — ' The Thetis sailed from Rio
Janeiro on the 4th of December, with a
million of dollars on board, besides
other treasure, and every prospect of a
fine passage, stretching away to the
south-east. The next day, the wind
coming rather favourable, they tacked,
thinking themselves clear of land ; and
so confident were they, that the topmast
studding-sails were ordered to be set,
the ship running at the rate of nine knots ;
and the first intimation they had of being
near land, was the jib-boom striking
against a high perpendicular cliff, when
the bowsprit broke short off, the shock
sending ail three masts over the side ;
thus in a moment bringing utter de-
struction on this fine vessel and her
valuable cargo.' Mr. Barlow shows, in
the paper above referred to, that the
deviation of the compass, arising from
the attraction of the vessel, was exactly
of the kind that was likely to occasion
this great mistake in the ship's reckon-
ing : for the distance run by the Thetis

being about eighty miles, if the local at-
traction of the vessel had been equal to
that of the Gloucester, she would have
passed five miles nearer to Cape Frio
than had been calculated" upon; an
error quite sufficient to account for the
fatal catastrophe.

(261.) It' is obvious that, when the
cargo of the ship consists chiefly of iron,
the error in the reckoning may be even
more considerable than what has been
now stated. The most fatal conse-
quences might arise in a few hours to a
vessel in the Channel, under these cir-
cumstances, in a dark and blowing
night, having for its only guide a com-
pass, subject to an error of fourteen de-
grees in opposite directions at. east and
west, the very courses on which she
would be endeavouring to steer. How
many of the mysterious wrecks that
have taken place in the Channel might
not be traced to this cause ! The loss
of the Thames Indiaman is given as an
example by Mr. Barlow *. This vessel,
besides the usual appointments of guns,
&c., had a cargo of more than four
hundred tons of iron and steel. The in-
fluence of such an enormous magnetic
mass would alone be quite sufficient to
explain the otherwise unaccountable cir-
cumstance, that after leaving Beachey-
head in sight at six o'clock in the even-
ing, the "ship was wrecked upon the
same spot between one and two o'clock
in the morning, without the least appre-
hension of being near the shore.

(262.) The practical application of
the principles above established to the
correction of the actual deviations of
the compass in a ship, being, as we
have seen, of such great importance
in navigation, Mr. Barlow bent his
mind to the discovery of a method of
effecting so desirable an object. His
first idea was, that since the guns and
other iron of a vessel must produce
exactly the same deviation of the needle
as a smaller mass of iron placed in a
similar situation, but as much nearer as
its mass is smaller, it might be possible
to place such a body of iron aft of the
compass, as would exactly counter-
balance the action of the guns, &c., for-
ward, and consequently leave the needle
as free to move, as if no such action
existed: but he soon found that, for
this purpose, the position of the compen-
sating ball of iron would require to be

* Essay on Magnetic Attractions, p. 36?.

MAGNETISM.

67

shifted for every different position of
the ship, which would, of course, be
impracticable. He therefore had re-
course to the following expedient, which
was found to answer perfectly under all
circumstances of situation. Since it is
possible to place a ball of iron in the
same line of direction, with regard to the
compass, as that in which the com-
bined action of the iron of the ship is
exerted, and to bring it to the exact dis-
tance at which its action shall be equal
to that of the ship's iron, it is obvious
that a ball so placed will, instead of de-
stroying the deviation of the compass,
double its amount; and that this will
be the case under all circumstances,
and in every part of the world. Instead,
therefore, of fixing the ball, let its pro-
per place be first determined, and the ball
itself laid aside; then, at any time when
it is desirable to ascertain what effect
is due to the magnetic attraction of the
ship, let it be applied in the situation so
determined, and observe how many de-
grees it draws the needle of the compass
from the direction it had previously
to the application of the ball. This will
be the amount of the actual deviation
produced by the iron of the ship ; and
the correction in the course of the vessel
may be applied accordingly. Strictly
speaking, it is not the angle of deviation
which is doubled by the action of the
ball, but the tangent of that angle ; but
as, in small angles, the tangents are
very nearly in the ratio of their arcs,
they may in most cases be taken, with-
out sensible error, as the same.

(263.) As the effect to be obtained
depends on the surface, and not on the
mass of the iron which acts, Mr. Bar-
low has found it more expedient to em-
ploy plates of iron, instead of balls. The
form he recommends is a double plate,
composed of two thin plates of iron,
screwed together in such a manner
as to combine any strong irregular
power of one plate, with a correspond-
ing weak part of another; by which
means a more uniform action is ob-
tained. These plates are of a circular
form, twelve or thirteen inches in diame-
ter, with a hole in their centre, through
which is passed a brass socket, with an
exterior screw; a brass nut, about an
inch and a half in diameter, screws on
the exterior of each end of the socket,
thereby pressing the plates together;
with an interposed thin circular piece
of board, which is intended to increase
in some degree the thickness of the

plate, without adding to its weight. It:
would appear also that the compound
plate is more powerful when the two, of
which it is formed, are thus separated
from each other *. The proper position
of the plate, with regard to the compass,
must be ascertained by trials on shore ;
comparing its effects, in different rela-
tive situations, with the observed devia-
tion of the compass on board the ship.

(264.) Although the method proposed
by Mr. Barlow be exceedingly inge-
nious, and will, no doubt, to a certain
extent, prove highly useful, several
causes exist in practice which must
interfere with the regularity of its ope-
ration. Changes of temperature will
probably affect the compass-needle, the
compensating plates, and the large
masses of iron contained in the ship, in
very different degrees; and many of
the latter bodies will be more or less
susceptible of acquiring permanent mag-
netism in the different circumstances in
which they are placed. In the course
of a long voyage, extending to very dif-
ferent latitudes, these causes are liable
to considerable variation, and must in-
troduce a degree of uncertainty in the
amount of the changes induced. Still,
however, the method of Mr. Barlow
will furnish a most valuable approxi-
mation to the correct determination of
the influence which the ship exerts on
the needle of the compass. Certain it is
that the proper estimate of the disturb-
ing force arising from this cause has, of
late years, acquired increased import-
ance from the very large proportion of
iron now employed in the construction
of ships of war, and of the machinery
for their guidance. Independently of the
guns, shot, and iron water-tanks, the
knees of the ship, the capstans, and
cables are now made of iron, so that the
whole forms a very large and powerful
magnetic mass.

(265.) In all situations, but more
especially in high magnetic latitudes,
experience has shown the advantage of
adopting an expedient originally sug-
gested by Captain Flinders: namely,
the selection of some particular spot in
the ship as the permanent position of a
standard compass, in which it should
be invariably placed for use, whether in
observing azimuths, or bearings of land,
or in directing the ship's course-: so
that if, on any particular occasion, it

* Barlow's Essay on Magnetic Attractions. Se-
cond edition, p, 100.

F2

68

MAGNETISM.

should be necessary to use a compass
in any other part of the ship, a refer-
ence should be made to the standard of
comparison, and the difference, if any,
in its pointing noted and allowed for ;
a certain degree of uniformity being
found to obtain in the effects of the
local attraction on a compass thus con-
fined to one spot, enabling a navigator
to form a sufficiently correct judgment
of the different amounts of variation to
be allowed with it on each change in the
direction of the ship's head *.

(266.) Not only are the compasses on
ship-board disturbed by the magnetic
attraction excited by the iron existing in
the vessel ; the chronometers also are
affected by the same influence. The
sudden alteration 'in the rates of chro-
nometers at sea had been frequently
noticed by intelligent seamen, but had
been generally ascribed to the motion
of the vessels. The true cause was first
pointed out by Mr. George Fisher, who
accompanied Captain Buchan in his
voyage to the Arctic Regions, in the
year 1818, and who gave an account of
his observations on this subject to the
Royal Society t. He found that the
chronometers on board the Dorothea
and Trent had a different rate of going
from what they had on shore, even
when these vessels had been frozen in,
and therefore when their motion could
not have contributed to that variation.
It appeared that this effect could be
attributed only to the magnetic action
exerted by the iron in the ships upon the
inner rim of the balance of the chro-
nometers, which is made of steel. A
similar influence was perceptible on
placing magnets in the neighbourhood
of the chronometers. This conclusion
was confirmed by the experiments made
for this purpose by Mr. Barlow, who
ascertained that masses of iron, devoid
of all permanent magnetism, occasioned
an alteration in the rates of chronome-
ters, placed in different positions in
their vicinity. The alterations varied
according to the positions of each chro-
nometer with relation to the magnetic
equator of the masses of iron to whose
influence it was subjected, and was al-
ways uniform in the same position. In
the case of the chronometers on board
the Dorothea and Trent, their rate was
always accelerated. Mr. Barlow found,
however, that this depends on the cir-

* Parry's Journal of a Voyage for the Discovery
of a North- West Passage, &c., Appendix, p. cxviii.
f Philosophical Transactions for 1820, p. 196.

cumstances of the case, for in other in-
stances they were retarded. He sug-
gests that great care ought to be taken
to keep the chronometers on board of
any ship out of the immediate vicinity
of any considerable mass or surface of
iron. They ought not, for instance, to
be kept in the cabins of the gun-room
officers, which are on the sides of the
vessel ; as probably a strong iron knee,
or even a gun, will be found at a very
inconsiderable distance from the spot
where the watch is deposited.

Mr. Barlow proposes to ' rectify this
error by a method similar to that which
he employs for the correction in the
compass, namely, by previously ascer-
taining what the effect of the ship's iron
is upon the rate of the chronometer.
This may be done : by means of a box
or pedestal, on the top of which is a
convenient receptacle for the chrono-
meter, and in the side of which a brass
pin is fixed, to carry the compensating
double iron plate, employed to repre-
sent the action of the ship's iron on the
compass. Then, having ascertained the
rate of the chronometer in the usual
manner, let the rate be again taken
while it is placed on the pedestal. The
plate should generally be kept at the
distance of about a foot from the ver-
tical line, through the centre of the dial ;
and its centre should be about the same
depth below the plane of its balance.
The rate thus obtained will be a very
close approximation to the ship's rate
of the instrument, provided care be
taken to keep it out of the immediate
action of any partial mass of iron, and
to place it in the same direction with
respect to the ship's head as it had
with respect to the iron plate when its
rate was determined *.

§3. Of the Azimuth Compass.

(267.) The purposes to which the
azimuth compass is applied, and the
general principles of its construction,
have already been stated in § 240 ; but,
for the sake of those who are desirous
of making practical use of it, it will be
necessary to enter into a fuller detail.

The ordinary azimuth compass is
represented in fig. 63. The semicircle
AB is fixed by a screw at its middle, or
lowest point, to a stand at the bottom
of the outer box, containing the whole
apparatus, in such a manner as to ad-

* Barlow's Essay on Magnetic Attractions, p.

MAGNETISM.

69

mit of its being turned round horizon-
tally, and placed in all azimuths. To
the upper extremities of this semicircle,
a brass circle, CD, is fixed by two

pivots GG, constituting; a horizontal
axis of motion ; while the inner cylin-
drical brass box, PQ, containing the
compass itself, is attached to the brass

Fig. 63.

circle, CD, by similar pivots, of which
one is seen at g, forming a horizontal
axis at right angles to the former, and
both together acting as gimbals. The
compass, with its card, is balanced in
the usual manner on a pointed pivot
rising from the centre of the bottom of
the inner box, the upper side of which
is covered with a circular piece of glass.
The two sights, E and O, are fixed ver-
tically on the upper side of the cylinder
of this box, diametrically opposite to
each other ; the one, E, to which the eye
is intended to be applied, consists of a
brass slip, having a narrow vertical
slit ; the other, O, which is turned to
the object, is a similar slip, having an
oblong aperture containing a fine
thread, or horse hair, passing along the
middle of the open space in a vertical
direction. Two vertical lines are also
marked on the inside of the box, which
are prolongations of the slit in the sight
for the eye, and of the thread in that
for the object. These lines are intended

as indexes for the measurement of the
angular distance in azimuth of an ob-
ject viewed through the two sights,
from the place of the magnetic meri-
dian, as shown by that portion of the
graduated edge of the card, which coin-
cides with the line with which it is com-
pared. The degrees are reckoned from
the north point of the compass, which
is marked zero, all round the circle, in
the direction from left to right, that is
from north to east, and thence to south
and west.

(268.) Sometimes a wire is placed be-
tween the two sights, stretching hori-
zontally from the foot of the one to that
of the other. This is intended as an
index to ascertain coincidences with the
degrees marked on the card, when they
are viewed from above : but as this is
a mode of using the instrument that is
seldom practised, this wire is usually
omitted, and observations made solely
by means of the vertical lines.

(269.) On one side of the box con-

MAGNETISM.

tain ing the compass there is frequently
inserted a nut or stop, which, when
pressed, bears, by means of a lever
within the box, against the card, and
arrests its movement : thus giving the
opportunity of reading off the number
of degrees more at leisure, and there-
fore more correctly, than could be done
at the moment of observation.

(270.) Sometimes the sights are, for
the convenience of carriage, made, by
means of joints at their feet, to fold
down over the glass which covers the
box, when the compass is not in use.
In other cases, they are united by a
transverse bar, and made so as to be
capable of being removed from the box
when the instrument is set aside.

(2 71.) The instrument, in its common
form, as above described, is still ex-
ceedingly defective, and incapable of af-
fording any very accurate observations.
Sufficient evidence of its imperfections
may be collected from the narrative of
Captain Phipps's voyage; from which
it appears that, although the observa-
tions were made with all possible care,
yet differences in the variation were fre-
quently occurring, in the same place
and at the same time, amounting to
two, three, and four degrees. In one
instance, the error was even five de-
grees ten minutes.

(272.) The azimuth compass contrived
by Captain Kater is a much more per-
fect instrument, combining all the ad-
Vantages of the ordinary construction,
together with those of being extremely
portable, and of being adapted to every
purpose of observation, whether at sea
or on land. A short description of this
compass, but without any figure, is given
in the Instructions for the Adjustment
and Use of the Instruments intended for
the late Northern Expeditions, which
were printed by order of the Royal So-
ciety in 1818. We have been enabled
to illustrate the following account of
this instrument by the annexed figures,
exhibiting its construction ; for the op-
portunity of drawing which we are
indebted to the kindness of Captain
Kater.

AB, fig. 64, is a brass cylindrical
box, containing the compass, of which
the card, CD, is five inches in diameter.
The needle, which is perforated in its
centre to admit of an agate cap, set in
brass, for the purpose (if suspension, is
fixed to a circular pir e of talc, over the
circumference of whi i a narrow circu-
lar ring of card is laid ; the outer mar-

gin of this card is accurately graduated
into half degrees. The breadth of the
cylindrical box is exactly one inch, and
it is covered as usual by a piece of

Fig. 64.

m

glass. A slanting piece of ivory is fixed
to the inner side of the box, so as just
to come over the outer edge of the gra-
duated circle of the card : a line, at right
angles to the circumference of this
circle, is marked upon the ivory, to
serve as an index for reading off the
degrees in the manner to be presently
mentioned.

To the opposite side of the box, at O,
a sight is affixed, consisting of a brass
frame, in the form of a parallelogram,
five inches long. To this frame is
adapted a shorter frame, Yf, two inches
in length, which slides upon it, and carries
the segment of a glass cylinder, ground
to a radius of five inches. By means
of this piece of glass, when presented to
the sun, the rays are collected into a
linear focus ; the line of light being
thrown on the index on the piece of
ivory, may be seen at the same time as
the degrees on the card.

This sight has a hinge where it is con-
nected with the box, by means of which
it can be folded down upon the glass
cover of the box, as seen in fig. 66 :
and when thus folded, it raises the
needle of the compass by means of a
lever under its centre, seen at L, so as
to press it against the glass cover, and
prevent its moving.

The sight, to which the eye is applied,
and which is shown separately in fig,
65, is an inch in height from its hinge
to its upper point ; but it may be raised
somewhat higher by means of an up-

MAGNETISM.

71

right piece, which slides between two
grooves in the side of the box ; it con-
sists of an upright plane, P, having a

Fig. 65.
S

narrow vertical slit, S, in its upper
part ; below this is a circular aperture,
in which a convex lens is placed : a ho-
rizontal plane, H, proceeding from the
lower edge of the former, and also fur-
nished with a convex lens, and a mir-
ror, M, is placed behind P, and inclined
at an angle of forty-five degrees. By
means of this combination of lenses
with the mirror, the degrees on the card
are seen by reflexion, considerably
magnified, and in a reversed position,
together with the index on the ivory,
which is contiguous to the part viewed.
The image of the card, beinij produced
by one reflexion, is reversed ; on this
account it is requisite that the figures
expressing the number of degrees should
themselves be reversed, so that they
may read correctly when viewed through
the lenses. This sight may be raised
or lowered as much as is necessary to
adjust its focus ; and the whole is made
to turn back, by means of a hinge, so
as to lie in Ihe same plane with the box
when the instrument is placed in its
case, as represented in fig. 66, which
admits of its being carried in the pocket.

Fig. 66.

(273.) The following is the method
of using this instrument, when the azi-
muth of the sun is to be taken. Ele-
vate the object- sight, and turning it to-
wards the sun, slide the glass along it
till the line of light is thrown on the
ivory index. The sight next the eye is

then to be adjusted, by raising it from
the position in which it lies when flat in
the case, till its horizontal lens is over
the edge of the card ; and the pro-
per focal distance obtained by sliding
it in the dove-tailed groove till the in-
dex line is seen distinctly. Next ob-
serve whether the line of light from
the sun seen on the piece of ivory
through the lens appears narrow and
well-defined ; and if it does not, incline
the sight furthest from the eye towards
the compass, till the requisite distinct-
ness is attained. Be careful that the
sight leans neither to the right nor left,
but is held perpendicular to the horizon,
in the direction between the sun and
the observer; for the neglect of this
precaution is the principal source of
error to be apprehended. Let the com-
pass be now inclined towards the ob-
server, so as to check the oscillations of
the card, by bringing it in contact with
the index and two pins fixed near it for
the purpose. Do this repeatedly, till
the card is steady, the compass being
sufficiently inclined from the observer
just to free the card from the index.
The line of light being then accurately
bisected by the index line, the degree
and fractional parts also indicated by
this line may be read off at the mo-
ment that an assistant takes the altitude
of the sun. If the card should happen
not to be perfectly steady, the mean of
its vibrations may be readily estimated.
The degrees on the card are read from
the north towards the east, and are car-
ried round to 360°, in order to obviate
the possibility of error in this respect. To
the degrees and minutes thus obtained,
must be applied the correction written
on the card, and the result will be the
true magnetic position of the sun, from
which, and the observed altitude, the
variation of the needle may be obtained
in the usual manner. When the varia-
tion is to be determined for the purpose
of correcting the ship's course, it is
sufficient, and indeed necessary, that
the magnetic azimuths should be taken
without any reference to the local at-
tractions which may affect the needle ;
but for scientific deductions, after a cer-
tain number of observations have been
obtained with the ship's head in one di-
rection, she should be put on an oppo-
site course, and another set of observa-
tions taken frc;a the same spot: the
mean of the twu. results will be the true
variation of the , ;edle.
(274.) Captain Kater's azimuth com-

72

MAGNETISM.

pass is also well adapted for surveying,
for which object, indeed, it was origi-
nally invented. To apply it to this pur-
pose, nothing more is necessary than to
slide the frame containing the segment
of the glass cylinder to the top of the
sight, when the hair will be seen, which
must be made to bisect the object viewed
by direct vision at the moment that its
bearing is also read off by reflexion.

(275.) There is also another mode of
using this compass, which may, per-
haps, be found more convenient and ac-
curate than that already described. It
is simply to turn back the reflecting
sight, and to view the line of light, and
read off the degrees by direct vision;
and it has this decided advantage, that
if the compass should not be in a hori-
zontal position, the observer may readily
perceive and correct the error. Some
care, however, is necessary not to mis-
take in reading the figures indicating
the degrees, they being inverted as
marked upon the card : this may be pre-
vented by viewing them also by re-
flexion.

(276.) On approaching the north
pole of the earth, the north end of the
needle will incline downwards ; but the
card may again be readily balanced by
taking out the ring and glass, and at-
taching a small bit of wax to the south
pole of the needle.

(277.) When it is considered how
great is the diminution of the power
with which the magnetism of the earth
acts upon the horizontal compass needle
in very high magnetic latitudes, the sa-
tisfactory results which have been ob-
tained, even under such extreme cir-
cumstances as those of the late arctic
voyages, — in Davis's Straits, for in-
stance, and Baffin's Bay, — from the em-
ployment of Captain Kater's azimuth
compass, which gave correct observa-
tions when other instruments became
useless, afford the best testimony of its
excellence, and of the precision which
may be expected from its employment
in the ordinary course of observation *.

(278.) In some azimuth compasses,
for which patents have been taken, a
triangular glass prism is substituted for
the mirror in the above instrument, act-
ing evidently on the same principle of
reflexion, and evidently borrowed, with
a trifling alteration of form, from the
original invention of Captain Kater.
Coloured glasses are sometimes pro-

* See Captain Sabine's observations in the Philo-
sophical Transactions lor 1819, p. HI.

vided for making observations on the
sun ; they are placed so as to be readily
interposed between the eye and the
nearest sight when wanted, and are
removeable at pleasure when not re-
quired.

$ 4. Of the Variation Compass.

(279.) A magnetic needle intended
to indicate the minute changes that
take place in the direction of terrestrial
magnetism, should, as already noticed
(§ 242), be of somewhat greater length
than an ordinary compass, in order that
the extent of the variations of angular
position may be more conspicuous.

(280.) For the same purpose, the fol-
lowing method was practised by Du
Hamel : — At each extremity of a long
needle, a slender, pointed piece of steel
was erected perpendicularly, which
served as sights for observing its posi-
tion with reference to the divisions of a
graduated limb, six feet in length, fixed
to a pillar at the distance of nearly sixty
feet from the needle, and in the direc-
tion of its axis *.

(281.) Analogous to this was the con-
struction employed by Mr. Prony, con-
sisting of a long magnetic bar, on which
was fixed a telescope, moving along
with it ; its motion being observed by
looking through it at a distant object,
the image of which would have a cor-
responding motion in the field of the
telescope. Humboldt, who made many
observations with this instrument,
considered it to be a very accurate me-
thod.

(282.) But a point of much greater
importance is that the magnetism of the
needle should be uniform, and that its
magnetic axis should remain perma-
nent. Hence its form should be of the
simplest kind, such as that of a slender
needle of nearly equal diameter through-
out, and magnetised with great care.
The observation of minute changes of
position may be made with sufficient
accuracy by means of a magnifying
glass; which expedient will supersede
the necessity of employing sights, or of
giving to the needle any extraordinary
length.

(283.) Another material point is to
obtain great delicacy of suspension, so
that the needle shall immediately obey
the slightest change of direction in the
force of terrestrial magnetism, or of

* Histoire de 1'Academie Royale des Sciences de
Paris, for 1772, part ii., p. 50.
f Biot, Traite de Physique, torn, iii., pp. 143, 144.

MAGNETISM.

73

any^other extraneous magnetic force.
This object can hardly ever be suffi-
ciently attained by balancing the needle
upon a point, as in the common com-
pass ; because, however small the fric-
tion may be, it is still a force which is
required to be overcome at the begin-
ning; of every new motion, and which
must even prevent all motion until the
moving power has increased to a cer-
tain amount. This objection does not
apply in the same degree to the suspen-
sion of the needle by a fine thread, which
is accordingly the best plan of construc-
tion for a variation compass. Care
should, of course, be taken that the force
of torsion be as small as possible. Mr.
Bennet proposed a spider's thread as the
best material for obtaining great delicacy
of suspension, and procuring the greatest
magnetic sensibility: for although twisted
through many thousand turns, it occa-
sioned no sensible deviation in a needle
suspended by it ; showing that its force
of torsion is insensible *.

(284.) The thread should be con-
tained in a vertical tube, fitted to the
middle of the upper side of an oblong
box, the remaining parts of which are
to be completed by glass plates, for the
purpose of protecting the needle from
agitation by the air. With a graduated
arc adapted to each end of the needle,
and magnifiers to observe the exact po-
sition of the extremities when referred
to these arcs, this simple form of the
instrument, which is the one employed
by Captain Kater, is calculated to an-
swer every practical purpose that can
be desired. Its superiority to the ordi-
nary construction was shown on the oc-
casion of the late Northern Expeditions,
when it was found that the friction on
the metal point in the variation needle
belonging to Mr. Browne, made by Dol-
lond, nearly a foot in length, and sus-
pended in the usual manner by an
agate cap on a metal point, was, in
the high magnetic latitudes reached
by Captain Parry, too considerable to
be overcome by the directive power of
the magnet ; and accordingly it hap-
pened that at Winter Harbour the in-
strument was quite useless, while the
one furnished by Captain Kater still tra-
versed t.

(285.) Considerable light, however,
may be thrown upon the causes that

* Philosophical Transactions for 1?92, p. 81.

t Parry's Journal of a Voyage for the Discovery
of a North- West Passage, &c. See Appendix, p.
cxvi.

produce the minute diurnal or 'monthly
changes in the variation, by adopting
the expedient suggested by Mr. Bar-
low, and which we have already slightly
alluded to, $ 122. From a variety of
considerations, we are warranted in_'con-
cluding that the direction assumed by
the magnetic needle is the result of a
great number of magnetic forces acting
upon it, some of which are of a nature
more permanent than the rest. Thus,
while the average direction is the re-
sultant of some cause of very general
operation, affecting extensive portions
of the globe, many occasional changes
are effected by causes of a more tran-
sient nature, some of which are periodi-
cal in their influence, but of which
others are, as it were, accidental, or
at least very irregular and fluctuating
in their action. Innumerable obser-
vations have proved that the compass-
needle is more or less agitated during
the prevalence of the aurora borealis *.
Its deviation from this cause has been
known to amount to six or seven de-
grees. Volcanic eruptions have been at
various times observed to occasion con-
siderable disturbance in the position of
the needle : this was particularly noted
during the eruptions of Mount Hecla
and of Vesuviust . Atmospherical
changes, such as violent winds, or a fall
of snow, have, in like manner, been
known to affect the needle. The elec-
trical conditions of the atmosphere, and
especially those connected with the ap-
proach or occurrence of thunder-storms,
have a powerful influence on magnetic
polarity.

(286.) It is evident that as the needle
in its ordinary states is urged to move by
a force resulting from the influence of
these variable forces, combined with
those that are of constant operation, the
effect of the former would be much
greater if the latter were withdrawn ;
and this can only be effected by neu-
tralizing the operation of these constant
forces. Mr. Barlow effected this by ap-
plying one or more magnets in the re-
quisite positions, so as to counteract
almost entirely the natural magnetic in-
fluence of the earth. In illustration of
which he gives the following example $.
Supposing that a finely suspended hori-

* See Wargentin's Memoir in the Philosophical
Transactions for 1?51, p. 126.

f The Abbe de la Torre observed changes of se-
veral degrees in the declination of the needle, during
an eruption of Vesuvius.
I t Philosophical Transactions for 1823, p. 327.

74

MAGNETISM.

zontal needle, under the natural in-
fluence of the earth, makes one vibra-
tion in two seconds ; and that by mask-
ing the terrestrial influence by magnets
properly adjusted, the time of vibration
is increased to eight seconds ; then it
would follow that the directive power
was reduced to one-sixteenth of the
former ; and, consequently, that any
lateral magnetic force acting upon the
needle would produce an effect sixteen
times greater than before; so that if
the former were twelve minutes, the
new effect on deviation might be ex-
pected to amount to between three and
four degrees, and therefore be such as to
admit of distinct and satisfactory obser-
vation. Thus he found that when the
needle was kept in its natural position,
and then deprived of nearly the whole
of its directive power by bringing a
magnet near it, the daily variation might
be magnified almost to any amount.
The same result was obtained when the
north pole of the needle was directed to
the south, east, or west, or, indeed, any
required position, at least within certain
limits. With this view, Mr. Barlow first
deflected the needle, by the repulsion of
a magnet, into a certain position, and
then, by means of another magnet, mo-
dified its directive power in the same
way as when it was in its natural posi-
tion in the magnetic meridian.

(287.) Thus, by combining different
sets of observations, in different posi-
tions of the needle, information may be
obtained as to the direction as well as in-
tensity of the extraneous forces that in-
terfere with the general directive in-
fluence of the earth. Mr. Christie, in
prosecuting these investigations, pre-
ferred applying two magnets, placed
one above and the other below, and on
different sides of the needle, in the line
of the dip, or that in which it would
arrange itself if freely suspended by its
centre of gravity, instead of retaining
them in the same horizontal plane with
the needle, conceiving that a more
equable distribution of the forces acting
on the needle would thus be obtained ;
for a portion of the forces acting upon
the horizontal needle in the line of these
magnets would be destroyed, and it
would still be acted upon by forces in
the same direction as before, but of less
intensity; whereas by even applying
the poles of two magnets to the corre-
sponding poles of the needle, and in the
same plane with them, the horizontal
directive force of the needle would be

diminished by increasing the angle which
the resultant of the terrestrial forces
and those of the magnet made with the
horizon, and which would be nearly
equivalent to increasing the angle of the
dip. This arrangement also procured
the further advantage of obtaining va-
rious modifications of effect, by altering
the distances of the neutralizing mag-
nets from the needle, whereby inferences
might be deduced as to the variations
in the intensities of the deflecting forces
occasioning the deviations of the needle
at different times. It would exceed the
limits of this treatise to attempt even a
short abstract of the mode of investiga-
tion pursued by Mr. Christie in this in-
quiry, and for the details of which we
must refer our readers to his paper in
the Philosophical Transactions*. The
general results to which he arrived have
already been given in §$ 123, 124.

(288.) In observations for determin-
ing the exact variation, great care should
be taken that the compass employed be
unaffected by any local causes of at-
traction from iron in the neighbour-
hood. In the account given of the me-
teorological instruments used at the
Royal Society's house t, Mr. Cavendish
points out the method he employed in
order to ascertain whether this cause of
error existed ; and if so, to determine
its amount. He removed -the variation
compass from the apartments of the
Society, into a large garden belonging
to a house in Marlborough- street,
about a mile and a quarter to the west
of Somerset House, where there seemed
to be no danger of its being affected by
any iron-work. Here it was placed ex-
actly in the meridian, and compared
for a few days with a very exact com-
pass, placed in an adjoining room, and
kept fixed constantly in the same situa-
tion. It was then removed back to the
Society's house, and compared again
with the same compass. By a mean of
these observations, the difference be-
tween the position in the two stations
was ascertained, indicating the amount
of the local influence of the iron in the
house and adjacent buildings, and con-
sequently the error of the instrument.

§ 5. Of the Dipping- Needle.

(289.) The principle on which the
dipping-needle acts has already been

* For 1823, p. 342.

f Philosophical Transactions for 1776, p, 391.

MAGNETISM.

75

explained ($ 97), as also the general form
of the instrument.

Fig. 67.

(290.) The simplest construction is
that represented mfg. 67. The needle
D d is a flat oblong piece of steel, broader
at the middle, and tapering to a point at
the extremities. A slender cylindrical
axis is passed at right-angles through
its centre, and moves freely in circular
apertures made in the middle of the
lateral horizontal bars, H h, fastened to
a vertical graduated circle, CC, indicat-
ing the angle which the needle makes
with the horizon. This circle is fixed
to a flat stand, ST, provided with one
or more levels ; the horizontally of
which is adjusted by means of screws
placed at the corners of the stand. The
usual mode of observing with such an
instrument is first to ascertain the di-
rection of the magnetic meridian by a
common compass, and then, removing
the compass to a sufficient distance, so
that it may not affect the position of the
dipping-needle, to fix the circle of the
latter in the plane''of this meridian, and
then to render it perfectly level by means
of the screws of the stand. For the ad-
justment of the instrument in the meri-
dian, in any particular place where the
bearing of a distant object is exactly
known, the frame containing the needle
is occasionally provided with two sight
vanes, placed on an index moving hori-
zontally on the top of the vane, and
which may be directed to that object.

(291.) Great care should be taken
that no iron or steel enters into the con-
struction of any part of the frame- work
of the apparatus, as such material
might produce a sensible action upon
the needle : great attention should even
be paid to the purity of brass that may be
employed in its construction, and to its
exemption from all magnetic properties.

(292.) It was formerly deemed an

advantage to make the needles of con-
siderable dimensions, so as even to ex-
ceed a foot in length. But experience
has shown that more is lost than gained
in point of accuracy by giving to them
a length greater than six or eight inches ;
and considerable convenience, of course,
results from this reduction of size, as
the instrument is thus rendered more
portable, as well as less expensive.

(293.) With a view to diminish fric-
tion, Mr. Mitchell, in the year 1772,
proposed that the two ends of the axis
of the dipping-needle should be sup-
ported on friction-wheels ; and two in-
struments with this improvement were
executed for the Board of Longitude
by Mr. Nairne. The needles were a
foot in length, and the ends of the axes
were made of gold alloyed with cop-
per, and the friction-wheels on which
they rested were four inches in diame-
ter, these wheels being themselves ba-
lanced with great care. The ends of the
axes of the friction-wheels were like-
wise made of an alloy of gold and cop-
per, and moved in small holes made in
bell metal ; and opposite to the ends of
the axes of the needles and of the fric-
tion wheels, were placed flat agates,
finely polished. Each magnetic needle
vibrated in a circle of bell-metal, divided
into degrees and half degrees ; and a
line passing through the middle of the
needle to the ends pointed to the divi-
sions. The needles were nearly ba-
lanced before they were rendered mag-
netical ; and by an ingenious contrivance
of Mr. Mitchell, of a cross fixed on the
axes of the needles, on the arms of which
were cut very fine screws, to receive
small buttons, admitting of being screwed
nearer to or farther from the axis, the
needles could be adjusted both ways, to
a great nicety, after being magnetised,
by reversing the poles, and changing
the sides of the needle. The frame of
the instruments were provided with
levels for the horizontal adjustment,
after they had been placed in the plane
of the magnetic meridian *.

(294.) In a subsequent volume of the
Philosophical Transactions t, a dipping-
needle is described by Dr. Lorimer,
calculated for making observations on
the dip at sea, where, from the un-
steadiness of the supports, the difficulty
of attaining any degree of accuracy is
very great. The needle was of the

• Philosophical Transactions for 1772, p. 4?6. a
t For 17/5, p. 79.

76

MAGNETISM.

usual shape and size," and moved verti-
cally on its axis, which had two conical
points, slightly supported in two corre-
sponding hemispherical sockets, inserted
into the opposite sides of a small up-
right brass parallelogram, about an
inch and a half broad, and six inches
high. Into this parallelogram was fixed,
at right angles, a slender brass circle,
about six inches diameter, silvered and
graduated to every half degree, on
which the dip is indicated by the needle.
This, for the sake of distinction, he
called the circle of magnetic inclina-
tion. This brass parallelogram and,
consequently, the circle of inclination,
also turned horizontally on two other
pivots, the one above and the other be-
low, with corresponding sockets in the
parallelogram. These pivots were fixed
in a vertical brass circle, of the breadth
and thickness of two-tenths of an inch,
and of such a diameter as to allow the
circle of inclination and the parallelo-
gram to move freely round within it.
This second circle fhe calls the general
meridian. It was not graduated, but
had a small brass weight fixed to the
lower part of it, to keep it in a vertical
position ; and the circle itself was
screwed, at right angles, into another
circle, of equal internal diameter, of the
same thickness, and twice the breadth,
•which was silvered and graduated on
the upper side to every half degree. It
represented the horizon: for it swung
freely in gimbals, and was, conse-
quently, always horizontal. The whole
was contained in a mahogany box, of
an octagon shape, with a glass plate at
the top, and one on each side for, some
way down. That part of the frame
which contained the glass could be lifted
off when requisite. The whole box
turned round upon a strong brass
centre, fixed in a double plate of maho-
gany, glued together cross-wise, to pre-
vent its warping or splitting ; and this
again was supported by three brass feet,
frosted so as to prevent their slipping
when the vessel rolled considerably.
When not wanted for use, it was en-
closed in an outer square box, in order
to preserve it effectually.

The peculiar advantage of this in-
strument consists in the freedom which
is allowed to the needle of obeying the
tendency, impressed upon it by terres-
trial magnetism, of placing itself in the
line of the dip, in consequence of the
power which it has of moving in dif-
ferent planes at right angles to one an-

other. Its position with respect to the
respective circles points out also, upon
simple inspection, not only the inclina-
tion, or dip, but also the magnetic
bearings in a horizontal plane. Hence
by directing the vertical circle to the
sun, or other object in the heavens, the
magnetic amplitude of the object is also
readily determined. Dr. Lorimer's com-
pass, though exceedingly plausible in
theory, presents such difficulties in its
practical execution, as can scarcely be
overcome by the most exquisite work-
manship.

(295.) The dipping-needle formerly
used by the Royal Society, and which
has been regarded as the model for the
construction of instruments of this
kind, is described by Mr. Cavendish, in
the 66th volume of the * Philosophical
Transactions *.' In this instrument, the
ends of the axis roll on horizontal agate
planes ; and a contrivance is applied, by
which the needle may, at pleasure, be
lifted off from the planes, and laid down
on them again in such a manner as to
be supported always by the same points
of the axis resting on the same parts of
the agate planes, the motion by which it
is let down being very gradual and with-
out shake. The general form of the in-
strument, the size and shape of the
needle, and the cross used for balancing
it, were the same as in the dipping-
needle constructed by Nairne°on the plan
of Mr. Mitchell, already described, §
293. The mode of using the instrument
was as follows : the dip was observed
first with its front to the west, and then
with its front to the east ; after^ which
the poles of the needle were reversed,
and the dip observed both ways as be-
fore. Care was taken that the needle
was rendered equally magnetical after
the poles were reversed, as it has been
before ; this equality being ascertained
by counting the number of vibrations
made by the needle in a given time in
both cases. The mean of these four
observations was the true dip.

(296.) In order to estimate the influ-
ence of the several causes of error which
might singly vitiate the result, but which
may be made to compensate one an-
other by combining these different modes
of observation ; let us suppose fig. 68
to present us with a front view of the
needle ; and S N to be the direction of
the magnetic axis, or line according to
which its magnetism is exerted ; and let

* For the year 1776, p. 375.

MAGNETISM.

77

M m be drawn at right angles to S N, senting therefore the axis of motion. If
and passing through the centres of the the needle were truly balanced, its
cylindrical ends of the axis, and repre- centre of gravity would coincide with

Fig. 68.

the intersection of these lines at c. But
supposing this not to the be case, and that
in consequence of an error in the sus-
pension the centre of gravib is aig ;
draw gf perpendicular to S N, cutting
it in n, and make nf equal to g n.
When the instrument is turned half way
round, so that the opposite face of the
needle is presented to us, the edge S M N
will now be in the place before occu-
pied by S m N, and the centre of gravity
will be situated at that part where
point /was before ; therefore the mean
between the forces by which the needle
is drawn out of its true position in these
two situations, in consequence of its not
being truly balanced, is accurately the
same"; and the mean between the two
observed dips is very nearly the same as
if the centre of gravity had been at n.
But if the centre of gravity were at n,
the dip would be very nearly as much too
great in the one position of the needle,
or it would be too little when the poles
are reversed ; or vice versa. Therefore
the mean of the observed dips in these
four situations will be very nearly the
same as if the needle were truly ba-
lanced.

(297.) In the second place, if the
planes on which the axis rolls are not
horizontal, the dip will be very nearly
as much greater than it would otherwise
be, when one face is turned to the west,
as it is less when the other is ; for if
these planes dip towards the south in
one case, they will dip as much towards
the north in the other, supposing the
levels by which the instrument is set to
remain unaltered. Consequently, the
mean of the two observations will be
very nearly the same as if they had
been placed in a truly horizontal plane.

(298.) The same method of reasoning
will show, in the third place, that the
mean of the two observations above-
mentioned will not be altered, although
the index-line joining the mark by
which we observe with the axis of mo-
tion be not parallel to the axis of the
needle ; that is, although the index line
do not coincide with the continuation of

the line SN ; or although the line join-
ing the two divisions of 90° be not per-
pendicular to the horizon ; or although
the axis of motion do not pass through
the centre of the divided circle, pro-
vided it be in the same horizontal plane
with it. Should it happen, indeed, that
the axis of motion is not in the same
horizontal plane with the centre of the
divided circle, the error thence arising
will not be compensated by this me-
thod of observing ; unless the position
of both ends of the needle be taken as
checks upon one another. This, how-
ever, is of no consequence, since it is
easy to examine whether or not they
are in the same horizontal plane.

(299.) But the error that is most dif-
ficult to be avoided in the construction
of the instrument, is that which arises
from the ends of the axis not being truly
cylindrical. It is, accordingly, essential
that the parts of the axis which rest in
the agate planes should be exactly the
same. The instrument, however, is so
contrived as to admit, on occasion, by
giving the axis a little liberty in the
notches by which it is lifted up and
down, of our making these planes bear
against a part of the axis distant about
a hundredth or a fiftieth of an inch from
their usual point of bearing. Mr. Ca-
vendish found that, when the axis is con-
fined, so as to have no such liberty,
and when care is taken, by previously
making the needle stand at nearly
the right dip, that it shall vibrate in
very small arcs when let down on the
planes : that then, if the needle be lifted
up and down any number of times, it
will commonly settle exactly at the same
point each time ; at least the difference
is so small as to be scarcely sensible.
But if it be not so confined, there will
often be a difference of twenty minutes
in the dip, according as different parts
of the axis rest on the planes ; and that,
although the greatest care be taken to
free the axis and planes from dust;
which can be owing only to some irre-
gularity in the axis. If the needle
vibrate in arcs of live degrees, or more,

78

MAGNETISM.

when let down on the planes, there will
frequently be as great an error in the
dip. It is true that the part of the agate
planes on which the axis rests when
the vibrations are stopped, will be a
little different, according to the point
at which the needle stood before it was
let down ; which will make a small dif-
ference in the dip, as shown by the di-
vided circles, when only one end of the
needle is observed, though the real dip,
or inclination of the needle to the hori-
zon is not altered ; but this difference
is far too small to be perceptible ; so
that the above-mentioned error cannot
be owing 'tto this cause. Neither does
it seem to arise from any irregularity
in the surface of the agate planes, for
they were ground and polished with
great accuracy ; but it most likely pro-
ceeds from the axis slipping in the
large vibrations, so as to make the
agate planes bear against a different part
of it from what they would otherwise
dp. Mr. Cavendish gives it as his opi-
nion that this irregularity is not owing
either to want of care or skill in the
execution, but to the unavoidable im-
perfection of this kind, of work. He
imagines, therefore, that this instrument
is at least as exact, if not more so,
than any which has yet been made.

(300.) Thus it appears, that in general
the indications afforded by the dipping-
needle are liable to two principal sources
of error : first, the axis of the magnet's
length may not be the exact axis of its
magnetic forces ; and, secondly, its point
of suspension may not, in every position,
be exactly coincident with its centre of
gravity. Different modes of observa-
tion must be resorted to, in order to
ascertain the amount of the errors aris-
ing from these causes. We must first
assure ourselves that the axis of rota-
tion of the needle is perfectly level, so
that the needle shall turn in a plane ex-
actly vertical : we must next see that it
is placed accurately in the plane of the
magnetic meridian ; and we have then
to observe carefully the positions at
which it settles, after being repeatedly
disturbed, and allowed to oscillate freely.
The mean of these positions may then
be taken as the true position of the
needle under these circumstances. We
are next to turn the whole instrument
horizontally till it has described a com-
plete semicircle, or 180°; that face of it
which was to the east being now to the
west, and vice versa: and then, taking-
similar observations on the dip, we get

a mean of these, for this new position.
Comparing these two means, we obtain
a resulting mean, which is free from
the first source of error. In order to
exclude the operation of the second cause
of error, we must now remove the
needle from its supports, and after de-
stroying its magnetism, magnetize it
again in the contrary sense; namely,
rendering that end a north pole, which
before was south, and vice versa;
then, replacing it upon its supports, we
must make with it similar sets of ob-
servations to those made before, turning
it first on one side and then on the other.
The mean thus obtained, combined with
the (former mean, will give the mean of
the whole ; which may be considered as
the true dip, at the place and at the
time of observation,

(301.) The error produced by the
want of coincidence between the axis of
motion and the centre of gravity of the
needle may be removed by the following
method, devised by Daniel Bernouilli,
and which, being easily executed, de-
serves to be generally known. Let a
dipping-needle be constructed with as
much correctness as can be effected by
the ordinary methods of workmanship,
and balanced as exactly as possible be-
fore it is rendered magnetic : when im-
pregnated, therefore, it will arrange it-
self tolerably nearly in the line of dip.
Carefully note the position it takes un-
der these circumstances, and then de-
stroy its magnetism. When it has thus
returned to its natural state, alter the
point of suspension, or adjust the centre
of gravity, in such a manner as that it
shall arrange itself in the same position
as that above noted by the sole in-
fluence of gravity. Now impregnate it
again, imparting to it the same poles as
before. It is evident that it will now ap-
proximate still more nearly to the true
line of the dip, since nearly the whole of
that portion of the force of gravity
which before produced a deviation from
the position no longer operates. If we
find that this approximated position dif-
fers several degrees from the former one,
the operation may be repeated, until we
have arrived so near to the true position,
that no further difference can be per-
ceived. It will rarely happen that the
third approximation will give an error
of half a degree.

(302.) This simple instrument was
adapted by its author to observation in
all situations, in the following ingenious
manner: — A very light brass graduated

MAGNETISM.

79

circle, ABC,/"-. 69, is fixed to one side as possible before impregnation. A very
of the needle SN, concentric with its light index, Rr, is then fitted on the axis
axis, and the whole is balanced as nicely so as to turn stiffly upon it. This will

Fig. 69.

destroy the equilibrium of the needle.
If the needle had been made with per-
fect accuracy, and perfectly balanced,
the addition of this index would cause
it always to settle with the index perpen-
dicular to the horizon, whatever degree
of the circle it might chance to point
at. But as this is not to be expected,
the index is to be set at various degrees
of the circle, and the position which the
unmagnetic needle takes, corresponding
to each place of the index, must be
observed, and the result of all these
observations recorded in a table. Sup-
pose, for example, that when the index
is at 50°, the needle inclines 46° from
the horizon : if ;in any place we ob-
serve that the needle, rendered magnetic
by juxtaposition between two powerful
magnets, having the index at 50°, has
an inclination at 46°, we may be certain
that this is the true dip at that place ; for
the needle is not deranged by magne-
tism from the position which gravity
alone would give it. As we generally
know something of the dip that is to
be expected in any place, we must set
the index accordingly. If the needle do
not show the expected dip, the position
of the index must be altered, and the in-
clination of the needle again observed.
Examine whether this second position
of the index, and this dip, form a pair
which is in the table : if they do, then
we have obtained the true dip; if not,
we must try another position of the
index. Noticing whether the agree-
ment cf this last be greater or less than
those of the former pair, we learn
whether to change the position of the
index in the same direction as before, or
in the opposite direction. Professor
Robison had a dipping-needle of this
kind, made by a person totally unac-
quainted with the making of philoso-
phical instruments. He used it at
Leith, at Cronstadt in Russia, at Scar-
borough, and at New York, and the
dip indicated by it did not in any single
trial differ a degree and a half from

other trials, or from the dip observed
by the finest instruments. He tried it
in Leith Roads in a rough sea; and
did not think it inferior, either in cer-
tainty or dispatch, to a needle of the
most elaborate construction. Professor
Robison deems it worthy of its inge-
nious author, and of the public notice,
because it can be made for a moderate
expense, and, therefore, may be the
means of multiplying observations on
the dip, which are of immense value to-
wards perfecting the theory of terrestrial
magnetism.

(303.) In a dipping-needle recently
made by M. Gambey, at Paris, intended
to be used at St. Petersburgh, the axis,
instead of being a cylinder, is a knife-
edge, as in a fine hydrostatic balance.
This edge is placed exactly in the
centre of gravity of the whole com-
pound needle, and is so fixed that when
the needle dips 71°, the edge rests per-
pendicularly on two agate plates. It is
evident that such a needle, however sen-
sible, is adapted for use only in those
situations in which the dip is nearly
71°. It is, however, well calculated for
ascertaining minute variations of incli-
nation in the same place *.

(304.) Another mode of dispensing
with the condition that the axis of mo-
tion should accurately pass through the
centre of gravity, a condition which it is
next to impossible ever strictly to fulfil,
is that adopted in the dipping-needle
invented by Professor I. Tobias Mayer,
in his treatise, De Usu accuratiori
acus indinatorice Magneticce t.

The centres of motion and of gravity
are, in this needle, designedly separated,
so that the inequalities of workmanship
in the axis, or in the planes of suspen-
sion, are rendered of less effect, being
opposed by thejoint influence of gravity
and magnetism; whilst, by a peculiar
process of observation, and an appro-

* Annales de Chimie etde Physique,
f Published in the Transactions of the Rojal So.
ciety of Sciences atGottingen, for 18 J 4.

80

MAGNETISM.

priate formula, the joint operation of the
two forces is resolvable, and the posi-
tion which the needle should assume
from that of magnetism alone is dedu-
cible with great precision *. This in-
tentional separation of the centres of
gravity and suspension is effected by
tapping the needle with a fine interior
screw, in order that it may receive a
fine steel screw, projecting at some dis-
tance from the needle, and on which a
small brass ball traverses. By this con-
trivance, the needle may be deflected
from the true dip, in any degree that may
be desired, and the terrestrial action,
which varies as the sine of the angle of
deviation from the line of dip, may be
increased in almost any proportion.

(305.) The following is the descrip-
tion of the needle constructed on this
principle, which was employed by Cap-
tain Sabine in the determinations of the
dip, as reported in the work already
quoted. This needle was a parallelopi-
pedon of eleven inches and a half in
length, four-tenths in breadth, and one-
twentieth in thickness ; the ends were
rounded ; and a line marked on the
face of the needle, passed through the
centre to the extremities, answering the
purpose of an index line. The cylin-
drical axis on which the needle revolved
was of bell metal, terminated, when it
rested on the agate planes, by cylinders
of less diameter ; the finer these termi-
nations can be made, as long as they
do not bend with the weight of the
needle, the more accurate will be the
oscillations. Small grooves in the
thicker part of the axis received the Y's,
which raised and lowered the needle on
its supports, and insured that the same
parts of the axis rested on the planes in
each observation.

A small brass sphere traversed on a
steel screw, was inserted in the lower edge
of the needle, as nearly as possible in
the perpendicular to the index line pass-
ing through the axis of motion ; by this
mechanism, the centre of gravity of the
needle, screw, and sphere may be made
to fall more or less below the axis of
motion, according as the sphere is
screwed at a smaller or greater distance
from the needle, and according as
spheres of greater or less diameter are
employed. The object proposed in thus
separating the centres of motion and of
gravity, was to give to the needle a force,
arising from its own weight, to assist

* See Captain Sabine's Account of Experiments
to determine the figure of the Earth, p. 46?.

that of magnetism in overcoming the
inequalities of the axis ; and thus to
cause the needle to return, after oscil-
lation, with more certainty to the same
point of the divided limb than it would
do were the centres strictly coinci-
dent.

(306.) The centres of motion and of
gravity not coinciding, the position
which the needle assumes, when placed
in the magnetic meridian, is not that of
the dip ; but the dip is deducible by an
easy calculation from observations made
with such a needle, according to the fol-
lowing directions : —

(307.) If the needle has been carefully
made, and the screw inserted truly as
described, the centres of motion and of
gravity will be disposed as in the lever
of a balance, where a right linejoining
them will be a perpendicular to the ho-
rizontal passing through its extremities,
that is, to the index line. This condi-
tion is not, indeed, a necessary one ;
but it is desirable to secure it, because
it shortens the observations, as well
as the calculation from whence the dip
is deduced. Its fulfilment may be as-
certained with great precision, by placing
the needle on the agate planes before
magnetism is imparted to it, and observ-
ing whether it returns to a horizontal
direction after oscillation, in each posi-
tion of the axis ; if it do not, it may be
made to do so at this time with no great
trouble.

(308.) With a needle in which this
adjustment can be relied on, two obser-
vations made in the magnetic meridian
are sufficient for the determination of
the dip. The two faces of the needle
are in succession turned towards the
observer, by reversing the position of
the axis on its supports, in such a man-
ner that the edge of the needle which is
uppermost in the one observation, be-
comes lowermost in the other. The
angles which the needle makes with the
vertical in these two positions being
read, the mean of the tangent of those
angles is the co-tangent of the dip.

(309.) But when needles are used in
which this previous adjustment has not
been made, or when its accuracy cannot
be relied on, four observations are re-
quired ; two being those that have been
already directed; and the other two
being similar to them, but made with
the poles of the needles reversed.

Calling, then, the first arcs F and/;
and those with the poles reversed G and
g, calling the dip £, and taking

MAGNETISM.

tans:. F-j-tancr./= A
tans. F - tanir./ = R
tang. G -f- tan:?. g1 = C

r. G - tang.# = D
Then the dip may be calculated by the
following formula: —
A . D , B . C

B+3 + B + D=2cotangtS'

(310.) In reversing the poles, it is not
necessary that the magnetic force im-
parted to the needle should be the same
in degree as it possessed previously to
the operation. The coincidence of the
poles with the extremities of the longi-
tudinal axis may always be insured by
adopting the precaution of placing the
needle in a groove, to prevent its lateral
motion, and by confining the sides of
the magnet by parallel strips of wood,
so that in moving along the needle they
may preserve its direction.

(311.) If the distance between the
centres of motion and of gravity be con-
siderable, the arcs in the alternate ob-
servations will be on different sides of
the vertical, especially when the dip is
great; in such cases the arcs to the
south of the vertical are read nega-
tively. The arcs in each of the four
positions, forming the data from which
the dip is deduced, are the arithmetical
means of several observations, usually
six, half of which should be made with
the face towards the east, and half with
the face towards the west; the needle
being lifted by the Y's and lowered gently
on its supports between each observa-
tion. The arcs indicated by both ends of
the needle should also be read, in order to
correct the errors arising from inequa-
lity in the divisions, or from the axis of
the needle not passing correctly through
the centre of the circle.

(312.) In order to insure the perfect
horizontality of the agate planes which
supported the axis of the dipping-needle
on Mayer's construction, employed by
Captain Sabine, a spirit level was at-
tached to a circular brass plate, of the
proper diameter to be placed upon the
planes themselves, with adjustments to
bring it parallel to the plate. The
errors of the level were shown by placing
the plate in various positions horizon-
tally ; and the errors of the planes by
turning the whole instrument upon its
horizontal centre. When these errors
were adjusted, and the planes and plate
perfectly horizontal, the apices of two
cones, which proceeded perfectly at
right angles from the plate uniting them
at their base, and were equal to the dia-

meter of the divided circle of the instru-
ment, ought to have coincided with the
divisions 90° and 90° of the circle;
when they did not, the cones afforded,
in this case also, the means of correct-
ing the adjustment.

(313.) The dipping-needle affords a
method of determining the position of
the magnetic meridian, independently of
the horizontal needle; for if we turn round
the whole instrument horizontally (so as
to place it successively in different azi-
muths), till we find that in which the
needle assumes an exactly vertical po-
sition, the plane of its motion is then ex-
actly at right angles to the magnetic
meridian ; and the latter may therefore
be determined from the former.

(314.) By comparing the inclination
of the dipping-needle to the horizon, in
two different positions, such that the
planes of its rotation are perpendicular
to each other, we may, by the following
trigonometrical formula, deduce the dip.
If the inclinations, observed in the two
azimuths, be represented respectively by
$' and $", and the dip itself (or the incli-
nation in the magnetic meridian) by £,
then,

Cot. 2$=cot.*S' + cot.*S"
By multiplying observations of this kind
in different azimuths, and taking the
mean of all, we may arrive at a very
accurate determination of the dip.

(315.) Mr. Scoresby has proposed an
ingenious method of finding the dip, by
observing the situation in which bar-iron,
void of permanent magnetism, loses all
power of affecting the compass placed
at a certain distance from it ; for, as
Mr. Barlow has ascertained, its position
must then be in the plane of the mag-
netic equator. The inclination of the
plane to the horizon is, of course, equal
to the complement of the dip. Mr.
Scoresby has described an instrument
calculated for making this species of
observation, in the Transactions of the
Royal Society of Edinburgh*.

(316.) Other methods, of a nature
somewhat more refined, exist for dis-
covering the dip, which depend on the
admeasurement of the intensities of the
magnetic forces by which the needle is
urged in different positions of the axis
and plane of rotation. The magnetic
force derived from the influence of the
earth, and acting in the direction of the
dip, may be resolved into other forces,
which will bear to one another the same
ratios as the sides of the triangles
* Vol. ix, p, 247.

MAGNETISM

which represent them. The angles of
these triangles, the dip being one of
these angles, may be determined by the
trigonometrical relations of these lines
when two of them are given. All that
is required for this purpose is to ascer-
tain the ratio of the forces which act in
directions parallel to these lines, and are
proportional to them. Of the methods
by which the intensity of these forces is
to be measured, we shall proceed to treat
in the next section.

§ 6. Methods of determining the inten-
sities of the Magnetic Forces.

(317.) When a magnetic needle is
moveable in any plane on an axis that
passes through its centre of gravity, so
that its movements are simply the ef-
fects of the magnetic forces of the earth
acting upon the two polarities of the
needle (which may be considered as
concentrated in its poles), it takes a
certain position, which is that in which
the forces are in equilibrium. Let
needle SN, fig. 70, for example, be
moveable on an axis at X, perpendi-
cular to the plane of the figure ; and let
Fig. 70. ;

\

NE be the direction in that plane of
the force of terrestrial magnetism acting
upon the pole N ; while S e, opposite
and parallel to NE, is the direction of
the force in the same plane, acting upon
the pole S. The position to which the
needle is brought by these forces is s n,
parallel to the common direction of
these forces, when they are in direct
opposition to each other, and therefore
in equilibrium. In order to estimate
the rotatory efficiency of the forces in ope-

ration in any "other position, as SN,we
must resolve the force represented by the
line NE into two others ; the one, N x,
in the direction of the radius of rotation
XN, prolonged ; and the other in the
direction NR, perpendicular to it. The
force N x, being opposed by the fixed
axis at X, contributes in no respect to
produce motion ; NR is the only part
of the terrestrial force that turns the
needle upon its axis. Now it is evi-
dent that NR is to NE, as the sine of
the angle NER, or its equal EN#, to
the radius; but the force represented
by the line NE being a constant force,
the rotatory force NR will, in every po-
sition of the needle, be invariably as the
sine of the angle EN#, made by the
needle, on its prolonged direction, with
the direction of the terrestrial force.
The same reasoning, in all respects, ap-
plies to the force S e acting on the pole
S ; but since it acts on the other side
of the axis in a contrary direction, it
will concur with the force acting on
the pole N, in giving the same rotatory
motion to the needle. The effect will,
therefore, be equal to the sum of these
two rotatory forces, and will be twice
as great as either of them taken sepa-
rately; and the resultant will still be
proportional to the sine of the angle of
inclination.

(318.) A little consideration will en-
able us to perceive that the condition of
the needle, with regard to the magnetic
forces, is analogous to that of a lever
moveable on a horizontal axis, and
acted upon by the force of gravity. If
we suppose a straight lever, AB, fig.
7 1 , moveable upon an axis X, at right
angles to it, to be raised from the ver-
tical position a b, which is that of equi-
librium, inasmuch as gravitation acts in
Fig. 71.

A <-

MAGNETISM.

'63

that direction, and placed in the inclined
position AB, it is well known that
the rotatory action of the force of gra-
vity acting upon all its particles is equi-
valent to a single force acting upon a
point, O, which is called the centre of os-
cillation ; and also that, in order to esti-
mate what portion of that force OE con-
tributes to its rotation on its axis, we
must resolve it into one in the direction
O xy and another in the direction O r,
at right angles to it ; this latter force
hein^ in all cases proportional to the sine
of the angle EO.r, or its equal EXb.
The only difference between this case
and the one we have been considering,
is that here the force is single, whereas
there are two forces acting upon the
magnetic poles.

(3 1 9.) These two forces being always
precisely equal and in opposite direc-
tions, perfectly balance one another
with reference to any motion of the
whole needle, either towards or from the
earth. This admits of experimental
proof ; for, in the first place, were there
any balance remaining in favour either
of the attractive or repulsive forces
emanating from the earth, the effect
would be shown by an apparent change
in the weight of the needle ; if, when
magnetised, it were on the whole at-
tracted to the earth, it would appear
heavier than before ; if repelled, lighter.
But no such change is observed to take
place. Neither is there any tendency
manifested in a magnetised bar to a
lateral or horizontal motion. This may
be proved by placing it at the end of a
light frame of wood, AB, Jig. 72, which
is suspended at its centre C by means
of a fine silk thread, T ; a weight, W,

Fig. 72.

being placed at the other end to act as a
counterpoise to the magnet NS. When
left to itself, it will be found that the
whole apparatus will turn round until
the direction of the needle coincides ex-

actly with the plane of the magnetic me-
ridian, just as if it had been suspended
by its own centre. Had there existed
any force impelling it horizontally, it
would have occasioned a deviation from
this plane, acting as it must have done
with the advantage of the lever AC. But
the two equal forces acting differently
upon the two magnetic poles, though
opposed with respect to any motion of
translation, yet concur in their rotatory
action, and may, consequently, as far as
relates to this action, be regarded as a
single force of twice the intensity of
either of them taken singly.

(320.) It is evident, then, that the
same dynamical laws which regulate
the motions of a compound pendulum,
actuated by terrestrial gravity, will also
regulate those of a magnetic needle, ba-
lanced on its centre of gravity, and ac-
tuated by terrestrial magnetism. The
same pendulum, it is well known, per-
forms all its vibrations in equal times,
whatever be the length of the arc in
which they are performed, provided
that arc be not too great. If we esti-
mate the length of a pendulum by the
distance between its centre of motion
and its centre of oscillation, then, in
pendulums of different lengths, and in
situations where the force of gravity is
different, the squares of the times of
performing a given number of vibra-
tions are directly proportional to the
lengths of the pendulums, and in-
versely proportional to the force of
gravity. Now the number of vibrations
performed in a given time is inversely
as the time employed in each vibra-
tion ; therefore, the square of the num-
ber of vibrations in a given time will be
inversely proportional to the length, and
directly proportional to the force of
gravity.

(321.) The same formula being ap-
plicable to the vibrations of magnets,
a very simple computation will enable
us to arrive at an estimate of the com-
parative forces acting on the same mag-
net in different inclinations of the axis,
and in different situations with respect
to the position of equilibrium in the
plane of motion. We have only to dis-
turb it slightly from this position, and
count the number of vibrations it makes
in a given time, a minute for example,
in different cases: then, taking the
squares of these numbers, they will be
proportional to the intensities of the ter-
restrial magnetic forces that are in ope*
ration in these several instances,

84

MAGNETISM

(3-22.) The preceding reasoning is
founded upon the supposition that
the axis of motion passed accurately
through the centre of gravity of the
magnet ; so that the effect of gravity
was removed, and could not in any way
interfere with the rotatory force of mag-
netism. This, however, is a condition,
which it is next to impossible practically
to fulfil ; and if it be not exactly fulfilled,
then, whenever the centre of gravity is
not in the precise line passing vertically
through the centre of suspension, the
effect of gravity is to impart to that
side of the magnet, on which the centre
is found, a tendency to preponderate ;
and its oscillations are no longer pro-
duced by the simple action of the mag-
netic forces, nor directed to the exact
line of their action. The only method
of correcting this source of error, when
it is not very considerable, is to reverse
the polarities of the magnet, and make
a new set of observations on the incli-
nation and intensity in this state of the
magnet ; and then to take the mean of
the corresponding observations, which,
in consequence of the compensation of
the opposite errors existing in the two
modes of estimation, will express the
true value of the quantity sought.

(323.) In order to compare the re-
sults of two sets of observations on
magnetic intensity in different parts of
the world, it is necessary to employ the
same needle in both cases ; since the
application of the various formulae ne-
cessary to be employed for comparing
the action of the same force on two dif-
ferent needles would be attended with
great difficulty and uncertainty.

(324.) The oscillations of the com-
pass-needle, which moves in a hori-
zontal plane, furnish data for the calcu-
lation of the intensity of that part only
of the terrestrial force which acts in that
plane; whereas those of the dipping-
needle, moving on a horizontal axis and
consequently in a vertical plane, when
that plane coincides with the magnetic
1 meridian, indicate the full amount of the
force of the terrestrial magnetism at the
place of observation. The ratio be-
tween these two quantities is that of the
base and hypothenuse of a right-angled
triangle, inclined at an angle equal to
the dip ; that is to say, the intensity of
the horizontal force is to the intensity
of the whole force as the cosine of the
angle of the dip is to the radius. Let
SN, fig, 73, be the horizontal needle ;

Fig.

NE the line of dip ; NR a horizontal
line perpendicular to SN. The force
NE is resolved into RE, perpendicular
to RN, and which being out of the plane
of motion, and perpendicular to it, does
not contribute to the motion of the
needle, and NR the horizontal force ;
which latter is to NE as the cosine of
the angle ENR is to the radius. This
force, NR, is constant in all positions
of the needle in the horizontal plane ;
and acts always in parallel directions.
Its rotatory action, however, will, of
course, depend upon the deviation of
the needle from the position of equili-
brium, that is, upon the angle which it
makes with the plane of the meridian ;
being proportional to the sine of that
angle. The oscillations are therefore
isochronous, that is, performed in equal
times, whether the arc be large or
small, like those of a pendulum ; and
are governed by the laws above stated
as applying to those of pendulums ; and
the same remark applies equally to the
oscillations of the dipping-needle per-
formed in the plane of the magnetic me-
ridian.

(325.) When the position of the axis
of the dipping-needle is changed, so that
the plane in which the needle moves is
no longer that of the magnetic me-
ridian, though still vertical, the force by
which it is actuated is in like manner to
be estimated by that portion of the ter-
restrial force which, on being resolved,
has the direction of that plane ; that
portion which is perpendicular to the
plane being of no effect.

Let the circle SANB, fig. 74, repre-
sent the plane of the motion of the
needle SN ; NE being the line of dip,
or the direction of the terrestrial forces.
The force represented by NE may be
resolved into the two forces NV and
NH, the one perpendicular to the ho-
rizon, and the other parallel to it. Let
the former be denoted by the letter v,
and the latter by h; and let us call the

MAGNETISM.

85

Fig. 74.

force of the earth e. Let I be the angle
of the dip ENH, and d the complement
of that angle, or ENV.

Then by the properties of the triangles
NVE, orNEH, we have

v = e cos. d; and h=e sin. d.
(326.) The force v, being vertical,
acts wholly in the plane of the needle's
motion, SANB : but the force h is out
of that plane, and must be decomposed
into two others; the one, HP, perpendi-
cular to that vertical plane, and which we
shall cally; the other NP, which we
shall call x, directed horizontally in
that plane. The angle HNP is equal
to the deviation of the vertical plane
SANB from the magnetic meridian;
let us call this angle a.
We shall thus have

y - h sin. a ; and x - h cos. a ;
or, substituting for h its value as ex-
pressed in the former equation, and
joining the value of v, we have,
v = e, cos. d.
y = e, sin. d. sin. #.
x-e, sin. d, cos.a.
Of these, the force y is destroyed,
being resisted by the axis of motion ;
and Ihe forces v and x are those only
which are effective in giving motion to
the needle. Let R express the re-
sultant of these forces, and <p the angle
which it makes with a vertical line. We
shall have

and tang. 0= — ,
v

or, substituting for x and v their respec-
tive values, as above found.
R = ecos. d *] 1 + tang. 2t/, cos.2a;

tang. (p=tang. d, cos. a.
(327.) From these equations many

important consequences may be de-
rived.

In the first place we may deduce, that
the intensity of the force 'R diminishes
as the angle a increases ; or in other
words, as the plane of motion deviates
more from that of the magnetic meri-
dian. It is greatest when these planes
coincide, being then equal to e; it is
least when they are at right angles to
one another, for then a=90°, and cos.
a = o, whence

R=e. cos. d.

(328.) The direction of the resultant,
and consequently the position into
which it brings the needle, also vary
in the different azimuths in which it is
placed. In proportion as the angle a
increases, the cosine of that angle dimi-
nishes ; and therefore the tangent of the
angle 0, which expresses the angle the
resultant makes with a vertical line,
also diminishes. Hence, in proportion
as the plane of motion comes nearer to
a position perpendicular to the magnetic
meridian, the position of the needle will
approach more nearly to the vertical
position ; and it is exactly vertical when
its plane of motion has arrived at that
situation. This has been already no-
ticed as affording a method of deter-
mining the position of the magnetic)
meridian, independently of the hori-
zontal needle (§ 313).

(329.) We may deduce also the for-
mula given in § 314, from the foregoing
equations ; for when the two azimuths
in which the observations are made
differ by 90 degrees, the tangents of 0
in the two cases will be respectively
tang. <p' = tang, d cos. a
tang. <p" = tang, d sin. a.
By taking the squares of each term of
these equations, and adding them, we
obtain

tang. \l = tang. V + tang- 2$";
which, when £, §' and S" express the
angles of the dip, or the complements
of d, <p, and (p", respectively, become

cot.2S = cot *§' + cot. 2S".
(330.) The same formula is derivable
more simply from the following consi-
derations : —

Let XD, Jig. 75, be the line of dip
in the magnetic meridian; and let
XVFA, XVGB, be the two vertical
planes at right angles to each other.
From D, draw the lines DF and DG,
perpendicular to these planes ; and also
FV and GV, perpendicular to XV. The
lines XF and XGwill be the positions of
the needles in these planes, according

MAGNETISM.

to the law already stated, $ 78. Taking
XV as radius, the lines VF, VG, and

v - e sin.

VD are the tangents of the angles
VXF, VXG, and VXD, respectively;

and in the right-angled triangle VDF,
VD^VF'-fFD8;

that is (FD being = VG)

VD2=VF2-f-VG2:

or tang. ZVXD = tang. B VXF -f- tang.
SVXG; which is the formula above
given.

(331.) On the other hand, the deter-
minations of the relative intensities of
the magnetic forces in different planes
furnish data for the computation of the
angles which those planes make with
the line of the dip, or the direction of
terrestrial magnetism. Thus the amount
of the dip may be determined by com-
paring the number of oscillations in a
given time made by the same needle,
when vibrating in the plane of the mag-
netic meridian, and also in a vertical
plane at right angles to it. For the
squares of these numbers being as the
intensities of the forces which respec-
tively act in these planes, and the force
in the former case being to that in the
latter as the radius to the cosine of the
angle d, which the line of dip makes
with a vertical line, we obtain the latter
by a simple proportion when the former
are given. Resuming the notation be-
fore employed, let e be the total terres-
trial force acting in the plane of the
magnetic meridian, and v that part of
it which acts in a vertical plane at right
angles to the magnetic meridian; and
let N and n express the number of os-
cillations, in a given time, which the
dipping-needle performs in these two
planes respectively : v = e cos. c^or,~d
being the complement of £ ;

sn. =

e

But v : e :: w2 : N2;

therefore

e N2

and sin. £ = ^-_

N2

(332.) We shall give the following
example of the application of these for-
mulae to the observations of magnetic
intensity, made by Humboldt, near
Quito, exactly at the terrestrial Equa-
tor, and at longitude 81° 2' west from
Paris. The number of oscillations made
by the dipping-needle vibrating in the
magnetic meridian, during ten minutes,
was 220 ; the number of oscillations,
made in the same time, when it vibrated
in a plane perpendicular to it, was 109.
Substituting these numbers in the for-
mula for N and n respectively, we ob-
tain

2202 48400

From log. 11881=4.0748530
Subtract log. 48400 = 4.6848454

there remains log. sin. £ = 9.3900076

whence we get $ = 14° 12' 35".
The direct observation of the dip, by
the dipping needle, was

5 - 14° 25' 5",

the difference between the two methods
being only 12' 30".

(333.) The angle of the dip with the
horizon may, in like manner, be job-
tained by comparing the relative inten-
sities of the forces, as determined by the
squares of the number of oscillations in
a given time, executed in the plane of the
magnetic meridian, and also in a hori-
zontal plane : for they are in the pro-
portion of the radius to the cosine of
the dip ; or, if we call the number of
oscillations made by the horizontal nee-
dle v, while N is that made by the same
needle, suspended as a dipping-needle,
and -placed in the plane of the magnetic
meridian ; then

(334.) Methods have been devised
for determining the dip, from the result
of observations made with the horizon-
tal needle alone, by comparing its num-
ber of oscillations with the weight of
the counterpoise necessary for maintain-
ing it in the horizontal position when
magnetized. But the formula and mode

MAGNETISM.

of computation are much more compli-
cated than those we have given; and
we must therefore refer our readers for
the details to Biot's Traite de Physique *.

(335.) The determination of the in-
tensity of terrestrial magnetism may, in
general, be made with much greater
accuracy, by observing the oscillations
of a needle moving horizontally, than
in any other way : because the mode of
suspension we can employ for obtaining
a horizontal motion is much more deli-
cate, and much less impeded by friction,
than any other motion or a fixed axis
can be. The greater duration also of
the period through which the oscilla-
tions continue enables us to ascertain
•with greater exactness the average time
of the vibration. Hence a silken sus-
pension is much to be preferred for
delicate experiments with horizontal
needles, to that of balancing them on
a point by an agate cup.

(336.) The following description of
the apparatus used by Captain Sabine,
in the voyages to the arctic regions in
the year 1822 and 1823, may furnish
valuable practical information to those
who may hereafter conduct experiments
of the same nature t. A mahogany
box was provided, made, for convenience,
of an octagonal shape, with a top of
stout glass ; its height was fifteen
inches, tand its diameter sufficient to
allow a horizontal bar of seven inches
in length to vibrate freely when sus-
pended by a silk line passing through a
brass button, inserted in a perforation
in the middle of the glass top. A metal
circle, fixed in the bottom of the box,
of rather more than seven inches dia-
meter, measured the arc of vibra-
tion. The bar was carried in a light
stirrup, into which it was slid until cor-
rectly balanced. The silk thread, from
which the stirrup was suspended, was
fifteen inches long, and consisted of a
sufficient number of silk fibres to sus-
tain the weight. In order to remove
all influence from the tendency in the
silk to untwist, a brass bar, equal in
weight to the magnetic bar, was first
introduced into the stirrup in place of
the latter, and the silk thread allowed to
untwist itself, and then adjusted by
turning the button in such a manner as
that the brass substitute should settle,
when at rest, in the magnetic direction.
This being now removed, the magnetic

* Tom. iii. p. 33. >

•j- Account of experiments to determine the figure
Of the earth, p. 477.

bar was replaced in the stirrup, and its
horizontally ascertained by its accord-
ance with the circle, the degree to which
it settled being registered as the zero.
It was then drawn about forty degrees
out of the meridian, and retained by a
copper wire passing through the glass
top, and capable of being moved in azi-
muth from its outside, and of being
raised so as to release the needle at
pleasure, in order to commence its os-
cillations. These were not noticed until
the arc had diminished to thirty degrees,
when the registry of them commenced,
and was repeated at the close of every
tenth vibration, until the arc had still
further diminished to ten degrees, when
the experiment was concluded. The
box was usually placed on the ground,
in a sheltered situation, far from build-
ings, or other sources of local inter-
ference ; the only adjustment required,
besides that of the silk thread, was to
•render the graduated circle horizontal,
which was accomplished by a pocket
spirit level, and wooden wedges placed
beneath the box.

(377.) Six bars were used in this ap-
paratus, differing from each other con-
siderably, both in rapidity of vibration,
and in the duration of the interval of
oscillation between thirty and ten de-
grees. They were seven inches long, a
quarter of an inch broad, 0.15 inches
thick, and strongly magnetized. When
not in use, they were kept in pairs, in
the usual manner, as described in J 218,
Jig. 60, being combined, with their op-
posite poles united, in separate boxes ;
and each bar was placed by itself in the
direction of the meridian for two or
three hours before its time of vibration
was ascertained. The times were re-
gistered to fractional parts of a second
by the beats of a chronometer, having
a rate inappreciable in the interval.

(338.) It should be observed, how-
ever, that comparative experiments on
magnetic intensities, made by the oscil-
lations of needles balanced horizontally,
are liable to a source of error when
they are made in places in which the
dip is considerably different. For one
of the poles having a tendency to in-
cline below the horizon, the axis of
suspension must, in order to cqmpen-
sate for this tendency, pass through a
point on that side of the centre of gra-
vity, so as to give an equal prepon-
derance to the other side. Hence arises
a difference between the two arms of
the lever, and consequently a difference

MAGNETISM.

in the effect of the force applied to
them. This circumstance has escaped
the attention of the most skilful ob-
servers, although it is of sufficient im-
portance to destroy all confidence in the
comparison of experiments made in
places where the dip is very different *.
(339.) We have seen the method by
which the amount of the dip may be
determined from the comparative inten-
sities of magnetic force in the plane of
the magnetic meridian, and in the hori-
zontal plane. It is easy to see how, by
reversing the process, we may arrive at
the knowledge of the force of terrestrial
magnetism in the magnetic meridian,
from observations of the intensity in the
horizontal plane, when the dip is pre-
viously known ; for we have only to
augment the former in the proportion
of the cosine of the dip to the radius :
or, what comes to the same thing, to
multiply it by the secant of the dip.
For since

it Mows that

cos.

h. sec.

(340.) The method of oscillations is
ttot the only mode of determining the
intensity of magnetic forces ; for the
same object may be attained by employ-
ing the balance of torsion : but in gene-
ral the former method is, on many ac-
counts, preferable.

(341.) It would conduce much to the
future advancement of the science of
Magnetism, if the absolute intensity of
the terrestrial magnetic forces could be
ascertained by a standard equally deter-
minate as those by which we estimate
the atmospheric pressure, or the tem-
perature of different climates; for by
repeating the same process of observa-
tion, in the course of successive centuries,
we might learn whether the intensity
of these forces experienced any variation
similar to what we know takes place
in their direction.

(342.) The method which first sug-
gests itself, would be to observe the
variation, the dip, and the intensity, by
means of three needles, respectively ap-
propriated to these objects, and carefully
preserved, with a view to a repetition of
the same experiments at distant periods
of time. As it is possible that their
magnetic power may become impaired

* Poniiiet, Elgmens de Physique Experimental®
torn. i. \\ 4S2,

by these long intervals of time, it would
probably be necessary to magnetize
them afresh, employing for that purpose
the most efficacious methods, so as to
induce a degree of magnetism which
shall at first be considerably greater
than what they are capable of retaining
permanently. When left for a certain
time to themselves, they will then return
to their natural state of saturation, and
in this state they may be subjected to
the experiments in question. Our as-
surance of their attaining this determi-
nate degree of magnetism may be in-
creased, by preserving a great number
of needles; brought to this state, and
noting their respective powers ; which
records may afterwards be compared
with similar observations made at a
future period; for if they should be
found to have preserved among each
other the same relative degree of strength,
we may confidently conclude that no
alteration has taken place in their mag-
netic constitution, and that they are well
adapted to the determination of the ab-
solute force of terrestrial magnetism.

(343.) If a method could be dis-
covered of obtaining a material for the
making of magnets of perfectly uniform
composition and qualities, there would
be no necessity to employ in this com-
parison the same identical magnets at
the several times of observation. Steel
would evidently be unfit for the purpose,
its composition being necessarily varia-
ble. Nothing would answer so well as
iron alone, wliich might be obtained in
a state of perfect purity by proper che-
mical processes ; for Coulomb found
that pure iron, however soft, may be
rendered nearly as retentive of magnet-
ism as steel, by merely twisting it. All
that would then be necessary, would be
to regulate the degree of twist, so that
it might be constantly the same for dif-
ferent pieces of iron : this might easily
be accomplished, by taking them exactly
of the same dimensions, and measuring
the number of turns in the twist given
to them. Each of the bars might then
be magnetized to saturation, and em-
ployed either separately or in defined
combinations, and their magnetic forces
examined, both by the torsion balance,
and by the method of oscillations*.

(344.) Poisson has also attempted to
resolve this problem by considerations
of a mathematical nature, for which we
must refer to his memoir, read to the

Biot, Traite" de Physique, torn,

MAGNETISM,

Academy of Sciences at Paris, in Nov.
1825, and of which an account is con-
tained in Pouillet's Elcmens de Phy-
sique *.

§ 7. Experiments on the Magnetic
Intensities at different Heights above
the Surface of the Earth.

(345.) In the year 1804, Messrs. Gay-
Lussac and Biot undertook, at the de-
sire of the French government, an aero-
static voyage, expressly for the purpose
of ascertaining whether the magnetic
force experiences any perceptible dimi-
nution at considerable elevations above
the surface of the earth. De Saussure
had inferred, from some experiments
which he made on the Col du Geant,
near Mont Blanc, the height of which
is 3435 metres (about 11,270 feet), that
the magnetic force of the earth was re-
duced to four-fifths of what it was in the
plains below. The instrument with
which he made these experiments was
simply a magnetic needle, suspended by
a very fine silk thread. Messrs. Gay-
Lussac and Biot carried with them a
needle, carefully constructed by Fortin,
and magnetized by Coulomb, according
to ^Epinus's process. No iron was
allowed to "enter into the construction
of the car of the balloon ; the only arti-
cles of iron they carried with them, were
a few knives and a pair of scissors,
which were suspended in a basket below
the car, at a distance of from twenty-five
to thirty feet, so that they could have
no sensible influence on the magnetic
needle. The continual rotation of the
balloon on its axis, during its ascent,
seemed at first to present an insupera-
ble obstacle to their observing the oscil-
lations of the needle. But, by bringing
themselves in a line with terrestrial ob-
jects and the sides of the clouds, they
perceived that they did not always turn
round in the same direction, the rotatory
motion gradually decreasing, and then
taking place in a contrary direction. By
watching the short intervals during
which they remained stationary between
these opposite motions, they were ena-
bled to observe five, or at most ten
oscillations at a time ; they were obliged,
however, to be very careful not to agi-
tate the car, for the slightest motion,
such as that produced by letting the gas
escape, or even that of the hand in writ-
ing, was sufficient to turn the balloon
aside. With all these precautions,
which required a great deal of time, they

• Tom, i.,.p, 491.

found means to make ten
in the course of the voyage,
ferent altitudes. The conclusion t
deduced from the average of all their
observations is, that the magnetic force
experiences no appreciable diminution
at any distance from the surface of the
earth, as far as 4000 metres, or 13,124
feet.

(346.) Many important facts relating
to this question have lately been estab-
lished by M. Kupffer, in the course of
a journey to the neighbourhood of
Mount Elbrouz, in the Caucasus, under-
taken by order of the Emperor of Rus-
sia, in the year 1829, and of which an
account has been given in a paper pub-
lished in the Memoirs of the Imperial
Academy of Sciences of St.Petersburgh*.
M. Kupffer found that the intensity of ter-
restrial magnetism really decreased as he
rose above the level of the sea ; and that
this decrease was much more considerable
than is conformable with the commonly
received hypothesis of a focus of mag-
netic forces situated at the centre of the
globe. He even thinks that the experi-
ments of Gay-Lussac and Biot, in the
voyage just mentioned, should have led to
the same conclusion, because, although
they could not detect any difference
in the apparent intensity, yet since the
temperature of the elevated regions of
the atmosphere in which they made their
observations is exceedingly low, it is
probable that the magnetic power of
the needle itself was greater than in the
warmer atmosphere at the surface of
the earth ; so that, unless the terrestrial
force had really diminished, an increase
of magnetic intensity would have been
apparent, and would have been indicated
by the increased frequency of the vibra-
tions in a given time. As this increased
frequency was not observed to take
place, M. Kupffer concludes that the
force of terrestrial magnetism is, in fact,
less at the height to which they had
reached, than it is at the surface of the
earth. It is certain that, in all ex-
periments of this kind, the temperature
should be accurately noted, as consti-
tuting an essential element in the rea-
sonings to be founded on them t.

CHAPTER VII.
Of the Magnetism of Bodies that are

not ferruginous.

(34 7.) It has long been suspected that
besides iron, other metallic substances

* For 1830, p. 69.

t See Journal of the Royal Institution, vol. i. p.

MAGNETISM.

are capable of exhibiting magnetic phe-
nomena. Nickel, and also cobalt, have
occasionally been found obedient to the
action of the magnet ; and sometimes to
possess considerable degrees of polarity.
Brass, which is a compound of copper
and zinc, has likewise been observed to
be magnetic under certain circum-
stances, especially after it has been ham-
mered. Cavallo states* that, when
quite soft, brass has generally no per-
ceptible degree of magnetism ; and even
those pieces which have acquired this
property by hammering, again lose it by
annealing or softening in the fire. He
seems to have ascertained that the mag-
netism acquired in the former state, is
not owing to any particles of iron or
steel imparted to the brass by the tools
employed in the hammering; and that
those pieces of brass which have that
property retain it without any diminu-
tion after having been hardened and
softened several times in succession. If
one end only of a large piece of brass be
hammered, that end alone is rendered
magnetic. He found, however, that the
magnetic power which brass acquires by
hammering has a certain limit, beyond
which it cannot be increased by further
hammering ; and that this limit is dif-
ferent in pieces of brass of different
quality or thickness.

(348.) Cavallo next examined various
pieces of copper by means of a delicately
suspended needle ; but never found them
magnetical, except occasionally in those
parts where a file had been applied, and
where, consequently, some particles of
steel, detached from the file, may have
adhered to the copper. On hammering
other pieces, both in the usual way, and
also between flints, he failed in obtain-
ing any decisive result. Zinc, whether
hammered or not, showed no sign of
magnetism whatever. Platina was found
to possess a degree of magnetic power
nearly equal to that of brass.

(349.) The magnetic power of brass
is sometimes so considerable as to in-
terfere very sensibly with the movements
of the needle in compasses, in the con-
struction of which brass is employed.
A remarkable instance of this is given
by Mr. Barlow f. Seebeck has recom-
mended an alloy of two parts copper
with one of nickel, as admirably adapted
for the manufacture of compasses, from

* Philosophical Transactions for 1786, p. 82; and
also in his Treatise on Magnetism.

t Essay on Magnetic Attractions, second edition,
J?-17.

its being 'entirely void 'of magnetism*.
We have seen, in the case of brass, that
two ingredients, which in themselves,
when separate, are devoid of magnetic
susceptibility, acquire that property by
combination. Mr. Hatchet ascertained
that a large proportion of either carbon,
sulphur, or phosphorus, combined with
iron, enables it fully to receive and to
retain the magnetic properties : but that
there is a limit beyond which an excess
of either of these substances renders the
compound totally insusceptible of mag-
netism t. On the other hand, instances
occur where the admixture of the mi-
nutest quantity of another body will
entirely destroy the magnetic power of
a metal possessing that power when in
a pure state. Mr. Chenevix found that
the addition of arsenic, in very small
proportion, deprived a mass of nickel,
which had previously manifested strong
magnetic power, of the whole of its mag-
netism $. Dr. Matthew Young states
that the smallest admixture of antimony
is sufficient to destroy the polarity of
iron §.

(350.) In the mineral kingdom a great
variety of substances, and even some of
the precious stones, as the emerald, the
ruby, and the garnet, exert a feeble yet
sensible attraction on the magnetic
needle; and sometimes even acquire a
slight degree of polarity ||.

(351.) Later inquiries appear to have
established the fact that all bodies what-
soever are, in a greater or less degree,
susceptible of magnetism. We owe this
discovery to Coulomb, who exhibited his
experiments in proof of it at a sitting of
the French Institute, in 1802. The
bodies examined were cut into small
cylinders, or bars, about a third of an
inch in length, and about the thirtieth
of an inch in thickness : but those which
were metallic were formed into needles
of about the hundredth of an inch in
diameter. Each of these cylinders was
suspended by a thread of taw silk, which,
being exceedingly fine, could scarcely
support more than from 100 to 150
grains without breaking: on this ac-
count it was necessary to reduce the
needles to very small dimensions. They
were placed, when thus suspended, be-
tween the opposite poles of two steel

* Annales de Physique, 1826.

t Philosophical Transactions for 1804, p. 315.

Nicholson's Journal, 8vo. vol. iii. p. 28?.'
§ Seebeck discovered that an alloy of one part
iron with four parts of antimony exercised no power
over the magnetic needle, even when in motion.
II Cavallo's Treatise on Magnetism, p. ?3.

MAGNETISM.

91

magnets, arranged in the same line, and
separated about a quarter of an inch
more than the length of the needle that
was to oscillate between them. What-
soever was the substance of which the
needles were formed, they always ranged
themselves accurately in the direction of
the magnets ; and if disturbed from this
position, returned to it with oscillations,
which were often as frequent as thirty
or more in a minute, and considerably
more frequent than when the magnets
were removed : thus indicating a very
decided force of attraction. Needles
made of tin, lead, copper, silver, and
gold, and cylinders of glass, chalk, bone,
and different sorts of wood, together
with a great variety of other organic
substances, both animal and vegetable,
were tried in succession, and with the
same result.

These experiments were repeated in
England, by Dr. Young, at the Royal
Institution, but with less decided suc-
cess: the force of attraction indicated
was estimated at rather less than the
two- thousandth part of the weight of the
substance employed.

(352.) There are but two ways of ex-
plaining these phenomena: they are
either owing to the presence of minute
quantities of iron entering into the com-
position of all the bodies which manifest
magnetic properties, or else they war-
rant the inference that all these bodies
possess a certain degree of inherent
magnetism. If the former mode of ex-
planation be the true one, we shall be
forced to admit that iron may exist in
bodies, in quantities so minute as to
elude detection by the severest chemical
examination, and yet have sufficient
power to be sensibly affected by a mag-
net. A set of very delicate experi-
ments was undertaken by Coulomb
with a view to determine this point : in
the course of which he satisfied himself,
that a smaller quantity of iron than can
be discovered by any chemical test yet
known, will, when added to a body, impart
to it a very decided magnetic suscepti-
bility. This is the case when a metal
contains only the 130,000th part of its
weight of iron. The magnetic powers of
different specimens of metals were found
to differ materially according to the
methods employed for their purification.
Hence he concluded that the greater
part, if not the whole, of the effect ob-
served, is to be ascribed to the presence
of iron. So confident was he of the truth
ot this theory, that he imagined the

magnetic action of all substances might
safely be taken as a criterion of the pro-
portion of iron they contain.

(353.) On the other hand, the indi-
cations of magnetic power given by
nickel and by cobalt are far too con-
siderable to be accounted for by the
agency of any ferruginous admixture.
Biot was in possession of a needle made
of nickel, which Thenard had exerted
all his chemical skill in rendering as
pure as possible : the directive force of
this needle, when magnetized, was not
less than one-third of a similar needle
made of steel *. Now the proportion of
iron which, if added to the nickel, would
be required to impart an equal degree of
magnetic power, is far beyond what can
ever reasonably be supposed to enter in-
to the composition of nickel so purified. It
is certainly just possible that nickel may
be a compound metal, containing iron
as one of its ingredients ; but we are not
justified in admitting an explanation so
extremely hypothetical, especially as
there are other facts besides those above
mentioned, which tend strongly to cor-
roborate the universality of magnetism.
Of this nature are the evidences of an
influence exerted by bodies not reputed
magnetic, in controlling the oscillations
of a magnetic needle placed in their im-
mediate vicinity; and also the pheno-
mena of the mutual influence exerted
between these bodies and magnets, when
the one is revolving rapidly, in exciting
sympathetic rotation in the other. These
very curious phenomena will be de-
scribed in the ensuing chapter.

CHAPTER VIII.
On the Magnetism of Rotation.

(354.) CONSIDERABLE .light has lately
been thrown upon the question of the
universality of magnetism, by the dis-
covery of the unexpected effects which
result from the reciprocal action of
magnets upon other bodies, when
the one or the other is maintained in a
state of rapid rotation. In the year
1 824 M. Arago showed that if a plate
of copper, or of any other substance, be
placed immediately under a magnetic
needle, it exerts sufficient influence upon
its movements to diminish sensibly the
extent of its oscillations, without, how-
ever, affecting their duration ; and the
needle is brought to rest in a shorter
time than happened when no such sub-
stance is placed under it The con-

* Traite de Physique, torn, iii., p. 126.

MAGNETISM:

verse of this experiment was attended
with still more striking effects. When
a plate of copper, for example, is made
to revolve with a certain velocity under
a magnetic needle, supported on its
centre, and contained in a vessel closed
on all sides, the needle is found to de-
viate from its natural position in the
magnetic meridian ; and the deviation
is greater in proportion as the rotation
of the plate is more rapid. If the ra-
pidity of revolution be sufficiently great,
the needle will be brought to revolve
also, and always in the same direction
in which the plate is made to re-
volve *. The experiment was varied in
the following manner : — A circular plate
of copper, balanced on a point at its
centre, was placed immediately under a
strong magnet, to which a rapid rota-
tory motion was given; the copper-
plate soon began to turn in the same
direction, and acquired by degrees a
very rapid ^velocity of revolution. It
was found, also, that the oscillations of
a copper-plate in a vertical plane, when
suspended by an axis which passed at a
small distance from its centre of gra-
vity, were much impeded, and soon de-
stroyed, when the plate was inserted be-
tween the two poles of a very powerful
horse- shoe magnet. M. Arago was of
opinion that these phenomena are inex-
plicable upon principles of ordinary
magnetism, and considered them as the
effects of some hitherto unknown power
in nature. But subsequent inquiry
seems to render it j probable, that they
are all reducible to the operation of
known laws of magnetism.

(355.) Mr. Christie having observed
a permanent change in the magnetism
of an iron plate in consequence of a
mere change of position on its axis, it
occurred to Mr. Barlow that this
change would be increased by rapid
rotation. But on trial this was found
not to be the case, the effect pro-
duced being merely temporary. The
first experiments were made with a
mortar-shell fixed to the mandril of a
powerful turning lathe, worked by a
steam-engine. When the ball was made
to revolve at the rate of 640 times in a
minute, the needle was deflected several
degrees from its natural position, and
there remained stationary during the
motion of the ball ; whenever the rota-
tion ceased, the needle immediately re-
turned to its original situation. On in-

* Bulletin Universe!, vol.; iii,, p, 328 ; Annaleg
de Chimie, xxviii., 325.

verting the motion of the shell, an equal
and contrary deflection took place. But
although numerous trials were made
under various circumstances, the law of
the phenomena could not be deduced
until the influence of the earth's action
had been neutralized by means of other
magnets perfectly adjusted. All the ano-
malies before met with now disappeared,
and the following law was rendered ma-
nifest. When the needle and ball are
both in the same horizontal plane,
whatever may be the direction of the
axis of rotation of the ball, if its mo-
tion at its upper part be made towards
the needle, the north pole of the latter
will be attracted ; and if the contrary
way, repelled. Hence he concluded,
that when an iron body is put in rapid
rotation on any line not coinciding with
its magnetic axis, a temporary derange-
ment takes place in its magnetic powers,
equivalent in effect to the influence of a
new axis of polarization, perpendicular
to the planes passing through its axes
of rotation and of ordinary polarization.
(356.) The next series of experiments
on this subject are those of Mr. Chris-
tie*, who ascertained that a plate which,
in a given position, produced a certain
deviation of the compass, no longer pro-
duced the same deviation, after it had
been carried round one entire revolution
in its own plane, although brought to
rest, and every part of the apparatus re-
stored to its former place. This change
in the directive power of the plate, pro-
duced by rotation, was greatest when its
plane was parallel to the line of dip, and
at the same time as little inclined to the
horizon as this condition would allow ;
or, in other words, when the axis of ro-
tation was in the plane of the magnetic
equator, and also in a vertical plane,
that is, in the magnetic meridian. Hence
he deduced a law which may be thus
expressed : — If we conceive a dip-
ping-needle to lie in the centre of an
imaginary sphere, having an equator
situated in a plane intersecting perpendi-
cularly the direction of the dipping-
needle, and a circular plate of iron to be
placed with its centre in the surface of
that sphere, its plane being a tangent to
that surface, when the plate revolves,
the effect of its revolution upon the dip-
ping-needle will be such that each side
of the equator of the latter (that is, the
portion of the equator which is situated
in a line at right angles to the line join-
ing the centres of the needle and plates)

Philosophical Transactions for 1825, p. 347. j

MAGNETISM.

93

\vill be deflected in a direction contrary
to the direction in which that edge of
the plate which is nearest to it moves.
The deviations of the horizontal needle
may be easily deduced from this law, by
reference to the motions of this imagi-
nary dipping-needle, for they will be
such as tend to bring them into the
same vertical plane with the latter,
which is the situation in which it makes
the nearest approach to its line of direc-
tion.

(357.) The investigation of this cu-
rious subject was further prosecuted by
Mr. Babbage and Mr. Herschel, who,
in conjunction, undertook to verify M.
Arago's experiments*. After a few
trials they succeeded in causing a com-
pass to deviate from the magnetic me-
ridian, and finally to revolve, by placing
under it plates of copper, zinc, or lead,
which were put into very rapid rotation.
In order to obtain more visible and re-
gular effects, however, they found it
necessary to reverse the experiment, by
setting in rotation a powerful horse-shoe
magnet with its poles uppermost, the
line joining them being horizontal, and
its axis of symmetry being placed verti-
cally ; while a circular disc of the sub-
stance to be examined was suspended
over this magnet. The disc was found
to follow the motion of the magnet with
various degrees of readiness, according
to the substance of which it was made.
They obtained in this way signs of
magnetic susceptibility from copper,
zinc, silver, tin, lead, antimony, mercury,
gold, bismuth, and carbon, in that pe-
culiar metalloid state in which it is
precipitated from carbonated hydrogen
in gas-works. Great care was taken, in
the case of mercury, to secure the ex-
clusion of iron. In other bodies which
were tried, such as sulphuric acid,
rosin, glass, and other non-conductors,
or imperfect conductors, of electricity,
rio positive evidence of magnetism was
obtained.

(358.) They next endeavoured to de-
termine the comparative intensities of
action of these different bodies. Two
methods were used for this purpose ;
first, by observing the deviation of the
compass over revolving plates of great
size, cast to one pattern ; and, secondly,
by the times of rotation of a neutralized
system of magnets suspended over
them ; and it is remarkable, that the
places of zinc and copper in the scale,

• Philosophical Transactions for 1825, p. 407,

according as the one or the other of
these two methods was employed, were
the reverse of each other ; although the
same order was assigned to all the other
bodies by both methods.

(359.) On trying the effect of the in-
terposition of different bodies as screens,
in cutting off or modifying the influence
of the rotating bodies, they could not
detect any interceptive power, except,
as might be expected, in the case of an
iron plate, which, when of sufficient
thickness, completely destroyed all per-
ceptible effect from rotation.

(360.) The magnetic energy deve-
loped by rotation was found to be much
diminished by any interruption of con-
tinuity in the plate which was acted
upon. This fact had previously been
noticed by Arago ; but the experiments
of Mr. Babbage and Mr. Herschel have
verified it in more detail, and have added
the curious circumstance, that re-esta-
blishing the metallic contact with other
metals, restores, in a great measure, the
force which had been lost by the divi-
sion of the substances ; and this hap-
pens even when the metal used for sol-
dering has, of itself, but a very feeble
magnetic power ; thus affording a means
of magnifying weak degrees of magnet-
ism. The reduction of the metals to
filings, or to powder, was found to pro-
duce a still more remarkable diminution
of their magnetic energy. The law of
diminution of force by increase of dis-
tance was next investigated ; but it
appeared to follow no constant progres-
sion according to any fixed power of
the distance, but to vary between the
square and the cube.

(361.) The explanation of these cu-
rious phenomena has been attempted
on the following principle — namely, that
in the induction of magnetism, time en-
ters as a necessary element ; or, in other
words, that a certain appreciable time
is required both for the acquisition of
magnetic polarity, communicated by
induction from a "magnetized body, and
for its loss, when the body in which it
has been induced returns to the neutral
state by the subtraction of all extraneous
influence.

(362.) In order to trace the operation of
this principle, let us conceive the north
pole of a magnet to move horizontally, at
a little distance above a plate of metal,
or other substance, having a very low de
gree of magnetic susceptibility, and also
a very low degree of retentive power.
The points over which it passes in sue-

MAGNETISM.

cession wni not instantly receive all the
magnetism which the magnet is capable
of exciting in them ; their state of maxi-
mum polarity, therefore, will not be at-
tained until the magnet has passed for
some small distance beyond them. Nei-
ther will they lose their polarity at the
same instant that the magnet is still
further removed. Thus, from both
causes, there will always be, in the rear
of the magnet, a space both more ex-
tensive and more strongly impregnated
with the opposite polarity than in ad-
vance of it. Hence, there will arise an
oblique action between the pole of the
magnet and the opposite pole of the
plate, thus lagging behind it, which will
urge it to move in the direction of the
magnet's motion. The development of
the more distant polarity, similar in
kind to that of the magnet, being more
diffused, that pole will be both weaker
and less oblique in its action, and will
be much inferior in power to that of the
nearer attracting pole. It is evident
that the converse will also be true when
the magnet is at rest, but free to move
horizontally; and when the different
parts of the plate are passed in succes-
sion under it, the latter will tend to drag
the former after it with a velocity con-
tinually accelerating, till they move on
together with the same velocities. It is
also manifest, that the greater the rela-
tive velocity, the more will the pole, de-
veloped in the plate, lag behind the mag-
net, or the magnet (in the reverse case)
lag behind the pole ; the more oblique,
therefore, will be the action, and the
greater the velocity produced. The
application of these principles to circu-
lar motion, or to the rotation of plates,
is sufficiently obvious ; but in this case,
if the velocity be excessive, compared
with the retentive power of the revolving
substance, the latter may have completed
a revolution before there has been time
for its being affected to a degree suffi-
cient to occasion motion in the magnet;
the induced polarity will then be weak-
ened, and its effects rendered insensible.
This diminution of the total effect by a
more general diffusion of polarity was
imitated by sticking a great number of
magnetized needles, vertically, through a
light cork circle, having their north poles
downwards, so as to form a coronet of
magnets. This apparatus, suspended
centrally over a revolving copper disc,
was not sensibly affected ; for the poles
of all the magnets, acting in rapid suc-
cession upon all the points below them,

produced nearly~"a uniform circle of
southern polarity, whose equal and con-
trary actions on the needles would de-
stroy one another.

(363.) Plausible as this theory may
appear, it is yet embarrassed with many
serious difficulties. It does not give any
satisfactory explanation of the mode in
which an attractive force, resulting from
induction, between one pole of a magnet
and the consequent polarity of the ad-
jacent parts of a piece of copper — a force
which is so feeble as not to produce any
sensible effects when both these bodies
are at rest— is immediately so greatly
increased on their separation and conti-
nued removal from one another, so as
to occasion a very considerable motion.
The force producing that motion is,
according to the theory, an attractive
force; but Arago has shown that the
general resultant of all the forces, which
operate between the pole of the magnet
and the plate, is a repulsive force, with
relation to the line' perpendicular to the
surface of the plate. The following ex-
periment proves this : — Suspend a long
magnet by a thread, in a vertical posi-
tion, to the beam of a balance, and coun-
terpoise it by weights on the opposite
side ; if the plate be then revolved
under the magnet, the equilibrium will
cease, and the magnet will rise, or ap-
pear to become lighter, indicating its
repulsion from the plate*.

(364.) The latest experiments on this
subject are those of Mr. W. S. Harris,
of Plymouth, of which he communicated
an account to the Royal Society, in June
1830f. Finding that the vibrations which
attend bodies in rapid rotation are pro-
pagated to a remarkable extent along
the solid parts of the apparatus by which
a magnetic needle is suspended, and that
they are also conveyed to great distances
by the surrounding air, even when
highly rarefied, he took great pains to
obviate these two sources of fallacy.
He accordingly conducted all his expe-
riments in an exhausted receiver, the
parts to be acted upon being effectually
secured from the influence both of vi-
brations from solids and of aerial vor-
tices. He conceives that, in general,
sufficient care had not hitherto been
taken to eliminate these several causes
of error, and that we cannot repose that
implicit confidence in the conclusions
deduced from them which is required in

* Annales de Chimie, xxxii. 213.
" f It has been since published ifl the Thilosophical
Transactions for 1831, P, 67.

MAGNETISM.

a subject of so delicate a nature. He
finds that the influence of a rotating
body on the magnet is real, but more
feeble than had previously been ima-
gined ; and that the law of intensity of
action is directly as the rapidity of ro-
tation, and inversely as the squares of
the distances between the attracting
bodies.

(365.) From some experiments which
Mr. Harris lately exhibited at the Royal
Institution of London, it appears, that a
magnetic needle, partly neutralized with
regard to the earth's action, and made
to vibrate while surrounded by a mas-
sive ring of copper, or other substance
of very weak magnetic energy, had its
oscillations sensibly repressed by the
presence of that substance, and that it
arrived much sooner at a state of rest
than when the ring was removed. The
magnetic influence of rotating bodies
was found to be intercepted by a va-
riety of substances interposed between
them.

(366.) His last communications to
the Royal Society contain an account of
experiments tending to show that every
substance susceptible of magnetism by
induction, when interposed as a screen,
tends to arrest the action excited by a
magnet on a third substance ; and that
this interceptive power is directly as the
mass of the intervening substance, and
inversely as its susceptibility to receive
induced magnetism. A single plate of
iron, for instance, about the sixteenth
of an inch thick, is found effectually to
intercept the action of a revolving mag-
net on a disc of copper ; but the same
result is not obtained when the disc
acted upon, instead of being copper, is
also of iron, unless the mass of inter-
posed iron be very considerable. He
afterwards determined that this inter-
ceptive influence depends, not merely
upon the surface of the interposed iron,
but is in proportion to its mass. Hence
he was led to suspect that indications
might be obtained of a similar influence,
exerted by substances not of a ferru-
ginous nature, if they were interposed
in considerable masses ; and this con-
jecture was verified on trying the experi-
ment. He found that the action of a
revolving magnet upon a disc of tinned
iron was completely intercepted by
masses of about four inches in thick-
ness, of either copper, zinc, or silver,
interposed between them.

(367.) It would appear, then, that
this intereeptive property is common to

all matter, though possessed, in various
degrees, by different kinds of substance;
and that, in order to render it sensible,
it is only requisite to employ them in
masses proportionate to their respec-
tive magnetic susceptibilities. Lead, for
example, which has a weaker magnetic
susceptibility than copper, must be em-
ployed in a larger mass in order to pro-
duce an equal effect. By an extension
of this principle, it would require a
thickness of above thirty feet of ice, in
order to render its interceptive power
sensible.

(368.) It is not necessary, however,
that the substance which exerts this
controlling power over the action of a
revolving magnet should be actually
interposed between the magnet and the
metallic disc. Mr. Harris found that,
in the case of iron, a mass of that
metal, when placed very near to that
surface of the magnet most distant from
the disc on which it was acting, had the
effect of neutralizing its power. With
regard to non-ferruginous bodies, it is
very difficult to render this influence
sensible, unless they are interposed in
the direct line of action.

(369.) The temporary induction of
magnetism may be conceived as taking
place in two different ways : first, by
the immediate action of the magnet
upon each individual particle of the body
on which magnetism is induced ; or,
secondly, by the action of each particle
on the next adjoining to it in succession,
constituting a continued and successive
propagation of magnetism from the one
to the other. Although both these kinds
of induction may take place at the same
time in the same substances, yet the de-
grees in which they are exerted seem to
be in some inverse ratio the one to the
other : for when the absorbing or reten-
tive power of the substance is consider-
able, the power of the magnet is soon,
checked: because the particles of that
substance first acted upon begin to
operate as screens to the succeeding
particles; and the induction, after a
certain point, proceeds entirely by com'
munication from particle to particle,
until the whole has attained a state of
permanence. When, on the contrary,
the retentive power of the given sub-
stance is small, little or no interceptive
influence can exist among its particles ;
and the induction will be produced
solely by the direct action of the magnet
on all the particles. Accordingly it is
only by the succession or multiplication

MAGNETISM.

of effect, resulting from the action of
a great number of particles, that the
controlling power of such a substance
becomes sensible. To this cause is ap-
parently owing the diminution of the
action of a revolving magnet on a disc
of copper, when the latter is intersected
by radiating grooves : since a portion of
the substance, every part of which is
concerned in the full development of
induced magnetism, is thereby removed.
In confirmation of this reasoning, Mr.
Harris found that the number of oscil-
lations made in vacuo in a given arc,
by a delicately suspended bar, sur-
rounded by several concentric copper
rings, did not materially differ from the
number performed, in similar circum-
stances, by the same bar, surrounded
by a solid mass of copper.

(370.) The effects of various metals
in diminishing the oscillations of a mag-
netic needle, have been determined with
considerable accuracy by Professor See-
beck. An account of his experiments
is given in the Annales de Physique for
1826*.

(371.) With regard to the theory of
magnetism, Poisson has remarked that
there is no necessity for supposing that
the phenomena of magnetism are pro-
duced in all bodies by a fluid, or fluids,
possessing everywhere the same inten-
sity of attractive or repulsive action,
and therefore requiring to be considered
as the same fluid in different substances.
No doubt can exist as to the identity of
the electric fluid; because we see it
passing from one conducting body into
another, and at the same time preserv-
ing all its properties, and exercising, in
like circumstances, the same attractions
and repulsions. But we have no evidence
of this kind in the case of the magnetic
fluids, because they are always confined

* See Quarterly Journal of Science for January,
1828, p. 456.

to the same particles ; and we cannot,
by mere reasoning, decide whether the
magnetism of two different bodies, such
as pure iron and pure nickel, should be
considered as the same imponderable
substance. It would assist us in the
determination of this question, were it
ascertained that similar and equal nee-
dles of iron and of nickel, when sub-
mitted to the magnetic influence of the
earth, or of any other magnet, would
make an equal number of oscillations.
An experiment was made by M. Gay-Lus-
sac with this view, from which it would
appear that the mutual action of the
magnetic fluids contained in steel and in
soft iron is decidedly greater than the
mutual action of the fluids belonging to
steel and to nickel. This experiment,
however, is not decisive of the question,
because we may still consider the mag-
netic elements of a body as not actually
in contact, but as constituting assem-
blages of particles, in which the two
fluids reside, and are separated by inter-
vals not greater than the dimensions of
those elements; and, as we formerly
observed (§ 165), the ratio between the
sum of the volumes of all these elements,
and the total volume of the body, may
be different in different substances, and
at different temperatures, in the same
substance. This diversity may explain
the result of Gay-Lussac's experiment,
without the necessity of the supposition
of a difference in the intensity of the
magnetic power in substances differently
susceptible of magnetism*. But the
interest which attached to speculations
of this kind, are now, in some measure,
superseded by the new theory of magnet-
ism, proposed by Ampere: for an ac-
count of which, we must refer to the
forthcoming Treatise on Electro- Mag-
netism.

* Memoires de 1'Acadcoiie Royale des Sciences,
torn. v. pp. 252, 254.

ERRATA.
Page 64, line 35, for sin, read cos.

37, .. to the to half the.

41,.. sin £sin.

ELECTRO-MAGNETISM.

CHAPTER I.
History of the Science prior to

Oers teds disco very .

(1.) THE analogies that exist between
the phenomena of magnetism and those
of electricity, in their general character,
in the laws which govern them, and in
the various combinations they present,
are so extensive and so remarkable, as
naturally to suggest the notion that the
agencies themselves from which they
proceed must be allied to one another
by some close and intimate relation.
Adventurous theorists have advanced
the doctrine, that each of these princi-
ples is merely a modification of the
other, and that both may be regarded
as ultimately identical in their nature,
constituting, instead of two separate
and primary powers, a single power of
a higher order of simplicity.

(2.) The connexion between mag-
netism and electricity was a favourite
subject of speculation and inquiry
among philosophers in the middle of
the last century. Many were the efforts
made to resolve this seductive problem,
which continued, however, to baffle the
labours of each succeeding experimen-
talist, who multiplied his attempts, and
varied his processes, without approach-
ing nearer to the point he aimed at ;
and also to elude the reasonings of
those who theorized upon every new
fact until they bewildered both them-
selves and their readers in the mazes of
visionary and conflicting hypotheses.

(3.) In the year 1774, the following
question was proposed by the Electoral
Academy of Bavaria as the subject of
a prize dissertation: — ' Is there a real
and physical analogy between electric
and magnetic forces ; and, if such ana-
logy exist, in what manner do these
forces act upon the animal body?'
The essays received by the Academy
on that occasion, were collected and
published, ten years afterwards, by
Professor Van Swinden, of Frantker,
the author of one of the essays for

which the prizes were awarded *. The
conclusion to which he arrived, after a
long and elaborate discussion of the
subject, was, that the similarity between
electricity and magnetism amounts
merely to an apparent resemblance,
and does not constitute a true physical
analogy ; whence he infers, that these
two powers are essentially different
and distinct from one another. The
opposite opinion, on the other hand,
was maintained by Professors Steigleh-
ner and Hubner, who contended that
so close an analogy as that exhibited
by these two classes of phenomena, in
(heated the effects of a single agent,
varied only in consequence of a di-
versity of circumstances. So many
new facts have been brought to light
since the time in which these authors
wrote, that the reasonings adduced on
either side in this controversy have now
lost their interest, excepting: that it is
still curious to observe by what devious
paths they were led away from the
truth, at the moment when they had
nearly reached it, and when a very slight
variation in the form of their experi-
ments would at once have disclosed it
to their view.

(4.) Subsequent discoveries relating
to the laws of electric and magnetic
action, both as respects attraction and
repulsion, and also induction, have
tended to confirm the analogy between
them, and to corroborate the opinion
that they ultimately emanate from a
common source. Electricity, it is true,
affects every species of matter with
which we are acquainted, in nearly an
equal degree; while magnetism, although
perhaps equally universal in its opera-
tion, yet acts very feebly, and probably
unequally, upon most kinds of matter,
and certainly exerts its principal energy
upon iron, a circumstance which has, to

« His work is entitled ' Recucil de M^moirea
sur 1'Analoi-ie de I'Eleotricitt' et du Magne"tisme
couronnes et publics, par i'Acadcmie de Bavi.Ve,
&c.' par J. II. Van Swiuden. En trois tomes. Svo
A la Haye. 1784.

13

2

ELECTRO-MAGNETISM.

this day, remained inexplicable, although
we have acquired the knowledge that
electricity, under certain modifications,
will produce every effect of magnetism.
Electricity, we know, may be transferred
from one body to another, but mag-
netism can be excited by induction only,
and is incapable of any similar kind of
transference. Still, however, there ex-
isted many positive facts, which, inde-
pendently of all analogy, demonstrated
that the magnetic needle was occasion-
ally influenced in its movements by the
action of electricity ; and that, in certain
cases, the magnetic properties could be
excited by electric explosions. The ap-
pearance of the aurora borealis, which
has all the characters of an electric phe-
nomenon, has been very frequently ob-
served to be accompanied by a disturb-
ance in the position of the compass ;
and a delicately suspended magnetic
needle has generally exhibited, on these
occasions, very frequent oscillations.
Lightning, which is still more decidedly
electric, has been known, in numberless
instances, to destroy, and sometimes to
reverse the polarity of the compass-
needle ; and many disastrous accidents
happening to ships, in consequence of
their mistaking their course, may very
probably have been owing to this cause.
In confirmation of this, we meet with a
narrative recorded in one of the early
volumes of the Philosophical Transac-
tions*, in which the ship Alexander,
being one hundred leagues from Cape
Cod, in latitude 48°, encountered a
violent thunder-storm ; the mast was
struck by lightning, which also reversed
the poles of all the compasses in the
ship, a change which was not discovered
till the ensuing night, when the stars
appeared, and it was found that they
had been steering in the opposite course
to that which they intended. It is also
stated, that in one of the compasses, the
end which had before pointed to the
north now pointed to the west. An-
other instance is recorded in the same
workf, where a stroke of lightning
passed through a box containing a
great number of knives and forks, melt-
ing some, and scattering the rest about
the room. It was found that all those
which were not melted had been ren-
dered strongly magnetic, so as to take
up large nails, and other pieces of iron,
placed near them.

(5.) Experiments were tried with the

* Vol. xiv. p. 520.

t Vol. xxxix. p. 74.

electrical battery, in imitation of these
effects, and in order to ascertain the
circumstances on which they depended.
But although steel bars were easily ren-
dered magnetic by passing strong elec-
tric shocks through them, yet the results
were by no means uniform, and no
general law could be traced as govern-
ing the production and distribution of
the polarity thus induced. A large pro-
portion of the effects appeared to be re-
ferable to the concussion which the par-
ticles of the bar received in consequence
of the violence with which the accumu-
lated torrent of electricity rushed through
them, thereby giving efficacy to the in-
ductive influence of the earth. This
influence, it is well known, depends al-
together, as was explained in the Treatise
on Magnetism (§ 109), on the position
of the bar with relation to the direction
of the dipping needle, which is the same
as that of the action of terrestrial mag-
netism. The experiments of Mr. Scores -
by*, made with a view of determining
the amount of this influence when aided
by electric concussion, fully confirmed
the principle upon which that mode of
explaining the phenomenon rests, by
showing that the action of a powerful
electric shock is, in a great measure,
similar to that of a blow from a ham-
mer, or to the forcible twisting of the
iron, or any other kind of mechanical
violence.

(6.) There still, however, remained
many anomalous appearances, to the
explanation of which this principle did
not furnish the most slender clue. These
anomalies presented themselves more
especially when the electric discharge
was made to pass transversely, or in
oblique directions, through the bar ; for
it was found, in those cases, impossible
to predict what direction the induced
poles would assume, or even whether
any distinct polarity would be commu-
nicated to the bar when so treated.

Nothing illustrates more forcibly the
proneness of the human mind to draw
general conclusions from insufficient
data, than the various opinions so con-
fidently maintained by different experi-
mentalists on this subject. D'Alibard
thought he had demonstrated, by his
experiments, that the electric discharge
imparts a northern polarity to that point
of a steel bar at which it enters, and a
southern polarity to that at which it

* Transactions of the Royal Society of Edin-
burgh, vol. ix.

ELECTRO-MAGNETISM.

makes its exit ; and this quite independ-
ently of the position of the needle with
respect to the magnetic poles of the
earth. Wilke, on the contrary, ima-
gined that he could establish the exist-
ence of an invariable connexion between
the negative electricity and the northern
polarity. The laborious series of expe-
riments which were undertaken by Van
Swinden, with the express view of re-
conciling these strange discrepancies,
instead of settling the dispute, seemed
only to leave it more than ever embar-
rassed with difficulties. About the year
1777, the celebrated Beccaria engaged
in a similar investigation ; and although
he, like his predecessors, failed in disco-
vering the true nature of the magnetic
influence of electricity, yet he noticed a
singular fact which occurred to him in
the course of his experiments, but of
which he does not appear to have ap-
preciated the value. He found that a
needle, through which he had sent an
electric shock, had, in consequence, ac-
quired a curious species of polarity; for,
instead of turning as usual to the north
and south, it assumed a position at
right angles to this, its two ends point-
ing to the east and west. There is little
doubt, that if he had followed up the
inquiry which this important fact had
opened to him, he would soon have ar-
rived at the great discovery which was
made about half a century afterwards
by Oersted, and which has dispelled the
whole mystery.

(7.) As nothing had been gained by
following the more violent operations of
highly condensed charges of electricity,
other philosophers occupied themselves
in the attentive study of the more tran-
quil influence of this agent, when merely
accumulated in insulated conductors,
and exerting simply its attractive and
repulsive powers in conjunction with
those of magnetism. But however these
actions mightbecombined,nothing could
be detected that indicated any interfe-
rence of agency or modification of effect
consequent on the combination. An
electrified body is found to exert the
same attractions and repulsions on a
magnetized needle as it does on the same
needle when devoid of magnetism ; nor
does it, like magnetism, exhibit any de-
cided preference for iron, compared with
its action on other metals. When the
two agencies are united in the same
body, as when bars of steel, already
rendered magnetic, are also charged
with electricity, and placed so as to act

upon one another, their electrical and
their magnetic actions appear to be per
fectly distinct, and in no respect to in-
fluence or modify one another.

(8.) The discovery of galvanism, and
the invention of the Voltaic apparatus,
opened a new field of inquiry ; for, by
furnishing the experimentalist with the
means of maintaining a continuous cur-
rent of electricity in very large quantity,
it enabled him to study the effects of this
powerful agent under circumstances of
a very different kind from those he had
previously had under his command. The
electro- chemical phenomena, brought
to light by its application to another
branch of physical science, for a long
time occupied the talents and absorbed
the attention of scientific men in every
part of Europe ; and many years elapsed
before Voltaic electricity was applied
with any success to determine the in-
fluence which it so directly exerts over
magnetized bodies. The few inquirers
who sought to establish a relation or
identity between these two powers, ex-
cited but little attention, in consequence
either of the obscurity of their reason-
ings or the inaccuracy of their experi-
ments. The various hints interspersed
among the journals of this period, re-
specting movements having been ob-
served in the magnetic needle by the
action of the Voltaic pile, were too vague
and uncertain to warrant any determi-
nate conclusion. The most definite and
authentic narrative relating to this sub-
ject was that of Ritter, who asserted
that a needle, composed of silver and
zinc, had arranged itself in the magnetic
meridian, and had been slightly attracted
and repelled by the poles of ta magnet.
He also stated,that by placing a gold coin
in the Voltaic circuit, he had succeeded
in giving to it positive and negative elec-
tric poles ; and that the polarity so
communicated was retained by the gold
after it had been in contact with other
metals, and appeared, therefore, to par-
take of the nature of magnetism. A
gold needle, placed in similar circum-
stances, acquired still more decided mag-
netic properties. These experiments
suggested to Ritter some vague idea
that electrical combinations, when not
exhibiting their electric tension, were in
a magnetic state ; and that there existed
a kind of electro-magnetic meridian de-
pending on the electricity of the earth,
at right angles to the magnetic poles.
But these speculations were of too crude
a nature to throw any distinct light on
B 2

ELECTRO-MAGNETISM.

the true connexion between magnetism
and electricity.

CHAPTER II.

Account of Oersted's Experiments.

(9.) The real discoverer of the mag-
netic properties of electric currents was
M. Oersted, Professor of Natural Phi-
losophy, and Secretary to the Royal
Society of Copenhagen. In a work
which he published in German, about
the year 1813, on the identity of chemi-
cal and electrical forces *, he had thrown
out conjectures concerning the relations
subsisting between the electric, galvanic,
and magnetic fluids, which he conceived
might differ from one another only in
their respective degrees of tension. If
galvanism, he argued, be merely a more
latent form of electricity, so magnetism
may possibly be nothing more than elec-
tricity in a still more latent form ; and
he therefore proposed it as a subject
worthy of inquiry whether electricity,
employed in this, its most latent form,
might not be found to have a sensible
effect upon a magnet. It is difficult
clearly to understand what he means by
the expression of latent states, as ap-
plied to electricity, but it may be suffi-
cient for us to know that in the various
endeavours he subsequently made to
verify his conjectures, he was led to
such forms of experiment as afforded
decisive indications of the influence of
Voltaic currents on the magnetized
needle. Yet, even after he had suc-
ceeded thus far, it was a matter of ex-
treme difficulty to determine the real
direction of this action, and it was not
till the close of the year 1819 that his
perseverance was at length rewarded by
complete success.

(10.) The first account of his disco-
very that appeared in England is con-
tained in a paper, which he himself
communicated, in Thomson's Annals of
Philosophy, for October, 1820f; and
in which the following experiments are
described : — The two poles of a powerful
Voltaic battery were connected by a
metallic wire, so as to complete the gal-
vanic circuit. The wire which performs
this office he called the uniting wire ;
and the effect, whatever it may be, which
takes place in this conductor, and in the
space surrounding it, during the pas-

* This work was translated into French by
Marcel des Serres, under the title of ' Reoherches
stir 1'ldentite des Forces Chimiqnes et Electriques.
I'aris 1813.' See the 8th Chapter of that work.

t Vol. XVI. of the first series, p. 273.

sage of the electricity, he designates by
the term electric conflict, from an idea
that there takes place some continued
collision and neutralization of the two
species of electric fluids, while circulat-
ing in opposite currents in the appara-
tus. Then taking a magnetic needle,
properly balanced on its pivot, as in the
mariner's compass, 'and allowing it to
assume its natural position in the mag-
netic meridian, he placed a straight
portion of the uniting wire horizontally
above the needle, and in a direction
parallel to it ; and then completed tha
circuit, so that the electric current passed
through the wire. The moment this
was done, the needle changed its position,
its ends deviating from the north ami
south towards the east or west, accord-
ing to the direction in which the electric
current flowed, so that by reversing the.
direction of the current the motion of
the needle was also reversed. The ge-
neral law he expressed as follows : —
* That end of the needle which is si-
tuated next to the negative side of the
battery, or towards which the current
of positive electricity is flowing, imme-
diately moves to the westward.'

(11.) The deviation of the needle is
the same, whether the uniting wire, in-
stead of being immediately above the
needle, be placed somewhat to the east
or west of it, provided it continue pa-
rallel to and also above it. This shows,
that the effect is not the result of a sim-
ple attractive or repulsive influence, fear
the same pole of the magnetic needle
which approaches the uniting wire when
placed on its east side recedes from it
when placed on its west side.

(12.) If the uniting wire be placed in
a horizontal plane under the magnetic-
needle the latter is affected to an equal
degree as in the former case, but the
motions are made in the contrary di-
rection ; for the pole of the needle next
to the negative end of the battery now
deviates towards the east.

(13.) The effects above described will
Fig. 1.

ELECTRO-MAGNETISM.

be more clearly understood from an in-
spection of Figs. 1 and 2, where N, S
present the two opposite poles of the mag-
netic needle, halanced upon its pivot ; and
p n the uniting wire ; the end p being
connected with the positive or copper
end of the simple galvanic battery, and
the other end, n, being connected with
the negative or zinc end of the same
battery. (See the Treatise on GALVA-
NISM, $ 15.) So that the direction of the
positive electric current is from p to nt as
described by the arrows in the figure.

Fig. 2.

When the uniting wire is above the
needle, as \njig. 1, the pole S, which is
next to the negative side of the battery,
or towards which the current of positive
electricity is flowing, will move towards
W, the western side of the horizontal
dotted circle; and the needle will as-
sume the position N' S'. When the
wire is below the needle, as mfig. 2, the
same pole S will move towards E, the
east point of the horizon ; and its new
position will be N' S', inclined in a di-
rection the reverse of that which it as-
sumed in the former case.

(14.) For the more easy retention of
these facts in the memory Oersted used
the following formula : namely, ' the
pole above which the negative electricity
enters, is turned to the west; under
which, to the east: Another, and more
convenient formula, however, will pre-
sently be given, comprehending not only
these but many other facts, which are
derived from a more universal principle
applicable to all of them.

(15.) When the uniting wire is si-
tuated in the same horizontal plane as
that in which the needle moves, and is
at the same time parallel to it, no decli-
nation takes place either to the east or
west ; but the needle is inclined, so that
the pole next to the end of the wire at
which the negative electricity enters is
depressed, when the wire is situated on
the west side, and elevated when situ-
ated on the east side.

Thus, if the uniting wire p n, fig. 3,

be placed on a level with the needle
NS and parallel to it, on its eastern side,
the pole S, next to the negative end of
the wire n, will be elevated, and the pole

Fig. 3.

N depressed, so as to assume the posi-
tion represented by the dotted needle
N'S'. If the uniting wire had been
placed on the western side of the needle
the pole N would have been elevated,
and S depressed ; and the axis of the
needle would have been in the position
N"S".

(16.) If the uniting wire, instead of
being parallel to the needle, be placed at
right angles to it, that is, extending from
east to west, whether above or below it,
the needle remains at rest, unless it be
brought very near to one of the poles ;
in which case the pole is elevated when
the entrance of the negative electricity

Fig. 4.

is from the west side of the wire, and
depressed when from the east side. Thus
the pole S, fig. 4, is elevated when the
current of positive electricity proceeds
from p to n ; that is, when the entrance
Fig. 5.

ELECTRO-MAGNETISM.

of the negative electricity is from the
west side of the wire.

(17.) When the uniting wire, instead
of being horizontal, is placed vertically,
as shown in fig. 5, either to the north or
south of the needle, and then brought
near to the adjacent pole, if the upper
extremity of the wire receives the nega-
tive electricity, that pole moves towards
the east ; but when the wire is brought
opposite to a point between the pole and
the middle of the needle, as in Jig. 6, the

Fig. 6.

n

same pole deviates to the west. When
the upper end of the wire receives posi-
tive electricity, the phenomena are re-
versed.

(18.) Oersted found that these expe-
riments succeeded equally well if t'he
uniting conductor consisted of one or of
several wires, or metallic ribbons, con-
nected together. Neither is the effect
altered in its kind, though it may vary
somewhat in degree, when different
metals are used : thus, platinum, gold,
silver, brass, iron, lead and tin, and even
mercury contained in a tube, when em-
ployed as the conductors of the electricity,
have a similar influence on the magnetic
needle. The conductor still exerts this
power, although it be interrupted by
water, provided the interval between the
metals does not extend to several inches
in length. The magnetic influence of
the wire on the needle is not prevented
by the interposition of glass, metals,
wood, water, resin, stones, or any other
substance that was tried. The effect
produced, nevertheless, is referable
purely to magnetism, for it is exerted on
magnetic bodies only, and has no in-
fluence on needles of brass, glass, or
gum lac. It appears to depend, not upon
the intensity of the circulating electri-
city, but solely on its quantity ; and ac-
cordingly Oersted found that he could,
with a single galvanic arc, repeat all the
experiments which he had at first made
with a compound Voltaic battery. In
nis way, also, he was enabled to detect

the reciprocal action which the poles of
a magnet exert on the conducting wire ;
for, by placing a plate of zinc, six inches
square, between two plates of copper
formed into a trough, in order to hold
the acid which is to act upon the for-
mer, but kept from touching them by
small pieces of cork interposed on each
side, on forming a communication be-
tween the two plates by an extended
wire, and then suspending the whole
apparatus by a thread, the effect of a
magnet in moving the wire could be
readily ascertained.

(19.) The announcement of the im-
portant discovery of Oersted excited the
greatest interest among all the philoso-
phers of Europe, and they immediately
occupied themselves in repeating and
extending his experiments. Among
those who were early distinguished by
their zeal and activity in this research
were Ampere and Arago, in France, and
Sir H. Davy and Faraday, in England.
So many were the cultivators in this
new field of inquiry, and so eagerly did
they pursue the path thus unexpectedly
opened, that a great number of interest-
ing facts were speedily brought to light ;
and where all were pressing forward in
the same career, it is scarcely possible
to adjust the claims to priority of dis-
covery, with respect even to the most
important facts. Instead, therefore, of
attempting to give a chronological view
of the progress of knowledge in this de-
partment of science, we shall adopt the
following more didactic, and, we trust,
more instructive plan. We shall first
state those general principles to which
philosophers have arrived by gradual
and successive inductions; secondly,
we shall trace the various combinations
of those principles in different ways, and
under different circumstances, and the
effects resulting from them ; and lastly,
point out the explanations which they
afford of particular phenomena, in the
order which appears most conducive to
clear and comprehensive views of the
whole subject of electro-magnetism.

CHAPTER III.
Fundamental Law of Electro-Magnetic

Action.

(20.) An attentive examination of
the facts described in the preceding
chapter will soon convince us that the
magnetic force which emanates from the
electrical conducting wire is entirely dif-
ferent in its mode of operation from all
the other forces in nature with which

ELECTRO-MAGNETISM,

we are acquainted. It does not act in a
direction parallel to that of the current
which is passing along the wire, nor in
any plane passing through that direction.
It is evidently exerted in a plane perpen-
dicular to the wire, but still it has no
tendency to move the poles of the mag-
net, in a right or radial line, either
directly towards or directly from the
wire, as in every other case of attractive
or repulsive agency. The peculiarity of
its action is that it produces motion in a
circular direction all round the wire;
that is, in a direction at right angles to
the radius, or in the direction of the
tangent to a circle described round the
wire in a plane perpendicular to it.
Hence, as Mr. Barlow has expressed it,
the electro-magnetic force exerts a tan-
gential action.

(21.) The direction, in the circumfe-
rence of these circles, of the action
exerted on any one pole of a magnet by
the electrical current which is moving
at right angles to the plane of the circles,

Fig. 7.

is determined by the direction of the
current. If we suppose the conducting
wire to be placed in a vertical situation,
as shown in fig. 7, p n, and the current
of positive electricity to be descending
through it, or moving from p to n (the
negative electricity moving, of course,
in the contrary direction, or ascending),
and if through any point C in that wire,
the plane NN be taken perpendicular
to p n, that is, in the present case, a
horizontal plane ; and lastly, if any num-
ber of circles be described in that plane
having C for their common centre, then
the action of the current in the wire
upon the north pole of a magnet, si-
tuated any where in that plane, will be
to move it in the line of the tangent to
the circle which passes through it, and
in the direction denoted by the arrows
in the figure ; that is, from left to right
in the remote part of the circle, and from
right to left in the nearer part. In other
words, the motions impressed will be in
a direction corresponding to those of

Fig. 8.

the hands of a watch having the dial
towards the positive pole of the Voltaic
battery.

(22.) When the direction of the cur-
rent is reversed, the wire still preserving
its vertical position, the direction of the
action is also reversed ; and the circular
motions produced correspond to the
movements of the hands of a watch with
its face downwards ; that is, still looking
towards the positive electrical pole.

(23.) The actions of either" the de-
scending or ascending electrical current
upon the south pole of a magnet are
exactly the reverse of those which are
exerted on the north pole. Fig. 8 re-
presents the action of the current mov-

ing from p to n, on the south pole ;
which is directed, as may be seen, from
right to left in that part of the circle
which is opposite to the wire, and which
would, therefore, impel the south pole
in a direction contrary to that of the
hands of a watch. On reversing the
direction of the current these effects will
again be reversed.

(24.) It is evident that in the course
of experiments on electro- magnetism,
the current and^ magnetic poles. may be
presented to our observation in a great
variety of relative positions ; and it will
be found not very easy to retain a per-
fect recollection of the way in which the
force should act conformably to the rule

8

ELECTRO-MAGNETISM.

above stated. Ampere has hit upon an
ingenious device for imprinting this
rule more firmly in the memory, and
enabling us to apply it under a great di-
versity of circumstances. The electric
currents are not only characterized as
positive and negative, and as flowing in
one or other of two directions along the
wire that conducts them, but may be
actually personified and conceived as
endowed with a head and feet, with a
face and back, and with a right and a left
hand. In order to turn this idea to the
best account, and being at liberty to
choose, with respect to the various kinds
of conditions belonging to the subject,
one or other of two alternatives, we shall
select in each case those which seem
naturally entitled to the preference : and
it fortunately happens that, on combin-
ing these conditions so selected, they
accord exactly with nature, and are
therefore well calculated to answer the
purpose of a kind of artificial memory.

(25.) First, it is more natural to fix
our attention on the current of positive,
than of negative electricity. Secondly,
in a vertical wire, a descending current
will occur to us more readily than an
ascending one : or, if we imagine our-
selves borne along by the current, it
would be more natural to conceive our-
selves moving with our feet foremost ;
but if, on the contrary, we suppose our-
selves to be at rest, we should conceive
the current to be passing from our head
to our feet. Our/ace would, of course, be
turned towards the magnetic pole to which
we are directing our attention ; we should
attend to the north pole in preference
to the south ; and the movement with
which we are most familiar, is that
which we perform with our right hand,
as in writing for instance, that is, from
left to right. Combining these condi-
tions, then, we may always recollect,

Fig. 10.

P

that if we conceive ourselves lying in
the direction of the current, the stream
of positive electricity flowing through
our head towards our feet, with the
magnet before us, tJie north pole of that
magnet will be directed towards our
right hand. If any one of these condi-
tions be reversed, the result is reversed
likewise.

(26.) The action of the conducting
wire on the pole of a magnet is necessa-
rily accompanied by a corresponding
and opposite action of the magnet on
the wire. When the wire impels the
pole from left to right, the pole impels
the wire from right to left, and vice
versa. Thus we have seen that a posi-
tive current, descending along a wire,
of which W, Jig. 9 A, represents the sec-

Fig. 9.

W W

tion by a horizontal plane, urges the
north pole N to the right, in the direc-
tion N n, but the wire Itself is also urged
in the direction W w, to the left. The
contrary action takes place between the
south pole S, jig. 9 B, so that if either
pole of a magnet were fixed, and the
wire moveable, the motion of the latter
would, as in the case we have already
considered, be circular, and the force
which impels it tangential. This is
shown in Jigs. 10 and 11, where pn, pn,
&c. show the successive positions of the
wire urged to move in the direction
shown by the arrows, by the influence

Fig.U.
I

of the north or south poles N or S. direction to that exerted by the other.
The influence exerted on the same cur- When the currents are reversed, all the
rent by the one being in the opposite effects just described are again reversed.

ELECTRO-MAGNETISM.

It is also to be observed, that the mo-
tion of the wire, whatever be its relative
position to the magnet, is always moved
parallel to itself; that is, in the direction
of a line at right angles to it.

(-27.) The direction of Ihe electro-
magnetic force being thus determined,
we have next to ascertain the exact
law, according to which its intensity
varies, with relation to the distance of
the electric current from the point on
which it acts. The most reasonable
conjecture we can form on this subject,
prior to experimental investigation, is,
that this law is the same with that which
is followed in the case of electric and
magnetic actions, namely, that the in-
tensity of the force is every where
inversely as the square of the distance.
But if this be the real law of action, it
must apply to the elementary portions
of the two agents which thus mutually
act apon each other ; or, to adopt the
more convenient language of theory, it
must obtain only among the elementary
particles of the electric and magnetic
fluids. In the magnet, the action of the
latter may be regarded as concentrated
in the points, which are the poles of the
magnet ; but in the conducting wire, the
electric fluid which is passing through
it, acts in an equal degree along the
whole line of its motion ; and admitting
the hypothesis of the action being in-
versely proportional to the squares of
the distances of each individual particle,
we have to deduce the law which will
result from the combined actions of all
the points of a line directed upon a
point out of that line. Now, it may be
mathematically demonstrated, that if the
line in question be perfectly straight,
and its length be exceedingly great in
proportion to the distance of the point
on which it acts, then the intensity of
action will be inversely proportional,
not to the square, but to the simple dis-
tance of the point, so that at three times
the distance, for example, the force
shall be one-third, at four times the
distance, one-fourth, and so on. That
this law is conformable to observation,
has been proved by the experiments
conducted by Biot and Savart, in which
the intensities of the force at different
distances were accurately ascertained,
by observing the number of oscillations
performed by the needle in a given time,
and taking the squares of those num-
bers.

CHAPTER IV.

Direct consequences of the Law of Elec-
tro-magnetic Action.

(28.) Let us now inquire into the
consequences of this law. So different
is the action of the electro-magnetic
force from that of the other forces in
nature, with the effects of which we are
more familiar, that a particular train of
investigation is required, in order to
trace its exact operation under every
combination of circumstances. It is
not easy, even in the simpler cases,
where a single magnetic pole is sub-
jected to the action of a conducting
wire, at once to pronounce upon the
precise motion that will result, espe-
cially if the motion of the magnetized
body is limited to a fixed plane, and re-
strained to mere rotation ; but the diffi-
culty is much increased, when, as most
frequently happens in actual experi-
ment, the investigation is complicated
by the necessity of including the com-
bined actions of several poles of dif-
ferent kinds. The only mode of ob-
taining clear views of the subject is to
examine the several cases in their order
of simplicity, commencing with each
force taken singly, and afterwards stu-
dying their several combinations.

$ 1. Effects on the Directive Property of
a Magnetic Needle.

(29.) Confining our attention, then,
for the present, to a single magnetic
pole, the north pole for instance, we
have to examine the effects produced
upon it by a conducting wire of indefi-
nite length, acting upon it with a tan-
gential force inversely proportional to
its distance, when the movements of
that wire are limited to the circumfer-
ence of a circle, in a given plane, per-
pendicular to the wire. The case under
consideration may, in a great measure,
be exemplified, by placing a magnet,
SN, fig. 12, on a flat support, AB,
resting, at its centre, on the pivot P,
and balanced by a counterpoise at the
opposite end, so that the south pole, S,

Fig. 12.

10

ELECTRO-MAGNETISM.

of the magnet may be exactly above
the centre of motion.

In this situation, the action of any
electrical current upon the pole S, can
have no influence in turning the bar,

AB, in any particular direction; and
the motion of the bar will be determined
solely by the action of the current upon
the north pole, N.

(30.) Let A B C D M N O, fig. 13,

Fig. 13.

be ihe horizontal circle in which the
needle NS revolves, S being the centre
of its revolution ; and let W be the ho-
rizontal section of the conducting wire,
which acts upon the needle, and along
which the positive electric current is
descending. In every position of the
needle, the tangential force, acting upon
the pole in the circumference of the
circle, takes the direction of a line to
the right hand, perpendicular to that
which connects the pole and the wire.
At D, for instance, it has the direction
of the line DC?, perpendicular to DW.
Its tendency to produce rotation in the
needle, by turning it round S, will
be proportional to the cosine of the
angle formed between WD and the ra-
dius DS ; or it may be represented by
the line S d, drawn parallel to WD, and
meeting the perpendicular D d, to which
it is, of course, also perpendicular ; for
it will readily be seen, that the rotatory
effect of the force we are considering
is the same, whether applied on the
needle at D, or at d, on the arm of a
lever S d, rigidly connected with the
needle. The needle, then, will be urged
by this force to move towards V, and as
the length of the lever by which it acts
continually increases until it reaches this
point, so also will the rotatory power
increase. After the needle has passed
V, it will again diminish ; when it comes
to M, its measure is S m, and on arriv-

ing at N, where the position of the
needle NS is at right angles to NW,
it is reduced to nothing. This, there-
fore, will be a position of equilibrium,
and the equilibrium will be a stable one,
for, on disturbing the position of the
needle by pushing it onwards to O, for
example, the rotatory force, in this new
position, acts upon it by the lever So,
on the opposite side of S m, and, there-
fore, tends to give it rotation in the con-
trary direction ; that is, to bring the
pole of the needle back again to N.
After performing a few oscillations, the
needle will, therefore, finally settle in
the position SN.

(31.) When the arcs of vibration are
small, the forces which tend to bring
the needle to its point of rest, are very
nearly proportional to the arcs them-
selves; so that, in this respect, its
movements are governed by the same
law as those of a pendulum. They ac-
cordingly furnish very accurate means
of determining the comparative intensi-
ties of the electro-magnetic forces which
act upon the same needle under different
circumstances of distance from the wire,
or of intensity of the electric current,
for the force will always be proportional
to the square of the number of oscilla-
tions which the needle performs in a
given time.— (See Treatise on Magnet-
ism, § 320.)

(32.) When the needle is still further

ELECTRO-MAGNETISM.

11

deflected towards the'wire, the force that
tends to bring it back to the position of
rest increases till it reaches its maximum
at the position SU, where the needle
points directly to the wire. Carrying it
still further to the left, the rotatory force
again diminishes, till it arrives at the
position BS, perpendicular to BW,
where, being directed to the exact centre
of motion, it is reduced to nothing. This
position of the needle, therefore, is also
one of equilibrium ; but it differs from
the former in being an unstable equi-
librium ; for if the needle be dis-
turbed ever so little from its position
on either side, it will acquire a ten-
dency to proceed onwards in that direc-
tion, and will move away from the point
B. At A, for instance, the rotatory
force acts upon it by the lever S a, urging
it towards U, and causing it ultimately
to settle at N. At C, again, it is urged
towards D by a similar force, propor-
tional to S c, and which, increasing as
the needle advances, carries it to V, and
finally brings it round to N.

(33.) It may here be remarked, that
the rotations of the needle are in oppo-
site directions in these two portions of
the circumference ; for, in the remote
part, BVN, the motion is similar to
that of the hands of a watch ; in the
nearer part, BUN, it is in the contrary
direction. The lines \VB and "WN,
drawn from W to the points where the
needle is in equilibrium, being at right
angles to the respective radii BS and
NS, are tangents to the circle at B and
N, and the circumference is divided by
these points into two unequal portions,
so that the needle, in passing from B to
N, by the operation of the tangential
force emanating from \V, as indicated
by the arrows in the figure, has to tra-
verse a longer distance when moving in
the remote than in the nearer part of
the circle. The disproportion between
these two arcs continually increases as
the wire is brought nearer to the circle.
When very near, as shown in fig. 14,

Fig. 14.

the arc BUN is very small, compared
with BVN ; yet, if the needle be placed
ever so little on the other side of B, it
will immediately recede from that point,
as if repelled by the wire, and will pro-
ceed to describe the larger portion of the
circle, in order to arrive at N, a position
which it might have reached by a much
more direct course had it described the
arch BUN.

(34.) The singular preference thus
shown by the needle for a very circuitous
path, in reaching its destination, when
it appeared free to take the shorter line
that leads to it, appeared exceedingly pa-
radoxical to those who first observed it,
and excited much astonishment. But
the explanation we have given shews
clearly that it is nothing more than the
direct result of the peculiar law of
electro- magnetic force, which is charac-
terized by the tangential direction of its
agency.

(35.) If the wire be supposed to pass
through the circumference itself, as in
fig. 15, that portion of the circumfer-

Fig. 15.

v\

ence BN, which was comprehended be-
tween the two tangents, and in which
the needle was urged to turn in a direc-
tion contrary to that of its revolution in
the rest of the circle, is now reduced to
a mere point ; and the needle, when
placed ever so little to the left of that
point, will move round the entire circle,
and even when it arrives at this point,
can hardly be said to settle there, for
the slightest movement in the same di-
rection will again place it under the
influence of the same impulse, which
will, therefore, carry it round a second
time. The very momentum it has ac-
quired in this motion will be sufficient
to transport it beyond this neutral point,
and to maintain it in a state of perpetual
revolution. Should the wire be actually
within the circle, as mfig. 16, then the
rotatory force will remain constantly in
the same^direction in every part of the
circle, and, according to theory, the

12

ELECTRO-MAGNETISM.

needle will revolve in perpetuity in the
same constant direction. It is obvious,
however, that in the circumstances under
which an experiment of this kind can
be made, this can never happen, because
the wire being a solid substance, and
passing perpendicularly through the
plane of the circle in which the needle
turns, its presence must arrest the mo-
tion of the needle as soon as it comes
in contact with it. The only position
which the needle can take, therefore, is
that of resting against the wire in the
manner represented in fg. 16. In any

Fig. 16.

other part of the circle, it will move on-
wards in the direction indicated by the
arrows.

(36.) Having thus investigated the
action of an electric current on a single
pole, we are now prepared for the consi-
deration of its combined action upon
the two opposite poles of a magnetized
needle, balanced in the ordinary way
on its centre. In this case, the current,
descending through the wire W, fig. 1 7,

Fig. 17.

•-.T)

exerts a contrary action upon the two
poles, N and S, of the needle. When
the needle is in the position PQ, that is,
in the same line with W, these two con-
trary forces, acting at right angles to
the radius, and on opposite sides of
the centre, concur in their rotatory ef-
fect, and the needle is urged by the sum
of these forces to turn in the direction

indicated by the arrows placed at
these points. When the needle is in the
position SN, at right angles to the line
WC, the rotatory forces, being directed
perpendicularly to WS and WN, as
indicated by the arrows, oppose one
another, and acting by the levers C* and
Cn, which are equal in length, are in
exact equilibrium. The equilibrium is
stable, as will be evident from consider-
ing that the displacement of S, in the
direction of D, increases the length of
the lever Cs, while the accompanying
motion of N towards E, diminishes the
length of Cn. The force represented by
the former, will, therefore, preponderate
over that represented by the latter, and
will carry back the pole S to its former
situation. The same would happen,
were the displacement made on the
other side of S, for in that case, the
force which impels the pole N would
have the advantage over that which acts
on the pole S, 'and would restore the
needle to its position of rest SN. This
opposition of forces occurs when the
needle is situated any where between the
lines AB and DE, which are respectively
perpendicular to the tangents to the
circle, WA and WE ; for, in either of
these situations, AB or DE, the rotatory
force exerted in one of the ."poles, is, as
we have before seen, § 30*, reduced to
nothing. Beyond these positions, the
rotatory force changes its direction,
so that in any part of the arcs APE,
and DQB, the forces acting upon the
two poles conspire in producing a si-
milar effect of rotation.

(37.) In proportion as the wire W,
fig. 18, is brought nearer to the needle,
the arcs ASD and ENB, in which the
two forces oppose each other, form a
larger portion of the circle, while those
in which they concur, AE and DB,
become smaller. Here it may also be

Fig. 18.

ELECTRO-MAGNETISM.

observed, that when the position of the
needle differs much from that oi' SN,
the two poles, N and S, will be at very
different distances from the wire, and
the intensity of the force being inversely
as the distance, the forces acting upon
the two poles will, in consequence, differ
materially. When the forces concur in
their rotatory effect, the result will not
be affected by this difference ; but when
they oppose each other, the increase of
force acting on the nearer pole, will go
far towards compensating for the greater
obliquity of its direction, and will bring
it more nearly to an equality with the
smaller force, which acts with greater
mechanical advantage on the distant
pole. This equality is actually attained
when the wire passes through the cir-
cumference of the circle ; for now the
force acting upon S,Jig. 19, in the direc-

Fig. 1 9.

tion RS, is to the force acting upon N,
in the direction RN, inversely as the
distances WS and WN ; that is, they
are as \VN to \YS : but as the former
acts by the lever C s, and the latter by
the lever C n, which are themselves in
the proportion of WS to WN, they must,
by the laws of statics, be exactly in
equilibrium.

To place the matter in another point
of view, the forces RS and RX, when
combined together, produce, as their re-
sultant, the force RW, which, being
directed to the centre of motion C, can
have no tendency to produce rotation.
Hence it follows, that the needle, what-
ever be its position in the circumference,
will appear to be totally uninfluenced by
Ihe wire : the action of the latter, on both
poles, exactly balancing each other.

(38.) This state of equilibrium no
longer remains when the wire is within
the circle, fig. 20. It will now be found,
that in no position of the needle do the
two forces conspire to produce the same
rotatory motion^and that they oppose one
another in every part of the circle. The

only position in which the equilibrium is
stable, is that of NS, the north pole
being to the left, and the south pole to
the right of the wire ; a position which,
it should be observed, is exactly the re-

verse of that which the needle assumes
when the wire is out of the circle, as
in figs. 17 and 18. When disturbed
from this position, and brought to n' s',
for example, the force urging the pole
n', which is nearest to the wire, becomes
more effective than that acting upon the
more distant pole s', and, therefore,
brings back the needle to its station.
But if the pole N were placed on the
opposite side of the wire, as at n', the
tangential force which carries it towards
the wire, is, here also, more effective
than that which acts upon the distant
pole s', and which tends to move it in
the contrary direction ; the needle, there-
fore, strikes against the wire, and being
unable to pass it, remains in contact
with it. If the. needle be carried still
further from the wire, however, the su-
periority of this force will continually
diminish, and cease entirely when the
needle is in the transverse position SN,
shown in fig. 21, where the two poles, S
and N, are equi-distant from W. Here
there is again an equilibrium, but it is of

Fig. 21.

14

ELECTRO-MAGNETISM.

the unstable kind, for as soon as N is
removed further from the wire, the force
acting on S gains the advantage, and
turns the needle round till its revolution
is arrested by its coming against the
wire, in the position s n.

(39.) Although, strictly speaking, the
tangential force exerted by an electrical
current upon either pole of a magnet,
has no tendency to cause the pole to
approach to, or recede from it, and,
therefore, does not possess the character
either of an attractive or of a repulsive
force, yet the movements of a needle, in
the circumstances we have just been con-
sidering, often resemble those of at-
traction and repulsion. But if viewed
with reference to such a cause, they
would appear exceedingly anomalous ;
and accordingly the sudden changes
from attraction to repulsion, which take
place from a slight alteration in the re-
lative positions of the wire and needle,
appeared to the earlier experimentalists
to be very capricious and unaccountable.

(40.) In order fully to understand
these transitions, we may arrange the
results of the preceding investigation,
as they refer to any one given position
of the needte SN, Jig. 22, varying the

Fig. 22.

a

position of the wire only, and we shall
then find that the lines which form the
boundaries between the positions of ap-
parent attraction and repulsion are the
circumference of the circle of which the
needle is a diameter, together with the
prolonged axis of the needle n s, and
another line crossing it, at the^centre, at
right angles, ef. The circumference
indicates the positions of the wire when
no apparent effect is produced on the
needle, or the positions of neutrality.

The line ef is that in which an equili-
brium obtains. When the wire is in any
continuous part of the line ef, namely
CF and E e, the equilibrium is stable ;
when in any of the dotted parts E C
or F/, of the same line, unstable ; on
the line s n, the action is at the maxi-
mum. The letters a, a, a, a, show the
spaces where an apparent attraction
takes place between the wire and the
nearest pole, when the former is situated
in the respective spaces bounded by the
above lines ; and r, r, r, r> the spaces
where there is apparent repulsion. These
latter spaces are shaded for the sake of
distinction. Thus within the quadrant
SCE there is apparent attraction of the
pole S ; in the shaded quadrant SCF
there is an apparent repulsion of that
pole ; in the shaded quadrant EON, the
pole N moves from the wire; in NCF,
towards it. In the spaces exterior to
the circle the actions are exactly the
reverse of those in the interior ; in the
shaded space bounded by s S, e E, and
the circumference, the action in S is ap-
parently repulsive; in the white space
on the other side of S, bounded by the
lines S s, Yf, and the circumference, it
is attractive ; and the contrary obtains
with regard to the spaces on the other
side of the line ef.

§ 2. Movements of the Magnetic Needle

in free space.

(41.) In the preceding investigation
our attention has been exclusively di-
rected to the determination of the effects
of the electro-magnetic forces on a mag-
netized needle, so restricted in its motion
as to be capable of only turning on its
centre ; and we have had to consider
only the forces which tended to produce
the rotation of the needle. A part of
the forces, however, which act on the
poles is exerted in another direction, and
would, were the needle at liberty to obey
them, occasion the displacement of the
whole needle, that is, would produce a
motion of its centre. The needle being
confined by its pivot, the only effect
produced by these forces is pressure upon
this pivot. But if this obstacle be re-
moved, and the needle be allowed to
move freely in any direction, the action
of these remaining forces will become
manifest ; the motion of the centre of
the needle being determined in its quan-
tity and direction by the magnitude and
direction of the resultant force estimated
by referring the two component forces
to that point.

ELECTRO-MAGNETISM.

(42.) When the conducting wire W,
•• 23, is situated in any part of the line

vs

\s

N

WC, at right angles to the axis of the
needle, the tangential force acting on the
pole S, in the direction represented by
the arrow, at right angles to WS, may
be supposed to be transferred to the
centre, C, of the needle, and to be re-
presented by the line C s. The force
acting upon N being in like manner re-
presented by C n ; the resultant of these
two forces will be a force represented
by the diagonal C a of the parallelogram,
having Cs and Cn for its two sides,
and tliese sides being equal, and equally
inclined to the line WC, this diagonal
will coincide with that line : hence the
force will be such as to move the centre •
of the needle directly towards W, that is,
the needle will appear to be attracted by
it. If either the current had followed
an opposite course, or the poles of the
needle had been reversed, the forces
would have acted in the opposite direc-
tion, and would have been represented
by the lines G s', C n', forming a paral-
lelogram, of which the diagonal is C r,
indicating a motion of the centre of the
needle from the wire, and resembling
repulsion. This effect also takes place
under the original circumstances of the
experiment when the wire is on the other
side of the needle, that is, in any part of
the line CW; so that the needle will
always appear to be attracted by the
wire on one side and repelled on the
other.

(43.) The intensity of the force which
thus impels the needle, either towards
or from the wire, diminishes as their dis-
tance is increased. Two causes con-
spire to produce this diminution ; the
one is that the component forces them-
selves are inversely proportional to the
distances of the points on which they

act from the wire ; and the other is that
the angles they form with one another
become more obtuse as that distance
increases. Mathematically speaking,
the tangential force applied to each pole,
when referred to the direction of the
line joining the wire and the centre of
the needle, is directly as the cosine of
the angle formed between the axis of
the needle and the line connecting the
pole and the wire; and it is also in-
versely as this line ; so that calling the
force referred to that direction a, the dis-
tance from the wire to the centre of the
needle d, the distances of the wire from
the respective poles S and N, * and ny
and the length of the needle m ; and « and
/3 being the angles between the axis of
the needle and the lines connecting the
respective poles with the wire, we have
the following equation :

_cos. u. cos. /3
a~ s + n

But as we have taken the case of W
being placed on the line drawn from the
centre of the needle at right angles to
its axis, the two angles above mentioned
are equal, and every part of the line is
equidistant from S and N, that is,
a = p, and s = n;

hence the equation becomes a=— '—.

Now

CS ±m

cos. *=_=*_;

which value of cos. « being substituted
in the former equation, the formula be-
comes

that is, the force of apparent attraction
is directly as the length of the needle,
and inversely as the square of the dis-
tance of the wire from each pole.

(44.) In order to estimate the attraction
with relation to the distance of the wire
from the centre of the needle, or d, we
must substitute for s2 its equal dz+^ m* ;
so that the formula becomes

But when the distance of the wire is
very great compared with the length of
the needle, the quantity £ m* may safely

be neglected ; and

— - may be taken
a2

without any sensible error as the ex-
pression of the attractive force.

(45.) This may be experimentally illus-
trated by suspending a magnetic needle,
SN,/g-. 24, from its" centre by a thread,

1G

ELECTRO-MAGNETISM,

so that it may be balanced horizontally,
and bringing it vvilhin a certain distance
of a vertical conducting wire P n; if the
electrical current be descending in that

•wire the needle will place itself so that
the north pole N will be to the right, and
the south pole to the left of a spectator
conceived to be placed in the situation
of the wire and looking towards the
needle, as shown in the figure ; whereas
if the needle be before the wire as at
N'S' the poles will have a reverse po-
sition. In both cases the needle will be
impelled towards the wire, as shown by
the inclination of the thread by which
it is suspended.

' (46.) When, on the other hand, the
needle is removed to a considerable dis-
tance from the wire, or what comes to

the same thing, when a very short
needle, s n, is taken and carried round
the wire P N,Jig. 25, in a circle, its poles
will always preserve the same relative
situation, as indicated by the letters in
the figure, each being turned in the
direction in which they are respectively
urged to move round the circumference
by the tangential force. But the ten-
dency to approach the wire will be quite
insensible, in consequence of the angle
formed by the directions of the two
forces being so nearly equal to two right
angles.

(47.) When the wire is placed in any
part of the circumference of the circle,
having for its centre the centre of the
needle, and passing ^through the poles,
the resultant of the two forces C s and
Cn (Jig. 26), has the exact direction
of the line CW; and therefore, neither

in this, nor in the preceding case, is
there any rotatory force in operation.
But, in the present case, the force CW
being oblique to the axis of the needle
SN, a part of that force is exerted in
moving the needle in the direction of its
length, from C towards S, and in bring-
ing the centre C opposite to W ; so that
it will not rest until that centre comes
in contact with the wire, as shown in
fg. 27. A similar tendency in the cen-
tre of the needle to move towards the
wire takes place in all other situations

Fig. 27.

ELECTRO-MAGNETISM.

17

of the wire on that side of the needle ; tion required. This will appear from
but the direction of the motion produced the following demonstration : —
is more or less oblique to the line con- //y<r. 29.

necting the centre of the needle with ' s

the wire. This direction may, in all
cases, be easily found by drawing the

lines C * and C n (fig. 28), respectively /' '•

perpendicular to WS and WN, and com-
Fig. 28.

\

pleting the parallelogram C s a n ; of
which the diagonal, C a, will be the
direction of the resultant force acting
upon C. For the forces at S and N,
being inversely as the distances WS and
WN, are in the ratio of WN to WS,
which is equal to the ratio of the sines
of the opposite angles WSC to WNC of
the triangle WSN ; that is, in the ratio
of C s to C n, which are the actual sines
of those angles with the equal radii SC
and NC. The lines C* and Cn will,
therefore, correctly represent, both in
their directions and in their relative pro-
portions, the tangential forces in ques-
tion.

(48.) The actions exerted between the
wire and the poles of the needle, are, as
we have seen, reciprocal ; the wire being
urged by a force equal in intensity, and
parallel in its direction, to that which
acts upon the centre of the needle ;
hence the determination of this resultant
force will also give us the measure and
direction of the resultant of the two
forces which act upon the wire. Thus
the needle SN, jig. 28, being urged by a
force represented by C a, the wire W
will, in like manner, be impelled by a
force represented by the line W w, equal
and parallel to C a, but having an oppo-
site direction.

(49.) The direction of the force im-
pelling the wire by the joint action of
the poles of the needle, may be found
geometrically, by describing a circle
W6 Sr Na,fg. 29, which shall pass
through the position of the wire, and
also through the two poles ; for the dia-
meter W r of that circle will be the direc-

Through S and N draw S r and N r,
respectively perpendicular to WS and
WN, and which will, of course, meet at
r, the extremity of the diameter W r;
and through W, draw W s and W n,
parallel respectively to r N and r S,
meeting them, when produced, in s and
n, and forming a parallelogram, of which
W r is the diagonal. The triangle W r s,
or its equal, Wrn, is similar to the
triangle WSN, because the angles WNS
and W r S, which subtend the same arc
W b S, are equal ; as also the angles
WSN and sWr, or its equal WrN,
which subtends the same arc W«N.
The sides of these triangles are, there-
fore, proportional ; that is,

WN : WS : : sr, or its equal Wn :
Ws.

But the tangential forces impelling W
in the directions W n and W s, from the
actions of the poles S and N, are in-
versely as the lines WS and WN ; that
is, directly as WN to WS, and therefore
in the ratio of the line W n to the line
W*. These lines will, therefore, repre-
sent, in their magnitudes as well as in
their directions, the two tangential forces
by which W is impelled ; and conse-
quently the diagonal Wrof the parallel-
ogram of which they are the sides, or the
diameter of the circle, will represent the
direction of the resultant force in ques-
tion.

C

18

ELECTRO-MAGNETISM.

(50.) Hence it follows that the wire is,
in all situations, impelled to move in the
direction of the tangent of a circle having
its centre in the prolongation of the axis
of the magnet, and of which the radius is a
mean proportional between the distances
of its centre from the two poles. Thus
the wire at W, fig. 30, is impelled by the
Fig. 30.

action of the two poles N and S, in the
direction of the tangent of the circle
of which the centre is at C, in the
line NS prolonged, and of which the
radius WC is a mean proportional be-
tween CS and CN. It will, therefore,
revolve in that circle, which will stand in
the same relation to the magnetic poles
N and S, with regard to the law of elec-
tro-magnetic action, that the magnetic
curves (See Magnetism, § 81) do with
regard to the law of magnetic action.

CHAPTER V.

Application of the principles to the ex-
planation of particular facts.

(51.) The principles we have derived
from the preceding investigation are the
foundations of the whole science of
electro- magnetism, and furnish the key
to the explanation of a vast variety of
facts, some of which might appear, with-
out an accurate attention to the circum-
stances of the case, exceedingly anoma-
lous and perplexing. It is evident that
they completely accord with the results
obtained in the original experiments of
Professor Oersted, which could not for
some time be clearly understood.

(52.) In these experiments it will be
recollected the wire was horizontal, and
applied either above or below the needle,
and in a direction parallel to. it. In this
case the action of the wire is exerted in
the tangent to the circumference of a
vertical circle, having the wire for its
centre ; and this action being in oppo-
site directions upon the two poles, con-
spire to give the needle a motion round
its axis. But the needle, having already

a tendency to place itself in the plane of
the magnetic meridian, in consequence
of the influence of the earth, will arrange
itself in a position intermediate between
this plane and the position to which it
tends by the action of the electric cur-
rent. The greater the intensity of the
latter force, the greater will be the devia-
tion of the needle from the magnetic
meridian ; and both the amount and
the direction of the deviation will be
found on an attentive examination of
the results of Oersted's experiments, as
already detailed, to be exactly conform-
able to theory.

(53.) When the wire, still kept in a
horizontal position, was placed by Oer-
sted at right angles to the needle, and
over its centre, no visible effect took
place, because the actions of the wire
upon the two poles were then exactly
balanced. But whenever it was brought
nearer to one of the poles than to the
other, the vertical action being more
strongly exerted upon that pole, occa-
sioned its elevation or depression, ac-
cording to the direction of that action,
precisely in the manner which the theory
would lead us to expect.

(54.) Mr. Barlow undertook a series
of experiments to determine the devia-
tions of a magnetic needle from its na-
tural position, produced by a vertical
conducting wire under different circum-
stances, and deduced from the theory
various formulae, by which its amount
may be calculated. For the details of
his researches, the reader is referred to
Mr. Barlow's Essay on Magnetic Attrac-
tions *.

(55.) Of the speculations and hypo-
theses to which these extraordinary facts
gave rise we shall defer the consideration
to a future place, and, confining our
attention to the facts themselves, we
should here notice the observations of
Mr. Faraday, which led to the more
striking illustrations of the theory of
tangential action we are about to de-
scribe. Mr. Faraday states t that on
placing the wire perpendicularly, and
bringing the needle towards it, in order
to ascertain its positions of attraction
and repulsion with regard to the wire,
instead of finding these to be four, one
attractive and one repulsive for each
pole, he found them to be eight; that is
two attractive and two repulsive for each
pole. Thus, allowing the needle to take

* Second edition, p. 240.
f Quarterly Journal of Science, vol. xii. p. 75.

ELECTRO-MAGNETISM.

19

its position of equilibrium across the
wire, and then drawing away slowly the
support of the needle from the wire, so
as to bring: the north pole, for instance,
nearer to it, there was attraction ; but
on moving it a little farther, so that the
end of the needle was the point nearest
to the wire, repulsion took place, al-
though the wire was still on the same
side of the needle. When the wire was
on the other side of the same pole of
the needle, it repelled it when opposite
to most parts between the centre and
the end ; but there was a small portion,
at the very end, where attraction took
place.

(56.) Fig. 31 exhibits a compendious
view of the relative situations of the
needle and wire in these experiments ;

Fig. 31.

S-=
R

/

•

r

/^

(I

/*

A

A

.K

the electric * current being supposed
to ( descend along the vertical wire,
p n, represented in eight different
positions ; the letters A, a, R, r, denot-
ing respectively the apparent action
(whether attractive or repulsive) ex-
erted in each of these positions. A
reference to Jig. 22, and the general re-
sults stated in § 40, will sufficiently ex-
plain the facts mentioned by Mr. Fara-
day, if we take into account a circum-
stance which very generally obtains in
needles of the pointed shape of those
employed in the experiment; namely,
that the centre of the active portion of
each half of the needle, or its true pole,
is not situated at the very extremity,
but at some point near it, and towards
the centre of the needle. Thus the wires
in the extreme positions at the ends of
the needle were in fact placed beyond
the poles, and corresponded in their si-
tuation to points out of the circle passing
through those poles, which is the circle
given in/g-. 22.

(57.) The reaction of the needle on
the wire in these situations was also
pointed out by Mr. Faraday, and illus-
trated by reference to the following
figure (32), which represents horizontal

sections of the wire in different positions
with regard to the needle, balanced in
its centre C. They are marked A or R,

Fig. 32.

As S »K

according as they appear to attract or
repel the adjacent poles S and N ; and
the arrow-heads indicate the directions
of the circular motion which resulted.

(58.) Mr. Faraday justly concluded
from these facts, that there is no real
attraction or repulsion between the wire
and either pole of a magnet, the actions
which imitate these effects being of a
compound nature ; and he also inferred
that the wire ought to revolve round a
magnetic pole, and a magnetic pole
round a wire, if proper means could be
devised for giving effect to these tenden-
cies, and for isolating the operation of
a single pole. For the first idea of the
possibility of the rotations of an electro-
magnetic wire round its axis by the ap-
proach of a magnet, we are indebted
to the sagacity of Dr. Wollaston*, who
did not, however, succeed in producing
this effect in the experiments which he
made for that purpose.

CHAPTER VI.
Electro-magnetic Rotations.

(59.) The continued revolution of one
of the poles of a magnet round a ver-
tical conducting wire was produced by
Mr. Faraday in the following manner •(• :
— That the action of the wire might be
limited to the pole in question, the whole
magnet, with the exception of that ex-
tremity in which the pole was situated,
was immersed in mercury, its lower end
being attached by a thread to the bottom
of the vessel which contained the mer-
cury, the conducting wire being made
to pass down into the mercury, imme-
diately above the place where the copper
wire was fixed to the vessel. This ap-
paratus is represented mjig. 33, and a
section of it shown in fig. 34.

For the purpose of directing the elec-
trical current through the mercury, a
hole was drilled at the bottom of the
cup, into which a copper pin was
ground tight, projecting upwards a little
way into the cup, and rivetted to a small

* Philosophical Transactions for 1823, p. 158.
t Quarterly Journal of Science, xii. p. 283,
C2

20

ELECTRO-MAGNETISM.

round plate of copper, forming part of
the foot of the vessel. A similar plate of
copper was fixed to the turned wooden
base on which the cup was placed, and

Fig. S3.

Fig. 34.

another piece of strong copper wire, at-
tached to it beneath, after proceeding
downwards a little way, was made to
turn horizontally. The surfaces of these
two plates, intended to come together,
were tinned and amalgamated, that
they might remain longer clean and
bright, and afford better contact. The
magnet used was of a cylindrical shape,
and very powerful, and had its lower
pole fastened by a piece of thread to the
copper pin at the bottom of the cup.
The height of the magnet and length of
the thread were so adjusted, that when
the cup was nearly filled with clean
mercury, the free pole floated almost
upright on its surface. The upright
wire, communicating with one of the
poles of the voltaic battery, and con-
ducting the electrical current intended
to act on the upper pole of the magnet,
passed downwards from the upper
branch of a stand, so as to descend to
a small depth below the surface of the
mercury. Its lower end was amalga-
mated, in order to ensure perfect con-
tact ; the circuit was completed by
making a communication between the
lower wire and the other pole of the
battery. As soon as the current is thus
established through the apparatus, the
upper pole of the magnet immediately
revolves round the wire which dips into
the mercury. As the force which impels
it continues to act without diminution,
notwithstanding the motion of the mag-
net, it operates as an accelerating force ;
but the motion of the magnet in a circle
giving rise to a centrifugal force, the
magnet is carried to a greater distance
from the wire, until its increased mo-
mentum is compensated by the increased

resistance of the mercury, at which pe-
riod the velocity becomes uniform.

(60.) The direction of the motion de-
pends on the direction of the current,
and on the denomination of the pole
that is moved by it. If the current
descends, the north pole of a magnet
revolves from left to right ; that is, in
the direction of the hands of a watch.
If the revolving pole be the south pole,
it moves in the contrary direction. All
this is in perfect conformity with what
has already been explained in $21, 22,
and 23, and illustrated by figs. 7 and 8.

(61.) With a view of diminishing the
resistance to the revolution of the mag-
net, which must necessarily take place
when it has to revolve in mercury, at-
tempts have been made to devise a me-
thod of suspending the magnet on a
pivot; but the difficulty has always
been to provide a proper channel for
carrying off the current after it has
acted upon one pole of the magnet.
It became evident that no solid con-
ductor would answer the purpose, as it
would always be in the way of the mag-
net during its revolution. This object
may, however, be accomplished by em-
ploying a magnet of the peculiar shape
represented in fig. 35, having a double
bend in the middle, so that this part is
horizontal while the two extremities are
kept in a vertical position. The magnet,
so shaped, is furnished with an agate
cap fixed to the lower side of the middle
horizontal portion, resting on a fine
point of an upright wire, which is fixed
to the base of the apparatus, and upon
which the magnet is balanced, so as to
allow of its turning freely round. In
order to steady its motion, however, a
wire loop is attached to the magnet lower
down, which embraces the upright wire,
and retains that part of the magnet in a
position nearly vertical. A small cis-
tern, holding mercury, is also fixed upon
the magnet at the middle of its upper
side, just above the point of suspension.
A bent wire, pointed and amalgamated
at the end, passes out from this cistern,
and dips into a circular trough of mer-
cury, which is open in the centre, to
allow the magnet to pass freely through
the opening, and which is supported on
a stage, sustained by means of legs con-
necting it with the base. A wire, pro-
ceeding from the interior of this circular
cistern, passes out of it, and terminates
in a cup with mercury. The electrical
current, intended to act exclusively upon
the upper half of the magnet, is to be

ELECTRO-MAGNETISM.

21

conducted by a vertical wire of sufficient
thickness, which is fixed so as just to
dip into the small cistern attached to the
magnet. Having reached this point, the
current is then diverted from its course
by the wire which dips 'into the large
cistern, and is thence carried away by the
wire which terminates in the cup last
described, to such a distance, and in such
a direction, as to prevent its acting on
the lower pole of the magnet. The
magnet will in this manner 'be made to
revolve with great rapidity. It is scarcely
necessary to remark, that the direction
of the rotation will depend both on the
direction of the current and on the na-
ture of the pole which is acted upon ; so
that reversing either of these conditions
will occasion a change in the direction
of the rotation. Mr. Watkins describes
an apparatus by which these opposite
rotations may be exhibited in two mag-
nets at the same time, and by the same
current, by placing the poles of the one
in a contrary position to those of the
other*. But it is unnecessary to dwell
upon these obvious combinations of the
more simple forms of the experiment.

(62.) In the preceding examples, the
wire was fixed, and the magnet at liberty
to move. But in order to exhibit the
revolution of the conducting wire round
one of the poles of a magnet, this ar-
rangement must be reversed, that is,
1he wire must have freedom of motion,
and the magnet must be fixed. This

* A popular sketch of Electro-magnetism and
Electro-dynamic.-*. By Francis WatKins. lsL'8.

Mr. Faraday accomplished by employing
the apparatus represented in fig. 36.
The glass cup holding the mercury is
shallow, and has a tubular stem ; but
instead of being filled with a plug, as
was the aperture in the former vessel, a
small copper socket is placed in it, and
retained there by being fastened by a
circular plate below, which is cemented
to the glass foot, so that no mercury can
pass out by it. This plate is tinned and
amalgamated on its lower surface, and
stands on another plate and wire, just as
in the former apparatus. A small cy-
lindrical magnet is placed in the socket,
at any convenient height, and then mer-
cury poured in until it rises so high that
nothing but the projecting pole of the
magnet is left above its surface at the
centre. The forms and relative positions
of these parts are seen in the section
Jig. 37. The wire which dips into the
mercury, and has its lower end amalga-

Fig. 36.

Fig. 37.

mated, may be suspended to a fixed wire,
either by a ball and socket joint, con-
structed so as to ensure a continuity of
metallic conductors, or more simply by
means of loops. The best mode of ob-
taining a perfect contact, is to make the
fixed wire terminate in a small cup con-
taining mercury, with its mouth upwards,
and to bend the moveable wire into the
form of a hook, of which the extremity
must be sharpened, and must rest in
the mercury on the bottom of the cup,
as shown in fig. 38. This latter wire,
having full liberty to move, revolves
round the pole of the magnet which is
above the surface of the mercury, with
an accelerated velocity, which afterwards

ELECTRO-MAGNETISM.

becomes uniform, from the increasing re-
sistance of the fluid ; the direction of
the motion being determined by the
principles already laid down in § 26, and
exemplified by jigs. 9, 10, and 11.

Fig. 38.

(63.) Mr. Faraday also contrived a
small apparatus, answering a similar
purpose with the last, and in which the
wire revolves very rapidly, with a very
small voltaic power. It consists of a
piece of glass tube, GG, fig. 39, the lower
end of which is closed by a cork,
Fie:. 39. through which a small
piece of soft iron wire is
passed, so as to project
above and below the cork.
A little mercury is then
poured in, to form a chan-
nel between the iron wire
and the glass tube. The
upper orifice is also closed
by a cork, through which
a piece of platinum wire
passes, and terminates be-
low by a loop ; another
piece of wire hangs from
this by a loop, and its
lower end, which dips a
very little way into the
mercury, being amalga-
mated, it is preserved from
adhering either to the iron
wire or to the glass. When
even a feeble voltaic com-
bination is connected with
the upper and lower ends of
this apparatus,and the pole
of a magnet is placed in
contact with the external
M end of the iron wire M,

the moveable wire within rapidly ro-
tates round the temporary magnet thus
formed by induction at the moment, and
by changing either the connexion or the
pole of the magnet in contact with the
iron, the direction of the motion itself is
changed. This apparatus has been
made so small as to produce rapid revo-

lutions, by the action of two plates of
copper and zinc, containing not more
than a square inch of surface each.

(64.) A still more simple mode of ex-
hibiting the rotation of the wire, is to
employ, instead of a pierced cup, a wide
and very shallow vessel, as a tea-saucer,
for containing the mercury, and to bring
a strong magnet underneath as near to
it as possible. It may even be placed
under the table on which the vessel is
laid. Under these circumstances, the
revolution of a wire, allowed to dip into
the mercury as before, will take place as
soon as it is placed in the voltaic circuit.
The effect is the same, whether the
magnet be held in a horizontal or vertical
position, or inclined at any angle, pro-
vided the magnet be of sufficient length,
so that the influence of the other pole
may not act sensibly upon the wire.

(65.) An apparatus was constructed
by Mr. Griffiths, for exhibiting, in like
manner, the simultaneous revolution of
two conducting wires round the opposite
poles of magnets. Two copper wires,
suspended so as to move freely, .were
made to dip into a shallow vessel con-
taining mercury, in which were fixed two
bar magnets, with their opposite poles
raised above the surface. On making
the connexion between the battery and
the apparatus, the wires revolved round
the magnets simultaneously, but in op-
posite directions*.

(66.) The two forms of electro-mag-
netic rotation which have now been
described, were exhibited at the same

* An apparatus of this kind was exhibited by
Mr. Barlow, at the London Institution, in 1823, in
a course of lectures which he there gave on Elec-
tro-magnetism.

ELECTRO-MAGNETISM.

time, by an apparatus contrived by Mr.
Faraday, in which the cups employed
in the two first experiments are both
acted upon by the same voltaic battery.
This compound apparatus is shown in
Jig. 40. The cups in which the lower
wires proceeding from the bottom of
each cup respectively terminate, are
made to communicate by means of
wires with the opposite poles of a bat-
tery. The upper wires communicate
by a cross wire, supported by an upright
pillar fixed in the middle of the stand.
The current of electricity, therefore, will
ascend through the mere iry and wire in
one of the cups, and descend in the other,
and produce at the same moment a re-
volving motion of the magnet in the one
case, and in the other case, a revolution
in the moveable wire. A cup is also
placed over the middle of the cross wire
lor the convenience of sending the elec-
tric current in the same direction along
both the wires, by making it communi-
cate with one of the poles of the voltaic
battery, while the lower cups both com-
municate with the other pole. The adop-
tion of this arrangement will produce a
corresponding change in the direction
of one of the rotations.
Fig, 41.

(67.) The two phenomena may even
be shown in the same vessel, if, in that
containing the moveable magnet, y?g\ 33,
the wire which dips into the mercury be
rendered moveable, as in fig. 38, by a
mode of suspension adapted to that
purpose. The wire and the magnet will
then both revolve in the same direction
round a common centre of motion, each

appearing to pursue and be pursued by
the other round the circumference of the
circle described by their revolution.
(See/o-,41.)

(68.) After the discovery of the revo-
lution of a magnet round a conducting
wire, and of the wire round a magnet,
many attempts were made to obtain the
rotation of a magnet, or of a conductor,
round their own axes. Ampere was the
first who accomplished the former of
these objects, which may be effected by
the following method : — The cylindrical
magnet seen in the section, /#. 4 2, termi-
nates at its lower extremity in a sharp

steel point, which rests in the centre of a
conical cavity of agate, in the bottom of
the vessel, which may be either of glass
or wood. The upper end of the magnet
is supported in a perpendicular position,
by a thin slip of wood, passing across
the upper part of the vessel and resting
against its sides, having a hole through
which the magnet passes freely. A
piece of quill is fitted on the upper ex-
tremity of the magnet, so as to form a
cup or reservoir above it for receiving a
small quantity of mercury. Into this
mercury is inserted the lower end of
a wire which is amalgamated, in order
to obtain a perfect metallic contact,
while its upper end terminates in a cup
holding a globule of mercury, for the
purpose of forming a communication
with one of the poles of the voltaic bat-
tery. The vessel being filled with mer-
cury, so as to cover the lower half of
the magnet, the galvanic circuit is com-

24

ELECTRO-MAGNETISM.

pleted by means of a thick copper wire
proceeding from the bottom of the vessel,
coming out through the side, and termi-
nating in another cup holding a small
quantity of mercury, by which a com-
munication may be established with the
other pole of the battery. As soon as
this connexion is effected, the magnet
begins to revolve round its axis with
great rapidity, the rotation continuing
as long as the connexions with the bat-
tery are preserved and the battery retains
its power.

(69.) In the original experiment of
Ampere, the magnet was allowed to
float without support in the mercury,
being kept in a vertical position by a
weight of platina attached to its lower
end. But this addition to the whole
mass to be moved occasions a great
diminution of effect, so that the appa-
ratus above described gives a much
greater velocity of motion with the
same galvanic power.

(70.) The same phenomenon has been
exhibited in various ways; the principle
on which it depends is that the electric
current should descend through the
upper half of the magnet only, so as to
act exclusively on the pole which is
situated in that half, and afterwards be
diverted from the magnet, and made to
pass away in such a direction as that it
shall not affect the lower pole of the
magnet. In the experiment above re-
lated, the electric current, after travers-

ing the upper half of the magnet, passes
into the mercury, and being diffused
through it, acts in no sensible degree
on the lower pole of the magnet, and
does not interfere with the rotation pro-
duced by its influence on the upper pole.
There are several circumstances, how-
ever, to be taken into account, in ex-
plaining this experiment, which cannot
now be easily rendered intelligible, and
the notice of which must be reserved
for a future part of the Treatise.

(71.) The same object is attained in
the following manner, by an apparatus
represented in Jig. 43, and in section
in Jig. 44. A magnet, pointed at both
ends, is supported below by an agate
cup fixed on a stem rising -.from the
bottom of the stand ; while its upper
point is lightly pressed upon by a screw,
with a milled head, passing through a
screwed hole at the top of an arched
beam, which forms part of the sustain-
ing frame- work of the apparatus. Near
the middle of the magnet, this frame
supports a stage in the form of a ring,
through the centre of which the magnet
passes freely, and carrying a circular
cistern of mercury, which also sur-
rounds the magnet, without touching
it. A similar cistern of mercury sur-
rounds the lower stem, which supports
the agate cup. A copper wire, project-
ing into the interior of each of these
cisterns, passes out through its sides,
and, being bent upwards, terminates in
a small cup, holding a little mercury, for

Fig. 44.

ELECTRO-MAGNETISM.

25

effecting: the communication with the
voltaic battery by wires, in the usual
manner. A small wire, pointed and
amalgamated at its end, is affixed to the
middle of the magnet, immediately above
the cistern, and is bent so as just to dip
into the mercury contained in the cistern.
A similar wire, proceeding from the
lower end of the magnet, is made to dip
into the mercury contained in the lower
cistern. The lower half only of the mag-
net beinir thus made to form part of the
galvanic circuit — which is continuous
from one cup through the cistern of
mercury, the wire belonging to the
magnet, the magnet itself, the other
wire, the other cistern of mercury, and
the wire terminating in the other cup —
receives the exclusive influence of the
electric current which passes through
it, and begins to rotate with considerable
velocity round the axis, which is con-
stituted by its upper and lower points
of support. The degree of rotatory
effect will depend very much on the
delicacy of the suspension of the mag-
net, so that the friction at the points
may be as small as possible.

(72.) When the magnet is large, it has
been proposed to gain additional rota-
tory power by directing another electri-
cal current to be supplied from a second
battery along the upper half of the
magnet, but in a direction contrary to
that which passes through the lower
pole. This might certainly be effected
by removing the milled head of the ver-
tical screw, and supplying its place by a
small cup to hold mercury, and by care-
fully amalgamating the lower end of the
screw where it touches the magnet. But
since the rotatory force is proportional to
the power of the voltaic battery used, it
is very doubtful whether the second
battery required in this latter method
might not be equally efficacious if it
were employed in increasing the strength
of the lirst battery, by being joined to
it, in the former mode of conducting the
experiment.

(73.) Having thus succeeded in mak-
ing the magnet revolve on its own axis,
it next became an object to effect, in
like manner, the rotation of a conduct-
ing body round its axis. As in the for-
mer case it was necessary to apply the
electric agency in the interior of the
magnet, so in the present instance some
means were to be devised for procuring
the action of the magnet from the in-
terior of the conducting body : hence it
was necessary to discard the wire, and

employ in its place a hollow cylinder of
metal, capable of receiving the pole of
a magnet in its axis. Such an arrange-
ment, which was devised by Mr. Barlow,
is exhibited by fig. 45, which repre-
sents a section of the apparatus. A
bar magnet is fixed upright in a solid
stand, which has a cavity adapted to
receive it, and which also supports a
circular trough of mercury, surrounding

the magnet as in the former instances.
C C is a light hollow copper cylinder,
the lower edge of which dips into the
mercury in the trough; and the upper
part is supported by an arch of the same
metal, from the middle of which there
proceeds a steel-pointed wire, passing
downwards so as to rest in an agate
cup fixed to the top of the magnet, and
also passing upwards and terminating
in a small cup P, holding a little mer-
cury, for the purpose of effecting a
communication with the voltaic battery.
A wire proceeds from the inside of the
trough, and passing out, is bent upwards,
so as to terminate in another cup with
mercury N, for establishing the con-
nexion with the other pole of the battery.
It is evident that, in this arrangement,
the electric current, which we may sup-
pose to descend from the positive wire
of the battery introduced into the, cup
P, being prevented from passing into the
magnet by the interposition of the agate
cup, can find no other channel than the
copper cylinder, down the sides of which

26

ELECTRO-MAGNETISM.

it "will descend into the mercury in the
trough, and thence, passing out by the
wire below, will proceed through the
eup N, and be received by the wire com-
municating with the negative end of the
battery. The cylinder may therefore be
regarded as consisting of a collection of
parallel wires, each of which receives
from the pole of the magnet placed in
the interior an impulse to move in a
direction parallel to itself.

Those on opposite sides of the mag-
net will be urged to move in opposite
directions; but as their forces act on
opposite sides of the axis of motion, they
will all concur in their rotatory effect.
The whole cylinder is accordingly found
to commence revolving as soon as the

Fig. 46 *.

electric current is sent through it ; and
the resistance it meets with being slight,
its velocity soon becomes very consider-
able. After what has formerly been
said, it is scarcely necessary to add, that
the course of its motion is from left to
right, or the same with that of the hands
of a watch, when the electric current is
descending along the cylinder, and when
the enclosed part of the magnet is its
north pole.

(74.) The motion is reversed when
either of these conditions is reversed.
This may be conveniently exemplified
in the two poles of the same magnet by
employing a horse- shoe magnet, sup-

• See Mr. Walking's Sketch, p. 74.;

ported vertically in a stand, as shown in
fig. 46. Two wooden circular troughs are
fixed upon the arms of the magnet,
and secured by binding screws. These
troughs contain the mercury into which
the lower margins of the hollow cylin-
ders dip. The upper part of each cylin-
der is formed into a hemispherical cup,
which is traversed in the middle by a
pointed wire, resting below in a small
cavity in the centre of the extremity of
the magnet contained within the cylin-
der, and having at its upper end a small
cup to hold mercury. Two other cups,
also containing a small quantity of
mercury, are supported upon the exter-
nal ends of bent wires, which pass
through the sides of the circular troughs
into the mercury contained in them.
Thus a continuous metallic communica-
tion is established from one cup to the
other, on each side, through each cylin-
der which surrounds the different poles
of the magnet. If a stream of electri-
city from a voltaic battery be made to
pass in the same direction in both the
cylinders, they will revolve in contrary
directions, being acted upon in an op-
posite manner by the two poles which
they surround. But if the two upper
cups be united by a short wire dipping
its two ends in the mercury they contain,
and the lower, cups be connected, the
one \vith the positive, and the other
with the negative poles of the battery,
the same stream will traverse both sides
of the apparatus, passing upwards in
one cylinder, and downwards in the
other ; and the rotations thence arising
will now, from the contrary influences of
the two poles, be in the same direction
in both the cylinders.

(75.) The rotation of a conducting
body round its own axis, as exhibited
in the experiments just related (§ 73),
throws considerable light upon the cir-
cumstances of the experiment before
described, in which a magnet was made
to rotate about its axis ; for the expla-
nation of that experiment will very much
depend upon the course which we sup-
pose taken by the electrical current
during its passage through the magnet.
If we supposed it to pass through the
interior of the magnet, that is along the
axis, and parts adjacent to it, it would
occasion rotation by its influence on the
parts of the magnet that are situated
nearer to the surface, and further from
the axis. On the other hand, if we
suppose the course of the electric cur-
rent to be nearly superficial, then it will

ELECTRO-MAGNETISM.

27

itself be influenced by the polarity of
those portions of the magnet which
lie near the axis, and the rotatory ten-
dency impressed upon it will produce
the rotation of the magnet, which will,
of course, be carried along with it. On
the latter supposition, it will correspond,
in all its circumstances, with the experi-
ment § 73, in which the conducting
body is urged to rotate by the influence
of a magnetic pole situated within it :
excepting only, that in the former case
the magnet and the conducting body
were one and the same, while in the
latter they were different ana separate.
Mr. Faraday has shown, however, that
the circumstance of the magnet and
conductor being immoveably joined to-
gether makes no difference in the results.
Thus let the magnet M, represented in
section, Jig. 47, be loaded at its lower
end with a platina weight, and fixed at
its upper end on a piece of card or wood,
having two branches of a strong wire,
W W, descending from its upper edge

Fig. 47.

munications with a voltaic battery by
means of the two cups containing mer-
cury. This experiment is important,
inasmuch as it appears to show that the
action is the same, whether the magnet
from which it proceeds be in motion or
at rest. We shall have occasion, how-
ever, in a future part of this treatise, to
point out another mode of explanation
arising out of a different view of the
subject.

(77.) On the other hand, when a hollow
cylinder of metal, balanced on a point
on the upper end of a vertical axis of
wood, and its lower edge dipping into a
trough of mercury, is acted upon by one
of the poles of a magnet placed on the
outside, and brought near it, as shown
in the section /#. 48, where M is the
magnet applied to the cylinder C, ba-
lanced in the wooden stand S, the rota-
tory force is very feeble, compared with
that which takes place when the mag-

Kg. 48.

along its two vertical edges, and termi-
nating below in points: so that the
whole may float, in a vertical position,
in a vessel full of mercury, from the bot-
tom of which a wire proceeds, support-
ing the cup N ; another cup, P, being
placed upon the upper edge of the wires
W W. The whole moveable part of
this apparatus will rotate by the trans-
mission of an electric current through
the wires, on making the proper corn-

net acts from within the cylinder, j The
reason is, that the tendencies to motion
of those portions of the moveable con-
ductor which are most remote from the
magnet, and of those which are nearest
to it, are in opposite directions with re-
spect to the centre of motion ; and, if
the conductor be cylindrical, and the
current equally distributed on every
side of it, must always exactly coun-
terbalance one another. This will be
evident when it is considered that,
although these latter portions are, in
consequence of their greater proximity to
the magnet, acted upon more strongly,
this advantage is compensated by the
greater extent of the portion on the re-
mote side, which is acted upon more
feebly. But ^his equilibrium will not
obtain if, as generally happens, the elec-
tric current be unequally distributed.
If, for instance, it pass along one side
only, the cylinder will revolve when the
magnet is brought opposite to it on that
side.

28

ELECTRO-MAGNETISM.

' (78.) It appears, by the result of the
experiment related in $ 73, that the
eleclro-magnetic influence of the con-
ductor takes place equally when the
current of electricity is diffused over a
considerable surface, as when it is con-
centrated in a slender wire. The effects
will, of course, be weaker in proportion
as it is diffused ; but when the whole of
these scattered forces can be brought to
bear in the production of any effect, the
amount will be the same as when they are
concentrated in a smaller space. Thus
every filament of which the cylinders in
these experiments may be supposed to
be resolved, conducts its respective por-
tion of the electric current, and con-
tributes its share in the production of
one common effect, namely, the revolu-
tion of the cylinder.

In like manner it has been found that
the stream of electricity, which is pass-
ing through the voltaic battery itself
from its negative to its positive pole,
exhibits the same electro-magnetic pro-
perties that it does while passing along
the wire which completes the circuit by
connecting the two poles; for a mag-
netic needle placed in the vicinity of the
battery, and in circumstances equally
favourable to the action of the current,
will be affected in the same way as it
is by the wire itself. Now as all action
implies a corresponding and equal re-
action, it is reasonable to infer that, as
the battery produces motion in the mag-
net, so the magnet might be made to
move the battery, if a sufficiently delicate
suspension could be contrived for the
latter, so as to render its motion sensi-
ble. This could scarcely be effected
with a compound battery of any size :
but by reducing it to a single plate,
making it as light as possible, and sup-
porting it on a single point, in the way
in which the cylinder was sustained in
the last experiment, this object has been
accomplished by Ampere.

(79.) The apparatus he employed for
this purpose is represented in section, in
fig. 49. It consists of a double cylinder
of copper, C C, about two inches and a
half in diameter, and the same in height,
closed at the bottom, so as to form a
vessel capable of holding diluted acid.
The whole is supported by an arched
plate of metal, which passes across the
upper orifice of the inner cylinder on
the upper end of a strong magnet, M,
which is introduced through the middle
of the cylinder. A cylinder of zinc,
Z Z, made as light as possible, and sup-

ported by an arched wire, A, having a
steel point proceeding downwards from
the middle of its curvature, is introduced
between the two plates of the double
copper cylinder, so that the steel point

may rest upon the arched plate of the
inner cylinder, and remain balanced in
this position. On introducing diluted
acid into the copper vessel, a galvanic
action immediately commences ; the
electric current passing from the zinc to
the acid, and ascending from the copper
through the pivot back again to the
zinc. Hence the zinc is in the situation
of a conductor conveying a stream of
electricity downwards, and under the
influence of the magnetic pole which it
surrounds. It will consequently revolve
with an accelerated motion, which is at
length rendered uniform by the friction
of the fluid.

Mr. Barlow states that he has fre-
quently, with this simple apparatus,
produced a velocity of one hundred and
twenty revolutions in a minute.

(80.) The theory just explained is pret-
tily illustrated by an addition to the pre-
ceding apparatus which was made by
Mr. J. Marsh ; and which consists in
having a second steel point fixed under-
neath the upper part of the arch which
sustains the copper cylinder : the copper
vessel may, by means of this point, be
itself balanced on the top of the magnet,
while the zinc cylindrical plate is ba-
lanced on the former; and each may
thus turn round its own centre indepen-
dently of the other. This arrangement

ELECTRO-MAGNETISM.

29

Fig. 50.

is represented in the section fig. 50. As
the electric current ascends in the
copper cylinder, while it descends along
the zinc, the former
will be urged by
the magnet in its
interior, to revolve
in a direction con-
trary to the motion
of the zinc cylin-
der. The velocity
of the copper vessel
is, however, much
smaller than that
of the zinc, not
only from its greater
weight, and from
carrying besides the
whole quantity of
acid, but also from
the friction of its
pivot being in-
creased by the
weight of the zinc
plate which that pi-
vot has to support.

^ ^^^^^^^ -_-____ In this double re-
volution, also, the

velocity of the zinc plate is further re-
tarded by the increased resistance it
meets with from the fluid which is
moving in a contrary direction.

(81.) Mr. Watkins has applied an ap-
paratus of this kind to each of the poles
of a horse-shoe magnet, firmly fixed in
a metal stand at its bent part, as shewn
in fig. 51. The upper ends of the mag-
net are furnished with agate cups for
receiving the steel points on which the
apparatus is supported. The wire it-
self traverses the arch affixed to the
copper vessel, and terminates in a point
at its upper extremity also, so that the
arch connected with the zinc plate rests
upon it. When the apparatus is brought
into action by charging the vessels with
acid, the four cylinders are seen to re-
volve on their axes, the two copper
vessels turning in opposite directions, and
the two zinc cylinders turning in direc-
tions opposite to these, and of course
also contrary to each other: the rapidity
of their revolutions depending on the
power of the magnet, on the strength of
the diluted acid, and on the delicacy of
their suspension.

(82.) Horse-shoe magnets may also
be conveniently employed for combin-
ing the, effects of both poles in giving
motion to a conducting wire. The
operation, of the two poles being in con-

trary directions at their opposite sides,
they will, on the other hand, conspire
in producing the same effect upon a
wire placed between them. Thus each

g. 51.

of the conducting wires p n, p' n', figs.
52, 53, in which the electric current is

Fis. 52.

Fig. 53.

descending' from p to s, when placed
between the magnetic poles N and S,
the former being north, and the latter

ELECTRO-MAGNETISM.

south, and which, for the sake of illus-
tration, we may conceive to be insulated,
will be urged by their united influence
to move parallel to itself, in the direction
denoted by the arrows a, a, in the figures ;
that is, from right to left, if the north
pole be behind, and the south pole
before, as in jig. 52 ; and from left to
right, if the poles are in a contrary posi-
tion, as \nfig. 53.

(83.) Several amusing experiments
have been contrived, in which vibratory
or rotatory motions of different kinds
are obtained by various applications of
this principle.

The following is the invention of Mr.
Marsh. A conducting platina wire W,
fig. 54, is suspended by a loop from a

Fig. 54.

metallic hook at the lower end of ano-
ther wire, which is fixed to the end of
the arm of a stand ; and which supports
above the small cup P, to contain mer-
cury. The lower end of the platina
wire, which thus hangs freely, dips into
a small cistern of mercury, Q, formed
out of the wooden base, and is just mid-
way between the two poles of a horse-
shoe magnet, M, laid flat upon the same
base.

The mercury in the trough is placed
in electrical continuity with another cup,
N, by means of a wire passing out from
the side, and supporting the cup. On
making a communication with the two
ends of the voltaic battery by means
of these cups, the current passing along
the loose platina wire, being influ-
enced by the magnet, urges the wire
either forwards towards Q, or back-
wards towards M, according to the

position of the poles, and the direction
of the current. In either case it is
thrown out of the mercury; and the
circuit being thus broken, the effect
ceases, until the wire falls back again
by its own weight into the mercury;
when the current being re-established,
the same influence is again exerted,
the phenomenon is repeated, and the
wire exhibits a quick succession of vi-
bratory motions.

(84.) This reciprocating movement of
the wire may be converted into one of
rotation, by adapting, as proposed by
Mr. Barlow, a spur-wheel, as shewn in
fig. 55, to the lower part of the upright
wire, which must then be firmly fixed
to the arm of the pillar. The wheel,
being constructed so as to turn round
freely, will revolve with great rapidity
as soon as the contacts are made with the
battery : for this purpose, however, the
wheel must dip so far into the mercury,
as that each of the rays shall touch the
surface before the preceding ray has
quitted the mercury. The direction of
the motion depends, of course, on the
same circumstances as were before men-
tioned : Mr. Barlow observes, however,
that in general the experiment succeeds
best when the wheel revolves inwards.

Fig. 55.

(85.) But it is not necessary to divide
the wheel into rays in order to produce
the effect above described ; for a cir-
cular metallic disc substituted for the
spur-wheel will revolve equally well,
when it is traversed by an electrical
current passing into mercury between
the poles of a horse-shoe magnet. For

ELECTRO-MAGNETISM.

31

this purpose the circumference or the
disc should merely touch the mercury in
the trough. It is necessary also that it
be well amalgamated ; this is best done
by removing it from its centres, and
cleansing the edge thoroughly by a file,
and then dipping a piece of wire into
nitrate of mercury, and taking up with
it a portion of the mercury contained in
the nitrate, transferring it to the edge of
the disc, by rubbing the wire, coated
with mercury, round it. This substi-
tution of a continuous for a divided disc
was suggested by Mr. Sturgeon,

(86.) The same current rr.ay be em-
ployed to turn two wheels with radii, by
disposing them in the manner shewn in
fig. 56, at the extremities of a horizontal
wire which is supported on two pillars
arising from the stand, and which serve
as the common axis of the wheels.

Fig. 56*.

energetic, nothing more will be required
than a single pair of plates. The most
convenient form of a battery of this kind,
is that described by Mr. Watkins, and
which is represented \nfig. 57.

It consists of a double cylindrical vessel
made of thin copper, with a bottom of
the same metal. A plate ofzinc rolled
into a cylinder, of a diameter interme-
diate between those of the copper cylin-
der, is introduced between them, but
prevented from touching them in any
part, by three wooden feet placed at the
bottom of the vessel, and also by pieces
of wood interposed as wedges between
the sides.

A copper wire is soldered to the inside of
the top of the outer copper cylinder, and

Fig. 57.

The lower ends of the rays dip into
troughs of mercury, each lying between
the poles of horse-shoe magnets. Each
trough has its respective wire and cup
P and N for making communications
with the voltaic battery. The current
passing from the one cup to the mercury
in the trough on the same side, rises
along the radius, which dips into it, and
passing along the axis, arrives at the
other wheel ; then descending along its
radius into the mercury, it makes its
exit by the cup on that side. The elec-
tric currents, moving in opposite direc-
tions in the two wheels, require a con-
trary disposition of the poles of the two
magnets by which they are to be acted
upon : that is, the poles of the two mag-
nets that are within the wheels must
both be of the same kind ; as must also
be those that are exterior to them. The
velocity of the wheels thus revolving by
the united action of both magnets is
very great.

(86.) The experiments on electro-mag-
netic rotation we have described, do not
require for their successful performance
a voltaic battery of any considerable size
or power. If the magnets be sufficiently

* The engraver has forgotten to insert the horse-
shoe magnets in this wood-cut. They should have
been placed aa the one in fig. 55.

has a small cup P, fixed at its extremity;
the wire passing through the bottom of
the cup in order to come in contact
with the mercury placed in it. Another
and similar wire N, is also affixed to
the upper edge of the zinc cylinder,
likewise terminating in a cup which
holds mercury. The battery is charged
by filling the copper vessel with diluted
acid ; and the electric current, which is
the effect of the voltaic action thence
arising, may be easily transmitted to
the apparatus where it is wanted, by
means of two bent copper conducting
wires, one end of the one being inserted
into the mercury contained in the cup
proceeding from the zinc cylinder, and
the other in the cup fixed to the copper
cylinder; while the other ends are im-
mersed in the mercury placed in the
cups attached to the apparatus. The
current may be arrested or renewed at

32

ELECTRO-MAGNETISM.

any moment, by removing one end of
either connecting wire from its cup, or
by replacing it. The direction of the
current may also be readily changed, by
merely exchanging the situation of the
wires in two of the cups. The extre-
mities of the connecting wires should
be made perfectly bright, and the ends
of the wire arms which support the
cups and enter the mercury in them,
ought also to be in a similar state, so
that a perfect metallic contact may be
preserved.

(87.) In making electro-magnetic ex-
periments, where numerous repetitions
of contacts between wires are often re-
quired, it is extremely useful, if these
wires are of copper, to rub the ends
over with a little nitrate of mercury ; an
amalgam is thus formed on the surface
of the copper, which does not oxidate or
become dirty, as copper itself does, but
remains bright, and fit for voltaic con-
tact for a considerable length of time.
For this useful manipulation we are in-
debted to Mr. Faraday.

(88.) The movement of currents by
the influence of the pole of a magnet
may be exemplified in fluid as well as
in solid conductors. Thus mercury,
while conducting a current of electricity,
is made to exhibit these motions with
the greatest facility. By immersing the
points of the positive and negative wires
into a shallow basin containing mercury,
a magnet held either above or below
the line of communication will cause
the mercury to revolve round the points
from which the currents diverge. This
motion may be rendered more evident
by covering the mercury with a very di-
lute acid solution, which occasions the
disengagement of bubbles of air which
are moved along with the mercury. The
same phenomenon may also be exhibited
in the following manner. If the positive
wire terminate in a steel point which is
dipped into mercury contained in a shal-
low basin, so as to convey into it an
electric current, which, passing in radi-
ating lines through the mercury, is re-
ceived by a copper ring surrounding the
steel point, and so tranferred to the ne-
gative pole, — by placing the pole of a
strong magnet underneath the basin
immediately below the steel-pointed
wire, the mercury will be seen to revolve
rapidly in a vortex round the point from
which the currents diverge. The revo-
lution is in the contrary direction, if
either the direction of the current be
reversed, or the opposite pole of the
magnet be applied.

(89.) Sir Humphry Davy found that
the arched stream of electrical light
which extends between two points of
charcoal that are placed in the voltaic
circuit, as described in the Treatise on
GALVANISM, § 27, is thrown into a rapid
rotatory motion by the action of the pole
of a magnet placed near it *.

CHAPTER VII.
Concentration of Effects.

(90.) We have already seen, § 82, that
when a conducting wire is placed be-
tween the contrary poles of a magnet, it
receives a similar influence from these
poles, and is urged to move in one particu-
lar direction by the united force of both.
A similar combination of powers will
occur when the pole of a magnet is
placed between two parallel conducting
wires, in which the electric currents are
moving in opposite directions. Thus, if
the needle N S, Jig. 58, balanced as a

Fig. 58.

dipping needle, be placed between the
two wires W, w, in the former of which
the current is ascending, and in the lat-
ter descending, the north pole of the
needle will be urged in the same direc-
tion, denoted by the arrow a, by both
the wires, in a plane parallel to the
wires, and at right angles to the plane
in which they are both situated. The
south pole will also be urged in the con-
trary direction by both wires ; and the
needle will, by the combination of these
forces, have a strong tendency to turn
upon its centre.

(91.) If the wires be joined together
at either end, or, what comes to the
same thing, if a continuous wire be bent
back upon itself, an electric current

* Philosophical Transactions for 1821, p. 427.

ELECTRO-MAGNETISM.

sent through such a wire wiil affect a
needle placed between its two branches
with twice the force that a single wire
would have exerted. This effect may be
exhibited by the following simple appa-
ratus, represented in Jig. f>9 ; where the
two cups terminating the bent wire
W A w B which passes above and be-

Fig. 59.

low a magnetic needle balanced on a
point, enable us to transmit through it
an electric current in any direction we
please. This current, moving in opposite
directions in the upper and lower hori-
zontal portions of the wire, will con-
spire, in bolh cases, to deflect it from
its natural position in the same direc-
tion, and to bring it into a position
nearer to a right angle to the plane of
the wires.

(92.) The force with which each pole
is impelled in a line at right angles to
the plane in which the wires are situated,
is directly as the intensity of the cur-
rent, (supposing it to be equal in both
wires,) and directly as the length of the
interval between the wires, and also in-
versely as the square of the distance of
the pole from the wires. This will ap-
pear from the following considerations.
Let A and B represent the sections
of two wires passing perpendicularly
through the plane of the figure, C being

Fig. GO.

A.

the middle point of the line A B, which
constitutes the interval between them.
Let the magnetic pole P be placed at
various distances along the line C R,
perpendicular to A B, and consequently
perpendicular to the vertical plane which

passes through AB, and comprehends
the two wires. Supposing the wires to
be of indefinite length ; the law of ac-
tion is such, that the intensity of the
tangential force exerted on the pole P
by the wire A, is inversely as the dis-
tance A P, which we shall call a, and is
in the direction P Q, perpendicular to
A P. In like manner, the wire B exerts
upon the pole P, a force in the direction
of P S, and which, on the supposition of
an equal intensity in the two currents,
is equal to the former force. If these
two forces be represented by the lines
P Q and P S, which we shall call q and
s, the resultant force will be represented
by the diagonal P R of the parallelo-
gram, having P Q and P S for its sides.
Calling PR, r, and AB, iy we have
this proportion,

: : q : r,

that is,

but, in different positions of P along the
line C R, q will vary inversely as a ; and

therefore r will be as — ; that is, the
a2

force by which the pole P is urged in
the direction of the line C R, by the con-
joined action of the two wires A and B,
varies, in different situations in that line,
inversely as the square of its distance
from either of the wires, and directly as
the length of the interval between the
wires.

(93.) In order to estimate the rota-
tory force exerted on a needle constrained
to move round a fixed axis in a plane
perpendicular to that of the wires, as in
the examples above given, $$ 90, 91 ; it
will be necessary to resolve the force
above found into one acting in the di-
rection of the tangent to the circumfe-
rence of rotation ; that is, to reduce it
in the proportion of radius to the cosine
of the angle which the needle forms with
the plane of the wires.

(94.) In the situation of the magnet
represented in fig. 59, where the wires,
instead of extending indefinitely in the
horizontal direction, enclose the magnet
also on the sides, the influence of the
lateral portions A and B require to be
taken into account in estimating the
effect produced. A little consideration
will satisfy us that the action of these
parts concur with those of the horizontal
portions in giving the same directive
tendency to the needle ; and that, in
fact, if we suppose the wire to be bent

ELECTRO-MAGNETISM.

info a circular form, as shewn \njig. 61,

the magnetic pole P, placed in the centre

Fig.Bl.

of the circle, or in a line passing through
that centre, and at right angles to its
plane, would be impelled in one uniform
direction by an electric current trans-
mitted thro'ugh the wire, in every part
of its course along that circular bend.
(95.) Supposing'it were possible for
the current to move in a perfect circle,
its direction being that indicated by the
arrows in figures 62 and 63, the north
pole of a magnet placed in its centre
would move to the right, and the south

pole to the left ; as shewn by the arrows
at N and S. If the north pole of a
magnet, therefore, were presented to the
right hand side of this circular current,
it would tend to move away from it,
having the appearance of being repelled :
and since a similar and reciprocal action
takes place between the magnetic pole
and the electric current, the latter,
together with the wire which conveys it,
will, if at liberty to move, recede from
the magnet, or appear to be repelled by
it. Just the contrary would happen if
the south pole of a magnet were pre-
sented on the same side ; that is, there
would be the appearance of a mutual
attraction between them. But when
either of these poles is presented on the
other side of the plane of the circular cur-
rent, effects of an opposite kind are pro-
duced : the north pole appears to attract,
and the south pole to repel. If the
north pole, which thus appears to attract
on one side, be brought nearer and nearer
to tho, plane of the circle, the apparent
attraction goes on increasing, till it

reaches that* plane ; but the moment it
passes through it and comes on the
other side, a repulsion equally strong
with the former attraction commences ;
gradually diminishing as the distance
from the plane increases.
v (9G.) This hypothetical case may in
some measure be realized in a very in-
genious apparatus invented by M. De
la Rive *, and which is shown in Jig. 64.
It consists of a small galvanic battery,
formed by a pair of zinc and copper
plates, Z and C, attached to a cork of

Fig. C4.

sufficient size to enable the whole ap-
paratus to float on the acidulated
water which is to act upon the zinc.
Each of the metallic plates is about half
an inch wide, and extends nearly two
inches below the cork, through which
its upper end is made to pass. A piece
of copper wire, W, covered with silk
thread, is affixed to the copper plate,
and passing upwards through the cork,
is bent into the form of a circle of about
an inch in diameter, so that the other
end returns into the cork and may be
soldered to the plate of zinc. In the
galvanic circuit which is thus formed by
the acid and the plates of zinc and cop-
per connected by the wire, an electric cur-
rent is determined from the copperplate,
along the circular wire, to the zinc plate,
as shown by the arrows ; and the mo-
bility of the floating apparatus affords
the best opportunity of exhibiting all
the effects of the attractive and repul-
sive tendencies we have just been de-
scribing, when a magnet is brought
near it on either side. It is proper to
remark that the instrument is rendered
more powerful by causing the wire to

* This apparatus is described in the Bibli-
otheque Universe!, vol. xvi. p. L'Ol ; and in the
Quarterly Journal of Science, vol. xii. p. J84»

ELECTRO-MAGNETISM.

make five or six turns in the circle, and
then tying the coils together so as to
form a ring, which being thus composed
of a number of concentric circles, the
action of each is combined, and the power
as it were multiplied by the number of
turns.

(97.) This difference in the effects
which the two sides of the plane of the
ring in this instrument have on the same
pole of a magnet, presents a very strik-
ing phenomenon, and exhibits a strong
analogy with the magnet itself. We
may in fact consider it as r; flat magnet,
having hYtwo poles in the centre of it,
two surfaces, the one on one side, and
the other on the other: so that if, on
looking at one of these surfaces, the
current is moving in the same direction
as the hands of a watch move when we
face the dial, then the side on which we
are looking may be regarded as having
the properties of the south pole; and
the other side that of the north pole.
The former attracts and is attracted by
the north pole of a magnet ; the latter
attracts and is attracted by the south
pole, and vice versa.

(93.) A very curious phenomenon is
seen when a magnet is presented hori-
zontally to the vertical electro-magnetic
ring of M. De la Rive ; supposing the
mRgnet to be sufficiently slender to pass
easily through the ring. If the pole be
presented to it on the side where attrac-
tion takes place, the ring will move
towards it, till it arrives at the pole, and
then proceeds onwards in the same
course, the magnet being held in the
axis of the ring, till it reaches the middle
of the magnet ; but there it seems in-
clined to stop ; and then, after a few
oscillations, it settles, as in a position of
equilibrium: for if purposely displaced
by bringing it forwards towards the
other pole, it returns with a force which
shows that it is repelled from that other
pole. Let us now withdraw the magnet,
and turning it half round, so that its
poles are in directions the reverse of
what they were at first, and holding the
ring in one hand, let us again introduce
the magnet into it with the other hand,
until it is half-way through. Under
these circumstances it is just possible
that we may have brought it into such
a situation as that the ring may again
be in equilibrium, undetermined in what
direction to move; but the slightest
change in this position causes it to move
with an accelerated velocity towards
that pole which is nearest to it j and

getting entirely clear of the magnet, it
is projected to a considerable distance
from it. At length, however, it stops,
and, gradually turning round, presents
the opposite face to the magnet ; attrac-
tion now takes place, and the ring re-
turns to the magnet with a force equal
to that with which it had before fled
from it ; and passing again over its pole,
finally rests in its position of equilibrium,
encircling the middle, or what may be
termed the equator of the magnet. In
the former position it was equally at-
tracted by the two poles of the magnet ;
in the latter it is equally repelled : and
accordingly the first was an unstable,
and the last a stable equilibrium. The
ring is represented in this last situation
injfig. 65, surrounding the middle of the
magnet, S, N.

Fi. 65.

(99.) M. De la Rive's apparatus may
be constructed so as not to require the
liquid in which it floats to consist of the
acid ; for if the copper plates be double,
and pass round the zinc plate, so as to
form a cell capable of holding the ncid
and the zinc plate, the whole combina
tion maybe enclosed in a glass cylinder,
which will enable it to float in water.
Both the surfaces of the zinc are thus
opposed to a surface of copper, as in the
construction proposed by Dr. \Vollaston.
(See Galvanism, $ 18.) This addition
was first suggested by Mr. Marsh, and
is represented in the preceding figure
(G">). The tube for this purpose, may
be made out of the neck of a Florence
flask.

(100.) The magnetic properties of
circular conductors may be exhibited in
a striking manner by bending the wire
D2

30

ELECTRO-MAGNETISM.

«r. GO.

into the form of a spiral, see fig. 66.
The upper end of the wire should be
bent downwards and terminate in a
point, for the purpose of being inserted
in a cup containing mercury, which
communicates by a wire with one of the
poles of the Voltaic battery. The coils
of the wire may be either secured from
contact by being wrapped round with
silk thread, or may be attached to one
surface of a card, while the wire which
proceeds from the centre of the coil
passes through the card, and descends
in a straight line on the opposite
side, so as to rest by its pointed extre-
mity on the inside of another cup, also
containing mercury, in order to form
a communication with the other pole
of the battery. A coil of this descrip-
tion, all the successive coils of which
conspire together in producing the op-
posite polarities on its two sides, imitates
still more decidedly the effects of a
magnet, whose poles might be supposed
to be situated in the centre of each disc.

(101.) A still closer imitation of a
magnet is obtained by making the turns
of the wire not in the same plane, as in
the spiral just described, but on a cylin-
drical surface, like the turns of a cork-
screw ; a figure which mathematicians
have termed a helix : an arrangement
which possesses many remarkable pro-
perties, both as regards its interior and
its exterior action.

In fig. 67, the several turns of the
helix are represented as separated to a
distance from each other, in order that
the direction of the turns, and the po-
sition of a magnet placed in the axis
may be distinctly seen. The electro-

magnetic influence exerted by each turn
is, as we have seen, to urge the north

Fig. 67.

pole of a magnet placed in its axis, to
move in one direction along that axis,
and the south pole in the contrary di-
rection. The force thus exerted is, of
course, multiplied in degree and in-
creased in extent, by each repetition of
the turns of the wire; and a magnetic
needle in every part of the interior of the
helix will have a powerful tendency to
place itself in the axis, and to turn its
poles in a manner conformable to the
nature of the force that is in operation.
(102.) Now this force depends on two
circumstances : first, the direction of the
current with reference to the axis of the
helix ; and, secondly, the direction of
the circumvolutions which compose it.
It is well known that screws are of two
kinds, distinguished as right-handed
or left-handed screws. In the former,
as shown in fig. 68, the turns proceed

Fig. 68.

Fig. 69.

downwards, (if the screw be placed with
its axis vertical) from right to left, on
that side which is next to the spectator.
In the left-handed screw (see fig. 69),
the turns proceed in the contrary direc-
tion. Now the magnetic polarity of the
electric helix, which is exerted in the
space it encircles, depends on the direc-
tion in which the current is moving with
reference to a plane at right angles to
the axis; for if the current be descend-
ing on the side next to the spectator,
(in the horizontal helix, fig. 67,) the
north pole of a magnet in the axis will

ELECTRO-MAGNETISM.

37

be determined to the ri^lit, and the
south pole to the left ; and this tendency
will be given in the right-handed helix
if the current be transmitted through it
from left to right ; but in the left-handed
helix from right to left. It requires but
a slight effort of attention to these par-
ticulars to perceive the influence they
have on the phenomena ; yet unless this
effort be made mistakes may easily be
committed.

(103.) When the needle lies exactly
in the middle of the axis of the helix,
the opposite forces which impel the two
poles in contrary directions, derived
from each coil of the wire, exactly
balance one another, and the needle re-
mains in equilibrium. When disturbed
from this position, by being pushed
nearer to one end, the forces derived
from the turns of the wire collectively
act with more power upon that pole
which is nearest to the middle point of
the axis, both because they are nearer,
and because they act less obliquely.
These forces will, therefore, prevail over
those that urge the more distant pole in
the contrary direction; and the magnet
will be brought back to its former po-
sition in the" middle of the axis. This
is illustrated mfig. 70, which represents
a section of the helix; S N being the

Fig. 70.
o. o o o o o. o o o_.o o o o o 0,0

secondly, because they act with less ob-
liquity: they will therefore impel the
whole magnet towards the middle of the
axis.

(104.) So powerful is the action of a
helix of this description, that if a small
magnetized needle, or bar, be placed
within it, so as to rest upon the lower
portions of the wire, the moment the
connexion is made with the Voltaic
battery, so that the electric current
circulates through the wires, the needle
is seen to start up, and place itself in
the axis, remaining suspended in the
air in opposition to the force of gravity.
This will even take place in a vertical
position of the helix, presenting the
singular spectacle of a heavy body raised
by an invisible power, and maintained,
like the fabled statue of Theamides, in
a situation totally free from any ma-
terial connexion and support.

(105.) The magnetic actions of a
helix at its two extremities, and at some
distance beyond them, agree with those
of the sides of a single circle, or spiral
coil already explained ; one end having
properties similar to the north, and the
other to the south pole of a magnet.
But the imitation may be rendered still
more complete if the two portions of the
wire which has formed the helix, and
are situated at its two extremities, be
bent back as shown mjig. 71, at N, S,
so as to return in a straight course
along the axis till they arrive at the
middle point, where they are again bent
at right angles, in order to pass out
between the coils, rising parallel to one

position of the magnet, a little to
side of the middle point of the axis,
will be evident that, in as far as the
S is acted upon by forces derived
the turns of the wire situated between
A a and C c, its tendency to move\ut
wards is exactly balanced by the
arising from the action of the wires be
tween B b and D d upon the pole N,
urging it in the contrary direction;
because these wires have exactly the
same relative situations to these re-
spective poles. But the pole N is be-
sides acted upon by all the wires that
are situated between B b, and the end
A a, and the pole S by all those situ-
ated between C c and D d. These two
actions are in opposite directions; but
the former is more powerful than the
latter; first, because the wires between
Aa and B b are nearer to N, than those
between C c and 1) d are to S ; and

another, and terminating in points for
the purpose of suspension in cups, as
already described in the case of the
spiral wire. Sometimes one of these
wires, instead of being bent upwards, is
made to descend vertically, arid ter-
minate in a sharp point below, where it
is inserted into a cup.

(106.) What constitutes the peculiar
excellence of this arrangement — which

ELECTRO-MAGNETISM.

has been termed by Ampere an electro-
dynamic cylinder, with a view to its
assimilation with the condition of a
magnetic cylinder — is this ; that what-
ever magnetic action the turns of the
heliacal part of the wire may have in a
longitudinal direction, (that is parallel
to the axis,) is counterbalanced by the
contrary action 'of the returning wire.
For the direction of each of the heliacal
portions of the wire, that of W w, for
instance,^. 72, being necessarily some-
what oblique, and the magnetic force it
exerts being along M m, at right angles

Fig. 72.

w

current through them, ceasing the in-
stant that current is arrested, and
capable of being suddenly reversed by
changing the direction of that current.

(108.) In order to facilitate the com-
parison of the properties of Voltaic
magnets with those of ordinary magnets,
it will be found convenient to adapt
them to the simple floating galvanic
apparatus devised by M. de la Rive.
Such is the one represented mjfg. 73.

Fig. 73.

to that direction, the whole of the force
is not exerted in the direction of the
axis A X, but only that part of it repre-
sented by C/, while another part, C e,
is directed at right angles to the axis.
But that portion of the straight wire
which passes along the axis, and cor-
responds in its length to the interval
between the two adjoining spiral turns,
exerts a force C d, precisely equal, and in
an opposite direction to G e. These two
forces, therefore, exactly destroy one
another; and there remains only the
force C/, in the direction of the axis.

(107.) Experiment has fully con-
firmed the accuracy of this theoretical
deduction: and the heliacal arrange-
ment just described is found to be a
tolerably exact representation of what
may be conceived to be a simple or
elementary magnetical filament ; for it
has opposite poles at the two ends, the
one being north, the other south. It
obeys the action of magnets that may
be presented to it, being attracted and
repelled, and assuming determinate po-
sitions with respect to the poles of the
magnet, just as if it were itself a magnet,
of which, indeed, it appears to possess
all the essential properties, and for
which it may be substituted in almost
every form of experiment. It is hardly
necessary to observe that the polarity of
these Voltaic magnets, as we may call
them, is entirely of a conditional nature,
dependant on the passage of the electric

Both the ends of the wires are here
made to descend through the cork, the
one being soldered to the zinc, and the
other to the copper plates; and the
whole being enclosed in a glass cylinder
adapted for floating it in water.

(109.) A very simple apparatus act-
ing on the same principle, is that of
Professor Vanden Boss, represented in
fig. 74. It consists of a plate of copper
about an inch square, and a similar
plate of zinc, placed parallel to the for-
mer, their contact being prevented by a
small piece of cork interposed between
them. To the upper part of one of
these plates a slender brass wire is at-

Fig. 74.

ELECTRO-MAGNETISM.

t ached, which ascends, and is inserted
into an opening made in the side of a
long quill, or a tube formed of portions
of quills inserted successively into each
other, and about six or seven inches long.
The wire, passing along the interior of
the quills, comes out at the end, and
being then wound round the outside of
the tube in a helix, along its whole
length, is made again to enter the quill
at the other end ; and proceeding back
along the axis, is brought out near the
middle, and made to descend till it meets
the other plate, to which it is soldered.
The whole apparatus is suspended at its
centre of gravity by a piece of untwisted
silk-thread. The plates being dipped
into dilute acid, while thus supported,
the galvanic action excited in them is
sufficient to render the helix magnetical.

Heliacal Rotations.

(110.) An ingenious mode of exempli-
fying the rotatory action of a magnet on
a conducting wire, when coiled into a
helix, was contrived by Mr.Watkins, and
described in his Popular Sketch of Elec-
tro-Magnetism*. The apparatus, re-
presented in/o-. 75, consists of a horse-
shoe magnet, h'rmly secured to a wooden
stand. Each of the poles of the magnet

Fig. 75. .

P. 73.

is encircled by a heliacal coil of copper
wire, having a slender bar across its top,
with a needle point in its centre, turning
in a conical hole drilled in the end of the
magnet, with a small platina cup above
it, in order to hold a globule of mercury.
The lower end of each of these coils ter-
minates in slender, pointed wires, which
are soldered to them, and which are in-
tended to dip into mercury contained
in a wooden cistern below it, .fixed by
screws to the leg of the magnet. A wire
also proceeds from the lower part of
each cistern, and, being bent upwards,
terminates in a small cup, also capable
of holding mercury. A brass standard
rises from the basis of the apparatus,
having a forked piece attached to its
upper end, with two points descending
into the two platina cups upon the tops
of the coils ; and there is also another
cup placed at the top of the forked piece,
holding mercury. The voltaic circuit
may thus be completed in various ways ;
either by placing wires in the mercury
contained in the small side cups, and
connected with one pole of the battery,
while other wires, communicating with
the other pole, are placed in the cup on
the top of the apparatus ; or else, direct-
ing one and the same stream of elec-
tricity through the whole of the appara-
tus, by joining one of the side cups with
the positive, and the other with the ne-
gative side of the battery. In the former
case, the current, passing in the same
direction, whether upwards or down-
wards, in the two coils, and being acted
upon by the different poles of the magnet,
will be urged to revolve in opposite di-
rections : in the latter case, the contrary
directions of the currents in both wires,
ascending in the one, and descending in
the other, being respectively acted upon
in opposite modes by the contrary poles
of the magnet, the combination of these
two contrarieties will produce rotations
in the same direction in both the wires.

CHAPTER VIII.
Galvanometers.

(111.) The action of a circular or spi-
ral coil has been applied to the construc-
tion of an instrument for detecting small
quantities of galvanic electricity, or Gal-
varioscope ; and also of a Galvanometer,
or instrument for measuring the inten-
sity of any galvanic current. For this
purpose, the diameter of the circle must
exceed the length of the needle which it
surrounds, in order to allow the latter to
place itself in the plane of the circle

40

ELECTRO-MAGNETISM.

which is to act upon it. Thus, if the nee-
dle n s, fig. 76, be placed in the same
plane with the wire W w, proceeding from
the two cups, P, N, and bent in a circular

Fig. 76.

or oval form, so as to enclose it, the in-
fluence of every part of the wire when
it so surrounds the needle, will be to
turn both its poles in the same rotatory
direction, until it takes a position at
right angles to the plane of the figure.
Let this plane be directed to the mag-
netic north and south — that is, coincide
with the direction which the needle
naturally assumes by the influence of
the earth when left to itself, and undis-
turbed by the action of any electric in-
fluence; and let a feeble current of
electricity be now sent through the wires :
the effect of this current will be to oc-
casion such a deviation of the needle
from the plane of the magnetic meridian
as will balance the force which the mag-
netism of the earth exerts in bringing it
towards that plane. In proportion as
the needle recedes from the meridian,
the terrestrial force increases in inten-
sity, while, at the same time, the electro-
magnetic force diminishes ; the number
of degrees at which it stops, and which
mark where the equilibrium between
these two forces takes place, will there-
fore indicate, with tolerable precision,
the intensity of the galvanic current cir-
culating through the wires.

(112.) The effect of a single turn, or
coil of the wire may be increased by
multiplying the coils; for in this way
the same current is made to act repeat-
edly, in its course through the convolu-
tions of the wire, upon the poles of the
same needle. It is true that the electro-
magnetic force of the current is some-
what weakened by such an extension of
the line of its course ; but its diminu-
tion from this cause will scarcely be
sensible, if the total length of the wire
be not very considerable in comparison
with the whole circuit of the current
including the voltaic battery. In order
to prevent the electric current from
taking a shorter course than the one
intended, it is necessary to secure the
adjacent portions of the wires from

coming in mutual contact ; for "such
contact would allow of the direct pas-
sage of the current from the one to the
other. For this purpose the wire must
either be wrapped round with silk
thread, or coated with sealing wax,
throughout the whole length of the coil.

(11.3.) A galvanometer, constructed
on this principle, was invented by Pro-
fessor Schweigger, of Halle, very soon
after the first discovery of Electro-
Magnetism, and was called by him an
Electro -Magnetic Multiplier. Various
forms have been given to this instru-
ment, either with a view to increase its
sensibility, or to adapt it to different
modes of application under particular
circumstances.

(114.) One of the simplest forms of
the instrument is that represented in
Jig. 77, in which a common compass-
needle is suspended on a pivot proceeding
from a wooden stand, and enclosed by a

Fig. 77.

great number of turns of wire, bent into
the shape of a vertical parallelogram,
and the two ends of which terminate,
as usual, in small metallic cups, con-
taining mercury, for the purpose of
establishing connexions with any gal-
vanic combination of which we are
desirous to ascertain and measure the
electrical state. A graduated circle,
having a dark line across it, coinciding
with the plane of the wires, is to be
fixed to the pivot, immediately under the
needle, in order to estimate its deviations
in either direction from that plane.

(115.) Greater mobility may be given
to the needle by the more delicate mode
of suspension employed in the balance
of torsion. With this view, it may be
suspended at its centre by a fine thread,
or, what is best of all, by a single fila-
ment of silk, enclosed in a tube, and
attached to the lower end of a short
metallic wire, passed through the cover
which closes the top of the tube, and
capable of being turned in the aperture
with some degree of friction, so as to
bring the needle to any required hori-
zontal position. The angular turning
requisite for this purpose is marked by
an index fixed upon the upper end of the

ELECTRO-MAGNETISM.

41

\vire, by reference to a small graduated
circle immediately below it, in the upper
side of the cover. All these parts are re-
presented in the vertical section,//^. 78.
The other parts of the apparatus, as far
as relates to the coils of wire which

Fig. 79,

Fig. 78.

_ ^..n — ^ n

^iiiiiiliiii

encircle the needle, are similar to those
of the former instrument : excepting
that the wires in the middle of the upper
part of the coil must be separated a
little, in order to leave an opening for
the free passage of the thread that sup-
ports the needle. A graduated circle,
equal in diameter to the length of the
needle, is placed, as in the former case,
immediately below the needle. The
compass, wire, and card, are enclosed in
a box, in order to secure them from the
agitations of the air; and the cover,
from the middle of which the upright
tube rises, should be of glass, in order
to allow of our seeing the position of the
needle.

(116.) Mr. Ritchie has lately proposed
a torsion galvanometer, in which he em-
ploys a thread of glass as the material
for suspending the needle*. Fig. 79 is
a vertical section of his instrument, of
which he gives the following descrip-
tion: — Take a fine copper wire, and
cover it with a thin coating of sealing-
wax. Roll it about a heated cylinder,
an inch or two in diameter, ten, twenty,
or any number of times, according to
the delicacy of the instrument required.
Press together the opposite sides of the
circular coil, till they become parallel,

* Philosophical Transactions for 1830, p. 218.

and about an inch, or an inch and a half
long. Fix the coil in a proper sole, and
connect the ends of the wires with two
small metallic cups, for holding each a
drop of mercury. Paste a circular slip
of paper, divided into equal parts, hori-
zontally on the upper half of the coil,
and having a black line drawn through
its centre, and in the same direction with
the middle of the coil. Fix a small
magnet, made of a common sewing-
needle, or piece of steel wire, to the
lower end of a fine glass thread, whilst
the upper end is securely fixed with
sealing-wax in the centre of a moveable
index, as in the common torsion balance.
The glass thread should be inclosed in a
tube of glass, which fits into a disc of
thick plate glass, covering the upper
side of the wooden box, containing the
coil and magnetic needle.

(117.) This instrument enables us to
estimate the comparative intensities of
currents of electricity circulating along
the wires of the coil. For this purpose,
the needle is to be placed directly above
the meridian line drawn on the paper
circle, and consequently directly above,
and in the direction of the wires form-
ing the upper side of the coil. As soon
as a current of electricity is made to cir-
culate along the wires, the needle will of
course be deflected. The glass thread
must then be twisted, by turning the
index, until the needle is brought to its
former position ; and the number of
degrees of torsion must be noted. A
similar experiment may next be made
with another current, the thread having

42

ELECTRO-MAGNETISM. '

previously been untwisted, so that the
needle is again restored to its former
position. The quantities of electricity
circulating round the wires will be, di-
rectly proportional to the number of
degrees through which the thread has
been twisted.

(118.) In the common galvanometers,
in which the force of the current is esti-
mated by the degrees of the deviation
of the needle, this deflecting force acts
with mechanical disadvantage as the
needle deviates from the coil. When it
has been deflected nearly ninety degrees
from its original position, an addition to
the power will produce scarcely any
addition to the effect ; and consequently
the instrument ceases to give indication
of a more energetic current. Hence
Mr. Ritchie's instrument is better enti-
tled to the appellation of a galvanometer,
or measurer of galvanic electricity, than
the former, which are mere galvano-
scopes, or indicators of the presence of
a galvanic current. It has, however,
the disadvantage of not being so sensi-
ble to the influence of feeble voltaic
electricity ; since the needle, being on
the outside of the coil, is acted upon
only by the difference of the two con-
trary electro-magnetic forces, arising
from the opposite currents in the upper
and lower parts of the coil. In the
former arrangement, the needle, being
between these two parts of the coil, is
deflected by the sum of these forces.

(119.) The sensibility of the galva-
noscope may be very much increased by
neutralizing the directive force of the
needle arising from the magnetic influ-
ence of the earth. Professor Gum-
ming employed for that purpose a mag-
netized needle placed immediately be-
neath the moveable needle *. Nobili
improved upon this idea by attaching
the neutralizing needle to the principal
one, placing them one above the other,
and parallel to each other, but with
their poles in opposite directions. They
are fixed by being passed through a
straw, suspended from a thread, as in
the apparatus formerly described. The
distance between the needles is such as
to allow of the upper coil of the wires
to pass between them, an opening being
purposely left, by the separation of the
wires at the middle of that coil, for
allowing the middle of the straw to pass
freely through it. A graduated circle,
on which the deviation of the needle is

* Transactions of the Cambridge Philosophical
Society, vol. i. p. 279.

measured, is placed over the wire on the
upper surface of the frame of the in-
strument, having an aperture in its cen-
tre for the free passage of the needle
and straw. The whole of this arrange-
ment may be understood by a reference
to/g-. 80, which represents a section of
the apparatus : s n is the lower needle

Fig. 80.

surrounded by the coil of wire, and con-
nected with the upper needle N S, by
the intermediate straw shaft which is
seen to pass through the upper horizon-
tal coil of the wires, and also through
the central aperture of the card imme-
diately above it, on which the graduated
circle is drawn. In Nobili's instrument,
the frame was twenty-two lines long,
twelve wide, and six high. The wire
was of copper, covered with silk, one-
fifth of a line in diameter, and from
twenty- nine to thirty feet in length ;
making seventy-two revolutions round
the frame. The needles were twenty-
two lines long, three lines wide, a quar-
ter of a line thick, and they were placed
on the straw five lines apart from each
other.

(120.) The adjustment of the opposing
polarities of the two needles should be
such, that the directive power of the com-
bination resulting from the magnetism
of the earth is very nearly balanced ;
the compound needle being allowed to
retain only sufficient power to bring
it to a constant position when unin-
fluenced by any electrical current. But
the peculiar excellence of the contrivance

ELECTRO-MAGNETISM.

is, that both needles are acted upon in
the same manner, as far as the rotatory
tendency is concerned, by the adjoining
wires. The lower needle, being in the
Situation similar to that in the simple
apparatus already described, § 91, is
acted upon by the sum of the forces of
the currents in every part of the coil.
The upper needle, placed, with regard to
the wires, in the same situation as in
Mr. Ritchie's galvanometer, is acted
upon by the excess of force in the upper
current which is nearest to it. This
force acts upon it in a direction the
reverse of that in which it acts upon
the lower needle, because it is situated
on the opposite side ; but since the poles
are also in a reversed position, the rota-
tory action becomes the same in its
direction on both needles. Hence, be-
sides the increase of sensibility in con-
sequence of the removal of the greater
part of the opposing force derived from
the magnetic influence of the earth, we
have also an increase of power from the
addition of the upper needle. There is
also a convenience in employing the
upper needle as an index ; for by allow-
ing of the graduated circle being placed
above, instead of within the frame, the
folds of the wire may be brought much
nearer to each other than in the com-
mon instrument : this renders it more
compact, and, from the greater approxi-
mation of the lower needles to the wires,
also more powerful. The estimation of
the deflection of the needle by reference
to the graduated circle, can also be more
conveniently made, from the view not
being obstructed by the presence of the
wires above the needle, as in the ordi-
nary construction. It is hardly neces-
sary to observe, that when fixing the
graduation, the zero point should be
placed so as to accord with the position
of the upper needle, when left to the
undisturbed action of the magnetism of
the earth.

(121.) It is evident that if the mag-
netic powers of the two needles employed
in Nobili's galvanometer were perfectly
equal, they would exactly neutralize each
other, as far as regards the directive
influence of the earth ; and the system
of needles would be indifferent to any
position. This would, however, defeat
the purpose of the instrument, the object
of which is to measure a feeble electro-
magnetic force, by putting it in equili-
brium with another force, likewise feeble,
but still acting, and susceptible of mea-

surement. If, 'therefore, a perfectly as-
tatic needle, that is, one which retains
no directive power whatever, be em-
ployed, it will be necessary to find some
other weak, but variable and easily-mea-
surable force to obtain this equilibrium.
Such a force is that of torsion ; and
accordingly the greatest degree of per-
fection attainable in the measurement
of minute electro-magnetic forces, would
appear to be obtained by applying to the
apparatus of Nobili, the needles being
previously rendered perfectly astatic,
the principle of the torsion suspension
adopted by Mr. Ritchie.

(122.) An extension of the principle
of Nobili's galvanometer has been pro-
posed by M. Lebaillif ; who employs a
combination of four needles instead of
two ; one pair being applied to the upper
part of the coil, and one pair to the lower,
in the manner exhibited in the section, y?g\
81, where N S, SN, represent the upper
Fig. 81.

pair of the magnets, having their poles
in opposite directions ; and S N, N S,
the lower pair, likewise reversed in their
polarities ; the two intermediate mag-
nets, which are within the coils of the
wire, having their poles similarly situ-
ated, as is likewise the case with the
uppermost, and lowermost magnets ;
and the whole being affixed to the same
vertical axis, which is a piece of straw,
passing freely through the wires, and
through the graduated circle, which
forms the top of the frame enclosing
both wires and needles. An index is
fixed in the upper end of the axis to
point out the positions of the needles.
The axis itself is suspended from the
end of a horizontal arm, proceeding from
an upright pillar at the side of the ap-
paratus. In order to form the coil, M.

ELECTRO-MAGNETISM. '

Lebaillif employs, instead of a single
wire, having, for instance, a length of
300 feet, five parallel wires, each sixty
feet long, the ends of which are stripped
of their silk coverings and united in a
bundle by being pressed together with
considerable force. In this way the
electric current which enters at one ex-
tremity is divided into five parts, and
made to flow, as it were, through five
separate channels. It is alleged, in
favour of this arrangement, that by thus
multiplying the channels of transmis-
sion, a proportionally larger quantity of
electricity is conveyed ; while the dimi-
nution of intensity arising from the
transmission of the same fractional part
of that current which passes through
one of the wires, along a great length of
wire, is avoided*. But experiments of
sufficient extent, and conducted with
sufficient care, appear to be wanting to
enable us to deduce any certain conclu-
sions with regard to this subject. The
only researches on this point, of which
we have been able to find an account,
are those of Dr. Kaerntz, who came to
the conclusion that the power of the
instrument to deflect the needle is
exactly in proportion to the number of
convolutions of the wire: six convolu-
tions giving six times the power of one
convolution t. But it would require a
much more extended investigation to
establish such a principle, and to fix the
limits of its operation.

(123.) The advantage arising from
the employment of four needles instead
of two, in Mr. Lebaillifs instrument,
appears extremely dubious ; for it
should be recollected that if, on the one
hand, greater power is gained by the
action of the wire on the additional
needle, an equal addition is, on the
other hand, made to the weight that is to
be moved ; so that probably nothing is
thereby gained as to the motion indi-
cating that power.

(124.) On account of the superior
conducting power of silver, wires of that
metal should be employed in preference
to those of copper; and they may then
be even as slender as the sixtieth of an
inch in diameter, which will allow of a
greater number of turns being included
in the same space.

(125.) For the purpose of comparing
the intensities of two electrical currents,
an instrument has been contrived, which

I * See Pouillet's El£mens du Physique Exncri-
loemale, tome i., p. G%.

t I'liilobophical Magazine, vol. Ixii., p. 441.

has been termed the Differential Galva-
nometer. Two wires of equal size are
twisted together, so as to form a com-
pound wire, which is coiled round the
compass needle, as in the instruments
already described ; and the four extre-
mities of the wires are immersed in four
cups filled with mercury. By this means
the two currents which are to be com-
pared with one another, may be trans-
mitted in opposite directions through-
out the whole extent of the coil. These
opposite currents, acting upon the needle
under precisely similar circumstances,
will, if they be equal, exactly counteract
each other, and the needle will remain
in equilibrio between the equal and con-
trary forces ; but if the currents be un-
equal in intensity, the needle will be
affected only by their difference, which
it will therefore indicate by its move-
ments.

(126.) When, on the contrary, we
wish merely to ascertain the existence
and direction of an electric current, it
becomes an object to bring the current
as near as possible to the needle, so that
its action on the poles may be extremely
powerful. The following form has, with
this view, been given to the Galvano-
scope. The needle is suspended from
its centre by a fine thread, between four
vertical spiral coils, the centres of which
are brought very near to the poles of the

Fig. 82.

needle. The same current is made to
circulate through all the four spirals,
the turns of which are directed so as to
produce repulsion of the contiguous pole

ELECTRO-MAGNETISM.

on the one side.and attraction of the ^ame
pole on the other side. This arrange-
ment is shewn in Jig. 82, where M is the
magnet suspended by the thread T, be-
tween the four spiral discs, composed of
the convolutions of the wire proceeding
from the cup P, and terminating in the
cup N. In each disc, the force acting
perpendicularly to the plane of the discs,
is multiplied in proportion to the num-
ber of the circumvolutions of the wire ;
and the spiral turns being made in the
same directions in all the discs, their
actions will concur in producing in the
needle a deviation in the same, direction ;
and the total force will be four times
that of a single disc. This arrangement
allows also of a very close approxima-
tion of the needle to the discs.

(127.) The lightness and extreme
flexibility of gold leaf have enabled
electricians to employ this material for
the construction of a very sensible elec-
trometer. (See Electricity, § 73.) The
same properties may be applied with
great advantage to the purposes of a
Galvanoscope, the electro-magnetic force
of the current being estimated, not by
the movements of a magnet on which it
is made to act, but by those of a move-
able conductor through which it is trans-
mitted, under the influence of a power-
ful magnet. The construction of the
Gold-leaf Galvanoscope is similar to that
of Bennet's electrometer, excepting that
the leaf is single, and there is added a
forceps to retain the lower end of the
gold-leaf, and complete the galvanic
circuit. The slip of gold-leaf g,fig. 83,
is suspended loosely from the forceps/",

while the lower end is laid hold of by
another forceps h ; each forceps termi-
nating in a cup, the one, P, being above,
and the other, N. below, for establish-
ing the communications by which the
current is transmitted through the gold-
leaf. The whole is enclosed in a cylin-
drical glass case, the middle of which is
placed between the poles of a strong
horse-shoe magnet M m, so that the gold-
leaf may be nearly equidistant from them.
When the circuit is completed through
the gold-leaf, the latter will be attracted
or repelled laterally by the poles of
the magnet, according as the current
is ascending or descending ; the broad
surface of the leaf becoming convex
towards the magnet in the one case,
and concave in the other. The curva-
ture of the gold-leaf may be viewed
through a lens in a direction at right
angles to the line of its motion, and may
be referred to a fine line drawn upon the
tube in the direction of its axis. This
instrument is, perhaps, the most delicate
test possible of the existence and direc-
tion of a weak galvanic current *.

CHAPTER IX.

Electro-magnetic Effects of Terrestrial
Magnetism.

(128.) Since the earth acts as if it
were endowed with a magnetic power,
or rather as if it contained a powerful
magnet in its centre, it naturally oc-
curred to those who explored the new
realms of science which the discovery
of Oersted had laid open, that a cur-
rent of voltaic electricity would itself be
influenced by the magnetism of the
earth. It was at first found extremely
difficult, however, to devise means of
rendering this action visible, in con-
sequence of the great feebleness of the
earth's action, compared with that of
such artificial magnets as we are in the
habit of employing. Ampere at length
succeeded in obtaining decisive evidence
that the conducting wire possessed a
directive power by the following con-
trivance. Two wires A and B,}lg. 84,
bent at right-angles, are made to pass
through a cylindrical piece of wood C,
fixed at the end of an arm proceeding
from the basis of the apparatus. They
are made to terminate at both their
extremities in small cups, designed to
hold mercury, the cups P and N being
intended to receive the wires communi-

* Curaraing's Manual of Electro-Dynamics, p. 178,

ELECTRO-MAGNETISM.

4G

eating with the poles of the voltaic bat-
tery ; and the others, n and p, which are
placed the one immediately above the

Fig. 84. i

P TST

other, receiving the two ends of the wire
W W w, which passes through a small
piece of wood at S and is bent below
into a square or rectangle R R. The
upper point of the wire rests on the
bottom of the cup n; the lower point
being merely made to dip into the mer-
cury in the cup p, without touching it,
so that the whole of the wire, with its
connecting piece S, has perfect freedom
of motion round a vertical axis passing
through the point of support in the up-
permost of the two cups, n. When a
connexion is made with the battery by
means of the cups P and N, so as to
direct an electric current through the
wire W W w, it will, from the extreme
delicacy of its mode of suspension, obey
the magnetic influence of the earth, and
arrange itself so that the plane of the
rectangle R R shall be perpendicular to
the plane of the magnetic meridian ;
and it will always return to this position
when turned aside from it by the hand,
or any other cause.

(129.) Another arrangement -which
exhibits the same effect is the one
already described, § 100, and represented
in fig. 66 ; where the electro-magnetic
force is increased by the number of coils
composing the spiral wire. This spiral,
as in the last case, immediately assumes
a position in a plane perpendicular to
the magnetic meridian, as soon as it is
made the channel for the transmission
of a current of voltaic electricity. The
mode of suspension here described is
that of Professor Van den Boss*.

(130.) The apparatus invented by M.
Be la Rive, and described § 96, with the

* Edinlnirgh Journal of Science, No, XII,

improvement described in $ 09, is also ex-
ceedingly well adapted for the exhibition
of the directive power of the galvanic
current ; for in consequence of the per-
fect freedom of motion allowed it, while
floating in a fluid, it very readily as-
sumes the position due to the magnetic
influence of the earth. \Vhenthe plates
are immersed in acidulated water, as in
M. De la Rive's original experiment, the
gas liberated by the action of the acid
on the plates, prevents them from tak-
ing a steady position ; but when put
into a little floating cell, the whole rea-
dily takes the position above mentioned,
and even slowly vibrates about it. The
same phenomenon is also obtained by the
arrangements described in § 108 and
§ 109, which have also the advantage
of exhibiting the strong resemblance
which these instruments, actuated solely
by electrical currents resulting from gal-
vanic action, have to artificial magnets.
For in consequence of their lengthened
cylindrical form, the magnetic forces are
directed along the axes, and the heliacal
cylinder places itself, like a magnet, with
its axis in the magnetic meridian ;
whereas, when a single circle, or com-
bination of circles in a single plane is
taken, that plane will arrange itself so
as to be at right angles to the plane of
the meridian, that is, will be in a plane
passing east and -west ; the face of the
plane only looking to the north and
south.

(131.) In all the cases above de-
scribed, the multiplication of the spiral
or circular turns of wire is not pro-
ductive of the advantage that might be
expected, because, as already remarked
with regard to the galvanometer of Le-
baillif, § 123, although the power is in-
creased, yet the weight to be moved by
that power is increased nearly in the
same proportion ; and the resulting
motion is therefore nearly the same.

(132.) The next point of comparison
between the action of the earth on the
conducting wire, and on the magnetized
needle, relates to the dip. Since the
direction of the force of terrestrial
magnetism 'is in a line situated in the
magnetic meridian, and inclined about
seventy degrees to the horizon, the ope-
ration of this force on a Voltaic wire,
bent in a plane so as to describe a
circle, square, parallelogram, or any
other figure which terminates where it
commenced, forming what has been
termed a closed circuit, is to bring it
into a position where it is perpendicular

ELECTRO-MAGNETISM.

fo Ibis direction ; that is, perpendicular
to the position of the dipping needle.
(133.) The following: was the appa-
ratus, by which Ampere succeeded in
exhibiting the effect now described. A

Fig. S3.

wire bent in the shape of a rectangle
R R, fig. 85, is supported by a tube of
wood, T T, passing directly across the
middle of its longest sides, and serving
as an axis. The two shorter sides or
ends of the rectangle are supported by a
lijjht wooden beam, B, in the form of a
lozenge, the middle of which is perfo-
rated by the tube just mentioned. One
end of the wire, W, which forms the
rectangle, is fixed to a steel pivot, which
turns horizontally on a small metallic
plate fixed upon the top of an upright
metallic pillar, rising from the side of
the basis of the apparatus. A little
mercury is laid upon the plate in order
to render the contact more perfect. The
wire, after it has completed a circuit of
the rectangle, and returned to the same
point W, where it had commenced it, is
bent so as to pass through the tube, and
to come out at the other end, w, where
it terminates in another steel point,
turning in a like manner upon a me-
tallic plate, fixed on the top of a pillar
on the other side of the apparatus. The
lower ends of both pillars, where they
are fixed to the stand, are continuous
with wires supporting cups with mercury,
P and N, in the usual manner. On esta-
blishing a communication between the
Voltaic battery and the cups, the electric
current will ascend in the pillar which
is next to the positive pole of the bat-
tery, and circulating along the rec-
tangular wire, will pass out by its other
extremity, descend by the other pillar,

and make its exit through the cup on
that side. As the rectangle is at perfect
liberty to move around the axis formed
by the two points by which it rests on
the plates, and this axis being hori-
zontal, it will be limited to a vertical
motion. If the axis of motion be placed
so as to be at right angles to the mag-
netic meridian, and the moveable part
of the apparatus be exactly balanced,
so as to retain any position in which it
may be placed, then on directing the
electric current through the wires, the
rectangle will, after a few oscillations,
place itself steadily in the plane of the
magnetic equator; that is, in a plane
perpendicular to the line of the dip;
being the exact position which the
theory would assign to it. On reversing
the direction of the current, the mag-
netic polarity of the wire becomes im-
mediately reversed, and turns completely
round, so as still to place itself in the
same plane as before, but with its faces
turned in opposite directions to those
they before assumed.

(134.) It is evident that, by adopting
a similar mode of suspension, a voltaic
magnet, formed by a heliacal coil of wire,
as described in § 105, would exhibit the
phenomena of the dipping needle, as
completely as a magnetized needle.

(135.) Thus has the analogy between
the action of terrestrial magnetism on
wires conducting an electric current, and
magnetized needles, been completely
established. We have next to inquire
whether a straight wire is affected by
the earth in the same manner as it
would be by the corresponding pole of
a magnet placed near it. In order to
make this comparison, we must first
clearly deduce from the theory formerly
laid down, what effects are to be ex-
pected on a straight conducting wire
from the magnetism of the earth, or
what is equivalent to it, from a south
magnetic pole, acting at an indefinite
distance, in the direction of the line of
the dip. The electro-magnetic force
being tangential, is exerted at right
angles to this direction, which is that of
the line connecting the wire with the
magnetic pole, or origin of the force.
Its action upon a current, whether as-
cending or descending, which moves in
this exact line, or the line of the dip, is
reduced to nothing : and it must act
with greatest intensity upon a current
which moves in a direction perpen-
dicular to the line of the dip. Now,
in order that a straight wire may be

ELECTRO-MAGNETISM.

perpendicular to this line, it must be
situated in the plane of the magnetic
equator. Such, then, is the direction
in which it receives the full influence of
the earth's magnetism ; and this influ-
ence is exerted in urging it to move in
a direction parallel to itself, and at the
same time perpendicular to the line of
the dip ; that is, it tends to continue in
the plane of the magnetic equator. The
direction of its motion to the one side
or the other must depend altogether
upon the course of the electric current
which is passing through it. If the
wire, for example, be placed horizontally,
and have the direction of the magnetic
east and west, and the current of posi-
tive electricity be flowing through it
from west to east, the tendency to mo-
tion in the wire, in consequence of the
influence of the earth, which acts like a
south pole, is towards the north, that
is ascending in the plane of the mag-
netic equator, which plane, it may be
recollected, dips downwards towards the
south, with an inclination to the horizon
of about twenty degrees, equal to the
complement of the dip.

This will be more clearly under-
stood by reference to fig. 86, in which
N, E, S, W represents a horizontal
plane. D d, which has an inclination
to this plane of 70°, is the line of dip,
to which the plane M M, representing
the plane of the magnetic equator, is
perpendicular. W E is a straight por-
tion of conducting wire, along which an

Fig. 86.

electric current is flowing in the direc-
tion from W to E. Under these cir-
cumstances, the effect of the electro-
magnetic force exerted by the earth is
to give the wire a tendency to move
parallel to itself, in the plane M JR, and
towards M, as denoted by the arrow ;
so that were it at liberty to obey this
impulse, it would next be found to

occupy the position marked by the dotted
line we. If the electric current had
been made to pass from E to W, the
direction of the motion would have
been altered, and the wire would have
moved downwards in the same plane,
still, however, preserving its paral-
lelism.

(136.) If the wire extend in the mag-
netic plane from north to south, as,
for instance, along the line M yE in
fig. 87, N S, as before, being the hori-
zontal plane: and if the electric cur-
rent move in the direction M M, that is,

Fig. 87.

from north to south, the wire will tend
to move towards the east, as shewn by
the arrow, still keeping in the same

flane, and remaining parallel to itself,
f, on the contrary, the current move
from south to north, the wire will be
impelled to move from east to west.

It need hardly be observed that all
these statements relate to what happens
in the northern magnetic hemisphere of
the earth, and when the dip is about 70
degrees, as is the case in England. In
the southern hemisphere, where the
northern polarity of the earth is in ac-
tivity, the effects are of course reversed.
At the magnetic equator, where the dip
is nothing, the plane M JE is perpendi-
cular to the horizon, and the tendency
to motion of a horizontal wire must be
directly upwards, or directly downwards ;
and the effect of the terrestrial mag-
netism must be merely that of opposing
or conspiring with gravitation, that is of
producing either an increase or a diminu-
tion in the apparent weight of the wire.
In these high latitudes the inclination of
the magnetic equator to the horizon is
too small to produce any very sensible
effect of this kind. It has, however,
been rendered perceptible in very nice
experiments.

(137.) In consequence of the plane of
the magnetic equator being not very far
removed from a horizontal plane, wires
placed horizontally, and being free to
move in a horizontal plane, may be
made to exhibit the actions of terrestrial
magnetism without much difficulty. Mr,

ELECTRO-MAGNETISM,

49

Faraday succeeded in obtaining this
effect in the following manner*. A
piece of copper wire, about .045 of an
inch thick, and fourteen inches long, has
an inch at each extremity bent at right
angles in the same direction, as shown
at W tv, in fig. 88, and the ends amal-
gamated ; the wire is then to be sus-

Fiff. 88.

•\v

ponded horizontally, by a long silk
thread *, from the ceiling. Two grooves
G g are cut in the sides of a rectangular
piece of hard wood, parallel to the sides,
and about half an inch in depth, and
filled with mercury. P and N are wires
fixed in the board, passing each into its
respective groove, so as to come in con-
tact with the mercury, and terminating
at their other ends in cups for making
the connexions with the voltaic battery.
The points of the wires are now to be
slightly immersed in the mercury con-
tained in the respective grooves ; and in
order to obviate the inconvenience aris-

ing from the film of oxide which is apt
to form upon the surface of the mercury,
and impede the motion of the wires, it is
advisable to cover the surface with a
stratum of diluted nitric acid, which, by
dissolving the oxide, removes this ob-
stacle to free motion. As soon as the
connexions are made with the battery,
and the electric current passes along the
wire, it will be seen to move laterally,
being carried across the field until the
points strike against the ends of the
grooves. On breaking the connexion,
the wire resumes its first position ; on
restoring it, motion is again produced.
On changing the position of the appa-
ratus with respect to the points of the
compass, the same effect still takes
place ; and the direction of the motion
is always the same relatively to the wire,
or rather to the current passing through
it, being at right angles to it. Thus,
when the wire is east and west, and the
electric current flowing from west to
east, the motion is towards the north ;
when the current passes from east to
west, the motion is towards the south.
When the wire hangs north and south,
and the current moves from north to
south, the wire is directed towards the
east, and when the current is reversed,
towards the west. In intermediate posi-
tions the motions of the wire are in in-
termediate directions.

(138.) These different motions cor-
responding to the different positions of
the wire, and directions of the current,
are exhibited by the lines in Jigs. 89, in
which N and S express the north and

Figs. 69.

Quarterly Journal of Science, &e., Vol. XII. n. 117.

50

ELECTRO-MAGNETISM.

south ; the arrow-heads at the end of the
lines shew the direction of the current
in the wire ; and the short arrows pro-
ceeding laterally from the middle, the
direction of the motion induced in the
wire.

(139.) It is evident that, in all these
cases, the wire is moving in obedience
to the same law, which produces the
revolution of a wire round a magnetic
pole in Mr. Faraday's first experiment
on magnetic rotations, already described
($ 59). It is a direct and neces-
sary inference from this law, that
were the two troughs of mercury
continued to ever so great a length,
and even were they carried round
the globe in a circle round the acting
magnetic pole of the earth, the wire
would continue to move along them,
and after describing the whole circle,
and returning from the point at which
it had set out, would resume its course,
and perform perpetual revolutions. In
the very limited space compatible with
actual experiment, the wire appears to
move in a plane ; but theory shews that
it is in reality a small portion of a cy-
linder, of which the radius is the dis-
tance of the magnetic pole of the earth
from the wire. It is amusing to compare
this incipient revolution of the wire with
the complete rotations effected in expe-
riments with artificial magnets ; and,
considering it as part of a similar expe-
riment upon a much vaster scale, to
view the wire as setting out on its
voyage of circumnavigation of the globe,
although it is in the next moment ar-
rested in its progress.

(140.) It is also a consequence dedu-
cible from the same law, that the force
by which the horizontal conducting wire
is urged is the same in all azimuths.

(141.) A real rotation, visible in all
its course, may however be exhibited,
as the effect of terrestrial magnetism.
This has been accomplished also by Mr.
Faraday, who, reflecting that in the ex-
periment of rotation round the pole of a
magnet, the pole is perpendicular to but
a small portion of the wire, and more or
less oblique to the rest, thought it pro-
bable that a wire, very delicately hung
and connected, might be made to rotate
round the line of dip by the earth's
magnetism alone ; the upper part being
restrained to a point, in the line of the
dip, and the lower being made to move
in a circle surrounding it. With this

view, a piece of copper wire, about 0.018
of an inch in diameter, and six inches
long, well amalgamated all over, was
hung by a loop to another piece of the
same wire, at W, see fig. 90, so as to al-
low of very free motion ; and its lower end
w, was thrust through a small piece of

90.

cork, in order to render it buoyant when
placed on mercury. A glass basin, ten
inches in diameter, was tilled with pure
clean mercury, and a little dilute acid
poured on its surface. The thick wire
which communicated with one of the poles
of the voltaic battery was then hung
over the centre of the glass basin, and
depressed so low, that the thin moveable
wire, having its lower end resting on the
surface of the mercury, made an angle
of about 40 degrees with the horizon.
On the circuit through the mercury
being completed, the wire immediately
began to move and rotate, and continued,
whilst the connexions were preserved,
to describe a cone, which though its axis
was perpendicular, had, evidently, from
the varying rapidity of its motion, re-
lation to a line W D, parallel to the
dipping-needle, as being that of the force
by which it was actuated. The direction
of the motion was, of course, the same
as that communicated in the expe-
riment described $ 64, when a south
pole is placed beneath the apparatus.
If the centre from which the wire hung
was elevated, until the inclination of
the wire was equal to that of the dip,
no motion took place when the wire was
parallel to the dip ; and if the wire was
less inclined than the dip, the motion in
one part of the circle capable of being
described by the lower end was reversed ;
results that necessarily follow from the

ELECTRO-MAGNETISM.

relation between the dip and the moving
wire *.

(14-2.) It is evident that by restraining
the motion of one of the ends of the
horizontal wire in the experiment de-
scribed in § 137, and represented in
fig. 88, so as to render that end a fixed
axis, and providing a circular mercurial
trough for the other end to move in,
the same force which produced a paral-
lel progressive motion in the former
case, will now produce a rotatory mo-
tion ; because the force producing a
horizontal motion is the same in all
positions of the wire. This equality is
proved by making the experiment with
two connected horizontal wires instead
of one ; placing them the one imme-
diately above the other, each being
furnished with its separate mercurial
trough, into which the moveable end
may dip. This arrangement is repre-
sented in fig. 91, where the current
entering by the cup P, and travers-
ing the mercury in the upper trough,
ascends through the wire A, passes
on through the upper horizontal wire
to the central wire C, placed in the

Fig, 91.

axis of suspension, (Ihe wire being hung
by the slender thread S,) alon<r which
it descends, and passes outwardly
through the lower horizontal wire, and
thence through the mercury in the
lower trough, to the cup N, whence it
escapes to the voltaic battery. When
this has been effected, it is found that
the suspended wires exhibit no tendency
to rotate, in whatever azimuth they may
be placed. Hence it may be inferred
that the tendency to revolution which
the earth communicates to the current

* Quarterly Journal of Science, XII, 413

moving from the circumference to the
centre in the upper horizontal wire, is
exactly counterbalanced by an opposite
rotatory force in the lower wire, in which
the current passes from the centre to
the circumference; and as this equality
is preserved in every azimuth, it follows
that the rotatory force is constant in
every position of the wire.

(143.) A vertical current in a con-
ductor moveable round a vertical axis
is also impressed by the influence of the
earth with a horizontal force, which
carries it towards the magnetic east,
when the current is descending, and
towards the west when it is ascending.
This will be made apparent by suspend-
ing a wire bent as shown in fig. 92, A,
and terminating above and below in the
points P and N, for the purpose of being
placed in their respective cups, and

Fig. 92, A.

balanced by a counterpoise, L. When
the current is passed through this wire,
the direction of its course along^he two
horizontal branches, H and h, being op-
posite, will counterbalance each other ;
but that in the vertical portion, V, will
take a position either to the east or west
of the axis, according as the current
descends or ascends through it.

(144.) If the wire, instead of having
the shape just shown, has the figure of
a complete square or parallelogram, as
shown in Jig. 92, B, the second vertical
branch will conspire with the first in
making the wire assume the same po-
sition, namely, that in which the current
descends, (as V,) to the east, and that
in which it ascends, (v,) to the west.

(145.) From an attentive considera-
tion of the facts that have now been
stated, we are enabled to understand why
a vertical circular current, urged by the
E2

ELECTRO-MAGNETISM*

the forces are in equilibrium. The
nature of this equilibrium will appear
from fig. 94, in which the circle is repre-
sented as seen from above ; and the
arrows point out the horizontal direction
of the forces, which when the circle is in
the line W E pass through the vertical
axis and are exactly balanced. The same
letters denote the corresponding points
in the two figures.

Fig. 94.
N

clecfro-magnefic force of the earth,
tends to arrange itself in the plane of
the magnetic equator. If at liberty to
do so, it will assume this position ; but
if its motion be restricted to any other
directions, it will place itself as nearly
as possible in that plane. If, for in-
stance, it be constrained by a vertical
axis to turn horizontally only, it will
place itself in a plane perpendicular to
that of the magnetic meridian ; that is,
directed east and west, or fronting to
the north and south. In its movements
to attain this position, it is urged by the
tendencies of the currents, in as far as
their motion is vertical, whether ascend-
ing or descending ; for the tendencies of
the horizontal portions exactly balance
one another, and produce no rotatory
effect. Thus if in the circle AD, Jig. 93,
which turns on the vertical axis, X Y,
the current descend in the branch D,
and ascend in the branch A, the former

will be impelled in the direction D E,
and the latter in the direction A W, by
forces which cease only when they have
attained the positions E and W. The
horizontal portions at X and Y neu-
tralize each other, with regard to their
rotatory tendency. Hence the circle
will revolve until it takes the position
shown by the circle at E and W, where

('Hf>.) When, on the other hand, the
axis of revolution is horizontal, and the
motion vertical, the position is deter-
mined by the forces that act. on the
horizontal parts of the circle, which now
conspire in determining a rotation to-
wards the plane of the magnetic equa-
tor, if the plane of motion coincide with
the magnetic meridian; or if not, as
near to it as the restrictions to the mo-
tion will allow.

(147.) On the whole, then, it appears
that a heliacal coil, such as that de-
scribed in § 105, balanced on its centre,
•will assume all the positions, and ex-
hibit all the directive properties of the
magnetic needle.

CHAPTER X.
Electro- Magnetic Induction.

(148.) THUS far we have considered
the electro-magnetic phenomena that
result from the reciprocal action of gal-
vanic currents on magnetized bodies.
We have next to examine that class of
effects which arise from the action of
the former on iron, or other ferruginous
bodies that have not previously been
rendered magnetic. Experiment has
proved that a conducting wire, during
the passage of an electric current
through it, tends to induce magnetism
in such bodies as are in the vicinity, and
in which that state is capable of being

ELECTRO-MAGNETISM.

53

excited. Induction, therefore, is one of
the phenomena of electro-magnetism,

and must be enumerated among the
properties of electricity in motion.

(U9.) It was discovered both by Sir
Humphry Davy and by Mr. Arago,
nearly at the same tirne^ that the con-
necting wire of a galvanic battery has a
sensible attraction for iron filings, and
that it will hold them suspended like
an artificial magnet, as long as the
electric current circulates through the
wire ; but the moment the galvanic cir-
cuit is interrupted, the action ceases,
and the filings immediately fall off. In
Sir II. Davy's experiments, the filings
adhered to the wire connecting the poles
of a voltaic apparatus, consisting of a
hundred pairs of plates of four inches,
in such considerable quantities as to
form a mass round it ten or twelve
times the thickness of the wire *.

The transverse magnetic action of the
electric current upon iron in its vicinity
is beautifully illustrated by the following
experiments, which were devised by
Mr. Watkins, and which he was so
obliging as to show us. A copper wire
of considerable thickness is extended
between the poles of a voltaic battery ;
and upon sifting overit very gently some
fine iron filings, they are observed to
adhere to the wire all round its circum-
ference, in the form of distinct trans-
verse bands, the particles of which mu-
tually cohere, as long as the current is
maintained. When a broad and thin
copper ribbon is substituted for the wire
as the conductor of the voltaic electri-
city, and iron filings are carefully sifted
upon it in small quantities, they are
seen to arrange themselves in parallel
lines at right angles to the length of the
ribbon ; and their magnetic properties
are further evinced by the quick changes
of position and general disturbance oc-
casioned by the approach of a magnet
brought underneath the conducting
plate.

(150.) It might naturally be expected
from these experiments with soft iron,
that steel, under the same circum-
stances, would receive a permanent
magnetism. Sh\Humphry Davy, hay-
ing fastened several steel needles, in
different directions, by fine silver wire,
to a wire of the same metal, of about
the thirtieth of an inch in thickness, and
eleven inches long, some parallel, others
transverse, above and below, in different

* Philosophical Transactions for 1821, p. 1>,

directions, placed them in the electrical
circuit of a battery of thirty pairs of
plates of nine inches by five, and tried
their magnetism by means of iron filings.
They were all magnetic ; those that
were parallel to the wire attracted
filings in the same way as the wire it-
self ; but those in transverse directions
exhibited each two poles. All the needles
that were placed under the wire when
the positive end of the battery was east,
had their north poles on the south side
of the wire, and their south poles on the
north side ; while those that were over
the wire had their south poles to the
south, and their north poles to the
north ; and this was the case whatever
was the inclination of the needles to the
horizon. On breaking the connexion,
all the steel needles that were on the
wire in a transverse direction retained
their magnetism, which was as powerful
as ever, whilst those that were parallel
to the silver wire appeared to lose it at
the same time as the wire itself.

(151.) All the needles placed trans-
versely under the communicating wire,
the positive end being on the right hand,
had their north poles turned towards
the face of the operator; and those
above the wire, their south poles. Con-
tact with the wires was not at all neces-
sary for the magnetization of the needles ;
for this effect is produced instanta-
neously, by the mere juxtaposition of
the needle in a transverse direction, and
that through very thick plates of glass.
A needle that had been placed merely
for an instant in this transverse direc-
tion with regard to the wire, was ren-
dered as powerful a magnet as one that
had long been in communication with it.

(152.) The intensity of the induced
magnetism was found to be propor-
tional to the quantity of electricity trans-
mitted through the wire in a given time.
Hence a wire electrified by a common
machine, however powerful, produces
no sensible effect ; a feeble magnetism
only is obtained by the reception of
large sparks : but on passing the dis-
charge from a Leyden battery through
the wire, the needles placed transversely
to the wire are rendered permanently
magnetic. The discharge of an electri-
cal battery of seventeen square feef,
highly charged, through a silver wire of
the twentieth of an inch in thickness,
rendered bars of steel two inches long
and from one-twentieth to one- tenth of
an inch in thickness, so magnetic, as to
enable them to attract small pieces of

ELECTRO-MAGNETISM.

54

steel wire or needles ; and the effect
was communicated to a distance of five
inches above or below laterally from the
wire, through water, or thick plates of
glass or metal electrically insulated.

(153.) The efficacy of electro-mag-
netic induction is, as might be expected,
greatly increased by employing a helia-
cal coil of wire, and placing the needle
or bar to be magnetized in the axis of
the helix, in the situation represented in
fig.M. Mr. Arago first employed this
method, and was enabled to produce the
maximum effect on the needle almost in-
stantaneously. It is not necessary, how-
ever, that the bar to be magnetized should
be exactly in the axis of the helix, as it
may lie in any situation within it, or be
inclosed in a tube of glass, or of any
other material which is not a good con-
ductor of electricity. Such a tube will
also be convenient as a support for the
coils of the wire, as well as for admitting
of the introduction of different needles in
succession. The needle should not be
allowed to remain beyond a moment in
the tube, for the magnetizing effects of
the helix are produced nearly instanta-
neously ; and it sometimes happens that
if the needle be left there a few minutes,
the polarity it had at first acquired be-
comes impaired, or contused, and even
occasionally destroyed.

(154.) If a long steel wire be placed
in the axis of a helix, the direction of
the turns of which change at different
points, the wire will be found to have a
number of consecutive points, corre-
sponding to those at which these changes
take place.

(155.) Mr. Watkins observes that the
needle to be magnetized, if it be not
very hard, need not have its whole
length inserted into the glass tube ; for
if held in the hand so that only half of
it is within the helix, it will become
magnetic equally with one that has
been wholly acted upon; because the
portion of the needle that has received
the magnetism communicates it to the
other portion. When a small part of a
needle, very highly tempered, is intro-
duced into the glass tube, the induced
magnetism will be found to extend to
about twice the length of the part so
introduced.

(156.) A very powerful temporary
magnet may be obtained by bending a
thick cylinder of soft iron into the form
of a horse-shoe, and surrounding it with
a coil of thick copper wire, secured from
communication among its several parts

by a covering of silk, or other non-con-
ducting material. When the wire is
made, ptart of the galvanic circuit of a
battery, even of moderate power, the
iron is rendered powerfully magnetic,
and will lift up a very heavy weight by
means of a piece of iron applied to its
poles, which act precisely like those of
a horse-shoe magnet. Fig. 95 exhibits
an arrangement of this kind ; W w being
the two ends of the wire, coiled round
the iron to be magnetized, and bent so
as to dip into the cups P and N, for
forming connexions with a battery.

Fig. 95.

(157.) This experiment has been
made upon a very large scale by Pro-
fessor Moll with an apparatus con-
structed by Mr. Watkins*. It con-
sisted of a cylinder of soft English
iron, an inch in diameter, bent into
the form of a horse- shoe, the interval
between the ends being eight inches
and a half. The copper wire forming
the spiral was one-eighth of an inch in
diameter, and made eighty-three convo-
lutions; the weight of the whole was
five pounds. A connecting piece of iron
was placed in contact with the two ex-
tremities of the horse-shoe ; and the
ends of the spiral wire dipped in mer-
cury, so as to form a voltaic circuit with
a simple battery, consisting of a zinc
plate, which exposed a surface of eleven
square feet to a very diluted mixture of
sulphuric and nitric acids, in a copper
cell. In the first experiment the ap-
paratus sustained, first, fifty pounds,
and afterwards, with care, seventy-six
pounds, by the magnetism induced upon
it.

(158.) When the weight suspended to

* Bibliotheque Universelle, 1830, p. 19.

ELECTRO-MAGNETISM.

the transverse bar of iron is small, it is
found that the iron retains its magnet-
ism for some time after the voltaic com-
munication is broken. If, instead of
merely breaking the connexion, the
electric poles are changed so as to re-
verse the direction of the current, then
the reversion of the magnetism takes
place with extraordinary rapidity. The
weight, indeed, falls ott', but is instantly
again attracted and sustained with the
same force as before. The rapidity of
this change is the more extraordinary
when it is compared with the slowness
and difficulty of changing the poles of a
magnet of equal force by the ordinary
method. If, instead of a heavy weight,
a light steel needle be in contact with
the poles of the electro-magnet, the
needle never falls off; the attractive
force being destroyed and re-established
before the weight of the needle has time
to effect its removal.

(1.59.) An extraordinary sensation is
experienced when the piece of soft iron
connecting the poles is held in the hand
during this change. At first a power-
ful attraction is felt ; this on a sudden
fails, and the iron gives way ; but the
force is so instantaneously renewed, that
the hand is violently drawn up again by
an attraction as strong as before. The
moment the voltaic circuit is completed,
the iron is magnetized to a maximum,
and sustains its greatest weight. No
increase of magnetic power is obained
by augmenting the force of the voltaic
battery.

(160.) With a larger horse-shoe mag-
net of soft iron, weighing twenty-six
pounds, and of which the diameter was
two inches and a half, the chord of the
arc being twelve inches and a half, and
the spiral wire being of brass, one-eighth
of an inch in diameter, and making
forty-four turns, and with the same
voltaic battery as in the former experi-
ment, the magnet supported 139 pounds.
When an iron wire was used, instead of
a brass one, this was increased to 154
pounds.

~(161.) However great these effects
may appear, they are much increased
by augmenting the number of coils,
without extending the length of the
wire. Professor Henry, of the Albany
Academy, in the United States, and Dr.
Ten Eyck, employed for the construction
of the magnet a soft iron bar, two in-
ches square, and twenty inches long,
having the edges rounded, bent into the

form of a horse-shoe, Five hundred
and forty feet of copper bell-wire was
wound round it, in nine coils of sixty
feet each. These coils were not con-
tinued from one end of the magnet to
the other, but each of them was wound
round a portion of the horse-shoe about
an inch in length, leaving the ends of
the wires projecting, and properly num-
bered. The alternate ends were soldered
to a copper cylinder, and the others to
a smaller cylinder of zinc, containing
only two-fifths of a square foot, and
forming a voltaic arrangement with
dilute ac-id. When the armature of soft
iron was placed across the ends of the
horse-shoe, it was found capable of sup-
porting 650 pounds; an astonishing
effect for so small a battery, which re-
quired a charge of only half a pint of
dilute acid. With a larger battery, the
weight sustained was 750 pounds, which
seemed to be the maximum of magnetic
power that could be developed in that
bar by voltaic electricity. It is remark-
able that when the ends of the wires were
united so as to form a continuous wire
of 540 feet, the weight raised was only
145 pounds.

In a subsequent experiment, a mag-
net was wound with twenty-six strands
of copper bell-wire, covered with cotton
thread, thirty-one feet long ; about
eighteen inches of the ends were left
projecting, so that only twenty-eight
feet of each actually surrounded the
iron. The aggregate length of the coil
was, therefore, 728 feet. Each strand
was wound on a little less than an inch ;
in the middle of the horse- shoe it formed
three thicknesses of wire ; and on the
ends, or near the poles, it was wound so
as to form six thicknesses. With a
battery nearly five feet square, this
electro-magnet suspended 2063 pounds,
or nearly a ton weight. This appears
to be the most powerful single magnet
ever constructed, either by the ordinary
modes of magnetizing steel bars, or by
the voltaic current.

(162.) Trials were also made to
procure a small temporary magnet,
which should raise the greatest weight,
compared with its own weight. A
small horse-shoe of round iron, slightly
flattened, one inch in length, and six
tenths of an inch in diameter, wound
round with three feet of brass wire,
raised, by means of a cylindrical battery,
420 times its own weight. Sir Isaac
Newton describes a magnet weighing

ELECTRO-MAGNETISM.

three grains, which he "wore in a ring,
and which is said to have raised 746
grains, or 250 times its own weight, and
this is the greatest relative strength of
any magnet yet recorded. It is evident,
therefore, that a much greater degree of
magnetism can be developed in soft iron,
by a galvanic current, than in steel of
the same dimensions, by the ordinary
processes of magnetizing *.

Mr. Watkins informs us that, in order
to obtain magnets of any power by the
above described method, great care
must be taken to ensure the purity of
the iron employed to form the horse-
shoe magnet ; and after it has been
welded and reduced to the proper shape,
it is advisable, in order thoroughly to
destroy any magnetism it may have ac-
cidentally acquired and retained during
the process, to heat it in a furnace, and
afterwards cool it very gradually, by
allowing it to remain undisturbed till
the furnace itself has grown cold. But,
even after every precaution has been
taken to ensure success, we are still lia-
ble to be baffled by causes which we
tannot explain, and which, when all cir-
cumstances seem to be the same, pro-
duce great and unexpected variations
in the results. It should be borne in
mind, indeed, that similar embarrass-
ments are often experienced in conduct-
ing almost every other experiment in
electro-magnetism, their results appear-
ing to be more or less capricious in
proportion as the conditions necessary
to be fulfilled, before uniformity can be
obtained, are numerous and delicate.

(163.) The best form of a conducting-
\vire for exhibiting its attraction for iron
filings is that of a flat spiral coil, similar
to what is represented in fig. 66, which,
however, for this purpose need not be
rendered moveable. A wire of this
form, through which an electric current
is made to circulate, will collect a pro-
digious quantity of iron filings, and
their relative positions and arrange-
ment, while they remain attached, pre-
sent many singular appearances. If the
rings of wire are not continued quite to
the centre, but leave an opening there,
the particles of iron are observed to
arrange themselves in lines, passing
through the ring parallel to the axis, and
then closing up as radii round the edge.
The particles of iron in the centre erect
themselves into a perpendicular fila-
ment, in the direction of the axis of the

Silliimin's Joutnal, quoted in the Journal of the
Royal Institution; 1, 609; and II. 182.

spiral, while the intervening particles
form filaments inclining from the centre
in proportion to their distance from it*.
The reason of this will be evident from
the principle explained in $ 50.

(164.) There appears to be a very
essential difference in the effects of the
shock of an electrical battery discharged
through a wire, and that of a voltaic
battery, in communicating permanent
magnetism to steel bars or needles. Mr.
Savary has brought to light several very
curious particulars relating to this sub-
ject, which have hitherto received no
explanation t. When the discharge
from a Leyden battery is made through
a straight wire, different needles, though
equal in size, and parallel to each other,
and placed transversely on the same
side of the wire, but at different dis-
tances, have their polarities not disposed
in all of them in the same manner. In
some the poles have the same relative
situation as those of a needle previously
magnetized, and free to move, which
has taken the position it would have
when under the influence of a continued
voltaic current passing in the same di-
rection along the wire.

But in others the position of the in-
duced poles is the reverse of this. For
the sake of conciseness of expression,
we shall call the action which produces
an arrangement of poles similar to that
resulting from a voltaic current, positive
magnetization ; the contrary effect being
that of negative magnetization. Thus,
in a series of experiments in which the
needles were placed at distances from
the wire which increased by equal in-
tervals, at the point of contact wilh the
wire the needle was magnetized posi-
tively, at a small distance negatively ;
a little further off it had acquired no
magnetism whatever ; at a distance
somewhat greater than this, it exhibited
positive magnetism ; and this effect con-
tinued for a certain interval, beyond
which the magnetization was again ne-
gative. When still more remote, it was
positive, and continued so to all greater
distances that were tried. Hence the
action appears to be periodical with re-
lation to the distance at which it is ex-
erted.

(165.) The number of periods in these
alternations, as well as the distances at
which they occur, appear to depend upon
a variety of circumstances of which it

* Watkins, Popular Sketch of Electro-Magnetism,
p. 46.

| Amwles de Chimie, tome, xxxiv,

ELECTRO-MAGNETISM.

57

is difficult to appreciate singly Ihe effect :
such as the intensity of the electric dis-
charge, the length of the straight wire,
its diameter, the thickness of the needles,
and their degree of coercive force. In
general, when the wires are very slender,
and the coercive force of the needles
feeble, the periodical alternations above
noticed arc less numerous ; and it
even frequently happens, with these
conditions, that the magnetization is
every where positive, and that the only
differences observable at different stages
of distance, are those of greater or less
intensity.

(1G6.) When the discharge from the
electric battery is transmitted through
a wire coiled into the form of a helix
around glass or wooden tubes, a similar
diversity is met with in the effects pro-
duced on different needles successively
placed in the interior of the tubes, and
in different situations relative to the
axis. By varying the intensity of the
charge of the battery, or the length or
thickness of the needles, the nature of
the result is changed. The maximum
of magnetic intensity which may be pro-
duced by a given wire, depends on the
ratio between its thickness and its
length ; so that the degree of magne-
tization amounting to saturation, bears
a relation to the value of this ratio. The
degree of magnetic power that a needle
receives fronTthe influence of an electric
discharge, and even the direction of .its
magnetization, depend also on the na-
ture and the dimensions of the bodies
that are in contact with it, or that sur-
round it.

(167.) The magnetizing influence of a
helix through which an electric dis-
charge is passed, is completely inter-
cepted by a cylinder of copper, of suffi-
cient thickness, inclosing the needle,
and introduced within the helix. When
the interposed cylinder is of less thick-
ness, some magnetic effect becomes
perceptible ; and when the thickness of
the copper cylinder is still farther re-
duced, the needle is rendered even more
strongly magnetic than when exposed to
the action of the helix without any inter-
posed substance. Tin, iron, and silver,
placed round the needle, produce a si-
milar modification of the electro-mag-
netic action of the helix ; when inter-
posed in very thin plates, they increase
this action ; when of a certain thickness,
they entirely intercept it. Cylinders
composed of metallic filings do not
produce this effect ; whereas we again

meet with the intercepting property, if
the interposed substance is composed
of concentric layers consisting alter-
nately of metallic and of non-metallic
bodies. It would thence appear that
solutions of continuity in a direction per-
pendicular to the axis of the needle, or
to the axis of the helix, have a consi-
derable influence on the magnetizing
effects of the latler upon the former.
An influence of a similar kind has been
observed from metallic plates of different
thickness, placed in contact with a needle
properly disposed with regard to a
straight conducting wire receiving the
discharge of an electric battery ; being
found, according to their size or position,
to modify the intensity and even the
direction of the magnetism acquired by
the needle.

(168.) All these phenomena appear
to depend on the suddenness of the
action exerted by the electric shock,
either directly on the particles it meets
with in its course, or on objects that are
situated at a distance in the surrounding
space. But the direction of the mag-
netization, as to its being of the positive
or negative kind, depends essentially on
the intensity of the discharge ; so that
discharges of different intensities deve-
lope in the metal a set of opposite
states analogous to the polarities of con-
trary signs, acquired at different dis-
tances from a conducting wire, or by
different intensities of electricity.

CHAPTER XI.
Mutual Actions of Electric Currents.

$ 1. — Action of Parallel Rectilineal
Currents.

(169.) THE discovery of Oersted, and
all the consequences we have developed
from the fundamental law that appears
to regulate the reciprocal action be-
tween electric currents and magnetic
bodies, belong to that division of the
subject to which the term Electro-Mag-
netism is more properly applied: for
they refer to the relation subsisting be-
tween the two agencies of electricity
and of magnetism, which we have been
accustomed to consider as distinct from
one another. But electric currents are
also found to have a mutual action on
one another: and this general fact,
which was ascertained by Ampere, soon
after the discovery of Oersted, esta-

ELECTRO-MAGNETISM.

Wished another great division of the
science ; and the law on which it is
founded is no less prolific in its con-
sequences, and important in its applica-
tions, than that we have hitherto heen
occupied in investigating. To this
branch of the subject Ampere has given
the name of ELECTRO-DYNAMICS.

(170.) When two conducting wires
are suspended or supported in such a
manner as to be capable of moving-,
either towards or from one another, at
the time that electric currents are pass-
ing through them, they manifest a mu-
tual attraction or repulsion, according
as the currents are moving in the same
or in opposite directions in the two
wires. This action is variously modi-
fied, when the relative inclinations and
positions of the currents are varied.

(171.) AVe shall begin by considering
the simplest case, which is that in which
the two currents are running in parallel
directions. The attraction or repulsion
of currents, under these circumstances,
admit of being exemplified in a great
number of ways, according to different
modes in which the conducting wires
are suspended and rendered moveable.
Thus the wires in the apparatus de-
scribed § 143 and 144, figures 92 and
93, may be employed for that purpose,
by bringing either the vertical or the
horizontal branches sufficiently near a
straight wire, through which an electric
current is also passing.

The following is also an apparatus
for the direct exhibition of this pheno-
menon. T,/g-. 96, represents a rectan-
gular table from which arise four up-
right pillars supporting two cross pieces
of wood, having a row of holes for re-
ceiving four cups, two on each piece, the

Fig. 96.

distances of which from each other may
by means of these holes be varied at
pleasure. Short wires, a, a, a, a, proceed
horizontally from the bottoms of the

cups, and serve as pivots round which
the two wires, W, w, bent twice at
right angles, are made to turn at the
upper part of their vertical branches,
having small holes drilled through them
for that purpose. These wires, thus
hung freely upon their pivots, carry on
their upper ends small weights, which,
bringing the centres of gravity as nearly
as possible in coincidence with the points
of suspension, enable them to be moved
by a very slight force. Conducting
wires, proceeding from a voltaic battery,
arc then inserted into the cups pre-
viously filled with mercury, in such a
manner that the galvanic current shall
pass in the same direction through both
the parallel wires; the moment this is
done, the wires move towards each
other, even from a distance of several
inches, exhibiting a powerful mutual
attraction. When the currents are trans-
mitted in directions opposite to each
other in the two wires, which they may
be made to do by transposing the com-
municating wires inserted into the cups
leading to one of the moveable wires,
while the others are left as before, the
moveable wires immediately recede from
each other, manifesting a repulsion as
powerful as the attraction was in the
former case.

(172.) The electro-magnetic forces ob-
tained from voltaic batteries of the ordi-
nary strength are so feeble when com-
pared with the force of gravity, that, in
devising experiments for exhibiting their
action, it becomes necessary, in order to
succeed, so to contrive the apparatus as
that the parts to be moved by these
forces may be as light as possible, and
be also suspended in such a way as to
occasion ihe smallest amount of friction.
Attention should be given not to en-
cumber the conductors, or the magnetic
bars which are to be set in motion, with
any superfluous materials capable of
adding to their weight ; and we should
avoid such dispositions as require them
to move in opposition to their own gra-
vity. The surfaces which are intended
to move through mercury should be as
much as possible reduced ; not only on
account of the friction which takes place
between the solid and the fluid, but also
because the surface of the mercury ra-
pidly oxidates, and the film of oxide thus
formed opposes considerable resistance
to the motion of a solid body, and greatly
impedes the mechanical action of the
apparatus. In general, it will be found i
that a vertical suspension by a point is j

ELECTRO-MAGNETISM.

preferable to suspension by a horizontal
axis, as the former occasions less fric-
tion. Thus Mr. Watkms finds, that the
attractions and repulsions of wires trans-
mitting voltaic currents are exhibited
with more facility when they are sus-
pended vertically, their two ends termi-
nating respectively in upper and lower
cisterns, than when turning horizontally,
as in the apparatus exhibited in the pre-
ceding figure.

These attractions and repulsions
may also be exhibited with a very fee-
ble current of electricity, by means
of the gold-leaf galvanoscore, already
described, § 1-27. By removing the
magnet, and inserting within the instru-
ment a thick wire, inclosed within a
glass tube, parallel and near to the gold
leaf, a strong current may be passed
through the wire, at the same time that
a feeble current is transmitted through
the gold leaf, which will then exhibit the
attractions or repulsions of parallel cur-
rents, according as they are in the same
or in opposite directions; for these
actions take place equally whether the
two currents are obtained from sepa-
rate voltaic combinations, or whether
they are merely two portions of the
same current in different parts of its
course.

(173.) This latter case occurs when-
ever a wire is coiled round in a spiral
or heliacal form, so as to bring different
portions of the same current, passing in
the same direction, very near to one
another. It occurred to the author of
this treatise, soon after hearing of Am-
pere's discovery of the attraction of
electrical currents, that it might be
possible to render the attraction between
the successive and parallel turns of a
heliacal coil very sensible, if the wires
were sufficiently flexible and elastic;
and, with the assistance of Mr. Faraday,
this conjecture was put to the test of
experiment, in the laboratory of the
Royal Institution. A slender harpsi-
chord-wire bent into a helix, being
placed in the voltaic circuit, instantly
shortened itself whenever the electric
stream was sent through it ; but re-
covered its former dimensions the mo-
ment the current was intermitted. It
was supposed that possibly some analogy
may hereafter be found to exist between
this phenomenon and the contraction of
muscular fibres, which seems to be
regulated by some properties of the
nervous system, not unlike those of
electric agency. Messrs. Prevost and

Dumas have advanced a similar theory
of muscular contraction, founded on a
supposed distribution of nervous fila-
ments, through which they imagine a
current of electricity is sent, for the
purpose of determining the action that
precedes contraction. This theory, they
conceive, is supported by microscopic
observations ; but it is far too hypo-
thetical in its present form to deserve
serious discussion.

(174.) The general fact of the mutual
action of electric currents being esta-
blished by these and similar experi-
ments, we must proceed to consider the
different modifications it receives by a
variation of circumstances regarding the
quality and direction of the currents.

(175.) We possess as yet but an im-
perfect knowledge of the peculiar affec-
tions which electricity experiences when
in motion, and which enable it to exert
the singular species of action we are
here investigating, so different from its
attractive and repulsive powers when at
rest. These two classes of effects obey
laws not only different, but in some re-
spects of an entirely opposite nature.
Accumulated electricity, when not in
motion, acts in a degree proportioned
to its tension ; but the wires, which are
silently conducting a current of elec-
tricity in motion, exhibit no sign what-
ever of electric tension ; they produce
no change in the electrometer, and
neither attract nor repel light, bodies in
their vicinity. The law of action in the
state of rest is, that dissimilar electrici-
ties attract, and similar electricities re-
pel one another. When in motion, on
the contrary, it is between similar cur-
rents, that is, currents moving in a
similar manner, that attraction takes
place; while a mutual repulsion is ex-
erted between dissimilar currents. The
electro-statical effects of electric tension
cease when the atmospheric pressure
is removed ; but the electro-dynamical
effects of currents take place equally
whether the conductor be surrounded by
the air or placed in vacuo.

(176.) Since the effects of electric
currents are the consequences of the
motion of the electricity, it is natural to
suppose that they will be in proportion to
the velocity with which it moves, as well
as to the quantity that is set in motion.
But we are in utter ignorance of the
real velocity with which the electrical
effects are propagated along a conduct-
ing body, during the completion of the
voltaic circuit: nor do we even know

GO

ELECTRO-MAGNETISM.

whether this velocity varies in different
cases, nor have we any distinct idea of
the causes that are likely to produce
such variation. We can perceive, how-
ever, that the mode of transmission has
a considerable influence on the results.
The currents transmitted by perfect
conductors are continuous; that is,
their intensity is either constant, or varies
insensibly during two consecutive in-
stants. When the conductors are im-
perfect, the currents are discontinuous;
for the electricity is allowed to accumu-
late for a certain time, and until the
insulating force is overcome, when it
escapes, and passes on with a sudden
impulse, analogous to an explosion.
The electro-motive power continuing to
act, gives rise to a second accumulation,
and a fresh explosion, and so on succes-
sively. These alternations may become
sufficiently rapid to escape our senses,
and thus produce the appearance of an
uninterrupted current, although it be
really discontinuous. The distinctive
character of such currents is, that they
are incapable of producing a deviation
in the magnetic needle. This is the
case with the current produced by the
common electrical machine, when a com-
munication is established between its
positive and negative conductors : and
also with the currents established in
what have been called the secondary
piles of Kilter (see Electricity, J 93),
or piles constructed with a series of
metallic discs separated by humid con-
ductors. Discharges from the Leyden
vial, in like manner, although they in-
duce a degree of permanent magnetism
in steel bars near which they pass, yet
scarcely leave any traces of their effects
on the needle of the galvanometer, when
transmitted through the wires of that
instrument.

(177.) The continuity of the electric
current being the quality most immedi-
ately concerned in the production of the
effects that are the subject of our present
consideration; and it being impossible
for us to discriminate differences of ve-
locity or of quantity, in any other man-
ner than by the total effects that result
from the passage of the current through
a conducting body, we shall distinguish
continuous currents only in respect to
their intensity, and pretend to judge of
the degrees of intensity solely by the
amount of the effects produced on the
galvanometer.

(178.) In order to arrive at the fun-
damental law of electro-dynamic action

of currents upon one another, it is
necessary to consider the total action of
each as resulting from the combined
actions of every one of its parts. As
it is not possible to institute a direct
measurement of those elementary forces
exerted by each indefinitely small por-
tion, the one upon the other, the inquiry
can only be made by assuming some
hypothesis relative to the law of diminu-
tion according to distance, and prose-
cuting the consequences of such an
hypothesis, when applied to such finite
portions of current as occur in our
experiments, and to compare them with
actual observation, their accordance
with which will be a test of the admis-
sibility of the hypothesis.

(179.) Guided by the analogy of all
the other known forces in nature, we
shall assume that the mutual actions of
the elementary portions of electric cur-
rents are inversely as the squares of
their relative distances ; and this is, in
fact, the supposition which agrees best
with all the facts that have hitherto been
ascertained. If we suppose A, for in-
stance (Jig. 97), to be an indefinitely
small] portion of the rectilineal current
P N, moving from left to right, it will
act upon another elementary portion, 13,

Fig. 97.

-N

A.

of a current placed at a given distance
with nine times the energy that it exerts
upon a similar portion of current placed
at C three times further removed from
it. If we call the force of attraction /,
the intensity of the first current a, and of
the second current b, and the distance
between them, or the line B A, d, the
following equation will express the law
just enunciated : —

/. a b

/=^

(180.) It may be demonstrated ma-
thematically, that such being the law of
elementary action, it will follow as a
necessary consequence that the total

ELECTRO-MAGNETISM.

61

action of a current, P N, extended to
an infinite length, upon an elementary
portion of a parallel electric current
placed at any given distance from it, is
in the simple inverse ratio of the shortest
distance between them — that is, of a
line drawn from the one to the other,
and perpendicular to both. Thus, in
the example just given, where the dis-
tance C A is three times the distance
B A, the action of the indefinite cur-
rent P N is three times greater upon B
than upon C. So that if this total
action be expressed by F,
F_ ab

~~d"

(181.) The action, whether attractive
or repulsive, of two elementary portions
of current, must be conceived as exerted
in the direction of the line which joins
them, and which, for the sake of distinct-
ness, we shall call the medial line. So
that in the case of the parallel currents
A and B, fig. 98, moving in the same
direction, an attraction denoted by the
short arrows a and a, takes place in the

Fig. 98.: i

«Y

«A

B

V
D

direction of the medial line AB; and
in the case of the currents C and D,
likewise parallel, but moving in opposite
directions, a repulsion takes place, as
shown by the short arrows, in the di-
rection of the same line. It should be
observed that, in both cases, the action
on each current is perpendicular to the
direction of that current.

§ 2. Action of inclined Rectilineal
Currents.

(182.) Let us next suppose that the
two currents, still remaining in the same
plan?, the direction of one of them, A,
fig. 9'J, is changed from parallelism to
the position C c, the action will still be
perpendicular to that position — that is,
the current A will be urged to move in
the line A a, at right angles to C c; but
the force which thus impels it will be
diminished in the ratio of radius to the
sine of the angle BAG, which the di-

. 99.

B

rection of the current makes xvilh the
medial line A B. This will readily ap-
pear by resolving the force A b into
the two forces, A a, A C, of which the
latter, acting counter to the direction of
the current, is destroyed, while the only
effective force is A a, which is to A b
as the sine of B A C to radius.

(183.) A similar diminution of the
force by which the current A reacts
upon B, takes place in consequence of
its obliquity ; for the portion C n, fig.
100, which, when it was parallel to B,
acted with its full power, has only the

'Fig. 100.

force of the portion m n, when acting in
the oblique position I) n, the diminution
being proportional to the cosine of the
angle C n D, or, what is the same, to
the sine of the angle D n B. The mu-
tual action of the current, therefore,
situated in the same plane, but in ob-
lique positions, may, in as far as this
obliquity is concerned, be expressed by
the following equation, in which « and (•>
denote the angles made by the directions
of the currents respectively with the me-
dial line.

/ = sin. «. sin. &

(184.) Let us now inquire into tho
modification the formula must receive
when the two currents are in different
planes, We have seen that in the last;

G2

ELECTRO-MAGNETISM.

instance, the effective force of an oblique
current, in its action on another current,
is obtained by reducing: that force to a
direction at right angles to the medial
line. Let, us suppose this to be done
with regard to both the currents A and
B,//£. 101, the former being; reduced to
the line a a, and the latter to the line b b,
situated respectively in planes perpendi-
cular to the medial line A B. The force

Fig. 101.

Fig. 102.

a A, estimated in the direction A c,
drawn parallel to B b, or the force B b
estimated in the direction of B d, drawn
parallel to A a, will, in either case, be
reduced in the proportion of radius to
the cosine of the angle a A c, or d B d,
which is equal to it, and which may be
defined, the angle between two planes
passing through the medial line and each
of the currents respectively. Calling
this angle p, the formula, including all
possible relative positions of the cur-
rents, either in the same or in different
planes, becomes when the intensities and
distance between the currents are also
taken into account,

/ = a b (sin. K. sin. /s.cos. ft.)

~~d*~~

which should express the action of the
currents estimated in the direction of the
medial line, provided the hypothesis with
which we set out were correct.

(185.) But experiment, the only cri-
terion of the soundness of physical theo-
ries, shows that an element is still want-
ing in this process for estimating the
value of the forces. It would follow
from the above formulae that when an
elementary portion of a current A, which
has the precise direction of the medial
line, that is, when it proceeds in a straight
line, either towards or from another ele-
mentary portion of a current H$,fg. 102,
no action can take place between them ;
for the angle « being reduced to nothing,
its sine likewise vanishes : and the same
result should also obtain when the
planes in which two currents are situated
are at right angles to one another ; for

B —

in that case the cosine of p. vanishes,
and the whole function expressing the
value of f is reduced to zero. This de-
struction of force ought to take place,
whatever be the position of the force B.

(186.) This, however, is not found to
be the case, excepting only when the
current B is at right angles to the me-
dial line. If it be in any other position,
and more especially if it also coincide in
direction with the medial line, a repul-
sion is manifested. This will appear
from the result of the following experi-
ment : —

(187.) Let a flat oval dish of glass, or
porcelain,//^-. 103, be separated into two
divisions by a glass partition, fixed with
cement, and the divisions filled with
mercury. Insert into each of these
troughs, the sides of a copper wire, bent
in the manner shown in fg. 103, so that
they may be parallel to the partition,

Fig. 103.

over which the arc passes, joining the
two parts of the wire which float in the
mercury. Every part of this wire, ex-
cepting the steel points soldered to ifs
extremities, is to be covered with silk.
Two wires, forming connexions with the
two poles of a powerful voltaic battery,
being inserted in the cups P and N,
which communicate with the mercury in
the basin, in the diiections of the
straight branches of the wire produced,
the current from the one will pass
through the mercury in the adjoining par-
tition to the steel point belonging to one
ofthe extremities of the wire, and then,
circulating through the whole extent of
the wire, will pass out into the mercury of
the partition, and be carried off' by a third
wire. It is found that at the instant this
current is established, the wire moves in

ELECTRO-MAGNETISM.

the direction of the long branches until
it is stopped by the end of the vessel *.
This repulsion is exerted, not only at the
indefinitely small distance that occurs
between the parts immediately in suc-
cession, but also at finite distances be-
tween all the parts of the same current.
(188.) Since it appears, from this and
other experiments, that portions of the
same, or of different currents, moving in
the same continuous line, or at oblique
angles, repel one another, Ampere found
it necessary to introduce another term in
the formula. Since the action due to
this cause is greatest when the two cur-
rents are in the same continuous line,
as A B,/g-. 104, and vanishes when the
medial line is at right angles to both of
them, as in the positions A and C, he in-
ferred that in all intermediate positions, as
at D, for instance, the action would be
proportional to the cosine of the angle
BAD, between the direction of the cur-
rent A, and the medial line A D, which
we have already expressed by the sym-
bol «. The same reasoning applies to

Fig. 104.

C

the current D, when the variations of
its position are taken into account. The
term to be added to the force, as for-
merly determined, must therefore be
some function of the cosines of the an-
gles a. and ft, and may be expressed by
prefixing to them the indeterminate co-
efficient k. So that we now have

ab
f = — - (sin . «. sin. /j. cos. ^ + k cos. «. cos. /s).

(189.) In his earlier speculations, Am-
pere had regarded the value of h as so
small that it might safely be neglected ;
but subsequent researches led him to the
conclusion that it is in reality equal to
— }2, so that the whole of this second
term of the formula is negative, when
the cosines of the two angles are them-
selves positive.

(190.) The electro-dynamic forces
which are thus called into action by vol-
taic currents, and of which the intensi-
ties and directions are determined ac-
cording to the laws above defined, arc
subject to the same laws of composition
and resolution as all other mechanical
forces, and present the same facilities
for the application of mathematical rea-
soning. It would of course be impos-
sible to attempt giving, within the limits
of this treatise, even an outline of the
analytical investigations by which various
important conclusions have been de-
duced. We must content ourselves with
stating merely results, referring such of
our readers as desire further information
on the subject to the works of Ampere,
Biot, and Savary.

(191.) It follows evidently from what
has been said, that when two electric
currents, situated in the same plane, are
inclined to one another at any angle, they
are always mutually repulsive when one
of them approaches to, and the other
recedes from, the summit of the angle ;
and, on the contrary, they attract one
another when they both approach to, or
both recede from, 'that angular point.
When the intensities and positions of the
currents are the same in the two cases,
and the only difference is in the change
of the direction of one of the currents,
then the attractive force in the latter
case will be found to be precisely equal
to the repulsive force in the former case.
And, universally, whatever be the action
of a system of fixed conductors upon a
moveable conductor, it is immediately
changed into an equal and contrary
action, by reversing the direction of the
current, through either the moveable or
the fixed part of the system. If the di-
rection of the current through both the
parts be reversed at the same time, the
original action is reproduced. In this
way we obtain a criterion by which the
mutual actions of different parts of any
electro-dynamical apparatus may be dis-
tinguished from those depending on the
influence of the earth. The former ef-
fects are permanent, but the character
of the latter is changed by reversing the
direction of the current throughout the
whole system.

(192.) It is hardly necessary to enter
into any minute description of the appa-
ratus by which these conclusions may
be experimentally verified. The modes
of suspension adapted to the particular
object of the experiment will readily
occur to any one vvho has made himself

ELECTRO-MAGNETISM.

acquainted with the details we have
already given of analogous experiments.
It will be sufficient to observe that there
are, in general, two kinds of rotation of
which the moveable parts of the appa-
ratus are susceptible, the one on a verti-
cal axis, as in figures 66, 71, 84, 91, 92,
93, and the other on a horizontal axis,
as in figures 85 and 96.

(193.) Care should always betaken,
in nice experiments, to guard against the
errors that might arise from allowing
the influence of the earth to interfere
with the actions we are examining. This
disturbing force may, in general, be
neutralized, and rendered ineffective by
particular dispositions of the conducting
wires. Thus the moveable conductor,
fig. 92, may be rendered astatic or inde-
pendent of terrestrial influence, by form-
ing a second parallelogram below the
first, composed of wires, so turned as to
oblige the current to pass in opposite
directions in corresponding parts of each.
This is shown in Jig. 1 05, in which P
and N represent the'steel points affixed
to the extremities of the wires for the
purpose of being placed in cups with
mercury ; and the directions of the cur-
rents in the several portions of the wire
are indicated by arrows, whereby it
appears that the action of the earth
upon any one part is neutralized by its
equal and opposite effect in another cor-

Fig. 105.v
-UP

responding part, similarly situated with
regard to the axis of rotation. The
wires should'be covered with silk thread,
in order to prevent metallic contact in
the parts where they are brought to-
gether. The shaded parts represent the
branches that may be conveniently tied,
or simply twisted together, after this
precaution has been taken, for the sake
of greater firmness. The weight W is
applied as a counterpoise on the other
side of the axis of rotation.

Fig. 107.

(194.) Fig, 106 represents another
form of an astatic conductor, having the
same properties as the preceding, but
better adapted for the examination of
the actions on the lower horizontal
branch.

(195.) Fig. 1 0 7 shows an arrangement
of a similar kind, in which the horizon-
tal branches neutralize one another, but

in which the interior vertical branches,
being in the axis of motion, have no
influence in producing rotation ; and the
exterior vertical branches are therefore
uncompensated, as far as relates to the
action of any current presented to them:
although being in opposite sides of the
axis, they neutralize one another as far
as regards the influence of the earth.

ELECTRO-MAGNETISM.

(196.) It follows, as a consequence of
the principles already laid down, that Ihe
action of a small portion of conducting
wire, bent into any number of flexures,
provided they extend to no considerable
distance, upon another current, any-
where situated, is equivalent to the
action of a similar wire proceeding in a
straight course between the two ex-
treme points of the contorted wire.
The action, for example, between a con-
ducting wire, A B, fig. 108, and an
elementary portion, C D, the one pur-
suing a sinuous course, the other recti-

Fig. 108.

lineal, is precisely the same as if the
former, instead of being contorted, had
passed in a straight line from the point
A to the point B ; that is, along the
dotted line A B.

(197.) By an extension of the reason-
ing which led to the conclusion just
stated, it may be proved that the action
of a current traversing the contorted
line A B, fig. 109, (and which we have
seen is equivalent to that of a current

Fig. 109.

of equal intensity passing in a direct
line from A to B,) on any elementary
portion m of a distant current G D, will
be to the action of a similar current
E F, (comprehended within the angle
A m B, which A B subtends at the point
m, the middle of the elementary cur-
rent,) in the inverse proportion of their
mean distances from the point m ; that
is, drawing G H, making equal angles

with A m and B m, through the middle
of E F, the action of A B is to the action
of E F, inversely as A m to G m.

(198.) If the electric currents be con-
ceived as spread over a given surface,
A, Jig. 110, instead of being confined
to a single line ; then the action of this
superficial stratum of currents on the
elementary current m, which is a part of

Fig. 110.

the current CD, will be precisely equal to
that of B, or E ; or of any other super-
ficial stratum composed of similar cur-
rents, situated at any distance from m,
and inclined at any angle, provided it
be comprehended by the sides of the
same pyramidal or conical figure, hav-
ing the surface A for its base, and m
for its apex. This, indeed, follows as a
necessary consequence of the preceding
law : the diminished influence of the
currents in A, resulting from their
greater distance, being exactly compen-
sated by their greater number. It may
be derived still more directly, indeed,
from the general law of the action of
electric currents being inversely propor-
tional to the squares of the distances.
We shall have occasion hereafter to
make important applications of this
principle.

§ 3. Action of Terminated Currents.

(199.) We have seen that the action
of a rectilineal current of infinite length
on a short portion of current at a dis-
tance, situated in the same plane, and
wholly on one side of the former, tends to
give it motion in a line perpendicular to
itself, and either in the same direction as
the extended current, or in the opposite
direction, according as it is receding from
or approaching to it. The same applies
to a current of any length, provided it be
situated wholly on one side of the cur-
F

ELECTRO-MAGNETISM.

rent that acts upon it, and which it will
be convenient to designate a terminated
current. Thus, the current A B, fig.
Ill, lying* wholly on one side of the ex-
tended current C D, and which is there-
fore a terminated current, will be urged

Fig. Ill
A

JV1

A.

in the direction of the dotted line M R.
For since the current A B is moving
from the angular point B towards A,
while the current C D is, in the part
C B, moving towards B, the former will
be repelled by the latter during the

whole of that part of its course. The
resultant of all the repulsive forces thus
operating on A may be represented by
the line M E. On the contrary, the
currents passing along the lines A B
and B D, are both moving in a direction
from the angular point B ; consequently
they attract one another; and the re-
sultant of these attracting forces may
be expressed by the line M F, which,
when combined with M E, gives the
total resultant M R, perpendicular to
AB.

(200.) Applying this principle to
various positions of a terminated cur-
rent with relation to the extended one,
we obtain results which are expressed
in the annexed diagram, fig. 112;
where, as before, C D is the extended
current passing from C to D, and the
upper lines represent various positions
of the terminated currents, of which the
directions are marked by the terminal
arrows ; while the dotted arrows point
out the directions of the resulting mo-

Fig. 112.

tions. As a general rule, whenever the
direction of the terminated current is
from the line of the extended current, it
is urged to move in the same direction
as the extended current moves ; when
its current is towards the extended cur-
rent, it is urged to move in the contrary
direction.

(201.) It is easy to perceive that, if,
instead of allowing the conducting wire
transmitting the terminated current to
obey its tendency to move parallel to
itself, its motion were restricted to ro-
tation round a fixed axis at one of its
extremities, as shown in fig. 113, the
force arising from the action of the in-

Fig. 113.

A

O

xr-.-

X- >^.../

definite current C D, will carry the ter-
minated current round the whole cir-
cumference of the circle. This rotatory
force is independent of the angle of
inclination of the currents, and is, con-
sequently, uniform in every position of
A 13 ; and will accordingly act as an
uniformly accelerating force, causing
the wire to revolve with continually
increasing velocity, until checked by
friction and other mechanical obsta-
cles.

(202.) The direction in which the wire
revolves depends, of course, upon the
relation between the directions of the
two currents concerned. "When the ter-
minated current is passing from the
centre to the circumference in the
shorter wire, its revolution will be as
represented in the figure ; that is, in a
direction contrary to that of the un-
limited current, in that part of the circle
which is nearest to it, but similar to it
in the more distant part of the circle.
The reverse takes place when the cur-
rent in A B is passing from the circum-
ference to the centre.

(203.) While such is the action of the

ELECTRO-MAGNETISM.

extended current upon the one that is
terminated, an equal but contrary re-
action is necessarily exerted by the ter-
minated one upon the extended current.
Thus, while the current A B, fig. 114,

Fig. 114.
A

UG.

]J

tends to move in the direction of the
upper arrow, its reciprocal action on the
current C D, urges the wire that con-
ducts it in the contrary direction, from
D to C, as denoted by the lower arrow ;
and the same principle applies to all the
other cases.

$ 4. Action of Diverging and Converg-
ing Currents.

(204.) Since the rotatory force is the
same in all positions of the wire, it is
evident that if wires, or other conducting
bodies, be so disposed as to cause the
currents to radiate from a centre in all
directions, they will tend to revolve in
the same manner as any one of them
singly would have done, by the influence
of a rectilineal current in the vicinity,
and in the same plane ; provided this
latter current be wholly without the
circumference of the circle of revolution.
The same thing will happen when the
currents converge from the circumference
to Hie centre, only the direction of the
motion will be reversed. These two
conditions of the experiment are repre-
sented in figs. 115 and 116, where the
arrow-heads in the paths of the cur-
rents denote their direction ; and the
exterior dotted arrows the direction of
the revolution of the conductors.

Fig. 115.

(205.) Examples of this kind of di-
vergence or convergence of currents
frequently occur in electro- dynamical
experiments. They are met with when-
ever a fluid conductor, such as mercury,
is the medium of communication be-
tween the point of a conducting wire
dipped into the fluid and a circular rim
of metal ; in which case there is always
more or less of diffusion of currents
while they are passing through the fluid ;
and generally there is a tolerably regu-
lar radiation or concentration of the
currents, of which some idea may be
formed by fig. 117; where the current
passing from P to N, through mercury
contained in the cylindrical vessel V,
will radiate from the point of the wire
towards every part of the circumference
of that vessel. If the current pass from
N to P, it will converge towards the
wire. In either case the action of a
strong current, passing along the straight
horizontal conductor C D, will give rise
to a revolving motion in the mercury, the
direction of which, corresponding with
the directions of the two currents, is
indicated by the arrows at w, D, and m.
(206.) It is not easy to exemplify by
direct experiment the theoretical de-
ductions applicable to the case of the
action of a straight current of indefinite

ELECTRO-MAGNETISM.

length on a terminated current; that is,
on a portion of another current, situated
on one side only. The difficulty arises
from the impossibility of limiting the
actions to those parts of the currents
to which we wish to confine them,
while studying their effects, and of ex-
cluding the action of the remaining
portions of the currents necessary for
completing the circuit. The only mode
of preventing the interference of the
latter, is to neutralize them by opposing
one part to another ; this may in a great
measure be accomplished by providing
for the subdivision and branching off of
the currents in different directions, so
that their actions may destroy one an-
other. If, for instance, the ends of the
smaller wire that is to be rendered
moveable, be made to dip into a vessel
of mercury, of sufficient width to allow
of the electric currents to diverge and
spread over a considerable surface, they
will not materially interfere with the
actions we are examining.

(207.) In the cases we have just
considered, (§ 204,) the axis of ro-
tation in the shorter wire was sup-
posed to be at its extremity. If
its situation were different — were it,
for example, in the middle of its length,
as at X, fig. 118, it is evident that when
the current is passing from' A to B, it
moves in the first portion towards the

Fig. 118.
A

XX

B

•^D

rotatory forces would oppose each other ;
and as the strongest would prevail, it
would bring the conductor into a position
where they are in equilibrium. Seey^.
119, where the portion /B of the limited

centre, and in the second portion from
it; hence the rotatory forces, denoted
by the arrows a, b, counteract one an-
other, and the wire is urged to revolve
only by the difference, if there be any,
between them.

(208.) If the current, which we have
hitherto supposed to be terminated, that
is, altogether on one side of the recti-
linear current, were prolonged so as to
cross the latter (without, however, join-
ing it), that portion which extended
beyond it would have a tendency to
move in an opposite direction to the
part on the other side; so that these

Fig. 119.

B

rectilineal current A B, turning on the
axis A, and crossing the unlimited cur-
rent C D, tends to move in a contrary
direction to the portion A/, the position
of equilibrium being that of A E.

(209.) If the current A B be traversed
by the current C D passing through the
axis itself, A, the position of equilibrium
is that of parallelism between the two
wires. In either case there can be no
revolution round the axis.

$ 5. Action between Currents situated
in different Planes.

(210.) Let us now consider what will
happen when the two currents A B and
C D, fig. 120, are in different planes,
both being of an indefinite length, and
extending on both sides of the perpen-
dicular line P Q, which is common to
the directions of both currents, and
divides each into two portions. At-

120.

traction will take place between those
portions in which the current is passing
towards P or Q, the points at which the
currents are nearest to each other ; that
is, in the present instance, between the

ELECTRO-MAGNETISM.

69

portions A P and C Q. The portions
P B and Q D will also attract one an-
other, because the currents are proceed-
ing from the points P and Q, in each
respectively. But the portions A P and
Q D will repel one another, as will also
the portions P B and C Q ; because the
currents are moving: in a different man-
ner in the two that compose each of
these pairs.

All these forces concur in producing
a rotatory motion round the axis P Q.
The wire A B will tend to assume the
position a b, parallel to C D ; and the
wire C D will be urged to take the posi-
tion c d parallel to A B. If only one of
these be moveable, it will place itself in
a line parallel to the other ; if both be
moveable, they both will take an inter-
mediate position ; so that, in either case,
they will become parallel to one another.
That part of the force which produces
this rotatory effect, and acts in a plane
perpendicular to the axis, may be termed
the directive force. It varies as the
sine of the angle A P a ; but another
part of the force still remains, namely,
that which acts in a direction perpendi-
cular to this plane — that is, parallel to
the line P Q, the nearest distance of
the wires. It is evident that this force
varies as the cosine of the angle A P a.
Whenever that angle is less than a right
angle, this force is attractive ; and as it
tends to bring the currents nearer to
each other, it may be distinguished from
the former by the designation of the
approximative force. Commencing
when the positions of the wires, from
being perpendicular, are slightly inclined
to each other, this latter force attains its
maximum when they have been brought
by the directive force into a parallel
position. When the corresponding por-
tions of the wires, on the other hand,
form an obtuse angle, the approxima-
tive force is negative, and is so in the
greatest degree when the wires are
parallel, so that their currents move in
opposite directions. This is an obvious
consequence of the change of sign which
the cosine of the angle A P a experiences
when the latter changes from an acute
to an obtuse angle.

(211.) If the movements of either of
the wires be restricted to rotation round
an axis different from P Q, such as X,
fig. 121, some part of the directive force
will be destroyed by the opposing action
of the current passing through the por-
tion X P, which intervenes between the
axis and the perpendicular. This oppo-

sition of forces will increase according
as the axis is further removed from the
perpendicular : so that all action may

Fig. 121.

come to be entirely neutralized, if its
distance is sufficiently great in propor-
tion to the length of the other branch,
P B, of the current situated on the other
side of P Q. In estimating the result-
ing effect, however, it is necessary to
take into account the mechanical advan-
tage which the rotatory force, impelling
the remote part PB, has over that which
impels XP, in consequence of the greater
length of lever by which it acts. When
the contrary rotatory momenta thence
arising are equal, the currents will be in
a position of equilibrium, which posi-
tion will tend the more nearly to coin-
cide with that of parallelism, in propor-
tion as the axis approaches to a coinci-
dence with the perpendicular P Q. This
result may be verified by employing the
apparatus fig. 106, which, from its con-
struction, is, as we have seen, astatic,
and by which we may study the action
of a transverse current upon either of
the horizontal branches to which it is
presented.

§ 6. Mutual Action of Rectilineal and

Curvilineal Currents.
(212.) If a terminated current, which
describes a curve line, be subjected to
the action of an unlimited rectilinear
current, its different portions will be
urged in different directions, each being
perpendicular to the respective portion
of the curve. Thus the different parts
of the circular conductors A B,/g-. 122,

Fig. 122.

A
A i

ELECTRO-MAGNETISM.

or E F,/£-. 123, through which an elec-
tric current is passing, will be urged by
the straight current C D, situated in the
same plane with it, in the various direc-
tions indicated by the dotted arrows.
If these forces were all equal, or nearly
Fig. 123.

so, which could only happen when C D
was at an infinite distance, or one in-
comparably greater than the diameter of
the circle, they would all be in equili-
brium. But, in all other cases, portions of
the circles nearest to the current will be
more powerfully acted upon than the
remoter parts, and the forces by which
they are impelled will therefore prevail,
and the whole circle will tend to ap-
proach or recede from C D, according
as the direction of the currents in that
part is similar or contrary to that of the
current C D. Thus it appears that the
approximative force is equal to the dif-
ference between the resultants of the
attractive and repulsive forces.

(213.) If the circular conductor be
now made to revolve on an axis X Y,
fg. 124, parallel to C D, and if it be

Fig. 124.
A

• E

turned on this axis so that its plane is
inclined to that which passes through its
centre O, and the rectilinear current
C D, the directions of the forces at A
and B, being out of the plane of the
circle, may be decomposed each into a
force in that plane along the radius of

rotation, and into one at right angles to
it. The latter of these forces will tend
to produce rotation, and to bring back
the circle into its position of stable equi-
librium E e in the plane common to it
and to the current C D. Hence the
directive force, or that which tends to
bring the circle into this position, by
turning it on its axis, is composed of
the sum of the resultant forces acting
upon the portions on each side of the
axis. As the two sets of forces con-
spire to produce the same rotation, it
matters not, as to the ultimate effect,
whether or not the axis pass through
the centre of the circle, provided it be
parallel to C D, and either within the
circle or beyond it, because, in the latter
case, where there is an opposition of
rotatory forces, the force acting on B
being greater than that which acts on A,
and also acting at a mechanical advan-
tage, will always prevail.

(214.) Let us next suppose the axis X Y
of the circular conductor to be at right
angles to C D, as represented in Jig • 125.
The position of equilibrium will, under

these circumstances, be precisely the
same as the former : for any disturbance
from the situation of the circle in which
its plane includes the current C D, would
give rise, in the portions of the circle
nearest to it, to rotatory forces that tend
to bring it back to that plane, as may
be understood from what was explained
in § 21 0. These forces will be aided by
those that act on the lateral portions of
the circle, in which an attraction exists
towards those portions of the straight
current where the directions correspond,
as far as regards the approach to, or
recession from the nearest points of the
two conductors. The only forces op-
posing these are the forces acting upon
the remoter parts of the circle, which are
of course too weak to change the nature
of the effect resulting from the former.

ELECTRO-MAGNETISM.

71

(215.) Whatever be the action which
the circular current receives from the
rectilineal one, a similar and opposite
action is exerted by it on the latter, which
is urged to assume a position in the'plane
of the circle, and such that the adjacent
currents in each may be moving in
similar directions.

(216.) The action of a circular cur-
rent upon a rectilinear, but terminated
current, situated wholly on one side of
the plane of the circle, and inclined to it
at a given angle, requires especial notice.
If the direction of the straight Current,
when prolonged, pass near ihe centre of
the circle, the forces that act upon it
are nearly balanced, and neither action
nor reaction is perceptible. If it be
near the circumference, the action of the
adjacent portion of that circumference
will predominate, and effects, similar to
those taking place between a terminated
and an unlimited current, will be pro-
duced, with this modification, however,
that the unlimited current being circular,
the motion of translation in a straight
conductor at right angles to the plane of
the circle, following the course of the
circumference, becomes itself circular ;
and if the conductor be attached to an
axis perpendicular to the plane of the
circle, and passing through its centre,
that conductor will be made to revolve
continually around that axis, as shown in
fig. 1 2 6 , where C D is the circular current,
and A B the straight moveable, but ter-
minated current. This rotatory force

Fig. 126.

extends its influence beyond the interior
of the circle to any distance, provided
the straight current do not pass beyond
the plane of the circle. The reaction of
the straight current on the circular con-
ductor impels the latter to revolve in thu

contrary direction, as marked by the
dotted arrows parallel to it.

(217.) All these effects will be con-
siderably increased if a great number of
similar currents be moving in the same
direction in the different parts of the
circuit, described by the straight current
in the last paragraph. This may be
obtained by making currents traverse
a number of wires placed in the surface
of a cylinder, and parallel to its axis,
which is also that of the rotation ; or,
what will be equivalent to this arrange-
ment, by making a current pass along
the surface of a hollow conducting cylin-
der, in the direction of its length ; for in
that case, the current may be considered
as dividing itself into an infinite number
of parallel filaments.

(218.) A similar augmentation of
power may be obtained by multiplying
the circular currents, either by employ-
ing a wire coiled into the form of a ring,
or into that of a flat spiral. When these
rings or spirals are combined with the
cylinders above mentioned, the effects
are again proportionably increased.

(219.) The modes of exemplifying
these conclusions experimentally are
various. Thus, a wire, as shown in fig.
127, consisting of two vertical branches,
united above by a transverse arch, to

Fig. 127.

the centre of which is affixed a steel
point turning downwards, for the pur-
pose of suspension, may be united below
to a circular rim, which dips into a
shallow trough of mercury, so as to en-
able us to transmit currents through the
wire, that will move in both the vertical
wires in the same direction; that is,
either upwards or downwards in both.
If, while so suspended and connected, a
circular current be made to act upon
it from below, whether by means of a
single circle, as shown in trie same
figure, or by a spiral coil, as that of
fig. 128, the wire will revolve round its
axis of suspension, in a direction deter-

ELECTRO-MAGNETISM,
mined by the relative direction of the round on an axis, A X, be subjected to

current.

Fig. 128.

(220.) A hollow cylinder, fig. 129,
balanced in a similar manner as that
of fig. 123, may be substituted for the
wire; but its revolution will in general

Fig. 129.

not be so rapid as the wire, because,
although it may convey a more powerful
current, the weight to be moved is
also proportionably increased.

(221.) The rotation of the circular or
spiral conductor may in like manner be
exhibited by suspending either of them
from a point, in the axis of the circle, as in
fig. 130, and subjecting it to the action of

Fig. 130.

a terminated vertical current, that does
not extend to both sides of its plane.

(222.) The effect is the same at what-
ever angle the direction of the straight,
current is inclined to the plane of the
circular current, provided the axis on
which it revolves be perpendicular to
that plane and pass through its centre.
Thus, if the wire AB,/g-. 131,moveable

the action of a circular current C D, the
plane of which is wholly below it, it will
revolve, describing a conical surface.

Fig. 131.

(223.) Pursuing this investigation, it
becomes evident that a revolving mo-
tion will equally take place, if the
straight conductor be parallel to the
plane of the circle, provided it does not
exceed in length the radius of the circle.
Thus, the straight conductor A B, fig.
132, which is wholly within the circle
C D, revolves round the axis A, in the

Fig. 132.

same direction as the current in that
circle, when its own current is passing
from the circumference to the centre.
When its current passes from the centre
to the circumference, it will revolve in a
direction contrary to the motion of the
current in the circle. On the other
hand, if the straight current be fixed,
and the circle moveable round its centre,
the action of the former will cause the
latter to revolve in directions opposite
to those which have been just stated.

(224.) It may be observed that we
have limited this proposition to the cases
in which the current A B does not ex-
tend beyond the circumference of the
circle; for if it did, as seen in/g-. 133,
the exterior portion A B, being affected
in an opposite manner from the interior
portion B C, there would arise an op-
position among the rotatory forces ;
and the amount as well as direction of
the resulting motion would be regulated
by the difference between them. This

ELECTRO-MAGNETISM.

contrariety of effect will, on the other
hand, be removed, if the straight cur-

Fig. 133.

rent be wholly exterior to the circle, as

A B, fig. 134, though still moveable

round the same axis as before. The

revolution will now be performed in a

Fig. 134.

A

direction contrary to that of the interior
current in the former case. The reaction
of an exterior current on the circular
conductor will likewise be in the oppo-
site direction to what it was before.

(225.) It is obvious that every thing
that has been stated with regard to
straight currents, the direction of which
is towards or from the centre of the
circular current, applies also to a num-
ber of radiating or converging currents.

Hence if the cylinder,}?^. 129, com-
municating by its point with one of the
poles of a voltaic battery, have its lower
rim immersed in a flat dish containing
mercury, communicating by a wire from
its centre with the opposite pole of the
battery, radiating or converging cur-
rents will be established in the mercury,
which, acting on the vertical currents
existing in the sides of the cylinder, will
cause it to revolve. If the currents
tending to or from the cylinder be ex-

terior to it, which may be obtained by
surrounding the cylinder with a metallic
ring of larger diameter, and making this
ring the medium of communication with
the battery, the revolution of the cylin-
der will be made in the opposite direc-
tion to what it was before.

§ 7. Reciprocal Action of Circular
Currents.

(226.) The mutual actions exerted
between two circular currents, may
readily be collected from the application
of the general law of attraction among
those parts in which the directions of
the currents are similar, and of repul-
sion where they are dissimilar. If one
of the circles be fixed and the motion of
the other be limited to revolution round
an axis, the effects of their mutual
action will depend on the position of
the centre of the moveable circle with
regard to the plane of the fixed circle,
and also upon the position and inclina-
tion of the axis with relation to the line
joining both centres.

(227.) If the centre of the moveable
circle be in the same plane with the
fixed circle, or not far removed from it,
whatever be the inclination of the axis,
a directive force will arise, tending to
bring the whole of the circumference
into that plane, and to make it assume
such a position as that the currents in
the adjacent portions of the circle shall
be in the same directions. Thus C D,
(figs. 135 and 136,) being the fixed,

Fig. 135.

and A B the moveable circle, on the
axis X Y, the directive force will bring
the latter into the position E e, that is,
in the same plane with C D ; and this will
happen equally, whether the axis be at
right angles to the line joining their

74

ELECTRO-MAGNETISM.

centres, as in fig. 135, or coincide with
it as in fig. 136, or have any other
inclination to it.

228. If the centre of the moveable
circle be any where in a line drawn
through the centre of the former, and
perpendicular. to its plane, the moveable
circle will tend to arrange itself in a
plane parallel to the fixed circle, and
having its currents moving in a similar
direction. This is evident from fig. 137,
in which the same letters as before are
used to denote the corresponding points.

(229.) In both cases an approxi-
mative force takes place, whenever the
moveable circle has arrived at its po-
sition of equilibrium; which force, in
the latter case, is particularly strong,
inasmuch as the attraction of the cor-
responding parts of the circles is uniform
throughout the whole circumference.

(230.) For each position of the centre
of the moveable circle, intermediate to
those above described, there exists a
particular position of equilibrium, the
line of which, if prolonged, would inter-
sect the plane of the fixed circle at a
certain distance beyond it.

(231.) All these positions of equili-
brium are determinate, and exclude the
possibility of any continued rotatory or
revolving motions.

(232.) When an electric current, after
traversing a certain line of conducting
bodies, returns upon itself, so as to
arrive at the point from which it had
set out, or very near it, it has been de-
nominated a closed circuit. Such is
the case with the circles we have been
considering. One of the most import-
ant facts on which the theory of electro-
dynamics rests, is that the mutual action
of two closed circuits cannot produce,
in either of these circuits, a continued
rotatory motion in an invariable direc-
tion ; and, consequently, no assemblage
of closed circuits can ever be made to
produce such rotatory motion, in what-
ever manner they may be disposed.

(233.) Experiments on the mutual

actions of circular currents, either on
each other or on straight conductors,
are most advantageously made by means
of a flat spiral rendered astatic, by op-
posing to it a similar coil on the oppo-
site arm of the lever, from the middle of
which they are both suspended, as shown
in fig. 138, the spiral turns being in

138.

different directions in each, so that the
rotatory influence of the earth on the
one shall be exactly balanced by its in-
fluence on the other.

§ 8. Mutual Action of Heliacal and
Rectilinear Conductors.

(234.) We have seen that the action
of conducting wires rolled into the form
of a flat spiral is similar almost in
every respect to that of a simple circular
wire; but when coiled round the sur-
face of a cylinder, so as to constitute a
helix, its action becomes much more
complicated. When the extremities of
the wire, after completing the helix, are
made to return along the axis, as de-
scribed in § 105, and shown in fig. 71,
constituting what has been termed by
Ampere an electro-dynamic cylinder,
the whole may be considered as equiva-
lent in effect to a succession of circles,
whose planes are perpendicular to the
axis, and occupy the whole length of the
cylinder. In determining the forces
that are called into operation by such
an apparatus, we may, therefore, put out
of consideration the slight obliquity
which the turns of the spires have to
the axis, and the effect of which is
completely neutralized by the corre-
sponding portion of the wire that passes
along the axis ; and we may regard the
whole as composed of currents circu-
lating at right angles to the length of
the cylinder.

(235.) Since we have seen that the

ELECTRO-MAGNETISM.

influence of a single circular current
C D, Jig. 126, on a straight terminated
current A B, perpendicular to the plane
of the circle, is to induce in it a ten-
dency to revolve with a motion parallel
to itself, round the line drawn from the
centre of the circle perpendicularly to
its plane, it is evident that the addition
of similar circles, placed in succession
exactly below the first, (supposing the
axis vertical, as in the figure,) uill tend
to increase this force of revolution. The
effect of each additional circle, it is true,
is less than the preceding, not only be-
cause its distance is greater, but also
because its action is more oblique, and
because the difference between the ac-
tions of the nearer and more remote
portions of the circle continually di-
minishes as the angle between the
lines drawn from the several points in
the straight conductor, and the centres
of the respective circles, increases, which
is the case as they are further removed
from the extremity of the straight con-
ductor. From all these considerations,
the force by which the current A B,Jfg.
139, is urged to revolve round the axis,

Fig. 139.
AA

X, in consequence of the action of the
lowermost circle G H, is plainly less
than that exerted by the circle F /, and
still less than that exerted by C D.

(236.) On the other hand, if we add
in succession a number of circular cur-
rents above C D, they will conspire with
the lower circles in their effect of pro-
ducing: a tendency to revolution in the
straight conductor; but each will do
this only by its action on that part of
the conductor that is above its own
plane ; for its operation in any portion

situated below that plane is to produce
a revolution in the contrary direction.
So that the action exerted upon any
elementary portion of a vertical current,
at E, for example, is, as far as it depends
upon the circles above that which is
nearest to it, exactly balanced by an
equal number of circles below it ; that
is, the circles lying between C and e are
counterbalanced by those lying between
e and F, the whole of that portion of
the cylinder between C and F being
neutralized ; and the only portion that
is active being that which lies beyond
F, that is, between F and G. 'This
active part of the cylinder becomes
smaller in proportion as the element is
situated nearer to the plane which di-
vides the cylinder into two equal parts ;
and at this point the action is reduced
to nothing: on the contrary, it increases
as the element comes nearer to either
extremity of the cylinder, where it is the
greatest of all. These extremities may
accordingly be considered as the active
poles of the cylinder, round which the
revolution of the conductor is made:
the resultant of all the forces called into
action by every part of the cylinder has
the direction of the tangent of the circle
of revolution ; that is to say, is at right
angles to the line joining the straight
conductor and the pole.

(237.) It is hardly necessary to re-
mark that the action in this, as in
every other instance, is reciprocal be-
tween the straight conductor and elec-
tro-dynamic cylinder; so that if the
conductor be fixed and the cylinder
moveable, the latter will revolve round
the former ; or, if restricted to a motion
round its own axis, it will perform a
rotatory movement round that axis.

(238.) The same tendency to revolu-
tion about the poles of an electro-dynamic
cylinder, arising from a force of a tan-
gential kind, apparently emanating from
these poles, is observed to take "place,
whatever be the angle of inclination
between the straight conductor and the
axis of the cylinder. In order to ex-
plain this curious fact, the application
of which is of considerable importance
in a theoretical point of view, we must
avail ourselves of the principle enun-
ciated above (§ 198), namely, that the
electro-dynamic action of currents that
occupy in a similar manner two different
surfaces, subtending the same angular
extent, and lying in the same direction
with reference to any point, on an ele-
mentary portion of current situated at

ELECTRO-MAGNETISM

that point, are equal. It will also be
convenient to analyze the actions of
each circular current into those exerted
in planes at right angles to each other,
a mode of viewing them which, being
analogous to the artifice of the re-
solution of forces constantly resorted
to in dynamics, will make no dif-
ference in the results. Conceive, then,
that the currents, instead of moving
in the circumference of a circle, tra-
verse the four sides of a square, and
that the cylinder is represented by a
square prism. S N, fig, 140, is in-
tended to convey the idea of a prism,
so constituted, the surface of which is

Fig. 140.

occupied by electric currents circulating
round each of the laminae, into which it
may be divided by planes perpendicular
to its axis, the direction of the currents
being marked in the two sides which
come into view, by the arrows. On the
sides opposite to them, and which are
not seen, the currents are, of course,
moving in the contrary directions.

(239.) Let us now examine the effects
of currents in each side upon a straight
conductor, whose direction is at right
angles to the axis, and which is placed
in various positions with regard to the
prism.

Let P Q R S, fig. 141, be the upper
surface of this prism, its axis being
horizontal ; let W be the section of a
vertical conducting wire, of indefinite

length, perpendicular to the plane of the
figure ; and let the current be moving in
this wire in the same direction as those
currents which traverse the adjacent ver-
tical side of the prism, of which the upper
edge is P Q. We shall suppose, for exam-
ple, that the currents are ascending in the
wire, and also in the side adjacent to it,
whence they traverse the upper side
from P Q towards R S (as denoted by
the arrows), descending again on the
side opposite to W, and of which the

edge is R S, and returning on the lower
side in a direction from R S towards
PQ.

Since the currents are passing in op-
posite directions in the upper and lower
surfaces of the prism, their effects on W
(as far as any horizontal motion is con-
cerned) are completely neutralized ; and
we need, therefore, only examine the ac-
tions of the vertical surfaces PQ and RS.
Let RW and S W be the sections of two
vertical planes, drawn from W to R and
to S, cutting P Q in U and V respectively.
It follows from the proposition above
referred to that the actions of the cur-
rents in that portion of the surface P Q,
adjacent to W, which is included be-
tween U and V, are exactly balanced
by the currents in the whole of the sur-
face R S, opposite to it, and which run
in contrary directions. The resulting
action, therefore, will be determined
only by the currents in the remaining
portions of the surface P Q, situated
between P and U on the one side, and
between V and Q on the other, both of
which attract the current in W: the
former in the direction of W u, the lat-
ter in the direction W v. These two
forces combine in giving a resultant in
the direction W r, indicating an attrac-
tion towards the centre of the prism.

(240.) In proportion as the situation
of the vertical conductor is taken nearer
to either of the extremities of the prism,
such as Q S, for instance, the portion
P U of the side P Q, intercepted between
the plane RW and the extremity P,
increases in extent, while the portion
VQ diminishes. Consequently the forces
arising from the attractions of the former
portion are proportionally increased, and
those from the latter diminished, and the
resulting force gradually becomes more
inclined towards P.

(241.) When W is situated in the
plane of the side Q S, as in fig. 142, the
force arising from the currents adjoining

W

Fig. 142.

TJ>"

to Q vanishes entirely, and the action
upon the wire depends solely upon the
currents in the remoter division of the
side PQ, namely, that comprehended
between P and U. The resultant force
will therefore be directed towards these

ELECTRO-MAGNETISM.

77

currents, being nearly at right angles to
the line W Q?

(242.) When W is situated on the
other side of this line, as shown in fig.
143, the extent of the active portion of
the surface PQ has increased consi-
derably, for it now occupies the large
space P U ; but its power has not in-
creased in the same proportion, because
143.

its action is more oblique, as well as
more distant than it was before. The
resultant of this action is in the direc-
tion W u. It is combined, however,
with another set of forces, those arising
from the unco mpen sated portion V S, of
the surface R S, situated between S and
the vertical plane W Q V. The cur-
rents in this portion are moving in a
direction contrary to that in W : their
action upon it is therefore repulsive, and
the force thence arising may be repre-
sented by W v, which, combined with
W u, gives, as a final resultant, the force
Wr.

(243.) When W is placed in the pro-
longation of the axis of the prism, as in
fig. 144, it is attracted by the whole of
the currents in the side P Q, and re-
pelled by the whole of those in R S,

'Fig. 144.

the former giving rise to the force Wu,
the latter to the force W v, their result-
ant being W r.

(244.) When W is in the situation
represented in Jig. 145, the currents
situated between V and S are neutra-
lized by those between P and U. Those

Fig. 145.

between R and V repel W in the direc-
tion of W v, while those between U and
Q attract it in the direction W u, forces
which produce the resultant Wr.
(245.) When W is in the prolongation

of the line Q S,fig. 146, the currents
between P and Q being neutralized by

those between V and S, the only active
currents are those between R and V,
which being repulsive, the resultant is
in the direction W r.

(246.) When the situation of W is as
shown in fig. 14 7, the active portions of
the currents are' those occupying the
spaces RU and V S, which being both
repulsive, and acting according to the
directions Wu and Wv respectively,
join in producing the resultant W r.

Fig. 147.

(247.) Thus it appears that, combining
all the results of this induction, the con-
ducting wire is, in every situation, urged
by a force impelling it in the direction of
a tangent to the circumference of a cir-
cle, C, fig. 148, round the extremity of
the prism, P, which may therefore be
considered as having the functions of

Fig. 148.

a pole. The force thus arising pos-
sesses the same character of being rota-
tory and tangential as that which was
exerted on the same wire when its direc-
tion was parallel to the axis : and if it
possess this character in two directions
that are at right angles to each other, it
may fairly be inferred that the law is
general, and that it applies to all the
intermediate inclinations.

(248.) The explanation above given
will be sufficient to convey a general
idea of the application of the theory to

ELECTRO-MAGNETISM.

the phenomenon in question. But the
subject has been investigated with all the
rigour of mathematical analysis, and the
results determined with all the precision
that can be required for comparison
with actual experiment. We have pur-
posely omitted several of the minuter
details which were even compatible with
the popular view we have presented, but
which would have required more com-
plicated diagrams for their exposition,
and considerably lengthened the inquiry,
but which have no material influence on
the ultimate conclusions. Thus, if the
straight conductor, instead of being
indefinite in length, were terminated,
and wholly above the horizontal prism,
it would be found that there is no longer
that exact compensation between the
currents in the upper and lower surfaces ;
for when the conductor is immediately
above the prism, the upper currents
have a much more considerable influ-
ence on it than the lower currents, and
urge it on its revolution in the same
direction as that in which it was moving
from the effect of the other forces in
operation. At the remotest part of the
circle, the lower currents come more into
operation from their acting with less
obliquity than the upper currents, and
concur, in their turn, in augmenting the
tendency to revolution in the same di-
rection.

(249.) The following laws are ob-
tained as the results of the mathema;-
tical investigation of the subject: —

i. The action of a very slender electro-
dynamic cylinder upon an elementary
portion of a current may be resolved
into two forces, acting in directions per-
pendicular to the direction of the cur-
rent, and also respectively perpendicu-
lar to the lines drawn from it to each of
the extremities of the axis of the cylin-
der; each of these forces being inversely
as the squares of these distances.

ii. The action of an electro-dynamic
cylinder upon an indefinite conductor,
perpendicular to its axis, may, in like
manner, be reduced to two tangential
forces, as in the former case ; but these
forces are in the simple inverse ratio of
the distances from the extremities of the
cylinder.

iii. If the length of the electro-dynamic
cylinder be supposed to be indefinite, its
action upon an elementary portion of a
current will depend entirely upon the
relative positions of the element, and
that extremity of the cylinder to which
it is referred, and is influenced in no

respect by the relative position of the
axis of the cylinder.

iv. The action of this cylinder upon
conductors, of whatever form or magni-
tude, is subject to the same conditions,
being dependent solely upon the posi-
tion of that extremity, which is referred
to the conductor, and remains the same
whatever be the direction of the axis of
the cylinder *.

(250.) The conclusions thus deduced
from the evidence of observation, com-
bined with the deductions from theory,
indicate the strongest analogy, and
almost perfect identity, between the
agency of electro-dynamic cylinders and
that of magnets. The law of their ac-
tion upon an electric current, and of the
reaction of the latter upon botb, is pre-
cisely the same ; so much so, that if we
had the command of sufficiently power-
ful currents, the electro-dynamic cylin-
der might be substituted for the mag-
net in all the experiments we have de-
scribed in the last Chapter, and the
same results, whether of attraction, re-
pulsion, or revolution, would be obtained
from them. The two extremities of an
electro-dynamic cylinder exhibit all the
properties possessed by the poles of a
magnet : that end in which the current
of positive electricity is moving in a
direction similar to the movements of
the hands of a watch, acting as the
south pole of a common magnet ; and
the other end, in which the current is
moving in a contrary manner, manifest-
ing a northern polarity.

(251.) It will be readily anticipated,
from the known resemblance between
the action of electro-dynamical cylin-
ders and of magnets on electric currents,
that two such cylinders will act upon
each other precisely in the same way
as magnets do. Theory confirms the
exactness of this general conclusion ; for
the following is the law to which mathe-
matical examination conducts us, namely,
that the mutual action of two electro-
dynamic cylinders may always be repre-
sented by four forces, having the direc-
tions of lines drawn from each extremity
of the one to both extremities of the
other, and being to one another in the
reciprocal ratio of the squares of. these
lines, provided these distances be not
exceedingly small t.

* .Ampere, Recueil d' Observations Electro-dyna-
miques, p. 343. See also Demonferrand's Manuel
d'ElectriciteDynamique ; or Cumming's Translation,
pp. 60, 67, 138, and 140.

f Ampere, Recueil d'Observations Electro-dyna*
rniques, &c. p. 343.

ELECTRO-MAGNETISM.

C2:V2.) The mutual action of electro-
dynamic cylinders on magnets is the
same as that of two magnets on each
other, so that in any experiment the one
may he Substituted for the other without
affecting the nature of the result.

CHAPTER XII.
Theories of Electro- Magnetism.

§ \.Electro- Magnetic Theory of Oersted.
(253.) THE discovery of the remark-
ahle phenomena of electro-magnetism
naturally gave rise to the invention of
a variety of hypotheses for their expla-
nation. Adopting the theory which
ascribes the electric phenomena to the
agency of two fluids, composing by their
union a neutral fluid, and exhibiting
their peculiar powers when that union
is decomposed, and when they are ob-
tained separately, Professor Oersted
conceived that a distinct class of effects
resulted during the act of their reunion ;
which was marked, not only by mecha-
nical agitations among the particles of
bodies, by the production of sound, by
the evolution of light, and by the dis-
engagement of heat, but. also by the
disturbance of the magnetic equilibrium.
These phenomena seemed to indicate
the occurrence of great and sudden
changes taking place in the conditions
of two powerful agents at the moment
of their coalescence, and suggested to
Oersted the idea that something ana-
logous to a shock takes place when the
fluids rush together from a distance.
During galvanic action, the separation of
the two electric fluids, proceeding with-
out intermission in one part of the ap-
paratus, and their reunion being in like
manner effected in perpetual sequence
along the conducting bodies which com-
plete^ the circuit, he conceived that a
continued series of electric shocks took
place throughout the whole line of con-
ductors ; a condition which he expressed
by the term Electric Conflict.

(254.) If these views be correct, it
must follow that the electric fluids,
which, whether at rest or in motion,
have, when isolated, no apparent in-
fluence on magnetic bodies, acquire,
during their conflict, the power of af-
fecting these bodies. This hypothesis
was expressed by Oersted in the follow-
ing words : " The electric conflict acts
only on the magnetic particles of matter.
All non-magnetic bodies appear pene-
trable by the electric conflict, while

magnetic bodies, or rather their mag-
netic particles, resist the passage of this
conflict. Hence they can be moved by
the impetus of the contending powers.
It is sufficiently evident that the electric
conflict is not confined to the conductor,
but dispersed pretty widely in the cir-
cumjacent space.

" We may likewise collect that this
conflict performs circles ; for without
this condition, it seems impossible that
one part of the uniting wire, when placed
below the magnetic pole, should drive
it towards the east, and when placed
above it, towards the west — (see $ 13,
figs. 1 and 2) : for it is the nature of a
circle that the motions in opposite parts
should have an opposite direction. Be-
sides, a motion in circles, joined with a
progressive motion, according to the
length of the conductor, ought to form
a conchoidal or spiral line ; but this,
unless I am mistaken, contributes no-
thing to explain the phenomena hitherto
observed.

" All the effects of the north pole
are easily understood by supposing that
magnetic electricity moves in a spiral
line bent towards the right, and propels
the north pole, but does not act on the
south pole. The effects on the south
pole are explained in a similar manner,
if we ascribe to positive electricity a
contrary motion and power of acting
on the south pole, but not upon the
north*."

(255.) The views entertained by Oer-
sted were very generally adopted by
philosophers who prosecuted the path
of discovery he had laid open. It was
the prevailing belief that electricity in
motion had magnetic properties, or
rather that it imparted to the body that
conducted it a species of transverse
magnetism. Some conceived that the
action resembled that of a series of
magnets placed around the axis of the
conductor, at riirht angles to each other,
their poles being situated in four lines
parallel to the axis, and forming a
square, as represented in Jig. 149, which

Fig. 149.

i:

exhibits a section of the conducting
wire, and four magnets with their poles

* Annals of Philosophy, vol. x vi. p. 2/6,

80

ELECTRO-MAGNETISM.

marked n, s, 'respectively, succeeding
each other in a regular order of alter-
nation round the wire. But this hypo-
thesis cannot be a faithful representa-
tion of the phenomena ; for it is found
on experiment that the action of the
conducting wire upon a magnetized
needle is exactly the same in every part
of its circumference. If the wire be
vertical, for instance, its effect is the
same in all azimuths, and has no rela-
tion to any rectangular planes passing
through the axis of the wire.

(256.) With this correction, the hypo-
thesis that a conducting wire acts as if
a series of minute magnets were placed
in succession round its circumference,
with their opposite poles facing each
other, will account for a large class of
phenomena. It explains why a com-
pass needle assumes its peculiar position
at right angles to the axis of the wire,
in obedience to the directive influence of
that particular portion of the imaginary
series of magnets which is the nearest to
the needle, ^see Jig. 150; and also the
mutual attraction between the needle

and the wire under these circumstances.
It also explains the other fundamental
fact in the science ; namely, the mutual
actions exerted between two conducting
wires : for when the currents are passing
in the same direction in both the wires
as in A and B, fig. 151, the polarities

Fig. 151.

*^A>** J/ c^*9*

*2i \f* \S

® J L • I

j\ S*

of the minute magnets on the sides adja-
cent to one another will be reversed, and
they will consequently attract one an-
other. The contrary will happen when
the currents are passing in opposite
directions in the two wires, as in A and
C ; for then the polarities of the mag-
nets belonging to each, which are ad-
joining to each other, are the same in
kind, and, therefore, repulsive of each
other.

(257.) But there is still one class of
phenomena which the hypothesis we
are considering is totally inadequate to
explain ; that comprising the rotatory
movements either of magnets or of con-
ductors, and which movements may be
maintained with uniform velocity not-
withstanding the retardation from fric-
tion, or the impediments of a resisting
medium ; exhibiting, in fact, the extra-
ordinary spectacle of a really perpetual
motion. The supposition of a series of
magnets encircling the conducting wire
will not account for this continued mo-
tion; for it is certain that no actual
combination of magnets, nor even any
conceivable arrangement of magnetic
particles, could ever, consistently with
the known laws of magnetic action,
produce any approach to perpetual ro-
tation. In order to obtain such move-
ments, the agent from which the force
emanates must itself be in motion, and
must revolve round the axis of the wire,
while traversing it from end to end, with
the utmost rapidity. Such was the pe-
culiar kind of movement, partly longitu-
dinal, and partly circular, which Dr.Wol-
laston attributed to the electro -magnetic
agent, and which he termed its verti-
ginous motion.

(258.) A further emendation must,
therefore, be made in the hypothesis in
order to adapt it to the phenomena, by
supposing that the two magnetic fluids,
which accompany the electric fluids,
when the latter are set in motion, and in
a state of conflict, (if we choose to adopt
the phraseology of Oersted,) acquire a
vertiginous motion in opposite direc-
tions transversely to the axis of the con-
ductor ; that is, the boreal fluid revolving
in one direction, and the austral fluid
in the other; these determinations being
given to them by the direction in which
the electric fluids are moving in the
conductor, dependent, of course, upon
the relative positions of the poles of the
voltaic apparatus from which they pro-
ceed. There will result from this pecu-
liar kind of movement, not only all the
effects that we have just seen to be the
consequence of quiescent circles of
magnets, but also those of a rotatory
nature, which nothing but an agent in
motion could produce. The tangential
action of a conductor upon a magnet is
a necessary consequence of the trans-
verse motions of the magnetic fluids in
the conductor; and the rotation of a
magnetic pole round that conductor, or
conversely, the revolutions of the con-

ELECTRO-MAGNETISM.

81

ducfor round a magnet, are phenomena
also naturally resulting from the verti-
ginous circulation of the two fluids.

(239.) The mutual attractions and re-
pulsions of parallel conductors, are at
once referred, as in the former case, to
the action of parallel magnets having
their poles in the same or in opposite
directions. If, for example, the electro-
magnetic current be moving in the same
direction in two parallel conducting
wires, the stream of austral magnetic
fluid belonging to one wire will be flow-
ing in the same direction as the boreal
magnetic fluid belonging to the other
wire, in that part which is adjacent to
it ; and, on the other hand, the direction
of the boreal fluid of the former will co-
incide with that of the austral fluid of
the latter wire, in the adjacent part.
According to the known laws of mag-
netic action, attraction must be the
result of such a state of things ; for the
boreal and austral fluids attract one an-
other. If W and w, Jig. 152, represent
sections of the conducting wires in both
of which the current of positive elec-
tricity is descending, the arrows in the
circumference of the outer dotted circles
Fig. 152.

A A, will point out the directions in
which the austral magnetic fluids cir-
culate on the surface of the wires ; and
those on the inner circles B B, the di-
rections in which the boreal fluids cir-
culate, and it will be seen that in the
parts p and q, when they are nearest to
each other, the austral fluid in the one
is moving in the same direction as the
boreal fluid in the other, and we may,
therefore, expect that they will attract
each other.

(260.) If the electric currents be

moving in contrary directions in the

two wires, as represented in a similar

manner \nfig. 153, opposite effects will

Fig. 153.

result ; for in that case both the streams
of austral fluid are moving in the same
direction in the adjacent parts of the
wires, and must consequently repel one
another ; and the same thing happens
with regard to the streams of boreal
fluid, which flow in the contrary direc-
tion to those of the austral fluid.

(261.) Such, then, is the hypothesis
that has been, after proper emendations,
made to correspond with the pheno-
mena, and which may be assumed as a
correct representation of them. It must,
at the same time, be admitted to be an
exceedingly strained and artificial hy-
pothesis, at variance with the analogy
of all other physical forces, and repug-
nant to our ideas of that simplicity
which seems to pervade all the opera-
tions of the material world. All other
known accelerating forces, emanating
from a certain point, and exerted upon
another point, act in the direction of the
line joining these two points. Such is
the case with the electric and with the
magnetic actions, in all the cases that
belong exclusively to the one or the
other of these two classes of phenomena.
When two conducting wires, bent into
helices, act upon one another, which
they do in a manner that imitates
very exactly the mutual action of two
magnets, the action is purely electri-
cal, and is exerted in the lines of di-
rection that join the acting points.
The same is the case with two magnets,
when magnetism alone is concerned.
But when a helix and a magnet act
upon one another, and present the very
same phenomena as in either of the pre-
ceding cases, the theory assigns a tan-
gential direction to the forces then called
into operation. That a mode of action
which is simple and intelligible in the
case of actions either purely electric or
purely magnetic, should be so suddenly
and so completely changed when the
electric and magnetic fluids act mutu-
ally upon one another, would be a
strange and scarcely conceivable ano-
maly in physical science.

(262.) We may avoid all these diffi-
culties by adopting the theory of electro-
magnetism devised by the genius of
Ampere, and ably supported by his
mathematical, in conjunction with his
experimental researches. Of this theory
we shall proceed to give an account.

§ 2. Electro-Dynamic Theory of
Ampere,

(263.) The phenomena relating to the
G

ElECTRO-MAGNETisM.

science of electro-magnetism may, as we
have seen, be reduced to three classes,
or general facts : the first being the evo-
lution of a tangential and rotatory force
usually exerted between a conducting
body and a magnet ; the second, the
transverse induction of magnetism by
the former in such bodies as are suscep-
tible of receiving it ; and the third, the
attractive or repulsive force exerted be-
tween two electric currents traversing
different conductors. In the magnetic
theory already discussed, the first of
these is considered as being the most
general fact, and the other two as being
merely its consequences. Ampere, on
the contrary, assumes the last of these
facts— that is, the mutual attractions and
repulsions of electric currents, as the

Enmary or fundamental fact, to which,
y the help of a particular hypothesis
as to the constitution of magnets, all
the other facts, not only of electro-mag-
netism, but of magnetism also, are
reducible.

(264.) His supposition is, that all
bodies that possess magnetic properties,
the globe of the earth being included
among the number, derive those proper-
ties from currents of electricity continu-
ally circulating among the parts of which
they are composed, and having, with re-
lation to the axes of these bodies, one
uniform direction of revolution, in planes
perpendicular to those axes.

(265.) The striking resemblance which
exists between the action of magnets,
and that of electro-dynamic cylinders
already described, and which extends
through a wide range of phenomena,
very naturally suggested the hypothesis
on which this theory is founded ; for
since the circular currents in the latter
are observed to produce effects similar
to magnetic polarity, it is but an exten-
sion of the analogy to consider a magnet
as deriving its properties from similar
currents continually circulating in its
substance.

(266.) In the account we have given
of magnetism it will be seen that the
phenomena attending the fracture of a
magnet oblige us to consider magnet-
ized iron as an aggregate of small par-
ticles of iron, each of which has the pro-
perties of a separate magnet (see MAG-
NETISM, § 141). In like manner, the
hypothesis just stated, relative to the
circulation of electric currents in the
substance of a magnet, must receive a
similar modification to that given to the
theory of magnetismt • Since the frag-

ments detached from a magnetic bar are
themselves complete magnets, the elec-
tric currents, from which it derives its
properties, must be conceived as circu-
lating round each of these fragments
separately, or rather round particles
smaller than any that can be obtained
by mechanical division. Each particle,
or magnetic element, may be regarded
as constituting a voltaic circuit, analo-
gous to a voltaic pile of which the two
ends are united by conductors ; the
vitreous and the resinous electric fluids
being separated at one point of the cir-
cuit, circulating in contrary directions
round the particle, until they meet toge-
ther, and by their reunion again forming
the neutral fluid. The course of the
fluids during this circulation is repre-
sented in fig. 154 ; V and R denoting
respectively the paths of the vitreous
and resinous electricities emanating from
the point E in the particle of iron P,
and flowing in the directions indicated
Fig. 154.
E

by the arrows, till they meet and coa-
lesce on the opposite side. But as the
effects of the one fluid are exactly the
reverse of those of the other, the result
is equivalent to the continued circula-
tion of one of the fluids, the vitreous,
for example, in one constant direction,
EV PR.

(267.) A magnet, then, is to be con-
sidered as composed of an assemblage of
parallel filaments, each of which is con-
stituted by a series of particles, round
which electric currents are circulating in
the manner just described, all of them
flowing in the same direction with re-
ference to the axis of the filament, and
moving in planes perpendicular to that
axis. That extremity of the filament in
which, when uppermost, the current of
positive electricity is moving in a direc-
tion similar to that of the hands of a
watch (the dial of which is also upper-
most), has the properties of a south
magnetic pole, and vice versa. If the
filament be placed horizontally, its north
pole pointing to the north, then the cur-
rents on the western side are ascending,
pass from west to east in the upper sur-
face, descend on the eastern side, and
return from east IQ west in the lower

!

ELECTRO-MAGNETISM.

part. This is shown in fig. 155, which
represents one of these elementary mag-
netic filaments, the eastern side being
presented to the spectator.

Fig. 155.

(268.) These currents which exist in
each particle of a magnet, may there-
fore be considered as constituting closed
circuits (see $ 232), the effects of which
on all bodies exterior to the circuit will
depend on the difference between the ac-
tions of the nearest and the most remote
parts of the circle described by the cur-
rent. The united effects of a great num-
ber of these circular currents will almost
entirely depend on these parts of the
current which occupy the exterior sur-
face of the mass.

(269.) Thus, supposing the magnet
to be cylindrical in its shape, and its
section shown in fig. 156, to consist of
the sections of each of its component fila-
ments a, b, c, d, &c., and round each of

Fig. 156.

which electric currents are circulating in
the directions indicated by the arrows, it
is evident that the currents of all the
interior parts will nearly, if not exactly,
compensate one another, and that their
action will be neutralized. But the cur-
rents that pass near the circumference
are differently circumstanced, inasmuch
as they are not compensated by any
others ; and their action is, therefore,
fully exerted on the bodies that are near
them, and is equivalent to that of a
single circular current flowing uniformly
round the circumference, p, p, p, of a
circumscribing circle, in the same direc-
tion. Hence, in estimating the effects
of the whole assemblage, we may con-
fine our attention to that of a superficial
current.

(270.) It is obvious that in order to in-

stitute an exact comparison between the
action of a magnet, and that of an artifi-
cial assemblage of electric currents simi-
lar to that which is supposed by the
theory to exist in the magnet, our imi-
tation must be made by collecting toge-
ther a great number of similar helices,
in parallel directions, and uniting them
in one mass» Such an arrangement is
called by Ampere an Electro-dynamic
Solenoid *.

(271.) The tendencies which a magnet
and conducting wire have to place them-
selves in positions at right angles to
one another, was deduced from the elec-
tro-magnetic theory as a consequence
of the supposed transverse situation of
magnetic fluids resulting from the elec-
tric conflict — that is, accompanying the
movements of the electric fluids. In
Ampere's theory the trans-verse direc-
tion of the action is ascribed to the
transverse movement* of the electric
currents in the magnet itself, which act
upon the current in the conductor, and
are also acted upon by that current, and
tend constantly to establish a parallelism
between them. Thus, since the currents
in the magnet N S, Jig. 157, move in
planes perpendicular to the axis of the
magnet, their action, being in those
planes, is transverse to the axis, and
tends to bring a straight conduct ing-

Fig. 157.

wire, PQ, into the transverse position
represented in the figure, in which the
direction of the current of the conduc-
tor is parallel to that of the current in
the nearest part of the magnet. On the
other hand, if the wire be fixed, and the
magnet moveable, the forces will tend
to bring the plane of that current, which
occupies the middle of the magnet, into
such a position as may include the
straight conductor ; and as the axis of
the magnet is perpendicular" to that
plane, so also must it be at right angles

* Tbcorie des Phenouilnes Electro-dynaimciuesj
p. 95,

84

ELECTRO-MAGNETISM.

to the wire which acts upon it. When
the magnet and wire have attained this
relative position, it is evident that, since
the adjacent currents move in the same
direction in both, an attraction will take
place between them. All this, as we
have seen, is in perfect accordance with
the observed phenomena.

(272.) It is unnecessary to pursue
the application of this theory to the
endless variety of cases of the mutual
actions of magnets and conducting
bodies, because, having already fully
gone into the details of the explanation
which is afforded of these facts by the
principle of a tangential force ema-
nating from both these agents, it will
necessarily follow that they are all
equally explicable on the electro-dyna-
mic theory, if it be once proved that
the basis of the former theory, namely
the tangential force, is itself a direct
consequence of the latter. Now this
has already been established experi-
mentally by the phenomena exhibited
by the helices and electro- dynamic cy-
linders described in a former Chapter,
§ 107, and the same has also been de-
duced from theory, according to what
was stated in § 249. It has been shown
that the same tangential force results
from the heliacal disposition of the cur-
rent, whatever be the position of the
axis of the helix relatively to the con-
ductor on which it acts. We are war-
ranted, therefore, in transferring this
conclusion to the action of the circular
currents assumed as existing in mag-
nets, and as being the sole source of
their activity.

(273.) Guided by these principles,
we find no difficulty in explaining the
phenomena of revolving motions so
frequently resulting from the mutual
actions of magnets and conducting
wires ; and which take place in exactly
the same manner when helices or elec-
tro-dynamic cylinders are substituted
for the magnets. It is instructive, how-
ever, to examine the particular cases
we have already given in exemplification
of the rotatory tendency arising from a
tangential force, by applying to them
the more general principles of electro-
dynamic action. In many instances it
will be found that the rotatory motions,
although in part produced by the action
of the currents in the magnet upon the
current in the straight wire, are also in
a still greater extentf dependant on the
influence of those portions of the cur-
rent that traverse the mercury into

which the conducting wires or the mag-
nets are immersed.

» (274.) This is exemplified, in the fol-
lowing arrangement represented in Jig.
158, where the bent wire, proceeding

Fig. 158.

from the positive cup P, terminates in a
steel point that is made to dip into the
surface of a quantity of mercury con-
tained in the vessel A B, in the centre
of which a magnet, M, is kept floating
in a perpendicular position by being
loaded with a weight of platina at the
lower end. A ring of copper is placed
on the surface of the mercury, from the
side of which proceeds a wire, which
terminates in the cup N. The electric
current, in passing from the steel point
to the ring of copper, traverses the mer-
cury, radiating from that point as from
a centre, and consequently giving a
revolving tendency to the currents in
the magnet below them. The magnet,
under these circumstances, revolves on
its axis. A similar effect, but in a con-
trary direction, takes place when the
course of the electric stream is reversed,
and is made to traverse the mercury
from the copper ring towards the steel
point, producing converging instead of
diverging currents. The explanation of
these phenomena is obvious, from what
has been said in § 203, 204. For let M
in fig. 159, be one of the currents at the

upper end of the magnet, and C D one
of the diverging currents ; the action of
the portion E D will be to produce a
revolving motion of the magnet in the

ELECTRO-MAGNETISM.

85

direction m E M, because the current
in m is attracted, and that in M -repelled
by the current in E D.

(275.) If the point P were inserted,
not, as before, in the centre of the fluid
above the magnet, but to one side of it,
the action of the currents would be
more complicated, some being attrac-
tive and others repulsive, according to
their situations relative to the magnet.
The resultant force will be one at right
angles to the line joining the centre of
radiation with the axis of the magnet,
and the effect of this force will be a
motion of translation of the whole mag-
net ; that is, of revolution round a line
parallel to its axis, and exterior to its
surface.

(276.) The presence of transverse
currents in every part of the surfaces of
magnets is well illustrated by their con-
joined influence, when a number of
magnets are placed horizontally, as in
jig. 160, like the spokes of a wheel, with
their similar poles turned towards the

Fig. 160.

centre C. In this situation all the cur-
rents on the upper sides of the magnets
are passing in the same direction with
reference to the circumference of circles
described from the centre C. They will
therefore produce continued rotation in
a vertical conductor, whose axis passes
through that centre, but is terminated —
that is, does not extend beyond that
side of the plane in which the magnets
are situated.

(277.) The theory of Ampere would
lead to the conclusion that no mecha-
nical arrangement of the parts of an
electro-magnetic apparatus can give
rise to rotatory movements, unless fluid
conductors form some part of the voltaic
circuit ; and accordingly no attempt to
obtain practically such movements has
ever been successful.

(278.) It is, accordingly, impossible

to obtain the revolution of a magnet
round its own axis, either by the action
of other magnets, or by that of an
electric current, which traverses neither
the magnet, nor a body thatjis so fixed
to it as to move along with it. This is
a direct consequence of the law derived
from the electro-dynamic theory, that
the mutual action of two closed circuits
cannot produce in one of these circuits
a continued rotatory motion in one con-
stant direction ; for it is evident that if
this be true with regard to two single
currents, it must also be true with re-
gard to any assemblages of such closed
currents, in whatever way they may be
arranged. The utmost that can result
is a tendency in one of them, if move-
able, to assume a fixed position of equi-
librium ; if, therefore, the system be so
constituted that it can only revolve
round an axis, about which the circuits
composing it are symmetrically ar-
ranged, it will acquire no motion what-
ever by the action either of a single
closed circuit, or of an assemblage of
such circuits. A magnet susceptible of
no other motion than rotation round its
axis is in this condition ; and hence, if it
derive its magnetic properties from elec-
trical currents, it must be impossible to
produce in it such a rotation by the
action of other magnets.

(279.) On the other hand, a detached
portion of a voltaic circuit moveable on
an axis that coincides with that of a mag-
net may be set in motion, and made to
revolve by the action of the closed cur-
rents, in the magnet itself. Thus let
V v, Jig. 161, represent a section of a
voltaic pile, with its positive and nega-
Fig. 161.

tive wires, W, w, proceeding from its
two poles, and inserted into the cups P
and N respectively ; the former being
placed at the top of an arch of wire, of
which the two branches descend on each
side, and terminate under the surface of

ELECTRO-MAGNETISM.

a quantity of mercury, contained in the
vessel, also seen in section ; and the
latter being at the end of a wire pro-
ceeding from the lower part of the
vessel, and in contact with the mercury ;
while a magnet is made to float in an
upright position in the axis of the vessel,
by a weight of platina fixed to its lower
end. The magnet, it will be seen, being
unconnected with the wires, forms no
part of the voltaic circuit, and remains
unmoved by it; yet it excites move-
ments in the conductors which surround
its upper portion. For since, in the
parts C and D, the currents of the wires
are approaching those of the magnet,
they will be impelled (see § 200) to turn
round it in a direction opposite to that
of the currents in the magnet : a reac-
tion is at the same time exerted by the
currents of the wires upon those of the
magnets, which therefore tend to move
the magnet progressivety, or in the
same direction as its own currents.
($ 203.) But the currents which pass
from the lower ends of the wires through
the mercury to the exterior of the ves-
sel, recede from the magnet, and tend
to impress on the mercury a motion of
revolution in the direction of the mag-
netic currents; and, consequently, by
the reaction of this force the magnet
receives a tendency to revolve in the op-
posite direction. The two forces result-
ing from these contrary tendencies of
the descending and the receding cur-
rents, oppose and partly destroy each
other, as far as regards their effect on
the magnet; and when the rotatory effects
of the whole of the remaining part of
the current composing the whole cir-
cuit, and including that of the pile itself,
and its two wires, W and w, are taken
into account, the compensation becomes
complete, and the total effect reduced to
nothing. Hence we see that, although
the wires are made to revolve in one
direction and the mercury in another,
the magnet itself, being acted upon by
equal and contrary rotatory forces, and
unattached to any part of the circuit,
remains perfectly unmoved.

(280.) But the case is altered if the
magnet be so connected with the appa-
ratus of the wires as to form a part of
the circuit, even for a portion only of
the current; for that portion of the
current which thus passes through the
magnet no longer exerts upon it any
rotatory tendency, and may, therefore,
be considered as suppressed : and since
the action of this portion exactly coun-

terbalanced the equal and opposite action
of the remainder of the circuit, that
equilibrium can no longer subsist, when
this portion is removed, and the remain-
der of the current becoming effective,
will produce a rotation of the magnet on
its own axis. The direction of this rota-
tion will be the same as that of the
descending wires; hence the magnet
may be connected with these wires,
without altering the nature of the action,
as in the experiment of Mr. Faraday,
described in § 76.

(281.) It follows, also, from the prin-
ciples of Ampere's theory, that when
the moveable portion of the circuit
which is attached to the magnet has
both its extremities in the axis, no mo-
tion of this kind will take place ; because
no action can result between a system
of closed currents and another current
terminating at both extremities in the
axis of the system.

» (282.) The theory of Ampere implies
a perfect identity in the mode of action
of a magnet and of an electro-dynamic
cylinder. A remarkable difference,
however, has been observed between
them. In the electro- dynamic cylinder,
the poles are situated at the very extre-
mities of the cylinder ; whereas, in ordi-
nary magnets", they are always found
to be nearer to the centre than the
ends ; the distance varying in different
magnets. This circumstance was long
considered as invalidating the truth of
the theory *. It may, however, be ex-
plained consistently with the hypothesis,
in two ways ; either by supposing that
the intensities of the currents gradually
diminish from the middle to the extre-
mities; or else by assuming that they
acquire a degree of obliquity when at
a distance from the centre of the magnet ;
that is, that they move in planes which
are not exactly perpendicular to the
axis of the magnet, but differently in-
clined in different parts. These effects
are, indeed, not only quite consistent
with Ampere's hypothesis, but follow as
the natural consequences of the esta-
blished laws of electro- dynamic action t-
These positions of the different cur-
Tents, according to their positions rela-
tive to the axis, will be best understood
h-Qmftg. 162, which represents a longi-
tudinal section of a magnet by a plane
passing through 1he axis ; the directions
of the currents being marked by short

* This was urged as an objection by Mr. Faraday,
in the Quaiterly Journal of iiciencp, vol. xii. p. 76.
f Ampere, Recueil, &c. pp. 257 and 310.

ELECTRO-MAGNETISM.

87

arrows. 'The elementary currents of
those particles of the magnet which are
situated in the axis, that is, along the
line X Y, will, of course, on account of
the symmetry of the figure, move in
planes perpendicular to the axis ; as
also those in the medial plane M m,

Fig. 162.

passing through the centre of the mag-
net. But with regard to the currents
nearer to the surface, they will, by the
action of the interior currents, be turned
towards the middle of the magnet,
while those parts of the same currents
that are nearest to the axis will be
repelled from the middle towards the
adjacent extremity ; and the planes of
their inclination will therefore be more
or less inclined to the axis, as they are
more or less remote either from the
axis, and from the middle of the magnet,
in the manner represented by the ar-
rows in the figure. The total amount
of inclination in the lateral currents will
be greater in proportion to the intensity
of the action of the interior currents,
and also in proportion to their number ;
it will, therefore, be greater in propor-
tion as the thickness of the magnet is
greater compared with its length. We
may conceive this relative thickness to
be so excessive as that the forces tend-
ing to produce this inclination of the
currents will at length overcome the
coercive force, and prevent the deve-
lopment of magnetism. This consider-
ation will easily explain the difficulty
that is experienced in magnetizing a bai-
rn such a manner as that the poles may
be in the direction of the shorter dia-
meter*; a remark which leads us to the
subject of the induction of magnetism.
Let us examine with what success the
hypothesis of Ampere may be applied
to this class of phenomena.

(283.) We have already seen that an
electric current communicates magnetic
properties by induction to such bodies
in the vicinity as are susceptible of
acquiring them. If these properties are
owing to electric currents circulating in

* See Cumming's Translation of Demonferrand,
Manual of Electro-Dynamics, p. 167 to 1JO, and also

p. 250.

the particles of the magnetized body, or
in the magnetic elements, as they have
been called, (see Magnetism, § 154,)
there are two suppositions, either of
which will account for this phenomenon.
The first is, that electric currents, which
did not before exist, are produced, or
called into action, by the influence of
another current in the vicinity. The
second hypothesis is, that the electric
currents pre-exist in all the particles of
iron, or other bodies susceptible of mag-
netism previous to their acquiring this
property, but without having any uni-
form direction ; under these circum-
stances, their actions upon any external
body counteract and balance one an-
other, so as to constitute the neutral
state. When, on the other hand, they
are under the influence of an external
electric current of sufficient power, they
are all turned by it towards the same
quarter, and assume a common direc-
tion ; they will now co-operate in their
action upon external bodies, and exhi-
bit magnetic properties. This change is
analogous to what takes place in the
rays of ordinary light when, from being
polarized in all possible directions, they
become suddenly polarized in one parti-
cular direction.

(284.) It is implied in the first of
these hypotheses, that every electric cur-
"rent tends to produce currents in a simi-
lar direction in other bodies. Ampere
has proved, by the following curious
experiment, that a powerful voltaic cur-
rent possesses this power of exciting
currents in neighbouring bodies that are
not generally considered as susceptible
of magnetism. A copper wire of consi-
derable length, covered with silk thread,
was rolled round a cylinder, so as to
form a coil of some thickness. Within
this coil, placed in a vertical position, a
copper ring of smaller diameter was sus-
pended by a fine silk thread, passing
through a small glass tube, which was
thrust between the threads of the cop-
per coil. The circumference of the ring
was thus brought, in every part, very
near to the conducting coil, through
which a very powerful voltaic current
was sent. When a magnet was pre-
sented to the ring, under these circum-
stances, the latter was attracted or re-
pelled in the same manner as if it had
formed part of the same circuit as the
coil. Hence it was inferred, that an
electric current tends to induce in con-
ductors, placed in its immediate vicinity,
currents that move in similar directions.

88

ELECTRO-MAGNETISM.

This tendency, indeed, is but feeble;
and the first endeavours of Ampere to
discover it failed, in consequence of his
employing inadequate means ; but, on
repeating the experiment with more
powerful batteries and magnets, he per-
fectly succeeded in rendering the action
sensible.

It were much to be desired that this
important experiment, upon the accu-
racy of which so much is made to
depend in accounting for magnetic
induction in Ampere's theory, were
carefully repeated, and with every pos-
sible variation in its circumstances, so
as to determine whether the effect which
he observed is uniformly sustained, is
invariably connected with its supposed
cause, and is always proportioned to it ;
or whether it be not dependent upon
some particular conditions in the current
with relation to its tension, velocity, or
intensity, or upon some temporary va-
riation taking place in these conditions.
In the particular form in which the ex-
periment has been tried, it seems scarcely
to warrant the very general conclusion
which Ampfere has deduced from it.

(285.) Even if we admit it to be esta-
blished as a general fact, that electric
currents, circulating in one body, are
attended by similarly directed currents
in neighbouring conductors, we are
still not in a condition to decide the ques-
tion, whether, in imparting magnetism
to metals, there is an, actual production
of electric currents, or simply a change
effected in the directions of currents
previously existing in their particles.
There is, however, no inconsistency in
the supposition that the effect may be
due to both these causes ; for the action
of an electric current may consist in
giving a common direction to pre-exist-
ing currents, while it, at the same time,
augments their intensity.

(286.) It is unnecessary to enter into
long details as to the modes in which,
according to the theory we are consider-
ing, an electric current, passing through
conductors of different forms, whether
straight, or bent into spirals, or helices,
or a magnet, in which currents are sup-
posed to circulate, induce magnetic po-
larity in the adjacent parts of pieces of
iron or steel brought within the sphere
of their influence, when we regard that
polarity as consisting in the establish-
ment of circular currents of the same
description as those of the inducing
magnet. It will be sufficient to show
ihat the fundamental fact, namely, that

either pole of a magnet tends to induce
the opposite polarity in the adjacent end
of a magnetizable body in its vicinity, is
the direct and necessary consequence of
the hypothesis. That this is the case
will readily appear from considering that
when the elementary magnetic filament
A B,/g-. 163, is brought near to a simi-
lar elementary filament, C D, in a neutral

Fig. 163.

B C D

state, the currents which circulate in the
former will excite in the latter a circu-
lation of currents in the same direction,
thereby rendering it magnetic. But since,
according to the theory ($ 267), the kind
of polarity manifested at either end of
a magnet depends altogether upon the
direction of the currents with respect to
the axis at the extremity, it is evident
that if the current at the end B revolves,
as seen by a spectator looking at that
end, in the direction of the hands of a
watch on the dial, constituting the
southern polarity, the current induced
at the end of the other piece, C, revolv-
ing in the same direction in space, will
appear to a spectator looking at that
end to move in the contrary direction ;
it will therefore have a northern polarity,
that is, one contrary to that of the ad-
jacent end B, but similar to that of the
remote end A. In like manner the
polarity of D, if the inductive influence
extend to that distance, will be the same
as that of B ; for the circumstances at-
tending the revolution of its current are
precisely the same in both.

(287.) When, on the other hand, the
neutral bar is placed near and parallel
to the inducing magnet, the action of
the currents on the adjacent side of the
latter will prevail over that of the cur-
rents on the remote side, on account of
their greater proximity to the bar, and
induce in its adjacent side currents
running in the same direction ; but these
two sets of currents, being situated in
different sides of their respective axes,
will constitute magnetic currents in con-
trary directions, and, therefore, of oppo-
site properties. Hence the poles of the
induced magnet are reversed with re-
lation to those of the inducing magnet.
This will readily appear from an inspec-
tion of Jig. 164. The same opposition
of direction takes place when two paral-
lel rollers turn upon one another, in con-

ELECTRO-MAGNETISM.

89

sequence of the parts in contact moving
in the same direction.

Fig. 164.

(288.) After the removal of the cur-
rent which originally determined them,
these induced currents continue to cir-
culate with more or less permanence,
according to the degree of coercive force
inherent in the body. In soft iron they
disappear almost immediately : in steel
they continue to maintain themselves,
and constitute permanent magnets. The
action of heat is either to weaken or
destroy the currents altogether, or else
to derange the uniformity of their direc-
tion, so that they cease to act in con-
cert, and the steel reverts to its neutral
state. It is found, in conformity with the
theory of Ampere, that all the effects of
magnetic induction are produced equally
well by electric currents circulating
through spiral or heliacal conductors, as
by artificial magnets.

(289.) The theory of Ampere fur-
nishes a key to the explanation of a
variety of facts attending the conversion
of steel bars into magnets by the ordi-
nary processes of magnetization, which
are not intelligible on any other hypothe-
sis. It accounts for the peculiar circum-
stances already noticed in the Treatise
on Magnetism, regarding the relative
advantages of the single or double touch,
according to the inclinations given to
the magnet when applied to the bar to
be magnetized ; and it more especially
explains the frequent occurrence of con-
secutive points when certain methods
are employed. Thus, let one of the
poles of a magnet, the north, for in-
stance, be placed on the middle of a
steel bar, at right angles to it : see fig.
165. The form of the steel bar will, as
Fig. 165.

transverse to its length ; and the cur-
rents in the magnet running in this di-
rection are those situated on the oppo-
site sides of the magnet, supposed to be
divided by the dotted line perpendicular
to the length of the steel bar. But these
portions of currents are themselves
moving in contrary directions ; the cur-
rents they respectively induce in the
parts of the bar which they touch, and
in the neighbouring parts, must there-
fore, in like manner, have opposite di-
rections, giving rise to opposite polari-
ties. Thus the two ends of the bar will
be converted into north poles, while the
point immediately under the centre of
the magnet will be a consecutive point,
or south pole.

(290.) The phenomena attending the
division or fracture of a magnet follow
very naturally from the constitution
assigned to it by Ampere's hypothesis ;
for, as the currents circulate in the
same direction in the divided ends while
they were united, they will appear to
circulate in opposite directions with
reference to the two sides of the plane
which divides them, and which become
the terminal planes of each fragment
when separate. The polarities of the
two ends must, therefore, be of opposite
kinds ; for the same reason that the
adjoining ends, B and C, fig. 163, of
two magnets placed in the same line,
with their currents having similar di-
rections, have opposite polarities. At
the poles of a horseshoe magnet, the
currents revolve in opposite directions
with respect to the two ends of the bent
axis ; but the directions of the adjacent
part of each current, as well as of the
remote parts, are similar. See ,/?£•- 166.

Fig. 166.

already remarked, give greater facility to
the induction of currents in a direction

90

ELECTRO-MAGNETISM.

(291.) If a steel bar, instead of being
bent into the form of a horseshoe, be
formed into a complete ring, fig. 167,

Fig. 167.

and then magnetized, it exhibits no
magnetic properties as long as the ring
is entire; but when broken into any
number of portions, each part has two
poles, and possesses all the properties
of an ordinary magnet. This experi-
ment suggested the theoretical investi-
gation of the properties of a system of
small circular currents situated in planes
perpendicular to another circle, passing
through all their centres. The result of
the investigation of this problem led to
a mathematical theorem exactly con-
formable to observation ; a ring so con-
stituted, or an electro-dynamic ring, as
it has been called, being found, both
from theory and experiment, to exert no
action upon a voltaic conductor or mag-
net, at whatever distance from it, or in
whatever situation it may be placed.

(292.) In viewing the application of
Ampere's theory to the mutual action
of two magnets, we might content our-
selves with the observations already
made as to the mutual action of two
electro- dynamic cylinders, which may
be taken as their representatives ; and
simply refer to the general principle
deduced from theoretical considerations,
§ 251, namely, that the resultants of all
the actions may be reduced to forces
emanating from the poles, and inversely
proportional to the squares of the dis-
tances. Yet as a more popular view of
the actual operation of the forces derived
from the attraction or repulsion of cur-
rents in the simpler cases may be more
satisfactory, we shall examine a few of
these cases.

(293.) It will be evident that when
two magnets are presented to each
other, with their axes in the same line,
it must depend upon the similarity or
contrariety of the directions of the cur-
rents at the adjacent ends, whether these
ends will attract or repel each other.
The former, it is well known, happens

when poles of opposite denomination
front each other ; the latter when similar
poles are brought together. The mo-
tion of the currents in the first case may
be aptly illustrated by two watches laid
the one above the other, so that the dial
of the one may be in contact with the
back of the other, for the hands in both
watches will then be moving in the same
direction. We may obtain a represen-
tation of the second case, by placing
the watches either face to face, or back
to back ; for in either of these situations,
the motion of the hands in the two
watches are in opposite directions. The
electric currents in the former case will
exert a mutual attraction; and in the
last, a mutual repulsion.

(294.) In estimating the attractive or
repulsive forces which arise in other
relative positions of the magnets, we
must take into account, not merely the
terminal currents, but those which exist
along the whole length of both magnets.
The general resultants of all the forces
thus arising may be reduced to attrac-
tive or repulsive forces between the
whole of each of the sides of one mag-
net, and the whole of each of the sides
of the other magnet. Thus, supposing
two magnets to be situated horizontally
nearly in the position to which they
would be brought by the influence of
terrestrial magnetism, the east side of
the one will attract the east side of the
other, and repel the west side ; the west
side will, in like manner, attract the
west and repel the east. Hence the
general tendency of all these actions is
to turn the magnets so as to bring
the two eastern sides, for example,
as near together as possible, and pa-
rallel to each other; that is, into a
relative position, such that the north
pole of each magnet shall be adjoining
to the south pole of the other ; and in
this situation the greatest amount of
attractive force will be exerted.

(295.) In positions intermediate to
these, and especially when much in-
clined to each other, the estimation of
the resultant force in each ; individual
case is often difficult, from the complex
operation of the numerous forces that
are in action in a variety of directions.
Thus, if one of the magnets, situated
as just described, parallel to each
other, and with their dissimilar poles
adjacent, be moved in the line of its
axis till the two ends, having similar
poles, are brought into the same plane,
as shown in fig. 168, a strong repulsion

ELECTRO-MAGNETISM.

91

lakes the place of the attraction before
observed, notwithstanding the similarity
of the currents in the two edges at S
and s that are nearest each other. The
reason is, that the attraction of the ad-

Fig. 168.

jacent sides is now much weakened both
by the greater distances of their remoter
portions at N and ?i, and also on ac-
count of the great obliquity of that
action. The repulsions, on the contrary,
exerted between the adjacent side of the
one and the remote side of the other
magnet, become very powerful, both
from their increased proximity and more
direct action ; and they predominate
accordingly. A similar account may be
given of the attraction which takes
place between dissimilar poles placed in
a similar situation. The reasoning in
both instances being analogous to what
was stated (§ 236) with respect to the
action of a helix upon an elementary
portion of current placed in different
situations with respect to the helix.

(296.) In the case of magnets that
are not of a prismatic or cylindric
shape, nor terminated by plane surfaces
perpendicular to their axes, the estima-
tion of the resultant force becomes much
more complicated. All that we have
now said on this subject, indeed, can
only afford approximations to the solu-
tion of the problem of finding this re-
sultant. The rigorous investigation of
this problem would involve mathemati-
cal considerations of too great an extent
for a treatise of this kind. The reader
who may wish to prosecute the inquiry
is referred more especially to the works
of Ampere, in which the subject is
treated with a masterly hand.

(297.) The magnetic influence of the
earth being so perfectly analogous to
that of other magnetic bodies, the theory
of Ampere with respect to the constitu-
tion of such bodies must, if founded in
truth, apply also to terrestrial magnet-
ism, which must, according to that
theory.be derived from electric currents
circulating in the globe from east to
west in planes parallel to the magnetic
equator. The united effect of such
currents would be to produce a southern
polarity on the northern side of these

planes, and a northern' polarity on the
southern side. It is scarcely necessaryfl £ r ^
to point out how exactly thu. phenomena • f '
described in Chapter IX. ($ f48't« 147,-)
accord with the consequences of siidh-iV ***•,,
an hypothesis. The magnetic axis of
the earth, according to this view of the
subject, is merely an imaginary line, per-
pendicular to the planes of the electric
currents circulating in the earlh, and
passing through the centres of the cir-
cles described by those currents ; and
the directive power of the globe which
acts on iron and on magnets on its sur-
face, is the result, not of any real influ-
ence proceeding from those portions of
the earth to which their poles point, but
of the electro-dynamic action of cur-
rents circulating in the plane of the
magnetic equator, in obedience to which
the corresponding currents which cir-
culate in the magnet place themselves,
so as to approach to parallelism with the
former; that is, to attain the position of
equilibrium between the forces in opera-
tion. This position is that of a plane
perpendicular to the line of magnetic
direction, or the line of dip : and ac-
cordingly, since the currents in the mag-
net are themselves perpendicular to its
axis, they will tend to bring that axis in
that very line. Hence the phenomena
of the clipping-needle, and hence the
position assumed by the compass-needle,
in the plane of the magnetic meridian,
as being the nearest approach which its
mode of suspension will allow it to
make to the line of dip.

(298.) All the effects of terrestrial
magnetism may be imitated by distri-
buting wires round the surface of an
artificial globe, so as to direct a galvanic
current through them. Mr. Barlow, in
a paper lately read at the Royal Society,
describes the following experiment which
he made with this view. A hollow
wooden globe, sixteen inches in diame-
ter, was furnished with copper wires
passing in grooves along each parallel
of latitude for every tenth degree. \Vhen
an electric current was made to pass
through these wires, in the same direc-
tion in each, it was found that a mag-
netic needle, properly neutralized with
regard to the earth's action, and sus-
pended in different situations near the
surface of the artificial globe, arranged
itself in positions perfectly analogous
to those actually assumed by the dip-
ping-needle in corresponding regions of
the earth. It is probable that if we
could indefinitely multiply these electric

02

ELECTRO-MAGNETISM.

currents on a globe so prepared, the
apparatus might be made to represent
with great accuracy every circumstance
of magnetic dip and direction ; and by
employing, instead of a magnetic nee-
dle, an electro- dynamic cylinder, all
the phenomena of terrestrial magnetism
might be exhibited, without the inter-
vention of magnetism, by means of elec-
tricity alone.

(299.) The origin of these electrical
currents permeating the interior of the
earth, and more especially its external
layer, may possibly be traced to the
action of the solar rays on successive
parts of the torrid zone, which taking
place from east to west, may excite cur-
rents of positive electricity in that direc-
tion, and in planes corresponding with
the magnetic equator. The probability
of such an effect being produced, and
the inference from analogy that similar
currents may be excited, and even exist
permanently in iron and steel, is greatly
increased by the recent discoveries of
Professor Seebeck, that electric currents
may be produced and maintained in cir-
cuits formed exclusively of solid con-
ductors, by the partial application of
heat. This discovery, which leads to a
separate department of this science, to
which the name of Thermo- Electricity
has been given, will be treated of in the
next Chapter.

(300.) A further confirmation of the
electro-dynamic theory of magnetism is
derived from its applicability to the cu-
rious phenomena of magnetic rotations,
which have been described in the eighth
chapter of the Treatise on Magnetism
(§ 354 to 360). Soon after the discovery
of this new class of facts, M. Arago
suggested to M. Ampere the substitution
of electro-dynamic cylinders for the
magnetic bars in these experiments on
the effects of rotation. The first trials
made by these two philosophers in con-
junction did not lead to any decisive re-
sult, in consequence of some defects in
the apparatus they employed ; but when
these defects were obviated in the sub-
sequent experiments which they made
with M. Colladon, in which a very short
double helix, forming a coil of about
two inches in diameter, was used, they
succeeded perfectly in obtaining the
same results as if magnets had been
employed*. Hence we may infer the
complete identity between all the effects

* Ampere, Theorie des Pbenomencs Elcctro-dy-
niiciues, p. 196.

of a common magnet and an electro-
dynamic cylinder.

(301.) We thus find that the theory
of Ampere satisfies every condition that
is required of a true theory, inasmuch
as it affords a complete explanation of
all the phenomena, even in their mi-
nutest details. It unites the character
of simplicity in principle, and compre-
hensiveness in its applications ; and by
suggesting new combinations, it has led
to the discovery of new facts. It also
has an important advantage over the
theory of tangential forces in presenting
greater facility of mathematical investi-
gation, and for the comparison of the
analytical formulae thence obtained, with
the results of experiment ; and thereby
affording the most severe test of its
accuracy. If the truth of the theory be
established, it will effect an important
step in the generalization of physical
phenomena, by showing that all those
formerly referred to the operation of an
unknown principle, considered as dis-
tinct from electricity and denominated
magnetism, are, in fact, essentially elec-
tric, and that the two principles are
identical, and instead of being the bases
of two separate departments of know-
ledge, are merely branches of a single
and more extended and comprehensive
science.

(302.) It must at the same time be
acknowledged that much still remains
to be done towards removing the diffi-
culties opposed to this as well as other
electro-magnetic theories, which are pre-
sented by the singular and apparently
capricious phenomena of the induction
of , magnetism by electrical currents
transmitted along conductors, and de-
rived either from the voltaic or the com-
mon electrical batteries. We allude
particularly to the results of the expe-
riments of Savary, already noticed in
$ 164 to 168, and which have-not yet
been sufficiently generalized to 'admit of
being explained on any hypothesis,

CHAPTER XIII.
Thermo- Electricity.

(303.) PROFESSOR SEEBECK, of Berlin,
discovered, in the year 1822, that cur-
rents of electricity might be produced
by the partial application of heat to a
circuit composed exclusively of solid
conductors. The original experiment
which established this fact was first an-
nounced in this country, in the Annals

ELECTRO-MAGNETISM.

93

of Philosophy*. A bar of antimony,
about eight inches Ions: and half an inch
square, was taken, and its extrem;tios
connected by twisting a piece of brass
wire round them so as to form a loop,
each end of the bar having several coils
of the wire. On heating one of the ex-
tremities, for a short time, with a spirit-
lamp, electro-magnetic effects were pro-
duced in every part of a circuit so
formed.

(304.) Thus it appears that for con-
stituting a circuit of this kind, two ele-
ments only are requisite ; 'which may
be represented in the diagram, fig. 169,
by the conductors A and B, consisting

Fig. 169.

of two different metals, in contact in
two points H and C, so that a circuit is
formed in H A C B.

(305.) The electrical current thus ex-
cited has been termed Thermo-electric,
in order to distinguish it from the com-
mon galvanic current, which, as it re-
quires the intervention of a fluid ele-
ment as one of its essential components,
was denominated a Hydro-electric cur-
rent. The term Stereo- electric current
has also been applied to the former, in
order to mark its being produced in
systems formed of solid bodies alone.
It is evident that if, as is supposed in
the theory of Ampere, magnets owe
their peculiar properties to the continual
circulation of electric currents in their
minute parts, these currents will come
under the description of stereo-electric
currents.

(306.) The chief evidence we possess
of the existence of thermo-electric cur-
rents consists in the production of elec-
tro-magnetic effects. A compass-needle
placed either within or without the cir-
cuit, and at a small distance from it, is
deflected from its natural position in a
direction conformable to its situation
with regard to the circuit. Still stronger
indications of electro-magnetic action
are obtained by placing two ends of one

• New Series, vol. iv., p. 318.

of the metallic arcs in contact with the
wires of a galvanometer. The thermo-
electric current has also been found to
excite contractions in the muscles of a
frog : but as far as experiments have
yet been tried, it is inadequate to effect
chemical decompositions, the ignition of
metals, or to exhibit sparks, or any
other of the phenomena of ordinary
electricity.

(307.) If the metallic arcs, through
which a thermo-electric current is made
to pass by the application of heat to one
of the points of contact of the different
metals, be delicately suspended, they will
obey the action of a magnet brought
near it. If the opposite poles of two
magnets be placed on the outside of a
circuit moving in a vertical plane, and
turning on a vertical axis, the conduc-
tors may be made to revolve by continu-
ing to apply the heat on the same side.
Thus if the circular arrangement of bars
represented infg. 169 be suspended by
a thread at A, and opposite magnetic
poles be applied at H and C, out of the
circle, while the flame of a spirit-lamp
is held steadily at H, the combined ac-
tions of the two magnetic poles upon
the adjoining ascending current at H,
and descending current at C, will be to
move the circle till its plane is at right
angles to its former position. But the
impulse it has acquired by the joint ac-
tion of the magnets is sufficient to con-
tinue the motion until the side C arrives
at the flame. This part of the circle
being thus heated, while the part H is
at the same time becoming cool, an
electric current is now determined from
G through A to H, and back again
through B to C, which direction, with
reference to the magnets, is the same as
before, and the circle is urged onwards
in its revolution. When it has com-
pleted an entire revolution, all the cir-
cumstances being the same as at first,
another impulse will be given, and the
circle will continue incessantly to re-
volve ; the current moving alternately
in opposite directions at every semi-re-
volution of the circle.

(308.) On the other hand, the pole
of a magnet placed within the circuit
will have no tendency to produce rota-
tion ; because the current in the oppo-
site branches of the circle moves in con-
trary directions; and being, therefore,
urged by the magnet to revolve in oppo-
site directions, the circle will remain in
equilibrium. But two systems of circles
supported each by a point on the ends

ELECTRO-MAGNETISM.

of a horseshoe magnet, placed vertically,
with the poles uppermost (that is, fixed
as in Jig- 50), and a lamp being placed
halfway between the poles, each of the
circles will revolve by the action of that
pole which is exterior to it.

(309.) It appears from these and
other experiments of a similar kind*,
that the mutual action of a magnet and
a thermo-electric current is subject to
the same laws as those of magnets and
galvanic currents ; and hence all the
phenomena of the attraction, repul-
sion, or rotation of conductors conveying
galvanic currents, may be exhibited by
a thermo-electric current transmitted
through the same conductors.

(310.) The two metals of which the
combination and contact produce the
most powerful thermo-electric currents,
are antimony and bismuth, which we
shall accordingly take as the representa-
tives of their respective classes. As in
the galvanic circuits, where the current
of positive electricity is flowing directly
from the copper to the zinc (see Gal-
vanism, $4), the latter is generally said
to be positive with respect to the former
(see Galvanism, § 73) ; so, in the ther-
mo-electric circuit, the bismuth is gene-
rally said to be positive with regard to
the antimony, because, in the colder
portion of the circuit, the electric cur-
rent is passing from the antimony to the
bismutht.

(311.) In the Treatise on Galvanism,
we have given ($ 73) a list of metallic
substances in the order of their oxidabi-
lities, or rather in the order of their
electrical relations, when united in gal-
vanic circuits, wilh interposed acids. A
number of experiments have been made
by Professor Gumming, for the purpose
of determining the comparative thermo-
electric relations of the different metals
by forming circuits of them taken in
pairs. From these the following series
has been deduced, descending from the
extreme positive, as we have already
defined it, which is bismuth, to the ex-
treme negative, which is antimony.

Bismuth,
Mercury, i
Nickel, }
PI at in a,
Palladium,

Cobalt, 1
Manganese, /
Tin,
Lead,
Brass,

* For a more detailed account of these, see Cum-
ming's Manual, already quoted.

t There is still some confusion in the application
of the terms positive and negative to the different
parts of the circuit, by different authors, and some-
times even, by the same author iu different i>Jaces,

Rhodium,

Gold,

Copper,

Silver,

Zinc,

Cadmium,

Charcoal, T

Plumbago,/

Iron,

Arsenic,

Antimony.

(312.) Tn this series every metal is
positive with respect to all those below
it, and negative to those above. Hence
any two metals occupying situations
intermediate between the two extremes
will together compose a thermo-electric
circuit similar to, though of less power
than that formed by bismuth and anti-
mony.

(313.) The order in which the metals
stand in the above series does not con-
tinue the same at all temperatures: thus
gold, silver, copper, brass, and zinc,
should be placed below iron in high tem-
peratures, though they rank above it in
a low heat.

(314.) On comparing the series above
given, which represents the thermo-elec-
tric relations of the metals, with that
given in the Treatise on Galvanism, §
73, which represents their galvanic re-
lations, it will be evident that there is no
correspondence between them.

(315.) The contact of the metals in
these experiments is most completely
secured by soldering them together ; but
in most cases it will be sufficient, if they
are in the form of wires, to twist them
closely together. Mercury may be con-
veniently employed as the intermedium
between other metals : the mercury be-
ing previously heated, the extremities of
each piece composing the pair being
dipped into it at the same moment, and
the other extremities being applied to a
galvanometer. Even a small fragment
of any metallic substance is sufficient to
afford indications of its thermo-electric
relations, if placed upon a disc of the
metal with which it is to be compared,
and touched with a hot wire ; the circuit
being completed through the wire and
the disc. But it is found that the results
of experiments so made do not always
accord with those obtained by employing
larger pieces, for it appears that the
effect is much influenced by the relative
dimensions of the heated surfaces. Even
\vhen the experiment is made by plung-
ing the metals to be examined in heated
mercury, the direction of the current
will be determined, in many cases, by
the order in which they have been im-
mersed.

(316.) Considerable diversities take

ELECTRO-MAGNETISM.

place in the directions of the current
when the metals contain any alloy, or
are not in a state of perfect parity.
Thus, although bismuth and tin are
each positive with regard to copper, yet
an alloy of the two former is found to
be negative with regard to the latter
metal.

(317.) We are yet far from possessing
any theory by which the whole of the
facts belonging to thermo-electricity
can be satisfactorily explained. The
most intelligible account of them appears
to be that given by Becquerel*, which
proceeds upon the hypothesis, that when-
ever a particle of a metal receives heat
from a body of a higher temperature
than itself, part of the neutral electric
fluid which is attached to it is decom-
posed, the vitreous fluid being retained,
and the resinous fluid driven off, and
passing into the adjoining particles of
metal. In proportion as the heat ex-
tends, by communication from particle
to particle, similar effects take place in
each of those that are acquiring heat,
while contrary effects are taking place in
all those that are losing heat. Thus, the
simple diffusion of that portion of heat
which was originally received by the
first particle, produces only an oscilla-
tory movement of the electrical fluids
between adjacent particles, attended by
a series of decompositions and com-
binations of the two electric fluids. But
if the source of heat be permanent, so
that the temperature of the first particles
which receive it be uniformly maintained,
the retrograde movements of the decom-
posed electric fluids are prevented, and
a continued current of each takes place
in opposite directions ; the negative
electricity being impelled forwards from
the parts where the temperature con-
tinues high to those which continue to
be colder, and a positive current moving
in the contrary direction. It follows,
from this hypothesis, that when two
different metals are placed in contact,
so as to constitute a circuit, the currents
from the heated parts that are con-
joined will be urged in opposite direc-
tions ; but the strongest will prevail,
and the thermo-electric current actually
observed is that which results, and of
which the intensity is equal to the differ-
ence between the two that are simul-
taneously developed.

(318.) Thermo-electricity does not
appear to have its source in any chemical

* See Annales dc Cbimie; tome xli. p. 353,

changes taking place in the materials
composing the circuit. Oxidation, at
least, has no share in the effect ; for
Becquerel has repeated the experiments
of Seebeck and others, relating to this
mode of action, when the apparatus was
surrounded by hydrogen gas, without
any sensible difference in the results.*

(319.) The great peculiarity which dis-
tinguishes thermo-electric currents from
those produced by galvanic action, is that
the quantity of circulating electricity is
much greater compared with its inten-
sity. They are, in this respect, still
further removed from the condition of
streams of electricity produced in the
common electrical machine, which pos-
sess amuchgreaterintensity.thoughthey
are much less considerable in quantity
than galvanic currents. Hence it is
chiefly by their effects in producing
deviations in the magnetic needle that
the existence of thermo-electric currents
is recognised. The low state of inten-
sity of these latter currents occasions
great loss of power whenever they have
to traverse any considerable line of con-
ductors,— even of metals, which are the
most perfect conductors. On this ac-
count it is that very little advantage is
gained by forming compound circuits ;
that is, arranging their elements in a
series of alternations analogous to those
of the voltaic pile. Messrs. Fourier and
Oersted made trials of this kind ; first
combining three bars of bismuth with
three bars of antimony, placed alter-
nately, so as to form the sides of a
hexagon, and with their contiguous ends
soldered together, thus composing a
thermo-electric circuit, which included
three pair of elements. The length of
the bars was about 'four inches and a
half, their breadth about half an inch,
and their thickness one-sixth of an inch.
This circuit was placed upon two sup-
ports, and in a horizontal position, one
of the sides of the hexagon being in the
magnetic meridian. A compass-needle
was placed below this side, and as near
to it as possible, arid was very sensibly
affected when one of the solderings at
the junctions of the bars was heated
with the flame of a lamp. The devia-
tion was considerably increased on heat-
ing two of the alternate angles of the
hexagon ; and a still greater deviation
was produced when the heat was applied
to the three alternate angles. Similar
effects were produced when, instead of

* Annales de Cbimie, tome xli, p. 359. .

ELECTRO-MAGNETISM.

applying heat, the temperature of one
or more of the other angles of the hex -
asjon was reduced by means of ice.
When the action of the ice was com-
bined with that of the flame, applied at
the same time to the alternate angles
all round the hexagon, the effect was
still more considerable, the deviation of
the needle amounting to sixty degrees.

(320.) By continuing these experi-
ments with more numerous alternations,
it was found that the total effect of a
compound thermo-electric circuit is very
inferior to the sum of the effects which
the same elements could produce when
employed in the formation of simple
circuits, so that the electro-magnetic
forces called into action increase in a
much less ratio than the number of
alternations constituting the series *.

(321.) The latest thermo-electric ex-
periments are those of Messrs. Nobili
and Melloni, of which an account was
read to the French Academy of Sciences
in September last (1831.)t A thermo-
electric pile, consisting of thirty- six pairs
of plates of bismuth and antimony, hav-
ing a galvanometer with two needles
attached to it, was found to be so sen-
sible as to be affected by the warmth of
a person at the distance of thirty feet.
A number of delicate experiments were
made with this apparatus on the permea-
bility of bodies to radiant heat, on the
temperature of insects, and on the pow-
ers of different bodies of emitting, re-
flecting, and absorbing heat. Consider-
able doubt is also thrown by these in-
quirers on the conclusions of Fourier
and Oersted with respect to the limited
effect of increasing the number of alter-
nations in augmenting the intensity of
the current.

(322.) It would appear also, from the
observations of Professor Gumming,
that although the hydro-electric and
thermo-electric currents may both be
regarded as continuous, when compared
with those of common electricity, excited
by the electrical machine, which are
manifestly discontinuous (see Galvan-
ism, § 21 and 94), yet when the hydro-
electric and thermo-electric currents are
compared with one another, the con-
tinuity of the latter is by far the most
complete. This will appear evident from
the consideration that, as it is necessary
that one of the three elements of a gal-
vanic circuit must be a fluid (see

* Quarterly Journal of Science, xvi. 126.
t Bulletin des Sciences, No. II. for 1831.

Galvanism, § 69), it will, therefore, be a
more imperfect conductor than the me.
tals, and will oppose some degree of
resistance to the passage of the electric
currents circulating through the whole
assemblage.

(323.) There are, therefore, strictly
speaking, three states of electricity. That
derived from the common electrical ma-
chine is in the highest state of tension,
and accumulates till it is able to force a
passage through the air, which is a per-
fect non-conductor. In the galvanic
apparatus the currents have a smaller
degree of tension ; because, although
they pass freely through the metallic
elements, they meet with some impedi-
ment in traversing the fluid conductor.
But in the thermo-electric currents the
tension is reduced to nothing, because
throughout the whole course of the cir-
cuit no impediment exists to its free and
uniform circulation.

(324.) These considerations serve also
to explain why the latter species of
current is inadequate to effect any kind
of chemical decomposition, or even to
produce any degree of permanent mag-
netism. It has hitherto been found
impossible to magnetize steel bars by
means of thermo-electricity, although
the apparatus employed for that pur-
pose was capable of producing a strong
effect on the magnetic needle.

(325.) It is probably owing to some
quality of this kind in the currents, which
theory assigns as the source of magne-
tism (namely, from deficiency in tension),
that all the endeavours which have so
many different times been made by va-
rious experimentalists to obtain from
magnetism any effects that may be con-
sidered as exclusively electrical, have
uniformly failed. Although a magnet
is powerfully affected by a current of
electricity proceeding from a voltaic
battery, it does not appear that the
magnet is capable of augmenting or
diminishing either the intensity or ve-
locity, or any other of the qualities of
the electric current. The only way in
which the reaction of the magnet is
shown, is by giving the electric current
a tendency to lateral deflexion, as if
urged by tangential forces proceeding
from the poles of the magnet. If the
wire of a galvanometer form part of the
voltaic circuit, and a powerful magnet
be applied to other parts of the circuit,
whether bent into a helix or not, no
indication of any action from the magnet
is afforded by the needle of the galvano-

ELECTRO-MAGNETISM.

97

meter.* We understand, however, that
Mr. Faraday is at present engaged in an
experimental inquiry upon this subject,
which cannot fail, in such able hands,
to lead to important results.

CHAPTER XIV.
Influence of Light on Magnetism.

(326.) PROFESSOR Morichini of Rome
announced, in the year 1813, his having
discovered that steel, exposed in a par-
ticular manner to the concentrated violet
rays of the solar spectrum, became mag-
netic; but the uniform failure of the
experiment, when tried by every other
person, had created great doubt of the
accuracy of the result as reported by
Morichini. In the course of some ex-
periments made by Mr. Christie, in the
year 1824, he was led to the conclusion
that the solar rays actually do exert a
sensible influence on magnetism, which
is shown by their affecting the vibra-
tions of a magnetized needle exposed to
them, quite independently of the effects
produced by the heat which they impart.
A needle, six inches long, contained in
a brass compass box with a glass cover,
was suspended by a fine hair, and made
to vibrate, alternately shaded and ex-
posed to the sun. He found, from a
number of trials, that the vibrations of
the needle, when exposed to the sun,
ceased in a much shorter time than when
they took place in the shade. That this
greater slowness of the vibrations was
not attributable to an increase of tem-
perature, was proved by the needle's
being observed to vibrate more rapidly
when its temperature was raised by
other means-f- .

(327.) In the summer of 1825, Mrs.
Somerville was induced, by the unusual
clearness of the weather, to investigate
this subject;|;. Having at that time no
information of the manner in which
Morichini's experiments had been con-
ducted, it occurred to her that if the
whole needle were equally exposed to
the violet rays, it was not probable that
the same influence which produced a
south pole at one end, would, at the
same time, produce a north pole at the
other. She therefore covered half of a
slender sewing-needle, an inch long, with
paper, and fixed it in such a manner as
to expose the uncovered part to the

* See Quarterly Journal of Science, xix. 338.

f Philosophical Transactions for 1826, p. 219.

J Ibid. p. 133.

violet rays of a spectrum thrown, by an
equiangular prism of flint glass, on a
pannel at five feet distance. As the
place of the spectrum shifted by the
motion of the sun, the needle was moved
so as to keep the exposed part constantly
in the violet ray. The sun being bright,
in less than two hours the needle, which
before the experiment showed no signs
of polarity, had become magnetic, the
exposed end having the properties of a
north pole.

The season continuing favourable,
afforded daily opport unities of repeat-
ing and varying the experiments with
needles of different sizes, and placed
in different positions with respect to
the meridian, and at different distances
from the prism. The results were nearly
uniform, and similar to that above
stated. It was not found necessary to
darken the room, provided the spectrum
was thrown out of the direct solar rays.

(328.) Mrs. Somerville next endea-
voured to ascertain whether the other
prismatic rays had the same property
as the violet. Needles, previously as-
certained to be unmagnetic, exposed to
the blue and green rays, sometimes
acquired magnetism, though less uni-
formly and less quickly than in the
violet ray : when magnetism was thus
communicated, it seemed to be equally
strong as in the former case. The in-
digo ray succeeded nearly as well as
the violet. The exposed end, in almost
every case, became a north pole. In no
one, instance was magnetism produced
by the yellow, orange, or red rays,
though in some instances the same
needles were exposed to their influence
for three successive days ; neither did
the calorific rays of the spectrum pro-
duce any sensible effect.

(329.) Pieces of clock and watch
spring were next tried with similar suc-
cess, and were found to be even more
susceptible of this peculiar magnetic
influence than needles, possibly on ac-
count of their blue colour, or greater
proportional surfaces. The violet rays
concentrated by a lens produced mag-
netism in a shorter time than the prism
alone.

(330.) Experiments were next insti-
tuted by transmitting the solar rays
through coloured media. Needles, half
covered with paper, were exposed on a
stone outside a window, under a blue
glass coloured by cobalt, to a hot sun
for three or four hours. They were
found to be feebly magnetic ; but their
H

ELECTRO-MAGNETISM.

magnetism " was not permanently re-
tained. In subsequent experiments,
by an exposure of needles under the
same circumstances, for six hours, a
very sensible degree of magnetism was
acquired, and remained permanent.
The rays transmitted through the blue
glass employed in this experiment black-
ened muriate of silver as powerfully as
those transmitted through uncoloured
glass ; thus proving that it was freely
permeable to the chemical rays of the
solar spectrum. Green glass was also
tried ; and the rays which had pene-
trated it were likewise found to commu-
nicate magnetism. The white light of
the sun produced no magnetic effect
whatever on needles exposed to its in-
fluence.

(331.) Although the experiments, of
which we have just stated the results,
are minutely detailed in the paper above
referred to in the Philosophical Trans-
actions, yet, in many trials made by other
experimentalists, no success has been
obtained. The experiments on the os-
cillations of the needle were repeated by
Messrs. P. Riess and L. Moser without
any satisfactory result*. We may,
therefore, consider the subject as still
open to inquiry, and as requiring a more
minute and scrupulous investigation.

CHAPTER XV.
Origin of Terrestrial Magnetism.

(332.) SEVERAL causes have been as-
signed for the magnetic influence which
the globe of the earth is found to exercise,
not only over the magnetic needle, but
also, as we have seen, over currents of
voltaic electricity transmitted through
conductors. (See Chapter IX. § 128,
et seq.~) Among the various substances
which occupy the interior of the globe
it is extremely probable that chemical
actions of different kinds are incessantly
occurring. These actions will, for the
most part, however, be very slow, and
will continue with a certain degree of
uniformity for very extended periods of
time. They will occur more especially
in the superficial strata of the earth,
where the combined agencies of water,
of atmospheric air, and of heat are in
constant operation. The influence of
the solar rays on a surface of such vast
extent must be very considerable : and
excepting in the vicinity of the poles,
every portion of that surface is exposed
in succession to their action, and acquires
* Aanales de Cliiuiie, xlii. 3U4,

during that exposure a certain degree of
heat ; which heat' is again lost by noc-
turnal radiation. Although the effect
of these alternate changes of tempera-
ture may extend only to a small depth
below the surface, yet considering their
immense superficial extent, they may be
sufficient to give rise to thermo-electric
currents of considerable power. It has
been conjectured, also, that these effects
may be combined with an influence of
another kind, more directly derived from
the rotation of the earth on its axis, on
the principle that all bodies have been
found to exhibit magnetic polarity by
rotation. t

(333.) That electric currents do really
circulate in different parts of the solid
strata of the earth, is not merely matter
of conjecture : the existence of such
currents has been lately proved, in the
most satisfactory manner, by Mr. Robert
Fox, in a paper " On the Electro-mag-
netic properties of metalliferous veins,"
which has been recently published in the
Philosophical Transactions*. Having
been led from theory to entertain the
belief that a connexion exists between
electric action in the interior of the earth,
and the arrangement of metalliferous
veins, he was anxious to verify this
opinion by experiment. The first trials
he made with this view were unsuc-
cessful : but by persevering in his at-
tempts, he soon obtained decisive evi-
dence of considerable electrical action
in the mine of Huel Jewel, in Cornwall.
His apparatus consisted of small plates
of sheet copper, which were fixed in
contact with ore in the viens by copper
nails, or else wedged closely against them
by wooden props stretched across the
galleries of the mine. Between two of
these plates, at different stations, a
communication was made by means of
copper wire, one twentieth of an inch in
diameter, which included a galvanometer
in its circuit. In some instances three
hundred fathoms of copper wire were
employed.

(334.) The intensity of the electric
currents was found to differ consider ably
in different places. It was generally
greater in proportion to the greater
abundance of copper ore in the veins,
and in some degree also to the depth of
the station. This curious fact may
possibly afford the miner some useful
indications as to the relative quantities of
ore which the vein contains, and also as

* Cumming's Manual, &c. p. 231,
i For 1830, page 3D3.

ELECTRO-MAGNETISM.

to the directions in which it is most pro-
ductive. The electricity thus perpetu-
ally in action does not appear to be in
any respect influenced by the presence
of the workmen and their candles ; nor
even by the explosions of gunpowder in
blasting.

(335.) Mr. Fox observes that ores
which transmit electricity have generally
some conducting material interposed in
the veins between them and the surface :
a structure which appears to bear some
analogy to the ordinary galvanic combi-
nations. These electrical currents which
pervade mines were found to have vari-
ous and frequently opposite directions in
different parts of the same mine.

(336.) The metals are probably not
the only substances capable of giving
rise to electrical currents in the earth ;
for it is well known that galvanic com-
binations may be formed by arrange-
ments of elements that are not metallic.
(See Galvanism, § 87.) The direction
of each current will of course be deter-
mined by the relative positions of the
elements* from which it is derived ; but
even if we suppose the arrangement of
these elements to be fortuitous, a pre-
vailing current will still result, arising
from the difference of their actions ; for
it is infinitely improbable that, without a
designed arrangement, the currents in
opposite directions should be exactly
equal, so as to destroy one another.
Irregularities of distribution probably
exist with regard to the materials com-
posing the interior of the globe ; the

resultant electro- magnetic action of the
whole combination being that of which
we witness the effects, and which may
be considered as due to electrical cur-
rents circulating in directions parallel
to the magnetic equator round the sur-
face of the earth.

(337.) Even in the irregularities in-
cident to the magnetic forces derived
from the earth we may discern the
operation of causes which are periodical
in their operation. Thus the diurnal
and annual changes of the variation
of the needle may be traced to cor-
responding changes in the position of
the different parts of the earth with
regard to the sun, in as far as these
electric currents are dependent upon
solar influence. The progressive changes
in the variation, which embrace longer
periods of time, are less easily accounted
for, and appear referable to causes which
act at greater depths below the surface
of the earth ; and are probably connected
with chemical changes taking place in
the interior of the globe, of which we
can possess no certain knowledge.

(338.) On the whole, then, it must be
allowed, that there are strong grounds
for the belief that there subsists some
mutual connexion, or rather an intimate
relation and affinity, between the several
imponderable agents, namely, Heat,
Light, Electricity, and Magnetism,
which pervade in so mysterious a man-
ner all the realms of space, and which
exert so powerful an influence over all
the phenomena of the universe.

NOTE. Since the above was sent to the press, a paper, by Mr. Faraday, has been
communicated to the Royal Society, disclosing a most important principle in elec-
tro-magnetism, of which, I regret, I can only give the following brief statement.

By a numerous series of experiments, Mr. Faraday has established the general
fact, that when a piece of metal is moved in any given direction, either in front of
a single magnetic pole, or between the opposite poles of a horse-shoe magnet,
electrical currents are developed in the metal, which pass in a direction at right
angles to that of its own motion. The application of this principle affords a com-
plete and satisfactory explanation of the phenomena observed by Arago, Herschel,
Babbage, and others, where magnetic action appears to be developed by mere
rotatory motion, and which have been erroneously ascribed to simple magnetic
induction, and to the time supposed to be required for the progress of that induc-
tion. The electro-magnetic effect of the elective current induced in a conductor
by a magnetic pole, in consequence of their relative motion, is such as tends con-
tinually to diminish that relative motion ; that is, to bring the moving bodies into
a state of relative rest : so that, if the one be made to revolve by an extraneous
force, the other will tend to revolve with it, in the same direction, and with the
same velocity.

H2

POSTSCRIPT.

THE design of the last four Treatises has been to offer a condensed and me-
thodical work on that important department of Natural Philosophy which com-
prises the diversified phenomena of Electricity and Magnetism. These phe-
nomena, which were formerly regarded as the effects of two perfectly distinct
agents, are now discovered to have an intimate relation to one another, and,
in all probability, to be dependent on one and the same principle : in like
manner, as it was found by Newton that the simpler mechanical phenomena
of the universe are the results of the single principle of gravitation. A suc-
cinct and connected account of the numerous discoveries which the exertions
of philosophers have recently brought to light on this highly interesting
branch of physical science, collected from the various scientific journals and
transactions through which they are dispersed, and digested in a didactic
order, seemed to be particularly wanting, and to be especially calculated
to further the objects of the Society for the Diffusion of Useful Knowledge.
In pursuance of this design, I have aimed at giving to the subjects treated as
much condensation as was compatible with perspicuity. I have endeavoured to
conduct the student, by a regular progression, from the simpler to the more
complex topics of research ; and I have also been anxious, by placing con-
stantly before his view the distinction which exists between ascertained facts,
and the hypotheses and theories devised for their explanation, to illustrate the
precepts of Bacon by examples, and to foster that genuine spirit of philosophi-
cal inquiry by which alone error can be avoided, and truth attained.

For the many deficiencies which I fear the reader will discover in the
completion of this design, I have to plead, in extenuation, the very scanty
portion of leisure, which the continual pressure of my professional duties
leaves at my disposal. When I undertook this task, at the request of the
Society, above four years ago, I was far from anticipating the extent of the
labour it has imposed upon me; and from the multiplied interruptions to
•which I have been subject, I have been compelled to prosecute the work in a
desultory manner, and at irregular and uncertain intervals.

Since the publication of the earlier Treatises, many valuable researches
have been made, both in Electricity and in Galvanism, which deserve to be
recorded in their proper places. This, however, is an inconvenience which,
in the present age of improvement, must be incident to every scientific
Treatise ; for while so many accessions are daily accruing to the stock of
information, it is hardly possible to keep pace with the rapid growth of
knowledge ; nor can we ever hope to incorporate the whole of the disco-
veries, which have been made up to the last moment of publication, in a
systematic work on any science. To wait till perfection is attained would be
vain and fruitless presumption ; for the architecture of science has this pecu-
liarity, that the foundations must be prepared, and the superstructure begun,
long before the plans and elevations are completed. To posterity will be left
the task of adding the key-stone, and of removing the scaffolding which
interrupts the symmetry of the perfect edifice.

P. M. ROGET.

39, Bernard- Street, Russell-Square,
December 12^,1831,

EXPLANATION OF SCIENTIFIC TERMS

MADE USE OF IN THIS VOLUME.

N.B. Many of the terms are common to both Volumes ; but those explanations that were given in the
former Glossary will be merely referred to in the present; except, in a few cases, where the definitions
were not supposed to be sufficiently explicit. Several omissions may be supplied by consulting the
Indexes.

ABERRATION.— See GLOSSARY I.

f SPHERICAL. —See

Gloss. I.

ABSCISSA. — See Conic Sections. Gloss. I.

ACCELERATION.— See Gloss. I.

ACCELERATED FORCE is the in-
creased force, or impetus, which a body
exerts when stopped, inconsequence of the
acceleration of its motion. Some object
to the expression, and propose to substi-
tute the term Accumulated Force.

ACCELERATED MOTION.— See Ac-
celeration.

ACCIDENTAL COLOURS.— See Gloss.
I.

ACHROMATIC.— See Gloss. I.

ACIDS. The term acid was originally
confined to denominate those bodies only
•which have a sour taste ; but recently
the name is given to other substances.
The present characteristic of an acid is,
that it 'changes the blue, green, and
purple juices of vegetables to a red
colour, and that it unites with alkalis
and metallic oxides to form salts.

ACID, CARBONIC.— See Carbon.

, NITRIC.— See Azote.

ADAMANTINE SPAR.— See Corundum,
Gloss. I.

AFFINITY.— See Chemical Affinity .

ACTING POINT.— See Machine.

ACTION.— See Gloss. I.

ACTIVE FORCE.— See Gloss. I.

ACUTE ANGLE.— See Angle.

ADULARIA.— See Feldspar, Gloss. I.

AERIFORM FLUIDS.— See Gas, Gloss. I.

AGATES.— See Gloss. I.

AIR, GENERAL. CONDENSED,ETHE-
RIAL, &c. See Gloss. I.

AIR, PRESSURE OF, a term sometimes
used in place of the weight or the pres-
sure of the atmosphere. — See Atmos-
phere.

AIR, INFLAMMABLE.— See Oxygen.

AIR-TIGHT, that degree of closeness in
any vessel or tube, which prevents the
passage of air, under the circumstances
in which it is placed.

AIR-VESSEL, a vessel in which air is
condensed by pressure, for the purpose of
employing the re-action of its elasticity
as a moving, or as a regulating power.

ALCHEMY, the name of that fanciful de-
partment of chemical science which, for
many centuries, was occupied in the
search for the philosopher's stone, and
the Elixir Vitce ; by the former of which

the baser were to be transmuted into the
precious metals, and by the latter, human
life was to be indefinitely prolonged.
Sir Richard Steele was one of the latest
of the followers of that phantom.
ALCOHOL. That portion of a vegetable
substance which is sweet to the taste, or
which is capable of becoming sweet under
certain circumstances, or by certain ma-
nipulations, is termed Succharum or
Saccharine matter. This, when sufficiently
fluid, readily enters into an intestine
motion called the Vinous fermentation,
emits Carbonic Acid Gas, and becomes of
less and less specific gravity during the
action. This lessening of weight is called
Attenuation, and the product is a Vinous
liquor. From this vinous liquor another
liquid is procured by distillation. It is
lighter than water, and called spirituous ;
and this, by re -distillation, (termed Rec-
tification,} is brought to the state of pure
spirit, rectified spirit of wine, or Alcohol,
which are different names for the same
thing. Alcohol always retains a portion
of water, and its purity is calculated by
its freedom from flavour and its lightness.
The strongest alcohol yet procured has
the specific gravity of 792, taking water
at 1000.

ALKALIES are substances that have an
acrid taste, change blue vegetable colours
to green, and form salts by their union with
acids. There are three alkalies: Potash,
called the Vegetable Alkali, and Soda
the Mineral Alkali are said to be Fixed
Alkalies ; and Ammonia, which is termed
the Volatile Alkali, because it exists in. the
form of a gas. Potash and Soda are un-
derstood to be the oxides of two substances,
called Potassium and Sodium, which are
classed among the metals. Lime and
certain other minerals, having some of the
properties of an alkali, are called Alkaline
Substances.

ALTITUDE of the sun or a star.— See
Horizon.

AMALGAM (from the Greek ama, toge-
ther, and 0 omeOj I marry) is a chemical
term signifying the union of any metal
with mercury, which is a solvent to Va-
rious metals.

AMETHYST.— See Corundum.

AMMONIA.— See Alkalies.

AMPLITUDE.— See Horizon.

ANALC1ME is a stone which is found in
grouped crystals, deposited by water, in,

2

EXPLANATION OF SCIENTIFIC TERMS.

the fissures of hard lavas. It melts
under the blowpipe into a semi-transpa-
rent glass. It is also called Cubizite.
— See Vol. I., Polarization of Light,
p. 39.

ANALYSIS is the separation of the parts
of a compound, so as to be able to examine
them apart. SYNTHESIS is the op-
posite, signifying the composition of parts
into one whole.

ANAMORPHOSIS (Greek ana, again, and
morphosis, a form) is a distorted repre-
sentation of an object ; but which is so
made as to appear in its proper shape by
viewing it in a particular direction, or
through a particular medium. The figure
is also restored, in some cases, by causing
the anamorphosis to be reflected from
specula with certain surfaces, such as
those of cones and cylinders.

ANGLE.— See Gloss. I.

ANGLES of Incidence, of Reflexion, and of
Draught. — See Gloss. I. for these terms
respectively.

ANHYDROUS.— See Hydrate, Gloss. I.

ANIMAL ELECTRICITY.— See Galva-
nism.

ANNEALING OF METALS consists in
making them red hot, and then letting
them cool gradually, in order to restore
their malleability, which they are apt to
lose under the operation of hammering.
The Annealing of glass is conducted in
the same manner, and is necessary to its
perfection ; for, otherwise, it would be
apt to fly into pieces by the slightest
scratch. — See Rupert's dr"ps.

ANTECEDENTS.— See Ratio.

APHELION is that point of the orbit of a
planet in which it is farthest from the
sun (Greek helios, the sun) ; and the
Perihelion is the point of the planet's near-
est approach.

APLANATIC (Greek a privative, and
plane, error) signifies free from error.
The term is applied to those optical in-
struments in which the spherical aberra-
tion is completely corrected. — See Aber-
ration, Spherical, in Gloss. I.

APOGEE (Greek apo, from, and ge, the
earth) is that point of the moon's orbit in
which she is at the greatest distance ; and
the Perigee is that point in which she is
nearest to the earth. At the time when
the earth was regarded as the centre of
the system, the terms apogee and perigee
were applicable to the places of all the
planets, and also to the sun, with respect
to their variable distances from the earth ;
but now they refer only to the moon.
What was then called the sun's Apogee is
now the earth's Aphelion, and the Peri-
gee of the former has become the Peri-
helion of the latter. — See Aphelion.
APOPHYLLITE,or FISH-EYE-STONE
is a scarce mineral, having a pearly lustre
like to the species of feldspar called
juoon-stone. Its crystals are various and

often tessellated with thick tables irre-
gularly piled and grown together. It
has a white milky colour, but in its di-
vided portions it is usually transparent.
This mineral is found in the iron mines
of Uto, in Sudermania, a province of
Sweden.

APPARENT TIME— See Time.

AQUA FORTIS.— See Azote.

AQUEOUS VAPOUR, the vapour of
water.— See Vapour.

ARC OF A CIRCLE.— See Angle.

AREA. The Roman area was a threshing
floor, but the word is used by geome-
tricians to denote any surface of a deter-
minate extent. This extent is ascertained
by calculating how many times larger the
surface is than that of another smaller
surface of the size of which we have an
idea, such as a square inch, a square foot,
&c., which we use as superficial measures.

ARITHMETICAL PROGRESSION.—
See Progression Arithmetical.

ARRAGONITE, an impure species of car-
bonate of lime, brought from Arragon in
Spain.

ARTIFICIAL MAGNETS.— See Magnet.

ASTRONOMICAL HORIZON. — See

' Horizon.

ASYMPTOTES OF AN HYPERBOLA.
— See Conic Sections in Gloss. I.

ATMOSPHERE.— See Gloss. I.

ATMOSPHERES. One, two, three, &c.—
See Gloss. I.

— REFRACTION OF.

The atmosphere, which thus surrounds
the earth like a shell, is more dense than
the medium (whether ethereal or a vacu-
um) that intervenes between it and the
celestial bodies; and, consequently, the
rays of light are refracted when entering
this shell. This refraction varies with
the density of the atmosphere, and also
with the direction in which the rays
enter. The atmosphere gradually in-
creases in density from the higher to the
lower strata ; and, therefore, a ray of
light will be more and more refracted in
its passage to the earth's surface, so as to
descend in a curve line. The curve, too,
varies with the direction of the ray ; that
coming from the zenith alone being a
straight line.

ATOM.— See Molecule.

ATTENUATION.— See Alcohol.

ATTRACTION IN GENERAL, CAPIL-
LARY, and of Cohesion. — See Gloss. I.

— ELECTIVE.— See Chemi-
cal Attraction.

• MAGNETIC.— See Mag-
netism.

AXINITE, a mineral commonly found in
crystals of four-sided prisms, so flattened
that some of its edges become thin and
sharp ; and hence its name from the
Greek axinc, an axe.

AXIS OF AN ELLIPSIS, PARABO-
LA, or CONE, &c.— See Gloss, I.

EXPLANATION OF SCIENTIFIC TERMS.

AXIS OF REFRACTION.— Seo Refrac-
tit-p Power.

• OF A MAGNET.— See Magnetism.

AZIMUTH.— See Horizon.

COM PASS.— Seo Horizon.

AZOTE, or AZOTIC GAS. (Greek a
privative, and xoe life,) so called because,
if inhaled alone it would be instantly de-
structive of life, is combined with oxygen
in the atmosphere, of which it constitutes
seventy-three parts in the hundred. This
gas is also called Nitrogen, because it is
believed to be the basis of Nitric acid or
*-/(jiia Fords.

BALANCE See Gloss. I.

BAROMETER.— See Gloss. I.

BASIL LEATHER, tanned sheepskin.

BATTERY, ELECTRICAL,— See Leyden
Final.

, VOLTAIC. — See Galvanic

Circle.

BERYL.— See Emerald.

BLACK-LEAD.— See Carbon.

BODY is any determinate portion of matter
which may act, or be acted upon, by other
bodies.

BOILING.— See Gloss. I.

BOILING-POINT.— See Gloss. I.

BURNING-GLASS.— See Gloss. I.

CAIRNGORM.— See Quartz.

CALAMINE, an ore of zinc.

CALCAREOUS SPAR.— See Gloss. I.

CALORIC, latent, specific, &c. — See
Gloss. I.

CALORIFIC RAYS.— See Gloss. I.

CALX.— See Oxygen.

CAOUTCHOUC, or INDIAN RUB-
BER, is an elastic resin or dried juice of
certain trees and plants, growing in South
America and the East Indies : the He-
vcea caoutchouc and latropha elastica of
the former; and the Urceola elastica of
the latter. In some parts of those coun-
tries boots and shoes are made of that
material.

CAPACITY FOR HEAT.— See Gloss. I.

CAPILLARY TUBE.— See Gloss. I.

. • ATTRACTION. — See

Gloss. I.

CARBON is the chemical name for char-
coal, supposing it to be pure ; but pure
carbon is understood to exist only in the
diamond. Charcoal (the carbon of the
arts) is the coaly residuum of vegetable
substances which have been burnt in
close vessels, and is an oxide of carbon.
Carbon unites readily with oxygen, form-
ing a gas called Carbonic Acid. It is this
•which is produced by the vinous fermen-
tation. The union of this acid with al-
kalies, and alkaline substances, or with
metals, forms what are called Carbonates :
thus we have Carbonate of Potash, Car-
Inmate of Lime, (Carbonate of Iron, &c.
Carbon enters into direct combination
with iron. Nine parts carbon to one of
iron is Carburet of Iron • what has erro-
neously been called Plumbago or Black-

lend. In the proportion of about a forty
fifth of the weight of the iron it forms
Cast-iron. Steel is a combination of about
one part of carbon to two hundred of
iron. This union of carbon and iron is
called Carbonization, and to decarbonize.
the cast-iron, or steel, is to drive off its
carbon, in the form of Carbonic A<-id (las.
Iron is said to be Case-hardened when its
outer surface is converted into steel. — See

CASE-HARDENING.— See Carbon.
CAST-IRON,— See Carbon.
CATOPTRICS, that part of the science of

optics which treats of the reflexion of

light.

CENTIMETRE.— See Metre.
CENTRE OF GRAVITY.— See Gravity.

• - OF GYRATION.— See Gyra-

tion.

- OF PERCUSSION.— See Per-
cussion.

-- OF PRESSURE.— See Pressure.

CENTRIFUGAL FORCE.— See Gloss. I.

CENTRIPETAL FORCE is that force
which draws a body towards a centre,
and thereby acts as a counterpoise to
the centrifugal force in circular motion.
Gravity is a centripetal force, preventing
the planets from flying off in a tangent,
as the stone does from the sling.

CHARCOAL.— See Carbon.

CHEMICAL ATTRACTION, CHEMI-
CAL AFFINITY, AND ELECTIVE
ATTRACTION are different names for
that unaccounted-for action by which the
particles of one class of bodies, when pre-
sented to those of certain other classes,
conjoin to form new compounds, making,
apparently, a choice or election, of those
with which they unite.

CHEMICAL COMBINATION is that
intimate union of two substances, whether
fluid or solid, which forms a compound ,
differing in one or more of its essential
qualities from either of the constituent
bodies.

CHORD OF AN ARC.— See Angle.

CHROMATICS (from the Greek chroma,
colour) is that division of the science of
optics which treats of the colours of light,
their several properties, and the laws by
which they are separated.

CHROMATIC VERNIER.— See Vernier.

CIRCLE OF GYRATION.— See Gyra-
tion.

CIRCUMFERENCE.— See Perimeter.

COHESION.— See Gloss. I.

-- , ATTRACTION OF. — See
Gloss. I.

COLOUR— .See Gloss. I.

COLOURS, PRIMARY.— See Gloss. I.

COMBINATION OF BODIES. — Sec
Chemical Combination.

COMMENSURABILITY.— See Ratio.

COMPASS, AZIMUTH.— See Horizon.

-- , MARINER'S.— See Mariner^
Compass,

EXPLANATION OF SCIENTIFIC TERMS.

COMPASS, VARIATION OF.— See fa-
rmer's Compass.
COMPOSITION OF FORCES. — See

Forces.
COMPRESSIBILITY.— See Gloss. I.

CONCAVE MIRRORS See Mirrors ^

LENSES.— See Lenses.

CONDENSATION.— See Gloss. I.

CONDUCTORS OF CALORIC. — See
Gloss. I.

IN ELECTRICITY. If

a body be over-saturated with the electric
fluid, that fluid will endeavour to escape
into other bodies ; or, if under-saturated,
the body will attract the fluid from other
bodies. If the positively, or negatively,
electrified body be surrounded by other
bodies through which the fluid cannot
pass, it is said to be insulated, — to be in
a state of insulation ; and substances
which thus oppose the egress or ingress
of the fluid are said to be Non-conductors.
Those through which the passage is un-
obstructed are Conductors.

CONE.— See Gloss. I.

CONJUGATE DIAMETERS.— See Gloss.
I. in Conic Sections.

HYPERBOLAS. — See

Gloss. I. in Conic Sections.

CONOID.— See Gloss. I.

CONSECUTIVE POLES.— See Magnet.

CONSEQUENTS.— See Ratio.

CONVERGING RAYS.— See Aberration.

CONVEX LENSES AND MIRRORS.—
See lenses and Mirrors.

CORUNDUM.— See Gloss. I.

CRYSTALLIZATION.— See Gloss. I.

CUBIZITE.— See Anakime.

CURVE AND CURVATURE. — See
Gloss. I.

CURVES, EVOLUTES, AND INVO-
LUTES.—See Gloss. I.

, EQUATION OF.— See Gloss. I.

, MAGNETIC.— See Magnetism.

CURVILINEAL.— See Gloss. I.

CYANOMETER, an instrument invent-
ed by Saussure, for comparing the dif-
ferent shades of blue, of which a good de»
scription is given in Optics, pp. 65, C6.

CYCLOID.— See Gloss. I.

CYLINDER.— See Gloss. I.

D'ALEMBERT'S PRINCIPLE. — See
Principle, &c.

DEAD LEVEL.— See Level.

DECAMETRE.— See Metre.

DECARBONIZATION.— See Carbon.

DECIMETRE.— See Metre.

DEGREES AN D MINUTES.— See Angle.

DENSITY (Latin densitot, closeness) is a
relative term, and denotes the compara-
tive quantity of matter in different bo-
dies, which is contained in the same space
(see Fblumej. Gravity is understood to
act in proportion to the relative quantity
of the matter of bodies ; and, hence, the
specific gravities of bodies are presumed
to be the measure of their densities. — See
Gravity.

DE-OXIDATION is the depriving a sub-
stance of the oxygen, or vital air, which
it contains. Concerning the de-oxidizing
power of the solar rays, see Optics,
p. 29.

DESCARTES'S OVALS.— See Ovals of
Descartes.

DIAGONAL, in geometry, is a straight
line, drawn through a figure from one
corner to another. A four-sided figure
has two diameters, which, when the angles
are right angles, are equal.

DIAMETER OF A CIRCLE, a right
line passing through the centre, and ter-
minating at both ends by the circumfer-
ence.

DIAMETERS, TRANSVERSE AND
CONJUGATE. See Conic Sections.

DIAPHANOUS (from the Greek dia,
through, and phaino, to shine) is that
quality of a substance which allows a pas-
sage to the rays of light. Translucent is
a more ordinary term, of the same im-
port. Transparent rises above translucent,
by admitting not only the passage of light,
but the vision of outward objects. Semi-
translunent and semitransparent are occa-
sionally written to express weaker degrees
of those qualities.

DIFFRACTION, or INFLEXION OF
LIGHT. Light, when it meets with no
obstacle, proceeds in straight lines ; but,
if it be made to pass by the boundaries of
an opaque body, it is turned from its rec-
tilineal course, which deviation is termed
diffraction, or inflexion. See Newton's
Optics, p. 10.

DIGESTER, a strong vessel of iron or
other metal, having a screwed-down and
air-tight lid, into which animal or other
substances are inclosed immersed in wa-
ter, and submitted to a higher degree of
heat than could be had in open vessels,
whereby the solvent power of the water is
so increased that bones (for which it was
originally invented) are converted into a

DIOPTRICS is that division of the science
of optics which treats of the refraction of
light.

DIPPING NEEDLE, a magnetic needle
poised so as to move freely in a vertical
direction, points downward, or dips, less
or more, towards the earth ; except it be
situated on some part of a line which
surrounds the globe, and is called the
magnetic equator. On this line the needle
remains horizontal. This equator, which
intersects the geographical equator at an
angle of about twelve degrees, has its
magnetic poles at a short distance from the
true poles of the earth.

DIRECT PROPORTION, or DIRECT
RATIO.— See Ratio.

DIRECTION, LINE OF.— See Force,
Direction of.

DIRECTIVE FORCE, in magnetism,is
the tendency in one magnet to assume a

EXPLANATION OF SCIENTIFIC TERMS.

particular position with relation to ano-
ther magnet.
DIRECTRIX OF A PARABOLA.—

See Omic Sections.

DISTILLATION is a process by which a
fluid, or portion of a liquid, is converted
into vapour by means of heat; and that
vapour returned into a liquid state by
cold, or, as some say, by the abstraction
of caloric. Distillation is evaporation,
that is, raising1 a liquid to the state of
vapour ; but the latter term does not
include the idea of preserving that vapour
and condensing it again into a liquid.
The chief object of distillation is either
to separate mixed liquids, where the
steam of one rises at a lower heat than
the other; or to free a simple liquid from
dregs or impurities. In the distillation
of vinous liquors, alcohol is produced,
which boils at a much lower heat than
water; but it is not yet perfectly ascer-
tained whether this alcohol existed ready
formed in the liquor, or owes its forma-
tion to a chemical action during the pro-
cess. ' The grosser liquor remaining in
the still, after the spirit is all exhausted,
is called spent wash, and is food for hogs.
DIVERGING RAYS (of light) are the
opposite of converging (which see). They
separate in their progress further and
further asunder, as the radii of a circle do
from its centre.

DODECAHEDRON.— See Rhombus.
DOGMATISM —See Empirical.
DOUBLE REFRACTION.— See Refrac-
tion.

DUPLICATE RATIO.— See Ratio, Com-
pound.

DYNAMICS (Greek, dynamis, force) is
that division of the science of mechanics
which considers bodies as acted upon by
forces that are not in equilibria. It,
therefore, treats of bodies in motion. — See
Equilibrium.

DYNAMETER (a Greek compound signi-
fying a measurer of power) is an instru-
ment so contrived as to measure, with
accuracy, the magnifying powers of mi-
croscopes and telescopes. It is described
at page 1 1 of the treatise on Optical In-
struments.

EBULLITION.— See Boiling.
ELASTICITY (from a Greek word signi-
fying to push, or drive back) is that qua-
lity of a substance, whether solid or fluid,
by which, when compressed, or when
forcibly expanded, it endeavours, in either
case, to reassume its former bulk.
ELASTIC FLUIDS.— See Fluids and Gas.
ELECTIVE ATTRACTION.— See Che-
mical Attraction.

ELECTRICITY. If a dry piece of amber
be rubbed by the hand, it will acquire the
property of alternately attracting and re-
pelling light bodies, such as bits of straw,
paper, or leathers. This power continues
only for a short time, when it becomes

exhausted and ceases to art. If the expe-
riment be made in the dark, sparks of
light will be observed, passing between
the light -bodies and the amber. Glass
and several other substances have similar
properties ; and the collection of pheno-
mena which have been observed concerning
them is called electricity, from the Greek
electron, amber. Bodies that are capable
of being endowed with this power of
attraction and repulsion and the emission
of light are termed electrics. They are said
to be excited by the friction ; and a ma-
chine contrived to produce this excitation
in an easy and rapid manner, by means
of a peculiarly-constructed cushion, called
a rubber, is an electrical machine. Bodies
that are not excitable by such means are
non-electrics.

ELECTRIC FLUID. In order to ac-
count for the various phenomena of elec-
tricity, it has been supposed that there is
a subtile fluid, identical with lightning,
which pervades the pores of all bodies,
and is capable of motion from one body
to another. When the natural quantity
belonging to a particular body (then said
to be saturated} is increased, that body is
said to be positively electrified ; and, when
that quantity is diminished, it is nega-
tively electrified. These two states have
also been called plus and minus electricity ;
and the former being obtained from glass
and the latter from sealing-wax, the two
are sometimes distinguished by the terms
vitreous and resinous electricity.

ELECTRICITY, ANIMAL.— See Ga/-
vanism.

, VOLTAIC.— See Gal-

} POSITIVE AND NE-
GATIVE.—See Electric Fluid.

- — , RESINOUS AND VI-

TREOUS.—See Electric Fluid.

ELECTRICS AND NON-ELECTRICS.
— See Electricity.

ELECTRICAL BATTERY.— See Leyden
Phial.

ELECTRO -MAGNETISM is a science
which comprehends the phenomena shew-
ing the connexion between Electricity
and Magnetism.

ELECTROMETER, an instrument nsed
for ascertaining the quantity and quality
of the electricity in an electrified body. —
See Index.

ELECTROSCOPE, an instrument for ex-
hibiting the attractive and repulsive agen-
cies of electricity. — See Index.

ELIXIR VIT.E.— See Alchemy.

ELLIPSIS.— See Cone and Conic Sections.

ELLIPSOID.— See Conoid and Spheroid.

EMERALD. The emerald is ranked
among the 'gems, and is now found only
in Peru. It is of a green colour, rather
harder than quart/, and always in crys-
tals, which are translucent and generally
transparent. What is called oriental

G

EXPLANATION OF SCIENTIFIC TERMS.

emerald is a green sapphire. The beryl is
a variety of the emerald, of a paler green,
frequently passing into blue, and is much
less prized. It is found in various coun-
tries, sometimes in Scotland. The eme-
rnld of Brazil is a tourmaline, which see.

EMPIRICAL (Greek en, and peirao, I try)
designates any assertion or act which is
made or done as an experiment, indepen-
dently of hypothesis or theory. The term
is generally used in a bad sense; especially
in the science of medicine, in which an
empiric is synoiiimous witli a quack. The
empiric is supposed to set up his own
short-lived experience to the collected
knowledge of ages, which is understood to
guide the regular practitioner ; and
which the empiric, in his turn, vilifies by
the name of dogmatism, a Greek deriva-
tive, denoting a fixed and positive opinion.

EQUATION OF A CURVE.— See Conic
Sections.

EQUATOR, MAGNETIC.— See Dipping
Needle.

EQUILIBRIUM. When two or more
forces, acting upon a body, are so opposed
to each other that the body remains at
rest, although either would move it if
acting alone, those forces are said to be
in equilibria, which is a Latin term signi-
fying equally balanced.

ERIOMETER, an instrument, invented by
Dr. Young, for the purpose of measuring
the diameters of minute fibres. It is de-
scribed in the treatise on Optics, pp.37, 38.

ETHER.— See Air, Ethereal.

— (in chemistry) is the name of an

extremely volatile liquid, formed by the
distillation of some one of the acids with
alcohol. There are, of course, several
modes of production, according to the
acid employed ; as nitric ether, sulphuric
ether, &c. ; but, when well rectified, the
ether is the same whatever acid has been
employed.

EUCLASE, a scarce species of emerald, re-
markably brittle ; and hence its Greek
name.

EVAPORATION is the conversion of
water or other liquids into vapour or
steam, which becomes dissipated in the
atmosphere in the manner of an elastic
fluid. When the evaporation is sponta-
neous, that is, at an ordinary tempera-
ture of the atmosphere, and without arti-
ficial heat, it is commonly called exhala-
tion, whereas evaporation is often under-
stood to be produced artificially, and pre-
ceded by boiling. — See Boiling, Steam^
and Vapour.

EVOLUTE OF A CURVE.— See Curves.

EXCENTRICITY. The distance between
the /bet of an ellipsis is called its excen-
tricity, and sometimes its ellipticity. It
is in this way that we speak of the excen-
tridly of the orbits of the planets which
are supposed to move in ellipses.

EXCITATION.— See Electricity.

EXHAUSTED RECEIVER. — See Va-
cuum.

EXPANSIBILITY.— See Gloss. I.

FA H RE N H KIT' S THE IIMOMETER.—
See Gloss I.

FELDSPAR.— See Gloss. I.

FIRST, or PRIME MOVER, in mecha-
nics See Machine.

FIXED ALKALIES.— See Alkali.

FLUID, ELECTRIC.— See Electric Fluid.

, MAGNETIC. — See Magnetic

Fluid.

FLUIDITY.— See Gloss. I.

FLUIDS, ELASTIC, NON-ELASTIC,
&c.— See Gloss. I.

FLY-WHEEL.— See Gloss. I.

FOCUS.— See Gloss. I.

REFRACTED AND GEOME-
TRICAL. The point in which the rays
of light (according to their known laws)
ought to be concentrated, when reflected
from a concave mirror, or refracted
through a lens, is termed the Geometrical
Focus,- that in which they are actually
found is the Refracted Focus. These
foci are separated from one another in
proportion to the degree of spherical
aberration .

FORCE.— See Gloss. I.

CENTRIFUGAL.— See Centri-

fugal,
petal.

CENTRIPETAL. — See Centri-

DIRECTIONOF.— See Gloss. I.

COMPOSITION.— See Gloss. I.

ACCELERATION OF. — See

Acceleration.

• DIRECTIVE.— See Magnets, Ar-
tificial.

FORMULA, a short general form, or rule,
easily to be remembered, directing how
certain things may be done. The word
is a Latin diminutive, and has often the
plural formula in place of formulas.

FREEZING-POINT.— See Gloss. I.

FRICTION.— See Gloss. I.

FRIGORIFIC.— See Gloss. I.

FULCRUM.— See Lever and Balance.

FUSION.— See Gloss. I.

GALVANISM, so named after its disco-
verer, Professor Galvani of Bologna,
" includes all those electrical '.pheno-
mena, arising from the chemical agency
of certain metals with different fluids*."
Volta discovered the means of multiplying
those effects : hence the science has also
been called J'oltaism ; and (from its action
on the muscles of animals newly killed)
Animal Electricity.

GALVANIC CIRCLE. If, between two
plates of different metals, a fluid be inter-
posed which has a chemical effect on
one of the metals, and little or none on
the other; and if a communication be
made between other parts of the plates,
by means of a conducting substance (as a

* For a definition which involves no hypothesis, see
Galvanism, § 1,

EXPLANATION OF SCIENTIFIC TERMS.

wire), a continued current of Electri -ity
will be produced, through the conductor,
from one plate to the other, as loner as
the chemical action is exerted. This is
the simple Galvanic Circle. A number
of similar pairs of plates, placed alter-
nately with an acting fluid intervening
between each, and the two ends of the
series connected by a wire, multiplies the
effect, and is called a Galvanic, or I'ultaic,
pile. For the construction of such piles,
and of batteries of various forms, see
Galvanism, Chapters II. and III. The
end of the galvanic pile which gives out
the electric fluid is called thj positive
pole, and the other end, in which the
wire terminates, and which receives the
electric matter, is called the negative pole
of the pile.

GALVANIC PILE.— See Galvanic Circle.

G A S._ See Gloss. I.

GASEOUS signifies that the suhstance
spoken of has the nature of gas ; and
thus gaseous fluids are distinguished from
other fluids.

GAS, AZOTIC.— See Azote.

CARBONIC ACID._See Carbon.

HYDROGEN.— See Oxygen.

NITROGEN. —See Azote.

GEOMETRICAL PROGRESSION.— See
Progression,

GLASS DROPS.— See Rupert's Drops.

A XN E A LIN G OF.— See Annealing.

GLAUBERITE.— See Gloss. I.

GONIOMETER.— See Gloss. I.

GOVERNOR.— See Gloss. I.

GRAVITY.— See Gloss. I.

CENTRE OF. The centre

of gravity of a solid hody is a point so
situated ivith respect to it, that if the
hody could he suspended from the point
in question, the whole body would remain
at rest (with respect to its tendency to
the earth) in whatever situation the
surrounding parts may be turned. Thus,
the centre of gravity of a globe, if of
uniform density, is its common centre ;
and that of a balanced beam is the pivot
on which it turns. See this further illus-
trated in the Popular Introductions, pages
xviii. xix. and xx.

~- RELATIVE AND SPECI-

FIC. The comparative or relative gra-
vities of different bodies towards the
earth are measured by a general standard
called weight, and one substance is said
to have a greater specific gravity than
another, when a less portion of its bulk
is of equal weight to that other. Thus,
a cubic inch of platina is nearly twice the
weight of a cubic inch of silver ; and,
therefore, is said to have double its speci-
fic gravity, — the specific gravity of platina,
relative to that of silver, is as two to one.
In this mode of comparison, the gravities
are relative ; but a standard is assximed
with which both may be compared. That
standard, for solids and liquids, is water,

•which is reckoned unity, and compared
with that fluid, silver is 10.5 and platina
'J1.4 : that is, equal bulks of water, silver
and platina, would have weights in these
proportions. In designating the specific
gravities of gases, the standard, or unity,
is atmospheric air.

GRAVITY, LINE OF DIRECTION OF,
is that right line which passes through
the centre of gravity of a body in a direc-
tion towards the centre of the earth.

GYRATION (Latin gyrus, a circle) is
the action of turning round a centre, in
the manner of a wheel.

, CENTRE OF, is a point in

a revolving body, into which, if all its
matter could be collected, it would con-
tinue to revolve with the same energy as
when its parts were in their original
places.

HALO, u a luminous and sometimes co-
loured circle, appearing occasionally
around the heavenly bodies, but more
especially about the sun and moon;"—
See Parhelia.

HARDNESS is the resistance to impres-
sion. It is incompressible, but limited to
solids. — See Compressibility.

HEAT.— See Caloric.

, CAPACITY FOR.— See Caloric,

Capacity for.

CONDUCTORS OF.— See Con-
ductors.

LATENT.— See Caloric, Latent.

RADIATION OF.— See Radiation.

. SPECIFIC.— See Caloric, Specific.

HECATOMETRE.— See Metre.

HELIX, the scientific name for a spiral.

HERMETIC SEAL.— See Gloss. I.

HEXAHEDRON.— See Rhombus.

HETEROGENEOUS.— See Gloss. I.

HOMOGENEOUS.— See Gloss. I.

HORIZON is a Greek word signifying a
boundary, and was employed to denote
the circle in which the apparent plane of
the earth terminated in the concave of
the sky. This is now called the sensible
horizon, to distinguish it from the true
or astronomical horizon, which is parallel
to the sensible, but is conceived to be a
plane passing through the centre of the
earth and dividing the whole celestial
sphere into the upper and lower hemi-
spheres. That the ancients believed the
earth to be an extended plain is obvious
from the words which they employed in its
description. They believed, too, that it was
longer from east to west than from north
to south : calling the former its length
and the latter its breadth, and hence we
have our terms longitude and latitude.
A line or plane in, or parallel to, the hori-
zon, is horizontal. Planes passing through
the zenith (or point which is directly
over our head) and the centre of the earth
are called vertical planes ; and the circles
in the heavens marked by such planes are
vertical circles, or azimuths. All these

8

EXPLANATION OF SCIENTIFIC TERMS.

circles cut the horizon at right angles ;
and it is in an arc of one which passes
through a star that the altitude of the
star above the horizon is measured. The
vertical circle which passes through the
place of the sun at noon, cuts the horizon
at the north and south points, and is called
the meridian of the place where it is
supposed to be drawn. Any other vertical
plane which may be imagined to pass
through the sun, or a star, when not in
the meridian, will make an angle with
the plane of the meridian, which will he
measured by the portion of the circle of
the horizon which it cuts off to the east
or west of that meridian. This angle, or
portion of the circle of the horizon is called
the Azimuth of the sun, or star, at that
time and place. The Magnetic Meridian,
as pointed out by the Mariner's Compass,
differs from the true (or real north and
south) by the amount of the Variation of
the Compass, and so does the apparent or
Mctgneiical Azimuth. The Azimuth Com-
pass is constructed so as to find easily
the magnetical azimuth. — See Magnetism,
pages 60, 68 — 72- An arch, intercepted
between the east or west point of the
horizon, and the point (of the same circle)
of the rising or setting of the sun or of a
star, on any particular day, is called the
Amplitude of the sun, or star, for that
day. The distances of the points of
rising or setting from the east and west,
as shown by the compass, is the Magneti-
cal Amplitude.

HYACINTH.— See Zircon.

HYDRATES. Chemical compounds, par-
ticularly salts, which contain water as
one of their ingredients, have been termed
Hydrates. If water is not a constituent,
they are said to be Anhydrous : a Greek
compound which signifies without water.

HYDROUS, watery, or containing water
in its composition.

HYDROGEN.— See Oxygen.

HYGROMETER, an instrument, of which
there are various forms of construc-
tion, for measuring the relative degree
of moisture which exists, at any par-
ticular time and place, in the atmos-
phere.

HYPERBOLA. — See Cone, and Conic Sec-
tions.

HYPERBOLOID.— See Conoid.

HYPOTHESIS.— See Induction.

ICELAND SPAR. — See Calcareous Spar,
and Spar.

ICOSAHEDRON.— See Rhombus.

IDOCRASE, a name sometimes given to
Fesnvian, which see.

IMPENETRABILITY.— See Gloss. I.

IMPULSE.— See Gloss. I.

INCIDENCE, POINT OF.— See Gloss. I.

INCOMMENSURABLE. -See Ratio.

INSTRUMENT.— See Machine.

INDEX OF REFRACTION.— See Re-
fracttve power.

INDIAN RUBBER.— See Caoutchouc.

INDUCTION, in philosophy, is the col-
lecting, or bringing into one focus, a mul-
titude of observations on any particular
subject, and drawing conclusions from
an examination and general survey of
the whole. It is the opposite of Hypo-
thesis, which is to lay down a theory in
the outset, and trusting to future experi-
ments, or example, for its proof.

IN ELECTRICITY is that

effect of an insulated electrified body which
tends to produce an opposite electric state
in neighbouring bodies.

IN MAGNETISM is analo-
gous to electric induction ; for illustra-
tions of which see Magnetism, page 4.

INERTIA.— See Ks Inertia.

INFLAMMABLE AIR.— See Oxygen.

INFLEXION OF LIGHT.— See Diffrac-
tion.

INSULATION.— See Gloss. I.

INVERSE PROPORTION, or RATIO.—
See Ratio.

INVOLUTE OF A CURVE.— See Curves.

IRON, CARBURET OF.— See Carbon.

ISOCHROMATIC, &c.— See Gloss. I.

ISODYNAMIC, having equal power.

JARGON.— See Zircon.

LATENT HEAT.— See Caloric.

LAW OF THE SINES.— See Refractive
Power.

LENS POLYZONAL.— See Polyzonal.

LEVEL.— See Gloss. I.

LEUCOCYOLITE, a name given to a va-
riety of Apophyllite, which see.

LEVER (Latin levare, and French lever,
to lift, or raise) is one of the mechanical
powers. It is an inflexible bar, supported
and moveable in one point of its length
on a pivot, or prop, called the fulcrum.
One end of the lever is applied to the
weight to be raised, while a force is ap-
plied to the other end. The power of
this instrument depends on the proportion
between the lengths of the parts of the
lever on each side of the fulcrum. — See
Balance.

For a plain account of the different kinds
and applications of levers, see Popular
Introductions, pages xx. — xxiv.

LEYDEN PHIAL. The Lcyden Phial, or
Leyden Jar, (so called because it was first
constructed in that city,) is a cylindrical
glass vessel, coated to a certain height,
inside and outside, with some conducting
substance, which is capable of being
charged with electrical fluid, accumulated
for various experiments. A combination
of such phials is an electrical battery.
See those words in the Index for more
complete explanation.

LIGHT, RAYS OF REFLEXION OF,
&c— See Gloss. I.

LIGNUM NEPHRITICUM (Latin %-
num, wood, and Greek nephros, a kid-
ney) is a bitter tasted wood, so called
because it was once believed to be a sove-

EXPLANATION OF SCIENTIFIC TERMS.

reign remedy in nephritis, or inflamma-
tion of the kidneys. It is chiefly im-
ported from Mexico. The wood, 'w'im
cleared of its bark, is of a dirty yellow
colour; but its infusion in cold w'aterhas
a sky-blue tinge, when viewed by a false
light, and a gold colour by a true one.
A small portion of any acid being mixed
with the tincture, both these colours dis-
appear ; but the sky-Wue may be restored
by means of potash.

LIMIT.— See Gloss. I.

LIQUIDS, and LIQUIDITY.-See Gloss. I.

LOADSTONE.— See Magnet, Native.

MACHINE (Latin machina, a frame or
contrivance). Any complication of arti-
ficial bodies acting upon one another by
contact, through the medium and motion
of which any effect is produced, is a ma-
chine. The initial force which puts the
machine in motion is called the First
or Prime mover. The point at which that
force is applied is the Acting point ; and
that in which the effect is produced is
the Working point : the machine being
the medium through which the power is
transferred, and by which it is modified
so as to answer the intended purpose.
When a simple body is the medium be-
tween the actiny and the working points,
it is an Instrument.

MAGNETISM " is that peculiar property
occasionally possessed by certain bodies
(more especially by iron and some of its
compounds), whereby, under certain cir-
cumstances, they mutually attract or re-
pel one another, according to determinate
laws."

MAGNET, NATIVE, or LOADSTONE,

is an ore of iron, found in the iron mines
of Sweden and other places.

MAGNETS, ARTIFICIAL, are made of
small bars of iron or steel, which, when
placed at perfect liberty, turn one end to-
wards the north, and the other, conse-
quently, in a southerly direction. These
two points are termed the North and the
South Poles of the magnet ; and a line,
supposed to connect these points, is its
Axis. The tendency to acquire a direc-
tion nearly north and south is its Pola-
rity. Either pole of the magnet attracts
iron. Slight poles formed at irregular
points of the bar, and which tend to dis-
turb the attraction of the real ones, are
termed Consecutive poles.

MAGNETIC CURVES. For the forms
of these curves, and diagrams illustrative
of their properties, see Magnetism, pages
19—22.

MARINER'S COMPASS, a well-known
instrument, consisting of a small mag-
netic bar, called a needle, poised on its
centre of gravity, so as to be enabled,
with the greatest ease, to turn every way
in an horizontal direction. After a few
vibrating motions, during which it is said
to traverse, the needle takes its direction

nearly north and south, which direction
is said to be in the plane of the Magnetic
Meridian of the place where the compass
happens to be. The true Geographical
Meridian is a plane passing through the
zenith and the poles of the earth. The
angle, which the magnetic meridian is
east or west from the geographical, or
true north and south, is different in dif-
ferent places and at different times ; aiid
this is called the Variation of the Com-
pass. — See Dipping Needle.

MAGNETIC FLUID is a hypothetical
fluid, by which the phenomena of mag-
netism have been accounted for. Some
have supposed two such fluids, a Boreal
or northern, and an Austral or southern.

MASS (of matter). — See Volume.

MAXIMUM. In a variable quantity or
effect, that quantity or effect which is the
greatest possible, under the circumstances
in which it is placed, is termed a maxi-
mum. Thus, in respect to the sails of a
windmill, they may be placed at any
angle, but there is one angular direction
on which the wind will have more power
than on any other, and this is, therefore,
termed a maximum. There are other
cases, in which we seek for a minimum,
that is, the least possible.

MECHANICS is that science which in-
vestigates the nature, laws, and effects of
motion and moving powers.

MECHANICAL POWERS are the simple
instruments or elements of which every
machine, however complicated, must be
constructed : they are the Lever, the
Wheel and Axle, the Pulley, the Inclined
Plane, the Wedge, and the Screw.

MELTING POINT. That point of the
thermometer which indicates the heat at
which any particular solid becomes fluid,
is termed the melting point of that solid.

MENISCUS (Greek mene, the moon) a
lens which is concave on one side, and
convex on the other ; and so called be-
cause it resembles the appearance of the
new moon.

MERIDIAN, MAGNETIC.— See Mari-
ners Compass.

METALS, annealing of. — See Annealing.

METAPHYSICS.— See Physics.

METRE. A metre is the French standard
measure of length, equivalent to 39.371,
or very nearly 39|, English inches. The
French measures ascend and descend in a
decimal progression. Thus,

English Inches.
A Millimetre is ... .03937

Centimetre 39371

Decimetre .... 3.93710

Metre 39.3/1

Decametre . . . 393-71
Ilecatometre . . 3937.1
Chiliometre . . 393? 1
&c.

MICA is a mineral of various colours, but
usually gray. It is chiefly distributed.

10

EXPLANATION OF SCIENTIFIC TERMS.

in very thin plates, through quartz, feld-
spar, and other fossils ; but it is also
found in the form of prismatic crys-
tals. It constitutes a principal ingredient
in many of what are termed primitive
rocks. The plates are sometimes large
(as much as eighteen inches diameter),

\ and extremely thin. They are employed
in Russia for window- panes ; and, in
that state, are called Muscovy Glass.

MILLIMETRE.— See Metre.

MINERAL ALKALI.— See Alkali.

MINIMUM.— See Maximum.

MIRRORS, PLANE, CONCAVE, and
CONVEX.— See Gloss. I.

MOCHO STONE.— See Agate.

MOLECULE is a diminutive, formed
from the Latin motes, a mass, and denotes
one of the minute particles of which the
mass or hody is composed. Molecules differ
from Atoms in this, that they are never
considered but as portions of some aggre-
gate. An Atom (from the Greek a priva-
tive, and tamno, I cut) is accounted as
one of the simplest particles in nature, —
what is incapable of further division.

MOMENTUM, or MOMENT, is the im-
petus, or force of a moving body. The
comparative momenta of bodies are in a
compound ratio of their quantity of mat-
ter and their velocity : that is, they are
in proportion to the products of the mat-
ter and velocity, when expressed in num-
bers. Thus a ball of four pounds weight,
moving uniformly at the rate of eighteen
feet in a second, would have double the
momentum, — that is, it would strike
against an object with twice the force
that a ball of three pounds weight, mov-
ing at the rate of twelve feet per second,
would do ; because the first product (4
multiplied by 18) is double that of 3 mul-
tiplied by 12. Momentum is the/orce of
percussion. — See Percussion.

MOONSTONE.— See Feldspar.

MOTION is the passing of a body, or any
parts of a body, from one place to an-
other : we say parts of a body, because
in the cases of a globe turning on its axis,
or a wheel revolving on a pivot, the parts
of the body change their situation, while
the bodies themselves are stationary.

MOVING POWER.— See Power.

MUSCOVY GLASS.— See Mica.

NATIVE MAGNET.— See Magnet:

NEGATIVE ELECTRICITY.— See Elec-
tric Fluid.

NITRIC ACID.— See Azote.

NITROGEN.— See Azote.

NON-CON D UCTORS.— See Electric Fluid.

NON-ELECTRICS.— See Electricity.

NON-ELASTIC FLUIDS.— See G«*and

Liquids.

NONIUS.— See Fernier'a Scale.
OBLATE and OBLONG SPHEROIDS.

— See Spheroid.

OBTUSE ANGLE.— See Angle.
OCTAHEDRON,— See Rhombus.

OCULAR SPECTRA.— See Accidental
Colours .

OPACITY (Latin o/?acws,dark) , a state im-
pervious to light.

ORDINATE, of an Ellipse, Parabola, and
Hyperbola. — See Conic Sections.

OSCILLATION (Latin, oscillatio, swing-
ing) is particularly applied to designate
the motion of a pendulum.

, CENTRE OF. « The

centre of oscillation in a pendulous body
is a point in the line passing through the
centre of suspension, and the centre of
gravity. If all the matter of the body
could be collected in that point, any force
applied there would generate the same
angular velocity, in a given time, as the
same force would generate in the same
time, by acting similarly at the centre of
gravity of the pendulum, when all the
parts thereof are situated in their respec-
tive places." This point differs from the
Centre of Gyration, because the motion of
the body is produced by the gravity of its
own particles, whereas in Gyration, the
body is put in motion by some other force,
acting at one place only. — See Centre of
Percussion.

OVALS. These figures have their name
from their resemblance to the transverse
section of an egg, Latin ovum. Ellipses
are ovals which are formed by a fixed law,
but the latter is a popular term for any
curved figure, approaching to that shape.
The carpenter's oval, for example, is made
up of circular arcs, that unite without
leaving any angular appearances at their
junctures.

OVALS OF DESCARTES. These, though
not Ellipses, are governed by a determi-
nate law, which constitutes them as va-
rieties of that curve. As in the Ellipse
the two lines drawn from the foci to any
point of the circumference vary, so that
the increment of one shall always be
equal to the simultaneous decrement of
the other ; so, in the Cartesian Ovals^
these increments are in an invariable ra-
tio. " These curves may therefore be
defined the locus (place) of the vertex of
a triangle, on a given base, one of whose
sides bears a given ratio to the sum or
difference of a given line and the other
side."

OVERSHOT WHEEL.— See Water-
Wheel.

OXYGEN, or OXYGEN GAS, is that
portion (something more than a fourth)
of the atmospheric air, which is capable
of supporting flame, and is essential to the.
respiration of animals. Oxygen is gene-
rally diffused throughout nature, but al-
ways in combination with other sub-
stances. United with Hydrogen Gas (In-
flammable Air} it forms water. It was at
one time supposed to be a necessary in-
gredient in the composition of all acids ;
and hence its name, which is derived:

EXPLANATION OF SCIENTIFIC TERMS.

11

from the Greek, and signifies a generator
of sours ; but acids have been sinco dis-
covered, which contain no oxygen. It
unites with metals and other substances
during their combustion, and forms
Oxides, what were formerly called Calces.
Hence we say that those; substances are
Ojrydabfe, and speak of the Oxides of iron,
of h-ad, tScc. — See De-Ofydalion.

PARABOLA.— See Cone and Conic Sec-
tions.

PA R A BOLOI D.— See Conoid.

PARALLEL LINES.— When two straight
lines, in the same plane, are so directed
that, however much they might be length-
ened, they would never approach nearer
to, nor recede from one another, they
are said to be parallel.

PARALLELOGRAM is a right-lined and
four-sided figure, whose opposite sides are
parallel, and consequently equal. When
its angles are right-angles, the figure is
usually called a Rectangle or Oblong. If
all its sides, as well as angles, are equal, it
is a Square. — See Rhombus.

PARALLELOPIPED.— A parallelepiped
is a solid having six sides, each of which
is a parallelogram, and consequently al-
ways equal to and parallel to its opposite.
It is a prism whose base is a parallelo-
gram.— See Prism.

PARHELIA, PARHELIUM, or PAR-
HELION (Greek para, near, and helios,
the sun), is a mock sun ; an appearance
similar to the sun, which occasionally ac-
companies halos (See Halo.) There have
been sometimes seen six or seven of these
mock suns at the same time, which, in
that case, are denominated by the plural,
Parhelia.

PENCIL OF LIGHT.— See Light, Ray

of.

PENUMBRA (Latin pene, almost, and
umbra, a shadow) is a partial shadow ;
that is, a shadow which only receives a
portion of the rays of a luminous body,
when that body has a measurable diame-
ter. This is well explained in Newton's
Optics, at page 23.

PERCUSSION, CENTRE OF. Percus-
sion is a forcible stroke given by a moving
body. In taking any .particular body,
such as a rod of equal thickness, held at
one end, and swung forcibly by the hand,
so as to strike upon a resisting object, the
forca of the stroke will be greater or less,
according to the part of the rod that shall
hit the object. There is one point of the
rod in which the whole force of the stroke
is concentrated, and the resistance to
which would neutralize the blow. That
point is termed the Centre of percussion.
When the percutient body revolves round
a fixed, the centre of percussion is the
same with the Centre of oscillation, and
if all the parts of the percutient body be
carried forward with the same celerity

(which is uot the case of tjie pendulum),

the centre of percussion is the same with
the centre of gravity.

PERCUSSION, FORCE OF.- See Mo*
mentum.

PERIGEE.— See Apogee.

PERIHELION.— See Aphelion.

PERIMETER. The length of the whole
bounding line of any plane figure, of
whatever parts or shapes that line may
consist, is termed the perimeter of the
figure. The length of the bounding line
of a circle (and, perhaps, of any curve
which returns upon itself) is its circum-
ference.

PERPENDICULAR.— See Angle.

PETUNTSE.— See Feldspar.

PHENOMENON (plural Phenomena} is a
Greek word, signifying an Appearance,
and is limited, in our language, to denote
those appearances in nature, whether dis-
covered by direct observation or experi-
ment, for which there is no obvious
cause. An Hypothesis is an assumed
cause, by which we endeavour to account
for a particular class of phenomena ; and
that hypothesis is best which solves the
greatest number.

PHLOGISTON (from the Greek phlego, to
burn) is a name given by the older che-
mists to an imaginary substance, which
was the principle of inflammability. Ac-
cording to them, every combustible body
was formed of an incombustible base,
united to this phlogiston, which escaped
into the atmosphere during the combus-
tion. This process is now attributed to
the union of certain known substances
(chiefly oxygen), which are therefore
called Supporters of Combustion.

PHOTOMETER (Greek phos, light, and
metron, a measure) is an instrument for
measuring the different intensities of
light. These instruments are variously
constructed ; but those of Rumford and
Leslie are the most generally known.

PHYSICS (Greek physis, nature) is that
science which is employed in observing'
the phenomena, and investigating the
constitution, powers, and effects of the
several bodies in nature. Aristotle, that
celebrated Greek, whose writings formed
the text-books of the schools for so many
centuries, after having treated of Phy-
sics, or Nature, added certain disquisi-
tions, concerning- Being in general, the
Soul of Man, and the Deity. These were
termed his Metaphysics (meta, beyond),
because they Avere distinct, or beyond
what he understood by Physics, or Na-
ture.

PILE, GALVANIC, or VOLTAIC.— See
Galvanic Circ/e.

PISTON.— See Gloss. I.

PLANE, TANGENTIAL— See Tangent.

, HORIZONTAL.— See Horizon

, VERTICAL.— See Horizon.

PLUMBAGO.— See Carbon.

PLUNGER.— See Piston.

12

EXPLANATION OF SCIENTIFIC TERMS.

POINT, ACTING.— See Machine.

BOILING.— See Boiling point.

FREEZING. — See Freezing

point.

OF INCIDENCE.— See Refrac-
tive Power.

WORKING.— See Working point.

POLARITY (of a magnet).— See Magnet,
Artificial.

POLES, of amagnet. — See Magnet. Artificial.

• OF A GALVANIC PILE.— See

Galvanic Circle.

POLYHEDRON.— See Rhombus.

POLYZONAL designates, literally, what
is composed of many zones, or belts.
The term is applied in this volume to
certain lenses, composed of pieces united
in rings which are therefore called Poly-
zonal lenses.

PORES (of matter.) — See Volume.

POWER is that principle which is capable
of effecting a change in the state or con-
dition of a body. When power is exerted,
as in mechanics, it is force, applied for
the purpose of producing or preventing
motion. In the former case it is termed
a moving .power, or force, and in the
latter a sustaining power , or force. Power
is latent force.

- ANIMAL, or ANIMATE, is the

power of a man, or other animal.

INANIMATE, is that of air,

Jire, water, or other inanimate bodies.

• MECHANICAL.— See Mecha-
nical Power.

, IN OPTICS expresses the effect

producible by lenses, or other instru-
ments, as magnifying power, healing
power, 8fc.

PRESSURE is the application of force to
a resisting body, when that force is in
continued contact with the body upon
which it is exerted. — See Impulse and
Percussion.

ATMOSPHERIC.— See At-

mospheric Pressure.

CENTRE OF. When a

fluid presses upon a surface, there is a
point in that surface, at which, if a force
be applied in the same line with the pres-
sure of the fluid, and equal to the whole
of that pressure, but in a contrary direc-
tion,— this counter-force will exactly
balance the whole pressure of the fluid,
— and that point is called the centre of
pressure.

PRIME MOVER.— See Machine.

PRIMARY COLOURS.— See Colours, pri-
mary.

PRINCIPLE, D'ALEMBERT'S, in Me-
chanics, is this : — If several non-elastic
bodies have a tendency to motion, with
velocities, and in directions which they
are constrained to change, in consequence
of their reciprocal action on each other,
then these motions may be considered as
composed of two others ; — one which the
bodies actually take; and the other such,

that, had the bodies been acted on by
such alone, they would have remained
in equilibrium. — See Equilibrium and
Forces, composition of.

PRINCIPLE OF VIRTUAL VELOCI-
TIES. " When a system of material
points, solicited by any force, is in equi-
librium, if the system receive a small al-
teration in its position, by virtue of
which every point describes an infinitely
small space, the sum of each force multi-
plied by the space described by the point
to which it is applied, according to the
direction of the force, is always equal to
zero." This is the general principle of
virtual velocities, referred to at page 2 of
MECHANICS, Treatise II.

PRISM. A prism is a solid contained by
plane figures, of which two are parallel,
and the rest are parallelograms.

PRISMATIC SPECTRUM is the various-
coloured appearance (Latin spectrum}
which a ray of white light exhibits when
separated by refraction through a glass
prism. The prism of the opticians is
triangular ; that is, its two ends are pa-
rallel, equal and similar triangles, and
conseqxiently its other faces are three pa-
rallelograms.

PROGRESSION, in a general sense, is
merely going forward, but as the word is
understood, it is presumed that the pro-
gress is in a determinate order. It is
motion measured by some scale whatever
that scale may be.

, ARITHMETICAL, is a

series of numbers, or quantities, in which
each term differs from that which pre-
cedes it by a fixed number, or quantity,
called the ratio. Thus 3, 5, 7, 9, 11, &c.
is an arithmetical progression having the
ratio, two ; that being the number by
which each term differs from the adjoin-
ing one.

GEOMETRICAL, is

series of numbers, or quantities, in which
every two consecutive terms differ by a
multiplier which is common to the whole
series : this multiplier is the ratio. Thus
2, 4, 8, 16, 32, &c. is a geometrical pro-
gression, of which every term is double
an adjoining one, and whose ratio (or
rate of increase) is 2. Whether arith-
metical, or geometrical, progressions in-
crease or decrease, that is, ascend or
descend, the principle, or order of pro-
gression, is the same.

PROPORTION, direct and inverse.— See
Ratio.

PROPORTIONALS.— See Ratio.

PYRAMID.— See Cone.

PYROMETER.— See Thermometer.

PYRO SCOPE.— See Thermometer.

QUADRANT.— See Angle.

QUARTZ.— See Gloss. I.

RADIATION— See Gloss. I.

RARITY, in bodies, is the opposite of Den-
sitfo which see.

.EXPLANATION OF SCIENTIFIC TERMS.

13

RATIO. In comparing two subjer's, -with
regard to some quality which they ha\ •<•
in common, and which admits of being
measured, that measure is their ratio.
It is the rate in which one exceeds the
other. Proportion is the portions, or
parts of one magnitude that are con-
tained in another. When the ratio is
commensurable, (that is, when it is redu-
cible to numbers,) it is equivalent to pro-
portion ; but the latter term is usually
employed in the comparison of ratios, in
which case, two equal ratios are said to
be proportionals. Thus 3 has to 4 (written
3 : 4) a certain ratio, or .proportion ; but
the expression 3 is to 4 in the same pro-
portion as G to 8, denotes that the ratios
of 3 to 4 and G to 8 are equal ; 3 being
the same proportion of 4 as 6 is of 8, that
is, three-fourths. Ratios, however, may
be unequal. Thus it is said that the
ration of 9 to 4 is greater than that of
7 to G, because | is greater than | : it
being thus that ratios are measured.

DIRECT AND INVERSE.

When two quantities, or magnitudes,
have a certain ratio to each other, and
are, at the same time, subject to increase
or diminution; if, while one increases, the
other increases in the same ratio, or, if
while the one diminishes, the other dimi-
nishes in the same ratio, the proportions,
or comparisons of ratios, remain unal-
tered, and those quantities, or magni-
tudes, are said to be in a direct ratio or
proportion to each other. Thus, if a
yard of cloth be worth a pound, ten or
any number of yards will be worth so
many pounds, and the proportion of value
continues unaltered.

But, if the magnitudes are such, that,
when one increases, the other necessarily
diminishes ; and vice versa, when the
one diminishes the other increases, the
ratio, or proportion, is said to be inverse.
Thus, there is, at any moment, a certain
ratio of the length of the day to that of
the night; but this is an inverse ratio;
for, in proportion as the length of either
increases, that of the other must diminish.

RATIO, Compound. A compound ratio is
made up of the product of two, or more,
simple ratios ; that is, of the product of
their first terms, which are called Ante-
cedents, compared with the product of
their second terms, called Consequents.
Thus 24 : 3 is a compound ratio of 4 : 1
and G : 3 ; this being made up of two
simple ones is called a Duplicate Ratio.
When three simple ratios are compounded,
they form a Triplicate Ratio ; if four, a
Quadruplicate Ratio, and so of other com-
pounds.

RAY is a single radiation from a body which
sends out emissions in all directions. —
See Radiation and Light.

RAYS, ABERRATION OF.— See Aber-
ration.

KAYS, CALORIFIC.— See Calorific Rays.

COLOURED.— See Colour* and

Prismatic Spectrum.

CONVERGENT.— See Reflexion,

Laws of.

DIVERGENT. — See Reflexion,

Laws of.

PARALLEL. — See Reflexion,

Laws of.

REFLEXION OF.— See Reflexion.

REFRACTION OF.— See Re-
fraction.

ORDINARY AND EXTRAORDI-
NARY.— See Refraction, Double.

RE- ACTION.— See Action.

REFLEXION.— See Gloss. I.

REFLECTING Microscopes, &c. — See
Gloss. I.

REFRACTION.— See Gloss. I.

— DOUBLE.— See Gloss. I.

REFRACTING Microscopes, &c. — See
Gloss. I.

REFRANGIBILITY.— See Gloss. I.

RELATIVE GRAVITY.— See Gravity.

REPULSION.— See Gloss. I.

RESINOUS ELECTRICITY.— See Elec-
tric Fluid.

RESISTING FORCE.— See Force.

RESOLUTION OF FORCES. — See
Forces.

RESULTANT.— See Force*, Composition of.

RHOMBUS.— See Gloss. I.

RIGIDITY.— See Gloss. I.

ROCHELLE SALT.— See Gloss. I.

RUBBER See Electricity.

RUBY. — See Corundum.

RUPERT'S DROPS (so called because
they were first brought to England by
Prince Rupert, a German prince, and
grandson of James I.) are a sort of glass
drops with long and slender tails. They
will bear a smart stroke of a hammer, but
burst into atoms with a loud report if the
surface be scratched, or the tip of the tail
broken off. They are made by dropping
melted glass into cold water, which con-
denses the outer surface, and imprisons
the heated particles while in a state of
repulsion. — See Annealing of Glass.

SAFETY-VALVE (a necessary appendage
to a steam-engine) is a valve opening
outwards from a boiler, and loaded with
a weight sufficient to withstand the pres-
sure of the steam until it rise to a certain
required height ; but which would be
forced open before the steam could burst
the boiler.

SAPPHIRE.— See Corundum.

SATURATED SOLUTION.-See Solution.

SATURATION.— See Electricity.

SCALE.— See Gloss I.

SCREW— See Gloss. I.

SECTOR. A sector is any portion of a
circle (less than a semicircle) contained
between two radii and a part of the cir-
cumference : such as the triangular spaces
A C B and A C D, the former of which is
an acute-angled, and the latter an obtuse-

14

EXPLANATION OF SCIENTIFIC TERMS.

f angled sector. When the angle contained
between the radii is a right angle, the
sector is a quadrant, and is so called. A
sector is also the name given to a mathe-
matical instrument, composed of two flat
rulers connected with a joint, which allows
them to open in the form of radial lines,
so as to include a sector.

SEGMENT OF A CIRCLE is any por-
tion cut off by a straight line. Thus, in
the preceding figure, E F G is a segment
cut off by the line E F. When this line
passes through the centre it divides the
circle into two equal segments, or semicir-
cles.

SEMI-TRANSLUCENT. — See Diapha-
nous.

SEMI-TRANSPARENT. — See Diapha-
nous.

SHEAR-STEEL (so called because fitted
for making clothiers' shears, scythes, &c.)
is prepared by laying several bars of com-
mon steel together, and heating them in a
furnace until they acquire the welding
temperature. The bars are then beaten
together with forge hammers, after which
they are drawn anew into bars for sale.

SILEX.— See Gloss. I.

SINE.— See Angle.

SINES, LAW OF THE.— See Reflective
Power.

SODA.— See Alkali.

SOLIDS.— See Gloss. I.

SOLID OF LEAST RESISTANCE.— See
Conoid.

SOLUTION—See Gloss. I.

SOUND.— See Gloss. I.

RAYS OF.— See Reflexion.

SPAR.— See Gloss I.

SPAR, ADAMANTINE.— See Corundum.

ICELAND.— See Calcareous Spar.

SPECIFIC denominates any property that
is not general, but is confined to an indivi-
dual or species.

GRAVITY.— See Gravity, Spe-
cific.

, HEAT.— See Caloric, Specific.

SPECTRA, OCULAR._See Ocular Spec-
tra.

SPECTRUM, PRISMATIC.— See Pri*-
mafic Spectrum.

SPECULUM.— See Mirror.

SPHERE.— See Gloss. I.

SPHERICAL ABERRATION. — See
Gloss. I,

SPHEROID.— See Gloss. I.

SPIRAL.— See Gloss. I.

SPIRIT OF WINE.— See Alcohol.

STATICS is that division of the science of
mechanics which considers bodies as in-
fluenced by forces that are in equilibrium.
It therefore treats of bodies at rest. The
word is formed from the Greek statos,
standing still. What belongs to statics is
statical.

STEAM is generally used to signify the
visible cloudy vapour arising from water,
and which at low temperatures is sup-
posed to be the consequence of a chemical
solution of water in air; but steam, as it
is used in the arts, denotes water, in an
elastic form, at or above the temperature
of the boiling point, when it is invisible.

STEAM-TIGHT is that degree of closeness
which, in any particular case, prevents
the escape of steam.

STEEL.— See Carbon.

TEMPER OF.— The different tem-
pers, or degrees of hardness, of rigidity,
or of elasticity, are given by means of the
different degrees of heat to which the
metal is exposed in the operation.

STEELYARD.— See Balance.

STRAIGHT LINE, the same as a right
line. — See Curve.

SUCTION. The action of sucking is per-
formed by the child's making a vacuum
in its mouth, which exhausts the air from
the pores of the nipple ; and the milk is
consequently ejected from the breast by
the unresisting elasticity of the air within.
The raising of liquids through a tube, by
means of a piston which lifts and sustains
the weight of the atmosphere from that
part of the well which is covered with the
tube, leaving it to press on the other parts
of the surface, is also, metaphorically,
termed suction.

SYNTHESIS.— See Analysis.

TABASHEER.— See Gloss. I.

TANGENT.— See Gloss. I.

TELESCOPE, ACHROMATIC. — See
Gloss. I.

TEMPERATURE.— See Gloss. I.

THERMOMETER.— See Gloss. I.

THERMOSCOPE, a name given by
some inventors to their particular kinds
of thermometers ; as pyroscopes are names
of some sorts of pyrometers.

TOPAZ.— See Corundum.

TORSION-BALANCE, a delicate elec-
trometer, so called because its principle
consists in the torsion or twisting of a silk
fibre.

TORRICELLIAN VACUUM. — See
Gloss. I.

TOURMALINE.— See Gloss. I.

TRANSLUCENT.— See Diaphanous.

TRANSPARENT.— See Diaphanous.

TRANSVERSE DIAMETER. — See
Gloss. I.

VALVE.— See Gloss. I.

VAPOUR.— See Gloss I.

EXPLANATION OF SCIENTIFIC TERMS.

15

VARIATION OF THE COMPASS.—

See Mariner's Compass.

VEGETABLE ALKALI.— See Alkali.

VELOCITY.— See Gloss. I.

VERNIER.— See Gloss. I.

CHROMATIC.— See Gloss. I.

VERTEX.— See Cone and Conic Sections.

VERTICAL CIRCLE.— See Horizon.

PLANE.— See Horizon.

VESUVIAN, or IDOCRASE, is a stone,
generally of a reddish-brown colour, simi-
lar in appearance to common garnet. It
is found, crystallized, among substances
thrown out of volcanoes ; and, as its
name indicates, particularly by Mount Ve-
suvius.

VOLTAISM._See Galvanic.

VOLTAIC BATTERY, &c.— See Gal.

VflHlC.

VORTICES, the plural of the Latin yortev,
a whirlpool. The primary hypothesis of
the natural philosophy of Descartes was
that the universe is a plenum (Latin

p/cnus, full); that is, without any vacuum,
or unoccupied space; and that the atoms
of matter moved in numerous vortices
which carried the heavenly bodies around
their several centres of motion ; such as
the planets about the sun, and, perhaps,
similar planets around the fixed stars.

WEATHER-GLASS. The barometer is
popularly termed a weather-glass, because
its variations are commonly believed to
prognosticate the approaching state of the
weather. In former times, the same appel-
lation was given to the thermometer.

ZENITH. — See Horizon.

ZIRCON is a heavy, hard, sparkling, and
transparent stone, susceptible of a fine
polish, and having a strong double refrac-
tion. It is usually divided into the two
varieties of hyacinth and jargon ; the for-
mer having a yellowish-red colour, and
the latter being most esteemed when co-
lourless.

12

GENERAL INDEX.

The references are given to the treatise, or treatises, in which the article is to be found. Of the con-
tractions, POP. IVT. stands for Popular Introductions; NE\V. OPT. for Newton's Optics; OPT.
INST. for Optical Instruments; THERM, for Thermometer and Pyrometer ; ELECT, for Electricity;
GALV. for Galvanism; MAGN. for Magnetism; and ELECT. -MAGN. for Electro-Magnetism.

ABERIIATION; spherical account and cause of . . . NEW. OPT.

effects of in lenses ....

effects of in mirrors . . . OPT. INT.

minimum of,' in lenses . . .

« . . . in inverse ratio of the magnifying

. . comparative in glass, sapphire, and

diamond .........

Academy del Cimento, their improvements on the thermometer THERM.

French, their attempted improvements on . . —

Account, Oersted's, experiments on electro-magnetism . ELECT. MAG.

Accumulated electricity, experiments on the motion of . ELECT.

. . . . by induction .... .

Accumulation of water in hills, cause and consequence of . POP. INT.

Achartfs pyrometer, description of .... THERM.

Achromatic telescopes, principles and description of . . OPT. INST.

the largest ever known*, described .

eye-pieces for astronomical telescopes . . .

opera glasses, construction of . . «

Acids decomposed by the voltaic pile .... GALV.

Action, chemical, connection between, and electricity . ELECT.

mutual, of two magnets, experiments on . MAGN. .

and reaction, their equality an established law of

nature ........

.. electro-magnetic, fundamental law of . . . ELECT. MAGN.
Action and Reaction, that their equality is one of the laws

of motion, illustrated POP. INT.

of a bird in flying described ....

of the oar in raising a boat described . . .

of the fly-wheel explained .....

of gravity, and the pressure of fluids, explained .

of sonorous bodies illustrated . . . .

Actions, electric and magnetic, emanate from a common
source .........

mutual, of electric current, concerning generally .
particularly, described.

(See Electric Currents.}

./EPINUS, his amendments of Franklin's theory of electricity
his theory of magnetism .....

corrections of his theory of magnetism . .

his method of making artificial magnets . .

Aerial vibrations, their reflexions the cause of echoes . POP. INT.

Age, the decay of sight consequent upon, how remedied .

. . its effect upon thermometers .... TKERM.

Air, in what it differs from liquids ..... POP, INT.

. . atmospheric, its mechanical properties . . .

. . elasticity of, experiments to prove ....

. . its enormous pressure on the human body . .

. . its resistance to falling bodies •

ELECT. MAGN.

ELECT.
MAGN.

33

12
35
. 36
47,48
. 72
97

. 12
1

. 7

65

65

7

GENERAL INDEX.

Air, its resistance to moving bodies proportionate to the

squares of their velocities ..... POP. INT. .

.. its tremulous motion the cause of sound . . . - .

. . means of weighing a small quantity of . . . -

Air-pump, experiments with, described .... -

Air-pyrometer, of Schmitz, construction of . . . THERM. .

Air-thermometer, Kinnersley's Electrical, described . . ELECT. .

Air, currents of, alway accompany the discharge of electricity - .
Alcoholic and mercurial thermometers, comparative table of

at different temperatures ..... THERM. .

Alkalis, decomposed by the voltaic pile .... GALV. .

Amalgams for the rubbers of electrical machines . . ELECT. .

Amician reflecting telescopes described .... OPT. INST. "

AM AX TON'S thermometer, description of ... THERM. .

objections to . . . . -

AJIPERE'S electro-dynamic theory .... ELECT. MAGN.

Analogy between the electric spark and lightning . . ELECT. .

Anamorphoses restored by reflection from cylindrical surfaces OPT. INST.

Anatomical description of the torpedo .... GALV. .

Anatomy of the eye explained ... ... POP. INT.

Ancients had no instruments for measuring heat . . THERM. .

their opinions respecting the cause of magnetic at-

traction ......... MAGN. .

Angle, the smallest under which an object is visible . . POP. INT. .

Angles of incidence and reflexion, equality of, known to Plato NEW. OPT.
. . . . known with precision by

Ptolemy ......... --

particular explanation of OPT. INST. .

defined . . . POP. INT. .

. . account of telescopes for measuring . . . OPT. INST.

Animal life equally destroyed by shocks from the electric or

the voltaic battery ...... GALV. .

Animals, dead, excited into convulsive muscular action by

galvanism ......... - .

warm-blooded and cold-blooded, galvanic effects of

the contact of parts of one class with those of the other - .

experiments of the effects of electricity upon . ELECT.

Annual motion of the earth explained . . • . POP. INT.

Antimony and bismuth, the metals best fitted for producing

thermo-electric currents ...... ELECT. MAG.

Aplanatic diamond lenses, Pritchard's .... OPT. INST.

telescopes, Clairault's and Herschel's, described -

table for object-glasses of .' . . --

Apparatus for producing electro-magnetic rotations . * ELECT. MAGN.

Apparent place and size of the image in plain mirrors . . OPT. INST.

dimensions of objects of sight .... POP. INT.

velocity of bodies, limit of . . . . . - — .

situation of the heavenly bodies . . . -

ARAGO'S experiments on the magnetism of rotation . . MAGN. .

ARCHIMEDES' burning mirrors, account of . . . NEW. OPT.

ARISTOPHANES, his description of burning glasses . . -
ARISTOTLE attempted to investigate the phenomena of the

rainbow ........ - •

Armatures for retaining and concentrating magnetism . MAGN. .

Arrangement of iron filings under magnetic influence . -

mechanical, of microscopes .... OPT. INST.

of the prismatic colours exhibited . . POP. INT.

Artificial cobwebs for micrometers ..... OPT, INST. .

magnets, various methods of making . . MAGN. .

general principles of . . . -

method of making by percussion . - .

juxtaposition . -

single touch . - > .
less retentive of their virtues than load-
stones

ready method of forming . . . . .

Ascent of fluids in capillary tubes, cause of, explained . . POP. INT.

. . of smoke, steam, &c., cause of . . . . -

of balloons ........ __

17
Page

17 — 18

. 71

. . 66

65, 66

. .31

. 43

. '26

. 54, 55

. 17

. 16

. 48

. 6

. 8

81 — 92

. 59

. .3

. 26

. 94

. 1

. 33

. 80

. .1

. 2

. 2
14,15

. 25

. 19
. 19

. 25

49, 50

. 30 — 33

. 94

. 13

. 20

. 21

. 22

2

. 79

. 80

. 87

. 91, 92

. 1

. 2

. 2

43, 54, 5,5

. 21

. 44

. 91

. 57

. 41 — 45

41

. 42

43

. 44

54

. \

. 5

. 8

3

18

GENERAL INDEX.

Astronomical telescopes, achromatic, described

eye-pieces for ....

Astronomy, popular introduction to ....

objections to the presently-received system an-
swered . . . . .

Atmosphere, experiments on the electricity of .

, . its pressure on the human body, how counteracted

tides in, as well as in the ocean

Atmospheric air, its mechanical properties . .
refraction observed by Ptolemy
pressure, variation of, its effects on the boiling
point of water .......

its effects on thehulbs of thermometers

Attraction in bodies denned and illustrated ...

. . of cohesion .......

of gravitation ......

of mountains upon a plummet
of houses and churches by neighbouring hills
. . and repulsion, electrical, explained . .

experiments on .
. . . . of magnetic iron

of magnetism analogous to that of
electricity . ........

of iron by the magnet, illustrations of

opinions of the ancients
respecting its cause .......

local, of vessels, general account of . . .

Barlow's experiments on that
subject .........

cause of the loss of certain ships
Barlow's proposed remedy for the
evil .........

chronometer disturbed by . .

Aurora Borealis affects the magnetic needle . . •
Axioms, Newton's, respecting light .....

Azimuth compass, construction of . . . . .

Rater's, described ....

BABBAGE and Herschel's experiments on the magnetism of
rotation .........

BACON, Friar, his description of magnifiers and combined

lenses

the inventor of the camera obscura
Balance, thermometer, description of .

electrometer .......

(torsion) .......

Balloons, cause of their ascent . . . .

the earth appears to sink from under them
BARLOW'S hypothesis concerning the variation of the needle
experiments on the local attraction of vessels
proposed remedy for

Barometer, table of the effects of its different states on the
boiling point of water .......

its principle and construction . . .
Barometrical thermometer described ....

BARTOLINUS'S experiments on Iceland spar . .

Battery electrical, description and construction of

and galvanic, different effects of
voltaic or galvanic. See Galvanic.
Beautiful experiment of the combustion of Mercury
BENNET'S gold-leaf electrometer described .
BERNOUILLI'S method of obviating an error in the dipping-
needle ...... ...

BIOT'S experiments on electricity ....

Bird, action of in flying ......

Bismuth and antimony best fitted for producing thermo-
electric currents .......

BLACKADDER'S Register thermometer described . .
Blind man, his feelings when first endowed with sight .

Page
NEW. OPT. 6. OPT. INST. 10

POP. INT.

- 27, 28
30—55

MAGN.

ELECT. MAGN.
NEW. OPT.
MAGN.

MAGN.

NEW. OPT.
OPT. INST.

THEHM.
ELECT.

THERM.
POP. INT.
THERM.
NEW. OPT.
ELECT.
GALV. .

ELECT.

MAGN.
ELECT.
POP. INT.

ELECT. MAGN.
THERM.
POP. INT,

3, 39
. 2,3

33
61—68

. 63—65
66

. 67
68

2
11

60, 68, 70
70—72

93, 94

» 3

. 52

47,48

38,39

19,21

8

38

30

63—65
67

15

66—67

48,49

. 9

36—39

9

13
19

78, 79

. 22

13

94

38,39
80,81

GENERAL INDEX.

Boat, action of the oar in rowing .... POT-. INT.

Bodies, their general properties explained . . . •

their impenetrability illustrated . . .

. . extension and figure, or shape . . .

. . divisibility, examples of . ' . .

. . inertia denned and illustrated . . .

. . attraction of ......

. . density and rarity of explained .... •

. . falling, resistance of the air to . . . —

. . experiments on .......

. . moving, general effect on the resistance of the air to

. . their elasticity illustrated ....

. . centre of gravity of, defined and illustrated . . .

. . their weight different at different parts of the earth's

surface ........

. . momentum of, denned and illustrated . . .

. . in motion, measure of the resistance of the air to .

. . specific gravity of explained .... —

. . their sonorousness owing to their elasticity .

. . at what degree of velocity their motion ceases to be

apparent ........

. . heavenly, why their apparent situation differs from

the real ........

. . various, tables of their expansion, by heat . . THERM.

. . what shape of, is most favourable for retaining elec- ELECT.
tricity ........

. . effects of electricity upon different

. . pointed, phenomena of entrance or issue of electricity in

. . mechanical effects of electricity upon

. . not ferruginous, on the magnetism of

. . of all kinds are, more or less, susceptible of magnetism

. . become more obscure by excess of light

. . reflect, more copiously, rays of their own colour .

.. magnitude of their corpuscles investigated . . •

. . colour or opacity of depends on their pores .

light not reflected from their solid surfaces . .

Boiling point of water affected by atmospheric pressure . THERM. .
table of its height in different states of

the barometer .......

Books upon optics and optical instruments", list of . OPT. INST.

Bores, elliptical, of thermometer tubes, their use . . THERM. .

BOYLE'S thermometer described ....

proposed scales for thermometers . . . .

Brass has sometimes a considerable magnetic power . MAGN.

Breezes, sea and land, accounted for . . . Poi>. INT.

BREGUET'S pyrometer described .... THERM.

BREWSTER'S improvements of lenses . . . OPT. INST.

. . lenses built of circular rings . . .

kaleidoscope, construction of . . . •

compound microscope for objects of Natural

History ......... •

reflecting telescope described . . .

telescope for measuring distances described . • •

BUFFON, his burning mirrors described . . .

Bulk, apparent, of images accounted for ...

Burning-glasses described by Aristophanes . . . NK\V. OPT.
lens, Parker's large one described . • • OPT. INST.

mirror of Archimedes .....

of Buffon and Villette with their experi-
ments ........

Calcareous spar, experiments on . .
Calorimeter, description and use of .

Hare's described

Camera lucida, description of . .

Camera obscura, accounts of the invention of
CANTON'S method of making artificial magnets

NEW. OPT.
. THERM. .

GALV.

. OPT. INST.
NEW. OPT. 3. OPT. INST.

MAGN.

19

Page

. 13

. 1—8

1

2

2

3

. 3,4
4
7

8,10

. 17, 18

11—13

9, 18—20

43

. 11

. 17,18

59,60

71

. 80

87
. 61,62

7

41—51
27

. 42 — 45

89—91

90

. 10

37

55

. 53

. 55

8

. 15

60

11

3,4

4

90

70, 71
32, 33
. 40
78
. 55

47
16

. 25
3
2
2
7
1

. 9
52,53

. 4

53,54

52

Caoutchouc, (India rubber) its use in making thread for
microscopes •••«»•••

OPT. INST,

20

GENERAL INDEX.

POP. INT.
NEW. OPT.
OPT. INST.
NEW. OPT.
MAGN.
NEW. OPT.
ELECT.

Capillary tubes, cause of the ascent of fluids in -
Cartesian ovals, account of .....

Cassegrainean telescope, description of ....
Cause of opacity in bodies, investigated

. . of magnetic attraction, opinions on ...

meaning of the word in Natural Philosophy
CAV ALLOTS electroscope described ....

CAVENDISH, his improvements on Franklin's theory of

electricity ........

. . his register thermometer described . THERM. .

CELSIUS'S thermometer described «... —

Reaumur's and Fahrenheit's scales respectively

converted into one another ..... .

Centering of lenses, directions for «... OPT. INST.

Centigrade (Celsius's) thermometer .... THERM.

Centre of gravity of bodies explained and illustrated . POP. INT.

Centrifugal force, explanation of .... •

Centripetal force, explanation of . ...»

Charcoal, splendid exhibition of electric light from . . GALV. .

galvanic ignition of, at the Royal Institution,

described ........ — — ,

Chemical effects of electricity, experiments on

and electrical action, connexion between

changes effected by galvanism

effects of galvanism exemplified in the case of cop-

pei and zinc . . . . . . . .

theory of galvanism ..... • .

properties of elastic fluids differ with their species POP. INT. .
changes in the materials not the cause of thermo-
electricity ELECT. MAGN.

CHILDREN'S galvanic battery, construction of . . GALV.

CHRISTIE'S experiments on the magnetism of rotation . MAGN.

Chromatic dispersion illustrated ..... OPT. INST.

Chronometers disturbed by the local attraction of vessels . MAGN.

Circles, galvanic, simple, formation of . . . . GALV.

compound ... . . . .

Circuit of electricity, meaning of the term . . . ELECT.

Circular conductors, exhibition of their magnetic properties ELECT. MAGN.

rings formed into lenses . . • OPT. INST.

CLAIRAULT'S aplanatic telescope described . . .

Cobwebs, artificial, their use in microscopes . . . NEW. OPT. .

Cohesion, attraction of, defined and illustrated . . POP. INT.

Cold, however intense, does not impair magnetism . . MAGN.

Coloured rings, phenomena of ..... NEW. OPT.

. . reflected from coloured plates, table of .

. . shadows, account and explanation of . .

. . . . hyperbolic curves ....

Colourless topaz of New Zealand used for prisms . . OPT. INST.

Colours, Newton's theory of . . . . . . NEW. OPT. ,

mixture of ....... • •

. . spectral, re-composition of ....

of transparent liquors vary with their thickness

reflected, Newton's scale for determining . •

reflected and transmitted by thick transparent plates

. . of natural bodies, cause of investigated . . •

of thin plates of air, water, and glass, tables of .

. . prismatic, obtained from voltaic light . . GALA'.

. . . . their arrangement . . . POP. INT. .

exist originally in the light . .

reflected, theory of ..... .

produced by the combustion of different metals . GALV.

of electrical light, various exhibitions of . . ELECT.
Combination of lenses, and their magnifying power, known to

Friar Bacon ........ NEW. OPT.

Combination of sulphur and silver promoted by galvanism GALV. .

Combustion of mercury, beautiful phenomena of . .

Comets, observations on ...... POP. INT.

Communication of the electric shock, extraordinary examples

of .... . ELJECT. •

Page
5
8

. 17

53, 54

33

. 44
3

12
. 34, 35

. 8

10

. 31
9

9,18—20

. 16,31

16,31

10

10, 11

. 47, 48

57,58

13

14

20
. 65

. 95
8

92
14

. 68

2—4

4—9

37

34

7,8

20

. 41

3,4

13

. 41
52

. 61
62
25
. 33
33
35

. 37
46

. 56
54

. 52
.11

. 91
90

. 92

13

. 24

. 3

24

13

. 36, 37

39, 40

GENERAL INDEX.

Comparative table of alcoholic and mercurial thermometers
at different temperatures •.*...
Comparison of the processes of making artificial magnets
Compass, mariner's, construction of ....

. . azimuth . . .....

variation . . ....

land ..

on the variation of .....

progressive changes of its dip and variation
Complex magnetic induction, experiments on . .

Component parts of bodies, investigation concerning
Composition of Forces* explanation of its effects .

which produce the earth's motion
round the sun ......

Compound microscopes, principles and theories of .

proper size of ....

for the purposes of natural history
galvanic circles, formation of .

battery ......

Concave and convex mirrors, concerning

popular explanation of their
effects on the rays of light ....

. . mirrors for burning glasses .....

. . images formed by, and curious experiments on
Concentration of the effects of electro-magnetism
Concord of sounds, how produced .....

Condenser, Volta's, described .....

Conductors and non-conductors of electricity distinguished .

metallic, for the safeguard of ships and buildings

Cones of electric fluid passing from a positive to a negative

point, representation of

Connecting wire of a galvanic battery attracts iron filings .
Consequences of the law of electro-magnetic action .
Construction of the Leyden jar .....

of an electrical battery ....

of the common thermometer, and history of the
improvements in . . .

of register thermometers, and history of
of thermometers, precautions to be observed in
of pyrometers, and history of their improvements
Convex lenses, radii of the surfaces of ....

mirrors, images formed by ....

Convulsive agitations of mercury under galvanic influence

muscular motions in dead animals excited by gal
vanism ......

Copper, effects of dilute sulphuric acid on .

. j . . nitric acid on .....

sheathing of ships prevented from being corroded by
sea water ......

Correspondence of different thermometric scales, tables of
COULOMB'S Electrometer, or torsion-balance, described
. . i . its extreme sensibility .

experiments on the magnetism of different bodies
method of making artificial magnets . .

lilic thermometer described . . .
rough battery, construction of .
•nqmeter described
Will thermometer described .

^ineter described .....

ttlectricity passing through the human
•duced by

mpany the discharge of electricity
Hpeir mutual action. [See Electrical

Hpansmitted by perfect conductors, are

•trie, distinguished from galvanic . •
chiefly recognised by their effects on

THERM.
MAGN.

MAGN.

NEW. OPT.
POP. INT.

OPT. INST. .

POP. INT. .
OPT. INST.

ELECT. MAGN.
POP. INT.
ELECT.

ELECT. MAGN.

OPT. INST.
GALV. ,

THEUM.
ELECT.

MAGN.

THERM.
GALV.

THERM.

GALV.

ELECT.

21

Page

54

. 50

55—60

68—72

72—74

60,61

22,23

29—31

7—10

53

16—18

. 30
43
48

. 47

4—9

5

. 2

83,84
3
3

32—38

73

52

5,6

59, 6Q

27
52

9—18

34,35

36—39

1—10

33—39

10—15

15—32

6

3

31

. 24

14

. 14

14

57—60
19,20
20, 21
90, 91
48—50
30,31
7
8

45—47
50,51

IS
. 26

ELECT. MAQN. . • 57 — 76

57

93

95

K

22

GENERAL INDEX.

Curvature of lenses, method of calculating their force .
Curves, magnetic, properties of, and means of delineating
Cushions for electrical machines, first used by Winkler

amalgams, recommended for ....

CUTHBERTSON'S balance electrometer described
Cylinder, reflecting surface of, ustid ire restoring anamor-
phoses . . .

DANIELL'S pyrometer, description of .

hygrometer ... ....

^ . table of temperatures- .....

Dark chamber (camera obscura), invented by Friar Bacon

Day telescopes, account of their construction

Day and night thermometer of II u therford . . .

Days and Nights, why they vary in their lengths

Dead animals, muscular motions excited in by galvanism

Decay of sight in age, how remedied ....

DE BUTT'S differential thermometers described
Decomposition of solar light, experiments on . . .

and re-composition of water by the voltaic pile
. . of water, why not effected by the common elec-
tric machine ........

. . of neutral salts by the voltaic pile . .

of metallic solutions by . - . .

of acids, alkalis, and earths, by . .

DE DOMINIS, his theory of the rainbow .

Definition of magnetism . i

DE LA LAND'S proposed thermometer ....

DE LISLE'S thermometer as used in Russia .

DE Luc's microscopic pyrometer .

metallic thermometers . . . •

electrical column described ....

Density and rarity of bodies denned ....

DESAGULIERS' pyrometer, description of ...

' . . his theory of the rainbow . . .

• his unfair plagiarisms from Snellius .
Development of Electricity by changes of temperature and
form .........

. . by contact, compression, and other

mechanical means .......

Deviation of the Compass on account of local attraction, ex-
periments and observations on .

Table of, observed in different ships

Dew, vapours, and rain, their distinction and causes . .
. . temperature at which it falls ....

DE WJLDT'S table of expansions of spirit and of mercurial
thermometers . . . . . . .

Diameters of the planets proportionally represented .
Diamond aplanatic lenses . . . . . .

microscopes, Pritchard's ....

Different effects of the electric and the galvanic batteries
Differential thermometers and their modifications

of Leslie ....

of De Butt ....

hygrometer and photometer, improvements sug-
gested on . . .

galvanometer, description of ...
Diffraction of light, account of • . . .

Grimaldi's discovery of . . .
Dip of the magnetic needle, observations on ...

its progressive changes . .
Dipping-needle, description and construction of, &c.

remarks on that formerly used by the Royal

Society

sources of errors to which it is liable
method of obviating those errors .
Gambey's, described ,

Mayer's » i , • •
Sabine's

OPT. INST.
MAGN.

ELEC. .

OPT. INST.
THERM.

OPT. INST.

THERM.
POP. INT.
GALV. .
POP. INT.
THERM.
NEW. OPT.
GALV.

NEW. OPT.

MAGN.

THERM.

GALV.
POP. INT.
THERM.
NEW. OPT.

ELECT.

MAGN.

POP. INT.
THERM.

POP. INT.
OPT. INST.

GALV. .
THERM.

ELECT. >i

NEW.'

Page
6

19—22
15

. 16

38, 39

16

49,50
32
52

. 10

36, 37

44—48

19

. 97
41

, 11,12
15

27

. 15
16

. 17
4
1
9
9

23,24
24

. 26
4

. 16

9

• . 7

53—55
55—57

61—65

66

5

49

55

35

13

. 39

GENERAL INDEX.

23

Dipping-needle, Scoresby's instrument for finding the dip
. . phenomena of, exhibited by voltaic magnets
Directive properties of the magnetic needle exhibited by
helical coil . . . .
Discharge of Electricity always accompanied with currents
of air .........

MAON. .
ELECT. MAGN.

Page
81
47

52

. 26
37
38
. 7
. 10
. 64
. 17
30,31

25

DiiH'fiaryt)- (universal) of Henley described .
Discharging electrometer, Lane's, described
Discovery of the laws of refraction ....
of the diffraction of light ....
of the polarity of light ....
of Saturn's sixth satellite, time of ...
Disguised electricity, described .....
Dispersive powers of different substances with respect to
light, tables of ........

NKW. OPT. .

OPT. INST. .
ELECT.

Distances, telescope for measuring ....
Distinction between the geometrical and the refracted focus .
Distribution of electricity, concerning
Diverging and converging electric currents, action of .
Divided object-glass of micrometers ....
Diving-bell, Halley's, observations on light in .
Divisibility of bodies, illustrations of ....
Diurnal changes of variation and intensity of the magnetic

25
7
20—23
64,65
57
38
. 2

. 31
25
. 9

53

. ' 2

58
27
. 27
3

45,46
35
.• 11

73
. 25

87
6
30—33
31
32
. 32
34
. 38
40 — 49
. 17
72
51,52
52
52,53
40,41
13
12
9—19
9
. 14
14

. 24

65
. 65
71
11—13

*

ELECT. . 4,11,
ELECT. MAGN. ;
OPT. INST.
NKW. OPT.
POP. INT. .

M.uix.
OPT. INST. ,
NEW. OPT. .

Double image telescope, description and use of . .
refraction, particularly of Iceland spar
Huygen's theory of .

ELECT.

DREBBEL'S improvement on the thermometer described .
Drops, Prince Rupert's, their fracture attended with electri-
cal light .........
Dry Pile of Hatchetts and Desormes, described ;

THERM.

ELECT.
GALV. .

MAGN.
NEW. OPT. .
OPT. INST. .

POP. INT.
MAGN. .

POP. INT. " .

DUHAMEL'S method of making artificial magnets
Duration of the impression of light on the eye ascertained
Dynameter, that instrument described ....

EAII-TRU 31 PET, principle and construction of . .
Earth, hypothesis of its magnetism . .
. . magnetic intensities at different heights above its sur-
face .........
rises to meet a falling stone ....
account and cause of its annual motion .
why it moves in an ellipsis ....
describes equal areas of its orhit in equal times
is nearer to the sun in winter than in summer
its threefold motion described ....
k under an ascending balloon .
nt of its motions and their effects
a of by the voltaic pile
;he reflection of aerial vibrations .
,id moon, their causes explained
?r's satellites described ....
method of finding the longitude by
odiac described ....
MRC pressure on the bulbs of thermometers

GALV.
POP. INT.

THEUM.
GALV.

anic and electric batteries, different
r. --huric acid on copper and on zinc
on ditto ditto
bites with a silver spoon by means of gal-

iyn their chemical but not in their mecha-

Beriments to prove . . .
K the cause of sound • • • •
- examples of • • • «

24

GENERAL INDEX.

Electric excitation, phenomena of . " .

attraction and repulsion, phenomena of
experiments on
similar to magnetic .

fluid, hypotheses concerning ....

whether there be one or two fluids
phenomena observed on its passage or trans-

EI.KCT.

MAGN.
ELECT.

ference

velocity of the transmission of

the sound which accompanies its transference

variously modified

. . representation of the pencils and stars of light,
visible on its escape or entrance from or to a pointed body
. . representation of the cones passing from a posi-
tive to a negative point ......

. . objections to the theory of a single fluid .

. . light has the same properties as common light

. . its brilliancy proportioned to the conducting
power of the body through which it passes

. . its colours vary with circumstances . .
. . whether it actually proceeds from the fluid itself
. . . . elicited by the fracture of Prince Rupert's drops

. . exhibited in the combustion of charcoal .
Electric spark, its zig-zag progress in a long passage

and lightning, analogy between .
shock, extraordinary examples of its communication
. . . . received from different species of fishes

battery, construction of .....

. . . . and galvanic battery, different effects of .

machines, various kinds of described

Winkler. the first who used cushions for
Nairn's described ....

• . . . Ingenhouz's described ....

peculiar odour proceeding from when
wrought ........

discharge through a steel bar renders induced mag-
netism permanent .......

induction analogous to magnetic induction
effects of galvanism .....

theory of galvanism .....

columns of De Luc and Singer

air-thermometer described .....

stone, or Lapus electricus, described

qualities of various stones ....

currents, mutual actions of ....

parallel rectilineal ....

inclined rectilineal ....

terminate .....

diverging and converging

situated in different planes

rectilineal and curvilineal .

circular . . . . . .

transmitted by perfect conductors are con-
tinuous . . . . . . . ' .

tend to produce currents in a similar direc-
tion in other bodies .......

Electricity, the phenomena first generalized by Gilbert
early cultivation of by the Royal Society .
distribution and transference of
conductors and non-conductors of . . .
insulation of ......

shape of bodies most favourable for retaining

vitreous and resinous distinguished

induction of explained .....

theories of ......

Franklin's theory, amended by ^pinus and Ca-

GALV.
ELECT.

GALV.
ELECT.
GALV.
ELECT.

Page
2

2, 3

4, 17—20

3,39

10

12—14

23—28
23

25

27

27

63,64
24

24
24
25
58

. 10
23
59

39, 40

26

36—39

. 9

. 14—17

15

15

16

2.-)

ELECT.'

vendish

positive and negative distinguished
resides wholly on the surface of bodies

r

GENERAL INDEX.

Efeetricity, transference of ...... ELECT.

phenomena of, in its passage through a vacuum

currents of air always accompany its discharge . .
Nicholson's instrument for distinguishing posi-
tive from negative .....

disguised, description of .... .

. . accumulated, experiments on the motion of . .

accumulation of by induction . . . ,

circuit of, meaning of the term . . . •

on the lateral explosion of ....

its effects upon bodies generally . . .

its mechanical effects on bodies . . .

its chemical effects ..... .

its effects upon animals ....

its effects upon vegetables .... — • .

evolution of heat by . . . .

instrument for collecting weak . . . •

doublers or multipliers of .... •

development of, by changes of temperature and

form .......... .

by contact, compression, and other

mechanical means .......

three states of distinguished . . . ELECT. MAGN.

. . and magnetism identical ....

of the atmosphere, experiments on . . ELECT.
metallic conductors of, for the safeguard of ships

and buildings from lightning ..... •

materiality of, experiments to prove . .

and magnetism found to be identical . . MAGN.

Electrometers, principles of those instruments . . ELECT.

Henley's described .....

Bennet's gold-leaf ..... -
Coulomb's, or torsion-balance . . . .

Cuthbertson's balance .... .

. . Lane's discharging ..... '

Electroscope, different kinds of ..... .

Henry's described ... .

Cavallo's ...... .

Efectrophorits, Volta's, described .....

Electro-magnetism, history of prior to Oersted's discoveries ELECT. MAGN.

experiment of Ritter, account of . .

Oersted . . . •

fundamental law of explained . .

direct consequence of the law of . .

its effects on the directive property of the

magnetic needle .......

., concentration of its effects . .

theories of . . . . .

heory of Oersted ....

j itions ......

i Itiplier ..... •

fleets of terrestrial magnetism . . : —

duction, concerning » • .

fects of galvanism .... GALV.

:ory of Ampere .... ELECT. MAGN.

enoid described .....

g, effect of •

I meaning of the term . . . GALV. .

|battery of Wollaston . . . •

described THERM.

lometer tubes, their use . . .

th revolves round the sun in that curve POP. INT. .
of incidence and reflection known to

. NEW. OPT. .
POP. INT. 11 — 20. MAGN.

25

Page

4,11,23—23
25
26

\A re-action, a law of nature

xplanation of the term . . .

lation of the phenomena of the rainbow NEW. OPT.

electricity, experiments on . . ELECT.

the voltaic battery * « • • GALV.

. phenomena of .... ELECT.

27, 28

30,31

39—41

31—36

37

41

41—51

42—45

47,48

49, 50

50, 51

45—47

51—53

53

53—55

55—57

96

92

59,60

59,60

60—62

1,88

19

19—37
19

19,20
38,39

51,52

. 1—4

3

. 4—6

6—9

9—18

. 9—14
32—38
75—88
75—78
19—32
40

45—52

52—57

13

81—92

83

90

31

3

16, 17

11

. 31

. 1

3

. 24
2

45—47
12

20 GENERAL INDEX.

v Page

Exhibition of electric light from charcoal .... GALV. * \ 10

Expansibility of glass rods and tubes, table of . . THERM. . 53
Expansion, relative, of different metals, from the freezing to

the boiling point of water, table of < , . . 20

of seven different metals, table of . . 17

. . . . of seven different solids, table of . . . 27

of spirit and mercurial thermometers, tables of , 53

of various bodies by heat, tables of . . . 61, 62

Experiments on Iceland spar by Bartolinus . . . NEW. OPT. . 9

on the decomposition of light . . . . 11,12

on the inflexibility of light .... . 20,22

on the inflexion of light ' . . . . . 60

. . on the images of concave mirrors . . . OPT. INST. .

... on burning mirrors . . 3

on the phenomena exhibited by thin transparent

plates NEW. OPT. . 40, 41

on galvanism bv Roget .... GALV. . 30, 31
on the mutual attraction of electric currents by

Roget . . ELEC. MAG. . 59

Explosion, lateral, of electricity ..... ELECT. . . 41

Eye, anatomical description of . . POP. INT. 94. NEW. OPT. . 3

. . preserves the impression of light for a time . . • . 35

. . how objects are rendered visible to the, illustrated . POP. INT. . "77, 78

Eye-piece, Ramsdeii's micrometical .... OPT. INST. . 14

FACTS and general principles of magnetism . . . MAGN. . . 1 — 16

FAHRENHEIT'S mercurial thermometer described . . THERM. . 7

scale, principle of its division . . .

Reaumur's and Celsius's scales, formulae for
converting into degrees of each other .....

Falling of a magnet on a stone pavement impairs its power
. . ^bodies, resistance of the air. on ....
. . . . experiments on . . . . .

the earth rises to meet . ...

FARADAY'S electro-magnetic experiments . .

Fatal experiment of Professor Richman . . .

. . consequences of the local. attraction of vessels .

FERGUSON'S pyrometers described ...

Ferruginous bodies, not the only ones susceptible of magnetism

Fibres for micrometers, various kinds of ...

Figures of bodies, illustrations of ....

Fishes, electric shocks received from several kinds of .

Fits of easy reflection and transmission of light

FITZGERALD'S metalline thermometer ....

Fixed stars, their distance and bulk unknown ,

Florentine Academy, their improvement of the thermometer

Fluid, electric. [See Electric Fluid.'] ....

microscope described ..,,.. OPT. INT.

Fluids, methods for inclosing objects in for observation .

.. magnetic, theory of two , . • MAGN, .{

. . heat considered as one of them . POP. 1

. . cause of their rise in capillary tubes . . • • '

. . mechanical properties of . . , . .

. . effects of gravity more observable in than in solid

/ todies . . . . . . . ., . .

.. gravitation and pressure of, explained . , .• . •

. . elastic, mechanical properties of . . .

.. differ in their chemical properties . .

Fly-wheel, its mode of action explained . ... . POP. L^

Foci of lenses, calculation of . . . . . OPT. I>

"• . . geometrical and refracted, distinction between .

Fog, why the sun appears red through one . . . POP. a

Force, definition of . . , . • r~

. . centrifugal, centripetal, and tangential, distinguished

Forces, composition of, laws of • Ti

how they produce the earth's annual

motion ....... ' -ft-

. . . magnetic, laws of ,...*• MAGNJI ^

«. .. intensity of, in relation Jo distance . • • — 4-.j

• t • . method of determining their intensities — — '

MAGN. .

12

POP. INT. .

7

8, 10

6

ELECT.MAG.

19—23,97,99

ELECT.

33,59

MAGN.

68

THERM.

21—23

MAGN.

87—91

OPT. INST.]

57

POP. INT. .

2

GAI.V.

. 26

NEW. OPT.

44

THERM.

20,21

POP. INT.

. 37

THERM.

3

GENERAL INDEX.

Forcing-pump, principle and construction of . . .

Form of the specula of reflecting telescopes . .

Fonnulee for converting the scales of Reaumur's, Fahrenheit's,
and the centigrade thermometers into degrees of each
other .......

Fountains and springs, origin and effect of ...

phenomena of intermitting, ac-
counted for ........

Fracture of magnets, experiments on the effects of

FKAXKLIX'S theory of electricity, as amended by yEpinus
and others . . . . .

discovery of the identity of lightning with
electricity . . .

French Academy, their attempted improvements of the ther-
mometer ......

Friction described and illustrated .....

Fringes (coloured) of shadows., concerning . . .
hyperbolical curves of .

Frogs, experiments upon by Galvani

FROTERINGIIAM'S pyrometer described

Fulcrum, explanation and application of • .

Functions and structure of the eye . ...

Fundamental law of electro-magnetism ...

GAL i LEAK telescope described . . . NEW. OPT. 6.

GALVANI'S experiments upon frogs . ' • ' . .

Galvanism first noticed by Sulzer ... .

experiments on the various effects of

electric effects of ....

luminous effects of ...

ignition of metals by . ....

heat more intense than by any
other means . . . . . .

electro-magnetic effects of

chemical changes effected by . .

physiological effects of ....

theories of . ....

combination of sulphur with silver by . .
Volta's theory of .
Galvanic battery, elementary, by Wollaston ...
of the London Institution described .
of piles, construction of ...
children's, described ....

Hart's .. ....

difference between, and electric battery
its power best determined by experiment
the connecting wire of, attracts iron filings
Galvanic circles, simple ones described .

compound ones, construction of
pile Volta's, described . ...

formed of animal substances
formed of vegetable substances .
' trough fiflSery of Cruickshank, &c.

> Hi Fay ......

iot dependent on the magnitude of the

POP. INT.
OPT. INST.

TIIEUM.
POP. INT.

THERM. .
POP. INT.
NEW. OPT.

GALV. .

TlIKIlM.

POP. INT.
NEW. OPT.
ELECT. MAG.

OPT. INST.
GALV. ,

ELECT. MAGN.
GALV.

27

Page
69
10

10
61—64

C4
15, 16

. 12
59

6

28,29
61
62
1

18*

20—24
3
6—9

. 24
1
1

. 9—19
9

10
. 12

12

. 13

. 14—17

17—19

3, 19, 20, 31

. 24

31, 32

3

4

5

8

. 8
9

29
52
2—4
4—9
4
25
25
7
3

s by Roget

pon mercury .....

inguished from the thermo-electric

iis kinds' of, described

tential .

lie's, torsion one ....

jtion of . .

!af, described .....

•edle described « .

inciples of magnetism . . .
artificial magnets .

trical phenomena, when first attempted
cted focus distinguished . ,

ELECT. .
OPT. INST,

28

GENERAL INDEX.

Georgium Sidus and his six moons, account of

GILBERT, the earliest scientific electrician

GIL PIN and Canton, their table of the variations of the

compass .... .

Glass, comparison of the aberration of light by, with that of
sapphire and diamond

. . globules substituted for small lenses, or microscopes
description of a thermometer with .

. . rods and tubes, table of expansibilities of
. . prism, on the transmission of light through .
Globe, terrestrial, representation and description of
Gold-leaf electrometer, description of ....

galvanoscope, description of ....
Graduation of the scales of thermometers, directions for .
Graphic telescope, description of . .

Gravitation defined and illustrated

Newton's, discovery of its universality
and pressure of fluids, explanation of their action
Gravity, centre of, explained and illustrated .

specific, of bodies, explained ....
its effects more obvious in fluids than in solid bodies
Gregorian telescope, description of ...

GRIMALDI'S discovery of the inflexion, or diffraction, of light
Grinding of specula and lenses ....
. . of microscopic lenses . . .

HATCHETT'S dry galvanic pile, account of .

HALE'S thermometer described ....

HAL LEY, Hooke, and Boyle, their 'proposed scale for thermo-
meters . . . . .

observations on the light in a diving-bell
Hardness of iron and steel favourable to their magnetic power
HARE'S calorimeter, description of .
Harmony of sounds, how produced

distinguished from melody

HARRIS'S experiments on the magnetism of rotation .
HART'S galvanic battery described ....'.

HAUY'S electroscope described . .
Heat, effects of, described . . . •

. . of the seasons, its diminution caused by the obliquity
of the sun's rays . . . . .

. . the ancients had no means of measuring .

. . imperfections common to all instruments for measuring

. . table of the expansion of bodies by .

. . table of various points of, according- to Wedgwood

. . its effects upon the powers of a magnet

. . favours the progress of magnetic induction, but impairs

its permanency ......

. . produced by the voltaic battery to a great intensity
. . experiments on its evolution by the voltaic battery
Heavenly bodies, on the apparent situation of .
Height of the boiling point of water in different states of the

barometer . . .".'..

Helical rotations, exhibition of .....

coil (balanced) assumes the positions and directive
properties of the magnetic needle . . .

and rectilineal conductors, mutual action of . •
Henley's universal discharger described

electrometer described . . • . .

HERSCHEL'S periscopic combination of lenses

front-view reflecting telescope . .
aplanatic telescope ....

. . table of dimensions of a double aplanatic object

glass ...

experiments on the magnetism of rotation .
Hitls, why tneir attraction does not draw houses towards
them . . ; ,

. . accumulation of water in .
History of tHe discovery of the local attraction of vessels

POP. INT.

ELECT. .

MAGN.

NEW. OPT.
OPT. INST.
THERM.

POP. INT.

ELECT.

ELECT. MAGN.
THERM. .
OPT. INST.
POP. INT.

NEW/ OPT.
OPT. INST.

GALV.
THERM.

NEW. OPT.
MAGN.
GALV. .
POP. INT.

MAGN.
GALV. .
ELECT.
Pop. INT.

THERM.

MAGN.

GALV.
POP. INT.

THERM.
ELECT.

ELECT.
ELECT. .

Pao;e

36, 37

1

32

13

. 38
3

. 63
90

. 40
19
45

13—15, 5G

26

5

39

57, 58

9, 18—20

59, 60

. 56

10, 16

10

. . 30
35

. 27
9

4,5
38

. 14
4

. 73
73

94, 95
8

. 3
4

46, 47
1

51—56

6J,62

29

12,13

12

. 10
12

87

15

OPT. I:

GENERAL INDEX.

23

Page
. 1-4
1—10
33—39
23
26
53
32,33
9
93
. 60
56—64
41,42
39, 40
45
. 49, 50
50
. 50,51
62
. 74, 75
. 25

28

30
30
33

. 9
88
10
. 12

2

. 3

91
. 1
51—56
26—30
35

1
2
. 28
. 9
57
9, 10, 12
28-31
31—36
4—7
. 7—10
11

. 12
. 87
2
52—57
3

. 60, 63
97,98
16

27,28

, 2
51—53
. 55—87
51^56

and construction of the common thermometer . THERM.

Ho mogeneous light, method of obtaining .... NEW. OPT.

Horse-shoe magnets and batteries described . . . MAGN. _.
HOURIET'S pyrometer, or metalline thermometer, described THERM.
HuGGiNVs theory of the double refraction of light . NEW. OPT.
Hydro-electric current defined ...... ELECT. MAGN.
Hydrometer, description of that instrument . . . POP. INT.
Hydrostatics, popular introduction to , . .
Hygrometer, thermometric, described .... THERM.
and photometer, Leslie's, described . . •
suggested improvements in. .

Hyperbolic curves formed by coloured shadows . . . NEW. OPT.
Hypotheses, different, on the nature of light . . . POP. INT.
Hypothesis of the earth's magnetism .... MAGN.
illustrated by means of
an artificial globe .......
Barlow's, concerning the variation of the mag-

Halley's curious one on the same subject . ••
invariably the precursor of truth . . . -

Identity of magnetism and electricity .... ELECT. MAGN.
Ignition of charcoal, by galvanism, splendid exhibition of . GALV. .
of metals, .. phenomena of . .
Images formed by plane mirrors, their place and bulk . OPT. [!NST.
by concave and convex mirrors, with curious

Imitation of terrestrial magnetism "by means of wires round an
artificial globe ... . ELECT. MAGN.
Impenetrability of bodies, illustrations of . . . POP. INT. .
Imperfections common to all instruments for measuring heat THERM.
of refracting telescopes .... NEW. OPT. .

Incidence and reflect ion, equality of the angles known to the

on the angles of POP. INT. 14, 15. OPT. INST.
Inclined plane, account of it as a mechanical power . POP. INT. .
Inconvenience of the plus and minus degrees of the thermometer THBRM.
India-rubber, its use for microscopes .... OPT. INST.
Induction in electricity, described .... ELECT.

accumulation of electricity by ... .
^ magnetic, analogous to electric induction . . MAGN.
complex experiments on . . • .

its progress favoured, but its permanency

.*. explained on the theory of Ampere . ELECT. MAGN<

as defined and illustrated .... POP. INT.
diffraction of light, observations and experi-
A ..... NBW.OPT.

> electrical machine described . . . ELECT.
wlson's for distinguishing negative from posi-
v . . . . . . ELECT.
•toted by Ptolemy to measure the degree of re-
NKW. OPT.
collecting weak electricity . . . ELECT.
•;netic, various ones described . . M.AOX. .
•^ measuring heat, imperfections common to all THKRM. .

30

Insulation in el

ectrici

GENERAL INDEX.
ty described ..... ELECT.

INTENSITY of terrestrial magnetism, variations of . MAGN.

of heat producible by the voltaic battery . . GALV.

of magnetic forces in relation to distance . MAGN. ,

method of measuring . .

methods of determining . •• .

at different heights above the

earth's surface ........

Intermitting springs, phenomena of, accounted for . . POP. INT. .

Introduction to mechanics ...... .

to astronomy ...... —

to hydrostatics ...... .

to pneumatics ......

to optics ....... •

Inversion of the images of objects on the retina, why not observed

/row, illustrations of its being attracted by the magnet . MAGN. .

. . magnetic, attraction and repulsion of .

. . and steel^ different qualities of, how they affect the mag-
netic power . . . . . . . .

retain magnetic power in proportion to their

hardness ........ . •

table of the relative magnetic power of dif-
ferent species of .......

. . why it is less retentive of magnetic power than steel — • • -

. . Scoresby's experiments on its magnetism . .

. . filings ) curious arrangement of, when under magnetic

influence ........

attracted by the connecting wire of a galvanic

battery ........ ELECT. MAGN.

JONES'S hygrometer described ..... THERM.

Jupiter and his four moons described .... POP. INT.
. . eclipses of his satellites, and method of finding the lon-
gitude by ........

Juxtaposition^ a method of making artificial magnets . MAGN. .

KALEIDOSCOPE, principle of its construction . . . OPT. INST.
KATER'S remarks on the processes for making artificial magnets MAGN.

azimuth compass described ....

KEITH'S register-thermometer described .... THERM.

KENNERSLEY'S electrical air-thermometer described . ELECT.

theory of the phenomena of the rainbow . NEW. OPT.

astronomical telescope . . . •

KEWLEY'S balance thermometer described . . . THERM.

KNIGHT'S method of making artificial magnets . . MAGN. .

LAND COMPASS, construction of .... MAGN.

LANE'S discharging electrometer ..... •

Lapis electricus, description of that stone ... ELECT.

Lateral explosion of electricity .....

JMW of induction, experiments for the development of .

. . of electro-magnetic action ..... ELECT. 3

direct consequences of . 1

its effects on the magnet needle

of inductions, its application to the explanation of

particular facts'' .... . . . .

of refraction of light discovered by Snellius . . NEW. (

of refraction of homogeneous light . . .

of magnetic forces ...... MAGN.

Layers, alternate, of muscle and brain from a galvanic pile GALV. . v.

LEBAILIF'S improved galvanometer described . . ELECT. 'M.«jl

Lenses defined and severally described . POP. INT. 89. OPT. IN>

effects of their combination .... '

tables of the magnifying powers .... . -4|

spherical aberrations of light in, and in mirrors . 1

minimum of spherical aberration in ... • \.

method of calculating the foci of ...

of grinding ...... *-?'

of polishing and centering . . . it ^

-. TTr

25

. 10

16

. 25

82—87

87
63

1—29

.30—55

56—64

65—73

74—100

78,79

. 2,3

3

10
. 14

15

, 11
14

. 21

52

50
36

52,53
43

55
51

70—72

37,38

43

. 5

6

47,48
45

55, 60, 61
38

. 53

41

28—31

: c-9

GENERAL INDEX.

Lenses, method of grinding for microscopes . . .
Brewster's improvements in ....

polyzonal, or built of zones . • • •

parabolic of Newton .....

pefucopic, combination of . . . • •
Parker's large one described ....

sapphire and diamond of Pritchard . . .

table of their magnifying power . .
LESLIE'S hygrometer and photometer described

differential thermometer ....

thermometrical hygrometer ....

photometers ......

pyroscope .......

Lever, principle of, and description of the several sorts of .
. . species of, distinguished according to the place of the
fulcrum .........

Leydenjar, construction of .....

Liyht, different hypotheses concerning its nature
.. why its rays are visible in their passage
. . refraction of, theory concerning ....

its effects on the apparent places of the

stars

through lenses of various forms . •
transmission of through a glass prism . *

. . prismatic colours exist in, and not formed in the me-
dium through which it passes .....

. . how its rays are refracted to the retina . .
. . nature and principles of, and axioms concerning
. . rays of, always proceed in straight lines
. . experiments on its composition and decomposition .
. . homogeneous, method of obtaining . .

law of its refraction ...

. . reflexibility of, general laws concerning
. . not reflected by infringing upon the solid parts of bodies
. . reflected by bodies chiefly of their own colour
. . duration of its impression on the eye . • .
. . Halley's experiments upon in a diving-bell .
. . its fits of easy reflection and transmission . .

. . diffraction of, explained, with experiments .

dispersive powers of by different bodies, table of
. . refraction of through different lenses .

rays of, their primary colours ....

their different refrangibilities
their interference . . . , .

, . undulations of, account of that theory .

its polarity suspected by Newton . , .

.. electric. [See Electric Light.]

its influence on magnetism ....

. . stars and pencils of, exhibited on the passage of the
electric fluid from or to a pointed body
. . electric, exhibited in the ignition of charcoal, &c., by
galvanism ......«•

Lnjhtinntj, 'analogy between it and the electric spark .
safeguard for ships and buildings from
affects the magnetic needle, and sometimes reverses
•its polarity ... .....

Line of no variation of the compass, traced .

s, in what they differ from air ....

v their undulation

colours vary with their thickness
(••• may-nets, where found
t-n- virtue more tenaciously than artificial

OPT. INST,

TJIKUM.

POP. INT.

ELECT.
POP. INT.

NEW. OPT.

OPT. INST.
NEW. OPT.

ELECT. MAON.

ELECT.

GALV.

ELECT. MAC;N.

M.UiN.

POP. INT.
Ni-:\v. OPT.

M.UiN.

ould be armed ....

, remarks concerning .

of its discovery ....

's experiments concerning

' deviation on account of, observed in

31

Page
35

. 41

7,8

. H

41

7

. 13, 39

. 40

39,40

40

41,42

. 42—44

44, 43

. 20—24

22—24

34, 35

, 74, 75

77

85,86

87

. 89
90

90

95

. 9,11

11, 12
2,3

26
20—22

55
. 37

35
. 38

44
. 60

25

13,35,40
11, 12, &c.

32

10
. 44

64

97,98
27

. 10, 11

59

. 59,60

2

23
1

. 71
37,38

54

. 54,55

. 61—68

fil— 63

. 63—68

G6

L 2

32

GENERAL INDEX

Local attraction, consequence of, in the wreck of vessels
Barlow's proposed remedy for .
chronometers disturbed by . . .

London Institution, galvanic battery belonging to

Longitude, method of finding from the eclipses of Jupiter's
satellites ........

Lucernal microscope, account of . . . .

Luminous effects of galvanism described . . .

MA CHINE s} considerations with regard to . - .

effect upon, of the medium in which they are
worked .........

electric. [See Electric Machines.']

Magic- Lantern, principles and construction of . •

Magnet, revolution of one of its poles round a vertical wire
. . made to revolve round its axis ...

voltaic, description of .....

made to show the phenomena of the dipping-
needle ........

method of making a very powerful temporary one
one which sustained 750 pounds . . .

Newton's small one, which raised 250 times its own
•weight ......

either pole of, tends to produce opposite polarity in
the adjacent end of a neighbouring magnetizable body
Magnets, native, or loadstones, where found

artificial, ready method of forming .
. . . . various methods of forming ...

. • . . method of forming by percussion .

by juxtaposition

less tenacious of their virtue than load-
stones .........

general principles of
impaired or destroyed by heat
impaired by falling on a stone pavement . ,
experiments on their fracture . .

lose their power by rubbing ....

. . on the preservation of . . . . .

polarity of, and other properties described and illus-

MAGN.

GAI.V.
POP. INT.

OPT. INST.
GALV.

POP. INT.

OPT. INST. .
ELECT. MAGN.

MAGN.

trated

horse-shoe, formation of ....

Magnetism^ definition of . , r . . .

general facts and principles of ...

(induced) rendered permanent by the electric dis-

MAGN.

charge

tures

globe

not impaired by cold, however intense .

of iron, experiments on .

terrestrial, concerning ....

variations in its intensity . .

hypothesis concerning

different theories of .....
mechanical theory of ' « . . .
of steel bars retained and concentrated by arma-

of bodies which are not ferruginous

all bodies in some degree susceptible of .

of rotation, various experiments on . .

excitable only by induction ....

induction of, explained in Ampere's theory

and electricity identical .....

terrestrial, exhibited by means of an artificial

influence of light upon
Magnetic theories, observations on

attraction and repulsion, similar to electric .
attraction, opinions of the ancients concerning
fluids, theory of two ....

Brewster's theory respecting
forces, laws of

ELECT. MAGN.

MAGN.

Paee
66
67
68

. 4
52,53

. 52
10

. 20
29

. 52

19—23

23,24

38

47

. 54
55

. 88
1

. 'i

10—15

42
43

54
41
12
12

15, 16
54
53

1—3

53

1

1—16

12
13
14

22—32
25
25

32—41
32,33

43
87—91

90

91—96
2

87
. 92

97, 98
96
3,39 >,

33

36-41

41

16— 2L

GENERAL INDEX.

Magnetic forces, relations of their intensities to distance .

methods of determining their intensities
« . method of measuring ....

at different heights above the earth's surface
. . induction analogous to electric induction
complex, experiments on .
means of quickening the progress of
iron, attraction and repulsion of ...
susceptibility and retentiveness, less in iron than in

33

M.u;x.

steel
cools

power of steel, below a certain heat, increases as it

of iron, or steel, retained in proportion to
the hardness ,

table of different species

curves, their remarkable properties and method of
delineating ........

batteries, concerning .....

instruments, description of various .
. . equator, explanation of the term . . .

needle, its proper proportion, weight, &c. .

on the dip off. .....

•• aifected by the Aurora Borealis

affected by lightning, so as to reverse its
polarity . . . . . . .

how affected by electro-magnetism
its movements in free space .
, its properties exhibited by a helical coil

suspended in the air without a visible agent
effects of thermo-electric currents upon .
properties of circular conductors exhibited
Magnifier*, Friar Bacon's description of ...

Magnifying j)Q\\eY of lenses, table of . . . .

of sapphire lenses, table of .
of telescopes and microscopes, method of
determining . . . . .

Magnitude of the corpuscles of bodies investigated .
Man, why he can see himself at full-length in a plane mirror,
which is only half his height .....

Mariners' compass, construction of .....

Mars, account of that planet . . . . •

Materiality of electricity, experiments in proof of
Materials, size and form proper for magnetic needles .
Matter is never annihilated ......

MAUROLYCUS, his description of the structure and functions
of the eye . .......

MAYER'S dipping-needle, described ....

Measures of heat, unknown to the ancients . .

imperfections common to all instruments con-
structed for that purpose .....

M-casuring of distances, telescope for ....

Mechanical properties of atmospheric air ...

. . powers, description of ....

properties of fluids, described ... .

of elastic fluids . .

[•magnetism ....

lectricity on bodies
it of electricity ....

Ferent in Voltaic from those of com-

ELECT. MAGN.

r introduction to ....

Nfcn, and the effect distinguished from

_

other metals by galvanism
s, described ....
lents of its combustion . .
tations under galvanic influence
ofRoemur ....

thermometers compared at different

NEW. OPT.
OPT. INST.

NEW. OPT.

POP. INT.
MAGN.
POP. INT.
ELECT.
MAGN.
POP. INT.

NEW OPT.
MAGN.

TUEKM.

OPT. INST.
POP. INT.

MAGN.
ELECT.

GAI.V.
POP. INT,

GALV.
OPT. INST.
GALV.

THERM.

16

82—87
25
87

4—7

7—10

11

3

10, 11
13

14
15

19—22
53

55—87
24

53—58

23—25
2

. 2

9—14

14—18

52

37

95

. 34
3

[35
40

11,45
. 55

. 81

55—60

36

60—62

56—58

2,3

. 3

79,80

51—56

25

. 7

20—29

56—60

65—69

32, 33

42—45

55—57

. 13
1-29

• 73

11—14
4
13

. 31
7

34

GENERAL INDEX.

Mercurial and alcoholic thermometers, by De Witt
Mercury, account of that planet .....
Meridians, denned and described ....
Metals, table of the relative expansion of different .

tables of their different degrees of oxidability .

ignited and melted by the galvanic battery .

colours produced by their combustion

tables of their comparative thermo-electric relations
Met llic solutions decomposed by galvanism

thermometers of De Luc .....
Regnier ....
Crichton .

THERM.
POP. INT.

ELECT. MAGN.

GALV.

THERM.

Page
55
. 35
41

• 17
22,23
11, 14
. 13
94

. 16
. 24
. 29
30,31
conductors for the safeguard of ships and buildings ELECT. . 59, 60

of Fitzgerald . 20, 21

Metalline thermometer, or pyrometer, of Breguet . . THERM. . 32, 33

of Houriet . . . . 33

Microscopes, their principles and construction . . POP. INT. . . 98

Micrometer-pyrometer of Smeaton, described . . . THERM. . 18 — 20

Micrometers of various kinds, construction of • . OPT. INST. . 56, 57

circular construction of . . . . . . 59, 60

Micrometrical eye-piece, by Ramsden, described . . . 14

Microscopes, on their invention and antiquity . . . . 33

theory of single ..... . % 33

. . of compound ..... . . 43

singly-reflecting ..... . 43

reflecting, proper form of their mirrors . . • . . 48

Amician reflecting ..... . 48

compound, proper size of ... • • • 48

mechanical arrangement of ... . 44

method of determining their magnifying power • . . 45

» . object-glasses for . . . . . . . . 46, 47

test-objects for ..... . . 49

fluid . . 42

small, made of glass globules ... .38

diamond and sapphire ..... . . 39

table . 36

for objects of natural history . . . . . 47

periscopic, of Dr. Wollastoii . . . . . 40

opaque ....... . . 50

solar POP. INT. 99. OPT. INST. 50, 51

lucernal ....... OPT. INST. . 52

smallest magnitude visible by • . . . . 32

Microscopic lenses, method of grinding .... . . 35

pyrometer of De Luc THERM. . 23, 24

ofRamsdeii . .24,27

Middletori's experience of the effects of cold on magnetism OPT. INST. . .14
Minimum of spherical aberration in lenses . . . MAGN. . . 13

Mirrors, description of the different kinds and their effects POP. INT. . 80, 85
Mirrors, plane, their images and reflections, concerning . OPT. INST. . 2

concave . . 2

curious experiments with their images . • . . 3

used as burning-glasses, Archimedes's, &c. — — . 1, 3, 7

convex, images of . . . . . . . . 3

of reflecting microscopes, proper form of . . — — — . . 48

telescopes . . . . . . . 15

spherical aberrations in ..... . 1 1

MITCHELL'S method of making artificial magnets by double

touch ......... — . 46, 47

Modifications of differential thermometers .... THERM. . 39 — 45

Momentum of bodies denned and illustrated . . . POP. INT. . . 11

. . measurement of .... . .11

Monsoons, description and cause of .... — • . . 70

Moon, general account of ...... \ . 49 — 55

.. her motion and phases explained , . . • . . 50,51

.. concerning the eclipses of . ~~ — . . 51,52

Moons of Jupiter, Saturn, and the Georgium Sidus . ^

MORTIMER'S pyrometer, described .... THOTM. . .17

MORVEAU'S platina pyrometer, description of . . . 29, 30

.. table of temperatures . . . . . . •—•. , 30

GENERAL INDEX.

M>th-, --of. Pearl micrometers, described ....
Mutual of /W/j-v. absolute, relative, uniform, retarded and
accelerated, defined ......

perpetual, exists only in the heavens

reflected, explanation, and illustrations of ,

compound, explained .....

the earth's annual, produced by the composition of
brces . . . . . ,

threefold, of the earth described

of the moon explained .....

of a body may be insensible on account of its great
velocity ........

Mountains, their attraction of a plummet

accumulation of water in ...

Moving bodies, resistance of the air to, how measured
Multipliers and doublers of ciectricity

MURRAY'S proposed scale for a thermometer . . .
JM USCHENBROCK'S pyrometer described
Muscle and brain, galvanic pile formed by alternate layers of
Muscular motions in dead animals produced by galvanism .

NAIRN'S electrical machine described

Wattare of light, theories of • . . .

Natural bodies, cause of the colours of ...

history, microscopes adapted to objects . .

magnets or loadstones, where found

Needle, magnetic, see Magnetic Needle, compass, and variation
Needle, Dipping, see Dipping-Needle.
Negative and positive electricity described

.. instrument for distinguishing

Near-sightedness, cause and remedy of pointed out
Neutral salts decomposed by the galvanic pile
New Holland, colourless topaz of, its use . . .

NEWTON'S experiments on the oil thermometer

axioms concerning light ....

reflecting telescope described . OPT. INST,

queries concerning the inflection of light .

scale for determining reflected colours, &c. .

suggestions of the polarization of light . •

table of the colours of thin plates and films .

theory of colours .....

theorems concerning refraction .

small, but powerful, magnet, account of .

discovery of the universality of the principle of
gravitation ........

NICHOLSON'S instrument for distinguishing negative elec-
tricity .........

Night telescopes, account of .....

Nitric Acid, its effects on copper and on zinc

NOBILI'S galvanometer described ....

Non-conductors of electricity described ....

OAR, its action in rowing a boat described
Objects of natural history, microscopes for ...

(test) for microscopes . . •
the shadowed side of, by what means visible .
manner in which they become visible to the eye .
their images inverted on the retina ...

. vi it dimensions of ...

jle under which they can be seen . .
f<ur ifiicroscopes, formation of .

•resent system of astronomy answered

's thermometer ....
theory of galvanism
ory of a single electric fluid
's rays a cause of the diminution of their

Magnetic theories ....

•lalley on light in a diving-bell

OPT. INST.
POP. INT.

ELECT.
THERM.

GALV.

ELECT. .
NEW. OPT.

OPT. INST.
MAGN.

ELECT.

POP. INT. .
GALV. .
OPT. INST. .
THERM.
NEW OPT. .
15. NEW. OPT,
NEW. OPT. .

ELECT. MAGN.
POP. INT. .

ELECT.
OPT. INST. .
GALV. .
ELECT. MAGN.
ELECT.

POP. INT. .
OPT. INST. .

35

Page

57

. 9, 10
. 10
13—15
15—18

. 33
. 34
50,51

. 80

. 6

. 62

.17,18

53

9

15

. 25
19

15

. 9
54

. 47
. 1

. 13

27,28

. 96

15

. 25

. 5

. 11

10—30

. 63

. 46

64

. 52

. 31

. 33

55

39

'27

POP. INT.

OPT. INST.
POP. INT.

THERM.
GALV. .
ELECT. .

POP. INT.
MAGN.
NEW. OPT,

28
. 10
14

42,43
5,6

. 13

• 47
. 49

• 77

. 77,78

. 78, 79

. 79

80

. 46, 47

37—39

8

32
£13,64

46,47
. 96
.38

36

GENERAL INDEX.

r, peculiar, proceeding from a worked electrical ma-
chine . . • • •.*.•.*.•
OERSTED, history of electro-magnetism prior to his disco-
veries ........

account of his discoveries ....

his electro-magnetic theory
Oi^-thermometer, Newton's improvements on
OPACITY of bodies, probable cause of ...

Opaque microscopes, description of ....

Opera-glasses, achromatic description of ...

Opinions of the ancients respecting the cause of magnetic aU
traction ........

Optics and optical instruments, books upon

popular introduction to .....

Orbits of the planets proportionally represented
Oxidability of different metals, tables of ...

PARABOLOIDAL surface proper for the mirrors of reflecting
microscopes ........

PARKER'S large burning-mirror described

Particles, minute, of bodies, separated by pores . .

Pearl (mother of) micrometer .....

Percussion, artificial magnets made by • ...
Periscopic combination of lenses ....

microscope described .....

spectacles .......

Perpetual motion exists only in the heavens
Phantasmagoria, that exhibition explained
Phenomena of the rainbow, different theories of

examined and investigated by
Aristotle and Euclid ......

of refraction accurately investigated by Ptolemy
of coloured rays, description and laws of .

prismatic, singular account of ...
exhibited by thin transparent plates . .

observable in the passage of the electric fluid

on the passage of the electric fluid
through a vacuum .......

of intermitting springs accounted for .
Photometer and hygrometer, description of ...

improvements in, suggested
PHOTOMETERS, different, by Leslie, described
Physiological effects of galvanism ....

Pile, galvanic, discovery of ......

. . formed by layers of muscle and brain

formed of vegetable substances . .
Piles, galvanic, secondary formation of ...

battery formed by a combination of .

Pith-balls, experiments with, on electrical attraction and
repulsion ........

Planets, probability of their being inhabited by rational beings
general account of .....

* . their orbits and diameters proportionally represented
primary, why the satellites revolve round them
vicissitudes of seasons in ....

Plates, galvanic electricity not dependent on the magnitude
of their surfaces .......

thin, thick, &c. colours of ....

tables of ...
Platina pyrometer described .....

phenomena exhibited on its combustion
PLATO, his knowledge of the equality of the angles of inci-
dence and reflexion. ......

Plummet attracted by mountains

Plus and minus degrees of the thermometer, inconvenience of
Pneumatics, popular introduction to ....

Polarisation of light first suggested by Newton . .
Polarity of a magnity described and illustrated
Pole of a magnet made to revolve round a vertical wire t

ELECT.
ELECT. MAGN.

THERM.
NEW. OPT.
OPT. INST.

MAGN.
OPT. INST.
POP. INT.

GALV.

OPT. INST.
NEW. OPT.

MAGN.
OPT. INST.

Page
. 25

. 1—4

. 4—6

79—81

. 5

. 63

50

. 24

. 33

. (JO

74—100

35

22,23

15
17
53

57

42
41

42

9

10

52

2
. 2

. 41

. 38

39—41

23—28

25

. C3

39,40

45

42—44

17

4

25

. 25

27

5

17—20

33,36

33—40

35

34, 35
48

GALV. . .10

NEW. OPT. 39—41,52—54

POP. INT.
OPT. INST.
NEW. OPT.

ELECT.

POP. INT.
THERM.

GALV.

GALV.

POP. INT.

THERM. .
GALV.

NEW. OPT.
POP. INT.
THERM.
POP. INT.
NEW. OPT.
MAGN.

. 29, 30
11

1

9

65—73

64

1,2

ELECT, MAGN. IS— 2

GENERAL INDEX.

Pole of a magnet tends to induce opposite polarity in adja-
cent bodies ........

sometimes reversed by lightning

Polishing of lenses and specula, method of ...
Polyzonal lenses described .....

PORT A said to have invented the camera obscura •
Forter, its taste when drunk out of a pewter pot .
Positive and negative electricity, concerning
Power of a magnet at 100° Fahrenheit increases as it cools
lost by rubbing ....

of a galvanic battery best determined by experiment
Powers, mechanical, individually described
Precautions in constructing thermometers .
Preservation of magnetic power, means for ...
Pressure of fluids, action of explained

of the atmosphere on a man's body, amount of
PIIEVOST'S theory of magnetic fluids ....

Primary colours of the rays of light .

Prisms, refraction of the rays of light through

Prismatic spectrum, arrangement of ...

observed by Seneca
phenomenon, singular account of .
colours, exhibited in the combustion of metals
. . . exist in the light, and are not produced by

the prism ........

PR IT CHARD'S diamond and sapphire lenses, account of
Progress of magnetic induction, method of quickening .

by heat, remarks on

Progressive changes of dip and variation of the magnetic needle
Properties of bodies in general .....

mechanical, of atmospheric air ...

of fluid "

PTOLEMY, his knowledge of the laws of refraction . .

Pulley, the principle of, illustrated .....

Pump, common, principle and construction of

forcing ... ....

. . air, description of, and experiments with
Pyrometer, history and construction of various kinds

by Achard described ...»

by Muschenbrock .....

by Desagulier .....

by Ellicot .......

Mortimer's ......

Froteringham's .

Smeaton's ......

Ferguson's ......

Microscopic, of De Luc ....

Ramsden's .......

Wedgewood's ......

Platina one, of Morveau ....

Schmidt's air-pyrometer
dell

It's, or metalline thermometer
Js, or metalline thermometer
Pyroscope, by tes^, described

ELECT. MAGN.
OPT. IN ST.

NEW. OPT.
GALV.
ELECT.
MAON.

GALV. .
POP. INT.
THERM.
MAGN.
POP. INT.

MAGN.

. NEW. OPT. .
POP. INT. 90, NEW. OPT.
91,

NEW. OPT.
GALV. .

POP. INT.

OPT. INST.
MAGN.

POP. INT.

NEW. OPT.
POP. INT.

THERM.

01 uiiiiu t a

.niell's
. . '''•''. 'l >• i

QUERIES, Newton's, concerning the inflexion of light * NEW. OPT,

e\r, and vapours, distinguished, and theif causes

1 out . . . . . POP. INT.

o\r, and hail . ...

. the phenomena of investigated by Aristotle and

different theories of
s reflecting telescope, description of
s 's micrometrical eye-pieces described .

microscopic pyrometer ..

gkt proceed in straight lines ..

primary colours of , f .

NEW. OPT.
OPT. INST.

37
Page

88
. 2

30, 31
. 7,8

3
24
13
13
54
29

20—29

10—15
53

57,58

65

41

15,33

12—31

15

1

38

11—13

90
13
11

. 12

29—31

1—8

. 7

56—60

2,3

24, 26

67,68

m

65, 66

15—32

9, 10

15

16

16, 17
17
81

18—20

21—23

23,24

24^27

27—29

29—30

31

31, 32

32, 33
33

44, 45

63

5
01

S3

GENERAL INDEX.

Page
37
1 1
10

7, 33
77
90

Rays of light, reflected from bodies of their own colour . NEW. OPT.

general laws of their reflexion . . . -

interference of ...... > .

refraction of and its laws ...

how they become visible in their passage . POP. INT. .

effects of their transmission through glass prisms - -

in what manner they are refracted to the retina - . . 95

Re-action and action, their equality a law of nature POP. INT. 11 — 20. MAGN. 3

REAUMUR'S thermometer, account of .... THERM. . . 8

Fahrenheit's and the Centigrade scales of ther-

mometers, formulae for converting into each other . - . 10

decomposition of the spectral colours produce white light . NEW. OPT. . 35

Reflected colours, cause of investigated . . . • POP. INT. . . 92

motion explained and illustrated . . . -- . 13 — 15

colours, Newton's scale for determining . . NEW. OPT. . . 46

Reflecting microscopes t OPT. IN ST. . 43, 48

telescopes . . . NEW. OPT. 10, 30. --- . 11,15—18

their peculiar advantages . . . POP. INT. . 100

.. surfaces of cylinders, their application . . OPT. INST. . . 3

Reflexibility of light, Newton's experiments on . . . NEW. OPT. 20 — 22

Reflexion of the rays of light, general laws of . . -- . .11

and transmission of light, fits of easy . . . -- . 44

of aerial vibrations the cause of echoes . . Pop. INT. . . 72

Refracted and geometrical focus, distinction between . . OPT. INST. . 7

Refraction of light, Snellius's discovery of its laws . . NEW. OPT. . 7

Newton's theorems respecting . . - . 33

theory of ..... POP. INT. . 85, 86

its effects on the apparent situation of dis-
tant bodies

through lenses of different forms described --- . . 89

of homogeneous light, law of ... NEW. OPT. . 26

double, explained ..... - • - . . 9

Refractive power of diamond and sapphire lenses . . OPT. INST. . 39, 40

and dispersive powers of different transparent sub-

stances ...... -- . 25

power and spherical aberration, in the inverse ratio

to each other ..... -- . .12

Refrangibility of the rays of light . . NEW. OPT. 32, 33

Register thermometers, history and construction of . THERM. . 33 — 39

of Cavendish .... - , 34, 35

Six's ..... - . . 35, 36

Black adder's .... - - 38, 39

Trail's ..... -- . 39

Keith's . - . 37, 38

REGNIER'S metallic thermometer .... THERM. . . 29

Relative magnetic power of iron and steel, table of . . MAGN. . 15

with respect to distance . . - . . 16

Representation of the proportion of the orbits of the planets POP. INT. .

of . . of the diameters . . - •

and description of the terrestrial globe . -- - .
of different appearances on the passage of the

electric fluid ........ ELECT.

Repulsion and attraction, electrical, concerning . . —

experiments on . \) 1 '

of magnetic iron . . . MAGN.

similar to electric . — . 3, 39

Resinous and vitreous electricity distinguished . . ELECT. . . 8, 9

Resistance of the air, its effects on falling bodies generally ^ POP. INT. . 7

on moving bodies . . -- - . 17, 18

Retina^ the images of objects inverted upon ... — .78,79

in what manner the rays of light are refracted to . - -- . . 95

Revolving camera obscura, description of ... OPT. INST. . 53

RICHMAN, fatal consequences of his electrical experiments ELECT. . . 33, 59

Ring, electro-dynamic, effects of .... ELECT. MAGN. . 90

. . Saturn's, described . . . . . . POP. INT. . . 36

Rings, coloured, phenomena of . . . . . NEW. OPT. . 41 — 43

RITCHIE'S torsion-galvanometer ..... ELECT. MAGN. . 41

HITTER'S experiments on electro-magnetism . . —— — — . , 3

ROBINSON'S method of finding the radii of the surfaces of

convex lenses ........ OPX, INST, 6

GENERAL INDEX.

39

ROCHOX'S telescope for measuring angles and distances .
ROE MUR'S mercurial thermometer described
KOGET'S galvanic experiments, account of . .

demonstrations of the properties of the magnetic
curves .........

invention of a machine for the mechanical delinea-
tion of the magnetic curves .....

experiments on the mutual attraction of the coils of
a helical conducting wire .....

Rotations caused by electro-magnetism ....

of a magnet round its axis ....

helical, by electro-magnetism ....

rendered visible in terrestrial magnetism

on the magnetism of .....

Rotatory and vibratory motions, experiments on
Royal Institution, voljtaic battery

experiments made at ...

Royal Society, their early attention to electricity

description of a dipping-needle belonging to them
RUMFORD'S thermoscope described ....

Russia, thermometer used in .....

RUTHERFORD'S day and night thermometer

SABINE'S dipping-needle described ....

Sapphire and diamond lenses, their aberration compared with
those of glass .......

lenses and microscopes .....

SAXTORIO, the maker of the first thermometer, described
Satellites, why they revolve round their primary planets

of Jupiter, their eclipses, and advantages of
Saturn, with his seven moons and ring, described . •
Saturn's sixth satellite, when discovered
Scale, Fahrenheit's, principle of its division

. . Newton's for determining reflected colours
Scales of thermometers, directions for graduating

. . for . . proposed by Boyle, Halley, and Hooke

. . . . by Murray

. . of Reaumur's, Fahrenheit's, and Celsius's thermome-
ters converted into each other .....

. . tables of the correspondences of different thermometrical
SCHMIDT'S air-pyrometer described ...»
SCLIWEIGGER'S electro-magnetic multiplier
SCORESBY'S experiments upon the magnetism of iron

instrument for finding the dip of the needle .
Screw, its effect as a mechanical power explained
Sea and land breezes, their causes described . . •
•Seasons, causes of their variation explained

their vicissitudes in the planets . . •

'art/ galvanic piles described .

;vhis knowledge of optics . .

by a current of voltaic electricity
ccasioned by the galvanic and electric fluid
n object, by what means visible

cnned and illustrated . %

omena of .....

illustrations of

prevented ....

OPT. INST.

THERM.

GALV.

MAUN.

ELECT. MAGN.

MAGN. .
ELECT. MAGN.
GALV.

ELECT. .
MAGN.
THERM. .

MAGN.
OPT. INST.

THERM.
POP. INT.

OPT. INST.

THERM.
NEW. OPT.
THERM.

13

NEW. OFT. 2

1 figure of
Ship's copper, its corrosi

Shock, electric. See E/ecMe shock.

communicable to vast numbers of persons at
a time and through great distances ....
Shocks, tipwerful, from voltaic and electric batteries, equally
destructive of animal life . . . .

electric, received from different species of fishes
Sidereal and solar time, distinction between
Sight acquired by a man who had been blind from infancy, its
effects upon him described . . •

cause and remedy of its decay in old age
Signs of the zodiac described .....

Silver and sulphur, their combination assisted by galvanism

ELECT. MAGN.
MAGN. .

POP. INT. .

GALY.

OPT. INST.
GALV.

POP. INT.

NEW. OPT.
POP. INT.
GALV.

ELECT.
GALV.
POP. INT.

GALV.

7
30,31

19
. 20

59

19—32

•23, 24

39

. 50

91—96

30—32

7

10—13
1

75—78

, 40,41

9

36,37

80,81

13

39,40
2

34,35

. 52

36

17

7

46

—15, 56
4,5
9

10
63
31
40
14
81
. 28

70,71

44—48

48

27

33

. 18
18
27

75,76

. 61

2

14

39,40

19
. 26

48,49

80,81

. 97

37, 42

24

40 GENERAL INDEX.

"Pape

Simple microscope, theory of ..... OPT. INST. . 33, 43

galvanic circles described .... GALV. . . 2 — 4

Sines, the law of, explained ...... NEW. OPT. . . 7

SINGE K'S electrical column ..... GALV. . . 26

Singular prismatic phenomenon, account of ... NEW. OPT. . . 38

Six's register thermometer ..... THERM. . . 35,36

SMEATON'S micrometer-thermometer described . . THERM. . 18 — 20

Smoke and steam, cause of their ascent . . . POP. INT. . . 8

SNELLTUS'S discovery of the laws of refraction . . NEW. OPT. . . 7

Snow, hail, and vapour, cause of . « POP. INT. . . 61

Solar microscope, description of . . POP. INT. 99. OPT. INST. . 50,51

. . rays, their influence on magnetism . . . ELECT. MAGN. . 97

. . and sidereal time distinguished . . . POP. INT. . 48, 49

Solenoid, electro-dynamic, description of . . . ELECT. MAGN. . 79

Solid parts of bodies not touched by the rays of light . NEW. OPT. . 55

. . bodies less affected by gravity than fluids . . POP. INT. . . 56

Solids, tables of the comparative expansions of seven, by heat THERM. . . .27

. . capable of conveying sounds by the vibration of their

particles ...... POP. INT. . . 71

Sonorousness of bodies owing to their elasticity . . --- • . 71

. . effects of, illustrated "... -- . . 71, 72
Sound which accompanies the transference of the electric fluid,

variously measured ....... ELECT. ... 25

. . various remarks on ..... POP. INT. . . 69 — 73

.. caused by the tremulous motion of the air . . • -- . . 71

. . conveyed by the undulation of liquids . . • --- . . 71

the vibration of the particles of solid bodies -- . . 71

.. its velocity about 1142 feet in a second . . -- — . . 72

not materially different, whatever is the

direction of the wind .... - . . 72

. . harmony, or concord of, how produced . . -- — . •

Spar, Iceland, its double refraction discovered by Bartolinus NEW. OPT. . 9
Spark) electric. See Electric spark.

Speaking trumpet, principle of its construction . . . POP. INT. . 72, 73

Specific gravities of bodies explained .... • -- . 59, 60

Spectacles, when invented ...... OPT. INST. . 8

periscopic, described ..... -- . 9

Spectral colours, when re-united, produce white . . . NEW. OPT, . . 35

Spectrum, prismatic, described . . . , . -- . 15

Specula, qualities and proportions of, metals for-*- • " . . OPT. INST. . 29

method of grinding and polishing . . . • -- . 30

Spherical aberration in lenses, cause of NEW. OPT. 7, 8, 27. - - - . s&\

, , in mirrors . . . . . — — — •—- -

is in the inverse ratio of the magnifying

power ........ ---

in lenses, minimum of

r comparison of, in glass, sapphire, and

diamond ...... • -

Springs and fountains, their origin, &c. .... POP.

.. intermitting, accounted for . . . •" V ^
Stands, or supports, for telescopes, construction of
Stars, fixed, their size and distances unknown
Statical thermometer, description of ...

Steel. See Iron.

Steel-bar, effects of a transmission of electric fluid throu
Steelyard, its principle and application . ^ ,
Stereo-electric currents, definition of . ^

Stone, falling, is met on its way by the earth .
Stones, electrical quality of some species described

Structure and functions of the eye . . POP. I> " 3

Siibslances, transparent, table of the refractive and disp£i;

powers of ........ ;^ .25

Sulphttric acid, dilute, its action upon copper and on zimf' . 14

SUTLER, the first author who noticed any of the effects ^T|

galvanism ...... .

Sun, the earth nearer to in winter than in summer . . vfcp. INT.

.. eclipses of, explained ...... -- . 51,52

. . why he appears red through a fog . . . .

Sun's rays, their obliquity the cause of the' diminution of

their heat «,.,,... -- . 46, 47

GENERAL INDEX.

Surfaces of bodies, light reflected from without touching them NEW. OPT.

of cylinders, their use in optical experiments . ()i-r. INST.

Syphon, description and principle of .... POP. INT.

System of astro/iomy, Copernican, objections to, answered .

TABLE of the height of the boiling-point of water in different
states of the barometer ......

. . of the relative expansions of different metals at differ-
ent heats ........

. . of the expansion of rods of different substances between
the freezing and the boiling point of water ...
. . of the expansion of seven different solids by heat .
. . of various points of heat according to Wedgewood
. . of temperatures according to Daniel

to Morveau

.. of expansibilities of glass rods and tubes
. . comparative, of alcoholic and mercurial thermometers
. . of correspondence of different thermometrical scales
. . of expansions of bodies by heat ....

. . of remarkable temperatures, according to Fahrenheit's
scale ......

. . of the refractive and dispersive powers of various sub-
stances .......

. . of the colours of thin plates of air, water, and glass
. . of dimensions of an aplanatic double object-glass .
. . of the magnifying powers of lenses ....

. . of the comparative thermo-electric relations of different
metals ......

. . of the relative magnetic powers of different species of
iron and steel ......

. . of changes of variation and intensity of terrestrial
magnetism ......

. . of the local attractions observed in different ships
Table-microscope, description of . . . . .

Tangential force described .

Taste of porter affected by galvanism ....

Teinu scope, description of ......

Telescope, different accounts of the invention of

astronomical, described . . NEW. OPT. 6.
night and day ......

Newton's reflecting . . NEW. OPT. 30,

achromatic, by Dollond, Blair, &c. .

theory of . . . . .

Cassograinean and Gregorian

front-view reflecting .....

reflecting, by Ramage

proper form of the specula of
•".v, method of ascertaining their power

for measuring angles and distances . .

THBRH.

OPT. INST.
NEW. OPT.
OPT. INST.

ELECT. MAQN.
MAGN.

OPT. INST.
POP. INT.
GALV. .
OPT. INST.
NEW. OPT.
OPT. INST.

irley's graphic
;--e .

- of refracting
-pieces for
for .

especting
iction of

of
of .

NEW. OPT.
OPT. INST.

•

tabk

table <>f, nmvdini; to Fahrenheit's
at which dew falls
Terrestrial globe, representation and description of .

magnetism, observations on ... •

variations in its intensities .
electro-magnetic effects of
exhibits a visible rotation . .
imitated by means of wires on an ar-
tificial globe ..... ...

POP. I NT.

MAGN.

ELECT. MAGN.

41

Tape

55

3

fi-J.T,:;
37,39

15
15

20
27

. 29
32

. 30

53

. 54, 55

57—60
. 61,62

. 63

25

. 52
21
35

94
15

32
66
36

17,31
24

54,55

5,4

10

10

. 15
23
19

16,17

. 17, 30

. 18

11

11

25

26

25,26

. 26—30

27,28

29

. 31

100

100

. 30

29

. 32
63

. 49
40

22—32
2.">

45—52
. 50

91

42 GENERAL INDEX.

Page

Test-objects for microscopes ...*., OPT. INST. , . 49

Theorems, Newton's, concerning refraction , . . NEW. OPT. . 33

Theories of the nature of light . . • • . 9

of the rainbow ....... . 4, 5

of electricity ELECT. 10 — 14, 60 — 62

of magnetism ....... MAGN. . 32 — 4 1

general observations on . . . . 96

of electro-magnetism ..... ELECT. MAGN. 79 — 92

Theory, magnetic ....... . 79 — 81

of the winds explained POP. INT. . 69

of the trade-wind ...... . . 70

of the Monsoons ...... . 70

of the sea and land breezes .... , 70 — 7 1

of the sonorousness of bodies .... . .71

of the nature of light ..... • . 74 — 75

of reflected colours ...... . .92

electro-dynamic of Ampere .... ELECT. MAGN. 81 — 92

induction of magnetism, explained by — . 87

of colours, Newton's NEW. OPT. . 31

of the undulations of light, Newton's . . . . 63

of achromatic telescopes ..... OPT. INST. . 19

of the single microscope ..... — » 33

of the compound microscope .... • . . 43

of galvanism, by Du Fay ..... GALV. . . 3

byVolta • . 31,32

Theories of Galvanism, electrical and chemical . . . . . 1 9 — 20

Thermometer, history and construction of ... THEKM. . 1 — 10

the first made by Santorio, described . . . . 2

improvement of, by the Florentine academy

byDrebbel . . . . . 2

with glass globules, described • . . . .

Boyle's, described ..... . . 3, 4

(oil) of Newton, described ' . . . . ,

Roemur's mercurial, described ... « . .

Amonton's . . . . v. , . . . 6

objections to . . \ . . 8

Craqtiius's ...... . . 8

centigrade, or Celsius's . . . , , . . 9

Reaumur's . . . . . . . . 8

Hales's . . . . . . . . 9

De Lisle's, used in Russia ....

De Laland's proposed ...» .

Thermometers, precautions requisite in the construction . . .

effects of age on .... . .

of atmospheric pressure on . .

directions for filling ....

graduating the scales . .

perfect vacuum in, not essential . .

tables of correspondences of various scales
Thermometer, Register, history and construction of .
Cavendish's described
Six's .

Keith's . ...

Black adder's
Trail's .

Differential, history and r
Leslie's <1<
DeF>i>-
METALLINE, 1>-

.

IA

by IJourtet J|f

peculiar applications yr^M — . "*

air of Kinnersiey , . .

statical by Gumming "& TJIKHM. . ».*<• — ±7

balance, by Kewley " , • • , . 47; 48

barometrical by Wollastou. . r . . 4», 49

GENERAL INDEX.

Thermometer, alcoholic and mercurial, table of comparison
at different temperatures ......
day and night, of Rutherford .
Thermometric Hygrometer, Leslie's, described . •
Thermoscopc, Rum ford's described ....
Thermo-electricity, definition of, and experiments- on
the phenomena of not, hitherto reduced
to a theory ... ...»

THERM. .

THERM. .
ELECT. MAGN

43
Page

54, 55
36, 37
. 41,42
. 40,41
. . 92—96

95

does not appear to arise from any chemical
change in the materials . . .

. 95

Thermo-electric currents distinguished from galvanic .

.

.93
. 94

chiefly recognized by their effects on
the magnetic needle . .

.95

Threefold motion of the earth described

POP. INT. .

. 34

. 54, 55

71

Time, solar and sidereal, distinguished ....
Topaz, colourless, of New Holland, use of double refracting

OPT INST.

. 48, 49
. 25

Torpedo, anatomical description of . .
Torsion-balance, description of .....

GALV.

ELECT.

12G
. 19—21
20 9l

Torsion-galvanometer, described . . . . •

ELECT. MAG.
ELECT. .

41

54

Trade-winds, description and cause of .
Transference of electricity ......
Transmission and reflection of light — easy fits of

POP. INT.
ELECT. 4,
NEW. OPT.

. 70

11, 23—28
44

5G

Transparent liquors, their colours vary with their thickness
plates, their phenomena ....
substances, tables of refractive and dispersive

. 37
39—41

4 — G— 25

POP INT

40 41

Trough battery, galvm 4 ...
i .

Trumpet, the sp< . mpet constructed
on the san:<> "...
Truth, invariably ^-t Rfe .
Tubes, capilh : . ft & • .

T.~ N'DUI * "\ '•

GALV.
POP. INT.

JVlAGN.

POP. INT.

7
7

72, 73
. 33
. 5

71

" ffir. . .
Universal discharge v .

^ V.u r r ^ ^Bity through

KiP ' •

.^ 'of ->i Ij^B boiling water
-d - . Mearth's surface

wi K" • •

•Ranges m
Bsm. .

NEW. OPT.
ELECT.

ELECT.
POP. INT.

THERM.

MAON.

• /A

03
37

. 25
5—61
44—48
8
22, 23
23
. 25
29—31

01

fil 7" 74

K • •

mft'of
m • '•

Hnfcmentum
: peonies

OPT. INST.
GALV.

ELECT.
POP. INT.

. 26
25
50,51
9, 10
. 11

72

Oft

Htnunds

ELECT.
POP. INT.

23

36

T\

•

OPT. INST.
NEW. OPT.

• 71
. 48
3
35

44

GENERAL INDEX.

Vitreous electricity distinguished from resinous . . ELECT. .

VOLTA'S galvanic pile, construction of ... GAT.V.

theory of galvanism, arguments for and against .

Voltaic battery of the Royal Institution • . .

Foliate battery, intense heat produced from . ... . GALV. .

experiments with .... .

pile, decomposition and recomposition of water by .

different decompositions by ... . ,

and common electricity, different mechanical effects of • • • - ••»
electricity, sensation of in passing through the

human body ........

transferable to the common electrical

Voltaic magnets, description of t . . . . . ELECT. MAGN.

made to exhibit the phenomena of the dip-
ping needle . ' . . . . . . .

WATER, accumulation of in hills and mountains . . POP. INT.

its boiling point affected by atmospheric pressure THERM.
height of its boiling point in different states of

the barometer . . . . . . . •-

decomposed and recomposed by the voltaic pile GALV.
. . why not decomposable by the common electrical

machine ........ .. . . . .

Water-level, description, of that instrument . . . POP. INT.

Wedge, a mechanical power ...... .

WEDGEWOOD'S pyrometer, description of . . THERM.

table of temperatures accord-
ing to .........

Weighing-beam, principle and construction of . . POP. INT.
Weight multiplied by the velocity, the measure of mo-
mentum ........ .

of bodies different at different parts of the earth's

surface ........ •

Wheel and axle, account of that mechanical power : . .

White produced by the reunion of all the spectral colours NEW. OPT.

. . light, decomposition of .....

Winds, general and particular remarks on ... POP. INT.

causes of, investigated ..... .

WiNKLER first used cushions in electrical machines . ELECT.

Winter, the sun nearer the earth in than in summer . POP. INT.

WOLLASTON'S barometrical thermometer described . TIIEI- i

elementary galvanic battery . . . GALV

periscopic spectacles described . , OPT. 1>

camera lucida, constructional' . . .

*

ZAMBONI, description of his dry galvanic pile •; . . GALV
Zig-zag progress of the electric spark when passing to along -

distance. ...... . . KLT.C

Zmeand copper, different effects of certain acids upon GAL\

Zodiac, signs of described . ..... Voi'.

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