Lectures on experimental philosophy, astronomy, and chemistry; intended chiefly for the use of students and young persons

LIBRARY

OF THE

UNIVERSITY OF CALIFORNIA.

GIFT OF

MRS. MARTHA E. HALL1DIE.
Class

LECTURES

ON

EXPERIMENTAL PHILOSOPHY,

LONDON

PRINTEn BY THOMAS DAVISON. WHITEFRIARS.

• LECTURES

ON

EXPERIMENTAL PHILOSOPHY,
ASTRONOMY,

AND •,,•' ; *>:

.1.

CHEMISTRY: :.,

INTENDED CHIEFLY

dFor tfjc $fee of j&tutontg anD footing $ ergon*.

BY G. OREGORY, D.D.

LATE VICAR OF WEST HAM J DOMESTIC CHAPLAIN TO THE LORD
BISHOP OF LLANDAFF; AND ADTHOR OF THE ECONOMY

OF NATURE, &C. &C.

OF THE

UNIVERSITY

TWO VOLUMES.

VOL. I.

SECOND EDITION, CORRECTED AND IMPROVED.

LONDON:

PRINTED FOR LONGMAN, HURST, REES, ORME,AND BROWN;
BALDWIN, CRADOCK, AND JOY j SCATCHERD AND

LETTERMAN; G. AND w. B. WHITTAKER;

AND
SHERWOOD, NEELY, AND JONES.

1820.

PREFACE.

THE object of this publication is, to
afford a useful companion to such Stu-
dents as may attend Lectures in the
Universities, at the Royal Institution,
or elsewhere ; and also to enable the
Masters of private Seminaries, with a
very moderate Apparatus, occasionally
to indulge their pupils with a practical
Course of Lectures on one or all of the
important branches of Experimental Phi-
losophy, Astronomy, and Chemistry.

Having published some years ago
" The Economy of Nature/' the author
thinks it necessary to state, that both
the plan and arrangement of that work
are essentially different from those of the
present. The Economy of Nature does
not contain Astronomy, nor, in fact,
Chemistry, as a distinct science ; on the
other hand, a very large portion of that
work is occupied with Mineralogy and
Physiology, which in this are purposely
omitted. Even the subjects which are
common to both will be found to be dif-
ferently treated in these Lectures.

February 20, 1808.

PREFACE

SECOND EDITION.

i THE first Edition of these Lectures
having experienced a very extensive cir-
culation, the Proprietors have thought it
their duty to procure for the present
such an entire and cautious revision as
should render it still more worthy pub-
lic favour.

Th« whole of the first volume, and so
much^of the second as relates to Astro-
nomy, has been carefully examined by
a gentleman whose different works on
Mathematics and several departments of
Natural Philosophy have acquired a high
reputation. He has made numerous ad-
ditions and improvements, correcting
errors, and carefully introducing as he
went along, the most important dis-
coveries both of English and of con-

IV

tinental Philosophers, down to the close
of 1819-

The chemical department has, in like
manner, undergone the careful revision
of a gentleman eminent in the science of
Chemistry. So numerous and important
have been the accessions to this region
of human knowledge, in the course of
the last twelve years, that a cautious
revision has, in fact, included the entire
re-composition of a considerable portion
of the second volume.

The Proprietors have every reason to
believe that the improvements thus made
to the Lectures will considerably aug-
ment their utility : and they humbly yet
confidently anticipate the reward of an
enlightened public, for the expense they
have incurred by engaging gentlemen of
such acknowledged competence to make
the volumes exhibit a correct yet popu-
lar view of the present state of Experi-
mental and Chemical Philosophy.

July, 1820.

DIRECTIONS FOR PLACING THE PLATES.

VOL. I.

Plate I. to face
II.
III.
IV.

VI.

VIL

VIII.

IX.

X.

XL

XII.

XIII.

XIV.

XV.

XVI.

XVII.

XVIII.

Plate XIX. to face

224

18

XX.

228

22

XXI.

223

36

XXIL

242

41

XXIIL

253

49

XXIV.

255

54

XXV.

271

68

XXVI.

284

•7K

XXVII.

307

/ O

102

XXVIII.

318

121

XXIX.

315

131

1 O 1

149

VOL. II.

166

Plate I. to face page

3

172

II.

12

185

III.

20

196

IV.

23

208

V.

108

219

VI.

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CONTENTS

TO

VOL. I.

EXPERIMENTAL PHILOSOPHY.

LECTURE I.

Page

General Objects and Principles 1

LECTURE II.
Attraction 10

LECTURE III.

Magnetism - - - - 21

LECTURE IV.

Hydrostatics - 35

LECTURE V.

Hydraulics - 48

VOL. I. b

CONTENTS.

Page
LECTURE VI.

Of Pneumatics - 6l

LECTURE VII.
The Phenomena of the Atmosphere - 78

LECTURE VIII.
Electricity - - - - <#

LECTURE IX.

Electrical Phenomena and Galvanism - 10Q

LECTURE X.

Light - 129

•

LECTURE XI.
The Refrangibility of Light . 148

LECTURE XII.
Reflexibility of Light, or Catoptrics \64

LECTURE XIII.
Vision and Optical Glasses - - - 178

LECTURE XIV.
Colours - 199

LECTURE XV.
The Laws of Motion - - - 216

CONTENTS.

Page
LECTURE XVI.

The Mechanic Powers - - 232

ASTRONOMY.

LECTURE XVII.

System of the Universe - - 250

LECTURE XVIII.
Of the Sun, and his real and apparent Motions 270

LECTURE XIX.

The Primary Planets; the Mode of calculating
their Distances, &c. - 280

LECTURE XX.

The Secondary Planets - -312

LECTURE XXI.

The Earth ... . 3?g

LECTURE XXII.
The Tides - - - - 343

LECTURES

ON I ; ;, ; • ^

EXPERIMENTAL PHILOSOPHY, fcc, , :».

LECTURE I.

EXPERIMENTAL PHILOSOPHY.

GENERAL OBJECTS AND PRINCIPLES.

You are, I presume, desirous, my young friends,
of acquiring knowledge, of satisfying your cu-
riosity, of storing your minds with useful ideas,
of fitting yourselves for company and conversa-
tion, and of enabling yourselves to proceed gra-
dually in the paths of science, till you arrive at
distinction and eminence.

Suffer me to ask you, if you do not feel a
strong curiosity to know the nature of all those
objects that you see around you ; to be informed
of the causes of those astonishing changes which
you observe every day produce. You see the
sun, which apparently rises every morning to
give light and heat to the world. You will be
surprised to be told, that it is not the sun that
moves upon these occasions, but it is the earth

VOL. i. B

2 Experimental Philosophy. [Lecture 1.

on which you stand, that revolves upon an axis,
and presents different parts of its surface to the
sun at certain hours of the day. Or, when you
:irc told tnis, do you not feel a wish to know the
proofs and the reasons of it; and why the sun
appears to n-iove, when in reality it is yourself,
or rather the earth on which you stand ? — Have
patience, and you shall know all this; and it
will be as clearly proved to you as any common
fact, or as the result of any arithmetical operation.

Again : You throw a stone, or shoot an arrow
upwards into the air ; Why does it not go for-
ward in the line or direction that you give it?
Why does it stop at a certain distance, and then
return back to you ? What force is it that presses
it down to the earth again, instead of its going
onwards ? On the contrary, Why does flame or
smoke always mount upwards, though no force is
used to send them in that direction ? And why
should not the flame of a candle drop towards the
floor, when you reverse it, or hold it downwards,
instead of turning up, and ascending into the air ?

You look into a clear well of water, or on the
surface of a looking-glass, and you see your own
face and figure, as if it were painted there, and
even more correct than the best artist could draw
it. Why is this? You are certain there is no
such figure, either in the well or behind the
looking-glass. You are told this is done by
reflection. But what is reflection? It must be
some property in light, which occasions its being

General Objects and Principles. 3

thus thrown back to your eyes, and which causes
you to see a figure as distinctly as if you looked
upon the figure itself. This shall also be ex-
plained to you ; as well as the reason why when
you look upon the ground, at a wainscot, or on
a rough unpolished table, you see nothing of the
kind.

When you look through some glasses you see
things much bigger than they really are, or mag-
nified; that is, made larger. When you look
through others you see them less than they appear
to your eyes, or diminished. What is there,
then, in the one glass that it should cause things
to appear larger than they do to your natural
sight: or, in the other, that they should seem
so diminished ? Yet this too will be explained ;
and you may, by certain rules, be taught to
calculate how much larger or smaller any glass
will make an object appear, before you look
through it.

You cannot be unacquainted with that tre-
mendous noise, which the ignorance of the an-
tients considered as an indication that their god
Jupiter was in a passion. We call it thunder.
But what is thunder ? You have also probably
seen fire descend in streams from the clouds, or
pass instantaneously from one cloud to another ;
and after darting first to one side, and then to
the other, several times, come to the earth with
a zig-zag kind of motion. This is lightning,
and it proves fatal wherever it strikes: it kills

4 Experimental Philosophy. [Lecture 1 .

men or cattle ; it sometimes levels to the ground
the proudest edifices, and sets on fire the loftiest
trees or buildings. You have probably never
once thought what can be the cause of this
thunder and lightning. But will you not be
astonished to see it imitated on a smaller scale,
the same noise excited, a rapid fire sent forth
like that, and producing similar effects ?

You see every day the clouds collected over
your heads, and passing hither and thither, as
directed by the wind. You see them assume
different shapes and forms ; sometimes gathering
into a large thick mass, at others breaking into
small divisions. What are the clouds made of,
think ye? Whence do they come? Why do
they appear and disappear ? Why do not they
fall down immediately upon the ground, as you
see other bodies ?

The clouds, you will probably guess, are water,
because you see rain occasionally fall from them,
and sometimes hail and snow. But how is water
supported in the air? Why do the clouds at
some times drop only rain, and at others hail or
snow ? You will say hail and snow fall only in
cold weather. But why is snow of that fine
flaky consistence like feathers? And why is
hail in little round balls ? All this may be ex-
plained.

You have doubtless observed that beautiful
coloured arch in the heavens, which, from its
appearance during rain, has been called the rain"

General Objects and Principles. 5

bow, and which Almighty God has made the
pledge, that he will not overflow the world with
another deluge. But do you understand how
this appearance is produced? It is, indeed, the
action of light upon the drops of the falling rain ;
but we can show you by what means this appear-
ance, and these vivid colours, are produced;
why it assumes the form of a bow ; why a se-
cond bow is often seen accompanying the first
or primary bow. We can measure the arch
which it inscribes, and explain the whole of this
wonderful spectacle.

It must be well known to some of you from
observation, and to most of you by the informa-
tion of others, that the sea, at certain hours of
the day, varying with the age of the moon,
approaches, and overflows, to a certain height,
the sandy beach by which it is surrounded. This
flux and reflux of the ocean, as it is termed,
is known by the common name of the TIDE.
Antient tradition tells us, that a philosopher put
himself to death, because he was unable to find
out the cause ; but modern philosophy has laid
open the whole theory of the tides, and can de-
monstrate the nature of them upon irrefragable
principles.

In some parts of the world there are fountains
of boiling water spouting from the earth. In
others, the earth itself opens and emits flames
and rivers of liquid fire, and throws out rocks
and stones of an immense size, with a force and

6 Experimental Philosophy. [Lecture 1.

velocity which are imitated in vain by the largest
pieces of cannon. Whole countries have been
swallowed up, and the proudest cities desolated
and destroyed by earthquakes. What is the
nature of these surprising operations? From
what immediate cause are they produced? On
what circumstances do they depend ?

You will answer, they are produced by that
Almighty Power which first created the universe.
It is the hand of God that can alone direct or
alter the course of nature. All this is true.
Nothing is done, nothing can be done, without
the agency, the direction of the Supreme Being.

Yet Providence acts by determinate laws in all
the arrangements of nature. It is not by chance,
nor by an arbitrary disposal of things, that
the operations of nature are effected. By the
Divine Wisdom all things are disposed in weight
and in measure; they are ordered on certain
principles, and effected in certain constant and
regular modes.

These modes, in conformity with which the
Divine Wisdom acts and governs the material
universe, are termed the laws of nature. We
cannot, it is true, account for every thing ; we
cannot trace effects to their remotest causes ; but
yet much is known by long observation, and the
discoveries of learned and ingenious men from
time to time. They have therefore referred what
they call the laws of nature, to a few principles ;
and these principles, when well understood, will

General Objects and Principles.

apply to the explanation of a long series
nomena, that is, appearances, from the Greek
word phainomai, to appear.

It is principally by experiment that all the
great discoveries of the moderns have^ been ac-
complished. This, indeed, forms the grand line
of distinction between the antient and the mo-
dern philosophy, and this constitutes the sole
merit and superiority of the latter. The antients
reasoned and conjectured about the nature of
things ; the moderns have submitted every thing
to the direct and positive test of experience : this
philosophy has therefore been termed experi-
mental philosophy, because all its doctrines and
principles are founded upon actual experiment,
in opposition to that philosophy which is founded
on fancy and conjecture.

It is, I believe, to the old alchemists, or those
who were engaged in the whimsical and visionary
attempt to discover the philosopher's stone, or a
method of converting other substances into gold,
that we are ultimately indebted for this excellent
philosophy. They engaged in various chemical
processes, or experiments, in order to effect this
grand discovery ; and from their patient and la-
borious endeavours many useful inventions pro-
ceeded, though often foreign from the particular
discovery they were in quest of. Our country-
man, Roger Bacon, a famous monk, who resided
at Oxford in the twelfth century, was one of
these; but one of the most rational and sagacious

8 Experimental PJiilosopJiy. [Lecture 1.

of the whole sect. He was soon convinced of the
difficulty of the research in which he was en-
gaged, that of transmuting or changing other
metals or substances into gold ; but he saw that
experiment, and the mode of analysing or dividing
bodies or substances into their constituen t parts, was
the true mode of investigating nature. He there-
fore ridiculed the idle conjectures and unmean-
ing jargon of Aristotle and his followers. In the
course of his researches he made that wonderful
discovery, the composition and use of gunpowder.
He had very nearly fallen upon that of air-bal-
loons. He made a number of excellent experi-
ments in chemistry and optics; and you know
that his only reward was to be accounted a ma-
gician by the ignorant age in which he lived,
and even by the unenlightened part of mankind
in succeeding times.

To another Englishman, of the same name, the
justly celebrated lord Bacon, philosophy is in-
debted for its next great improvement. He fol-
lowed the footsteps of his namesake and prede-
cessor ; he reduced his principles to a system ;
and laid it down as a maxim, that it was by
experiment alone that any thing in philosophy
could with certainty be known. He therefore
traced out the way in which future experimental-
ists might proceed, and afforded a variety of
hints, on which they afterwards improved.

The good and the illustrious Boyle, however,
may be justly termed the father of modern phi-

General Objects and Principles. 9

losophy. He adopted the Baconian principle of
conducting all inquiries by experiment alone. He
effected much in the analysing of bodies, and the
examination into the principles of which they
were composed. He is by many said to have
invented that curious and useful instrument, the
air-pump ; and his experiments on the nature of
air have laid the foundation for ah1 the modern
doctrines concerning it. His discoveries on light
and colours were an excellent introduction to the
grand theory of Newton on that subject, and,
possibly, served as the basis or foundation, of
them. In short, there was scarcely a topic of
natural philosophy to which he did not bend his
attention, and scarcely one which he did not
more or less improve : but still the facts educed
were insulated.

Such was the state of philosophy when Newton
appeared. He reduced, into one grand scheme,
all the scattered discoveries of his predecessors.
He explained the motions of the heavenly bodies
on a principle entirely new, and established that
beautiful planetary theory which is now univer-
sally received. He developed, with mathematical
precision, all the phenomena of light and colours,
the nature of vision, and the use of optical glasses
and instruments, which last he greatly improved.
In short, he gave body and consistency to natural
philosophy, and made it, what it never was be-
fore, a coherent system of truth, illustrated and
proved by experiment.

1

LECTURE II.
EXPERIMENTAL PHILOSOPHY.

ATTRACTION.

BEFORE we proceed to the higher branches of
science, it will be necessary to explain what is
usually meant by attraction, and the different
kinds which have been distinguished by modern
philosophers. In the first lecture I called your
attention to the effect which follows when you
throw a stone, or shoot an arrow upwards into
the air. Instead of proceeding according to the
direction in which you sent it, you see its force is
quickly spent, and it returns to the earth with a
velocity increasing as it descends. Now it is easy
to conceive that the resistance of the air may
stop it in its progress ; But why should it return ?
Why should not the resistance of the air stop or
impede it in its return ?

The answer you will think very plain — It is
its weight that brings it back to the earth, you
will say, and it falls because it is a heavy body.
But what is weight? Or why is it heavy? It is,
in truth, the earth which draws or attracts the
stone or the arrow towards it ; this overcomes the
force with which you sent it from you at first,
and the resistance which the air would otherwise
make to its falling. It is the force required to

Attraction. 11

overcome this attraction, which causes a body to
be heavy (gravis) ; and hence comes the verbal
noun gravitation.

To illustrate these matters, drop a little water
or any other liquid on a table, and place upon
the liquid a piece of loaf sugar, the water or
fluid will ascend, or, in vulgar language, be
sucked up into the pores of the sugar ; that is,
the one is attracted by the other. Again, if you
take two leaden bullets, and pare a piece off the
side of each, and make the surface, where you
have taken off the piece, exceedingly smooth, and
then press the two balls together, you will find
them adhere strongly together ; that is, they are
mutually attracted by each other.

If you take a piece of sealing-wax or amber,
with a smooth surface, and rub it pretty quickly
upon your coat sleeve till it becomes warm, you
will find that if straws, feathers, hairs, or any
very light bodies, are brought within the distance
of from an inch to half an inch of it, these light
bodies will be drawn to the sealing-wax or amber,
and will adhere to it. Thus, in philosophical
language, they are attracted by it.

This last effect is very similar to what you
have heard of the magnet or loadstone, or what
many of you may have seen performed by the
little artificial magnets, which afford a very
rational and pretty amusement to young persons.
You have seen needles, steel filings, or even
knives or keys presented to the magnet, and at-

12 Experimental PhilosopJiy. [Lecture 2.

traded by it. On this circumstance an amusing
story in the Arabian Nights Entertainments is
founded. A' rock of loadstone (adamant it is
called by an error of the translator) is supposed
to exist in a certain part of the ocean ; and when
a vessel approaches it, all the iron bolts and nails
are attracted by it, and the vessel consequently
goes to pieces and is wrecked.

But I can show you a still more surprising
(and to most of you, I dare say, new) effect of
attraction. I take two phials, which I number
1 and 2, filled each of them with a fluid perfectly
colourless ; you see they appear like clear water :
on mixing them together the mixture becomes
perfectly black. I take another phial, No. 3,
which contains a colourless fluid also, and I pour
it into this black liquor, which again becomes
perfectly clear, except a little sediment which re-
mains at bottom. Lastly, I take the phial No.
4, containing also a liquid clear like water, and
by adding a little of it, the black colour is re-
stored.

All this may appear to you like magic, but it
is nothing more than an effect of attraction. Phi-
losophy keeps no secrets, and I will explain it to
you. The colourless liquor in the phial, No. 1,
is water in which bruised galls have been steeped
or infused ; that in No. 2, is a solution of sul-
phat of iron, the name now given to the copperas
or green vitriol of commerce. In plain terms,
it is water in which common copperas or green

Attraction. 13

vitriol is dissolved. The iron which this salt
(green vitriol) contains, has a strong attraction
for the gall water; and when they are mixed
together they unite, and the mixture becomes
black ; in fact, is made into ink. But when the
phial, No. 3, which contains aqua fortis (the
nitric acid, as it is called by chemists), is poured
in, the iron, which has a stronger attraction for it
than for the galls, unites with it, and having left
the galls, the liquid is again clear. Again, the phial
No. 4, contains potass, formerly called salt of tar-
tar, or of wormwood. It is the vegetable alkali
of chemists. The aqua fortis, or nitric acid, has
a stronger attraction for this alkaline matter than
it has for the iron ; it therefore drops the iron,
which again unites with the matter of the galls,
and the fluid resumes its black complexion.

You may amuse yourselves with the same ex-
periment in another way. If you write a few
words with common ink (which you now know
how to make) upon a thick paper, and let them
dry,' and then take some aqua fortis diluted or
weakened with water, and with a feather drop or
rub it upon the letters, the writing will totally
disappear. When this is dry, with another fea-
ther smear it over with some of the solution of
potass or salt of tartar, and the writing will be
restored.

These several kinds of attractions which I have
now mentioned, philosophers have ranged under
five distinct heads. The^r^, that, I mean, of

14 Experimental Philosophy. [Lecture 2.

the stone or arrow falling to the ground, they
have called the attraction of gravity, or gravity
tion. The second, that of the two leaden balls
adhering together, and of the water ascending
into the pores of the sugar, they call the attrac-
tion of cohesion, and also capillary attraction.
The third is electrical attraction, because the
sealing-wax, when chafed or warmed by rubbing,
is in an electrified or excited state, like the glass
cylinder of an electrical machine when rubbed
against the cushion, and therefore attracts the
hair, feathers, 8tc. The fourth is the magnetic
attraction ; and the fifth is called chemical attrac-
tion, or. the • attraction of combination, because
upon it many of the processes and experiments in
chemistry depend; and because by this means
most of the combinations which we observe in
salts, the ores of metals, and other mineral bodies,
are effected.

On the two first of these species of attraction
only I shall at present enlarge ; because it will be
necessary to treat of the others when we come to
investigate those branches of science to which
they properly belong.

First, therefore, of gravitation. It requires no
experiment to show the attraction of gravity;
for since the earth is in the form of a globe, it is
manifest that it must be endued with a power of
attraction to retain upon, its surface the various
bodies which exist there, without their being
hurjed away into the immensity of space in the

Attraction. 15

course of its rotatory diurnal motion. The earth
has therefore been compared to a large magnet,
which attracts all smaller bodies towards its cen-
tre. This is the true cause of weight or gravity
(which are correlatives). All bodies are drawn
towards the earth by the force of its attraction ;
and this attraction is exerted in proportion to the
quantity of solid matter which any body contains.
Thus, when two bodies are placed in opposite
scales, and we see one preponderate, we say it is
heavier than the other ; in truth, that it contains
a greater quantity of solid matter. For as every
particle of matter is attracted by the earth, the
greater number of such particles any body con-
tains the more forcibly it will be attracted.

The attraction of matter is universal : so that
not only does the earth attract all bodies upon it,
or near it ; but all such bodies reciprocally at-
tract the earth. Nay, farther, the earth attracts
all bodies in the universe, and they, again, all
attract the earth. Every particle of matter exerts
an attractive energy upon every other particle ;
and each of the bodies into which particles are
grouped attracts every other body. Thus, the
sun attracts all the bodies in the planetary system ;
and they, in their turn, attract the sun and each
other. The fixed stars, again, attract each other,
and our sun ; they also attract, and are attracted
by, the several bodies to which they probably
form distinct centres. The attractive forces of
bodies upon each other, are directly proportional

16 Experimental Philosophy. [Lecture 2.

to their quantities of matter, and inversely pro-
portional to the squares of their distances. This
is the first grand deduction of the Newtonian
philosophy, established upon indubitable prin-
ciples, and on which all the momentous facts of
physical astronomy depend. The tides, the pre-
cession of the equinoxes, the irregularities of the
moon's motion, the mutual perturbations of the
planets, and many other interesting phaenomena,
all receive a satisfactory explication upon the
principle of mutual and universal attraction.

But to proceed : we know by experience that
the weight or gravity of a body or thing is not
in proportion to its bulk. A bullet of lead, of
the same size as one of wood or of cork, will
weigh considerably heavier, and one of gold
would be heavier still. It is reasonable, there-
fore, to suppose that the ball of gold or of lead
contains a greater number of solid particles, which
are united or pressed closer together than those
of the wood or cork; the latter being more porous,
and its particles lying less closely compressed or
compacted together. One body containing more
solid particles within a certain compass, size,
bulk, or space, than another, gives origin to the
terms specific gravity and density, which are
greater or less in proportion as there are more or
fewer constituent particles comprised within a
given apparent bulk.

II. The attraction of cohesion is observable in
almost every natural object, since in reality it is

Attraction. 17

that which holds their parts together. It has
been already made evident in the experiment of
the two leaden balls, and the same effect will be
proved by pressing together the smooth surfaces
of two pieces of looking-glass, particularly if a
little moisture is dropped between them to ex-
clude the air more perfectly. The adhesion or
tenacity of all bodies is supposed to depend on
the degree of this attraction which exists between
their particles ; and the cohesive power of several
solid substances has been ascertained by different
courses of experiments, in which it was put to
the test what weight a piece of each body of a
certain diameter would sustain.

In the following table the numbers denote the
pounds avoirdupois, which, at a mean, are just
sufficient to tear asunder a rod of each of the
bodies, whose base is an inch square.

Metals.

Steel, bar 1 35,000 Ibs. Tin, cast 4,440 Ibs.

Iron, bar 74,500 Bismuth 2,900

Iron, cast 50,100 Zinc 2,600

Silver, cast 41,500 Antimony 1,000

Copper, cast 28,600 Lead, cast 860
Gold, cast 22,000

Woods.

Locusttree 20,100lbs. Teak, Orange 15,000 Ibs.

Box 20,000 Alder 13,900

Jujeb 18,500 Elm 13,200

Ash 17,000 Mulberry 12,500

12,000 Ibs.

Walnut

8,130 Ibs.

11,500

Mahogany

8,000

10,000

Poplar

5,500

9,800

Cedar

4,880

9,250

1

18 Experimental Philosophy . [Lecture 2.

Fir

Beech

Oak

Pear,

Lemon

The direct cohesive strength of a body is in
the joint ratio of its primitive elasticity, of its
toughness, and the magnitude of its section.

Cohesion is also visible even in fluid substances,
the particles of which adhere together, though
with a less degree of tenacity than solid bodies.
" The pearly dew*" is a well known phrase in
poetical language, and the drops of rain or of
dew upon the leaves of plants assume this round
or pearly appearance by the attraction which the
particles have for one another. In the same
manner quicksilver, if divided into the smallest
grains, will appear round, like small shot, because
the particles attract each other equally in every
direction, and thus each particle draws others to
it on every side as far as its power extends. For
the same reason two small drops of quicksilver,
when brought near to each other, will seem to
run together and unite.

The attraction of cohesion exists between fluid
and solid bodies. Thus a plate of glass or metal
(Plate I. fig. 1.) which has been immersed in
water or mercury, will retain some drops hanging
to it, even when turned upside down, or inverted.
Again, if two plates of glass, A. A. (fig. 2.), a
little wetted on the surface, and separated on one

Attraction. 19

side by any small interposing body B., about the
thickness of a shilling, are immersed in water,
the water will rise between them in the curve
C. D. E., that is, highest on that side where the
plates touch each other, and at a moderate height
near the surface of the fluid. The same effect
was instanced in the water or liquor rising in the
piece of lump sugar ; and it may be seen every
day, when a piece of blotting-paper is used to
suck up a drop of superfluous ink. Another easy
experiment will further illustrate die nature of
this attraction. Suppose A. B. C. (fig. 3.) two
glass plates a little moistened with oil of oranges,
and placed upon each other, so as to touch at
the end A. B. Let them be kept open at the
other end by a small body C. If then a drop of
the same oil is introduced at the end which is
open, while the plates are kept in a horizontal
position, the drop will proceed with an accelerated
motion towards the end A. B. If the end A. B.
is then a little raised, the drop will be suspended
in its course, and, if raised to a considerable
height, it will return, but slowly ; in which case
the attraction of the plates is, in some degree,
overpowered by the weight or gravity of the drop.
This peculiar kind of attraction has received the
name of capillary attraction, from the experiment
having been made with small tubes as fine as a
horse-hair (capillus^ Latin), in which the water
will rise to a considerable height ; and upon the
same principle, water or any other fluid will rise

20 Experimental Philosophy. [Lecture 2.

in the cavities of a sponge. These experiments
will succeed equally in a space which is void of
air (such as the vacuum made by an air-pump)
as in the open air ; so that the effect cannot pro-
ceed from any pressure of the atmosphere, but
must be caused by attraction alone.

Some bodies, however, in certain circumstances,
appear to possess a power the reverse of attrac-
tion; and this is called, in philosophical lan-
guage, repulsion. The repulsion of electricity
and of magnetism will be evinced when we come
to treat of those subjects ; and the -same feathers,
which were at first attracted by the excited or
electrified body, will be repelled or driven from
it; the magnet will repel at one end the same
bodies which it attracts at the other. Upon simi-
lar principles, if a small piece of iron is laid on a
bason of mercury, it will not sink, but will be
supported by it, while the mercury will be de-
pressed on each side ; and thus it is that a small
needle will swim upon the surface of water.

LECTURE III.

EXPERIMENTAL PHILOSOPHY.

MAGNETISM.

IN my last lecture I endeavoured to make you
acquainted with the nature of attraction in ge-
neral. There is, however, scarcely any instance
in which the principle of attraction is displayed
in a more striking manner than in that of the
MAGNET, or LOADSTONE; so called, as Mr.
Adams conjectures, from load, the Saxon word
for lead, that is, the leading-stone, from its
proving a guide to seamen by means of the com-
pass, or magnetic needle, which always points
towards the north.

The loadstone, or natural magnet, is an ore of
iron, found more or less in every iron mine.
Loadstones are of a dull brownish black colour,
and most of them are sufficiently hard to afford
sparks like a flint when struck with steel. They
differ very much both in form and in weight.
There was a very large one in the Leverian Mu-
seum, but it did not appear to be very powerful.
I observed in my second lecture, that the earth
itself has been compared to a large loadstone ;
and this opinion is countenanced by the immense
quantity of iron which is contained within its
bowels, or which indeed, more properly speaking,

22 Experimental Philosophy. [Lecture 3.

is diffused through all nature. In a part of Vir-
ginia there is a magnetic sand, the grains of
which exhibit all the properties of larger load-
stones, and indeed are loadstones in miniature.

The great and distinguishing property of the
magnet is its attraction for iron; and this at-
traction is mutual between them. Thus, if a
magnet and a piece of iron are placed each of
them on a small piece of wood, in a bason or
tub of water, so as to float on the surface, (see
Plate II. fig. 4.) the magnet will approach the
iron as well as the iron the magnet; and if either
of them is held steady, the other will move to-
wards it. Muschenbroek, by a series of experi-
ments, endeavoured to ascertain the degree of
force with which a magnet would attract at dif-
ferent distances. He suspended a magnet two
inches long, and sixteen drachms in weight, to
one of the scales of an accurate balance, and
under it he placed a bar of iron, while the
weights were put in the opposite scale.

At 6 inches it attracted 8 grains.

5 - - 31

4 4i

3 - - 6

2 - - 9

1 - 18

And in contact 87

From subsequent experiments, it has been
proved that the magnetic force diminishes as the

Magnetism. 23

square of the distance increases ; in this respect
being analogous to gravity.

Some natural magnets are much more power .
ful than others; and it is remarked, that the
smaller possess the power of attraction in a greater
degree, in proportion to their size, than the larger.
It indeed frequently happens, that a small load-
stone, cut off from a large one, will lift a greater
weight of iron than that from which it was cut
off. This can only result from the large stone
containing a considerable portion of matter not
magnetic, which rather impedes the action of the
magnetic part than otherwise. Loadstones have
been found of not more than twenty or thirty*
grains in weight, which would lift a piece of iron
forty or fifty times heavier than themselves ; and
we even read of one of only three grains, which
lifted a weight of iron of seven hundred and
forty-six grains, that is, two hundred and fifty
times its own weight.

This property, however, which is possessed
by the natural loadstone, it will communicate to
any piece of iron by only touching it ; and the
piece of iron thus converted into a magnet will
communicate it to others, and these again to other
iron, without losing any part of, their magnetic
virtue, which seems rather increased than dimi-
nished by action. Magnets made by being
touched by a loadstone, or by other iron which
has been touched by it, are called artificial mag-
nets, and are commonly sold in the shops of those

24 Experimental Philosophy. [Lecture 3.

who deal in mathematical and philosophical in-
struments. Soft iron acquires magnetism with
more ease than hard iron or steel, but the latter
will retain it much longer. A well tempered bar
of steel will retain the magnetic virtue for many
years without diminution.

The magnet which has the strongest power of
attraction does not always communicate it most
freely to iron or steel. This circumstance has
occasioned a distinction between the different
kinds of magnet. Those which communicate
most freely and in the greatest degree the mag-
netic virtue, are called generous; those which
raise the greatest weight in proportion to their
size, are called vigorous magnets. The magnetic
virtue is not diminished, but is rather increased,
by communication. Though however it may be
communicated by simply touching the bar of
iron or steel, yet it is augmented by repeatedly
touching or rubbing it with the magnet : but it
must be always rubbed one way only, that is,
either from left to right, or from right to left ;
for if the magnet is drawn backward and forward
on the iron the power will be destroyed, for rea-
sons that will be hereafter explained, treating of
the poles of the magnet.

The magnetic virtue is found to be the most
active at two opposite points of each magnet,
which have been termed its poles, from their
correspondence with the poles of the earth, as is
found by placing the magnet on a small piece of

Magnetism. 25

wood floating on water, or in any situation in
which it may turn freely, when the magnet will
arrange itself nearly in that direction, namely,
from north to south. To find the poles of a
magnet, place it under a smooth piece of glass, or
a piece of white paper, and sift or shake some
steel or iron filings on the paper or glass, and
you will find them arrange themselves in beauti-
ful curves, as represented in PL II. fig. 5. E E.
At each pole, however, the filings will take a
straight or rectilinear direction, as at A. B. and
those which happen to be situated at a small di-
stance from the poles will assume more or less of
the curve in proportion to their distance from
them. Some natural magnets are found to have
more than two poles ; in which case they may be
considered as two or more magnets united toge-
ther, and, in fact, have been sometimes separated
into so many distinct magnets.

In England we call that the south pole of the
magnet which points towards the north, and that
is termed the north pole which is directed to the
south. The foreign philosophers, on the con-
trary, naii^e them according to the pole to which
they point. That is, the north pole of the mag-
net is that which is directed to the north or arctic
region, and the contrary.

The principle of repulsion is also very strik-
ingly exemplified by the magnet; for if the same
pole of two magnets is presented one to the other,
that is, the north pole of one magnet to the north

26 Experimental Philosophy. [Lecture 3.

pole of the other, they will mutually repel or
drive away each other: if, on the contrary, the
south pole of the one is presented to the north
pole of the other, they will be mutually attracted.
It is on this account that it is necessary, in mak-
ing artificial magnets, to draw the magnet, with
which they are rubbed or touched, always one
way. It is most effectually done also by applying
one of the poles of the magnet to the bar or piece
of iron which is to be rendered magnetic, and
drawing it slowly along several times. It is ex-
traordinary that the end of the bar which is first
touched with the magnet will have the contrary
property to the end of the magnet with which it
is touched or rubbed. If, for instance, the end
with which the bar is touched is the north pole
of the magnet, the end of the bar to which it is
first applied will be a south pole, and the con-
trary.

It is obvious that the directive power of the
magnet, or that which causes it, when placed so
as that it can freely turn of itself, to take always
a position nearly north and south, is the most
useful property of the magnet. This is practi-
cally applied by means of the mariner's compass,
in which a fine needle, index, or piece of steel-
wire, formed like the index or hand of a clock or
watch, is so balanced as to turn horizontally with
great ease on the prop which supports it. The
needle or index is fixed in a box ; and under-
neath it the points of the compass, or the different

Magnetism. 27

quarters of the horizon, that is, east, west, north,
and south, with their intermediate points, are
marked on a card. As the magnetic needle al-
ways points nearly towards the north, by observ-
ing the course or direction of the ship, that is,
which way her head is turned, it is easy to know
to what point she steers ; and by keeping a regular
account of the distance she traverses, the sea-
man can go with considerable exactness from one
place to another. Before this great and import-
ant invention, seamen usually steered by ob-
serving the fixed stars, and particularly the polar
or north star. But as this could only be done in
fine weather, and when the stars were visible,
they frequently lost their way and suffered ship-
wreck. Indeed few of them dared to sail out of
sight of land. But when they had a tolerably cerj
tain means of knowing one point of the heavens,
it was easy to know the others ; and it became,
after this invention, neither necessary to observe
the stars, nor to be afraid of the open sea, out of
sight of the shore. It was by means of the mariner's
compass that Columbus was enabled to make the
great discovery of the American continent, and
by means of it subsequent voyagers have sailed
quite round the globe.

Though the position of the magnetic needle,
when it comes to rest on a vertical pivot, is, as
we have remarked, nearly north and south, or
coincident with the meridian, yet it is not exactly
so, nor is it the same at different places, or in the

28 Experimental Philosophy. [Lecture 3.

same place at different times. In some parts of
the North American continent, the needle now
points north and south ; at others, it deviates or
varies from this position, the variation or de-
clination, as it is technically called, being in some
places westerly, in others easterly. At London,
the declination of the needle in the year 1580,
was 11° 15' towards the east. From that time
the declination, easterly, gradually diminished
until the year 1658, when the position of the
horizontal needle at London was precisely north
and south. From that period to the present
the north end of the needle has deviated more
and more from the true north towards the west,
until now (in the autumn of 1819), the declination
at London is 24° 19' W. In like manner at
Dublin, Edinburgh, Paris, Copenhagen, and
other places, where the declination has been long
observed, it is found to increase westerly: though
in none of those places is the declination the same
at it is at London. In all of them, however, it
has increased but ittle during the last ten or
fifteen years. In 1800, the declination at London
was 24° 3' ; hence, during the last nineteen years,
the declination has not, on the average, varied a
minute in a year : and, it is exceedingly probable,
that it has nearly, if not quite, attained its greatest
western limit in England.

Besides this constant variation in the decima-
tion, as referred from year to year, there are
minor variations in different parts of the year,

Magnetism. 29

and, indeed, in different parts of the day. Mr.
Gilpin found by a mean of twelve years, from
1793 to 1805, that the declination" appeared to
increase, or go westward, from the winter solstice
to the vernal equinox O'.SO ; to diminish, or go
eastward, from the vernal equinox to the summer
solstice 1'.43; to increase again, from the summer
solstice to the autumnal equinox, 2'. 43 ; and to
decrease only OM4 from thence to the winter
solstice. These minute changes were observed to
take place at London : corresponding mutations
have been noticed in different parts of the conti-
nent of Europe.

With regard to the diurnal variation, Colonel
Beaufoy, whose observations have been carried on
for some years, at Bushey-heath, near Stanmore,
finds the maximum variation to occur at about
half an hour past one o'clock in the afternoon.
The mean of his observations for May, 1819,
give, at 8h. 37m. A. M. 24° 32' 42" W.
at 1 h. 24 m. P. M. 24° 41' 22",
at 7h. 26m. P.M. 24° 34' 10''.

The mean for June, 1819,

give, at 8h. 40 m. A. M. 24° 31' 28" W.
at Ih. 29m. P.M. 24° 41' 41".
at 7h. 47 ra. P. M. 24° 35' 09".
No satisfactory theory of these variations has
yet been adduced.

Magnets, while they attract other bodies, appear
to be themselves subject to the attraction of the

30 Experimental Philosophy. [Lecture 3.

earth ; for the magnetic needle, when it is so sus-
pended as to move freely in a vertical plane, ge-
nerally assumes a position with one of its poles
elevated and the other depressed. This, how-
ever, varies in different latitudes: near the equator
it is in a position almost horizontal ; as it ap-
proaches the northern regions, the south pole is
depressed, or drawn towards the earth ; and on
the other side of the equator, in the southern la-
titudes, the. north pole is depressed. This is
called the dip of the needle, and is subject to
periodical variations. In 1720, the dip at Lon-
don was 75° 10'; in 1775, it was 72° 30'; in
1805, 70° 20' ; now, in 1819, it is 70" 32'.

Iron may acquire the magnetic virtue by other
means than communication with a magnet. 1st.
If a bar is kept for a long time in a vertical po-
sition, or, still better, in the direction of the
dipping needle. Thus old iron bars in windows
are often found strongly magnetic. 2d. If iron
is heated and suffered to cool quenched in water,
holding it in the position of the dipping needle,
the same effect is produced. 3d. If it is rubbed
hard in the same position by any steel instru-
ment. 4th, A few strokes of a hammer, first at
one end of a bar, and then at the other, while
held in the position of the dipping needle, will pro-
duce the effect. 5th. A shock of electricity passed
through the bar will gf ten render it magnetic.

Many entertaining experiments are performed
by means of magnetism. In the shops, little

Magnetism. 31

swans made of tin, or more properly of iron tinned
over, are sold, which, when put to swim in a basin
of water, will, when one end or pole of an arti-
ficial magnet is presented to them swim after
it ; and when the other end or pole is turned
towards them, they may be chased round the
bason. If a small piece of bread is stuck on the
end of the magnet which attracts them, an igno-
rant person will suppose that they are following
the bread as if to eat it.

A small fish may also be made in the same
manner to swim in a basin of water, and will
follow a magnetic hook, or be lifted out of the
water by it.

Sometimes an artificial pond is made, about an
inch in depth, and seven or eight in diameter,
with the hours of the day marked about its edge.
One of the magnetic swans is then put to swim in
the pond ; and if a watch is placed underneath,
with a small magnet fixed to the end or point of
its hour hand, the swan, guided by the magnet
beneath, will then swim to the hour, and show
the company the time of day.

But there are not any of the magnetic experi-
ments more interesting or entertaining than that
of the divining circles. They are drawn on paper,
pasted on the top of a thin box, fig. 6. Pi. II.
The index a, is fixed on 'the axle of the toothed
wheel c, which works into the pinion d. On the
axle of d is another pinion of the same numberof
teeth, that puts in motion the wheel g, of the

32 Experimental Philosophy. [Lecture 3.

same size and number of teeth as the wheel c.
On the axle of g is fixed the bar magnet qq, and
they turn together. Over this axle (but inde-
pendent of it) is fixed a point in the top of the
box for th^ loose needle xx to turn upon, and
which is the centre of the pasted circle F. In
the compartments of this circle are written an-
swers to the questions asked in the compartments
of the circle G. A circle of strong paper, of the
size of F, should cover the pasted circle, and
turn easily on the centre ; it should have one of
the triangular pieces cut out, in order to see the
answers. If then the needle xx is taken off its
point, and a person wishes to ask some of the
questions on the carton G, the person must turn
the index to the question, and then place the
needle on its point, giving it a whirl round, when
it will stop over the answer. The open part of
the loose circle being turned to that place, will
exhibit the answer.

Itinerant jugglers often attract considerable
notice by exhibiting a number of these experi-
ments ; and there are several very amusing toys
constructed upon magnetic principles, and sold
in the shops of the makers of mathematical in-
struments.

After all, however, the theory of magnetism
is but imperfectly developed ; nor, indeed, have
its leading phenomena been very cautiously traced.
Very imposing formulae have been published,
especially by continental mathematicians, includ-

Magnetism. 33

ing, as is pretended, all the phenomena of terres
trial magnetism in different latitudes ; but when
applied to recently ascertained facts, their in-
accuracy is at once detected. There is reason
to hope that the cloud which has long hung over
this department of science will speedily be dis-
pelled.

Hitherto the effect of magnetic attraction has
only been stated in very general terms, and no
attempt has been made to estimate the quantity
of 'that effect under different circumstances.

Mr. Barlow, of the Royal Military Academy,
was the first who undertook a regular series of
experiments with a view to this determination,
and he soon found that there were three distinct
conditions to be attended to, viz. the position of
the needle and compass, with respect to the
attracting body, the mass, or rather the surface
of that body, and the distance at which the ac-
tion took place. With respect to position, he
discovered that a plane may be conceived to be
drawn through the centre of attraction of any
mass of iron, inclining from north to south at an
angle equal to the complement of the dip, in
which plane the iron has no effect on the needle ;
that is, while the pivot of the compass is found in
this plane, the needle will have its true magnetic
bearing the same as if no iron were in its vicinity.
He also discovered the law of deviation out of
that circle, showing it to depend upon the angle
which the compass formed with the above plane,

34 Experimental Philosophy. [Lecture 3.

and another passing vertically through the north
and south points: helikewise found that atdifferent
distances, the position being the same, the tangents
of the angles of deviation were inversely propor-
tional to the cubes of the distances, and directly
proportional to the cubes of the diameter of the
attracting ball.

But the most remarkable result obtained in
the course of these experiments (with the excep-
tion of the discovery of the plane of no attraction
above referred to) was, that the poicer of an at-
tracting body is independent of the mass of that
body ; a simple tin spherical shell of any given
dimension, acting equally as powerful as a solid
iron ball of the same diameter ; which is another
striking instance, in addition to many others,
of the analogy that subsists between the mag-
netic and electric attractions. Mr. Barlows ex-
periments, we understand , are not yet completed :
but it is hoped he will soon lay his most interest-
ing results before the woVld; as they will, doubt-
less, admit of an important practical application, to
the magnetism of iron in ships, and its effect upon
the direction of the needle in the ship's compass.

LECTURE IV.

EXPERIMENTAL PHILOSOPHY.

HYDROSTATl CS.

THE word which stands as the title of this lec-
ture, implies simply the science which relates to
the weight of water compared with that of other
bodies ; but the science, as now taught and cul-
tivated, treats not only of the weight and pres-
sure, but of every thing relative to the action
and mechanical properties of the dense or in-
compressible fluids, such as water, &c.

Though water is generally regarded as in-
compressible, yet it is not entirely so, since it is
capable of transmitting sound, which proves that
it is elastic, and every elastic body must be com-
pressible. To prove the fact, however, the Floren-
tine academicians filled a globe of gold perfectly
full with water, and afterwards closed the orifice
by a tight screw. The globe was then put into
a press of considerable force ; it was a little flat-
tened at the sides by the force of the press, but
was proportionably extended in other parts of its
surface, so that it was concluded that the water
did not occupy less space than before. On press-
ing it still harder, the water was made to exude
through the 'pores of the gold, and adhered to

36 Experimental Philosophy. [Lecture 4.

its surface like drops of dew. From this expe-
riment it may be inferred, that if water is indeed
capable of compression, it is so only in a very
slight degree, since, instead of yielding to the
force of pressure, it found its way out through
the pores of the metal. The same has been
proved more scientifically by subsequent philo-
sophers.

The first principle that may be laid down with
respect to the pressure of fluids is, that the sur-
face of all waters which have a communication
whilst they are at rest will be perfectly level.
To explain this more fully, observe the three
united tubes (Plate III. fig. 7). It will be seen
that if water is poured into the perpendicular
tube A, it will run through the horizontal tube
C, and rise in the opposite perpendicular tube B
to the same height at which it stands in A.

Hence appears the reason why water, con-
veyed under the earth through conduit-pipes,
will always rise to the level of the reservoir
whence it is drawn. It is in this manner that
the cities of London and Westminster are sup-
plied with water, either from London Bridge
water-works or the New River. In the former
case, water is raised from the Thames by immense
pumps worked by wheels, which are turned by
the tide, to the highest part of the town whither
water is to be conveyed by pipes ; and, in the
latter, it is well known that the reservoir of the
New River stands on a rising ground near Isling-

Hydrostatics. 37

ton, which is higher than any of the places where
the pipes terminate.

It is surprising that the antients should have
been totally ignorant of so simple a principle as
that of water rising to its level ; yet it is to this
ignorance that we owe those stupendous works
of art, the antient aqueducts, the ruins of which
we still behold with admiration. Thus, for in-
stance, in Plate V. fig. 195 an arch or arches
would have been built to carry the water from
the spring head at the side «, across the valley,
to supply the house on the other side; whereas a
simple pipe of lead, iron, or wood, carried under
ground across the valley, will answer every pur-
pose, and supply the house and ponds about it
as amply as if an aqueduct had been constructed
on the antient plan.

The reason why water thus rises to its level,
is because fluids press equally on all sides : thus
(in fig. 7.) if the tube B were taken away, the
water would still press at b with equal force as
before; and if the tube C were taken away, the
water would press against the part a as forcibly
as it would if it had remained. Thus, if with
the thumb we stop the end of the crooked tube
b (fig. 8.) at a, when full of water, the water
will press against the thumb with a force pro-
portioned to the height of the water in the tube
above a; and, if we remove the thumb, it will
run over at a, and fall in b to the level of a.

To explain this in a popular way, without the

38 Experimental PhilosopJiy. [Lecture 4.

aid of mathematical theory, fluids have been sup-
posed to be constituted of small globules, as re-
presented in fig. 10. If therefore any one of the
columns, 1, 2, 3, 4, or 5, be removed, its place
will be immediately supplied by a number of
small globules, which will roll from, the other
columns and fill up the vacancy, and consequently
the superficies of the whole will presently sink to
the same level; as will be found to be the case in
a vessel filled with shot, with bullets, or any
small round and smooth bodies. On the other
hand, supposing these particles to have a very
smooth and slippery surface, so as to move with
very great ease upon one another, if the vessel
which contained them were not full, and any ad-
dition were made to the quantity, this addition
would displace a number of other particles, which
would roll round, and restore the level at the
surface. Thus, in fig. 9, we will suppose a per-
pendicular pressure to be made by the column
ik, opposite to the point d; but as it, can proceed
no further than that point, because of the bottom
of the vessel, the pressure will be directed late-
rally towards the sides efof the vessel, in such
a manner that, if there were any aperture then
in the vessel, the fluid would flow out : as that
however is not the case, the particles g and h
being restrained by the side of the vessel, those
which compose the lateral column force them-
selves between these particles g and h, and h
will be raised towards the surface of the fluid,

Hydrostatics. 39

unless a column equal to i k press against it, and
keep it in its place. Since therefore the particle
h would be raised towards the top of the vessel,
unless restrained by a pressure quite equal to the
column i A;, it follows, that two columns of water,
to be in equilibrium, must be perfectly on a level
at their surface.

On this principle we are enabled to account
for springs, which are sometimes found on the
tops of mountains. They, in fact, come from
some waters which are situated upon mountains
higher still, and flow through canals or natural
pipes, which proceed under ground, perhaps for
the distance of miles.

It is upon these facts the maxim is founded,
which has led to the hydrostatic paradox, and
that is, that the pressure of fluids is not in pro-
portion to their quantity, but in proportion to
their perpendicular height; and from this the
supposed paradox follows, that a given quantity
of water may exert a force two or three hundred
times greater or less, according to the manner in
which it is employed.

To make this plain, we will take three vessels
of the same height, and the same base, though
differing materially with respect to their forms,
and the quantities they contain, viz. A, B, C, D,
%. 13. E, F, G, H, fig. 11. L, M, N, O, P, Q,
fig. 12. Now it may very easily be understood,
that the vessel fig. 15, is pressed at the bottom
B, C, by the whole mass of water it contains, and

40 Experimental Philosophy. [Lecture 4.

that the pressure there must be equal at every
part. The vessel fig. 1 1, however, is of a differ-
ent shape, and will hold more than three times
the quantity of water ; yet the pressure at the
base is still the same as in the former instance,
because the bottom F, G, supports only the
column of water I, F, G, K, which is the same as
that contained in the vessel fig. 6. All this may
be easily comprehended ; but the great difficulty
lies in understanding how the very small tube in
fig. 12. can exert a pressure at the bottom or
base of the vessel equal to that in the preceding.
Here it will be necessary to remember the maxim
that was laid down, That the pressure of fluids is
in proportion to their height, and not to their
quantity. Thus we may observe the column of
water in fig. 12. is equal in height to the columns
in fig. 11. and 13; and if we advert to what was
said, when speaking of fig. 9, we shall perceive
that the small column L, M, P, Q. displaces a
quantity of water contained in the lower part of
the vessel M, P, N, O, and forces it to rise to the
top of the vessel at s, for instance, which, if
strong enough, will cause a re-action equal to the
pressure of a column of water M, P, r, s. The
same will take place at the other side, and at
every part of the vessel which is covered, so that
in effect the pressure at the bottom N, O, will be
the same as if the column of water were equal in
size from the bottom N, O, to the top of the
tube, as shown by the dotted lines. All this may

Hydrostatics. 41

be proved by experiment, having a false bottom
to each of the vessels supported by an iron rod
fixed to a balance, as in fig. 13 ; in which case it
will be found that the same weight, at the oppo-
site end of the balance, is necessary to support
the bottom in each.

The hydrostatic bellows is a very pleasing
machine, constructed upon this principle. It
consists of two strong boards, united by leather,
almost in the manner of a common bellows, only
that for convenience its form is round (see
Plate IV. fig. 14.) In this figure a is a pipe,
which goes into the inside of the bellows, and
u is a weight laid upon the upper board. If
then water is poured into the pipe «, the weight
will be lifted up ; and if the pipe was still taller,
a greater weight would be raised. By a very
small force exerted in this manner, that is, by
water conveyed through a very small perpendi-
cular tube, Dr. Goldsmith relates that he has
seen a very strong hogshead burst in pieces, and
the water scattered about with incredible force.

To show that this principle in hydrostatics is
not without practical utility, it is only necessary
to mention, that upon the plan of the hydro-
static bellows a press has been constructed of
immense power, see fig. 15, in which a is a
strong cast iron cylinder, ground smooth on the
inner side, and e is a piston or moveable plug,
fitting very tight within it. c is a common forcing
pump, in which the water ascends through a

42 Experimental Philosophy. [Lecture 4.

valve at its lower end, and is forced through at o
into the cylinder. This forms a pressure at m, by
the action of one man working at s, which
squeezes cotton bags, hay, or other packages,
into twenty times less compass than they would
otherwise occupy. The effect would be the same
if c\ instead of a pump, were a slender tube, pro-
vided it was long in proportion to the pressure
which was required.

From all these experiments it is easy to con-
ceive why the banks of ponds, rivers, and canals
blow up, as it is called. If water can insinuate
itself under a bank or dam, even to the thick-
ness of a shilling, the pressure of the water in
the canal will force it up. In fig. 1 8, a is the sec-
tion of a river or canal, and c is a drain running
under one of its banks. Now it is evident
that if the bank g is not heavier than the co-
lumn of water de, that part of the bank must
infallibly give way. This eifect is prevented
in artificial canals, by making the sides very
tight with clay heavily rammed down, or by cut-
ting a trench, n, from two feet to eighteen
inches wide along the bank of the river or canal,
and a little deeper, which being filled up with
earth or clay well moistened with water, forms a
kind of wall when dry, through which the water
cannot penetrate.

Another maxim in hydrostatics, of equal im-
portance with the former, is, that every body
lighter than water, or, in other words, which

Hydrostatics. 43

swims in it, displaces exactly as much of the
water as is equal to its own weight.

This fact is proved by a very easy experiment.
Put a small boat, #, (fig. 17.) in one scale, and
balance it with water in the opposite scale, b.
If then the boat is put into the basin, fig. 16,
exactly filled with water, it will be found that a
certain quantity of the water will run over the
brim of the basin, which water, on taking out the
boat, you will find will be exactly replaced by the
water which before balanced the boat in the op-
posite scale, b, fig. 17.

Hence it is plain, that a boat or other vessel
sailing upon the water, displaces exactly as much
of the fluid as is equal to the vessel and its lad-
ing, and, if more weight is added, it will sink
deeper in the same proportion, or, in other words,
a weight of water equal to the added lading will
be displaced ; whence a laden ship is said to draw
more water •, that is to sink deeper, than when it
is light or unloaded.

Every body, on the other hand, which is hea-
vier than water, or which sinks in it, displaces
so much of the water as is equal to the bulk of
the body sunk or immersed in the water. Thus
it is plain, that if a leaden bullet is dropped into
a vessel of water, it will take up just as much
room as a small globe of water of equal dimen-
sions. On this principle are computed the tables
of specific gravities, by means of what is called
the hydrostatic balance ; for since every body

44 Experimental Philosophy. [Lecture 4.

that sinks displaces a quantity of water exactly
equal to its own bulk, it follows, that every body
when immersed in water loses so much of its
weight as is equal to the weight of an equal bulk
of water. Thus, if the body, when weighed in
air, is two ounces in weight, and an equal bulk of
water is one ounce, it will of course lose, when
weighed in water, one ounce of its apparent
weight. It is by this means .that adulterated
metals or coins are distinguished from the true
ones : thus copper is bulk for bulk heavier than
tin, and gold is heavier than copper or brass,
which last is a mixture of copper and zinc. If
therefore a brass counter is offered for a guinea, if
of the same weight, though it may not to the eye
appear much larger than a real guinea, yet you
may depend upon it that it is so in fact. We will
then take a guinea, which we are sure is real
gold, and weighing it first in air, and then in
water, we shall find it loses about one-nineteenth
of its weight in the latter. We then weigh the
brass counter in the same way, and find it loses
about one-eighth, which we find is much more,
and therefore we cannot doubt but the coin is
made of base metal. When we look at tables of
specific gravities, we see the specific gravity of
gold put down at about nineteen one-half, of
mercury at about thirteen one-half, lead eleven
one-quarter, silver ten one-quarter, copper eight
one-half, iron seven one-half, tin seven one-
quarter, &c. ; that is, gold is nineteen times one-

Hydrostatics. 45

half heavier than its bulk of water, and conse-
quently loses more than one-nineteenth of its
weight in that fluid.

This mode of ascertaining the standard value
of metals was invented by the famous philosopher
Archimedes, who made use of it to detect a fraud
in the golden crown of Hiero, king of Syracuse.
This king had given a certain weight of gold to
be made, by a goldsmith of that place, into a
crown ; the weight of the crown was exactly the
same as the weight of the gold he had received ;
but Hiero still suspecting an imposition, Archi-
medes was requested to detect the fraud ; and he
was led to make the trial in this way, without
melting the crown, or destroying the workman-
ship, from the resistance which he found was
made by the water to his own body upon his
going into the bath. A quantity of fine gold was
therefore brought, and equally balanced in a
scale against the crown ; but when both came to
be weighed in water, it was found that the crown
was much lighter ; whence not a doubt could re-
main but that it was made of adulterated metal.

It is upon the same principles that the density
of different fluids is put to the test. It might,
it is true, be ascertained by weighing them
against each other in different scales ; but it may
be done in a more easy and expeditious manner
upon the hydrostatic plan, since the same body
that will sink in one fluid will swim in another,
and the same body will sink to different depths in

46 Experimental Philosophy. [Lecture 4.

different fluids. Thus I have known good house-
wives in the country try the body of their mead
and other liquors, by observing whether an egg
will swim in them, which, we know, will sink in
common water. The exact relative weight of
fluids may be ascertained by suspending from
one end of an accurate balance (such as that fig.
17.) either a small globe, or a conical piece of
glass. Its weight in water being previously
ascertained, which suppose to be two hundred
and twelve grains ; if it is immersed in a fluid
heavier than water, some weights must be added
in the opposite scale ; as for instance, if it is sea
water, then ten grains must be added, which
will make the relative weight of sea-water to
common water as four hundred and twenty-two
to four hundred and twelve. If, on the con-
trary, it is immersed in brandy, which is less
dense, and consequently lighter than water, you
will find it necessary to take out of the opposite
scale about forty grains, and then the relative
weight of brandy to water will be as three hun-
dred and seventy-two to four hundred and twelve,
or about one-tenth lighter.

A very convenient instrument is made use of
by excisemen, officers of the customs, and all
whose business it is to ascertain the density* or
strength of liquors. It is called an hydrometer,
and is nothing more than a small hollow globe
of glass or metal with a stem to it, like the han-
dle of a teetotum, but longer, which stem is

Hydrostatics. 47

marked or graduated. The instrument is made
so that the ball sinks in water, but not entirely,
and therefore a part of the stem is always above
the surface. If it is immersed in a fluid lighter
than water it will sink, and less of the stem will
be above the surface ; if in a heavier fluid, it will
rise higher, and more of the stem will be visible.
This instrument is fully described, and its theory
explained more at large in the first vol. of Gre«
gorv's Mechanics.

LECTURE V.

EXPERIMENTAL PHILOSOPHY.

HYDRAULICS.

HYDROSTATICS, we have seen, is that science
which relates to the weight and pressure of fluids ;
the science of hydraulics teaches us what respects
the motion of fluids, and the means of raising
them by pumps, and conducting them by pipes
or aqueducts from one station to another. This
branch of science is, also, called Hydrodynamics.

It was laid down as a principle, in the preced-
ing lecture, that of all waters which communicate
with each other, the surface will be level, or, in
common language, that water will rise to its level,
or to the same height as its source. The reason
of this was not fully assigned then, because it was
not necessary ; it was observed, that fluids press
equally on all sides; but another reason which
partly operates to produce the level surface of
water is the pressure of another fluid, that is, the
air or atmosphere, which, as it bears equally on all
points of the earth's surface, must equally press
the source from which water is derived and the
orifice of the tube or pipe in which it rises, as
was evidenced in the three united tubes, which
were exhibited as explanatory of this fact.

That a reservoir of water, less than S3 feet in

Hydraulics. 49

height, will not flow unless exposed to the pres-
sure of the atmosphere, will be plain from filling
a cask or other vessel full of this fluid. If the
bung is perfectly tight, and there is no aperture
above for the air to press upon it and force it out,
it is in vain that we shall attempt to draw it off
by opening a passage for it below. Hence the
use of vent-holes, and vent-pegs in casks: by
raising the vent-peg air is admitted, which forces
the liquor to flow out at the cock or faucet, where-
as if the vent-peg were kept tight no liquor
whatever could be obtained. The Valencia is a
common instrument made of tin, the lower part
of which is in the figure of an inverted cone, (see
PI. V. fig. 22.) with an orifice at the bottom «,
and one at the top b. It is used for taking sam-
ples of liquors out of the bung-holes of casks. In
order to use it, the operator puts it into the
bung-hole with both orifices open, and the liquor
rises through the orifice at bottom to the top of
the instrument ; he then puts his thumb on the
hole or aperture at top, so as to exclude the air'
completely, and the liquor will not run out at the
bottom till the air is admitted by the thumb be-
ing removed, which is done in order to let it flow
into the cup or vessel which is to receive it.

Thus it is plain that fluids, circumstanced as
above, are put in motion, or caused to flow, by
the pressure of the atmosphere ; and it will be
shown, that whenever that pressure is removed,
they will rise above their natural level, and flow

VOL. i. D

50 Experimental Philosophy. [Lecture 5,

where they otherwise would not. The syphon or '
crane, is a bent tube, of which one leg is longer
than the other (fig. £1). With this instrument
we want to draw off the fluid contained in the
vessel D, which we will suppose immoveable, as
a well or a heavy cistern. We know that if the
instrument is put into the vessel, without some
particular management the fluid can never be made
to flow over the bent part B ; for the air which
presses on the surface of the fluid will also press
through the bore of the tube, and prevent its
pursuing that course. In order to use it, there-
fore, we fill the syphon with water or some other
fluid, and stopping both ends, immerse the
shorter leg in the vessel D. The stoppage be-
ing removed, the water will flow out at the leg C
by its own gravity, and, by the pressure of the
atmosphere on the surface, will continue to flow
while there remains any fluid in the vessel. If a
vacuum is made in the syphon, by drawing out
the air with one's mouth, or in any other way, the
same effect will take place.

The syphon fountain is a beautiful example of
the effect from the pressure of the atmosphere.
In fig. 20, a is the long or outer leg of the
syphon, which is inserted by a brass or wooden
cap in the glass vessel c; the inner leg b also
passes through the cap,»and terminates in a spout-
ing pipe of an extremely small bore. To make
it act, we must first put it in a position the reverse
of what it stands in at present, and through the

Hydraulics. 51

leg a pour in at d a quantity of water, which will
force the air out of the vessel through the leg b.
We then stop both orifices with the finger, as in
the common syphon, and immerse the leg b in
the vessel e filled with water. The water in the
glass will then flow out through the leg a ; and
the glass being vacant of air, the water from the
vessel e will ascend through the leg 5, and form a
most beautiful jet or fountain within the glass
vessel.

The syphon may be disguised in such a man-
ner as to produce many entertaining effects. The
cup fig. 23, is called Tantalus's cup, from the
celebrated fable of Tantalus, who is represented
by the ancients as suffering continual thirst, and
though he is in the midst of water, is unable to
assuage it —

" E'en in the circling Hoods refreshment craves,
And pines with thirst amidst a sea of waves;
And when the water to his lips applies,
Back from his lips the treach'rous water flies.''

In the cup there is a figure of Tantalus, and if
we pour water into it, so that it shall nearly reach
to the lips of the image, the water immediately
sinks, and is drawn off again. The truth is, there
is a syphon concealed within the image; and
when the water is poured into the cup, so as
nearly to reach the lips, the fluid is then raised
above the bend of the syphon, which of course
then begins to act, and the water is drawn off by
the longer leg in the manner already described.

52 Experimental Philosophy. [Lecture 5.

Sometimes the syphon is concealed in the handle
of the cup (see fig. 23.) in such a manner, that
when a person raises it to his lips to drink out of
it, the fluid which it contains shall be carried
over the bend of the syphon, and it will then be
drawn off by the longer leg, so that the person shall
not only be disappointed of his draught, but will
have his clothes well splashed, to the great
amusement of the by-standers.

In some parts of the world there are what are
called intermittent springs, or wells which seem
to ebb and flow like the tides. This we shall
perceive is usually caused by a natural syphon.
In fig. 24, A is a well of this nature, B is a ca-
vity or reservoir of water under ground, with
which it communicates, by means of the pipe or
syphon C. It is obvious, that unless the water
in the reservoir rises above the height of the
bend of the syphon C, the well cannot be filled ;
but if by considerable rains, or any other cause,
the reservoir should become full, then the syphon
will begin to act, and the water will run into the
well as long as there remains any in the reservoir.
It will then cease to receive any more, and the
drain from the well will empty it in its turn. At
Gravesend there is a pond of this kind, which
ebbs while the tide is coming into the adjacent
river, fills after the tide has risen to its height,
and all the time that it is ebbing in the river. At
Larntown, in Worcestershire, there is also a brook
which, in summer, has a stream sufficient to turn

Hydraulics. 53

a mill, and the greater part of the winter is desti-
tute of water. This probably communicates by
a syphon with some cavity in the earth, which is
filled by the melting of the snow to a certain
height, and after that it will continue to be drawn
off by the brook, so as to furnish a stream till the
reservoir is entirely emptied.

It is by the pressure of the atmosphere that
the common or sucking pump is enabled to act.
It is said to have been invented by a mathemati-
cian of the name of Ctesebes, about one hundred
and twenty years before Christ ; but the principle
on which it acted was unknown till the 17th cen-
tury. Mankind, perfectly ignorant that the air
had weight, attempted to account for these effects
by a maxim not only unfounded, but even desti-
tute of meaning. This was, " that Nature ab-
horred a vacuum." What they meant by Nature
is as little to be understood as when the same
word is used by those ignorant sciolists who affect
to deny the existence of a God. Absurd, how-
ever, as this maxim was, it remained uncontra-
dicted till within one hundred and sixty years,
when it met with a practical refutation. About
that time some workmen were employed by the
duke of Florence, to raise water by a common
sucking pump to the height of fifty or sixty feet.
A pump was accordingly constructed for that
purpose; but, after all their efforts, they were
unable to raise it above the height of thirty-two
feet. It was then found either that Nature had

54 Experimental Philosophy. [Lecture 5.

not this horror of a vacuum, or at least, that it ,
was a very limited kind of a horror; for why
should Nature have a horror of a vacuum at one
height and not at another ? The matter was re-
ferred to the famous astronomer and philosopher
Galileo ; but in his time philosophical knowledge
was not sufficiently advanced to solve the diffi-
culty.

The difficulty is, however, now explained,
through principles furnished by Galileo's pupil
Torricelli. We knoAv that a pump is a hollow
piece of timber or metal, to the bore of which a
piston, bucket, or sucker, is exactly fitted. That
the piston has a valve in it made with leather,
like the clapper of a bellows. When the piston
is pushed down, therefore, the air, or any fluid
contained in the pump, will force it open ; and
when the piston is drawn up, the pressure of the
air or water, which has been admitted in that
way, will keep the valve down. But to make the
matter perfectly clear, let us represent the opera-
tion in a glass model. In PI. VI. fig. 25, is a
pump constructed on the plan of a common, or
as it is usually called sucking pump. Let this
pump then, D, C, B, L. be immersed in water at
K ; in which case you will see the water rise as
high as L in the pipe or body of the pump. G is
the piston, sucker, or bucket, as it is sometimes
called, in which a is the valve ; and at H is a box
made similar to the bucket G with a valve in it
£, with this difference, that the box H is immov-

Hydraulics. 55

able, and fills the bore of the pump. D is tli£
rod (which is generally of iron) by which the
piston is raised. When, therefore, by drawing up
the rod B the piston or bucket is raised from B to
C, the valve and pisjton being perfectly or nearly
air-tight, it is obvious that a vacuum is created,
that is, there is a space from B to C, from which
the air is drawn out. This, however, is in some
measure supplied by the air from below, which
enters through the valve b, which it opens by its
force. It is evident, however, that this air must
be exceedingly dilated, by its now occupying so
much more space than it did before. The force or
spring of the air, within the pump, is so much
weakened, that it is not able to resist the pres-
sure of the external air upon the water. The ex-
ternal air, therefore, pressing upon tjbe surface of
the water, forces it to ascend through the notched
foot of the pump A, perhaps as high as e in the
body or bore of the pump. By another stroke of
the piston G, or by causing it to descend, the
upper valv,e a is again opened by the force or
spring of the air, and the valve below (b) is shut
by the same pressure. Thus by the descent of
the piston, all the air which was included be-
tween the box H and the space C, to which the
piston was before raised, will rise above the valve
a in the piston, and by drawing it up, the valve
a will again be shut, and a second vacuum
created as before, which again will be filled by
the air from below, ascending through the lower

56 Experimental Philosophy. [Lecture 5.

valve b. The spring of the air being thus weak-
ened by this second motion, the pressure of the
atmosphere without the pump will cause the
water again to ascend within it, we will suppose
to F. By the next stroke the air will be almost
entirely exhausted, and the water will rise in the
body of the pump above the boxll, perhaps as
high as B. On forcing down the piston or bucket
again, the valve b in the box H will be shut by
the pressure as before, and the valve a in the
piston G will be opened by the same pressure,
and consequently water instead of air will now be
raised by the elevation of the piston. When the
piston is thus raised, it is evident that a vacuitm
will again be produced between the box H and
the piston C, which will instantaneously be filled
up by the water flowing through the valve b, as
before described. Thus, by the continual work-
ing of the pump, the water will be raised by the
piston into the wider space, and caused to flow
through the spout I. Every time the piston or
bucket is raised, the valve b is lifted up by the
water beneath, and every time the piston or
bucket is forced down, the valve a rises, and the
valve b is depressed. For the easiness of work-
ing in common pumps, the rod D is fixed to a
handle, which acts as a lever, and turns on a pin
in the body of the pump.

We have not yet, however, explained the diffi-
culty respecting the pump of the duke of Flo-
rence ; and you do not yet understand why the

Hydraulics. 57

water would rise in it no higher than thirty-two
feet. We must recollect what was said respect-
ing the cause of the water's rising in the body of
the pump. We know it was the pressure of the
atmosphere on the surface of the exterior water
that forced it to rise. From this circumstance it
is evident that the air has weight. But again,
as the atmosphere, or that mass of air which
surrounds the globe, is only of a limited height
(supposed about forty -five miles) and that of
gradually diminishing density, it follows that its
weight or pressure must be limited also ; and it
is found that a column of water of thirty- two or
thirty-three feet high is, at a medium, equal in
weight to a column of air of the same diameter
or thickness the whole height of the atmosphere.
Consequently the pressure of the atmosphere can
never force water through any vacant space
higher than about thirty-three feet. By the ac-
tion of a common pump of four inches bore and
thirty feet high, a single man can discharge
twenty-seven gallons and a half of water in a
minute ; if the pump is only ten feet above the
surface of the well, the quantity discharged in
that time may be eighty-one gallons six pints.

The forcing pump is upon a different plan.
Here the piston is without a valve, and the water
which rises through the valve in the box is
forced out by the depression of the solid piston.
Thus, in fig. 29, when the piston or plunger g* is
lifted up by the rod D, the water beneath forces

D 5

58 Experimental Philosophy. [Lecture 5.

up the valve b in the box H, and rises into the
body or barrel of the pump above H. When the
piston g, therefore, (which we must observe has
no hole or valve in it) is depressed to H, the valve
b being closed by this action, the water in the
barrel of the pump, finding no other vent, is
forced into the pipe M M, and so up through the
pipe. If there is no occasion for a continued
stream of water, the pipe M is continued to any
given height, and then the water would be thrown
out like a jet-d'eau at every stroke of the piston.
But to make a continued stream a further con-
trivance is necessary.

To this end an air vessel, such as K K, is an-
nexed to the pipe M, and into this air-vessel the
water is forced by each stroke of the piston.
When therefore the water, by this action conti-
nued, gets above the lower end of the pipe GHI,
which is fixed air-tight, in the top of the vessel,
the air in the upper part is proportionably con-
densed. The action of the pump being then
continued, in proportion as the vessel K K is
filled with water, the air above it is compressed,
and in return presses on the surface and drives
out the water through the pipe at the orifice in its
end in a continual stream, and with great force.

It is upon this principle that the famous and
truly useful invention of the fire-engine is found-
ed. It consists of two forcing pumps, and a large
air vessel which communicates with the pipe. In
fig. 27, A B is the body of the engine, in which

59

the water is contained ; D and E are two forcing
pumps, wrought by the lever FG, moving on the
centre h. The easiest mode of supplying the en-
gine with water, is that which is usually employed
in London in cases of fire, when a leather pipe
communicates with the orifice of one of the pipes
which supplies the city with water. When this
cannot be done, the water is poured by .-buckets
into the vessel AB, and being strained through
the wire grating N, is, by the pressure of the
atmosphere, raised (as before described in treat-
ing of the forcing pump) through the valves at
the lower end of the barrels D and E, when
either of the forcers ascend, and at their descent
it will be forced through the other valves alter-
nately, into the air vessel C : the air, therefore,
in this vessel being very strongly compressed, by
its spring it will force the water up through the
metal pipe within the air vessel; the part Q of
which being flexible, its end may be directed to
any part of the building where the flames predo-
minate.

By the means of forcing pumps water may be
raised to any height above the level of a stream
or spring, provided the machinery is sufficiently
powerful to work them. The London Bridge
water-works, which supply the city of London
with water, consist of a certain number of forcing
pumps, which are worked by large wheels turned
by the tide. There is also a beautiful engine of
this kind at the duke of Marlborough's at Blen-
heim.

60 Experimental Philosophy. [Lecture 5.

The most powerful forcing pumps, however,
are wrought by steam engines, for steam is one
of the strongest powers in nature. The steam
engine consists of a large cylinder or barrel, in
which is nicely fitted a solid piston, like that of
a forcing pump. The steam is supplied from a
large boiler close by, and is admitted into the
cylinder by an orifice, which can be occasionally
shut. The force of the steam lifts the piston, to .
the top of which is affixed a long lever to work a
forcing pump, or for any other purpose; and
when the piston is lifted a certain height, it
opens a small valve in the bottom of the cylinder,
through which a small quantity of cold water be-
ing admitted the steam is condensed, and thus a
vacuum being created, the piston again descends,
and is again lifted up by the force of the steam.
For a detailed description of this invaluable en-
gine, however, our readers must consult the En-
cyclopaedias or Pantologia, and our best treatises
on Mechanics.

LECTURE VI.
EXPERIMENTAlTPHILOSOPHY.

OF PNEUMATICS.

THE air we breathe is an heterogeneous mix-
ture, that is, a matter composed of different sub-
stances, and not of particles of perfectly the
same nature. This is one of the secrets which
the wonderful discoveries of modern chemistry
have revealed to us. According to this system,
caloric, or the matter of fire, is the basis of all
fluidity, and therefore air may be considered as
consisting of very minute particles, which swim,
or are suspended in a mass of that very subtile
and active fluid. The properties of caloric are
not, however, perceptible in this mixture ; for on
account of the attraction which subsists between
those particles of which air is composed, and
those of caloric, the latter is rendered latent, as
Dr. Black expresses it, or, in other words, in-
active. The matter of atmospheric air is therer
fore composed of caloric as its basis, and some
other matters. Or the other matters may be
considered as dissolved and floating in the mass
of fire, like salt, or gum, or any other substance
in water. The nature of these matters will be
explained in the chemical lectures, and would be

62 Experimental Philosophy. [Lecture 6.

improper at present, since it is of the general
properties of air of which I am now to treat, or
rather of its mechanical and not its chemical pro-
perties.

Fluids are divided into two classes ; the incom-
pressible, and the elastic. That branch of science
which is called hydrostatics treats of all the
known qualities of the former, and that of pneu-
matics embraces all which respects the general
properties of the elastic fluids. The elastic fluids
are again divided into two classes, those which
are condensible, such as vapour, which is easily
condensed by cold; and the permanently elastic
fluids, of which there are many, such as oxygen
air or gas (the word gas being an old German
term signifying spirit * ), nitrogen or azotic gas,
or phlogisticated air, as it was first called, carbo-
nic acid gas or fixable air, hydrogen gas or in-
flammable air ( that which is used to inflate bal-
loons), nitrous gas, hepatic gas, &e. But of
their general or mechanical properties the com-
mon air will serve to give a perfect idea.

The properties of air of which the science of
pneumatics particularly treats, are its weight,
pressure, and elasticity or spring.

That air, like all other bodies, is possessed of
weight or gravity many obvious facts will serve to
convince us ; and, in truth, it may be reduced to
the simplest of ah1 experiments, for air may be ac-

* Whence our word ghost.

Pneumatics. 63

tually weighed. If, for instance, a bottle which
holds a wine quart is emptied of its air, either by
the action of the air pump, or by filling it with
quicksilver, and emptying the quicksilver out,
taking care that, in corking it, no air is suffered
to enter, it will be found to be sixteen grains
lighter than it was before it was emptied of its air.
A quart of air, therefore, weighs just sixteen
grains ; a quart of water weighs fourteen thou-
sand six hundred and twenty-one *., which, di-
vided by sixteen, gives a result in round numbers
of nine hundred and fourteen ; so that water at
a medium is nine hundred and fourteen times
heavier than air.

This, however, is only to be understood of air
near the surface of the earth ; for, in fact, as air
is a body possessed of gravity, that which is near-
est the earth sustains a greater pressure, and is
consequently more dense or compact ; and it is
rarer or more thin and light in the higher regions
of the atmosphere, being less pressed with the
weight of air which is above. The atmosphere,
I observed in my last lecture, is that mass of air
which surrounds the globe, and which is gene-
rally computed to be about forty-five miles in
height. If altitudes in the air are taken in arith-
metical proportion, the rarity of the air will be
in geometrical proportion ; and therefore sup-
posing that the atmosphere extended to the height

* A quart of water is generally calculated at two pounds,
bat it is in fact something less.

64? Experimental Philosophy. [Lecture 6.

of five hundred miles, it has been computed that
one cubic inch, such as the air we breathe, would
be so much rarefied at that height, that it might
fill a hollow sphere equal in dimensions to the
orbit of Saturn.

We need not, however, have recourse to cal-
culations to prove a fact so generally understood.
All persons who have visited the tops of high
mountains know by experience that the air is
thinner or rarer at those altitudes than below.
-As they ascend their breathing becomes quicker,
the atmosphere is clearer, neither clouds nor va-
pours can rise to such heights ; and it is common
in these situations to see the lightning below pass
from one cloud to another, while all above is clear
and serene. Ulloa, who went to take the mea-
sure of a degree upon the earth's surface, informs
us, that while he stood on the top of one of the
Andes in Peru, the clouds, which were gathered
below the mountain's brow, seemed like a tem-
pestuous ocean, all dashing and foaming, with
lightnings breaking through the waves, and some-
times two or three suns were reflected from its
bosom. " In the mean time he enjoyed a cloud-
less sky, and left the war of the elements to the
unphilosophical mortals on the plain below
him."

The reason of all this must be evident. The
clouds are vapour, that is, water rarefied by heat ;
vapour is lighter than air near the surface of the
earth, but in the higher regions the air is thinner

Pneumatics. 65

and lighter than these vapours, and consequently
they can only ascend to a limited height. What
Ulloa observed on the Andes, has been confirmed
by the adventurers in balloons, and particularly
by Mr. Baldwin, who ascended from Chester in
the year 1785. The earth was entirely hid from
his view by the immense mass of vapours : he
compares them to a sea of cotton, tufting here
and there by the action of the air, and soon after
the whole became an extended floor of white
cloud.

To prove the weight and pressure of the atmo-
sphere I shall mention an easy experiment, which
the student may make himself, without any phi-
losophical apparatus. If we nearly fill a com-
mon saucer with water, and then take a tea-cup,
and burn in it a piece of paper ; while the paper
is yet burning, turn down the cup, paper and all
into the saucer, we shall soon see that the pres-
sure of the air upon the water contained in the
saucer will force it up into the cup. To under-
stand the nature of this experiment it is necessary
to anticipate in some degree what will be the
subject of future lectures. Heat, caloric, or
fire, is now known to be a real substance;
when, therefore, the paper is burned in the tea-
cup, the air is driven out by another fluid (viz.
caloric) taking its place. Caloric, however, pe-
netrates all substances ; and therefore when the
flame is extinguished, it is dissipated through the
pores of the cup, leaving almost a perfect vacuum^

66 Experimental Philosophy. [Lecture 6.

to fill which the water is pressed up, as before de-
scribed. It would rise, if there were no impedi-
ment, to the height of thirty-two feet, because,
as I explained in my last lecture, a column of
the atmosphere is at a medium equal in weight
to a column of water of that height.

The weight of the air, or rather of the atmo-
sphere, is, however, exactly determined by the
following experiment.

Take a glass tube about three feet long, open
at one end ; fill it with quicksilver, putting the
finger upon the open end, turn that end down-
ward, and immerse it into a small vessel of quick-
silver, without admitting any air : then take away
the finger, and the quicksilver will remain sus-
pended in the tube twenty-nine inches and a half
above its surface in the vessel ; sometimes more,
and at other times less, as the weight of the air
is varied by winds, vapours, and other causes.
That the quicksilver is kept up in the tube by
the pressure of the atmosphere upon that in the
bason, is evident ; for, if the bason and tube are
put under a glass, and the air is then taken out
of the glass, all the quicksilver in the tube
will fall down into the bason ; and if the air is ad-
mitted again, the quicksilver will rise to the same
height as before. The air's pressure therefore on
the surface of the earth, is equal to the weight of
twenty -nine inches and a half depth of quicksilver
all over the earth's surface, at a mean rate.

A square column of quicksilver, twenty-nine

Pneumatics. 67

inches and a half high, and one inch thick,
weighs just fifteen pounds, which is equal to the
pressure of air upon every square inch of the
earth's surface ; and one hundred and forty-four
times as much, or two thousand one hundred
and sixty pounds upon every square foot ; be-
cause a square foot contains one hundred and
forty-four square inches. At this rate a middle-
sized man, whose surface may be about fourteen
square feet, sustains a pressure of thirty thou-
sand two hundred and forty pounds, when the
air is of a mean gravity ; a pressure which would
be insupportable, and even fatal to us, were it
not equal on every part, and counterbalanced by
the spring of the air within us, which is diffused
through the whole body, and re-acts with an
equal force against the outward pressure.

Now, since the earth's surface contains, in
round numbers, 200,000,000 square miles, and
every square mile 27,878,400 square feet, there
murst be 5,575,680,000,000,000 square feet oa
the earth's surface ; which, multiplied bv 2,160
pounds, (the pressure on each square foot) give*
12,043,468,800,000,000,000 pounds for the pres-
sure or weight of the whole atmosphere.

The above experiment on the quicksilver,
which is called the Torricellian experiment, after
its inventor Torricelli, who made it about tin?
year 164r5, is the foundation of that instrument
which is called the barometer, so useful in fore-
telling changes of the weather. In the common

68 Experimental Philosophy. [Lecture 6,

barometer the quicksilver in the ball below is
left open to the pressure of the atmosphere, which,
according as it increases in weight or density,
presses on the surface of the quicksilver, and
forces it into the vacuum in the glass above.
When the air is dense or heavy it supports the
clouds and vapours ; when it is rarefied and thin
it is unable to support them, and they fall in the
form of mists, rain, hail, or snow. When, there-
fore, the quicksilver rises in the glass, we say it
is a sign of fair weather, when it falls it prognos-
ticates foul.

That the air is elastic is easily seen from various
experiments, particularly when it is confined in a
bladder or any flexible substance, we then find it
may be compressed by force into a narrower com-
pass, and that it will expand again when that
force is removed. But of all instruments for
showing the elasticity as well as all the other
properties of the air, the air-pump is the most
complete. It was invented nearly simultaneously
by our illustrious countryman, Mr. Boyle, and a
celebrated German, Otto Guericke.

Whoever is acquainted with the construction
of a common water-pump, can have no difficulty
in comprehending the nature and action of the
air-pump ; the principle is exactly the same, and
we may therefore, without further preface, refer
immediately to the Plate VII. fig. 28, to explain
its operation.

Having put a wet leather on the plate L L of

Pneumatics. 69

the air-pump, place the glass receiver M upon
the leather, so that the hole i in the plate may be
within the glass. Then, turning the handle F
backward and forward, the air will be pumped
out of the receiver; which will then be held
down to the plate by the pressure of the external
air or atmosphere. For, as the handle F is turn-
ed backward, it raises the piston d in the barrel
B K, by means of the wheel E and rack D : and,
as the piston is leathered so tight as to fit the
barrel exactly, no air can get between the piston
and barrel ; and therefore all the air above d in
the barrel is lifted up towards B, and a vacuum is
made in the barrel from b to d, upon which, part
of the air in the receiver M, by its spring, rushes
through the hole i, in the brass plate L L, along
the pipe G, which communicates with both bar-
rels by the hollow trunk I H K, and pushing up
the valve 6, enters into the vacant jplace b d of
the barrel B K. For wherever the resistance or
pressure is taken off, the air will run to that place,
if it can find a passage. Then, if the handle F
is turned forward, the piston d will be depressed in
the barrel ; and, as the air which had got into the
barrel cannot be pushed back through the valve
b9 it will ascend through a hole in the piston, and
escape through a valve at d, and be hindered by
that valve from returning into the barrel, when
the piston is again raised. At the next raising of
the piston, a vacuum is again made, in the same
manner as before, between b and d ; upon which

70 Experimental Philosophy. [Lecture 6.

more of the air that was left in the receiver M
gets out thence by its spring, and runs into the
barrel B K, through the valve b. The same thing
is to be understood with regard to the other bar-
rel A I ; and as the handle F is turned backward
and forward, it alternately raises and depresses
the pistons in their barrels, always raising one
wffile it depresses the other. A vacuum being
made in each barrel when its piston is raised, the
particles of air in the receiver M push out one
another by their spring or elasticity, through the
hole i, and pipe G, into the barrels ; until at last
the air in the receiver becomes so much dilated,
and its spring so far weakened, that it can no
longer get through the valves, and then no more
can be taken out. Hence there is no such thing
as making a perfect vacuum in the receiver; for
the quantity of air taken out at any one stroke
will always be as the density of it in the receiver:
and therefore it is impossible to exhaust it entire-
ly, because, supposing the receiver and barrels of
equal capacity, there will be always as much
left as was taken out at the last turn of the
handle.

There is a cock & below the barrels, which
being turned, lets the air into the receiver again ;
and then the receiver becomes loose, and may be
taken off the plate.

There is also a glass tube m n (fig. 29.) open
at both ends, and about thirty-four inches long ;
the upper end communicating with a hole in the

Pneumatics. 71

pump-plate, and the lower end immersed in
quicksilver at n in the vessel N. To this tube is
fitted a wooden ruler m m, called the gage, which
is divided into inches and parts of an inch, from
the bottom at n (where it is even with the sur-
face of the quicksilver), and continued up to
the top, a little below, to thirty or thirty-one
inches.

As the air is pumped out of the receiver M, it
is likewise pumped out of the glass tube m n, be-
cause that tube opens into the receiver through
the pump-plate; and as the tube is gradually
emptied of air, the quicksilver in the vessel N is
forced up into the tube as in a barometer, by the
pressure of the atmosphere. And if the receiver
could be perfectly exhausted of air, the quick-
silver would stand as high in the tube as it does
at that time in the barometer : for it is supported
by the same power or weight of the atmosphere
in both.

The quantity of air exhausted out of the re-
ceiver on each turn of the handle, is always pro-
portionable to the ascent of the quicksilver on
that turn ; and the quantity of air remaining in
the receiver, is proportionable to the defect of
the height of the quicksilver in the gage, from
what it is at that time in the barometer.

By means of the air-pump all the mechanical
properties of air are, as before observed, most
completely ascertained. Thus the weight and
pressure are clearly proved by a very easy and ob-

7£ Experimental Philosophy. [Lecture 6.

vious experiment. If we take a vessel of a long
or cylindrical shape, (fig, 30.) which is open at
the top, and place it on the pump, where the
receiver stands in fig. 28, then press it on the
top with the hand so as to exclude the external
air, we shall find, as the vessel begins to be
exhausted of air, a considerable pressure on the
back of the hand ; and if the operation is conti-
nued, that pressure will even become painful, and
we shall perceive it impossible to remove the
hand. This evinces that the weight of that co-
lumn of air which is above must be considerable,
and that the calculation above stated, of the
weight which a man's body usually bears, is not
overrated. If, instead of the hand, a piece of
bladder is tied over the open top of the vessel,
we shall see the bladder gradually sunk in like a
jelly-bag, and at length burst with considerable
force by the pressure of the external air ; a flat
piece of thin glass, placed in the same situation,
will be broken in pieces. Why then is the glass
receiver, which, we see, is placed on the pump in
fig. 1, not broken ? The reason of this is, first, the
shape of the glass, which is globular or arched at
top, and this is found, by long experience, to be
the best form for supporting a weight ; secondly,
these receivers are generally made of thick glass,
and with particular care, so as to sustain a
greater pressure than that of fifteen pounds on
a square inch without any danger of breaking.
A beautiful experiment to evince the pressure

Pneumatics. 73

of the air, is this. Let a metallic cup be provided,
in whose bottom shall be fixed a cylinder of thorn,
or some other wood, about three inches long;
and let this cup and attached cylinder be placed
at the top of the receiver of the air-pump, so as
to exclude all external air. Then let quicksilver
be poured into this cup, and let a glass to re-
ceive it be placed within the receiver. Then, as
the rarefaction of the interior air proceeds, the
quicksilver will be forced, by the external pres-
sure, through the pores of the wood, and will be
seen to descend in a beautiful shower.

Various facts in nature are explained by under-
standing the pressure and force of the air. The
word suction is founded on a vulgar error, for, in
fact, there is no such thing. In all cases where
suction is supposed, a vacuum or void is created,
and the pressure of the atmosphere forces the
fluid to fill up this void. Thus when children
suck at the breast, the mouth and lips of the
child act as an air-pump. The child swallows
the air in his mouth, while he holds the nipple
fast in his lips, so that none can come in that
way. A vacuum, of course, is created, and the
external air pressing on the breasts of the mother,
squeezes the milk into the infant's mouth. The
action of cupping glasses is explained on the
same principle. The air is driven out of the
cupping glass by means of heat, (as in the expe-
riment with the tea-cup,) that part of the body
where the glass is applied has therefore no pres-

VOL. I. E

74 Experimental Philosophy. [Lecture 6,

sure of air upon it, and the fluids of the body are
driven to that part where there is least resistance.

By the air-pump we are also convinced more
clearly of the elasticity and compressibility of
the air. Take a bladder from which the air is
almost totally exhausted, and which appears
(juite flaccid and compressed, tie the neck of it
tight as it was when full, and put it in an air-
pump. As the air is exhausted we shall see the
bladder gradually inflate, till, at length, it will be
puffed out to the full size it was before we had
expelled the air. Mr. Boyle relates that, by
means of the air-pump, he had rarefied common
air so as to make it fill nearly fourteen thousand
times the space it did before.

A similar effect would take place with a blad-
der, by carrying it to the higher regions of the
atmosphere, where, as before explained, the air
is thinner and lighter, and consequently its pres-
sure less. If a bladder half full is carried up to
the top of a high mountain, it will gradually di-
late to its former size.

If, instead of a bladder almost empty, a full-
blown bladder, or a thin glass bubble filled with
air. and closely stopped, is put into the ah -pump,
as soon as the air is exhausted, the bladder or the
bubble will burst in pieces.

The air is also capable of being rarefied by
heat. If a bladder, half blown and tightly tied
at the neck, is held to the fire, we shall find that
it will dilate to nearly its full size ; and if either a

Pneumatics. 75

full-blown bladder or a thin glass bubble filled
with air is held close to a strong fire, it will burst.
That air is a compressible fluid must be
evident, when we consider that it is elastic ; and
it must be further evident from what was said in
the last lecture on the use of the air vessel an-
nexed to the forcing pump and common fire
engine. There is, however, a beautiful experi-
ment expressive of the effects from compressed
air, which, with the aid of the plate, I shall
endeavour to describe. It is a kind of artificial
fountain, which is made to send out a stream or
jet of water by means similar to those employed
in the fire engine, that is, by a body of compressed
air forcing the water contained below it through
a small pipe, and out of the jet or orifice of the
pipe. In Plate VIII. fig. 31, ABCD, is a
copper vessel, which may be made of any con-
venient form ; within the vessel is a small pipe
or tube N O open at bottom, and with what is
called a stop cock *, such as R, at the upper end
to keep in the air when it is necessary. To
make the fountain play, we first fill it about
two- thirds full, with water, then screw in the
pipe, which must be made air-tight by oiled
leather. The air contained between the surface

* A stop cock is exactly like the common cocks used in
beer barrels, &c. — When turned one way there is an
orifice through the stopple, which then admits the air, or
any fluid ; when turned the other way it is solid, and stops
the passage.

76 Experimental Philosophy. [Lecture 6.

of the water and the top of the vessel is then of
the same density with that of the atmosphere.
We then take the condensing syringe, fig. 32,
and screw it above the stop cock, and force a
quantity of air into the vessel, which, as it can-
not return, forces its way through the water into
the upper part of the fountain, where it remains
in a condensed state ; while the air in the foun-
tain or vessel is condensing, we turn the stop
cock R to prevent the escape of the water. We
then screw on a jet or pipe with a small aperture
at top, and when we turn the stop cock again,
the condensed air above, by its expansion, forces
the water through the pipe, and out at the jet, in
a beautiful fountain.

The condensing syringe, fig. 32, is made like
a common squirt or syringe ; but it has a valve
at bottom, which, instead of opening inwards as
the valve of a pump, opens outwards at R. Near
the top of the syringe there is a small hole P.
When, therefore, the condensing syringe is
screwed on the vessel, if we draw up the piston
(which is solid, as in a squirt, and not with a
valve, like the piston of a pump) there will be a
vacuum left between that and the valve, till we
draw up the piston as far as the little hole P, near
the top. When it gets past the hole, the exter-
nal air will rush in and fill up the vacuum ; when
we push the piston down again, by which ac-
tion the valve below is opened, and the air
forced into the vessel — the valve shuts, and re-
strains the air from returning.

Pneumatics. 77

Air, it is said, may be thus compressed into fifty
thousand times less compass than its natural bulk,
provided the apparatus is strong enough. On
this principle of condensed air is constructed the
air-gun, a very dangerous and destructive in-
strument. It was formerly a very complex ma-
chine, from having the chamber for containing
the condensed air within the body or rather the
butt end of the gun. That which is how in use
was invented by the late ingenious Benj. Martin :
see fig. 33. It is in shape exactly like a common
gun. Just below the lock, a copper ball A,
fig. 34, screws on, which is charged or filled
with condensed air by a condensing syringe, ex-
actly as we charge the brass fountain, only that
the ball contains no water; the ball has a stop
cock a, which is turned or shut when it is not on
the gun : the bullet is rammed in a<= w« ~i— 0- --
musket, V»»* «~"^ nt me barrel very exactly. By
drawing the trigger, a small valve is opened at the
bottom of the barrel, and it is so contrived as to
let out only one charge of condensed air at each
pull of the trigger ; the bullet is discharged with
a force sufficient to kill an animal at the distance
of sixty or seventy yards. The copper ball con-
tains about ten charges. There are generally
two of these to each gun, and that which is
not immediately in use may be carried in the
pocket.

In the next lecture we shall treat of the atmo-
spherical phenomena.

LECTURE VII.

EXPERIMENTAL PHILOSOPHY.

THE PHENOMENA OF THE ATMOSPHERE.

THE word phenomenon, the plural of which
stands at the head of this lecture, and which we
shall frequently have occasion to use, means
simply an appearance. It is derived from the
Greek verb PHAINOMAT, which signifies to ap-
pear; but it is generally used to imply any
striking or remarkable appearance. The atmo-
sphere was before explained t'o mean that mass of
air which surrounds the earth. Various con-
iectures have been made with respect to the
neigm 01 me CILUIOO^U.^ . nn(^ as we know to a
certainty the relative weight of a column of ti*e
atmosphere by the height to which its pressure
will raise water or mercury in an empty tube, so
different calculations have been founded on these
data, to ascertain its extent as well as its density
at different heights. If the air of our atmosphere
were indeed every where of an uniform density,
the problem would be very easily solved. We
should, in that case, have nothing more to do,
than to find out the proportion between the
height of a short pillar of air, and a small pillar
of water of equal weight ; and having compared

The Phenomena of the Atmosphere. 79

the proportion the heights of these bear to each
other in the small, the same proportion will be
certain to hold in the great, between a pillar of
water thirty-two feet high, and a pillar of air
that reaches to the top of the atmosphere, the
height of which we wish to know. Thus, for
instance, we find a certain weight of water
reaches one inch high, and a similar weight of
air reaches seventy-two feet high : this then is
the proportion two such pillars bear to each
other in the small. Now, if one inch of water is
equal to seventy-two feet of air, to how much air
will thirty-two feet of water be equal ? By the
common rule of proportion we readily find, that
thirty-two feet, or three hundred and eighty-four
inches of water, will be equal to three hundred
and thirty-one thousand seven hundred and
seventy-six inches, which makes something more
than five miles, which would be the height of the
atmosphere, were it homogeneous, or its density
every where the same as at the earth's surface,
where seventy-two feet of air were equal to one
inch of water.

But this is not really the case ; for the air's
density is not every where the same, but de-
creases as the pressure upon it decreases; so
that the air becomes lighter and lighter the
higher we ascend ; and in the upper regions of
the atmosphere, where the pressure is scarcely
any thing at all, the air, dilating in proportion,
must be expanded to a surprising degree; and

80 Experimental Philosophy. [Lecture 7.

therefore the height of the atmosphere must be
much greater than has appeared by the last cal-
culation, in which its density was supposed to be
every where as great as at the surface of the
earth. In order, therefore, to determine the
height of the atmosphere more exactly, geometri-
cians have endeavoured to determine the density
of the air at different distances from the earth. —
The following sketch will give an idea of the
method which some have taken to determine this
density.

If we suppose a pillar of air to reach from the
top of the atmosphere down to the earth's sur-
face ; and imagine it marked like a standard by
inches, from the top to the bottom ; and still
further suppose, that each inch of air, if not at
all compressed, would weigh one grain. The
topmost inch, then, weighs one grain, as it suffers
no compressure whatsoever ; the second inch is
pressed by the topmost with a weight of one
grain, and this added to its own natural weight
or density of one grain, now makes its density,
which is equivalent to the pressure, two grains.
The third inch is pressed down by the weight of
the two inches above it, whose weights united
make three grains ; and these added to its natu-
ral weight, give it a density of four grains. The
fourth inch is pressed by the united weight of the
three above it, which together make seven grains ;
and this added to its natural weight gives it a
density of eight grains. The fifth inch, being

The Phenomena of the Atmosphere. 81

pressed by all the former fifteen, and its own
weight added, gives it a density of sixteen grains;
and so on, descending downwards to the bottom.
The first inch has a density of one, the second
inch a density of two, the third inch a density of
four, the fourth inch of eight, the fifth of sixteen,
and so on. Thus the inches of air increase in
density as they descend from the top, at the rate
of one, two, four, eight, sixteen, thirty- two, sixty-
four, &c. which is called a geometrical progres-
sion. Or if we reverse this, and begin at the
bottom, we may say, that the density of each of
these inches becomes less upwards in a geometri-
cal progression. If, instead of inches, we sup-
pose the parts -into which this pillar of air is
divided to be extremely small, like those of air,
the rule will hold equally good in both. So that
we may generally assert, that the density of the
air, from the surface of the earth, decreases in a
geometrical proportion.

This being understood, should we now desire
to know the density of the air at any. certain
height, we have only first to find out how much
the density of the air is diminished to a cer-.
tain standard height, and thence proceed to tell
how much it will be diminished at the greatest
heights that can be imagined. At small heights
the diminution of its density is by fractional or
broken numbers. We will suppose at once that
at the height of five miles, or a Dutch league,
the air is twice less dense than at the surface of.

8£ Experimental Philosophy. [Lecture 7.

the earth : at two leagues high, it must be four
times thinner and less dense, and at three leagues
eight times thinner and lighter, and so on. In-
stead of Dutch leagues, suppose we took a Ger-
man league of seven miles, and that it was four
times less dense at the height of the first German
league, then it would decrease in the same pro-
portion, and be four times less dense than the
first at the second league, that is, sixteen times;
and four times less dense than the second at the
,third league, that is, sixty-four times ; and four
times less dense than the third at the fourth
league, that is, two hundred and fifty-six times
less dense than at the surface. In short, what-
ever decrease it received in the first step, it will
continue to have the same proportion in the
second, third, and so on, and this, as was ob-
served, is called geometrical progression.

Upon the same principle it was attempted to
calculate the height of the atmosphere. By carry-
ing a barometer to the top of a high mountain,
the density of the air at two or three different
stations was easily ascertained. — But, alas ! so
feeble are human efforts in endeavouring to com-
prehend and measure the works of the Creator,
that this theory was soon demolished. It was
found that the barometrical observations by no
means corresponded with the density which, by
other experiments, the air ought to have had ;
and it was therefore suspected that the upper
parts of the atmosphere were not subject to the

The Phenomena of the Atmosphere. 83

same laws or the same proportions as those which
were nearer the surface of the earth; or that,
changes of temperature might operate with other
causes to change the law. Another ingenious
method was subsequently devised.

Astronomers know, to the greatest exactness,
the place of the heavens in which the sun is at
any one moment of time : they know, for instance,
the moment in which it will set, and also the pre-
cise time in which it is about to rise. However,
upon awaiting his appearance any morning, they
always see the light of the sun before its body, and
the sun itself appears some minutes sooner above
the mountain top, than it ought to do from this cal-
culation. Twilight is seen long before the sun ap-
pears, and that at a time when it is eighteen degrees
lower than the apparent horizon, or verge of the
sky. There is then, in this case, something which
deceives our sight ; for we cannot suppose the sun
to be so irregular in his motions as to vary every
morning : for this would disturb the regularity of
nature. The deception actually exists in the at-
mosphere. By looking through this dense, trans-
parent substance, every celestial object that lies
beyond it is seemingly raised up, n a way similar
to the appearance of a piece of money in a bason
filled with water. Hence it is plain, that if the
atmosphere were away, the sun's light would not
be brought to view so long in the morning before
the sun itself actually appears. The sun, without
the atmosphere, would appear one entire blaze
of light the instant it rose, and leave us in total

84 Experimental Philosophy. [Lecture 7.

darkness the moment of its setting. The length
of the twilight, therefore, at a given time, is in
proportion to the height of the atmosphere : or let
us invert this, and say, that the height of the
atmosphere is in some proportion to the length of
the twilight. This consideration led to an inves-
tigation (to which we shall recur when we treat
of astronomy) from which it has been inferred
that at the height of 45 miles, the atmosphere has
sufficient density to bend the rays of light. At
greater altitudes, the density is not sufficient to
occasion any perceptible effects.

The density of the air, however, depends not
merely on the pressure it sustains, but on other
circumstances ; so that it varies even at the same
height in different parts, and in the same place at
different times, as is seen by the mercury in the
barometer rising to different heights, according
to the state of the weather. Heat in particular
was mentioned as a very powerful cause in rarefy-
ing the air. From this circumstance arises one of
the most striking and formidable of the atmo-
spherical phsenomena — the WIND. Wind is no-
thing but a strong current or stream of air.
Whenever the air is heated by the sun, or by any
other means, it will be rarefied, and less able to
resist the pressure of the adjacent air, which will
consequently rush in "to restore the equilibrium,"
to speak in the technical language of philosophy,
or, in plain terms, to reduce the rarefied part to an
uniform density with the other. This current of
air is sensibly felt near the door of a glass-house,

The Phenomena of the Atmosphere. 85

or wherever there is a large fire. A current of
air is also to be perceived rushing through the
key-hole, or any chink or crevice, into a heated
room. This may serve to give a general idea of
the causes of winds.

This principle we consequently find realised
on a great scale, in what are called the trade
winds, which blow constantly from east to west
near the equator. When the sun shines intensely
upon any part of the earth, it is plain that, by the
immense accession of heat, the air must be greatly
rarefied. The cold air will therefore rush from
the adjacent parts to that where there is little
resistance, and consequently cause a stream or
current of air, in other words, a wind, towards
that quarter. The sun rises in the east, and sets
in the west, consequently the air will be heated
gradually from east to west, and the wind will
blow in that direction. Near the equator, there-
fore, where the surface of the earth is heated in
succession from east to west, there will be a
constant wind from the east, but on the north
side of the line it will incline a little to the north,
and on the south side a little to the south, for an
obvious reason, because it is colder towards each
pole, and therefore the mass of cool air will be
principally drawn from these quarters.

The same cause will explain, in a popular way,
the land and sea breezes in the tropical climates.
In islands, and small tracts of land which run into
the sea in those regions, it will generally be found

86 Experimental Philosophy. [Lecture 7.

that, during the day, there is a current of air to-
wards the sea, and at evening the current sets in
from the sea to the land. The reason of this is,
that water is always of a more even temperature,
that is, of a more equal heat, than land. During
the day, therefore, the land becomes considerably
heated, and the air is rarefied ; the consequence
is, that in the afternoon a breeze sets in from the
sea, which is less heated. On the contrary, dur-
ing the course of the night the land loses its heat,
while that of the sea continues more nearly the
same. Towards morning, therefore, a breeze re-
gularly proceeds from the land towards the ocean,
where the air is warmer, and consequently more
rarefied, than on shore.

The monsoons are periodical winds which
blow between the tropics, and which, though
the theory of them is rather more complicated,
originate in the same cause. They depend, in-
deed, upon large tracts of territory being heated
during the warm season, by which the general
course of the trade winds is partially interrupted.
Thus, when the sun approaches the tropic of
Cancer, the soil of Persia, Bengal, China, and
the ad joining countries, is so much more heated
than the sea towards the southward of these
countries, that instead of the usual trade wind,
the current of air proceeds at that season from
the south to the north, contrary to what it would
if no land was there. But as the high moun-
tains in Africa, during all the year, are extremely

The Phenomena of the Atmosphere. 87

cold, the low countries in India to the east-
ward of it become hotter than Africa during the
summer, and the air is naturally drawn thence
to the eastward. From the same cause the trade
wind in the Indian ocean blows, from April to
October, in a north-east direction, contrary to
the general course of the trade wind in the open
sea in the same latitude ; but when the sun re-
tires behind the tropic of Capricorn, these north-
ern parts become cooler, and the general trade
wind assumes its natural direction. In the north-
ern tropic the monsoons depend upon similar
causes.

In our climate the winds are more variable,
because the rarefactions which take place in the
air are here more partial, more frequent and sud-
den, than in the tropical regions. I have suf-
ficiently explained, that whatever dilates or rare-
fies the air in any part must produce a wind or
current of air towards that part. Among the
most pewerful causes of winds, therefore, we
must account the electricity of the atmosphere,
which (as will be explained hereafter) is the
cause of thunder and lightning. A thunder
storm, therefore, is commonly either preceded or
followed by a smart gale of wind. The rays of
the sun are also sometimes partially interrupted
by clouds or mists in particular places, conse-
quently the earth will be more strongly heated in
one part than another, in which case there will
always be a current of air from the colder to the

88 Experimental Philosophy. [Lecture 7.

warmer region. The fall of rain too, and many
other circumstances, may produce an alteration
in the temperature, which will be followed by a
change in the wind.

The velocity of the wind has been frequently
measured with great accuracy, and varies under
different circumstances. It has been said of
swift horses, such as Childers and Eclipse, that
they outstripped the wind, and so they did
at its mean rate. But we ourselves can even go
faster than the wind in some states ; for in calm
weather, when its motion is just perceptible, its
velocity is not more than one or two miles in an
hour, and even a brisk wind Joes not travel at
the rate of more than 15 or 20 miles an hour.
Childers, on the contrary, is known (o have run
at the rate of nearly one mile in a minute, that is
at least 50 in the hour, which is equal to the ve-
locity of a storm.

The storms which we experience in these happy
climates are as nothing when compared with those
dreadful convulsions of nature which are occa-
sionally felt in warmer latitudes, where the fruits
of a whole year's labour are often destroyed by a
single hurricane. These terrible phenomena
happen in the West Indies, generally in the rainy
season, about the month of August. They are
always preceded by an unusual calm ; but the
storm comes on suddenly, commonly accom-
panied with rain, thunder, and lightning, and
sometimes with an earthquake. Whole towns

The Phenomena of the Atmosphere. 89

are made a heap of ruins by one of these hurri-
canes ; fields of sugar-canes are whirled through
the air ; the strongest trees are torn up by the
roots and tossed like stubble ; nor can any build-
ing be constructed strong enough to afford a
shelter from the beating of the storm, and the
deluge of wet with which it is accompanied.
The island of Jamaica was visited in the year
1780 by this fatal calamity, and the damage
which ensued is not to be calculated. The hur-
ricanes in the West Indies have been attributed,
with great probability, to some occasional ob-
struction in the usual and natural progress of the
equatorial trade winds.

The harmattan is a wind which prevails oc-
casionally during the months of December,
January, and February, in the interior parts of
Africa, and always blows towards the Atlantic
ocean. There are generally three or four returns
of it every season; it blows with a moderate
force, not quite so strong, indeed, as the sea
breeze. A fog or haze always accompanies the
harmattan, so that the sun is concealed the greater
part of the day, and the largest building cannot
be seen at a quarter of a mile distance. The
particles which constitute this fog are deposited
on the leaves of trees, and on the skins of the
negroes, making them appear white. But the
most extraordinary property of this wind is its
extreme dryness. No dew falls during its con-
tinuance (on the average about a week), and the

90 Experimental PhilosopJiy. [Lecture 7.

grass is parched up like hay. Household furni-
ture is cracked and destroyed, the pannels of
wainscots split, the joints of a well-laid floor of
seasoned wood will be opened so as to admit the
breadth of a finger between them, and the covers
of books, though shut up in a close chest, are
bent as if they had been exposed to the fire. Nor
does the human body escape ; the eyes, nostrils,
lips, and palate are parched up, and made very
uneasy. Though the air is cool, there is a prick-
ling heat all over the skin ; and if the harmattan
continues four or five days, the scarf skin peels
off*. . This wind, though fatal to vegetable life, is
said to be conducive to the health of the human
body. It stops -all epidemics; indeed no infec-
tion can be communicated, even by inoculation,
during its continuance. It relieves patients la-
bouring under fevers, and is remarkable for the
cure of ulcers and cutaneous diseases.

The sirocco is as deleterious as the harmattan
is salubrious. It is common in Italy and the
south of France. In the former it is called the
sirocco, from a common opinion that it blows
from Syria ; in the latter it is called the Levant
wind. The medium heat of the atmosphere while it
it blows, is one hundred and twelve degrees. It is
fatal to vegetables, and often destructive to the
human species. It depresses the spirits in an un-
usual degree ; it suspends the power of digestion,
so that those who eat a heavy supper, while it
continues, are often found dead in their beds in

The Phenomena of the Atmosphere. 91

the morning. The sick, at that afflicting period,
commonly sink under the pressure of their dis-
eases ; and it is customary in the morning, when
this wind has blown a whole night, to inquire
who is dead.

The sarnie^ or mortifying wind of the deserts
near Bagdat, is also dreadful in its effects. At
its approach the camels instinctively bury their
noses in the sand, and travellers throw themselves
as close as possible to the ground till it has passed
by, which is commonly in a few minutes. As
soon as those who have life dare to rise up, they
examine how it fares with their companions, by
plucking their arms and legs ; for if they are
struck by the wind they are often so mortified
that their limbs will come asunder. The fatal
effects of this wind must depend upon a quantity
of putrid vapour with which it is charged, pro-
bably from passing over stagnant lakes, or
marshes loaden with putrid matter.

Whirlwinds, which are so sportive in their
appearance in this country, carrying up straws
and other light bodies a considerable height in
the air, have been known in the tropical countries
to produce most tremendous effects. It is
probably a description of them which is known
there by the name of t^wiados ; these carry up
with them the whole materials of a cottage, or
even large trees, with the same velocity as our
whirlwinds do straws and the lightest bodies.
A whirlwind at land is a water-spout at sea ; at

92 Experimental Philosophy. [Lecture 7.

least, botli seem to proceed from the same cause.
Wherever the air is suddenly rarefied in a par-
ticular spot, from electricity or any other cause,*
a kind of vacuum is created, and the circum-
ambient air rushing at once from every quarter,
a conflict of winds takes place, and the circular
motion, already noticed, ensues. It is to be ob-
served that, in water-spouts at sea, the water
ascends, and does not descend (according to the
vulgar notion) from the cloud, which is formed
at the extremity of the spout. The water in
this case rises, where the vacuum is created by
the whirlwind, by the pressure of the atmo-
sphere, as in a common pump. Only the vacuum
not being quite perfect, it rises in small drops,
and forms the cloud at the upper extremity of
the phenomenon. An artificial water-spout may
be made in a very easy way. In a stiff paper or
card make a hole just wide enough to insert a
goose quill, then cut the quill off square at both
ends ; place the card at the top of a wine glass or
tumbler filled with water to within about a quarter
of an inch of the lower orifice of the quill. Then
apply the mouth to the upper part of the quill,
and draw out the air. The water in the glass
will then be seen raised in the form of an in-
verted cone like a water-spout, and not in a con-
tinued stream, but broken into drops, and mingled
with particles of air.

It is by the agency of the air that water is
raised in vapour from the earth to form clouds.

The Phenomena of the Atmosphere. 93

You need not be told,. I. presume, that clouds
are water in a suspended state, and so is the
common smoke which ascends from our chim-
neys, the columns of which, in fact, are so
many clouds. Vapour is water expanded by
heat or fire to the state of an elastic fluid,
and it rises in the atmosphere*, because va-
pour is lighter or less dense than our common
air (it is, in fact, fourteen hundred times lighter
than the water of which it is composed, whereas
common air is only about nine hundred times
lighter than water) ; and it is a rule in philosophy,
depending on the principle of gravitation, that
when two fluids of different densities are brought
together, the lighter will always rise to the sur-
face. It is, however, only near the surface of
the earth that the air is denser and more heavy

* There is a constant process of evaporation going on
from all bodies on the surface of the earth which contain
moisture. In a dry atmosphere the evaporation from the
human body is very considerable, but the heat which that
carries ofi" is continually recruited by the vital principle,
which is wonderfully adapted to resist, to a certain extent,
the eflecti both of a hot and a cold medium, keeping the
blood in either, very nearly at the same temperature.
When, however, this principle is roused by exercise, and
a warm and moist air, or a spasm on the skin obstructs the
free passage of the perspirable matter, the blopd becomes
over-heated, and we feel oppressed. On the other hand,
exposure to a keen dry wind, without sufficient exercise,
endangers delicate persons, from the too great cooling of
the blood.

94* Experimental Philosophy. [Lecture 7.

than water. The vapours, therefore, can only
rise to a limited height; and it is generally
agreed that there are no clouds at the height of
four or five miles in the atmosphere : their usual
height, indeed, seldom exceeds a mile, nor very
often half a mile. Vapour, by coming in contact
with a cold body, can be deprived of its heat, and
is suddenly condensed into water again, as in the
refrigeratory of a still, where the vapour, confined
in a spiral tube, is made to pass through cold
water, and is condensed, as in the steam engine,
which was noticed in a former lecture.

If, therefore, "the vapours in the atmosphere,
by ascending into the colder regions of the air,
by electricity, or by meeting with cold winds,
are deprived of the heat which keeps them in
the vaporific state, they will of course be con-
densed to clouds, and will fall down in the form
of ram. Perhaps the attraction of the earth,
when they approach it, may, in many cases, serve
to draw off the superfluous heat, or electricity, and
condense the vapours; which may account for its
generally raining on the tops of mountains, and
for the changes of the weather predicted by the
barometer. For when the air is so far rarefied as
not to be able to support the column of mer-
cury to a certain height in the tube of the ba-
rometer, it is generally regarded as a sure pro-
gnostic of rain.

The air in the higher regions being sometimes

The Phenomena of the Atmosphere. 95

intensely cold, the vapours immediately after
condensation are frozen, and the frozen particles
in their slow descent unite at a determinate
angle, forming the beautiful feathery flakes of
snow, each of which is, in fact, a very compli-
cated group of little crystals. Hail is sometimes
an entire drop frozen in its descent through a
colder region, or by means of a rapid evaporation,
in which case it is a transparent globule; but
much more frequently a common snow flake rolled
up in a manner by whirling between two cur-
rents forming an opake nucleus, which by its ex-
treme coldness encrusts itself with clear ice out
of the vapours it meets with in falling. These
rolled snow flakes often fall unencrusted before
a severe frost. Angular hailstones are the frag-
ments of larger spheres which have broken in
their fall, probably by the expansion of air en-
veloped in the spongy nucleus.

The dew, which falls in a summer evening, is
part of the vapour which is raised in the course
of the day by the sun's heat ; but not being com-
pletely dissolved or dispersed in the atmosphere,
it is condensed, and falls with the evening's cold.
In cool nights the dew often becomes frozen in
the form of hoarfrost.

The atmospherical phenomena will be further
explained when we treat of electricity.

LECTURE VIII.

EXPERIMENTAL PHILOSOPHY.

ELECTRICITr.

IP the electrical fluid is not caloric, or the matter
of fire, it resembles that element in so many of
its phsenomena and effects, that there is reason to
believe it a combination of it with some other
substance. But of the nature of that combina-
tion we are at present ignorant. To mortify the
pride of man, philosophy leaves some things
unexplained : the really ignorant are those who
think they can penetrate into every secret of
nature ; whereas the truly wise will see that there
is much placed out of the reach of human com-
prehension, and many things yet left to be disco-
vered by the industry and the patience of man.

The electric matter resembles caloric or fire
in its most usual effects, the power of igniting or
setting on fire inflammable bodies; in melting
metals; in the emission of light; and in the
velocity of the electric spark. Friction, which
is known to produce heat and fire, is also the
most powerful means of exciting electricity;
heat also extends itself most rapidly in humid
bodies and metals, and these are the best con-
ductors of electricity ; and as caloric is the most

Electricity. 97

elastic of all fluids, and perhaps the great cause
of repulsion, so the electrical repulsion may,
perhaps, be referred to the same principle.

On the contrary, there are some facts which
seem to prove that the electric matter is some-
what different in its nature from caloric. The
electric matter affects the organs of scent; its
progress may also be arrested by certain sub-
stances which, on that account, are called non-
conductors; glass, in particular, which admits the
passage of both heat and light, stops the course
of the electric matter: on the contrary, the
electric fluid will adhere most tenaciously to
some other bodies, without diffusing itself even
to those which are in contact with them : thus
an electric spark has been drawn by a wire-
through the water of the river Thames, and has
set fire to spirit of wine on the opposite side.

The principal phenomena of electricity are
first, The electrical attraction and repulsion.
Secondly, The electrical fire rendered visible:
and, thirdly, The power which certain substances
possess of conducting the electrical matter;
whence arises the distinction between con-
ductors and non-conductors, or non-electric and
electric bodies. The electric are those which are
capable of being excited, such as glass, amber,
&c, but do not conduct; the non-electrics are
such as conduct the electric matter, but cannot
be excited to produce it, such as metals, stones,
and all fluids.

VOL i. r

98 Experimental Plnlosopliy. [Lecture 8.

These phaenomena were not, however, all dis-
covered at once ; on the contrary, it was by slow
degrees that philosophy became acquainted with
the properties of this surprising fluid. It was,
however, long known that amber* and some
other matters, when rubbed on a soft and elastic
substance, had a power of attracting feathers,
straws, or other light bodies. We may, without
either pains or cost, make the experiment: by
taking a piece of sealing-wrax, and rubbing it
quickly upon a coat sleeve, or any piece of woollen
cloth, we shall find that it will readily attract
hair, feathers, chaff, &c. A smooth bubble of
glass will answer still better.

Sulphur is also a body that is capable of
exercising this power of attraction ; and to observe
more perfectly its effects, Otto Guericke, burgo-
master of Magdebourg (the same who is men-
tioned in a preceding lecture, as having afforded
hints for the construction of the air-pump), made
a large globe of sulphur, which he fixed in a
wooden frame, and, by whirling it about rapidly,
and rubbing it at the same time with his
hand, he was enabled to perform several experi-
ments. This may be regarded as the first elec-
trifying machine. He observed that a body
which was attracted by his globe was afterwards
repelled by it, but that if it touched another body,
it became after that capable of being attracted
again. Thus he was able to keep a feather sus-

* Amber, electron in Greek, whence the name electricity.

Electricity? 99

pended over his globe ; but if he drove it near a
linen thread, or the flame of a candle, it in-
stantly recovered its propensity to approach the
globe again. This fact is now explained; the
feather, by being attracted by the globe, and
especially when in contact with it, becomes
charged, or loaded with the electric matter;
when it touches or comes very near a body which
is not charged with electricity, it parts with its
share to that body, and returns again to receive
a fresh supply, if " within the sphere of attrac-
tion," that is, within those limits whither the
attractive powers of the globe extend.

This philosopher was enabled to remark the
hissing noise which a stream of the electric mat-
ter produces, and he had a glimpse of the elec-
tric light ; but Dr. Wall, an English philosopher,
observed it more clearly. By rubbing amber upon
a woollen cloth in the dark, he found that light
was produced, attended by a hissing or rather a
crackling noise. Mr. Hawksbee, another of our
countrymen, observed the same thing of glass ;
and he constructed a kind of machine, which
enabled him to put a glass cylinder in motion.

Thus the electric attraction and the electric
light were proved by experiment; but it was
reserved for Mr. Grey, a pensioner of the Char-
ter-house, to make the distinction between those
bodies which are capable of being excited to
electricity, and those which are only capable of
receiving it from others. After attempting in

100 Experimental Philosophy. [Lecture 8.

vain to give the power of attraction to metals, by
rubbing, hammering, and heating, he conceived
a suspicion, that as a glass tube, when rubbed in
the dark, communicated its light to other bodies,
it might possibly be made to communicate also
its power of attraction. He provided himself,
therefore, with a glass tube three feet five inches
long, and near an inch and one-fifth in diameter.
The ends of the tube were stopped with cork,
. and he found that when the tube was excited by
friction, a feather was attracted as powerfully by
the cork as by the tube itself. To convince him-
self more fully, he procured a small ivory ball,
which he fixed to a stick of deal four inches long,
and thrust into the cork ; and he found that it
attracted and repelled the feather even with more
vigour than the cork itself. He afterwards fixed
the ball to a longer stick, and even to a piece of
wire, with the same success. Lastly, he attached
it to a piece of packthread, and hung it from a
high balcony, where he found that, by rubbing
the tube, he enabled the ball to attract light bodies
in the court below.

His next attempt was to examine whether this
power acted as well horizontally as perpendicu-
larly. With this view he made a loop of cord, which
he hung to a nail in one of the beams of the ceiling,
and ran his packthread, which had the ivory ball
at the end, through the loop ; but in this state he
found, to his utter mortification, that his ball
had totally lost the power of attraction. On

Electricity. 101

mentioning his disappointment to a friend, it
was suggested, that the cord which he employed
for the loop, through which the pac-kthf^cl rah,
might be so coarse as to intercept the electric
power. To remedy this, they made , the hWp of
silk, which they considered as stronger, in pro-
portion to its thickness, than the former. . With
this apparatus they succeeded beyond expectation.
As they attributed their success entirely to the
fineness of the silk of which the loop was made,
they thought they would perform still better by
supporting the packthread by a very fine brass or
iron wire ; but to their utter astonishment, the
electric virtue was entirely lost; while, on the
contrary, when the apparatus was supported
by the silk loops, they were able to convey the
power of attraction along a packthread of seven
hundred and sixty-five feet in length. It was
evident, therefore, that these effects depended
upon some quality in the silk, which disabled it
from conducting away the electric power, as the
hempen cord and the wire had done; and,
by subsequent experiments, this hypothesis was
amply confirmed.

This little narrative may serve to give a tole-
rably competent idea of non-conducting and con-
ducting bodies; and we must remember, that
those bodies which do not conduct the electric
fluid are most capable of exciting it, and are sup-
posed to be naturally charged or loaded with a
quantity of it. They have, therefore, been called

102 Experimental Philosophy. [Lecture 8.

electrics ; such are amber, jet, sulphur, glass, and
all precious stones ; all resinous substances ; and
t)-e clvjj-1 parts of animals (except the bones),
such (as ,ha.ir, wool, silk, &c. On the contrary,
stony sijbst<mc^s in general, fluids in general,
alum, pyrites, sulphuric acid, black lead, char-
coal, and all kinds of metals are among the non-
electrics, or those which conduct the electric fluid.

Soon after the discoveries, as above related, of
Mr. Grey, both the English and German philo-
sophers contrived means of accumulating the
electric matter and increasing its effects. Not
only the electric fire was rendered visible, but it
was made to pass from one conducting body to
another. Spirits and other inflammable matters
were easily set on fire by the electric spark ; and
animal bodies were made to feel what is called
the electric shock — that is, the uneasy sensation
felt on the electric fluid passing through any part
of our bodies.

The machines at first constructed for pro-
ducing the electric fire were made in a very com
plex form. It is now found that it may be ex-
cited by much simpler means; and the machine
exhibited in plate 9 (fig. 35.), though extremely
simple, is very powerful. In this figure ABC
represents the board on which the machine is
placed. D and E are two vertical supports,
which sustain the glass cylinder F G H I. The
axis of the cap K, in which the cylinder is fixed,
passes through the support D, and it is turned

Electricity. 103

by a winch or handle, as represented in the plate.
The axis of the other cap is inserted in the sup-
porter E; O is the glass pillar to which the
cushion is fixed. At the bottom of the pillar O
is a brass screw T, which brings the cushion at
the top of the pillar nearer to the cylinder or re-
moves it further, at the discretion of the ope-
rator, when he wishes to increase or lessen the
pressure.

Y Z is the prime conductor, which by means
of metallic points takes the electric matter imme-
diately from the cylinder ; and in order that the
electric fluid may be accumulated upon the con-
ductor, and not run off to the earth, the con-
ductor is insulated, that is, placed upon a non-
conducting body, which will not attract the fluid
away from the conductor. ' The insulating sub-
stance, in this case, is a glass pillar, L M (glass
being the most convenient substance for this
purpose), and VX is the wooden foot or base of
the glass pillar. The conductor is always of
metal, at least externally, as metals are found to
be the most powerful ,of the conducting bodies.
They are commonly made of wood, and cased
over with tin-foil.

When electrical machines were first constructed,
instead of a cylinder, a glass globe was made use
of; and when this was turned, the hand of the
operator was applied to it, and afterwards a piece
of glove leather ; but the most effectual and easy
means is now found to be a leather cushion,

104 Experimental Philosophy. [Lecture 8.

covered or smeared over with what is called an
amalgam^ or a mixture of tin and mercury. A
small chain is also annexed to the apparatus, in
order to make a communication with the earth ;
which is always necessary, as the electrical fluid
is all supposed to be ultimately derived from the
earth. When the chain is laid over that con-
ductor which communicates with the cushion,
then that conductor is no longer insulated, but
an immediate communication is established with
the earth : if, on the contrary, the chain is taken
from it, and laid over the prime conductor, dif-
ferent effects are produced, which we shall en-
deavour hereafter to explain.

It is scarcely necessary to add that the elec-
trical power is excited by turning the cylinder
pretty quickly round, while it rubs against the
cushion. On turning the cylinder for a little
time in this manner, we find that sparks may be
drawn by the knuckle from the prime conductor,
which is then charged or loaded with the electric
matter, and this matter has a kind of sulphureous
smell. Again, if a metallic plate is placed at
some distance beneath the conductor, and some
light bodies, such as feathers, straws, or little
images of men and women cut in paper are pre-
sented to it, they will be first attracted to the
conductor, they then become in effect conductors
themselves, and, as soon as charged with the
electrical matter, they will be repelled; they will
then fly to the plate, and discharge the electricity

Electricity. 105

they have received, and then be in a state to be
attracted again, when they will again fly up to
the conductor ; and a very curious effect is pro-
duced by the little images being thus put in mo-
tion, as if by a kind of magical power.

The human body itself may, in this manner,
be made a conductor; but to enable it to accu-
mulate any quantity of the electric matter, the
man must be insulated, that is, some non-con-
ducting substance must be placed between him
and the earth, and he must stand upon a cake of
rosin, wax, or sulphur, or upon a stool with
glass legs. If, then, he lays his hand upon the
conductor, his body will be filled with the elec-
trical matter, and sparks may be drawn from any
part, upon being touched by another person;
and each spark will be attended with a crackling
noise, and a painful sensation to each party. If,
in the same circumstances, spirit of wine is
presented to the man in a metal spoon, when
he touches it with his finger it will be set on
fire ; and gunpowder, or any other very inflam-
mable substance, may be kindled in the same
manner.

As metals are the most powerful conductors
of electricity, if a wire of iron or any other metal
be suspended by silken cords (that is, insulated),
the electric matter may be conveyed to an im-
mense distance through dry air; for air is a non-
conducting substance when not moist, and there-
fore will not draw away the electric matter. In

F 5

106 Experimental Philosophy. [Lecture 8.

this manner some French philosophers conveyed
the electric fire through a circuit of three miles.
Though water is a conductor, yet, not being so
powerful as metals, the late Dr. Watson con-
veyed (as has already been observed) the electric
fire, by means of a wire, through the Thames,
and it set fire to spirit of wine on the opposite
side.

The most powerful means, however, of accu-
mulating the electric fluid is found to be the
Leyden phial. This discovery was made about
the year 1745, by Mr. Von Kleist, dean of the
cathedral of Camnin. He found that a nail or a
piece ,„ of iron wire, inclosed in an apothecary's
phial, and exposed to the prime conductor, had
a power of accumulating the electric virtue, so as
to produce the most remarkable effects ; and he
soon after ascertained that a small quantity of
fluid added to it increased the power. The fact
is, that if glass is coated on one side with any
conducting substance, that substance will accu-
mulate the electrical matter, because it is inter-
cepted by the glass, and prevented from diffusing
itself; the form of the glass is of little conse-
quence. The Leyden phial or jar, as at present
employed, is a thin cylindrical glass vessel, such
as fig. 39, about four inches in diameter, and
coated within and without, to within two inches
of the top, with tin-foil or any conducting sub-
stance. Within the jar is a metal wire, with a
knob at the top of it, which wire communicates

Electricity. 107

with the inner coating of the jar. To discharge
the phial, a communication must be made (either
by what electricians call a conducting or dis-
charging rod D, or any other fit instrument) be-
tween the inner and outer coating of the jar. Its
effects may be proved by placing the phial or jar
(fig. 39.) on an insulated stand, bringing the
coating in contact with the conductor, and then
turning the machine. If in this case we apply
the discharging rod D, we shall find there will
be no explosion, because both sides being insu-
lated, the phial was not charged ; but if a small
chain is suspended from the brass knob of the
phial, and communicates with the table, the
phial will then be charged, and the explosion
will be considerable. The reason of this has
been explained before, as it was proved that the
electrical matter is derived from the earth.

The shock which is given by the Leydcn phial
is much more powerful than that from the largest
conductor; but this power is greatly increased
by uniting together the force of several jars, in
what is called an electric battery (see fig. 40.).
The bottom of the box in this apparatus is co-
vered with tin-foil, to connect the external coat-
ings of the jars ; and the inside coatings are con-
nected by the wires a, &, cy d, e,j\ which meet
in the large ball above, There is a hook at the
bottom of the box, by which any substance may
be connected with the outside coating of the
jars ; and a ball B proceeds from the inside, by

108 Experimental Philosophy. [Lecture 8-

which the circuit may be conveniently com-
pleted. By the discharge of an electrical battery
a large dog may be killed in an instant, and
the strongest man will be knocked down and
deprived of sensation; a wire of some mag-
nitude may be melted, and most of the phaeno-
mena of lightning are produced, but on a smaller
scale.

LECTURE IX.

EXPERIMENTAL PHILOSOPHY

ELECTRICAL PHENOMENA AND GALVANISM.

SOME of you will, I doubt not, be disposed to
remind me, that I have neglected to explain why
the electrical machine exhibited different effects
when the chain, which communicates with the
earth, was put over the prime conductor, from
those which take place in its ordinary mode of
operation, when the chain was connected with
the cushion.

In a very early stage of the science, two kinds
of electricity were observed, or, according to Dr.
Franklin's theory, two different effects from the
same cause. A ball of rosin or sealing-wax, and
a globe of glass, when excited, will each of them
electrify ; but the electricity produced from each
will differ in some of its effects. Thus, if we
electrify two cork balls, suspended by silken
threads, with the same substance, either glass or
sealing-wax, they will mutually repel each other ;
but if one of them is electrified with glass, and
the other with sealing-wax, they will be mu-
tually attracted. From this circumstance it was
conjectured at first, that there were two kinds
of electricity ; that from glass was called the

110 Experimental Philosophy. [Lecture 9.

vitreous, and that from resin vus substances or
sulphur was termed the resinous electricity.
Another circumstance which served to distin-
guish them, was the different appearance of the
electric light. A divergent cone of light, re-
sembling a painter's brush, distinguished the
vitreous electricity, while a single globe or ball
of clear light was the mark of the resinous. In
process of time, however, it was discovered that
these different phenomena depended rather on
the surface than the composition of the electric ;
for glass, when the smooth surface was de-
stroyed by being ground with emery, and being
rubbed with a smooth body, exhibited all the
appearances of the resinous electricity ; yet after-
wards, when it was greased and rubbed upon a
rough surface, it resumed its former property.
It was therefore concluded, upon various experi-
ments, that the smoother of two bodies, upon
friction, exhibits the phenomena of the vitreous
electricity, and the contrary.

M. Coulumb proposed another theory. He
considered the electric matter as composed of two
distinct fluids, which are neutralized the one
by the other in the ordinary state of bodies, but
which separate when the bodies are electrified.
Such a theory, however, only serves as a vehicle
for reasoning: the experiments establish two
distinct modes of operation ; and they may be
explained with nearly equal facility by either of
the hypotheses.

Electrical Plicenomena. Ill

When any body contains a superfluous quan-
tity of the electric fluid, it is (according to the
Franklinean theory) said to electrify positively or
plus ; when it contains less than its proper share,
it is said to be negative or electrified minus,
that is, some of its electricity is taken from it.
That electricity, therefore, which was before
called the vitreous, Dr. Franklin calls positive
electricity; and that which was termed the re-
sinous, he considers as negative electricity. If,
therefore, a rough and smooth body are rubbed
together, the smooth body in general will have
the positive electricity, and the rough the nega-
tive. Thus, in the ordinary operation of the
electrical machine, the cylinder is positively
electrified or plus, and the rubber negative or
minus ; and the redundancy of the positive elec-
tricity is sent from the cylinder to the prime con-
ductor. This, however, is supposing the chain,
which communicates with the earth, to be at
the same time in contact with the rubber; for
as the earth is the great repository of electrical
matter, if the chain is removed, and put over
the prime conductor, these effects will be re-
versed, and the prime conductor will then be
negatively electrified or minus, and the rubber
will be plus or positive *.

* Whether the theory of Franklin be adopted, or
whether the hypothesis of two distinct fluids be retained,
signifies nothing as to the fads, it simply regards the
manner of explication. On either hypothesis, the fact

Experimental Philosophy. [Lecture 9.

That the electrical matter is possessed of force,
even while it proceeds in a stream imperceptible
to our senses, is evident from an easy experi-
ment. To the under part of the Leyden phial
an apparatus is often adapted, as in fig. 38. It
consists of the wire b c, and a brass fly at the
top. While the bottle is charging the fly will
turn round, and when it is charged it will stop.
If the top of the bottle is touched with the
finger, or any conducting surface, the fly will
turn again till the bottle is discharged. The fly
will electrify cork balls positively while the bottle
is charging, and negatively while it is discharging.
A similar effect is observable in what is called
the electrical bells (fig. 37.). In this apparatus
three small bells a b c are suspended from a nar-
row plate of metal, the two outermost a c by
chains, and that in the middle b (from which a
chain passes to the floor) by a silken thread.
Two small knobs of metal d e are also hung by
silken threads on each side of the bell, in the
middle, which serve for clappers. When this
apparatus is connected with an electrified con-
ductor, the outermost bells, suspended by chains,
will be charged, will attract the clappers, and be

remains, that electric action follows the inverse ratio of
the square of the distance ; as has been decisivelyproved by
Coulomb and others. It is also an established fact, that the
whole fluid of a conducting body is diffused about its sur-
face. Electrical facts are well confirmed; but the theory,
like that of magnetism is, as yet, uncertain.

Electrical Phcenomena. 113

struck by them ; and the clappers then becoming,
in their turn, electrified, will be repelled by these
bells, and attracted by that which is in the mid-
dle, and their electricity will be then attracted
away by the chain which passes to the floor.
After this the clappers will be again attracted by
the outermost bells, and thus the ringing will be
continued as long as the conductor is charged.
An apparatus of this kind is usually attached to
the conducting rods, which are fixed to the
gable-ends of houses to protect them from light-
ning, and thus serve to give notice of a thunder
storm.

The instrument called an electrometer (fig.
36.), which is commonly used for measuring
the quantity of electricity contained in any body,
is constructed on a similar principle. It consists
of a vertical stem L M which terminates in a
round top L like a ball. It may be fixed in one
of the holes of the conductor, or at the top of a
Leyden phial. , To the upper part of the stem a
graduated semicircle is fixed, as well as the index,
which consists of a very 'slender piece of wood,
which reaches to the centre of the graduated
arch, and at its extremity there is a small pith
ball. When the body is electrified, the index
recedes more or less from the pillar, and the de-
gree is ascertained by the gradations on the arch.

Electricity accelerates the evaporation of liquors
and the perspiration of animals. There is reason
also to apprehend that it is not without effect

114* Experimental Philosophy. [Lecture 9.

upon the vegetable creation, as from some ex-
periments we are led to conclude that plants
which have been electrified vegetate earlier and
more vigorously than those which have not been
subjected to its influence.

Electricity is, indeed, a most powerful agent
in nature, and we are probably not yet ac-
quainted with all its effects. It is, however, in
the atmospherical phsenomena that these effects
are most apparent and most tremendous. It is
to Dr. Franklin that we are indebted for the
interesting discovery, that the cause which pro-
duces THUNDER and LIGHTNING is precisely the
same with that which produces the ordinary
phsenomena of electricity.

This eminent philosopher was led to the dis-
covery by comparing the effects of lightning
with those produced by an electrical machine,
and by reflecting that if two gun-barrels when
electrified will strike at two inches with a loud
report, what must be the effect of ten thousand
acres of electrified cloud ? After much thought
upon the subject, he determined to try whether
it was not possible to bring the lightning down
from the heavens — a thought at once daring
and sublime ! With this view he constructed a
kite, like those which are used by school boys,
but of a larger size and stronger materials. A
pointed wire was fixed upon the kite, in order
to attract the electric matter. The first favour-
able opportunity he was impatient to try his ex-

Thunder and Lightning. 115

periment, and he sent his kite up into a thunder
cloud. The experiment succeeded beyond his
hope. The \vire in the kite attracted the elec-
tricity from the cloud ; it descended along the
hempen string, and was received by an iron key
attached to the extremity of the hempen string,
that part which he held in his hand being of
silk, in order that the electric fluid might stop
when it reached the key. At this key he charged
phials, with which phials thus charged he kindled
spirits, and performed all the common electrical
experiments.

Thus it became evident that the cause of those
terrible convulsio.ns of nature, which, in warm
climates especially, are attended with such tre-
mendous effects, is no other than a superfluous
mass of electrical matter, collected in those immense
watery conductors, the clouds; and that this matter
is discharged when an electrical cloud meets with
another which is less powerfully charged, or when
it is brought sufficiently near to the earth to be
within the sphere of the electrical attraction. This
fact may be proved at almost any time, but par-
ticularly in a sultry summer's evening, by repeat-
ing Dr. Franklin's experiment with the kite.
Some caution, however, must be used in making
ihe experiment; and it will succeed better if a
small wire is twisted in with the hempen string
by which the kite is held ; indeed Mr. Walker, in
his Lectures, recommends to fly the kite with

116 Experimental Philosophy. [Ilecture 9.

wire instead of a string, which, he observes, may
be coiled upon a strong rod or bar of solid glass,
held in both hands. Sparks may, in this manner,
be taken from the wire or string, as from a com-
mon electrical machine. For security, however,
a key must be suspended by a wire from that
which is coiled up, so as to touch a half-crown, or
a plate of metal lying on the ground. If the key
is then lifted a little from the plate, a stream of
fire will be seen proceeding from the key to the
plate; but if a sensation like a cobweb on the
face takes place, it will be prudent to throw down
the glass bar, and leave the kite to itself*. Elec-
tricity may be again attracted from the atmo-
sphere, if a long wire screwed into the knob of a
Leyden bottle, and pointed at the extremity, is
held aloft in the air ; and if this experiment is
made in the night-time, when thunder and light-
ning are near, a star will appear at the point of
the wire, and if the bottle is touched with the
other hand, a shock will be received. A man also
standing upon a glass stool, and holding in his
hand a fishing-rod coated with tin-foil, or any
long metal instrument, aloft in the air., will gene-
rally be more or less charged with electricity, in
proportion to the state of the atmosphere, and

* Professor Richmann, of Prtersbnrgh, in consequence
of disregarding the due precautions, was killed while he
was conducting the experiment of drawing electricity from
a thunder cloud.

Galvanism. 117

sparks may be drawn from his body as if he had
been electrified in the usual manner.

Thunder storms in this country are seldom
attended with fatal effects, yet it is desirable to
be made aware of their approach. They are ge-
nerally observed to happen when there is little
or no wind, and are preceded by one dense cloud
or more, increasing very rapidly in size, and
rising into the higher regions of the air. The
lower surface is black and nearly level, the upper
parts are arched and well defined; sometimes
many of them appear piled one upon another,
all arched in the same manner. At the time
this cloud rises, the air is generally full of small
separate clouds, motionless, and of whimsical
shapes. These gradually are drawn towards the
thunder cloud, and when they come near it their
limbs mutually stretch towards each other, and
then coalesce. Sometimes, however, the thunder
cloud swells and enlarges without the addition of
these clouds, from its attracting the vapours of
the atmosphere, wherever it passes. When the
thunder cloud is grown to a great size, the lower
surface becomes rugged, parts being detached
towards the earth, but still connected with the
rest. About this time also it seems to sink lower,
and a number of small clouds are driven about
under it, in very uncertain directions. It is while
these clouds are most agitated that the rain or
hail falls in the greatest abundance.

While the thunder cloud is swelling, and ex-

118 Experimental Philosophy. [Lecture 9.

tending its branches over a large tract of country,
the lightning is seen to dart from one part of it
to another, and often to illuminate its whole
mass. When the cloud has acquired sufficient
extent, the lightning strikes between it and the
earth in two opposite places. As the lightning con-
tinues, the cloud dilates, till at length it breaks
in different places, and displays a clear sky.

The clouds, however, are sometimes nega-
tively electrified with respect to the earth, and in
this case the lightning is supposed to proceed
from the earth to the cloud ; but the mischievous
effects are the same, and, in fact, there is reason
to think that this is a rare case.

During a thunder storm the safest place is in a
cellar; for when a person is below the surface
of the earth, the lightning must strike it before it
can reach him, and its force will therefore pro-
bably be expended on it. When it is not possi-
ble to retreat to a cellar, the best situation is in
the middle of a room, not under a metal chande-
lier, or any other conducting surface ; and it is
adviseable to sit on one chair, and to lay the feet
up on another; or it would be still better to
lay two or three beds or mattresses, one upon
another, in the middle of the room, and place
the chairs upon them, the matters (viz. hair and
feathers) with which they are stuffed being non-
conductors. Persons in fields should prefer the
open parts to any shelter under the trees, &c.
The distance of a thunder cloud, and conse-

Galvanism. 119

quently the degree of danger, is not, however,
difficult to be estimated. As light travels at the
rate of seventy-two thousand four hundred and
twenty leagues in a second of time, its effects
may be considered as instantaneous within any
moderate distance ; but sound, on the contrary,
is transmitted only at the rate of three hundred
and eighty yards in a second. By accurately ob-
serving the time, therefore, which intervenes be-
tween the flash, and the noise of thunder which
succeeds it, a very near calculation may be made
of its distance. Or, the distance may be very well
estimated by means of the pulsations in the wrist,
allowing five and a half to a mile ; and in the same
proportion for any other number of pulsations in
the interval between the flash and the thunder.

The discovery of Dr. Franklin, which ascer-
tained the identity of lightning and the electric
fluid, suggested to the same philosopher the
means of preserving buildings from lightning, by
means of metallic conductors attached to the out-
side of high buildings. As these are now com-
mon, it is unnecessary to describe them. The
principle on which they are constructed rests on
the well-known fact of metallic bodies being better
conductors of the electrical fluid than any others.
The conducting rod is pointed at the top, in order
the more gradually to attract the electricity from
the clouds and the atmosphere ; and the upper
part should be made of copper, to prevent its
rusting, and the remainder painted. The con-

120 Experimental Philosophy. [Lecture 9.

ducting rod should not be too slender, and should
extend in the earth beyond the building, to con-
vey the electric matter clearly away ; and if it
terminates in a pool of water, which is one of the
best conductors, it will be still safer.

I shall conclude this lecture by a short view of
that branch of science (for such it is now uni-
versally allowed to be) which has been termed
GALVANISM, or VOLTAISM.

It was long known that common electricity
could excite a tremulous or convulsive motion in
dead animals; but about the year 1791 it was
discovered that these effects could be produced
without the aid of an electrical apparatus, and
apparently by different means, and hence they
were at first ascribed to a different power in
nature.

This discovery, like some others of importance
in philosophy, was the effect partly of accident.
Dr. Galvani (whence the term Galvanism), pro-
fessor of anatomy at Bologna, having observed
certain involuntary motions or contractions in
the muscles of some dead frogs, which had been
hooked by the back-bone and suspended from
the iron palisades of his garden, was induced to
examine more minutely into the cause of these
motions; and he found that he could produce
them at pleasure, by touching the lifeless animal
with two different metals, provided the metals
were, at the same time, in contact with each other.
From latter observations it appears that these

Galvanism. 121

contractions may be excited by one metal, as-
sisted by other substances, or even without any
metal whatever. The metals, however, are the
most certain agents, but they will produce no
effect without the intervention of some fluid which
has a chemical action on one or both of the
metals.

The experiment may be tried upon any animal
recently dead; but what are called the cold-
blooded animals, that is, those which have their
blood of a temperature not higher than that of
the atmosphere, such as reptiles and fishes," retain
this sensibility much longer than others ; dead
frogs for instance will retain it for several hours,
and sometimes for a day or two.

To give the experiment proper effect some
preparation is however requisite; and as the gal-
vanic influence acts principally on the nerves, it
is necessary that they should be exposed to one
of the metals: it is made most successfully on
the hind legs of a dead frog. — To this end we
have only to cut them off with a small bit of the
spine attached to the nerves of the thigh, as in
plate X. fig. 41, where GH are the lower limbs,
thus adhering to a small piece of the spine AB, by
means of the crural nerves CD. The legs must
be skinned in order to lay bare the muscles, and
a small piece of tin-foil wrapped round the spine
A, B. If we then hold one of the legs in our
fingers, and let the other be suspended with the
bundle of nerves and spine hanging upon it, and

VOL. i. G

Experimental Philosophy. [Lecture 9-

then interpose a piece of silver, as half-a-crown,
between the lower thigh and the nerves, so that
it may touch the former with one surface, and
the tin-foil which is wrapped round the spine
with the other, we shall find the lower leg con-
vulsively agitated, so as even sometimes to strike
against the hand which holds the other.

Living animals, when thus placed between two
different metals which touch each other, will also
be convulsively agitated. Or you may make the
experiment upon yourselves in a very innocent
way, so that the taste and even the sight may be
affected by it. Take, for instance, a piece of
metal (zinc is the best), and lay it on^your
tongue, and another piece of metal, as a shilling
or half-crown under it, make the edges of the
two metals touch, and you will immediately ex-
perience a kind of irritation and a taste like cop-
per in your mouth. If, again, in a dark place
one of the metals is applied to your eye and the
other up your nostril or in your mouth, upon
bringing the metals in contact a faint flash of
white light will appear before your eyes. Nay
the same effect will be produced, and the light
will still appear, if one of the pieces of metal is
put up your nostril, and the other upon the
tongue ; or even if one is put between the upper
lip and the gums, and the other on the tongue ;
only remarking that the metals must be different
— silver and zinc are the best for the purpose.

These experiments have served to explain

Galvanism.

many facts which were well known, but the rea-
son of which was not before discovered. It had
been long observed, that porter and ma t liquors
have a different and a pleasanter taste when drunk
out of metal than out of glass or earthenware ;
and on the contrary that water out of a metallic
cup has a disagreeable and metallic taste ; these
effects are now known to be owing to a slight
galvanic shock, such as is experienced by placing
the tongue between two metals in contact.

Mixtures of metals have been long known to
corrode each other, while pure metals have re-
mained unchanged ; — thus the Etruscan inscrip-
tions engraven on pure lead are preserved to
this time, while medals of lead and tin of no great
antiquity are much defaced. The copper sheath-
ings of vessels when fixed on with iron nails be-
come very soon corroded ; and I believe it is now
customary to fix them to the bottoms with copper
nails. These effects are owing to the action of
the metals on each other, or rather on the mois-
ture which is interposed, which, being decom-
posed by the action of the metals, is separated
into its constituent parts (oxygen and hydrogen),
and one or both of the metals become oxidated,
rusted, or corroded.

The conductors of electricity are also con-
ductors of galvanism : — these are divided into two
classes; the .dry, such as metallic substances and
charcoal 5 and the wet, as water and certain other
fluids.

G2

124 Experimental Philosophy. [Lecture 9-

The galvanic influence cannot be powerfully
excited without a combination of three con-
ductors, two of one class and one of another.
When two of the three bodies are of the first class
(as two metals, zinc and silver, or zinc and copper
with water or an acid), the combination is said to
be of the first order. But it is an indispensable
requisite that one of the three conductors should
have a chemical action on one or both the others :
thus water, as containing oxygen, has an action
on the metals ; if it is impregnated with oxygen
gas its action is increased, and much more power-
ful than that of water deprived of air by boiling ;
and if a small quantity of any of the mineral acids
is added, the effect will be still greater. Thus
the agitation ore xcitement occasioned by the
action of an acid principle is the source of gal-
vanism, as the excitement occasioned by friction
is of electricity.

Yet it will appear by an easy experiment that
the galvanic influence has a powerful agency in
directing and increasing this chemical action.
A glass tube (fig. 42.) about 4 inches long has its
extremities completely stopped by two corks, A,
and B. An oblong piece of zinc, CD, is thrust
through one of the corks, and projects within and
without the tube. In the other cork is fixed a
silver wire projecting with the extremity F, within
the tube, while its other extremity is bent so as to
come near the projecting part of the zinc C. If
then the tube between the corks is filled with

Galvanism. 125

water impregnated with a small quantity of mu-
riatic acid, the zinc will be immediately acted
upon by the diluted acid, and bubbles of gas will
be seen to proceed from it, but the silver wire EF
remains untouched. If then you bend the silver
wire FG so that its end may touch the zinc at C,
you will find not only that the fluid acts more
strongly upon the zinc at D, but that the silver
at F is also strongly acted upon, as appears by
the evolution of gas, &c. This is what is called
a galvanic circle, and this circle is completed, in
the technical language of this science, by bringing
the silver wire in contact with the zinc at C.

The effects from simple galvanic circles, and
the analogy between the phaenomena of galvanism
with these of electricity, suggested the idea of
extending the combinations, and forming what
are now called galvanic batteries. The first and
simplest of these were formed of round pieces of
zinc and silver with pieces of cloth or leather
rather smaller, and moistened with water or diluted
acid, interposed in the manner of fig. 43, where
the silver, zinc, and wet cloth are marked by the
letters S, Z, W. This was at first called the gal-
vanic pile, from its form.

The most convenient form for a galvanic bat-
tery, however, was soon afterwards found to be
that represented in fig. 44. It consists of an ob-
long vessel or trough of baked wood of different
sizes, according to the strength of the intended
battery. In the sides of the trough there are

126 Experimental Philosophy. [Lecture 9-

grooves, in each of which are placed a double
metallic plate, commonly of zinc and copper sol-
dered together, thus dividing the whole of the
trough into a number of distinct cells, so cemented
that no fluid can pass from one to another. The
cells are afterwards filled with water (to which at
present a small quantity of nitric or muriatic acid
is added to increase its action on the surfaces of
the two metals thus presented to it in each cell) :
two or more of these batteries may be joined by
connecting them with a piece of wire.

If, when the battery is thus charged and the
diluted acid begins to act, you apply a finger of
each hand (a little moistened, the better to con-
duct the electricity) to each extremity of the
trough, a shock will be felt such as that com-
municated by a Leyden phial, in proportion to
the extent of the battery. The mode of apply-
ing its power to other purposes is as follows:
ACDEF is a wire which communicates with the
last plate of the battery at A. BKIGH is an-
other wire which communicates with the last plate
at B. DEHI are two glass tubes through which
these wires pass to enable the operator to direct
the ends of the wires without drawing off the
electricity. If a thin metallic body, as gold or
silver leaf, or tin-foil, is placed between the ends
or extremities of the wires, it will be melted; gun-
powder will be exploded, or combustible bodies
will be set on fire; the muscles or limbs of dead
animals will also be convulsively agitated.

Galvanism. 127

To prove that the agency of electricity and
galvanism is essentially the same, it is only neces-
sary to mention that a common coated jar, or
even an electrical battery, may be almost instan-
taneously charged from a galvanic battery. It is
however to be remarked that the electrical virtue
seems to be more diffused, but more permanent,
in a galvanic, and more concentrated in a com-
mon electrical battery.

The electrical energy is not confined to the
substances we have already specified. In the
mineral kingdom, the tourmalin, a stone found in
the East Indies, by being merely heated, exhibits
most of the electrical phaenomena.

In the animal kingdom it has long been known
that rubbing the back of a cat will produce
sparks in the dark. But however this effect may
be deemed superficial, and attributed to the hair,
there are some other animals which have this
virtue more extensive and more powerful. The
torpedo, a kind of ray, communicates a strong
shock when touched, and the shock is greatly in-
creased by touching it with both hands, and thus
completing the circle. The gymnotus, or elec-
trical ee^ found in the rivers of Guiana, pos-
sesses the same power, but in a superior degree.
It seems also to depend on the will of the animal.
The electric organs both in this and the torpedo,
each of which is furnished with a pair, bear a
strong resemblance to the galvanic trough or
battery.

128 Experimental Philosophy. [Lecture 9-

In point of theory, galvanism is as much
afloat as either magnetism or common electricity.
Three different theories of the galvanic battery
have been proposed. 1 . That the galvanic pile is
entirely electrical. 2. That it is altogether che-
mical 3. That electricity produces the phaeno-
mena, but is, itself, evolved by chemical action.
The first of these theories was advanced by
Volta; the second by Donovan; the third by
Wollaston, and defended by Dr. Bostock.

It has been ascertained by unequivocal expe-
riments that the galvanic pile never acts unless
when one of the metals which compose it has
been oxydized ; and that its energy only conti-
nues as long as the oxydizing process goes on :
hence Volta's theory is evidently imperfect.

The most cursory attention to the galvanic
pile will suffice to demonstrate that it never acts
except the circle be completed ; that is, unless
there be a current of electricity : and this seems
to set aside Donovan's theory. Whence it would
seem to follow, that both chemical decomposi-
tions and a current of electricity are necessary to
constitute the galvanic pile. They who wish
farther to investigate this curious subject may
advantageously consult Dr. Bostock's History of
Galvanism.

LECTURE X.

EXPERIMENTAL PHILOSOPHY.

LIGHT.

IN considering the nature of light, a difficulty
presents itself similar to that which occurred with
respect to the electrical fluid. Some philosophers
have been disposed to consider the matter of light
as essentially different from elementary fire, while
others have regarded them as intrinsically the
same matter, only exhibited in different states.
A late writer on these subjects conjectures that
light is diluted fire, that is, fire weakened and
diffused as spirits when mingled with water ; and
another terms it fire in a projectile state, that is,
its particles are separately projected, and, in
truth, at an immense distance from each other,
whereas in culinary fire it is collected and con-
densed. It is a circumstance which not a little
favours this latter opinion, that light may be col-
lected and condensed by what is called a burning-
glass, so as to burn like the fiercest flame. On the
contrary, flame itself may be so diluted or diffused
as to be perfectly innoxious. " The flame,*" says
Dr. Goldsmith, ei which hangs over burning
spirit of wine, we all know to scorch with great
power; yet these flames may be made to shine
as bright as ever,yet be perfectly harmless. This

ISO Experimental Philosophy. [Lecture 10.

is done by placing them over a gentle fire, and
leaving them thus to evaporate in a close room
without a chimney: if a person should soon after
enter with a candle, he will find the whole room
filled with innoxious flames. The parts have
been too minutely separated, and the fluid, per-
haps, has not force enough to send forth its
burning rays with sufficient effect."

It is not, however, my intention in these lectures
to involve you in the intricacies of theory, or to
pursue speculative inquiries at the expense of
useful facts. It will be more profitable to detail
and explain the properties of light than to waste
our time in conjectures on its essence. The most
remarkable properties of light, then, are, first,
itsveloci/y; secondly, its rarity; thirdly, its force
or momentum; fourthly, the property of being
always detached in straight lines ; fifthly, refrac-
tion; and, sixthly, the reflection of light.

I. The velocity of light is such as may well
astonish the inexperienced student, when he is
told that in the very short space of a moment, or
a second of time, a ray of light travels the im-
mense extent of one hundred and seventy thousand
miles. The manner in wliich the velocity of
light is calculated is not less ingenious than the
discovery is surprising. It was by observing the
eclipses of Jupiter's satellites, and it will be
amusing to you to observe the process by which
the calculation is accomplished. When the earth,
in going its annual revolution round the sun, is

Light. 131

at C (plate XI. fig. 45), an eclipse is observed of
one of the satellites of Jupiter, which thus re-
gularly suffers eclipses, at intervals of about forty-
two hours and a half. If the earth never left C,
but continued there immbveable, we should re-
gularly see the satellite eclipsed at the expected
interval of forty-two hours and a half; and also
in thirty times that number the spectator would
see thirty eclipses. But the earth is not fixed ;
let us, then, farther suppose that the earth in
moving through half its orbit from C, the place
of conjunction, has just placed itself in opposition,
near D, that is, where it would be situated be-
hind the sun relatively to Jupiter. If light had
no progressive motion, a spectator on our globe
would see the first satellite of Jupiter emerge
from the shadow after a period equal to as many
times 4>2f hours, as there would be eclipses
after the moment of conjunction. But this does
not happen : for the spectator at D sees the ter-
mination of the eclipse about sixteen minutes
later than the calculation predicts ; so that, in
all the intermediate positions between C and D,
the difference as far as this limit has been con-
tinually increasing. Now C D, the rectilinear
distance between these two positions, is equal
to the diameter of the earth's orbit, that is,
to about 190 millions of English miles. This
space, therefore, is passed over by light in 16
minutes; so that, assuming it to move uniformly,
we find, by an easy proportion, the space passed

Experimental Philosophy. [Lecture 10.

over by light in a second to agree with what we
have just stated. This discovery we owe to
Roemer, a Danish astronomer, and it is extremely
interesting and important.

Such, then, is the rapidity with which these rays
are darted forward, that the journey they per-
form thus in less than eight minutes, a ball from
the mouth of a cannon would not complete it
in several weeks. But here it may be said, If
the velocity of light is so very great, how is it
that it does not strike against objects with a mon-
strous force? If the finest sand (the objector
may continue to observe) was thrown against our
bodies with the hundredth part of this velocity,
each grain would be as fatal as the stab of a
stiletto : How then is it, that we expose, without
pain, not only other parts of our bodies to the in-
cursions of light, but our eyes, which are a part
so exquisitely sensible of every impression ? To
answer this objection, experiment will inform us,
that the minuteness of the parts of light is still
several degrees beyond their velocity ; and they
are therefore harmless, because so very small.
A ray of light is nothing more than a constant
stream of minute parts still flowing from the
luminary, so inconceivably little, that a candle,
in a single second of time, has been said to dif-
fuse several millions of particles of light. The
sun furnishes them, and the stars also, without
appearing in the least to consume by granting us
the supply. Musk, while it diffuses its odour.

Light. 133

wastes as it perfumes us; but the sun's light
is diffused in a wide sphere, and seems inex-
haustible.

That the motion of light is inexpressibly rapid
you may easily convince yourselves, by only
giving attention to the firing of a cannon at a con-
siderable distance, and observing the time that
elapses between your seeing the flash and hearing
the sound. It has been calculated from some very
accurate experiments, that sound travels at the
rate of one thousand one hundred and forty-two
feet, or three hundred and eighty yards, in a
second of time ; and if you remark, as was before
observed, the time which intervenes between your
seeing the flash and hearing the noise of the
cannon, you will soon perceive how infinitely
more rapid light must be in its motions than
sound.

II. It is a principle in mechanics, that the force
with which all moving bodies strike is conjointly
in proportion to the size of those bodies, or the
quantity of matter which they contain and the
velocity with which they move. Now if we con-
sider the amazing velocity of light, it is evident,
that if the separate particles of it were not in-
finitely smaller than we can conceive, they would
be destructive in the highest degree. To illus-
trate this by a plain examplej: A few grains of
shot, fired out of a musket or fowling-piece, will
deprive a large animal, or even a man, of life.
How is this? If the shot were thrown by the hand,

134 Experimental Philosophy. [Lecture 10.

it would hurt neither the man nor the animal.
It is the velocity, the swiftness, with which it is
impelled by the force of the powder, that enables
it to penetrate solid substances. Now it has been
demonstrated that light moves at least two millions
of times faster than a cannon-ball; and conse-
quently if the particles of light were only equal
in size to the two millionth part of a grain of sand,
we should be no more able to withstand their
force than we should that of sand shot point
blank from the mouth of a cannon. How in-
finitely small must these then be, when it is more
than probable they are not equal to a twentieth
that size, that is, not equal to theforty millionth
part of a grain of sand ! What an idea does this
give us of the works of our infinite Creator, and
how little must we seem in our own eyes ! O Phi-
losophy, how is it that thou dost not always teach
mankind humility !

But we have other proofs not less decisive than
this, of the extreme minuteness of the particles
of light. When we observe with what facility
they penetrate the hardest bodies, glass, crystal,
precious stones, and even the diamond itself,
through all which they find an easy passage, or
those bodies could not be transparent, How ex-
tremely small must these particles be ! When a
candle is lighted, if there is no obstacle to ob-
struct its rays, it will fill a space of two miles
round with luminous particles in an instant of
time, and before the least sensible part of the

Ligttt. 135

substance is lost by the luminous body. If the
whole space were filled with men, every eye would
see the candle the moment it was posited in a
visible situation. Farther, how small must the
particles of light be, when they pass without re-
moving the minutest particles of microscopic dust
that lie in their way, and even these minute par-
ticles are rendered visible, by reflecting back the
particles of light that strike against them !

Small as the particles of light are, it is highly
probable that, though diffused through all space,
they are separated from each other by distances
of much more than a thousand miles. This may
be inferred as follows : It is an obvious fact, that
the effect of light upon our eyes is not instantane-
ous, but that the impression remains for some
time. You may easily satisfy yourselves of this,
by shutting your eyes after having looked for
some time on a candle, a star, or any other lumi-
nous body, when you will perceive that a faint
picture will remain of the object for some time.
The smallest division of time, that we can well
conceive, will be the one hundred and fiftieth
part of a second. If, therefore, one lucid part of
the sun's surface emits one hundred and fifty par-
ticles of light in a second of time, we may con-
ceive that these will be amply sufficient to afford
light to the eye without any intermission. You
will remember, then, that light travels at the rate
of about one hundred and seventy thousand miles
in a second ; so that, the sun emitting one hun-

136 Experimental Philosophy. [Lecture 10.

dred and fifty particles in that space of time, each
particle must be more than one thousand miles
distant from the other*. Indeed it is reasonable
to suppose that they must be at great distances
asunder, or they could not pass so continually as
they do in all directions, without interfering with
each other.

If, in fact, light were not thus thinly diffused
it must be extremely injurious to our organs,
since we find that when it is condensed or com-
pressed, as in the focus of a burning-glass, there
is no substance that can withstand its force.
Gold, when exposed to its influence, is instantly
melted, and even the diamond itself, which re-
sists a very intense chemical heat, is suddenly
dissolved. To show, however, still more de-
cisively, that the particles of light are naturally in
this extremely rare or diffused state, or, in other
words, follow each other at an immense dis-
tance, it is a well-known fact, that the rays of
light, even when collected in the focus of the
strongest burning-glass, will not inflame spirit of
wine, or any other combustible matter, while they
merely pass through it. To make you com-

* This is, in truth, quite an extreme estimate. It ap-
pears from the accurate experiments of M. D'Arcy (Mem.
Acad. Par. 1?65), that the impression of light upon the
retina continues two minutes and forty seconds : and as a
particle of light would move thirty-two millions of miles
during that interval, constant vision would be maintained
by a succession of luminous panicles, thirty-two millions
of miles distant from each other.

Light. 137

prebend this fact more clearly, I must observe,
that whatever light passes through is called a me-
diiim, and those substances which do not reflect
the rays, but which may be seen through, are
called transparent ; those, on the contrary, which
intercept or reflect the rays, are called opaque.
Now a phial in which spirit of wine is contained
is a transparent medium, and in that state the
spirit will not be set on fire : if, on the other hand,
the spirit is poured forth into a spoon, or any
opaque vessel, which, in fact, intercepts the rays
of light, stops them in their progress, and thus
collects them in a mass, it will immediately be
inflamed. This, I think, proves, that the par-
ticles of light must follow each other at a great
distance, and that they must be in the first place
compressed together by the force of the burning-
glass, and then stopped and condensed by an
opaque body, to enable them to produce a consi-
derable degree of heat.

That light may be exceedingly diluted, as well
as condensed, we may easily perceive ; for the
light of the glow-worm, of rotten wood, and of
what are called the solar pJwsphori, can never be
condensed by any burning-glass, so as to pro-
duce the slightest degree of heat. The expe-
riment has also been made with the light of the
moon, and that has been found too faint and
rare to be condensed into a burning focus.

The principle upon which the rays of light are
collected in the focus of a burning-glass NY ill be

138 Experimental Philosophy. [Lecture 10.

explained hereafter, when we treat of lenses,
and of mirrors. But I do not wish to pass over
any thing that I mention, without an attempt to
render it clear to your comprehension. I men-
tioned the solar phosphor i , of which it is pro-
bable that very few of you have heard before.
They are certain substances which, when ex-
posed for a little time to the strong rays of the
sun, are found to imbibe a large quantity of light,
so that they will shine, or appear luminous, if
immediately carried into a dark place. The
most remarkable of these is the Bolognian phos-
phorus. It was accidentally discovered by a
shoemaker of Bologna. This man had collected
together some stones of a shining appearance at
the bottom of Mount Peterus, and being in
quest of some chemical secret (probably the
philosopher's stone, which was to turn every
thing into gold), he put them into a crucible to
calcine them, or reduce them to the state of a
cinder. Having taken them out of the crucible,
they were exposed to the light while he was
examining them, and afterwards he happened to
carry them into a dark place, probably to throw
them away; when, to his utter surprise, he
observed that they possessed a self-illuminating
power. Baldwin, of Misnia, another chemist,
observed some time after, that chalk, dissolved
in aqua fortis (after the aqua fortis had been
evaporated by heat, and the matter reduced to a
perfectly dry state), exactly resembled the Bo-

Light. 139

lognian stone in its property of imbibing light,
and emitting it after it was brought into the
dark, whence it has been termed Baldwin's
phosphorus. In truth, .the same effect may be
produced from calcined oyster-shells, and from
all the varieties of that mineral called ponderous
spar, of which the Bolognian phosphorus is a
species. Diamonds ateo, and some emeralds,
and other precious stones, will emit light when
carried out of^a light into a dark place. The
light emitted by these phosphor! always bears
an analogy to that which they have imbibed. In
general it is reddish ; but when a weak light only
has been admitted to them, or when it has been
received through white paper, the light which
they give out is pale or whitish.

III. Notwithstanding the rarity of light, how-
ever, and the smallness of its particles, it is not
destitute of force or momentum. To prove this,
a most ingenious experiment was made by the
late Mr. Mitchell. He constructed a small vane
in the form of a common weathercock, of a very
thin plate of copper, about an inch square, and
attached to one of the finest harpsichord wires,
about ten inches long, and nicely balanced at
the other end of the wire by a grain of very
small shot. The vane was supported in the
manner of the needle in the common mariner's
compass, so that it could turn with the greatest
ease; and to prevent its being affected by the
vibrations of the air, it was enclosed in a glass

140 Experimental Philosophy. [Lecture 10.

case, or box. The rays of the sun were thrown
upon the broad part of the vane, or copper plate,
by a burning-glass of two feet diameter, in con-
sequence of which it was observed to move re-
gularly at the rate of about one inch in a second
of time. Upon this experiment a very curious
calculation is founded. The instrument or vane
weighed about ten grains, and the velocity with
which it moved was at the rate of one inch in a
second. The quantity of matter therefore con-
tained in the rays of light which struck against
the vane in that time amounted to about the
twelve hundred millionth part of a grain: the
velocity of light exceeding the velocity of the
instrument in about that proportion. The light
in this experiment was collected from a surface
of about three square feet, and as it was from a
concave mirror *, only half the quantity was re-
flected. The quantity of light therefore incident
upon a square foot and half of surface is no
more than one twelve hundred millionth part of
a grain. But the density of the rays of light at
the surface of the sun is greater than at the
earth, in the proportion of forty-five thousand
to one. From one square foot of the sun's sur-
face, therefore, there ought to issue, in the space
of one second, one forty thousandth part of a
grain of light to supply the consumption. More
than two grains a day therefore is, according to

* Mirrors or looking-glasses reflect about half the light
that fulls on them perpendicularly.

Light. 141

this hypothetical computation, expended from the
sun's surface, or six hundred and seventy pounds
in six thousand years, which would have shortened
his diameter about ten feet, if it were formed
of matter of the density of water only. From
all this you will conclude that I have adopted
the common theory, that the sun is the great
source of light ; and if his diameter is rightly
calculated (of which there can be no doubt) at
eight hundred and seventy-eight thousand eight
hundred and eight miles, we see there is no
ground for any apprehensions that the sun will
speedily be exhausted by the waste or consump
tion of light. The matter will not be widely dif
ferent, if we imagine, as is now generally believed,
that the particles of light are emitted from a
luminous atmosphere which surrounds the body
of the sun.

IV. Another principle to which I proposed to
call your attention is, that light always moves in
straight lines. This is evident from an experi-
ment which any person may easily make, viz.
that of looking through a bent tube, when no
light whatever will be apparent. As a further
proof it is only necessary to mention, that when
light is intercepted by any intervening body, the
shadow is bounded by straight lines.

It is generally supposed, according to this
principle, that those bodies only are transparent
whose pores are such as to permit the rays of
light to pervade them in a rectilinear direction ;

Experimental Philosophy. [Lecture 10.

and they act like a straight tube, which allows
them a free passage ; and those bodies are opake
whose pores are not straight, and which there-
fore intercept the rays, like ,the bent tube already
mentioned.

If the rays of light proceed in straight lines, it
is obvious that they must be sent from every
visible object in all directions. It is however
only by those rays which enter the pupil of our
eye that they are rendered visible to us ; but,
being sent in all directions, it is evident that
some rays from every part must reach the eye-
Thus the object ABC (pi. XI. fig. 46) is rendered
visible to an eye in any part, where the rays Aa,
Ab, Ac, Ad, Ae, Ba, Bb, Be, Bd, Be, Ca, Cb,
Cc, Cd, Ce, can come ; and these affect our
sight with the sense of different colours and
shades, according to the properties of the body
from which the light is reflected, as will be ex-
plained when we come to treat of colours.

Of the refraction and refaction of light I shall
hereafter treat more at large ; but, ip the mean
time, it will greatly facilitate the study of optics,
if you will carefully peruse, and still more if you
will commit to memory, the following principles
and definitions.

1. Light is a substance, the particles of which
are extremely minute, which, by striking on our
visual organs, gives us the sensation of seeing.

2. The particles of light are emitted from what
are called luminous bodies, such as the sun, a

Light. 143

fire, a torch, or candle, &c. &c.: they are re-
flected or sent back by what are termed opdke
bodies, or those which have no power of affording
light in themselves.

3. Light, whether emitted or reflected, always
moves in straight or direct lines ; as may easily
be proved by looking into a bent tube, which
evidently obstructs the progress of the light in
direct lines ; and proves that the theory of recti-
linear emission is free from the objections which
lie against the hypothesis of the undulatory mo-
tion of light.

4. By a ray of light is usually meant the least
particle of light that can be either intercepted or
separated from the rest. A beam of light is ge-
nerally used to express something of an aggregate
or mass of light greater than a single ray.

5. Parallel rays are such as proceed equally
distant from each other through their whole
course. The distance of the sun from the earth
is so immense, that rays proceeding from the
body of that luminary are generally regarded as
parallel.

6. Converging rays are such as, proceeding
from any body, approach nearer and nearer to
each other, and tend to unite in a point. The
form of rays thus tending to a union in a single
point has been compared to that of a candle-ex-
tinguisher ; it is in fact a perfect cone.

7. Diverging rays are those which, proceed-

144 Experimental Philosophy. [Lecture 10.

ing from a point, continue to recede from each
other, and exhibit the form of an inverted cone.

8. A small object, or a small single point of
an object, from which rays of light diverge or
indeed proceed in any direction, is sometimes
called the radiant, or radiant point.

9. Any parcel of rays, diverging from a point,
considered as separate from the rest, is called a
pencil of rays.

10. rFhe focus of rays is that point to which
converging rays tend, and in which they unite
and intersect or cross each other. It may be
considered as the apex or point of the cone ; and
it is called the focus (or fire-place), because it is
the point at which burning-glasses burn most
intensely.

11. The virtual or imaginary focus is that
supposed point behind a mirror or looking-glass,
where the rays would have naturally united, had
they not been intercepted by the mirror.

12. Plane mirrors or speculum? are those re-
flecting bodies, the surfaces of which are per-
fectly plain or even, such as our common look-
ing-glasses. Convex and concave mirrors are
those the surfaces of which are curved.

13. An incident ray is that which comes from
any body to the reflecting surface ; the reflected
ray is that which is sent back or reflected.

14. The angle of incidence is the angle which
is formed by the line which the incident ray

glit. 145

describes in its progress, and a line drawn per-
pendicularly to the reflecting surface; and the
angle of reflection is the angle formed by the
same perpendicular and the reflected ray. Thus,
in fig. 47, AB is the reflecting surface, CG is
a line drawn perpendicularly to that surface, e is
a ray of light incident at G, and reflected tof;
and the angle CGe of incidence is evidently equal
to the angle CGjfof reflection.

15. By a medium, opticians mean any thing
which is transparent, such as void space, air,
water, or glass> through which consequently the
rays of light either may or do pass in straight
lines.

16. The refraction of the rays of light is their
being bent, or attracted out of their course in
passing obliquely from one medium to another
of a different density, and which causes objects
to appear broken or distorted when part of them
is seen in a different medium. It is from this
property of light that a stick, or an oar, which is
partly immersed in water, appears broken.

17. A lens is a transparent body of a different
density from the surrounding medium, com-
monly of glass, and used by opticians to collect
or disperse the rays of light. Lenses are in gene-
ral either convex, that is, thicker in the middle
than at the edges, which collect and, by the force
of refraction,, converge the rays, and consequent-
ly magnify; or concave, that is, thinner in the
middle than at the edges, which by the refrac-

VOL. i. H

146 Experimental Philosophy. [Lecture 10.

tion disperse the rays of light, and diminish the
objects that are seen through them. The va-
rieties of these will be described in a subsequent
lecture.

18. Vision is performed by a contrivance of
this kind. The crystalline humour, which is
seated in the fore part of the human eye, imme-
diately behind the pupil, is a perfect convex lens.
As therefore every object is rendered visible by
beams or pencils of light which proceed or di-
verge from every radiant point of the object,
the crystalline lens collects all these divergent
rays, and causes them to converge on the back
part of the eye, where the retina or optic nerve
is spread out ; and the points where each pencil
of rays is made to converge on the retina, are
exactly correspondent to the points of the object
from which they proceed. As, however, from
the great degree of convergence which this con-
trivance will produce, the pencils of light pro-
ceeding from the extreme points of the object
will be made to cross each other before they
reach the retina, the image on the retina is always
inverted.

19. The magnitude of the image painted on
the retina will, therefore, it is evident, depend on
the greatness or obtuseness of the angle under
which the rays proceeding from the extreme
points of the object enter the eye. For it is
plain, that the more open or obtuse the angle is,
the greater is the tendency of these rays to meet

Light. 147

in a point and cross each other: and the sooner
they cross each other, after passing the crystal-
line lens, the larger will be the inverted image
painted on the retina. The visual angle, there-
fore, is that which is made by two right lines
drawn from the extreme points of any object to
the eye ; and on the measure of that angle the
apparent magnitude of every visible object will
depend/

20. The prism used by opticians is a piece of
fine glass, in form of, a geometrical triangular
prism ; it has the power of separating the rays of
light.

LECTURE XI.

EXPERIMENTAL PHILOSOPHY.

a

THE REFRANGIBIL1TY OT LIGHT.

THE natural progress of light, we have already
seen, is in straight lines; yet it is found to be
subject to the laws of attraction, as well as all
other bodies; and, under the impulse of that
power, it is sometimes turned out of its direct
course. This only happens when it passes out
of one medium into another of a different den-
sity, as from air into water or glass, or from
water or glass into air; and this property of
light is called refraction. A very easy experi-
ment will show you what is meant by refraction ;
for. if you put one end of a straight stick into
water, it will appear at the surface as if it were
broken, that is, refracted, from the Latin verb
refrangv, to break.

It is evident that this effect can only arise from
the rays of light being drawn or attracted out of
their direct course ; and this I shall prove by a
very common and a very easy experiment. Put
a shilling, or any other conspicuous but small
object, into a bason or other vessel, and then re-
tire to such a distance, as that the edge of the
vessel shall just hide it from your sight. If, then,
you remain motionless while the vessel is filled with

Rtfrangibility of Light. 149

water, you will find that the shilling will be ren-
dered perfectly visible, though in fact neither you
nor it have changed places in the slightest degree.
Let it be remembered, that it is only the rays
which fall obliquely that are thus refracted; for a
ray which falls perpendicularly is equally attract-
ed on all sides, and therefore suffers no refrac-
tion at all. To illustrate this by the experiment
which has just been mentioned. You must know
that it is by light reflected from it to your eye
that any object is rendered visible : you see the
shilling in the bason, therefore, by rays of light
which are reflected from its surface. Now the
angle of incidence and the angle of reflection are
equal ; and as you stand in an oblique direction
to the shilling, you see it, while the bason is
empty, by rays of light which fall upon it in a
direction exactly as oblique as that in which your
eye is situated towards it. The shilling, then,
which before was hid from your sight, is ren-
dered visible by pouring in the water, because the
rays of light, which serve to render it then visi-
ble, are bent out of their course. Thus the ray
of light, AC, pi. XII. (fig. 48), which passes ob-
liquely from the air into water at C, instead of
continuing its course to B, takes the direction
€4, .and consequently an object at a would be
Tendered visible by rays proceeding in that direc-
tion, when they would not have touched it, had
.they proceeded in their direct course,

150 Experimental Philosophy. [Lecture 11.

By this figure you will understand that the
angle of refraction PCa is not so large as the
angle of incidence pCA, but bears a certain pro-
portion to it ; and this proportion or ratio varies
with respect to different mediums. Thus, when
a ray passes from air into water, the angle of
incidence is to that of refraction in the ratio of
about four to three ; from air into glass nearly as
three to two; from air into diamond nearly as
five to two; and the contrary proportion holds
in passing back again ; as when light passes from
water into air, the ratio is as three to four, &c.
From all this you will clearly understand, that
the more obliquely a ray falls, the greater is the
refraction. It is also necessary that you should
remember, that light is refracted or drawn towards
the perpendicular, (as in fig. 48), when it passes
out of a rare into a denser medium ; and it is re-
fracted from the perpendicular, or in a more ob-
lique direction, when it passes from a dense me-
dium into one which is rare ; and the denser the
medium, the greater is the refraction : thus the
diamond is found to refract most powerfully.

This principle will explain several of the com-
mon phsenomena of nature. Mr. Walker ob-
serves, that " many a school-boy has lost his
life by supposing the bottom of a clear river to
be within his depth, as (when he stands on the
bank) the bottom will appear one-fourth nearer
the surface than it really -is. w In this case, the

Refrangibillty of Light. 151

rays proceeding out of the denser medium (the
water) into the rarer (the air), they are bent out
of their course more obliquely towards the eye
of the spectator. Have you ever seen a skilful
marksman shoot a fish in the water with a bullet?
If you have, the sportsman could tell you that
he took his aim considerably (perhaps a foot)
below the fish as it appeared, because it seemed
much nearer the top of the water than it really
was. The distortion of objects through a wrinkled
or crooked pane of glass, arises also from the
unequal refraction of the rays that pass through
it. When light passes out of pure space into
air, it is also refracted ; and therefore the sun is
visible, by means of the refraction of our atmo-
sphere, some minutes before he rises above the
horizon in the morning, and some minutes after
he sets below it in the evening. It has been cal-
culated that, in looking through the common
glass of a window, objects appear about one-thir-
tieth part of an inch out of their real place by
means of the refraction.

But the most excellent use to which this prin-
ciple has been applied is the construction of op-
tical glasses ; for, by grinding the glass thinner at
the edges than in the middle, those rays of light,
which would strike upon it in a straight line, or
perpendicularly if it were plain, strike upon it ob-
liquely, and consequently suffer a refraction, and
are made to converge ; and? on the contrary, by
making the glass thinner in the middle than at

152 Experimental Philosophy. [Lecture 11.

the sides, the rays are refracted the contrary way,
and are made to diverge.

The reason of this will be sufficiently evident,
if it be recollected that all curves or segments of
a circle may be conceived as formed of a number
of straight lines infinitely short, and inclining to
each other like the stones in the arch of a bridge,
or the bricks at the top of an arched window-
frame. It is evident, therefore, that in fig. 49,
where parallel rays are supposed to strike a sur-
face of this form, those only which enter the mid-
dle part will go in a straight direction, whereas
those which strike the sides will strike them ob-
liquely, and will consequently be refracted. If
the surface, then, be a perfect curve, as in fig. 50,
it is plain that only the ray which strikes the
centre point of the curve will enter it in a straight
direction, and consequently all the rest which
strike it obliquely will be more or less refracted,
according to the degree of obliquity, and will
consequently be made to converge.

Glasses are usually ground for optical purposes
into seven different shapes (see fig. 51). First,
the glass may be flat on both sides, as the com-
mon pane of a window, No. 1, Or, secondly,
it may be flat on one side and convex on the
other, plano-convex. No. 2. Or, thirdly, it
may be convex on both sides, like our ordinary
reading-glasses, No. 3. Or, fourthly, it may be
flat on one side and concave on the other, plano-
concave, as No. 4. Fifthly, it may be concave

Refrangibility of Light. 153

on both sides, like the glasses near-sighted peo-
ple generally use, as No. 5. Sixthly, it may be
concave on one side and convex on the other,
like the crystal of a watch, though not in such a
degree, as No. 6 ; this is usually called a menis-
cus. Seventhly, it may have one side, which
must be convex, ground into little facets, like
those of some jewels, while the other side is plain.
Children know it by the name of a multiplying-
glass, as Ncu 7.

The effects of these different glasses will be
easily understood from what has been premised.
A ray entering the plain glass, No. 1, will indeed
be refracted by the glass, but it will suffer another
refraction on going out of it, which will nearly
rectify the former; the place of the object will,
therefore, as was before stated,, be a little changed,
but its figure will remain unaltered.

If^ again, several parallel rays enter the glass,
No. 2, plain on one side and convex on the other,
as in figure 50, they will be differently refracted,
in proportion to the obliquity with which each
of them falls upon the surface. The middle
ray, for instance, which passes perpendicularly
through, will not be refracted at all, but go on
straight forward. All the other rays, howeyer,
will suffer refraction. The ray CE., fig. 50, will
be refracted upwards to F ; the ray A D will be
refracted downwards to the same point. There
they will cross, and then go onward, diverging
or separating from each other for ever ; that which

H5

154? Experimental Philosophy. [Lecture II.

came from the bottom going upward, and that
which came from the top downward. The figure
given there is flat, but it must be supposed
spherical, the glass being represented edgeways.
If so, therefore, the collected bundle of rays,
passing through the glass, unite and form a cone,
or a figure like a candle extinguisher, the bottom
of which is at the glass, and the point at F. This
point, as I once before had occasion to mention,

. is called the focus of the glass. From a calcula-
tion in geometry, we learn that the distance from
this point is always equal to the diameter of the
circle which the glass would make if its convexity
were continued.

When the rays of the sun fall directly upon a
glass DE, (see fig. 52) equally convex on both
sides, they will be refracted still more abruptly,
and meet sooner in a point or principal focus at

f. The distance of this focus is, we are informed
by the same calculation, equal to the semi-dia-
meter of the circle, which the convexity of the
glass continued would make. Either this glass
or the former, as they collect the rays of the sun
into a point, will burn at that point, since the
whole force of the rays is concentrated there.
The broader the glass in these instruments, the
greater will be its power, from its collecting a
greater number of rays.

It is to be observed, that they are only parallel
rays? or those which proceed in a direct line to
the surface of the glass, that are thus converged

Ref i eligibility of Light. 155

to a point or focus; the rays of the sun, how-
ever, come from so great a distance, that they
are always regarded as parallel. Divergent rays,
such as proceed from a point, as the flame of a
candle, will be refracted parallel. If, therefore,
we place a candle exactly at a focal distance from
one or both of these glasses, as at^J its rays will,
upon going through the glass, all run parallel to
each other. If the candle is placed nearer the
glass than its focal distance, the rays, after passing
through the glass, will no longer run parallel, but
separate or diverge : if it is placed farther off,
the rays will then strike the glass more parallel,
and will therefore, upon passing through it, con-
verge or unite at some distance behind the glass.
After the rays have united or converged to a
focus, they will cross each other, and form an
inverted picture of the flame of the candle, as
may be seen on a paper placed, at the meeting
of the rays behind. How the image is inverted,
therefore, is easy to apprehend ; for the upper
rays, after refraction, were such as came from
the under part of the luminous body ; and the
under rays, on the contrary, came from its top : .
so that the rays are turned upside-down, and So
consequently is the image. It is very pleasing
to view a picture of this kind thus formed, each
ray preserving the colour it had in the luminous
object with the utmost imitative precision. The
shadings of the little piece are far beyond the
reach of art, and the design far more correct

156 Experimental Philosophy. [Lecture 11.

than that of the finest painter. I mention the
candle as being an obvious luminary ; but if any
object whatever is placed at the proper distance
from a convex glass, its picture will be, in the
same manner, thrown behind, and may be re-
ceived upon paper, or any other body, in all its
natural proportions and colourings. The nearer
the natural object is to the refracting glass, the
farther off will this picture be behind it ; be-
cause, as was said before, the rays which form
it do not then converge or unite, but at a great
focal distance. The farther off the natural ob-
ject is, the nearer will be the focal distance it
makes, and consequently the nearer will be the
picture behind the glass ; for, wherever the focus
is, there will the perfect picture be. When
however the rays come from several objects at
a moderate distance, they may be considered as
all parallel, and this difference of focus is then
imperceptible.

To put what has been said in other words.—
As the rays of the sun may be all considered as
falling parallel upon every glass of the convex
kind, so they must always unite behind it in a
focal point. As all the rays flowing from other
objects are not always parallel, when placed too
near the glass, they separate after refraction, and
run off divergent ; when placed at a proper dis-
tance, they unite or converge in a focal point,
and there imprint a picture, if there is any thing
properly placed to receive it, in which the natural

Refrangibility of Light. 157

figure will be represented, its motions, its colours
and shadings.

The whole of the preceding theory may be
illustrated by means of a common reading-glass.
If a candle is held so near it, as that the rays
passing through shall strike the wainscot of the
chamber with a bright spot, just as large as the
glass itself, the candle is then at the focal dis-
tance; and rays, striking the glass divergently,
are refracted through it, parallel to each other,
neither spreading nor drawing together as they
proceed. If the candle is held nearer than the
focal distance, the rays will fall then more di-
vergent upon the glass, and will consequently be
refracted more divergent, so that they will form
a very broad spot of light upon the wainscot.
If the candle is placed at a much greater distance
than the focus, the rays fall upon the glass more
nearly parallel, and consequently, when they
are refracted will tend to unite and converge
behind the glass, and will form but a small speck
of vivid light on the wainscot. This speck, if
closely examined, will appear a perfect picture of
the candle.

Every visible point, in any body whatever,
may be considered as a candle sending forth its
rays, which split and pencil out into several other
rays before they arrive at the eye. Each body is
as if composed of an infinite number of splendid
points or candles, each point with its own radi-
.ance, .and diffusing itself on every side. Instead

158 Experimental Philosophy* [Lecture 11.

of one body, the eye, in fact, is impressed with
thousands of radiant points sent out from that
body, which being grouped at the bottom of the
eye, imprint the picture of the object whence
they flow. Each point sends forth its own rays.

It is upon this principle the camera obscura is
constructed. If we take a double convex glass
and adapt it so as to fit a hole in the window-
shutter of a darkened chamber, so that no light
shall come into the room but through the glass ;
then let us place a sheet of white paper behind it
at the proper distance, we shall thus have a ca-
mera obscura ; for a picture of every external ob-
ject will pass through the glass, and be painted
upon the paper in the most beautiful colours that
imagination can conceive, and all the motions of
those objects also. It is necessary, in this ex-
periment, that the window should not be opposite
to the sun ; for then we should see no image but
that of his brightness : and yet it is necessary
also, that while we make the experiment, the sun
should shine and illuminate the objects strongly,
which are to paint themselves within. Without
this strong illumination, the rays will be sent so
feebly from every object, that we shall have but
a faint picture, if any at all.

Painters and architects often make use of a
similar contrivance, or portable camera obscura, to
take a draught of landscapes or buildings : their
glass is fixed in a box, and by means of a mirror,
on which the diminished pictures fall, they are

Refrangibility of Light. 159

reflected upon oiled paper or polished glass
properly placed, upon which the artist sketches
his draught. With regard to the contours, or
outlines, which this picture gives, nothing can
be more exact ; but, so far as respects the shading
and colouring, the artist can expect but little as-
sistance from it : for, as the sun is every moment
altering its situation, so is the landscape every
moment varying its shade; and so swift is this
succession of new shades, that, while the painter
is copying one part of a shade, the other part is
lost, and a new shade is thrown upon some other
object.

If such a glass, that is, double convex, is so
fitted to a hole in a dark lantern, that little
pictures, painted in transparent colours on pieces
of glass, may be passed successively along be-
tween the gloss and the candle in the lantern, we
shall thus have a magic lantern. The pictures,
striking the glass very divergent, will be refracted
very divergent also, and will be painted upon the
wall of the chamber in all their colours, as large
as we please to make them ; for the farther the
wall is from the glass, the more room will the
rays have to diverge. As these figures would be
painted on the wall reversed, if the picture were
held upright, it is necessary to turn them upside
down, when we would exhibit the shadows on the
wall erect. The same kind of contrivance is
now employed, with great success, to elucidate
the principal phenomena of astronomy.

160 Experimental Philosophy. [Lecture 11.

In looking through a glass of this description,
that is, a convex or double convex lens, the ob-
jects which we look at will appear magnified;
for it is a rule in optics, that we see cvi'ry tiling
in the direction of that line In which the rays ap-
proach us last. When I come to treat of the eye,
the reason of this will be explained. Suffice it
to say for the present, that the larger the angle
under which any object is seen, the larger will
any object appear. The convergence of the rays
of the convex lens, therefore, enlarges greatly
the angle of vision, as must be evident if we
continue the lines/D,/E,/T, and/G, fig. 52,
in the direction to which they point, and therefore
in proportion to the distance the appearance
of the objects will be enlarged. The jcommon
spectacle-glasses and reading-glasses are of this
description.

The effects of the plano-concave and double
concave lenses, No. 4 and 5, are directly op-
posite to those of the convex lenses; for the
thick parts of these glasses, you see, are towards
the edge, and therefore their attractive and re-
fractive powers are not towards the centre, but
towards the circumference. Parallel rays, there-
fore, striking one of these glasses are made to
diverge, or are dispersed. Rays already divergent
are rendered more so ; and convergent rays are
made less convergent. Hence objects seen
through these glasses appear considerably smaller
than they really are. To prove this, let ab (fig. 53)

Rcfrangibility of Light. 161

represent an arrow, which would be seen by
the eye, if no glass were between, by the con-
vergent rays, ca and db ; but if the concave lens
D be interposed between the object and the eye,
the line ac will be bent towards g*, and the line
bd will be bent towards k9 and consequently both
will be useless, as they do not enter the eye. The
object then will be seen by other lines, such as ao
and 6r, which, on entering the glass, will be
refracted, and bent in the directions oc and rd.
According to the rule just now laid down, there-
fore, every object is seen along the line which
enters the eye last. The arrow is seen according
to the angle or, which is much smaller than the
angle db ; consequently it will appear considerably
diminished, and at the distance of nm.

The spectacles which are used by near or short-
sighted persons consist of concave lenses; for
the reason of short sight is, that, the form of the
eye being too convex, the rays are made to con-
verge before they reach the optic nerve ; and there-
fore the concave glass, causing a little divergence,
.assists this defeat of sight. But this matter will
be still further explained when we treat of vision.

The meniscus, No. 6, is properly like the
crystal of a common watch, and it neither mag-
nifies nor diminishes. Sometimes, however, it is
made in the form of a crescent ; that is, thickest
in the middle; and in that case it acts like a
double convex lens.

It is evident that all lenses, as to their surfaces,,

162 Experimental Philosophy. [Lecture 11.

whether concave or convex, are segments of dif-
ferent circles, the radii and diameters of which
may vary almost to an infinite extent. The
distance of the principal focus, or focus of parallel
rays, that is, the point where all the parallel rays
meet, as the point/; fig. 52, will vary in different
lenses, according to their respective degrees of
convexity. Hence, when opticians speak of the
radius of a lens, when they say it is three or six
inches, they mean that the convex surface of the
glass is that part of a circle, the radius (that is,
half the diameter) of which is three or six inches.
The axis of a lens is a straight line drawn through
the centre of its spherical surface.

The principal focus, or focus of parallel rays,
in convex lenses, is ascertained (as was before
intimated) upon mathematical principles. It may
however be found with sufficient accuracy for
common purposes, by holding a sheet of paper
behind the glass, when exposed to the rays of the
sun, and observing when the luminous spot is
smallest, and when the paper begins to burn.
Or when the focal length does not exceed three
feet, it may be found by holding the glass at
such a distance from the wall opposite a window
sash, as that the sash may appear distinct upon
the wall.

You will observe, that in a double convex lens
the rays of light are twice refracted ; first, on
entering the convex surface of the dense medium,
the glass; and, secondly, on going out of the

Tte/mngiUlity of Light. 163

same dense medium, and entering the rare me-
dium, or the air, which, from the form of the
glass, you know must present a concave surface.
Now rays are equally converged by entering a
convex surface of a dense medium, and a concave
surface of a rarer medium. The focus of a
double convex lens, then, is at only half the
distance of the focus of one which has only one
convex surface, that is, a plano-convex. The
focus of a double convex lens, therefore, as you
have already seen, fig. 52, is the length of the
radius, or semi-diameter of that circle, which is
formed by the convexity of either of its surfaces.
That branch of optics which respects the re-
frangibility of light is usually called dioptrics,
from the Greek dia, through, and optomai, to
*ee ; so that it means to see through.

LECTURE XII.

EXPERIMENTAL PHILOSOPHY.

RF.FLEXIBIL1TY OF LIGHT, OE CATOPTRICS.

THERE is no part of the science of optics more
amusing, or indeed more astonishing, to un-
scientific readers, than that which regards the
reflection of light. How a looking-glass comes to
reflect images without their touching it ; how the
whole figure of a man, six feet high, shall be
seen in a glass not above three feet ; how, when
we look at some polished surfaces, as a watch-
case, for instance, a man's face seems not bigger
than his finger-nail ; while, if we look on other
surfaces, the face shall be of gigantic size ; these
are all wonders that the curious would wish to
understand, and the inexperienced to examine.

The property which polished surfaces possess
of reflecting light, is referred by Newton to the
principle of repulsion. For it is justly remarked
by him, that those surfaces, which to our senses
appear smooth and polished, are found, when
viewed through a microscope, to be still rough
and uneven. It will, however, suffice for our
purpose, in describing the effects of reflection, if
we consider every particle of light as rebounding
from the surface of a mirror, like a tennis-ball
from the wall of a tennis-court.

ReflexiUlity of Light. 1 65

It is, in truth, by reflection that all objects are
rendered visible. Even glass, crystal, and water
reflect a part of the rays of light, or their forms
and substance could not be distinguished ; but
those bodies which transmit it copiously, are
called clear or transparent ; those which do not
transmit it, are termed opake. The whole of the
light which falls upon bodies, is not, however,
reflected. On the contrary, it is calculated that
the smoothest and most polished surfaces do not
reflect above half the light that falls upon them.
Those bodies with polished surfaces, which re-
flect most copiously the rays of light, are called
mirrors; by the ancients they were made of
metal, as iron, tin, or copper, and exquisitely
polished ; those in general use among us are made
of glass, rendered opake at the back part by an
amalgam or mixture of tin and quicksilver, or
mercury. Mirrors are made in various forms;
plane, that is, with a smooth and level surface;
convex, concave, or cylindrical. The most com-
mon are the plane mirrors.

A ray of light striking perpendicularly, in a
direct line, upon a plane mirror, is reflected in
exactly the same direction. Those rays which
strike it obliquely, are reflected back in an op-
posite direction, but^, with exactly the same degree
of obliquity. Hence the great law of reflection
is, that the angle of reflection's exactly equal to
the angle of incidence. This was explained to

166 Experimental Philosophy. [Lecture 12.

you in the tenth lecture, fig. 47, and it will serve
to elucidate all the phsenomena of reflection.

Lest you should, however, have attended to
the maxims and definitions subjoined to that
lecture less assiduously than you ought, I shall
refer you to another figure. In PL XIII. fig.
54, ?io may be considered as a ray of light striking
perpendicularly on the surface of the mirror a &,
and it is consequently reflected back in the same
line. The ray d o, coming from the luminous
body d, strikes the mirror obliquely, and is re-
flected to the eye in the line o e, in such manner,
that the angle e o n is equal to the angle o d n ;
in other words, the angle of reflection is equal to
the angle of incidence.

This, you will answer, is sufficiently clear ; but
how comes it that I do not see the object at o,
since it is there that the rays strike the mirror ?
And why is it, that, on the contrary, the object
appears behind the glass, and in the situation of
s ? This has been partly explained by a rule
which I formerly laid down ; namely, that we
see every thing in that line in which the rays
last approached us. Now an object is rendered
visible, not by single rays proceeding from every
point of its surface, but by pencils of rays, or
collections of divergent rays issuing from every
point, as was explained in the preceding lecture.
These pencils of rays are afterwards, by the
refractive powers of the eye, converged again to

Reflexibility of Light. 167

points upon the optic nerve, which lies at the
back of the eye ; and these points of convergent
rays on the optic nerve, are correspondent to the
points of the objects from which the rays diverged.
Now the pencils of rays strike the mirror, while
they are in their divergent state ; and as the
angle of reflection is equal to the angle of in-
cidence, they are reflected' back in the same state,
and converge exactly as they would have done
had they not been intercepted by the mirror.
As, therefore, we always see objects in the line in
which the rays approached us last, the two lines,
viz. that which goes from the object towards the
mirror, and the reflected line, are united in the
mind of the spectator, and the object is con-
sequently seen at s, at an equal distance behind
the mirror, as the object was before it. To make
this clear, however, I shall present you with
another diagram. The lines D c, (fig. 55.) are
the lines of incidence, c B are the lines of re-
flection, and these form equal angles on the
surface of the polished mirror; so that all the
ray scorning from the object, and falling upon the
mirror at c, will strike the eye at B, and the
reflected image will thus become visible. Now
no object can be seen that does not lie in a
straight line from ttfe eye, or, at least, appear to
do so. The body D, therefore, when it comes
reflected to the eye, will appear to lie in the
straight line AA, which, since the angle of in-
cidence is equal to that of reflection, will be

168 Experimental Philosophy. [Lecture 12.

exactly in the two lines D c and c B. The rays,
therefore, going from D to c9 will seem to have
proceeded to A, and consequently the picture
will be there. For, as the rays have diverged in
going from the object at DD, and diffused them-
selves upon the surface of the glass, they will be
again converged into an equal focus, by the time
they arrive at B 5, and they will therefore paint
the object at A A.

Hence we may learn, that if a man sees his
whole image in a plane looking glass, the part of
the glass that reflects his image, need be but one-
half as long, and one half as broad as the man.
For the image is seen under an angle, as large as
the life ; the reflecting mirror is exactly half-way
between the image and the eye, and therefore
need be but half as large as the object, to sub-
tend an angle as large as the image ; or, in other
words, it is just half as large as the image, which
is of the same size with the man. Thus the man
AB, (see fig. 56) will see the whole of his own
image in the glass CD, which is but half as large
as himself. His eye, at A, will see the eye of
the image at an equal distance behind the glass
at E. His foot at B, will send its rays to D ;
these will be reflected at an equal angle, and the
ray will therefore seem to have proceeded in the
direction of FDA, so that the man will see his
foot at F ; that is, he will see his whole figure
atEF.

It is thus that plane mirrors reflect. The

Reflexibility of Light. 169

nature of those which are convex or concave is a
more difficult study, though the same law pre-
vails with respect to them as with respect to the
others. To understand the principles on which
they act, it will be expedient to call to your
recollection what was said in the former lecture
on spherical surfaces. All curves or arches may
be considered as composed of a number of small
flat planes, lying obliquely to one another. Pa-
rallel rays, therefore, striking an object opposed,
to them in this position, will strike it more or less
obliquely. Thus, in fig. 57, the rays a, 6, c, d,
which would fall perpendicularly on a horizontal
surface, strike obliquely upon those which are
opposed to them ; and, instead of being reflected
parallel, are reflected divergent. For the same
reason, convergent rays would be reflected less
convergent by such a mixed surface as this, and
divergent rays would be rendered still more
divergent. Fig. 58, you see, is the reverse of the
preceding, and it serves very well to represent the
effects of a concave mirror. By this you must
perceive that the parallel rays a, b, c, d, which
would have been reflected parallel by a plane
mirror, are made to converge, because, instead of
striking this mirror in a direct line, they strike it
obliquely ; and you may easily conceive, that by
the same rule, convergent rays will be reflected
still more convergent, and divergent rays will be
made to converge less.

As by a mirror of the convex kind convergent

VOL. I. I

170 Experimental Philosophy. [Lecture 12.

rays are rendered less convergent, you will easily
comprehend why objects are diminished by it.
By the rays being made less convergent, the
visual angle is diminished; for, you know, we
see every object in the line in which the rays of
light last approached the eye. By the same rule,
a concave mirror magnifies or enlarges the image
of an object ; for the visual angle is enlarged or
rendered more obtuse, and consequently the image
is magnified in proportion to the curvature of the
. concave surface.

To prove what I have just now laid down with
respect to convex mirrors, in fig. 59, a b is a
dart, which is seen in the convex mirror c d.
Now, though rays issue from the object a b in all
directions, as was explained in the tenth lecture,
Plate XI. fig. 46, yet it is seen only by means of
those which are included within the space between
o and 7i, because it is only those which can be
reflected to the eye at r. Now you will easily
perceive that if these rays had gone forward in
the direction in which they were proceeding,
they would have united at p, and the object
would have been seen of its full size. As it is,
however, the rays are reflected less convergent
than they were in their natural course, and the
angle o r n, being less than the angle a p 5, the
image at s appears smaller than the object, and
nearer to the surface of the mirror. The reason
of this last effect has been already explained,
when I said that objects are rendered visible, not

Reflexilility ofLigU. 171

by a single ray, but by pencils of divergent rays
proceeding from every point of the object. Sup-
pose, therefore, G (fig. 60) a radiant point of any
object, from which a pencil of divergent rays
proceeds, and falls on the convex mirror a b.
These rays (agreeably to the rule laid down
above, that convex mirrors cause divergent rays
to diverge still more) will be rendered more
divergent, and will have their virtual or imaginary
focus at g, that is, much nearer to the surface of
the mirror than if it were plane.

For these reasons, a person looking at his face
in a convex mirror, will see it diminished. Thus,
in fig. 61, though rays proceed from every part
of the face, it is only the rays that touch the
mirror within the space between c and r that can,
agreeably to the great law of reflection, (the
angle of incidence being equal to the angle of
reflection) be reflected to the eye. The rays c
and r being therefore rendered less convergent
(as in the former instance in fig. 59), he will
see the chin along the line o r s, and the forehead
along the line o c n, and the angle of vision being
thus diminished, all the rest of the features will
be proportionably reduced. Large objects, how-
ever, placed near a convex mirror, will not only
appear reduced, but distorted; because, from
the form of the glass, one part of the object is
nearer to it than another, and consequently will
be reflected under a different angle.

Convex mirrors are at present a very fashion-

172 Experimental Philosophy. [Lecture 12.

able part of modern furniture, as they exhibit a
large company, assembled in a room, in a very
small compass. Globes lined with amalgam used
to be formerly hung up in the middle of a room,
by which the whole company were exhibited at
one view, seated at a dinner-table, or dispersed
about the room.

The phenomena of concave mirrors are still
different. By them convergent rays are ren-
dered still more convergent, and consequently
the visual angle is enlarged. Their general effect
is therefore to magnify. This will be sufficiently
exemplified by PL. XIV. fig. 62. In this, as in
the former instance, a face is looking at itself;
and I take the extreme of those rays which can
be reflected to the eye, one from the forehead
and one from the chin. These lines, ac, and
mn, are reflected to the eye at o, which con-
sequently sees the image in the lines of reflection,
and in the angle odq, and therefore evidently
magnified beyond the natural size, and at a small
distance behind the mirror.

This effect, however, will only take place
when the eye is between the mirror and its prin-
cipal focus, that is, the focus or point, where
rays falling parallel or perpendicular on the glass,
will unite after reflection ; the point where the
rays of the sun (which are always considered as
parallel) will unite and burn: for a concave
mirror acts as a burning-glass. By the great law
of reflection, the principal focus of a concave

Reflexibitity of Light. 173

mirror, is at one-fourth of the diameter of that
sphere, of which the concave surface is a section,
which is therefore sometimes called the centre of
concavity. At this point the rays reflected from
the mirror, are converged and cross ; and if the
spectator's eye is beyond this point or focus, he
will not see the image behind the mirror, but
before it, a shadowy form, suspended in the air ;
but, from the crossing of the rays, it appears
inverted.

In fig. 63, a b is a concave mirror, cd is a
hand held up before it. The image, therefore,
you see is not placed behind the mirror, as
happens in every other case, but the hand seems
to hang suspended in the air at m. The reason of
this very extraordinary and striking phenomenon
is to be found in what was already intimated.
Objects are rendered visible, not by single rays,
but by pencils of divergent rays, proceeding
from the different points of the object. If these
pencils of divergent rays should happen by any
cause to be united, the object will in that point
cease to be visible. This happens in the focus
of a concave mirror, where, by the law of re-
flection, they are all united. If the eye, there-
fore, is placed in that point, it will see nothing of
the image. It must recede to a sufficient distance
to permit the rays to cross and again becpme
divergent. In that case the image will be seen,
not behind the mirror at the virtual or imaginary
focus, as it is in plane and convex mirrors, but

1 74 Experimental Philosophy. [Lecture 12.

suspended in the air between the eye and the
real focus, for every image is seen about that
place, whence the pencils of rays begin to diverge.
In plane mirrors the rays have only diverged
from the luminous points of the object itself; and
as the eye cannot see behind, it sees the image
in a straight line, but joins the line of incidence
and that of reflection together. The image there-
fore appears at the same distance behind the
glass, as the object stands before it. In concave
mirrors the case is entirely diff erent ; for in them
there is an actual focus, where the rays are con-
verged to a point, and from which they begin
«gain to diverge. The image is therefore seen
there, but in an inverted position, for reasons
already given. Thus, in fig. 63, the rays c and d
go diverging from the two opposite points of the
object; by the action of the mirror they are
again made to converge to a point at o S9 where
they cross, and again proceed divergent to the
eye.

It will, however, render this interesting part
of optics still clearer, if I present you with an-
other diagram, similar in some degree to the
preceding. In fig. 64,'AcB is a concave mirror.
The centre of concavity is at C. From the points
of the dart D, we suppose a pencil of divergent
rays emitted, which you see touch the mirror at
AcB. These rays are reflected, according to the
general law of reflection, (the angle of reflection
being ecjeiial to the angle of incidence) which is

Reflexibility of Light. 1 75

proved by drawing the dotted lines C A, Cc, CB,
from the centre of concavity to the points whence
these rays are reflected, which are therefore per-
pendiculars to the surface of the mirror. The
angle C Ad, or the angle of reflection, you see, is
equal to DAC, the angle of incidence, and so
you will find it of the rest. The reflected rays
then, you see, converge to a point, and form the
extremity of the dart (which is now inverted)
at d. In the same manner every other pencil of
rays emitted from the object, will be converged
at or near the principal focus, and the image will
be formed at e d. For you wih1 perceive that if
the rays Et/J T5g*, ' E A, were continued to the
mirror, they would be reflected and converged at
e^ forming the opposite extremity of the dart.
When the object is further from the mirror than
the centre of concavity C, the image wih1 be
nearer the mirror, and smaller than the object;
when the object is nearer than the centre of con-
cavity, the image will then be more remote, and
larger. Thus, if e d was the object, DE would
be the reflected image.

It is not many years since a person derived
considerable emolument from exhibiting in the
metropolis some optical deceptions of this kind,
with concave mirrors. A ghastly apparition was
sometimes made to meet the ignorant spectator,
and from its shadowy appearance it was evidently
nothing human ; sometimes a hand was held out
in the air, with every possible mark of friendship,

176 Experimental Philosophy. [Lecture 12.

but when he approached to unite it with his own,
a drawn sword was instantly presented to his
breast. A nosegay, or a piece of fruit was
offered, but when he attempted to seize it, a
death's head snapped at him.

I mentioned that concave mirrors were fre-
quently used as burning-glasses, and a curious
experiment may be made by means of them, to
show that common culinary fire may be reflected
in the same manner as the rays of the sun. If
two large concave mirrors are placed opposite to
each other, as in fig. 65, at almost any distance,
and a red-hot charcoal is held in the focus of one
at a, and a match, or any combustible matter, in
the focus of the other at b, the match, &c. will be
presently set on fire by the reflected flame of the
charcoal.

You have seen, I dare say, the distorted figures
which are sometimes painted on boards, and ex-
hibited in the shop- windows of opticians. They
look like a mere splash of a painter's brush ; but
when a mirror of a cylindrical or conical form is
set in the middle of the board, a beautiful figure
is reflected from it. This shows that what ap-
pears to be a casual dash of paint on the board
is, in fact, a figure drawn with the nicest mathe-
matical precision. When the image is to be
rectified by a cylindrical mirror, the lines are
only extended, and, by the great law of reflection,
the rays from the picture are reflected by the
mirror less convergent, and the figure is con-

Reflexibility ofLiglit. 177

sequently rectified. A little consideration on
this subject, applying the principles which have
been laid down in the course of this lecture, will
easily enable you to see the theory on which
these mirrors act, particularly if you have the
objects before you : without which, indeed, an
infinity of words must be expended in describing
and explaining them.

LECTURE XIII.

EXPERIMENTAL PHILOSOPHY.

VISION AND OPTICAL GLASSES.

IT has already been explained, that objects
are rendered visible not by single rays, but by
small bundles of rays diverging from every point
of the object, like an inverted cone, or like a
painter's brush or pencil, and therefore called
pencils of light. It has also been intimated, that
these pencils of light are, by the refractive powers
of the eye, again made to converge upon the back
part of that organ, in points corresponding to
those from which they proceeded, so as to form
there a complete image of the object. In the
tenth lecture, fig. 46, it was further shown, that
pencils of light are sent forth in all directions,
from every part of a visible object; so that an
eye, when placed in any situation that light can
travel to it from the object in a straight line,
(whether above or below, or at either side) shall
be able to perceive it.

In describing the nature of refraction, enough
has been said to show you that it is the property
of every convex glass to cause the rays of light
to converge. In this respect the eye is to be

Vision and Optical Glasses. 179

considered as a convex lens, constructed with
such admirable skill by the great Author of
Nature, that the rays converge to a point exactly
in the proper place ; so that if the humours were
otherwise disposed, even to the breadth of a horse-
hair, the effect would be totally destroyed. But
you will understand the subject better, by con-
sidering the structure of this curious organ ; in
describing which, I shall adopt the simple, but
expressive language of Mr. Ferguson.

The eye is nearly of a globular form. It con-
sists of three coats and three humours. (See
fig. 66.) The part DHHG of the outer coat is
called the sclerotica ; the rest, D E F G, the
cornea. Next within this coat, is the choroides,
which serves for a lining to the other, and joins
with the iris mn, mn. The iris is that coloured
circle which gives the character, as to colour, to
the eye, and is composed of two sets of muscular
fibres; the one of a circular form, which con-
tracts the hole in the middle, called the pupil,
when the light would otherwise be too strong for
the eye ; and the other of radial fibres, tending
every where, from the circumference of the iris,
towards the middle of the pupil ; which fibres,
by their contraction, dilate and enlarge the pupil
when the light is weak, in order to let in more of
its rays. The third coat is only a fine expansion
of the optic nerve L, which spreads like net-work
all over the inside of the choroides, and is there-
fore called the retina ; upon which are painted

180 Experimental Philosophy. [Lecture 13.

the images of all visible objects, by the rays of
light which either flow or are reflected from
them.

Under the cornea is a fine transparent fluid,
like water, which is therefore called the aqueous
humour. It gives a protuberant figure to the
cornea, fills the two cavities mm and nn, which
communicate by the pupil P, and has the same
refractive power as water. At the back of this
lies the crystalline humour R, which is shaped
like a double convex glass, and is a little more
convex on the back than the forepart. It con-
verges the rays, which pass through it from every
visible object, to its focus at the bottom of the
eye. This humour is transparent, like crystal,
and is much of the consistence of hard jelly.
It is inclosed in a fine transparent membrane,
from which issue radial fibres, called the ligar
mentum ciliare, all around its edge ; and join to
the circumference of the iris. These fibres have
a power of contracting and dilating occasionally,
by which means they alter the shape or convexity
of the crystalline humour, and also shift it a
little backwards or forwards in the eye, so as to
adapt its focal distance at the bottom of the eye,
to the different distances of objects; without
which provision, we could only see those objects
distinctly, that were all at one distance from the
eye.

At the back of the crystalline lies the vitreous
humour KK, which is transparent like glass, and

Vision and Optical Glasses. 181

is the largest of all in quantity, filling the whole
orb of the eye, and giving it a globular shape.
It is much of the same consistence as the white
of an egg, and very little exceeds water in its
refractive power.

As every point of an object ABC, sends out
pencils of rays in all directions, some rays, from
every point on the side next the eye, will fall
upon the cornea between' E and F ; and by
passing on through the humours and pupil of
the eye, they will be converged to as many points
on the retina or bottom of the eye, and will there
form a distinct inverted picture cba of the object.
Thus, the pencil of rays qrs, that flows from the
point A of the object, will be converged to the
point a on the retina ; those from the point B
will be converged to the point b ; those from the
point C will be converged to the point c ; and so
on of all the intermediate points : by which means
the whole image abc is formed, and the object
made visible.

That vision is effected in this manner, may l?e
demonstrated experimentally. Take a bullock's
eye while it is fresh, and having cut off the coats
from the back part, quite to the vitreous humour,
put a piece of white paper over that part, and
hold the eye towards any bright object, and you
will see an inverted picture of the object upon
the paper.

It has been a matter of inquiry among scientific
persons, why the object appears in an upright

182 Experimental Philosophy. [Lecture 13.

position, while the image on the retina is inverted.
In truth, we know nothing of the connexion
which exists between the thinking faculty and
the organs of sensation. It may, however, suf-
fice to answer the present question, if we say
that the mind certainly does not look upon the
image which is painted on the optic nerve. That
nerve is sensible of the impression, from the rays
of light being reflected upon it, as the organs of
touch feel the impression of any external object,
by coming in contact with it. Nor is there any
reason why the mind should not perceive as ac-
curately the position of bodies, if the rays reflected
from the upper parts of those bodies are made to
touch the lower parts of the eye, as if they had
been directed to the upper parts. Suffice it, that
such a correspondence is established between the
parts of the eye to which the rays are converged,
and the different parts of the object, that we do
not find that persons blind from infancy, who
have been restored to sight by the operation of
couching, have been led into the smallest mistake
as to this point*.

To very perfect sight the three humours of
the eye appear necessary. Yet by a very bold
experiment (for such it undoubtedly was at first),
it is found that we can see tolerably well, even
though one of them should be taken away, par-

* For an elaborate disquisition on this subject, the
reader may consult the Rev. A. Horn's Essay on Vision.

Vision and Optical Glasses. I8C»

ticularly if we assist the sight by glasses. It very
often happens that the crystalline humour loses
its transparency, and thus prevents the admis-
sion of the visual rays to the back parts of the
eye. This disorder is called by the surgeons a
cataract. As we know that the crystalline hu-
mour stands edgeways behind the pupil, all
then that we have to do, is to make it lie flat in
the bottom of the eye, and it will no longer bar
out the rays that come in at the pupil. A sur-
geon, therefore, takes a fine straight awl, and
thrusting it through the coats of the eye, he de-
presses the crystalline humour into the bottom of
the eye, and there leayes it. Or sometimes he cuts
the coats of the eye, the crystalline and the
aqueous humour burst out together; in some
hours the wound closes, a new aqueous humour
returns, and the eye continues to see, by means
of a glass, without its crystalline humour. This
operation is called couching for the cataract.
Cheselden once couched a boy who had been
blind from his birth with a cataract. Being thus
introduced, in a manner, to a new world, every
object presented something to please, astonish,
or terrify him. The most regular figures gave
him the greatest pleasure, the darkest colours
displeased, and even affrighted him. The first
time he was restored, he thought he actually
touched whatever he saw; but by degrees his
experience corrected his numberless mistakes.
More recently an interesting case of this kind

184< Experimental Philosophy. [Lecture 13.

has been described in the Philosophical Trans-
actions by Mr. Ware.

The eye may be remedied when the crystalline
humour onJy is faulty ; but when there happens
to be a defect in the optic nerve, then the disorder
is almost always incurable. It is called the gutta
serena^ a disorder in which the eye is, to all ap-
pearance, as capable of seeing as in the sound
state ; but, notwithstanding, the person remains
for life in utter darkness. The nerve is insensible,
and scarcely any medical treatment can restore
its lost sensations. This is the disorder so
pathetically described by Milton in his lamenta-
tions on his own blindness.

In the course of the preceding lectures it was
necessary to mention the angle of vision. But
you will now be able better to understand why
an object seen under a large angle, as near objects
are, appears larger than the same object would
at a distance. Thus men and women, when
you meet them in the street, appear of their na-
tural size, but if you look down upon them
from the top of St. Paul's, they appear as small
as puppets ; and thus if you look from one end
towards the other of a long and straight row of
trees, you will see them gradually diminish, as
they are further removed from your eye, though
on a near inspection you would find them all
of an equal size. The reason of this can be no
longer a secret. You are already informed, that
rays (or rather pencils of rays) are sent forth

Vision and Optical Glasses. 185

from every visible object, in all directions, some
more and some less convergent. When you are
near, therefore, you see the extreme points of
any object by pencils of rays, which converge or
meet in an angle more obtuse than when it is
at a greater distance; and as the rays cross each
other in the eye, a larger image is of course painted
on the retina. Thus, in PL XV. fig. 67, the ob-
ject ABC is seen by the eye at D, under the angle
APC. and the image upon the retina cba is very
large ; but to the eye at E, placed at double the
distance, the same object is seen under the angle
A/?C, which is only equal to half the angle APC.
The image cba, therefore, is only half as large in
the eye at E as in the eye at D ; and this will
sufficiently explain why objects appear smaller
in proportion to their distance from the ej/e.
Observe, however, that this proposition will admit
of some exceptions, where the judgment corrects
the sense. Thus, if a man six feet high (and not
far distant from the spectator) is seen under the
same angle with a dwarf two feet high (say at
the distance of three feet from the spectator),
still the dwarf will not appear as tall as the man,
because the sense is corrected by the judgment,
which makes a comparison of both with sur-
rounding objects of known size. These ex-
ceptions will, however, in general, only take
place with respect to near objects, and those
with whose forms we are well acquainted.

From what has been said of the structure of

186 Experimental Philosophy. [Lecture 13.

the eye, you will also perceive the causes of
distinct and indistinct vision. To see an object
distinctly, it is necessary that every pencil of
diverging rays, which reaches the eye from the
object, should be converged to a point on the
optic nerve, corresponding to that from which
the rays have diverged. If, on the contrary they
are brought in an unconverged state to the retina,
you may easily conceive that the particles of light
will be so scattered and dispersed, as to make an
indistinct impression. This last defect takes
place when the eye, by age or infirmity, is made
flat, and consequently is not sufficiently convex to
cause the rays to converge in their proper place ;
persons with this defect can often see objects
better at a great distance than very near. The
opposite fault to this is when the eye is too convex,
when the rays will be made to unite too soon,
before they reach the retina ; persons with this
defect, therefore, are called short sighted because
they can only discern objects which are very near
to the eye.

I have seen a very pretty contrivance in the
shop of an optician, illustrative of the causes of
weak and short sight. Two eyes were made of
glass, as fig. 68 and 69, and the pencils of diverg-
ing rays, issuing from three points, were repre-
sented by threads of silk of three different colours.
Thus in fig. 68, which represents weak or in-
distinct vision, you see the rays are not united
in points when they reach the back of the eye,

Vision and Optical Glasses. 187

where the retina is situated ; but if they were
suffered to pass on without interruption, would
converge in some part behind it. On the con-
trary, in figure 69, you see that, from the great
convexity of the cornea, the rays are made to
converge too soon, and, in effect, the perfect
and distinct image is formed in the midst of the
vitreous humour, and before it reaches the retina.

From what you have already learnt of the na-
ture of lenses, you will be able to comprehend
that the remedy for the former of these defects,
that is, where the eye is too flat to cause the rays
to converge in their proper place, is a double
convex lens, the property of which is to increase
the convergency of rays. The focus of this glass,
however, must be exactly adapted to the wants of
the eye for which it is intended. As therefore
the eye grows flatter from age and infirmities,
this will explain what is meant by " spectacles
for all ages." Where the defect of sight is not
great, as in younger persons, spectacles not very
convex will suffice; but where the eye is very
flat, as in old persons, glasses of a stronger mag-
nifying power will be required.

On the contrary, near sighted eyes (such as
fig. 69) being too convex, it is necessary to pre-
vent the rays from converging too soon, which
can only be done by means of a concave glass,
which renders convergent rays less convergent.
This glass, however, must also be exactly adapted

188 Experimental Philosophy. [Lecture 13.

to the necessity of the eye, otherwise the rays will
not converge at the proper point.

I cannot quit this subject without noticing the
gross stupidity of the atheist. Can any persons
in their senses conceive that so nice, so exquisite
an organ as the eye should be formed by chance !
That by chance the humours should be disposed
with the most perfect mathematical precision,
so that a mistake to the breadth of a hair would
be sufficient to defeat the purpose of vision ! Yet
these are the men, my young friends, who without
understanding any principle of any one science,
have the impudence to call themselves philo-
sophers* ! though in what their philosophy can
consist, would require more than Newton pos-
sessed to be able to discover.

There is reason to believe, that the use of
convex glasses, both as burning glasses and mag-
nifiers, was not unknown to the antients ; and,
in the twelfth century, Alhazen, an Arabic philo-
sopher, treated at some length of the magnifying
power of these glasses. He was followed by our

* Why they have chosen to adopt this name no man
can possibly devise. They might as well have called them-
selves architects, heralds, antiquarians, or by any other de-
nomination with which they have no connexion what-
ever. Ask any of these pretended philosophers why a
convex lens causes the rays of light to converge, or any
similar question, and you will soon see whether they have
any pretension to the name of philosophers,

Vision and Optical Glasses. 189

truly illustrious countryman Roger Bacon, who
demonstrated by experiment that a small segment
of a glass globe would assist the sight of old
persons. Thus he may be regarded as the person
who first discovered the theory of spectacles,
though they were not brought into use until the
following century.

The telescope was invented about the end
of the sixteenth century, and the discovery is
commonly supposed to have been casual. The
account which is generally received is, that the
children of Zacharias Jansen, a spectacle-maker
of Magdeburgh, trying the effect of a convex and
concave glass united, found that when placed at
a certain distance from each other, they had the
property of making distant objects appear nearer
to the eye ; but the reason of this effect was not
discovered till the time of Kepler.

The microscope was also an invention of Jansen
or his children: and as it is rather a simpler
instrument than the telescope, it will serve to
introduce you very properly to a knowledge of
these kinds of glasses. You already know that
the nearer any body is to the eye, the larger is
the angle under which it will be seen ; but if
placed too near, the image will be confused,
because the divergence of the rays is then too
great to admit of their being properly converged
on the retina by the humours of the eye. In fact,
an eye which is not near sighted cannot discern
any object clearly at a shorter distance than six

190 Experimental Philosophy. [Lecture 13.

inches ; and many objects are too small to be
seen at that distance. This deficiency is supplied
by the microscope.

The single microscope is only a small convex
glass cd, (fig. 70,) having the object ab placed in
its focus, and the eye at the same distance on
the other side ; so that the rays of each pencil,
flowing from every point of the object on the side
next the glass, may go on parallel in the space
between the eye and the glass; and then, by
entering the eye at C, they will be converged to
as many different points on the retina, and form
a large inverted picture AB upon it, as in the
figure.

If, as in fig. 71, which represents the effect of
this microscope, the object AB is in the focus of
the lens DE, and the eye is in the other focus F,
as much of the object will be visible as is equal
to the diameter of the lens ; for the rays AD and
BE proceed through the extremities of the lens,
and are united at F. Hence a maxim in optics —
that when an object is placed in one focus of a
lens., and the eye in the other ^ any lineal dimen-
sion of the object appears just twice as large as it
would to the naked eye, whatever the size of the
lens. For the lines FD and FE, if protracted as
far as A and B, would form an image exactly
twice as large. If the eye is nearer to the lens
than the focus, it will see the object still larger;
and if it is further off than the focus, it will not
see it so large.

Vision and Optical Glasses. 191

To find how much this glass magnifies, divide
the least distance (which is about six inches) at
which an object can be seen distinctly with the
bare eye, by the focal distance of the glass ; and
the quotient will show how much the glass mag-
nifies the diameter of the object. The most
powerful single microscopes are very small globules
of glass, which any person may make for himself
by melting the ends of fine glass threads in the
flame of a candle.

The double or compound microscope consists
of an object-glass cd, (fig. 72,) and an eye-glass ef'.
The small object ab is placed at a little greater
distance from the glass cd than its principal focus,
so that the pencils of rays flowing from the dif-
ferent points of the object, and passing through
the glass, may be made to converge and unite in
as many points between g and h, where the image
of the object will be formed: which image is
viewed by the eye through the eye-glass ef. For
the eye-glass being so placed that the image gli
may be in its focus, and the eye much about the
same distance on the other side, the rays of each
pencil will be parallel, after going out of the eye-
glass, as at e and^ till they come to the eye at A:,
where they will begin to converge by the re-
fractive power of the humours ; and after having
crossed" each other in the pupil, and passed
through the crystalline and vitreous humours,
they will be collected into points on the retina,
and there form the large inverted image AB.

Experimental Philosophy. [Lecture 13.

The magnifying power of this microscope is
as follows. Suppose the image gh to be six times
the distance of the object db from the object-glass
cd ; then will the image be six times the length
of the object : but since the image could not be
seen distinctly by the bare eye at a less distance
than six inches, if it is viewed by an eye-glass ef,
of one inch focus, it will be brought six times
nearer the eye ; and consequently viewed under
an angle six times as large as before ; so that it
will be again magnified six times ; that is, six
times by the object-glass, and six times by the
eye-glass, which multiplied into one another
make thirty-six times ; and so much is the ob-
ject magnified in diameter more than it appears
to the bare eye; and consequently thirty-six
times thirty-six, or one thousand two hundred
and ninety-six times in surface.

The solar microscope is constructed upon si-
milar principles. Two convex glasses are in-
closed at their proper distances in a brass tube.
This tube being fixed in the window-shutter of
a dark room, the object is put between the two
glasses, when a very large inverted image of
it will be exhibited on the opposite wall, pro-
vided the sun shines sufficiently bright and clear
upon the microscope. This instrument bears a
strong analogy, therefore, to the camera obscura
already described. Sometimes, three lenses are
employed, and the magnifying power of the mi-
croscope proportionally increased.

Vision and Optical Glasses. 193

What microscopes effect upon minute bodies
very near, telescopes effect with regard to great
bodies very remote; namely, they enlarge the
angle in the eye under which the bodies are seen ;
and thus, by making them very large, they make
them appear very near: the only difference is,
that in the microscope the focus of the glasses is
adapted to the inspection of bodies very near ; in
the telescope, to such as are very remote. Sup-
pose a distant object at A B (see fig. 73), its rays
come nearly parallel, and fall upon the convex
glass cd; through this they will converge in points,
and form the object E at their focus. But it is
usually so contrived, that this focus is also the
focus of the other convex glass of the tube. The
rays of each pencil, therefore, will now diverge
before they strike this glass, and will go through
it parallel ; but the pencils all together will cross
in its focus on the other side, as at e, and the
pupil of the eye being in this focus, the image
will be viewed through the glass, under the angle
geh, so that the object will seem at E under the
angle DeC. This telescope inverts the image,
and therefore is only proper for viewing such
bodies as it is immaterial in what position they
appear, as the sun, the fixed stars, &c. By add-
ing two convex glasses, the image may be seen
upright. The magnifying power of this, which is
called the dioptric telescope, is found by dividing
the focal distance of the object-glass by the focal

VOL. I.

194 Experimental Philosophy. [Lecture 13.

distance of the eye-glass, and the quotient ex-
presses the magnifying power.

The greatest inconvenience attending dioptric
or refracting telescopes was found to be that
which arises from what is called the aberration
of light, which, when high magnifiers were used,
that is, lenses much thicker in the middle than
at the sides, produced often a confused, and
sometimes a coloured image. This effect is the
result of refraction, and it consists in different
rays, according to their obliquity, uniting in dif-
ferent foci, though proceeding through the same
lens. This will be easily understood by fig. 74.
Suppose, then, PP to be a convex lens, and E e
an object, the point E of which corresponds with
the axis of the lens, and sends forth the rays
EM, EN, EA, EM, EN, all of which reach the
surface of the glass, but in different parts. The
ray EA, which penetrates the centre of the glass,
suffers no refraction ; the rays EM, EM, which
pass near EA, will be converged to a focus at F —
But the rays EN, EN, which strike more ob-
liquely near the edges of the glass, will be differ-
ently refracted, and will meet about G, nearer to
the lens, where they will form another image Gg.
In this manner several images will be formed in
different foci ; and though to the eye which looks
through the lens one image only will be apparent,
yet that image, from being composed of so many
combined, will be confused and distorted.

Vision and Optical Glasses. 195

What is thus established in theory may be de-
monstrated by experiment, and that experiment
is easy to make. Cover one side of a glass globe
or of a thick lens with a piece of brown paper,
making a row of pin-holes across the diameter of
the lens very accurately at equal distances. Let
the light which passes through the lens fall upon
a sheet of white paper, and you will find that
when the paper is held near the lens the spots of
light will be nearly at equal distances ; but if the
paper is further removed, the intervals between
the exterior spots become less than the intervals
between the interior, and soon unite.

But there is a still further aberration, which is
productive of even a greater inconvenience than
this which I have now specified. When I come
to treat of the prism and the prismatic colours,
you will find that each particle of light is suscep-
tible of a different degree of refrangibility, and
consequently that every lens (especially high
magnifiers) acts in some degree as a prism in
separating the different coloured rays — Hence, if
we suppose PP (fig. 75) to be a double convex
lens, and oo an object at some distance from it,
if the object oo were red, the rays proceeding from
it would form a red image Rr ; if it were violet,
an image of that colour would be formed at \v
nearer the lens ; and if the object were white, or
any other combination of different coloured rays,
these rays would have their respective foci at dif-
ferent distances from the lens, and form in fact

196 Experimental Philosophy. [Lecture 13,

a succession of images, in the order of the pris-
matic colours from Rr to Vv. As in the former
case, these different images will form but one to
the eye of the spectator ; but it will be imperfect
and coloured at the edges, as well as the field of
view. Various remedies were devised for this
defect. At length Mr. Dollond, finding that flint
and crown glass had different refracting powers,
and that crown glass (the common window glass)
dispersed the rays of light less than any other,
adapted two convex glasses of crown glass to a
double concave of flint glass (which has the great-
est dispersive power), so as exactly to fit, and by
that means made them counteract each other, so
that the field of view is presented perfectly colour-
less. These telescopes, therefore, are called achro-
matic (or colourless) telescopes.

The reflecting telescope accomplishes- by re-
flecting the rays issuing from any object, what
the last did by refracting them. Let ab, (PL
XVI. fig. 76) be a distant object to be viewed ;
parallel rays issuing from it, as ac and bd, will be
reflected by the metallic concave mirror, cd to sty
and there brought to a focus, with the image a
little further and inverted, agreeably to the effect
of a concave mirror on light, as formerly described.
The hole in the mirror cd does not distort or hurt
the image st9 it only loses a little light ; nor do
the rays stop at the image st ; they go on, and
cross a little before they reach the small concave
mirror en : from this mirror the rays are reflected
nearly parallel through the hole O, in the large

Vision and Optical Glasses. 197

mirror, to R ; there they are met by the plano-
convex lens hi, which brings them to a conver-
gence at S, and paints the image in the small tube
of the telescope close to the eye. Having by this
lens, and the two mirrors, brought the image of
the object so near, it only remains to magnify this
image by the eye-glass Jcr ; by which it will ap-
pear as large as zy.

To produce this effect, it is necessary that the
large mirror should be ground so as to have its
focus a little short of the small mirror, as at q ;
and that the small mirror should be of such con-
cavity as to send the rays a little converging
through the hole o ; that the lens hi should be of
such convexity as to bring those converging rays
to an image at S ; and that the eye-glass Icr should
be of such a focal length, and so placed in the
tube, that its focus may just enter the eye through
the small hole in the end of the tube.

To adapt the instrument to near or remote ob-
jects, or rather to rays, that issue from objects
converging, diverging, or parallel, a screw, at the
end of a long wire, turns on the outside of the
tube, to take the small mirror nearer to, or fur-
ther from, the large mirror ; and so as to adjust
their foci according to the nearness or remoteness
of the objects. The sun-glass at the end of the
small tube should be unscrewed, when any other
object, except the sun, is looked at. This pecu-
liar construction of the reflecting telescope is
called the Gregorian telescope, from the name of
its inventor.

198 Experimental Philosophy. [Lecture IS.

To estimate the magnifying power of the Gre-
gorian telescope, multiply the focal distance of the
large mirror by the distance of the small mirror
from the image S ; then multiply the focal dis-
tance of the small mirror by the focal distance of
the eye-glass Tcr ; lastly divide these two products
by one another, and the quotient is the magnify-
ing power.

Sir Isaac Newton formed his telescope upon
a somewhat different principle from that of
Gregory. In his instrument, still known by the
name of the Newtonian telescope, instead of the
small concave mirror en, there is placed diago-
nally a plane mirror, on which the spectator looks
through the side of the telescope by means of an
eye-glass adapted to that purpose. The cele-
brated Dr. Herschel commonly uses the New-
tonian telescope on an improved principle, and
through that makes most of his observations.

Dr. HerschePs great telescope is however of a
different construction. It has only one large con-
cave reflector at the bottom of the tube ; and the
spectator stands with his back to the object, and
looks in upon the reflector through an eye-glass.
The magnifying power of this is the same as that
of a Newtonian telescope would be of the same
sized reflector; but, there being only one re-
flector, the quantity of light is less diminished.
A minute description of this curious telescope is
given under the word TELESCOPE in that uni-
versal dictionary called the Pantologia*

LECTURE XIV.

EXPERIMENTAL PHILOSOPHY.

COLOURS.

I HAVE explained the nature of vision, and
that it is by means of the rays of light which
are sent from the different objects that sur-
round us to our eyes that they are rendered
visible. But you are yet at a loss to understand
whence proceed the infinite variety of colours in
which the whole creation is superbly arrayed.
You must be rendered sensible of these colours
by means of the light : but you will be surprised
to learn that the colours are not in the things,
but in the light itself; and that every beam or
pencil of light is composed of particles of different
colours. " The blushing beauties of the rose,
the modest blue of the violet," says Goldsmith,
"are not in the flowers themselves, but in the
light that adorns them: odour, softness, and
beauty of figure, are their own ; but it is light
alone that dresses them up in those robes which
shame the monarch's glory."

You must have observed yourselves, that the
colours of objects are essentially altered by the
light in which they are seen. The colours of

200 Experimental Philosophy. [Lecture 14.

various pieces of silk or woollen stuff are not
the same by day as by candle light ; but there
is a common experiment which will yet more
forcibly illustrate what I have been observing,
and prove that colour is not in the objects, but
in the light by which they are seen. Let a pint
of common spirit, the cheapest will answer as
well as the best, be poured into a soup-dish, and
then set on fire : as it begins to blaze, let the
spectators stand round the table, and let one
of them throw a handful of salt into the burning
spirit (still keeping it stirred with a spoon). Let
several handfuls of salt be thus successively
thrown in ; the spectators will see each other
frightfully changed, their colours being altered
into a ghastly blackness. It is plain, then, that
the solar rays are composed of matter different
from the light which is emitted by this flame ;
and the truth is, that the light of a candle is
somewhat different from both.

But the genius of Newton has enabled us to
go still further in ascertaining the nature of
light. He has analysed it with as much expert-
ness as a chemist analyses any physical sub-
stance, and has divided it into its component
parts. To this noble discovery the great philo-
sopher was led rather by accident than by de-
sign ; but a mind such as Newton's was able to
improve whatever hint chance submitted to his
view. It was in attempting to rectify the errors
arising from the aberration of light in the glasses

Colours. 201

of the telescope, that his attention was directed
to the wonderful effect which is produced by a
prism.

The prism of the opticians is a triangular pris-
matic piece of glass, usually of the length of
about three inches. If a small hole ~Fr fig. 77,
is made in the window- shutter, EG, of a dark
chamber, and a beam of light, SF, proceeding
directly from the sun (for the experiment will
only succeed when the sun shines), is made to
pass through the prism, ABC, an image of the
sun, PT, will be represented on the sheet of
paper, MN, fixed to the opposite wall. But
you will observe two very extraordinary cir-
cumstances attending this representation of the
sun. The first, that the figure is not round but
oblong; and, secondly, if you will observe the
figure in the plate, you will see that it is intended
to represent different colours, and in the real
image these colours will be found extremely
vivid. On measuring the image, which philo-
sophers have agreed in calling a spectrum. Sir
Isaac Newton found that, at the distance of
eighteen feet and a half from the prism, the
breadth of the image was two inches and a half,
and its length ten inches and one quarter, that
is, nearly five times its breadth. The sides were
right lines distinctly bounded, and the sides were
semicircular, as in the plate. From this it was
evident that it was still the image of the sun,
but elongated by some refractive power in the

202 Experimental Philosophy. [Lecture 14.

glass. In the image PT the- colours succeeded
in this order from the bottom at T, to the top
at P, namely red, orange, yellow, green, blue,
indigo, violet*.

Unable as yet to account for the phenomenon,
he was induced to try the effect of two prisms,
and he found that the light, which by the first
prism was diffused into an oblong, was by the
second reduced to a circular form, as regularly
as if it had passed through neither of them.
After various conjectures and experiments, he
had recourse, at length, to what he calls the
experimentum crucis. At the distance of about
twelve feet from the prism, which was close to
tiie aperture F, he placed a board which might
receive the image in the same manner as the
sheet of paper MN. In this board there was
also a small hole, through which some of the
light might pass ; behind this hole, then, he
placed a second prism, and, by moving the first
prism, he made the several parts of the image
cast by it on the board to pass successively
through the hole, so as to be refracted again
upon the wall by the second prism. He found
then, that the different colours of the spectrum,
when permitted to pass through the hole in the
board, were incapable of further decomposition :

* These, taken in an inverse order, are readily called
to mind, by means of the word vilgyor, formed of the
successive initials of violet, indigo, Mue green, yellow,
orange, red.

Colours. 203

that the red rays continued red, the orange
the same, he. The cause of the phenomenon,
therefore, was no longer a secret. It was plain
that every beam of light consisted of particles
different in colour, or which rather have the effect
of producing different colours, and that all of
them blended together formed white. It was
further evident, that the particles of one colour
were more refrangible than those of another ;
and therefore those which formed the upper part
of the image or spectrum suffered a much greater
refraction than those at the bottom; in other
words, were more under the influence of the at-
tractive powers of the glass. Hence it was further
evident why the figure or spectrum was of an
oblong form instead of round ; for the particles
of light, being differently refrangible, were spread
out longitudinally by the action of the prism.

Various experiments will convince you that
white light is no more than a compound of
these parti-coloured rays or particles. Thus, if,
instead of the sheet of paper MN, you sub-
stitute the large convex glass D, see fig. 78, in
its place, the scattered rays will be converged and
united at W, where, if the paper is placed to
receive them, you will see a circular spot of a
lively white. At W also the rays will cross
each other ; and if the paper is removed a little
further, you will see the prismatic colours again
displayed as at RV, only in an inverted order,
owing to the crossing of the rays.

204 Experimental Philosophy. [Lecture 14.

To show further in what manner white is
produced. Let two circles be drawn, as in fig.
79, on a smooth round board ABCDEFG, and
the outermost of them divided into three hundred
and sixty equal parts or degrees : then draw seven
right lines, as A, B, &c. from the centre to the
outermost circle; making the lines A and B
include eighty degrees of that circle; the lines
B and C forty degrees ; C and D sixty ; D and
E sixty ; E and F forty-eight ; F and G twenty-
seven; G and A forty-five. Then, between
these two circles, paint the space AG red, in-
clining to orange near G ; GF orange, inclining
to yellow near F ; FE yellow, inclining to green
near E ; ED green, inclining to blue near D ;
DC blue, inclining to indigo near C ; CB indigo,
inclining to violet near B; and BA violet, in-
clining to a soft red near A. This done, paint
all that part of the board black which lies within
the inner circle; and putting an axis through
the centre of the board, let it be turned very
swiftly round that axis, so that the rays pro-
ceeding from the above colours may be all blended
and mixed together in coming to the eye; and
then the whole coloured part will appear like a
white ring, a little grayish ; not perfectly white,
because no colours prepared by art are perfect.

Any of these colours, except red and violet,
may be made by mixing together the two con-
tiguous prismatic colours. Thus, yellow is made
by mixing together a due proportion of orange

Colours. 205

and green ; and green may be made by a mixture
of yellow and blue.

The theory of colours is therefore now un-
folded. Those bodies, or those parts of bodies,
which have the property of reflecting only the
red-making rays, will appear red; those which
reflect the violet will be violet, &c. ; and those
which reflect some rays of one colour and some
of another will be the intermediate shade or colour
between both ; and as white is a compound of all
the seven primary colours, so black is an entire
deprivation of them all; and when an object
appears black, the light is completely absorbed,
or at least not reflected by it. To prove, however,
still more forcibly that colour is not in the objects,
but in the light itself; no object whatever can
reflect any other kind of light than that which is
thrown upon it ; and when any one of the pri-
mitive rays has been separated from the rest,
nothing can change its colour. Send it through
another prism, expose it in the focus of a burning
glass, yet still its colour continues unaltered ; the
red ray will preserve its crimson, and the violet
its purple beauty ; whatever object falls under
any of them soon gives up its own colour,
though ever so vivid, to assume that of the
prismatic ray. Place a thread of scarlet silk
under the violet-making ray, the ray continues
unaltered, and the silk instantly becomes purple.
Place an object that is blue under a yellow ray,
the object immediately assumes the radial colour.

206 Experimental Philosophy. [Lecture 14.

In short, no art can alter the colour of a
separated ray ; it gives its tint to every object,
but will assume none from any ; neither reflec-
tion, refraction, nor any other means can make
it forego its natural hue ; like gold, it may be
tried by every experiment, but it will still come
forth the same.

In whatever manner we consider the colour of
a single prismatic ray, we shall have new cause
to admire the beauties of nature. Whatever
compositions of colouring we form, if examined
with a microscope, they will appear a rude heap
of different colours unequally mixed. If by
joining, for instance, a blue with a yellow, we
make the common green, it will appear to the
naked eye moderately beautiful; but when we
regard it with a microscopic attention, it seems a
confused mass of yellow and blue parts, each
particle reflecting but one separate colour : but
very different is the colour of a prismatic ray ;
no art can make one of equal brightness, and
the more closely we examine it the more simple
it appears. To magnify the parts of this colour
would be but to increase its beauty.

The red and orange rays, you have seen, are
least subject to refraction, or are least turned
out of their way by the interposition of the
glass; they are therefore, we may conclude,
either larger than the rest, or propelled with
greater force ; in technical language, they have
the greatest momentum. Agreeably to this we

The Rainbow. 207

find, that when the eyes are very weak they can
scarcely support a scarlet colour; its impres-
sions are too powerful, and, next to the solar
beam itself, dazzle and disturb the organ. On
the contrary, the more refrangible the rays (the
violet for instance), the less forcibly they strike
the eye; and green, the intermediate colour,
is the most agreeable, and is that in which
Providence has chosen to array the meadows
and the woods, in a delightful variety, the di-
versities of green being greater than those of any
other colour.

Of all the objects of nature the rainbow ex-
hibits the prismatic colours in the greatest per-
fection. It is, indeed, a natural prism, and
separates the component particles of light with
the same accuracy and precision.

The rainbow was one of those phsenomena
which astonished and perplexed the antients;
and, after many absurd and unsuccessful con-
jectures, their best philosophers, Pliny and
Plutarch, relinquished the inquiry as one which
was above the reach of human investigation. In
the year 1611 Antonio de Dominis made a con-
siderable advance, however, to the true theory,
by suspending a glass globe in the sun's light,
when he found that, while he stood with his
back to the sun, the colours of the rainbow
were reflected to his eye in succession by the
globe, as it was moved higher or lower. He
was, however, unable to account for the pro-

208 Experimental Philosophy. [Lecture 14.

duction of the different colours, as the experi-
ments with the prism had not yet been made,
and it was reserved for Newton to perfect the
discovery.

To begin, however, with the experiment of the
former philosopher, let us suppose ourselves in
his place. Let A, (PL XVII. fig. 80,) be a glass
globe, and ScZ a ray from the sun, and falling
on the globe at d ; it will, in that place, suffer a
refraction, and instead of going on to c will be
bent to n. From n a part of the light will be
reflected (for a part will necessarily pass through),
and falling obliquely at o, it will again be re-
fracted. In this case you see that the globe,
from its form, will act in some measure like a
prism, ^and the ray will be separated into its
component parts. An eye, therefore, situated
at g, w7ill see the red rays at the line just above
the orange, Sec. and so on to the violet. Now
you wilf recollect, that in a shower of rain there
are drops at all heights, and therefore the eye
situated at g will see all the different colours.

This will account for the first or primary
bow, which you see is thus formed by two re-
fractions and one reflection; but there is often
a second bow on the outside of the other,
which is rather fainter, and which is made by
two refractions and two reflections. To ex-
plain this, take a similar glass globe, B, fig. 81.
Let the ray T in that enter at the bottom of the
globe at r, where it is refracted, and part of the

The Rainbow. 209

light will escape at *, and the rest, instead of
escaping to w9 will be reflected to t ; from this,
part will escape to x, and part will be again re-
flected to u9 where it suffers another refraction,
and is sent to the eye at g, where the violet rays
will be first visible, and then the others in suc-
cession.

Now each drop of rain may be considered as
a small globe, and within a certain range will
refract and reflect the light in the manner above
described. To make the matter still plainer,
therefore, let us for the present imagine only
three drops of rain, and three degrees of colours
in the section of a bow (fig. 82). It is evident
that the angle CFE is less than the angle BFE,
and that the angle AFE is the greatest of the
three. This largest angle then is formed by the
red rays, the middle one consists of the green*
and the smallest is the purple. All the drops
of rain, therefore, that happen to be in a cer-
tain position to the eye of the spectator, will
reflect the red rays, and form a band or semi-
circle of red; those again in a certain position
will present a band of green, &c. If he alters
his station, the spectator will still see a bow,
though not the same bow as before; and if there
are many spectators, they will each see a different
bow, though it appears to be the same.

The phsenomenon assumes a circular appear-
ance, because it is only at certain angles that the
coloured or refracted rays are visible to our eyes,

210 Experimental Philosophy. [Lecture 14

as is evident from the experiment with the glass
globe, which will only refract the rays in a certain
position. The least refrangible, or red rays, make
an angle of forty-two degrees two minutes, and
the most refrangible, or violet rays, an angle of
forty degrees seventeen minutes. Now if a line
is drawn horizontally from the spectator's eye, it
is evident that angles formed with this line, of a
certain dimension in every direction, will produce
a circle, as will be evident by only attaching a
cord of a given length to a certain point, round
which it may turn as round its axis, and in every
point will describe an angle with the horizontal
line of a certain and determinate extent.

From an analytical investigation (which, how-
ever, it would not be consistent with our plan to
introduce here* ) it results that the total breadth
of the interior bow is 2° 15', that of the exterior
bow 5° 407, and the distance between them 8? 25'.

We see a greater or a less part of the rainbow,
according as the sun is more or less elevated above
the horizon. When die luminary is near the
plane of the horizon, then the axis of vision (as
EF) which is at the same time, that of the cone
formed by all the effectual rays, coincides with
the horizon ; and the rainbow, in this case, is a
•emkarcle. In proportion as the sun is elevated,
the axis EF sinks below its first position, and the

• It ma? be seen in a note at page 21 8, rol. ii. of Gregory's
translation of Hauy's Philosophy.

Colours. 211

bow regularly diminishes. Lastly, when the sun
is 42° above the horizon, the axis being sunk the
same number of degrees below that circle, the
summit of the rainbow touches the horizon : so
that, when the sun is higher than this no primary
bow can be seen. A portion, however, of the
exterior or secondary bow, may be seen, if the
sun have any elevation between 42° and 54°.

If we stand on an eminence, when the sun is
at the horizon, a rainbow exceeding a semicircle,
(and, indeed, in favourable circumstances, ap-
proaching to an entire circle), may be seen,

As the cause of colours must be now apparent
to you, and as it is evident that they must pro-
ceed from some quality in bodies or their surfaces,
which causes them to reflect rays of a particular
hue, you will easily understand why some bodies,
which are called semipellucid, afford one colour
by transmitted, and another by reflected light.
The truth is, the beam of light in passing through
them is dissected and separated, and part of one
colour is permitted to pass through, and part is
sent back. If a solution of a wood called lignum
nephriticum is put into a clear phial, when viewed
only by the reflected light which falls upon it, the
solution will appear blue ; but if held up against
the light, and seen through, the colour will be a
fine yellow. The same is found to be the case
with some precious stones, and some glass compo-
sitions. Thus, if a small quantity of arsenic is
mixed in the composition of glass, the mass will

Experimental Philosophy. [Lecture 14.

appear bluish white by the reflected light, but
orange by that which is transmitted through it.

The blue colour of the sky may be accounted
for upon this principle. The atmosphere may
be considered as a semipellucid medium, which
is loaded with small and light particles of va-
pour ; and these particles may be compared with
the particles of arsenic, which are mingled in the
glass above mentioned. If the air is very heavily
charged with these vapours, therefore, a large
proportion of the light will be reflected, and
that dusky whiteness appears which distinguishes
mists and fogs ; but in a clear state of the atmo-
sphere only the weaker and more refrangible rays,
such as the blue, violet, &c. are reflected, and
hence proceeds the blue colour of the sky.

On the same principle depends the green
colour of the sea. It is a mixed mass, charged
with heterogeneous particles. All the more re-
frangible rays, therefore, are reflected, while the
stronger rays, the red, orange, &c. are trans-
mitted. Thus Dr. Halley, in a diving-bell, sunk
many fathoms deep in the sea, observed, that
when he extended his hand out of the bell into
the water, the upper part of it was red, and the
lower part a blueish green. The redness was
occasioned by the strong red rays, which in
their progress through the mass of water were
intercepted and reflected by his hand ; while,
on the contrary, the heterogenous particles dis-
persed through the water reflected only the re-

Colours. 213

frangible rays, so as to afford the appearance of
green. These principles applied to many other
of the phenomena of nature will serve to explain
their causes ; and if they excite you but to use
your own understandings, and to think for your-
selves, this sketch of the phenomena of light and
colours may be of as essential service to you as
the most laboured detail.

Since the former editions of this work were
published, philosophers have entered into a new
field of investigation in the region of optics. Be-
sides the properties of light indicated by the
words reflection, refraction, and Inflection, there
has recently been discovered another, denomi-
nated polarization. Dr. Sebeck in Germany, Dr.
Brewster in Scotland, and M. M. Malus and
Biot in France, are the philosophers to whom we
owe the principal discoveries in this new track of
inquiry.

When the particles of light traverse crystal-
lized bodies, endowed with a double refraction
(such, for example, as Iceland spar), they expe-
rience about their centre of gravity divers mo-
tions, which depend upon the nature of the forces
which the particles of the crystal exercise upon
them. Sometimes the effect of these forces is
limited to disposing all the moleculae of the same
ray similarly the one to the other, in such manner
that their homologous faces are turned towards
the same parts of space. This is the phenomenon
to which Malus gave the name of polarization,

Experimental Philosophy. [Lecture 14.

assimilating the effect of the forces to that of a
magnet, which should turn the poles of a series
of magnetized needles all in the same direction.
When this disposition obtains, the luminous par-
ticles are retained in the whole extent of the
crystal, and experience no farther motion about
their centre of gravity. But there exist other
cases where the particles which traverse the
crystal are not fixed to a constant position.
During all the time of their passage, they oscil-
late about their centre of gravity with computa-
ble velocities and periods. Sometimes, again,
they turn upon themselves, as it were, with a
continued motion of rotation.

The various phenomena, thus briefly alluded
to, are classified under the terms fixed and movea-
ble polarization. The philosophers above named
have established, illustrated, and confirmed them,
by a great variety of striking experiments ; and
some new instruments (such, for example, as the
calorigrade, now sold by opticians) have ori-
ginated from these researches. The train of dis-
coveries connected with polarization is by no
means completed. It has, however, already fur-
nished a most striking confirmation of the New-
tonian theory of colours, and of the rainbow,
establishing their correct accordance with nature
and truth, even in the minutest particulars.

The best account which has yet been given to
the world of the discoveries relating to polariza-
tion, may be found in the fourth Vol. of Biot's

Colour*. 215

Treatise on Natural Philosophy. This philoso-
pher, however, has fallen into some strange errors
in his explication : we, therefore, hope that Dr.
Brewster, whose researches into the nature of
polarization have been extensive, elaborate, and
successful, will speedily favour the world with a
connected view of the whole subject.

LECTURE XV.

EXPERIMENTAL PHILOSOPHY.

THE LAWS OF MOTION.

EVERY thing in mechanics depends upon very
simple principles, and may be resolved ultimately
into the power of gravity and the laws of mo-
tion.

In treating of gravitation, in our second lec-
ture, it was shown to be that kind of attraction
which subsists between the mass of the earth and
all those bodies which are on its surface. It
is that which, in the stated revolutions of this
planet, prevents us, and all the bodies which
surround us, from falling into infinite space ; and
which draws so forcibly every thing whatever
towards the centre of the earth.

That this attraction is greater or less at different
distances is generally allowed ; a body which at
one semidiameter of the earth weighs one pound
will have four times less weight at two semidia-
meters, and nine times less at three. At small
distances, however, we are not sensible of this
difference in weight; for though we could be
elevated a mile above the earth's surface, when
we consider that its diameter is about eight thou-

The Laws of Motion. 217

Band miles, we shall easily see that the small dif-
ference which this would produce is scarcely to
be estimated.

Falling bodies, however, we know, acquire an
accelerated or increased force, according to the
height from which they are precipitated; but
this mast be accounted for from different prin-
ciples. Every man is sensible that the fall of a
stone is to be dreaded in proportion to the height
from which it descends. If it falls from only a
foot above his head, it is not likely to be so fatal
as if it fell from the parapet of a high house.
The falling body, therefore, must of necessity
acquire an increase of velocity in its descent ; and,
in fact, it is said that a leaden bullet let fall from
one of the steeples of Westminster Abbey ac-
quired velocity sufficient to pierce through a deal
board.

This effect must therefore be referred to the
law of acceleration conjointly with the first law of
motion, as laid down by Sir Isaac Newton, which
is, that " all bodies are indifferent to motion and
rest : in other words, a body at rest will continue
in that state, unless put in motion by some exter-
nal impulse ; and a body in motion will continue
that motion for ever, unless stopped by some ex-
ternal obstruction." This property of matter is
termed, in the technical language of philosophy,
its vis intrtice.

To apply this to the case immediately in point,
it is evident that the bullet which is dropped

VOL. i. L

218 Experimental Philosophy. [Lecture 15.

from the steeple of Westminster Abbey, having,
by the power of gravity, once acquired a certain
degree of motion, would continue to fall, by the
motion it had received by the first impulse, even
if the cause were to cease. For instance, if when
it had fallen halfway it were possible to deprive
it of gravity, it would still, by the above law,
continue its motion, and in the direction in which
it was sent, as a stone continues to proceed, when
thrown by the hand, without any new impulse.
The power of gravity, however, does not cease,
and therefore every inch the bullet falls it re-
ceives an increase of motion. Thus, if in the
space of one second it falls one pole (sixteen feet
and a half), it will then have acquired as much
swiftness or velocity as will carry it through three
poles in the next second, through five in the
third, through seven in the fourth, and nine in
the fifth. This. will account for its accelerated
motion, and for the increased force with which
it falls near the bottom. Thus the time which
bodies take in falling is easily calculated ; for, if
they fall about one pole in the first second,
which is what they nearly do by the force of
gravity, they will then fall three in the next, and
in five seconds they will fall about twenty-five
poles, or three hundred feet. These spaces, how-
ever, are a little diminished by the resistance of
the air.

As heavy bodies are uniformly accelerated in
their descent, they are as uniformly retarded by

The Laws of Motion. 219

the power of gravity in their ascent. Thus, if I
were to throw the bullet up to the steeple of
Westminster Abbey, I must give it just as much
force as it acquired in its descent. Thus again,
the body D in rolling down the inclined plane,
A B (Plate XVIII. fig. 83) will acquire suf-
ficient velocity by the time it arrives at B to carry
it up nearly to C ; and if the plane were per-
fectly smooth, and the air gave no resistance, it
would carry it up quite to that point: it is upon
this principle the pendulum is constructed. You
all know, I conceive, that a simple pendulum
consists of a bob or ball fixed to a small string or
wire. If therefore the bob (fig. 84) is let go at
a, it will fall to d, and by the velocity it acquires
in the fall it will rise to c : this is called an oscil-
lation ; and if a pendulum were put in motion in
a space quite void of air, and free from all resist-
ance from friction on the point of suspension, it
would move for ever. Pendulums vibrate in pro-
portion to the square roots of their lengths, and
the vibrations of the same pendulum are always
performed in the same space of time. Hence
their great utility in measuring time ; for a pendu-
lum of thirty-nine inches, one-fifth will vibrate an
aliquot part of the time the earth is turning on
its axis, that is, l-86400dth part, or sixty times
in a minute. Near the equator, however, pendu-
lums move slower than near the poles ; and they
are also subject to variations and irregularities
from heat and cold, which causes the metals, of

220 Experimental Philosophy. [Lecture 1 5,

which the rods are usually formed, to lengthen of
contract.

It is from that sluggishness of motion, which
is called the tis inertice of bodies, that there
proceeds something like an endeavour in all bo-
dies to preserve the state in which they are ;
when at rest to continue in a state of rest, and
when in motion to continue in motion. This
position may seem abstruse, but it will admit of
illustration by the most common facts. If I push
a bowl of water with my hand, the water flies
backwards over the edge upon my hand, for it
endeavours to continue in the state of rest in
which it was. But if I take the bowl in my
hand, and run along with it, and suddenly stop
short, the water flies forward the way I was run-
ning, from its vis inertice, or tendency to continue
in the same state of motion. In the same man-
ner, if I am sitting in the front of a carriage,
which, after going very fast, stops suddenly, I
am jolted from my seat, and my head will, with-
out care, drive through the front glass of the
carriage.

It is a plain and obvious principle, that the
greater the quantity of matter is which any body
contains, the greater will be its vis inertias. The
heavier any body is, the greater is the power
which is required, either to set it in motion or
to stop it. So again, the swifter any body moves,
the greater is its force ; as was sufficiently exem-
plified in the case of a bullet, which was supposed

The Laws of Motion.

to fall from the steeple of Westminster Abbey.
But to make the matter still plainer: if the
roller a (fig. 85) leans against the obstacle b, it
will be found incapable of overturning1 L v but if
a is taken up to c, and suffered to roll down the
inclined plane against #, it will overturn it in-
stantly. It is plain, therefore, that by its conti-
nued motion the roller a has acquired a force
which it had not in itself. The stroke which a
strikes at b is called its momentum. Hence re-
sults the well-known maxim in philosophy, which
I have before had occasion to repeat to you—
" That the whole momentum, or quantity of
force, of any moving body, is estimated by the
quantity of matter multiplied by the velocity or
swiftness with which it moves." When the pro-
ducts, therefore, arising from multiplying the
quantity of matter in any two bodies by their
respective velocities, are equal, we say their mo-
menta, or moving forces, are the same. Thus,
if a body, which I call A, Aveighs forty pounds,
and moves at the rate of two miles in a minute ;
and another body, which I call B, weighs only
four pounds, and moves at the rate of twenty
miles in a minute, the entire force with which
these two bodies will strike each other would be
equal, and each of them would require an equal
force to stop it. For forty multiplied by two
gives eighty, the force of A ; and twenty multi-
plied by four is eighty, the force of B.

Upon this easy principle depends much of

Experimental Philosophy* [Lecture 15.

practical mechanics : and it holds universally
true, that when two bodies are suspended on
any machine, so as to act contrary to each other ;
if the machine is put into motion, and the per-
pendicular ascent of one body multiplied into
its weight is equal to the perpendicular descent
of the other body multiplied into its weight,
those bodies, how unequal soever in their weights,
will balance "one another in all situations : for, as
the whole ascent of one is performed in the same
time with the whole descent of the other, their
respective velocities must be directly as the
spaces they move through ; and the excess of
weight in one body is compensated by the excess
of velocity in the other. Upon this principle it
is easy to compute the power of any mechanical
engine, whether simple or compound; for it is
but only finding how much swifter the power
moves than the weight does (i. e. how much
further in the same time), and just so much is
the power increased by the help of the engine.

The second law of motion laid down by Sir
Isaac Newton is — " That the alteration of the
state of any body from rest to motion, or from
one motion to another, is always in proportion
to the force which is impressed, and in the direc-
tion of that force."

All motion is naturally rectilinear. A bullet
projected by the hand, or shot from a cannon,
would for ever continue to move in the same
direction it received at first, if no other power

The Laws dfMotfai.

diverted its course. When therefore we see a
body move in a curve of any kind whatever, we
conclude it must be acted upon by two powers
at least ; one putting it in motion, and another
drawing it away from the rectilinear course in
which it would otherwise have continued to move :
and whenever that power, which bent the motion
of the body from a straight line into a curve,
ceases to act, the body will again move on in
a straight line touching that point of the curve
in which it was when the action of that power
ceased. For example, a pebble moved round in a
sling ever so long a time, will fly off the moment
it is set at liberty, by slipping one end of the
sling cord : and will go on in a line touching the
circle it described before; which line would
actually be a straight one, if the earth's attraction
did not affect the pebble, and bring it down to the
ground. This shows that the natural tendency of
the pebble, when put into motion, is to continue
moving in a straight line, although by the force
that moves the sling it is made to revolve in a
circle.

From this maxim it will evidently appear, that
when two forces act at once upon the same body,
in different directions, it will go in neither, but
in a course between both. If the billiard ball a
(fig. 86) is struck at once by the two cues b and
f, it will be impelled forward in the diagonal or
middle line, whereas b would have impelled it in
the line e, and c in the line d.

Experimental Philosophy. [Lecture 15.

Or if a boat (fig. 87) is drawn up the stream
by two men on the opposite banks, it will follow
the direction of neither exactly, but will proceed
directly in the middle of the stream.

Suppose again (PL XIX. fig. 88) the body A
to represent a ship at sea ; and that it is driven
by the wind, in the right line AB, with such a
force as would carry it uniformly from A to B
in a minute : then suppose a stream or current
of water running in the direction AD, with such
a force as would carry the ship through an equal
space from A to D in a minute. By these two
forces, acting together at right angles to each
other, the ship will describe the line AEC in a
minute ; which line (because the forces are equal
and perpendicular to each other) will be the
diagonal of an exact square.

If the acting forces are equal, but at oblique
angles to each other, so will the sides of the
parallelogram be : and the diagonal run through
by the moving body will be longer or shorter,
according as the obliquity is greater or smaller.
Thus, if two equal forces act conjointly upon the
body A3 (fig. 89) one having a tendency to move
it through the space AB in the same time that
the other has a tendency to move it through an
equal space AD ; it will describe the diagonal
AGC in the same time that either of the single
forces would have caused it to describe either of
the sides. If one of the forces is greater than
the other ; then one side of the parallelogram will

The Laws of Motion. 225

be so much longer than the other. For if one
force singly would carry the body through the
space A E, in the same time that the other would
have carried it through the space A D, the joint
action of both will carry it in the same time
through the space A H F, which is the diagonal
of the oblique parallelogram A D E F.

If both forces act upon the body in such a
manner, as to move it uniformly, the diagonal
described will be a straight line ; but if one of
the forces acts in such a manner as to make the
body move faster and faster, then the line de-
scribed will be a curve. And this is the case of
all bodies which are projected in rectilinear direc-
^tions, and at the same time acted upon by the
power of gravity, which has a constant tendency
to accelerate their motions in the direction wherein
it acts.

This last is an observation of great importance,
as it is the foundation of the beautiful system of
Newton concerning the planetary motions. The
force which impels these bodies forward in a rec-
tilinear direction, is called the projectile or the
centrifugal force, as driving them from the centre ;
and the force which draws it towards the centre,
or the power of gravity, is called the centripetal
force. Thus, if the body A (fig. 90) is projected
along the straight line A F H in open space, where
it meets with no resistance, and is not drawn aside
by any power, it will go on for ever with the
same velocity, and in the same direction. But

L5

226 Experimental Philosophy. [Lecture 1 5.

if, at the same moment the projectile force is
given it at A, the body S begins to attract it
with u force duly adjusted*, and perpendicular
to its motion at A, it will then be drawn from
the straight line AFH, and forced to revolve
about S in the circle ATW; in the same manner,
and by the same law, that a pebble is moved
round in a sling. And if, when the body is in
any part of its orbit (as suppose at K), a smaller
body, as L, within the sphere of attraction of
the body K, is projected in the right line LM,
with a force duly adjusted, and perpendicular
to the line of attraction LK; then the small
body L will revolve about the large body K in
the orbit NO, and accompany it in its whole
course round the yet larger body S. Here S
may represent the sun, K the earth, and L the
moon. But of this we shall treat more at large
in the lectures on astronomy.

These principles will serve to explain many
facts which will come from time to time under your
observation. Thus if a leaden ball is dropt from
the mast-head of a ship, under swift sail, you
would suppose, before the ball would reach the
deck, the ship would be slid from under it, and
that it would fall behind the ship into the sea,

* To make the projectile force a just balance to the
gravitating power, so as to keep the planet moving in a
circle, it must give such a velocity as the pl.met would
acquire by gravity, when it had fallen through half the
femidiameter of that circle.

The Laws of Motion. 227

This is not the fact ; for the ball falls down by
the side of the mast, as if the ship were at anchor.
Why? Because the ball is under the influence
of two forces ; one horizontal, by the motion of
the ship, which is the same as if you had sent it
forwards from your hand with the same degree
of velocity as the ship moves at ; the other force
is perpendicular, by the power of gravity : so
that though it appears to fall perpendicularly,
it does not, but describes, in space, the same
kind of semi-parabola as a ball shot from a gun.

If I throw a log of wood into the Thames,
when the wind is across the river, the log will
not obey the current, by going down the river,
nor the wind, by going across the river, but will
go in an oblique direction made up of the two.

The third law is, that " re-action is always
equal to action." Thus, in consequence of this
principle, the resistance of a body at rest, which
is acted or pressed upon, acts against a moving
body with a certain degree of power, and produces
the same effects as an active force would have
done in the same direction. Thus, if I strike
an anvil with a hammer, the anvil exerts against
the hammer the same force with which it is struck
itself. Hence a common trick in the country,
of a man lying on die ground with a large anvil
on his breast, and suffering a strong man to
strike it with a sledge hammer with all his
might. If the anvil be very large, its vis inertke
resists the force of the blow, and the man is

228 Experimental Philosophy. [Lecture 15.

perfectly safe. If the anvil were very small,
only the weight of a pound or two, the first
stroke would kill the man.

A pretty experiment of Mr. Walker's will
serve also to illustrate this part of the subject.
" Let a be a little cannon, (PI. XX. fig. 91.)
and b a hollow piece of iron or brass, to slip on
pretty tight upon c c, and of the same weight as
a. Now if half a thimbleful of gunpowder be
put in a, and b shut upon it, both being sus-
pended by two strings ; if the powder is fired,
the parts a and b will be thrown equally distant
from r, the center where they hung; showing
the re-action to be equal to the action. Hence a
heavy gun seems to recoil less than a light one,
on account of its greater vis inertice ; otherwise
its re-action is the same, with the same charge."
Hence it is evident, that when a load is drawn
by a horse, the load acts against the motion of
the horse, and the action of the animal is as much
impeded by the load, as the motion of the load
is promoted by his efforts. Many other illustra-
tions of these laws may be seen in the larger
treatises of mechanics.

Before I proceed to the consideration of the
six mechanic powers, it is necessary to say a few
words on what is called the centre of gravity.

The centre of gravity is that point of a body
in which the whole force of its gravity or weight
is united, and to which its action may usually
be referred. Whatever, therefore, supports that

The Laics of Motion.

point, bears, in fact, the weight of the whole
body ; and while it is supported the body cannot
fall, because all its parts are in perfect equilibrium
about that point. Thus, if I endeavour to balance
my cane, by laying it across upon my finger,
after some time I find a place where neither
end will preponderate. The part, then, which
rests upon my finger is the centre of gravity. An
imaginary line drawn from the centre of gravity
of any body towards the centre of the earth, is
called the line of 'direction , and it is in this line
all heavy bodies will descend.

The difficulty of sustaining a tall body upon a
narrow foundation will be evident, if you attempt
to balance your cane with its small end upon
your finger. Its centre of gravity is somewhere
about the middle of the cane, and unless you
have sufficient dexterity to keep the foundation
on your finger perpendicular under the centre of
gravity, it will undoubtedly fall. In this consists
the great difficulty of posture-masters and rope-
dancers. The dancer on the rope balances him-
self by a long pole loaded at both ends with
lead, and keeps his eye steadily on some point
exactly in the line of the rope, by which he can
see whether his centre of gravity is either on one
side or the other of his slippery foundation, and
if any irregularity takes place he rectifies it by
his balancing pole.

Every body stands firm on its base, when the

Experimental Philosophy. [Lecture 15.

£ direction falls within such base ; for in this
/the body cannot be made to fall, without
first raising the centre of gravity higher than it
was before. Thus, the inclining body ABCD,
(fig 92.) whose centre of gravity is E, stands
firmly on its base CDIK, because the line of
direction EF falls within the base. But if a
weight, as ABGH, is laid upon the top of the
body, the centre of gravity of the whole body
and weight together is raised up to L; and
then, as the line of direction ID falls without the
base at D, the centre of gravity I is not sup-
ported ; and the whole body and weight tumble
down together.

As a practical illustration of this, I shall
mention that the tower of Pisa (fig. 93.) leans
sixteen feet out of the perpendicular, and stran-
gers are consequently afraid to pass under it. If,
however, the materials will hold together, there
is no necessity for any such apprehension. For
if the plummet c is let fall from its centre of gra-
vity, you will see that the line of direction is
within its base or foundation, and therefore it
has stood without a miracle these three hundred
years.

The nearer the centre of gravity and the line
of direction coincide, the firmer any body stands
upon a horizontal plane. If the plane is inclined
a body will slide down it, if the line of direction
falls within the base; but it will tumble down

The Laws of Motion. 231

when that line falls without the base. Thus the
body A (fig. 94.) slides down the inclined plane
C D, while the body B rolls down upon it.

The broader the base the firmer any body
stands ; thus you find you stand firmer with your
feet a little asunder than when close together ;
and in the former case it will require a much
greater force to push you down. Hence the advan-
tage of walking with the feet rather wide asunder,
on a slippery pavement in frosty weather. When-
ever the line of direction, however, falls without
the base of our feet, we necessarily fall ; " and
it is not only pleasing," says Mr. Ferguson,
" but even surprising, to reflect upon the various
and unthought-of methods and postures which
we use to retain this position, or to recover it
when it is lost. For this purpose we bend our
body forward when we rise from a chair, or when
we go up stairs: and for this purpose a man
leans forward when he carries a burden on his
back, and backwards when he carries it on his
breast ; and to the right or left side as he carries
it on the opposite side." A thousand more in-
stances might be added, but they will readily
suggest themselves to the mind of reflecting
persons.

/ LECTURE XVI.

EXPERIMENTAL PHILOSOPHY.

THE MECHANIC POWERS.

MAN, considered as to his bodily structure, is
but a feeble creature ; it is mind which gives
him a superiority over other animals. Con-
trivances to assist his natural powers we have rea-
son to believe took place at a very early period
of society, as we find few nations, even in the most
savage state, which are entirely without them.
It is philosophy, however, which explains their
theory and uses, and which extends their appli-
cation.

When we survey the vast variety of complex
machines, which one of our great manufactories,
for instance, exhibits, we are struck with astonish-
ment, and the creative genius of man appears to
the greatest advantage ; but the surprise of the
unscientific person will be increased, when he
learns that this vast assemblage of mechanism is
reduced into six simple machines or powers, from
which, and their different combinations, the most
stupendous works of human art are produced.
These machines are ; 1. the lever ; 2. the wheel
and axle ; 3. the pulley ; 4. the inclined plane ;
5. the wedge ; and 6. the screw.

1. The lever is, perhaps, the simplest of all

Mechanic Powers.

the mechanic powers, and was probably the first
which was brought into use. It is a bar of iron
or wood, one part of which is supported by a
prop, and upon that prop all the other parts turn
as on their centre of motion. You see the lever
made use of in one form or other every day when
a labourer takes a hand-spike, or large stake,
and placing a stone under some part near the
end, by putting the extremity under a cask, a
piece of timber, or any other body, and attempts
to move it, by pulling at the other end, he makes
use of a lever. The handle of a pump is a lever
also ; even the poker with which I raise the fire
is a lever, the bar of the grate is the prop, and
at the end which I hold in my hand is applied
the strength or power. This is, however, not
the only kind of lever, for in fact there are three
different sorts or orders of these instruments.
The first is that which I have been describing,
viz. when the prop is placed between the weight
to be raised and the power (see fig. 95.) In this
figure ABC is the lever; D is the fulcrum or
prop; and the part AB and BC, on different
sides of the prop, are called the arms of the lever.
It is demonstrable that in this instrument the
nearer the prop is to the end A, and the longer
the arm BC is, the less force will be required to
effect any given purpose. This is, indeed, re-
duced to a matter of experiment. For let P repre-
sent a power, whose gravity is equal to one ounce;
and W a weight, whose gravity is equal to twelve

534* Experimental Philosophy/. [Lecture 16.

ounces. Then, if the power is twelve times as
far from the prop as the weight is, they will ex-
actly counterpoise ; and a small addition to the
power P will cause it to descend, and raise the
weight W; and the velocity with which the power
descends will be to the velocity with which the
weight rises, as twelve to one : that is, directly as
their distances from the prop ; and consequently,
as the spaces through which they move. Hence
it is plain that a man who by his natural strength,
without the help of any machine, could support
a hundred weight, will by the help of this lever
be enabled to support or rather raise twelve hun-
dred. If the weight is less, or the power greater,
the prop may be placed so much farther from the
weight, and then it can be raised to a prpportion-
ably greater height. For, universally, if the in-
tensity of the weight multiplied into its distance
from the prop is equal to the intensity of the
power multiplied into its distance from the prop,
the power and weight will exactly balance each
other ; and a little addition to the power will
raise the weight. Thus, in the present instance,
the weight W is twelve ounces, and its distance
from the prop is one inch ; and twelve multiplied
by one is twelve; the power P is equal to one
ounce, and its distance from the prop is twelve
inches, which multiplied by one is twelve again ;
and therefore there is an equilibrium between
them. So, if a power equal to two ounces is ap-
plied at the distance of six inches from the prop,

Mechanic Powers. £35

it will just balance the weight W; for six multi-
plied by two is twelve, as before. And a power
equal to three ounces placed at four inches dis-
tance from the prop would be the same ; for
three times four is twelve; and so on, in pro-
portion.

The statera, or Roman steelyard, is a lever oif
this kind, and is used for finding the weights of
different bodies by one single weight placed at
different distances from the prop or centre of mo-
tion D. For if a scale hangs at A, the extremity
of the shorter arm, AB, is of such a weight as
will exactly counterpoise the longer arm EC ; if
this arm is divided into as many equal parts as it
will contain, each equal to AB, the single weight
P (which we may suppose to be one pound) will
serve for weighing any thing as heavy as itself, or
as many times heavier as there are divisions in
the arm BC, or any quantity between its own
weight and that quantity. As for example, if
P is one pound, and placed at the first division,
one in the arm BC, it will balance one pound in
the scale at A ; if it is removed to the second
division at two, it will balance two pounds in the
scale ; if to the third, three pounds ; and so on
to the end of the arm BC. If each of these in-
tegral divisions is subdivided into as many equal
parts as a pound contains ounces, and the weight
P is placed at any of these subdivisions so as to
counterpoise what is in the scale, the pounds and
odd ounces will by that means be ascertained.

236 Experimental Philosophy. [Lecture 16.

To this kind of lever may be reduced several
sorts of instruments, such as scissars, pincers,
snuffers, which are made of two levers acting
contrary to one another, their prop or centre of
motion being the pin which keeps them together.

The second kind of lever has the weight to be
raised between the prop and the power. Thus,
in raising the water-plugs in the streets of Lon-
don, you will see the workman put his iron crow
through the hole of the plug till he rests the fur-
ther extremity of it on the ground, and making
that his prop, he raises the lever or crow, and
draws out the plug. In this lever, as in the for-
mer, the longer the arm of the power is, or the
greater the distance of the workman from the
weight, the more is his natural force assisted by
the machine. To estimate this, if A B (fig. 96.)
is a lever on which the weight W of six ounces
hangs at the distance of one inch from the prop
G, and a power P equal to the weight of one
ounce hangs at the end B, six inches from the
prop, by the cord CD going over the fixed pulley
E, the power will just support the weight ; and
a small addition to the power will raise the weight
one inch for every six inches that the power
descends.

This lever shows the reason why two men car-
rying a burden upon a stick between them, bear
unequal shares of the burden in the inverse pro-
portion of their distances from it. For it is well
known, that the nearer any of them is to the

Mechanic Powers. 237

burden the greater share he bears of it ; and if
he goes directly under it, he bears the whole. So
if one man is at G, and the other at B, having
the pole or stick AB resting on their shoulders;
if the burden or weight W is placed five times
as near to the man at G, as it is to the man at B,
the former will bear five times as much weight as
the latter. This is likewise applicable to the
case of two horses of unequal strength to be so
yoked, as that each horse may draw a part pro-
portionate to his strength ; which is done by so
dividing the beam they pull, that the point of
traction may be as much nearer to the stronger
horse than to the weaker, as the strength of the
former exceeds that of the latter.

To this kind of lever may be reduced oars,
rudders of ships, doors turning upon hinges,
cutting-knives which are fixed at the point of
the blade, &c.

The third kind of lever is when the power is
placed between the weight arid the prop. An
example of this kind of lever you see when a
man raises a long ladder to place it against a
wall. It is obvious that this kind of lever, so
far from assisting human strength, requires a
power much greater than the weight to be raised.
For let E (fig. 97.) be the prop of the lever AB,
and W, a weight of one pound, placed three
times as far from the prop, as the power P acts
at F by the cord C going over the fixed pulley

238 Experimental Philosophy. [Lecture 16.

D ; in this case the power must be equal to three
pounds, in order to support the weight.

Disadvantageous as this kind of lever appears,
it is upon this principle the human arm is con-
structed ; for the muscle which moves the arm,
and which is inserted in the bone below the
elbow, may be considered as the power, which
you see is placed between the weight to be raised
by the hand and the prop, or place where the
muscle is inserted above. To compensate for
this disadvantage, these muscles are made unusu-
ally strong, and we may judge of their immense
power by the weights which athletic persons are
enabled to wield. The same power exerted
only on equal terms ought to raise a weight of
ten thousand pounds.

II. The wheel and axle (fig. 98.) is the next
in order of the mechanic powers. The power is,
in this machine, applied to the circumference of
the wheel, and the weight to be raised is fastened
to one end of a rope, of which the other end
winds round an axle that turns with the wheel.
This instrument is more commonly used with a
handle : thus, to wind up a common kitchen
jack, I turn the handle, which coils the cord
round the axle in the middle : to wind a bucket
from a well, I do the same thing ; to wind up
my watch, the same : the handle in all these is
in the place of a wheel, and the farther this
handle is from the centre, the axle, on which the

Mechanic Powers. 239

whole weight is sustained, the more powerful
will it be. Or if it is a wheel, the more its dia-
meter exceeds the diameter of the axle, the
greater will he its power. Thus, if the diameter
of the wheel is eight times as great as that of the
axle, it will have eight times the power ; and a
man who by his natural strength could only lift
a hundred weight, by this machine will be en-
abled to lift eight hundred.

Of this kind are the machines called cranes,
which you see employed at the water-side, for
winding up bales of goods out of ships. The
large circular crane, in which a man or horse
walks and turns it horizontally, is also a machine
of this nature; and the capstan^ which draws up
the cables of ships, and is turned by hand-spikes
inserted in holes at the end of the roller or cap-
stan. The windlass, also used in warehouses
for raising goods, is the wheel and axle ; and,
indeed, many more complex machines may be
resolved into this principle.

The spokes of the wheel, or the winch which
turns the axle, may be considered as levers, and
therefore by some the wheel and axle are referred
to the same principle.

III. The pulley is usually considered as the
third mechanic power, though, in truth, the
single pulley AA (fig. 99.) gives no mechanical
advantage, and only enables us to change the
direction. This is evident from the figure, where
the two equal weights W and P balance each

£40 Experimental Philosophy. [Lecture 1(5.

other as exactly as the arms of a balance or scale
beam, which are of equal lengths. Thus it
gives a man no advantage, except that he can
apply his weight as well as his strength in rais-
ing a body from the earth, and then he can lift
more than his own weight.

With a combination of pulleys, however, the
case is different. For if a weight W hangs at
the lower end of the moveable pulley D, and
the cord GF goes under the pulley, and is fixed
at the top of the hook H on one side, and nailed
to the block C on the other ; it is evident that
H and C between them support the whole weight
W ; H supports one half, and C the other half.
Now suppose I take the support of one of their
halves upon myself, but merely change the direc-
tion of my power, and instead of holding up the
cord at C, throw it over the immoveable pulley
fixed there, and exert my strength below at P;
it will be evident that I support one half the
weight W, and the hook H supports the other.
If therefore I draw the cord at P, the weight W
will continue to rise, but wherever it rises, I con-
tinue to support but half its weight while H sup-
ports the other. Thus, one single moveable
pulley diminishes one half of the weight to be
raised ; if we should add another, it would di-
mmish the half of that which remained, and so
on. For instance, if a weight of eight hun-
dred pounds is to be raised, I use one moveable
pulley, and that will lessen the weight one hah0,

Mechanic Powers.

that is, to four hundred : I add another move-
able pulley, and that will lessen the remaining
four by one half, which is two hundred ; if I
still add a third, that will lessen the remaining
two by one hah0, which is one ; so that if I use
three moveable pulleys in raising eight hundred
weight, I shall be able to raise it with as much
ease as one hundred without them.

As systems of pulleys have no great weight,
and lie in a small compass, they are easily car-
ried, and can be used in many cases where more
cumbrous engines cannot. They have much
friction, however, because the diameter of their
axis bears a very considerable proportion to their
own diameter, because they are apt to rub
against each other, or against the sides of the
block, and because the rope that goes round
them is never perfectly pliant. Still they are
highly useful, and their combinations may be
varied at pleasure, to suit the case in hand, whe-
ther at land or sea.

IV. The inclined plane is very justly regarded
as the fourth mechanic power, though some have
rejected it altogether. The advantage of this
machine (if you will admit of that term) is, that
by means of it a heavy body may be made to
ascend a given height with much less power than
it would require to raise it the same height if it
were perpendicular. This is a very common mode
of assisting human strength ; you will every day
see porters, when they have to roll a cask or

VOL. i. M

Experimental P?iilosophi/. [Lecture 16.

bale up the step of a warehouse, place a board
along from the step to the ground, which ren-
ders the ascent gradual and easy. The power of
the inclined plane is as great as its length exceeds
its perpendicular height. For instance, let AB
(PI. XXII. fig. 100) be a plane parallel to the
horizon, and CD a plane inclined to it ; and sup-
pose the whole length CD to be three times as
great as the perpendicular height AC ; in this
case the cylinder E will be supported upon the
plane CD, and kept from rolling down upon it
by a power equal to a third part of the weight of
the cylinder. Therefore, a weight may be rolled
up this inclined plane with a third part of the
power which would be sufficient to draw it up
by the side of an upright wall. If the plane
were four times as long as high, a fourth part of
the power would be sufficient ; and so on, in pro-
portion. Or, if a weight were to be raised from
a floor to the height AC, by means of the ma-
chine ABCD, (which would then act as a half
wedge, where the resistance gives way only on
one side) the machine and weight would be in
equilibrio when the power applied at AC was to
the weight to be raised as AC to AB ; and if
the power is increased, so as to overcome the
friction of the machine against the floor and
weight, the machine will be driven, and the
weight raised ; and when the machine has moved
its whole length upon the floor, the weight will
be raised to the whole height from A to C.

Mechanic Powers. 243

V. The wedge is nearly allied to the inclined
plane ; indeed it may properly be considered as
two equally inclined planes joined together. You
know that its uses are to cleave or separate wood
or stone, or any heavy bodies that adhere toge-
ther. The power of the wedge is as its length
to the thickness of its back. To show how we
may calculate the force of a wedge, let a (fig.
101) be a wedge, which is interposed between the
two cylinders c and w, which are pulled against
the wedge by the two weights r and s, represent-
ing the resistance to be overcome by the force of
the wedge. If then r and s influence the cylin-
ders each with a force equal to two pounds, the
resistance to be overcome will be equal to four
pounds. Now the length of the wedge a is
twice the thickness of its back, and the weight
o, suspended to it, is two pounds. Here, then,
is a resistance equal to four pounds overcome by
a weight of two pounds, by means of a wedge,
the length of which is double the thickness of
its back. This explains sufficiently what a wedge
will be able to effect by simple weight or pres-
sure ; but we see every day, where a hard stone
or a piece of tough wood is to be cleft by a wedge,
that a ton weight would not force it in, when a
smart stroke of a hammer, which has not a for-
tieth part of that weight, will effect it at once.
In this case we are to have recourse to what was
said in the last lecture on the momentum or force
which is gained by the velocity of a moving

244 Experimental Philosophy. [Lecture 16.

body, and consider that the momentum of a
hammer consists of its weight multiplied by the
velocity with which it moves (which is consi-
derable), and then the effect will appear less ex-
traordinary. It is by means of the momentum
of the hammer striking with considerable ve-
locity, that the wedge is driven in ; and then its
friction keeps it from slipping out again.

VI. The screw (fig. 102) may properly be con-
sidered as an inclined plane wrapt round a cy-
linder. The power of the screw is therefore as
the length of each spiral or thread is to its height,
or, in other words, as the circumference of the
threads to their distance from one another. The
screw, however, can only be wrought by means
of a handle or winch, which is, in fact, a lever,
and it may, therefore, be regarded as a com-
pound machine. To estimate its force, then,
let us suppose that I desire to screw down the
press G upon B ; every turn I make once round
with both handles, I shall drive the press only
one spiral nearer to B ; so that if there are eleven
spirals, I must make eleven turns of the handles,
FL, before I come to the bottom. In pressing
down the screw, therefore, I act with a force as
much superior to the resistance of the body I de-
sire to press, as the circumference of the circle,
which my hands describe in turning the machine,
exceeds the distance between two little spirals of
the screw. For instance, suppose the distance
between the two spirals to be half an inch, and

Mechanic Powers.

the length of both handles twelve inches. My
hands placed upon them in going round will de-
scribe a circle, which, upon calculation, will be
found to be seventy-six inches nearly, and con-
sequently this will be an hundred and fifty-two
times greater than half an inch, which was the
distance between two of the spirals. Thus, if a
bodyjis to be pressed down with this machine,
one man will press it, with this assistance, as
much as an hundred and fifty-two men without
it. Or if the screw were so contrived as to raise
the weight instead of pressing it, which sometimes
is the case, the human force would be assisted in
the same proportion with the same instrument.
But we here only speak as if the handles of the
screw were but twelve inches across, and the
spirals a whole half inch distant from each other ;
what if we suppose the handles ten times as long,
and the spirals five times as close ; the increase
of the human force then would be astonishing.

The power of the screw may, however, be still
more correctly estimated by t what is called the
perpetual screw. To explain this, let the wheel
C (fig. 103) have a screw db on its axle, work-
ing in the teeth of the wheel D, which suppose
to be forty-eight in number. It is plain, that
for every time the wheel C and screw ab are
turned round by the winch A, the wheel D will
be moved one tooth by the screw; and, there-
fore, in forty-eight revolutions of the winch, the

246 Experimental Philosophy. [Lecture 16.

wheel D will be turned once round. Then, if
the circumference of a circle described by the
handle of the winch A is equal to the circum-
ference of a groove e round the wheel D, the
velocity of the handle will be forty-eight times
as great as the velocity of any given point in the
groove. Consequently, if a line goes round the
groove e, and has a weight of forty-eight pounds
hung to it below the pedestal EF, a power equal
to one pound at the handle will balance and sup-
port the weight. To prove this by experiment,
let the circumferences of the grooves of the
wheels C and D be equal to one another ; and
then if a weight of one pound is suspended by a
line going round the groove of the wheel C5
it will balance a weight of forty-eight pounds
hanging by the line g ; and a small addition to
the weight H will cause it to descend, and so
raise up the other weight.

If the line g, instead of going round the
groove e of the wheel D, goes round its axle I,
the power of the machine will be as much in-
creased as the circumference of the groove e
exceeds the circumference of the axle: which,
supposing it to be six times, then one pound at
H will balance six times forty-eight, or two hun-
dred and eighty-eight pounds hung to the Jme
on the axle ; and hence the power or advantage
of this machine will be as two hundred and
eighty-eight to one. That is, a man who, by

Mechanic Powers. 247

his natural strength, could lift a hundred weight,
will be able to raise two hundred and eighty-
eight hundred weight, or 1 4 tons 8 hundred, by
this engine.

But the following engine is still more power-
ful, on account of its having the addition of
four pulleys ; and in it we may look upon all the
mechanical powers as combined together, even
if we take in the balance. For as the axle D of
the bar AB (fig. 104) enters its middle at C, it
is plain that if equal weights are suspended upon
any two pins equi-distant from the axis C, they
will counterpoise each other. It becomes a lever
by hanging a small weight P upon the pin n9 and
a weight as much heavier upon either of the pins
b9 dy or e, as is in proportion to the pins being
so much nearer the axis. The wheel and axle
FG is evident ; so is the screw E which takes in
the inclined plane, and with it the half wedge.
Part of a cord goes round the axle, the rest
under the lower pulley K, over the upper pulley
L, under AT, over /, and then it is tied to a hook
at M in the lower or moveable block, on which
the weight W hangs.

In this machine, if the wheel F have thirty
teeth, it will be turned once round in thirty re-
volutions of the bar AB, which is fixed on the
axis D of the screw E : if the length of the bar
be equal to twice the diameter of the wheel, the
pins e and n at the ends of the bar will move
sixty times as fast as the teeth of the wheel do ;

248 Experimental Philosophy. [Lecture 16.

and, consequently, one ounce at P will balance
sixty ounces hung upon a tooth q in the ho-
rizontal diameter of the wheel. Then if the
diameter of the wheel F be ten times as great as
the diameter of the axle G, the wheel will have
ten times the velocity of the axle ; and therefore
one ounce P at the end of the lever AB will
balance ten times sixty, or six hundred ounces
hung to the rope H which goes round the axle.
Lastly, if four pulleys are added, they will make
the velocity of the lower block K, and weight
W, four times less than the velocity of the axle ;
and this being the last power in the machine,
which is four times as great as that gained by the
axle, it makes the whole power of the machine
four times six hundred, or two thousand four
hundred. So that if a man could lift one hun-
dred weight in his arms by his natural strength,
he would be able to raise two thousand four hun-
dred times as much, or 120 ton weight, by this
engine. But it is here as in all other mechanical
cases ; for the time lost is always as much as the
power gained, because the velocity with which
the power moves will ever exceed the velocity with
which the weight rises, as much as the intensity
of the weight exceeds the intensity of the power.
The friction of the screw itself is very consi-
derable ; and there are few compound engines
which will not, upon account of the friction of
the parts against one another, require a third
part more of power to work them when loaded.

Mechanic Powers. 249

than what is sufficient to constitute a balance be-
tween the weight and the power.

Some philosophers have considered the wheel
and axle, and the system of pulleys, as only mo-
difications of the lever; and the wedge and the
screw as (modifications of the inclined plane.
If this be admitted, we shall then have, instead
of six, only two mechanical powers. The mo-
difications and combinations of these are, how-
ever, almost endless, and wonders are performed,
when to these means of increasing force are
added the most powerful agents in nature, wind,
water, and steam, as exemplified in the wind-
mill, the water-mill, and, above all, the steam-
engine. If the simple and obvious principles I
have here elucidated shall assist the student in '
estimating the advantage of the more common
machines, and stimulate him to pursue his re-
searches into the manner of operation of the
more complex engines to which I have just ad-
verted, these explications will not have been
given in vain.

M 4

LECTURE XVII.

ASTRONOMY.

•

. SYSTEM OF THE UNIVERSE.

ASTRONOMY is that science which treats of
the heavenly hodies.

It is by means of this science that we know
the movement of those bodies, the duration of
their revolutions, whether apparent or real, their
position, their respective distances, &c.

The origin of astronomy is very obscure, and
appears to be also very antient. " There is no
doubt," says Cassini *, " but that astronomy was
known almost from the beginning of the world.
It was not only curiosity which led man to the
study of astronomy, but it may be said that ne-
cessity itself obliged him to it. For if he did
not observe the seasons which result from the
apparent changes of the sun's place, it would
be impossible to succeed in the practice of agri-
culture and other useful arts."

Astronomy, even if it could be considered as
useless to man, derives from its very nature a cer-
tain degree of dignity. But let it be remembered,
that upon it navigation, geography, and chrono-
logy greatly depend. By its aid man passes the

* Memoirs of the Academy of Sciences, vol.vni. page 1.

System of the Universe. 251

seas, and penetrates into foreign climates, be-
comes acquainted with those which he inhabits,
and regulates the dates of ages past.

Hipparchus laid the principal foundations of
a methodical system of astronomy one hundred
and forty-seven years before Christ. On the ap-
pearance of a new fixed star, he took occasion to
make a general catalogue of the stars, assigning
to each its place in the heavens, and its mag-
nitude, so as to enable posterity to ascertain,
whether any new star had appeared, or any of
those which he had observed had suffered any
change. Ptolemy, about two hundred and eighty
years afterwards, added his observations to those
of Hipparchus ; and by the natural advantage
which he possessed over his predecessor, he was
enabled to rectify greatly the observations of the
former philosopher. Ptolemy was the last of the
Greeks who made any considerable improve-
ments in the science of astronomy. It was after-
wards cultivated by the Arabians with great assi-
duity, and success, but did not meet with any
encouragement in Europe till about the middle
of the 13th century. At this period Alphonsus
the Tenth, king of Castile, became its zealous
patron, and immortalized himself by a series of
astronomical tables, which were published under
his direction, and were distinguished by the
name of the Alphonsine tables.

It was not, however, till the sixteenth century
that astronomy was placed upon its proper basis

252 Astronomy, [Lecture IT.

as a science, by the system of Copernicus*, pub-
lished at Nuremberg in 1543, and afterwards
brought to perfection by Kepler, Galileo, and
Newton : — a system so bold and daring, that it
produced general astonishment, and yet its truth
has been confirmed by the observations of every
succeeding age.

The surface of the heavens seems to us to be
studded with stars ; between the fixed stars and
us there seem to be other stars which change
their situations respectively one towards another,
and these all astronomers have agreed in calling
planets^ or wandering stars.

The antient philosophers, who knew very
little even of the movements of the planets, had
no means of knowing the true disposition of
their orbits ; and this is the reason they vary so
greatly in their opinions. They supposed, at
first, the earth to be immoveable, as the centre
of the universe, and that all the celestial bodies
turned about her; which, indeed, was natural
for them to believe, without having discussed the
proofs to the contrary.

It is asserted, however, that the Babylonians,
and afterwards Pythagoras and his disciples,
considered the earth as a planet, and the sun
as immoveable, and the centre of our planetary
system.

Plato is said to have been the reviver of the
system of the immobility of the earth ; and many
* Born at Thorn, in Royal Prussia, in 1472-

System of the Universe. 255

philosophers followed his opinion ; among others
was Claudius Ptolemy, the celebrated astronomer
and mathematician of Pelusium in Egypt, already
mentioned, who lived in the beginning of the
second century of the Christian sera. It is, how-
ever, incredible that, the true system of the world
having been once discovered, the hypothesis by
which the earth is supposed to be the centre of
the celestial movements should have again pre-
vailed ; for though this hypothesis accords with
some of the most obvious appearances, and seems
to agree at first with the simplicity of nature, yet
it is impossible on that system to account for all
the celestial phsenomena.

Ptolemy, who has given the name to this system,
endeavours to prove that the earth T ( PL XXIII.
fig. 105) is immoveable as the centre of the
universe ; and he places the other planets round
about her in the following order, beginning with
those which he believes the next to the Earth :
the Moon D , Mercury $ , Venus ? , the Sun 0,
Mars $ , Jupiter 1£, and Saturn T?5 till he comes
at length to the fixed stars. When, however,
astronomers had begun to observe the planets,
they remarked that Mercury and Venus are
sometimes nearer and sometimes farther from
us than the Sun ; and that Venus never departs
from the Sun more than about forty-seven de-
grees and a half; and Mercury about twenty-
eight degrees and a half, and sometimes much
less. But it is evident that if these two planets

Astronomy. ^Lecture 17,

were turned about the Earth, as they supposed
the Sun himself turned, they would sometimes
appear opposite to the Sun, or more distant from
him than one hundred and eighty degrees;
which never happens. This is the reason why
the Egyptians regarded these two planets as
satellites of the Sun, and thought that they
turned about him, their orbits being carried with
him in his revolutions about the Earth. They
therefore supposed the Earth T (fig. 106) im-
mov cable, as the centre of the system ; and they
supposed the other celestial bodies to turn round
her : first, the Moon D ; secondly, the Sun 0 ;
about which they made Mercury $ and Venus
£ to revolve, till they came to Mars $ , Jupiter
i;, and to Saturn T?; and lastly to the fixed
stars.

At the present day, however, when we know the
immense distance at which the stars are placed,
both these systems become insupportable. They
require that all the heavenly bodies should go
through the whole course of their orbits in about
24 hours, which would give to the fixed stars a
rapidity of motion that exceeds all belief: — nay,
the Sun himself would in a single second have to
describe a space of more than two thousand five
hundred miles.

Copernicus, with a view of obviating the
inconveniences of the imaginary systems that
preceded him, commenced at first by admitting
the diurnal motion of the Earth, or her motion

System of the Universe. 255

round her own axis, which rendered useless that
prodigious celerity in the motions of the heavenly
bodies, of which I have just spoken, and by these
means simplified the system. This motion once
admitted, it was no violent step to admit of
a second motion of the Earth in the ecliptic.
These two motions explain, with the utmost faci-
lity, the phenomena of the stations and motions
of the planets. According to Copernicus, then,
the Sun S (PI. XXIV.%. 107) is the centre of our
planetary system, and the planets turn about him
in the order following ; Mercury g , Venus ? ,
the Earth J, Mars £, Jupiter 1£, Saturn T? ,
(to which we may add Ceres, Pallas, Juno, Vesta,
and the Georgium Sidus $) at a distance from
the Sun, nearly as the numbers 4, 7, 10, 15, 52,
95, 191* The Moon, also, he supposed to be
carried round the Earth in an orbit which goes
along with the Earth in her annual revolution
round the Sun. In like manner about Jupiter,
Saturn, and the Georgium Sidus, are the -four
satellites of the first, the five satellites of the
second, and the two satellites of the third ; none
of which, however, were known to Copernicus.

Although the celestial phenomena explain
themselves with the greatest facility according to
the system of Copernicus, and though observa-
tion and reason are equally favourable to it, yet
it was rejected by an able astronomer who flou-
rished soon after his own time. Tycho-Brahe,
from the experiment that a stone thrown from a

256 Astronomy. [Lecture 17.

high tower fell at its foot, argued that the Earth
must be without motion, never reflecting that
the Earth, in that case, is like a vessel in full
sail, when if a stone is thrown from the mast,
it would fall at the foot of that mast, provided
the motion of the vessel was neither accelerated
nor retarded during the fall. Tycho-Brahe,
therefore, invented a system between that of
Ptolemy and that of Copernicus. He supposed
that the Earth was at rest, and that the other
planets revolving round the Sun, turned also
with him round the Earth in twenty-four hours.
It was towards the end of the sixteenth century
that he proposed his system. He placed the
Earth (fig. 108) immoveable, as the centre, and
made the Moon turn round her, as well as the
Sun S, and the fixed stars : the other planets, viz.
Mercury, Venus, Mars, Jupiter, and Saturn,
turning round the Sun, in orbits which are carried
with him in his revolution round the Earth. As
the system of Tycho-Brahe requires the same
rapidity of motion as that of Ptolemy and of the
Egyptians, it is at once annihilated by the same
arguments.

Leaving, however, for the present the history
of astronomical discoveries, I shall request your
attention to the celestial phenomena.

There are evidently two sorts of stars ; the one
luminous of themselves, and throwing light on
every object which surrounds them to a certain
distance ; such as our Sun, and those which we

System of the Universe. 257

call fixed stars. The others are opake bodies,
as the Earth which we inhabit, not luminous
of themselves, but which shine by a borrowed
light ; in few words, luminous by reflecting that
light which comes from a luminous star: such
are the planets of the first and second order, and
the comets.

The stars of the firmament are said to be fixed,
because they have been generally observed to pre-
serve the same distance from each other : they do
not all appear to us of the same magnitude,
whether they are really different in size one from
the other, or whether they appear so to us in
consequence of their different distances. It is
probable that both these causes operate to exhibit
the fixed stars of such various magnitudes. Be
this as it may, astronomers have agreed in distri-
buting the fixed stars into six different classes,
according to their relative magnitude, inde-
pendent of those small stars which compose the
white and brilliant spaces in the heavens, which
are denominated nebulae, and that bright band
which extends across our hemisphere, and which
from its lucid appearance is termed the milky way.
Those which are distinctly visible are fewer in
number than might be supposed. The British
catalogue, which, besides the stars visible to the
naked eye, includes a great number which can-
not be seen without the assistance of a telescope,
contains no more than three thousand in both he-
mispheres. The number of stars discoverable,,

258 Astronomy. [Lecture 17.

in either hemisphere, by the naked eye, is not
above a thousand. From what we are able to
judge by computation and observation, it is con-
cluded that none of the fixed stars can be at a less
distance than 32,000,000,000,000 of miles from
us, which is further than a cannon-ball would
fly in 7,000,000 of years. The famous French
astronomer Lalande, indeed, makes the distance
by a late computation to be 7,086,760,000,000
leagues.

Though the number of the fixed stars is less
than common observers might imagine, yet it
is still too great, from their resemblance to each
other, to enable us to distinguish them by giving
each a particular name, as has been done with
regard to the planets. Astronomers therefore
have found a commodious method of arranging
them under various figures, called constellations.
They have given to these constellations the names
and figures of various personages celebrated in
antiquity, and even of many animals or of inani-
mate bodies, as instruments, machines, &c. which
fable has feigned to have been carried from earth
to heaven. Ptolemy has enumerated forty-eight
constellations; and there are upon our globes
about seventy. On Senex's, Jones's, and Gary's
globes Bayer's letters are inserted* ; the first in

* In the best of Jones's and Gary's globes, the double,
triple, quadruple, and nebulous stars are indicated by
appropriate characters.

System of ike Universe.

the Greek alphabet being put to the largest star
in each constellation; the second to the next,
and so on ; by which means every star is as
easily found as if a name were given to it. Thus
if the star a, in the constellation of the ram, is
mentioned, every astronomer knows as well what
star is meant, as if it were pointed out to him in
the heavens.

The constellations which surround the ecliptic,
or apparent annual path of the Sun, and which
fill that zone of the heavens which is called the
zodiac, are the twelve following :

Aries, or the ram, <Y»
Taurus, the bull, 0
Gemini, the twins, n
Cancer, the crab, as
Leo, the lion, SI
Virgo, the virgin, «R,
Libra, the balance, &
Scorpio, the scorpion, Wf
Sagittarius, the archer, £
Capricornus the goat, Jcf
Aquarius, the water-bearer^ X
Pisces, the fishes, X .

The zodiac goes quite round the heavens; it
is about sixteen degrees broad, so that it takes in
all the orbits of the old planets, and likewise the
orbit of the Moon.

After having divided the ecliptic into twelve

260 Astronomy. [Lecture 17.

equal parts, which are each thirty degrees, they
have assigned a mark to each of these distances,
and they have given to it the name of the con-
stellation which it contained. The first of these
signs begins always at the point of intersection
of the ecliptic with the equator, in which the Sun
is found at the vernal equinox.

The twenty-one constellations enumerated by
Ptolemy in the northern part of the heavens are,

Ursa minor, the little bear.

Ursa major, the great bear.

Draco, the dragon.

Cepheus.

Bootes.

Corona Borealis, the northern crown.

Hercules, Hercules kneeling.

Lyra, the harp.

Cygnus, the swan.

Cassiopeia, the lady in her chair.

Perseus.

Auriga, the waggoner.

Serpentarius,

Serpens, the serpent.

Sagitta, the arrow.

Aquila, the eagle.

Delphinus, the dolphin.

Equulus, the horse's head.

Pegasus, the flying horse.

Andromeda.

Triangulum, the triangle.
The fifteen constellations described by Ptole-

System of the Universe. 261

my towards the southern part of the heavens
are,

The whale. The cup.

Orion. The crow.

Eridanus, the river. The centaur.
The hare. The wolf.

The great dog. The altar.
The little dog. The southern crown.

The ship. The southern fish.

The hydra.

The stars which could not be comprehended
in these constellations were called unformed stars ;
but several new constellations have been made
out of them by the moderns. The following
have been added to the northern constellations :
The camelopard. The lizard.

The greyhounds. The little triangle.

The little lion. Cerberus.

The lynx. Mountain Menalus.

The fox and goose. The fly.'
Those which follow. have been added to the
constellations in the southern hemisphere :
Noah's dove. The phoenix.

The unicorn. The sword-fish.

The cross. The flying fish.

The sextant. The water-snake.

Sobeiski's shield. The cameleon.

The royal oak. The fly.

The peacock . The bird of Paradi se .

The crane. The south triangle.

The American goose. The Indian.
Notwithstanding these additions, there yet re-

262 Astronomy. [Lecture 17.

main in this hemisphere a very great space, and
a great number of unformed stars, of which the
Abbe de la Ca-lle, a very learned and a very la-
borious astronomer, has formed fourteen new
constellations, which he has dedicated to the arts,
in giving them the figures and the names of the
principal instrument. The following is the list
of these, in the order of their right ascension :
The carver's workshop. The air-pump.
The chemical stove. The octant.

The clock. The compass.

The rhomboid reticule. The square and ruler.
The graver. The telescope.

The painter's easel. The microscope.

The mariner's compass. The mountain near

Table Bay.

1 have already noticed that there is a remark-
able track round the heavens, called the milky
way, from its peculiar whiteness, which is found,
by means of the telescope, to be owing to a vast
number of very small stars that are situated in
that part of the heavens. There are also several
little whitish spots which appear magnified, and
more luminous when seen through telescopes,
yet without any stars being distinguishable in
them. One of these is in Andromeda's girdle,
and was first observed in the year 1612 by Simon
Marius ; it has some whitish rays near its middle,
is liable to several changes, and, according to some
astronomers, occasionally disappears. Another
is near the eliptic, between the head and bow of
Sagittarius ; it is small but very luminous. A

System oftlie Universe. 263

third is on the back of the Centaur* which is too

x *

far south to be seen in Britain. A fourth, of a
smaller size, is before Antinous's right foot, having
a star in it, which makes it appear more bright.
A fifth is in the constellation of Hercules, be-
tween the stars £ and TJ, which spot, though but
small, is visible to the naked eye, if the sky is
clear, and the Moon absent. It is also found
that several of the stars, which appear single to
the naked eye, are double, triple, or even qua-
druple, when viewed through a good telescope.
Dr. Herschell and other astronomers have classi-
fied these.

Dr. Herschell has discovered other appear-
ances in the heavens, which he calls nebulae or
cloudy stars. They are stars surrounded by a
faint luminous substance of a considerable extent.
What the nature of this substance may be we
cannot easily conjecture, but the phaenomenon is
certainly very curious and interesting *.

* Before I proceed any further in explaining the solar
system, it seems proper to make the student acquainted
with the principal words and phrases which are appro-
priated to this science.

The poles are the extremities of the axis on which the
globe turns.

The globe or sphere is divided into two equal halves
or hemispheres by one great circle, perpendicular to the
axis, which for that reason is called the equator or equi-
noctial.

The sensible horizon is a circle which separates the
visible from the invisible hemisphere, or that which is

264 Astronomy. [Lecture 17.

the boundary of our sight, and which seems to bring the
apparent arch of the heavens in contact with the earth.

The rational horizon is a great circle, parallel to the
former, but which would divide the globe into equal
portions.

A parallel sphere is so called because under it the equa-
tor coincides, or is parallel to the horizon. The poles
are in the zenith and nadir ; that is, one pole is directly
over the head of the spectator, and the other directly
under his feet. The inhabitants of this sphere would be
those, if it were habitable (which, however, we may ven-
ture to decide in the negative, from the extreme cold),
that lived under the poles, who could have but one day
and one night in the year. The day continues six months
•while the sun appears to pass through six signs of the
zodiac, and the night six months, while he appears to
pass through the other six. The day, under the north
pole, begins when the sun enters aries, and continues till
he reaches libra ; when night commences, and continues
the other six months.

Under the south pole the direct contrary happens, it
being day there when it is night in the former situation,
and the contrary. But at both the poles there is a long
continuance of twilight, both after the sun has departed,
and before he appears.

The polar inhabitants (if there are any) see the sun for
half the year, moving continually round above the hori-
zon, in a spiral line ; the first round skimming the skirts
of the horizon ; the second, higher ; and so on, till, by
ninety revolutions, he has reached the tropic, his utmost
declination; after which, by ninety more revolutions, he
again reaches the horizon, and then rong winter night
begins.

A right sphere is so called, because under it the equator
cuts the horizon at right angles. The poles will lie or be
in the horizon. The equator will be in the zenith and
nadir.

System of the Universe. $65

The inhabitants of this sphere are those who live under
the equinoctial line, and have their days and nights
always equal, viz. twelve hours each ; because not only
the equator but also all the parallels of latitude are cut
into two equal parts by the horizon. And therefore, as
the sun's diurnal arches are equal to the nocturnal, each
day must be equal to the night, viz. twelve hours each.
The sun rises and sets nearly in a vertical direction.
He comes to the meridian \vith the same degree of the
equator with which he rose ; and hence there can be no
ascensional difference. He i$ half a year on one side of
their zenith, and as much on the other ; passing over
their zenith but twice a year, viz. at the equinoxes.

An oblique sphere is so called because in it the equator
cuts the horizon obliquely. This position of the globe is
common to all the inhabitants of the earth, except those
who are situated under the poles, and under the equinoc-
tial. The properties of this sphere are as follow : the
pole is elevated to any degree less than ninety, the axis of
the earth always making an acute angle with the horizon.
A ' . the parallels to the equator cut the horizon obliquely,
nnking the diurnal greater or less than the nocturnal
arches j and consequently producing an inequality in the
days and nights, which are never equal but when the
sun is in aries and libra, which happens in March and
September, when he moves in the equator, making equal
days to alF the inhabitants of the earth, except those
under the poles. The inhabitants of this sphere, who live
without the tropics, never have the sun in their zenith,
but under the tropics he is vertical once, and between
the tropics and the equator twice, every year. The stars
rise and set obliquely in this position ; and the nearer the
observer is situated to the equator, the greater number
of them will be visible. The length of the twilight is
longer or shorter in this position, according as the lati-
tude is greater or less.

VOL. I. N

266 Astronomy. {Lecture 17.

The Anlcecii, or Antoecians, are those inhabitants of
the globe, who have the same longitude with us, but are
as far to the south of the equator as we are to the north.
Their hour is the same as ours, it being noon, &c. with
both at the same time. Their days are equal to our
nights, and the conirary. And their summer is our winter.

The Pcricecii, or Perioecians, are those that lie under the
same parallel of latitude with us, on the same side of the
equator, only are distant one hundred and eighty degrees
of longitude, viz. a semicircle.

They have contrary hours, it being noon with them
when it is midnight with us. Their days and nights are
of the same length with ours. Their season or time of the
year is also the same cs with us.

The Antipodes are such inhabitants as have the same
latitude south as we have north, but diffetr one hundred
and eighty degrees in longitude j that is, they and we have
opposite parallels and opposite meridians. Their hour is
directly the reverse of ours, it being noon with them when
it is midnight vviih us. Their longest day is our shortest
day, and their longest night our shortest night. The four
seasons are contrary, their summer being our winter, &c.
They are called Antipodes because iheir feet are opposite
lo our feet ; that is, they go with their heads downwards
in respect of us.

The Amphiscii are so called because their shadows are
cast different ways at noon at different times of the year ;
that is, their shadow sometimes points to the north, and
sometimes to the south: therefore it 'is easy to perceive
that these people live in the torrid zone, that is, between
the tropics.

A great circle is one the plane of which passes through
the centre of the spheres.

A secondary to a great circle of the sphere is a great
circle passing through its poles.

The angular distance of a heavenly body from a great

System of the Universe. 267

circle is an arch of the secondary to the great circle passing
through the body and intercepted between it and the great
circle.

Altitude is the angular distance of a heavenly body
from the horizon. The meridian altitude of the sun is the
height of it from the horizon at twelve o'clock.

Declination is the angular distance of any heavenly body
from the equinoctial or equator, and is called north or
south, according to the side of the equinoctial on which
the declination is.

Right ascension is an arch of the equinoctial contained
between the first of aries <Y» and the point of it that is cut
by a secondary to the equinoctial passing through the hea-r
venly body.

Oblique ascension is that arch of the equinoctial which
is contained between the first of aries and the point of the
equinoctial which is cut by the horizon at the rising of the
heavenly body.

Ascensional difference is the difference of degrees between
the right and oblique ascension, which converted into time,
by allowing fifteen degrees for every hour, shows how
much the sun or star rises or sets before or after six ; that
is, subtract the less from the greater number, and the re-
mainder will give the ascensional difference.

Amplitude is an arch of the horizon contained between
the true east or west points and that point of the horizon
where the heavenly body rises or sets, and is called north
or south amplitude accordingly.

Azimuth is an arch of the horizon intercepted between
the north or south points and that point of the horizon to
which the heavenly body is referred by a secondary passing
through it.

Almacanthers are less circles parallel to the horizon.

The latitude of a heavenly body, is its angular distance
from the ecliptic, and is called north or south latitude ac-

N2

268 Astronomy. [Lecture 17

cording as the body is on the north or south side of the
ecliptic.

The longitude of a heavenly body is an arch of the
ecliptic intercepted between the first of aries and the point
of it, which is cut by a secondary to the ecliptic passing
through the heavenly body.

The armillary sphere is an instrument composed of the
principal circles which are usually drawn upon an artificial
globe.

The colures are two secondaries to the equinoctial ; the
one passing through the equinoctial points, and called the
equinoctial colure, the other passing through the solstitial
points, and called the solstitial colure.

The ecliptic is a great circle of the sphere, in which the
sun always appears to move, so called because eclipses ge-
nerally happen when the moon is in or near this circle.
The obliquity of the ecliptic is the angle it makes with
the equator, which is now about twenty-three degrees
twenty-eight minutes. This angle varies within very
narrow limits.

The equinoxes are the two points where the ecliptic
cuts the equator, so called because when the sun is in
either of these situations the days and nights are equal to
each other all over the globe.

The geocentric place of a planet is that position which
it has when seen from the earth, or, strictly from the
earth's centre.

The terminator is that great circle which divides the
enlightened hemisphere from the dark hemisphere of any
planet.

The heliocentric place of a planet is that in which it
would appear to a spectator placed in the sun's centre.

The sextile is an aspect of two heavenly bodies when
they are sixty degrees distant from each other, and is de-
noted in an ephemeris by #.

System of the Universe. 269

Trine is an aspect of two planets when they are a hun-
dred and twenty degrees distant from each other, and in
an ephemeris it is denoted by A. In like manner quartile,
marked D , is when two heavenly bodies are 90° asunder
in longitude ; opposition^ marked § , when they are 180°
asunder ; and conjunction, marked £ , when two heavenly
bodies have the same longitude. Thus at the time of new
moon, the sun and moon are in <J ; at the time of full
moon they are in g; and in the first and last quarters they
are in n or quartile aspect. These aspects for all the
planets are shown in Partridge's Almanac.

The diurnal Parallax of a heavenly body is the angular
distance between the places of the body, when referred to
the heavens, as seen from the centre and the surface of the
earth j or it is the angle^which the earth's radius would
subtend at the heavenly body.

The annual Parallax, or the parallax of the earth's or-
bit, is the angular distance between the different places of
the body as seen from opposite points of the earth's orbit.

Apogt is that point of the orbit of a planet or the ima-
ginary orbit of the sun which is farthest from the earth.

Perigt is that point in the orbit of a planet, &c. when
it is nearest to the earth.

Aphelion is the point of an orbit most distant from the
sun.

Perihelion is that point of an orbit, wheiher planetary
or cometarv, which is nearest the sun.

LECTURE XVIII.

ASTRONOMY.

OF THE SUN, AND HIS REAL AND APPARENT
MOTIONS.

THE sun with the planets and comets which
move round him as their centre constitute what
is called the solar system. Those planets which
are near the sun not only finish their circuits
sooner, but likewise move faster in their re-
spective orbits than those which are more remote
from him. Their motions are all performed
from west to east in orbits nearly circular, but
in truth elliptical, except so far as they are
effected by each other's disturbing forces.

The sun, the centre of the system, has been
generally considered as composed of the matter
of light and heat, whether these are to be re-
garded as essentially the same or not; perhaps
it will be speaking more correctly to say, that
he is the source of both, and that he both warms
and enlightens the bodies which surround him,
probably by means of perpetual emanations from
a luminous atmosphere. The sun has two ap-
parent motions, the diurnal and the annual. In
the first he appears to revolve round the earth

The Sun and his real and apparent Motions. 271

in the course of a solar day, or about 24 hours ;
by the other he appears to traverse that circle in
the heavens which is called the ecliptic, in the
course of the solar year. It is almost un-
necessary to tell you that neither of these mo-
tions is real. For the first depends upon the
eartirs rotation on its own axis, and the second
on her annual revolution round the sun. This
deception of our senses with respect to the gun
and heavenly bodies appearing to move, may be
compared to that which we experience, when
sailing in a vessel within sight of the shore, when
the trees and villages appear all moving in a con-
trary direction, and we ourselves to remain at rest.
But though the vulgar language of astronomy
is thus, as M. Voltaire observes, a tissue of
falsehood, it yet conveys no deception to those
who are once acquainted with the true prin-
ciples. Thus, though we know that the sun
does not change his place in the heavens, and
that it is the earth only which moves, yet it is
no absolute solecism to say that the sun is in
aries, or any other point of the heavens ; for with
respect to us he is to all intents and purposes
apparently there. To make this clear by a very
easy diagram : Let us for a moment suppose the
earth the centre of the system at S, (PI. XXV.
fig. 109.) and the sun to revolve round it in the
orbit ABCD ; and let EFGH represent what
appears to us the concave sphere of the starry
heavens. As the sun moves in this supposed

Astronomy. [Lecture 18.

orbit, when he is at A he will appear to a
spectator at S to be at E among the fixed stars,
when at B he will appear at F, when at C at
H, &c.

Now let us reverse the supposition, and con-
sider the place of the sun as it really is at S,
and let us regard ABCD as the earth's orbit,
and we shall find the result substantially the
same as to the appearance of the sun in the
heavens. That is, when the earth is at A, the
sun will appear among the stars at H; when
at B, the sun will appear at G; when at C,
the sun will be at E. Though the sun there-
fore does not in reality change his place, you
must perceive that to a spectator on the earth
he will in fact appear to describe the same circle
EFGH in the starry heavens, as if he had been
the moving body instead of our earth. v

The earth's orbit being an ellipsis, the sun
is not always at equal distances from it. When
in his apogtj, the sun is about 1171468 leagues
further from us than when in his perige\ In
this last case then not only must he subtend a
greater angle, butj it would appear that we
should derive from him a greater degree of
heat. The difference of temperature between
summer and winter does not, however, depend
solely on our proximity to the sun or our
distance from him, though this Cause is not
without its influence ; for in truth the sun is in
his apoge in our summer, and in his perige in

The Sim and his real and apparent Motions. 273

winter. The heat of summer depends chiefly
on three other causes.

1st. In summer the solar rays strike less
obliquely upon the earth than in winter ; and
it may be demonstrated on the principles of
mechanics, that a body which acts perpendi-
cularly upon another acts with all its force;
whereas if it acts obliquely, its force is less in
proportion to the degree of the obliquity. The
rays of light follow the same laws as other
bodies, and consequently their action might be
measured by the sine of their angle of incidence.
There is no necessity for a diagram to explain
what is now laid down, since it is obvious that
as the equator divides the earth into two equal
parts, when the sun is on this, that is, the north
side of it (as he is in summer) his rays must
strike more vertically, or more in the perpen-
dicular line, than when he is in the southern
tropic. 2d. In summer also, the rays falling
more vertically, have less of atmosphere to pass,
and that atmosphere is usually less clouded.
3d. In summer the sun continues a longer time
above the horizon than below it; and conse-
quently there is time for the earth to accumulate
a greater portion of heat than in the days of '
winter.

Since the sun is further from us in summer
than in winter, it follows that the inhabitants
of the opposite (the southern) hemisphere must
have (all other circumstances equal) more heat

Astronomy. [Lecture 18.

during their summer, and more severe cold
during their winter, than we have ; and this is
found to be the case.

In the last lecture I mentioned the signs of
the zodiac, or those which the planets traverse
in their revolution about the sun, and through
which the sun himself apparently passes in con-
sequence of the annual revolution of the earth.
To these 12 signs the names of the 12 constella-
tions of the zodiac are given ; we must, however,
not confound these signs in the heavens with the
constellations which bear these names. In the
time of Hipparchus the sign and the constella-
tion were nearly the same, and each of the con-
stellations occupied with sufficient exactness that
12th part of the zodiac which bore its name.
But at present this is not the case; the sign
Aries, which is the first, denotes the first portion
or 12th part, that is, the first 30 degrees on the
circle of the ecliptic, counting from that point
where that circle intersects the equator ; but the
constellation Aries is an assemblage of stars
which formerly corresponded with the place of
the sign, but which is now advanced about
30 degrees, so that in fact the constellation Aries
now occupies the place of Taurus ; Taurus that
of Gemini, &c.

The first point of the zodiac, or, as it is called,
the first point of Aries, is at the point where
the equator intersects the ecliptic. It is from
this point that astronomers begin to count the

The Sun and Solar System. 275

longitude of the fixed stars ; and this point also
constitutes the vernal equinox. This point, how-
ever, is found to recede westward every year about
50 seconds of a degree. The fixed stars, of course,
appear to have advanced every year in the same
proportion, by a movement which is general and
common to all, about the poles of the ecliptic.
Their longitude is therefore annually augmented
in this proportion.

This general movement of the fixed stars, and
this difference of longitude, depend upon what
is called the precession of the equinoctial points ;
and this precession, physical astronomers say, is
produced by the modified attractions of the sun
and moon upon the spheroidal figure of the
earth, which is known to be not a perfect globe,
but rather flatted at the poles. By means of
these attractions acting more powerfully upon
the equatorial regions, the poles of the equator de-
scribe circles about the poles of the ecliptic, in
the long period of 25,748 years. Hence, if the
sun is one year in conjunction with a particular
star at the instant of the equinox, he ought the
succeeding year to be at the equinox before he
comes in conjunction with the same star. The
arrival of the sun at the equinoctial point there-
fore precedes the termination of his revolution,
and hence is derived the phrase the precession
of the equinoxes. The complete explication of
this interesting phenomenon is too recondite to
admit of introduction into a popular treatise like

276 Astronomy. [Lecture 18.

the present. It is very well done, though not
in an elementary manner, in Laplace's elegant
Systeme du Monde.

The fixed stars appear every day to make an
entire revolution round the earth. The sun, I
have said, makes the same apparent diurnal re-
volution. But the diurnal motion of the sun
is apparently slower than that of the fixed stars.
It is almost needless to repeat to you that these
appearances are caused by the daily rotation of
the earth upon its axis, which is accomplished
in 23 hours 56 minutes and 4 seconds. If,
however, the earth only turned upon its axis;
and if while it turned in this manner it did not
advance in its orbit, the apparent diurnal move-
ments of the sun and fixed stars would always be
the same. The stars which had passed once
over the same meridian , with the sun would
constantly repeat the same movement in the
same time; the winter and the summer nights
would at the same place present the same con-
stellations. But because of the annual motion
of the earth from west to east round the sun, in
which it advances about 59 minutes and 8
seconds o^a degree in a day, the sun appears to
advance in the same proportion in the ecliptic.
This constitutes the difference between solar and
sidereal time, in explaining which I shall make
use both of the figure and the words of Mr.
Ferguson.

" Let ABCDEFGHIKLM be the earth's

The Sun and Solar System. 277

orbit, (PI. XXV. fig. 110.) in which it goes
round the sun every year, according to the order
of the letters, that is, from west to east; and
turns round its axis the same way from the sun
to the sun again in every 24 hours. Let S be
the sun, and E (in fig. 109) a fixed star at such
an immense distance, that the diameter of the
earth's orbit is but a point in proportion to that
distance. Let N m be any particular meridian
of the earth, and N a given point or place upon
that meridian. When the earth is at A the sun
S hides the star E, which would be always hid
if the earth never removed from A ; and conse-
quently, as the earth turns round its axis, the
point N would always come round to the sun
and star at the same time. But when the earth
has advanced, suppose a twelfth part of its orbit
from A to B, its motion, round its axis will
bring the point N a twelfth part of a natural
day, or two hours, sooner to the star than to
the sun, for the angle N B n is equal to the
angle ASB : and therefore any star which comes
to the meridian at noon with the sun when the
earth is at A, will come to the meridian at 10 in
the forenoon when the earth is at B. When the
earth comes to C, the point N will have the star
on its meridian at 8 in the morning, or four hours
sooner than it comes round to the sun; for it
must revolve from N to n before it has the sun in
its meridian. When the earth comes to D, the
point N will have the star on its meridian at 6 in

5278 Astronomy. [Lecture 18.

the morning, but that point must revolve six
hours more from N to n, before it has mid-day by
the sun: for now the angle A S D is a right
angle, and so is N D n ; that is, the earth has
advanced 90 degrees in its orbit, and must turn
90 degrees on its axis to carry the point N from
the star to the sun : for the star always comes
to the meridian when N m is parallel to R S A ;
because D S is but a point in respect to R S.
When the earth is at E, the star comes to the
meridian at 4 in the morning ; at F, at 2 in the
morning; and at G, the earth having gone half
round its orbit, N points to the star R at mid-
night, it being then directly opposite to the sun.
And therefore, by the earth's diurnal motion,
the star comes to the meridian 12 hours before
the sun. When the earth is at H, the star
comes to the meridian at 10 in the evening ; at
I it comes to the meridian at 8, that is, 16 hours
before the sun; at K 18 hours before him; at
L 20 hours ; at M 22 ; and at A equally with
the sun again.

" Thus it is plain, that an absolute turn of the
earth on its axis (which is always completed
when any particular meridian comes to be parallel
to its situation at any time of the day before)
never brings the same meridian round from the
sun to the sun again; but that the earth re-
quires as much more than one turn on its axis
to finish a natural day, as it has gone forward
in that time; which, at a mean state, is a 365th

The Sun and Solar System. 279

part of a circle. Hence, in 365 days, the earth
turns 366 times round its axis; and therefore,
as a turn of the earth on its axis completes a
sidereal day, there must be one sidereal day
more in a year than the number of solar days,
be the number what it will, on the earth, or any
other planet, one turn being lost with respect
to the number of solar days in a year, by the
planet's going round the sun ; just as it would
be lost to a traveller, who, in going round the
earth, would lose one day by following the
apparent diurnal motion of the sun; and con-
sequently would reckon one day less at his
return (let him take what time he would to go
round the earth) than those who remained all
the while at the place from which he set out. So,
if there were two earths revolving equally on
their axes, and if one remained at A until the
other had gone round the sun from A to A
again, that earth which kept its place at A
would have its solar and sidereal days always of
the same length ; and so would have one solar
day more than the other at its return. Hence,
if the earth turned but once round its axis in a
year, and if that turn was made the same way
as the earth goes round the sun, there would be
continual day on one side of the earth, and con-
tinual night on the other.""

The sun is unquestionably to us the most
interesting of all the heavenly bodies. The heat
which he diffuses animates our world, and his

280 Astronomy. [Lecture 18.

light is the source of all our purest pleasures.
His power reaches to a most extended sphere,
the more active in proportion to the nearness.
Our water would be in a boiling state at Mercury,
and frozen at Saturn. Yet the beings who exist
in those worlds are undoubtedly accommodated
to the climates they inhabit.

The sun is of a form nearly spherical. He
however appears to us only as a circular disc.
This is because all the parts of his surface are
equally luminous; and consequently there is
nothing which can suggest to us that the cen-
tr^cal parts are more prominent than the sides,
though in reality they are nearer to us by 160,000
leagues.^ In the same manner the full moon
appears to us a flat surface, but a good telescope
corrects the deception.

So early as the year 1611 spots were discovered
upon the disc of the sun. The discovery was
claimed both by father Scheiner and by Galileo.
These spots consist, in general, of a central part,
which appears much darker than the rest, and
seems to be surrounded by a mist or smoke;
and they are so changeable in their situation
and figure as frequently to vary during the time
of observation. Some of the largest of them^
which are found to exceed the bulk of the whole
earth, are often to be seen for three months to-
gether, and when they disappear they are gene-
rally converted into faculse or luminous spots,
which appear much brighter than the rest of the

Spots on the Sun. 281

sun. About the time that they were first dis-
covered by Galileo, forty or fifty of them might
be frequently seen on the sun at a time, but at
present we can seldom observe more than thirty ;
and there have been periods of seven or eight
years in which none could be seen.

The speculations and opinions of philosophers
concerning the nature and origin of the solar
spots are various, and perhaps all erroneous,
since we are in truth unacquainted with the
materials of which his body is composed. One
of the most popular conjectures is, that they
are occasioned by the smoke and opaque matter
thrown out by volcanos, or burning mountains,
of immense magnitude; and that when the
eruption is nearly ended, and the smoke dis-
sipated, the fierce flames are exposed, and ap-
pear like faculae or little torches. M. de la
Hire imagined the sun to be in a continual state
of fusion, and that the spots which we observe
are only the eminences of large masses of opaque
matter, which by the irregular agitations of the
fluid sometimes swim upon the surface, and at
other times sink and disappear. Nearly akin to
this is the more recent hypothesis of Herschel,
who supposes the sun to be itself opaque, but
surrounded by a phosphoric or luminous atmo-
sphere, beyond which the tops of mountains on
the sun's body sometimes project, and appear
to the telescopic observer as black spots.

Whatever may be the nature of these spots.

Astronomy. [Lecture 18.

the observance of them has produced a discovery
of some importance. It was early observed that
they ceased to be visible at certain intervals, and
again at stated periods reappeared. The apparent
motion of the spots is from the eastern to the
western side of the sun ; and as they are observed
to move quicker when they are near the central
region than when they are near the limb, it fol-
lows that the sun must be a spherical body, and
that he revolves on his axis from west to east.
The time in which he performs this revolution,
as observed by Cassini, is twenty-five days,
fourteen hours, and eight minutes; and from
the time of the motion of the spots, which is
sometimes straight, but more frequently curved
or elliptical, it is discovered that his axis is not
perpendicular to the plane of the ecliptic, but
inclined to it, so as to make an angle with the
perpendicular of about seven degrees and a half.
The zodiacal light, as it is called, is another
striking phenomenon connected with this glo-
rious luminary. In explaining it the sun is
supposed to be enveloped with a fluid matter,
luminous in itself, or only enlightened by the
solar rays, and which constitutes a higher atmo-
sphere. This matter is more abundant and
more extended round his equator than else-
where, and gives to the solar atmosphere an
appearance resembling that of a double convex
lens, the diameter of which is in the plane of the
sun's equator. It was discovered in 1683, by

Zodiacal Light. 283

Cassini, who observed it for about 8 days. It
has obtained the name of the zodiacal light,
because it appears along the zodiac in the form
of a lance or pyramid. It is of a faint whitish
colour resembling the milky way.

The zodiacal light is more or less visible ac-
cording to circumstances. It is most apparent
when it has a sufficient extent along the zodiac,
and when the obliquity of the zodiac to the
horizon is not too great, for otherwise its faint
light will scarcely be distinguished from the
twilight, whether previous to the rising of the
sun, or after his setting.

The zodiacal light appears generally in a
conical form, having its base always directed to-
wards the body of the sun, and its point towards
some star in the zodiac. It is thus it appears
in the evening in the spring, and in the morning
in the autumn. Its eastern point being dis-
played in the evening, and its western in the
morning. The two points may sometimes be
seen in the same night, as at the solstices, and
particularly at the winter solstice, when the
ecliptic makes, in the evening and the morning,
angles, almost equal with the horizon, and suf-
ficient to leave a considerable part of the point
above the line of twilight. The summer solstice
has the disadvantage of the too great obliquity
of the ecliptic with respect to the horizon and of
a long twilight.

In the evening and morning observations,

284 Astronomy. [Lecture 18.

only the superior parts of the phenomenon,
with respect to the horizon of the observer, are
apparent. For, as the sun rises and approaches
the horizon, or again before he has descended
many degrees below it, it becomes lost in the
twilight. This circumstance is usually thus
explained— Let IKOA (PL XXVI. fig. 111.)
be the zodiacal light in one of the most favour-
able positions for observing it, that is about
the latter end of February or beginning of March,
when the first point of Aries may be supposed
in K, upon the plane of the horizon HR, and
the sun being in S, about the 10th degree of
Pisces upon the boundary CP of twilight, 18
degrees below the horizon. The ecliptic TKZ
is here confounded with the axis AZ of the
zodiacal light, and forms with the horizon an
angle of about 64 degrees. The point A of
this light falls between the stars of the neck and
head of Taurus, and terminates about the 10th
degree of Gemini, whence it follows that the
distance from its point to its base at the sun is
about 90 degrees.

The same figure represents the situation AEZ
which this light would assume, the morning of
the same day just before day-break. The angle
N t z of the ecliptic with the horizon being
about 26 degrees, supposing only that the spec-
tator, who had in the evening the north-pole B
on his right, and the meridian M at his left,
being turned towards the east, shall have on the

Zodiacal Light. 285

contrary the north at his left, and the south at
his right. It is plain, from what has been said,
that the part of the zodiacal light which is near
the sun cannot be seen upon the horizon, because
the twilight will cause it to disappear, or at least
render its borders very indistinct. It is only
a total eclipse of the sun which can show it at
the base, and in its densest part ; in that case,
as soon as the disc of the moon has completely
obscured that of the sun, there appears round
the moon an enlightened border, and a kind of
beam ; it is more or less dense, according to its
distance from the edge of the moon.

The zodiacal light must be more easily and
more frequently perceived in the tropical cli-
mates, and particularly near the equator, than it
can here; first, because in those parts the ob-
liquity of the equator and the zodiac to the
horizon is less ; and secondly, because the dura-
tion of the twilight is much shorter. This curious
light was observed by Cassini in 1683 ; and there
is reason to suspect that earlier astronomers ob-
served it, but did not describe it with sufficient
precision.

LECTURE XIX.
ASTRONOMY.

THE PRIMARY PLANETS ; THE MODE OF CALCU-
LATING THEIR DISTANCES, &C.

THE planets, I have already intimated, are
opaque bodies, very nearly spherical, and we
have reason to believe much like the earth.
They are not luminous of themselves ; and be-
come visible only by reflecting the light which
they receive from the sun. Kepler discovered
some of the principal laws by which the motions
of the planets are governed. He was the first
that demonstrated, by calculations equally diffi-
cult and laborious, that they must revolve in
elliptical, and not in circular orbits. He calcu-
lated by the observations of Tycho, the distance
of Mars from the Sun in different parts of his
orbit, and proved that it could not possibly be
adjusted to the circumference of a circle. New-
ton showed afterwards, by the theory of attrac-
tion, that the curve which a planet describes
would be strictly an ellipsis, of which the central
star (or sun) occupies one of the foci, were it not
for the slight irregularities occasioned by the at-

The Primary Planets. 287

tractions of the other planets. Let A E P G
(PL XXVI. fig. 112.) be an ellipsis, or the course
of a planet. The central star or sun is at S,
which is one of the foci.

The second law of Kepler is, that the squares
of the times of the revolutions of the planets are
as the cubes of their mean distances from the sun.
That is, if we compare the square of the time
which any two of the primary planets occupy in
completing their orbits, we shall find between
these two squares the same proportion as between
the cubes of the mean distances S E of these two
planets from the sun. Thus, if we know the
times of the revolution of two planets, we can
thence compute what are their respective dis-
tances from the sun ; and if we are made ac-
quainted with the true distance of the one, we
shall easily find the true distance of the other, as
indeed the distances of all of which we know the
time of their periodical revolutions.

Thus, if we suppose the planet Venus to re-
volve round the Sun in 224 days, and the Earth
in 365 ; and if we admit the mean distance
of the earth from the sun to be 95 millions of
miles — then, as the square of 365 is to the square
of 224, so will be the cube of 95,000,000 to a
fourth number, which will show the cube of
Venus's mean distance from the sun ; and if the
cube-root of this number is found, it will give
about 68 millions of miles for the mean distance
of Venus from the Sun.

288 Astronomy. [Lecture 19.

The third law of Kepler is, that the areas are
in proportion to the times :• — That is, that the
time occupied by a planet in passing the different
arcs AD, DE of its orbit are to one another, as
the areas of the trilineal spaces A S D, D S E ter-
minated by these areas, and by the right lines AS,
DS, and DS and ES ; these areas are, by the
same reasoning, to one another, as the time which
the planet employs in passing through the arcs
which terminate them. Hence we see that these
times are shorter in proportion as the planet is
nearer the sun, for then the area of the triangle
is so much smaller. Newton has proved that
these three laws are necessary consequences of
the projectile force combined with the centripetal
or attractive force, which retains the planets in
their orbits; and the demonstration, now much
simplified, finds a place in all our higher treatises
of mechanics and astronomy.

Astronomers have divided the planets into two
classes ; the first class they call primary planets,
principals. They are eleven in number, viz.
Mercury, Venus, the Earth, Mars, Ceres, Pallas,
Juno, Vesta, Jupiter, Saturn, and the Georgium
Sidus or Uranus. Those of the second class
they call secondary planets or otherwise satellites
or moons.

The primary planets are such as revolve round
the sun only. These are also divided into supe-
rior and inferior; those being called superior
planets whose distance from the sun is greater

The Primary Planets. 289

than that of the earth, and those inferior planets
whose distance is less than that of the earth.

The superior planets are, Mars, Ceres, Pallas,
Juno, Vesta, Jupiter, Saturn, and Uranus, which
are further from the sun than the earth is, and
which, consequently, environ the latter in their
revolution : it is for this reason we see them some-
times on one side of the sun, and sometimes on
the other. The inferior planets are, Mercury
and Venus, which are nearer the sun than the
earth, x and which, consequently 5 never environ
the latter in their revolution. On this account
we see them always on the same side as the sun,
and never in opposition, because this earth is
never between them and the sun,

It has been already stated that the apparent
diameter of the sun, viewed at his mean distance
from the earth, is 32' 3' '3. The apparent dia-
meters of the planets seen from the earth bear a
relation to their real size, and the distance of
<each. But, in comparing these diameters with
one another, or with that of the sun, they are
supposed to be seen all at a distance equal to the
mean distance of the earth from the sun, as in
the following table.

VOL. I.

290

Astronomy.

[Lecture 19.

A TABLE of the mean apparent diameters of the
sun, and of the primary planets, seen from the
earth ; and of the comparison of these diame-
ters with that of the sun.

The Names of
the Planets.

Apparent
Diameters.

Min. " *'

Diameters of the
Planets compared
with that of the
Sun.

The Sun
Mercury
Venus
The Earth -
Mars
Ceres
Pallas
Juno
Vesta
Jupiter
Saturn
His ring
Uranus

32 3 18

070
0 27 0
000

010
005
030
005
0 38 12
0 18 0
0 37 0
0 3 12

One
l-274th
1 116th
l-113th
l-168th

l-10th
1-llth
l-5th
l-25th

When once the apparent diameters of the
planets are known, seen all at the same distance,
it is easy to determine the size of each planet in
terrestrial diameters. And as the real diameter
of the earth is known in leagues, we may thence
calculate the number of leagues which the real
diameter of each planet contains. This may be
seen by the following Table, in which the terres-
trial diameter is taken for unity.

The Primary Planets.

291

TABLE of the diameters of the sun and the pri-
mary planets in terrestrial diameters, and in
leagues of 2283 French fathoms each.

Names of the
Planets.

Si;
In Terrestrial
Diameters.

e of the Planet
In Leagues.

t.
English Miles.

The Suu

112 l27-34ths

323,155

813,246

Mercury

0 7-17ths

1180

3,224

Venu*

0 S3-34ths

2784

7,867

The Earth

1

2865

Mars

0 2-3ds

1921

4,131)

Ceres

.

....

160

Pallas

_ - .

- - .

80

Juno

Vesta

•

Jupiter

11 2 5ths

32C44

89,170

Saturn

10 l-10th

289391 -halt

79,042

His ring

23 1 half

67512

Uranus

4 1-half

12892

85,112

The magnitude of the planets compared with
one another, are as the cubes of their diameters.
We have seen in the preceding table the size of
their diameters compared with that of the earth ;
by cubing them, therefore, we shall have the size
of the planets themselves, compared with that of
the earth, which is regarded as unity.

o2

Astronomy.

[Lecture 19-

TABLE of the magnitude of the sun and primary
planets compared with that of the earth.

Names of the
Planets.

Magnitude.
Almost { In Decimals.

The Sun
Mercury
Venus
The Earth
Mars
Ceres
Pallas
Juno
Vesta
Jupiter
Saturn
Uranus

1435023
0 3 43ds
0 10-1 Iths
1
0 1-1 Oth

1435022,666239
0,078372
0,917559
1 , 000000
0,301445

1479 l-4th
1030
91 l-4th

1479,231780
1030,173430
91,250000

The density of the planets is calculated in the
same manner as that of the sun ; by the quantity
of their action one upon another. They are
found to be such as are expressed in the follow-
ing Table, and are compared with the density of
the earth taken for unity.

The Primary Planets.

293

TABLE of the densities of the sun and of the
primary planets, compared with that of the
earth.

Names of th«
Planets.

D

Almost.

easitie-;.
In Decimals.

The Sun
Mercury
Venus
The Earth -
Mars
<fWps

0 l-4th
2 2-53ds
1 ll-40ths
1
0 3-4ths

0,254630
2,037700
1,276000
1,000000
0,729170

Pallas

Vpctri

Jupiter
Saturn
Uranus

0 l-9th
0 2-19ths
0 2-9ths

0,229840
0,104500
0,220401

Since the magnitude of the planets, and also
their densities, are known relative to the earth,
it is easy to know their solidity, by multiplying
these two quantities the one by, the other, re-
lative to that of the earth, which is taken for
unity.

Astronomy.

[Lecture 19,

TABL E of the solidity of the sun and of the pri-
mary planets compared with that of the earth.

Names of the
Planets.

Solidity.
Almost. | In Decimals.

The Sun

365400

365399,82 1 504

Mercury

0 15-94ths

0,159699

Venus

1 l-6th

1,169388

The Earth

1

1 ,000000

Mars

0 2-9ths

0,219805

Ceres

.,

. —

Pallas

.

.. —

Juno

.

.

Vesta

_______ .

Jupiter

340 ,

339,98(S632

Saturn

108

107,653123

- Uranus

17 3-4ths

17,740612

The proper motion of each of the primary
planets is from west to east in an elliptical orbit,
(PL XXVI. fig. 112.) AEPG, the sun forming
one of the foci. The plane of the orbit of the
earth is called the ecliptic, as I have before ex-
plained. The orbits of all the other planets are
differently inclined to it, but there is not any of
tlie old planets which departs more than eight
degrees from the ecliptic ; so that they are all
contained within the zodiac. It is this departure
from the ecliptic, which is called the latitude of
the planets, in like manner as the latitude of the
stars denotes their distance from the ecliptic.

The Primary Planets.

295

TABLE of the inclination of the orbits of the pri-
mary planets from the plane of the ecliptic.

Names of the Planets.

Inclination.
Degf. Min. "

Mercury

709

Venus

3 23 32

The Earth -

000

Mars

1 51 3

Ceres

10 37 34

Pallas

34 50 40

Juno

21 0 0

Vesta

7 8 46

Jupiter

1 18 52

Saturn

2 29 38

Uranus

0 46 26

These orbits differ greatly in extent in pro-
portion as the planets are respectively more or
less distant from the central star of the system,
the sun. The means by which these distances
are ascertained have been mentioned before,
when we spoke of the second law of Kepler.
But it must be evident that we must know the
distance of some one planet from the sun, before
we can compute the distance of any other by com-
paring the time of its orbit with that, the distance
of which from the sun is known.

As we exist upon the earth, our calculations
must originate from the planet we inhabit.
Here only we have certain grounds, and, what-
ever we measure of the arch of the heavens

Astronomy. [Lecture I£

must have something relative here to serve as
the basis of our operation. The horizontal pa-
rallax, as it is called, has therefore been a com-
mon basis employed for measuring the distances
of the heavenly bodies from the earth. With
respect to the moon, this method answers with
great accuracy, but with respect to the sun it is
liable to great error, for reasons which I shall
afterwards state, and as to the fixed stars, it is
altogether inapplicable. Indeed, from their great
distance they can have no parallax of this kind.
To explain what I have now remarked, I
must refer to the diagram (PL XXVI. fig. 118.)
.where BAG represents one half of the earth,
A C its semidiameter, S the sun, supposed at an
immense distance, m the moon, and E K O L a
part of the moonrs orbit. C R S is a line repre-
senting the rational horizon of an observer at A
extended to the sun ; H A O his sensible horizon
extended to the moon's orbit. A L C is the
angle under which the earth's semi-diameter AC
is seen from the moon at L. AS C is the angle
under which it is seen from the sun at S. Now
it is evident that the angle A L C is equal to the
angle O A L, and the angle A S C to the angle
O A,/*; and consequently, as the angle O Ay is
much less than O A L, the earth's semidiameter
appears much greater as seen from the moon at
L than from the sun at S, and therefore the earth
is at a much greater distance from the sun than
from the moon.

The Primary Planets. 297

If then we can measure either of the angles
A L C or O A L, which are in effect the same,
we shall have the moon's distance from the
earth.

To effect this operation, take a graduated in-
strument DAE, having a moveable index with
sight-holes, and let it be fixed so that its plane
surface may be parallel to the plane of the equa-
tor, and its edge AD in the plane of the meridian.
So that when the moon is in the equinoctial, and
on the meridian A D E, she may be seen through
the sight-holes, when the edge of the moveable
index cuts the beginning of the divisions at O on
the graduated limb D £, and let the precise time
when she is thus seen be carefully noted. Again,
when the moon has reached the sensible horizon
at O, let her be viewed in the same manner
through the sight-holes, and the time be precisely
noted, making proper allowance for the refrac-
tion. Then, as the moon makes her apparent
revolution from the meridian to the meridian
again on an average in £4 hours and 43 minutes,
deduct the time in which she passes from E to
O, from 6 hours 12 minutes, and then you will
have the time in which she describes the arc OL,
and this will enable us to measure the moon's
horizontal parallax, or angle O A L. For as the
time of the moon's describing the arc E O is to
90 degrees, so is 6 hours 12 minutes to the de-
grees of the arc D d e, which measures the angle
Ji A L, from which subtract 90 degrees, and

o 5

298 Astronomy. [Lecture 19.

there remains the angle O A L, equal to the angle
ALC, under which the earth's semidiameter AC
is seen from the moon.

Now, since the sum of the angles of a plane
triangle makes two right angles, or 180 degrees,
and the sides of a triangle are always proportion-
ed to the sines of the opposite angles, say, as the
sine of the angle A L C at the moon L is to its
opposite side A C, the earth's semidiameter, or
8985 miles* so is radius the sine of 90 degrees,
or of the right angle A C L to its opposite side,
which is the moon's distance at L. from the ob-
server's place at A — Or, so is the sine of the
angle C A L to its opposite side C L, which is
the moon's distance from the earth's centre, and
which will prove to be about 240,000 miles.
The angle C A L is equal to what the angle
O A L wants of 90 degrees.

The sun's distance cannot so easily be deter-
mined, since his horizontal parallax, or the angle
O A S, equal to the angle A S C, is so small as
to be scarcely perceptible, being not more than 8
seconds and a half, whereas the moon's horizon-
tal parallax, or the angle O A L, is very discern-
ible, being at a mean 57' 18", which is more than
400 times greater than that of the sun.

The sun's horizontal parallax, therefore, for
these reasons, could not be ascertained with
any degree of accuracy till the transits of Ve-
nus over the sun's disc, which happened in
the years 1761 and 1767, for at such an im-

The Primary Planets. 299

mense distance, and in so small an angle, the
error of one second will create an error of seven
millions of miles. Hence the amazing difference
in the calculations of different astronomers.
Ptolemy and his followers, as well as Tycho
Brahe and Copernicus, conceived the sun's di-
stance to be 1200 semidiameters of the earth ;
Kepler nearly 3500, and Ricciolus doubles that
distance.

The celebrated Dr. Halley first pointed out
the means of solving this difficult problem, which
he terms " the most noble in the sciences," upon
theoretical principles, though in the course of
nature he could never expect to see them re-
duced to practice.

Venus passes the sun, or is, in the astro-
nomer's phrase^ in conjunction with it, very
often ; and if the plane of her orbit were coin-
cident with the plane of the ecliptic, she would
on such occasions appear like a spot on the sun
for about seven hours. But the orbit of Venus
only intersects the ecliptic in two points, which
are called its nodes. Venus, therefore, can never
be seen on the sun but at those inferior con-
junctions which happen in or near the nodes of
her orbit ; and though this circumstance seldom
happens, the time of its occurring is easily cal-
culated by astronomers. The last transit before
the time of Dr. Halley was in the year 1639, and
he calculated that one would again occur in 176J,
and another in 1769.

SOO Astronomy. [Lecture 19.

Though the sun's distance, therefore, is so
great that the earth's diameter is only a point in
comparison, and his parallax, for the reasons
already assigned, could not be determined with
accuracy, the case is very different when Venus
is perceptibly between the earth and the sun, for
her distance is between three and four times less
than that of the sun. If, therefore, when Venus
in her transit enters upon the sun's disc, she is
observed by two different spectators on different
parts of the earth's surface, she will appear to
each of them at the same instant on different
parts of the sun. Dr. Halley, therefore, re-
commended that some scientific men should be
sent to different parts of the world, where the
transit could be observed with accuracy ; that
the precise times of her entrance and egress from
the face of the sun should be carefully noted by
each ; and from these observations, compared
with the time which she would occupy in passing
over the sun's surface, as seen (by supposition)
from the earth's centre, he demonstrated that
not only the parallax of Venus but that of the
sun might be found.

I shall not trouble you with the detail of this
problem. It is founded on the principles al-
ready explained in treating of the moon's hori-
zontal parallax, and is explained at large in
different treatises on Astronomy *. Let it suffice

* To those who wish to enter more deeply into the sub-

The Primary Planets. 301

to say, that the transits in 1761 and 1769 were
carefully observed by very eminent astronomers

ject, the following extract from Mr. Nicholson's Astro-
nomy will be satisfactory.

*' The planet Venus passes the sun twice in revolving
from any position of elongation to the same position again.
At those times this planet is said to be in conjunction
with the sun.

" When the planet Venus is situated in a line between
the sun and the earth, it is said to be in its inferior con-
junction ; and when it is in the opposite part of its orbit,
the sun being in a line between it and the earth, it is
said to be in its superior conjunction. If the orbits of
the earth and Venus were in the same plane, it is evident
that Venus would pass behind the sun with a direct
motion every superior conjunction, and would pass over
its disc, or before it, with a retrograde motion every in-
ferior conjunction. But as Venus's orbit is inclined to
the ecliptic in an angle of about 3? degrees, this planet
will, in general, pass to the northward or southward of
the sun, and will only be visible on its disc when the in-
ferior conjunction happens at or near one of the nodes.
This happens but once (or sometimes twice at an interval
of about 8 years) in more than 120 years.

" To show how this transit is applied to the purpose of
finding the sun's distance, we shall pass over those ele-
ments that enter into the computation previous or subse-
quent to actual observation, and shall only explain the ge-
neral principles on which the method is founded.

" Let s (PL XXVIII. fig. 117.) represent the Sun, E
the earth, V, U, W, the planet Venus in different posi-
tions, the arc L N a part of the earth's orbit, and the arc
O M a part of the orbit of Venus. Then, because the an-
gular velocities of Venus and the earth are known, as also
their proportional distances, it will be easy to compute
the time Venus will employ in passing through the arc

30£ Astronomy. [Lecture 19.

in different parts of the world, and the sun's
horizontal parallax was determined to be about

V W, which when viewed from the earth, is equal to the
known chord of the sun CD; the heliocentric value or
length of the arc V W may likewise be readily found.
Suppose then an observer at A on the earth's surface to
view the planet Venus at V, it will appear just entered
within the sun's disc at C, and passing in the arc V W,
will appear to describe the line CD, arriving at D at the
end of the computed time. But during this time the ob-
server will, by the earth's diurnal revolution, be carried
from A towards P ; and arriving at P at the same instant
that Venus arrives at U, will behold the transit just finish-
ing atD : consequently it will be of a duration proportion-
ally as much shorter than the computed time, as the helio-
centric arc V U is shorter than V W. The arc V W is
known by cqmputation, therefore, since Venus's motion
may in very small arcs be reckoned uniform,
" As the computed time

Is to the computed arc V W,

So is the observed time

To the arc V Uj

which being taken from V W, leaves the aic U W, that
subtends the angle U D V. This last angle is the parallax
of the base A P; and the base. A P is found by the analogy
" As one day or 24 hours

Is to the circumference of the earth (or
parallel of latitude)

So is the observed time

To the arc A P, whose chord is the base.
" But because the minutest errors in a business of this na-
ture are of very great consequence, and because the length
of the arc VW, depending on the sun's diameter, can
scarcely be obtained by calculation to that extreme degree
of exactness, which is requisite, it is advisable to take an-
other observation on a place so situated on the earth, that,

The Primary Planets. 303

8 seconds, as already intimated, and his distance
from the earth to be about ninety-five millions
of miles.

The distance of the sun from the earth being
well ascertained, the distance of the other planets
may be easily calculated by the second law of
Kepler ; as their orbits or rather the time occu-
pied in traversing their orbits, is known by
observation. The following table will be found,
believe, to exhibit a fair statement of their re-
spective distances.

the observer being carried in a direction apparently con-
trary to the former, the errors may counteract each other.

'* Let the representations be as in the last figure. If the
sun has declination at the time of the transit, B (fig. 1 18.)
will represent the pole towards which the sun declines.
The observer at A, if at rest, would behold the transit
during the time Venus passes from V to Wj but being by
the earth's diurnal revolution carried from A through the
arc A E P to P, and arriving at P at the instant in which
Venus arrives at U, he will perceive the transit just finish-
ing at D ; consequently its duration will be as much longer
than the computed time as the heliocentric arc V U is
longer than V W. V U being found by the before-men-
tioned analogy, the difference between V U and V W is
W U or the parallax of A P, as before.

" Now, in these two cases, a similar error will have a
contrary effect in the first to that which it has in the lat-
ter. For, if, by any error, the computed arc V W
(fig. 117-) be taken too large, the arc U W, and conse-
quently the parallax, will come out too great. But in the
latter observation, if the computed arc V W (fig. 1 18.) is
taken too large, the arc W U, and consequently the paral-
lax will come out too little. Therefore the mean between
two such observations will be much more to be depended
on than either singly.

304

Astronomy.

[Lecture 19.

TABLE o° the mean distances of the primary pla-
nets from the sun, in French leagues of 2283
fathoms each, and in English miles in round
numbers.

Names of the Planets.

MeanD
In Leagues.

istances.
In English Miles
in round numb.

Mercury
Venus
The Earth
Mars
Ceres
Pallas
Juno
Vesta
Jupiter
Saturn
Uranus

13,156,246
25,144,166
34,761,680
52,966,024
86,904,200

37,000,000
68,000,000
95,000,000
144,000,000
260,000,000
266,000,000
253,000,000
225,000,000
490,000,000
900,000,000
1800,000,000

180,794,802
831,628,860
663,315,425

The revolutions of the planets may be consi-
dered as relative to the sun, or as relative to the
earth. In the first case they are called periodi-
cal revolutions ; that is, the time which the pla-
nets employ in revolving about the sun in coming
again to a fixed point in the heavens. In the
second, they are called sy nodical revolutions ;
that is, the time which the planets seen from the
earth employ in returning to the sun ; or the
time which passes between the mean conjunction
and the next following. This time is very dif-
ferent from that of periodical revolutions, as may
be seen in the following table.

The Primary Planets.

305

TABLE of the duration of the synodical revolu-
tion of the primary planets, compared with
that of their periodical revolutions.

Names of the
Planets.

Duration of the Sy-
nodical Revolutions.

Duration of the Pe-
riodical Revolutions.

Mercury -
Venus
Mars

PPTP*

About 116 Days
1 Year 219 -
2 59

About 88 Days.
224
1 Year 321

Ahmir Ififtl

1 *5Ol

I'allas
Vpsfra

Unknown Period.

1682
13 J5

Jupiter

I Year 84 Days.
i i '•*

1 1 Years 3 1 3
on — 1 r\A.

Uranus

83 130

The two inferior planets, Mercury and Venus,
as well as three of the superior, Mars, Jupiter,
and Saturn, were known to the early astrono-
mers. The Georgium Sidus, or Uranus, was
discovered in the year 1781, by Dr. Herschell ;
Ceres was discovered the first day of the present
century, by Mr. Piazzi, an Italian astronomer;
Pallas, by Dr. Olbers of Bremen, in 1802 ; Juno,
by Mr. Harding, at Lilienthal, in 1804; and
Vesta, by Dr. Olbers, in the spring of the year
1807.

The general character and appearance of the
principal planets will be best understood by a
reference to Plate XXVII. and therefore few
observations will be necessary on this subject.

MERCUBY, from his nearness to the sun, is

Astronomy. [Lecture 19,

but seldom viable. No spots bare as jet been
discovered on his surface, and therefore bis ro-
tation on bis axis is not known. Mercury and
Venus, being inferior planets, can never appear
quite at the full to us, but must show phases
analogous to those of the moon, according to
their relative positions as to the sun and the
earth.

Yews is the most brilliant in appearance of
all the planets; and she is called the morning or
evening star, according as she precedes or fol-
lows the sun; m the first case she appears^ to the
right, in the second to the left of that luminary.
Some spots bare been discovered on her surface,
yet her rotation on her axis has not been posi-
tively ascertained. She is said to be surrounded
by an atmosphere of about fifty mile* in height,

MA*S, the first of the superior planets, is dis-
tinguishable from the rest by the red appearance
of his disc, which all agree in attributing to the
density of his atmosphere. His figure is an ob-
late spheroid, like that of the earth, which in-
deed be resembles most in all circumstances.
Spots have been observed on bis surface, from
which bis diurnal rotation has been ascertained,
as well as the inclination of his axis to the eclip-
tic, which is 59* 4£. Two large white circular
spots are observed at bis poles, whence it is con-
jectured that they are continually covered with
snow.

PALLAS, Jrao, and VESTA, are too

The Primary Planets. SOT

small, the diameter of none of them probably
exceeding 100 miles, to admit of any accurate
observations by the best instruments now in use,

JUPITEE is by far the largest planet in our
.svstem, and the brightest next to Venus in ap-
pearance. When viewed through a good tele-
scope, several belts, or bands, darker than the ge-
neral surface (see PL XXVII. figs. 115 and 116.)
are observed across his disc parallel to his equa-
tor, which, as they are constantly van-ing, are
supposed to be a series of clouds in his atmo-
sphere. Spots have also been seen on his disc
between the belts ; and from their disappearance
and reappearance, his diurnal rotation on his
axis has been computed at about 9 hours 55 mi-
nutes. His axis is nearly perpendicular to his
orbit ; his figure is an oblate spheroid, much flat-
tened at the poles.

SATURN, when viewed through a good tele-
scope, is the most extraordinary and interesting
of all the planets. He is surrounded by a flat,
circular, broad, and luminous ring, (see fig, 114.)
which does not touch the planet, but casts a
shadow upon it, and is itself divided into two
parts. With respect to the nature of this extra-
ordinary phenomenon, no probable conjecture
has yet been formed.

The GEORGIUM SIDUS, or URANUS, is too far
distant to admit of such accurate observation as
could be wished. It may sometimes be seen as a

SOS Astronomy. [Lecture 19.

star by the naked eye ; but its moons, or satel-
lites, can only be seen by a good telescope.

Besides these, there are other bodies attached
to our system, which, although their orbits are
singularly eccentric, have yet many things in
common with those which we have been describ-
ing ; they are called COMETS.

They are not luminous of themselves, but,
like the planets, are opake bodies, shining only
by the light of the sun, which they reflect to-
wards us. All the comets revolve round the
sun in a manner peculiar to themselves, that is,
in elliptical orbits exceedingly long and eccentric,
yet regulated by laws similar to those of the pla-
nets themselves, each describing equal areas in
equal times, about the sun as a centre of force.
On this principle astronomers have attempted to
calculate the period of their return, and in one
case at least with success, since it is generally
agreed that the comet which appeared in 1759 is
the same which was observed in 1531, 1607, and
1682. Its periodical revolution is therefore com-
pleted in about 76 years, and it may be conse-
quently expected again in the year 1835.

Some of the comets move from West to East,
like the planets, while others proceed in a con-
trary direction from East to West, and in the
contrary order of the signs of xthe zodiac. Some
pass nearly in the line of the ecliptic, and some
almost perpendicular to it. These orbits being ex-

The Primary Planets. 309

tremely protracted and eccentrical, the aphelion
of a comet is consequently at an immense distance
from the sun. In that case the light which they
receive from him is too feeble to be reflected to
us, and they are only visible when they approach
their perihelion. The time of their appearance
is, therefore, very short, compared with the time
of their disappearance. In order to describe the
course of a comet, let ABPC (PI. XXIX. fig.
120.) be the very long orbit of a comet, in one
of whose foci S is placed as the sunv; the aphelion
in A ; the perihelion in P. The comet is not
visible to us but when it approaches towards B,
and during the time which it passes the arc BPC
of its orbit. But the time is considerably shorter
than that which it employs to pass the other por-
tion of its orbit CAB, for these two reasons : first,
because the arc BPC is much shorter than the
arc C AB ; and in the second place, because the
comets, like the planets, are slower in their
course while they depart further from the sun ;
and, on the, contrary, they are swifter as they ap-
proach the sun. It requires much less time to
pass over the portion BPC of their orbit which
is visible to us, than the other portion CAB.

The most luminous part of the comet is com-
monly surrounded with a kind of atmosphere,
which again seems to emit from it a fainter
light, somewhat resembling the Aurora Borealis.
The interior part is called the nucleus, and the

310 Astronomy. [Lecture 19.

exterior the beams, or hair, in Latin coma, whence
the name comet, or hairy star.

It happens commonly, that a comet is accom-
panied by a train of light, sometimes very long,
as at L, and always directed to that part of the
heavens which is directly, or nearly, opposite to
the sun; this is called the tail of the comet.
Newton attributes the rise and the direction of
the tails of comets to the levity of certain par-
ticles, which the sun raises, by its heat, from the
atmosphere of the comet, when it approaches its
perihelion. He compares it to the smoke from
a burning body, which rises perpendicularly if
the body is at rest, or obliquely if the body is in
motion. In fact, the tails of comets, which al-
ways rise from the side which is opposed to the
sun, have a degree of curvature which is turned
from the side towards which the course of the
comet is directed. M. de Mairan attributes the
formation of the tails of comets to a part of the
solar atmosphere, with which he supposes the
comets to be charged, and which they draw
along with them in approaching their perihelion.
Other philosophers have supposed the tails of
comets to be collections of electric fluid, rendered
at once luminous and stationary. But all this is
mere conjecture.

The number of the comets is certainly very
considerable. Riccioli enumerates 154, others
assert that 450 had been seen previous to the

The Primary Planets. 311

year 1771. The tables of Berlin estimate them
at 700 ; and some have even supposed that there
are millions. They differ greatly in size : some
are so small as to appear like the fixed stars,
others not larger than Venus; while Hevelius
observed one in 1651, which was equal in ap-
parent magnitude to the full moon ; its light was,
however, much more pale and dim, and its as-
pect, on the whole, dismal. The nucleus of the
planet which appeared in the year 1807 was very
large; while the comet of 1811 had scarcely any
perceptible solid nucleus. The beautiful comet
of the summer of 1819 had a very evident nu-
cleus : its tail, also, was for a few evenings very
splendid.

LECTURE XX.

ASTRONOMY.

**-

THE SECONDARY PLANETS.

THE Secondary Planets are those which per--
form their revolution round other planets, which
themselves make their revolutions round the sun.
They are reckoned eighteen in number, viz. the
moon, the four satellites of Jupiter, the seven sa-
tellites of Saturn, and the six satellites of Uranus.

I shall first speak of the moon ; since, from
her proximity to the earth, we have a better op-
portunity of observing her motions and phaeno-
mena, than we have of the other secondary pla-
nets.

The apparent diameter of the moon, if seen
at the same distance from the earth as the sun,
would be little more than four seconds. Whence
we may conclude that her diameter is at least
390 times less than that of the sun. The moon's
diameter is about iiths that of the earth, or about
2170 miles. The whole bulk of the moon is
about Jr of that of the Earth.

The moon being much nearer to the earth than
the planets are, and having an apparent diameter
of more than half a degree, has been known ever

The Secondary Planets. 313

since the creation ; whereas the satellites of the
other planets have only been known to astrono-
mers since the invention of telescopes.

The moon completes her resolution in some-
what less than a month, during which period she
is once in conjunction with the sun, and once in
opposition. While the earth traverses not quite
a twelfth part of her orbit, that is, not the whole
of one of the signs of the zodiac, the moon com-
pletes her revolution or orbit round the earth.

Since the moon has no other light than what
she receives from the sun, it follows that she can
never have more than one half of her surface
enlightened ; but it depends upon the relative
position of the spectator with regard to the sun,
whether more or less of the face of the moon
will appear enlightened. For, being of a globu-
lar figure, it depends upon this position what part
of her orb shall receive the rays of the sun in
such a manner as to reflect them back to the eye
of the spectator. These different appearances of
the moon are called her phases*.

Thus, when the spectator is placed at T, be-
tween S5 the sun, and moon, at L, (PL XXVIII.
fig. 119.) the whole side of the moon which is
opposed to him will be enlightened, and she is

* These appearances will be pretty correctly represented
by moving an ivory ball suspended from a siring round
the flame of a candle, and observing in what manner the
light is reflected from different parts of its surface, accord-
ing to the position in which it is held.

VOL. I. P

314 Astronomy. [[Lecture 20.

then said to be at thet/w//. In proportion as she
approaches the sun, only a part of her surface
will be enlightened, as at P, when not more than
half will be in that state. She is then said to be
in her last quarter. In fine, the enlightened parts
become less and less* to a spectator on the earth
as she advances towards the sun, till at last she
comes between the sun and the earth at N, when
she is altogether invisible, and this last phasis
is called the new moon. She has not long passed
this point before she begins to present a small
portion of her surface enlightened. When she
is at Q, she is said to be in her first quarter,
and the enlightened part continues augmenting
till she is again at the full.

When the moon is placed between the four
parts A, B, C, D, and at an equal distance from
each point, she is said to be in her octants. In
the first A, and in the fourth, D, she presents
only one-eighth of her surface enlightened, and
in the second, B, and the third, C, three-eighths
of her surface are enlightened.

In the phases A, Q, B, which are between the
new and full moon, the convexity of the en-
lightened part is turned towards the west, and
in those of C, P, D, which are between the full
and the new moon, this convexity is directed
towards the east. All these changes, or phases,
will be rendered more evident to the student,
if he will in every position of the moon, imagine
tangents to the moon's orbit, drawn through her

The Moon. 315

centre, and observe what portion of the illu-
minated portion of the moon comes below that
tangent, with regard to the earth.

About the first octant and the fourth, the en-
lightened portion of the moon is in the form of
a crescent. The rest of the body of the moon
is then seen pretty distinctly. This results from
the light which is reflected upon the moon from
the surface of the earth. — For, as we have the
light of the moon, so the moon has the light of
die earth. In other words, the earth is a moon
to the moon, and with similar phases.

The revolution of the moon round the earth
measured by any fixed point in the heavens is
27 days 7 hours 43 minutes and 11 seconds.
This is called a periodical month. But the time
which intervenes from one conjunction with the
sun to another is 29 days 12 hours 44 minutes
and 3 seconds, and this is called a synodical
month or lunation. The reason of this dif-
ference is that, during the synodical revolution
of the moon, the earth advances on an average
about 27 degrees on the ecliptic.

To render this sufficiently intelligible we must
have recourse to a diagram. In fig. 123. (PL
XXIX.) let S represent the sun, FC a part of
the earth's orbit, or ecliptic, M D a diameter
of the moon's orbit when the earth is at A, and
m d the same diameter when the earth is at B.
While the earth is at A, if the moon is at D3 she
will be in conjunction, and if the earth were to

p2

316 Astronomy. [Lecture 20.

continue at A when the moon had completed its
orbit from D, through M, and to D again, it
would be exactly in conjunction, and the pe-
riodical and synodical month would be the same.
But as the earth does not continue at A, but
moves to B, and as the moon's orbit moves with
it, the diameter of that orbit will then be in the
position m d, and the moon will be at d. If
then the moon is at d, while the sun is at S, it
will be seen by the figure, that it cannot be in
conjunction, but must move to e9 in the diameter
fe, and consequently describe the arc d e to bring
it in conjunction with the sun. To do this oc-
cupies at a mean about 2 days 5 hours and 51
seconds; and the synodical is just so much
longer than the periodical month.

It is almost unnecessary to mention to you
that the diurnal potation of the earth about its
axis occasions an apparent daily revolution of
the moon from east to west, or, in common
language, the rising and setting of that lumi-
nary. But, during this apparent revolution of
the moon from east to west, she in reality ad-
vances in her orbit about 13 degrees from west
to east. There is therefore an apparent daily
retardation in the course of the moon, as she
rises and sets each day about 49 minutes later
than the preceding. This, however, is strictly
true only as to the equatorial regions, and under
circumstances to be afterwards explained. The
moon turns round on her own axis in the same

The Moon.

time that she makes her periodical revolution
round the earth*. On this account she always
presents to our view the same part of her surface,
or nearly the same face. There may, however,
be observed a little variation in the situation of
her spots, or in the position of her face ingeneraJ,
as presented to the spectator. This is called a
Vibration, and depends on the different aspects
which the moon assumes in consequence of the
diurnal motion of the earth on its axis, and of
the inclination of the axis of the moon in de-
scribing her elliptical orbit.

In the course of a year the moon makes 13
and £ revolutions upon her axis ; and as in eacli
of these revolutions the sun enlightens suc-
cessively every part of her surface, it follows
that the inhabitants of the moon, if there be
any, would enjoy about 13 days and a third.

The phenomenon of the harvest moon is not
generally understood. I shall endeavour to
explain it, following chiefly Mr. Ferguson, and
deviating but little from the simple language of
that justly popular philosopher.

It has already been stated that the moon rises
about 49 minutes later every day than on the
preceding; but this is strictly true only with
regard to places on the equator. In places of
considerable latitude there is a remarkable dif-

* The inhabitants of the moon, therefore, if we suppose
there are any, would have but one day and one night in
the course of a month.

318 Astronomy. ' [Lecture 20,

ference, especially in the time of harvest, with
which fanners were better acquainted than as-
tronomers till of late; and they gratefully ac-
knowledged the goodness of God, in giving
them an immediate supply of moonlight alter
the setting of the sun, for their greater con-
veniency in reaping the fruits of the earth, with-
out understanding the means by which this was
effected. About the equator, where there is no
variety of seasons, and the weather changes
seldom, and at stated times, moonlight is not
necessary for gathering in the produce of the
"earth. At the polar circles, where the mild
season is of very short duration, the autumnal
full moon rises at sunset from the first to the
third quarter. And at the poles, where the sun
is for half a year absent, the winter full moons
shine constantly without setting from the first to
the third quarter.

It is easy to state in general terms that these
phasncmena are owing to the different angles
made by the horizon and different parts of the
moon's orbit; and that the moon can be full
but once or twice in a year in those parts of her
orbit which rise with the least angles. But to
explain this subject intelligibly, I must dwell
somewhat longer upon it. The plane of the
equinoctial is perpendicular to the earth's axis ;
and therefore, as the earth turns round its axis,
all parts of the equinoctial make equal angles
with the horizon both at rising and setting ; so

The Harvest Moon. 319

that equal portions of it always rise or set at
equal times. Consequently, if the moon's motion
were equable, and in the equinoctial, at the rate
of 12 deg. 11 min. from the sun every day, as
it is in her orbit, she would rise and set about
49 minutes later every day than on the pre-
ceding; for 12 deg. 11 min. of the equinoctial,
rise or set in about that time in all latitudes.

But the moon's motion is so nearly in the
ecliptic, that we may consider her for the pre-
sent as moving in it. Now the different parts
of the ecliptic, on account of its obliquity to the
earth's axis, make very different angles with the
horizon as they set or rise. Those parts or signs
which rise with the smallest angles set with the
greatest, and the contrary. In equal times,
whenever this angle is lost, a greater portion of
the ecliptic rises than when the angle is larger;
as may be seen by elevating the pole of a globe
to any considerable latitude, and then turning it
round its axis. Consequently, when the moon
is in those signs which rise or set with the
smallest angles, she rises or sets with the least
difference of time; and with the greatest dif-
ference in those signs which rise or set with the
greatest angles.

In northern latitudes, the smallest angle made
by the ecliptic and the horizon is when Aries
rises, at the time when Libra sets ; the greatest
when Libra rises at the time Aries sets. From
the rising of Aries to the rising of Libra the

320 Astronomy, [Lecture 20.

angle increases; and from the rising of Libra
to the rising of Aries it decreases in the same
proportion. By this it appears that the ecliptic
rises fastest about Aries, and slowest about
Libra. On the parallel of London, as much of
the ecliptic rises about Pisces and' Aries in two
hours as the Moon goes through in six days ;
and therefore, while the moon is in these signs,
she varies but two hours in the time of her rising
for six days together; that is, she rises about
twenty minutes later every day or night than on
the preceding, at a mean rate. But in fourteen
days afterwards the Moon comes to Virgo and
Libra, which are the opposite signs to Pisces
and Aries; and then she differs almost four
times as much in rising ; namely, one hour and
about fifteen minutes later every day or night
than the former, while she is in these signs.

The ecliptic, together with the fixed stars,
make 866^- apparent diurnal revolutions about
the earth in a year, the sun only 365^. There-
fore the stars gain three minutes fifty-six seconds
upon the sun every day ; so that a sidereal day
contains only twenty-three hours fifty-six mi-
nutes of mean solar time ; and a natural or solar
day twenty-four hours. Hence twelve sidereal
hours are one minute fifty-eight seconds shorter
than twelve solar hours.

The sun advances almost a degree in the
ecliptic in twenty-four hours, the same way that
the moon moves ; and therefore the moon by

The Harvest Moon. 321

advancing 13 l-6th degrees in that time, goes
little more than twelve degrees farther from the
sun than she was on the day before. The moon
goes round the ecliptic in twenty-seven days
eight hours ; but not from change to change in
less than twenty-nine days twelve hours; so
that she is in Pisces and Aries once in every
lunation, and in some lunations she is twice in
one of these signs.

As the moon can never be full but when she
is opposite to the sun, and the sun is never in
Virgo and Libra but in our autumnal months, it
is plain that the moon is never full in the oppo-
site signs, Pisces and Aries, but in the harvest
and hunter's moon. And therefore we can have
in a year only two full moons, which rise so near
the time of sunset for a week together, as above
mentioned.

Here it will probably be asked, why we never
observe this remarkable rising of the moon but
in harvest, since she is in Pisces and Aries twelve
times in the year besides; and must then rise
with as little difference of time as in harvest?
The answer is plain; for in winter these signs
rise at noon ; and being then only a quarter of
a circle distant from the sun, the moon in them
is in her first quarter ; but when the sun is above
the horizon, the moon's rising is neither regarded
nor perceived. In the spring these signs rise
with the sun, because he is then in them ; and
as the moon changes in them at that time of the

Astronomy. [Lecture 20.

year, she is quite invisible. In summer they
rise about midnight, and the sun being then
three signs, or a quarter of a circle before, them,
the moon is in them about her third quarter;
when rising so late, and giving but very little
light, that rising passes unobserved. In autumn
these signs, being opposite to the sun, rise when
he sets, with the moon in opposition, or at the
full, which renders her rising very conspicuous.

Hitherto, for the sake of being perfectly in-
telligible, I have supposed the moon to move in
the ecliptic, from which the sun never deviates.
But the orbit in which the moon really moves is
different from the ecliptic; one half being ele-
vated 5 l-8d degrees above it, and the other half
as much depressed below it. The moon's orbit
therefore intersects the ecliptic in two points
diametrically opposite to each other ; and these
intersections are called the moon's nodes. So
the moon can never be in the ecliptic but when
she is in either of her nodes, which is at least
twice between every two successive changes, and
sometimes thrice. For, as the moon goes almost
a whole sign more than round her orbit from
change to .change; if she passes by either node
about the time of change, she will pass by the
other in about fourteen days after, and come
round to the former node two days again before
the next change. That node from which the
moon begins to ascend northward or above the
ecliptic, in northern latitudes, is called the

The Harvest Moon. 32S

ascending1 node, and the other from which she
begins to descend below the ecliptic southward,
the descending" node.

The moon's oblique motion, with respect to
the ecliptic, causes some difference in the times
of her rising and setting, from what, for the
sake of perspicuity, I stated in the preceding
paragraphs. When she is northward of the eclip-
tic, she rises sooner, and sets later, than if she
moved in the ecliptic ; and when she is to the
southward of it, she rises later, and sets sooner.
This difference is variable, even in the same
signs, for the nodes recede about 19^ degrees in
the ecliptic every year. When the ascending
node is in Aries, the southern half of the moon's
orbit makes an angle of 5-J- degrees less with the
horizon than the ecliptic does when Aries rises
in northern latitudes. In fact, the angle is then
only 9 j degrees on the parallel of London. The
moon consequently rises with less difference of
time while in Pisces and Aries than if her track
was exactly in the ecliptic. But in the course of
9 years and 112 days the descending node is in
Aries, and then the moon's orbit makes an angle
of 5-J- greater with the horizon when Aries rises?
than the ecliptic does at that time, that is, about
20y degrees on the parallel of London ; and this
causes the moon to rise with greater difference
of time in Pisces and Aries than if she moved
in the ecliptic. The shifting of the nodes,
however, scarcely ever affects the moon's rising

Astronomy. [Lecture 20.

so much, even in her quickest descending lati-
tude, as not to allow us still the benefit of her
rising nearer the time of sunset for a few days
together about the full in harvest, than at any
other time of the year.

The moon, when viewed through a telescope,
presents a vast irregularity of surface. These
inequalities are most apparent at the edge of her
enlightened part, when she is not at or near the
full ; for the sun's rays are intercepted by the
hills or prominences, so as to give that part of
her surface a jagged appearance : and sometimes,
to show the luminous tops of mountains, at a
considerable distance from the illuminated disc.
Upon mathematical principles, some of these
prominences have been measured, and one of
them is computed to be at least three miles in
height.

.Maps of the moon, have been published, and
her surface fancifully divided into lands and
seas, and names were even assigned to both.
The more correct discoveries, however, made
with the powerful glasses of Dr. Herschell, have
dissipated these pleasing illusions. Those parts
which were formerly supposed to be seas are now
found to be only cavities or valleys, which re^
fleet the light less strongly than the more ele-
vated parts. Through these instruments, in
fact, the moon appears a mere volcanic mass,
without water or atmosphere. That the moon
has no atmosphere has by many been thought

The Harvest Moon, 325

proved ; for, say they, if she had, the edge of her
disc would never appear so clear or well defined
as it does ; and when any of the fixed stars dis-
appear behind the moon, they retain their full
lustre till they touch her very edge, and then
vanish in a moment. These circumstances, they
affirm, could not take place if the moon had an
atmosphere; for she would then have always
round her a kind of mist or haze, and the stars
would appear fainter when seen through it. Still,
it must be acknowledged, that these reasons,
though feasible, are by no means decisive.

This account of the moon may serve to give a
general idea of a satellite, or secondary planet,
particularly as to its orbit and phases; but
whether or not, the satellites of the other planets
exactly resemble our moon in the other cir-
cumstances which have been just mentioned,
their immense distance will not allow us to de-
termine.

The four satellites or moons of Jupiter were
discovered by Galileo in the year 1610. The
sixth and largest satellite of Saturn was dis-
covered by Huyghens in the year 1655; three
others by Cassini ; the third in 1671 ; the fifth in
1672; the fourth in 1684; and the first and
second, by Dr. Herschell, in 1789. The six sa-
tellites of Uranus or the Georgium Sidus were
discovered by Dr. Herschell, who discovered the
planet. Astronomers denominate the satellites
with relation to their distances from the principal

826 Astronomy. [Lecture 20.

planet; they therefore call that the first satel-
lite which is nearest the planet, the second sa-
tellite that which is nearest to the former, &c.

From the continual changes of their phases
or appearances, it is evident that these secondary
planets are also opaque bodies like the planets
themselves, and shine only by means of the
borrowed light which they receive from the
sun.

The angles under which the orbits of Jupiter's
moons are seen from the earth, at their mean
distance from Jupiter, are as follow : the first
8' 35"; the second 6' 14"; the third 9 58"; and
the fourth 17' 30". And their distances from
Jupiter, .measured by his semi-diameter, are
thus : the first 5 2-3ds ; the second 9 ; the third
1 4 23-60ths ; and the fourth 25 1 8-60ths. This
planet, seen from its nearest moon, would appear
a thousand times as large as our moon does to
us ; waxing and waning in all its monthly shapes
every 42^ hours.

Jupiter's three nearest moons fall into his sha-
dow, and are eclipsed in every revolution ; but
the orbit of the fourth moon is so much inclined,
that it passes by its opposition to Jupiter, with-
out falling into his shadow, two years in every
six. By these eclipses astronomers have not only
discovered that the sun's light takes up eight
minutes of time in coming to us ; but they have
determined the longitudes of places on this earth
with considerable certainty, and with much

Of the Secondary Planets.

327

greater facility, than by any other method yet
known.

0

TABLE of the mean distances of the secondary
planets from their principal planet?.

Names of the
Planets.

Mean Dista

noes.
In French

leagues.

In Radii of

the Earth.

The Moon

59

84515

In Radii of

Jupiter.

1st Satellite

of Jupiter

5,67

—

92540

2d - -

9

—

1 46^98

3d - -

14,38

—

23471O

4th - -

25, .SO

—

4-12Q46

In Radii of

Of the Ring

Saturn.

1st Satellite

of Saturn

4,70

1,93

65149

2d - -

5, 12

M7

83377

3d - -

7, 16

3,45

1 16)58

4th -

18, 00

8,00

270048

5th - -

52, 50

23,23

884152

6th

*

7th

In Radii of

Uranus.

1st Satellite

ofUranus

16,50

—

106I65I

2d - -

JQ Q[

_

1 2640 1 r

3d

1

4th

5th

6th

328

Astronomy. [Lecture 20.

The secondary planets, like the primary, finish
their revolutions in longer times, in proportion
as they are further from the centre of their orbits,
the relation of the square of the times, and the
cubes of the mean distances obtaining equally
with all, as may be seen by the following table.

TABLE of the duration of the periodical revolu-
tions of the secondary planets round the prin-
cipal planet.

XT c ,, „, Duration of the Revolutions.

Names of the Planets. /doyg hrs< ?*« ,„ InSeconds.

The Moon by affinity \
with the Stars. /

27 7 43 11 36 or

2360591

— — by affinity with \
the Equinox. J

27 7 43 5

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1st Satellite of Jupiter

1 18 2/33

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2d

3 13 13 42

3OS822

3d ...

73 42 33

618153

4th

16 IQ 32 8

1441923

1st Satellite of Saturn

1 21 18 27

163107

2d -

2 17 44 22

236662

3d ...

4 12 25 12

390312

4th

1522 3438

1377278

5th - - .

79 7 47 o

OS53620

6th

7th

LECTURE XXI.

ASTRONOMY.

THE EARTH.

THE earth is nearly of a spherical figure.
The truth of this without having recourse to
scientific principles, will appear sufficiently evi-
dent from the voyages of celebrated navigators
such as Magellan, Sir Francis Drake, Lord An-
son, Cook, &c., who all set out, at different times,
to sail round the world, and by steering their
course continually westward, arrived at length at
the exact place whence they departed: which
could never have happened had the earth been
of any other form than spherical.

This form is also apparent, from the circum-
stances which attend large objects when seen at
a distance on the surface of the sea. For when
a ship proceeds to sea, we first lose sight of the
hull or body of the vessel ; afterwards of the
rigging ; and at last discern only the top of the
mast ; which is evidently owing to the convexity
of the water between the eye and the object ; or
otherwise the largest and most conspicuous part
would have been visible the longest, as is mani-
fest from experience, in other cases.

330 Astronomy. [Lecture SI .

Again, the earth is proved to be nearly sphe-
rical in this manner : its roundness permits us to
see only a very little extent of its surface : for
upon a level space, for instance, a calm sea, the
eye elevated six feet above it, cannot perceive an
object placed upon it at a distance greater than
2551 fathoms; that is, it cannot discern more
than the extent of a circle of 5114 fathoms dia-
meter. But the circumference of this circle ap-
pears to touch the heavens, and the plane of this
circle extended to the starry heavens is what is
called the horizon. If the observer were placed
in the centre T (PL XXIX. fig. 122.) of the
earth, the horizon H H would divide the sphere
into two parts ; but being placed at the surface a,
the superior and visible hemisphere h Z ?i is
smaller than the inferior h N h, which is invisible.
It may yet be observed, that the radius of the
earth T a being infinitely small, compared with
the imaginary radius of the starry heavens T H
or T Z, the difference between the two horizons,
with respect to them, is not perceptible. When
the first, therefore, is called the rational, and the
other the sensible horizon, which are the names
by which they are distinguished, it must be with
reference to nearer objects.

Many other proofs might be adduced to show
that the earth is nearly spherical ; nor are the
little unevenn esses on its surface, arising from
hills and valleys, any material objection ; since

The Earth. 331

»

the highest mountains with which we are ac-
quainted bear a less proportion to the whole bulk
of the earth, than the small protuberances on the
coat of an orange bear to that fruit. And accord-
ingly we find that these trifling protuberances
occasion no irregularities in the shadow of the
earth during the time of a lunar eclipse ; but that
the circumference of it always appears to be even
and regular, as if cast by a body perfectly globu-
lar ; and this also affords a further proof of the
spherical form of the earth ; since no body but a
sphere can in all positions project a shadow with
a circular boundary. In speaking of the earth,
however, when I use the term spherical, I would
not be understood to indicate that it is a perfect
globe or sphere. The most correct observations,
on the contrary, prove that it is an oblate spher-
oid, that is, a little flattened at the poles, and
larger about the equatorial regions, somewhat re-
sembling (if we may use so homely a comparison)
the form of a turnip.

The earth's axis makes an angle of nearly 23 \
degrees with the axis of its orbit, and keeps al-
ways the same oblique direction inclining towards
the same fixed stars throughout its annual course;
and this causes the return of Spring, Summer,
Autumn, and Winter, as will be shown in a
future lecture.

The mensuration of the earth has been attempt-
ed by different persons, with different degrees of
precision. Mr. Richard Norwood, in the year

332 Astronomy. [Lecture 21.

1635, took the sun's altitude when it was in the
summer solstice, both at London and York, with
a sextant of five feet radius, and by that means
found the difference of latitude between these two
cities to be two degrees and twenty-eight minutes.
He then measured their distance in as exact a man-
ner as he was able ; and having taken into the
account all the windings of the road, with the as-
cents and descents, he reduced it to an arc of the
meridian, and found it to contain twelve thousand
eight hundred and forty-nine chains ; and this
distance, being compared with the difference of
latitude, gave him five thousand two hundred
and nine chains to a degree, or about fifty-seven
thousand three hundred French fathoms or toises.
This method requires no explanation, if the
two places are considered • as lying under the
same meridian, which indeed is nearly the case.
The same operation may also be easily performed
by trigonometry, when the two places lie under
different meridians; for if we measure the di-
stance of any two objects and take the angles
which each of them makes with a third, the tri-
angle formed by the three objects will become
known ; so that the two sides may be as accurate-
ly determined by calculation, as if they had been
actually measured in the same manner as the first.
And by making either of these sides the base of
a new triangle, the distances of other objects may
be found by trigonometry as before ; and thus,
by a series of triangles connected together at their

The Earth. 333

bases, we might measure the whole circumference
of the earth. But this would be an enterprise
as useless as it is laborious ; for, since we know
the relation which any part of a circle bears to
the entire circumference, the measure of a few
degrees or even of one single degree, will give
the measure of the whole. But by applying the
telescope to the quadrant, and furnishing it
with a micrometer, we are enabled to correct a
great many inaccuracies attending this kind of
mensuration. The Academy of Sciences at
Paris, perceiving from these considerations the
necessity of a new measure of the earth, repre-
sented the execution of it as a measure of
national honour and importance. Monsieur
Picard was the person employed to perform this
business. He began by measuring the distance
between Villejuif and Juvisy ; and this base,
which he found to be five thousand six hundred
and sixty-three fathoms, was that to which he re-
ferred all his calculations. He next placed him-
self at Juvisy, and by directing the telescopic
sights of his quadrant, the one to the windmill
at Villejuif, and the other to the spire of the
church at Brie, he measured the angle subtended
by these two objects. Leaving his present
station, he removed himself to Villejuif, and
by measuring the angle between Juvisy and
Brie, the distance between Villejuif and Brie
was found by calculation to be eleven thousand
and twelve fathoms. This distance he made a
new base ; and by forming a second triangle be-

334 Astronomy. [Lecture 21 .

tween Brie, Villejuif, and Monthleri, he found
the distance, in like manner, between Brie and
Monthleri to be thirteen thousand one hun-
dred and twenty-one fathoms. He then formed
a third triangle between Monthleri, Brie, and
Monjay; a fourth between Monthleri, Brie, and
Malvoisine ; and a fifth between Monthleri,
Monjay, and Maree ; and from all these mea-
sures, the distance between Mareil and Mal-
voisine was found to be thirty-one thousand eight
hundred and ninety-seven fathoms French.

In a similar manner, by means of thirteen
triangles, he proceeded as far as Sourdon, near
. Amiens, and found the distance between Sour-
don and Malvoisine to be sixty-eight thousand
four hundred and thirty fathoms. But as cal-
culations are less subject to errors than me-
chanical operations, Mons. Picard, in order to
avoid every inaccuracy of this kind, took a new
base near Sourdon, and found its length, both
from a continuation of his trigonometrical ope-
rations, and from an actual mensuration ; and
as these exactly agreed, he could no longer doubt
of the truth of his former calculations. For the
two bases were separated by so large a distance,
that it was impossible for them to correspond,
except by a perfect exactitude in all the interme-
diate steps.

This part of his project being finished, he had
now to reduce the distance between Sourdon and
Malvoisine to an arc of the meridian.

The Earth. 335

Having obtained this terrestrial distance to a
great degree of accuracy, he had only to find the
celestial arc which corresponded with it. This
he did by observing the meridian distances of the
same star, both from the zenith of Sourdon and
Malvoisine, and taking their difference ; and as
this difference, which he found to be one de-
gree, eleven minutes, and fifty-seven seconds,
answered to a distance of sixty eight thousand
four hundred and thirty fathoms upon the earth,
he concluded, by the rule of proportion, that the
length of a degree, in that latitude, must be fifty
seven thousand and sixty-four fathoms. But
having connected Amiens to his series of triangles,
and finding from this new measure that a degree
would be fifty-seven thousand and fifty-seven
fathoms, he took a mean between the two, and
fixed his degree at fifty-seven thousand and sixty,
or about sixty-nine and a "half English miles.

The surveys were all taken upon a supposition
that the earth was a perfect sphere ; but the
truth of this doctrine was soon called in question
as the science advanced. Newton and Huyghens
had shown, from the known laws of gravitation,
that the true figure of the earth must be that of
an oblate spheroid, flattened at the poles, and
protuberant at the equator. Dominique Cassini,
on the other hand, depending more upon the
accuracy of his measures, than upon deductions
drawn from theoretical reasoning, asserted it to
be that of a prolate spheroid, flattened- at the

336 Astronomy. [Lecture 21.

equator, and protuberant at the poles. To de-
cide this important question which had now be-
come a national dispute, it was ordered by the
French king that a degree should be measured,
both at the equator and polar circle, so that from
a comparison of these with that in France, the
true figure of the earth might be determined in
as exact a manner as possible.

For this purpose, Messieurs Maupertuis, Clai-
raut, Camus, Le Monnier, and Outhier, were
sent to the north of Europe to measure the re-
motest degree they could reach ; and Messieurs
Godin, Bouguer, and La Condamine, to Peru, in
.South America, to measure a degree near the
equator. The first of these companies began
their operations at Tornea, near the Gulf of
Bothnia, on the 8th of July 1736, and finished
them about the beginning of June 1737. M.
Maupertuis, soon after their return to France,
published an exact and interesting account of all
their transactions.

The result of this measurement was found to
be, that an arc of the meridian contained between
the parallels of Tornea and Kittis was equal to
fifty-five thousand twenty-three and a half fa-
thoms. And as the magnitude of this arc was
found, by means of the zenith distances of cer-
tain fixed stars, to be 57 minutes 28 and 2-3ds
seconds, it was determined, after proper correc-
tions, that the true length of a degree of the
meridian which cuts the polar circle is fifty-

The Earth. 337

seven thousand four hundred and twenty-two
fathoms.

Those who were sent to Peru, in South- Ame-
rica, had still greater difficulties to encounter
than their friends in Lapland, and were a longer
time employed in their operations. They set out
upon their expedition, about twelve months be-
fore the former, and did not finish their survey
till the year 1741. The province of Quito was
the place determined on as the most proper for
their purpose. Here they measured an arc of
the meridian, of three degrees seven minutes and
one second, and found it to contain 176,950
fathoms ; which being reduced to the level of the
sea, and properly corrected, the first degree of
the meridian from the equator was found to be
equal to 56,753 fathoms. These measures afford
a decisive demonstration that the earth is flat-
tened at the poles, and protuberant at the equa-
tor. For had the figure of it been a complete
globe, as was formerly imagined, a degree of the
meridian in every latitude would have been found
the same ; and had the figure been that which
was given to it by Cassini, a degree at the polar
circle would have been less than a degree at the
equator. But as a degree at the equator appears
to be about 307 fathoms less than a degree in
France, and about 669 less than a degree at the
arctic circle, it is easy to show that the figure of
the earth must be nearly the same as was as-
signed it by Newton.

VOL. i. Q,

338 Astronomy. [Lecture 21. -

Subsequent admeasurements carried on upon
a large scale, and with great accuracy, in Eng-
land and Scotland, by Roy, Mudge, and Colby ;
in France by Delambre, Mechain, Arago, &c. ;
in Denmark by Schumacher ; in Lapland kby
Swanberg ; and in India by Lambton. Though
they are attended by certain minute irregularities,
all tend to confirm the general result that the
axes of the earth are in about the ratio of 304
to 305.

Experiments on the pendulum in different
places, as, by Bouguer at the equator, Campbell
at Jamaica, Ciscar at Madrid, Borda and Biot
at Paris, Whitehurst and Kater at London, Biot
at Leith and Unst, Dr. Olinthus Gregory at
Woolwich, and in Balta Isle, Zetland, and Lord
Mulgrave at Spitsbergen ; all prove, generally,
that the equatorial axis exceeds the polar axis.
A synoptical account of the results, agreeably to
this latter method, is given in Tilloch's Philoso-
phical Magazine for June, 1819.

There is nothing of more importance to a naval
people than the power of ascertaining the Longi-
tude at sea. This problem is ultimately resolva-
ble into that of knowing the precise hour at the
place where the mariner is, and the precise hour
at any other place the longitude of which is well
ascertained — London, for instance. It is easy to
find the hour at any place where the mariner may
happen to be, by observing the height of the sun
or of any fixed star ; and observations on the

The Earth. 339

eclipses of the satellites of Jupiter show the hour
by the clock of London at the time when they
are observed ; the difference, then, between the
times observed at the different places, will give
the difference of longitude. This is the reason
why a clock or time-piece which does not vary
at all, and which is set to the time of the place
from which a vessel sails, will always serve to
show the difference of time between whatever
place it may be at, and that of the place which
it has left, and consequently will indicate the
longitude, provided it goes accurately.

To render this matter still more familiar, a*
the sun appears to move uniformly round the
earth, and to describe a circle, which contains
360 degrees, in twenty-four hours, he will of
course move through an arc of 15 degrees in an
hour. When it is noon, therefore, at London
and at all other places which lie under the same
meridian, it will be one o'clock in the afternoon
at all those places which1 lie under the meridian
15 degrees to the east of that of London ; and
eleven o'clock in the morning, at all those places
which lie under the meridian 15 degrees to the
west of that of London. If the distance of the
meridians are 30 degrees, it will make two hours
difference in the time ; if 45 degrees, three hours,
&c., reckoning according to the situation of the
places.

From these circumstances you will readily ob-
serve, that as places differ in longitude, or are

Astronomy. [Lecture 21.

situated under different meridians, so the clocks
and watches of those places, supposing them to
be well regulated, will show different hours at
the same moment of absolute time ; a difference
of 15 degrees in longitude always producing a
difference of one hour in the time shown by those
machines.

In the Nautical Almanac, a work printed un-
der the authority of the Commissioners of Longi-
tude, for the purpose of facilitating astronomical
computations, the distances of the moon from
the sun, and from certain fixed stars, are ready
computed for every day at noon, and every three
hours afterwards, for the meridian of Greenwich ;
with a rule for finding the time, answering to
any given distance whatever. Suppose now that
the pupil was at sea, and wanted to find the
longitude of the place he was in : he chooses some
remarkable fixed star, whose name and situation
are known, and finds with a quadrant the angu-
lar distance between that star and the moon ;
and by a watch, previously regulated for that
purpose, the exact time when the observation was
made : this being done, he looks into the alma-
nac, and finds what time it is at Greenwich when
the moon and star have the same distance ; and
this time, being compared with the time of obser-
vation, will, by allowing 15 degrees to an hour,
give the longitude of the place required. The
names and places of the brightest fixed stars are
to be found in the " Tables requisite to be used

The Earth. 341

with the Nautical Almanac ;" together with the
methods made use of for obtaining their true
distances from the moon at the time of observa-
tion. For it is to be observed, that the distance
found by the quadrant is not that which is to
be used in determining the longitude, but the
distance as it would appear to a spectator placed
at the earth's centre. This is the distance as it
is computed for Greenwich ; and in order that
they may agree, it must be determined in the
same manner for the place of observation.

The last method of finding the longitude,
which is founded upon observations of the moon,
is, by the general consent of astronomers, the
best that has yet been discovered. And though
it may not be easily practised by every common
mariner, yet by a person of skill and abilities the
operation will be performed in a few minutes.

In the first place, then, it may be observed, that
the moon's daily motion in her orbit being about
13 degrees, her hourly mean motion is about
half a degree, or one minute of a degree in two
minutes of time ; so that, if an error of one mi-
nute is committed in calculating the place of the
moon, it will produce an error of two minutes in
time, or half a degree of longitude.

The late Professor Mayer, of Gottingen, fol-
lowing the theory of Newton, formed a set of
lunar tables which gave the moon's place in the
heavens to a great degree of accuracy ; and these
were afterwards improved by Mr. Charles

342 Astronomy. [Lecture 21.

Mason, so as to determine the distance of the
moon from the sun or a fixed star at any given
time within little more than half a minute of a
degree. — This difference from the truth cannot
subject us to an error in longitude of much more
than a quarter of a degree, or 15 geographical
miles.

It will conduce to a greater degree of accuracy,
if the moon's distance is taken from two stars,
or jfrom the sun and a star on each side of her
as often as opportunity permits : for as the im-
perfections of the instrument, as well as unavoid-
able small errors which attend the use of it, have
a natural tendency to correct each other, the
mean result, arising from these different observa-
tions, will generally be much nearer the truth
than if either of them is taken separately.

Observations upon the eclipses of Jupiter's
satellites, the times of which are recorded in the
Nautical Almanac, and in that much more cor-
rect Almanac, White's Ephemeris, serve likewise
to determine the longitude with considerable pre-
cision. But, for a minute explication of these
and other methods, the reader will do well to
consult Dr. Mackay's work, written expressly on
the subject.

LECTURE XXII.

ASTRONOMY.

THE TIDES.

As a phenomenon affecting this earth, the
consideration of the tides will properly follow
what we have advanced on that subject. It is
almost unnecessary to explain to you what is
meant by the word tide. If a definition were
called for, it might be said that it is a daily
regular and periodical rising and falling of the
waters of the sea.

In great oceans this rising and falling, in
other words the flux and reflux of the sea, take
place twice a day. That is, about every six hours
the waters of the ocean extend themselves over
its shores : this is called \hejlux or flood ; in this
state they remain a short space of time, after
which they retire or fall back ; and this is called
the reflux, or ebb tide.

During the flood tide the waters of those
rivers which communicate with the ocean are
stopped in their course by the advance of the sea
water ; the rivers swell, and overflow their banks ;
during the reflux or ebb tide the stream resumes
its usual course.

344 Astronomy. [Lecture 22.

Where the motion of the waters is not re-
tarded by capes, islands, or straits, or other
similar obstacles, three periods are remarkable in
the tides — The daily period, the monthly, and
the annual.

The mean daily period is 24 hours 49 minutes,
during which there are two flood and two ebb
tides. This interval of 24 hours 49 minutes
is the time in which the moon performs her
mean apparent daily revolution round the earth.
During this diurnal period we observe,

1st, That the high tide reaches the Eastern
harbours and roads, sooner than those to the
West.

2dly, That between the tropics the tide always
seems to proceed from East to West.

3dly, That in the torrid zone, unless there is
some particular obstacle, the flood tide comes
regularly at the same time to all places under the
same meridian. On the contrary, in the tempe-
rate zones it comes sooner to a lower than to a
higher latitude ; but beyond 65° of latitude the
tide is not sensible.

The monthly period is distinguished, 1st, by
this circumstance, that at the new and full moons
the tides rise much higher than at other periods ;
and these are called spring tides ; and when the
moon is in the quarters, the tides are lowest, and
are called neap tides. The new and full moons
are called the syzigies, the quarters, the quadra-

The Tides. 345

tures : the tides go on increasing from the quadra-
tures to the syzigies, and decreasing from the
syzigies to the quadratures.

2dly, When the moon is in the syzigies
or quadratures, the tide is at the highest three
hours after the moon has passed the meridian.
When the moon is going from the syzigies to the
quadratures, the time of high water is rather
sooner than these three hours. The contrary hap-
pens when the moon passes from the quadratures
to the syzigies.

Bdly, Whether the moon be in the southern
or the northern hemisphere, the time of high
tide does not happen any later in northern cli-
mates.

The annual period is distinguished by these
circumstances : — 1st, That at the time of the
equinoxes the spring tides are higher than at any
other season of the year, and the neap tides the
lowest, because at these periods the sun and
moon are in the equator. At the solstices, on
the contrary, the spring tides are not so high as
in other lunations; nor the neap tides so low
as at other periods. The tides also are higher
at the winter than at the summer solstice.

2dly, The tides are higher in proportion as
the moon is near the earth, that is, when she is
in her perige. They are also higher when the
moon is near the equator, and has of course less
declination. In general, then5 it may be said, the

Q5

346 Astronomy. [Lecture 22.

highest tides are when the moon is at once near
the equator, in perige, and in the syzigies.

8dly, In northern climates the spring tides
are higher in the evening during winter ; and in
the summer they are higher in the morning *.

It is evident from the detail of these phe-
nomena, that the tides have a marked connexion
with the motions of the moon; and that they
are also in some degree governed by those of the
sun. Whence we may fairly conclude that these
luminaries, and particularly the former, are the
principal natural causes of the phenomena of the
tides.

Kepler had long ago conjectured that the gra-
vitation of the earth towards the sun and moon
was the cause of the tides. " If the earth ceased,"
said he, " to attract the waters of the ocean, they
would be elevated towards die moon; for the
moon's sphere of attraction extends to our earth,
and evidently acts upon the waters." What was
mere conjecture in this great astronomer was
reduced to certainty by the superior genius of
Newton: upon his principles, therefore, I shall
endeavour to exhibit a popular view of the theory
of the tides.

* The days on which the highest tides may be expected
are always given in White s Ephemeris before mentioned.
That very useful almanac also exhibits the time of morn-
ing and afternoon high water daily, as computed accurately
for London Bridge; with subsidiary rules, by which the
respective times of high water at several other ports may
readily be found.

The Tides. 347

The surface of the earth and of the sea is so
nearly spherical, that it may for the present be
regarded as such. This being granted, if we ima-
gine the moon A (PI. XXIX, fig. 121) situated
in any part above the surface of the sea at E, it
is evident that the water E will be attracted by
her more in that point than any other in the
whole hemisphere PEH ; there will of course
be a tide at E.

For the same reason, the water at G will be
less attracted by the moon than any part of the
sea in the hemisphere PGH. The water then at
this part will be less affected by the moon than
at any other ; it will be therefore elevated on the
opposite side, and this will make a tide at G.

By these means the surface of the whole ocean
will assume an oval form, the longest diameter
of which is EG, and the shortest PH. As the
moon then changes her position, by the earth's
diurnal motion, this oval figure will follow the
apparent place of the moon ; this therefore will
produce two tides in the course of 25 hours, as
before established.

Such is the general theory of the tides. But
to explain it more fully, let us suppose the moon
to be at rest, and let us imagine the earth to be a
solid globe also at rest, covered however to a
certain depth with a homogeneous fluid, the
surface of which shall also be spherical.- — Suppose
the particles of this fluid to gravitate, as in fact
they do, towards the centre of the earth, at the

348 Astronomy. [Lecture 22.

same time that they are attracted by the moon.
It is then certain that if all the particles of
the fluid with which the globe is covered were
attracted by an equal force and in a parallel
direction, the action of the moon would produce
no other effect than to move or displace the
whole mass of the globe and of the fluid to-
gether, without causing any other derangement
in the respective situation of their parts.

But, according to the laws of attraction, the
parts of the superior hemisphere, that is, of that
portion which is nearest the moon, are more
forcibly attracted than the centre of the globe ;
and on the contrary, the parts of the inferior
hemisphere are less forcibly attracted. It follows,
then, that the centre of the globe being moved
by the action of the moon, the fluid which
covers the superior hemisphere, and which is
attracted more forcibly, must have a tendency to
move more than the centre, and consequently to
rise with a force equal to the excess of this
attraction above that which acts upon the centre.
On the contrary, the fluid which is expanded
over the inferior hemisphere being less attracted
than the centre of the globe, will have a less
tendency to the same point. It will of course
have a kind of centrifugal force, nearly equal to
the force which attracts that of the superior
hemisphere. Let us* then suppose that the
moon A, by the force of her attraction, draws
towards her the centre T to the extent of 20

The Tides. 349

feet, and brings it to t ; that the part E being
nearer to the moon, and still more forcibly at-
tracted, is carried to the extent of 30 feet ; and
that the point G being more distant from the
moon and more feebly attracted than the centre T,
is only drawn as far as g to the extent of 1 0 feet ;
it is evident that the radii * t e and t g must be
longer by 10 feet than the radii TE and TG.
The waters therefore must appear elevated to
that extent, while they are lowered at p and h.
Thus the fluid (as appears evidently by the
figure) will be elevated at two opposite points
in the line AG, in which line are the centres of
the earth and the moon. -If further the at-
traction of the sun is added to that of the moon,
the former being about a third of the latter,
the effect will be proportionably greater ; but if
these two attractions are placed in counterpoise
to each other, the effect will be proportionably
less.

The motion of the waters of the sea (at least
that of which we are sensible, and which is not
common to them with the whole mass of the
terrestrial globe), is not the effect of the entire
action of the sun and moon, but of the difference
between the action of these luminaries upon the
centre of the earth, and upon the fluid with
which it is covered, as well on the upper as the
lower surface. It isv this difference which we '

* The radius is a line from the centre to the circu.m*
ference of any circular figure.

350 Astronomy. [Lecture 22.

call action, force, or attraction, solar or lunar.
The lunar action, as just noted, is thrice as
energetic as that of the sun.

I shall now deduce from the doctrines which
have been advanced, what I hope will be found
a clear and convincing explanation of the principal
phsenomena of the tides.

We have seen that the waters of the ocean
must rise at the same time at that part of the
ocean which is immediately under the moon,
and at the opposite point. Consequently, at
ninety degrees from these points on each side,
the water must be lowered. In the same manner
the solar action must elevate the waters in that
part which is immediately under the sun, and at
the part diametrically opposite. Combining the
two actions, we shall find that the elevation of
the water at the same place must be subject to
some variations both with respect to quantity
and time, according as the solar and lunar actions
are combined; or according as these forces act
differently, or against each other.

In general, in conjunctions and oppositions of
the sun and moon, their forces are combined.
In conjunctions these bodies act on the same
meridian; and in opposition, they still act in
the same line, and each raises the water on that
side which is immediately under it.

In the quadratures, on the contrary, the water
which is elevated by the sun, is depressed by the
moon's attraction, for the moon is then ninety

The Tides. 351

degrees from the sun. This, then, is the time
of the lowest or neap tides ; and the highest or
spring tides happen at new and full moon, when
the two luminaries are in conjunction or op-
position.

In the course of every natural day there are
two tides, which depend upon the action of the
sun, as in every lunar day there are two which
depend on that of the moon ; all follow, how-
ever, the same laws. In general, the nearer the
moon happens to be to the earth, the greater is
its attraction, and the same may be said of the
sun.

Laying aside for the present the action of the
sun on the ocean, the highest tide would be at
the moment when the moon passed the meridian,
if the waters had not, like all bodies in motion, a
vis inertice, by which they are inclined to retain
the impression they have received. But this
force must necessarily produce two effects. It
must retard the time of high water, and it must
in general diminish the height of the tide. As
a proof, let us for a moment suppose the earth
at rest, and the moon above it in a certain point.
Abstracting, then, the action of the sun, the
force of which upon the tides is much less than
that of the moon, the water would unquestion-
able rise in that part which was under the moon.
Let us suppose again that the earth turns upon
its axis : on one side it turns very rapidly as to
the motion of the moon ; and on the other, the

352 Astronomy. [Lecture 22.

water which has been raised by the moon, and
which turns with the earth, endeavours (if we
may use the expression) to preserve by its vis
inertias the elevation which it has acquired,
though in withdrawing from the moon it loses
somewhat of that elevation. Thus the water
carried forward by the motion of the earth on its
axis will be elevated more to the east of the moon
than it would have been without this motion ; yet
it will at the same time be less elevated than it
would have been directly under the moon, had
the earth continued immoveable. The motion of
the earth on its own axis, then, has in general a
tendency to retard the time of high water, and
to lessen its elevation.

Both after the flux and reflux, the ocean con-
tinues some time quiescent, neither disposed to
rise nor fall, because the waters have a tendency
to preserve the state of rest and equilibrium in
which they are at the flood and ebb tide ; and
because the motion of the earth, displacing the
waters with relation to the moon, lessens the
intensity of the action of that luminary. These
two efforts counterbalance each other for some
moments. We must add also, that the attrac-
tion of the particles of the fluid to each other,
and obstacles of different kinds, which must
retard their motion, prevent them from passing
all at once from a state of flood to that of ebb.

The moon passes above the eastern parts of
the globe before the western. The flood tide,

The Tides. 353

therefore, always proceeds in this direction. But
the general motion of the sea between the tropics
from east to west is more difficult to explain.
This motion is evinced by the direction in which
all floating bodies proceed there. It is observed
also that, all other things being equal, it is much
easier to navigate towards the west than in the
contrary direction. M. D'Alembert has de-
monstrated, in his Inquiry into the Causes of
Winds, that the action of the sun and moon
must cause a motion in the waters under the
equator from east to west. This action must,
according to the same writer, equally affect the
air, and is one of the principal causes of the
trade-winds.

If the moon remained always in the equator,
it is evident she would then be always ninety
degrees distant from the poles, and that there
could be there neither flux nor reflux ; for the
waters at the poles would always be low. Though
the moon, however, is not always in the equator,
she is never more distant from it than twenty-
eight degrees. We are not to wonder, therefore,
that near the poles, and even at the latitude of
sixty-five degrees, the tide is not perceptible.

As it only happens twice in a month that the
sun and moon are in the same line or direction,
(that is, when they are in conjunction or opposi-
tion,) the elevation of the water ought in general
to take place neither immediately under the sun
nor under the moon, but in a point between the

354 Astronomy. [Lecture 22.

two, as in truth we find to be the case. Thus,
when the moon passes from the syzigies to the
quadratures, (that is, when she is not ninety
degrees from the sun,) the highest elevation of
the waters ought to take place at the setting of
the moon; — the contrary happens when the
moon passes from the quadratures to the syzigies.
In the first case the time of high water ought to
precede the three lunar hours : for on one side
the vis inerticc of the waters produces the
elevation three hours after the moon passes the
meridian ; and on the other, the relative situation
of the sun and moon affects this elevation before
the moon passes the meridian. On the contrary,
in the second case, and for similar reasons, the
time of high water must happen rather after the
three hours.

As there is some retardation of the jtide by
the vis inertias of the waters, and their tendency
to preserve an equilibrium, the highest tides do
not take place exactly at the time of the op-
positions and conjunctions of the sun and moon,
but two or three tides after. In the same manner,
the lowest neap tides happen a little after the
quadratures.

Since in the winter the sun is a little nearer
the earth than in the summer, it is observed
that, when all other circumstances are equal, the
tides about the winter solstice are rather higher
than those of the summer solstice.

Such would be the regular phaenomena of

The Tides. 355

the tides, if the sea were, in all parts, of the
same depth ; but the shoals in certain parts, and
the narrowness of some of the streights and
channels, cause a great variety in the height of
the tides ; of which it is impossible to give an
account, without an exact knowledge of all these
irregularities, the relative situation of the shores,
the depth of the channels, &c.

At the mouths of rivers, the flood tide and
the tide of ebb exhibit different phsenomena.
The current of the river resists the flux of the
sea, but aids its motion at the reflux; whence
the tide of ebb lasts considerably longer than
the tide of flood. This is the reason, too, why
high water takes place at a later hour in great
rivers than elsewhere. But the diversities of
ebb and flow in different localities are too
numerous to be traced in our narrow limits.

END OF VOL. I.

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