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NATtfkAL AND EXPEKIMEJ^TAL

PHILOSOPHY:

BY THE

REV. DAVID BLAIR,

rTHOR OF A GRAMMAR ON CHEMISTRY, VNlVBRSj^L
PRECEPTOR, &C. &C.

EIGHTH AMERICAxV,

FROM THE TWELFTH LONDON EDITION,

J IMPROVED JiND ENLARGED. \t-

"'4

. » <**»

GRAM

OF '■'

PHILOSOPH^^

INCLUDIITG

PHYSICS,

ACOUSTICS,

DYNAMICS,

OPTICS,

MECHANICS,

ASTRONOMY,

HYDROSTATICS,

ELECTRICITY,

HYDRAULICS,

GALVANISM,

PNEUMATICS,

MAGNETISM,

ACCORDING TO THE LATEST DISCOTERIES,

WITH ONE HUNDRED ENGRAVINGS ON W001>.

BY THE

REV. DAVID BLAIR,, .

4^kor of the Class Book^ Universal Preceptor y "EngHsk
\ Qrammar, Reading Exercises, Models of

1 TWENTY-FIRST EDITION,

from the Twelfth London Edition, Improved and Enlarged.

Letters. Sec. '-

HARTFORD,

O. D. COOKE & CO.

. A""

■y THE

i6682

DISTRICT OF CONl!?ECTICUT, as.

BE IT REMEMBERED, That on the fourth du
Xf* ^- of February, in the forty-eixth year of the Independi
ence of the United States of America, Samuel G.
Goodrich/ of the said District, hath depoisited in this Office, thi
title of a Book, the right whereof he claims as Proprietor, ia
the words following — to wit : ^ A Grammar of Natural and '
Xxperimental Philosophy; including Physics, Dynamici,
Mechanics, Hydrostatics, Hydraulics, Pneumatics, Acoui-:
tics. Optics, Astronomy, Electricity, Galvanism, Magnetisnv
ftccorduig to the latest discoveries. Wi£h one hundred t»
gravings on wood. By the B^v. David Blair, Author of th|
Class BoqJi^lJniversal Preceptor, English Grammar, Read^
ing £«if», Models of Letters, &c. From the twelftir
London Sditron, improved and enlaced."

lO conformity to the Act of ^ Congress of the United
StuJkiBs, entitled, ^^ An Act for the encouragement of learnii^,
by securing the copies of Maps, Charts, and Books, to the Au-
thors and Proprietors of such copies, during the times therda
mentioned."

CHARLES A. INGERSOLL,

Clerk of the District of Connecticut.
A true copy of Record, examined and sealed by me,
CHARLES A. INGERSOLL,

Clerk of the District of Connecticut;

S,& H. Clabk^ Print JVfiddletoun.

FROM THE author's

PREFACE ^:^.

TO THE TWELFTH LONDON EDITION.

The Author of the following pag;es is aware that there al-
ready exists several valuable but expensive books on Natural
and Experimental Philosophy ; and he shoald not have pre-
sumed to add to their number, except for the purpose of re-
ducing an important branch of knowledge, in bulk and price,
to the level of the business of schools, and adapting the whole
to the present state of knowledge.

Every instructor of youth must be aware, that mere die*
quisitions are of no use in the art of teaching ; and that no
science can be taught, if the student does noi work or perform
operations in it ; or answer questions which involve the con>
qideration of its various detaUs.

He who only reads about a science, can be nothing more
.than a smatferer ; whibt he who commits its terms and ele-
mentary principles to memory, and applies them by some act
of his own mind, to the various combinations of -tM science,
toon becomes a master of it.

In strict conformity with this principle, this Grammar of
Batural Philosophy has been compiled. All the definitions
«&d elementary principles have been written with studied
brevity, so that they may be learned by rote. With these
have been intermixed such easy and familiar Experiments,
Observations, and Illustrations, as will enable the young stu-
#feat to work in each science, while at the same time copious
iti^BSTioNS have been annexed, for the purpose of adapting the
l*ok to the system of teaehing by questions without answers.
, The author cannot let pass this favourable opportunity, to
; express his sense of the honour which has been rendered to his
\ humble endeavours, in the five works which he has already
i submitted to the public. Fie alludes to his CLASS BOOK,
bsUxNIVERSAL PRECEPTOR, his Exercises iiv Read-
iHc, his English Grammar, and his IModels of Juve-
nile Letters, in all of which he laboured diligently to give
* popular feature to useful knowledge, and has met wUiv \\\s.
, '«wardintheupparaUeledsuccc» oi'(Wfe>i^<^»&»

r- '

GRAMMAR

PHILOSOPHY.

OF MATTER AND ITS PROPERTIES.

1. Matter is the general name of everything or
' substance, that has length, breadth, and thickness.

Obs. Philosophers have in all ages discussed the generif
nature of matter, but without arriying at any satisfactory re-
tolt. This is certain, that all we know of matter is merely
relative to our own powers and senses ; and those relative
properties, being all we can know, are the proper objects of
Philosophical inquiry.

2. The properties of all matter or substance, are

SOLIDITY, DIVISIBILITY, MOBILITY, and INERTNESS.

3. Solidity is that property which every substance
possesses, of not permitting any other substance to
occupy the same place at the same time.

JUuAration 1 . If a piece of wood or metal occupy a certain

rie, before any thing else can take possession of that space,
wood or meted must be removed.

% Water and even air, have this property.

Experiment 1. If some water be put into a tube closed at
one end, and a piece of wood be inserted that fits the insid^ of
tiie tube very accurately, it will be impossible by any ibi)ie to
get the wooden piston to the bottom of the tube, unle& the
irater is first taken away.

2. The ejperiment^ Wpty tf^ be tii»j3b^ -^irEiib.^ \s^s^ ^^^
water*

THE MbOS'S PHiSK.

OF MATTER AND ITS PROPERTIES. 9

Hhu. It is found from experiment and observation, that all
matter is capable of being moved, if a sufficient force can be
applied for the purpose.

6. Inertness, or inactivity, is that property of matter
by which it would always continue in the same state of
rest or motion, in which it is put, unless changed by
some external force.

lUtu. 1. It is evident that matter, as a stone, can never pat
itself in motion, unless it be in some way acted upon.

2. Bodies in motion, as a bowl on the ground, or a cannon-
ball passing through the air, fall from motion to a state of
rest, either by the friction of the earth, by the gravity or
weight of the body, or by the resistance of the air.

Exp. 1. A marble shot from the fingers would run but a
small distance on a carpet : its motion would be continued
much longer on a flat pavement; and longer still on fine
smooth ice. Here the friction is greatest on the carpet, and
least on the ice. If the firiction were quite removed, and the
resistance of the air also, the marble once put in motion
would continue in that state for ever.

2. If a ball were fired from a cannon with a certain veloci-
ty, and there was no resistance from the air, it would circulate
round the earth perpetually, and never come to a state of rest,

3. If a person were standing in a boat at rest, and the boat
be suddenly pushed from the shore, he will be in danger of
ftUing backwards. And if the boat in swill motion be stopt
before he is aware, he will fall forwards, because his tenden-
cy will then be to continue in the same state of motion.

QUESTIONS ON MATTER AND ITS PROPERTIES.

Whxd is matter?

What are the properties of matter?

VnoLBt is solidity?

WhaX is divisibilitp ?

Grvft^an-example of the divisibility of matter.

What is mobiHty f

In what does thuproperty of matter consist?

What is inertness ?

What effect does inertness have on matter ?

Why does not a body once put in moUQiv«XNT%.'^^ ^^Aiis^fcVa

»0V€?

10 MECHANICAL AFFECTIONS OF MATTER.

OF THE MECHANICAL AFFECTIONS C

MATTER.

7. By Attraction is meant the tendency t
dies have to approach each other whatever
cause of such tendency.

8. There are five kinds of attraction : viz.
traction of cohesion ; of gravitation ; of electrii
fnagnetism; and o{ chemical affinity .

9. The attraction of Cohesion is that by wh

constituent particles of bodies are kept togethe

this principle they preserve their forms and a

vented frofai falling to pieces.

Illus. The attraction of cohesion takes place bet^
dies or atoms, only when they are at very small distai
each other.

iLxp, 1. If two leaden bullets are scraped very cl<
squeezed tog^ether, they will adhere so firmly as to i
considerable force to separate th%m.

2. If two globules of quicksilver, be placed near e
cr, they will run together and become one large drop.
The result of sundry experiments made by profess
schenbroek, to shew the cohesive power of different sol
be seen in the following table In estimating the
cohesion of solid bodies, he applied weights to separ
according to their length ; the pieces of wood which
were parallelopipedons, each side of which was 27-
aninch, and the metal wires made use of were 1-1
Rhinland inch in diameter, and they were drawn asi
the Ibllon^ng weights. —

lb.
Fir . . 600 Copper ^ 2

Elm - - - 950 Brass . - 3

Alder - - 1000 Gold - - 5

Oak - - - 1150 Iron - - 4

Beech - - 1250 Silver - 3

. Ash . . - 1250 Tin . .

Lead - 29 1-4

10. Capillary attraction is reckoned a species
sion. The suspension of the fluid in capillarr

MECHANICAL AFFECTIONS OF MATTER. 1 1

owing to the attraction of the ring of glass contiguoas
to the upper surface of the fluid ; and in capillary
tubes, the heights to which the fluid rises are inverse-
ly as the diameter of the bores.

Exp. 1. If a small glass tube open at both ends, be dipt in
Water, the water will rise in the tube, higher than its level in
the basin. The smaller the bore of the tube, the higher will
the water rise.

2. Take two pieces of glass five or six inches square, jdin
any two of their sides, and separate the opposite sides with a
small piece of stick, so that the surface may form a small an-
gle ; then immerse them about an inch deep in a basin of
Coloured water, and the water will rise between the glasses
end form a beautiful curve.

3. A piece of sugar or sponge, will draw up water or any
other fluid upon the same principle.

1 1. It is, probably, owing to the various degrees of
cohesion, that some bodies are hard, and others soft ;
that some are in a solid, others in a fluid state.

Obi As it is by the attraction of cohesion that the parts of a
body are kept together ; so when a body is broken, it is this
Attraction that ia overeome. Hence the reason of soldering
of metals, gluing of wood, &c. Hence also may be explained
why some bodies are hard^ others nofU and others^uttf, which
properties may result from the different figures of the parti-
cles, and the greater or less degree of attraction consequent
thereupon. £/a«/ift^|may arise from the particles of a body,
when disturbed, not being drawn out of each other's attrac-
tion ; as soon, therefore, as the force upon it ceases to act,
they restore themselves to their former position.

12. Repulsion is a force that is supposed to extend
to a small distance round bodies, to as to prevent
them from coming into actual contact.

Obi. 1. The repelling force of the particles of a fluid is but
fOUiU, and, therefore, if a fluid be divided, it readily unites
again. But, if a hard substance, as glass or sealing wax be
broken, the parts cannot be made to adhere, unless they are
moistened in one instance, or melted in the o^er.

2. Sir Richard Phillips who ascribes attraction to bodies mu-
tually intercepting the impulse of a uni'vem\.Tik«^\]CEC^^<c>ci«^
i^ugh all spaoe^ ascribes RijtuUifm to vot^^iea «A cAaL-^^Ysv^^
iSreumamhieat xneth'mn, produced by tixoae ^^ctevi^Ax «>^>a^

12 MECHANICAL AFFECTIONS OF MATTER*

which always accompany high degrees of repulsive action. Th
repulsion of electricity he considers as merely relative ; be
cause every electrified surface has within a given distance i
contrary electricity, and light bodies when apparently repelli^
from one surface, are, in truth, but attracted by the othd
sur&ce ; and perhaps all repulsion is produced by a counteft
attraction.

Exp. 1. Water repels most bodies till they are wet. Asmill
sewing needle will swim in a basin of water.

2. Drops of water will roll on the leaves of many T^;etM
bles without wetting them.'")

3. If a ball of light wood be dipped in oil, and put into I
pan of water, the water will be repelled from the wood, ani
will form a channel round it.

13. The attraction ofGravitoUiony or gravity^ is thl
name of that force by which distant bodies tend toward!
one another.

Obs. 1. All bodies on or near the surfece of the earth teol
towards its centre by a power called the ailraciion ofgraviU'
tion^ or according to the writer above mentioned, by iniereepi'
ed pressure of an elastic raffdium, which fills all space, an!
seeks to pervade all matter; and this seems a reasonaUa
cause of the phenomenon. Monthly Mag, Oct, 1811.

2. A stone, or other heavy body let fall, will move towaidl
the earth till it meet with some other body to obstruct il
course . And bodies move in lines perpendicular to the surftM
because the point to which they ultimately tend is the centli
of the earth, and the line of direction produced coincides vn$
the radius, and is at right angles with the sur&ce, which i
nearly spherical. Some bodies ascend, because they are Mlh
ed upon by a force greater than the attraction of gravitatHl^
and in a contrary direction. Vapours, smoke, &c. do not (Sfi>
scend, because they are lighter than the airi <^d supported JH
it. '

3. When we speak of attracting powers^ we do not attoHJ
to explain their nature or assigh their causes. Having ^
rived general principles or laws of nature, from phenomeii
we only g^ve a name to these principles, in order to expol
other appearances by them.

4. The tendency of all bodies towards the earth really A
suits from their tendency towards the several parts of tt
eartli. For, by an experiment made by Dr. Maskelyne

the side of the mountain Schebal\\etv,Vvc fo\xTvdW\e «A.t
of that mountaSn sufficient to draw Vhe ^Yaxcfe-Vvcv^
^€J perpendicular. 5ce Hixttcn?* D4rUowirp.

MECHANICAL AFFECTIOITS OF MATTER. 13

\ 14. By gravity, a stone dropped from a height faHs
i'tothe surface of the earth ; and hy it the heavenly bo-
Ljdies are retained in their orbits.
^ 15. The planets gravitate towards the sun, and to-
' Wards each other, as well as the sun towards them.
1 6. By gravity all terrestrial bodies tend towards the
centre of the earth, and in all places equally distant from
the centre of the^arth, the force of gravity is equal.
Obt. See the Monthly Magazine, May 1, 1813, for an account
of the effects of Pressure of all terrestrial substances on each
other ; the above writer observes, that the power of pressure
nets from the surface to the centre of all planets, or indepen-
flent totalities of paatter itecbsbarilt and without inter-
^iisaioN : and is,: or has been, the great instrument or bahd-
^AiD of NATURE, by which most of its varieties . of sub-
stance are, or have been produced. It is synonymous with
the action or momentum of the weight of bodies in their en-
<}eavour to fall to the centre of phmetary spheres, and with
the impulse called by astronomers the principle of gravitation.
It is evidently one of the primary principles of nature, and
'Would drive all atoms of matter into solid and immoveable
contact, but for another power called repulsion, synonymous
to elasticity, or expansion, producing varied degrees of densi-
ty. To press and to resist appear to be the conflicting princi-
plee or agencies, to which we may ascribe all the phenomena
of nature ; and in the degree in which pressure overcomes re-
sistance, or resistance counteracts pressure, heavy and light
bodies, inert minerals, or active org^anizations, become the ac-
cidents, or necessary varieties, of those active powers. To
press and to resist appear then to be the active principles of
all matter, or in other words. Union by gravity^ and Expan-
sion by keat^ seem to be the great secondary causes of all phe-
nomena. The Elastic ITT of a universal medium producing
action from without and substantial compression towards a .
centre ; and the Elasticity of heat producing action from
fffilhin and Expansion /rom its respective centres, point out
Ulabtigitt as the generic moving power of Nature. And if
Elasticity and its synonyme expansion, be a mere result of
BEAT, and HEAT llself be merely a phenomenon of mo/ton, then
it would appear, that m OTipN itself, is the primary cause of all
things ! Nor is there any inoongruitj in refemTvglQ ^3c^.^ ^'^xsv^
pruBMry cause, the pressure of gravity > ax\5 \3afc'KW?K.^-
w^jT wkich opposes oravity, because the e\»A\icW5 rs^. 'Cv\<--'K\«i^

14 MECHANICAL AFFECTFONS OF MATTER^

um of space producing gravity is universal, and the elastid
producing expansion, is but iocal and relative V

17. The force of gravity is less at the equator tin

it is at the poles, because the equatorial diameter

24 miles longer than the polar diameter, and becaa

the swing, or centrifugal force of the earth at tl

s equator, diminishes the gravity.

Ohs, Hence, seconds' pendulums, which in this latito
must be 39,2 inches, require to be 1-iOth shorter^ or but 31
at the equator.

18. The force of gravity is greatest at the earth's 8Q

face, from whence it decreases upwards and dow

wards. It decreases upwards as the square of tl

distance from the centre, and downwards simfly

the distance.

Obs, 1. The power of gravitation is greatest at thd suifi
of the earth, from whence it decreases both upwards and ddm
wards ; but not in the same proportion. The force of giw#i
upwards^ decreases as the square of the distance from the 01
tre. That is, gravity at the surface of the earth, whiol^
about 4000 n^es from .the centre, is four times more poweil
than it would be at double the distance, or 8000 miles fromt
centre. Gravity and weight may be taken in particular e
cumstanees, as synonymous terms. We say a piece of Iff
weighs a pound, or sixteen ounces, but if by any means it ooi
be carried 4000 miles above the surface of tho earth, it W(M
weigh only 1-4 of a pound, or four ounces ; and if it couU
transported to 8000 miles above the earth, which is three tiv
the distance from the centre that the surface is, it would wd
enly I -9th of a pound, or something less than two ounces.

2. It is demonstrated, that the torce of gravity downwii
decreases, as the distance from the surface increases, so that
one half the distance from the centre to the surface, the sal
weight, already described, would weigh only 1-2 a poni
and so on.

Thus, a piece of metal, &c. weighing, on the surface oft
e&rth, one pound, will at

f The centre weigh - - ^
1 1,000 miles from the centre, - 1-4 pound.
I 2,000 « « - . 1-2

at-J 3,000 " u . . 3.4 ,

4,000 « « - . 1

8,000 w tt - . t-4

J2,0Q0 " u , , V^W

eP MOTIOH. 15

tipiKl it the distance of the moon from the earth, which is
mO^OOO miles, it would weigh only the 3,6(KHh partof apound,
tl^eeaase the distance is 60 times further from the centre of
Jfte earth than the surface.

^QUESTIONS ON THE MECHANICAL AFFECTIONS

OF MATTER.

What is meant by attraction f

How many kinds of attraction are there ?

What is cohenve attraction?

What parts of bodies are affected by this Idnd of attraction?

What effect does it have on solid bodies ?

Define what is meant by capiUary attraction f

What is the rule in regara to the rise of fluids in capillary
tubes?

How may this kind of attraction be illustrated ?

What effect does the various degrees of eohesive attractioB
have oo bodies ?

W]bat is the cause of elasticity in bodies ?

"What is repulsion ?

Does this force act strongest in solids, or fluids ?

What simple experiment will show that bodies repel each
other?

What is grmvity^ or the attraction of gravitation f

To what point do bodies tend by this attraction ?

Whafis the cause of this power?

What does the experiment of Dr. Maskelyne prove ?

Is there any difference inthe gravity or weight of the same
^t^y at different places, and why?

Why is it necessary that seconds' pendulums should be short-
er at the equator than at the poles ?

Where is the power of gravitation greatest ?

In what proportion does it vary upwards, or downwards
firom the sur&ce of the earth ?

What would be the weight of a pound of metal here, when
Slurried to the distance of the moon ?

TftE LAWS OF MOTION.

1 9. Motion is the continued and succe^^vv^cViasi^^^
place of any body. Nothing can be t^io^mc^^ ^t '^^

16 OF MOTION.

stroyed without motion, and every thing that happens ;
depends upon it

20. Primary Laws of Motion are,
First, Tliat every body 'mil continue in its state

resty or of uniform motion, in a right line, until it is \
compelled by some external force to change its state.

Secondly, That the change of motion is always prtrj
portional io the moving force by which it is produetifl
and it is made in the hn^ of direction in which ikttt\
force is impressed, , w^

Thirdly, That action and re-action are always tqm\
and contrary. ^

21. We are chiefly concerned with two kinds oflet
motion. .||^

1. That by which an entire body is transferRdp^
from one place to another.

2. The motion of the partd of bodies among theoi-^
selves, supposed to be the cause of fluidity and vapOQr>

Illiis. By the first kind of motion, a heavy body falls to dMt
surface of the earth, a carriage moves, and a ship sails. ^
the second^ plants aod animals grow, and the compositioosutf
docompositioDs of bodies take place.

Exp. Take a decanter of clear water, and bold it in thft
rays of the sun, and you will sec that the light particles oof
tained in it are in perpetual motion.

2. Let the rays of the sun pass throQgh a small hole ia
window shutter, and you will observe the particles floating
the atmosphere are in constant motion, of whose ezhti'
3iV)U were not before aware.

22. Several things require notice with reg;
motion :

1. The force which impresses the motion.

2. The quantity of matter in the moving bo

3. The velocity and direction of motion.

4. The space passed over in the moving I

5. The time employed in going o/er this sp;

6. The force with which it strikes anotl
that may be opposed to ft.

OF MOTION. 17

2S. Every body, by its inertness, resists all change
i of state ; therefore, to put a body in motion, there

East foe sufficient cause.
Obt. Any body at rest on the sur&ce of the Earth will al-
lys continue so, if no external force be impressed upon it to
re it motion, and if the obstacle which hinders the attraction
j of gravitation from carrying it towards the centre be not re-
I Akoved. A body being put into motion by some external im-
: pulse, if all external obstructions were removed, and the at-
traction of gravitation suspended, would move on for ever in
a right line ; for there would be no cause to diminish the mo-
tion, or to alter its direction. This cannot bo fully established
hj experiment, because it isjmpossible entirely to remove all
obstructions ; but, since the less obstruction remains, the longer
motion continues, it may be reasonably inferred, that if all ob-
stacles could be removed, motion once communicated to any
body, would never cease.

Jllus. 1. It is plain that a mass of matter, as a stone, cannot
put itself in motion ; it therefore would have for ever remain-
-ed at rest, unless acted on by some power.

2. When a cannon ball is first discharged, it may be said to
move in a straight line ; and it is plain, that this would always
be, its direction, unless some power turned its course. It is
also as evident, that it would always continue its motion for-
ward, did net the friction of the air, or its own gravity, or
some jDther cause so impede its motion, as to bring it to the
j^ound.

24. The causes of motion are called motive pow-
ers, and are called muscuLir or mechanical : as the
action of men and other animals, the force of wind,
water^ gravity, the pressure of the atmosphere, or
any elastic medium, and steam. '

25. The change of motion produced in any body*
is proportional to the force impressed, and in the di-
rection of that force.

Obf» "EiSectB are proportional to their adequate causes. If,
therefore, a given force will produce a given motion, a double
force will produce the .double of that motion. If a new force
be impressed upon a body in motion, in the direction in which
it moves, its motion will be increased proportionable to the new
force impressed: if this force acts in a OCvtecWcvw ^«»Nx'sct^ \a
that in which the body moves, it will lose ^^o^Y>ass«?\\«sX<i

18 OF MOTION.

its motion ; if the direction of this force be oblique to the
rection of the moving body, it will give it a new direction.

26. To every action of one body upon another^

there is an equal contrary re-action ; or mutual

tions of bodies on each other are equal and in contn<

ry directions, and are always to be estimated in the

same right line.

Obs, Whatever quantity of motion any body commiuiicatl*
to another, or whatever degrees of resistance it takes amy
from it, the acting body receives the same quantity of motiflB|
or loses the same degree of resistance, in the contrary direc-
tion : the resistance of the body acted upon producing tti
same effect upon the acting body, as would have been prodn^
ed by an active force equal to, and in the direction of, thit^
resistance. Henc3 it appears, that one body acting upon an-
other, loses as much motion as it communicates ; and that tbe
sum of the motions of any two bodies in the same line of dine^
tion, cannot be changed by their mutual action.

27. The velocity of motion is estimated hy the tiiae
employed tt. moving over a certain space, or by the
space passed over in a certain time. The less the
time, and the greater the space moved over in that
time, the greater is the velocity.

lUus, 1 . To ascertain the degree of velocity^ the space ruA
over must be divided by the time.

2. To measure the space run over, the relocily must be mulH*
plied by the time ; for it is evident, that if either the velocity
or the time be increased, the space run over will likewise b^
increased.

3. If the velocity be doubled, then the body will move ovar
twice the space in the same time : if the time be twice as great,
then the space will be doubled : but if the velocity and tius
be both doubled, then will the space be four times as g^eat.

Exam, 1. If a ship sail at the rate of 12 miles in an hour,
or sixty minutes, then the velocity is equal to one mile in fivt
minutes.

2. If two persons set out together on a journey, and one walks
two miles and a half, and the otiier walks five miles, an hooft
the velocity of the latter, will be double that of the former.

28. A body in motion must every instant tend to some
particular point. In which case the motioii\v\U be in a

straight Hoe, oritmnj be contmw^tty chw^vn^VXtfi^^

OF MOTtOK. 19

.h its motion is directed ; and this will produce
linear, or circ€lar motion.
If a body is acted upon ooly by one force, or by
[ forces in the same direction, its motion will be
same direction in which the tnoving force acts.

I. The motion of a boat, which a man at a g;iven place
) him with a rope, is of this kind.

Equable motion is either simple or cofhpound.
motion is that which is produced by the action,
ressed force, of one cause. C&mpound motion is
lich is produced by two or more conspiring
}, i. e by powers whose directions are neither
te nor co-incident.

If two or more forces, differently directed, act
le same body at the same time, as it cannot obey

II, it will move in a direction somewhere be«
them. This is called the composition and re-
% of motion. *

Ulus, Suppose a body a to be acted upon
^ ^ by another body in the direction a 6, while
"^^f^ at the same time it is impeUed in the direc-
^ i tion a c, then it will move in the direction a d.
f j^ If the lines a ft, and a c, be made in propor-
tion to the forces and ^e lines e <f, and d ft,
m parallel to them, so as to complete the parallelo-
len the line which the body a will describe, will be in
onal a (/, and the length of this line will represent the
th which the body will move.

1. 1 . There are many instances in nature, of motion pro-
7 several powers actings at the same time. A ship ^v-
le wind and tide is one : so also is a paper kite, acted
the wind in one direction, and by the string in another.
3all fired from a cannon is acted upon by two forces,
is that occasioned by the powder, the other is the
gravity.

The force, or power of overcoming resistance,

moving body, is as its momentum, or quantity of

Since a body having a certain degree of motion is able
!ome a certain degree of resistance, it is raamfest^tk«.t
increased momentum, it ^W \>e ^\e \a ^'n«c^^x&& ^
resistance. Hence the momenXxxTA. ^^ «k^ Xi^sfte^ '-»
i h7 its power* of oYeTComingte«s^»3cos^«

30 OF MOTIOlf. I

33. In moving bodies, if th^ quantities of mattm^'

be equal, tbe momenta^ or quantity of motion, will Hr?^

as the velocities. I in

Obs, If the body A be equal to the body B,but Ahafltwioil
the velocity of B, A has twice as much motion as B. I

34. The velocity of two bodies being equal, thdrl*,
momenta will be as their quantity of matter. JL

Obs, If the bodies a and 6, fig. 1. move with equal Tdo^I
ties, since every portion of matter in a has as much motinn W^
an equal portion of 6, it is evident, that if a has twice the q M» fc t
tity of matter of 6, it must have twice as much motion. kj

35. The momenta of moving bodies, are in the co»l B
pound ratio of their quantities of matter and velocitiM|~

Obs, The greater quantity of matter there is in any bo^'Ml
and the greater velocity it moves with, the greater fnll en'Kr
dently be its quantity of motion, and the reverse. If^ fiir flifr 1(5
ample, tlie body A be double of ihe body B and moves wilhj «
twice its velocity, the nuDmcntum] of A will be quadrapila afi^
that of B : for it will have twice the momentum ofB fixunlbff^
double quantity, and also twice the momentum of B fromiklff
double quantity of matter. Hence, if in two bodies, the pt'lR
duct of the quantities of matter and velocities are equal, wM
mpmenta are equal, or as the products. ' m

QUESTIONS ON THE LAWS OF MOTION.

What is motion ?

What are the primary laws of motion ?

What are the several things to be noticed in regard to S
tion ?

What are the causes of motion ?

How do moving bodies gain a new direction ?

What is meant by reaction ?

How is the velocity of motion estimated ?

What is the rule for ascertaining the degree of veloci

What is the rule for measuring the space run over ?

What is understood by simple and compound motion ^

\yhat is understood by the composition and r^solu
motion?

How is this land of motion illustrated?

What proportion is there between the force or p
moving body, and its momentum ?

Illustrate this law.

OF ACCBLERATBD MOTION. 21

What proportion do tfie momenia ofmtxving bodies bear to
beir velocities ?
Jllastrate the law.
When are the momenta of moving^ bodies.eqaal ?

OF ACCELERATED MOTION.

36 . Accelerated motion is that in whic h the Telocity
3 continually increasing from the continued action of
he motive power. Uniformly accelerated motion, is
hat in which the velocity increases equally in equal
imes.

lUiu. 1. The increasing velocity with which a body falls
o the earth, is an instance of accelerated notion, which is
rauied by the constant action of gravity.

S. A cannon ball is acted on by a single impulse of the pow-
ler and the accelerating fifirce of gravity, it therefore de-
cribes a curve. This is the foundation of the art of gunnery.

37. A new impression heing made upon a falling

iody, at every instant, hy a continued action of gravi-

y, and the effect of the formei;* still remaining, the

relocity continually increases. :

lUtu. Suppose a single impulse ofgfavitation, in one instant,
.0 give a falling body one degree of velocity ; if after this the
brce of gravi^tion were entirely suspended, the body would
continue to move with that degree of velocity, without being
iccislerated or retarded. B ut, because the attraction of gra-
citation continues, it produces as gre^t a velocity in the se-
cond instant, as in the first; which being added to the first,
nakes the velocity in the second instant, double of what it was
in the first. In like manner, in the third instant, it will be
tripled ; quadrupled in the fourth ; and in every instant one
i^pree of velocity will be added to that whioh the body had
before ; that is, the motion will be uniformly accelerated.

38. Motion is said to be retarded , if its velocity
continually decreases ; and to be uniformly retarded^
if its velocity decreases equally in equal times.

Obi. The student who has not learnt some Algebra and
Geometry, may go to Article 40 without disadvantage.

39. The velocities of falling bodies, feire in propor-
tion to the spaces run over, and the spaces passed
over in each instant, increase as the odd uumhe.^ \^
3, 5, 7, P, &c. ^

OF ACCELERATED MOTIOK.

liiut. The ip&CB deicril
fBLUingfrooa ^. 3, in the I
ed bj a i, with s uniformlj
Telocity, represented by t^
which the lut degree is e:
c, will be reprownted by
the tciuigle afrc. lfgra<
act, the tpace passed ovei
portion of time 6/, would
„ . .Aby fr/, moltipiied into the
thtt ii by the rsctangle J c gj-^ which u equal I
triai^leubc. But if^rftvityrtill sets, then the 1
mint be added ; of course, the bodv moves ovei
the space in the second inatont that it did in On
next portion of time it would move over five tim
repmented by the two rectangles and triangle ;
Ibarth portion of tine, seven times ; and so on i
eel propwBuoD.

It fbUJowi, that the vihiiU space described, is i
ofthe timii; thatis, in twice the time, it will &1I
tinges the space ; in thrice the time, nine ttmes tb
K I

laia. TheUmeofi
&lling body being re]
any portion A B of th(
uigje, the velocity wi
tional to B C, which i
. nd the apace deid
the time D E, auppoi
short, will be proper
area D E F G, which
by the product B C, an
A UISE H queotly the whele arei

represent the spaee described in the time A E a
space described in ths time A H ; but A H I i
square HK,andAEF()fEL;lhe space is
ways as the square of the time and is equal to h
which would be described in the same time with

Oit. All bodiea descending in vacuo, are f
through 16.1 feet in one tecm\d, and to acquire
fidling which would carry them uniformly throi
in the next second, and an increaae of velocity, ■
ii fmiiid to be added to every suoceeding second <

OF ACCELBBATED MOTION. 23

3. In the first instant there is one space run through ; at
M end of the second, there are four ; at the end of the third,
ine; and BO on.

40. It has been found by experiment, that a body
illing from a height, moves at the rate of about 16
let in the first second of time ; in the next 48, in
le third 80, in the fourth 112 feet, and so on.

ExmM, The space will therefore be 16 in theirs/ second ;
S-|^8, or d4 equal to 16X4 : 4 being the square of 2, in the
ttoni seomd ; 16-|-48-|-80,or 144 equal to 16X9 : 9 being
lie square of 3, in the third second : 16- 1 -48- 1 -80- 1 -1 1 2, or
56 equal to 16X16; 16 being the square of 4, in me fourth
Boood. And so on, because 4, 9, 16, &c. are the squares of
; 3, 4, &c.

41. The force with which a body moves, or which
t exerts upon another body, is always in proportion to
ts velocity multiplied by its weight, and this force is
sailed the momentum of the body.

IlUu, If two equal bodies move with different velocities,
beir forces or momenta are in proportion to their velocities.

Exp. 1. If two equal cannon-bsdls be projected by differ-
Kt quantities of powder, so that the velocity of the one is
'double that of the other, then the force or mcvnentum of the
ilmer will be double that of the other.
L S. If two stones, one of two pounds, and the other of six
MndB be hurled with equal velocities, the force or momen-
hn of the latter will be three times greater than tiiat of the

nier.
CoroL In all cases, the momenta of bodies must be as tlic

{lutities of matter multiplied into the velocities.

' QUESTIONS ON ACCELERATED MOTION.

What is accelerated motion ?
' What is meant by uniformly accelerated motion ?
W.hat causes acceleration of motion?
Why does a ball shot out of a cannon describe a curve f
When is motion said to be retarded ?
Deknonstrate the laws of falling bodies by a diagram.
What luws do falling bodies observe in a vacuum ?
At what rate are the velocities of falling bodies increased ?
What is the rule for estimating the foroe c»^ ?l YEiWvw5»\^Q?v-^^
Jlhwtrate thi« n/lc.

OF CENTRAL FORCES.

OF CENTRAL FORCES *

i. All motions produced upon a body, by on

e only, must be made in a rigbt line.

herefore, a body moving in a curvilinear direction moi

icted upon by two forces at least ; and when one of thM

ses to act, the body will move a«ain in a strai^t line.

llliu. A stone in a sling is moved round by the hand, whil

is pulled towards the centre of the circle which it deflcxiM

f the string. But when the string is let loose, the itooa jBi^

Jin a tangent to the cirde.

43. The force whicb impels a body towards a cent!
gvhen it revolves in an orbit, or circle, is called the ca
tripetal force ; that by which it endeavours to r6ced
from the centre, is called the centrifugal force ; an
these combined forces are called central forces.

Obs, The projectile and centrifugal ferces differ from mi
other, as the whole from the part. The projectile fores ii tb
by which a body would move forward in a tangent to its ort
if there were no centripetal force to prevent it: thecentri/
^] force is that part of the projectile force which carries
body off from the centre while it is moving in the tangent.

2. When bodies revolve in a circular c
•^^ — -^ ''"*»- about a centre, the centripetal and ccntrif
forces are equal; because the periphei
the circle is in the middle between the
where the body would have been if i
JT \ ^^^^ moved in a right line either towards the «
or in the tangent. If a body revolve in t'
cle b d, fig. I. in the time in which it describes the arc 6 n<
have been impelled towards the centre, through the spai
for by the projectile force alone, it would have been
from 6 to a. The line a n is then the space descr
means of the centripetal force, and this force is proper
an. But if, when the body was at 6, no centripetal f

* The doctrine of central forces will be studied ^
advantage in connexion with Astronomy, and may '
over by the junior student in this place, particularl
not learned the first six boqloi of EacUd, md tl»
riflcB of Algebra;

OF CENTRAL FORCES. 25

)on it insUad of describing the arc b n, it would have
ilongthe tangent b a and the line n a would have been
e through which it would have parted from the cen-
Brefore the centrifugal force is proportioned to n a. —
ese forces being then proprotional to the same line n a,
3 equal to one another.

A body revolving in an orbit, describes by a ra-
'awa to the point towards which the centripetal
lets, equal areas in eqoal times, and in unequal
it describes areas proportional to the times.
1. The student will readily understand this proposi-
fcien he understands its terms. By areas is meant the
bounded by the orbit and lines joined to the centre,
elocities are equal, i. e. if the parts of a circular orbit
al, the areas must be equal.

be velocity of a body, revolving freely about an im-
lie centre is inversely, as a perpendicular let fall from
ntre on a right line that touches the orbit. For the
es will be as the lines moved over, which being the ba-
]ual triangles, must be inversely as the heights of the
is; therefore the velocities are as inversely as the
, which are measured by perpendiculars, let fall from
imon centre.

'hen a body describes equal areas in equal times, about
loveable point, or proportional ^reas in unequal times,
t be impelled towards that point, by the centripetal
ehtch retains it in its orbit.
The centripetal forces of bodies, revolving in
int circular orbits about the same centre towards
they tend, are as the squares of the arcs describ-
he same time, divided by the radii of the circles.
/. Therefore, the centripetal forces of equal bodies
ng in circular orbits, are as the squares of the velocities
Y, and as the radii of the orbits inversely.
. Because the length of arcs described in the same time,
3 in the proportion of the velocities, and the centripetal
are as the squares of the arcs described in the same time,
Iby the radii f these forces are also as the squares of the
ies divided by the radii ; that is, as the squares of the ve-
I directly, and the radii of the orbits inversely. Hcmce
itripetal forces of equal bodies, at equal distances from
itre, arc as the squares of the nambcr of revolutioDS^ in

2G OF THE CENTRE OP GRAVITV.

any given time ; for this number i£ as the velocity with *
the body moves.

46. The centripetal forces of equal bodies re
ing in equal circular orbits, are inversely sa
squares of their periodical times.

Obt. The circnliur orbits or spaces being equal, the
in which these are described, or the periodical times^ ai
Tersely as the velocities ; and therefore, the squares c
periodical times are inversely as the squares of the velo
or the squares of the velocities are inversely as the sq
of the periodical times ; but the centripetal forces are i
squares of the velocities; therefore, these forces are ini
ly as the squares of the periodical times.

47* The centripetal forces of equal bodies,
volving in unequal circular orbits, if the perio*
times are equal, are as the radii of the circles.

48. The centripetal forces of equal bodies,
volvingin circular orbits, are as the radii of the or
or distances, directly, and as the squares of the ]
odical times inversely.

lUus, If the periodical times are equal, and the radii
qual, the force is ae the radii. If the radii are equal, an
periodical times unequal, the forces are inversely m
squares of the periodical times. Therefore, if both the
and periodical times are unequal, the forces will be i
compound ratio of both, or as the radii directly, anc
squares of the periodical times inversely.

OF THE CENTRE OF GRAVITY.

49. The centre of gravity of a body, is that pji
about which all its parts do in any situation exactly
lance each other, so that if a body be suspendei
supported by the centre of gravity, it will rest in
position.

50. Whatever supports the centre of gravity b
the weight of the whole body ; therefore, the w
weight of a body may be considered ns balanced ix

point.

OF THE CENTRE OF GRAVITY. 27

51. The common centre of grarity of two or more
bodies, is the point upon which they would rest in
= aay position.

[ Illut. If the centres of gravity of two bodies,

[ ^ j i AB.fig.3.beconnectedwiththerig:htline AB,

f ^J the distances AC, and EC, from file common

[ Motve of gravity, C, are inversely as the weight of the bodies
f A and B ; that is, th^point of C will be as much nearer to A

tiiao to B, as A is heavier than 6 ; that is AC : BC : : B : A.
Exp. Suppose A to be a ball of 12 DOi:ftids, and B to weigh

o^ 4 pounds, and the length of AC to be five inches : then

BCwiU be 15 inches: for it will be5:BC::4: 12, or4X

fiCai&X 12a««0, and BC»20.4=:15.

62. The centre of motion is the point about which
the body moves ; and a heavy body suspended on a
centre of motion will be at rest, if the centre of gra-
vity is directly under, or above, the centre of motion.

llku. If a heavy body £, fig. IL
Jft hangs by a string on a centre of mo-

^r I tion C, the action of gravitation at ^ ^

^r I is in the direotion EL, contrary to the '

9 ^f I A direction in which the string acts to

'^ yL^W I ^^^ prevent the body from fidling. In this
1 ^^^^-*6**^^ position^ therefore, the opposite for-
I Y ces being equal in contrary direc-

I \ fy tions, destroy each other, and the

I I body is at rest. Butifthebodyisatp,

. I I one q{ the forces acts in the direction

JQ ^ pC : and the other in the direction

7L, that is, in direction oblique to each other, whence the body
Krill move in the diagonal of the parallelogram formed bypC^
>L. And in all cases, since (witiiout the aid of mechanical
lowers afterwards explained) the force which sustains any
)ody must be equal to its weight, the centre of gravity can
>nly be at rest when these forces are in the same line of di-
ection, that is, when the centre of gravity is directly under,
r directly above the centre of motion.

63. If a Hue be drawn perpendicular to the horizon,
rom the centre of gravity of a body, it is called the line
f direction^ because it is the line which the centre of
ravity would d^icril^e if the body were suffered to fall.

^8 OF THE CEKTBB OP

54. While the line of directioD falU within thebMl

upon which the body stands, the body cannot fiA|

but if it fall without the base, the body will ' "*

t^^^^M '""'- '^^'^ inclined bod]' ebei,

A^^^fl^F whoH centre of graritj ia t, itani

_HK^^M '^^'^■'■^ ^' l>ii^ °f diractioQ c/fUli

a^^^H^ thebue. But if the bodjrofr^ be ,

^ upon it, the centre of gnivitrwUl ben

^^^^K lA 'i *^ tb«" ^^ li"" of direotitHi U

g^gS ISfKs &U out of the bue ; of coane tba oc

i iWi^Wy ^— * of^vity ii not aupported, aod the w

^ £J « muit jaU.

^ Oij. Thia provei the injnrtoni oSa

mingm a coach or boat in danger nf oTcnettillf , tiH» o«i
gravity being thereby raised, and the line of directian thi
out of the base. Whereas, in aueh circiimstanoea, tha profV'
course ie to lie down in (he bottom, u ai to bti)^ Qm Inikif
direction, and coiuequently the centre of graT i ty, withii
kase, and thereby remore the danger df oTenetting; -

56. The broader the base, and the nearer tha
of direction is to the centre of it, the more findf
does a body stand ; and the narrower the base of I
body, and the nearer the line of direction is to dfe
side of it, the more easily it is overthroHTi.

Obs. Hence a sphere i> easily rolled nlong; aotl k nBIlMf
pointed bully is with difKculty made to stand.

56, If a plane be inclioed on which a heavy bo^
is placed, the body will slide down upon the plae
while the tine of direction falls within the base ; bv*
will roll down, when that line falls without the *

<V^^ .^Ci^ "'"'■*■ The body., fig. IV. I

>MF ^I^ thelineofa,rect;oD ca within tlu
^Sv IBHII will rnly^hije down: butthelina
a ^^mMj rectlon ba o." the body b blling i
— _, ^^^ the base, tlini body rolla down tbe ■
**^ (\i ^ ^'^'''" *^^ ^'"^ °^ directjr

a * within the base of our fcet, w
anil most Urmly, when it ii in the middle ; but whe'
of thp base, we fiiU imlew we slop out, and this ia Iht
•fnalkin;;.

ON PfMTDULUMS. 2B

Rope-dancers are able to perform their feats by knowing
exactly to keep the common centre of gravity, of them-
i-and their pole, just within the extended base.
We apply this principle in the common actions of life ;
we bend our bodies forward when we rise Jbrom a chair,
up stairs ; — so a man leans forward when he carries a
%n on his. back, and to the right and left as he carries k
2 opposite side.

OF PENDULUMS.

^ A Pendulum is a keayy body hauging by a
g or wire, which is moveable at a centre, and
swing is called a vibration or oscillation.
9. The vibrations are produced by the falling of the
it to the lowest part of the circle, and by the force ac-
d in the fall.

I. All the Yibrations of the same pendulum^ ^iMt)^-
"eat or small ,^ are performed in equal times ^md
onger a pendulum, the slower are its vibrations,
quares of the times being inversely as the lengths.
}, A pendulum that vibrates in the latitude ofLon-
n n second of time, is thirty-nine inches two- tenths
; but a pendulum that vibrates seconds at the
tor, must be but thirty -niuQ^ inches one-tenth.
). A pendulum, by 59, to vibrate half seconds must
le-fourthpart as long, aaoi^e that vibrates second?^
a pendulum to vibrate once in twoseconds must be
times as long, as one which vibrates seconds.
«. 1. As it is found by experiment that a pendulum which
tes 60 times in a minute is 39.13 inches nearly, therefore
d how long pendulums must be to vibrate, 30, 50, and 120
in a minute,, we say, by 50,

Inches. Inches.
as 302 : qq^ . ..39.13 ; 156.62
60^: 603:: 39.13: 66.34
1203 : 603 : : 39.13 : 9.78
As heat expands and cold contracts b^ metals, a pendu-
rod is longer in warm weather than in cold ; and irregu-
' hence takes place in clocks.

OF PROJECTILES (iUESTio..^.

The vibrations of pendulums are subject to many im

les, for which no effectual remedy has yet been devi

se are owiii§f partly>to the variable density and temp<

i of the air, partly to the rigidity and friction of thib rod

.ch they are suspended, and principally to the dilatatioo

traction of the materials, of which they are formed. '

tal rodsof penduluzns are expanded by heat, and contr

by cold : therefore, clocks will go slower in tummer,

«ter in winter. The common remedy for this inconvenia

the raising or lowering the bob of the pendulum by m^

r a screw.

OF pro/ectiles.

Gl. Bodies thrown horizontally or obliquely i
the air, have a curvilinear motion, and the p
which they describe is the curve, called a parabi

Qbs. 1 . Very dense bodies moving with small yelocitias
M^l^ the parabolic track so nearly, that any deviatioi
scarcely discoverable; but with very considerable velod
the resistance of the air will cause the body projected to
scribe a path altogether different from a parabola, whioh '
not appear surprising, when it is known, that the resittanf
the air to a cannon-ball of two pounds weight, with the ^
city of 2000 feet per second, is more than equivalent to
limes the weight of the ball.

lUus. Thehor^ontali
fig. 4, AB, of a bo<ly pT
ed at an elevation of
greater than AC or A'
ranges of bodies pr
with the same veloc*
greater or less elevaf
the parallel lines EF
A D C B always as the square

AG, the curve AFH will be a parabola ; and such is
of a projected stone or cannon-ball.

QUESTIONS ON CENTRAL FORCE?

What is meant by central forces?
When a body is acted on by only one force, in v
lion does it move ?
What is meant by centripetal force ?
What is meant by eentrijfugalforr^ ?

QUESTIONS. 31

What k the difierence between the projectile and cehtriiu-
^^ forces?
^ When bodies revolve in an orbit, "what proportions do the
-'^centrifugal and centripetal forces bear to each other?
^ • ninstrate this by a diag^ram

>Vhen do revolving bodies describe equal areas, and when
do they describe nnequal areas ?
niostrate these laws.

How are the centripetal forces of bodies, revolving in dif-
ferent circular orbits, about the same point determined ?

How do you determine the centripetal forces of equal bo-
dies, revolving in equal orbits ?

How do you determine the centrifugal forces of equal bo-
dies, revolving in unequal orbits, their periods being equal ?

OF THE CENTRE OF GRAVITY.

What IS the centre of gravity of a body ?

How can the centre of gravity be found ?

What is the common centre of gravity, of two or more
bodies?

Illustrate this by a diagram.

What is the centre of motion ?

Why will a heavy body, suspended on the centre of motion
be at rest?

Demonstrate this by a diagram.

What is meant by the line of direction ?

W here must the line of direction fall, to prevent a body fron
&lling? .

Illustrate this.

What practical inference may be drawn from this Ulustra
tion ?

Why is a.body more apt to keep its perpendicular directioi
when set dh a broad base, than on a narrow one?

How do rope-dancers contrive to Jceep their balance ?

OF PENDULUMS.

What is a pendulum ?

How are the oscillations of a pendulum produced ?

What is tile rule for calculating the time in which pendu
lums of different lengths vibrate ?

What is tlie len^ of a pendulum vibrating seconds a
London ?

Wbjrmast it be shifttet at the eatnilot^ Vl^ T^st^Ni^Vcw^
same time,*

32 THB MBCHAiriGAL POWE&S.

How loog^ oiiMt a pendalam be to yibratehtlf saooni

Lot'doD? .... *

HowloiigBiiittapeidkQiaiiibeta Yibnteoiwftiii M
coods at the tamm {ilaoa f ^

How lonf miif t a peodaliui be, whi^
iu a minate at London.?

Why do clooks go ftiiter in winter than ftnomiaer f *'^

■ ■■■'■-•■," .i

OF PROIISCTILEB. -^

What 11 the path called, which a. body defdribl^' i«

thrown horiaoataUjr, or obliquely into the air? f

Is there any diffdredce in the caries dewiribed hflit

moving at ^;reater or len Toleeitief ? \ y -)

Illastrate thit difference by a <tiagramv ' "*- ■'^j
What renitance does the air oppose to ax»nnoii MUI

pounds weight looving 9X the rate of SOOC^feet jMt'm&ati

OF THE MECHANICAL POWER& ii^S^]

6!S. The MfccHAi7icA£ Powers are simple eop

founded on the principles of the laws of Ittpi

which enable men to raise heavy weights^ i|

heavy hodies, aad overcome resistance. ' ^

Obt. 1. The principal moyiug powers are — fln(f=]
strength of animab, chiefly that of men and horses; seoM
the force of ranning waters-and of winds'; thirdlv, theM
steam ; fourthly, the force of springs ; $(thly> the wei|iP
heavy bodies.

2. The simple weight, at applied to clocks, ja^CL
other machines, is the power which can be mi^ eanqf
plied as a first mover, and its action is almoft nirilbmi^i
this power requires to be renewed after a certain peris
is mostly used for slow movements.

3. The spring is a useful mpving power, but lilce the wd
it requires to be wound up after a certain time, whanea*
also chiefly used for slow movements. ^

&S^ Three circumstances are to be considen
treating of mechanical contrivances:

1 . The weight to be ndsed, or the reKttoneeio be overc

2. The jiower by which it is to be raited ; and
'3« The withtmenii employed.

OF THE LEVER. 33

64. There are six mechanical powers; vie. the
EVER ; the PULLEY ; the wheel and axis ; the in-
lived PLANE ; the WEDGE ; and the screw ; the
bject of which is to increase the effect of a given
ower, so that the momentum of the power may^

xceed that of the resistence.

Obt. 1. If the power be 100 poaadi, and the weight 1000
onnds, powers to move the weig;bt mpst be made to move
ith above ten times the velocity of the weight, and this is
fected by means of the mechanical powers.
2* If a man can raise by asingle fixed pulley, a be^m toh the
»p of a house in two minutes ; he will b® able to raise six
ich beams ip twelve n|inutes ; but with a tackle haviqg three
wer pulleys,^ he will raise six beans with, the same iease at
ice ; bat he will be six times as long* about it, that is, twelve
inutest because his hand will have six ^^t9 as much space
) pass over.

3. Canooa balls do much more mischief, than the battering
ims of ancient times ; suppose the weight of a ram to be
),000 pounds, and to move at the rate of one foot in a second ;
id the weight of a cannon ball to be 24 pounds, and to move
. the rate of 1000 feet in a second, then the momentum or
oving force of the ram, will be 20,000 XI ==20,000,' and that
•the eannon will be'^4 XI, 000=24,000 ; of course the effect
the latter will be one fifth greater than that of the former.
bus has a small body a greater momentum than a large one,
Hj/vided the velotiljf afths small one be made lo cgmpensatefor
e greater qrmniity oft(tnlter in the other,

65. The power of a machine is calculated, when it
in a state of equilibrium, that is, when the power

ist balances the resistance opposed, and the momen-
im of each is equal.

OF THE LEVER.

66. The Lever is a bar of iron or wood, supported

y and moveable on a round centre called RfulcrMm^

aving the resistance at the short arm, and the power

t the long arm.

Ohs, Levers of continuous matter as wood or metal arc in
.ngle ratio ; but levers of elastic media which disuse the
iroe arc a duplicate ratio.— P/it//i/)5.

OP THE LBVER,

67. There are three kinds of levers, dis
according to the different positions of the/t
the moYxng power with respect to each oth<

68. In all kinds of continuous levers, the p
the resistncee, as the distance of the resisi
the fulcrum is to that of the power from th

69. A lever ofihejirst kind, is when ll
is placed between the weight and the mov
as in fig. 5.

Exam. Ifitbereqi
the stone a, which i
pounds, by the streng
[ tf equal to 100 pounds w<
ce, which rests on th
f placed with one endun
and the man presses it
other end e. As the man's strength is only equal
part of the weight of the stone, the arm of the le*^
oe ten times as long as the arm 6f , in order tha
and weight might balance each other.

Illus. 1. A balance is a lever of th^ first kind
arms, see fig. 6.

2. A steelyard^ fig, 7, is also the first kind of ]
moveable weight

3. A poker, in the act or stirring the fire, is a
kind : the bar of the grate upon which it rests is t
the coals, the weight' to be orercome ; and the
power.

Obs, 1. To this kind of lever, may be refen
pincers^ snvffers. Sec. which are made of two le
contrary to each other. The fulcrum in these <
pin which keeps them to^^ether.

% The lever of the first kind is chiefly used i
large stones ; or to raise great weights to smal] he
der to get the ropes uude^ them.

OF THE LCVEl^. 35

The second kind of lever, is when the fulcrum

le end, the power at the other, and the weight

in them.

I. See fi^. B, where a is the fulcrum, If the weight,
le power.

le advantage gainedCby this lever is as great, as the dit-
rtbe power from the fulcrum exceeds the distauce of
ght from it ; thus if the hand at e be nine times as far
as the point X on which the weight acts, then the force
ound at c, will balance the weight b of nine pounds.
tk /2/uf. 1. This kind of lever cx-

M ^A^ plains why two men carrying a

W S 3^ burthen, as a cask, upon a pole,

\gm^mmmmJmflfmm^ may bear unequal sbares,accord-
4t ^ ing to the strength, by placing it

^W nearer to the one than the other;

see fig. 9. Here the weight vt^ is
learer to a than ^; of course, a would bear twice as
reight as h,

his is applicable to the case of two horses of unequal
\u where the beam may be so divided, that the horses
aw up in proportion to their respective ability.

this kind of lever may be referred oars, riidders of ■
loors turning on hinges, and cutting knives which are

1 one end.

A lever of the third kind is when the prop is at
id, the weight at the othet, and the power applied
en them. Here the power must exceed the
t in the same proportion, as the distance of the
t from the prop^ exceeds the distance of the

"^Ijgll^ Illus. 1 . Let/ fig. I0,be the prop or

^■B^ ^^ fulcrum, p the power, and w the

mmmmmJ^lfJtKK Weight; if the distance />/ be only

^ Y/) ' three inche?,and wf be twelve, then

for the hand;? to balance the weight
)8. will require a force 'of four times 20, ©r 80 lbs. be-
.he weight is at four times the distance from the ful
bat the power is.

. 1. A ladder, which is to be raised by the strength of a
irms, represents a lever of this k ind,where the fulcrum is
d which is fixed against the wall, or upon which another
ands ; the weight may be considercdM ^\.VVvft\»^'^^\V ^S.

36 OF THE LEVER.

ih» ladder, vad the power is the strength applied tc
ing^ofit

2. The wheels in clock and watch work, may
ed levers of this kind, because the power that n
aicts near the centre of motion, by a pinion, and the
it has to overcome, acts ag;ainst the teeth at the
enee.

3. The bones of a man's arm, and the greatest
the moveable bones of animals, are levers of the thi

To take the arm, fig. 1
stance — d the elbow, is th
motion, the power, i^ the
serted at c, about one tei
fiir below the elbow as t
and a is the weight to be ]
muscles must according!

power equal to one hundred pounds to raise a we;

pounds.

CoroL Hence, in natural levers, the power is t

ge6u8ly situated, owing to the power being so near

of motion, but the loss of powe^ is comp^isated by

and compactness of the limb.

69. A hammer-lever differs only in its for
lever of the first^kind.

lUus. Let a e 6, fig. 12, represec
this kind, bended at c, which is tl
p is the power acting upon the long«
Dy meaas of the cord a d going over
d ; and the weight %o acts upon 1
arm c 6, of the lever. As a e is fiv
long as e 6, a weight of five pounds
balance ^ pounds at w.

iiT

Exp, If the shaft of a hammer is f
long as the iron part that draws tl
lower part c resting on the board i^ as a fulcrun
polling at a, a man will draw a nail with one-sixth
power that he must use to pull it out with a pair
in the latter case, the nail would move as fast as th<
in the former, the hand would move over six time
space as the nail daring the time of drawing it
weo4.

OF THE PULLSr.

3fl

OF THE WHEEL AND AXIS.

•

73. The wheel and aart 9, though made in many forms,
consists of a cylinder, and a wheel fastened to it, (Jig,
13,) or of a cylinder with proj ectings pokes, (^^. 14.)

74. The advantage gained is in proportion as the
circamference of the wheel is greater than that of
the axis ; or as the diameter of the wheel is greateir
tUanthe diameter of the axis.

lUus. If the diameter of the wheel,
fig. 13, or the length of the spoke,
fig. 14, be four feet, and the diame-
ter of the axis only 8 inches, then
the power P, of one hundred lbs. or
the strength of a man aj^plied to
the spokes S, equivalent to a hun-
dred pounds,will balance a Weight
W of six hundred pounds.

In this case as in the le-
ver, the power will travel
over six . times as much
space as the weight, when
the machine is put in mo«t
tion.

Exam. 1. To this en^
gine, cranes of all kinds
for raising heavy weights
may be referred.

2. Sometimes the axis is turned by a winch fastened to it,
which serves for a wheel, and the power gained is in propor-
tion as the winch is larger than the axis.

3. A capstan is a cylinder of wood, with holes in it ; into
these, bars are put to turn it round, The bars are made to
act something like the spokes, fig. 14.

OF THE PULLEY.

76. The pulley is a small wheel turning on an axis
with a rope passing over it. See Jig. 15.

lUus, The small wheel x is called a sheeve, and is so fixed[
to a block a, as to be moveable round a pin passing through
the centre.

38 OF THE INCLINE]^ PLANE.

76. Pulleys are either fixed or moveable.
Obt. 1. The fixed pulley gives no mechanical advai

but is used only to chang^e the direction of a power. B
man may raise a weight to any height, without moving
the place in which he is, as a stone to the top of a boi
otherwise he must ascend with the weight.

77. The moveable pulley represented by a:, fi^
is fixed to the weight, and rises and falls with it,
the advantage gained by it is as 2 to 1.

1. The reason of this is evident, for in raising the "W
one inch, foot, or yard, both sides of the rope must be
tened as much, that is, the hand h must move tiiroug]
inches, feet, or yards ; which shews,' as before, that the
through which the power moves, must always be in pi
tion to the advantage gained.

2. When the upper^cd bio

fig. 16, contains <t;o - pulleys, i

only turn on their axis, and the ]

moveable block x contains also

which turn and rise with the w

W, the advantage gained is ai

Ml nM\ ^ to one. For each pulley in the 1

H f£H I ■ block will be acted upon by an

T ' " \ H^ part of the weight, and since in

^kff! ■ pulley that moves with the we:

^^ 9. double increase of power is ga

therefore the advantage gainied is safour to one.

76. In general the advantage gained by pulle
found by multiplying the number of moveable pul
by 2.

Obt. 1. A weight W of 72 lbs. may be balanced b;
moveable pulleys, by a power of nine pounds, because 1
Tided by 8 gives 9 ; but in this case the power, when p
motion, will pass ever 8 times as much space as the we
that is, to raise the weight one foot, the hand must move
eight feet

2. A pair of blocks with a rope is called a iackU.

OF THE INCLINED PLANE.

79. The inclinell plane is merely a plane sui
indiDed to the horizon, and is used to move wei
/ivm Me level to another, see jig. 11 .

OF THK WEDGE.

^09 Obt, It is often made byplacing^ '
boards, or earth, in a rioping direc-
^tion, and is of great importance in
rolling up heavy bodies, as casks,
wheel-barrows heayily loaded, &c.

80. The force with which a hody descends upon an
inclioedplane, is to the force with which it would de-
scend perpendicularly, as the height of the plane is
to its height.

Ilhts, If the plane a 6, fig. 18 •
be parallel to the horizon, the
cylinder r will rest on any part
of it whereyer it is laid. But
if the plane be placed perpen-
dicularly as a 6, fig. 19, the
cylinder will descend with its
whole weight, and would require a power equal to its weight,
to keep it from descending. Or, if the plane be inclined to
the horizon as a (/, fig. 17, and three times ihe length of the
perpendicular b tf, the cylinder e will bo supported by a pow-
er eqaal to a third part of its weight. And if the plane be 20
feet long, and the perpendicular height be 4 feet, or ont-fifth^
then SOOlbs. would be balanced upon it by lOOlbs. because
the plane is five times the length of the perpendicular height to
-which the weight is to be raised. To the inclined plane may
be reduced hatchets^ chisels, and other edge tools, which are
eloped only on one side.

OF THE WEDGE.

81. The Wedge may be considered as two equally
inclined planes united at their bases. The advantage
gained is in proportion as the length of the two sides
of the wedge is greater thiin the back, or as the length
of one side is greater than half the back,
d x V h lUus, The wedge a b c dx, (see fig 20,)

«may be divided into two inclined planes, a v
e ar, and b vd zx, which may be used sepa-
rately, ai\d will gain advantage as such ;
therefore, when united at s x, the advan-
glf tage gained will be in the same proportion,
as when th^y were used in. CAfiwcwX.'^xNa^.

Obs . When the wood c\evj ©& ^V ^ ^"«kV%!w:

OF THE aCRClY.

before the wedge, the advantag^e then gained is in proporti
as one side of the deft ii greater than half the length of t
back.

82. The wedge is a very important mechanic
power, used to split rocks, timber, &c. which cov
not be effected bj any othfer mechanical power. ^

Obt, All instruments, and some sorts of ch^els ehanii
ed on both aides, are to be referred to the principle id X
wedge.

OF THE SCREW.

83. The Screw is an inclined plane used with a 1
yer or winch to assist in turning it ; and then itbecoin
a compound engine of great force, either in preM
bodies closer together, or in raising great wei^ts.

Obt. The screw may be conceived to be made, by cutt
a piece of {)aper into the form of an inclined plane, and tl
wrapping it round a cylinder ; the edge of the paper will ib
a spiral line round tiie cylinder, w^ch will answer to 1
thread of the screw.

84. The advantage gained by this mechanical po
er, is in proportion as the circumference of the c
cle made by the lever or winch is greater than t
distance between the threads of the screw.

Illus. It is evident that the wii
or lever will turn the cylinder a
round, whilst the weight, or the :
sistance, can be moved from <
spiral winding to another, as froi
to s, see fig. 21. If the distapee
tlie spirals s or, is half an inch, i
the lever a, three feet, or 36 inches long, then the circle
scribed by the lever will be about 228 inches, or 456 half in
cs, consequently a force at the end of the lever, equal to o
one pound, would balance a resistance at the thread of *
pounds. Hence it appears, that the longer tho winch of
ver, and the nearer the spirals, the more advantage is gain
But in the screw there is great loss of power ; for a screw a
be moved upward or downward in a fixed nut, as in fig.
or the nut may more on a screw, as in fig. 22.

86. Almost all kinds of presses,common corkscrew
&c. act upon the principle of this mechanical pow<

or FRICTION, -. 41

When a screw turns in awheel, it is called an endless
screw.

OF FRICTION.

86. In the application of ail the mechanical pow-
ers, one-third must be allowed to overcome the Fric-
TioN of the surfaces, and the various other obstacles
to which all machines are liable.

Obs. 1. IfdOlbs. are required to balance any weight with
a mechanical power, 80 lbs. will be wanted, owing to/ric/ton,
to put the machine in motion.

2. Friction is the resistance a moving body meets with from
the surface over which it passes ; it is of two kinds, the rub-
bing by friction, and the friction by contact. The former is
represented by a locked waggon wheel going down a hill, the
second by the wheel toucliing the ground in its usual motion.
The force of, friction varies in proportion to the different sur-
faces in contact ; thus a marble passing on a smooth pavement
suffers less from friction than it would from gravel, s^nd it
would be impeded in its motion still less if it were driven over
ice. But the hardest and most polished bodies are not free
from inequalities that retard their motion vrhBxjt they act upon
one another. The smallest impediment from friction is, when
finely polished iron is made to- rub on belUmetal, but even
these are said to lose about one-eighth of their moving power.
The friction between rolling bodies is much less than in those
that drag; hence, ia certain kinds of wheel-work, the axle is
made to move on small wheels, or rollers, in the inner circum-
ference of the nave. These are denominated friction rollers,
and are so placed together in a box, ai^ fastened in the nave,
that the axle of the carriage may rest upon tliem, and they
turn round their own centres as the wheel continues its motion.
Friction roUets do not answer in very heavy machines, as the
pressure is apt to wear the naves into notches ; but in light
and rapid motions they are extremely useful. Larger metal
balls, on the same principle, are made use of m moving immense
blocks of stone.

3. After a great variety of experiment^ made with the ut-
most care and attention, Mr. Vince deduces the following con-
clusions, which may be considered as established facts ;

Ilhis. 1 . That friction is an uniformly retarding force in
hard bodies, not subject to alteration by the velocity, except
when the body is covered with woollen cloth, &c. and in that
case the friction increases a little with. \]laft v^^a^vV"^ .

42 QUESTioFS ON mcBiiricAi. ArrscnoHs.

3. Friction inoreasM in a leM ratio tbntte wejglit «
body, being different in diffisrent bodies. It is nol ^et i
Giently known for^any ona body, what proportion the bm
of friction bears to the inoreaae of weight.

3. The smallest siufa< e has the least friction; ihm yt
being the same. But the ratio of the friction to die iwi
not accurately known. '

QUESTIONS ON THE MECHANICAL AFFECTl

OF MATTER,

"What are the Mechanical potpcrs? *

What are the principal moving powers ?

AVhat is the power most easily applied^ and "whoae •
most uniform? • , . « .

What are the three circumstances to be considered inl
ipg of Mechanical contrivances f • '

What are the six mechanical powers ? '

How can the force of a small body be made equal to H
a large one ? ' •

Whatisthefefer.?

What is the fulcrum ?

Where is the resistance placed ?

On what part of the lever is the power applied ?.• . .

How many kinds of levers are there ? .

How are they distinguished ?

Describe the^ri^ kind of lever. •

Wbatare the' uses of this kind of lever? . .'

Draw a diagram, pointing oat the resistance^ ihort arm.
arm^ fulcrum., and power.

Describe the iecond kind of lever.

Where is the (ulcrum^ power, &c. ?

What are the uses of the second kind of lever ? .

Describe the third kind of lever.

Where is the prop ? • .

Where is the weight, and where the power ?

Give a natural illustration of this kind of lever.- .

What is the hammer lever?

What are its principles and uses?

Of what does the wheel and axis consist?

How is the power gained by the wheel proportioned 'l
relative diameters of the wheel and axis ?

On what mechanical principle does the wheel and wad
power?

HYDROSTATIC^. 43

*■:

%. WlifttisthejNiZieyJ^

I \ 'WfaatutheiueofthelizedptiUey?
as;i Wliat proportioa of advantajgpe ii gmined by the moyeable
ijttlley?

Explain the reason why a we%ht can' be raised more easily
^Vith a pulley than without ooe.

vrti' What is the rule for finding the advantag^e gained by pulleys ?
What is a pair of blocks with a rope called?

• What is an inclintd plane ?

TS Of what aFery day use is the inclined plane ?

How is the velocity with which a body descends on an ia-
dined plane estimated ?
£i^lain the diagram.
^ On what princij^e does the wedge act ?

• What is Uie screw /. ♦

g. What is the rule by which the force of a screw and lever is
estimated? •
Explain the principle of the screw.
^. .' Illustrate this principle by an explanation of the figure.
^ III the appUdation of the mechanical powerS| what allow-
ance must be made for friction ? ,
' What is friction ?
What are the kinds of friction ?
What are the conclusions of Mr. Vince on this, subject ?

HYDROSTATICS,

OR THE LAWS OF FLUIDS.

87 « Hydrostatics treat of the nature, gravity
pressure, and motion of fluids in general, and of the
methods of weighing solids in them. And its mecha-
nical pvactiqe, called hydraulics, relates particulari^
to the motion of water through pipes, kc,

86. A fluid is a body, the parts of which yield t<
any impression, and are easily moved among eacl
other. Fluids are either non-elastic and incompres*
sible, as water, oil, mercury, &c. or elastic and com*
pressible, as air, steam, and the different gases.

06f . Heat, or motion, is supposed to be the causa of fluldl
ty; for example,' ice^ without heat^ ii a^ vj^^-^-tni^^Mbu

44 HTDEOSTATICS.

becomes a fluid, in t^o/er— and with more heat, in^%
fluid, in steam. In the fint state, the atoms «re fiked jp
tals — in the second, are thrown into intestine motiioiA-Hi
the third state, are forced asunder with an amazing^ ^Sqfi
force. - *:?

2. Philosophers have usually assumed, that the pai^
fluids are round and smooth, since they are so easily i
among one another. This supposition will account fiir
circumstances belong^ing; to them. If the particles are i
there must be vacant spaces between them, in the same
ner as there are vacuities between cannon balls that are
togfether; between these balls smaller shot may be p
and between these, others still smaller, or gravel, or
may be difinsed. In a similar manner, a certain qattn
particles of sugap can be taken up in water without incH^
the bulk; and when the water has dissolved the ragtti
may be dissolved in it, and yet &e balk remain the same
admitting that the particles of water are round, this it
accounted foa.

3. Others have supposed, that the cause of fluidib^
mere want of cohesion of the particles of water, oil, mn
from this imperfect cohesion, fluids in small quantitiei
xmder peculiar circumstances, arrange themselves in a v
cal manner, and form drops.

89. Fluids are subject to the same laws of gr
with solids ; but their want of cohesion occasions
peculiarities. The parts of a solid are so connect
to form a whole, and their weight is concentrate
single point, cailed the centre of gravity : be
atoms of a fluid gravitate independently of each o

90. Fluids press not only like solids, perpen
larly downwards, but also upwards, sidewajrs, a
every direction.

Exp. Take a glass tube open at both ends, put a tt
one end, and immerse the other in water. The fluid w
rise far in the tube ; but the moment the cork is tal^en
will rise to a level with the surrounding water; which ]
the pressure upwards.

91. A fluid kept in an open vessel, or vei
will assume a flat surface parallel to Jhe horizoo
will remain at rest, or rise to a common level.

HrOBOSTATICfl.

Exp. If aTerael j^. 23i coniiiti sfpipu
VKriouily inclined, communicating with
each other at b, nnd open at the top, wa-
ter poured into any one of thetn mil riie
to the nine level >/, in all.

92. Thepresflure ofthesame fluidisinproportion
to the perpendicular height, and is exerted in ever;
Erection ; bo that all the parts, at the same depth,
preoa each other with equal force in eTery direction,

&SJ). 1. Ifa bladder fuU of air be immersed in water, Iheo
the perpendicular presaiu-a is manifeat, for the deeper the
bladder is immened, the more will its bulk ba contrBCted.

S. An emptj bottle beii^oorked, and bjmeuuofawnght,
fat down a certain de^ into the kb, it will be broken, or the
cork win be driven inte it by the perpaidicnlar prenure. —
But a bottle flUed with water, wine, fee. may be let down to
wif depth, withont dunage, became in this caie the internal
proaure ii equal to the external.

3- It ii evident, that the quantitiei of water ^ the different
pipei, fig. 33, whaleTer be Iheir size, press equally against
each otiier, for IT the water be luddenly taken out of the pipes
t, *, or/, the lurfiice of the water will instantly descend to a
lower lerel b all the other pipes,

93. The horizontal bottoDi of a vessel suslaius the
pressure of a column of the fluid, the baaeofnhichis
the bottomof the vessel, and the perpendicular height
eqaal to the depth of the fluid.

-van

Exp. 1. In the veisal o bijig. 94, the bottom c b does not
iiutain « ppesaure equ&l to the quantity of the whole fluid, but
imly of a column, whate base c b, and hei^t b a.

46 HTDROSTATIOS.

2. In the vessel fg^Jig. 25, the bottom g sostain^a pA
•qaal to what it would be if the vessel yns as wide iff
as the bottom. ' - -' Ji

94. The pressure of a fluid upon any gf"
of the bottom or sides of a vessel, il e
weight of a column of that fluid, having o^baae
that part of the bottom or side, and ai^
to the perpendicular height of the fluid.

Obs, 1. Hence nUi^ be calculated the pressufe il^

the streng^ required for dams, cisterns, (npes, Sl[c, V.j

2. The presiurt of fluids differs from their gr^vi^ tt f

in this; the i^e^Ai is according to the 9tf<in^,biiixbiiifN|;

U according to the perpendicular height. >. . ^

3* From this property also, we ascertain the |H'!nii|

spouting fluids. If a hole is b<»^ in the side of^wa if

/ pipe filled with water, the fluid will spout out, wl^kb-M

* the lateral pressure, and this pressure is so mwAkmom^

proportion as the ho\p is &rther remoTed frdm vbm MM

that is, a hole three feet below the surface pf a veaa^ oC

will throw out, in the same time, much more wat«r4liil

only a single foot below. • . ■

95. The hydrostatical paradox is this : liMil

quantity of fluid, however small, may fie

counterpoise any quantity, however large.

Exp. If to the wide vessel a byjig. 26, a tube e d
tached, and water be poured into either of them, it wi
at the same height in both. Of course, the small qui
c d, balances the large quantity in a b. But "this '
paradox in terms, because the action of the fluid is do
not upward.

96. The upper pressure of fluids is shewj

hydrostatical bellows.

Exp. 1. This machine consists of two oval boards
in leii^;th, and about 14 in width, covered with '
rise and &U like the common bellows, but without
pipe three feet long is flxed to the top board ; let
ter run into the bellows to separate the boards, f
to the amount of two or three hundred pounds m
the upper board; after which, if the pipe be v
water, it wiU, by its upper pressure, sustain ar
weights.

2. Upon the principle of the upward pressure
metal may be made to swim iu water. Into a

HYDROSTATICS* 47

ge a glass tube, op^ throughout ; but by a string hdd a
piece of lead (1-4 of an inch thick^ fast to the bottom of
tube to prevent the water from getting in between the lead
the glass, in this situation, if the tube is immersed in the
el of water to about three inches depth, the string may be
o, but the lead will not fall ; it will be kept adhering to it
le upward pressure below it. The lead being about eleven
8 heavier than water, and the three inches being eleven
fi the thickness of the lead, is the reason why that depth is
1 on. Had iron been used, the depth must have been less

two inches, because iron is 7 or 8 times heavier than wa-

and if the plate had been of gold, the depth to which it
t have been plunged, wo^d have been nearly 5 inches,
Lusegoldis 18 or 19 times iPLvier than water.
7. A fluid specifically lighter than another fluid

float upon its surface. For the lighter fluid will
ess powerfully acted upon by the force of gravi-
m than the heavier ; whence, the heavier will
: the lower place.

vp. 1. Let a small and open vessel of wine be placed with-
large vessel of water, the wine will ascend.

Let mercury, water, wine, oil, spirits of wine, be put into
lial in the order of their specific gravities ; they will re-
1 separate.

3. If a body floats on the surface of a fluid speci-
ly heavier than itself, it will sink into the fluid
t has displaced a portion of fluid equal in weight
le whole solid.

>ff. A body, floating on a liquor specifically heavier than
', wiU sink into it, till the immersed part takes up tiie
i of so much fluid as is equal to it in weight. For, in that
, that part of the surface of the fluid upon which the body
, is pressed with the same degree offeree, as it would be
the space full of the fluid ; that is, all the parts of the sur-
are pressed alike, and, therefore, the body, after having
: into the fluid till it is in equilibrio with it, will remain at

z^y. 1. Place a cube of wood in a small jar, exactly filled
I water ; a part of its btflk will be immersed, and will dis-
e a quantity of the water. Take the cube out of the water,
put it into a scale, with which an empty vessel in the other
I stands balanced. Then pour water mto that vessel till the
libriom ii restored, aad Uiat portion of ^«ta>t ^inUL«»s^

48 OF SPECIFIC OltlVITIES.

fill up the jar in which the cube was placed. Coaaeqi
the weight of the water diq^ced is ezitotly equal to tibe ^
of the wood.

2. Let a glass jar, with a weight floffident to mmkb i
in water to about two-thirds of its length, be plaoed fin
larg% vessel of water, and afterwards in one which laTnc;
wider than the jar^ and which has in it a null qiua
water; the jar will sink to the same depth in both ToiMil
is, till so much of the yessel is under water as it equal i
to any quantity of fluid whose weight is equal to that
whole vessd^

OP SPECIF^ GEAVmr.

99* By the- specific gravities of bodies, is i
the relative weights which equal bulks of difl
bodies have in regard to each other.

Obt. 1. Thus a cubic foot of cork is not of eqnalj^
with a cubic foot of water, or marble, or lead ; biit ^is
is four times heavier than the cork, the marble 1 1 tini
the lead 46 times ; or, in other words, a cubic foot o
would weigh as much as 45 of cork, &c. Sic,

2. The terms abtoluie gravity Bnd 8pee\/ie gravity ym
quently occur in physics. The first is what we exp
common life by the word weight., and signifies the whole
power, with which a body presses downward to the
Every particle in every substance is heavy ; that is, i1
tendency to fall toward the earth, or is attracted by the
Now. the greater the number of particles a substanc
the more powerful will be its tendency toward the earth
we express the d^pree of this tendency by certain quai
on which we have fixed as standards, by ounces, pound
Thus we say, righUy, this stone weighs sixteen times as
as this ounce. It is very common, however, to ws.j
stone is as heavy again as this ; that pound weight is a
times as heavy as this ounce ; but this, in a physical it
improper ; the two stones, if they be of the same kin
equally heavy^ bulk for bulk. Their heaviness or gm
the same, but their weight is difierent

100. It is usual to compare the weight of b
with that of water, as it is by weighing them in ^
that their various specific gravities are most coi
ientlj found.

OF BPKCIFrC GlUyiTICS. 49

Oltr. The method of aaeertainiii^ the specific gravities of
lK>die8 was discovered accidentaUy by ArchiBiedes. He had
been employed by the king of Syracuse to investigate the
metals of a golden crown, vmich he suspected, had beeo adul-
terated by the workmen. The philosopher laboured at the
problem in vain, till, going one day into the bath, he perceiv-
ed that the water rose in ttke bath in proportion to the bulk of
his body ; he instantly saw that any other substance of equal
lise would have raised the water just as much, though one of
equal weight and of less bulk could not have produced the
s^e effect He immediately felt that the solution of the
king's question was within his reach, and he was so transport-
ed with joy, that he leaped from the bath, and running naked
through the streets, cried out, '' Eupipca, £up7]xa,'' — ^^ I have
found it ou/, — I have found it out!"** He then got two masses,
one of gold and one of silver, each equal in weight to the
crown, and having filled a vessel very accurately with waW,
he first plunged the silver mass into it, and observed the quanti-
ty of water that flowed over ; he then did the same with the
gold, and found that a less quantity had passed over than be-
fore. Hence he inferred that, though of equal weight, the bulk
of the silver was greater than that of the gold, and that the
quantity ol water displaced was, in each experiment equal to
the bulk of the metal. He next made a like trial with the
crown, and found it displaced more water than the gold, and
less than the silver, which led him to conclude, that it was
neither pure gold, nor pure silver.

101. A body immersed Id a fluid will smk to the
Jiottom, if it be heayier than its bulk of the fluid : and
if it be suspended in it, it will lose as much of what it
weighed in air, as its bulk of the fluid weighs.

102. All bodies equal in bulk, which would sink
an fluids, lose equal weights when suspended therein ;
and unequal bodies of equal weights lose in propor-
tion to their bulks.

Obs. This is th'^ foundation of the whole doctrine of specific
gravities. The fluid is a common and uniform measure of
weight, with which the other bodies are compared and con-
trail.

103. If the same body be successfully placed in flu-
ids of different specific gravities, it will displace more of
the lighter than the heavier fluid ; and if the weight oC

50 ov tMOfrR; mutitt.

the bod; be eqnal to that ofthe «ame balk
th«a it mil remria at r«it id any part of t
1(M. TfacHTDKOSTATicBALAircE, tue
specific graritiea of bodies* differs but lit
conimoa balance, (see &g. S?.^ It ba> a boc
torn of one of tfae KdeSjOnwekdi differei
that are to be examined mi^ be hong bv
iitito.Uabod72ll|;.37,i
an tba Mala ba Ant couni
b^ wevIiU io tli« opporite
mcnaa in water, the e^tiili
I leiEnjad; tben ifs veigbt'
I mUfram which the bodVb
It tfaseqailibriDm, that waieh
\ to the weight of water HU
menad booj.
Obi, The inttnimeDtiiM
'ing^tha ipeciBc gnritei <rf'
■d the HTDaoHKTm ; •
_ prove or aacertain the atrei

trytbeitrenglhofworljaiidez&niiDelhetatm
in Mtt-work*. The deeper tbe hydrometer ■:
tlie better they are ; in worti and brioe the «
drometen are cammonly boUow balla ofglan,
ball, containiog qaickiilverat the bottom, and
at the top. The tube or itetn ii graduBted, tht
which itiinki may be known.

105. Tbe specific gravity of all bodiei
water may be found, first by neigbiogth^
then ip water, and dividing the weight
lou of weight in the water.

EUeam. A guinea wei^ 129 graiatia air; b;
•d io water it lona Tl-4£raiDi, whicbihewB, i
of water of equal bulk with the galDea, weigh
iivide 139 by 7 1-4, or 7.26, and the qaolieat <
which prove* the ^iaaa to be 17.793 timet lu
balk 01 water.

CanL 1. Wahenoeeaiilydedacethemetbo
the *pMUc gravitiei of alt bodies, taking ra
itandard, a cobio foot of which beiuf naiGx
weixh 1000 avoinlapoli onnoei.

Aa waigftt which a body Ioim in a floid,
irafflituUieipecifio travit; ofthe flaidia to tti

OF SPECtFIC <»ftAiriTIB8.

If a ifuiiiea weigh in air 129 g;raiii8, and in being immersed in
water loae 7 1-4 of its weight, the proportion will be 7 1-4; 129
: : 1000 to the specific gravity of a guinea. By this method^
the specific gravities of all bodies that sink in water may be
found and expressed in a table.

2. Hense, if difierent bodies be weighed in the same fluids
Aeir specific gravities will be as their whole weights directly,
and as the weights lost inversely.

If a body to be examined consist of small fragments, they
may be pat into a small bucket and weighed ; and then if from
the wei^t of the bucket and body in the fluid, we subtract
the weight of the bucket in the fluid, there remains the weight
of the lM>dy in the fluid.

3. If the same body be weighed in difierent fluids, the speci-
fic gravity of the fluids will be as the weights lost.

The loss of weight sustained by a glass ball in water and
milk is respectively 803 and 831 grains, therefore the specific
gravity of water is to that of milk as 803 : 831, that is, as 1.000
: li)34. By the same method, the specific gravity of water, is
\6 that of spirits of wine as 1 .000 to 857.

SPECIFIC GRAVITIES OF VARIOUS BODIES.

Flatina

Finegold

Mercury •

Lead

Fine sUver

Copper

Iron

Diamond

Marble

Glass

Flint

Chalk

Coal

Mahogany

06f. 1. The above table shews the specific weights of the
various substances contained in it, and the absolute weight of a
cubic foot of each body is ascertained in avoirdupois ounces^
by multiplying the number opposite to it by 1000, the weight
of a cubic fitot of water, thus the weight of a cubic foot oji
mercury is 14019 ounces avoirdupois, or 876 lbs.

23.000

Milk

1.034

19.640

Box-wood

1.030

14.019

Rain water

1.000

11.325

Oil

- .920

11.091

Ice

.908

9.000

Brandy

.820

7.645

Living men

.891

3.517

Ash

.800

2.705

Beech

.700

3.000

Elm

.600

2.570

Fir

> - .550

1.793

Cork

.240

1.250

Common air

.00112

1.063

Hydrogen gas

.000105

iti weight in TnqroQneMWil]^ be feoad m maltiliyinR ifci
.01145. AiidiftlMW«i|^tb«|ivenmTn7oaDOta,tt«l
IbuBd in miiUiplyu% it i^ 1 j007i.

3. Mr. RobcrtMD, late librarian to the B4i9ral Boma0$s
YestigatedtheqwoificgraTiiyAfUviiii^aien, in order told
what quantitj of wood w«h^ be Mflmnttokeni aaanirf
in water, lapposini; tittt mort aien were.flpeeueeUgElMV
than river water, but the eontrarj appeared totelli*<
from triab whkh he made upon ten di fl w r en * paiiuwi^
mean specific gravity i^as, as eoLpteased in the tables OhBit
about l-9th leas than oonunon wat^r. Ut however, «vine
cases, they are heavier, and, indeed, as the wei^ ti i
heads out of the water would generally sii^ them, ital^
worth while to be able to asoertaiB the wiefght of omA
ficient to buoy up any man«

If, then, as ah example, the absolute weight of a nl
sized man be 135 pounds, the weight of two cubia Ibe
water be 123 lbs the specific gravity of oork be tmefcuM ol
of water, and the weight of the cork required be lepieMi
by w ; then the x lbs. of cork, when wholly immened*
displace 4 x lbs. of water ; and the weight of the perseii
the cork, 135-1-2:, must of course be equal to that of the
ter displaced by both 123-|-4a:, in order that the petatB-i
flwim as required. Hence we have the following e4;a|t
123-|-4a:, =» 135-l-a;. Consequently, m-|-3««
Therefore 3a:= 135— 123=».12 Whence we find ap«*s4. J
pounds of cork, therefore, will keep such a person IroA i
uig, so that he may remain with his head completely el
water.

HYDRAULICS.

106. The science of Hydraulics teachea hoi
estimate the velocity and force of fluids in otMit
Upon the principle of this science all machines wt
ed by water are constructed, as engines, millSy p^nt
fountains, &c.

107. Water can be set in motion only by its omi
rity ; as when it is allowed to descend from ahj^

1 lower level : by an increased pressure of the air,
}r by remoTing the pressare of the atmosphere, it
rill rise above its nataral level.

06t, 1. In the fanner case it will iieek the loweat situatioD,
n aMtMtr, it may be Ibrced almiHt to any heifht.

3. Any put of a Aoid at rest presses, and ia pre»eil, eqiuny
a ■tldirwtioDa. Foe each particle ia digpoaed to give wa; on
he lU^tlart difference of preMure : coruequentl}', it presses
qnaUy in all dii-ections. Hence the lateral preoare of a Suid
% eqinl to the perpendicolar pressure, ^nd Ihii it one of
he nuM txtruoriituary propertia offiaidtf and can be concaiTed
o ariie only from the extreme Ikcility with which the compo-
MDt putioles move among each other.

106. A Syphon is a bent tube, one of whose legs is
onger than the other. The shorter leg is immersed in
he Uqnor to be drawn off, and the pressure of the air
teing takenfrom that part of the surface of the liquor
rittunthe tube, the liquor will rise above its natural lev-
il in Uie vessel, and will flow off through the loDger teg,
Obt. 1. A syphon ia used bj filling it
I with water or some other Suid, then itop-
J pmg both ends with the finger, and in this
I state immersing it in the ressal. The fin-
gers being removed the water flowi out of
Uie longer leg, by its own fravity, and
aAerwimls by the pressure of the atmos-
phere on the liquor in the vessel.
2. Intermitting iprings are caused by the principle of the
lyphon, the water flowing through the natural pipes or sy.
'ji the earth, which fill only at

lUiu. If C bo a cavem

in a moitntain, receiving

wat«r which escapea by

the channel A B C, it is

evident that it will flow

only when it rises in D to

IhelevelN. B. D.

109. The velacity with which water spouts out at a

bole in a side or bottom of a vessel, is in proportion to

tkt Bqtucre root of the distance from the bole below the

HTDRAULXCt.

surface of the water ; and the pressure of wat<
the sides of a vessel is as the square of the d

Exam, 1. If at the distance of one foot from tfa
the Telocity is 1, another hole four feet from the 8ur£
give the velocity of 2, and at 9 feet deep there woul
locity of 3, 2 and 3 being the square root of 4 and 9.

2. If a vessel be three feet deep and filled with i
pressure upon the udes of the first foot will be 1, c
and second will be 4, and on the whole side it will b<

110. Fluids may be conveyed over hills an

in bent pipes, to any height which is not gre

the level of the spring from whence they fli
Obs. All water finds its own level. But from igi
this single principle, the ancients constructed vast
across valleys to convey water across them ; wh
modems effect the same purpose by means of wood<
iron, or stone pipes.

111. Fountains are formed upon the san
pie : if, near the bottom of any vessel, a si
bending upwards be fastened, the water ^
out through the pipe, and rises nearly as hi|
surface of the water in the vessel.

112. The Common Pump consists of a p
at both ends, in which is worked a moveahl
that fits the bore exactly, and is provided wit

Illus. Fig. 30 is the representation of
pump; a 6 is called the 6arre/ which c(
piston ; b d the pipe communicating wit
tcr below. At the junction of these ■
there is a fixed valve or little trap-door
fig.) d opening upwards.

The mode of oper9,tion is as follows :
is fixed in water, and the piston e ^ is t
down upon the valve d. In drawing up
from d to c, a vacuum of air is form
space, consequently, the air in the rest <
from d\xi b wiU force its way through
jiy and fill the part which had been ezhi
wiU therefore, be rarer than before
being equivalent to the pressure of this atmospheri
water at d% iB which the pomp is immenied, the i

e ibned or prcBud up into the pipe nluaa x, until the up-
er air wittiin be u dsoie u belbre.

Upon depreuiB^ Ibe piitoa a aecoiid time, tlie lane efiect
I pr«dBoad, till at length the water is breed, by the prenure
r the air b, op into the pipe above if. When tbe piatOQ now
esceada, it ie forced into the water, which, ai it caoDot re-
au Ihrongh the valve d, mail, tbererore, riie nbave the
iitwi by paMing tfarougli iU vahe c; aed when the piiton ia
eit ratoed, all the water above it will be lilted up, and will
an off by the pipe.

113. The Forcing Pump coDsuts of a barrel, a
lunger, and two fixed valvei, that should be air
ight, and bo disposed aa to let the water freely rise,
iut preveot its return.

Illui. Id fig. 31, ■ 6 ii the barrsi : e a aolid
pistoa or plUDger ; at J i> ooe valve opening up'
wardi, the other a the btanchiag pipe t. When
[hepiitouia Gnt moved upwarda in the barrel,
the air below will be rorefind and the water riie
up in b : and by repeated atrokm of the pialoTiB,tbe
water will be brought up between the fixed
^valved and (.

cannot, therefore, descend by rf, bat maat
FiDBheita way through the upper valve at t, which
hutftlheiDomeDtthe waiter baa pasaed by ita own
veight.
ti IS a BtroDs air vessel ctoied at the lop by a
_ smiU pipe, that reachea nearly to the bottom.
"* The water i> forced aloD°; Ibe naing pipe t, gela
to the veaael, and risea above the tower part of
B pipe. The air which is above tba water in
e vaaiel being can&ned, and condensed iota a
mailer bulk tlian its natural apace, presaea by ita elasticity
ipoo the BOrface of the water, and forcaa it violently up the
lipe iu a continual stream. Tbis ia the principle of Um En ■
lurxibr extinguishing Grea.

1 14. The water ia raised by pumps owing to the
dastici^ or prensure of the atmosphere ; it can be
-aised only 33 feet, because the force of the atmos-
>bere is equal only to a column of water 33 feet high.

Obi. I. The forcing pump ia unlimited in regard to tlie
leight to which it can throw water. '1' be air-i easel added tt>
he tbrcin^ pump, gives the water a more «cv^^^^^ tfr^A.^^.

o«

HYDRAULICS.

2. A constant itream may be prodaced by two bar
piatons moving np and down al|temately, as is th
many pamps.

115. The Steam Engine consists of a laq
dcr or barrel, in which is fitted a solid piston
of the forcing pump. Steam is thus supplier
large boiler, which, in forcing up the piston, i
opens a valve, through which cold water rushe
principle of the common pump.

Other steam is then introduced above thi
which forces it down again, and drives the w
of the pipe with immense force.

Steam then raises the piston again, and agai
it fall, and by this alternate motion the grand
rations are performed. The action of thi
moves up and down a large beam, and th
communicates to other machinery, the powe:
or 200 horses i

Obs, t. The power of seme of the steam engines c
ed by Messrs. Boulton and Watt, is thus describ^,
by actual experiment. An engine having a cyUndei
ches in diameter, and making 17 double stroim pei
performs the work of 40 horses, working night aoc
which 3 relays, or 120 horses must be kept, and bar
pounds of Staffordshire coal per day. A cylinder ol
es, making 25 strokes, 4 feet each, per minute, per
work of 12 horses working constantly, and burns 37(
of coals per day. These engines will raise more ths
cubic feet of water, 24 feet high, for every 100 weigt
pit coal consumed by them.

2. This cut represent
mon steam engine. A i
cylinder, containing t
which works up and d
moves the beam and t
ance B . The steam pi
the boiler, at the pipe £
cylinder under the pist
it, and the jet at C th
water, condenses the si
allows the pipton to fa!

HYOaAOLiea.

3. Tbi* i* Watt't improved mgine. The principle i* the
tne ai the pieceAivg, but the econoai; is greater. The
■am which it below the piilpn eicapea iaio tbe condeoaer A,
' tfaa cock B, wbicb iiopsnedbj the rod C, and at the rame
Be tbe iteam la admitted by the cock D into the upper part

the cflioder ; when the piitoo had Ueicendad, the cook* E
lA F Oct in a liiailar maaner in letting out the iteam from
love, aod admiltiog it balow Ibe piiton. Tbe jet ia auppli-
I bf tlie water of (he cistern G, which is pumped up at H,
MD a reeervoir -. it ia drawn out, lof;elher with the air Uiat

extricated from it, bj the air pump I, which throwa it into
« cietem K, whence the pump L raiiei it lo the cillem M ;
id it entsra the boilera through a ralve which openi wben-
rer the float W deicenda below ita proper place. The pipei

and P lerre alao to ascertaia tbe qnantit; of water in tbe
>iler. The piston rod ii cDnfined to a motion nearly rectili-
lar by the frame Q. Tbe tly-wheel R ia turned b; the aun
itl^ planet wheel S T, and the ttrap Uluroa tbe centrifngal
igalalor W, which goTeraa the supply of aleam bj the valre
: (top cock X.

4. Steam engines have been adrantageouily applied lately
limpelTcneliiaimoDlfa watert,ai riven, canalt, £:c.

58 quESTiONS.

QUESTIONS ON HYDROSTATICS.

Of what do hydrottatics treat f f '

What is the difference between hydrostoHes and hffir
lies .?

Whatis^floid?

What is the tniiise of fluidity ?

Of what shape are the particles of floids ?

In what direction do fluids press ?

Ilustrate the iq>ward'pnasuTe of a fluid by an ezperinaM

What governs the pressure of a fluid upon the bottoaii
vessel ?

What relation is there between the
a fli^id, and its pressure ?

Give illustrations of these principles.

How is the pressure of the fluid on the bottxAia of tfa« vi
oalculatedf

How does the pretsure of a fluid differ from its grayity f

Vflmt JBi^hydrosiaiiealftaradoxf

Explain its principle.

Describe the hydrostatioal bellows, and explain en iR
principle it acts.

Explain the manner in which lead may be made to t
upon water.

'Explain the reason why a body specifically heavier
water will not sink to any depth in that fluid.

Describe some experiment to illustrate this doctrine.

What is meant by the specific gravities of bodies ?

What is the difierence between absohUe and spee\fie

Describe the method by which the specific gravity of a 1
is taken, and explain the principle.

What is the construction of the hydrostatic balance f

Explain its use.

Explain the action of the hygprometer.

What is the rule for finding the specific gravity of a 1
after weighing it in air and in water ?

How is the specific gravity of a fluid taken ?

What were the results of Mr. Robertson's experimen)
•the specific gravities of men ?

What does the science of %flfrau/i(;5 teach?

What useful machines are constituted on the prinoipli
Ihis science ?

What are the different ways in which water can be ■
motion ?

What are the direetions in which fluids press ?

PNEUMATICS.

6^X^

In whatpropordon is the lateral to ihe^perpendieular pres-
sure of a fluid?

What is a syphon ?

Illustrate the principles of the syphon.

How are intermitting springs caused?

How is the velocity with which wat^ spoutsout at a hole
in the side of a vessel calculated ?
. Give an exan^le.

On what principle may water be conveyed over hills in bent
pipes ?

On what principles are fountains formed ?

Describe ibe common pump^.

What are the different parts of a forcings pQmp,and the prin-
ciples upon which it acts f

How b%h can water be raised by the conmon pump, and
on what principle,?

Describe the steam eogine, and gpve- an example of its-
powers.

PNEUMATICS.

1 16. The science of Pneamatics treats of the me-
daaDical properties of elastic or aeriform fluids ; such,
as their weighty density^ compressibility^ and elasticity.

117. The air in which we live surrounds the earthy
and extends to a considerable height above it. The
air, together with the clouds and vapours that float
in it, is called the atmospliere.

Obt, 1. This atmosphere is necessary to animal and vege-
table life, and te combustion : it is a very heterogeneous mix-
ture, beings filled with vapours of all kmds. it consists how-
ever of two g;reat principles called oxygen in 28 parts, and
axote in 72 parts, of 100.

2. Theheightto which the atmosphere extends has never
been ascertained ; but at a greater height than 45 miles, it
ceases to reflect the nys of li^it from the sun.

118. The air is not visible, because it is perfectly
transparent : but it may he felt on moving the hand in
it. or when it move's and produces what we call mnd^

.' ' / FITEUMATICS.

. ' Exf. 1. The existence of the air may be ascertauied Jjy
^ swinging the hand edgeway? swiftly up and down, which girv

the idea of separating the parts of some resisting medium.
2. Any swill motion, as of a stick, or whip, or fiua^ proni

the existence of air as a resisting medium.

119. Air is 900 times lighter than water ; Irat the

whole atmosphere presses on all sides like other

fluids, upon whatever is immersed in it, and in propQi-

tionto the depths.

Exam. 1. It is known that the pressure of the aftmoiphete
M less upon a mountain than in the plain or valley beneatli.

2. The pressure of the air may be thus shewn : cover t
iHne glass complet(.ly filled with water, or wine, with a pieet
of writing paper ; then place the palm of the hand over fhs
paper, so as to hold it tight and accurately even. The fjiam
may then be turned upside down, and the hand removed
without the water running out. The pressure of the air upon
the paper sustains the weight of the water.

3. It is the pressure of the atmosphere which sustains the
mercury in the barometer tube. On ascending a mountain the
mercury sinks. This shows that a part of tlie pressure is taken
off.

On the surface of the earth the water beils at 212 degrees;
on Mount Blanc, it boils at 187 degrees. These and many
other experiments shew, the higher we ascend from the surfiioe
of the earth, the less is the atmospheric pressure.

120. The air can be compressed into a less space
than it naturally occupies.

Exp. 1. Take a glass tube open only at one end, and it is
ef course full of air. Plunge the open end into a bowl of wa-
ter, and you see the water rises an inch or so in the tube ; the
air, therefore, which before filled the whole leng^ of the tube,
is com'^ressed by the water into a smaller space.

2. Take a cork swimming on a basin of wa^er, cover it With
an empty o;lass tumbler, which force down tJirough the wattf.
The cork evidently shews, that the surface of the water within
the tumbler is not on a level with the surface without. TWi
experiment proves that air is a body which prevents mAtX
from occupying: the same space with itself; it proves also thit
the air is compressible, because the water does not ascend in
the glass.

121. The air is of an elastic or expanding natore
and the force of the spring is equal to what ig common-

pnQiuTccs. $%

■ culled iti weight. The spring, howeTer, operates in
1 di recti oos, and ie as poweHbl in small as Inrge bulks,
Exp. 1. rill B bladder -with air by blowing into it, and in
is a^te the bladder is highly elastic ; it proTes also that air
as much a tubilance as vodS or metal, for do force can,
ithout breakiDg- the bladder, bring the sides together,
' "le parts of an cmptj bladder mayba squeezed into

3. Opea a pair of cominoa bellows in the usual manner,
td thea it(^ the norzle securely, and no force on bring the
irt* together, without first unstopping the nozzle, or bnrstiiig
le leather : another proof that atr is a solid sabatance.

1Z2. When air is in motioo itcouBtitates what We
ill wind; which is nothing more than a current of
r, raryiDg its force, according to the Telocity with
hich it flows.

123. AiR-PTTMPs are machines made for ex ban st-
r irom certain glass vessels adapted to the

62 PVBVMATICS.

a a two brass barrels each containing a piston, with ftvib^
opening upwards. The pistons are worked by means of tk
winch 6, which moves them up and down alternately. Q
the wooden frame d e, there is a brass plate g^ g^ond psi
fectly flat and even ; and also a brass tabe, commmiMati^
with the two cylinders beneath, and the cock i, and <lpwiB|
iato the centre of the brass plate at a.

k is the glass receiver wluch is to be exhausted of air« Midi
made to fit very accurately on the brass plate, purtiMibd||
when a wet piece of leather is laid between them.

Having shut the cock t, the pistons are worked nam
down, and the air is'drawn from the glass receiver throqn tin
pipe, and is suffered to escape ; when the piston is foreed&ini
the air rises through it, because the valve opens npwaidi
but it is prevented from returning into the vesssel fiir the sm
reason. The air being gradually exhausted from the nam
er, it becomes immoveably fixed by the pressure oi. flu lar
reunding atmosphere.

Upon opening the cock t, the air rushes again violflntly, am
with a noise into the oeceiver.

124. The Air-pump is the grand machine by whici
experiments on air are made. By its means the M-
lowing important properties of air are demonstrated

1 . The air has weight.

Exp. 1. The air being exhausted by an air-pump, fixim a
glass receiver, the receiver will be held fast by the prenun^oi
Uie external air.

2. If a small receiver be placed under a larger, and both be
exhausted, the larger will be held fast, while the smaller will
be easily moved.

3. If the hand be placed upon a small open vessel, in sucli i
manner as to close its upper orifice, it will be held down with
great force.

4. The upper orifice of an open c^ceiver being closely co-
vered -with a piece of bladder, upon exhausting Sie receiver,
the bladder will be pressed till it burst.

5. Let the air be exhausted from a glass vessel, kuid by
means of a cock, let the vessel be kept exhausted ; weigh the
vessel whilst it is exhausted, and when the air is to be re-»d-
nutted, the difference is the weight of so much air as the ves-

se2 contains; which difference will be a\)o\x\.^%^ ^cvott Wt ^
*«i7c/ ctthic inches.

lato the raeavar «, fif . 3S, put a nniill T«g««l
of qniekiilTer, uid Ihrougb the collu' of leather ■■
■tA, lOBpend ■ glu9 tube, cloaei] atthe upper end,
over the quicksilver. TheapparBtua thus situat-
ed ia to be placed on the brass plate of the air-
pomp, and Uie air completely eihausted from the
■jl reoeiver,the tube iitheii to be letdown into the
■■j qniekrilver, which will not riee in it aa long as the
reeeiTer continuea empty ; but as soon as the air
ii re-admitted, all the Biirfnce of the quicksilver is
. pr«ned apon by the air.eioept that portion which
liea above the orifioe of the tube-, it willthereibre
1 lAe tvbe, until the weight of tha elevated q^cksilver
■ as forcibly on that part of it which lies beneath the
tube, u the weight of the air does on every other equal portion
withoat the tnbe.

7 . A common experiment atntHi^ boys is on tho same prin*
mple. Take a pieee oT thick spongy soal leather, cut it into
a circular form, lod through the centre pass a string ; wet it
tborooghly, and place it Bat on a smooth sur&ce ; then try to
pnll it up in a petpendicular line A vacuum ia formed in
the oentrs, while the et^es are pressed down by the weight of
the atmiMphere. In this way, a smooth stone of many pounds
weight may be lifted.

(Ai. 1. Hence the pressure of the atmosphere on or near
the snr&ce of the earth is known ; the weight of any column
of air being eqaal to the weight of the column of mercury, of
the same diameter, snpportel in the barometer. And, since
the height of this column varies with the weight of the atmos-
jhere, ^etween 28 and 3 1 inches, equal to 23 feet of water,)
the Taneliee in the weight of the ahnospbere are known by
file BAROMETEK. The most usual altitude of the barometer,
in Londrai, ii Ivetween 28 and 31 inches, but it is seldom seen
below 28 1-2 or 30 1-3 inches

2. In calm weather, when it is inclined to rain, the mercury
is commonly low. In serene settled weather, the mercury is
generally bigb. During very great winds, though unaccom-
panied with rain, the meroujy sinks lowest of all.

2. The air prtists equally in all directions. '
Exp. 1. If a glass vessel be filled with water, andcovered
with a looee piece of paper, on inverting the glass, the water
will be kept &om &Uing by the upward pressure of the air.

S. If a vessel be perforated in miell holes at the bottom, but
cloMd at the top, the apwud prenure of the air nUl, keei; the

64 nrEiniATic9.

water within tiie v«nel; a» will «ppe(ar by tnocessiTi
ping and anstoppii^ a imall hole in the top of the yeas
drawings beer from an air-tight caik.

3. Two brass hemispherical oaps put cloee togethe:
the air between them it exhausted, will be preraed t

with considerable force.

4. A syringe being fastened to a plate'of lead, and tfa
of the syringe being drawn upwards with one hand, ^
lead is held in the oth^, the air, by its upward preeeu
drive back the syringe upon the piston ; whereas, if th<
syringe be hung in a receiver, and the air being ezl
the syringe and lead will descend ; but upon re-admit
air, they will again be driyen upwards.

5. If a thin ^ass yessel, whose aperture is <dosed, be
under the receiyer of an air-pump, and the air ex
from the receiyer, the vessel will be broken by the pre
air within.

3, The air is an elastic fluid, or capable ofco
Hon or expansion,

Exp. 1. A blown bladder, pressed with the hand,
turn into the form which it had before the pressure.

2. An empty bladder put under a receiver, when th
nal air is exhausted, becomes extended by the elasticit
internal air.

3. A bladder suspended within the receiver, with
weight hanging from it, which touches the bottom, w
external a^r is exhausted, by the expansion of the :
air, will raJse the weight.

4. The bladder being put into a box, and a weight ]
on the lid, the lii^, on exhausting the air, will be lifed i

5. If a tube, closed at one end, be inserted at its oj
in a vessel of water, the fluid in the tube will not rise
level of the water in the vessel, being resisted by th<
force of the ar within the tube. On this principle thi
bell is formed.

6. This bent tube ab cd^ fig. 36, is open

I ends. I have poured mercury into the tul

to rise in both s'des of the tube r and b ; i

^° from c rf, is fiill of air at the common densit]!

U» up rf, so as to make it air tight, and pour n
C into fl, so that the column of mercury a b t
C eq ual in length to the height at which it stan<
barometer at the time. The air in the sho
will now be compressed by the weight of the
phere, and aho with an additional weight of a col

PNBVMATieS.

jmercorj; and the mercury in the shorter leg: ^iU be riflen to
e, and « e is the only half of d e; that is, the pressure of a
doable atmosphere compresses the air to half the space which
it natarally ocoapies. If another equal column of mercury
were added to the length a 6, the air in d e would be reduced
into one fourth the apace that it formerly occupied.

7. a fig. 37, is a stfong copper vessel, having a
tube that screws into the neck of it, so as to be
air-tight, and long enough as nearly to reach the
bottom ; x is the handle of a stop-cock. Having
poured some water into the vessel, and screwed in
the tube/the condensing syringe is to be adapted,
and the air condensed. The stop-cdck is to be
shut, while the syringe is unscrewed, then, on
opening the cock, the air, by its great density
acting upon the water in the vessel, will force it
out in a jet to a considerable height. This is called
the artiiSicial fountain.

4. The elastic spring of the air is equivalent to the
force which compresses it.

Exp. Let the air be exhausted from an open tube, whose
lower part is inserted in a vessel containing a small quantity
of mercury, and let the air within the vessel be prevented
from escaping ; this air, by its elasticity, will force the mer-
cury up the tube nearly to the height to which it would be
raised by the pressure of the atmosphere.

Ohs, If the spring with which the air endeavours to expand
itself when it is compHsssed, were less than the compressing
force, it would yield still farther to that force; if it were
greater, it would not have yielded so far. Therefore, when
any force has compressed the air so that it remains at rest, the
spring of the air rising from its elasticity must be equal to the
pressure.

5. The elasticity of the air is increased by heat.

Exp. 1. To the bottom of a hollow glass ball, let an open
bended tube be affixed. Let the lower part of the bended
tube and part of the ball be filled with mercury ; the exter-
nal smiace will be pressed by the weight of the atmos-
phere ; and the internal surface will be equally pressed by

66 PNEUMATICS.

the spring of the air enclosed within the vevsel. If the btU b
immersea in boiling water, the increased elasticity of the ii
eluded air will raise the mercury in the small tube. I'he sam
may be shewn by immersing in boiling water, a tube, closedi
one end, into which a small quantity of mercury has beeo nd
mitte J, inclosing a portion of air witiiin the tube.

2. The wind is no other than the motion of the air upontfa
surface of the globe. The principal cause of wind, is, thi
the atmosphere becomes heated over one part of the eart
more than over another. For, in this case, the warmer a
being rarefied, becomes specifically lighter than the rest ; it
therefore overpoised by it, and raised upwards, the upper par
of ii diffusing themselves every way over the top of ttie atmo
phere ; wliile the neighbouring inferior air, rushes in from a]
parts at the bottom ; which it continues to do until the" eqa
librium is restored. Upon this principle it is, that most wim
may be accounted for.

3. Fill a large dish with cold water ; into the middle of th
put a water-plate, filled with warm water. The first wi
represent the ocean ; and the other an island, rarefying tli
air above it. Blowout a wax candle, and if the air be stil
on appl3ring it successively to every side of the dish, the smok
will be seen to move towards the plate. Again, if the a'mbiei
water be warmed, and the plate filled with cold water, I
the smoking wick of the candle be held over the plate, and tl
contrary will happen.

6. T/ie pressure of the atmosphere varies at djffi
rent altitudes,

Exp. Put a glass tube, open at both ends, through a cori
into a lar^e phial containing a small quantity of coloured wi
ter ; let the lower end of the tube be in the water, and let tl
cork and tube be closely cemented to the neck of the bottl
Then blow thrcigh the tube, till the^uantity of the air witi
in the phial h ao increased, that the water will rise above H
neck of the phial. Lot this phial be placed in a vessel of aan
to keep the air within of the same temperature ; tlie wat<
will stand at different heights in the tube, according to tl
elevation of tlia place where it is placed ; from whence is a]
pears, that the pressure of the atmosphere varies at differei
altitudes.

Curol. Hence the proportion of the specific gravity of a
to that of water may be determined. If the difference :
height of the two places where tlie above experiment
made be 54 feet, and that difference cause a difference

PNEUMATICS* 67

%^ of an inch in the height of the water ; it follows, that a
eolamll of water of 3*4 of an inch, or 1-1 6th of a foot, is equi-
pon<}erBDtto a column of air of 54 feet, having the same base ;
therefore, the gravity of air to that of water, is 54 to 1-1 6th,
or 864 to 1 . In ascending the mountain of Snowden, in Wales,
which is 3720 feet perpendicular height, it was found, tliat
the barometer sunk 3 inches and 8-10tiis.

PNEUMATIC INSTRUMENTS.
126. The Syringe is a hollow tube with a small ori-
fice at one end : at the other end is inserted a solid
cylinder, so exactly fitted to the tube that no air can
pass along its sides, and a fixed handle to the solid
cylind<Br. If that end of this instrument which has
the smaller orifice, be drawn back, a vacuum will be
produced within the syringe ; and the pressure of the
atmosphere on the surface of the water, meeting' with
no opposite pressure, will force the water into the
tube, from whence it m^y be forcibly expelled by
pushing down the piston.

126. The Condenser is used to force air into any
vessel ; it is a syringe, having a solid piston, and a
valve in the lower part of its barrel which opens
downwards. By thrusting down the piston the air is
forced through the valve, which is afterwards held
close by the elasticity of the condensed air. When
the piston is lifted up, a vacuum is produced, till it is
raised above a small hole in the barrel, when the air
rushes in, and is again dischnrged through the valve.

127. The AiR-GuN, is an instrument, in the form
of a gun, by which a quantity of condensed air is sud-
denly set free, and drives a ball through the barrel
with great force.

128. The Barometer is a very useful instrument
for determining the variations of the weather.

Exp. 1. If a glass tube of about 22 or 33 inches long, herme-
tically sealed at one end, be filled witli mercury, and then in-
verted into a basin of the same fluid, the mercury in the tube
will stand at an altitude above the surface of that in the %ki&\BL

68 pxcuiUTica.

batireenaSBudSliDi^iBL A tab* thai illad,iod^BiWi
from28to31iiMliM,H«d)Bd alMraiiwtar. HM«Ni*ifta
bioiiialiofirttorini^XG3.t8KiminiTi«]r,> aafaiBkA'
ftir wei^Q^W gtwini, andif mM«iit7b*14 timMhWri
thaa water, tb* ipMifiggnvitrof •»!■ taflMtafa««V|
o 886X14—14300.
S. NowttiaaereaiTiiitbebaroButgrtQbewiaHMlDl
, ttascolumnbeaqninlnttothe wM^afthaaHanHlBbl
■' on the 9ari>M of th« mutmff id tM buiB, «id ■ Aw^km
trne criterion to meuure thst weight, and diiaSy Jiianhll
that purpMc, ID order to fiiretal die cbaogai inllM iraattK,
Oil. I. IfeaofainchintfaaHalaof variatiaabediViiWfc
ten eqaal paiti, marked 1, 2, 3,iDcr««iiDg l^pwudi, nfl
wtmier whoae length it 1 1-lOth of an iDCh, be libmiMdli
ded Into ten equal parti, iocreaiiag doirDwardi, and an p^l
ai to elide along- ttiegnidaatedaealB of the baMaMtMv UM|
titudeofihe mercury in the tube, above the inrfcoe oftet
the basm, may be tbmid, io inches and hundredth pari! of
inch, by this tcmier. If the surbce of the nuronrjin *
tube does nrft essctly coincide with a division in tlw eoala
variation, place the index of the sernter even with Ibti N
bee, and obseri ing where a division of the vernier enwl
cniiicidea with one of the scale, the fignre in the rentier m
shew whathundredlli parts of an inch are to be added ta t
tenth immediately be1o« the index.

2. If the atmosphere were homogeneoos, its altibidB WWi
be easily found. For vhen the mercury stood at 39 l-XlNA
the density of the airlieingto that of mercury as 1 to 1938
conteqiienlly, the altitude of an homi^eneous alBUMplM
would be equal to 12390X29 1-2=5.77 mJIea.

3. The barometer has been applied to the m
heigMi of lowen, mounlaini, kc. Birice 12390 inetie«~iir i
near the surface of the earth, are equal to one inch of mat)
rj-, 1939 inches, or about 103 feet of air, must ba equal
l-IOth of an inc'i of mercury. Therefore, if a haromaler
carried up any great eminence, the roerooiy win dam
1-lOth of an inch for every 103 feet that the banoMlar

1 39. The Therhoheteh a an instrument colcal
ed for measuring the temperature of the air, bdcI t
dies, :ind is usually n cylindrical glass tube, cmtanu
spirits of wine, mercury, kc. which fluids are fin
tosirell and occupy different portionB vftbetnbe
different temperatures.

PNBimATJC«« 69

06*. TIm bull!, and part of the tabe it filled with quicksil-
ver ; thb oootracts aad expands as the iostrumeot is exposed
to more or less heat, and coaseqneotly the temperature of
oontif^aoas bodies is shown by its rise and fall in the tabe,
which is measured by a scale .

2. The Thermometer chiefly used in Great Britain, is that
constmoted by Fahrenheit; in which there are 180 divisions
between the freezing point being reckoned 32^ above aero,
or the commencement of the scale ; consequently the boiling
water point is 212^

3. The scale on Reaumur's thermometer, which is prin-
cipally used on the Co*itinent, begins at the freezing point,
and proceeds both ways, frpm to zero. From freezing to
boiling water are 80 degrees.

4. Since the thermometers of Fahrenheit and Reaumur
are thoee mostly in use, it will be often found convenient to
be able readily to convert the degrees on Fahrenheit's scale
into those of Reaumur, and vice versa : and as one degree
on Reaumur's scale is equal to 2.25^, or to 90- 4th of Fah-
renheit; and as the former scale places the freezing point at
zero, and the latter places it as 32 ; the following canons will
reduce the degrees on the one to the corresponding one on
the other.

Obt. To convert the degrees of Fahrenheit into those of Reau-

mar ; F— 32 X 4=R : thus the 167"^ of Fahrenheit answers

9
to the 60^ of Reaumur.

To convert the degrees of Reaumur to those of Fahren-
RX9

heit: |-32=:F.

4
Thus the 40** of Reaumur answers to the 122® of Fahren-
heit.

5. Mr. WeJgewootl contrived a thermometer for measur-
ing higher degrees of heat, by means of a property of argilla-
ceous bodies, viz. the diminution of their bulk by fire. This
diminution commences in a dull red hcnt, and proceeds regu-
larly as the heat increases, till the clay becomes vitrified.

Each degree of VVedgewood's thermometer answers to 130
degrees of Fahrenheit; and he begins his scale from 1077 o
Fahrenheit.

70 QUESTIONS ON PNEUMATICS.

DegfUMof Degnm^ i
Fahrenheit. IVtdgmml
Cast iron melts - - 21877 160

Fine gold melts - - 5237 SB

Fine silver melts - - 4717 28

Mbrcurt boils - - 600

Cow's milk boils - - 213

Water boils - - 212

Heat of the human body - 92 to 90

Watkr freezes and snow melts 32

Milk freezes > - 30

Strong wine fireezes - 26

Mercury freezes - — 39 or 40.

130. The HYGROMETER is an instrament fbrinea'
suring the degrees of moisture in the air ; of whidi
there are various kinds ; for whatever contracts aad
expands by the moisture and dryness of the atmoiK
phere, is capable of being formed into a hygrometer.
Such are most kinds of wood ; catgut, twisted cord;
the beard of wild oats ; the weather house, &c.

QUESTIONS ON PNEUMATICS.

What is the object of the science of pneumatics?

W hat is the aimnsphere ?

To what height does the atmosphere extend ?

How may the existence of the air be ascertained f

How much lighter is air than water?

How is the pressure of the air shown ?

How is it known that the pressure of the atmosphere de-
creases upwards ?

W hat is said concerning the elasticity, or expanding iiatine
of air ? '

How is tliis illustrated?

What is wind ?

What is an air-pump?

How is it demonstrated that the air has weight?

What is said concerning the variation of the mercury in the
barometer tube?

What experiments show that the air presses equally in all
directions ?

How is it proved tliat the air is elastic ? mention the esqpt-
rjments.

What is fatd of the Mm 6f the elastic spring of air ?

V^hat experiments-fll^kistrate that the elasticity of air is
increafled by heat? ^'

What simple experiment proves that the pressure of the at-
mosphere Taries at different altitudes .'

What is a syrinf^e ?

Describe its action.

Describe the eondemer.

How is the air-gun constructed?

lYhat is the use of the barometer ?

How is it constructed ?

What is the rule for measuring altitudes with the barometer ;

What is the thermometer ?

On what principle does it show the temperature of air, aqtl
of other bodies ?

What thermometers are chiefly used in England and in thi.*^
country?

How is it graduated ?

What are the freezing and boiling points ?

What is the hygrometer ?

Of what substance may it be constructed ?

ACOUSTICS.

131. Acoustics is the science which treats of the
nature, pheQomeDa, and laws, of the sense of sound.
It extends to the theory of musical concord and harmo-
ny, and is therefore, a valuable and interesting science.

132. Sound is considered as arising from vibrations
in the air, communicated to it by vibrations of the
sounding body, acting in pulsations or concentric
waves, like the surface of water when a stone is
thrown into it.

Obt. If when a pie^^e of artillery is fired at a distance, some
dust floating in the air, or a cobweb be closely inspected, it will
be seen to be agitated at the instant when the report is heard.
This poves that the vibrations of the air travel with the same
velocity that sound does, and that it is by means of these .vi*
brations striking on the ear-drum that sounds are conveyed.

rd3. A sonofrous body, whilst soundins, is unques-
tionably in a state of vibration, and the air, by simifar

72 ACOUSTICS.

Tibratiohs, communicates and propagates these ^bn*
tions.

134. The chief causes of the variety of sounds, are,

Firsif the greater or less frequency of the vibration.
Secondii/t the quantity or force of the vibrating^ materials.
And, Jiiirdly^ the g;reater or less simplicity of the sounds,

Hence arise the height, the strength, and the modi-
fication of sounds.

Oh$. When sounds are equally acute, they are said toba?(
the same pitch ; but when they differ in acuteness, thatsoaod
which is shriller is said to be acute, or to have a higher pitch;
and that which is less shrill, is said to be ^aver, or to haTei
lower pitch, or a deeper tone. A difference in pitch, form
the chief character by which musical sounds are duting^uis^s^
from each other, and is the foundation of their use in music

135. The vibrations of a sounding body, continw

for a longer or shorter time, according as the hody k

more or less elastic, or as it is thicker or thinner.
^^ ^ iJrram.l. When a string of unilbn

shape and quality is stretched be
I < ween, and fixed to, two steady pint
as a 6, fig:. 38, if it' be drawn out a
O its natural or quiescent position a ft.

into the sitnation a c 6, and then be let go, it will in consfr
qu6t)ce of it5i elasticity, not only come back to its positioi
a b, but it will go beyond it, to the situation adb, or nearl]
as far from a 6, as a c 6 was on the other side. All the motioi
one way, is called one vibration : after this, the string wil'
go again nearly as far as c, making a second vibration; thei
nearly as far is d, making a third vibration, and so on ; di'
minisbing the extent of its vibrations gradually, until it set
ties again in its original position a b,

Obs. 1. During the whole of these vibrations, the striof
will forcibly acton the air and produce corresponding vibra
tions in it, which, reaching and entering the ear, produce oi
the nerves therein, the sense of sound.

2. The following experiment indicates a curious accord-
ance of vibration, and proves that the air re-acts in the exad
law of the origiual vibration.

^0 Exp, If you divide a string as a 6, fig

g 3 39, into three equal parts, a6, 6c, e^ ^

, J"]!! Wiu^ M 1 placing dots at e and b ; place a br^

like a vioUn brid^^Q^ ^\. b, t^a^ "^Vittf

light bodies, i\lc\i aa «m-aX\\^SXA ^\«\m

■*■.

AGOUSTI^S^ 73

at C} and other plaees of the pnrt b d. Then draw a violin bow
over the part ab; you will find that all the bits of paper will
be thrown off from the part 6 d, excepting; the opo at c; shew-^
ingf that the point e remains at rest^ whilst the remainder qf
the string is vibrating, jast as thoagh c also had a stop, as at h.

136. Sodods in general are conveyed to the ear
by means of the air : but water is also a good con-
ductor of sound ; as are timber and flannel.

Evp. 1. A bell rung under water returns a tone as distil^
as if rung in air.

2. If you stop one ear with a finger, and the other by press-
ing it close to along stick or piece of deal board, and a watch
be held at the other end of the wood, the ticking will be bear(},
be the stick or board ever so long.

3. If you tie a poker or any piece of metal on the middle
of a strap of flannel, about two or three feet long, and then
press with the thumbs or fingers the ends of the fiannel in your
ears, while you swing the poker against an iron or steel fender,
you will hear a sound like that ofa very heavy chorch bell.

4. If two persons stop their ears, they may converse with
each other, by holding the two ends ofa stick between their
teeth, or, only resting the ends of the stick against their teeth.
The same may be done by a series of sticks, with the ends
touching each other. The same efiect is also produced if the
end of the stick rest on the throat, or breast, or if one end of
it touch a vessel into which the other speaks. In the last in-
stance the sound is most distinct if the vessel is capable of a
tremulous motion, as one of glass, bell metal, or copper.

Sound may also be conveyed from one person ^another by
a string stretched between their teeth.

137. Sound travels at the rate of 1142 feet in a
second, or about 13 miles in a minute. This is the
case witb all kinds' of sounds: the softest whisper
flies as fast as the loudest thunder.

Obt. The velocity of sound has been applied to the measure-
ment of distances.

1. A ship at sea in distress fires a gun, the light of which is
seen on nhore 20 seconds before the report is heard, therefore
it is knowb to be at the distance Of 20 times 1142 feet, or little
more than 4 1-3 miles.

2. I see a vivid flash of lightning, and if in three seconds I
hearatremendoai clap of thunderi I imlMitly kifiiow that the

74 ACOUSTICS.

thunder cloud is only two-tiiiitis of a mile distant, I aho
ttiereiboe retire instantly firom any exposed situation.

3. The pulse of a healthy person beats iabout 76 times i
minute ; it, therefore, between the flasli of lightning^ and
thunder I can fed 1, 3, 3, 4, &o. beats of my pulw, I la
the oloud is 900, 1800, 3T00, 36,000 feet from me.

138. Sound, like light, afler it has been reflec
from sereral plnces, may he collected into one pi
as a focus, where it will he more audible than in i
other part ; and on this principle whispering g
LERiES are constructed.

Ob$. In the reflection of sound as well as of light, the ang]
reflection is equal to the angle of incidence. By the same-
therefore, sound may be coUected into a focus.

Exp. 1. If the pulses of air conveying sound be rufferet
impinge on a oonoave surface, the reflected vibrations
converged into a focus.

2. The same efiiect is produced whenever a number of p
surfaces are so situated that the reflected sounds meet,
cross each other at a certain point. If the ear be placec
this point, the sound will be audible in proportion to the ni
ber of sur&ces so placed. The famous whispering galler
St. Paulas is on this principle.

139. Speaking Trumpets, and those made to as
the hearing of deaf persons, depend on the reflect
of sound from the sides of the trumpet, and also by
heing confined and prevented from spreading in ev
direction.

Obs. 1. A speaking trumpet, to have its full effect, mus
directed in a line towards the hearer ; the report of a gu)
cannon is much louder when fired towards a person, than
placed in a contrary direction.

S. The human voice is produced by the expulsion of
firom the lungs, and by the vibrations excited in that air, \
rery small membrane called the glottis^ in its passage thro
the trachea or windpipe ; and by the subtle mo£ficatio
the mouth, tongue, and lipi.

3. Singing is performed by a very delicate enlarg«men
contraction of the |;lotti8, aided likewise by the mouth
tongue for articulation.

4. In stringed initnimentstlie air is struck by the strin|^,
the Til>r«Uoi9 of tin air produce corresponding aounds n

ACOUSTICS. 75

■» epjT ; bttft ia pipes the air is forced ufainst the sides Vy th^
breath, and its vibrations or tones produced by the re-action of
the sides.

140. An ECHO is the reflectioQ of sound striking
against a surface fitted for^he purpose, as the side of
a house, a brick wall, hill, &c. and returning back

' again to the ear, at distinct intervals of time.

Oht, t . If a person stand about 65 or 70 feet from such a sur«
lace, and perpendicularly to it, and speak, the sound will strike
agaiimt the wall and be reflected back, so that he will hear it
as it goes to the wall, and ag;ain on its return.

2. If a bell situated in the same way be struck, and an ols
server stand between the bell and the reflecting surfiuse, he
will hear the sound goii^ to the wall, and again on its return.

3. If the sound stnke the wall obliquely, it will go off* ob«>
liquely, so that a person who stands in a direct line between

I the bell and the wall, will not hear the echo.

141. Concord is any succession of sounds that ex-
cite in the ear certain agreeable sensations. Sound is
therefore the subject matter of musical science. Har-
mony is thQ coincidence of two or more sounds, which
by their union afford to the mind pleasure and delight.

06t 1. Concord arises from the agreement of the vibrations
of two sonorous bodies ; so that some of tl^e vibrations of each
strike upon the ear at the same instant.

Thus if the vibrations of two strings are perAirmed in equal
times, the same tone is produced by both, and they are said to
be in unwun. If the vibrations strike the ear at different times,
there iy no unison, and consequently a dUeord is produced.

Obs. 2. Concord is not conftned to unison. In this case no
variety of tones would be produced. It is the effect of agret'
meni between vibrations.

lUuf, If the vibrations of one strin^f are double those of ao-
other in the same time, the second vibration of the latter will
strike upon the ear at the same instant with the first vibraticm
of the fi^rmer : this makes tlie concord of an octave.

142. Two strings of equal length, tension and thick'
ness,by performing their Fibrai ions together,will sound
the same note, or be iu unison. Two pip^s of the same
length and diameter will ugree in the same manner.
Large instruments & long strings produce grave or dee^

dCESTIOSS OK MIOUSTICS.

■51

tittU: small ingtruments and short strings produce
tarts and bigh tones.

tDte la Ihe case r>l the striogj, the air is slrnck bj flit
feo^Vad the Boiiiid is oxciled by the rihratiaoa : in that of
Uw pipes tbe body ii >'.rurl( by the air, but as action ani) ft-
]|4tiMi are equal, the eflect la tbe same.

S. Let a musical atring of any len'lli be divided into
tVO •q'lal part5 hy a briilgs ia the middle j and tbe sound of
NWh bairis eigbt oates, or ao octaye, higlier than the Lone of
fka Wboln itring'.

' Onau pipe; produce grave or acute tonei in proportion la
lb*i{l«ng(haDiI size. It is the ihortetl stripg ofa harpaichard
itUoi yielda the highest Datei.

1*3, Sounds maybe conveyed to a much greater
dbtnce through a continuous tube, than through the
6peD air.

lUut. Pipea are aMdin t&rerii«,raDnii>sfraiBaiiaea«wilO
UWtlierto conve^ordenta tbe aervaoti.

Dr. HereQhel employ a aimilar tube attaohad tpU^fatj
net teleaoope tor oommuaieatinj; hit obaerratioDt to 'in Mifit*
ant who aita in a amall faouie near the inatrDmonf ; 'tiolt ttlU
mder cover note*' tbtn down the ptrtieular tilW at WW
tlwywere made.

Oil, The tubal oied to eoDTey aowkli ira oaDad mmMI*
tiobet. ' .

It ii by aeaiu of inch tubes that the deotpUon of Tbtt k
called the invitibU lady ia carried on. la thii azhibttini a
iqaare railing at wood ii fiied in the middle of the rooM, aad
within (he railiDg a globe is fixed, having bar trumpati Imart-
ediotoilfOne oppoaita to each lideoftba raflinf. Thaipaat|'
tun aik a queition b; apeak^ag into one of Um troHpata^ tmi
iben on holdinfr the ear to tbe aaae trumpat, tliMf rtlMia
tlieanawer. This deoaptlon ia perbrmed bj oeaMgii|1ta
tonnd by tubea which are carried IVoin ooe rooai la Smit
vader the flooring, and witfainlhabatof tba rafllnCloK^iil
aperture opposite U> the aiouth of tbe tninpat, Wbai^Ma.
qoeition la acbed it ii wmreyed bjtUi plpa to a..p«aaA'
placed in the next room, and the replj it oonTt^radbaAtVflH
trumpet by the nidetaba. <' ;, :, ' ■

QUESTIONS ON ACOUSTICS. V -.

OfwIuitdoestheiaieBeeofaeatiffto (Mat? ' '' '■' ■ '' i^

1 — •* n^iiiiiii niiilliiiii lilinni iiliiJalir ; |ii

How ii Ihii praT«tf t ■ ''.- ? ,

OP OPTICS. 77

What tMthie principal causes of the varieties of sounds?

"What is said aboat the pitch of a sound f

On what 'does the continuance of vibrations depend, for a
longer or shorter time ?

What other substances besides air conduct sound ?

Give illustrations of the'power of liquids and solids to con-
duct sounds.

AtPwhat rate does sound travel through the air ?

What is said about measuring distances by the velocity of
sound P

On what principle are whispering galleries constructed?

What is said of plain and concave sur&ces in converging
sound into a focus ?

On what principle are speaking irump$ts constructed ?

How is the human voice produced f

What parts are concerned in the modulation of the voice'
in singing?
\ How does the production of sound by pipes differ from that
I by strings ?

What is an Echo ?

How is this accounted for ?

What is said concerniug the reflection of light and sound fol*
lowing the same laws ?

What is concori^.^

From what does concord arise ?

When will two strings, or two pipes be in unison ?

How may sounds be conveyed to a greater distance than^
through the open air ?

What is said of Dr. Herschel ; the invisible lady^ &c. ?

OF OPTICS ;

THE LAWS OP LIGHT AJ^D FISIOJ^.

144. Light consists, either o£ small particles ema-
nating from aluminous body, or of vibrations excited
by combustion in an universal medium, which, pro-
ceeding to the eye, produce the perception of vision.

Obt, 1. It u evident we cannot see any object by willing or
wishing to see it; but that something must pass £rom the ob*

eOect can take place vithaal

_. Buppoted rBysoClight to consiil of e:

ceedingly small p»rticU>, iDflailel; Etnailer tkau noi, morii
from lutniuoiis boUies; but the modern? suppose thein to co
EL!t of the aoJolatioDs of au elastic meJium, which fills i
(pace, anJ which produces the sensation pf light to the fj
jii9i as the TJbratioDi of air produce the sen aalion of sound
the ear We shaU not prefer either hypothesis in tbe
pages, bpcauee either of them will accouot eqnall; for alltl
pti«iocMiiatfU|U> -

. 14i^ A Rttjfi or fmuS Iff LiglU,u my 9
wnaU porUoD of tbpit which comes fnta a 1
body. A Btam oflii^t, ia h bodj of ponOel tiji {
fMKtfofn7k4>«bodyofcUi;ergiiigor<Knif«igUi^ng

146. Any body which is tFsnflpareiit, or wbielt i
fords a ready pasHge for li^t, is called a transpare
Medium, as air, glass, water, &c. Bodies wbkh <
not allow the passage of light through them, ■:
called O^Mi^iM, as stone, wood, &c.

147. Raysofbght which coming from a point, CO
tiuually separating as they proceed, are caQe^ JQ*iMr
in^ Rays, Rays which tend to a common point ■
called Converging Rayi. When the lines in whi
they move are parallel, they are called parallel nf]

148. The pointfrom which dirergiDgraysprocei
is called the radiiuU point ; that to which conTei
ing rays are directed is called the foew. .Anv
li^t, bentfroni a straight course in the same meAw
is said to be inJUcttd ; when turned back on the mifi
of a body, it is said to be reflected; and, when titn
out of ite course in passing out of one nedinm it
another, it is said to be refracted,

149. Everyrisiblebodyemits particlesofligU^
reflects vibrstionB from its sur&ce in all direiAMN
which, passing without any obstruction, more witl
the same medium in right Unes.

06t, WhareTeraspeol>tori>plao«dinr«qwcttotlM.la
nous body, erer; point of that partof tbesniAoa wUife
tnnud towards him ii vinbla to bim i the pirtidM of tOi

OF OPTICS. 79

lions of lif^t are, tberafore, emitted in all direotiont, and
those raya 6nly are intercepted in their paatage by an inter-
posed o^ect, which would be intercepted upon the lappoei-
tion that the rays move in right lines.

Exp. 1. Let a portion of a beam of lig;ht be intercepted by
any body ; the shadow of that body will be bonnded by right
lines passing from the luminoas body, and meeting the lines
which terminate the opaqae body.

2. A ray of light, i»ssing through a small orifice into a
dark room, proceeds in a straight line.

3. Rays will not pass throu^ a bended tube.

4. Rays of light may, therefore, be properly represented
by right lines.

150. Rays or vibrations of light move with great
velocity ; for the flash of a gun, fired at a considera-
ble distance, is seen some time before the report is
heard. The clap of thunder is not heard till some
time after the lightning has been seen.

06t. This proposition is proved by observation made on the
satellites of the planet Jupiter, and od the aberration of the
rays of light from the fixed stars, from whence it will be seen,
that the velocity is at the rate of 200,000 miles in one second
of time.

151. The particles of light must be exceedingly
small, if they are particles ; or the force of the vibra-
tions most be very delicate ; otherwise their velocity
wonld render their momentum too great to be endured
by the eye without pain.

Exp. 1. If a candle be lighted, and there be no obstacle
to obstruct the progress of its rays, it will fill all the space
within two miles every way before it has lost the least sensi-
ble part of its substance.

2. Rays of light will pass without confusion through a
small puncture in a piece of paper, firom several candles in
a line parallel to the paper, and Ibrm distinct images on a
sheet of paste-board placed behind the paper.

152. The quantities of light, received from a lu-
minons body upon a given snr&ce, are inversely as
the squares of the distances of the surface from the
himiiioii0 body.

to OF BEFJlACTIOtf,

Obi. The direct Ughtoftbe sua iacakulatect to be eq
t)iB.t ofg&GOcaiidlu plnceJ at the dUluiceofonc foot &>
•bject : and lh«l ofthe mo™ to the lighl of one candle w
feet dutance, of J Upiter at 1 6'20 feet, and of Venue at 4'^

Exp. The light, paanng from a candle or any lui
bodA wiUdiva^e bi it proeeeda, and will illiuDiiute si:
lAiah fdriBM «ai bt to mA ottw H the wjiuiW of thi
tinfiiT frill Hw rrniill- nutif attha- dMHiMvf «■
-" U)dleilliuiaKlwa^M>rfkM,atd< "'

oTtbaduUDM*, l,S,«nd3. - .

153. If rays proceed tromat^^ntwuntflf.
Unite dubuKe. their divergeoc]' is so tnung, tlu
' may b^ considered as parallel. ..

Obi. 1. Hence fix the nja nhicb could come from U
tre, or anj other given point, of tbe sun's surfiu:a, are co
•d as parallel at the umnenae distance of the earth.

S. To rnideraland the natare of the conrergencjr, dil
C7, and paialleliam of rayi of light, see fig;, 40. A 1)1
C dirtrgrt nya ofli^t towards x. They are said to 'm
when coDiidered ai floiring from x towardi C. Aid
paralld ai flowing from x towards a and b. C is the /
the einuKTgingn.ja; and the imaginary focDi of tbe di'
raya.

154. When rays of lig^t pass obliqueli/ oat 4
traospareat medium into another, which is eithfe
debseior rabrerare, they are bent oat of their i
course, and they ace then said to be EEmiCTBt

I6fi. Rays of light are always refracted ton
perpendicular to tbe surface in a denier medivo
Ibis refraction la, more or legB,in proportion as d
^, more or less, obliquely on the refraotjog 8ti

OF OPTICS. 81

Expj, Let B C« % 41« b« supposed to be a ray -of light pass-
ii^ out of air into water or g^lass, L G, at the point C ; K K
is a line drawn perpendicular to L G.,and the ray B C, instead
of proceeding along C H wHl in so passing, be bent towards
the perpendicular C as long as C 1.

156. On the contrary, when light passes out of a
denser into a rarer mediom, it moves in a directiod
farther from the perpendicular.

Exp. 1. Thus if the ray C I, fig. 41, pass out of the glass in-
to air, it will not proceed in C x but in the direction C B, fur-
ther from F C than C.

2. Takeapan A B DC, fig. 42, with an upright side, into a
dark room ; let in, by means of a small hole in a window shut-
ter, a ray of Ugfat G B, so as to fall upon the bottom of the pan
at E ; mark the spot E ; then without moving the pan, fill it
with water, and the ray now will not pass on to £, bat will be
refracted down to F. The candle G will answer as weU as
the direct rays of the sun.

3. Ifa shilling be stuck on the part F with wax, so that an
eye at G cannot see it when the pan is empty, it will become
visible the moment the pan is filled with water.

4. Take a glass goblet half full of water and put a shilling
into it, then pat a saucer or plate upon it, and holding it tight
on, turn plate and glass together ; a by-stander unacquainted
with the laws of refraction, will suppose that he sees a shilling
and a half crown ; the one is seen by refraction through the
water, the other by the rays after refraction at the surface.

167. The Angle of Incidence^ is that which is con-
tained between the Lne described by the incident ray,
and a line perpendicular to the surface on which the
ray strikes, raised from the point of incidence. Thus
in fig. 41, the angle B C K is the angle of incidence.

158. The«^fi^/e of Reflection, is that which is con-
tained between the line described by the reflected ray
and a line drawn perpendicularly to the reflecting sur-
face at the point in which the ray passes through that
surface. Thus, in fig. 41, £ C K is the angle of re-
flection.

1 69. Availing themselves of the principle of refrac-
tion, philosophers have so contrived surfaces, that the
perpendiculars to them constantly Nttrs^^aftL^^^^^^^-

8t on tmtif . '.^

new and ivfortant effects. Vhig they liire'dti
means of conrez nd conciiFeg^assiMtM, so ai(1
lect or dUsperse tbe mi of l^^t which pass tbvc

160* ThereererariooskiiKis. of lenses im
eording to their Arms.

4S ^ W g^ W' \ Apbifio-eoffoesbNtl

.side flat, and the odM
Vex,'as A, fig. 49. .

ApUmo-wneave is flat on one side^ and bo&ci
the otheis as B, fig. 43.

A doubUretmvex is cOnrex on hoth sides,es 6,1

A cfoifMe-coficapebcQncaTe on both sids^asJO;

A memicui is conFei on one side, aad cooci
die other, as E, fig. 48. .<

The axti of a lens, is a line passing Huroai
centre t thas F G is the axis to all the five leis

161. If parallel rays fall upon a plano-conrc
they will be so refracted as to unite in 9^ point I
called the principal focos, or focus of paniUe

Exam. Thas the parallel rays a 6, fig, 40, fidling v
lent are refracted towarcU the perpendicular C x^ and
a focus at C.

162. The distance from the
.of the glass to the focus, is cal
[ ^oecd distance ; which focal d
:iD a plano-conyex lens is eq^is
"diameter of the sphere of Wh
lens is a portion, fig. 40,and t|
distance of a double-convex lens, is equal to tb
us of a sphere of which the lens is aportion^ 1
1 63. All the parallel rays of the sun wUc
through a convei glass as D£, are collected ii
cus / and the force of the heat at the focus il
common heat of the sun, as the area of the gla
the area of the focus.

' ■ ' OF omc>. 83

td%u, Ifs lens four incbM in diamttcreollect the nin'anTi
into ■. biea» nt the diiUnce of twi-lve inchei, the inuge wiU
not be more than ane-tenth of an inch in diameter, the lurfiMC
of this little circle ii 1600 times lea than the lor&oe of the
leiu, sod coiueqiieatl^ the heat will be 1600 timea greater at
the focus than at the leru.

Car. I ■ Hoioe the conatraction of comnlon bunung-glatieit
which are all double convex leruei.

3. Hence the rearan why furniture has bem set eo fire by
leBTiDg a globul&r decanter of water incantiotuly expoMd to
the rays •! the lun, which acta as a double convex lens.

164. If another double convex F 6, fig. 44, be
placed in the rajs at the same diglaoce from the {or.m,
it will BO refract the rays back again, that Ibejr mil
go out of it parallel to one another.

lUui. It ii evident that all the rays, esoept the middle one,
(TOM each ether in the focus/, of course the ray DA, which ii
uppermost in going in, ie the lowest in going ont, oi G e.

Exp.t. If acandle be placed at/, the diyetging rays between
F G, will, upon going out of the lens, become parallel at d e.
3. If a candle be placed nearer the glass than the fbcni, the
rays will dnergt, after going through 5ie lens.

3. If the caniUe be placal larther from the glan than the
(jcus, the rays will cimvtrge, after passing throngh the gtasi,
and meet in a point which will be more or leu distuit from
th« glass, tt the caOdle is neafer to, or farther from, its focus.
4i When the rays meet, they will form an inverted imaga of
the flame oflhe candle. Suppose B.acamUe, fig. 46, and C a
convex Inu, then on a dark ikisen D, the image A of the can-
lUe will be produced, and will be reversed, Iwouse the r«y«
cnM each other in passing through the lens.

: lfaiiobjectABC,fig.4
' lie pluwd beyond the focui F
I Ihe filBSsd f/, Bomeof then
i which Hair from every point
1 the object on the side neii I
1 '^last, will TbII Upon it. and afl
I passing tiirotigh il they will
_ l^converged into as meny poii
on Ihe oppoEite aide of (he gliis, where the image of the whi
will be formed, which wiJI be inverted, Thoa the rays 80"
ing from A, at A d, A t. A /, will converge in the space d t
and by meeting in a will there form the inukge of the point 1
nnd 50 of Ihoae rays Howing from B and C,Biid of course of 1
the intermediate parts.

6. If the object A B Che brought nearer the glsM, the pi
ture a 6 c will be removed to a greater distance from it.

7. "nn picture Will be bi much lai^er or le« thui Qm c
ject, u iti distance from the glav a greater or l««i thia t
dialanee of the object

166. When Dorallel rays pass throogh a donb
concaje lens, tney will divei^e after passiDg throu)
the guss, as if they bad come from a pointin the c«
tre of the concavity: of Ute glam.

Exam. If tbe rayi ait, fto. flg, 4
Z put throng^ A B, and C Im tba onI
T of concfmty, then the ray a aAw pn
ring the glass, will go on id thedinttt

uifithadct

^frnmctndM^

■ in the fray: therajftwillgDHrfafl
direction m n, and la on.

166. When paTBtlel rayspass throughaplriiiotca
cave leoB, they diverge after passing tibroiigh it, W
they had come from a point at the distance ofairiwi
diameter of the gjany concsTity.

167. The/ollowing are the principal p
of rays in coimenon with various leuea ; ■

Oit. I. Throogh aeaNMc turfatt, puling ont «r»nMr W
a danser madinm, jwralbl r<y will bewane co P TwAg. .

OF OPTICS. 85

CofiwfV^ mjft towards the centre of comradtj, will suf-
fer no revaetion.

Converging rays to a point beyond the centre of conyezity,
will be made more converging.

Coneerging ragft towards a point nearer the sor&ce than the
centre of ccHiyezity, will be made less converging by refrac-
tion.

But when the rays proceed out of a denser into a rarer me-
dium, the reverse occurs in each case.

% When rays proceed out of a rarer into a denser medium,
through a cohoavb surface h£ parallel before refraction, they
are miade to diverge.

If they are dieergent, they are made to diverge more, to suf-
fer no refraction, or to diverge less, according as they proceed
from some point beyond the centre, from the centre, or from
some point between the centre and the sur&ce.

If they are convergent, they are either made less converging,
parallel, or diverging, according to their degree of convei|^noy
before refraction :

And the reverse., in passing out of a denser into a rarer me-
dium.

Exp, Most of the preceding propositions may be confirmedf
in a room from which all external light is excluded, by placing
a convex lens, or concave, fixed in a frame which moves per-
pendicularly upon an oblong bar of wood, or table, at different
distaneee from a lighted candle placed perpendicularly on the
same bar of wood, and receiving the images upon white paper.
Upon this bar of wood, on one side of a line over which the
convex lens is placed, let a line perpendicular to the last men-
tioned line be divided into parts, 1, 2, 3, 4, &c. each equal to
the distance of the focus of parallel rays; and on the other side
of the lens, let a line be divided in the same manner, and let
the first division, which is farther from the lens than the focus,
be subdivided into parts respectively, equal to 1-2, 1-3, 1-4*
te. of the distance of the focus of parallel rays : if a can^e be
placed over the division 2, it will form a cQstinet image on a
paper held over the division 1-2 : if a candle be over 3, the
image will be at 1-3, &c. whence it appears, that the distances
of the correspondent foci vary reciprocally ; or by holding
alarge double convex lens, or burning glass, in the sun's rays,
and receiving the image on white paper, or other substance at
different distenoes.

168. When rays of light strike against a smooth sur-
fac^i bdA are sent back from it, they are said to be Rfi-

■^'1 |l ^W f ^

M ' ' OP MrtBCTiov.

iTLKCTEo^ mi the j^y that comes from anj
body, andiUb iiqpoa the reflecting siu^Gice,
OieiiUcideiUraw*

IQmm. If L O, ilf • 4ti*hMi vafleetiiigr tnrfiifie, i
^u^ tiM B C i» tlM intitat ray, and C £ M the n

159. The angU of inxidenct is that whii
tsfaied between the iiicident ray, and jst per]
to tiie^reflectiog surfiice in the point of re<
B C Ki fig. 41. The omgU of rtjkditm is
tained beUreen the perpendicular and the
ray, as K C ^, fig. 41. And the angle of
and reflectipn are always eqoal.

^ 06it. 1* Let the lines C A and
be (]b«wn, which are perpeadic
cpDcaTe tar&ce a e, and it w
that the angle of incidence 4 «
to the angle of reflection Cam
• u equal ioCem.

y 2. Sir Isaac Newton explains the cause of n
supposing^, that li^ht, in its passage from the lun^n
diwpr>ff^ to be alternatelj reflected by, and transmit
any refracting surfeioe it may meet with ; that tl
tionS|^ which he calls fits of easy reflection and ea
lion, return successively at equal intervals ; and tl
communicated to it, at it* first emission out of tl
body It proceeds firom, probably by some very sub
tie substence diffused through the universe, in t£
manner : As bodies falling into water, or passing:
air, cause undulations in each, so the rays of light
Tibrations in this elastic substance. The quicki
Tibrations depending on the elasticity of the medi
quickness of the vibrations in the air, i^hich props
depend solely on the elasticity of the air, and n
quickness of those in the sounding body,) the m
particles of it may be quicker than that of the
flierefore, when a ray, at the instant it impinge
aur&ce, is in that part of a vibration which is cc
aotio», it may be reflected. He farther suppose
light fidls upon the first surface of a body, none
thera, but all that happens to it there^is, that eveir
aot in a fit of easy transmission, is there put into
trbm they come at the other side, (lor this elasti
'wily penrading the pores of bodies issapaU^ofl

OF OPTICS. 87

brations within the body as without it) the rttyi of one kind
shall be In a fit of easy transmission, ai>d those of another in
a fit of easy reflection, according; to the thickness of the body,
the intervals of the fits being different in rays of a different kind.
3. This doctrine of fits does not accord with the g;eneral
simplicity of nature, or of the other parts of this great man's
philosophy. It seems &r more reasonable to consider the
phenomena of light, as being produced by vibrations of an
universal medium, the intervals between the vibrations cor-
responding with the fits above supposed. Such ideas of vibra-
tions correspond too, with the general analogies of nature in
other particulars.

170. The following are the chief phenomena of

reflected rays :

1. Parallel rays reflected from a coircAVJB sor&ce, ar«
made converg^ing.

Converging rays falling upon a amcaxt surface, are made to
converge more.

Diverging rays falling upon a concave surface, if they ^
verge fi^m a focus of parallel rays, become parallel.

If from a point nearer to the surface than that focus, they
diverge less than before reflection.

If from a point between that focus and the centre, they
converge alter reflection to some point, on the contrary side
of the centre, and farther from the centre than the point from
which it diverged.

If from a point beyond the centre, the reflected rays will
convolve to a point on the contrary side, but nearer to it than
the point from which they diverged.

If from the centre, they will be reflected thither again.

Exp. Place a concave mirror at proper distances from an
open orifice, or a convex, or a concave lens, through which a
beam of solar rays passes, and verify the preceding propositions,

2. Parallel rays reflected from a cobvex surface, are
made diverging.

Divertj^mg rays reflected from a convex surface are made
more diverging.

CJonvergins^ rays reflected from a convex surface, if they
tend towards the focus of parallel rays, will become parallel.

If to a point nearer the surface thin that focus, will con-
volve less than before reflection.

If to a point between that focus and the centre, will diverge
as fitmi a point on the contrary side of the centre fiirther fircm
it than the point towards which they converged.

lifto a point beyond the centre, tkeywW\^^«^^^^so^^

«M«tlMiiiMU!eHtfwthDb«ak. IiidtlNB,«MlmMk
the oyalwTC M UMe eoanzl^t OMt Am fbad ptU^W

thereliMa; ^rtwiBe,iiiilf ttwot|}ao>iT— Wtdto k^
£ttK>sBlhiiBttB«T>t thavJiidtwailMindMiMt

176. When the dianeter of ao object n^Mi,
^^wreat diamBter to tbe eye, ia ioreiielj m ito i
taoee inm the eye, and the appannt diamter in
ngg^e which it aabtenda ta4hee]re; aoAatita^
redt sixe U ai tbe ao^e which it lobtends to the f
Obt. 1. llia'uiglenbUDcMuUMltMtvmUtalifMlit
MbjthaWritsr«nopt>a,flieM»iiMUM«MUf,««Baatbe«
nttij uoertBined, u it dependi apoti the odma cf lb* iri(
•od Uie gtDtmd npao ^rbiek it ii mmd ; H dnwMh •!■> ■
tbe eTe. Mr. Harm tbinki tha leaat angle lor ai^ (it|f*a
be about 40aeci>ndB; and at a medium, not lets than two
Dutei. To the generality of efei, the neareat cbalaoce td
tinct riuoD, is about 7 or eight inchea. Tajdng 3 inctMi
that diatanoe, and 3 minutes Tor the letut vinble angle, ai
bular otiject of leia than the three-hundredth part of an i
cannot be leen.

2. The apparent diameter of an object ia ai the Oiamate
its image upon the retina ; and the diameter of the tnu
when the ol^ect is given, is inoeruti/ as the distaaoe of th*
ject ; therefore the appamit diameter of the object ia abo
etnc^ at the distance of the objeet. The same mtf be prtt
of an; apparent length whatsoever.

3. Hence the apparent diameter of an object may be m
nified in an; proportion; for the leii the diatance <^ tb« i
from the object, the greater willbeits appareot diameter. ]
without the help of gluscB, an object brought neaMT tb* <
than about five inches, though it appean laiger, wiH at
«ame time appear oonfutedlf .

4.* ManjaeceitionsiDTiaioa arise from the abora oodikU
tioii. We Jodge of the diilartee of anj object bj tha *ira
Itt^th o< the pUne, which lies between the eye andtbabltf)
When this inethad fails as, we compare tha known nugnitl
of the object with its present apparent magnitodai or
•ompere the degrees of diatinctnesi with which W* ••• I
Mveral parts <rf' an object ; or weoburvewhethfraM dwi
of the apparent place of an object when viewed friMi •SAr
atatioDt, or ita paraUaa, be great or imall, thii cbfUg* be
■Iwaji in proportiea to the dirtonce of tba olgaot Or t
fdneipla, w« luj' jttdg« Qf tba diKuisa af a naw fititet,

OF COLOURS. 9^1

obtenrfni^ theohang^e which is made in its apparent sitaationy
Opon viewing it sacoessively with each eye singly. Or, since
it is the difference of th& apparent place of an object, as
viewed by each eye separately which makes an object to be
seen double unless we tarn both eyes directly towards it,
and since in doings this, where the distance is very small, we
tarn the eyes very much towards each other, and less at a
greater distance ; the different sensations accompanying; the
different degprees in which the eyes'are turned towards each
other, afford by habit, a rule for judg;ing; of the distance.

5. In objects placed at such distances as we are used to,
and can readily allow for, we know, by experience, how
much an increase of distance will diminish their apparent
magnihide^ and, therefore, instantly conceive their real mag;-
fiitade, and neg^lecting the apparent, suppose them of the
size they would appear if they were less remote ; but this
can only be done, where we are well acquainted with the
real magpaitude of the object; in all other cases, we jqdge of
magpaitudes by the angle which the object subtends at the
known, or supposed, distance; that is, we infer the real
magnitude from the apparent magnitude in comparison witl^
the distance of the object.

OF COLOURS.

176. Rays of light are differently refrangible, or

refrcLctible, that is to say, some are more easily turned

out of their course than others ; — and are differently

reflexibhy when some are more easily reflected than

o&ers.

Obs, The powers of rays in regard to their refrangibility
or reflexibility, are ascribed, on the hypothesis of their beicg
particles like sand, to the different momenta of the particles ;
and on the hypothesis of their being simple vibrations like
undulations of water, to the vivacity or acuteness of the vi-
brations. It seems, indeed, evident, that the rays which are
turned the most out of their course by a refrangible medium
have lett powers or momenta, than those which are the least
diverted from their course. Different colours are therefore
simply the effect of the different perceptions produced oo the
optic nerves by the different forces or the action of the rays,
the red being the most forcible, and the violet the least.

177. Light 18 called homogeneous^ when all the rays
are eqnaOy refrangible ; dXkd heterogeneous ^when some

n . OP THE PBISU.

nfyiareniDrercrrangiblelhan other?. The eolsurupBh
ducedhy homogeneous rays, are calledpn'mary or «'m-
phcoloan; tha&e ofheletageneous, secondari/ or mixed.
173. To examiae ihe different colours of a ray of
light, a smBll hole must be made in the shutter of a
dark rooiQi and the ray must full upon a prism in an
oblique direction.

Oil. Friiow are cammonl; made of solid glasSn but students
wbudo not po^aesa one nf this kinil, oib; eesiiy make b substi-
tate. Take three piccea ol plate glass, (the kind of which
looking glasMt are made) cncb four, or six inches long and
two' or ^ee inches wide ; get mude of tin a frame canuiting
of the two enda in the enact shape of the three pieces nf g^Iaffl
pltced in the form of a triangle, with b strip of tin raonnig
from each angle af one end to the angles or corners of the
Other. These strips are beat to as to receive the two tdgm
of the glass plates. When the frame a comptate, «xo^Bib
soldering in of one of the ends, fix the glass pl^e* ia QkUt
places, and then bare the end soldered. The tin fomting O*
ends is turned up so aa to receive the plates, ' and ooe dt flu
ends is furnished with a spout to pour in water. When tint
ia done, the vessel is made water bgfat, by stopiniir wiUi pot
ty all the crevices between the glass and tin. The prina fe
^eo filled with clear water by the spout and corked up.

Ilba. I. Let A B, figure 50, represent part of Qi> jdtj
dteteiiidowofa room, in which no light enters evmafi
the hole C. If the light of the sun be received u—^-^
«I any distance from the hole, as at E, a ~
spotwill befjrmed Qp<ni the semen, which ii latj
•meter than the liide at C. Place a glan pritiB J
the hole, so that the Iwbt may pasi through h tt d
perpeadicnlu to the axis of the prism i ar ' -
straight fnan E to F, the light wiiiiA c
hole, will, bj pMBng thnHi^ the priam, be btnt wAib*

OF OPTICS* < 95

image G H apon a screen which may be situated at any dis-
tance from the prism, but below the straight direction C F.
The Ieng;th of the spectrum G H is equal to about five times its
breadth, and is terminated by semicircular ends. The highest
part G is of a beautiful red colour, which, by insensible shades,
degenerates into an oranjgey then a yellow^ a green^ a blue^ an
indigo^ and a vioUU which is the colour next to H, viz. at the
lowest part of the spectrum.

2. Or in fig. 5 1, let E G be a shutter, F a hole, S S a ray or
direction of solar light, A B C a glass prisma then the light fill-
ing on B C will be unequally refiracted to the wall M N, and
produce the coloured spectrum P T. The violet being the
most, and the red the least turned out of their course.

179. If the whole spectrum be divided into 360
parts, the re(2 will occupy 45 of them, the orange 27,
the yellcfos 48, the green and the blue 60 each, the
indigo 40, and the violet 80. By mixing the seven
primitiye colours in these proportions, a dusky white
is obtained. And the seven colours are reducible to
three, viz. the red, blue, and yellow.

Exp» Paint on a circular board the seven colours in their
proper proportions, and then whirl the board with great ve-
locity. It will appear of a dirty white. If the colours were
more perfect and accurately defined, the white would appear
more perfect also.

180. The colours ofhomogeneous light can neither
be changed by refraction nor reflection, and the white-
ness of the sun's light arises from a due mixture of all
the primary colours.

Exp* 1. Let a beam of homogeneous light pass through a
round hole in a pasteboard, and then be refracted by a prism
on the other side, the colour of the rays will remain the same.

2. Red lead, viewed in homogeneous red light, will be red,
but if placed in green, or any other homogeneous light, it will
take decolour of the rap which fell upon it

181. The colours ofall bodies are either the simple
colours of a homogeneous light, or such compound co-
lours as arise from the mixture of homogeneous light.

Obi, 1. Each sort of light having a peculiar colour of its
own, fduch no refiraction or reflection can alter, since bodies
appear coloured only by reflected light, their colours can be
DO other than the colour of some single homo^neouR U^Kt^oc
of amizti&v of different sorts of light.

94 ^ OF oPTicir.

2. When the thiokneM of the particlMof a bodyif Mdi.
that one sort of colour b reflected, other oolourB will be tnat
mitted, and therefore the body will appear of the first odkmr
And, in g;eneral, a less thickness is foxind to be necessa r y to r»
fleet the most refrangible rays, as yiolet and indigo, than thou
which are least refrangible, as red and orange. Sir I. N6W<
ton, from a variety of experiments on light and coloorst con
eludes that every substance in nature is transparent, providei
it may be sufficiently thin.

3. Mr. Delval has, however, by a great -rariety of wd
conducted experiments, shewn that colourB are exhibited
not by reflected, but by transmitted li^t This he prorai
by covering coloured glass, and other transparent coloorei
media, on the further sur&ce, with some substance perfeatl:
opaque, when he found that they reflected no ooloiiF, but af
peared perfectly black. He concludes, therefore, that,- i
the fibres of mineral and animal substances are fotmd, wfae
cleared of heterogeneous matters, to be perfectly white, th
rays of light are reflected from these white particles, throng
the coloured media with which they are covered ; that thes
media serve to intercept and impede certain rays in thai
passage through them, while a free passage being left t
others, they exhibit, according to these circumstances, diffe
rent colours.

4. iVIr. Dftlval concludes, (1,) That the colouring particle
do not reflect any light. (2,) That a medium, such as Sir ]
Newton described, is diffused over the anterior and furthe
surfaces of the plates, whereby objects are reflected eqoall
and regularly as in a mirror.

5. To determine the principle on which opaque bodies a|
pear coloured, it must be recollected, first, that all the cidou
ed liquors, appear such only by transmitted light ; and, sc
condly, that these liquqrs, spread thinly upon a white groonc
exliibited their respective colours; he Uierefore condude
that all coloured bodies, which are not transparent, consist of
substratum of some white substance, which is thinly ooverc
with the colouring particles.

6. On extracting, by means of spirits of wine, the colom
ing matter from the leaves, wood, and other parts of vegeti
bles, he found that the basis was a substance perfectly whit
He also extracted the colouring matter from diflerent anim;
substances, as flesh, feathers, &c. when the same conclasic
was obtained. Flesh consists of fibrous vessels, containii
blood, and is perfectly white when divested of blood by abli
lion, and the red colour proceeds from the light which ii r

OF Tin KAIIHOW.

beted from ths white flbrontiabiUoe* throogh the rsd traoa-
paraot covering formad b^ tha blood. The reiult wm tha
Nju* teott an •xBmination of the mineral kingdom.

OF THE RAINBOW.

16C. The SaMtm is a meteor in form of a parfy-
coltmred arch, or semicircle, eshibited only at the
time whea it rains. It is always seen in that point
of the beavenH which is opposite to the sun, and is
occsaioned by the refraction and reflection of his nja
in the drops of fuUingrain. There is likewise, though
not always distinctly visible, a secondHry, or much
fiunter rainbow, inresting the former at some distance.

Oil, When the lun ahinas upon drops of r»ia as they are
' Aninfilbe nji which come from thne drops to the eye of
IIm tpeotUor, after ooe reflectioa and two refractiDus, pro<
dues the innarniatt, or phihart rauibow.

Fig. St, repreeeptiag the rr/racJion and njiutian of light
JUHngondr^o/vati '" "^' ' ' '"' —

3' ^ -V which TFT is red, and
7B ^-^theiQnerpurtVOXFio-
V let.andlbBiaterniediata
parts recbooicg troin
the red to the violet, or-
ange, yellow, green,
blue, indigo. 3app«e
thespectator'aeyeatA,
and let AI bean imagin-
ary liae from tha centre
I ol the !aB to the eye of
the BpaEtator. Ifabeam

of lighl 3 coming from

Qu (HI), falli npoa uiy drop of rain F, and the rayi which
•inwge fttF, make an angle FAl of 42 degrees £ minutes
with dw Iia« AI, thew rftyi make the same asgle with the

i «*Mm U H, adw M aaU> of «< ir Wi
.<^&2n^Q«iBt*tfB«alM«doiir, U

' tbv «HM te At ay^ My nftMtcd Jm Hmd tlw
n^tWl paH tifltow a«-q«^' Hw IntanndMtodn
twMQ F ud D wm, fir tha aM iMMom ba «r tk» ii

dUaealooH And tlwt .whnk bU bai^jpimdegH
th* dropi in th« litM ,P D, in>;bailMrwnBi uroQifi
dropi in wtiioh Qie u^si mad* b; tlie MMlpng and il
nkjn u* BqiMl. .Tlrai, ifhM^nr ■ drop of rut li pU
ftaan^ wtueh flwdtiDtiHlMjviMike with A lua^
the H^ F AI, or u43'3', our sntli drop will appc
If F A I ware turned round upon the line A I, ao til
end of IhiB line abeuld Blwajs be at tlie e;e, ind the o
I opponte to the lun, in this rsrolotiDD the drop F won
•cribe a circle, of whi*h I would be the ceotre, and^
■n ue. 'And rince in thia revolution th« ai^le F' A
tinuei the wme, if the ann wbi to ihine upoa thii dm
iwndTM, the eflsctual nji would make tbo game angl
theinddeiit ray>,inwhaterer part of the arcT P T0
maj happen to be ; end, comequently, in whatsrer ;
the arc the drop F is, it will appew red. Now ai inni
ble dropa are &lliiis at once in right lines from the
whilst one drop ii at F, there will be others at T T, and
other part of tiie ara, which irill appear red in the nmi
aer that F woold have done in the supposed ciroolar i
tion. Therefore, when the sun shines upon the rain
will be a red BIO T FT produowl opposite to the an
like muuier a riolet arc V D X will be [vodncad, taA
bltermediata odourt, irtiich will tc^ther nuke up tl
maryraiabcnr. At the eye at A the rsTsare dividei
8.red toPpDq>le,JDit as thej present tl^nuelTea to tl
In oatore.

Eicp. l,tt a ^t»m globe, fllled with water, b« axaeMd
m^aftheaim: let ue eye of the spectator be so ntosta
the least refracted raj from the drop, oomiog' to the ejK
maka an angle of abMit4£° wiQi ^ellnepasDni^tiumi

'The angle whidi the effectual red rays make with tli
il«4 )V1* a i^mit to be 4r' W, that of Oa Tiolet i«7i 4f

OP OPTICS. 97

9je and the son, the red rays only will be seen ; if the place
of the eye be changed, so as to enlarge this angle, the red will
disappear ; but if ^e angle be lessened, the colours of the more
refrangible rays will appear.

06f. 1. Cascadeii and fountains, whose waters, in their fall,
are divided into drops, will exhibit rainbows to a spectator*
property situated, during the time of the sun's phining. 1'his
appearenoe is also seen by moon-light, though seldom su^
ficMotly yiyid to render the colours cUstinguishable. Colour-
ed bows have been seen on grass formed by the refraction of
the son's rays in the morning dew.

3. Artifi^al rainbows may be produced by a candle light
on the drops of water ejected by a small fountain, or jet d'eau,
or from the stream emitted from an eolipile. But the most
natural and pleasing is by means of the air fountain, the jet
of. which is perforated with a great number of very fine holes
from which the water spouts so as to form a kind of fluted co-
lumn. The rainbow is formed by the sun's rays, for the spec-
tator has to place the spouting streams directly in the sun'k
beams, with his own back to the sun, and being in a direct
line with the sun and the centre of the jet, by stooping his
head to a certain degree, he will discover the beautifril ap-
pearance of the natural prismatic colours, and a small rain-
bow, on the same principle as those which are seen in the
time of rain and sunshine.

183. The primary rainbow can never be a greater

arc than a semicircle.

Since the line a t fig. 52, is drawn from the sun through the
eye of the spectator, and through t the centre of the rainbow,
this centre is always opposite to the sun. And since the an-
gle/a t is an angle of 42^ Sf^ /, the highest part of the bow is
ASt^ 3' from t its centre. If, therefore, the sun b more than
4S? V above the horizon, t, which is opposite to it, must be
more than Af? ^ below the horizon, and no primary rainbow
will be seen. As much as the altitude of the sun is less than
430 S^, so much will the highest point/ of the rainbow be
above the horizon; and when the sun is in the horizon, t, the
centre of the bow will also be in the horizon on the opposite
side, and half the circle will be visible ; but when the sun is
set no bofw can be seen.

1 84. When the light falls upon the under parts of drops
of ndn, some of it, after two reflectionjs and two refrac-
tiona^comes to the eye of the spectator who had his back
towafdsthesaniBDdhis&ce towards th& dxo\l« ^^

•duM rtyi Much wepanHel t6 .4cil luoAei
^1^ )iB*e T^tta jmtx refracted, jud oace i^e&a
a drogof mn, *rtn be coloured, and pEodnce ai
rior fil^wir, when tfaey«iiK^ after two'i
tioiii.ad two reflefitxMH

'-HA-J

' Of S4»7'-wll
liQB drawn fi
■anthrodi^t '
of tbs ipeotal
therefore ma

■ame aii|;]e with the incident ra; S B Therelbre if tb
tator'a ejre uat A, all the ra;9 except the violet, will
len aneie with A I than E A, and fall above the apet
e;e. In like manaer it ma; be ahewn, that from the <
only red rt;> will come to the spectator'! e;e, the re
ingbelow it; andtbatthe rays emerging from the in
diate drope between E and F, and comiag to A, will i
at intermediate angles, and pretcnt to the eye the in
diate coloara. If EAi he conceived to turn rofind n|
line A I, in snch a revolaCion of the drop G, the aogl*
wonld remain the same, and causequentl; the emargii
would make the lame angle with the incident njt,
■Qob a revolution the drop E would describe a circl«,ol
I would be the centra, aodCBD an arc. Couieqi
liocfl the emarpng rays make the same angle with th
dent ones whoa the drop is at any other partoT the ai
Ettheeolonr of the drop will be violet to an ays place
in whatever part<rf' the arc the drop is placed. Non
Uura ara isnnmerable dropi of rain fallii^ at once,
one Jropii atE, there will be others in all pacts of (1
which will all appear violet-coloured, for thejUM rMi
E would have appeared of this colour in any other part
arc In like manner at the drop F, appears red *t F,
any part ^ the arc FD, to will any other falling drop 1
comes to any part of that arc. - The intenawIiUe u
fatmti Id Um same manner with the violet an CBD, i

OF OPTICAL INSTRUMENTS. 99

r^ arc FD': and 'thus the whole secondary rainbow ii pro-
duced.

i85. The colours of the secondary rainbow are
fainter than those of the primary, and are ranged in

the contrary order.

06f . 1. The ang^le which the violet rays make with the in-
cident ooei is found to be 54^ 7', and that of the red rays 53*^
57'. At every reflection many njs pass out of the drop with-
out being v^ected; consequently, the secondary rainbow
which is produced after two reflections, is formed by fewer
rays than the first, which is produced after one reflection.

2. Again, in the primary bow, the Tidet rays, when they
emerge eflbctnally, make a less angle with the incident rays,
and, therefore, with the line AI, than the red rays. But the
rays are here only once reflected, and the angle which the ef-
fectual rays make with A I is the distance of 3ie coloured drop
from I, the centre of the bow. Therefore, the yiolet arc in the
primary bow will be nearer to the centre of the bow than the
red arc, that is, the innermost colour will be violet, and the
outermost red. But in the secondary rainbow, the rays are
twice reflected ; and the violet rays, which emeige so as to be
effectual after two reflections, make a greater angle with the
incident rays, that is, with the line A I, than the red ones :
which angle is the distance of the violet arc from I the centre of
the bow. Therefore, the violet arc in the secondary bow will
be fiuiher from the centre of the bow than the red arc ; that
is, the outermost colour is violet, and the innermo9t red.

OF OPTICAL INSTRUMENTS.

186. A mirror or speculum is an opaque body,
whose sur&ce is finely polished, so that it will reflect
the rays of light which fall upon it, and thus repre-
sent the im{)ges of objects.

Obs. They are made of metal, or glass polished on one side
and silvered on the other. There are three kinds of mirrors,
▼iz, the plarUi the convex^ and the concave*

187. Concave glasses are necessary to those whose

eyes are too convex.

Exp. If the parallel rays, fig. 48, (f a, C m &, and e c, fall upon
the concave ndnor A B, then d a will be r^ected along a m, C
b will be reflected along b m, and « § along em; of course they
aH meetinm : and m 6 is found to be equid to m C, or half C ft.

100 OP OPTICS.

lOut' When the eye is too round, the rayi proeeeding fan
objects are convergedto a focas before they get to the ntrnt;
to remedy this, a concave glass is used, because the p f u paity
of this is, to disperse the rays which prevent them finom eon-
ing to a focus so soon as they otherwise would.

188. Convex glasses are necessary to those whose

eyes are too^a^

IHus, When the eye is too flat, the rays prooeeding froa
objects do not converge to a foeus so soon as they reach the re-
tina ; a convex glass has the property of converging the rayii
and of course bringing them to a focus sooner uian £ey other-
wise would.

Obs, Were there no other use of the science of didptrin .
than that of spectacles, the advantage that mankind reonve
thereby is inferior to no other benefit, not absolutely requisitB
to the support of life. For as the sight is the most noble and
extensive of all our senses ; as we make the mioBt frequent nie
of our eyes in all the actions and concerns of life : surely that
instrument which relieves the eyes when decayed, and sup-
plies their defects, must be estimated as the greatest of all ad-
vantages. Forlorn, indeed, must have been the situatien of
many young, and almost all old people, before this admirable
invention.

189. Microscopes are instruments for viewing smaH
objects ; and they apparently magnify objects, because
they enable us to see them nearer than with the nak-
ed eye, without affecting the distinctness of vision.

Exp, Take a piece of brown paper and make a pin-hole in
it, then bring the eye close to the hole, and the paper within
two or three inches of any small print or object, and it will be
apparently much ma^ified, though without the paper the
letters would at that distance be wholly illegible.

Obs. All that the hole or microscope effects is to enable us
to see an object disliricfh/ much nearer to the eye than itoonld
be seen by the eye unaided. The mof^nifi/ing power is as the
proportion of the distance, at which we usually view objects,
to that at which the microscope enables us to see them or their
true images.

190. There are three kinds of microscopes, the
single f the compound, and the solar.

The single microscope, is a small double convex lenS,
having the object placed in the focxiSy^xidVVv^ e^^'aJ^.^-e,
same distance on the other side. \\a m^g[vv^:j\Ti%V=^^

OF MICROSCOPES. 101

is found by dividoig seven inches, the least distance
at which an object can be seen distinctly by the
naked eye, by the focal distance of the lens.

Exp, If the focal distance of the leoa be cuoly the 1-4 of an
inch, then the diameter of an object will be magnified 28 times
(because 7 divided by f-4 is the same as multiplying 7 by 4,)
and the surfkce will be magnified 784 times.

The compotmd microscope consists of an object-glass
and an eye-glass. Its power is in proportion as the
image is larger than the object, and also according as
we are able to view it at a less distance. There are
generally two eye-glasses, by. which means the object
is less magnified, but more of it is seen.

fi^ .^ H j^S^^^ lUtu, 1 . The object to be viewed is
"^ ' |^a6, fig. 54, e({ is ue object glass, and

!/the eye glass. The small object a
'6, is placed a little beyond the focus
ofd e, the rays will convei^ge and the
image be formed at g h. The image, therefore, and not the
objMt, is viewed bv the eye, B A E, through the lens « f,
which is so placed that the image g h may be m its focus, and
the eye about the same distance on the other side ; the rays .
on eadi pencil wiU be parallel after going out of the eye-glass
at e/, tUl they come to the eye at A;, where they will l^gin
to converge by the refractive humour of the eye, and having
crossed ^ch other and passed through the chrystalline ana
vitreous humours, tl^y will form the inverted image A B on
the retina.

5. If the image g /i is 4 times larger than the object a fr, and
by the help of the eye-glass we can view it 7 times nearer
than we could by the nt^ed eye^ on both these accounts the
diameter of the object wiU be magnified 4 times 7, or 28 times ;
and the surfiice 28 times 28, or 7^ times,

The solar microscope depends on the sun-shine,
and is used in a darkened room. It is composed of a
tube, a looking-glass, a convex lens, and a single
microscope. The sun's rays are reflected by Uie
looking'glasa through the tube upon the object, the
image of which is thrown upon a white screen, sheet}
kc» placed at a distance to receive it.

9»

102 or OPTICS.

Obi, The magnifying power of the instrument is in propor-
tion as the distance of the image from the olorject-glassis greater
than the.object itself is from it. If the distance of the olnrject
from the object-glass be 1-4 of an inch, and the distance of the
picture be ten feet, or 120 inches, then the object is magnified
in length 480 times, or in surface 230,000.

191. Telescopes are used for viewing objects at a
great distance ; of these, there are two kinds, the
refracting and reflecting,

192. It is the sole business of all Telescopes to ena*
ble the eye to see the object under an enlarged angle.
For this purpose, a new image of an object is produc-
ed by the object-glass of the telescope, and tben this
image is viewed by means of the eye-glasses.

Obs. As the object or its image is seen by the eye nnder an
enlarged angle, or in the same way in which it would be seen
if much nearer to the eye, so the first impression conveyed to
the mind by a telescope, is that of bringing the object nearer,
which is only another mode of declaring that it is enlarged, or
seen under a large angle.

193. The apparent diameter of an object seen through
a telescope, is to the apparent diameter of the same ob-
ject seen by the naked eye, as the distance of the image
from the object-glass is to its distance from the eye-glass.

Illus. If the image, formed by the object-glass, were receiv-
ed upon paper, the apparent diameter of the object seen by
the naked eye at the station of the object-glass, would be
equal to the apparent diameter of the image seen from the
same station, and the apparent diameter of the image will of
necessity be inversely as the distance of the eye from it, or as
the focal distance of the object-glass. If the eye then be plac-
ed at the station of the eye-glass, consequently, the image will
appear to the eye in that nearer station bi<j:ger than at the ob-
ject-glass in the inverse ratio of the distances. Therefore the
apparent diameter of the object seen with the telescope, is to
the diameter of the same object seen by the naked eye at the
station of the object-glass, as the distance of the distinct image
from the object-glass is to the distance from the eye-glass, that
is, as the focal distance of the object-glass is to the focal dis-
tance ol" the eye-glass ; consequently, if the former be divided
by the latter, the quotient will express the magnifying power;
thus, if the telescope will magnify ten times in diameter, the
focal (7i.<:fance of the objeet-gYai^^ is (enixK.he«^and tha^of the
^ye-glassone inch.

OF TELESCOPES. 106

Obi, 1. Consequently, a telescope will not magnify an ob-
ject, unless the fix^ distance of the object^lass is greater than
the focal distance of the eye-glass. And of course the object-
glan of a telescope should be less con vex than tho eye-glass.

2. An object will be equally magnified by two tdescopes of
▼ery different lengths, if the ratio of the focal distances of the
object-glass and the eye-glass be the same in each.

3. And if a telescope is inverted, objects seen through it
will be diminished ; for the object-glass which has the g^reat-
er focal distance then becomes the eye-glass, and it reverses
the proportion.

4. The visible area, or space which may be seen at one view
through a telescope, is as Uie area of the eye-glass.

$. The brightness of an object seen through a telescope de-
pends upon the area of the object-glass, but not the visible area.

6. The distance of the eye from the eye-glass should be
equal to the principal focal distance of the eye-glass.

194. A Telescope, to shew objects in their natural
posture, has three eye-glasses. The two additional
lenses simply give an erect position to objects. The
three eye-glasses have all their focal distances equals
and the magnifying power is found as before, by di-
viding the focal distance of the object-glass jby the
focal distance of one of the eye-glasses.

1 95. Galileo^s telescope consists of a convex object-

glass'and a concave eye-glass,so placed that the distance

between them is the difference of their focal distances.
Obs, From a distant object, rays fall upon the convex ob-
ject lens, from which they will proceed towards the focus of
that lens. But the concave eye-glass renders the converging
rays parallel when they reach the eye ; whence an image will
be formed upon the retina. And the pencils of rays being
made more diverging by passing through the concave lens, the
visible image is seen under a larger angle than the object^
and appears magnified. Also, because the pencils which form
the image only cross one another once, the image appears
erect

196. Newton's telescope consists of a tube, towards
the end of which a concave mirror is placed. The con-
verging rays, before they reach the focus^ are made to
fall upon a plane mirror placed at an angle of 45 deg.
and thrown upwards to the focus of acQuxc:Sw\&^GE&^^'^

^§t hebschel's telescope.

in tbe upper side of the telescope, througli which
&« iye looks down od the object.

.1^7. GregoTij's Ulescope consists of a tube on which
a copcuve mirror, huving a hole in iu centre is plac-
ed. Any parallel rays from an object falling upon
thia mirror, wilt, after reflection, form an inverted
image at its focus. This image, however, is inter-
cepted by a sumller mirror, which reflects it back to
an eye-glass in the hole of the large mirror, throu{^
w£ich the observer views the object.

Oh. I. l>i tile teleB(io|ieB malle by Dr. BbrSCHKL, the ob-
j«cr:^ reSecled by a mirror, aa in tbe Georgiao teleicope,
aadthe ray^ are imercepled bj a Xem at a proper dislanoe, n
that the obBerver ha: his back to (be object, iiiid liM>ks througb
the leas Bt Uie mirror. The pagDifyioi; power will be the
. Mme at in tha Nawtonian talncope, bat than buni nd ••-
QODil reflector, tba brightaetB of tbe object viamad in Iba
Henchel teleicope it ^^ater than that in the Newtoaiu ta-
Atcopa.

2. TheLtAe of Dr. Henchel'i grand teletcope^ ii 39 ttat 4
incbci in len|tli, 4 feet 10 indiei in diameter, ararr pnrt of
which it nade of iron. Tbe concave inrraca of Uw ffmt
mirror ia 48 inches of poliihed lurTace in diameter, ill tUdt-
ne«B3 l-2iacbe9, anditi neightiiupwarcbofSOOOlbai TUa
noble iDttrumeot wai, in all ita parti, Gcastmcted nndar ^
aole dirsclioD of Dr. Herschel : it w»i bagun in th« vaai
1785, and coaplelod Au;. 38, 1789, on which dajr wudtiea-
vered the lizth gateUita of Satnro, It uajpiiGai MOO tiSMt.

3. ABC ia a ny oriight, reSected bjr the grsit Speculum
B, to the ef e-glass C. D ia a cbnir far the obaerver. E a
moveable gallery for ipectaton. FG a imooth baas for the
frame to turn on. II and I pultsji to move the JoatrumeDt.
K are rooma for auistanla.

4. Dr. Prieitler obaerves, that the easieat methodof finding
Ihe magnifying power of any telescope, by experiment, ii to
measuretha diameter of the aperture of the objecl-glasa, and
that of Iha little ima^ of it, which ia formed at the place of
the eye. Another method, is to observe at what distsncs

.you can read any book withtbe naked eye; and then remoT*
log the book la the farthest distance at which yon can dia-
tioctly read it by the help of the teleacope, Ibe greater dil-
taaca divided by the leu, gives the power of the teleacope.

6. Dolland'a achromatic tcleicope by a diapoaitioii and
mixture of crown BadBiatobject^lanai, destroys the colours
, which ariiB in any single object-glaaa,

198. The Camera Obscvra ia made by fixing a con-
vex gtass in a hole of a windofF shutter, and, if ao light
enters the room butthroughtheg;la!9,the pictures of all
objectBOatheoutsideiDHybeseeDinaninverted position
on a white paper placed in the focns of the lens. If th*

t06 OF TUE MAGIC LANTEKir.

cDUTCxlensbe placed in atnbe intheioaide of abc
within which is a looking-glass Eloping backwardi
a obseura.

Itlw. Fif . 56, repr«B

box ctHiauting of two pi

Tho outer ABCDEFG

I ■ >hi]tter or coTsr LNI

kwhichmovei roand a hi

' PQ, BiidwIutnopeii,Miii

_ „ ^ figure, it curies two bt

boards wbicb aerrs to exclude the external l^t u mud

nible from the rough gUn O, upon which the ot»em
mk. The Ibreaide of the box ia wKotii^, sBd in <
Aperture another narrower box EHIEG ilidei: thii
wants the inner side, uid hai a convex ffan leuaflxed a
ir this machine be turned with the lent 1 towardi anjotg
that are well illuminated, an inverted picture of thmn wil
Armed within the box on the side ABCD, and that pid
Bb; be rendered diitinct by moving the gliding box EH
in or out, in order to a^jiut the focut according to the
tance of the ohjecta. At the bock part of the box a Bat p
of looking f^au u litoated at an inclinatitra of halT a right
gle, as it IS shewn b; the dotted lines B R : in consoqueni;
which the rays of light till upsn the looking glan, and
reflected upwanls to the rough glats O. The picture th«
formed upon that rough glnaa, and will appear erect to a s]
tutor situated behind the box, and loolong down upon
glass O, from which a drawing ma; be made.

199. The Magic Lantern is an instrument used
oif^nifjiog paintings on glass, and throwing their
ages upoaawhite screen in a darkened chamber. (I
figs. 57 and 58.) And the magic lantern becomes
Phantaimagoria when the glass slides are rende
opaque between the objects.

OF THE MVLTIFLTING GLA^^* 1,Q7

IS, Fig;. 57 represents the machioe with the effect it
ices. Fig. 58 shews the internal parts of the machine
(I at their proportionate di9tanc%«. The lantern con*
a candle A, a reflector M N, which is so situated as to
the light, A in its focus. On the (ore part of the lantern,
is a thick doable convex lens C D, or a plane-convex
Uy called a buirs eye,) of short focus. The lantern is
I on every side, so that no light can come out of it, ex-
what passes through the lens C D. In the direction of
ms, there is a tube x fig. 57, fixed in the lantern, which
lateral aperture from side to side ; through this the
slider aa with the painted small images, is moved in an
ted position. GH, fig. 58, represents one of these ima-
The fore part of the tube x contains another sliding
which carries the double convex lens EF. The effect
Me parts is as follows: — The thick lens CD throws a
deal of light from the candle A upon the image GH,
> increase that light still more, the reflector .MN is of-
mt not always, placed in such lanterns; for, as the
is in the focus of the reflector, the light proceeds in
lei lines from the reflector to the lens CD. The imaga
leing thus well illuminated sends forth rays from every
, which, by passing through the lens EF are converged
focus upon the wall, and form the large images as is
1 in fig. 57. In some magic lanterns, instead of the sin-
us EF, two lenses are used of less curvature, and set at
e distance from each othei^, and act rather better than a
tlens.

0. The Multiplying Glass, is made by grind-
own the side of a convex glass into several flat
ces.

Illus, Fig. 59, is the reprcfledta*
tion of a multiplying glass with
the flat sidethbjbdydk. The ob-
ject C,4een by the eye at H, wiU
.appear multiplied into as many
^ different objects as the gluss con*
tains plane surfaces. For since
rays flow from the object C to all
parts of the glass, and each plane
surface will refra<!t these rays to
the eye,the same object will appear
) eye in the direction of the rays which eater it through.
furface. Tb us the rays falling ia Ibe ditecWoxvC \^ '^''^
Ae obiect in its true place atC,beca\x^^ \\\^t^>3a«^ vaS&st

106. . QUESTIONS Off OPTICS.

. [ bol Ihe iTiysfalling upon tbeBUrfacsp.ft iand

<Jf willbc nfmcteJlo land B, and, therefore, to auercat
H, Ihe object C will appear io Iho direction H c E, andfiSD,
M wall u Unt of H i C. The lamc thing wtll happen, if, in-
.ftttd of thraa, tkece be aaj nomber of flat aurfacBi.

(QUESTIONS ON OPTICS. ^E

Wbatil&frlMacNswtDa'a theory of liglit? ^^M

Vbkt ii k ny or pencil of light ? ^^P

VhatbaiMNioflight, and what apenfilof ra^/ ^'
Whatii t. tmMpareot medium ?
What labitBDoes are called opague I
Wlimt ii tb« diflereDce between divtrgxng and canctrgine

rtn>

Vlwtii

%t it tha radiant point ?
' Whatii the/acu>?
Wheo ia a ray of light laid to be infieeleJ, whan rt f Uttii,
and when refraeled ?

Whatia itudeancaroingthe emiaaion ofpartiolaof Ii^IIb
all direotiooa ?
What i« tha rate of reloci^ with which light m»jmi
"-— '- it proTedthat tbeparticleaof light(if thaj'an pU>

tedbyaluminoat body

Give an eiample of thit rule-
Explain the diagram illaitratii^ th* dicafenes and jmmI-
Iditm of the cayn.

What bappeni irtian a ray of light paaic* out of a lanr
into a deoaar mediomf

When a ray panaa obliquely froDi a rartr to a daDMV ■•■
dian, which way in regard to a perpendicnlar ia it refraioMf

When a ray paatea ont of a denaer into a rartr madbH^
which way ii it refracted .'

ninatratfl Iheie tawa by a iketcb.

What ia meant by tha angle of uicUmce, and what by Ow
ancle of reflection ?

D«*cribe the rarioaa kiads of lauai. ;

What it tha axia of a len* ?

Ifparallalrayifall nponapAmB-eonoetlehi, wfapttliMl .

What i« maaot byyboaJ rfutonct 7

What ii Uwrsh&raadiiistlu/MdfitteiiMirKplaiK

(QUESTIONS'. 109

How do you find the focal distance of a doubk convex lens f

What proportion is there between the heat of the foous of a
glass and the heat of the sun itself.^

What kind of a lens is a common burning-glass ?

Explain and illustrate proposition 164.

Explain fig. 46, illustrating proposition 165.

What are the principal phenomena of the rays in oonnec-
tion with the various lenses f

What difference in quantity is there between the angle of
incidence^ and that of rfjlection ?

How does Sir Isaac Newton explain the cause of reflection^

What are the chief phenomena of reflected rajrs?

How many coats are proper to the eye, and what do they
enclose ?

What are the names of the coats, and how are they situated
in respect to each other ?

Mention the names of the humours.

Where is the sense of vision situated ?

Prove that the images of objects are painted on the retina in
an inverted position.

How is it proved that these images are the cause of vision P

What causes dinmess of sight in old age ?

What is the cause of short-sightedness.'*

Explain proposition 175.

Howdo we judge of the distance of an object?

Are all the rays of light refrartible or reJlexiOfe alike ?

Explain the reason why they arc not.

When ifi light called homngeiico'iis ? and when heterogene-
ous.^

What are the colours produced by homogeneous rays caU-
ed?

What are those produced by heterogeneous ray? called ?

How is a ray of light separated into its primary colours ?

What are the colours of the spectrum, and in what order do
they succeed each other ?

What is ftaid of the colours of honibgencous light ?

How are the various colours accounted for ?

What colour is the colouring matter of various substance??
as the leaves of trees, &c. ?

What is the cause of the rainbow f

Illustrate by a diagram, how the light is refracted and re-
fleeted to produce the rainbow ?

What produces the secondary rainbow ?
What is the mirror or speculum ?
How do concave glasses assist the sight ?

110 OF ASTRONOMY.

How do convex glasses assist the sight ?

Why do small objects appear larger under the iiiicro6cop<
than with the naked eye ?

What laws govern Uie magnifying power of the microKope

How many kinds of microscopes are there?

Describe eadh, and the method of using them.

By what rules are the magnifying powers of each of thesi
instruments estimated ?

Point out the difference between the refraeiing and r^Ue^
ifig telescope.

What is the sole business or effect of telescopes ?

Explain why the image of an object is seen through th
telescope under an enlarged angle.

Explain and illustrate proposition 193.

How many eye-glasses have telescopes \i^iich shew object
in their natural positions ?

Explain the difference between the several telescopes dc
scribed.

What are the dimensions of the several parts of Dr. Hei
flchePs grand telescope f

What is the principle of the camera obscura ? Describe i1
operation and use.

What is the magic lantern ^

Point out the principle of its construction, and describe i(
effect.

What is fhe phantasmagoria?

Describe the multiplying-glass.

OF ASTRONOMY.

201. Astronomy is the science which teaches th

motions of the Earth, the Sun, Moon, Planets, Cometi

and Stars, and explains the phenomena occasioned b

those motions.

Obs, 1. The student in the day time may observe one of tt
chief of these motions in the rising, ascent, exaltation, di
clension, and setting of the sun. In the morning he will sc
it rise in the eastern part of the heaveas, ascend in this h<
misphere towards the South, attain its greatest height at nooi
and then descend again, till it sets in the West, as far froi
the South as it rose in the morning. This is the jfirtt praetk
lesson in Astronomy.

, OP ASTROVOMr. Ill

t. In the night time he may observe the stan rise and set
in the East, ascend towards the South, and decline and set to
the West ; and this will be the teeond lesson.

3. He may, however, observe that one star, viz, that over
the North Pole, never moves, and Uiat all the others move
aroiind it, and those within a certain distance never set ; and,
in short, in this third lesson, which is worthy of beings pursued
through successive evenings, he will become master of the ge«
neiU motions of the heavens.

4. He will observe, in a fourth lesson, what is also to be
pursued through successive nights, that the moon changes its
place with r^ird to the stars, that she increases in light in
proportion to her increased distance from the sun, till she ar-
rives at the opposite, or rises as the sun sets ; and that the
light increases on one side, and decreases on the other, being
alwajrs towards the sun.

5. He will in like manner observe, that the planets change
their motions slowly in regard to the fixed stars, and that both
moon and planets move in the same line, or nearly so, among
the fixed stars ; and this./S///i lesson may be mixed with others
through successive weefci.

5. He will be highly gratified by applying any telescope,
(the larser the better, but the smallest will afibrd much grati-
ncation,j to the moon, planet?, and stars. He will observe the
decrease and increase, and change of the shadows of the Moon's
pits ; the satellites of Jupiter ; the Moon-like appearance of
Venus ; the ring and moons of Saturn ; and many ef the con-
stellations and nebulous stars.

7. By mixing such observations on the heavens and heaven-
ly bodies, with the results given in the following paragraphs,
of observations made during many thousand years, the stu-
dent will soon become expert in this most sublime of all the
seienees*

SOS. The jSno/ari^s^em consists of the SuD in the
centre :

OfsevenprimaryplanetSyMeTcury 9 , Venus Q ,the
Earth ® , Mars i , Jupiter U , Saturn h * Herschel J^ :

Of four Jiiteroids^ or minor planets, Ceres V 9 Pal*
las 9 , Juno 9 9 and Vesta 9 :

Of eighteen secondary planets, the Earth's Moon,
Jupiter's four Satellites, Saturn's seven, and six be-
longing to Herschel :

And of a conjsiderable number oi ComeU.

112 80LAR SYSTEM.

Obs. 1. PfOLEMT supposed the earth to be perfectly at rest,
and the sun, moon, planets, comets, and fixed stars, to revolve
about it every day ; and that, besides this diurnal motion, the
sun, moon, planets, and comets, had a motion in respect to the
fixed stars, and were situated, in respect to thejearth, in the
following order : the Moon, Mercury, Venus, the Son, Mars,
Jupiter, Saturn*

2. The system received by the Egyptians was, that the earth
was immoveable in the centre, about which revolvedf in or-
der, the Mooa, Sun, Mars, Jupiter, and Saturn ; and aboat
the Sun revolved Mercury and Venus. This dispoeit&on will
account for the phases of Mercury and Venas» but not for the
apparent motibns and retrogradations of Mars, Jiipiter, Setun)«
and Herschel.

3. In the system of Tycho Brahe, a Danish nobleman, the
earth is placed immoveable in the centre of the orbits of the
sun and moon, without any rotation about its axis ; bat he
made the sun the centre of the orbits of the other planet^
which therefore, revolved with the sun about the earth,

4. The system whi(^ is now universally received is called
{he Copemiean^ or Solar System, It was taught by Pythago-
ras, 500 years before Christ ; and afterguards rejected, till re-
vived by Copernicus, in the sixteenth century. In this sys-
tem the Sun is placed in the centre of the system, about whuh
the planets revolve from west to east, in the following order :
Mercury, Venus, the Earth, Mars, Jupiter, Saturn, and the
Herschel planet: beyond which, at immense distances, are
placed the fixed stars. The Moon revolves round the earth ;
and the earth turns about its axis. The other secondary
planets move round their respective primaries from west to
east, at different distances, and in different periodical times.

5. In the fig. the solar system is represented. 8 is the
Sun in the centre, surrounded by circles, representing in suc-
cession the orbits of Mercury, Venus, the Earth and Moon,
Mars, the four Asteroids; C Ceres, P Pallas, J Juno, V Ves-
ta, Jupiter and his four Moons, Saturn, his ring, and sevea
moons, and Herschel and his six moons. The student w31
find it a pleasant exercise to draw the system on larger pepeT)
in which the distances should be in exact proportion.

BDLIK STSTCH.

- 6. TherCBrenveral wftysofdemonitntiiiKttwttheiilueta
UOFB round the lun ; oiie or two of vhii£ ahall be men-
tioMd : thoi Mercniy uid Ventu alwaji appear in the neigh-
bourhood of the Bnu, and thererore, if the lun revolved round
the eulh ai ■ centre, lo muit tho« planeti ; but if they didr
then tho motiOD of each would alwayi appear to the inbftbitanti
of the earth nearly equable, and in the lame directioo;
wliereat now tbey are Bometiines ilationaiy, or appear to hate
no proper motion : lometintet they more eartward in refarence
to the llzeditara, and their motion is then called direct, pro-
(rwriT*, or in eaniejumlia ; sometimea they move weatwatd,
or hava a retrograde motion, and arq then mid to move in an-
UeeiaMa ; all which appearanoei are necessary when we ad-
nit the ran to be the centre oftheir orbits and of the earUia,
but wholly irroconcilable with any other hypothesij.

Aim, when Mercary and Veoui appear in conjunction with

the iiui, tbey are sometiiiiei hid behind the body <^ the sun, and

fomrliatm paia between it aod the cartbt appeuine like « dark

10*

114

' AaTROMOMr.

gpoton theiun's disc or Tace; bnt if they have lititade, whco
ill ita luperior conjunction, Ibst is, when beyond the lun, ths;
ihine with a lace perfectly ciraulsr, like a fidl moan. Bml thV
bc« disappean in it> inferior conjunctton, that ii, wbeo it a
beLween ua ojid the biui, oa the moon da« at her change;
whence il is evident that their orbits are betweeo the sun and
the orbit of the earth. Mars loraetimea appean in oppoaition
to the sun, which proven that iti orbit includes that of tb*
earth ; and thai it includes the Sun is [dain, otberwisa Man
would in ila conjunction with the Sun disappear, like Haroii-
t7 and Venus, which uever happ^u : the same may be ob-
served of Jupiter, Saturn, and Herschel.

7. The motions of the earth in its orbit are proved, bj ths
eSect of its motion on the apparent motions of the leTeral
jdanetarj bodies. I'heso, as the earth happens to t>a ntuatad,
become stationary, rBtrog-rade, or direct, and the TariatioiK
are exactly meaaured by motions referred to the earth, Uke
the motions of objects aahore, when we are moving in a boat.
903. The earth is of a glubular form, because,
1. The shadow of the earth projected on the moon
in an eclipse is always circular. — 2. Theconvexityof
the siirlacu of the sou is visible ; the mast of an ap-
proachin^shipbeing^eenbeforeilshull. — S.Thenorth
polar stur becomes more elevated by trarclling north-
ward in proportion to the space passed over. — 4. Navi-
gtitors, byrteering their course continually westwiird,
ariive again at the place from whence they departed.
DEFiNiTionS »-- -

06s. The oxii of the
earth is an imaginary line
passi ngthrou|;hthecentre
north and south, about
wliicli the diurnal revo-
lution ii performed. It is
represented by the line
between A B, fig. 63.

2. The Pola of the

^^^ ofthiaas

B.TheKqiiBtorCD.k
an imaginary line passing
rounil the earth east and
wef>. at equal dirtaneet

DEFINITIONS.

116

4. The flnall drcle £ F, is called the Arctic circle ; the
cnde G H, is called the Antarctw circle.

6. The circle north of the equator I K, is called the tntpic
of Cancer^ that south of the equator L M, the tropw of Capri-
com.

6. The spaces between the ends north and south of the equa-
tor are called zonci ; that space between the tropics is called
the torrid tone ; between the tropics and the polar circles are
called the temperate sonet, and north of the arctic and south of
the antarctic ciroles are called the frigid zones,

7. Latitude is distance north or south of the equator ; lon-
gitude is distance east or west measured upon the equator,
from any assumed point.

Astronomical circles, whether larger or smaller, are divided
into 360 degrees : the length of a decree of course depends up-
on the ma^pnitude of the circle ; a degree on the earth is about
69 1-2 miles.

DKFiirrnovB rblativg to thb celestial globe.

Obs. 1. The surface of the Celestial Globe represents the
internal surfiuse of the imaginary hollow sphere of the hea-
vens. The lines and figures on the globe are of course ima-
ginary.

2. ThelineEB,isthe
tropic of Cancer, The line
I D, is tiie tropic o£ Capri-
corn. The sun never goes
north of Cancer nor south
of Capricorn.

3. The line CF, is the
Equatfir or EquinoctitU
line,

4. The line B I, is the
Ecliptic^ and indicates the
path that the sun appears
annually to pursue in the
heavens. It is divided
into 12 equal parts, called
signs of the zodiac.

5. The points at which the ecliptic cuts the equator G H,
are called the equinoctial points,

6. Those two points of the ecliptic farthest from the equator
are called SoUtices.

7. That space in the heavens about 16 degrees in width
tiireusfa the middle of which parses the ecliptic, i0 called the
todiae*

''^"^s. ^^

H_ f

^ v"^^

•I

1 16 OF A8TRONOMV.

8. The latitude of a heavenly body is distance from the
ediptic : UmgUude is distance from the first degree of Aries.

9. The smtible Harison is an imaginary circle, which ay-
pears to touch the surfiice of the earth, and separate the vigi-
ble part of the heavens from the invisible. The rational Hq"
riz9n is a circle parallel to the fonner, the plane of which
passes through the centre oi the earth, and cuts the heavcnfl
into two eqiml hemispheres.

10. The Poln of the Horixon are two points, the ooe oi
which, over the head of the spectator, is called the Ztnith ;
the other, which is onder his feet, is called the Jfadirm

U. A circle which passes from north to south throueh the
zenith of any place is called a Jfmtftan, and is said to be the
meridian of tiiat place. The meridian of any place passing
through the poles, and foiling perpendicnlaiiy upon the hori-
zon, cuts it in two opposite cardinal points, called /forth and
South.

12. The Altitude of any heavenly body above the horiioo ii
the part of a vertical circle intercepted between the body and
the horizon, or the angle at the centre of the earth measured
by that arc,

13. The Azimuth of a heavenly body, is the arc of the hori-
zon intercepted between the meridian and a vertical circle
passing through that body ; it is eastern or western as the body
is east or west of the meridian.

14. The Amplitude of a heavenly body at its rising or set-
ting, is the arc of the horizon intercepted between the point

. where the body rises, and the east or west.

15. The Declination of any heavenly body, is its distance
from the equator ; and is either northern or southern.

16. The Right Ascension of any heavenly body is its dis-
tance from the first of Aries reckoned upon the equator.

17. A planet's place, as seen from the sun, is called its HeliO'
centric place, and as seen from the earth, its Geocentric place.

18. Two planets are said to be in Conjunction with each
other, when they have the same longitude, or are in the same
degree of ecliptic on the same side of the heavens, though
their latitude be different. They are said to be in OppotUion
when their longitudes differhalf a circle, or they are in op-
posite sides of the heavens.

19. The celestial sphere is called rtgA/,o62t9U«, or paratteA
as the equator is at right angles, oblique, or parallel to the
horizon.

204. As the earth revolves round its axis daily from
west to easfcjthe heavenVybodveft «^^^ot \,<;> %%^^ctatop on

OP THE ECLIPTIC. 117

the earth to revolye in the same time from east to
west, and the alternate guccession of day and night
is the effect of the reyolution of the earth towards
and from the sun.

Obi. For all the heavenly bodies appearing^ to move fix>m
east to west, while the eardi revolves firom west to east, the
sun will appear, in each revolation, to rise above the horizon
in the east, and after describini": a portion of a circle, to set in
the west, and will continue below the horizon, till, by the re-
volation of the earth, it again appears in the east; and thus
day and night is alternately produced.

206. As the earth revolves round the sun in 365
days, 6 hours, 56 minutes, 4 seconds, the sun appears
to revolve round the earth in the same time, but in
the contrary' direction.

06«. 1. It IS manifest that the circle in which the snn ap-
pears to move, is the same in which the earth would appear
to move, to a spectator in the sun. Hence the apparent place
of the sun being found, the true place of the earth in its orbit
is known to be 180** distant.

2. The orbit in which the earth revolves round the sun is
not a circle but an ellipse, having the sun in one of its foci. —
For the computations of the sun's place, upon this supposition,
allowing for the disturbing forces of the planets, are found to
agre« with observations.

206. The stars round the Zodiac are classed in 12

Signs: Aries, Taurus, Gemini, Cancer, Leo, Virgo,

Libra, Scorpio, Sajgittarius, Capricorn, Aquarius,

Pisces.

Ote. 1. The names and characters of the signs are, Aries,
Y ; Taurus, ^ ; Gemini, q ; Cancer, q i Leo, ^ ; and

Vii^go, HD ; all northward of the equator.

Libra, w^. ; Bcorpio, iq, ; Sagittarius, f ; Capricomus, yf ;

Aquarius, t^ ; and Pisces, ^ ; are tmiihward of the equator*

a. The circle E, 6g. 64, reprnenti that ctrcU in tttabOM

3. The circles F G, represent the broad bait oi

the heareoB occapied by the 13 ooiuttlhttioiu ollkd tht

4. The ligns of the utdifto are claiten of Htn, mud •»
represented bj Ggurei drawa on the celestial glob*. Ttaaf
are so situated that tlie earth in its aniHialreT^atkiD pUMt
directly between them and the sua.

The :jua as he nppean to more roood in the eoljptio, mmh
to eater these cluiters of slan, and is therefore sud tS b* ift
' this or that siin or coaitellBtion. Thos, ifarigbtlin^tft
fi4, be drawa from the earth paistiif through the inn UlLit
reaches one of the coostellationi, the son ii said la be lattaia-
constHllation ia nrhioh the line tenninates.' ThuwlMBtlM.
■arth is at A, the tuo is in the coostellalioi], or tigm Ariwf ..
wbea the earth is at B, the sao isio Caooar; when tkaawtk-
ii at C.the sud is ia Litnn; whea tha earth tj kt D,thaMim
is in Capricoro.

TfRRUTRIAXi PHENOMENA. 119

5. TIm pupil will bearinmindthat the obseryation founded
on the idea that the sun and start revolTe round the earth, are
founded opon apptaranet only and not upon fo/ott. He will
therefore not torget while be is imagining that the heavens
moye around the earth that in fact they are stationary as it re-
spects the earth, and it is the earth that moyes round the sun.

207. The axis of the earth in every part of the
earth's revolution about the sun, makes with the plane
ofits orb, that is, of the ecliptic, an angle of 66^ degrees^
consequently the planes of the equator and ecliptic,
make with each other an angle of 23^ degrees nearly,

being the compliment of 90 degrees.

06t. 1. The obliquity of the ecliptic is not permanent, but
is continually diminishing by the ecliptic approaching nearer
to a parallelJIm with the equator*, at the rate of about half a
second in a year, or from 50'' to 55" in 100 years. — The in-
clination at this time is 23^ 27' 46" nearly. The diminution
of the obliquity of the ecliptic to the equator, is owing to the
action of the planets upon the earth, especially the planets
Venus and Jupiter. 1'he whole diminution, it is said, can
never exceed one degree, when it will again increase.

2. The diminution of the obliquity of the ecliptic is a con-
sequence of the approach of the earth's axis towards a per-
pendicular direction to the plane of the ecliptic ; but the
earth's axis has, besides the progressive motion, a tremulous
one, by which its inclination to the plane of the ecliptic variee
backwards and forwards some seconds ; the pericni of these
variations is nine years. The tremulous motion is termed
the mutation of the earth^s axis. Both these motions of the
terrestrial axis are occasioned by the action of the sun, moon,
and planets, on the earth.

208. The difference of longitude at two places may
be found by observing, at the same time from both
places, some simultaneous appearance in the heavens.

Obs. If the eclipse of Jupiter^s innermost satellite, on the
very instant of its immersion into the shadow of Jupiter, be
observed by two persons at different places, it will be seen by
both at the same instant. But if this instant, with reference
to the day, be half an hour, for example, sooner at cue place
than at the other, because the places differ half an hour in
their reckoning of time, their difference of longitude must be
70^ 30'; because the whole 360^ are equal to 24 hours, and
consequently every 15? are equal to an hpur.

120 OF ASTRONOMY.

209. Those who Hv^e on opposite sides of the
earth, but in the same parallel of latitude, have oppo-
site hours of the day, but the same seasons.

Obs, Beingp both on the same side of the equator and at th^
same distance from it, when the sun's decUnatiQii makes it
summer or winter in one of the places, it will be the same at
the other ; but because they are distant from each other 180^
of longitude, when it is noon at one place, it is midnight at the
other ; these are called Ptriaci.

210. Those who live ia opposite parallels of lati-
tude, but under the same meridian, have opposite sea-
sons of the year, but the same hour of the day.

Obs. When the sun has declination towards the north pole,
it will be summer to those who live in the northern parallel of
latitude, and winter to those who live in the southern parallel
of latitude. But having the same longitude, their hours of the
day will be the same ; these are called ^niaei.

211. Those who live in opposite parallels of latitude
and opposite semicircles of the meridian, have oppo-
site seasons of the year, and opposite hours of the day.

Obs, 1. Because they are in opposite latitudes, they will
have opposite seasons ; and because they are in opposite semi-
circles of the meridian, they will have noon when it is mid-
night at the other place ; these are called Antipodes.

2. These and many other propositions will be better under-
stood by means of the problems on the Globes and Maps, to
be found in OoldsmitKs Orammar^ and his Royal AlUu.

2 1 2. The axis of the earth, in its circuit round the
sun, being inclined to the plane or level of its orbit, this
inclination occasions the succession of the four seasons.

Obs. 1. The earth's axis makes an angle of 66^ 2Qf with its
orbit, that is, with the ecliptic, and always preserves its paral-
le^jsm, or is directed towards the same point, at an infinite dis-
tance, in the heavens ; hence during one half of the year the
north pole is continually illuminated by the sun, and the
south pole is all that time in darkness : and during the other
half oi' the year, the south pole is constantly in the Ught, and
the north pole is in darkness : and other parts in a proportion-
al degree partake of this vicissitude, and create the variety of
the seasons.

2. The difference in the degrees of heat, is owing ohiefly to
the different he^hts to which the «uti x\sfc% iJa^iN^ ^^Xiwais^

titeatont'lengdHirtlM^Tk IfteillMnmriietbigli-
Mt Ib iniMaiir iti ngn lUl Ifn oliliqiielj,. and eoosaqaently,
mare of them &11 on any given portion of the earth's tur&oe
thA is winter, when the rajn nil obtiqpxely; and when the
days ari kiB|f, and tiie nkfati short, the earth and air are more
heaUid lb the day than &ey are oookd in the night, and the
tewMM'irtMD tte days are short and the nights loi^.

^'hiiM<<sfK»^>

IDhi. Thelei^^enmg and shortening of the days, and the
iliffbiUnt seasons, are produced hy the motion of the earth T,
%• 8&S !n its orbit round the sun S. The axis of the earth
if 8 iddiDes to the plane of the orbit, and is parallel to itseli
in all parts of the orbit. In Jane the north p^ N inclines to
UMei^ and it is summer in the northern parts of the earth ;
in Deieember the north pole declines from the sun, and it is
'winter in the northern and smnmer in the soutiliem hemis-

.. S19w The orbit in which the earth rerolyes about
liie'««n, 18 not a circle but an ellipsis or oval.

CMf . 1. The sun's diameter being on the thirty-first of De-
mmmh&r 31' 45"; and on the second of July 39' 45^ or nearly
ene thirtieth logger, and consequently so much nearer, and
^lereasing and decreasing gradually, it is evident the orbit oi
liie earth is an ellipsis. Newton found the mean diameter oi
^e sun, to be 33' 12'% but as above it is 32' 15".

% In January therefore, the earth is in its perihelion^ snd in
JiAf fn ill ig»MsQfuand having a anller cirde te travetie in

1S€ •' AfTROHOMT*

its perihelien-lialf tiian its aphelion^iali; it ii ttg^ (Ujm kB|-
er in performing the aphelion-half of its orbit than tha pan.-
helioB-halfl

3. The motion of the aphelion point is 1^ 44' in 100 yean;
so that it will pass round the ecliptic in 90700 years.

4. in the perihelion, the earth moves in its orbit 61' ll"ptf
day, and in the aphelion, bat 57' 10'% being ooe-fifteenth less.
Its mean daily motion is 59' 10 1-2".

214. Twilight is occasioned by the atmosphere
above the horizon reflecting rays of the son, when
the sun itself is below the horizon.

Obs, 1. When the sun is at any point below the boriion, it
cannot be directly seen by a spectator. But, because lays
from the sun can pass to the part of the atmosphere above the
head of the spectator, this part of the atmoephiBre will be illa-
minated before the sun rises, or after it sets, and will beoome
visible by reflection to the spectator ; that is, twUighi will be
produced.

2. It is entirely owing to the reflection of atmosphen that
the heavens appear brig^ht in the day time. For without it,
only that part would be luminous in which the sun is placed ;
and if we could live without air, and should turn our backs to
the sun, the whole heavens would appear as dark as in the
nig^ht. In this case also we should have no twilight, bat a
sudden transition from the brig;htest sunshine to dark night
immediately upon the setting of the sun.

3. The twilight is longest in a parallel sphere, and shortest
in a right sphere : and in an oblique sphere, the nearer the
sphere approaches to a parallel, the longer is the twilight, be-
cause twilight lasts till the sun is eighteen degrees perpendi-
cularly below the horizon.

215. The atmosphere also refracts the son's rays in
such a maoner, as to bring him into sight, every clear
day before he rises in the horizon, and to keep him in
view for some minutes, atler he is really set below it.
The effect of this refraction is about six minutes of
time, or 33' of space, being rather more than the di-
ameter of the sun or moon.

Obs, From the same cause, the heavenly bodies appear
higher than they really are, so that to bring the apparent alti-
tudes to the true ones, the quantity of refraction must be sub-
ttacted. The hig her they rise the lesa ate ^Jtv^ t^^ x^^^^taiv

£16. A ^aiural iby is the time the sim takes in
pening fiom the merkUaa of any place, till it cornea
round to the aame meridiaii again ; Iwt the natural
dajjvaM not eqoal to one another : and the Efiuttum of
time^.iajh® difference between the mean length of the
mrlnm i&7 {ot 24 hours) and the length of any sin^e
day OMMflred by the son*8 motion, or between mean
tmi weAmparefd Hme.

Pif.l. ra aay natural day is tha time inwhidi theaarth
l^^fittTT.^y^ reyolutioii round its axis, and such a portion of
a saqpnd' as is equal to Ibe sun's increttents of ririit aseensiaa
Int flpii}9ay i but the suu's daily inereoMUts of i%lit ascension
sfii 'upaiiisl :' HanrsfnTTi ttiri additional ]portion mt the second
rafalntiin wiQ sonietimes be greater' and sometimes less, and
riiBiamfsptflT. the times in ivuch the natural days are oqb&-
flaliad wS M unequal.

S* If tito siut were to move uaifiHnhr round the 'equator in
the jnalflt tiaia in which it aimesirs to describe the ediptie, its
ttppaJMitdaitjrBUitionwouldbeameasi^ For

ue aatittal days in that ease beii^ liable to no variation, ei«
tharfiroMithe mdination of the sunVr orbit, or the irreg^oUurity
of Ha motfon^ must be equal.

or THE ainr 0.

217. The sun is a spherical bodT, situated near the
centte-of frainty of thesystinn of planets of which our
Euih is one; its diameter is 877547 English miles ;
and R rey b l ye s romiid its aiis in 25 dqrs and 10 hours.

Ilif. l.'FlMiii several phenomena His oondoded that there
is aft. atBMiphare whidi environs the sun^ and extends to a
«sngidarable distance firom it It seems likely also that its
l%fa|; and heai arsr created by gaseous combiution. Enler
inaiiM the t^tit equal to 6500 candles at a foot distance, while
Oa moon would be as one oandle at 7 1*2 feet ; Venus at 4S1
leet t and Svmitet at 1390 feet Consequently the sun would
appear like. Jupiter at 130,000 times his present distance.

$. The period'of the sun's revolution aSout its axis has been
detemiaea by means of several dark spots of various %ures,
whidmn^eoBUMDly beseenwith telescopes; in the same

124 OF ASTROirOMY.

manner have the periods of the revolatioa of Mara, Vemi^
and Jupiter, about their axes, been determined ; wheocA iiu
inferred that this motion is general, and belongi to all the
planets.

3. In the year 1779, there ynta a spot on the sun, iHiieh
was lai^e enough to be seen by the naked eye. It wm divid-
ed into two parts, and must have been 50,000 miles in diame-
ter. Dr. Herschel supposes, that the spob in the son are
mountains on its sarface, which he thinin may be more than
300 miles high. He examined the sun with several powen
from 90 to 500, and it appeared, that the blade spoti are the
opaque ground or body of the sun; and that the laminoas
part is an atmosphere, which, being intercepted or broken,
gives us a glimpse of the sun itself.

4. He concludes, that the sun has a very eztensiTe atmos-
phere, which oomdsts of elastic fluids that are vuom qit less
lucid and transparent ; and of which the luc«d. odm fomish
us ¥rith light. This atmosf^iere, he tiiinlB, is not Urn than
1843, nor more than 3765 miles in height; and he 8i^ipo6es
that the density of the luminous solar clouds need sot be
greater than that of our aurora boreaUs^ to produce the effects
with which we are acquainted.

5. If one of these spots appears upon the eastern limb or
edge of the sun^s disc, or face, it moves from thence towards
the western edge in about 13 1-2 days. Here the spot disap-
pears ; and in about 13 1-2 days more, it is seen again upon the
eastern edge ; and so continues to go round, completing its
revolution in 27 days ; during one half of which time we see
it on the disc of the sun, and during the other half it disap-
pears.

6. The quantity of matter in the sun, is to that in Jupiter
nearly as 1100 to 1, and the distance of that planet from the
sun, is in the same ratio to the sun's semi-diameter ; conse-
quently, the centre of gravity of the sun and Jupiter is nearly
in the super/ices of the sun.

7. By the same method of calculation, it will be found, that
the common centre of gravity of all the planets cannot be
more than the length of the solar diameter distant from the
centre of the sun.

8. The sun''s diameter is equal to 100 diameters of the
earth, and therefore its cubic magnitude must exceed that of
the earth one. million of times : and its mass is 329,620 times
that of the earth, therefore, the sun would move but one foot
by the action of the earth, while the earth would move 329i

6J20 feet by the action of the sun.
19. Its similarity to the other g\oV)«a oi VSae wJi»x v3^\.«SbsHs

<biaity,1iWioipliir>t ■trftun flif jgrfttot ^riOi lawatoiMy mud
TBll#fi^ Md Totationr on ui^ toad at to tuppoie, thatit is
MMlp^Mdy irittbilid, tilw tlitt rwt •TtlieptoiMtiyby Mngf
iviioM pifMit art ad a ptod to tlieir pecaliar oireim^toiic^i.

10« ^fbaagh it mxf be olj acted, from tha aflbcti prodocad
at Itedirtaaoaaf 95,000,000 mnat, that av«rj thins: muHba
iciotehad iy at itt tQrftoay|r«t many ftcti ifaaw timt beat If
pndQMdl^ tba ran'i rays wAj whan tiMj act on a saitabla
madhna ; or vlian radiatad and roflaotad by tnitablo sarfa-
oai. On tfaa tofw of moaataiM of uiAeiont haight, we al-
irayitlid tagiontof ioa andflMwrtbongbif theaolarrayt
Himmal i n caaveyed all the heat w And on this g^lobe it
4Night to ba the boltast whara tfaair oonna iM the least inter-
raptod.

11* If w« ara- to ooMideraU light as analogoiis, firomiti
aqnid pnwaia oo the aya, it teamt liMy that the lig^ and
hMt atthamn are oooMloned by the oombmtion of gas in the
fqi|iar raghmiof iti atmoiphare,aot diMnmilar perfaape, tothe
aaaabwitiett eCaarbniatted hydrogen gat, with which we hare
lately fflhuirinatad our rtreets and bonset. • The timaltancons
prodintiBttanii^ and heat, leema in tmth, in all caiet to be
prodnoad hythe eombnition of gat.

t& Dr. Hanobel coaoaiTet flie ran and planets to have a
gaMtval'lBOtion at the rate of the earth's motion in its orbit,
wiA rriation to the fixed stars; bat at this rato, if the dis-
4MM#of Utoitors ^ S00»000 times that of the diameter of the
isarth*! orbit, tfaa son would be 0(^000 yeafi in moving over
ibm diitnnoa of tiie nearest fixed star.

OF THE PRIMARY PLANETS.

tl8. The primary Planeto are those which regard
die mm » their proper centre. The Bomber already
knows 18 seven : Hercary, Venus, the Earth, Mam^
Jmiter, Saitnni, and Herachel.

196

SLCMBSpSiM n* FLAVETS.

061. 1^.6$

^y^ o o

Q-.

■rj

219. Four romll telegcopic planet8» cdl^'iW|]i Wt
haye lately been^iscorereali^weenthep^bitl^^ril^
aod Jupiter ; called Ceres» Jbino, PaUaa, . a»dl!'.V«ilt«

ELEMESTB (HP THE FLANBTflb

I., •

DBFinnoirs.

1. The San being placed in the fociu of the elUpftidUl oitit
of a planet, the planet cannot always be at the same diftvnoe
from the sun ; but will be farthest from' it when in tlie ex-
tremity A of the greater axis, most distant from the jfoeai 8*
in which the sun is ; and nearest to it when in P.

The point A is tenjaued flw
BiGHsa AP8i8,orthe apbx*
Lioir; and the point P, the
f.owsR APSi8,or the pbbi-
BBLioir: these points are
constantly varyiqg, aqd their
motion in a century, is called
the secular motion.

The distance between Ihe
oeirtre of the ^pse C, an)
the* sun or focus S, is called

SXCBWTRICITT.

The greater axis A P, is

the LINK of vtPSIDXS.

The straight line S £, drawn
from the extremity of the less
axis £ C to the sun, is the

AIEAir DISTANCR of tho

pVanel trom Xh^^ vuv.

MramtMrn mKtAmcm.. :.
And liMJMUkdiitMietiMP.bsrtlit taBoaiitrioily is eqOal |o

tibe nUHMLIOII PltTAVC*.

'WliM il»fimorBMMniiattli6g;i«MMt diitaiu^ firon the
«Mrtb,fliftttidtolMiBit8iLPO«BK} uid when at the least du-
taDoei in its fbbxqsa/

d. Aj^laiMtdoei not proceed in its orbit with an equable

■lotieB; lMitin«ndi«manner,tbat.aUnei]rawn train the sun to

the plaaMUdaperibes an ana always pvqportionate to the time.

ForeaBU»ple|Suiqpoteaplanettebein A* whence in acertain

Hmm it anires at B; the space or area whkh the line S A,

called the EJJMnrs Tscvom, desoribfiB, is the triangle ASB»

Bn-^fpomag the planet to be in P, let the straight line SD be so

drawn -tfaatthe area PSBlvajbiB equal to the area ASP; than

the phnet will move throug^h the arcs AB and PD Id equal

t]mii^' HhhiI^ these ares are unequal, beaag^ nearly to each

€4urteeqiroeally as their distsnces from the sun; fiirtfaeareas

being «qiMl, 4he are fD mm much in prepoction greater than

the Mie AB, as SP is less than 8A.

9. A plaae^'t asoicalt is its distance from the aphelion.
'Whtiiaiflaaet is supposed to 9iove in a drde in the centre of
whMh ik tfaci son, tilie portion FG of the circle bears the same
xalio to the whol4 circumference, that the time since the planet
passed its i^helion 4oeB to the time of its whole revolution, axid
ia tanBcd the hxav avomalt. If the eUiptical orbit of a
planity be so divided that the area ASO ahall have the same
Mde Wfhe whde ellipse ABED, whidithe timesince the planet
pMSed d|S aphelion, bajt to its whole period, then is the angle
ASO^bto measure of tiw {danet's distance fr<»n the aphelion,
at the time the planetis in G; this angle is called the trujv
AiKtMAiif. The deference between the mean anomaly, and the
tme anomaly, is called the souatiov of* the Planetis Centre.

4. .^hen the motion ef a planet is reckoned from the equi-
noctial pcmt>it is called its losoitvds. There are tables of
esuefa jjjtiinetV mean motion, and of the equation of its centre,
with its relative distance from the sun, and Us latitude in any
part of its orbit ; whence its true place may be calculated at

anytime.

5. Let S6 be a mean proportion between the semi-axis ma-
jor CA, and the semi-axis minor C£, then, when the planet
comes to the point 6 or L, the equation ef its Centre will be
the greatest, and this grbatbst EavATies varies according
to the excentricity of the orbit. The points G and L may be
found by observation, and as A lies equally bet^ciea UiASOL^^laSb
fyMiifl 4^r ^^^^ them be dietenm^^

Vti

9V HERCUaV.

9. A |i1ri»> iKLOKOATioa, or itasn^lst diilsnce from x^

•on, ii u B ionned at the earth by two lines, one dra.-^^

from tlie Lo the sun. and one from the earth lo thspIsTkef

7. A p I PERIODIC TiBfK, or the time it takes id b rw.

Tolution I the sun, ia found by observing when it is la any

point of i >it, and alter any nQmbcr of revalations, ob>

■erring K. comeb to the same point ag;ain : that interval

of time di' bj Ihe number of revolutiang, j^rei the time

of one. ' is called a thofical betoldtion ; but ai

while th et is making this revolution, the equinoctial

paint is n by the precession of the equinoxes, tiiere will

be lui aduii 1 time re.iuired for the planet to move, beliiTe

it will be ■ with, when the former

it will a

will percoive that a pla.
in Ihe order of the signi,
ary diredion, or ustrO'
An inierior

aamii, and to be aomeliu^c.

planet's motion ii iirttt through

relrograde through its inferior ; netnecn wnicn anuauons it is

tlalionan/, A superior planet i9 relrograde when in <^^)osJ-

tion to Ihe sun, and dtrecl when is conjunction with it ; and

between those situations it is alalianari/.

9. The plans* or leiek of the planet's orbits are.vacioudy
inclined to that of the ccliptio. The eppOEite paints in which
the plane of en orbit crouea the plane of the ecliptic, are
called NODBa : that at which the planet rises north of the
ecliptic, Is culled the aicendij^ nodt, and the other the de-
leending node. The line which joins the nodes, pasiinf
through the sun, is called the line of Ike nodii.
OF Mercury 9 .

220. Thediamelorofthis planet is 3180 miles.

Its sidereal revolution 87d. 23h. 15'. 44".

Aphelion Place
Secular motion of aphelion
Ascending node
Secular motion of the node
Inclination of orbit
Greatest equation .

EiceotriQity ia mileE

83°

85'.

4fi

1 A'i^A'M.

at TwKm.- !•»

Relative mean distance from tbfl Mm 38710

JHeaa dUlnnce in miles - - 88,841,468

Obt. 1. The grealtil elo»gali»n of Mcicor; from the nut
bein^ but as*! 30' it in tnosl^j above the horiloo when ths lun
is ; uid, Iherefure, is seldmn •een. Wheo it u niible it ii ■■
the east jtiit berore Bun-riss, or in Om WMt teaa after inn-tMi
accordioglv u its pUc(! faUvwi orprccMttti thktof UMniB.

i. WheD viewed with B talawope, it tffam witb phtwt
«iinilHr tn the moon. When in iti ia&rior caqfiueliBn, if id
latitude be less thnn the aiaii-dianiat«r of 11m ton, it pMUi
Over the gun'a lace; this is sdlcdatmiBt ofHwoviT-

3. The densitj of (he sDot liMt, whieh b ip th* wtte pr»-
portion fts bi» light, is seven timai m great in tS*reuty m with
m; 90 that water there, woold be MOMd off io the riwpa of
•team : for by eiperimenti wtib the thenncoHter, it appMll
that a heat 7 times greatar tban that et tbt avn'* beami i>
■moBM't will lerve to make vatsr bnL Thia bowenr cle>
panft on Um wm^ at the aJuw^hew of Meraary.

OF Veksi 9 .

SSI. lltedianKter ofthw {dmet is 9496 milti,
Itrerdrea round its ni« in 23fa. 20'.
Sidereal revolution S«4d. 16h. 49' 10". 6.
liOi^tode, Jan. 1,1815 . 9i.1lo.45'.
AmnaliBolioo . . . 7 14 47 90

Aphelion . . . . Ip 6 3fi 18
gecnlMriBMioD of aphelion , 121

AMendiAKDode . S 14 57 18

'Secular motion of node . . 61 40

Inclination of orbit . . . 3 S3 S6
Oreatat equation ... 47 20

Rehtire etcentricit7 . . , 498

Excratricity in milei . 495887

RelatiTe mean distance from the aon 72333

Mob diaUace in miles . . . 08,891,486

The greateat elongation, ia 47° .48'.

Ote. 1. Whan Oie doii{^tioD of Vaooi it 30° 44' betwaM
lb infcrior eoi^nDotion and grestait elct^tioo, it appetn
krMUMt ; for thM, thotuh iti phaiit ia bat be 9S.S00tii> of a

1S9 or VHt BJUITB.

dnle, it'MMBnA Mtnr totha Miflt'lllulnfti 'pl^Hif
couunction, iriUQ it xman wifli a jinfirt ffi», tt#W
want of tiU'bee li Mom Oud osupannled h^ flNntanipiirf
<J the light.: in Ovt rftoatian Vmai i* af^i ■!■■ tij ttJijip
anitted aif* !■ broMl da^Ii|^

- ' '"- "-- it MSatiHM

ao upliad to _ _^ _„

problem in mitmu^tMl^ it, the trsadfatuMM^'tlM ^
■Mt* from tha tu tasn I>mb ditennined

3. WhenT^nw hta flMWtotof Iheiun, it riao belort
Am aim, uid b MlM » mornuit itar ; this appearuice con-
timiM mbodttM dBjitogctlMr: when this planet is to ttae
Mttoraienm,ft ■>(*. ilterflieiuii, andia called an eveniiig
itar far about flu MBa pariod, £90 days. Venus appears the
tn-uffatot at tha phbcta : it hw a coosiderabie atmospbere,
•ad Kime uMoolBen aaert, thu they have disoovored moiu-
tiini OD ita nntw*.

- ' or TBI.ElKTH 9. ■ ' ■■ ■>

se2. The diameter of the eartfi is TSSS nulci.
It revolres roubd its axie in tSb. 66'. 4''.
Sidereal rerolutioa 366d. 6h. Q". 11". 6.
Longitude, Jan. 1st, 1816, 3s. 10°. 16'. 64".

Aphelion . . . , 3 9 39 S6;

Secular motioo of aphelion . 1 43 S5

Greatest equation ... 1 65 36.&

Horizontal Parallai, or angle of it! se-

mi-djameterat the sun, . 8". US".

Inclination of axis, Jan. 1SI6, . 23°. 27'. 46".4

Relatiye excentricitj . . . 1681.395

ExcentPid^ in miles . . l,d7M86

Mean distance from the sun . . 95,173,1X7

223. In the daily revolatien of the earth round its
axis, the centrifugal force diminishes the weight of bo-
dies more at the equator than in any other place on the
, surface of the earth, in the duplicate ratio of the se-
mi-diameter to the cosine of the latitude of the place.

Oil. 1. A< the earth rerolT«> upon ita azii, eTerrplaiM oa
its lurface, except the two poles, deicribei a circle, the jHim
o/iriiKsbisperpeadictiiarlo.the ixiituid tha ndisi of irtuoh
A fA« lirieuica of tbat luriitce from ttie a^.

OF THE'' EARTH. IM

2. Whence a body at the equator has its centriibgal force
as much greater than a body between it and the pole, as the
radius of the circle of the equator is greater than that radius ;
and universally, the centrifugal force at the equator, i» to the
centriiiigal force at any o&er place on the surfkce of the
earth, as the semi-diameter of the earth is to the cosine of the
latitude.of the place. And since it is manifest, that the gra-
Tity must be diminished as much as the centrifugal force is
increased, the gravity of a body at the equator, is as much
less than that of a body at any other place on the earth, as
the semi-diameter of the earth is greater than the cosine of
the latitude of the place. ^

3. It is found by calculation from this prop, thatjravity at
file equator is diminished by the centrifugal force, in the ra-
tio of &8 to 289, and if the diurnal motion of the earth round
its axis was about 17 times fiister than it is, the centrifugal
force would at the equator, be equal to the power of gravity,
and all bodies there would entirely lose their weight. But if
the earth revolved still quicker than this, they would all fly
off.

4. Since a place in the equatm* describes a circle of 24,930
miles in 24 hours, it is evident that the velocity with which
that place moves, is at the rate of 17.3 miles per minute. The
velocity in any parallel of latitude decreases in the proportion
of the cosine of l&titude to the radius. Thus, for the latitude
of London^ say, as rad. : cos. 51^. 30^ :: velocity of the equa-
tor ; velocity of London ; by logarithms, as 10.00000 :
9.794150:: 1.232046 : 1X)26 196= 10. 6 miles; that is, London.
moves about the axis of the earth at the rate of more than
10 1-2 miles in a minute of time.

5. Lagrange calculates that the obliquity of the ecliptic
has diminished 2000 years, and will diminish 2000 more, and
Schubert determines its limits at 20'' 34' and 27<' 48'. Its
variation in a century is 60''. On January 1, 18 15, it was 23^
27' 46" 4.

224. The earth is an oblate spheroid, elevated at
the equator and depressed at the poles.

Obs. 1. It has been found by observation, that a pendulum,
ehorter by 2.169 lines, is required to vibrate seconds at the
equator than at the poles ; but the length of pendulums vibrat-
ing in the same time are as the gravities of the places where
they vibrate ; therefore the gravity at the poles is greater
than at the equator. And it has been found by Sir I. Newton,
'titiat this difference of gravity is so much greater than would
arise from the centrifn«:al force aloue^ l\\?tX. \V\ftt^^^^ ^^

13S OP THE SARTK.

•qiialorial il> etcr of I
bea.-23fl(o , wliitb
the ,if>i» ul 31 ni.let.

i, H' ler near the polei nre heavier IhRa Ihe sama

' boilieal heoqiiWor; (i) Beoau^o theyare ni^rorthe

earlh'i =, There (he uholeforceof Ihecarlh'ealtra. Hon

ii Bi;cuiu^ia>e<' (3) Bnuan^e tneirire.ilTiru^l Tor-ce is leetaa

acrnunl ol' the diunial motion being slower. For both IheH

i tram the jioles tAwarill the equator,

i»ll« li

3. And|^<

degreea of latitndo npon the earth's surface ai

ridUn nt)r tlie pi thui near the equnlor ;

that is, it is ■□ an whence ■ d(^r?e mea-

■iirad upon that Hi than upon an arc of the

^^5 Tbeuqui ■vcinantecedentia,OT

go b;ickwjrds :oDtriiry to the order

of th« Bigiw. ^ . .- Ded tbe preCMnon of

the tijuinoxe), because it curries .he equinoctial poitil*
forward in regard lo the sitjns. And tbepreeeision of
Ifte ejui'jio.-ce* Qiiike the tropical year shorter than the
periodical year.

Obs. If, while the lun movei in the orler of the Bi;ii«, the
•quinoctiel p«itit uidves in the contrarf direction, it a mani-
fest, (hat the >un miAsL arrive a: the aoletntial or aquinoctial
Boint from which it tet out, before it arr veiat (he raise pl*ae
ID the zodiac, or mu&( complete the tropical fenr sooner than
file periodical year ! he Iropiual year it obaersed to bo 366
days, 5 hours, 49 minutes ; the periodical year, 386 days, 9
houn, 4 minutes, .j6 aeeonjs.

£tJ6. The preceaaioD of the eqi<inoxe« ie caused by
the action of the »an und moou on thut PiceHS of mat-
ter about the equatorial parts of the earth, by which,
from a perfect sphere it heconip? an oblate spheroid.

Oil. 1. If theeicB?! of matter al Lhe equator he considered
as a ring encompauing the eartb at any distance, hi Sat'jin
is encom|iaspad by its ring; if it be Eupposed that this riiff
morea round its centre, the same vay in which the mooo
tnoFe round the eartji; it ii ohvio'i!: Oiat e^ery point of
this ring will hr acted upon by the diali-irliini^ fierce of the tun
iu the sarac manner as the monn is ai^t^'l upon Par-
tiaalaflf, the motion o[ the nad«a of tht^ rm^, and c((i>

THC £ARTH. 133

seqaently, ofthe whole riog which moyes with these nodes,
and its inclinatioo to the plane in which its centre moves will
be affected in the same manner with the orbit of the moon :
whence, its nodes when in syzyg^ies will stand still, and its in-
clination will be greatest; but in all other situalions, the
nodes will go backwards, and fastest of all when in the quad-
ratures, at which time the inch nation of the ring will be the
least. This will be the case whatever be the thickness ofthe
ring, or its distance from the centre. If this ring be supposed
to adhere to the earth, it is obvious that it will still have the
motions described above, and that, in this situation, the earth
itself must participate of these motions.

2. Hence the axis of the earth, being perpendiculrir to
the plane ofthe equator, changes there with its inclination to
the plane of the ecliptic twice in every revolution of
the earth about the sun. For instance, it increases whilst
the earth is moving from the solstitial to the equinoctial, and
diminishes as much in its passage from the equinoctial to the
solstitial points : which phenomenon is called the nutation of
the poles.

3. This precession of the equinoxes is found to be 50 seconds
of a degree every year, westward or contrary to the sun's an-
Doal motion : so that with respect to the fixed stars, the equi-
noctial points fall backwards 30 degrees, in 2160 years, whence
the stars will appear to have gone 20 degrees forward, with
respect; to the signs of the ecliptic, which are reckoned from
the equinoctial point. Thus, the stars which were formerly
in Aries, are now in Taurus, ice. This period is completed
in 25,920 years.

4. The student should be aware that the precession of the
equinoxes which merely changes the position ofthe stars with
reference to the nodes on the earth's orbit, is different from
the progression of the line of Apsides, or of the Aphelion or
Perihelion points at the rate of 1" 44' in a century, or round
the ecliptic in 20700 years. Sir Richard Phillips, deduces
from this motion, and from the varied action ofthe Sun in both
hemispheres, a theory, by which he accounts for the present
aa;gregation of water in the southern hemisphere, and con-
cludes, that the same aggregation will take place in the
northern hemisphere, when^the Perihelion has its utmost
northern declination.

227. There are two tides every twenty-four hours,

and they are caused by the attraction of the moon and

ofthe suiii

134

OF ASTRONOMY.

1. Let ApLn fig. 68, be the earth, and C it
dotted circle FN represent a mass of water a
fiice of the earth ; let M, m, be the moon : S, i
ferent situations. Because the power of g^rav
the squares of the distances increase, the wa
of the earth A are more attracted by the moo
central parts of the earth C, and the central
attracted than the waters on the opposite sid<
L ; consequently, the waters on the side L \
less than the centre, or will recede from ther
while the moon is at M, the waters will rise
on the opposite sides of the earth A,L ; while, I
traction of the moon, the waters at P and N w

2. Or thus; because the moon and earth an
volvir^ about their common centre of ^avity.
points A, C, L, describing circles about this ci
the same periodical times, the forces acquired
these circles, will be to each other as their di
centre aA,aC,aL. Consequently, the point L
er force than C, and C than A, to retain it in
these points are retained in their respective cir
at M ; and, consequently, the point L, which
and therefore requires the greatest force, is at
whilst A, the nearest po4nt, is attracted the d
water about A being attracted too much, ar
too little, both will hare their gravity diminish
oCthe moon, and will endeavour to leave the

THE TIDES. 135

•

die water ti P and N, having their gravity increased by the
same eaose, will subside. Hence the form of the water on
the sar&oe of the earth will become an oblong spheroid.

3. T^iis oval of waters keeps pace with the moon in its
monthly course round the earth ; while the earth, by its daily
rotation about its axis, presents each part of its surface to the
direct action of the moon, twice each day, and thus producei
two floods and two ebbs. But because the moon is in the
mean time passing from east to west in its orbit, it comes to the
meridian of any place later than it did the preceding day ;
whence the two floods and ebbs require nearly 25 hours to
complete them. The tide is at the greatest height, not when
the moon is in the meridian, but some time afterwards, be-
cause the force by which the moon raises the tide continues to
act for some time after it has passed the meridian.

4. As the moon thus raises the water in one place, and de-
presses it in another, the sun does the same ; but in a much
less degree, on account of the small ratio of the semi-diame-
ter of tibe earth to the distance of the sun ; for, as it was shewn
of the moon, that the force of the smi by which it disturbs iti
motioD 18 as the distance of the moon from the earth to that
oi the sun from' the same, so, in this case, the force of the sun
to disturb the waters is as the semi-diameter of ^e earth to
the distance of the sun, which ratio is very small.

228. The tides are greatest at the new and full
moons, and least at the first and last quadratures, and
the highest tides are near the time of the equinoxes.

When the moon is in conjunction or opposition with the sun,
as M^ in, S, the tides which each endeavours to raise are in the
same place ; whereas, when the moon is in the first or last
quarter, the sun being in the meridian when the moon is in
Uie horizon, as M, Q, depresses the water where the mooK
raises it; whence Uie tides are then the least of all. On the
full and new moons, which happens about the equinoxes, when
the luminaries are both in the equator or near it, the tides are
the greatest of all : for first, the two eminences of water are
at the greatest distance from the poles, and hence the difiler-
enoe between ebb and flood is more sensible; for if
those eminences were at the poles, it is obvious we
should not perceive any tide at all; secondly, the equa-
torial diameter of the earth produced passes through the
moon, which diameter is longer than any other, and conse-
quently, there is greater disproportion between the dis-
tances of the zenith, centre, and UiB^dAi^ %(QOi ^<& ^«o^x^ ^^

136 OF ASTRONOMY.

gravity of tba earth and moon, in this sitaatioii, than iai
other : and thirdljr, the water rising higher in the open m
rashes to the shores with greater force, wheee heibgitopp
it rises higher still ; for it not only Hses at the shpr^ in g
portion to the height it rises to the open seas, but alsq
oording to the Telocity with which it flows from thence afal
the shore. The spring tides which happen a little hefi^m
▼ernal and after the aatamnal eqainoz, are the graatM
all, because the san is nearer the earth in winter &a« in
eummer.

229. When the moon is in the northern henisplifl
it produces a greater tide while it i§ in the merid
above the horizon, than when it is in the nieridiaiq 1
low it ; when in the soathem hemisphere, thet r^T^

Obi. For the ^l^e reason, when the moon is in-tiif ton
cirn signs, the greatest tides on t^e other side of the ej^ui
will be wli^n it is helow pt^r bopi^n, and the Xe^^t tid»%ir!
U is abore it.

2. What hath been said of the titles, must ^e an4fV||
upon supposition, that the globe of the earth is entirely oov
ed with water to a considerable depth ; but continents wli
stop the tide, streights between thenar islands, and the si
lowness of the sea in some places, whi^h are all impedim<
to the course of the water, cause many exceptions which
only be explameJ from particular observations on the nat
of tides at different places.

OF Mai^s i .

230. The diameter of Mars is 5400 miles. --It :

volves about its axis in 24h. 40'.

o. 1 - 1 1 .• < 686d. 23h. 30'. 35" 6.

bidereal revolution { - ooi j

^ or 1 year, 321 days.

Longitude, Jan. 1, 1815 . . 7s. 28^ 1

Annual motion . . . 6 11 17

Aphelion .... 5

Secular motion of aphelion

Node .... 1

Secular motion of node

Inclination of orbit

Greatest equation

Relative excentricity

14183

ASTEROIDS. 137

Excentricity in miles - • 13,254,852
RelatiTemean distance from thesun 152,369.27
Mean distance in miles - - 145,014,148

Obi, 1. When Man ii in opposition to the son, it is nearest
Id tfia earth, and its diameter consequently appears the g;reat-
«at It never shines with a brig;ht light, but has a red ap-
peannoe, whence it is conclnded it has a dense atmosphere.
It appears with different phases according to its position,
tiioiigh never homed, its ffuse being always greater than a
lemi-circle. /

The ftiUowing particulars respecting Mars are given by Dr»
Horschdl, after long and accurate observations.

The axis of Mars is inclined to the ecliptic 59^ 42'.

The node of the axis is in 17^ 47' of Pisces.

The obliquity of the ecliptic on the globe of Mars, is 28<^ 42^

The point Aries on the martial ecliptic answers to our 19^
28' of fiStgittarius.

The &are of Mars is that of an oblate spheroid, whose
•qnatinul diameter is to the polar one as 1355 to 1272, or as
16 to is nearly.

The equatorial diameter of Mars, reduced to the mean dis-
tance of the earth from the sun, 9" 8'".

And that planet has a considerable, but moderate atmosphere,
CO that its inhabitants, probably, enjoy a situation, in many re-
speota, limilar to ours.

Op Vesta p .

231. Diameter 1800 miles.

Distance from the sun - 224,145,000.

Sidereal revolution - - 4 years, 4 months.

Inclination of orbit - - 16^

Excentricity in miles - - 30,000,000

^/Ucending node - - 5s. 11® 6'

Of Juno or Harding 9-

Diameter 1500 miles.

Distance from the sun - - 253,541,009
Sidereal reTolution - - - 5 years.

Inclination of orbits - - - 21^
Sxcentricity in nQes - - • 60,000,000

138 OF ASTROirOlfY.

OF CERES OR PIAZZA f •

233. Diameter 1700 miles.
Sidereal revolution 1680 days^ or 4 years 266 days.

Distance from the sun , • • 263,344,042

Excentricity • . - - . 21700

Inclination of orbit - • - 10^ 36*

Ascending node - - - 25 21 6

Longitude, Jan. 1, 1801 - 25 17 19

Aphelion - - - 10826 9

OF PALLAS OR OLBERS $ •

234. Diameter 2000.

Sidereal revolution \ 1682 days,or 4 years,? months,

I and 1 1 days.
Distance from the sun - 263,153,691.

Excentricity - • - 24640

Inclination - - . 340 39'

Ascending node - - - 6s. 22 28

Longitude, Jan. 1,1804 - 9 29 63

Aphelion - - - 10 1 7

Annual motion - - - 2 18 11

Obs. Tliesc anomalous bodies, so unlike the other primary
planets. Dr. Ilerschel has denominateil Asteroids, Probably
thny are the fragments of some comet; or perhaps other si-
milar bodies abound in the solar system, thou«^h they have
hitherto from their smalluess or darkness, escaped obsenra-
tioQ.

OF JUPITER !!.•

285. Diameter 94000 miles.
Diurnal rotation 9h. 56'.
Sidereal revolution 4332d.14h.27' 10.8,or lly.315d.

Geo. longitude, Jan. 1, 1815

6s. 9*^.

20'.

Annual motion

Aphelion ...

6 11

Secular motion of Aphelion

Node - ^ -

3 8

Secular motion of node

Jnclhiution 0/ orbit

BATVRH. 139

Greatest equation ^ - • 5 31

Relative excentricitj - - 23013.3

Excentricity ia miles • - - 23375454

Mean distance in miles - 495,990,976

Obt. 1. Jupiter is the largest of the planets, and has a brig^ht
appearance ; but less 80 than Veuas, ou account of its mucU
gre'^ter distance. Like Mars, it appears largest when in op-
position to the sun; when it is to the west of the sun, it is a
moroiog star, and when to the east of it an evening star. Its
axis being nearly perpendicular to the plane of the orbit,
there is but little diversity of seasons. It is surrounded by
many faint substances, called belts, which are jnrallel to its
equator ; their numbers are variable ; between them are fre-
quently seen dark spots. Jupiter has four satellites.

2. Jupiter is surrounded by cloudy substances, sal ject to
frequent changes in their situation and appearapoe, called bis
Belts. The Belts are sometimesof a regular form ; sometimes
interrupted and broken ; and sometimes not at all to be seen.

OP SATURN h.

236. Saturn's diameter is 78000 miles.

Diurnal rotation lOh. 16'.

^.j , , ,. S 10759d. Ih. 51. 11.2 or

Sidereal revolution ^ 2 ^^ j^^^

Geo.. longitude, Jan. 1, 1815
Annual motion - -
Aphelion - - ..

Secular motion of aphelion
Node . - -

Secular motion of node
Inclination of orbit
Greatest equation - *

Relative excentricity

Excentricity in miles

Mean distance in miles ^

237. Saturn is a beautiful object for a good telescope,
Iiaving SEVEN moons,a double RiNG,and substances si-
milar to the belts of Jupiter. Saturn's ring is a thin,
hroady opaque circle, encompassing the planet without
touching it ; and is very similar U) the horizon of a globe
surrounding the globe : and asS^lvMV?^^^^&^'^^^^\^'^^

10s.

O'^.

32'

•

6364042

60127670

907,956,130.

14*

diciilwto itnio^ if tke ^obe be rectified for
it will aptly represent the planet and the rin
Obt. 1 . Tha distance of the inner ring from Sntiil
miles, and iti breadth 19034 mile! : between the t«
jpace oC 2997 miles ; and the breadth of the outer r
loiles. Saturn has an alnoaphere extending ta the i
light of this planet is p&le and feeble, on uccount i
distance-

S. Thelutter discoveries of Dr. Herschel have I
what was supposed to be u ein;le broad flat ring ■
divided into Ino porta, lying eiactly in the sama
revolving about an axis perpendicolar to that plai
3!' 15".

Of HBAtauBi. V-

-^^ «S& Thsdlametw of Ob planet is SilOf

iileiMRrriatisBQtnSM^ «r 8»r. S21d.

Dm. ^lytade, Jan. 1. 16I0. ' • 9»:ff

ilnliail owtion - • •. -

Apbelioa - - • - 11 16

Secularmotion of Aphetion - - 1

Ascending node - - - 2 12

Secatar motion of node - -

Inclination of orbit ...

Greatest equation ... 5

Relative escentricity

Excentricity in miles - - 84

Relatire mean distance from the son 1

Mean distance in miles - , - 1,816,

O61. I. This planet wu discovered b; Dr. Hi

Varch 13, 1731. Its appearaooe is like thatof a i

tWMD the aisth and seventh magnitude, and tht

wueelr bt seen without a teleacape. The Henet

tatalUta- He modestly called it OeBrgium Sidui.

t. If the planets Mercury, Ventu, the Earth, Ma

and Batnni, be in conjunction at any time ; in tlM

tSOfiOH J»H thej will be veiy nearly in coi()unQ

tUsoluliatu. St

MerciiTj after mtldnc ■• 1163577 in 8S3fl

Temii .... 455133 — 8S3S

The Euth ■ . - 380000 — 8835

Man , . , * 149ST8— S83S

SECONDART FLANfiTS. 141

Revolutions, Seconds.
Japiter «. * . . 23616 in 8835946544448
Saturn . - - - 9516 - 8835946558608
3. Aa easy distinction between a plauet and a fixed star is
this : the former shines with a steady light, but the latter is
constantly twinkling^. This twinklin^ or scintillation of a
star is occasioned bj the irregular progress of the light from
such distant bodies to the eye.

OF THE SECONDART PLANETS.

239. The secondary planets are those which move
round some primary planet, as their centre,in the same
manner as the primary planets move round the sun.

The earth IS thus attended by one secondary pla-
net, called the Moon ; Jupiter by four, Saturn by se-
ven, and Herschel by six.

240. The motion of a secondary planet in its orbit
is not nearly so uniform as that of a primary ; because
though every secondary gravitates chiafly towards its
primdry as a centre, yet its motion is much disturbed
by the unequal action or influence of the sun.

OF THE MOON ••

241. Its diameter is 2180 miles.

It revolves about its axis in 29d. 12h. 44'.2". 8283.

Mean distance from the

earth

. 236347 miles.

Mean excentricity

13035'

Greatest equation

6^. 18'. 32".

Greatest inclination of orbit

6 18

Least inclination of orbil

Greatest diameter

33 24

Least diameter

29 22

Tropical revolution

27d

. 7h. 43'. 4".6796

Sidereal revolution

7 43 11 5259

Synodic revolution *

•

12 44 2 .8283

The excentricity

13 700 miles.

Horizontal parallax

63'

46"

to 61' 26".

Sidereal revolution of the

apogee

fty;

2>\^ \\ \V ^^A^«v^

142 OP ASTRONOMY.

Sidereal reyplution of

the node - - 18y,223 7 13 17.744
Longitude, Jan. 1, 1815 Ss. 26*. 13'. 63^

Diurnai motion, in respect

to the equinox • - 13 10 36.0271

New moon January, 1815, at lOd. Ih. 57^

Dtf, 1. The tropical revolution signifies the complete Te9>
lution of 12 signs, performed round the earth, and is sometinMi
called a periodical month.

The «i(/er«a/ signifies a completion of the motion to the sanw
star, and is something longer than the tropical on accomitflf
the precession of the equinoxes.

The synodic r^olntion is the time firom one eonjonetioB
with the sun, or one new moon to another, this exeeeds tiie
sidereal, and it will be found that in 29d. ISh. 44'.2^.8283 thst
the moon will be again in conjunction with the sun, title earth
being at the distance of 29*^. 6' 20^.2 from the place of the finv
mer conjunction.

2. When the moon is at its greatest distance fit»m tiie earth
in its orbit, whieh is ecliptical, or at its higher apsis, it is said
to be in its Apogte ; when at its least distance, or lower apsis
in its Perigee.

3. When the moon is in conjunction with the sun, it is Aew
JHoon ; when in opposition, it is Full Moon : its conjunction
and opposition are called by the common name of its Sytsygia*

242. The moon at its conjunction, is invisible ; at
its opposition, its whole disc is enlightened ; at its
quadratures, it is half enlightened ; between the con-
junction and quadrature, it is horned ; and between
the quadrature and opposition, it is gibbous.

(See Fig. Sd—'Frontispiece.')

lllut. S is the sun, T the earth, ABC, &c. the moon in its
orbit. One half of the moon is always enlightened by the sun.
At A, the moon is between the earth and sun, it is then new ;
and is invisible as represented at a : at B the enlightened part
X zh turned to the earth, and she appears homed as at 6 ; at C
the half of the enlightened side is turned to the earth, and she
appears a half moon as at c; at D the part x z it turned to the
earth, and it appears as at (/ ; and at E the whole of the en-
lio^htened part of the moon is turned to the earth, and we have
full moon as at e. As she passes throuf^h the rest of the orbitt
file puts on th0 same phases as before^^uXm ^ wiVcvn ^x^et

THE ItOON. 143

Obi. The earth must be a satellite to the moon, and subject
to the same changes, but more than 13 times larger than the
moon appears to us. At new moon to us, the earth appears
full to her.

243. The moon always has nearly the same side
towards th^ earth.

Obi, 1. Hence if the moon revolves about its axis, its perio-
dical time must be equal to that of its revolution in its orbit
round the earth. This is also found to be the case with the
filUi satellite of Saturn as it regards the primary.

2. Though the year is of the same absolute length, both to
the earth and moon, yet the number of days in each is very dif-
ferent; the former having 365 1-4 natural days, but the latter
ohlyabout 12 7-19, every day and night in the moon being as
long as 29 1-2 OB the earth.

^244. The moon appears to have two librations,

one upon a line perpendicular to its axis, called its

libration in latitude ; the other upon its axis, called

its libration in longitude.

Obi. This appears from observation ; some small portions
of the surfiuse of the moon being visible in some parts, and
invisible in other parts, of its orbit ; that is, in consequence
of her libration in latitude we sometimes see one pole and
sometimes the other. And by this libration in longitude,
more of the western limb is sometimes seen, and at others
more of the eastern. The inclination of her axis to her orbit
is 60^ 49'.

245. The nearer the moon is to its syzygies,* the
greater is its velocity ; and the nearer it is to its quad-
ratures, the slower it moves.

246. When the earth is in its perihelion, the pe-
riodical time of the moon is the greatest ; when the
earth is in its aphelion, the periodical time of the

moon is the least.

Obt. Since all the irregularities of the moon's motion pro-
ceed firom the action of the sun, it follows, that where the action
of the 9un is greatest, the irregularities arising from it will be
greatest too. But the nearer the earth is to the sun, the greater
will be the action of the sun upon the moon ; and the more the
moon tends towards the sun, the less will it tend towards th^

♦ The line joining the centres of the 8un^ea^t\v,^wv^\s^Rl«^^
tl the new and full moon, is called tbe Uxkft q»1«t**1^^*

mrtfa. Wh«n, therefore, the earth is -dt (be perihelion, ;
coDsequentlf at its least distaucc from the Bun, the acttaf:
the luo opan Ibe moon will be grcaleit, anil (leifroy mori
iU teDdenc; lowards Ibe earth than at any other distac
Therefore, irfaao the esrifa » at Die perihelion, Uie aieoni
describe ■ larger orbit about the earth, than whcu the b>
i> nl anf other ilistance from the luii, anJ, FODieqoeiitlir, !
periodical time will then be the Iniigett. But (he earlh it
ite iierihelioo in the winter, and coDeequentiy, then tbe Uu
will describe the ojlermost circle stbaut the eailh,aiiil.
periodical tima will be Ibe longeit-, which ugreea with

nphelioD, the lendeac]' of the moon (ownrila tbe earth wilt
the great eat, and, consequeiill^, her periodical lima Ui»lei
And in thi>Da», which will be iolhetummer, itwiildaicr

the innermoiit circle about the earlh.

247. The excenlricitj of (he moon's orbit is vu
ed ID every revolutioo of the moon, and ia great
when tlic moon is in Bjzygy, and least when it is
quadrature : and the orbit is most of all excentri
when the line of the apsis ia in the sjzygies, and le
of iill CKcentiicnl when this line is in the qtindratur
. And ifwe cainparaier'«ral ravoltKIonof Um^

D that litQBtioD, the diSereaca between the t«
which the moon has to the earth in one of tbe apm, ud i
wtiicb it hai in theoppoaita one, ii the greateit ^ all ; Wht
M, whee the Una of the apiiM iaiu the qDadratDre^ (bk dil
eaca ia the leaat, and therefore the lamr ezoeatricity will

When the gnrity of llie moon towtu-da Uw aajrtb dawrM
too hit, the exe«ntric)ty of her orbit will inereiaa, and w^
her KTiTitj towarda the earth inereuaa too fait, the k«
tricitjof herorbit willdeoreBH! and tba orbit it«tf«fll
prokcb nearer to a cirola.

3. All thairregalaritieaof IbamoeaaramMtar wk«B
aarth ii in iti parihdioD, than when it ia in rta a plialia B ,
Moia the aA>ct erf' lb* mm'* aetioa, wbarabr thaf km pM
oad.ia inTaraely aa tfaea]iaraof :tadiatancelfnaillMa«
They are abofraatarwhendia»oaaii idsaqJoetioBV
the inn, than in qipoaition, for^M aaawraaaeat ftrthaal
and noon, taken to|Mher, ara naarar tiie aonin UM M
*llaaUoa of tka moon, ttkntiu^ «I«^IL^!M^■!aM.

TH£ MOOK. 146

248. The moon is a dark, opaque globe, and its sur-
face is irregular.

Obi, 1. The face of the moon, a^ seen through a telescope,
appears diyersified with hilU aod vaUeys. This is proved by
Tiewing her at any other time than when she is fuU ; for thcu
there ia no regular line bounding light and darkness; but the
ed^ or border of the moon appears jasfged ; and even in the
dark part near the borders of the lucid surface there are seeu
•ome amall spaces enlightened by the sun's beams. Besides,
it is Bioreover evident, that the spots iu the moon taken for
moantains and valleys, are really such from their shadowa.
For in all situations of the moon, the elevated parts are con-
stantly found to cast a triangular shadow in a direction from
the SQO, and the cavities are always dark on the side next to
the SQQ, and illuminated on the opposite side.

2. By the observations made by Dr. Herschcl in November,
17799 and the four following months, we learn, that the alti-
tude df the lunar moantains has been very much exaggerated.
His observations were made with great caution, by means of
a Newtonian reflector, 6 feet 8 inches long, and with a mag-
nifying power of 222 times, determined by experiment ;'and
the method which he made use of to ascertain the altitude of
those mountains, which during that time, he had an opportu-
nity of examining, seems liable to no objection. The rock
sitaated near Laew niger, was found to be about one mile ia
height, but none of the other mountains, which he measured,
proved to be more than half of that altitude ; and Dr. Her-
sohel concludes that, with a very few exceptions, the gener-
ality of the Ulnar mountains do not exceed half a mile in their
perpendicular elevation.

3. To Dr. Herschel we are also indebted for an account
of several burning volcanoes, which he saw at different times
in the moon. In the 77th vol. of the Phil. Trans, he says,
"I perceive three volcanoes in different places of the dark
part of the new moon. Two of them are nearly extinct ; or,
otherwise in a state of going to break out. The third show-
ed an actual eruption of fire, or of luminous matter.' ' On the
next night. Dr. Herschel saw the volcano burn with greater
violence than on the preceding evening. He considered the
eruption as resembling a small piece of burning charcoa.
when it is covered by a thin coat of white ashes, which fre-
qaently adheres to it, when it has been some time ignited, and
it had a defi;ree of brightness, about as strong as that with
which such a coal would be seen to glow iu faint da.^«\v^v.

l^S OF .^TAOSOHY.

249.. \n Eclipse o/thc Moon happansviheDthi
passing betwet^a the aun aad mooD, caats its )
on the moon,aDil of course (he moon cant
eclipsed at the full, or in opposition, and whet
full, it is in or near one of its nodes.

Oil. 1. For it ia only when the mmia it id opposit
it can come within tbe sbadow of the earth, which i
ways bo an that side of the esrtb whir.h is from theaui

S. The earth betug ■□ tbe plane ol'the ecliptic, thi
of il9 shadow is always in that plane ; it therefore tt
be iu Hi nodes, that i!, in the plane of the ecliptic, i
dow of the Girth will fall upon it: alio, since this sh
of conaiderable breadth, it is partly above and parti
the plane of the ecliptic; if therefore the moon io op
be ao near one of its nodes, thai its latitude is less tl
the breadth of the shadow, it will be ectiiised. But I
the plane of the mooa's orbit makes an angle of more
degrees with the plane of the ecliptic, it will frequent
too much latitude at iU opposition to corns within the
of the earth.

3. Let S repreienlthesQi), fig. 70, mthe laooo 1
t)M earth and the sun, a £ G b a portion of the earl
Int, e aod / two places on tbe larface of tbe eartt
^tk pvt Df the moon's shadow is called the umf
tb* liAt part the pmumbra ; now it is eTident th»t if
tatMCn siloated ia that part of the earth where tbe
fidll. that is, between e and/, there will be a total ei
the Inn at that place; at* and/, in the pwimbn
will be a pariiai •clipse ; and b^ond the paDumbr
will be no a(Jip*e. As the earth ii not alway* at tfa
dirtanoe from the moon, if an eclipse sboold ha^^en w
•uth ii u far trom tb* moon that th* lines F e and C
Mch other btfot* they coma to the earth, a spectatw
ti oa tlM earth, in a diraot li>e between the oentrei
don Mid MOM) wmU H« a Ttac of light niind the da

X)F ECLIPSES. 147

of the mooD, called an annular eclipse ; when this happeds
there can be no total eclipse any where, because the moon's
umlyra does not reach the earth. People situated in the pe-
nanabra will perceive a partial eclipse. According to M« de
Sejonr, an eclipse can never be annular longer than 12 mii).
24 sec. nor total longer than 7 min. 58 sec. The duration of
an ecHpseof the sun can never exceed two hours. Keith, 168.

250. The san being larger than the earth, the sha-
dow of the earth is a cone, the base of which is on the
sarface of the earth, and the moon is eclipsed by a
section of the earth's shadow.

Obs. If the earth were larger than, or equal to, the sun, it
is manifest that its shadow would either perpetually enlarge,
or be always of the same dimensions ; but in this case the su-
perior planets would sometimes come within it, and be eclip9-
ed, which never happens. Therefore the sun is larger than
the earth, and produces a shadow from the earth of a conical
form, which does not extend to the orbit of Mars.

261. An eclipse of the moon is partial, when only
a part of its disc is within the shadow of the earth ;
it is total, when all its disc is within the shadow ; and
it is central, when the earth's shadow falls upon the

centre of the moon's disc.

Obt. 1. Let S, represent the sun, fig. 70, EG the earth, and
m the moon in the earth's umbra, having the earth between
her and the sun ; DEP and HGP the penumbra. Now, the
nearer any part of the penumbra is to the umbra, the less
light it receives from the sun, as is evident from the figure ;
and as the moon enters the penumbra before she enters the
umbrai she g^dually loses her light and appears less brilliant.

2. The duration of an eclipse of the moon, from her first
toachingthe earth's penumbra to her leaving it, cannot ex-
ceed five honrs and a half. The moon cannot continue in the
earth's umbra longer than three hours and three quarters, in
any eclipse, neither can she be totally eclipsed for a longer
period than one hour and three quarters. As the moon is
actually deprived of her light during an eclipse, every inha*
bitant npon the face of the earth, who can see the moon will
see the eclipse. Keith, 169.

262. An Eclipse of the Sun happens when the moon,
passing hetween the sun and the earth, intercepts the
sun's light, and the sun can only be ecli\j%e,d^KVvft.\iR:^

148 OF ASTRONOMY.

moon, or when the moon, at its conjunction^ is ii
near one of its nodes.

Obs, For unless the moon is in or near one of its nodi
cannot appear in or near the same plane with the inn : v
oat which, it cannot appear to us to pass over the disc oi
ran. At every other part of its orbit, it will have to u
northern or southern latitade, as to appear either aboy
below the san. If the moon be in one of its nodes havin
altitude, it will cofer the whole disc of the aan, and pro(
a total eelipse, except when its apparent diameter is less
that of the sun : if it be near one of its nodes, haying a s
deg;ree oi latitude, it will only pass over a part of the s
disc, or the eclipse will be partial.

253. In a total eclipse of the sun, the shadow
the moon falls upon that part of the earth where
eclipse'is seen.

. Obi. A spectator, placed any where in the centre^ wil
see any part of the sun, because the moou will intercepi
the rays of li^ht which come to him directly irom thei
and it is manifest that, in this situation, the moon, bein,
opaque body, will cast its shadow upon that part of the e
where the eclipse is total.

254. In a partial eclipse of the sun, ^penumbra
imperfect shadow of the moon, falls upon that pai
the earth where the partial eclipse is seen.

255. If the moon, when new, is in one of its no4
the eclipse of the sun will be central.

Obs. 1. For then the centres of the earth, sun, and mc
being; all in the plane of the ecliptic, the centre of the n:
will pass between the sun's centre and that of the earth.

2. The penumbra of the mooo, in a central eclipse,
not cover the whole disc of the earth. The 8emi-diamet<
the moon's penumbra, being; equal to the sum of the appai
semi -diameters of the sun and moon, that is, about
23''. X 15'. 37". or 32'. at the medium ; its diameter is al
64'. whereas the diameter of the earth's disc is about 1
whence the penumbra cannot cover the whole disc.

3. The heig^ht of the shadow of the moon is about 60 1-2 si
diameters of the earth. The semi-angles of the earth's i
dow and the moon's shadow, being; each equal to the sun's
parent semi .diameter, the angles are equal to one another,
these cones are similar. Therefore as the semi-diameter of
base of the earth's shadow (that is, of the earth) is to the sc

OF ECLIPSES. 149

diameter of tlie base of the moon's shadow, (that is, of the
moon,) so is the height of the earth's shadow to the hcig^ht of
the moon^s shadow. Now the semi-diameter of the earth is
to that of the moon nearly as 100 to 28, and the heig;ht of the
earth's shadow is about 217 semi-diameters of the earth ;
iviience the height of the moon's shadow is equal to about 60 1-2
semi-diameters of the earth ; for 100 : 23 : : 217 60 1-4 nearly.

256. An eclipse of the sun is said to be annular^
when at the time of the eclipse a ring of the sun ap-
pears round the edges of the moon ; and a central
eclipse of the sun will be an annuLir one, if the dis-
tance of the moon from the earth at the time of the
eclipse be greater than its mean distance.

257. If the orbit of the earth and that of the moon
were both in the same plane, there would be an
eclipse of the sun at every new moon, and an eclipse
of the moon at every full moon. But the orbit of the
moon makes an angle of about five degrees and a
quarter, with the plane of the orbit of the earth, and
crosses it in two points called nodes.

06«, 1. Astronomers have calculated that, if the moon be
lest than 17° 21' from either node, at the time of new moon,
tho sun may be eclipsed ; or if less than 11° 34' from either
node, at the full moon, the moon may be eclipsed ; at all other
times there can be no eclipse, for the shadow of the moon
will fall either above or below the earth at the time of new
moon ; and the shadow of the earth will fall cither above or
below the moon, at the time of full moon.

2. To illustrate this, suppose the right hand part of thr^
moon''s orbit; fig. 7Q, to be elevated above the plane of the
paper or earth's orbit, it is evident that the earth's shadow, at.
full moon, would fall below the moon ; the left hand part of
the moon's orbit at the same time would be depressed below
Uie plane of the paper, and the shadow of the moon, at tho
time of new moon, would fall below the earth. In this cafe,
the moon's nodes would be between E and a, and between (1
and i, and there would be no eclipse, either at the full or new
Dioon ; but, if the part of the moon's orbit between G and h
be elevated above the plane of the paper, or earth's orbit, the
part between E and a will be depressed, the line of t}\c moon>

13*

1'50 OF ASTRONOMY.

nodes will then pass through the centre of the earth and th
of the moon, and an eclipse will ensue.

3. An eclipse of the sun begins on the western side of 1
disCi and ends on the eastern ; and an eclipse of the moon b
gins on the eastern side of her disc, and ends on the wetter

4. The average number of eclipses in a year is/otir, tv
of the sun, and two of the moon; and, as the sun and mo
are as long below the horizeo of any particular place at th
are above it, the average number ot visible eclipses in a ye
is two, one of the sun and one of the moon. Keith,

258. When the moon is near the first of Aries, ai
is moving towards the tropic of Cancer, the time
its rising will vary but little for several days toget
er, and produce the phenomena of a Harvest moon.

Oht, 1 . If the moon were to move in the equator, its moti*
in its orbit, by which it describes a revolution, in respect
the sun, 29 days, 12 hours, would carry it every day eaf
ward from the sun about 22^ 11', whence, its timeofrisii
would vary daily about 50 minutes. But because the mooc
orbit is oblique to the equator, nearly coinciding with t!
ecliptic, different parts of it make different angles with t!
horizoD, as they rise or set ; those parts which rise with tl
smallest angles, setting with the greatest, and the reven
Now the less this angle is, the greater portion of the orbit i
ses in the same time. Consequently, when the moon is
those parts which rise or set with the smallest angles, it ria
or sets with the least difference of time, and the reverse. B
in northern latitudes, the smallest angle of the ecliptic ai
horizon is made when Aries rises aod Libra sets, and tl
greatest when Libra rises and Aries sets ; and therefor
when the moon rises in Aries, it rises with the least diffe
euce of time. Now the moou is in conjunction in or nc
Aries, when the sun is in or near Libra, that is in the autut
nal months ; when the moon rises in Aries, whilst the si
is setting in Libra, the time of its rising is observed to vai
only two houE| in 6 days in the latitude of London. This
called the harvest moon.

2. This circumstance takes place every month ; but as
does not happen at the time of full moon, there is no notii
taken of it. When the moon's right ascension is equal to si
signs, that is, when she is in or about the beginning of Libr
there is the greatest difference of the times of rising,viz. aboi
an hour and 15 minutes. Those signs which rise with the lea
an^Jeset with the greatest, and the contrary ; therefore, whi
there ia the least dilTerenco in the Ume^ o^ TVl\xx^^^^[l«x^ \\ ti

OF THE SATELLITES. 151

greatest in settings, and yioe rtrm. All this may be pleas-
ingly exemplified by means of a celestial globe.

OF THE SATELLITES OF JUPITER.

259. The following table gives the periodical
times and distances of Jupiter's satellites, and the an-
gles ander which their orbits are seen from the earth,
at its mean distance from Jupiter.

SateUiiet. Days, h. min. Dis. in miUi, Angles of orbits

1— 1 18 26.6 — 266.000— 3' 55''

2—3 18 17.9 — 423.000— 6 14

3—7 3 59.6 _ 676.000 — 9 68

4—16 18 5.1—1.189.000— 17 30

Obs. The third satellite is the largest of all ; the first and
(burtb are nearly of the same size ; the second is the smallest.

260. These satellites of Jupiter are of great use in
astronomy. (1) In determining the distance of Jupi-
ter from the earth. (2) They afford a method of de-
monstrating that the motion of light is progressive.
And (3) from the eclipses of the satellites of Jupiter,
we ascertain the longitude of different places.

261. OF THE SATELLITES OF SATURN.

Satellites. Periodical terms.

Distance in miles.

1 — 1 day.

8h. 53'

8'' 135.000

22 37

22 107.000

3 — 1

21 18

27 — 170.000

4 — 2

17 41

22 — 217.000

5 — 4

12 25

12 — 303.000

6—15

22 41

13 — 704.000

7—79

7 48

— —2.050.000

06#. The Ist and 2d satellites were discovered by Dr. Her-
•ohelyin the years 1787 and 1788. To prevent mistakes he call-
ed them the 6th and 7th, though nearer to the planet than the
other five. Dr. Herschel observes, that Saturn has probably
a coneiderable atmosphere. It turns on an axis perpendicu-
lar to the ring, in lOh. 16' 0.44'' and is flattened at the poles.
80 that the equatorial diameter is to the polar as 11 to 10.

OF THE SATELLITES OF HERSCHEL.

282. Dr. Herschel has at different times, discover-
ed six satellites belonging to his new ^l^nAt*

J5t

SttaHim. tnmJbtimtta. . ' PanbajfealTii
1 — Jan. t«, 1790 — £d. tlh. 9^- '
« — J«n. 11, Itti 1- -8 W. - I 1
3— HM-.26. n94~ 10. ».:X*
4 — Jin. 11, I78T,-- 13 11 .«
6 — Feb. 9» 17M — i& T 49
6 — Feb. 28, 1794 ^ IM IjB W
. 268.: A nj of Ju^ iltbaat eight nufotei -n

ii^ fromtbe snn to 9t« eirtli'

Oil. From coiopatins the times of the apparent en
Hnd GOiersion of Jupiter's latellites, ivitb tnbles culciilsl
the mean diatancea of the earth ffOM the mteilite, tho '
Emersion bI the least ilistance of the planets is CooaJ, to h
about 8 minutes sooner, and at the greatest distance al
minntea later, than by the tables: coruequi^ntly, the i
light is about 16 minutes in paMiii|; throiisli tho enrth'a
or 8 minutea in coming from the mis to the earth. T
amctcr ofthe earlh'! orbit being 1 D J.OQO.OQO milej, the y
of light will bo 194,000,000

■ ■ ■ -^02 JBB miles tDBtecand ol
18-1-SO

26i. CometsnreopaqueandseliiibodieB. Ac<
tit R girea distance from the enrth, shitiea bri
when it is on the same side of (he enrth as the
than when it is oti the contrary side ; from whei
appears that it owes its brightness to the sun.

Oil. 1. Of all the coroota, the periods of only thw
known with any degree of certainty. The first of these
etsappearedintheyears 1531,1607, and 1782; aod is e:
ed to appear every 75th year. The second <^ them aj
edinlSSa-ana 1661, and was expected to return in 178
every lS9th year afterwards. The third, having last aj
ed in 1680, and its period being no leas thao 575 year
uot return until the year 2225. This comet, at its gt
distance, is about 11^00,000,000 of milej from flle siin
its least distance from the sun's centre, which is 49,000
is withinleisttian a third part of tb« sun's semi-dismete:
his surface. Id that part of its orbit which i$ nearest tl
it mores at the rate of 080,000 miles an hour.

THE PAAAZXAXES, &C«

2. The Chinese astronomical books record the appearance
of 2 or 300 comets.

3. The tail of the comet of 1680 was at least 100 millions
of miles long^; and that of 1812 was 30 millions of miles. Sir
Richard Philips published in' the Monthly Mag^ine the opin-
ion that this wonderful appendage of comets, is occasioned
by the refraction, and consequent condensation of the sun's
li^t throug^h the dense ^atmosphere of the comet : hence the
tail is always in an exact rig;ht line opposite to the sun : and

ji hence on the principle of a convex lens, the tail lengthens as
- it approaches the sun, and shortens as it departs.

OF THE PARALLAXES, DISTANCES, AND MAGNITUDES
OF THE HEAVENLY BODIES.

266. The Parallax of the heavenly bodies, is the
change of their apparent situation with respect to each
other, as the spectator views them from different sta-
tions entire earth, or parts of the earth's orbit.

266. The Diurnal Parallax is the distjmce be-
tween the apparent place of a heavenly body, as
viewed from the wr/ace of the earth, and its apparent
place, as viewed from the centre of the earth.

Obs, 1. Let DAB in fig. 71, be the earth, C its centre, A
the station of a spectator en the surface of the earth ; and F,
G, H, different places of the moon, or any other heavenly bo-
dy ; TO, NM, LI, are its different parallaxes, and THO, or
AHC ; MGN, or AHC, &c. angles of parallax.

•f.
■ "i

2. If a spectator in his first station at A* figf. 7] sees a
et at G, its apparent place in the heavens will be N ; if
by the diurnal rotation of the earth, be comes into the s
P, the planet will appear at M, which is the place in
it would have appeared if viewed from C the centre :

i 3 in all cases, the parallax which arises from the diarn

tioD, is the same which would arise from a chang^e of s

\ from the surface to the centre; for, in either cas

' 3i chang;e of the spectator's line of view is the same. 1

appears the propriety of the above definition of the d
parallax.

267. The parallax of any planet is always pr
tlonal to the angle which a semi-diameter of the <
drawn from the station of the spectator upon the si

i to the centre, would suhtend, if viewed from the p

J j: Obs. 1. If the planet be at H, fig. 71, and the spectato

AHT will be his line of view ; on changing; the station

spectator from A to C, the line of view will become (

j , whence TO will be the parallax. But TO subtends

proportional to THO, or AHC, the angle which the <

i semi-diameter would subtend, if viewed from the pla

2. The parallax of a planet depresses its apparent pla
the parallactic arc. Thus, if the planet be viewed from
71, its apparent place is O ; if from A, its apparent plac
fartherfromZ the vertex, thanO, by the parallactic ai

3. When the altitude of a body is observed, it must I
rected by parallax or refraction, adding the former, an

tractin^ the latter, in order to ^et the true altitude,
altitude above the rational horizon at the centre of the

268. The diurnal parallax of any planet, at a
distance from the earth, is greatest when the pk
in the horizon, and it decreases as the altitude
planet increases.

Obs. I . The parallax is proportional to the angle whi<
fig. 71, would subtend, if seen from the planet H : bi
given line, viewed from the given distance of the |
would continually diminish in its apparent magnitude,
degreft of obliquity at which it is viewed increases, tha
the planet advances from II towards E ; therefore, the
lax is greatest in the horizon, and decreases as the
approaches the vertex. The parallactic angle AGC
than AHC, and AFC less than AGC.

2. The moon's mean parallax i? 57Mr',

THE PARALLAXES, &C. 155

3. At the same altitude of different planets, their diurnal
parallaxes are inversely as their distances from the centre of
the earth, because the angles subtended by the semi>diameter
"vriXL be mostly as the distances.

269. To measure the distance of the moon from
the earth.

£[«/. The moon^s horizontal parallax is the angle which a
semi-diameter of the earth would subtend, if viewed directly
firom the moon.

lUtu. Let H be the moon in the horizon observed by a
spectator at A, fig. 71, and C the centre of the earth. In the
triangle AHC, let the angle AHC, the moon's horizontal par-
allax, be found. The angle HAC is a right angle, and AC,
the semi-diameter of the earth, is known to be 3985 miles. —
Hence, AC the sine of AHC, 57'. 11". is to 3985, as AH, tak-
en as a radius, to the number of miles in HC, the moon's dis-
tance Irom the earth. The moon's mean distance is thus
found to be 240,000 English miles.

Obs, According to \I. de la Lande, the horizontal semi-di-
ameter of the moon, is to its horizontal parallax for the mean
radius of the earth as 15'. is to 54'. 57". 4 or very nearly as
3 to 11 : hence, the semi-diameter of the moon is Spilths of
the radius of the earth. And as the magnitudes of spherical
bodies are as the cubes of their radii, the magnitude of tlie
moon is to that of the earth as 33 to 113, that is, as 1 : 49.

270. To determine the relative distances of the in-
ferior planets from the sun.

IlluM. If the elongation of Venus, or the greatest angle of
Venus' distance, be found by observation ; Uien, as a radius
is to the sine of the angle, so is the distance of the earth to
the distance of Mercury. If the sun's distance from the earth
be supposed to be divided into 1000 equal parts, then the dis-
tance of Mercury, will in this manner be found to be 387, and
that of Venus 723.

171. To determine the relative distances of the
superior planets from the sun.

Obs. If die angle apparently performed by a superior planet,
while the earth is moving frcxn one end to the other of its axis,
l>e determined by observation, then half that angle is the angle
formed by the dutance of the earth from the sun seen from the
planet. Hence there is given that angle, the right angle at the
sun, and the complement to 180^, i.e. three angles and the base
lOOOt the earth's distance, to find the perpendicular in a right

that of Jupiter 5201, and thst of Satum 9638.

272. To find Ihe paraliai of the sun by tl
of Venus.

Obi. 1. This is one of the mnat important proble
to astronomy; because when the precise anglais 1
(ler which the aemi-dluneter of the earth is seen at
triangle 13 giv*n of which one angie is 90" al the ea
Ire, (he other the pmrallait at the aun, (being the ai
which the earth's semi-Jiameler is seen,) »nd the
complement of the siiuie to 90", and the base is
lemi-diamster known b; measurement

2. Venos would be seen hke a dark apot on the I
sun, aa often as she pa>5eB in hev orbit between O
the enrth, but that the plane of her orbit does ni
with the plane of the earth's orbit, and she panes il
the eye of a •pectator at the earth, above or belol
exoept only when he happens to be in or near her ni
time of the conjunoliOLi. This, however, happeiisb
the nearest transits to our time being in IG39, 1'
•Dd 1814.

3. Nothing a more easy to be onderatood than tl
lion of the phenomenoa of a transit to ascertain the
the Earth from Vena* and the Sun. AU that ii wa
angle pmented at Venus, by any known portion of
■unbce, and this is determined, by obierving the
at which Veniu enters or leaves the ton's lace or
two places, and then converting the differente of til
ing ibr geographical difference,) into degrees, nii
■econds, whicfa is of conrae, the parallax of Venui
the general principle ; but the details of the calci
rentbred oaai{dez by the varions distances of otx
their opportunities of observing, and by the com)x>i
of the Earth and Venus, in the intervals of obaervi
these points are, however, susceptible of unarring <

4. The parallax of Venus being thus determioei
parallas is easily determined by the proportioni i:
article 3T3, and it appears to be 8 2-3 seconds, i. ,
diameter of the earth equal to SSSS miles subtendi
an angle of S.65 secoodj, and by trigonometrioa) <
the perpendicular of the triangle, or tiie line jiHiiin
and the earth's centre, is 95,173,000 milei, thni br
we have S.63t914(tiusiiieaf 8".S5.) : lOJOOOOpO :

PLANETABY MOTIONS. 157

Qqf. of 3986 :) 95,173,000 milet. But some astronomers
Buuce the pftrallax somewhat more, and make it but 93 or 94
millions, while others make it less, or 96 millions.

273. To measure the distance of any planet from
the sun.

lUui, Because the real distances of the planets from the
nm are as their proportional distances ; as Uie proportional
distanoe of the earth from the sun is to the proportional dis-
tance of any other planet from the sun, so is the real distance
•f the earth from the sun in miles, to the real distance of anj
planet from the sun in miles.

Hence are found the distances of the planets from the sun in
Ei^lish miles. Mercury, 36,841,468; Venus, 68,891,486;
Mars, 145,014,148 ; Jupiter, 494,990,976 ; Saturn, 907,956,-
139 ; and the Herschel 1800,000,000, all agreeable to the fact
of the' sun's parallax bein§^ 8 2-3 seconds.

274. To find the periodical time of a planet.

Obi, Because the squares of the periodical times of the pla-
nets were found by Kepler to be as the cubes of their dis-
tances, the periodical times of any two planets beings known,
and the comparative or real distance of one of them from the
sun being given, the distance of the other may from this pro-
portioD be found.

CAUSES OF THE PLANETARY MOTIONS.

27^- According to the Greeks, and the demonstra-
tions of Newton, the planets are retained in their orbits
by gravitation i which draws or impels them towards
the centre of motion,and carried forward by a projectile
force f which tend to carry them off at right angles to
the other force, or in a tangent to their orbit.

//fitf. 1. The motion of the primary planets is very simple
and uniform, beings compounded only of a projectile motion for-
wards in a straight line, which js a tangent to the orbit : and a
gravitation towards the sun in the focus. The power which
occasions the former is called a centrifugal force, and that
which occasions the latter a centripetal one ; and though g;ra-
vitation is mutual between all the planets, being directly as
the quantities of matter they contain, and inversely as the
squares of tlioir distances from each other ; yet the motions of
the planets are not much affected by it ; for their quantities
of matter are but very small when compared with that of the
inn, and therefore its attraction, or their gravitation toward^
it, nearly destroys that of the planets one towards Q.ivc^t}3ksx .

158

OS ASTRONOKY.

lUtu. 2. Suppose P, fi|
^ planet at rest, acted up<
F force which tends to can
wards G, which force is <
towards a centre, and ca]
gravitating force ; but at t
instant another force, ac
rig^ht angles to the former,
(T carry it towards F; the tiPi
will of course, by the '.
mechanical forces, carry it from P to C, in the diagi
square. If then the distances FP and PG are iuppos<
infinitely small, or to represent the powers acting in th
est conceivable portion of time, then the diagonal PC
conceived to be an infinitely small portion of a circle, oi
of a circle so small that it coincides with a circle.

3. If then the actio
same forces be renew
in the directional CG
the centre, and froii
right angles, the plane
turned into the diarc
forming another small
chord, or increment oi
and the repetition of t
p pulses continued with.
Ji) i^tTi will, of necessit
the planet round the
body, in a circular or1
Obs. I. The cause
tation or of projectile
not affected to be expl
the Newtonian philosophy; it is simply assumed, t
forces act according to certain laws, let their causes
they may ; and it is certainly more safft to treat of eff(
of causes, though to investigate the latter is the proj
ness of philo'-ophy.

276. If a body by an uniform motion, descri
side of a parallelogram, in the same time that ii
describe the adjacent side by an accelerated for
body, by tne joint action of these forces, wo
scribe a curv^^ terminating in the opposite i
the parallelogirm.

OF THE PLANETARY MOTIONS.

159

Let A B C D, % 74, be a parallelo-
g^ram, and suppose the body A to be
carried through A B by an uniform
force, in the same time thai it would be
carried through AC by an accelerated
force, then by the joint action of these
«l forces, the body would describe a curve
•^ A G I D. For by the preceding illustra-
tion, if the spaces A£, £K, and KB, be proportioned to each
sther, the spaces AF, FH, and HC, will be in the same pro-
portion, and the line AGID will be a straight line when the
body is acted upon by uniform forces ; but in this example,
the force in the direction AB being uniform, would cause the
body to move over equal spaces, AB, £K,and KB, in equal
portions of time ; while the accelerative force in the direction
A.C, Would cause the body to describe spaces A F, F H, and
ETC, increasing in magnitude in equal successive portions of
time ; hence the parallelograms AEGF, AKIH, &c. are not
about the same diagonal, therefore AGID is not a straight
line, bat a curve.

277. The curvilinear motions of all the planets,
arise from the uniform projectile motion of bodies in
straight lines, and the universal power called attraction,
which draws them off from these lines.

Illus, IfthebodyE.fig.75,
be projected along the line
EAF, where it meets with
no resistance, and is not
drawn aside by any other
M force, it will (by the laws of
motion) go on for ever in the
same direction,and with the
same velocity. For, the
force which moves it from
E to A in a given time, will
carry it from A to F in a
successive and equal por-
tion of time, and se on ; there being nothing either to ob-
struct* or alter its motion. But if when the projectile force
has carried the body to A, another body, as S, begins to at-
tract it, with a power duly adjusted and perpendicular to its
motion at A, it will be drawn from. \3[ie «\.t«I\^^\^^ "^K^ >

reo

%V AST&ONOUr.

and revolve about S in the circle* AGOOA. When the bo
£ arrives at 0, or any other part of its orbit, if the small bo
M, within the sphere of E's attraction, be projected ai in 1
straight line M n, with a force perpendicular to the attract!
of E, it will go round the body £, in the orbit m, and aoco
pany E in its whole course round the body S. — Here S m
represent the sun, E the earth, and M the moon.

2. If tiie earth at A be attracted toward the sun at S, so
to fall from A to H by the force of gravity alone, in the saj
time which the projectile force singly would have carried
from A to F ; by the combined action of these forces it n
describe the curve A G ; and if the velocity with which £
projected from A, be such as it would have acquired by fi
ing from A to V (the half of AS,) by the force of grav
alone, it will revolve round S in a circle.

278. If one body revolve round another (as tl
earth round the sun,) so as to vary its distance fit
the centre of motion, the projectile and centiipe
forces must each be variable, and the path of the i
volvingbody will differ from a circle.

//. Fig. 76. Ifwh
a projectile force cai
E from A to F the su
attraction at S woi
bring it from A to
the gravitating pow
2 would be too great :
the projectile fore
tlie planet, therefo
instead of procecdi
in tlie circle ABC
in the preceding p:
blem) woulc' descH
the cur 76 AO, and t
proach nearer to <
sun; SO being less til
SA. Now, as the a
tripetal force or gra
tating power, alwi
increases as the squi
of the planet's distai

• If any body revolve round another in a circle, the revolv
body must be projected with a velocity, equal to that whicl

OF THE PLANETARY MOTIONS. 161

from the sun diminishes, when the planet arrives at O, the
centripetal force will be increased, which will likewise in-
crease the velocity ef the planet, and accelerate its motion
from O to P ; so as to cause it to describe the arches OP, PQ,
QR, RD, DT, TV, successively increasing in magnitude, in
equid portions of time.

The planet being thus accelerated, it gains such a centri-
petal force, or tendency to fly off at V, in the line of VW, as
overcomes the sun^s attraction ; this centrifugal or projectile
force being too great to allow the planet to approach nearer
the sun than it is at V, or even to move round the sun in the
circle tab cd^ &c. it flics off in the curve XZM A, with a ve-
locity decreasing as gradually from V to A, as if it had re-
turned through the arches VT, TD, DR, &c. to A, with the
same velocity which it passed through these arches in its mo-
tion from A towards V. At A the planet will have acquired
the same velocity as it had at first, and thus, by the varied
centrifugal and centripetal forces, it will continue to move
round S.

But if the action of gravity be too great for the projectile
force at O, why does it not draw the planet to S ? and if the
projectile force at V be too great for the centripetal force, or
griLvity, at the same point, why does it not carry the planet
farther and farther from the sun, till it is beyond his attraction ?

Firtl^ If the projectile force at A were such as to carry the
planet from A to G, double the distance, in the same time
that it was carried from A to F, it would require four times
as much gravity to retain it in its orbit, viz. it must fall through
Al in the time that the projectile force would carry it from A
to G, otherwise it would not describe the curve AOP. But
an increase of gravity gives the planet an increase of velocity,
and an increase of velocity increases the projectile force;
therefore, the tendency of the planet to fly oS from the curve
in a tangent Pm, is greater at P than at O, and greater at
Q than at P, and so on ; hence, while the gravitating power
increases, the projectile power increases, so that the planet
or comet cannot be dr^^wn into the sun.

Secondly^ The projectile force is the greatest at, or near the
point V, and the gravitating power is likewise the greatest at
that point. For if A S be double of V S, the centripetal force
at V will be four times as o^rsat as at A, being as the square of

wo'il -I have acquired, by falling through half the radius of the
circle towards the attracting body.

Emerson's Cent. FoTtw^^^^.^^.

14*

162 OF ASTIIOVOMY.

the distance from the tan. If the projectile forc« at V bi
double of what it was at A, the spaoe V W, which is the double o
AF, will be described in thesame time that AF was described
and the planet will be at X in that time. Now if the action o
gravity had been an exact counterbalance for the projectili
force during; the time mentioned, the planet would have beei
at t instead of X, aod it would describe the circle /, a« 6, e
&c. ; but the projectile force being too powerful for the cen<
tripetal force, the planet recedes fi-om the sun at S, and at<
ceuds in the curve, YZM, &c. Yet it cannot fly offin a tan*
gent in its ascent, because its velocity is retarded, and con-
sequeutly its projectile force is diminished, by the action oi
gravity. Thus when the planet arrives at Z, its tendency
to fly off* in a taogent Zn, is just as much retarded, by the ac-
tion of gravity, as its motion was accelerated thereby at Q,
therefore it must be retained in its orbit.

OF THE FIXED STARS.

279. Those luminous points or bodies which always
appear in the heavens at the same distance from each
other, are called Fixed Stars ; because they do not
appear to have any proper motion of their ov^n.

Obs. 1. The fixed stars are luminous bodies. Because they
appear as points of small magnitude, when viewed through a
telescope, they must be at such immenso distance, as to be
invisible to the naked eye if they borrowed their light; as is
the case with respect to the satellites of Jupiter and Saturo^
although they appear of very distinguishable magnitude
through a telescope. Besides, from the weadcness of reflected
light, there can be no doubt but that the fixed stars shine wilh
their own light. They are easily known from the planets, by
their twinkling.

2. The number of stars, visible at any onetime to the nak-
ed eye, is about 1000 : but Dr. Ilerschel, by his skilful im-
provements of the reflecting telescope, has discovered that the
whole number is great beyond all conception. The compara-
tive brightness of the stars is Sirius 100, Canopus .98, Cen-
tauri 96, Acherni .94.

3. The magnitudes of the fixed stars appear to be different
from one another, which difference may arise either from a di-
versity in their real magnitudes, or distances; or from both
these causes acting conjointly. The difference in the apparent

ma^aituUe of the stars is euchas lo ai^tmloC \.VL«\t being divided

OF THE^ FIXED STAR1S. 163

into six olaMt, th« lai^est beings called etan of the first mag-
Ditad«y and the least, which are visible to the naked eye,
•tan of the sixth magnitude. Stan only visible by the help
af glanet, are called telescopic stan. Bode's catalogue con-
tains 17,000 stars. Dr. Halley very justly remarks, that the
■tan must be infinite in number to maintain, their equilibri-
un in space. And Dr. Herschel thinks he has seen stan
4Sy000 times as far off as Sirins. In one instance a duster of
5000 Stan, in a mass, were barely visible in (he 40 foot telcs-
•ope, mnd consequently must have been 11 trillions of miles off!

4. It must not be inferred that all the stan of each class
appear exactly of the same magnitude : there being great
latitude given in this respect ; even those of the fint magoi-
t(ftle appear almost all different in lustre and size. There
are also other stan of intermediate magnitudes, which, asas-
tronomen cannot refer to any one class, they, therefore, place
them between two. Procyon^ for instance, which Ptolemx
makes of the first magnitude, and Tyeho of the second, Flam-
8TSAD lays down as between the first and second. So that,
instead of six magnitudes, we may say that there are almost
as many orden of stan, as there are stan ; such considerable
varieties being observable in their magnitude, colour, and
brightness.

5. To the bare eye the stan appear of some sensible mag-
nitude, owing to the glare of light arising from the number-
less reflections of the rays in coming to the eye ; this leads
ns to imagine that stars are much larger than they would ap-
pear, if we saw them only by the few rays which come direct-
ly from them, so as to enter the eye without being intermix-
ed with othen. Examine a fixed star of the fint magnitude
Ihrougfa a long and narrow tube, which, though it takes in as
much of the s^ as would hold a thousand such stars, scarce-
ly renden that one visible.

6. There seems but little reason to expect that the real
magoitades of the fixed stars will ever be discovered with
certainty, we must, therefore, be contented with an approxi-
mation, deduced from their parallax, Of this should ever be
Moert&ined,) and the quantity of light they afford us compar-
ed with the sun. To this purpose, Dr. Hxbschel informs us,
that with a magnifying power of 6450, and by means of bis
Dew micrometer, he found the apparent diameter of a, Lyrae
to be 0^.335, or the third of a second.

V. The ingenious observations of Kepler upon the magni-
tudes mnd distance of the fixed stan, deserve to be introduce^
as ha has been £[>llowed in the con^eclVLt q V^^j T>t «VLfeAAis:v. « '^^

164 OF ASTRONOMY.

observes that there can be only 13 points upon the snrjace of
a sphere as far distant from each other as from the centre ; and
supposing the nearest fixed stars to be as far from each other
as from Uie sun, be concludes there can be only 13 stars of ths
first maj>^nitude. Hence at twice that distance from the iim»
there may be placed four times as many, or 52 : at three timai
that distance, nine times as many, or 1 17 ; and so on. Thew
numbers will give pretty nearly the number of stars of the firstf
second, third, &c. magnitudes. Dr. Halley farther remarta,
that if the number of stars be finite, and occupy only a part of
space, the outward stars would be continuaUy attracted to
those within, and in time would unite into one. But if the
number be infinite, and they occupy an in finite space, all the
parts would be nearly in equilibrio, and, consequently, eiAh
fixed star being drawn in opposite directions would keep its
place or move on till it had found an equilibrium.

280. The ancients^ that they might the better dis-
tinguish the stars with regard to their situation in the
heavens, divided them into several constellations, that
is, masses of stars, each mass consisting of such as are
near each other. And to distinguish these groups
from one another, they gave them the names of such
men or things as they fancied the space they took up
in the heavens represented.

Obs. 1. T'he following table contains the names of the con-
stellations, and the number of stars observed in each by Flam-
stead,

FkmtBtead^

Ursa minor

The Little Bear - - 24

Ursa major

- The Great Bear - 87

Draco -

The Dragon - - 8©

Cepheus

- Cepheus - - - 35

Bootes

Bootes - - - 54

Corona Borealis

- The Northern Crown - 21

Hercules

Hercules kneeling - - 113

Lyra

- The Harp - - 21

Cy^nus

The Swan - - - 81

Casgiopea

- The Lady in her chair 65

Perseus

Perseus - - - 59

Aurii^a

- The Wa<r?:oner - - 66

Serpentarius -

►Serpentarius - - 74

S<?r/>r«*

- T\iefeeT^e\\\. - - ^^w

OF THE FIXED 8YA118.

16^

liU

•

The Arrow -

ila

The Eagle - )
Aniinoas - )

- 71

inoiis

phiotts

The Dolphin

. 18

lulot

•

The Horse's Head

afoi

The Flying Horse -

Iromeda

•

•«

Andromeda

iDg^almn

The Triangle

. 16

The Ram •

iras

•

The Bull

141

aiui

•

The Twins -

* 85

icer

The Crab -

•

The Lion

" 95

aa Berenices

«•

Berenice's Hair

- 43

•

The Virnn -
The Scales -

110

- 51

rpios »

«tf

The Scorpion

ittarias

•

«•

The Ardher -

- 69

iricornas

•

The Goat

lariuB

•

The Water-Bearcr

- 108

:e»

The Fishes

113

The Whale -

- 97

»n

4»

Orion

slanus

. '

Eridanus J-

- 84

•

The Hare - . -

lis major

«•

The Great Dog

- 31

lis minor

The Little Dog -

Navis

•

The Ship

. 64

ira

•V

The Hydra

iter

• •

- .

The Cap

- 31

•vds

•

The Crow

itauras

•

The Centaur

. 12

pat

The Wolf

The Altar -

•ona Aastralis

The Southern Crown

- 12

56fl Auetralis

The Southern Fish

umbia Noachi

•

Noah's Dove

- 10

bur Carolinum

The Royal Oak

•

The Crane -

. 13

Bnix

The Phoenix

- 13

The Indian

i'O

•

The Peacock

- 14

is .-

••

The Bird of Paradise

- 11

„„.„„,• 1

Flamtni. t

Ap=i. - - - .

The Bee or Fly - - * -"

Cba>n.leon - - -

TheCameleoQ - . Jft

TriunguliuQ Auitralii

The South Triuigle - f *■

PiKis Valani

The Flying Fish - S

Dowlo - - ' -

The Sword Fish - - •

Touon . - -

The AmericBn Goow - 8

Hydriia -

The Water Snake - - 19

The Lynx . . - «

1*0 minor - .

The LitUe Liot. - - Sj

Aaterion &. Clara -

The Greyhound - U

Corberpua

Cerberua

VulpeuulBJE Anier

The Foxsnd Goose - 3»

Scotom Sobieaki

Sobieiki'a Shield ■ • -

LsoreU

TbeLiiard - - IB

C«inelo|»r<lolua

The Caraelcopard - ftS

Monocer™ -

The Unicorn - - 31

B«lton» - -

ThebexULot ... 41

i. Stan not included in a

f constellation ere oaUed unfirvt-

Cdalua. Besides the name

> of the conalellations, the tiMUtnt

Greehi gave particular nameg to some single at&n, or email
collections of stars : thus the cluster of small stora in Iho neck
of the bull, VBE called the Pleiades ; live stars in the bull's face,
the Hsadtt ; a bright ilu- in the breast of Lis, the Ltea't
Searl; and Hlarg:e star between theknees of SoDtevfrefttnH.
3 Greek letters have been added by Ba^er ta stan in th*
several conateUations of hia catBlogue (a bemg affixed to the
largest star) by means of which any atar may be eaaily femid.

4. Twelve of these constellations lie upon the ediptiii, in-
cluding a space about GAeen degrees broad, called the Zadimc,
within which all the planets move. The coaatellatioii AiiM,
about 2(KI yean ago, lay in the first sign of the ecliptie t bst
on Bccoimt of the preceadon of the equinoxei, it now IiM ia
the second. In the foregoing table^nfinmu was made out of
the anformed stars near Aqvila ; and Camo BtrtnU** oOf sT
the unformed stars near the Lien'tiaiL They are'botltnMH
tioaed by Ftolkht, but as unformed stars. The aaoitalla-
tioEis as for as the triangle, with Come Berenieei, are aarAvM;
thoae after PiKtf, are t(mthrm.

5. The luminous part of the hearens, called the Jfi^ff*^
consists of Gied stars too muH to be seen by the naked m.
In a paper on the oonatmctians of the heaTeni, Dr. HerHMl
says it il very probable, that the great stratum called tba
milky way, is that in which the ion u placet), though pw-

OF THE FIXED STARS. 167

t in the centre of its thickness, but not far from the place
some smaller stratum branches from it. Such a suppo-
rill satisfactorily, and with great simplicity* account for
phenomena of the milky way, which, accordii.g to this
esis, is no other than the appearance of the projection of
rs contained in this stratum, and its secondary branch.

1 another paper on the same subject, he says, — We will
treat to our own retired station in one of the planets at-
^ a star in the great combination with numberless others ;
order to investigate what will be the appearances from
tntracted situation, let us begin with Uie naked eye.
3krs of the first magnitude, being, in all probability, the
^ will furnish us with a step to begin our scale ; setting
erefore, with the distance of Sirius or Arcturus, for in-

as unity, we will at present suppose, that those of the

magnitude are at double, and those of the third at tre-
3 distance, and so forth, leaking it then for granted,
star of the seventh magnitude is about seven times as far
Ls as one of the first, it follows that an observer, who is
ed in a globular cluster of stars, and not far from the
, will never be able, with the naked eye, to see the end
for since, according to the above estimations, he can
xtend his view about seven times the distance of Sirius,
lot be expected that his eyes should reach the borders of
ter, which has^ perhaps, not less than fifty stars in depth

where around him. The whole universe, therefore, to
nrill be comprised in a set of constellations, richly oma-
id with scattered stars of all sizes. Or if the united
tness of a neighbouring cluster of stars should, in a re-
!ible clear night, reach his sight, it will put on the ap-
nce of a small, faint, nebulous cloud, not to be perceived
lUt the greatest attention. Allowing him the use of a
ton telescope, he begins to suspect that all the milkyness

bright path which surrounds the sphere may be owing
rs. By increasing his power of vision, he becomes cer-
hat the milky way is, indeed, no other than a collection
*y small stars, and the ncbuls nothing but clusters of stars.

Dr. Herschel then solves a general problem for computing
jngth of the visual ray : that of the telescope, which he
will reach to stars 497 times the distance of Sirius. Now
3 cannot be nearer than lOO.OOOX 1 90.000,000 milea,therc-
Dr, Uerschel'g telescope will at least^rcach to 100.000

or ASTBOirotrv.

X190.000.000.X497nii1«f. Aud Dr. HerscheU
llu mott crawdcil pari oi Ihe mllk^ ira]', be hat
view Ihat coatBJoed no leat Ihaa 683 stars, and
aaiiliaueLl for mnQV niinutea, so ti>at in > quartei
hehasMea 116,CKK)Btan pasi tliraugh the fielt
telescope efoal; 15'. a]ierlijrc : am] at anolher
minntei, hesRW 2.'i8,000 elan paii Ihroi^b Uii
telescope. Ever; improTameaCiD hi) tekscaiie t
ed itare not seen beliiri, » that tbare appears i
Ibeir Dumber, er to Ibe extent of Ibe aaivcrie.

a. There are spots in tbe beaveos, called .Veb
which coDsist dC clusters of telescnpio stars, othei
luminous spots of differeot formi. The laast coi
one oftbo mill way between the two stars on tbe 1
on') mord, markeJ j by Bayer, and discoi-ered
Iflofiby Uuygecs; it contain! oaly seven start, a
part is a bright spot upon a dark ground. To U
ws are indebled for catalogues of 2000 iiebuls an
stars which he bimsGirhad discovered. Some ol
round compact system, otbers are store irregula
brms, and some aro long and narrow. Tbe ti|
luminous spaces in the heavens Sir Richard Phil
arise from hght per se ; but ascribes the luminosi
spiicei lo the multitude of plauots, aaleroidi, f.
conetary bodiea, witb which thoie spaces are fill

9. New start sonetimet appear, while other
Sareral stars, mentioned by the aucieQt astrouot
now to bafimnd: several «ra now visible to thi
which are not meationeA in the aui.'ieat catali^ue
stars have snddeulj appeared, and again, after a i
iuterval, vaqiibed : also a change of jil&re has bf
ia some start. Dr. U^chsl has obserred Ihi
nearly filly tlan, with lefereiwe lo others in thi
Caitothe Gads has a period of 34e years; Gamm
1300 yean; Epiilra Bootee, of IfiDlyean; Del
ofSTSyeue; Gammft VirpBia of 70S years; ai
trt: but the lUa of ana man 1* evidently too sho
correct rMolti, in reigard to periods to disproport
Barrawipaceof exiitenoe.

ID. These motiaot of tke start among Ihemteli
parent to ohearvMioni ttedootrioe of Dr. flench
Mlronomarsis rendwed probable, tlut tbe laa lut
orbit of iti own MjMK the fixed stars of the milk]
rate of the earth's fiotion, car>ring with it «I1 Um
•stheplaHtsthemuUM, cmrryiritkltaMBthMr^

at THE FIXED STIRS. 169

UUitea in Ibcir own orbili. Tbe rotHtion of the aun on its in-
I clined >xu. uwordlngto the theory of 8ir RirJiard Philtpa,
tecmis to indicate the sction of « centrifugal force in the !iin,
•nd to render the notion, thst the whole solar eystem is analo-
gous to B prinary and ita silellite?, exceedingly provable.
'. It. From an attentive eiamination of the «tars with good

' telesooiiea, man}' which appear only angle 1o the naked eye,
[' are found to oonsist of nnmerouj rtan. a Hercvlii, is a dou-
-* ble itar, « Bvoli), end Dr. Henchel, hj hie highly improTed

lalescopea, has fouod about TOO.
'~ IJ. In February, 1814, Dr. HerK*el, the prince of aslrono-

- merg, read to the Royai Society, Uie resulta of thirty yean'

- obnTTBtioni on f^ebulffi, with the best telescopes e vr pwses!-
ed bj man. He conceit^s (hat the stars form independent
^Btems among themselvei. He considers our sun as part of

. tint shoal or system which we call the milky way, and that all
_ the Stan of the firal, second, and IhirJ magnitude, belong to
. that vast cluster. The stars, he remBrk", are not spread in
cqaal portions over thehcrizon, but are foundin patches, each
Oflnt»ining many thousands, and many more than the eye can
Npante from the mass. These he c&lls clusters ; and he con-
•aivea they have a constant disposition to unite more closely,
bya power which he calls the clustering power; doubtlees the
BMe power which is described in the observation to article
we. He gives an account of eighty of these clusters, some of
the drawings of which are copied here.

Tha fUlowii^ figure represents a comfrkibxs cldbtbk
of BTARI, the centre part 8' long 2' broad.

2ftl. TbeJhMMdPm
rf a hearmlj Iwdj, .
change of its apparent '

iaTtewed from the
I ia its annaal niotioii ; i
( the angle which the dia
^of the earth's orbit
*" Bubteud, if thiU diameti
-, -.Tf^jt,-^!"' viewed from that body,
*--''iiiM-" 282. Iftlie distance

object be greater than 100,000 times the bas(
angles at the stations will not senaiblj differ fron
angles ; consequently, the lines drawn from th
ject to the stations, are, physically speaking, pa
and the parallax of an object, the distance of w)
Hbove 100,000 timesgreaterthxn that between tl
stations of observation, is consequently inaensibi
Obi. ir the object be >t n greater distance frmn eitl
tion thui 100.000 timea the base, the angle at one of the !
being 90°, the angle at the other vill be mora than t
57". B, the diOerence of which angle and 90" being ei
more than 3", is too »iaaU to brconio <>eii;iMe by obter

OF tHE FIXED STARS. 171

CII5

$S3. If the parallax of an object (observed with an

i^Ktrument sufficiently exact to measare an angle of

^\) be insensible, the distance of the object from

^ther station cannot be less than 100,000 times the

!, yet it may be greater in assignable ratio.

Obt. Lines drawn from any given points in a base, to an ob-

ct, may he esteemed in practice, parallel, without any sen-

^ble error, if the distance of the object be more than 100,000

'%ime8 the base. Rays, therefore, diverging from any point

jn the sun's disc upon the surface of the earth, may be esteem-

«(i parallel, if their distance from each other do not exceed

«bout 970 miles at the earth's surface : because 970 is to the

distance of the earth from the sun in a proportion of 1 to

100,000.

284. The fixed stars have no sensible annual paral-
lax, because when the place of any star is observed by
the best instruments, from opposite points or ends oi
the earth's orbit, its apparent place in the heavens re-
mains the same, which could not be the case, if the
angle of its parallax were so much as two seconds,

Ohs, 1. Hence, it appears that the fixed stars are so remote,
that a diameter of the earth's orbit bears no proportion to
their distance, or that a diameter of the earth's orbit, if view-
ed from one of the ilxed stars, would appear as a point.

3. The distance of the stars must be greater than 100,000
times the base,' from the extremities of which it is observed^
that is, greater than 100,000 times the diameter of the orbit oi
the earth, or greater than 100,000X190,000,000, which ia
nineteen billions of miles as the least possible distance of the
nearest fixed star.

3. The parallax of a fixed star, being not more than 2". the
fUD, when viewed from that star, would appear under an angle

32' 6" 1"

less than or less than and, therefore, could not be

200,000, 100,

distinguished from a point.

4. Since bodies equal in magnitude and splendour to the
sun, being placed at Uie distance of the fixed stars, would ap-
pear to us as the fixed stars now do, it may be supposed proba-
ble, that the fixed stars are bodies similar to the sun, which u
the centre to the solar system. This being the case, the reasoc
will appear, whja fixed star, whenvieweCLVSaxotW'^^Vs^

magnifying 200 times, appears no oftiex \3dlmv ^ "^VjdX.- T
nppwent diameter of the star being \e^ Vbaxv VV^ "V

tbat Sr'. at the «7e cf the spectator^ olnervuif it in w-tik

5. The ipanillix of theftied stir, ivMi iiiB#ed finan 4
oppwt)t pwtg of tha €«rth^ «%it, i» hfi iMmijitd y. hM
is prob»bl« tiiat tlufr yawJtoof th» oi M w t ^itMi innoBMa
ani aoateqaentljr tha^UituiM greatar, in tha tuammNKpotlm
astheparaUasitleok .Qiv Bi»^jthDi«i*tMiMii*yitn

las ofa fiaaivfliar haA'^vw^dmfiivaiiMM Jta«Nd4'lMi«#te
al)Ia todataetit. Tha &qMtar af tiia.aarth's wwWti^^ii
Ibra, bat a poiBt attiptafixaflitan, . RogJatliipufjWiltrfiM
tba immiiwa ■ dintaaoa .a(. tiia atuv >t3ia|Difllvaa. a( Imtvfcpii
ooo&pirad with the tf^^ncte a<tenriQP,af op a t B,. ^^ijlNtiMxti
alaraia thacauftan saea .bj Hurtmhali arr«i 4iiM(t JiM^
other, as the fixed stars of oar duster of tha mOkj WHgk *
jbofidtoba firan eaeb otlier, jrat thosaiclaitani ppifiiaiij fl
aarth inabat points I 17o agosidantioQB eanbg ■p^.'^^aide
lid oc sabUtta I - V! /;

^^uiSTioiisr Off ^Aflrkomic^ / 1. : ..

What is astronomy ?

"What constitutes the solar system ?

What are the names of the primary planets composing t
solar system ?

What are the names of the asteroids ?

What is the number of the secondary planets, and to wl
primary planets do they belong ?

Explain the solar system.

What was the system of Ptolemy ?

How is it demonstrated that the planets move roand t
sun?

How are the motions of the earth in its orbit proved ?

How is it proved that the Earth is of a globuUir form ?

What is the axis of the earth ?

What are the poles ? and what the equator ?

What are the arctic and antarctic circles ?

What are the tropics ?

What are the sottes?

What is latitude^ and what is longitude ?

Explain the %ure of the celestial globe ?

What are the poles of the Aorison, and what ara they caHe

What is meant by meridian?

What is altitude^ what azimuth^ and what amplitude ?

What is meant by dectinationy and what by righi asctns%$i

n are two planets said to be in eof^unefion and when in
a is the celestial sphere said to be rightj obliquey or pa-
is daif and nighi produced ?
long is the earth in performing a revolution round the

t are the 12 signs of the sodiae ?

t are the names of the sig;n9, and which are north and

With of the equator?

lin fig^ure 64. What is the eclipiie ?

t is the quantity of the angle made by the indinatixm

cliptic to the equator ?

'hat is the diminution of the obliquity of the ecliptic

t is the nutation of the earth's axis, and to what cause
ng?

may the difference of longitude in two places be foun^ ?
iin this.

un prop. 310, also 211, and 212.
t causes the succession of seasons ?
t is the figure of the earth's orbit ? I

t occasions twilight ?
lin.

is it that we see the sun before he actually rises ^Jbore
zon ?

t is a natural day ?
t is equation of time ?
t is meant by mean, or apparent time ? .
t is the shape of the sim^ and where is he pla/ced ?
t is bis diameter ?
he sun an atmosphere ?

t is the calculation of Euler in regard to the compara-*
it of the sun?

has the period of the sun's revolution about its axis
termined ? What is this period ?
: is Dr. HerschePs opinion of the height of the son's at-
re?

t comparative quantities of matter does the ran and
contain ?

much larger is the sun than the earth ?
: is the probability that the sun is inhabited ?
: is the distance of the sun from the earth ?

16*

W)i7 are tbe mteroids called Iclacopie plnneb .'

Esplain figure bl.

What is meant by opftcft'un and ;l(riAe((undtBlance ?

What is apogee and perigee .'

How will you ehhw that a planet does not pcocaed ia ii
bit with an equable laorion .'

What is a planet's anomnly ?

CBlculate the true [jlace of a planet.

What 19 meant by the e7Uttfionbf a planet's cenlre '.

What is a planet's datgaUOfi .'

What is periodic linte ? and what is meant by trapital
aidereat rerBlulion J

What is H planet's (/i>«f motion, aod why docs it samel
appear flalioaary, end mnetimes to more in a conlrary iJ

WhatarethenorfM.'

What is the diameter ofjtfcrcun/?

Whatii tbe period of it) lidereal rcvolutjoa >

What is the greatest elongation of this planet inw ttie

What!" meant by itj transit ?

How much greater is the sun's heat and light at Hei
than with ua?

What h l]ie diameter of Vcjius ?

What is the period of it^ sidereal rerolution ?

What is its period of revolution on itaown ajfis f

When does Venus appear brightest?

When is this plaiiet the mnmiiig and wbea tb« eri
star?

What is the diameter of the earth !

What are the periods of its two revolution* ?

What effect does its ctnlnfugal force have on the 'weig

If the motion of the earth was «eraite«i times p«at«r
it is, what would be the effect on the weight of bodies f

What is the Telocity with which a pUc« movta perivj
at the equator ?
Calculate the velocity of PfaHade^hia.
What is the shape of the earth ?

What is the difference between tbe tgaiilarial mai tbe ]
diameter of the earth ?
How is it determtnedlhattheearlbii flattened ai the p
Why are boiUes heavier M the polot thui at Ihc eqiu
Why is the petiodital year longer thaa tbe trvpieat f
Vtbat caiues the precaiion irf &« equinosei !

QUESTIONS. 175

How often does the tide rue andiall f

What is the cause of tides ?

Draw a %ure illustrating the theory of tides.

Why are the tkles greatest at the new and full moons ? and
why least at the first and last quarters, and why are they
hig^hest of all about the time of the equinoxes?

Explain prop. 220.

What is the diameter of Man?

What is its period of revolution about its axis, and what is
its sidereal period ? i

What are the names and diameters of the several asUroidt ^

What is the sidereal period of each ?

What is said of the origin of asteroids ?

What is tlie diameter of Jupiter ?

What is its didrnnl, and what its sidereal period ?

How many satellites has this planet ?

What is 3aiJ of Jupiter's belts ?

What is the diameter of Saturn ?

What is its diurnal, and what its sidereal period ?

How many nutois has Saturn ?

What is said of Saturn's ring; ?

To what distance does^the atmosphere of Saturn extend ?

What is the diameter of Herschel, what its sidereal revolu-
tion ?

When and by whom was this planet discovered ?

Mention the names of tlie planets in succession, beginning
with that nearest the sun.

What is an easy distinction between a planet and a fixed star ?

^Hiat are secondary planets ?

Why are the motions of the secondary planets less unifiarm
than those of their primaries ?

What is the diameter of the moon ?

What it" mean distunoo from the earth ?

What is the period of its sidereal revolutions ?

What is meant by iyikodie revolution ?

What is meant by the moon's Syzy^ia ?

Explain prop. 243, making use of the plate whioh il-
lustrates it.

Why is the moon sometimes invisible when it is above the
horizon ?

How many days complete the year at the moon ?

How many of onr days is equal to one at the moon ?

Explain prop. 247,

What is said of the excentricity of the moon^s orbit ?

How if it pnMd UmI tiM aiboa if dtonillid i»^ J

Talleys ?
What dtM Pe. fiMMbfl^l^iB rq^ud te tii»

th» MooQ, <nJ tt> ynJhahnwf wiMiih hmwKW Qmf ?
What oe oM Jo of aecgittwfcf the mooaf.
lUufltntte this by mlMniiijp the il^^
If this eurth w«ni «slaif« w tiie nm, what di l hw p t

Ipena waokl happen in iwiad fi» ad^aef f j

When if an aol^pfa of te vutoD filial, and wliMi Mi^

wbat oooafvnf thif diiBranoaf '
Whataratha iMl^TpoMamd^fSb^fiimtm^

when an aol^paa fl€ the niii happens ?
When does a total eelipfe of the son happen, and whiAJn*

casipnsitf ■. ;. •*. ■.-1

Why is tiliete not an eclipse of the sdn at every navTM^
What is the averafaniiwbeBef eelipsea pery«rit,«f «i#|«

and meoa? ' '■>:■'. i

Explain ^phenoniina of the totert BMwipu-
'Whatai^'etaMto^ .',j..is>'

How many eometi syre known to vetara at eerlai* W/tMd
What if Hieant by the p»raVa» of a heavenly body r -
What if the diurnal parallax ?
How are the parallaxes of the planets calculated ? XSi^lak

the figure.
How are the. reladTe distanees of tiie planets frons tfaasB

determined ?
How do yon measure the distance of any planet from flu

sun ? What is the rule ? ft.

Find the periodical time of a planet ^'

Account for the curvilinear motioas ofthe i^anets.
Explain prop. 279. ^

Why are some of the stars called/d?ed ttan .' ^

How is it proved that ^e fiaed stars shine «4th fUt evh

light?
What number of stars are visible at any one time to thi

^riaked.eye ?
What is said of the distance and number of the fixed alnt
What is the use of dividing the stars into coostdlatieiif f
Are all the stars included m the several consteUatiQiiif 9
What gives the ajfpearance called the miBm way f
What are the spots in the heavens called n^ufo /
Wlutt is said cqncemiiiif the appearaooe of new stars ^

i)P ELECTRrciTT. 17f

are the principles on which the parallax of a heaveit-
is determined f
have not the fixed stars a sensible annoal parallax ?

inference is drawn from the lact, that the fixed stars
annual parallax ?

OF ELECTRICITY.

The surface of the earth and of all the bodieft
bich we are acquainted, is supposed to contain
ess a power of exciting op exhibiting a certain
y of an exceeding subtile agent, called the elec-
id or power.

The qumtity usually belonging to any surface,
id its natural share, and then it produces no
e effects ; but when any surface becomes poa-
of more, or of less, than its natural quantity, it
rifled, and it then exhibits certain appearances
d to the power, called electric.

'•imeni I. .Take a stick of sealing wax and rub it with
id, or with a piece of flannel, or on your coat sleeve,
ill have the power of attracting small bits of paper, or
rj light substances, when held near them.

L clean and dry glass tube be rubbed several times up-
nd downwards, and then presented to any small light
:e, it will immediately attract and repel them alter-
)r a considera'tle time. The tube is then said to be

L glass tube be rubbed several times in the dark, and
tught within about half an inch of the finger, a lucid
'ill be seen between the finger and the tube, accompa-
:h a snapping noise, and the finger at the same time
(;eive the sensation of a prick from a pin. The attrae<<
lulsion, sparkling, and noi«e, are the effects of electrHl^
ire denominated electrical appearances.

the following figure represents the apjieB ranee of

1, If an oblong pii
, »uch as a poke
l^iended by meviB of
Tdlring, and (he eic
■:ube be presented tc
lend, then the lower
■metallic body nill e

lb6U^ Whi<^ ehcwi Ibat the electric fluid poBses thi

meU.

.$. KfjutMd«fthau«UIliebod]r,Kitiokaf gU|h
TU. be niiiNad«i, Mu.of thwe i^Moaawna wiUbi

8f&7.' All thou bodici whtcfa truuumt or
dectrici^ fimn one nir&ce to iDother, are «
(hwfon ; lad tboie surikces that will not tm
electtie VBmer, are called deetria or non-coi
'268. Tlie me^s, i^mi-metali, and nietal
are conductors of electricity ; so are charcoa
and other fluJda, except the aerial fluids and
most all saline, and taaay earthy substances,
wise nan-electrici of condiictori.

iB9: The following substances ere elec
non-condvctort, viz. vitrified suhstances, am
phur, resinous substaMes, was, silk, cotton,
wool, hair, paper, elastic fluids, sugar, oil,
osydcs, animal and vegetable asbea, dry v
substances, kc.

290. When a surface is supposed to have n
its nataral quaatily oftbis fluid, it is said to be
ly electrified ; when it is sappoeed to have 1
)ta natural share, it is said to be negatively el<
^91. When any electrified conductor u
surrounded by iton-cojtductori, so that the
fluid cannot pass from the conductor alongcc
to the earth, it is said to be ttUTilated.

Etp. Tho human body is ». good condaotor of «
buC iia peraon stfuul oa •.cal^oSv^ui, qt oa «.i.\b

by glass legs^ the electric fluid cannot pass from bim to the
rth ; and if he is touched by another standing; on the ground,
-will exhibit sparks.

292. The principal method of exciting the electric
lid iB by contact, pressure, or friction. When an
ectric and non-electric are rubbed against each
her, electricity is excited, and the electric power
isses from the non-electric or conductor, to the
ectric or non-condiictor.

Exp. 1. If a smooth glass tube be rubbed with the hand;^
B electric fluid will leave the hand, and pass upon the tube^
lich will then have more than its natural quantity.
2. And if the finger, or any conducting substance, be pre-
ated to the tube, the electricity will then pass to it.
Obs. Certain changes in the forms of substances, are al-
fiys connected with electrical effects. Thus, when vapour i^
rmed, or condensed, the bodies in contact wiUi the vapour he-
me electrified. If, for instance, a plate of metal be strongly
sited, and a drop of water be then poured upon the plate, at
e moment the water rises in vapour, the gold leaves of the
ectrometer will diverge with negative electricity. Sulphur,
ter being melted, becomes strongly electrical during the
ne of congelation.

29 J. Two surfaces, both positively, or both nega-
rely electrified, mechanically repel each other ; and
ro substances, of which one is positively , and the
her negatively electrified, mechanically attract each
:her.

294. If any person who is insulated, rubs a glass
ibe, the person and the glass tube will become elec-
ified, and be capable of attracting and repelling light
>dies ; but the electricity of the person will differ
om that of the tube.

Exp, 1 . Let two cork balls, connected by a linen thread, be
5ld by a silk thread, attached to the middle of the former, at
me distance from a wall ; then bring the excited tube near
le balls, and it will first appear to attract, and soon after repel
lem ; this apparent repulsion will continue for a considera-
.e time, though the tuije be removed.

2. Let another pair of cork balls be brought in contact with
le insulated person, and they will appear to repel each other.

3. But if the two pair of balls bebxQXi^Vwe^iXN'vicv^-^ ^"^
Arnr^t onoh oth^i", and the e\ec\ri<isx\ "^cwfiv ^'C^'^v^'5v\i^«^'^^

nbioh ihews [bat tbere are tvo electrieilieF, one
rereno of the other, and aeeming to hare what

4. irthe iuiulated porsoa rob a click of salpliur
or eealiiig-irai, tbat sobslaDce will acquire tbe
which, ia the [ireoeding experiment, wasacqaired
9iilate<l person.

5. Uenoe poiitiie and ntg/ilire electricity bnve
been called vitreom and reiinnus.

295. Opposite electricities always accomp
other, for if any Borfiice become positive, th
with which it la rubbed becomeB negative ; i
surface be rendered positive, the neiireit c(
surface will become negative,

296. When one side ofa metallic or othei
tor receives the electric fluid, its whole surf
staatly pervaded ; whereas when an electil
gented to an electriSed body, it becoincB e
on a small spotooly.

Obi. There is a stone found in maoy pirb of
called lourntalin, nbich it Mmatimes cTjitalised
lided priBtn, terminated by a three-sided and a >iz-i
. mid ; when Ihii subttaace is geally bested, it becc
trical, BDdoDe.sKtremitr, tbat termiDated by tbe
pynuuid, is poaitive, the other is nesative ; to a c
taol, iti eleclricilieg are eialled by increasing tbe
ture ; when it liegina to coot, it is still found eleci
the eleotricities ars cbaoged ; the pyramid, befor
b now aegatJTe, uid vice reria. When Ihe stone
iidarable siie, Bashes of ligb't may be seen along I
There are other genis and crjstalized lubstaDceg, t
foa • property •imilar to that of the touraialiD.
noiu appearaace of tome diamonds, when heated,
depends upon their eloctrical excitation. The
called tba ioraeile, which is • cabc, having its ed^
glee defectiTe, becomei eleotrical by beat, and in oi
pTMenti no lass than eight sides, in diSerent st
ikaitiv*, fb^ir negatlrv ; and the opposite poles are
leetion of tbe axil of tbe oryital.

S9T. If to one side of an electric, viz. a
glau, yon eonununtcate poeitiTe electrici^. t
flite Me will become cegatiTelv electrified,
plate IB diea end to ^e ehorgEik

! OP ELECTMCTTT. 181

Ob$, The pnitiTe and nef^tire electricitieB, in the above
' eue, cannot come together, unless acommumce^ioii,b7 meam
of tonductOTB, 19 nude between the sides of tbe glnss ; and in
'. &B minner ai a plate of g'lHw is charged, !0 the plate of air
^rfag between any electrified Burtkcai is sin-ays char;^.
298. When two surfnces oppositely electrified, ar«
■ nited, their powers are destroyed, and if Iheir union
J. be made through the humnii body, it producei an af-
; fection of the nerves called on electric shock,
: 299. Machines have been contrived for rubbing to-
I gether the surfaces of electrics and non-eleclrics,aDd
1 n>r collecting the electric fluid, when so excitc-d.

Jllus. I. Fig.31,repre-
eents ane}tft'ieal m{:chint,.
G F, F is a strong board,
which supports all tha
parts of (lie machine, and
wO which may befastenedloa
table by means of one or
more iron,or brass clumps,
,n?Q. TheelaunfLndtrd
B i< supported by the Imo
SI glnuhgi.G andE. IRis
the rubber of leather, and
silken Bap. The rubber is
spread with an amalgHm
of mercnr; and zinc.or tin.
The rubber or cushion is

_ fastened to a spring.wbioh

, Is from a socket cemented on-the lop of the slaa jiitlar

i. The lower part of this pillar k fixed into a small boai^,
•faiah slides u|Hin the bottom hoard of the machine, and bj
^ Hanaqf aicrew-nut andaslitat H.majbG fixed more or lea*
. ftmrd, in order (hat the rubber may press more or less upon
(be <7liiider. NF is aglanp.War. which is fixed in the bot-
tcm board, and supports tlie prime cmulutUir iVlL of hollow
Iwn, or tin plate, or coated wood, which has a collection of
pstnferf aim at L, and knobbed wire at M. From the *riiiJ
huh O B longer spark may be drawn with the hand than from
my other part of the conductor.

a. When the cylinder is turned swiftly, the friction of the

dta airBinBt the rubber causes the electric fluid which was op-

on the rabber to pass to the gloss, I'rom whencie il is conct
to the pointa of the prime conductor, whioh are preaente
every part of the cylinder b succeaBioii. If one end of
chain be put on the knob x and the other end han^ oo
ground, there will then be a coostBDl supply of Ibe eltt
Quid to the prime conductor, which will be discharged in epi
toanybodjrpresentedtoit. TheruOftfrisre-suppUedby mi
dfthe Bur&ces in immediate contact, und these ngsin areinp
e^ by the general mass of the fluid that ii lodged on the ea

300. Bodies or surfaces, ihnt are chained with
same electricily, appear to rtpel eacii other ; bu
one have more and the other less than its share, tt
will appear to attract one another.

Exp- 1. Ifa tuft of feathers be hungon IhepriniBOondU'
LM, as- ai, the moment they are electrified by_ turning
wheel of the machine, they will endeavour to avoid one ant
er and stand erect : because, being; all electrified byUie »
eleotricity, they repel each other.

2. A large feather will, if placed in the hole n, when
roacbine ii worked, become beautifully turgid, expanding
fibres in all directions : and they coHapae when the eleetri
13 taken off, by presenting any conducting eubitance to th

3. Excite a. gloss tube eighteen or twenty inchea long, t
present to it a small feather, which will first have the nppi
ance of being attracted by it, and afterwards jump froit
If no other body happens to be in the way, it wiU tend lowi
the ground; but if the tube be held under it. it will be
repelled, and may be driven about for a considemble timi

4. Suspend a plate uf metal from the conductor, and nii^
neath it, at the distance of about three or four inchea,
another plate of the same lize ; upon the lower one u
featheri, pieces of paper, &c. may be placed ; these will
soon as the machine ii worked, jump to the plate, from wl
they will be repelled and fiy to discharge tbemselres a
the lower plBtfl, after which they win be attracted and
pelled agua, and id continue till Ute electricity of the op
plate is comptelely discharged.

5. If two bells made of cork, or the pith of elder, ab
the size of lai^ pMs, bs fastened to silk threads, they i
hang parallel to each other, and be in contact; bat w)
brouj^t near the electrified prime condacbH-, OMf will «tra
ly r^d Mcib other.

OF ELECTRICITY. 183

6. These balls, in their electrified state, shew whether the
electricity is positive or negative ; for if it be positive, by ap-
plying an excited stick of sealing-wax, the thread will col-
lapse ; but, if it be neg;ative, the sealing-wax will make theln
recede still &rther.

30 i. A pair of cork or pith balls, or pieces of gold*
leaf, are used to discover the presence or strength o^
electricity, and denominated an electrometer.

Exp. Fig. 82, represents a quadrant electrome-
ter, which may be fixed in the hole z of the prime
conductor, fig. 81. It consists of a very light
rod, and pith ball A, turning on the centre of p.
semicircular B. According to the strength of
the electricity the pith ball flies up, and the scal^
marks the degree in which the prime conductor
is electrified.

302. If a surface, containing only its natural share
of electricity, be brought near a body that is electri-
fied, positively or negatively, a part of the opposite
electricity, in the form of a sparky will force itself
through the air, from the latter to the former.

303; When two surfaces, one electrified positive-*

ly the other negatively, approach each other, the

superabundant electricity rushes violently from one

to the other, to restore the equilibrium.

Ohs. It rushes, says Sir Richard Phillips, through the near■^
est point of physical contact, usually some spicula on the sur-
fiice, and in this spicula or point is consequently concentrated
tiie entire power of the opposing surfaces. Hence the positive
ride exliibits brushes at the points, diverging, difiusing, and
Tanishing ;. and the negative a concentrated and uniting star.

304. If an animal be placed so as to form part of this
circuit, the electricity in passing through it produces
a sudden and violent sensation,called the electric shocks

306. The motion of electricity, in passing from a
positive to a negative body, is so rapid, that it ap-
pears to be, in truth must be, instantaneous.

Oht. The writer referred to above, says it is analogous to
light and shade, and therefore necessarily coincident.

30S. Whcnaiiyptirfofonesitleofagljiasispreseal
ed to .1 body elecfritiod poeitively or negatiyely, th)
side of the glass becomes puaseased of the control
kiud of electricity to the side of the body it ia [iresenl
ed to, and the other side of the glass is possessed (
the same kind of electricity as the other body.

Kzp. irtheknobO oftteprioieeDnduotor, fig. 54,beeUi
trilled positiTely. and a paiu oi glnsg be presented to the nd
ncit to O, it will be negativelj electrified, and the othsr nil
will be poiitively electrified.

307. Electricity may be communicated to th
whole surface of glass, or any part of it, if it be c<
vered mtb a metallic substance, as tin-foil ; and tb.
ia called coating the glass.

308. If a conductiog coinmuni cation be made b(
tween both sides of the glass thus tnated and chargt
with electricity, a discharge or explosion takes plac<

Obt. Glass of any rorm. provided it be samid, will txam
the purpose ; but cylindrical jars are chiefly uacJ.

309. A glass bottle, or jar properly coated for el«i
trical purposes, is called a Leyden phial, or jar, froi
the city where this property was first discoTercd.
^^ Illui. Fig. 83, repreiente a Leyden ji

^^^^v>JL coated with tin-foil on the inside aitd oatdd

nfcc ^T withinsbout three inebea of the topofitse:

^\^^^P lindrical part, and bavii^ ■ wire with » roiu

S3 ^fl^B ^Tt£s knob, or ball, A, at iti extremity. Th

^raH '"'''^ ptsiei through the cork, or woodi

H^B stopper, and at itg lowest extremity is a piei

Iffl^P ofchaiii that touchci the inside coating in »

veral parts. To charge this jar, a communication is made hi

tween the electrical machine and the brass knob A, while U

outride Of the jar communicBtea with the earth by the tab

or the hand.

Exp. 1. Bring the knob A of the jar near the prime ooDduc
or,aiid at^er a few turns of the madiine the jar will be chMgec
that is, the inside of the jar wdl be poaitively, and the outM
negatively electrified; orif the inside ia oegatiTelr, the on
aide will be poaitirely electrified. R is a djschargfing rod.iAii
ti osed to convey ttie superabundant eleotrioily froiB tf

OF ELECTRICITY. lOO

■ide to the nther, where tbere is len Ihaa the natural shue.
The duGharging rod congists of two bmu knobg a a atlBched
to wirei, wtuch move rounds joints, fixed toBglau handle R.
S. When ooe of the kitobii is applied to the ball A, uid the
other to the outside coating, a camntuniaation jg made be-
twe«n the outside and inside of the jar, by which the equili-
brium it instantly restored by Ihe auperabundant electricity
passing froip one side lo the other, appearing in the form of ^
TJTid flash, accompanied with a laud report.

3. A shock may be taken by putting one hand to. the out-
side coating, as at a, and bringing the other to thebnobA.

4. Any number of persons may reoeive the shock together
by laying hold of each others' hands, the person at one end
looclung the outside of the jar, and the person at the other
endbringing his hand near the knob A. If there were a
hundred persons so situated, they would every one leel the
shock at the same instant. The electric fluid may be thus
(xaiveyed.many oiilea in a moment of time.

310. Sereral Leyden jars, connected together by
making a communication between all the rmtgides, and
another communication between all their inndes,
iijrm an electrical battery.

JlUu. Fig. 84, represents a battery, consisting of sixteen
jar^coated with tin-foil, and disposed in a proper box. The
wires, which proceed from the inside of every four of those
jars, are screwed, or fastened, to a common horizontal wire E,
wbieh is knobbed at eacheilremity, and l>y means of the wires
FjF.F, the inside coating of 4, 8, or 13, or of aU th^ sixtee^

pircuit, will, by the dischargee of the battery, in
come red hot. It sometimes melts into small globi
fereot sizes.

2. If between two slips of window-g^lasses sonn
"be placed, and the slips of glass be pressed firml;
and the shock from a battery be sent throug;h then
leaf will be forced into the pores of glass.

3. If the gold4eaf be put between cards, and a sti
be passed thi'ough them, it will be completely fase<

4. Gunpowder may also be fired by the electric

312. Metallic points attract the Qlectri<
bodies silently, which renders them of a sup]
in defending buildings from lightning.

313. When electricity enters at a poir
pears in the form of a star ; but when it goes
a point, it puts on the appearance of a brush

Obt. Delicate apparatus may be put in motion b
trie fluid when issuing from a point ; hence we hav
omsriesj mills^ kc,

314. Lightning is the rapid motion of va
of electric matter, and Thunder is the noise

1 il- . ? J A? - ^ - /•!! -.1- j.„ : - - _1 i._! _ .

QUESTIONS ON ELECTRI0IT7. 187

(htmder^ which iamore or leu intense, and of longer or £hort-
«r daration, according; to the quantity of air acted upou, and
the distance of the place where the report is heard from the
point of its discharge.

316. When the electric fluid passes through highly
rarefied air, it constitutes the aurora bortalisy or
fwrthern lights. Most of the great convulsions of na-
ture, such as earthquakes, whirlwinds, hurricmes,
&c. are generally accompanied by, and often depend-
ent upon, the power of electricity.

Obs. The water-spout is probably the result of the opera-
tion of a weakly electrical cloud, at an inconsiderable elera-
tion abore the sea, brought iuto an opposite state ; and the
attraction of the lower part of the cloud, for the surface of
the water, may be theammediate cause of this extraordinary
phenomenon. The corruscations of the aurora borealis and
aostralis, precisely resemble strong artificial electricity, dis-
charged through rare air ; and as the poles are non-conduc-
tors, being coated with ice or snow, and as vapour must be
constantly formed in the atmosphere above them, the idea of
Franklin is not improbable, that the auroras may arise from
a discharge of electricity, accumulated in the atmosphere
near the poles, into its rarer parts; though other solutions of
the phenomena may be given on the idea, that the earth itself
is endowed with electrical polarity; or that the motions of
the atmosphere produce the effect.

QUESTIONS ON ELECTRICITY.

What parts of bodies contain the electric Jluidy internal or
external ?

Under what circumstances are the effects of the electric
fluid shown ?

When a piece of sealing-wax, or glass, is rubbed, why does
it gain the power of attracting or repelling light bodies?

How is it proved that the electric fluid will pass through
a metal ?

How is it proved that it wiU not pass through glass, or
sealing- wax?

What substances are conductors and what are non-^ondwt-
ori of this fluid ?

When a surface is aupposed to have more than its natural
quantity of this agent, what is said of it ?

188 ltV£STlpXS OK ELECTAfCITX.

Wben HWr&Be ^ leas of the fluid lv>w
from ODC wbicb tut more ?

When a a bodjtiuita he intulaledf .

Under what cinunuUnces can the human bodj he im
to give an <l«ctric shock » ^ ^^l

What are the principal methods of excitine the '^jH
Quid ? *•*■

When do two electrified bodies repel each other ? andwW
do they attract each other ?

How i; it shown that when on iiiiulsted person rubs a gljj»
tube the penon is electrified (lUferantlj from the tabe!

What u meant bj vitrentt and rennouf electricity T

When electricity is excited bj rubbing two
l^ether, are they both in the rame electrical slates i

What is meant by tleclria .>

Whnt is said of the stoue called

iitheeffeot; '
EzpUin ^ niBi of Um diflerest pHU of Oie «Ueifi^tkki

What is an tltelronuler f

If a surface containing its nataral share of eledricilf Im
brou°;ht near one which is potitiTely oi n^atively eleotnfitd,
what is the effect?

Does the electric Said spread on glasa !

Haw can it be commanioatad to a part of the mriwe ooly!

When is a piece of glass said to be eoattd f

What happens when a oommiinication is mada batWHi
both sides of a coated and chained glttu f

What shaped glasses answer best fiir theie e^perimeots ?

Whatisaitjrdoipftial?

Explain iti ilTOcture and the method of obaifiiig raddB-

What cDostitutei an tlielrieat batten/ 7

Mention some of the striking ezperimenti made bjQii
battery.

How are baildii^ defended from the eSecta of lightning .'

When does electricity appear as a itar, and whiB HK
bruih ?

How can machinery be put in motiw by eleetrkit]' f

Whatis/ig/ilnine?

Whnt is tkunderJ

What constitutes the durara iorMlii ?

What is the caiue of the uvfn-ipouf/

OF GALVAHIC ELECTRICITY. 189

aiLVANIC ELECTRICITY,

316. Galvanism is another mode of exciting elec-
tricity. In electricity the effects are chiefly excited
fcjr mechanical action ; but the effects of galvanism
are prodnced by the chemical action of bodies upon
«ach other.

317. The nerres and muscles of animals are most
«sily affected by the galvanic fluid ; but combined
m the voltaic battery, it possesses surprising poweri
of chemical decomposition.

^J: !• In 1791, Galvani, of Bologna, discovered that &
dead frog may have its muscles brought into action by very
amall quantities of electricity. He also discovered that the
same inotjons may be produced in the dead animal, merely
by making a communication between the nerves, and the
masdei, by means of conducting substances.

2. Some fishes have the property of giving shocks analogous
to those of artificial electricity ; namely, the torpedo, the gym-
nohu eUelricw, and the siiurus tUetrieut. If the torpedo,
whilst standing in water, but not insulated, be touched with
one hand^ it generally communicates a trembling motion or
sUghtshook tothe hand. If the torpedo be touched with both
handsat the same time, one hand being applied to its under and
the other to its upper surface, a shock will be received exactly
like thateccasioued by (he Leyden phial. The shock given by
the torpedo when in air, is about four times as strong as when
in water; and when the animal is touched on both surfaces by
the same hand, the thumb being applied to one surface, and the
middle finger to the opporite, the shock is felt much stronger
than when the circuit is formed by both hands. The gymnotus
electricus, or electrical eel, possesses all the electric properties
if the torpedo, but in a superior degree. When small fish are
p«t ioto the water wherein the gymnotus iskept, they are gen-
erally stunned or killed by the shock, and then they are swal-
lowed if the animal he hungry. The strongest shock of the
gymnotus will pass a very short interruption of continuity in
the circuit. When the interruption is formed by the incision
made by a penknife on a slip of tin-foil that is pasted on glass,

that tAip isput into the circait,the shock ^vi ^«M\.\y^ih^<^^<^

made
and

IM OF ELECTKICITV.

thst iatermption, will shew n small but vivid sp«rk(|
to bsseen in a dark room. The gymnotus seemed also
poKCSsed of a sort of ncur seme, by n-hich he knows \l
the bodiei presented to him are ccmductora or naL V.
has been ascerlaincil by a great nuiaber uf experiment
Erp. 1. If a living' fri^, or a live fish, ai a flounder,
■ slip of lin-foU pastedoTt its back, be placed upona n
line, whenever a comma nlcation is formeil between t
and tin'foil, the spa^m-i of the muscles aie excited.

2. Ifaperson place a piece of one metal, as a halt
above, and a. piece of some other metal, as zinc, be
tongue, by bringing the outer eJgcs of theee pieces i
tact, he will perceive a peooliar ta^te, anil in the dv
see a flash of light.

3. If a pernon in ^ dark place put a slip of tin-foil qj
bulb of one of his eyes, and a piece of silver in his nw
caii<iing these pieces to communicate, a bint flash will,
before hia eyes. ,

318. The conductors of the galrnDic fluidi
ricled into the perfect and impcrftci. The ^
conductors consist of metRlIic substances ant
coal. — The imperfect conductors are water ani
dated fluids, as the acids, and all the subBtanci
contain these fluids.

319. The simplest galranic combinations mu!
sist of three different comtuctors, not wholly ■
clues. When two of the three bodies are oft!
class, the comhination is said to be of the firBt <
when otherwise, it is said to be of the second.

320. it seems to be iodispensably requisite, :
pie galvanic circles, that the conductors of on
shall hare some chemical action upon those ofthc

Exp. If a piece of line be laid upon a piece of copg
upon the copper a piece of cord or Qanne), moistened
■oiotion of salt water, a nrtU of the first class is formi
then if three other pieces be laid on tbeee in the sam
and repeated several times, the whole will Ghiu a pile ■
ry of the jirrt orderj

321 . When the three bodies which form 3 g-
circt« of the first er4«r are laid on ooe anotberi I

OF GALVANIC ELECTRICITY. 191

ind the under one not touching, these two ex-
es form opposite electric states.

2. The galvanic effects may be increased to any
3e, by a repetition of the same simple galvanic
>ination, and these repeated combinations are
d galvanic piles or batteries, which may be cod-
ted of various forms.

p, 1. Take a number, say twelve plates of silver, and
ime number of pieces of zinc, and also of woollen cloth,
ist having; been soaked in a solution of sal ammoniac in
* ; with these a pile is to be formed, viz. a piece of silver,
ce of zinc, a piece of cloth, and thus repeated. These
> be supported by three glass rods placed perpendicular-
th pieces of wood at the top and bottom, and the pile is
lete, and will afford a constant current of electric fluid,
gh any conducting substance ; thus, if one hand be ap-
to the lower plate, and the other to the upper one, a
I will be felt, which is repeated as often as the contact is
fired. But the plates will soon become oxydaied^, and re-
i cleaning in order to make them act.

2. Another, battery consists of a
row of glasses of any shape, a a, fig.
86, containing a solution of salt and
water; into each of these, except
the two on ^e outside, is put a plate
of zinc «, and another of silver x ;
i plates communicate by means of the wires lo tr, and are
stened, that the silver, a;, in one glass is connected with
dnc, sr, in the other : when one hand is dipped into the
glass, and another in the last, a shock is felt. The glasses
be of any number.

/19\ 3. The most convenient

/ kind of battery consists of

a trough B, ^. 87, made

of baked wood, three in«

ches hroad, and about ai

^ deep ; in the sides of the

M trough are grooves oppo-

to eaOu viuw y into each pair of grooves b^fizcti by ceme^il

I9S OP ELSCTIHCITt.

apliilH of zinc and silver soldered logellier, an J in.
ofsilTer and line ; the cement Htitet be filled in to i
rent nny communication between [bo dilferenl ck
cells are lo be filled with water, or nith a lolnliori
and nitrouB acid, when, n communicatiaD beiu^ I
twieii the first and last cell, b; means of the hanJ^
shock ii felt, and will be repealed ai often aj the o

4. Sereral perrons, by joining hands, having fin
them with water, may receire the ahoch.

6. Ifiilates of copper and ziac, two or three inntM
and pieces ol clolh of the ^me size, Boahed in a n
talti of sal ammoniac, or nitre, be arranged in tha
copper, ziuc, moistened cloth, and lO on,3Dd made it
■Dialed pile, of which the series are two hundred, se<
markahle phenomena willeccur. Whoa one haud|
to the bottom of the pile, and Ibe other to the top, ba
being moistened, a shock will be perceived. Wheft
lie wire, baring a bit of well burned charcoal at ill i
ty, is maile to connect the twnextremilies of the pil^
will be perceived, or the point ofthecbnrcoal will bei
nited. A wire connected with the Ioj> of the pile, br
contact with a sensible eler.lro meter, will cause the 1
diverge ; and, b; removing (he wire aod appljing
glass lo the electrometer, it will be foand that the elt
i> poaitive ; a wire coDuecLed with the boltoai ofthe |
albct it with negative electricity ; a wire frpm the a
the pile will have no inSueace on (he Instrument,
of platina from the extremities* of the pile be ir.trodui
water, or into two portions of water connected by mc
stances, oxygen gas will separate at (he wire exhihjl
positive elec&icity; and hydrogen gas at the wire exi
the negative electricity ; and the proportions are sue!
the proper circumitancei exist, that Ihey wdl produ
ter when exploded by the electricalipark,thatis, the
of hydrogen will be to that of oxygeu, as two to one.
same wires bo introduced into n strong solution of sn
or phosphoric acidjOr inlometallic solutions, oxygen «i
rate at the positive surface, the inflammable, or I
mattercontainedintbc ?nlutioo, at (he negative surfai
323. The spark Irom a powerful galvuDJc b
acta upon and indames gunpowder, chiircoal, t
and other mflammable bodies, melts all metals, di
rs 4JianiODds> &c.

OF GALVANIC ELECTRICITY. 193

Ejrp» Fillfhe battery (fig;. 87,) with water and nitrous aoid,
1 the proportion of nine parts of water and one of acid, and
ripe the edges of the plates very dry, then the wires w w are
be ^tened to pieces of copper, and put into the outer
ells : a a are little glass tubes to hold the wires by. Bring the
aids of the wires together on the plate of glass Xj and a spark
wtM be perceived : if gunpowder belaid on the glass between
he points of the wires, it will be exploded.

2. Gold and silver leaf may be inflamed in this way ; Dutch
pld burns with a beautiful green light ; silver with pale blue;
jold with yellow light.

3. The most powerful combination that exists, in which the
greatest number of alterations is combined with extent of sur-
bce, is that in the laboratory of the Royal Institution, It con-
dsts of two hundred instruments, connected together in regu-
lar order, each composed of ten double plates arranged in cells
of porcelain, and containing in each plate 32 square inches ; so
that the whole number of double plates is 2000, and the whole
sarfiice 128,000 square inches. This battery, when the cells
were filled with sixty parts of water, mixed with one part of
mtric acid, and one part of sulphuric acid, affords a series of
brilliant and impressive effects. When pieces of charcoal
about an inch long, and one-sixth of an inch in diameter, are
brought near each other, (within the thirtieth or fortieth part
of an inch) a bright spark is produced, and more than half the
volume of the charcoal becomes ignited to whiteness, and by
withdrawing the points from each other a constant discharge
takes place through the heated air, in a space equal at least to
four inches, producing a most brilliant ascending arch of light,
broad, and conical in form in the middle. When any substance
18 introduced into this arch, it instantly becomes ignited ; pla-
tina melts as readily in it as wax in the flame of a common
candle ; quartz, the sapphire, magnesia, lime, all enter into
fusion : fragments of diamond, and points of charcoal and plum-
bago, rapidly disappear and seem to evaporate in it. Such are
the decomposing powers of electricity, that not even insoluble
compounds are capable of resisting their energy: for glass, sul-
phate of baryta, fluor spar, &c. when moistened and placed in
contact with electrified surfaces from the voltaic apparatus,
are slowly acted upon, and the alkaline, earthy, or acid mat-
ter carried to the poles in the common order. Not even the
most solid aggregates, nor the firmest compounds, are capable
of resisting this mode of attack ; its operation is slow, but the

z. Anomcr ^ivanic circie is seen uy uie uiscoi
a silver spoon in eating; eggs ; the saliva and fluid e
ductors of the second class, and the silver pf the fir

3. Pure mercury retains its splendour a long; tii
it be amalgamated with tin, and it is quickly oxyd

4. Worto of metal, the parts of whicU are solden
soon tarnish in the places where the metals are join

5. The nails and the copper in the sheathing; o
soon corroded about the place of contact. These
fects of galvanism.

325. The effects of galvanism on metal
are greatly increased by using plates of a h
and on the contrary the shock Is increased
plying the pairs of plates.

Obs, 1 . The shock of a battery containing 80 or
plates, of three or four inches in diameter, is such i
sons would be willing to bear more than once. A
time, such a battery produces but feeble effects "w
through a metallic wire. On the contrary, if one o
of plates containing the same extent of surface h*
sensation it gives is hardly to be felt, while it will (
metallic wire of considerable size.

2. Professor Hare, of Philadelphia, has invented

OK GALVANISM. 195

galvanic influence and the^ nervous influence. The
ISalvanic influence being capable, in some instances of
'Hnpplying the place of the nerves.

Obs. 1. Dr. Wilson Phillip in his inquiry into the laws of
efthe vital functions has shown, that if a nerve be diTided, and
rklBtreamof g^\ranism be directed along; and through the part,
^riiose functions depend on this, that the function was per-
Gonned as usual. Thus on dividing the nerves which are dis-
tributed to the stomach, the process of digestion ceases though
4he animal continues to live for some time. But on supplying
r^ie place of the nervous power by the galvanic influence, di-
C^<^ was performed as usual.

2. In the same way it was found, that when the nerves dis-
: tnbuted to other parts were divided, and the part became
r pdflied for want of the nervous power, a stream of galvanism
• Tightly directed would again in a good degree restore the ac-
ItioD of that part.

} 3. Dr. Phillip having conjectured that the heat of animal»
~ pended on the influence of the nerves, wished to observe
w far the galvanic power might produce this effect. For
this purpose some blood was drawn from two animals of the
lame kind and temperature into two small cups. The blood
in one of the cups was submitted to the galvanic influence, while
the other was placed under the same circumstances, except
in this respect. Now if the galvanized blood remained warm
the longest, or had its temperature increased, it would show
ao additional analogy between the galvanic and nervous ener-
gy, if indeed animal heat depends on the latter. The experi-
[Mentgave a decided proof that the galvanic power had some
finflaence on the temperature of newly drawn blood, for that
Iportion which was submitted to its action, not only remained
r ▼arm longer than the other, but its temperature was actually
rused several degrees.

4. From the difficulty in breathing, which animals experi-
enee on depriving the lungs of a portion of their nervous in-
"flaenoe, Dr. Phillip was led to make trial of galvanism in
aUhma^ suspecting in this disease there might be a want of
^nervous power. The inference which he draws from a very
^considerable number of trials on persons afflicted with this
disease, is, that there is a difficult transmission of the ncr-
Vous influence, through the nerves which supply the organs
~) of respiration, and that in a great majority of cases the asth-
I aa may be permanently cured, or at least relieved by gal-
1 Tanism.

196 on GALVAHiSM.

337. Galiranistn hHs a peculinr ami moat @urpm
ing effect on the muscles of dead noimiils, their limb
being thrown into FJolent motion by it a conslderabl
time atler the life of tlie aDimnl la extinct.

5. The most sinking effects ot pilvmiism oa the ham
frome after Jeath, 'vereashibited at Gla^on a few years ainci

The BUbject on nhith these eKperimeiiLs were mide, wi
the body erf' the murderer Clydesdale, who was hanged at thi
city. He WM scispended an honr,aiidthe first eiperimentWf
mtide in about ten minutes after he was cut down. The gul
yanio battery coniisled ofS70 (iiiirB of four inch plates.

The subject was prepared for the first eiperiment by mat
inr an InoUion into the nape of the neck, and removing
part of the nllat perlebre,sa as to bring the spinal marrow iiil

left hip !o as to lay above the icialie nerve and another ima
one in the heel. The pointed rod connected with one end ■
the battery was now made to touch the spinal marrow, whil
the end of the other was placed in contact with the Bclat
nerve. Every moBclo of the body was immediately agititi
with convulsive movement? resembling a violent shudderit
from coM. On moving the rod to the heel, the kiiec heii
previously bent, the leg was thrown oat with such violence,
nearly to overturn one of the aasistanla, who in vain attempt!
to prevent its extension.

I'he next experiment was made by directing the galvan
power in the coarse of the phrtnie nerve, which goei to tl
principal muicle of respiration, the diaphragm. The eflec
were fur more striking than before. " Full, nay, laborio
breathii^," layi Dr. Ore, " inatanOy commenced. The chi
heaved and fell ; the abdomen was protruded, and again ix
lapsed with the relaxing and retiring diaphragm."

In the jud-tnent of many scientific gentlemen who witne>
ed the scene, this reipiralory experiment was, perhap*, t
most striking evermadewithaphilosophicalapparatoi. T
next experiment was made by applying one of the wirei tot
supra abilal nerve under the eye brow, and the other to t
heel. Most eitraordinary grimaces were made ; " evt
muscle in the couotenance was simultaneously thrown h
fearful action ; rage, horror, despair, anguish, and ghM
smiles, united their hideous eipressions in the murderf
face." At this period, several of the spectators were forced
leave the room from terror or lickness, and one gentian
tainted, " In the last experiment, one of the wires wunii

QUESTIONS ON GALVANISM,. 197

^* to touch the spinal marrow at the nape of the neck, and the
JBlfaer an incision in the top of the fore finger; the first being

-.^nnously clenched. The finger extended instantly, and
ntmi Uie conTulsive agitation of the arm, the finger seemed to

,,feointoat the different spectators, some of whom thought he

' mtA reallf come to life."

QUESTIONS ON GALVANISM.

What is meant by Galvanism ?

What parts of- animals are most easily afiected by the gal-
vanic fluid?

When, and by whom was the galvanic principle discovered ?

What effect does it have on dead animals ?

What animaiR have the power of giving electric shocks ?

What is the experiment with the Uving irog^ or fish?

How are galvanic conductors divided ?

What are ihe perfect, and what the imperfect conductors ?

What are the most simple galvanic combinations ?

When is the combination of the first, and when of the second
order?

Do the conductors of one class have any chemical action on
those of the other ?

How ia a battery of the first order constructed ?

What parts of a pile are in opposite electric states ?

How can galvanic effects be increased ?

fizplain the mode of constructing a galvanic battery.

What substances can be set on fire by the galvanic shock :

What fluid is used to fill a galvanic trough ?

What &cts in common life are explained by galvanism ?

{n what respect is it increased by multiplying the pairs of
small plates ?

What is Professor Hare's invention ?

What analogy is there between galvanism and the nervous
influence ?

What effect does galvanism have on the temperature of
. warm blood?

What is said of its effects on asthma ?

What is said of the effects of galvanism on the body of Cly-
desdale?

17 *

198 or MAsnmM.'

OF MAGNETISM.

328. Maortish explaina the properties
loaditone, ornatnral mi^et, which is a dark
e6 and hard mineral bodj, and is found to be
of iron, being genetally found in iron mines.

The ma^etic properties of the natoral magni
m^ be commnnictited to other bodies, which -■
then called artifidal magneti. Theae propertiesi
however, be coniunnmcated to no other sabstancei
than iron or nickel.

329. The following are the churacteristic proper-'
ties of a magnet 1. The magnet attracts iron aori
steel, i. Amagnet, if left at liberty, >vill point to-
wards the poles of the earUi, or very nearly so, aail
each end always pobts to the siime pole. 3. When
the north pole of one magnet is presented within a
certain distance to the south pole of another, they
will attract each other. But if n north pole of one
be presented to the north pole of another, or a south
to a sooth, they will repel each other. 4. The two
poles of « magnet, left at liberty, do not lie in the
same horizontal direction ; one of them inclines to-
wards the horizon, and of course the inclination of
this causes an elevation of the other pole above it^
This is called the inclination or dipping of the msgntt
5. Aay magnet may be made to impart those proper,
ties to iron or steel.

All natural and artificial magnets, as well as the bo-
dies upon which they act, are either iron in its puTB
state, or such compounds as contain it All m^oeti
attract iron and nickel.

Obt. 1 . I'he action and re-action of the maenqtie powsr M«
mntunl and equal j &r if a piace of iron, oriteel, or other 6r-
ruginous substance, be brought within a certain diitaaee ot
one of the poles of a magnet, it is attracted by it, so M to ad-
here to the magnet, and not attffei itseWto b« w^anted witfc-
out an evident edbn.

i OF MAGNETISM. 199

% The attraction is mutual, for the iron attracts the mag-
net as much as the magnet attracts the iron ; since if they be
placed on pieces of wood or cork, so as to float upon the sur-

. kce of water, it will be found that the iron advances towards
the magnet as well as the magnet advances towards the iron :

. or, if the iron be kept steady, then the magnet will move to-

; wards it,

?i 330. When a magnet is at liberty to move itself
f freely, it constantly turns the same end towards the
north pole J and of course the opposite part towards the
■ southpole, of the earth.

331. Those parts of the magnet's surface which it
tarns towards the poles of the earth, are called the
north and south poles of the magnet.

332. The property of pointing to these poles is
called its directive power y and when it moves to place
itself in that direction, it is said to traverse,

1 333. The magnetic meridian passes through the

I poles of the magnet when standing in their natural

' direction. The declination of the magnet, or of the

mimetic needle, is the angle which the magnetic me-

- vidian makes with the meridian of the place.

\ S34. The north or south poles of two magnets re-
pel each other ; but the north pole of one attracts the
south pole of another.

Exp. Place a magnetic needle upon a pin stuck on a table,
and wnen it stands steady, place an iron bar, eight inches long,
and half an inch thick, upon the table, so that one end of it
may be on one side of the north pole of the needle, and near
enough to draw it a little out of its natural direction. In this
titnation approach gpradually the north pole of a magnet to the
oflier extremity of the bar, and you will see the needless north
end will recede from the bar, in proportion as the magnet is
. inrongfat nearer to the bar.

335. The inclination, or dipping, of the magnetic
I needle, expresses the property which the magnet pos-

- sesses of inclining one of its poles towards the hori-
[ zoD, and elevating the other pole above it.

336. Any magnet may, by proper methods, be
made to impart its properties to koiv^ ?sl^t\. ^^ w:.^^

200

337. When apiece of iron 19 brought will
Inin distance of one of tbe poles of !i magnc
tracteii by it ; the attraction is stroi^eBt at t.

338. The magnetic attraclion is not in th«
minishedbythemferporih'onofanybodiescxi

Exp. 1. Suppose a m^Qet placed at an inch (Us
a piece of iroD, requires an ounce of force to ron
nhicb is the same thing, sappose that the altracli'
each other is equal to one ounce : it will be foun
same degree of attraction rccD^iias constantly
lhou°'h a plate of other metal, glass, paper, iic, be
bet ween the magaet and the iron, or though tbe; I
inteparate boxes of glass or other matter.

2. Move steel filing! placed on a brass plate, ia '
by holding a magnet under the yfifsel.

3. 9lrew on a sbect of paper, some iron RHngs
small magnet amnng- them; then shake (he table a
the TiltngB will arraugs themselves in the ivay rcpi
%. 90. But if iron filings are ihakeo through a g
upon a paper tbat covers a bar magnet, the fill

4. Spriuhle steel duEton a a sheet of paper, unJc
placed a magnet, or two magncla, having tlieir pel
to each other, and at IbeJislance of aljout an inrh.

5. A needle uDdcraoeshauated receiver, will
ed at (he same distance as in tbe open air.

339. SoA iron is attracted by the magnet i
cibly than steel, but it is not capable of pi
the magnetic property so long.

340. Heat weakens the magnetic power, ar
heat destroys it. The gradual addition of w
magnet kept in its proper situation, incn
magnetic power.

Obi. Among natural mai^ets, the smallest genan
a greater attractive power, in proportion to their
those which a re larger. Tberehavebeenaaturalm
^ceedJDg twenty or thirty grains, which would lif
iron that would weigh forty or My times more I
selves. A small magnet, worn by Sir Isaac Newtoi
weighing about three grains, is said to have taki
grains, or near 260 times its owD weight, andMr.t

\ OF MAGNETISM. 201

^ teen one of six or seven grains weight, which was capable of
lifting a weight of 500 grains. But magnets of two pounds and
upwards, seldom lift up ten times their own weight of iron.
341. The north pole of a magnet is more powerful
'- in the northern, and the south in the southern parts of

the world.
i 342. When a magnet with two poles is freely sus-
> pended, or floats upon water, widi no iron near it,
f it places itself in the magnetic meridian, and it is this
. principle of polarity that makes it so useful to navi-
I gate at sea.

Obs. 1. ,When a magnet is kept freely suspended, so that it
may turn north or souUi, the pilot, by looking at its position,
can steer his course in any required direction at sea.

2. An artificial magnet, fitted up in a proper box, is called
the magnetic needle, and the whole together is called the
1 mariner^ s compass See figs. 90, 9 1 .

\ 3. Though the north pole of the magnet always points to-
' ward the northern, and the south toward the southern parts,
yet their direction is seldom in the exact direction of the poles
of the earth, that is, the magnetic and the real meridian sel-
dom coincide, and the angle which they make is called the
angle of declination, or variation of the magnetic needle.

4, This declination is said to be east or west, according as
the north pole of the needle is eastward or westward of the
true meridian of the place.

5. At present the declination, or variation, of the magnetic
\ needle, is about twenty -four degrees westward at London, and
t the dipping seventy degrees.

343. When a piece of iron is brought sufficiently
near a magnet, it becomes itself a magnet. Bars of
iron that have stood long in a perpendicular situation,
are generally found to be magnetical.

06*. 1, If a long piece of hard iron be made red hot, and
then suffered to cool in the direction of the magnetical line, it
becomes magnetical. The electric, shock will often render
iron magnetical ; so also will lightning.
\ i 2. Artificial magnets are made by applying one or more
; powerful magnets to pieces of hard steel. The power of a
magnet is not diminished by communicating its properties to
other bodies.
3. Two or more magnets joined together may communicate

which cannot be accounted for upon this hypothesis
the Doctor supposes may'arise from an unequal and ir
distribution of the magoetical matter. The irregulai
bution also of ferruginous matter in the shell may li
cause some irregularities.

5. Mr. Cavallo's opinion is, that the magnetic of tt
arises from the magnetic substances therein contains
that the magnetic poles may be considered as the cei
the polarities of all the particular aggregates of the m
substances ; and as these substances are subject to <
the poles will change. Perhaps it may not be easy
ceive how these Substances can have changed so mat
as to have caused so great a variation in the poles, tt
tion of the compass having changed from the east tow:
wert about thirty-three degreee in two hundred yean
the gradual, though not exactly regular, change of v)
shews that it cannot depend upon the accidental <
which may take place in the matter of the earth.

6. Mr. Churchman, of America, says tb^re are ivf
netic poles of the earth, one to the north and the othe
south, at different distances from the poles of the ear
revolving in different times ; and from the combinei
ence of these two poles, be deduces rules for the po:
the needle in all places of the earth, and at all tim

or HAQHETtSM.

98 •^.

lUiu. 1. A magnetic oeedle ia
■■repreBented in figs. 88 and 89,
tbe Gnt of which sbotra the up-
per EJJe, and the leeaai] a tide
■ view of the needle, having- a
preKy large hole in Ibe middle,
; lo which a conical piece ii adnpled bj ineaniof a bran piece
A 0, into this tbe agate cap (aa it is called) is fattened. The
■ \ apex of tbe hollow uap reste upon the point of a pin F, which
- ii fixed in the centre of the box.aod upon which the needle
■ , b«ii)g properly balanced, turns Terynirably.
i iliui.2. AmariDer'scompassisrepreseatedinfig.gO; the
■^ bos which CDotaina tbe card, or fly, with the needle, ii made
S ofa circular form, either of wood, orbrtus, or copper. It is

luipemled within aeqaare wooden box B, fig. 91, by means of
two concentric circles, called gamboli, so fixed by cross axes,
B, a, a, a, fig. 90, to the two boxes, that the inner one, or
conpBss box, shall retain an horizontal position in all mnUoas
of a ship, whilst tbe outer or square box is fixed with respect
to tbe aliip. The coinpass Irax is covered with a pane ofglai!,
that tbe motions ofthj card may not be disturbed by the wind.
What is called the card is a circular piece of paper, which ia
fastened upon tbe needle, and mores with it. The outer edge
trf this card is divided into 360 equal parts or degrees, and
within the circle of these divisions it is again divided into 33
equri parts, or arcs, which are celled tbe points of the compass,
or rbiunbg, each of which is often subdivided into quarters.

;, There seems to be a similarity between ma^etiam and
electricity. If two pieces of soft iron wire beled each to a
separate thread, and they are hung freely, and if the north end
of a magnet bar be brought under them, liie wires will repel
f each other as in electricity. Tbe same result would happen if
the sooth pole oflKe magnet be presented instead of the north.

miCSTIOlTS OS MIDNETISM.
QUESTIONS ON MAGNETiaM.#

What are Ibo ciisracterialic propertioa of the magnet ?

What are the suhstajicas atlracted by the magnet ?

What are tbe poles of the tnagDHl !

Wbat 19 the magnrlic merifJton .'

What ia the dectination of the magnetic tieciUe?

Which of tbe magnetic polec ailracl, and whicb npel i

Wbatbmcaiit by tbe dipping of the needle ?
Is tbe attractjoa of a magnet diminished by the interpoai
of any body ?
How ia this proved ?

Whicb 19 B.t1nLGted moat powerfully, iron or steel.'
How is the magnetic power weakened or destroyed ;
How i< the magnedc power increased !
When Joeithoma^et plaueilaelfin the magnetii^ merid
WhuiBmeantbyfheune'^eo/dec/tiuiIian, orrariationf
Whot U a raariner'5 Pompfts? ?

a magnet !
How are artificial magnets made f
Where does the catae of magneliam cxiat?
What is Mr. Cavallo's opinion of the cause ofmagnetiaa

What is Mr, Churchman
How is a magnetic needle i
Explain the figure of the m

GLOSSARY

OF SCIENTIFIC TERMS AND TECHNICAL WORDS.

iceelerated motion^ is motion added to moiioa, by the conslatnt

action of an orig^inal force.
Acousiics, the science of sound and hearing^.
Air-Pumpy a machine for making experiments on air.
Amplitude, the point of the compass at which a heavenly bo-
dy rises or sets.
Aphelion^ the point of the orbit of a planet which is the most

distant from the sun.
Apogee, the sun^s or moon's greatest distance from the earth.
AHnnomy^ the science which treats of the planets, stars, and

olestial motions.
Attraction, the phenomena of bodies falling together, without

sensible cause.
Atmosphere, the fluid or air in which men and other animals

live, and which surrounds the earth to a considerable

height.
Atimuih, the bearing of any heavenly body which is above

the horizon.
Barometer, an instrument for measuring the dasticity of the

atmospheric air.
"^knue, that which produces or appears to produce an effect.
'^(tpillary ttibet, which resemble hair, and exhibit peculiar

phenomena of attraction on fluids.
"Central forces, that composition of forces, by which bodies

move in circles or curves.
Centripetal force, the force which acts from the circumference

towards the centre.
"Centrifugal force, the force which acts in the direction of a

tangent to the circle or orbit.
Centre ^Gravity, the point of any body about which all its

parts balance each other.
Cohesion, the power which binds particles of matter into so-
lid masses.
^'oats of the eye, the sclerotica, the choroides, and the retina.

206 GLOSS AST.

Candiic/ii-i, classes of bodies wliicb condact, wilh diflerea

grees of &cility, Uie powers of heat and el«ctricil;.
CaTicaee, a boUow smtSocb.
Caraiex, a projecling surlaoe.
Cimtergeticy, tending towardi a point.
dmiltllaliam, figures which the variooi groups of stan

tuppmed to resemble.
Dtcnmpoie, to take to pieces, or analyze.
Dcelinadmi, the distance of a heavenly body, north or loi

the equinoctial.
Diagttnat, the line drawn from one angle to another ol

figure.
DUc, the face or surlaceof the sun or aioon.
Divtrgemii, spreading Irom a point.
iJiruiiility, the power which cKists of dividing particl

matter indefinitely.
Dynatnici, Uie science of motion, forces, or momenta.
SxcetUTicHy,m the earth's orbit; the excentricity is equ

168 ten thousandths, or 0168 of themean distance.
Exceutricils of Ihcplanet't erbil, is the distance of the

Iroin the centre of the orbit.
Ecliptic, the line in which the mn eppearFto move.
Eclipse of !hf Sun, an interception of the light of the su

the earth by the intervening moon.
Eclipse e/lhr JIfoan, the interception of the light of then

the moon by the intervening earth.
£/ajCici^, the disposition and power which bodies posse

returning to their original position and shape.
Ekmetili, any substances which cannot be dccamposei

oxygen, nitrogen, and hydrogen ; phosphorus, sulp

carbon, and the earths; the metnl?. and the alkalies.
kquinptlial or Equator, the circle of the earth, which is :

way between the north and south pole.
Equalion of lirM, arises from various unequal motions of

n,bDlchielly from theqaick motion in the earth'
ilion, which is 61' ID" per day, on Januat^ U.mi
BT motion in Ihe aphelion, which is about S'Tfl.

, , _. _bout6T'£;

per day, or onefifleenCh less, on July 1.
EqiaoBxti, the beginning of Aries and Libra, or equal

and n^ht.
EuiittmeieT, an instrument for measuring the purity of air
EvapOTOtiim. the passing of fluids into a state of vapour.
Foe\a, a central point, the place where rayi ofU|ilt ora n

raT power converge. Its plural is/oci.
FuSerxaa,*. prop on which a lever acts.

OLOSSART.^ 207

IWion,tlie rendering^ of a solid body fluid, by the application

of beat.
j^asj the elastic or expanded state of any fluid or substance

formed by the action of heat.
Uahotmism, the science of chemical or animal electricity, in

which electrical effects are permanent without friction.
Oeieenirie phccy the places of the planets, as seen from the

earth.
pravtfy^ the tendency of masses of matter to fall together.
Ipr&t^'fy, specifier the relatire weight of difierent bxUes in re-
f gard to some standard, as water.
^ekieeniric plaee^ the places of the planets as seen from the

son.
^riMont the line that bounds the view, where the earth and

heavens seem to meet, and which cuts the heavens into

two equal parts.
Bmuntn of the ^e, the aqueous, the crystalline, and the vit-
reous.
^lArottaiies^ the science which teaches the laws of pressure

and motion in fluids.
^/irauiictf the science whidi teaches the construction of wa-
ter engines.
^iroge1^ the bases of water, 15 parts with 85 of oxygen,

forming 100 parts of water, and combined with carbon, it
' produces the gas lights.
Jflfirogen gat^ the same as inflammable air, and fourteen times

lighter tban atmospheric air.
heriness^ the disposition which all matter has to remain in its

aetual state.
hjbuie tpaet^ a description applied to space; because no

bounds or limits can be conceived to it.
RHwne, contrary-wise, an opposite proportion.
hddenee, the point at which a ray of ^ght strikes the surface

of any body,
/ir, eUetrieaU formed so as to condense the electrie power.
Imiitude of places on the earth, is their distance in degrees

from the equator, northward or southwanl.
Laliludey in astronomy is the distance of a heavenly body in

degrees from the ecliptic, northward or southward.
Ltnsy tbe name of any transparent body, the sides of which

are convex or concave, for the purpose of converging or

diverging rays of light.
Ught, the affection or power by which distant objects are

brought in contact with th* eye, either by particles in

rapid motion, or by vibrations of a uuiv^xvoixc^^dKnctsv,

Ligttid, a sUte oTiabetiincs id vhicbi by the b

powenofbeBtitbepartioleB slide euiljitmoDg one aoo'

er ; without haat, all liquidi become GkbiI or solid.

Lin^itiideol places dq the earth, j> theclutaace of the ate

' JiOD of the place from the fir»( ineridiaii, east or wnt

Longtiude, in astraDom;, ia the dietance of a heavenly bo

from the beg in ding of Aries mEafUrcdoa the ecliptic.
Malric, the BubstaDce in which metsllic ores are imbedJt
Mailer, the bsjis of all suhstances, constantly obanging

far[n», but always maintaiaiDg its eiisleuQe.
Jttagniltnn, tbe science which treats of (he pheaameDB. of t:

loBd-etOQG.

Xechanical Patntri, inslruDients for adding to aoimaliforc

as the lever, pulley, wed^ screw, be.
Xenilruum, any liquid which dissolves and aati chemicallji

an; solids.
Meridian, a line passing h'om the north pole to the south poi

through the zenith of any place.
JUetali are distinguisbedbytheirweighl, splendour, ftoil dm

tility, and modero chemistry has delected oearl/lorl

Idnds, of which platina, Ihe-tieayisst, is 33 limeabeaTii

than water.
MicTDicopc, any instrument hy mean? of which an objeBt,C

its (rue image, may be seea much nearer, and coiw

qnend/ much larger.
Mirror, a reflecting surfi^ce, plant, coneoM, or eoKOtx, I!

means of irhich bodies may be seen of the Datanl^ Mt

enlarged, orof a diminished size.
MobUity, tbe power of msviug matter by a saffioiaDt fcrtx

under certaio taws of motion.
Mamentuia, the force acquired by different mauei of maltM

moved with different velocities.
Jfadir, the point under foot, opposite the zenith.
JVadt, Uie point of a planet's orbit at which it crow** (k

ecliptic, or earth's orbit
JVon-Conditclars, surfacesof bodies which da not raoeiTe Ml

transmit the electric power.
Obliiuili/, of the ecliptic, is the angle made by tin plaaM

(he earth's orbit and the equator, in 181^ 33° Zl l-t
Opacilj/, non-1 raasparency. . !.

Optics, (he delightful science which treats of the lam of li

sioD, light, aod colours. <

Parallai, horizontal, the angle which theearth'iiMai<diMtl

ter subtends at a distant heavenly body.
Parallax, annual, the angle viYucblbc e»nh'i orbit mbled
to disfant heavenly bo^n.

GLOSSABY. 209

iriheUon^ the place i^ a planet's orbit neareit to the sun,
constantly in progression.

itrifaction^ the depositing of earthy matter, from water, on
the surface of leaves, and other substances, forming an
earthy or stony encrustation.

mdulums^hehyy bodies suspended from a point, and perform-
ing vibrations backward and forward.

hyties^ the science of matter and bodies, in the abstract.

lenumt space filled with matter, sensible or insensible to man.

neumaticSf the science which treats of the properties of air.

owers oppotingf elasticity producing expansion; and gravi-
tation, producing Compression.

rism, a glass wedge, used to refract light, and separate its
different rays of colours.

rojectHeSr^ bodies thrown into the air, having a parabolic
motion.

ays^ chemical, those ofthe violet end of the spectrum.

ainbow^ an effect produced by the reflection and refraction
of light on drops of rain.

arefactionj expansion, dilatation, increase of bulk.

ay of light, a single beam or impulse of light, sometimes cal]^
ed a pencil of light.

adius, the line which extends from the centre to the circum-
ference of a circle.

adiuB'Vector, the line which extends from the solar centre
of the ecliptical orbits of the planets, to the circumfe-
rence ofthe orbit; and the areas described by the radi-
us-vector are proportioned to the times, in which- they
are described.

tpvdsion, an opposite power to that of attraction, exerted at
very short distances.

'jtfittiixniofrayi of Light^isih^ rebounding on striking a
polished surface, which they always leave with the same
inclination or angle as that with which they fell on it.

'.efraetion of rays of' Light, is the new direction which they
receive on passing out of one medium into another.

\etina, is the net of nerves which is spread at the back ofthe
eye to receive the excitement ofthe rays of light.

aturation, when a fluid will hold no more of any substance
in solution.

atelliteSf moons attached to planets.

ecular Motion, the motion in a century.

incy the line drawn from the circumference of a circle, per-
pendicularly on its diameter, being half the diord of
double the arch.

18*

compound.
Syzygies^ the moon's place, when the earth, sun, s

are in a straigpht line.
Tangent, a line touching a circle at one point.
Telescope, an arrangement of lenses or mirrors, by

observer is enabled to see an image of an obj<

a larger angle, than he could with the naked c
Thermometer, an instrument to discover the heat

from many degrees below freezing to that
• water or 212**.

Vacuum, a space unoccupied by sensible matter.
Vapours, the minute particles of fluids separated b]

floating in the air.
Velocity, the space which any body moves through

time.
Vibration, the action of a musical string, or of t

surfaces of bodies when struck.
Volatile, the disposition of bodies to evaporate.
Zenith, the point over head.
Zodiac, a space in the heavens, extending eight c

each side of the ecliptic within which space

moon, and planets, move.
Zone, spaces on the earth's surface in which dista

mena appear, in re^rd to the seasons and lens

INDEX.

te gravity

rated motion

ics

ic tubes

^eigfht

ia^licity
mp

ude

identia

tic circle

des

of incidence

reflection

circle
ion
ids
phere

60,62

116

113, 132

115

120

127

120

81,86

127

127, 142

115

116

126, 137, 187

Camera obscura
Central forces
Centre of g^ravity

of motion

Ceres

Cohesion

Colours

oflig^ht

Comets

Compound motion
Concord
Condenser
Conductors

-galvanic
of sound

pressure of 60,63,66
•sun's rays refract-
ed by 122
iion 10
a borealis 187
-australis 187

33
-earth's 119

th 116

3e, hydrostatic 50

leter 67

7, electrical 165

-galvanic 191

-its power 193

Conjunction
Consequentia
Constellations
Curvilinear

Day, natural
Day and night
Dimness of sight
Discharging rod
Distance of planets
Divisibility

Earth

globular

—•revolution of

axis of

orbit of

Earth, shape of

Earthquakes

Echo

Ecliptic

Eclipse, of the moon

— ■ of the sua

Elasticitj

105
24
26
27

138
10
91
93

152
19
75
67

178

190
73

116

113

118, 166

30,159

123
121
89
184
153

114,130
114
117
119
121
131
187

95
118
146
147

motion of
galvanic
Eleclnc ahock

ElectriciJ machine

bktterj
Electrics
Eloctrometer
Elements orpUneta
Equation
Equator
Equin,

prcoeeaioi

64 GalranifDii its ejects
la ananuoals IE

177 Gravitation 10,]

3 FpeciGc

9 ateotate

1 Gymnotm elcclrici

] Heatenlifbadiea m
3 Hersohel ^

7 sateUitea at^

t Hydraulics
B Hyilrostatica |

Hydroatalical paradox^

Fixed Btan

16a bellows

Fluidity, causo of

riuidi

-13,44 lljgrometer

preseure of

45, S3

aeriform

59 Inclined plane

Focus, of light

7B Inertoesa

Focal distance

Force

23 stringed

centripetal

24 wind

24 optical

projectile

157 Invisible lady

granlating

158

Forces, centra!

84 Juno

Forcing pump

58 Jupiter

Fountains

54 Jupiter.satellitegor

Fricti.^n

190 Lantern, magio

circle.

189, 190 Latitude

battery

191 Lamofmotioni

pile

191 of Quids

tt^ugh

191 ofTidon

shook

IM of light

Galvani

fialTanism

189 Lenses

itseffiwtioi

1 met- Leyer

193,194 Vf^^fani

77 HultiplTiDgrilMi

107

78 Nadir

11«

78 NebdiB

ted

78, 80 Wigbt and day .

1«

78 S6dtM

19S

lesof

79 Kon-oondnohm

17«

•eneoui

91 Nortbn-D lieht
91 01b«r.

187
13»

iOf

93 OpUc*

1B6 Optical rMtnwwnB

iction

t7 Orbit, «arth'i

116, 119 moM's

laro

106 P^M

131

198 Parabol.

201 Parallax, aDonal

176

,'of

199 Parallaxe. 153,17ft

.eridi»ll

199 Psoduiumi

301 Paaaaibra

I4>

19» Perigw m»14t

-yof

303 Pwiheliw

i*r

ompaai

aOI.MB P«t(*oi

itt

136 Kw«i

13a

powen

196

128 Blameoti of

100 al.u.e^'iMof

1«

166 panDdioaltinMrf

lit

99 McoDdaiT,

99 di.tanMaof

IM-

99 matioDef

154

8 Planetary motioo

157

141 Piaae.i.icfiaBd

148 PDeun.alic»

5»

144 P»le«

iof

17 Pmsure of fluid.

'Of

IB ofalmosphere

itionof

19 Primary plaaati HI

on of

19 Pri«n

ated

21,28 Pwjectiloi

31 Projectilafiirce

157

S7 Pullay 33,91-

lear

90,159 Pump, fordoK

fi&

*«•

ntDSx.

Rainbow

95 Telescopes

Rays of light

76 Therm oroBter

converging

78 Resumiir's

crinsins of

parallel

rsfrocted

78, BO Thunder

refleottd

1 diverging

Ee-actioa

IB apparent ,

n mean

21 periodical , ,

Torpedo g.;

Sateililes

151 Tounnilin 1

Satan,

133 Tropics 0\

Entellitee of

151 TTUmpets M

Screw

33, 40 Twilight i

Sig^t

Silamus etectricni

lfl9 Velocity of motion J

Singing

74 ofMand i

Bolidity

7 Venus J

SolBTByitem

111,113 Veita 1

Solstices

Velocity of

conductors of

Speaking truiDpet
Specific gravity

Speotrum

Spring, infermittfng

Slars, filed

cluslera of

Steam engine
Sun

t Water spout

I We%e

i Weight

i Wheel

i Whispering f

J Whirlwind

S Wind

3 Wind imtrumenls

eclipse of

Syphon

Syringe

) Zfnith
3 Zodiac

SCHOOL GEOGRAPHY,

ojv j3jv improved PLAJV.

RECBNTIiT PUBLISHED BY

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Rudiments of Geography,

BY WM. C. WOODB RIDGE, A. M.

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fication which peculiarly distinguishes your work, is ezoM

lently adapted to facilitate the acquisition of knowledge, in

to impress it on the memory, especially when applied to M

maps, and connected in early life with strong and interettin

associations. The general views which you have taken cl

the Arts and Sciences, Literature, Climates, &c. are higm

valuable ; presenting within a narrow compass, iafon^j

tien, which is rarely accessible to the younger student" I

Hj* A larger and more extensive system of "UNIVER-
SAL GEOGRAPHY," Ancient and modem, on the plan of
the *' Rudiments of Geography." — Modern Geografiiy hf
W. C. Woodbridge, A. M., Ancient Geography by E. WilUiOi
has recently been published, designed especially for the vt
of the higher olasses.

■4.

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