SOUTHERN BRANCH,
UNIVERSITY OF CALIFORNIA,
LIBRARY,
JLOS ANGELES, CAUF.
FIRST PRINCIPLES
NATURAL PHILOSOPHY.
A TEXT-BOOK
FOR COMMON SCHOOLS
ELROY M. AVERY, PH.D.,
AUTHOR OF A SERIES OF TEXT-BOOKS ON PHYSICAL SCIENCE.
NEW YORK . CINCINNATI -.- CHICAGO
AMERICAN BOOK COMPANY
67361
DR. AVERY'S
PHYSICAL SCIENCE SERIES.
I St.
FIRST PRINCIPLES OF NATURAL PHILOSOPHY
THE ELEMENTS OF NATURAL PHILOSOPHY.
3d-
THE ELEMENTS OF CHEMISTRY.
4 th.
THE COMPLETE CHEMISTRY.
Containing the ELEMENTS OF CHEMISTRY, with an additional chapter or
Hydrocarbons in Series or ORGANIC CHEMISTRY. It can be used in the sair
clss with THE ELEMENTS OF CHEMISTRY..
CajyH&f, 1884,'"$ 'Sheldon & Co.
A.
THIS book is the result of an attempt to meet the
wants of schools which cannot give the time
required for the completion of the author's Elements of
Natural Philosophy. No effort has been spared by
author or publishers' to make it worthy of the place it
is intended to fill.
Especial care has been taken to provide simple, teach-
ing experiments which do not require expensive ap-
paratus.
The latest developments of science, such as the intro-
duction and use of electrical units, have been freely
utilized ; the mere polemics of physics has been ignored.
Any teacher or pupil using this book and finding any
error therein is requested to communicate the same to
the publishers or author.
If, in its study, any person encounters difficulties not
easily removable by other means, he may feel free to
write to the author (in care of the publishers) for fur-
ther information. Such letters should be accompanied \
with addressed envelopes for replies.
CHAPTER
MATTER.
SBC. PACK
I. MASSES, MOLECULES AND ATOMS.., 1
II. PHYSICAL PROPERTIES OF MATTER 10
III. THREE CONDITIONS OF MATTER... . 20
CHAPTER II.
MOTION AND FORCE DYNAMICS GRAVITATION
ENERGY.
I. MOTION AND FORCE 26
II. GRAVITATION 36
III. FALLING BODIES 45
IV. PENDULUM 51
V. ENERGY... .... 67
CHAPTER III.
SIMPLE MACHINES.
I. PRINCIPLES OF MACHINERY LEVER 67
II. WHEEL AND AXLE PULLEY 78
III. INCLINED PLANE WEDGE SCREW 80
vi CONTENTS.
CHAPTER IV.
LIQUIDS.
SBC PAGE
I. LIQUID PRESSURE 96
II. EQUILIBRIUM BUOYANCY 107
III. SPECIFIC GRAVITY 113
IV. HYDROKINETICS 117
CHAPTER V.
PNEUMATICS.
I. ATMOSPHERE ATMOSPHERIC PRESSURE 122
II. PUMPS SIPHON 129
CHAPTER VI.
ELECTRICITY AND MAGNETISM.
I. GENERAL VIEW 140
I!. KRICTIONAL ELECTRICITY 151
III. VOLTAIC AND THERMO-ELECTRICITY 180
V. MAGNETISM 207
V. INDUCED ELECTRICITY 227
CHAPTER VII.
SOUND.
I. NATURE, REFRACTION AND REFLECTION OF SOUND... 245
,'l. TELEPHONE COMPOSITION AND ANALYSIS OF SOUNDS 259
CHAPTER VIII.
HEAT.
I. TEMPERATURE THERMOMETERS EXPANSION 281
II. LIQUEFACTION AND VAPORIZATION .. 289
CONTENTS. Yii
SEC. PACK
III. LATENT AND SPECIFIC HEAT 397
IV. MODES OF DIFFUSING HEAT 308
V. THERMODYNAMICS 817
CHAPTER IX.
LIGHT.
I. NATURE, VELOCITY AND INTENSITY OF LIGHT 328
II. REFLECTION OF LIGHT 336
III. REFRACTION OF LIGHT 347
IV. CHROMATICS AND SPECTRA 365
V. OPTICAL INSTRUMENTS 373
CONCLUSION ENERGY 382
APPENDIX 887
INDEX... .. 393
CHAPTER !*
MATTER.
SECTION I .
MASSES, MOLECULES AND ATOMS.
" Read Nature in the language of experiment."
Experiment I. Place a cork on the surface of water. Invert
a glass tumbler or jar over the cork and push the glass down
into the water. Notice that the water cannot enter far into
the vessel, although it enters far enough to show that its tendency
is thus to enter, but that something prevents it. The air in the
glass prevents the water from entering. Air and water cannot
be in the same place at the same time.
Experiment 2. Try to get your two hands in the same place
at the same time. Repeat the attempt with two books and other
articles, until you are sure that no two things that you can handle
can be in the same place at the same time and that every one of
them takes up room.
I. What is Matter ? Matter is anything that
occupies space or "takes up room."
Mind, truth and hope do not take up room and so we
know that they are not forms of matter. The earth, the
air, a table, an orange, a slate or a raindrop does take up
room and so we know that each is a kind or form of
matter.
% NATURAL PHILOSOPHY. 2
2. Divisions of Matter. Matter exists in
atoms, molecules and masses.
It is very important that we clearly understand what
these words mean or we shall have trouble in trying to
understand much that is to follow.
3. What is an Atom? An atom is the small-
est quantity of matter that can enter into com-
bination and thus form molecules and masses.
In nearly every case an atom is a part of a molecule.
(a.) We may say that atoms are the smallest particles of matter
that can exist. They seldom exist alone, but quickly unite with
others like themselves to form elementary molecules, or with others
unlike themselves to form compound molecules. For example, one
atom of oxygen combines with another like itself to form an ele-
mentary molecule of oxygen, while one atom of oxygen combines
with two of hydrogen to form a compound molecule of water.
There are sixty-six kinds of atoms now known.
4. What is a Molecule ? A molecule is a
quantity of matter so small that it cannot be
divided without changing its nature.
The word molecule means "a little mass," but in
Natural Philosophy we must be very careful to use the
word with accuracy that is, in accordance with the
above definition.
(a.) A molecule is so very small that a total of 8,000,000,000
molecules of water is barely visible in the best of modern micro-
scoi>es. ll a drop of water could be magnified until it appeared to
Vie as large as the earth on which we live, each molecule in the drop
thus magnified would still look sma'ler than a base-ball. But
while the pupil may thus get the idea that a molecu.e is very
small, he most not think that, like the fairies, it exists only in
5 MASSES. 3
fancy. Molecules are as real as base-balls and much more numer-
ous. If a molecule, by any means, be divided, its nature will be
changed. A molecule of water may be changed into two atoms of
hydrogen and one of oxygen.
(6.) In an elementary molecule the atoms are alike ; in a com-
pound molecule the atoms are of two or more kinds. Some com-
pound molecules are very complex. The common sugar molecule
contains forty-five atoms of three kinds. Elementary molecules
make elementary masses or substances. Compound molecules
make compound masses or substances. The nature of the mole-
cule determines the nature of the substance.
5. What is a Mass ? A mass is any quan-
tity of matter that is composed of jnolecules.
If a quantity of matter is large enough to be seen, even
with a powerful microscope, you may know that it is a
mass. If it may be divided without changing the nature
of the substance, you may be sure that it is a mass.
Masses are elementary or compound. An elementary
mass is called an element. There are as many elements
as there are kinds of atoms. Compound masses or sub-
stances are innumerable.
(a.) We may take a lump of salt, which is a mass, and break
it into many pieces ; each piece will be a mass. We may take one
of these pieces and crush it to finest powder ; each grain will still
be a mass. We may imagine one of these grains of powdered salt
to be divided into so many parts that any further division will
change them from salt to something else. These particles of salt,
which are so small that further division would change their nature,
are too small to be called masses ; they are molecules. If one of
these molecules be divided, it will cease to be salt. Instead of
salt, we shall have an atom of chlorine gas and an atom of the
metal sodium. Two molecules would make a mass, but the mass
would be so small as to be invisible, except in imagination.
4 XATtrRAL PBILOSOP&Y. 6
Experiment 3. Heat the mercury in the bulb of a common
thermometer. The bulb remains full, but the liquid rises in the
tube. There seems to be more mercury than there was before
How can this be ? There must be a greater number of molecules,
the molecules must be larger or they must be farther apart.
Experiment 4. Heat some water in a glass tube without boiling
it. Bubbles arise and attach themselves to the sides of the tube.
These are bubbles of air. This air came from the water. The
air particles were either in the same place with the water mole-
cules, at the same time, or were in otherwise vacant spaces between
the water molecules. Can you believe that they were in the
same place at the same time ?
Experiment 5. Carefully pour alcohol into a test tube about
three quarters full of water until the tube is full. Close the mouth
of the tube with thumb or finger and shake the liquids until they
are mixed. Notice that the tube is not full now. Some of the
molecules have been destroyed or dropped into the spaces between
other molecules. Can you believe that any of the molecules
were destroyed ?
Experiment 6. Make a common goose quill pop-gun. Notice
that when you use it the air confined between the two wads is
compressed or made to occupy about half its original space. The
air particles were reduced in size or in number, or were crowded
together more closely. Perhaps the matter of which a body
is made does not actually fill all the space which the body
seems to occupy.
6. Continuity of Matter. When you look at a
brick wall from a considerable distance, it has an appar-
ent uniformity of structure; you cannot see that it is
made of many bricks, separated by mortar-filled spaces.
This is the fault of your sense of vision and not the
fault of the wall. As yon come nearer, you see whaf
7 FORMS OF ATTRACTION. 5
you did not see before the individual and separated
bricks. But such is the structure of the wall whether
you can see it or not.
Rub the smooth handle of a fine awl over a piece of
fine wire gauze and the gauze seems to present a con-
tinuous surface. It is the fault of the instrument in
your hand and not the fault of the gauze. Rub the
point of the awl over the gauze and you soon find open-
ings between the metal threads. But the openings are
there whether you can feel them or not.
Rub the point of a fine sewing needle over the surface
of a window pane. The glass seems to be continuous in
its structure and the needle cannot get through. It ia
the fault of the instrument you are using. Try one
more delicate. Let a ray of light fall upon the glass
and it easily finds a passage way between the solid mole-
cules. Rays of light are often used by scientific men as
instruments for their work.
There are spaces between the molecules of every
form of matter, whether you can detect them by
any of your physical senses or not.
7. Forms of Attraction. Each of these three divi-
sions of matter has its form of attraction.
The attraction of masses is called gravitation.
The attraction of molecules is called cohesion
or adhesion.
The attraction of atoms is called chemical
affinity.
6 NATURAL PHILOSOPHY. 8
8. Forms of Motion. It is probable that each of
these three divisions of matter has its own form or mode
of motion.
The motion of a mass is often called mechan-
ical motion. The motion of a hammer or of a flying
bullet is an example.
The motion of the molecules in a mass consti-
tutes heat or light or, probably, electricity or
magnetism. If a flying bullet strike the target, we may
imagine that the shock which destroys its mechanical mo-
tion, produces or increases the vibration of the molecules
among themselves. We ought to have little difficulty in
imagining these little molecules rapidly swinging to and
fro within the mass. These molecular vibrations consti-
tute heat. We know that when a bullet is stopped by
the target, the bullet is heated and the production of
heat is to be explained only in this way.
The motion of atoms within the molecule is
probable, but has not yet been proved.
(a.) Fancy a million flies surrounded by an imaginary shell. If
each fly be allowed to represent a molecule, the contents of the shell
will represent a mass, because it is composed of many molecules.
We may imagine this shell to be thrown through the air. The
motion of the shell would represent mass or mechanical motion.
As the shell is moving through the air, the flies are moving slowly
among themselves within the shell. This motion of the flies
represents molecular motion and is a very different thing from the
motion of the shell. When the shell strikes the ground, the
mechanical motion is destroyed but the molecular motion is
increased, for the flies are set in much more rapid motion by the
shock. This is just about what happens when the bullet is fired
against the target.
10 CONSTITUTION OF MATTER. 7
(6.) These motions of atoms, molecules and masses give to mattet
the power of doing work, the scientific name of which power is
Energy.
(c.) Try to get a clear mental picture of theso molecules, each
separated from its neighbor by a distance much greater than its
own diameter. If a mass could be magnified until its molecules
were as large as worlds, the spaces between the molecules would
be as great as the spaces between the planets. Try to imagine a
creature small enough to live on a molecule as we live on the
earth. Imagine this creature looking at the neighboring molecules
as we look upon the stars at nigrht. Remember that each molecule
is in motion back and forth among its neighbors, sometimes strik-
ing them, perhaps, and rebounding from them. When we heat the
mass, we make each molecule move faster, strike harder blows
and push its neighbors further away, thus producing expansion of
the mass.
9. The Constitution of Matter. Every body of
matter is made of a vast number of minute par-
ticles called molecules, no two of which are in
contact. Every molecule is ceaselessly vibrating
to and fro among its neighbors, often hitting
them and rebounding from them.
10. Physical Science. Physical science com-
prises Physics and Chemistry.
Physics deals with masses and molecules; chemistry,
with atoms and combinations of atoms.
Experiment 7. Bring a piece of warm sealing wax near some
small bits of paper, but without touching them. Notice that the
paper bits do not move. Briskly rub the sealing wax with a piece
of warm flannel and quickly bring it near the paper bits as before.
Notice that the sealing wax now produces very peculiar motions in
8 NATURAL PHILOSOPHY. II
the paper bits. Notice also that you have wrought an important
change in the sealing wax, but that still the stick is sealing wax.
Experiment 8. Rub a brass button on the floor or carpet until
it is uncomfortably warm. Notice that while you have produced a
change in the button, it is still brass; you did not change the
substance of which it is made.
11. What is a Physical Change?.^ physical
change is one that does not change the nature
of the molecule.
(a.) A piece of marble may be ground to powder, but each grain
is marble still. Ice may change to water and water to steam, yet
the identity of the substance is unchanged. A piece of glass may
be electrified and a piece of iron magnetized, but they still remain
glass and iron. These changes alter the distances and arrange-
ment of the molecules, but leave the molecules themselves un-
changed ; they are physical changes. A change like the rusting
of iron or the burning of wood, changes the nature of the molecule
itself, and, consequently, of the substance. Stick a change as this
is called a chemical change.
12. What is a Property of Matter? Any
quality of matter is called a property of matter.
Lead is heavy ; heaviness is a property of lead. It
may be manifested without changing the lead to any-
thing else, and is, therefore, called a physical property.
Sulphur or brimstone is brittle. The brimstone may
axhibit this property and still remain brimstone. Hence,
brittleness is a physical property of brimstone.
Sulphur is also combustible it may be burned. But
this property of sulphur (combustibility) can not be
shown without changing the sulphur to something else.
Such are called chemical properties of mqtter*
14 DEFINITION. 9
13. Definition. Physics, or Natural Philosophy,
is the branch of science that treats of the laws
and physical properties of matter, and of those
phenomena that depend upon physical changes.
14. Recapitulation. To be reproduced and ampli-
fied by the pupil for review.
2
w S
> i
10 NATURAL PHILOSOPHY. 15
SECTION II.
THE PHYSICAL PROPERTIES OF MATTER.
15. Division of Physical Properties. The phys-
ical properties of matter are divided into two classes,
universal and characteristic.
16. What are Universal Properties ? Uni-
versal properties of matter are such as belong to
matter of every kind.
17. List of Universal Properties. The prin-
cipal universal properties of matter are extension,
impenetrability, weight, indestructibility, inertia,
divisibility, porosity, compressibility, expansi-
bility and elasticity.
18. What is Extension ? Extension is that
property of matter by virtue of which it takes
up room.
It is involved in the definition of matter given in 1.
(a.) In this country and in England, the foot and yard are units
commonly used at the present time. But in most other civilized
countries of the world, the international or metric units are used.
These units are almost universally adopted hy scientific men even in
England and America.
In this book, frequent use will be made of the terms meter,
liter and gram, their multiples and divisions. The pupil should
l8 THE PHYSICAL PROPERTIES OF MATTER. 11
familiarize himself with these units. He will find further informs
tion concerning them in Appendix B. at the end of this volume.
Experiment 9. Pass a bent tube and a funnel, as shown in
Fig. 1, or a funxxel tube as shown in Fig. 2,
through the cork of a bottle. Be sure
that alt joints are air tight. The delivery
tube is best marte of glass which may be
bent when headed to redness in an alco-
hol or gas flaine. Place the end of the
delivery tube in a tumbler of water.
Pour water through the funnel. As it
runs into the bottle, air will be forced
out and may be seen bubbling through
the water in the tumbler. A bottle con-
venient for this and other experiments may be prepared by per-
forating the cover of a glass fruit jar, as
shown in Fig. 2. The holes carry cork or
caoutchouc stoppers, through which the
tubes pass. Full directions for bending
glass tubing, boring holes, etc., may be
found in Appendix 4, of Chemistry.
FIG. 1.
JIMi
FIG. 2.
Experiment 10. Thrust a lamp chimney
into water. The water will rise inside the
chimney, entering at the lower end and
pushing the air out at the top. Repeat the
experiment, closing the upper end of the
chimney with the hand (or use an inverted
tumbler). The water can not rise as
before because the vessel is filled with air
that can not escape.
Experiment II. Drive a nail into a piece
of wood ; the particles of wood are either crowded more closely
together to give room for the nail, or bume of them are driven
12 NATURAL PHILOSOPHY. 19
oat before it. Clearly, the iron and the wood are not in the
same place at the same time.
19. What is Impenetrability ? Impenetra-
bility is that property of matter by virtue of
which two bodies can not be in the same place
at the same time.
20. What is Weight ? Weight is the measure
of mass attraction or gravity.
(a.) The word gravity generally points out the tendency ot
bodies, not supported, to fall to the ground.
(6.) A body is heavy or light according to the amount of attrac-
tion ; the greater the attraction, the greater the weight.
(c.) If an apple be held in the hand, the attraction between the
earth and the apple produces pressure upon the hand. This pres-
sure is not the attraction, but it is the measure of it and is called
weight.
(d.) If the same apple were upon the moon, its weight would
be the measure of the attraction between the apple and the moon.
But as the moon has less matter than the earth, the attraction
between the apple and the moon would be less than that between
the apple and the earth and the weight would, consequently, be
less.
21. What is Indestructibility? Indestructi-
bility is that property of matter by virtue of
which it can not be destroyed.
(a) No human being can create or destroy a single atom of
matter. Water evaporates and disappears only to be gathered in
clouds and condense and fall as rain. Wood burns, but the ashes
and smoke and the invisible gases formed, contain the identical
22 tNERTTA. 13
atoms of which the wood was composed. In a different form, the
matter still exists and weighs as much as before it was burned.
The experiment is difficult, but has been repeatedly performed.
The universe contains the same atoms to-day that it did at th
close of the creation not one more, not one less.
Experiment 12. Upon the tip of the fore-finger of the left
hand, place a common calling-card. Upon this card and directly
over the finger, place a cent.
With the nail of the middle
finger of the right hand, let a
sudden blow or " snap " be
given to the card. A few trials
will enable you to perform the
experiment so as to drive tlu
card away and leave the coin
resting upon the finger. The
card flies away on account of
the force of the " snap." The cent remains on the finger because
the blow is so quick that the card has no time to give any of
its motion to the coin.
Experiment 13. To n.ake the experiment still more interest
ing, use a bullet instead of the cent and the open top of a bottle
instead of the finger-tip. Keep trying until you succeed in drop
ping the bullet into the bottle. The experiment illustrates the
inertia of the bullet, the card being driven away before it lias
opportunity to impart its motion to the bullet.
22. What is Inertia? Inertia is that prop-
erty of matter by virtue of ichich it has a ten-
dency ivhen at rest to remain at rest or when
in motion to continue in motion.
(a.) A ball cannot put itself in motion. When the ball is thrown
through the air, it has no power to stop and it will not stop until
Borne external force compels it to do so. This external force may
14 NATURAL PHILOSOPHY. 22
be the bat, the catcher, the resistance of the air or the force of
gravity. It must be something outside the ball or the ball will
move on forever.
(&.) Illustrations of the inertia of matter are so numerous that
there should be no difficulty in getting a clear idea of this property.
The "running jump" and " dodging " of the playground, the fre-
quent falls which result from jumping from cars in motion, the
hackward motion of the passengers when a car is suddenly started
and their forward motion when a car is suddenly stopped, the diffi-
culty in starting a wagon and the comparative ease of keeping it in
motion, etc., etc., may be used to illustrate this property of matter.
Experiment 14. Strike a piece of loaf sugar or of brick with a
hammer. The sugar or brick is separated into many parts.
23. What is Divisibility ? Divisibility is that
property of matter by virtue of ivhich a body
may be separated or divided into parts.
(a) The divisibility of matter may be carried to such an extend
as to excite our wonder and test the powers of imagination itself.
It is said that the spider's web is made of threads so fine that
enough of this thread to go around the earth would weigh but
half a pound and that each thread is composed of six thousand
filaments. A single inch of this thread with all its filaments ma;r
be cut into thousands of distinct pieces and each piece of each
filament be yet a mass of matter composed of molecules and atoms
We may consider that the atom marks the limit of divisibility.
Experiment 15. Fill a test tube with water. Slowly add
sugar. A considerable quantity may be added without increasing
the bulk of the liquid.
Experiment 16. Take a test tube three quarters full of water
and carefully add alcohol until it is filled. Close the tube with the
thumb and shake. Notice that the tube is no longer full. We
know that the water and the alcohol cannot be in the same plac
25 COMPRESSIBILITY, 15
at the same time and are forced to the conclusion that some of
the water molecules have been received into the space
between the alcohol molecules or vice versa.
24. What is Porosity ? Porosity is that prop-
erty of matter by virtue of which spaces exist
between the molecules.
(a.) When iron is heated, the molecules are pushed further
apart, the pores are enlarged and we Bay that the iron has
expanded. If a piece of iron or lead be hammered, it will bo
made smaller because the molecules are forced nearer together,
thus reducing the size of the pores.
(6. ) These pores are very large in comparison with the size of
the molecules. As was said a few pages back, if a race of persons
could be imagined small enough to live on a molecule as we live
on our earth, we might fancy them looking across the space around
it and seeing the nearest molecule as we look off into the sky and
see the moon or stars.
(c.) Cavities or cells, like those of bread and sponge, are some-
times improperly spoken of as pores.
25. What is Compressibility? Compressibility
is that property of mat-
ter by virtue of u-hich
a body may be reduced
in size.
Experiment 17. Invert a
thin glass bottle over a plate of
water, as shown in Fig. 4. The
heat of the hand will expand
the air in the bottle and some
of it will escape in bubbles.
If no bubbles appear, pour warm
water over the bottle.
16 XATtfoAL PtitLOSOPXT. 26
26. What is Expansibility ? Expansibility is
that property of matter by virtue of which a
body may be increased in size.
(a.) Compressibility and expansibility are the opposites of each
other, resulting alike from porosity. Let each pupil prove, by
experiment with an ordinary pop-gun or other apparatus, that air
Is compressible and expansible.
Experiment 18. Provide strips of rubber, whalebone, wood,
iron, steel, brass, copper, zinc and lead. Stretch the piece of rubber
and notice what takes place when the stretching force ceases to
act. Bend each of the other strips and notice what takes place
under similar circumstances.
27. What is Elasticity ? Elasticity is that
property of matter by virtue of which bodies
resume their original form or size ivhen that
form or size has been changed by any external
force.
All bodies possess this property in some degree. Solid
bodies have elasticity of form; liquids and gases have
not.
(a.) Different substances possess this property in different
degrees.
Solid bodies are not perfectly elastic. They may contract after
being stretched or expand after being compressed but they do
not return to exactly their former size.
Liquids and gases are perfectly elastic. No matter how great
the pressure that may be exerted upon them, they return to
exactly their former size when the pressure is removed.
The ordinary spring-balance owes its value to the elasticity of
the spiral steel spring, which is stretched by the weight.
30 COHESION AND ADHEStON. I?
28. What are Characteristic Properties?
Characteristic properties of matter are such as
belong to matter of certain kinds only.
They enable us to distinguish one substance from
another. Glass is brittle, and by this single property
may be distinguished from India-rubber.
29. List of Characteristic Properties. The
characteristic properties of matter are numerous. They
depend chiefly upon cohesion and adhesion. The most
important are hardness, tenacity, brittleness, mal-
leability and ductility.
Experiment 19. Take a sheet of gold-leaf in your fingers and
try to pick the metal off with the fingers of the other hand.
Some of the gold will " stick to your fingers."
30. What are Cohesion and Adhesion? Co-
hesion is the force that holds together like mole-
cules ; adhesion is the force that holds together
unlike molecules.
(a.) Cohesion is the form of molecular attraction that holds most
substances together and gives them form. If you pull on an iron
wire and are unable to break it, you are to understand that the
cohesion of the iron particles is stronger than you are.
(6.) Adhesion is the form of molecular attraction that causes the
penci/ or crayon to leave marks upon the paper or blackboard and
makes paste, glue, mortar and cements " stick."
(c.) In a brick wall, cohesion binds together the molecules of the
mortar layer into a single, hardening mass ; adhesion reaches out
and grasps the adjoining bricks and holds them fast a solid wall.
Each acts only through distances too small to be measured.
18 NATURAL PHILOSOPHY. 31
Experiment 20. Get pieces of chalk, glass, iron, lead, copper
and marble. Try to scratch each one with each of the others.
Make a note of the result of each experiment, thus :
Glass will scratch copper ; copper will not scratch glass. Deter-
mine which is the hardest substance in your collection and which
is the softest.
31. What is Hardness? Hardness is that
property of matter by virtue of which some
substances resist any attempt to force a passage
between their particles.
It is measured by the degree of difficulty with which
one substance is scratched by another. Fluids are not
said to have hardness.
32. What is Tenacity ? Tenacity is that prop-
erty of matter by virtue of which some sub-
stances resist a force tending to pull their par-
ticles asunder.
(a.) Like hardness and the other characteristic properties of
matter, it is a variety of cohesion which is the general term for
the force which holds the molecules together and keeps masses
from crumbling into dust.
33. What is Brittleness tBrittleness is that
property of matter by virtue of which some
substances may be easily brolcen, as by a blow.
(a.) Glass furnishes a familiar example of this property. The
idea that brittlenoss is the opposite of hardness, elasticity or
tenacity, should be guarded against Glass is harder than wood,
but very brittle : it is very elastic, but very brittle also. Steel is
far more tenacious than lead and far more brittle.
36 RECAPITULATION. If
34. What is Malleability ? Malleability it
that property of mutter by virtue of which some
substances may be rolled or hammered into
sheets.
(a.) Gold is the most malleable metal. It has been beaten so
thin that a pile of 282,000 leaves would be but an inch high.
Experiment 21. Heat the middle of a piece of glass tubing
about six inches long, in an alcohol flame, until red-hot. Roll the
ends of the glass slowly between the fingers and, when the heated
part is soft, quickly draw the ends asunder. That the fine glass
wire thus produced is still a tube, may be shown by blowing
through it into a glass of water and noticing the bubbles that
will rise to the surface.
35. What is Ductility ? Ductility is that prop-
erty of matter by virtue of which some sub-
stances may be drawn into wire.
(a.) Platinum wire has been made OTF<TT of an inch in diameter
Glass, when heated to redness, is very ductile, as was shown in
the last experiment. All ductile substances are tenacious, but a
tenacious substance is not necessarily ductile.
36. Recapitulation. To be reproduced and ampli-
fied by the pupil from memory.
PHYSICAL
GENERAL.
Extension, Impenetrability, Weight
Indestructibility, Inertia, Divisi
bility, Porosity, Compressibility
Expansibility, Elasticity.
PROPERTIES
OF MATTER.
( ADHESION. Tenacity.
CHARACTERISTIC. \ \ Brittle*e*s.
\ COHESION. MatteaM ity.
[ Ductility
20 NATURAL fjfiLosoPalr. 3?
SECTION III.
THE THREE CONDITIONS OF MATTER.
37. Conditions of Matter. Matter exists in
three conditions or forms the solid, the liquid;
and the aeriform.
Ice is solid ; water is liquid ; steam is aeriform.
38. What is a Solid ? A solid is a body whose
molecules move among themselves with difficulty.
Such bodies have a strong tendency to retain any form
that may be given to them. A movement of one part of
such a body produces motion in all of its parts.
Experiment 22. Place your finger in a vessel of water and
move it about. The watery particles easily flow over and around
one another ; there is great freedom of molecular motion. Remove
your finger, holding the tip downward. The water molecules in-
istantly glide into the space lately occupied by your finger, which
leaves no hole behind it. Notice that a drop of water hangs
upon your finger tip. That drop contains many molecules which
cling together, held by the force of cohesion, while the drop clings
to your finger, held by the force of adhesion.
Experiment 23. Suspend a glass or metal plate, of about four
inches area, from one end of a scale-beam and accurately balance
the same with weights in the opposite scale-pan. The support-
ing cords may be fastened to the plate with wax. Beneath the
plate, place a saucer so that when the saucer is filled with watel
40 AERIFORM BODY. 21
the plate may rest upon the liquid surface, the scale beam remain-
ing horizontal. Carefully add
small weights to those in the
scale- pan. Notice that the water
beneath the plate is raised above
its level. Add more weights
until the plate is lifted from
the water. Notice that the
under surface of the plate is
wet. These water molecules on
the plate have been torn from
their companions in the saucer.
The added weights were needed
to overcome the tendency of the water molecules to cling to-
gether.
NOTE. After seeing a physical experiment, always ask your-
self " What was the object of that experiment ? What does it
teach?" Never allow yourself to look upon an experiment as
being simply entertaining ; thus reducing the experimenter, so far
as you are concerned, to the level of a showman.
39. What is a Liquid? A liquid is a body
whose molecules easily move among themselves,
yet tend to cling together.
Liquids adapt themselves to the form of the vessel
containing them but do not retain that form when the
restraining force is removed. They always so adapt
themselves as to have their free surfaces horizontal.
Water is the best type of liquids.
40. What is an Aeriform Body? An ae'ri-
form body is like air. Its molecules easily move
among themselves and tend to separate from
each other almost indefinitely.
22 NATURAL PHILOSOPHY. 40
A vessel may be half full of a solid or a liquid but not
of an aeriform substance. Atmospheric air is the best
type of aeriform bodies. Aeriform means "having the
form of air."
Experiment 24. Place n piece of ice in a large metal spoon.
Hold the bowl of the spoon in the flame of a lamp. Notice that
the solid ice changes to liquid water and finally disappears as a
vapor.
Experiment 25. Heat half a brick in the stove, place it on any
convenient support, drop a few scales of iodine (which you can get
at the chemist's) upon the brick and cover the brick with a large
bell glass. The glass will quickly be filled with the beautiful
violet colored vapor of iodine. Notice whether the iodine changes
to a liquid before it becomes a vapor, as the ice did.
41, Gases and Vapors. Aeriform bodies are of
two kinds, gases and vapors.
Gases remain aeriform under ordinary conditions,
although they may be changed to the liquid form by
intense cold and pressure. Oxygen is a gas.
Vapors are produced by heat from substances that
are generally solid or liquid. They resume the solid or
liquid form at ordinary temperatures. Steam is a vapor.
42. Changes of Condition. The same substance
may exist in two or even three of these forms. Most
solids, as lead and iron, may be changed by heat to
liquids ; others, as iodine, may be apparently changed
directly to vapors ; still others, as ice, may be easily
changed first to the liquid and then to the vapor form.
It is probable that our present inability to liquefy and
vaporize certain substances arises from our limited means
43 FLUID. 23
for the production of heat Many substances that
formerly could not be melted are easily melted in the arc
of the now common electric lamp. With this idea in
mind, we may say that a solid is frozen matter; that a
liquid is melted matter ; that a gas is vaporized matter.
43. Ultra-Gaseous Form of Matter. Recent
experiments with electric discharges in high vacuums
[ 187, 239, 306,] have yielded remarkable results
which, in the opinion of many, prove the existence of
a fourth condition of matter. For matter in this ex-
tremely thin or attenuated form, the name "Radiant
Matter " has been proposed.
(a.) In a very remarkable lecture on this subject (August 22,
4879), Prof. COOKES said :
" Gases are considered to be composed of an almost infinite num-
ber of molecules, which are constantly moving in every direction
with velocities of all conceivable magnitudes. As these molecules
are exceedingly numerous, it follows that a molecule can not
move far in any direction without coming into contact with some
other molecule. But if we exhaust the gas contained in a closed
vessel, the number of molecules becomes diminished and the dis-
tance through which any one of them can move without coming
into contact with another is increased. The mean free path is
inversely proportional to the number of molecules present. The
further the exhaustion is carried, the longer becomes the average
distance a molecule can travel before entering into collision. By
thus lengthening the mean free path of the remaining molecules,
we obtain phenomena so distinct from anything which occurs in air
or gas at the ordinary tension that we are led to assume that we
are here brought face to face with matter in a fourth state or con-
dition, one as far removed from the state of a gas as a gas is from
a liquid."
NATURAL PHILOSOPHY.
44
44- What is a Fluid? A fluid is a body
whose molecules easily change their relative po-
sitions.
The term includes liquids, gases and vapors.
45. Recapitulation. To be reproduced, upon paper
or the blackboard, by eaoh pupil.
MATTER.
\ SOLIDS.
Molecules change
their relative po-
sitions with diffi-
culty
LIQUIDS.
Molecules cling to-
gether feebly.
FLUIDS.
GASES ; ordinar
Molecules change
their relative po-
sitions easily.
AERIFORM BODIES.
Molecules tend to
separate.
rily aeriform.
VAPORS ; ordina-
rily liquid o:
tfli.
45 QUESTIONS FOR REVIEW. 25
QUESTIONS FOR REVIEW.
1. How should the " Book of Nature " be read?
2. (a.) What term is applied to anything that you can see, feel,
touch or taste ? (6.) What is energy ?
3. What limits the number of eleme^ r .
4. A stone that measures eight cubic inches is quietly placed in
a bowl full of water. How much water will run out ? Why ?
5. What is an element ?
6. Given a lamp chimney, a small cork and a pail of water;
how will you illustrate the compressibility of air ?
7. (a.) What is the smallest possible division of an element?
(6.) Of a compound substance?
8. Two hydrogen atoms make a hydrogen molecule. Is hydro-
gen an elementary or a compound substance ?
9. Two hydrogen atoms and one oxygen atom make a water
molecule. Is water an elementary or a compound substance ?
10. (a.) If you thrust a knitting needle into a mass of dough, is
the hole thus made a pore? (b.) What is a pore ?
11. Are the molecules of water larger or smaller than those of
steam ?
12. Name two classes of fluids ?
13. Give an illustration of molecular motion in a mass that is at
rest.
14. Which are the greater, the diameters of molecules or the
distances between molecules ?
15. Are intermolecular spaces greater in water or in steam?
16. What is molecular attraction called ?
17. Give an illustration (not contained in this book) of one of
the universal properties of matter.
18. What is the difference between a fluid and a liquid ?
19. One sixteen thousandth of a cubic inch of indigo dissolved
in fuming sulphuric acid will give a perceptible color to two or
three gallons of water. What property of matter may thus be
illustrated?
CHAPTER U.
MOTION AND FORCE. DYNAMICS.-
GRAVITATION, ETC. ENERGY.
SECTION I.
MOTION AND FORCE.
46. What is Motion? Motion is a changing
of position.
No body can move or be moved from one place to
another without motion.
47. What is Force? As generally used, force
signifies any cause that tends to produce, change,
or destroy motion. All physical phenomena are caused
by the action of forces upon matter.
(a.) There are many kinds of force. We often hear and speak
of the force of muscular action, the force of the wind, the force
of gravity, etc. We shall soon see that heat, light, magnetism,
electricity, etc., may exercise force.
48. What is Dynamics ? Dynamics is that
branch of Natural Philosophy or Physics which
treats of forces and their effects.
(a.) The word "mechanics" was formerly used in the sense
that we now use the word "dynamics." Mechanics properly
denotes the science of machines and is a branch of dynamics.
49 MOTION AND FORCE. 27
Experiment 26. Place on the floor a croquet ball and a heavj
iron ball. Strike them equal blows with a mallet. Notice that
equal forces do not always produce equal velocities.
Experiment 27. Roll the iron ball and the croquet ball with
equal velocities. Find out which requires the greater force to
stop its motion.
49. Momentum. The momentum of a body is
its quantity of motion.
(a.) Momentum depends upon the weight of the moving body
and its velocity or rapidity of motion.
Examples. An iceberg moves very slowly but almost irresisti-
bly, or with great momentum, because it is tfry Jieary. A ballet
is not very heavy but, when fired from a rifle, it has a great mo-
mentum because it moves with great Telocity.
(6.) Momentum is generally measured by the product of the
numbers repi 3senting the weight and the velocity. The unit of
momentum has no definite name.
M= W x V.
Examples. The momentum of a body having a weight of 80
pounds and a velocity of 15 feet, is twice as great as that of a body
having a weight of 5 pounds and a velocity of 30 feet. The mo-
mentum of the former is 300 ; that of the latter, 150.
(<?.) In comparing momenta, the pupil must be careful that the
units of weight used are alike. The units of velocity also must
be alike. If the weight of one body be given in ounces and that
of the other in pounds, they must be reduced to the same denomina-
tions. If the velocity of one body lie given in feet per minute and
that of the other in miles per hour, a similar reduction must be
28 NATURAL PHILOSOPHY. 50
50. Laws of Motion. The following propositions,
known as Newton's Laws of Motion, are so important
and so famous that they ought to be remembered by
every pupil.
(1.) Every body continues in its state of rest
or of uniform motion in a straight line
unless compelled to change that state by
an external force.
(2.) Every motion or change of motion is in
the direction of the force impressed and
is proportionate to it.
(3.) Action and reaction are equal and oppo-
site in direction.
51. The First Law. This is called the law of
inertia, because it results directly from inertia ( 22).
It is impossible to furnish perfect examples of this law,
because all things within our reach or observation are
acted upon by some external force. A base-ball when
once set in motion has no power to stop itself. If its
motion was not interfered with, it would move in a
straight line but the force of gravity is ever active,
turns it from that line and compels it to move in a
graceful curve instead.
Experiment 28. Rapidly whirl a pail, partly filled with water,
in a vertical circle. Not a drop will fall from the bucket even
when it is upside down because the water seeks to move in a
straight line and thus to move away from the center about
which it is swung.
52
MOTION AND FORCE.
52. Centrifugal Force. Although it is impossible
to give any experimental proof of the first law of motion,
we sec many illustrations of the tendency of moving
bodies to move in straight lines even when forced to
move in curves. Examples, such as water flying from a
Fio. 6.
revolving grindstone or mud from a carriage-wheel, arc
familiar to all. A wagon in rapidly turning a corner is
likely to be overturned for the same reason. This fact
explains why the outer rail on a railway curve is laid
higher than the inner one. A stone is thus shot from a
sling.
The school-boy is sent rolling in the game of " crack-
the-whip " because, while ilie mentalpart of the boy may
SO NATURAL PHILOSOPHY. $ 52
seek to move in a curve in obedience to the pull of hig
leader, the material part of the boy seeks to move in a
straight line in obedience to Newton's First Law of
Motion. The fun of the game arises from the fact that
the matter often triumphs over the mind.
This tendency of matter to move in a straight
line and, consequently, further away from, the
center around which it is revolving, is called
Centrifugal Force.
(a.) The laws of centrifugal force may be studied or illustrated
by the whirling table and accompanying apparatus, represented in
Fig. 6.
53. The Second Law. The second law of motion
is sometimes given as follows :
A given force will produce the same effect
whether the body on which it acts is in motion
or at rest; whether it is acted on by that force
alone or by others at the same time.
(a.) If a ball that is moving with a velocity of 50 feet a second
be hit with a force that, acting alone, would produce a velocity
of 25 feet a second, the ball will have a velocity of 75 feet a
second.
(6.) If a boat be pulled northward with a force that would give
it a velocity of 3 miles per hour and, at the same time, pulled
eastward with a force that would give it a velocity of 4 miles per
hour, it will move nearly northeastward and with a velocity of
5 miles per hour.
At the end of the hour, the boat will be at the place where it
would be if the first force had really moved it 3 miles northward
and the second force had then moved it 4 miles eastward.
54 MOTION AND FORCE. 31
Experiment 29. Float upon water two blocks of wood, one of
which is twice as heavy as the other. Connect them by a stretched
rubber cord. Release the blocks and they will move toward each
other, but with unequal velocities. Determine how much faster
one moves than the other, and compare their momenta.
54. The Third Law. Examples of the third latf
of motion are very common. When we strike an egg
upon a table, the action of the egg may make a dent in
the table, while the reaction of the table breaks the egg.
The oarsman urges
the water backward with
the same force that he
urges his boat forward.
In springing from a
boat to the shore, mus-
cular action tends to
drive the boat adrift ;
the reaction, to put the
passenger ashore.
Experiment 30. Hang
two clny balls of equal
wsight by strings of equal
lengths so that they will
just touch each other. If
ouo be drawn aside and let
fall against the other, both
will move forward, but
only half as far as the first
would have moved had it
met no -esistance. FIG. 7.
62 NATURAL PHILOSOPHY. 54
Experiment 31. Place two ivory balls, which are elastic, as
you did the clay balls of Experiment 30. Repeat the experiment.
The first ball will give the whole of its motion to the second
and remain still after striking, while the second will swing about
as far as the first would have done if it had met no resistance.
In this case, as in the former, it will be seen that the first ball
loses as much momentum as the second gains.
Experiment 32. Make a railway of two wooden strips,
1-| inches by \ inch and about six feet long, fastened together by
three or five cross-pieces, as shown in Fig. 8. The distance
between the rails should be about an inch. Place the railway on
a board and fasten down the middle cross piece with a screw.
Spring up the ends and support them by books or wooden blocks.
At the toy shop, get several large glass " marbles " and place them
on the middle of the railway. Bring one ball to the highest point
of the track and let it roll down against the others. Ball No. 1
gives up its motion to No. 2 and comes to rest ; No. 2 gives it to
No. 3 and, in turn, comes to rest. The energy is thus passed
through the line -to No. 7, which is driven some distance on
^"Ar-, i:;,,::
76 f
FIG. 8.
the up grade, as to the position shown by the dotted line at 8.
From 8, this ball rolls down grade and passes its energy along the
/ine, forcing No. 1 up the grade to a lesser distance than before.
The balls will repeat their motions several times until they are
finally brought to rest by friction, etc.
57
MOTION AND FORCE.
33
Experiment 33. This action of ivory or glass balls is due t
the fact that they are elastic, and
are flattened by the blow. To
show that this is so, smear a flat
stone or iron plate with paint.
Before the paint becomes dry,
ylace one of the glass balls on the
meared surface and notice the
ize of the round spot thus made.
Then drop the ball from a height
of several inches and notice that
the spot made is larger tJuin before.
This shows that the glass ball was
flattened just as truly as if it were
made of rubber. Elasticity at
once restores the original shape.
FIG. 9.
55. Effect of Elasticity upon Reaction. The
effects of action and reaction are largely modified by
elasticity, but never so as to destroy their equality.
56. Reflected Motion. Reflected motion is the
motion produced by the reaction of a surface
when struck by a body, the surface, or the bo* 111
or both being elastic.
A ball rebounding from the wall of a house is an
example of reflected motion.
57. Law of Reflected Motion. The angle in-
cluded between the line in which the body moves before
it strikes the reflecting surface and a perpendicular to
.that surface drawn from the point of contact, is called
the angle of incidence. The angle between the line
NATURAL PHILOSOPHY.
57
in which the body moves after striking and the perpen-
dicular, is called the angle of reflection.
FIG. 10.
The angle of incidence is equal to the angle
cf reflection.
A ball shot from A will be reflected at B back to C,
making the angle of incidence ABD, equal to the angle
cf reflection, B D.
58. Recapitulation. To be amplified by the pupil
for review.
DYNAMICS.
FORCE.
DEFINITION.
FORMS.
"CENTRIFUGAL."
MOMENTUM.
MOTION. \ NEWTON'S LAWS.
REFLECTED MOTION.
58 EXERCISES.
EXERCISES.
1. What is the momentum of a 100-pound ball moving 275 fee*
a second ? Ans. 27500.
2. A 25-pound ball is moving 100 feet a second. A two-pountf
bird is flying at the rate of 50 feet A second. The momentum ol
the ball is how many times as great as thst of the bird ?
Am. 25 timea
3. A 50- pound body has a momentum of 1000. What r'lmbei
will represent its velocity ? Ans. 20.
4. Which has the greater momentum, a steamboat at rest or r
canoe in motion ? Why ?
5. A boat that is moving at the rate of 5 miles an hour weigta
4 tons; another that is moving at the rate of 10 miles an hour
weighs 2 tons. How do their momenta compare ?
6. A stone weighing 12 ounces is thrown with a velocity of 23
feet a second . An ounce ball is shot with a velocity of 15 miles a
minute. Which will have the greater momentum? How many
times as great ? Ans. 5.
7. Can an angle of incidence be greater than a right angle ?
8. (a.) When water is heated it becomes steam. Is this a
physical or a chemical change? (6.) When steam is intensely
heated it is changed into a mixture of two gases, oxygen and
hydrogen. This mixed gas will not condense to water, but will
burn and even explode. What kind of a change is this? (c.) What
was divided in the latter case that was not in the former ?
9. Bend a twig and tell what change is thus wrought upon the
molecules on the convex side of the twig and what upon (hose on
the concave side ?
10. (a.) Why are pile drivers made very heavy? '(6.) Why are
they raised to considerable heights?
11. A man weighing ICO pounds stands in a boat that weighs
1000 ]x>unds and pulls on a rope held by a man weighing 200
pounds and standing in a boat weighing 2000 pounds. Compare
the velocities and momenta of the two boats.
36 NATURAL PHILOSOPHY. 59
SECTION II.
GRAVITATION.
59. What is Gravitation ? Every particle of
matter in the universe has an attraction for
every other particle. This attractive force is
called gravitation.
60. Laws of Gravitation. (1.) Gravitation va-
ries directly as the product of the masses.
For example, doubling either mass doubles the attrac-
tion ; trebling either mass will multiply the attraction
by three.
(2.) Gravitation varies inversely as the square
of the distance between the centres of gravity
( 65).
For example, doubling the distance quarters the at-
traction ; trebling the distance will divide the attraction
by nine.
Doubling both the product of the masses and the
distance, will halve the attraction ; trebling both the
product and the distance will divide the attraction by
61. What is Gravity ? Gravity is th attrac-
tion between the earth and bodies upon or near
its surface.
64 GRAVITATION. 37
It is the kind of gravitation with which we are most
familiar. It acts in a vertical di- .
rection, as shown by the plumb
line, Fig. 11.
62. What is Weight?
Weight is the measure of
gravity.
The attraction between two
pounds of iron and the earth is
twice as great as the attraction
between one pound of iron and
the earth. Because its gravity is
twice as great, we say that its
Fio. 11.
Weight is twice as great or that it is twice as heavv.
63. Law of Weight. Bodies weigh most at
tne surface of the earth.
Below the surface, the weight decreases as the
distance to the centre of the earth decreases.
Jlbovc the surface, the weight decreases as the
square of the distance from the centre of the
earth increases.
64. Examples. How much will a pound of iron
weigh one thousand miles below the surface of the earth ?
Ans. As the iron would then be only three-fourths
s far from the earth's centre, it would weigh only three-
fourths as much, or twelve ounces.
67361
38 NATURAL PHILOSOPHY. 64
How far below the surface of the earth will a ten-pound
ball weigh only four pounds ?
Ans. As it is to weigh only f as much, its distance
from the earth's centre must be only f of the earth's
radius : of 4,000 miles is 1,600 miles. It will be 1,600
miles from the centre or 2,400 miles from the surface.
A body at the earth's surface weighs 900 pounds :
What would it weigh 8,000 miles above the surface ?
Ans. It would then be 12,000 miles from the earth's
centre or three times as far. The square of three is
nine. According to the law above given, it would
weigh only as much, or 100 pounds.
Experiment 34. Six inches from the upper end of a string
about two feet long, tie a small loop. Fasten any convenient
weight, as an apple or large key, to the lower end of the string.
Stick two stout pins or tacks into adjacent corners of the frame of
your slate. Slip the loop over one pin and support the slate by the
short part of the string, allowing the weight to hang free. When
the slate and string have come to rest, mark a line on the slate,
showing exactly the position of the string. Support the slate in a
similar manner from the other pin and draw another line on the
slate, again showing exactly the position of the string. Place your
finger at the point where these two lines on your slate cross each
other and balance the slate horizontally upon your finger tip.
65. Centre of Gravity. The centre of gravity
of a body is the point about which all the mat-
ter composing the body may be balanced.
A body thus balanced is said to be "in equilibrium."
The weight of a body may be supposed to be concen-
trated at the centre of gravity.
66
GRAVITATION.
be Outside
66. The Centre of Gravity may
of the Body. The centre of grav-
ity may be outside of the matter
of which a body consists, as in the
case of a ring, hollow sphere, box,
or cask. The same fact is illustrated
by the "balancer," represented in
Fig. 12. The centre of gravity is in
the line joining the two heavy balls,
MM, and thus under the foot of the
waltzing figure.
(a.) The "balancer" may be bought at
the toy store for a few cents. The pupil
may better make one, using a large cork for
the body and a smaller one for the head.
The neck, arms and legs may be made of
Btout pins, parts of hair-pins or other wire.
The heavy balls may be two small potatoes. . The uplifted arm
may be made of extra length and serve as a
staff for a paper flag, neatly colored with red
and blue ink or pencil. Instead of using
the heavy balls, the prongs of two forks or
the open blades of two penknives, as shown
in Fig. 13, may be thrust into the cork, the
only thing necessary being to bring the centre
of gravity lower down than the foot of the
figure. It is far better for the pupil to mnk
his apparatus, when he can, than to buy it.
He will understand more dearly, learn mort
rapidly, and soon enjoy the exercise of his in-
gem.ity. The remark applies to girls as well as to boys, and
in many classes it happens that the girls are more ingenious and
enterprising than the bov^
Fio
40 NATURAL PHILOSOPHY. 6"/
67. Equilibrium. When the centre of gravity
is supported, the whole body will rest in a state
of equilibrium.
The centre of gravity will be supported when it and
the point of support are at the same place or in the
same vertical line.
(a.) A yard-stick may be supported by a needle thrust through
its middle ; the point of support and the centre of gravity are
together.
(6.) It may be balanced with one end resting on the finger ; the
point of support and the centre of gravity are in the same vertical
line, the latter being directly over the former.
(e.) It may be supported by hanging it by a string ; the point of
support and the centre of gravity are again in the same vertical
line, the latter now being directly under the former.
(d.) The yard-stick in these three positions illustrates the three
conditions or kinds of equilibrium.
68. Stable Equilibrium. A body supported in
such a way that, when slightly displaced from
its position of equilibrium, it tends to return to
that position, is said to be in stable equilibrium,
Such a displacement raises the centre of gravity.
Examples : a disc or round flat plate supported above the
centre; a semi-spherical oil-can; a pendulum or plumb-
line. The cavalry-man represented in Fig. 14 may rock
up and down balanced upon his horse's hind-feet, because
the heavy ball brings the centre of gravity of the com-
bined mass below the points of support. The " balancer"
(Fig. 12) affords another example of stable equilibrium.
70 GRAVITATION. 4i
(a.) This apparatus is easily made. A piece of board an inch
thick may be fashioned into a
shape something like the body
of a horse. The head and
neck may be made of paste-
board. A feather makes a
good tail and small nails
answer admirably for legs.
A potato or apple may be
used for the heavy ball. The
support may be made by plac-
ing a lath across th3 backs of
two chairs. The " balancer "
previously made may ride this
bare-backed horse standing
wherever he is placed, the '
weight of the apple and the length or curvature of the wire being
varied to suit the circumstances.
69. Unstable Equilibrium. A body supported
in such a way that, ivhen slightly displaced
from its position of equilibrium, it tends to fall
further from that position, is said to be in un-
stable equilibrium.
Such a displacement lowers the centre of gravity.
The body will not come to rest until the centre of
gravity has reached the lowest possible point, when it
will be in stable equilibrium. Examples : A disc sup-
ported below its centre ; an egg standing on its end ;
a stick balanced upright upon the finger.
70. Neutral Equilibrium. A body supported in
such a way that, when displaced from its posi-
tion of equilibrium, it tends neither to return tq
42 NATURAL PHILOSOPHY. JQ
its former position nor to fall further from it,
is said to be in neutral or indifferent equi-
librium.
Such a displacement neither raises nor lowers the
centre of gravity. Examples : A disc supported at its
centre ; a sphere resting on a horizontal surface.
71. Line of Direction. *4 vertical line drawn
downward from the centre of gravity is called
the line of direction.
It may be considered as a line connecting the centre
of gravity of the given body and the centre of the earth.
72. The Base. The side on which a body
rests is called its base.
If the body be supported on legs, as a chair or table,
the base is the figure formed by joining the points of
support.
73. Stability. When the line of direction
falls within the base, the body stands; when
without the base, the body falls.
When the body rests upon a point, as does the sphere.,
or upon a line, as does the cylinder, a very slight force is
sufficient to move it, no elevation of the centre of gravity
being necessary.
The broader the base and the lower the centre
of gravity, the greater the stability.
(a.) These facts explain the stability of leaning towers like those
of Pisa and Bologna. In some such towers the centre of gravity
is lowered by using heavy materials for the lower part and light
materials for the upper part of the structure.
74
GRAVITATION.
43
(6.) It is difficult to stand upon one foot or to walk upon a tight
rope because of the smallness of the base.
PIG. 15.
(c.) A porter carrying a pack is obliged to lean forward : a man
Carrying a load in one hand is obliged to lean away from the load,
to keep the common centre of gravity of man and load over the
base formed by joining the extremities of his feet.
74. Recapitulation. To be amplified by the pupil
for review
f DEFINITION.
GRAVITATION.
GRAVITY
DEFINITION.
WEIGHT {S"''"*'
CENTRE OF GRAVITY. -j n-L* r e
( Stable.
EQUILIBRIUM -(Unstable.
( Neutral.
44 NATURAL PHILOSOPHY. 74
EXERCISES.
1. Why can a child walk more easily witl- ane than without 1
2. Why will a book placed on a desk-lici stay there while a
marble would roll off ?
3. Why is a pyramid a very stable form of structure ?
4. Why is a ton of stone on a wagon less likely to upset than a
ton of hay similarly placed ?
5. If the weight of a body be doubled, what will be the effect
on its attractive force ?
6. If two bodies attract each other with a force of four units,
what will be their attractive force when the distance between them
is doubled?
7. How far does the earth's attraction extend?
75
FALLING BODIES.
SECTION III.
FALLING BODIES.
Experiment 35. Drop a feather and a cent at the same tim
from the same height and notice that the cent will reach tin
ground first.
Experiment 36. Take an iron and
a wooden ball of the same size, drop
them at the same time from an upper
window and notice that they will
strike the ground at nearly the same
time. You will find that it requires a
little practice to drop them at exactly the
tame time.
75. Velocities of Falling
Bodies. In Experiment 35, the
cent fell faster than the feather,
because it met with less resist-
ance from the air than the feather
did and not because it was heavier.
In Experiment 30, we made this
resistance of the air nearly equal
and, probably, convinced ourselves
that the velocity of a falling body
does not depend upon its weight,
unless it has to overcome resist-
ance, or perform work, while it
is falling.
FIG. 1G.
46 NATURAL PHILOSOPHY. 75
When the resistance of the air is removed, the feather
and the cent will fall with equal velocities. This resist-
ance may be avoided by trying the experiment in a glass
tube from which the air has been removed. The experi-
ment is difficult to perform and requires expensive
apparatus. Fig. 16 shows the cent and the feather
falling with equal velocities in a long tube from which
the air has been removed with an air pump, an instru-
ment that we shall soon consider. 187.
76. Reason of this Equality. The cent is heavier
than the feather and is, therefore, pulled downward by a
greater force. The irovi ball has the greater weight, which
shows that it is acted upon by a greater force than the
wooden ball. But this greater force has to move a
greater load, has to do more work than the lesser force.
For the greater force to do the greater work
requires as much time as for the lesser force to
do the lesser work.
A regiment can march no further in an hour than a
single soldier can ; a thousand molecules can fall no
further in a second than a single molecule can.
77. Gravity is a Constant Force. In these ex-
periments, the feather, the cent and the balls could not
change from their condition of rest to that of motion by
their own power ( 22)*. It was necessary that some
force act upon them to produce their motion. The force
of gravity is what did the work.
During the first second, the force of gravity gave the
falling ball a certain velocity ; it gave the ball just as
79 FALLING BODIES. 4?
much more velocity during the next second and just aa
mucli more during the third second. At the end of the
third second, the ball was moving just three times as fast
as it was at the end of the first second.
</l force that thus continues to net uniformly
upon a body, even after the body has begun
to move, is called a constant force. Gravity is
a constant force.
If gravity or any other constant force gives a body a
velocity of 32 feet in one second, it will give a velocity
of 64 feet in two seconds and a velocity of 96 feet in
three seconds. The increase of velocity is 32 feet in
each second.
78. Freely Falling Bodies. When a falling body
meets with no resistance, it is called a freely falling
body. For heavy bodies, the resistance of the air is so
little that it is generally left out of the account. A ball
rolling down an inclined plane, can not be considered a
freely falling body.
(a.) The laws of freely falling bodies have been very carefully
studied. The apparatus used for this purpose is somewhat expen
sive. It is carefully described in the section on " Falling Bodies '
In the author's Elements of Natural Philosophy.
79. Laws of Falling Bodies. These laws are a?
follows :
(1.) The velocity of a freely falling body at
the end of any second of its descent is
equal to 32.16 feet or 9.81 meters mul-
tiplied by the number of the second.
48 NATURAL PHILOSOPHY. 79
(2.) The distance traversed by a freely fall-
ing body during any second of its de-
scent is equal to 16.08 feet or Jj.,9
meters multiplied by one less than
twice the number of seconds.
(3.) The distance traversed by a freely fall-
ing body during any number of sec-
onds is equal to 16.08 feet or Jf.9
meters multiplied by the square of tht
number of seconds.
80. Initial Velocity of Falling Bodies. We
iave been considering bodies falling from a state of rest,
gravity being the only force producing the motion. But
a body may be thrown downward as well as dropped.
In such a case, the effect of the throw must be added to
the effect of gravity. If a body be thrown downward
with an initial or starting velocity of fifty feet per sec-
ond, we must add fifty feet to the result obtained under
the first or second laws above or fifty feet multiplied by
the number of the seconds to the result obtained under
the third law.
81. Increment of Gravity. Since gravity
the velocity of a freely falling body 32. 1C feet or 9.81
meters each second, this quantity is called the increment
of velocity due to gravity or, more simply, the increment
of gravity. In physical mathematics, it is generally
represented by the letter g. It must be remembered
that g = 32.16 feet or 9.81 meters.
FALLING
49
82. Formulas. If we represent the velocity at the
end of any second by v, the number of seconds by t, the
distance passed over each second by s, a,', d the total space
fallen through by S, we shall have the following formulas
for freely falling bodies :
With 50 feet initial velocity.
v = gt + 50.
s = \g (%t 1) + 50.
rr
With no initial 'velocity.
(1.) v=gtorlg x
(2.) s = $g(2t-l).
(3.) S=lgt*.
83. Recapitulation. To be amplified by the pupil
for review.
IMPEDED ; VELOCITIES VARY.
( DEFINITION.
FALLING BODIES.
FREELY FALLING.-
( VELOCITIES EQUAL.
GRAVITY.
( A CONSTANT FORCE.
( CAUSES INCREMENT OF VELOCITY
EFFECTS OF INITIAL VELOCITY.
LAWS AND FORMULAS.
50 NATURAL PHILOSOPHY. 83
EXERCISES.
1. How far will a body fall in 10 seconds? Ant. 1608 ft,
2. What velocity will a freely falling body attain in 4 seconds'
Ana. 128.64 ft.
3. How far will a body move during the fourth second of itg
fall? Ar,. 112.56ft.
4. How far will a body move during the fifth second of its fall,
if it starts with a velocity of 25 feet per second ?
5. (a.) If a body rolling down an inclined plane gains a velocity
of 10 feet in the first second, what will be its velocity at the end
of the tenth second ? (6.) What is its increment of velocity ?
Am. (a.) 100ft.
6. (a.) What is the increment of gravity in meters? (6.) In
centimeters ?
7. Two balls are dropped, one 3 seconds after the other. When
the second one has fallen for two seconds, how far is it from the
first? Ant. 387.68ft.
8. A falling body has a velocity of 98.1 meters. How long has
it been falling? Ans. 10 sec.
9. Show that the formula, S = i gt*. given in 82, is the sam
in meaning as the third law given in 79.
85 * HE PENDULUM. 51
SECTION IV.
THE PENDULUM.
84. The Pendulum. A common pendulum is
a weight so suspended as to be capable of swing-
ing to and fro.
It appears in many forms. The most common form
consists of a steel rod, thin and flexible at the top, carry-
ing at the bottom a heavy mass of metal known as tn*,
bob.
85. Motion of the Pendulum. When the sup-
porting thread or bar of the pendulum is vertical, the
centre of gravity is in the lowest possible position. The
pendulum then remains at rest, for the force of gravity
tends to draw it downward, thus producing pressure at
the point of support but no motion. When the pendu-
lum is drawn from its vertical position, the centre of
gravity is raised. Gravity draws the pendulum back to
a vertical position, when inertia carries it beyond until
it is stopped and drawn back again by gravity. It thus
swings to and fro in an arc.
Experiment 37. In an open doorway or other convenient
place, suspend a weight by a string or fine wire, four or five feet
long. Swing this pendulum through an arc about a foot long
and count the vibrations that it will make in a minute. Then
swing the same pendulum through an arc two or three feet long
and again count the vibrations that it will make in a minute.
NATURAL PHILOSOPHY.
86
FIG. 17.
86. First Law of the Pendu-
lum. The vibrations of a given
pendulum, at any given place,
are performed in equal times
whether the arc be long or short.
Experiment 38. Provide another pendu-
lum of the same length as the one used in the
last experiment, but make it considerably
heavier or considerably lighter. Remember
that the true length of a pendulum is the
distance between the point of support and a
point near the centre of gravity, which latter
point is called the centre of oscillation. Set
both pendulums in vibration at the same time
and notice whether the heavy or the light one
vibrates the more rapidly.
87. Second Law of the Pendulum. The time
of vibration is independent of the weight or ma-
terial of the pendulum, depending only upon the
length of the pendulum and the intensity of the force of
gravity at any given place.
Experiment 39. Prepare two pendulums of such lengths that
one shall vibrate just twice as often as the other. One will be
several times as long as the other. Find just liow many times as
long it is.
Then prepare two pendulums, one of which shall vibrate just
three times as often as the other and determine the ratio between
their lengths as before.
Do the same thing with two pendulums, one of which vibrates
four times as fast as the other. Place the ratios that you have
found in the places of x, y and z in the following table :
89 THE PENDULUM.
Numbers of Vibrations. Lengtht.
1 1
2 *
3 y
4 z
5 f
6 f
Can you see any law or rule governing in such cases? Try, with-
out experiment, to put the proper numbers in the places of the
two interrogation points. Notice that the greater the length,
the less the number of vibrations.
88. Third Law of the Pendulum. The vibra-
tions of pendulums of different lengths are performed in
different times. The lengths are inversely propor-
tional to the squares of the numbers of vibra*
tions in a given time.
L : I :: n* : JV.
89. The Second's Pendulum. The length of a
second's pendulum, at the level of the sea, is 39
inches at the equator; 39.2 inches, near the
poles and about 39.1 inches or 993.3 milli-
meters or .9933 meters, in this latitude.
As such a pendulum would be inconveniently long,
use is generally made of one one-fourth as long which,
consequently, vibrates half seconds.
The length and time of vibration of the second's
pendulum being thus known, the length of any other
pendulum may be found when the time of vibratipo is
54 NATURAL PHILOSOPHY. 89
given. The time of vibration may be found when the
Length is given.
(a.) The third law may be used in solving such a problem. It
is interesting to notice how little difference there is between the
length of a second's pendulum and the meter.
90. Use of the Pendulum in Time-pieces. The
motion of a clock is due to the force of
gravity acting upon the weights, or to
the elasticity of the spring. But the
weights have a tendency toward accel-
erated motion (increasing velocity),
while the spring would give an exam-
pie of diminishing motion. Either
defect would be fatal in a time-piece.
Hence, the properties of the pendulum
set forth in the first and third laws are
used to regulate this motion and make
it available for the desired end.
If the clock gains time, the
pendulum is lengthened by low-
ering the bob ; if it loses time, the
pendulum is shortened by raising
the bob.
Remember that the pendulum does
not make the clock go ; it simply de-
termines how fast it shall run. It does
'FIG 18 ^ n * s ^7 means f the escapement, shown
in Fig. 18. Every vibration of the
lendulnm works the crutch, n m, and allows the wheel,
/?, to more forward one tooth,
91
THE PENDULUM.
55
91. Recapitulation. To be amplified by the pupil
for review.
DEFINITION.
CAUSE OF VIBRATIONS.
THE PENDULUM.
LAWS
f FIRST.
SECOND.
{ THIRD.
THE SECOND'S PENDULUM.
APPLICATION TO CLOCKS.
56 NATURAL PHILOSOPHY. QI
EXERCISES.
1. If a pendulum swings through an arc two feet long, how
many more times will it vibrate in a minute than when it swings
through an arc four feet long ?
2. One pendulum is 10 inches long and vibrates four times as
fast as another. What is the length of the other?
3. Two pendulums are of the same length. The bob of one
weighs a pound ; that of the other, half a pound. Compare their
rates of vibration.
4. One pendulum is 16 inches long ; another is 64 inches long.
How many times will the short one vibrate while the long one is
vibrating four times ?
5. What must you do to a pendulum to make it vibrate three
times as fast?
6. A clock gains time. What is the trouble with its pendulum?
7. Two pendulums are 4 and 9 feet long respectively. How
many vibrations will the short one make while the long one is
vibrating twice?
8. Two pendulums are respectively 49 and 64 inches long. How
will their times of vibration compare?
9. How long must a pendulum be to vibrate once in tw
econds ?
10. How long must a pendulum be to vibrate twice a second ?
94 ENERGY. 57
SECTION V.
ENERGY.
92. Work. In physical science, the term work
signifies the overcoming of resistance of any kind.
Whether this overcoming of resistance is pleasant or not
does not enter into consideration here, all play being a
species of work. The word is here used in this enlarged
sense.
93. Energy. Energy is the power of doing
work.
If one man can do more work than another, he has
more energy. If a horse can do more work, in a given
time, than a man, the horse has more energy than the
man. If a steam-engine can do more work than a horse,
it has more energy. If a moving cannon-ball can over-
come a greater resistance than a base-ball, it has more
energy.
94. Elements of Work Measure. Imagine a
flight of stairs, each step having a rise of twelve inches.
On the floor at the foot of the stairs are a one pound
weight and a ten pound weight. Lift the first weight
to the top of the first step. How much work have you
performed ? .Perhaps you will answer, one pound cf work.
Now place the second weight beside tho first. How
much work did you perform in so doing : Perhaps you
will say ten times as much as before or ten pounds.
58 NATURAL PHILOSOPHY. 94
Now lift each of them another step, and then another,
until they rest on the top of the tenth step. To lift the
heavier weight the second, third and following times
involved as much work each time as to lift it the first
foot, but you would hardly say that you had lifted a
hundred pounds.
Still it is sure that to place it on the tenth step
required just ten times as much work as it did to place
it on the first step or just one hundred times as much
work as it did to place the one pound weight on the
first step.
It is evident that the two elements of weight
and height are necessarily considered in meas-
uring the work performed.
95. Units of Work ; the Foot-pound. It is
often necessary to represent work numerically and so we
need a unit of measurement. The unit commonly in
use, for the present, in England and this country is the
foot-pound.
A foot-pound is the amount of work required
to raise one pound one foot high against the
force of gravity.
The work required to raise one kilogram one meter
high against the same force is called a Mogram-meter.
(a.) To get a numerical estimate of work, we multiply the num-
ber of weight units raised by the number of units of length in the
vertical height through which the body is raised. A weight of 25
pounds raised 3 feet, or one of 3 pounds raised 25 feet, represents
75 foot-poun4f.
98 ENERGY. 59
96. Rate of Doing Work. In measuring work
done, the time employed is not taken into consideration.
In estimating the power that is to do the work, the time
is an important element. Lifting a ton 100 feet high
involves 200,000 foot-pounds, whether the work be done
in ten minutes or ten hours. But the engine that can
do this work in ten minutes is more powerful than one
that requires ten hours.
97. Horse-Power. <A horse-power represents
the ability to perform 33,000 foot-pounds in a
minute or 550 foot-pounds in a second.
An engine that can do 66,000 foot-pounds in a minute
or 33,000 foot-pounds in half a minute is called a two
horse-power engine. To compute the number of horse-
powers represented by an engine at work, multiply the
number of pounds raised by the number of feet, and
divide the product by 33,000 times the number of
minutes required to do the work.
Experiment 40. Into a pail full of moist clay or stiff mortar,
drop a bullet from, a height of one yard. Notice the depth to
which the bullet penetrates. Drop the bullet from a height of
four yards. It will strike the clay with twice the velocity ( 79)
and penetrate four times as far as it did before.
98. Relation of Velocity to Energy. If the last
experiment be continued by dropping the bullet from a
height of nine yards, it would strike the clay with a
three-fold velocity and penetrate to nine times the depth.
The work done by a moving body will vary as the mass
and as the square of the velocity.
60 NATURAL PHILOSOPHY. 98
(a.) The energy of a moving body may be computed in foot-
pounds by multiplying the number of pounds in the moving body
by the square of the number of feet it is moving per second and
dividing the product by 6432 (or twice the increment of velocity
due to gravity, 81).
Kinetic Energy = ^.
NOTE. For a fuller discussion of this subject, see the author's
Elements of Natural Philosophy, 157.
99. Two Types of Energy. There are two kinds
of energy. One is called energy of motion ; the other,
energy of position.
A falling weight or running stream possesses energy
of motion ; it is able to overcome resistance by reason of
its weight and Telocity.
But, before the weight began to fall,, while it was at
rest, it had the power of doing work by reason of its
elevated position with reference to the earth. When the
water of the running stream was at rest in the lake
among the hills it had a power of doing work, an energy, .
which was not possessed by the waters of the pond in
the valley below. This energy or power of doing work
results from its peculiar position.
The weight or the water, when thus elevated and mo-
tionless, has no working power in the sense with which
we are most familiar with it. Yet it is very clear that
it is possible, under certain conditions, for it to do work.
We might, therefore, use the term "possible energy" to
denote the power in question and, as a matter of fact,
100 ENERGY. 61
the term " potential energy," which means the same
thing, is thus used.
Energy of motion is called kinetic energy;
energy of position is called potential energy.
ioo. Convertibility of Kinetic and Potential
Energies. We may at any moment convert kinetic
energy into potential, or potential energy into kinetic.
One is as real as the other and, when it exists at all,
exists at the expense of a definite amount of the other.
Imagine a ball thrown upward with a velocity of 64.32
feet. As it begins to rise it has a certain amount of
kinetic energy because it has weight and velocity. At
the end of one second it has a velocity of only 32.16 feet.
Consequently, its kinetic energy has diminished. It has
jost some of its velocity but it has gained a better posi-
tion for doing work. Having risen 48.24 feet, it has
gained a considerable potential energy. All of this po-
tential energy results from the kinetic energy which has
disappeared, or from the work that has been done.
At the end of another second the ball has no velocity ;
it has reached the turning-point and is at rest. Conse-
quently, it has no kinetic energy. But the energy with
whicli it began its flight has not been destroyed ; it
has been stored up in the ball at a height of 64.32 feet
as potential energy. If at this instant the ball be caught,
all of the energy may be kept in store as potential
energy.
If now the 'ball be dropped, it begins to lose its poten-
tial and to gain kinetic energy. When it reaches the
NATURAL PHILOSOPHY.
ioo
ground at the end of two seconds it has no potential
energy, but just as much of the kinetic type as was
given to it when it began to rise.
In a simple way this illustrates the important fact
that energy may be changed from one form to
another without any change in its quantity.
101. Energy a Constant Quantity. In the case
of the ball thrown upward, at the start, at the finish or
at any intermediate point of either its ascent or descent,
the sum of the two types or kinds of energy is the same.
It may be all kinetic, all potential, or partly both. In
any case, the sum of the two continually varying
energies is constant. Just as a man may have a hun-
dred gold dollars, now in his hand, now in his pocket, now
part in his hand and the remainder in his pocket ; chang-
ing a dollar at a time from
hand to pocket or vice versa,
the amount of money in his
possession remains constant,
viz., one hundred dollars.
102. Pendulum Illus-
tration. The pendulum af-
fords a good and simple illus-
tration of kinetic and poten-
tial energy, and of their power
of changing, one to another,
without loss. When the pen-
dulum hangs at rest in a vertical position, as P a, it has
no energy at all.
FIG. 19.
103 ENERGY. 63
Considered as a mass of matter, separated from the
earth, it certainly has potential energy ; but considered
as a pendulum, it has no energy.
If the pendulum he drawn aside to b, we raise it
through the space a h ; that is, we do work, or spend
kinetic energy upon it. The energy thus used is now
stored up as potential energy, ready to be changed back
into energy of the kinetic type, whenever we let it drop.
As it falls the distance ha, in passing from b to a,
this change is going on. When the pendulum reaches a,
its energy is all kinetic and just equal to that spent in
raising it from a to 6. This kinetic energy now carries
it on to c, lifting it again through the space all. Its
energy is again all potential just as it was at b.
If we could free the pendulum from the resistances
of the air and friction, the energy first given to it would
swing to and fro between the extremes of all potential
and all kinetic; but at every point of the arc traversed,
the total energy would be an unvarying quantity, always
equal to the energy first used in swinging it from a to b.
103. Indestructibility of Energy. Were it not
for friction and the resistance of the air, the pendulum
would vibrate forever; its energy would be indestruc-
tible. Energy is withdrawn from the pendulum to over-
come these impediments, but the energy thus withdrawn
is not destroyed
What becomes of it will be seen when we come to
study heat and other forms of energy, which result from
NATURAL PHILOSOPHY.
103
the motions and positions of the molecules of matter.
The truth is that energy is as indestructible as
matter. For the present, we must admit that a given
amount of energy may disappear and escape our search,
but it is only for the present. We shall soon learn to
recognize the fugitive even in disguise.
104. Recapitulation. To be amplified by the pupi]
for review.
ENERGY.
DEFINITION.
WORK.
Wtigkt.
Height.
f Foot-pound.
UNITS \ Kilogram-Metev
1 Horse-Power.
RELATION TO VELOCITY,
( KINETIC. )
TYPES \ \ Convertikiliiy.
[ POTENTIAL. )
INDESTRUCTIBILITY.
104 EXERCISES. 66
EXERCISES.
1. What is the necessary horse-power of an engine that is
required to raise 100,000 pounds 198 feet high in one hour?
An*. 10 H. P.
2. How long will it take a 2-horse-power engine to raise 10 tons
50 feet high ? Ans. 15 min., 10 sec., nearly.
3. How much must you increase the velocity of a moving body
to quadruple its energy ?
4. How does the energy which a moving body must expend
before it can come to rest compare in amount with the energy pre-
viously required to put the body in motion ?
5. Define force and energy.
6. What amount of work is needed to lift 20 bricks weighing 5
pounds each 50 feet high ?
7. (a.) How much work could the bricks mentioned in Exer-
cise 6 perform in falling back to the ground ? (6.) Where did they
get that energy ?
8. How much work may be done by a ball weighing 64,32C
pounda and striking with a velocity of 200 feet a second ?
Am. 40,000,000 foot-pounds.
NATURAL PHILOSOPHY. 104
HE VIEW QUESTIONS.
1. If a cork be released at the bottom of a vessel of water, it
quickly rises to the surface. Explain the upward motion of the
cork.
2. Give some illustration of unstable equilibrium not mentioned
in this book.
3. Why does a wagon with a ton of hay overturn more easily
than a similar wagon with a ton of stone ?
4. In loading a wagon, where should the heavy articles be
placed? Why?
5. Why do quadrupeds leam to walk more easily than bipeds ?
G. Imagine a man suspended in otherwise empty space. Could
he put himself in motion ? Why?
7. If the distance between two bodies be doubled, how will
their attraction for each other be affected ?
8. What is meant by " increment of gravity " ?
9. Why have liquids no permanent shape ?
10. What property of steel fits it for use in pens ?
11. What property of matter is illustrated in the action of the
part of a watch that makes the wheels move ?
12. What force resists our attempts to draw a nail out of wood ?
13. What is the object of ballast in a vessel ?
14 Tli rust your hand into water and it comes out wet. What
property of matter is illustrated by the experiment ?
15. What is the cause of weight?
CHAPTEfi Hi*
SIMPLE MACHINES.
SECTION I.
PRINCIPLES OF MACHINERY. THE LEVER.
Experiment 41. Arrange two small pulleys and a spring-bal
ance as shown in the figure. The pulleys and balance may bt
bought at the hardware store and will be
much used. If W weighs 10 pounds, the
spring-balance at P will show that the hand
exerts a pull of 5 pounds. If the hand moves
two feet it will be noticed that W moves one
foot. No matter how far P moves ; W will
move just half as far in the same time. Under
all circumstances, the product of the number
representing the weight of W into the number
representing the distance it moves, will equal
the product of the number representing the
weight (or pull) at P into the number repre-
senting the distance it moves. Change the
positions of P and W and see if this is so.
FIG. 19.
NOTE. In this case, if the apparatus be
delicate, it will be necessary to make an allowance for the weight
of the cord and movable pulley. This allowance may be made
by hanging just enough weight at P to keep the apparatus in
68 NATURAL PHILOSOPHY. 105
equilibrium before W is attached. In all experiments with
pulleys, levers, and other machines, it is proper to see that the
machine itself is in equilibrium before attempting to determine the
relations of power to weight.
105. What is a Machine ? A machine is a
contrivance by means of which a given power
may be used to overcome a given resistance with
certain advantages.
Its use is to transform the intensity of energy, so that
an energy of small intensity, acting through a consider-
able distance, may be made to do the same work as a
]arge power, acting through a small distance, or rice
versa.
106. A Machine can not Create Energy. No
machine can create or increase energy. In fact, the use
of a machine causes a waste of power, for a part 'of the
energy must be used to overcome the friction of the ma-
chine itself, thus diminishing the amount that can be
used for doing useful work.
107. Of what Use are Machines ? Some of the
many advantages resulting from the use of machines are :
(1.) They enable us to do work more quickly
than we otherwise could, as in the sewing-
machine or spinning-wheel.
(2.) They enable us to do work that we other-
wise could not do at all, as in lifting a large
stone with a crow-bar or pulleys.
108 PRINCIPLES OF MACHINERY. 09
(3.) They enable us to change the direction of
our force, as in hoisting a flag on a flag-staff.
It would be inconvenient to climb the pole and
then draw up the flag.
(4.) They enable us to employ other forces than
our own, as the strength of animals, the forces
of wind, water, steam, etc.
108. General Laws of Machines. The work to
be done by a machine is generally called the weight or
load. The work of the power (e. </., foot-pounds) is
always equal to the work of the load, the power expended
in the machine itself being disregarded. The following
are called the general laws of machines :
(1.) What is gained in intensity of power is
lost in time, velocity, or distance; and
what is gained in time, velocity, or distance, is
lost in intensity of power.
(2.) The power multiplied by the distance
through which it moves, equals the weight
multiplied by the distance through which
it moves. For example, if the power moves
five times as far as the weight, the power will
be only one-fifth as great as the weight.
(3.) The power multiplied by its velocity, equals
the weight multiplied by its velocity.
For example, if the power moves five times as
fast as the weight, the power will be only one-
fifth as great as the weight,
70
NATURAL PHILOSOPHY.
109
109. What is a Lever ? A lever is an inflex-
ible bar capable of being freely moved about a
fixed point or line, called the fulcrum.
Every lever has two arms. The power-arm is the
perpendicular distance from the fulcrum to the line in
which the power acts; the weight-arm is the perpen-
dicular distance from the fulcrum to. the line in whioh
the weight acts.
1 10. Classes of Levers. There are three classes
of levers, depending upon the relative positions of the
power, weight, and fulcrum.
(1.) If the fulcrum is between the power and
weight (P. F. W.), the lever is of the first class (Fig. 20);
fl F e. g., crow-bar, balance,
steel-yard, scissors, pincers.
(2.) If the weight is
between the power and
the fulcrum (P. W. F.), the
lever is of the second
class, (Fig. 21) ; e. g., cork-
squeezer, nut-cracker, wheel-
barrow.
FIG. 30.
FIG. 21.
( 3.) If the power is be-
tween the weight and the
fulcrum (W. P. F.), the
lever is of the third class
(Fig. 2?) ; e. g., fire-tongs,
sheep-shears, human fore-
arm.
III THE LEVER. 71
Experiment 42. Bore a small hole 18 inches from one end of
a yard-stick and nearer one edge than the other. Thrust a round
metal pin through this hole into a firm vertical support, which
may be made by nailing an inch board, 6 inches by 4 feet, to the
side of a soap box and placing bricks in the box. Such a support
will be convenient for many purposes. The pin should be of
such a size as to move easily in the hole and yet have strength
enough to carry a load of several pounds. It will be well to put a
email metal washer on the pin between the yard stick and the
support. Be, sure that tlie stick is in equipoise (or balances on the
pin), shaving off the upper edge of the stick near one end if neces-
sary for this purpose. Borrow, buy or (best of all) make two sets
of weights of \ lb., 1 Ib. and 2 Ib. respectively. The weights
may be provided with suspension loops of linen or silk thread,
the weight of which may be disregarded.
(d.) Hang an 8 oz. weight on each side of the pin, 6 inches
distant. Notice that this lever of the first class balances;
then remove the weights.
(6.) Hang a 1 lb. weight on each side of the pin, 12 inches
distant. The lever balances. Remove the weights.
(c.) Hang a \ lb. weight on one side of the pin 16 inches
distant and a 1 lb. weight on the other side of the pin, 8 inches
distant. The lever balances. Remove the weights.
(d.) Hang a lb. weight on one side of the pin 16 inches
distant and a 2 lb. weight on the other side of the pin, 4 inches
distant. The lever balances.
in. Static Laws of the Lever. It will be
clearly seen from the last experiment, that the following
statements are true :
(1.) The power multiplied by the power -arm
equals the weight multiplied by the
weight-arm; or,
72 NATURAL PHILOSOPHY. 1 12
(2.) A given power will support a weight as
many times as great as itself, as the
power -arm is times as long as the
weight-arm.
NOTE. A static law expresses the relation between the power
and weight when the machine is in equilibrium. In order that
there be motion, one of the products mentioned in the law above
must be greater than the other. The lever itself must be in equi-
librium before the power and weight are applied. It is to be
noticed that when we speak of the power multiplied by the power-
arm, we refer to the abstract numbers representing the power and
power-arm. We can not multiply pounds by feet, but we can
multiply the number of pounds by the number of feet.
Experiment 43. Make two scale-pans by hanging two tin can
covers (each by three or four stout threads about a foot Icng) from
the balanced yard stick of Experiment 42. The upper ends of
the cords of each pan may be knotted together and provided with
a loop that will easily slide over the end of the yard-stick. Place
one of these loops an inch from each end of the yard-stick and be
sure that your scales balance well. Place an 8 oz. weight in one
scale pan and weigh out ^ Ib. of sand.
112. The Balance. The balance is essentially
a lever of the first class, having equal arms.
Its use is to determine the weights of bodies. The
lever itself is called the beam. The ends of the beam
carry two pans, one to support the weights used, the
other to support the article to be weighed. (Fig. 23.)
(a.) That the balance may be accurate, the arms must be of the
same length. To make these arms exactly equal is far from an
easy task. Balances are made so delicate that they may be turned
by less than a thousandth of a grain. A really good balance is an
expensive piece of apparatus and requires great care.
THE LEVER.
FIG. 23.
113. False Balances. False balances (levers
of the first kind with unequal arms) are some-
times used by dishonest dealers.
When buying, they place the goods on the shorter
arm ; when selling on the longer. The cheat may be
exposed by changing the goods and weights to the oppo-
site sides of the balance. The true weight may be found
by weighing the article first on one side and then on the
other, and taking the geometrical mean of the two false
weights ; that is, by finding the square-root of the pro-
duct of the two false weights.
For instance, if the article appears to weigh eight
pounds on one side and six pounds and two ounces on
the other, the true weight is seven pounds.
8 x 6^ = 49. VT9~= 7,
74 NATURAL PHILOSOPHY. 114
In buying seven pounds of butter, the dealer would
pay for only six pounds and two ounces and, in selling
the same butter, he would get pay for eight pounds,
thus gaining, by fraud, the price of one pound and four-
teen ounces, in addition to his legitimate profit.
114. Double Weighing. The true weight of a
body may be found with a false balance in another way.
The article to be weighed is placed in one pan, and a
counter-weight, as of shot or sand, placed in the other
pan until equilibrium is produced. The article is then
removed and known weights placed in the pan until
equilibrium is again produced. These known weights
will be the true weight of the given article.
115. Load between Two Supports. If a
beam rest on two supports and carry a load
between them, the beam may be considered a
lever of the second class.
The part carried by either support may be found by
considering it as the power and the other support as the
fulcrum. (Fig. 24.) For instance, if the string of fish
weighs 15 pounds and is placed two feet from the boy in
front and eight feet from the boy in the rear, we may
want to know how much of the load thi latter is carry-
ing.
In this case, the shoulder of the boy in front is
called the fulcrum. The weight- arm is two feet and
the power-arm is ten feet. As the power-arm is five
tiroes as long as tb.e weight-arm, th weight will be five
"6
THE LEVER.
75
times the power or the power will be one-fifth the weight,
viz., three pounds. Of course, the boy in front carries
the other twelve pounds of the load.
FIG. 24.
If W8 prefer, we may call the shoulder of the boy in
the rear the fulcrum. In this case, the weight-arm is
eight feet and the power-arm is ten feet. As the weight-
arm is four-fifths as long as the power-arm, the power
will be four-fifths of the weight, viz., twelve pounds.
Of course, the other three pounds is borne by the ful-
crum, or the boy in the rear.
116. Compound Lever. Sometimes it is not con-
venient to use a lever sufficiently long to make a given
power support a given weight. A combination of levers
palled a compound lever may then be used.
76
NATURAL PHILOSOPHY.
n<5
may be mentioned as a familiar illustration of the com-
pound lever. In this case we
have the following:
Statical Law. The con-
tinued product of the power
and the lengths of the alter-
nate arms, beginning with
the power -arm, equals the
continued product of the
weight and the lengths of
the alternate arms begin-
ning with the weight-arm,
(a.) If the arms of the lever, a, (Fig. 25) be 1^ feet and 6 feet ;
of lever b, 1 foot and 3 feet ; of lever c, 2 feet and 5 feet, a pull of
7 pounds at P will support a weight of 210 pounds at W.
7x6x3x5 = 210 x2xlxU.
FIG. 25.
117. Recapitulation,
for review.
-To be amplified by the pupil
MACHINES.
DEFINITION.
NO CREATIVE POWER.
USES.
LAWS.
LEVER...
DEFINITION.
CLASSES
LAW.
COMPOUND.
... J False.
FIRST: Balance
[ Double Weighing.
SECOND : Load between Two Supports
THIRD.
EXERCISES. ??
EXERCISES.
1. The power-arm of a lever must be how many times as long
as the weight-arm to have 100 kilograms support 1,000 kilograms'!
2. Why is the short arm of a steelyard larger around than the
long arm?
3. In Fig. 20, if the weight-arm is one foot and the lever is five
feet long, what weight at W will be supported by a pull of seven
pounds at P ?
4. In Fig. 21, if the weight-arm is one foot and the lever is five
feet in length, what weight at W will be supported by a pull of
seven pounds at P ?
5. In Fig. 22, if the power-arm is one foot and the length of the
Jever is five feet, what pull at P will be needed to support seven
pounds at Wt Ans. 35 Ib.
6. A lever is 75 inches long. It enables a weight of one pound
to balance a load of two pounds. Is the fulcrum in the middle 1
Why ? If not, how far must the fulcrum be from the end of the
lever?
7. The weight-arm of a lever is 6 feet long ; the power-arm is
12 feet long, (a.) What is the length of the lever if it be of the
first class? (6.) If of the second class? (c.) If of the third class?
8. Where must a straight bar be supported so that a pound
weight hung at one end will support two pounds at the other endl
Ans. At | its length from one end.
9. The toggle joint shown in Fig. 26 and
similar in action to those commonly used on
carriage tops is used for punching iron. While
the joint is moved from c to d, the punch b is
moved forward until the distance ab becomes ac
+ bc. If e d 10 inches ; nb, 99 1 inches; ac
and be each, 50 inches and a power of 1,000
pounds pull c toward d, what force will be ex-
erted upon the punch at 6 ?
Ant. 20,000 Ib.
FIG. 26.
?S XATTJRAL PHILOSOPHY. Il8
SECTION II.
THE WHEEL AND AXLE AND THE PULLEY.
118. The Wheel and Axle. The wheel and
axle consists of a wheel united to a cylinder in
such a way that they revolve together about a
common axis.
The power being applied to the circumference of the
wheel, the load is carried by a rope wound around the
axle.
119. Advantages of the Wheel and Axle.
The ordinary crowbar or other lever of the first class
can lift a load only a short distance
at one time. In order to raise the
load higher tban the vertical dis-
tance through which the weight
end of the lever passes, it is neces-
sary to support the load and re-
adjust the fulcrum. The motion
is irregular and time is lost. These
difficulties are obviated by using the wheel and axle.
120. Law of the Wheel and Axle. The power
multiplied by the radius, diameter or circum-
ference of the wheel equals the weight multi-
plied by the corresponding dimension of the axle,
or,
The power will support a weight as many
TfrHEEL AtfD AXLE.
times as t'reat as itself, as the radius, diameter
or circumference of the wheel is times as great
as the siivilar dimension of the axle.
Example. - If the radius, diameter or circumference of the
wheel is t< times as
great as th similar di-
mension of ae axle, the
power will move ten
times as far and ten
times as fast as the
weight and the weight
will be ten times as
great as the power. A
power of 15 pounds will
support a load of 150 FIG. 28.
pounds. See 108, (2) and (3).
121. Various Forms of V/heel and Axle.
The wheel and axle appears in various forms. It is not
necessary that an entire wheel be present, a single spoke
or radius being sufficient for the application of the power,
as in the case of the windlass (Fig.
28) or capstan (Fig. 29). In all
such cases, the radius being given,
the diameter or circumference of
the wheel may be easily computed.
(See Appendix A.}
In one of the most common
forms, the power is applied by
means of a rope wound around the circumference of the
wheel. When this rope is unwound by the action of the
power, another rope is wound up by the axlo and the
weight thus raised.
FIG. 29.
so
NAfUttAL PHILOSOPHY.
122
122. Wheel-work. Another method of securing a
great differeno in the in-
tensities of bala iiced forces,
is to use a con bination of
wheels and axles i f moderate
size. Such a combina-
tion constitutes a train.
The wheel that imparts the
A motion is called the driver ;
< | that which receives it, the
follower. An axle with
teeth upon it is called a
pinion. The teeth or cogs
FIG. 30.
of a pinion are called leaves.
123. Ways of Connecting Wheels. Wheels
may be connected in three ways :
(1.) By the friction of their circumferences.
(2.) By lands or belts.
(3.) By teeth or cogs.
The third of these methods has been already con-
sidered.
124. Uses of the First Two Ways. The first
method is used where no great resistance is to be over-
come but where evenness of motion and freedom from
noise are chiefly desired. It is illustrated in some
sewing-machines.
The second method is used when the follower is to be
at some distance from the driver. The friction of the
127
THE PULLEY.
81
belt upon the wheels must be greater than the resistance
to be overcome. It is il lustrated in most sewing-machines,
in the spinning-wheel and, on a large scale, in every
machine shop.
125. Relative Velocities Determined. Tlie fol-
lower ivill revolve as main/ ihucs as fast as tlie
driver, as its radius, diameter or circumference
is contained times in that of the driver.
(a.) If the driver have a circumference of 10 feet and the fol-
lower a circumference of 2 feet, the follower will revolve 5 times
as fast as the driver.
126. What is a Pulley ? A pulley consists of a
wheel turning upon an axis and having a cord
passing over its grooved, circumference.
The frame supporting the axis of the wheel is called
the block.
127. A Fixed Pulley. If a cord
be passed over a pulley fixed to the
ceiling, a weight being at one end and
the hand applied at the other, as at
P in Fig. 31, the hand will have to
exert a force equal to the weight of
the load. If the weight be moved,
the hand and weight will move equal
distances. It is evident, then, that
the fixed pulley affords no in-
crease of power, but only change
of direction.
FIG. 31.
IfATtTRAL PHILOSOPHY.
128. A Movable Pulley. If one end of the cord
be fastened to the ceiling, the load suspended from the
pulley, and the other end of the cord
drawn up by the hand or passed over
a fixed pulley, as shown in Fig. 32, it
will be evident that the fixed support
(the hook) carries half the load and
the hand the other half.
To raise the weight one foot, the
hand must pull up two feet of the
cord ; that is to say, each section of
the cord carrying the weight must be
shortened one foot. Thus the hand,
by pulling 50 pounds two feet, is able
to raise 100 pounds one foot.
129. Advantages of the Pulley. It is to be
very carefully noticed that the pulley does not create any
energy or do any work. It simply enables us to exchange
velocity for intensity of work, and to change the direc-
tion in which the power acts.
If the hand at P pulls down with a force of 50 pounds
and moves through a distance of four feet, it performs
200 foot-pounds of work. At the same time, the weight
of 100 pounds will be raised two feet and this work is
also represented by 200 foot-pounds.
It also requires some additional poiver to overcome the
friction of the machine and to bend the ropes, but we
probably would be glad to pay this price for the ability
to exchange velocity which we can produce for the in-
tensity which we desire.
130
THE PULLET.
130. A Combination of Pulleys. By the use ot
several fixed and movable pulleys in
blocks, the number of parts of the
cord supporting the movable block
may be increased at pleasure.
In all such cases, the part of
the cord to which the -power is
applied, will carry only a part
of the load. The value of this
part of the load depends upon the
number of sections into which the
movable pulley divides the cord.
In Fig. 33, the movable pulley
is represented as dividing the cord
into six such parts. Then the power
applied at P will support a load six
times as great as itself.
In practice, the several wheels of
each block are made of the p IG
Wsame size and placed side
by side, turning upon the same axis. The sev-
eral wheels in a block are often called sheaves.
131. Law of the Pulley. With a
puller/ having a continuous cor a, a
given power will support a weight as
many times as great as itself as there
are parts of the cord supporting the
movable block.
132. Concerning: the Number of Parts
FIG. 34. of the Cord. Look at the several figures of
84
NATURAL PHILOSOPHY.
132
pulleys. You will see that when the fixed end of the
cord is attached to the fixed block, the number of parts
of the cord supporting the weight is twice the number
of movable pulleys used.
When the fixed end of the cord is attached to the
movable block, the number of parts of the cord is one
more than twice the number of movable pulleys used.
133. Recapitulation. To be amplified by the pupil
for review.
WHEEL
AND AXLE.
DEFINITIONS.
ADVANTAGES.
RELATION TO THE LEVER.
LAWS.
FORMS.
DRIVER.
FOLLOWER.
WHEEL WORK...
CONNECTIONS.
RELATION OF P TO W.
I Modes.
\ Uses.
PULLEY...
DEFINITION.
1 FIXED.
MOVABLE.
COMBINATIONS.
LAW.
NUMBER OF PARTS OF THE CORD.
132 EXERCISES. 85
EXERCISES.
1. If the wheel (Fig. 27} be 5 feet in diameter and the axle be
1 foot, what power must be exerted by the hand at P to support a
load of 125 pounds at IF? An*. 25 Ib.
2. If the handle of a windlass (Fig. 28) describes a circle 9 feet
in circumference and thus causes the axle to wind up 3 feet of
rope, what weight at W will be supported by every pound of force
applied at P ?
8. A ship's anchor, weighing two tons, is to be hoisted by a
cable wound around the barrel of the capstan (Fig. 29). The
barrel is two feet in diameter, A man pushes at the end of each
of the four capstan bars (radii) which are eight feet long. With
what force must each man push ? Ana. 125 Ib.
4 (a.) With such a pulley as is represented in Fig. 31, how
great a load will a pull of 10 pounds support ? (6.) How much
with such a pulley as is represented in Fig. 32 ? (e.) How much
with such a pulley as is represented in Fig. 33 ? (d.) How much
with such a pulley as is represented in Fig. 34?
5. With such a pulley as is represented in Fig. 32, how great a
load can a 100-pound boy raise ?
Ana. A little less than 200 Ib.
6. A man who weighs 180 pounds lifts a weight of 100 pounds
by means of a fixed pulley over his head. What is the man's
pressure on the floor ? Ana. 80 Ib.
7. A weight of 4 pounds is hung from the wheel which is 5 feet
in diameter. A weight of 21 pounds is hung from the axle which
is 1 foot in diameter. Which will descend, assuming that tb
wheel and axle works without friction,?
86
NATURAL PHILOSOPHY.
133
SECTION III.
THE INCLINED PLANE, WEDGE, SCREW, ETC.
133-
What is an Inclined Plane ? The in-
clined plane is a sur-
face sloping so as to
make an oblique angle
ivith the direction of
the force to be over-
come.
FIG. 35.
against the force of gravity.
In most cases, it is used
to aid in lifting bodies
134. Law for the Inclined Plane. In Fig. 36,
the plane is twice as long as it
is high. As there indicated, a
force of ten kilograms will sup-
port a weight twice as heavy. This
result may be easily verified by ex-
periment. We may establish the
following law :
Wlien a given power acts
parallel to the inclined plane, it will support a
u'eight as many times as great as itself as the;
of the plane is times as great as Us ver*
10 Kg.
FIG. 36.
138
THE WEDGE.
87
135- What is a Wedge ? A wedge is a mov-
able inclined plane.
136. Its Use. The
wedge is used for mov-
ing great weights short
distances.
A common method of
moving bodies is to place
two similar wedges, with FlG - 37 -
their thin ends overlapping, under
the load. Blows of equal force
are struck upon the heads of the
wedges at the same time and the
load is thus raised as shown by the
arrows in Fig. 38.
FIG. 38.
137. A More Common Use. A more common
kind of wedge consists of two inclined 'planes
united at their bases.
Such wedges are used in splitting timber,
stone, etc. The power is given in repeated
blows instead of continued pressure. No
definite law of any practical value can be
given for the wedge, further than that, with
a given thickness, the longer the wedge the
greater the gain in intensity of power.
FIG. 39.
138. What is a Screw ? A screw is
a cylinder, generally of wood or nwfal, with
NATURAL PHILOSOPHY.
138
spiral groove or ridge winding about its cir*
cumference.
The spiral ridge is called the thread of the screw.
The thread works in a nut, within which there is a
corresponding spiral groove to receive the thread.
(a.) The power is used to
turn the screw within a fixed
nut, N, or to turn the nut
about a fixed screw. In either
case, a lever or wheel is gen-
erally used to aid the power.
Every turn of the screw or nut
either pushes forward the
screw or draws back the nut
by exactly the distance be-
tween two turns of the thread,
this distance being measured,
in the direction of F W, the
axis of the screw. The weight
or resistance at W is moved this distance, while the power at P
moves over the circumference of a circle whose radius is P F.
FIG. 40.
(6.) The circumference of the circle, or the distance that the
power moves while the weight is being moved the distance
between two turns of the same thread of the
screw, may be found by multiplying two times
PF (the diameter of the circle) by 3.1416.
See Appendix A. The difference between
these two distances is generally very great.
Hence, this machine affords great intensity of
power with a corresponding loss of velocity.
(c.) Figure 41 shows that the screw is only
a modified inclined plane.
FIG.
;<j.i THE SCREW. 89
139. Law of the Screw. With the screw, a
given power will support a weight as many
times as great as itself as the circumference
travelled over by the power is times as great as
the distance between two adjoining turns of the
thread.
140. Compound Machines. We have now con-
sidered each of the six simple machines. One of these
may be made to act upon another of the same kind,
as in the case of the compound lever or wheel-work ;
or upon another of a different kind, as in the case of
the endless sciew.
When any two or more of tfiese machines are com-
bined, the effective force may be found by computing
the effect of each separately and then compounding
them; or by finding the weight that the given power
will support, using the first machine alone, considering
the result as a new power acting upon the second ma-
chine, and so on.
141. An Example. A horse is harnessed to the end
of a capstan bar (Fig. 29) at a distance of 5 feet from
I,bo centre of the capstan barrel which is 15 inches in
diameter. The rope wound upon the capstan barrel
belongs to a system of tv.'o fixed and three movable
pulleys. If the horse exerts a force of 500 pounds and
we allow 25 per cent, for loss by friction, what force
will be exerted upon the building which is being thus
qaoved?
90 NATURAL PHILOSOPHY. 141
Solution. The horse travels over the circumference of
a circle 10 feet in diameter. In the mean time, the
capstan will wind up a length of rope equal to the cir-
cumference of a circle 15 inches (1 foot) in diameter.
As the diameter or circumference of this larger circle is
eight times as great as the diameter or circumference of
the capstan barrel, the power moves eight times as far
and eight times as fast as the weight does. Therefore
the force exerted by the horse will be increased eightfold
and the end of the rope will be pulled with a force eight
times 500 pounds, or 4,000 pounds. This increased in-
tensity of effort is the effect of the capstan alone.
By drawing a sketch or diagram of the pulleys, it will
be seen that the fixed end of the rope must be attached
to the fixed block and that there will be six parts of the
rope acting upon the movable block and, thus, upon the
weight. Then the pulley will increase the effect of the
capstan sixfold.
4,000 Ib. x 6 = 24,000 Ib.
Deduct i for loss by friction, 6,000 Ib.
And we have left 18,000 Ib.
8 the force exerted by the compound machine.
(a.) The solution may be simplified as follows .
100 3
JL *- X -l*-*-6 x = 18,000 Ib.
144 FRICTION. 91
142. What is Friction ? Friction is the re-
sistance which a moving body meets from the
surface on which it moves.
143. The Cause of Friction. It is impossible, by
any known means, to produce a perfectly smooth sur-
face. Even a polished surface contains minute projec-
tions which fit into corresponding depressions on the
opposing surface. To produce motion of one surface
on the other, these projections must be lifted out,
bent down or broken off.
Friction is generally lessened by polishing and lubri-
cating the surfaces that move upon each other and often
by making the two bodies of different material. The
axles of railway curs are made of steel, the boxes in which
they turn are made of brass, the surfaces are made smooth
and kept oiled. In spite of all of these precautions, the
axle often becomes heated by friction to such an extent
as to render it necessary to stop the train.
144. Friction is a Transformer of Energy.
Friction always develops heat : in other words, it con-
verts mechanical energy into a familiar form of mo-
lecular energy. Whenever we find a loss of power
through friction, we should bear in mind that the miss-
ing energy has not been destroyed. It has simply been
transformed and still exists somewhere in the form of
heat.
Energy, as well as matter, is continually
changing its form but it can not be destroyed
92
NATURAL PHILOSOPHY.
145
145. Recapitulation. To be amplified by the pupil
for review.
INCLINED PLANE.
(" DEFINITION.
[ LAW.
WEDGE.
DEFINITION.
TWO USES.
SCREW.
DEFINITION.
LAW.
COMPOUND MACHINES ; RELATION OF p TO
FRICTION.
DEFINITION.
CAUSE.
REMEDY.
[BFFBCT
145 EXERCISES. 93
EXERCISES.
1. A boy who can lift only 100 pounds wishes to put a barrel ol
flour (196 pounds) into a wagon box 5 feet above the ground. He
backs the wagon to one end of a plank 20 feet long and weighing
125 pounds. Show that he can, without help, use the plank as an
inclined plane for his purpose and state how much force he exerts
(a.) in getting the plank into position and (6.) how much in lifting
the flour? Ans. (a.) 62 lb.; (6.) 49 Ib.
2. How long must an inclined plane be that a force of 20 pounds
may support a weight of 60 pounds, one end of the plane being
10 feet higher than the other end ?
3. With apparatus arranged as shown in Fig. 33, the spring
balance reads 15 pounds. What is the value of Wt
4. A given screw has 4 threads to the inch. It is worked by a
power that moves around a circle of 40 inches circumference. I
wish to exert a pressure of 1,600 kilograms. How much power
must 1 use ? Ans. 10 Kg.
5. With the screw described in the last exercise, what pressure
can be exerted by a power of 30 pounds, allowing -fa of the same
for friction 1 Ant. 4,320 lb.
94 NATURAL PHILOSOPHY. 145
REVIEW QUESTIONS.
1. How does the rising of a man in a small boat affect the st
bility of the boat ? Why ?
2. Why does a hand saw become warm when it is used?
3. On what two things does momentum depend ?
4. Define the term machine.
5. What advantage is gained by the use of a fixed pulley ?
6. What is the difference between adhesion and cohesion.
7. Tell what property of matter is affirmed in the declaration
of " The Cloud " :
' I pass through the pores of the ocean and shores,
I change but I can not die."
8. (a.) What two studies constitute physical science? (6.) Of
what two subjects do they treat ?
9. State the relative positions of the point of support and the
centre of gravity in each of the three kinds r>r equilibrium.
10. Explain why a "running jump" if onger than a "stand-
ing" one.
11. (a.) Which is the more valuable, a lump of gold weighing,
on a spring balance, one pound at the surface of the earth or a
lump that would similarly weigh as much 1,000 miles above the
surface ? (6.) Why would not a lever balance (Fig. 23) answer for
the comparison as well as a spring balance ?
12. If the molecules of your body do not touch one another,
why is it that the wind does aot blow you away in the form of
fine dust (
13 What force must be overcome in order to scratch a sub-
stance?
14. Dip a glass rod into mercury and tell which is the stronger,
the adhesion of the liquid for the rod or T,he cohesion of the liquid
molecules ?
145 REVIEW QUESTIONS. 9$
15. Show that in lifting at the end of the plank mentioned in
Exercise 1, page 93, the plank represented a lever of the second
class, and indicate the positions of P, F and W.
16. (a.) When water is poured from a jar, it often runs down
the inclined side of the vessel instead of falling vertically. What
force draws the falling water from a vertical course? (b.) Can
you suggest a way to prevent such a result ?
17. If a stone weighs 10 pounds at the level of the ocean, how
much will its weight measure, by a spring balance, 1,000 miles
nearer the centre of the earth ?
18. Why is it easier to roll a sphere than a cube ?
19. What is a foot-pound ? A kilogram-metei ? A horse-
power?
20. Find the kinetic energy of a 100-pound bal) owiutf with
a velocity of 2,000 feet a second.
CHAPTER
LIQUIDS.
SECTION I.
LIQUID PRESSURE.
experiment 44. Fill a small bottle with water, hold a
Prince Rupert drop in its mouth and
break off the tapering end of the
" drop." The whole " drop " will be
instantly shattered and the force oi
the concussion transmitted in every
direction to the bottle which will be
thus broken.
These "drops" are not expensive
and may be obtained from JAMES W.
QUEEN & Co., Philadelphia.
146. Transmission of
Pressure. Fluids transmit
pressure in every direction,
upward, downward, and
sidewise at the same time.
(a.) This property of liquids may
>o be illustrated by the apparatus repre-
sented in Fig. 42. The globe and
cylinder being filled with water and the several openings in the
globe closed by corks, a piston is pushed down the cylinder. The
pressure thus receded and transmitted by the confined watei
LIQUID PRESSURE.
expels the cork and throws a jet of water from each aperture and
not merely from the one opposite the piston.
(6.) Figure 43 represents a corked bottle of
water. When the cork is forced downward, it
exerts pressure upon all the water molecules in
contact with it. These transmit the pressure to
every part of the inner surface of the glass. Every
part of this surface equal in area to that of the
nd of the cork will be subjected to a pressure
like that exerted by the cork. The riressure acts
in a direction perpendicular to the surface of the
gla^s, as shown by the arrows in the figure.
(c.) It must be remembered that fluids include
both aeriform and liquid bodies. Aeriform bodies
are largely compressible; liquids are nearly in-
compressible.
147. Pascal's Principle. Wlien fluids are sub-
jected to pressure, the
pressure sustained by any
part of the restraining
surface is perpendicular
to it and proportional to
its area.
This may be shown by ex-
periment as follows :
Provide two communicat-
ing tubes of unequal sectional
area. When water is pourecl
into these, it will stand at
the same height in both tubes. If the water in the
smaller tube be subjected to pressure by means of a
5
FIG. 44
98 NATURAL PHILOSOPHY. 147
piston, the water will be forced back into the larger tube.
To prevent this result, a piston must be fitted to the
larger tube and held there with a force as many times
as great as the force acting upon the other piston as the
area of the larger piston is times as great as the area of
the smaller one.
If, for example, the smaller piston have an area of 1
square inch and the larger piston an area of 16 square
inches, a weight of 1 kilogram or ounce may be made to
support a weight of 16 kilograms or ounces. Of course,
the weight here referred to includes the weight of the
piston itself in each case.
(a.) It is evident that in this experi-
ment it will be difficult to get the pistons
to work without considerable friction.
For this reason, the experiment is some-
times modified and simplified by filling
the lower part of the tubes with mer-
cury, which will stand at the same level
in both arms. If water be poured into
the smaller tube it will depress the
mercury surface in that tube ; 16 times
the weight of water must be poured into
the larger tube to restore the two mer-
cury surfaces to the same level.
148. Pascal's Experiment.
Pascal firmly fixed a very nar-
row tube about 30 feet high into
the head of a stout cask. He
then filled the cask and tube with
FIG. 45. water. The weight of tb; small
amount of water in the tube actually burst the cask.
ISO LIQUID P".
149. The Hydrostatic Bellows. The hydro-
static bellows consists of two boards fastened to-
gether by a broad band of stout leather
and a small vertical tube communicat-
ing with the interior.
In the figure, the tube that bears the funnel
is to be joined to the tube passing upward
from b. This vertical tube may be filled with
water and the pressure thus exerted will lift
a heavy weight (e. g., several boys) placed
upon B.
If the 'board, B, has a
surface of 66 square inches
exposed to the upward
pressure of the water in the
bellows and the tube have a
sectional area of \ square FIG. 46.
inch, every pound of water in the tube will support 264
pounds at B.
150. The Hydrostatic Press. The hydrostatic
press acts upon the same principle. It is represented in
perspective by Fig. 47 and in section by Fig. 48. Pres-
sure is produced by the" force- pump A. The substance
to be pressed is placed between K, the head of the
piston, and an immovable plate M N. The reservoir, B,
uiid.the cylinder of the pump, are connected by the tube
d. By the action of the pump, the water in the cylinder
A is subjected to pressure and this pressure is trans-
mitted undiminished to the water in B. The piston, '
100
XATVRAL PHILOSOPHY.
150
is generally worked by a lever of the second class, result-
ing in a still further gain of intensity of power.
If the power-arm of the lever be ten times as long
as the weight-arm, a power of 40 pounds at the end of
FIG. 47.
the lever will exert a pressure of 400 pounds upon the
water in A.
If the piston in A have a sectional area of 1 square
inch and the piston in B have an area of 500 square
LIQUID PRESSURE.
101
inches, then the pressure of 400 pounds exerted by the
small piston will produce a pressure of 400 pounds x
500 or 200,000 pounds upon the lower surface of the
large piston. Hence the following rule :
Multiply the pressure exerted by the piston of
the pump by the ratio between the sectional
areas of the two pistons.
FIG. 48.
Experiment 45. Over the opening of a wide mouthed bottle
or fruit jar, tie a piece of thin sheet rubber. Hold the bottle in a
tub of water with the mouth downward, sidewise and upward.
In each case, the pressure of the water will force the rubber inward.
Try the experiment at different depths of water and notice that
the pressure will vary with, the depth.
151. Liquid Pressure Due to Gravity. The
pressure exerted by liquids, on account of their weight,
may be downward, upward, or lateral. We shall now
briefly consider these three kinds of liquid pressure,
102
NATURAL PHILOSOPHY.
152
Experiment 46. Into a bent glass tube, ABC, pour mercury
(quicksilver) until it covers the bend and rises two
or three inches above it. The mercury will stand
at the same level, a c, in the two branches of the
B tube. If pressure of any kind be exerted upon the
surface of the mercury at a, it will be promptly
shown by the movement of the mercury and a
consequent difference in the heights of the two
mercury columns. The greater the pressure, the
greater will be the elevation of c above the lvel
of a.
A common glass funnel or a piece of glass
tube may be joined to A by a short piece of
snugly fitting rubber tubing. Suppose the funnel
to be thus connected and the apparatus supported
by any convenient means in an upright position.
Pour water into the funnel and mark the level of
the water by a suspended weight or other means ;
mark the level of the mercury in the long arm of
i 11 the tube by a string or small elastic band.
Remove the funnel and connect the straight
FIG 49 glass tube above A. Fill this with water to
the same height as before, as indicated by the
suspended weight. Although the quantity of water used is much
less, the depression of the mercury at a and its elevation at c
will be the same as before, showing the downward pressure at a
to be the same in both cases Directions for bending glass will be
found in Chemistry, Appendix 4.
152. Downward Pressure. The pressure on the
bottom of a vessel containing a liquid depends
upon the depth and density of the liquid and
the area of the bottom.
The quantity of the liquid and the shape of the vessel
niake no difference,
154 LIQUID PRESSURE. 103
153. Rule for Downward Pressure. To find
the downward pressure on a horizontal surface,
find the weight of an imaginary column of the
given liquid, with a base the same as the given
surface and an altitude the same as the depth
of the given surface below the surface of the
liquid.
NOTE. A cubic foot of water weighs about 1,000 ounces, 62
pounds (more exactly 62.42 pounds).
154. Example. A cask has a base of 3 square feet
and a height of 2 feet. A tube is fitted into the upper
head. The cask is filled with water ; more water is then
added until the tube is filled to a height of 3 feet above
the upper head. What is the pressure on the lower head
of the cask ?
Solution. Our "imaginary column" of water has a
base of 3 square feet and a height of 5 feet. It therefore
has a volume of 15 cubic feet. The weight of the
"imaginary column" of water would be 15 times ^2.42
pounds or 936.3 pounds. This is the pressure exerted
upon the lower end of the cask.
Although the quantity of water actually used is less
than half that of our "imaginary column," the pres-
sure exerted by it is the same as explained in 152.
If a liquid like alcohol had been used, the pressure would
have been only about -fa of 936.3 pounds, for alcohol is
only about -^ as heavy as water. If the liquid used had
been mercury, which is 13.6 times as heavy as water, the
pressure would have been 13.6 times 936.3 pounds or
12,733.68 pounds more than six tons.
104 NATURAL PHILOSOPHY. *55
NOTE. In all such cases the pupil may be allowed to use 62|
pounds as the weight of a cubic foot of water if he prefers to do
BO. That value (1,000 ounces) is more easily remembered.
Experiment 47. Make a small hole in the bottom of a tin fruit
ean or similar vessel. Push the can downward into water until
the open mouth of the can is " near the water's edge." The liquid
Will spurt upward through the hole in a little jet.
155. Rule for Upward Pressure. To find the
upward pressure on any horizontal surface, find
the weight of an imaginary column of the given
liquid with a base the same as the given sur-
face and an altitude the same as the depth of
the given surface ' below the surface of the
liquid.
156. Example. What will be the upward pressure
on a horizontal plate a foot square when placed 10 feet
beneath the surface of water ?
Solution. The volume of our "imaginary column"
of water is 10 cubic feet. It would weigh 10 times
62.42 pounds or 624.2 pounds. Consequently, the up-
ward pressure would be 624.2 pounds.
157. Rule for Lateral Pressure. To find the
pressure upon any vertical surface, find the
weight of an imaginary column of the liquid
with a base equal to the given surface and an
altitude the same as the depth of the centre of
the given surface below the surface of the
liquid.
159 LIQUID PRESSURE. 105
158. Example. A dam is 25 feet high and 30 feet
long. Water stands at a height of 20 feet on one side
of the dam. What is the liquid pressure on the dam ?
Solution. The dam may be vertical or sloping, but in
either case, the surface that we have to consider is 20 feet
high and 30 feet long. The depth of its centre below
the surface of the water is 10 feet. Our "imaginary
column " will, therefore, have a volume of 6,000 cubic
feet and a weight of 374,520 pounds. The pressure will,
therefore, be 374,520 pounds.
159. Recapitulation. To be amplified by the pupil
for review.
TRANSMITTED EQUALLY IN ALL DIRECTIONS.
PASCAL'S EXPERIMENT.
EFFECT ON RESTRAINING SURFACE. \ HYDROSTATIC BELLOWS
HYDROSTATIC PRESS.
f DOWNWARD.
PRODUCED By GRAVITY J UPWARD.
I LATERAL.
106 NATURAL PHILOSOPHY. 159
EXERCISES.
1. Referring to Fig. 48, suppose the area of the end of piston, a,
to be 1 square inch and that of piston, C, to be 10,000 square
jaches and the two arms of the lever, 0, to be 1 foot and 10 feet
respectively, what pressure at K will be produced by a pressure
of 100 pounds by the operator? Am. 10,000,000 Ib.
2. Where will water issue from a water pipe with the greater
force, in the basement or near the top of a house ? Why ?
3. Find the pressure on a dam 25 feet long, the water being 10
feet deep ? Am. 78025 Ib.
4. A tank has a base 6 feet by 8 feet. What is the pressure on
that base when the water in the tank is 4 feet deep ?
Ans. 11984.64 Ib.
5. A tank has a base 2 meters by 3 meters. What is the pres
sure on that base when the water in the tank is 1.5 meters deep ?
Ant. 9000 Kg.
l6l EQUILIBRIUM. 107
SECTION II.
EQUILIBRIUM. BUOYANCY.
160. Equilibrium of Liquids. A liquid of small
surface is said to be level when all the points of its sur-
face are in the same horizontal plane. The central idea
is expressed in the familiar saying, water seeks Us
level. This is true whether the liquid be placed in a
single vessel or in several vessels that communicate with
each other.
Experiment 48. Incline a tea-pot that is nearly full of water
until the liquid begins to run out at the spout. Notice that the
end of the spout and the water surface in the vessel are at
the same level.
161. Communicating Vessels. When a liquid is
placed in one or more of several vessels communicating
with each other, it will not come to rest until it
stands at the same height in all of the vessels.
Experiment 49. From one end of a scale-beam, suspend a
cylindrical bucket of metal, b, and below that a solid cylinder, a,
winch accurately fits into the bucket. Counteqwise with weights
in the opposite scale-pan. Then place a vessel of water, as shown
in Fig. 50, so as to immerse and the counterpoise will descend,
showing that a has lost some of its weight. Carefully fill 6 with
water. It will hold exactly the quantity displaced by a. Equili-
brium will be restored. The bucket and cylinder may be had
of dealers in philosophical apparatus.
108
NATURAL PHILOSOPHY.
i6i
Experiment 50. Insert a short spout in the side of a vessel (as
tin fruit-can) about an inch below the top. Fill the vessel with
water and let all above the level of the spout escape. This is to
replace the vesrel of water in which a (Fig. 50) is immersed.
Instead of the bucket, ft, use a cup placed on the scale-pan. Instead
i>f a, use any convenient solid heavier than water, as the fragment
of a stone, which may be suspended by a horse-hair or a fine
thread. Counterpoise the cup and stone in the air. Immerse the
FIG. 50.
stone in the water and catch, in any convenient vessel, every drop
of water that overflows. This will be the fluid that the solid
displaces. The equilibrium is destroyed, but may be restored by
pouring the water just caught into the cup on the scale pan. It
may sometimes be necessary to make allowance for the small
quantity of water that adheres to the cup in which the overflow
was caught, but with a good balance and good work the result will
clearly show the truth of Ardiiinetfw? Principle.
163 BUOYANCY. 109
162. Archimedes' Principle. It is a familiar fact
;hat a person may easily raise to the surface of the water
a stone which he can not lift any further. When an arm
or leg is lifted out of the water of a bath-tub, it suddenly
feels heavier. Many such facts are explained by the
important principle here given :
The loss of weight of a body immersed in a
fluid equals the weight of the fluid which it
displaces.
Experiment 51. Place the tin vessel with a spout, mentioned
in Experiment 50, upon one scale-pan, and fill it with water, some
of which will overflow through the spout. When the spout has
ceased dripping, counterpoise the vessel of water with weights in
the other scale-pan. Place a floating body on the water. This
will destroy the equilibrium but water will overflow through the
epout until the equilibrium is restored. This shows that the
floating body has displaced its own weight of water.
Experiment 52. Place a fresh egg in a vessel of fresh water;
it is a little heavier than the water and will sink. Place it in salt
water ; it is a little lighter than the brine and will float. Carefully
jiour the fresh water on the salt water in a tall, narrow vessel like
that shown in Fig. 162. Place the egg in
the water; it will descend until it reaches
a layer of the liquid with a density like its
own and there it will float.
163. Floating Bodies. A
floating body displaces its own
weight of the fluid on which U
floats.
FIG. 61. Th - g may be g ] lown experimentally
by filling a vase with water. When a floating body is
110 NATURAL PHILOSOPHY. 1 63
placed on the surface, the water displaced will overflow
and may be caught. The water thus caught will weigh
the same as the floating body.
(a.) If a body placed on the surface of a fluid can not displace
its own weight of the fluid, it will sink and not float. The buoy-
ant effect of the fluid is then less than the weight of the body.
Sometimes a heavy substance has such a shape that it will dis-
place enough of a lighter fluid to float thereon. For this reason,
an iron kettle or an iron ship will float on water, although iron is
several times heavier than water. In all such cases, the buoy-
ant effect of the fluid equals the weight of the floating body,
(6.) Just as the gravity of a body may be considered as acting
upon a single point called the centre of gravity, so the buoyant
effort of a fluid may be considered as act
ing upon a single point called the centre
of buoyancy. The centre of buoyancy
is situated at the centre of gravity of
the displaced fluid.
(c.) The centre of buoyancy may be
considered the point of support of the
floating body and the principles of 65- FlG - 53 -
C8 applied anew. For example, if the
boat's centre of buoyancy be at C and its centre of gravity b
at b (as it may be if its occupant is sitting down), the boat
will be in stable equilibrium, while if its centre of gravity be at
B (as it may be if its occupant is standing up), the boat will b
in dangerously unstable equilibrium.
RECAPTTULA r/O.V.
Ill
164. Recapitulation. To be amplified by the pupil
for review.
(XI
g al
I I
11
H
11.3 NATURAL PHILOSOPHY. <j 164
EXERCISES.
1. Where are water pipes of uniform strength connected with
the same reservoir more likely to burst, on a hill or in a valley 1
Why?
2. What weight of water will a 50-pound canoe displace?
8. What weight of water will a cubic foot of iron displace ?
4. How much weight will a cubic foot of lead lose when it is
placed in water ?
5. Do you know anything about the personal history of Arch\-
medes?
6. How much water will a board displace, (a.) when it floats on
the surface ? (6.) When it is held under the surface?
7. When is a boat in stable equilibrium ?
8. In lifting a pail of water from a cistern, it does not seem
ieavy until it is raised out of the water. Why is this ?
9. Why is it difficult to stand in water reaching to your chin ?
107 SPECIftC GRAVITY. 113
SECTION III,
SPECIFIC GRAVITY.
165. What is Specific Gravity ? The specific
gravity of a body is tlie ratio between its weight
and the weight of a like volume of some other
substance taken as a standard.
It is an abstract number and shows how many timei
the weight of a body will contain the weight of the same
volume of some other substance that is taken as a
standard.
166. Standards of Specific Gravity. For solids
und liquids, the standard adopted is distilled
water at a temperature of Jf @-> or 39.2 F.
For aeriform bodies, the standard is air or
hydrogen.
167. Elements of the Problem. For solids or
liquids, the dividend is the weight of the given
body ; the divisor is the weight of the same bulk
of water; the quotient, which is an abstract
nuniber, is the specific gravity.
The weight of the same bulk of water is found some-
times in one way and sometimes in another but, in every
case, it is tlie divisor. By grasping and keeping thi?
idea, you will avoid much possible confusion. Of course,
when any two of these three are given, the third can be
found.
114
NATURAL PHILOSOPHY.
168
168. To Find the Specific Gravity of Solids.
The most common method of finding the specific gravity
of a solid heavier than water, is to find the weight of
the body in the air (= W), then suspend the body by a
light thread and find its weight in water (= W), and
divide the weight of the body in air by the weight of the
same bulk of water ( 162, Archimedes' Principle).
Sp. Or.^
(a.) The method is illustrated by the following example ;
Weight of substance in air = 58 oz.
Weight of substance in water = 51 oz. (Fig. 54).
Weight of equal bulk of water
Specific gravity 58 1 oz. -*- 7| oz.
170 SPECIFIC GRAVITY. 115
169. To Find the Specific Gravity of Liquids.
The principle is unchanged. A sirhple method is as
follows:
Weigh a flask first empty; next, full of water; then,
full of the given liquid. Subtract the weight of the
empty flask from the other two weights ; the results
represent the weights of equal volumes of the given
substance and of the standard. Divide as before.
(a.) A flask of known weight, graduated to measure 100 or 1,000
grams or grains of water is called a specific gravity fla&k. Its use
avoids the first and second weighings above mentioned and simpli-
fies the work of division.
(6.) The specific gravity of a liquid may he easily determined
as follows :
Find the loss of weight of any insoluble solid in water and in
the given liquid. Remember ( 162) what these two losses repre.
sent. Di vide as before. The solid used is called a specific gravity
bulb.
(c.) The determination of the specific gravity of gases presents
many practical difficulties which can not be considered in thla
place.
170. Recapitulation. To be amplified by the pupiJ
for review.
DEFINITION.
_ STANDARDS.
SPECIFIC
DIVIDEND AND DIVISOR.
GRAVITY.
OF SOLIDS.
( SPECIFIC GRAV
OF LIQUIDS \
( SPECIFIC GRAV
vrrv FLASK
ITY BULB.
116 NATURAL PHILOSOPHY. 170
EXERCISES.
1. A body weighs 150 pounds in air and 100 pounds in water,
What is its specific gravity ?
2. A body weighs 75 ounces in air and 60 ounces in water.
What is the weight of an equal bulk of water?
3. Lead has a specific gravity of about 11 J. Mercury is a liquid
having a specific gravity of about luf . Will lead sink in mercury?
4. Sulphuric acid has a specific gravity of 1.8. Will a glass
ball lose more weight in the acid than it will in water ?
5. In which will a body weigh more, in fresh water having a
specific gravity of 1 or in sea water having a specific gravity of
6. Is it, then, easier for a person to float in fresh water than it
JB in sea water ?
7. A piece of brass weighing 41.9 ounces was placed in a vessel
full of water. The overflowing water was caught and found to
weigh 5 ounces. What is the specific gravity of brass ?
An*. 8.38
8. A liter bottle holds 1,000 grams of water or 800 grams of
alcohol. What is the specific gravity of alcohol ?
9. Let each pupil experimentally determine the specific gravity
of some solid that will sink in water.
174 HTDROKINETIC& 117
SECTION IV.
HYDROKINETICS.
171. The Flow of Liquids through Horizontal
Pipes. When liquids from a reservoir are made to flow
through pipes of considerable length, the discharge is
far less than that due to the head. The diminution is
chiefly owing to friction against the sides of the pipe.
The "head" is the vertical distance from the centre
of the orifice to the surface of the liquid.
172. The Flow of Rivers. The friction of a
stream against its solid bed fortunately retards the ve-
locity of the water. Otherwise the velocity of the cur-
rent at the mouth of a river, whose head is elevated
1,000 feet above its mouth, would be about 170 miles per
hour. Such a current would be disastrous beyond de-
scription. The ordinary river current is from three to
five miles per hour.
173. Water-power. Water may be used to turn a
wheel and thus move machinery by its weight, the force
of the current or both.
174. The Turbine Wheel. The turbine wheel, of
which there are many varieties, is the most effective
water-wheel known.
118
NATURAL PHILOSOPHY.
174
FIG. 54.
(a.) Figure 54 represents one form in perspective and in hori-
zontal section through the centre of the wheel and case complete.
The wheel, B, and the enclosing case, D, are placed on the floor of
a penstock wholly submerged in water, under the pressure of a
considerable head. The water enters, as shown by the arrows,
through openings in D, which are BO constructed that it strikes
the buckets of B in the direction of greatest efficiency.
(b. After leaving the buckets, the " dead water " escapes from
the central part of the wheel, sometimes by a vertical draft tube,
best made of boiler-iron. The weight of the water in this tube
increases the velocity with which the water strikes the buckets.
(c.) A central shaft, A, is carried by the wheel and communi
cates its motion to the machinery above. The wheel itself rests
upon a central pivot carried by cross-arms from the bottom of the
outer case. The case, D, is covered with a top, T, which protects
the wheel from the vertical pressure of the water. The axis of
the wheel passes through the centre of this cover.
175. The Overshot Wheel. In the overshot
wheel, the water falls into buckets at the top and, bv
HTDK O KINETICS.
119
its weight aided by the force of the current, turns the
wheel. As the buckets are gradually inverted, the water
is emptied aud the load thus removed from the other
side of the wheel.
PIG. 55.
Such wheels require not much water but a consider-
able fall. It is said that they have been made nearly
100 feet in diameter. The water is led to the top of
the wheel by a sluice, 8 V. The power may be com-
municated to the machinery by the shaft, A, or by the
rim of the wheel, as at c, according to the desire for
intensity of power or for velocity. The water supply
may be controlled by a gate at F.
176. The Breast Wheel. In the breast wheel,
the water acts upon float boards fixed perpendicular to
the circumference. The stream being received at or
near the level of the axis, both the weight of the water
and the force of the current may be turned to account.
120
NATURAL PHILOSOPHY.
177
FIG. 56.
177. The Undershot Wheel. In the undershot
wheel, the stream strikes, near the bottom of the wheel,
against a few float boards. It is turned by the force of
the current.
FIG. 57.
NOTE. In point of efficiency, these wheels rank in the ordei
ibove given,
177
REVIEW QUESTIONS.
121
REVIEW QUESTIONS.
1. In what direction do liquids at rest exert pressure?
2. Upon what three things does the pressure of a liquid on the
bottom of the vessel holding it depend ?
3. How may a little water be made to exert a great pressure ?
4. Show two ways in which a small weight may be made to
balance a heavy one?
5. State two points of difference between solids and liquids?
6. If a pendulum of given length swings through an arc five
inches long in one second, how long will it take the same pendulum
to swing through an arc of ten inches ?
7. Show that the appara-
tus represented in Fig. 58
is a lever, tall of what class
it is and indicate the posi-
tions for P, W and F.
8. Define malleability.
9. In what direction do fluids transmit pressure with the great-
est facility?
10. State the principle that underlies the action of the hydro-
static press ?
11. What was Pascal's experiment? Find out who Pascal was.
12. It is stated that the specific gravity of silver
is about ten. This means ten of what f
13. What is the velocity of an ordinary river
current ?
14. Describe the hydrostatic bellows. What is
meant by the centre of buoyancy ?
15. Describe the use of the specific gravity bulb.
16. Experiment shows that it is difficult to
balance a lead pencil on the finger. Why is it
e:isier thus to balance it with the additional load
of two penknives, as shown in Fig. 59 ?
- 58 -
FIG. 59.
CHAPTER V.
PNEUMATICS.
SECTION I.
THE ATMOSPHERE AND ATMOSPHERIC
PRESSURE.
178. What is Pneumatics ? Pneumatics is
that branch of Physics which treats of aeri-
form bodies, their mechanical properties and
the machines by which they are used.
179. Tension of Gases. However small their
quantity, gases always fill the vessels in which
they are held.
If a bladder or India rubber bag,
partly filled with air and having
the opening well closed, be placed
under the receiver of an air-pump
( 187), the bladder or bag will be
fully distended, as shown in the
figure, when the air surrounding
the bladder is pumped out.
The flexible walls are pushed out
by the air confined within. This
tendency is called elastic force or tension,.
FIG. 60.
l8l ATMOSPHERIC PRESSURE. 123
We may imagine the countless molecules of air in the
bladder or bag as being in constant motion and continu-
ally striking against the walls that confine them. These
molecular blows push the walls outward and their total
effect constitutes tension.
This conception is generally referred to as the
Kinetic Ttieory of Gases ( 43 ; a).
180. The Type. As water was, for obvious reasons,
taken as the type of liquids, so atmospheric air will
be taken as the type of aeriform bodies.
Whatever mechanical properties are shown as be-
longing to air may be understood as belonging to all
gases.
181. Weight of Air. Being a form of matter, air
has weight. This may be shown by experiment. A
hollow globe of glass or metal, having a capacity of sev-
eral quarts or liters and provided with a stop-cock, is
carefully weighed on a delicate balance. The air is then
removed from the globe by an air-pump, the stop-cock
closed and the empty globe weighed carefully. The
second weight will be less than the first, the differ-
ence between the two being the weight of the air
removed.
(a.) Under ordinary conditions, a cubic inch of air weighs about
0.31 grains. A liter of air weighs about 1.293 grams, being thus
about T ^5 as heavy as water.
(6.) Measure the length, breadth and height of your school-
room and find the weight of the air that it contains.
124 NATURAL PHILOSOPHY. l8l
Experiment 53. Fill a tumbler with water, place a slip of
thick paper over its mouth and hold it
there while the tumbler is inverted : the
water will be supported when the hand
is removed from the card, as is shown in
Fig. 61.
Experiment 54. Plunge a small tube,
or a tube having a small opening at the
lower end, into water, cover the upper
end with the finger and lift it from its
bath. The water is kept in the tube F IO . 61.
by the upward pressure of the atmos-
phere. Remove the finger and the downward pressure of the at-
mosphere, which was previously cut off, will counterbalance the
upward pressure and the water will fall by its own weight. Such
a tube, called a pipette, is much used for transferring small quan-
tities of liquids from one vessel to another.
Experiment 55. Make a "sucker" of a circular piece of thick
leather and fasten a string to its middle. Soak
it thoroughly in water and firmly press it upon
a flat stone to drive out all air from between the
leather and the stone. Pull the string gently so
that a vacuum may be formed, as shown in Fig.
62. If the stone be not too heavy, it may be
lifted by the string. We shall soon see that, in
reality, the stone is pushed up by the air
instead of being pulled up by the string.
Experiment 56. Suck the air from the hol-
low stem of a common key and quickly press the
FIG. 62. OP 6 " end of tne stem against the lip where it
will be held by the pressure of the air.
Experiment 57. For a few cents you can buy a four inch
test tube of a dealer in chemical glass ware (see Chemistry,
AfMOSPBERtC PRESSURE.
125
Appendix 7). Holding it by the open end, heat the tube quite hot
in the flame of a lamp or candle. When the heat has expanded
the air and driven part of it out of the tube, press the mouth of
the tube against the fleshy part of the hand or thumb where it
will be held by atmospheric pressure.
Experiment 58. Vary the last experiment by placing the
tube, mouth downward, in a saucer of water. Atmospheric
pressure will force water upward into the tube.
Experiment 59. Secure an empty tin fruit can with a hole
about two inches in diameter
in one end. Smoothly stretch
a piece of clean mosquito net-
ting over this end of the can,
as shown in Fig. 63. Fill
the can with water, place a
piece of writing paper over
the mosquito netting and
hold it there while you invert
the can. Draw the pa per hor-
Fio. 63.
meshes of the mosquito netting.
izontally from the end of the
can. Atmospheric pressure
will prevent the water from
running out through the
Experiment 60. With a small nail, punch a hole in the
middle of the closed end of the can used in the last experiment.
Repeat that experiment, keeping the nail-hole covered by the
forefinger, as shown in Fig. 63. Remove the finger for an instant,
quickly covering the hole again. When the atmosphere has a
chance to press downward through the nail-hole upon the water
in the can, the water runs out through the mosquito netting.
When this opportunity is removed by closing the nail hole, the
water is held in the can by the upward pressure of the at-
mosphere.
126 XATUtlAL PHILOSOPHY. l82
182. Atmospheric Pressure. The atmosphere
exerts a great pressure upon the surface of the earth and
all bodies found there. This atmospheric pressure de-
creases as we ascend from the earth's surface.
The weight of a column of air one inch square and
extending from the sea-level to the upper limit of the at-
mosphere is about fifteen pounds; a similar column, a
centimeter square, weighs about one kilogram.
We express this by saying that the atmospheric
pressure at the sea-level is fifteen pounds to the
square inch, or one kilogram to the square cen-
timeter.
(a.) Several illustrations of atmospheric pressure will be given
after we have considered the air-pump.
183. Torricelli's Experiment. The intensity of
atmospheric pressure may be measured as follows :
Take a glass tube a yard long, about a quarter of an
inch in internal diameter. Close one end and fill the
tube with mercury. Cover the other end with the
thumb or finger and invert the tube, placing the open
end in a bath of mercury. Upon removing the thumb,
the mercury will sink and come to rest at a height of
about 30 inches, or 7GO millimeters, above the level of
the mercury in the bath.
The apparatus used, when properly graduated, becomes
?. barometer.
184. Pascal's Experiments. Pascal repeated Tor-
ricelli's experiment on the top of a mountain and found
that the mercury column was three inches shorter, show-
186
ATMOSPHERIC PRESSURE.
127
ing that as the weight of the atmospheric column dimin
ishes, the supported column of mercury also diminishes.
He then took a tube forty feet long, closed at one end.
Having filled it with water, he inverted it over a water
bath. The water in the tube came to rest at a height
of 34 feet. The water column was 13.G times as high as
the mercury column, but as the specific gravity of mer-
cury is 13.6, the weights of the two columns were equaL.
Experiments with still other liquids gave correspond-
ing results, all of which strengthened the theory that
the supporting force is due to the weight of the atmos-
phere and left no doubt as to its correctness.
185. Pressure Measured in Atmospheres. A
gas or liquid which exerts a force of 15 pounds upon a
square inch or one kilogram upon a square centimeter of
the restraining surface is said to exert a pressure of one
atmosphere. A pressure of 60 pounds to the square
inch, or 4 kilograms to the square centimeter would bo
called a pressure of 4 atmospheres.
186. Recapitulation. To be amplified by the pupil
for review.
DEFINITION.
TENSION.
TYPE IS AIR.
PRESSURE.
Per square inch.
Per square centimeter.
Measured in atntospkeru.
Torricelli.
Paseat.
128 NATURAL PHILOSOPHY. l86
EXERCISES.
1. The downward pressure of the atmosphere on the hottom of
an ordinary wooden pail is about a ton. Why is not the bottom
forced out and how can any person carry the pail ?
2. Suppose a bottle to be tightly corked at the top of a high
mountain, carried to the sea-level and opened with its mouth under
water. Would air bubble out or water rush in ?
3. What is the pressure on one side of a window 2 feet by
6 feet? AM. 25920 Ib.
4. What is the pressure on one side of a door 1 meter by 2
meters ?
5. Why is a barometer tube closed at the top ?
6. Why is a barometer tube not closed at the bottom ?
7. When the barometer stands at 28 inches, at what height
would the water in the tube of Pascal's experiment come to rest ?
8. A steam-boiler was tested " at a pressure of 10 atmospheres/
What does this mean?
187
AIR-PUMPS.
SECTION II.
AIR-PUMPS. LIFTING AND FORCE-PUMPS.
SIPHON.
187. The Air-Pump. The air-pump is an in-
strument for removing air from a closed vessel.
The essential parts are shown in section by Fig. 64.
FIG. 64.
The vessel, R, is called a receiver. It fits accurately
upon a horizontal plate, through the centre of which is
an opening communicating with a cylinder, C, by means
of a bent tube, t. An accurately fitting piston moves in
the cylinder. At the junction of the bent tube with the
cylinder and in the piston, are two valves, v and v',
130
NATURAL PHILOSOPHY.
187
opening from the receiver but not toward it. When the
piston is raised, v' closes and the atmospheric pressure
is removed from v. The tension of the air in R opens v.
The air which was in R and t expands and fills R, t and
C. When the piston is pushed down, v closes, v' opens
and the air in C escapes from the apparatus.
NOTE. A. person having an air-pump has the means of per-
forming almost numl>erless experiments, some amusing and all
instructive. A cheap and efficient air-pump may be bought of
JAMES W. QUEEN & Co., Philadelphia. Several experiments
with the air-pump are given below. Others will be found in
The Element* of Natural Philosophy.
Experiment 61. The hand-glass is a receiver open at both
ends. See that the lower end fits accurately
upon the plate of the air-pump. (It is well
to smear the plate with tallow in this and
similar experiments.) Place the hand over
the other end. When the pump is worked,
the pressure of the atmosphere is felt, and
the hand can be removed only by a consider-
able effort. The appearance of the palm of th? hand at the
end of this experiment is due to the
tension of the air within tie tissues
of the hand.
Experiment 62. Perform the experi-
ment described in 179.
Experiment 63. Ov:>r the upper end
of a cylindrical receiver, tie tightly a wet
bladder and allow it to dry. Then ex-
haust the air. The bladder will be
forced inward, bursting with a loud
noise. It may be necessary to prick a
FIG. 65.
FIG. 66
187 AtR-P0Mt>S. 131
pin-hole through the bladder after the receiver has been ex
hausted.
Experiment 64. Replace the bladder with a piece of thin india.
rubber cloth. Exhaust the air. The
cloth will be forced inward by atmos-
pheric pressure and nearly cover the
inner surface of the receiver.
NOTE. Ths hand-glass, used in Ex-
periment 61, will answer for the two
experiments last given, by placing the
small end upon the pump-plate.
Experiment 65. The "fountain in
racuo " consists of a glass vessel, through
the base of which passes a tube ter-
minating in a jet within, and provided
with a stop-cock and screw without.
By means of the screw, it may be at- -.
tached to the air-pump. Remove the
air, close the stop-cock, place the lower end of the tube in water
open the stop-stock ; a beautiful fountain will be
produced (Fig. 67).
Experiment 66. The Magdeburg hemispTieres
are made of metal (Fig. 68). They are hollow and
generally three or four inches in diameter. The
edges being greased and placed together, the air is
exhausted from the hollow globe through a tube
provided with a stop-cock and screw. When the
air has been pumped out, close the stop-cock and
remove the hemispheres from the pump. It will
l>e found that a considerable force is necessary to
poll the hemispheres asunder. This force is
equal to the atmospheric pressure upon the
circular area inclosed by the edges of the hemispheres. U
PHILOSOPHY.
188
ihis area be ten square inches, it will require a pull of 150 pounds
to separate the hemispheres.
Experiment 67. Partly fill two bottles with
water. Connect them by a bent tube which fits
closely into the mouth of one and loosely into the
mouth of the other. Place the bottles under the
receiver and exhaust the air. Water will be driven
from the closely stoppered bottle into the other.
Readmit air to the receiver and the water thus
driven over will be forced back.
FIG. 69.
188. The Condenser. TJie condenser is an in-
strument for compressing a large amount of gas
into a closed vessel. The chief difference between
it and the air-pump is that its valves open
toward the receiver.
189. The Lifting - pump.
The lifting-pump consists of a
cylinder or barrel, c, a piston, p,
two valves, v v, and a suction
pipe, s, the lower end of which
dips below the surface of the
liquid to be raised. The arrange-
ment is essentially the same as in
the air-pump.
As the piston is worked, the air
below it is gradually removed.
The downward pressure on the
liquid in the pipe being thus re- =2
moved, the pressure of the at- 0:
mosphere, exerted upon the
FIG. 70.
191
LIFTING A\D FORCE-PUMPS.
133
surface of the liquid, pushes the liquid up
through the suction pipe and the lower valve
into the barrel.
When the piston is again pressed down, the lower
valve closes, the reaction of the water opens the piston
valve and the piston sinks below the surface of the liquid
in the barrel. When next the piston is raised, it lifts
the water above it toward the spout of the pump. At
the same time, atmospheric pressure forces more liquid
through the suction pipe into the barrel.
190. Practical Points. The cistern or well con-
taining the liquid must not be cut off from atmospheric
pressure, i. e., must not be made air-tight. For water
pumps, the suction pipe must not be more than 34 feet
high. Owing to mechanical imper-
fections chiefly, the practical
limit of the water pump is
about 28 vertical feet.
191. The Force -Pump. In
the force-pump, the piston is often
made solid, *. e., without any valve.
The upper valve is placed in a dis-
charge pipe, d, which opens from
the barrel at or near its bottom.
When the piston is raised, water
is forced into the barrel by atmos-
pheric pressure. When the piston
is forced down, the suction pipe valve
is closed, the water being forced
FIG. 7t
134
NATURAL PHILOSOPHY.
192
through the other valve into the discharge pipe. When
the piston is raised again, the discharge pipe valve is
closed, preventing the return of the water above it, while
atmospheric pressure forces more water from below into
the barrel.
Sometimes the upper valve is placed in the piston, as
in the ordinary lifting-pump, the discharge pipe opening
from the upper part of the cylinder which is closed at
the top.
192. Direction of Valve Openings. In the case
of the air-pump, the condenser, the lifting and force-
pumps, the valves open in the direction in ivhich
the fluid is to move.
193. The
FIG. 72.
Air-Chamber of a Force-Pump.
Water will be thrown in spurts from
a pump like tli at represented in Fig.
71. A continuous flow is secured
by connecting the discharge
pipe with an air-chamber.
This air-chamber, c, is provided
with a delivery pipe, b or s, which
reaches below the surface of the wa-
ter in the air-chamber.
When water is forced into the air-
chamber, it covers the mouth of the
delivery pipe and compresses the air
confined in the chamber. This less-
ening of the volume of the air causes an increased ten-
194 THE SIPHON. 135
sion ( 179), which soon becomes sufficient to force the
water through the delivery pipe in a continuous stream.
A pump may have both delivery pipes but one of them
must be closed by a cock, as shown at *,
Experiment 68. Set a pail of water on the table. Place one
end of a piece of rubber tubing in the
water and let the other end hang over
the edge of the pail reaching below
the top of the table. Suck some
water through the tube. Water
will continue to flow until the pail is
emptied or the water surface falls
below the end of the tube in the pail.
194. The Siphon. Tlie
siphon consists of a bent
tube, open at both ends,
having one arm longer than the other.
It is used to transfer liquids from a higher to a lower
level, especially in cases where they are to be removed
without disturbing any sediment they may contain.
The siphon may be first filled with the liquid and then
placed with the shorter arm in the vessel, care being hud
that the liquid does not escape from the tube until the
opening, C, is lower than m n, the surface of the liquid ;
or it may be first placed in position and the air removed
by suction at the lower end.
The pressure of the atmosphere will force the liquid
up the shorter arm and fill the tube. In either case, a
constant stream of the liquid will flow from the vessel
136
NATURAL PHILOSOPHY.
195
until the surface line, m n, is brought as low as the
opening in the shorter arm or, if the liquid be received
in another vessel, until the level is the same in the two
(a.) The action of the siphon is explained in the author's larger
work. If the liquid to be transferred is water, the height, a b,
must be less than thirty-four feet.
195. Recapitulation. To be amplified by the pupil
for review.
PUMPS..
DEFINITION.
CONSTRUCTION.
AIR
LIMITATIONS.
EXPERIMENTS.
CONDENSER.
WATER.
! LIFTING.
FORCE.
Valve Openings.
THE SIPHON.
195
EXERCISES.
137
EXERCISES.
1. What is the total atmospheric pressure (in pounds) upon the
surface of a wooden cu be one inch on each edge ?
2. If mercury is 13.6 times as heavy as water, and a given
pump will lift water to the height of 28 feet, how high will the
same pump, other conditions being similar, lift the mercury ?
3. How can you arrange a single suction or lifting-pump to
raise water from the bottom of a well 50 feet deep ?
4. Construct the apparatus shown
in Fig. 74, filling each of the three
bottles half full of water. Blow in
the tube,/, until a jet is formed at n.
Explain the continued action of the
apparatus.
Be sure that all joints made by the
corks of the three bottles are air-tight.
For directions in working glass tubing,
see Chemistry, Appendix 4.
5. How many valves are there in
a force-pump? Where are they placed
and in what direction do they open ?
6. What is the thing sometimes
erroneously called "the force of suc-
tion"? FIG. 74.
7. The volume of a given quantity of r, gas or vapor will be
inversely proportional to the pressure exerted upon it. A cubic
foot of steam (measured under a pressure of one atmosphere) will
have what volume under a pressure of 8 atmospheres ?
An*. 216 cubic inches.
138 NATURAL PHILOSOPHY. 195
REVIEW QUESTIONS.
1. (a.) When an inverted bottle is held under water, why does
not the water fill it ? (b.) Why is it that any water enters the
bottle ?
2. (a.) When the sails of a ship are taken down, why does not
the boat suddenly stop ? (b.) What finally stops the ship ?
3. Why does not a stone move in a straight line when thrown
horizontally ?
4. What is the difference between the words " vertical " and
perpendicular " ?
5. When a bullet is flattened by the target, what " law " is
thereby illustrated ?
6. Why does a freely falling body increase in velocity ?
7. What has become of the energy expended centuries ago in
building the still remaining
parts of the Egyptian pyra-
mids?
8. What kind of a lever is
represented in Fig. 75? In-
dicate the positions for P, W
and F.
9. What is always the dividend in problems in specific gravity ?
10. If a pendulum, having a certain length and a weight of 2
ounces, vibrates once a second, how often will a pendulum having
the same length and a weight of 6 ounces vibrate ?
11. State the law of weight.
12. State, in ordinary language, the meaning of the formula,
= \gt*.
18. What is the difference between Kinetic and Potential En-
ergy?
195
REVIEW QUESTIONS.
139
FIG. 76.
14. A stratum of sand or gravel through which water can
easily work its way, comes to the surface of the ground at a, Fig.
76. This stratum is inclosed between two curved strata imper-
vious to water. When a hole is bored at c until it strikes the
stratum, a, an artesian well is formed. Explain the biiion of the
artesian well
CHAPTER VI.
ELECTRICITY AND MAGNETISM.
SECTION I.
GENERAL VIEW.
196. Importance of the Subject. We are now
to begin the study of a class of phenomena of intense
interest and almost unlimited practical importance.
Every pupil has seen the lightning flashing in the stormy
sky and wondered at the cause of the terrible, yet beauti-
ful, display. Later in life, he was told that lightning is
electricity. He has seen or heard of trees and houses
shattered by the lightning stroke and may now remem-
ber that if electricity can do this work, it must
be a form of energy.
He is told that the telegraph is electricity under con-
trol and working at the will of man. When he reads
^he daily paper and learns of events that took place only
a few hours before in Europe, Asia or Africa as well as
in every part of his own country, he must, it seems,
think of the wondrous speed of this form of energy as it
courses under the waters of the sea and over the valleys,
plains and mountains of the land. With the aid of ttig
Ip7 GENERAL VIEW. 141
telephone, he talks with friends and recognizes their
voices though they be miles away. He is a fit subject
for pity if he has no desire to know how these things
are done.
He hears of the mysterious power that points the
needle of the mariner's compass to the north and guides
the ship across the trackless waters. He is told that
this power is called magnetism, that it is closely asso-
ciated with electricity and is frequently produced by it.
He sees the electric light and is surprised to find that,
in turn, magnetism produces electricity. He talks with
the merchant or manufacturer about these and other
things, that were lately wonders and conveniences but
have now become common-place necessities of business
life, and finds that the so-called "practical " man of the
world is forced to acknowledge his great and ever in-
creasing obligation to modern science. He probably
gets the idea that it iv ill pay for him to learn more about
these things.
197. Simple Apparatus. Provide two stout sticks
of sealing-wax and one or two pieces of flannel folded into
pads about 20 centimeters (8 inches) square ; two glass
rods or stout tubes closed at one end, rs Q
30 or 40 centimeters in length and
about 2 centimeters in diameter (long IG '
"ignition tubes" will answer) and one or two silk pads
about 20 centimeters square, the pads being three or
four layers thick ; a few pith balls about 1 centimeter in
diameter (whittle them nearly round and finish by roll-
ing them between the palms of the hands); a silk ribbon
142 NATURAL PHILOSOPHY. IQ7
about an inch wide and a foot long ; a balanced straw
about a foot long, represented in Fig. 77. The ends of
the straw carry two small discs of paper (bright colors
preferable) fastened on by sealing-wax. The cap at the
middle of the straw is a short piece of straw fastened by
sealing-wax. This is supported upon the point of a
sewing- needle, the other end of which is stuck upright
into the cork of a small glass vial. From the ceiling or
other convenient support, suspend one of the pith balls
by a fine silk thread.
(a.) The efficiency of the silk pad above mentioned may be
increased by smearing one side with lard and applying an amal
gam, made of one weight of tin, two of zinc and six of mercury.
The amalgam which may be scraped from bits of a broken looking-
glass answers the purpose admirably. A piece of cat-skin or other
fur may be used instead of the flannel pads. See that the sealing-
wax and glass rods, the flannel and silk pads are perfectly dry.
Have them quite warm, that they may not condense moisture from
the atmosphere.
Experiment 69. Draw the silk ribbon between two layers of
the warm flannel pad with considerable friction. Hold it near the
wall of the room. The ribbon will be drawn to the wall and
held there for some time.
Place a sheet of paper on a warm board and briskly rub it with
india-rubber. Hold it near the wall as you did the ribbon.
Experiment 70. Briskly rub the sealing-wax with the flannel
and bring the wax near the suspended pith ball. The ball will be
drawn to the wax. Bring the wax near one end of the balanced
straw ; it may be made to follow the wax round and round. Bring
it near small scraps of paper, shreds of cotton and silk, feathers
and gold leaf, bran and sawdust, and other light bodies; they are
attracted to the wax. (Fig. 78.)
197
GENERAL
143
Experiment 71.
Repeat all of these ex-
periments with a glass
rod which has been
rubbed with the silk
pad.
Experiment 72.
Make a light paper
hoop or an empty egg-
shell roll after your
rod.
FIG. 78.
Experiment 73. Place an egg hi a wine-glass or an egg-cup.
Upon the egg balance a yard-stick or a common lath. The end of
the stick may be made to follow
the rubbed rod round and round.
Place the blackboard pointer or other
stick in a wire loop or stiff paper
stirrup suspended by a stout silk
thread or narrow silk ribbon. It
may be made to imitate the actions
of the balanced straw or lath.
FIG. 79.
Experiment 74. Suspend the
rubbed sealing-wax or glass rod as
you did the blackboard pointer in
the last experiment. Hold your
hand near the end of the rod. It will turn round and approach
your hand.
NOTE. The pupil may be ingenious enough to invent new
experiments for himself and the class. The ability to invent is
often very valuable and may be acquired early in life. Most
of the great inventors began making experiments when mere
children.
144
NATURAL PHILOSOPHY.
198. Electric Attraction. The attractions man-
ifested in these ex-
periments were due
to electricity that
was developed by
friction.
Experiment 75. Bring
the rubbed sealing-wax or
glass rod near the pith ball
again. It will attract the
ball as in Experiment 70.
Allow the ball to touch the
rod and notice that in a mo-
ment the ball is thrown off.
If the ball be pursued with
the rod, it will be found
that the rod which at-
tracted it a moment ago,
now repels it. Evidently
FIG. 80.
the ball has acquired a new property.
Experiment 76. Touch the ball with the finger. It seeks the
rubbed rod, touches the rod, flies from the rod. Repeat the ex-
periments with the sealing-wax after it has been rubbed with
flannel.
Experiment 77. Rub the glass rod with silk and bring it over
the small scraps of paper, as in Experiment 71. Notice that after
the attraction the paper bits do not merely fall down, they are
thrown down.
199. Electric Repulsion. The repulsions mani-
fested in these experiments were due to elec-
tricity that was developed by friction. Such
199
GENERAL V7EW.
145
electricity is called frictional or static elec-
tricity,
The glass or wax is said to be
electrified by friction. The
ball, after obtaining its new
property of repulsion by com-
ing into contact with the glass
or wax is said to be electrified
by conduction. The sus-
pended pith ball is called an
electric pendulum.
Experiment 78. Prepare a bat-
tery solution according to the recipe
given in 258, 6, using only half the
quantity of each substance as therein
directed. While the solution is cool-
ing, provide a piece of sheet copper
and one of sheet zinc, each about 10 centimeters (4 inches) long
and 4 centimeters (1 inches) wide. To one end of each strip,
solder (see Appendix) or otherwise fasten a piece of copper wire
about 15 centimeters (6 inches) long and 1 millimeter ( T V to %
inch) thick. Place the zinc strip in a common tumbler about
three-fourths full of the battery solution. Notice the minute
bubbles that break away from the surface of the zinc and rise to
the surface of the liquid. These are bubbles of hydrogen, a com-
bustible gas. The formation of the gas is due to chemical
action between the zinc and the liquid.
Experiment 79. Take the zinc from the tumbler and, while
it is yet wet, rub a few drops of mercury (quicksilver) over its
surface until it has a brilliant, silver-like appearance. Replace the '
zinc, thus amalgamated, in the solution and notice that no bub-
bles are given off.
FIG. 81.
146
NATURAL PHILOSOPHY.
200
Experiment 80. Place the copper strip in the liquid, taking
care that it or its wire does not touch the zinc or its wire. No
bubbles appear on either the zinc or the copper. It may he
convenient to place a narrow glass strip be-
tween the ends of the metal strips in the tum-
bler to keep them apart.
Experiment 81. Bring the upper ends of
the strips together, as shown in Fig. 82, or, still
better, join the two wires, as shown in Fig. 102.
being sure that the wires are clean and bright
where they are united. Notice the formation
of bubbles on the surface of the copper,
where none previously appeared. ( 248.)
FIG. 83.
20O. Suspicion. It certainly seems that the connect-
ing wire is an important part of the apparatus as now
arranged and we are led to suspect that something un-
usual is taking place in the wire itself. It is evident
that we have a complete " circuit " through the liquid,
the metal strip and the wire.
Experiment 82. Untwist the wires or, in other words, "break
the circuit." Connect the copper wires with a short piece of very
f.ne iron wire. The connections should be made so that the cir-
cuit shall include about 2 centimeters (f inch) of iron wire. The
ron will become hot enough to burn the fingers or to ignite a
small quantity of gun cotton twisted around it.
Experiment 83. If one of the
copper wires be twisted around
one end of a small file and the
other wire be drawn along its
rough surface, a series of minute
sparks will be produced as the
circuit is rapidly made and broken.
FIG.
201
GENERAL VIEW.
147
Experiment 84. Place the cell so that the joined wires sliaT.
run north and south, passing directly over the needle of a small
compass ($ 295, ft.) and near to it. The needle will instantly turn
as though it were trying to place itself at right angles to the wire
Break the circuit and the needle will swing hack to its north and
south position.
FIG. 84.
201. Certainty. We now feel sure that something
unusual is taking place in the wire of our complete cii>
cuit for we have seen the wire become hot, explode gun-
cotton, yield sparks and exert a very mysterious influence
upon the magnetic needle. As a matter of fact, we now
have a current of electricity flowing through a galvanic
cell.
Electricity thus produced by chemical action
is called galvanic or voltaic electricity. It is one
form of current electricity.
Experiment 85. Wrap a piece of writing paper around a
large iron nail leaving the ends of the nail bare. Wind fifteen
or twenty turns of stout copper wire around this paper wrapper,
taking care that the coils of the wire spiral do not touch each
other or the Iron, It is well to use cotton covered or " insulated *
148 NATURAL PHILOSOPHY. 202
wire. Connect the two ends of the wire spiral with the two wires
of the galvanic cell or, in other words, put the spiral into the cir-
cuit. Dip the end of the nail into iron filings. Some of the
filings will cling to the nail in a remarkable manner. Upon
breaking the circuit, the nail instantly loses its newly acquired
power and drops the iron filings.
If the experiment does not work satisfactorily, look carefully to
all the connections of the circuit, see that the ends of the wires
are clean and bright and that they are twisted together firmly. It
may even be necessary to wash the plates, rub more mercury on
the zinc and provide a fresh battery solution.
202. Temporary Magnets. You ha -e probably
satisfied yourself that the nail has the power A attract-
ing iron filings while the electric current is flow-
ing through the wire.
You have made an electro-magnet and its
power of attracting iron is called magnetism.
Satisfy yourself, by trial, that the nail loses its mag-
netism as soon as the circuit is broken or the current
ceases to flow around it and remember that your electro-
magnet is a temporary magnet.
Experiment 86. While the nail is magnetized, draw a sewinjr-
needle four or five times from eye to point across one end of the
electro-magnet. Dip the needle into iron filings ; some of them
will cling to each end of it.
203. Permanent Magnets. When steel is treated
as in the last experiment, it becomes permanently mag-
netized.
Experiment 87. Cut a thin slice from the end of a vial cork
and, with its aid, float your magnetized needle upon the surface of
207 GENERAL VIEW. 149
a bowl or saucer of water. The needle comes to rest in a
north and south position. Turn it from its chosen position and
notice that after each displacement it resumes the same position
and that the same end of the needle always points to the
north.
204. A Simple Compass. t^ small magnetized
steel bar freely suspended, is called a compass.
The one that you have made may be less convenient
but is as reliable as the compass of the mariner or the
surveyor.
205. Artificial Magnets. The electro-nrugnet and
the permanent magnet that you made are, of course,
artificial magnets. There is a natural magnet
known as lodestone.
206. Other Forms of Current Electricity.
Electric currents may be generated by the action of
other currents of electricity or by the action of magnets.
Electricity thus developed is called induced electricity.
A current of electricity that is generated by heating
the junction of two metals that form part or all of a cir-
cuit is called thermo-electricity.
207. The Different Forms of Electricity are
Identical. So far as experiment can show, one form of
electricity may have a particular property in greater
degree than some other form but all are identical, each
having all the properties of any of the others.
150
NATURAL PHILOSOPHY.
208
208. Recapitulation. To be amplified by the pupil
for review.
All Forms of Electricity are Iden-
tical in Nature.
s
W M
H P-
$210 ELECTRICITY. 151
SECTION II.
FRICTIONAL ELECTRICITY OR ELECTRIC
CHARGES.
209. The Nature of Electricity. But little is
known concerning the real nature of electricity. It is
easier to tell what electricity can do than to tell what
it is.
The majority of modern physicists consider that elec-
tricity is a form of energy producing peculiar
phenomena; that it may be converted into other
forms of energy and that all other forms of
energy may be converted into it.
Several theories have been advanced to account for
electrical phenomena but none of them is satisfactory.
210. Electric Manifestations. Electricity may
reveal itself as a charge or as a current.
By means of friction, the glass rod or the sealing wax
( 198, 199) acquired an electrical charge and, conse-
quently, the power of attracting and repelling light
bodies: by means of chemical action, the galvanic cell
generated electricity that manifested itself as a current.
In this section, ^ve shall consider electricity that appears
as a charge, i. e., static electricity.
Experiment 88. Prepare two electric pendulums. Bring the
electrified glass rod near the pith ball of one , after contact, the
NATURAL PHILOSOPHY.
210
ball will be repelled by the glass. Bring the electrified sealing.
wax near the second pith ball ; after contact it will be repelled by
the wax. Satisfy yourself that the electrified glass will repel the
first ; that the electrified sealing-wax will repel the second. Let
the glass rod and the sealing wax change hands. The first ball
was repelled by the glass ; it will be attracted by the sealing-
wax. The second ball was repelled by the sealing-wax ; it will be
attracted by the glass.
Experiment 89. Suspend two pith balls, as shown in Fig 85,
and touch them with a
rubbed glass rod. Instead
of continuing to hang side
by side, they repel each
other and fly apart. If
the electrified glass rod
be held near them, they
separate still further. If
the electrified sealing-
wax, instead of the glass,
be held near them they
will fall nearer together.
If the rubbed glass rod be
suspended, as shown ia
Fig. 79, it will be repelled
by another rubbed glass rod but attracted by rubbed sealing-wax.
211. Two Kinds of Electricity. The electricity
developed on glass "is different in kind from that
developed on sealing-wax.
They exhibit opposite forces to a third electrified body,
each attracting what the other repels. The self-repul-
sion of the parts of an electrified body may be beautifully
illustrated by electrifying a soap-bubble which will thu&
be made to expand..
FIG. 85.
215 FBICTIONAL ELECTRICITY. 153
212. Only Two Kinds of Electricity. All elec-
trified bodies act like either the glass or the
sealing-wax.
213. The Two Electricities Named. The elec-
tricity developed on glass by rubbing it with silk
is called positive or + .
The electricity developed on sealing-wax by
rubbing it with flannel is called negative or .
The terms vitreous and resinous respectively were
formerly used.
214. The Law of Electrostatics. The most im-
portant electrostatic law may be stated thus :
Electric charges of like signs repel each other;
electric charges of opposite signs attract each
other.
215. Electroscopes. An instrument used to
detect the presence of electricity, or to determine
its kind, is called an electroscope.
The electric pendulum ( 199) is a common form of
the electroscope. Two strips of the thinnest tissue paper
hanging side by side constitute a simple electroscope.
It is well to prepare the paper beforehand by soaking in
a strong solution of salt in water and drying.
The balanced straw (Fig. 77) or, better yet, two gilt
pith balls connected by a light' needle of glass or sealing-
wax balanced horizontally on a vertical pivot or a goose-
quill balanced on the point of a sewing-needle, makes $
convenient electroscope.
J54
NATURAL PHILOSOPHY.
215
The gold leaf electroscope is represented in Fig. 86.
A metallic rod, which
passes through the cork
of a glass vessel, termi-
nates below in two narrow
strips of gold leaf and
above in a metallic knob
or plate. The object of
the vessel is to protect the
leaves from disturbance
by air currents. The
upper part of the glass is
often coated with a solu-
tion of sealing-wax or
shellac in alcohol, to
lessen the deposition of
moisture from the atmosphere. This instrument may
be made by the pupil and, when well made, is very
delicate.
Experiment 90. From a horizontal glass rod or tightly-
stretched silk cord, suspend a fine copper wire, a linen thread and
two silk threads, each at least a yard long. To the lower end of
each, attach a metal weight of any kind. Place the weight sup-
ported by the wire upon the plate of the gold leaf electroscope.
Bring the electrified glass rod near the upper end of the wire ; the
gold leaves instantly diverge. Repeat the experiment with the
linen thread : in a little while the leaves diverge. Repeat the
experiment with the dry silk -thread ; the leaves do not diverge
at all. Rub the rod upon the upper end of the silk thread ; no
divergence yet appears. Wet the second silk cord thoroughly
and, with it, repeat the experiment ; the leaves then diverge
PIG. 86
217 FRICTION AL ELECTRICITY. 155
216. Conductors. Such experiments clearly show
that some substances transmit electricity readily
and that others do not.
Those that offer little resistance to the passage
of electricity are called conductors; those that
offer great resistance are called non-conductors
or insulators.
A conductor supported by a non-conductor is said to
be insulated.
(a.) In the following table, the substances named are arranged
in the order of their conductivity :
Conductors.
1. Metals.
2. Charcoal.
3. Graphite.
4. Acids.
5. Salt wa'ir.
6. Freshwater.
7. Vegetables.
8. Animals.
9. Linen.
10. Cotton.
11. Dry wood.
12. Paper.
13. Silk.
;14. India rubber. Insulators.
15. Porcelain.
16. Glass.
17. Sealing-wax.
18. Vulcanite.
The fact that a conductor in the air may be insulated, shows
that air is a non conductor. Dry air is a very good insulator, but
moist air is a fairly good conductor for electricity of high potential.
All experiments in frictional electricity should, therefore, be
performed in clear, cold weather when the atmosphere is dry,
for a moist atmosphere renders insulation for a considerable length
of time impossible.
Experiment 91. Support a yard-stick or common lath upon
a glass tumbler. Bring the glass rod, electrified by rubbing it with
silk, to one end of the stick and hold some small pieces of paper
under the other end of the stick. The paper will be attracted and
repelled by the stick as it previously was by the glass itself. The
electricity passed along the stick from end to end.
217. Tension. Electricity exists under widely dif-
ferent conditions with respect to its ability to force its
way through a poor copductor or to leap across a gap.
156 NATURAL PHILOSOPHY. 217
The electricity developed in a galvanic cell will not
pass through even a very thin piece of dry wood ; the
electricity developed by rubbing the glass rod will pass
through several feet of dry wood.
It would require a battery of many cells to force a
current across a gap of y^ of an inch. It is not diffi-
cult to force frictional electricity across a gap of several
inches while we all know that, in the case of lightning,
electricity leaps across a gap of many hundred feet.
In the one case, the electricity is said to be of low
potential; in the other case, it is said to be of high po-
tential. We are now dealing with electricity of high
potential.
The terms, "low tension" and "high tension" are
often used in the same sense.
218. Potential. The term electrical potential (or
simply potential) has reference to the electrical condi-
tion of a body or to its degree of electrification. If the
potential of A be higher than that of B, and the two
bodies be connected by a good conductor, an electric
current will flow from A to B until the poten-
tials are alike.
Difference of potential is somewhat analogous to differ-
ence of liquid level and gives rise to electromotive force.
219. Electromotive Force. Electromotive force
(often written E. M. F.) is the mysterious power which
causes electricity to move from one point to another.
It is somewhat analogous to hydrostatic pressure. Wbcr-
S 220 FRICTION AL ELECTRICITY. 151
ever there is difference of potential, there is E. M. F. t
but the terms are not synonymous.
The unit of electromotive force is called a volt.
A volt is a little less than the E. M. F. of a Danieli
dell ( 260).
220. Resistance. Every electric circuit offers a re-
sistance to the passage of the current. This resistance
will, of course, depend largely upon the conductivity of
the material used for the circuit.
With a given material for the conductor, the
resistance varies directly as the length and in-
versely as the weight of a given length.
If one conductor is twice as long as another made of
the same kind of wire, the resistance of the longer will
be twice as great as that of the shorter.
If one conductor be twice the diameter of another made
of the same length and material, the weight per foot or
yard will be (2 Z =) four times as great and the resistance
of the first will be one-fourth as great as that of the
second.
If they be made of the same material and length, one
weighing twice as much per foot as the latter, the resist-
ance of the former will be half as great as that of the
latter.
The unit of resistance is called an ohm.
A galvanized iron (telegraph) wire, 4 millimeters in
diameter and 100 meters long, or a pure copper wire,
1 millimeter in diameter and 48 meters long, has a re-
sistance of about one ohm.
A megohm is a million ohms.
158 XAWRAL P&ILOSOPHT. 221
221. Charging by Contact. If an insulated, un-
electrified conductor be brought into contact with a
similar conductor that is electrified, or near enough to
it for the passage of an electric spark, electricity will
pass from the latter to the former until the two con-
ductors are equally charged with the same kind of elec-
tricity. The former is said to be charged by con-
duction.
222. Induction. Actual contact with an electrified
body is not necessary for the manifestation of electric
action in an unelectri-
q. - fied body. When an
ft B | \\J\c electrified body, C, is
oo vb\ brought near an insu-
.^ %
*^ with electric pendu-
lums, as shown in the
figure, the latter shows
electric action. The
electricity of C rebels
one kind of electricity in B and attracts the other, thus
separating them. The second body, B, is then said to
be polarized.
The two kinds of electricity in J9, each of which a
moment ago rendered the other powerless, are still there
but they have been separated and each clothed with its
proper power. This effect is due to the action of the
electrified body, C, which is said to produce electric
^Pj\ lated unelectrified con-
m^\ ductor, B, provided
g 223 PRICTIONAL ELECTRICITY. 159
separation by induction. When C is removed, the sep-
arated electricities of B again mingle and neutralize
each other.
(a.) Conductors for the purposes of this and similar experiments
may be made of wood covered with tin-foil, gold leaf or Dutch
leaf. They may be insulated by fastening them on top of long
necked bottles or sticks of sealing-wax or by suspending them
by silk threads.
(6.) Prick a pin hole in each end of a hen's egg and blow on
the white and the yolk. Paste tin-foil smoothly over the whole
surface of the egg. Fasten one end of a white silk thread to the
egg with a drop of melted sealing wax so that the egg may hang
suspended with its greater diameter horizontal. Three or four
such insulated conductors will be found convenient. Sometimes
it is convenient for each egg to have two thread supports. Place
a loop or ring at the free end of each thread. When the loops
are placed on a horizontal rod (e. g., a piece of glass tubing), the
greater diameters of the suspended eggs should lie in the same
straight line. An elongated conductor, like A B of Fig. 88, may
be made by hanging two or three egg conductors so that they are
in contact
223. A Neutral Line. If an insulated conductor,
bearing a number of pith ball (or paper) electroscopes,
be brought near an electrified body, C, (Fig. 88), but not
near enough for a spark to pass between them, the pith
balls near the ends of the conductor will diverge, show-
ing the presence of separated or uncombined electricity.
The pith balls at the middle of the conductor will not
diverge, marking thus a neutral line.
This action will take place across a considerable dis-
tance even if a large sheet of glass be held between A
and C.
160 NATURAL PHILOSOPHY. 223
If C has a positive charge, the charge at A will be
negative and that at B will be positive, as may be shown
by charging an electric pendulum and testing at A and
B, as shown in Fig. 88.
FIG. 88.
If C be removed or " discharged " by touching it with
the hand, all traces of electrical separation in A B will
disappear. The charged pith ball will be attracted at
every point of A B.
224. Charging a Body by Induction. If the
polarized conductor be touched with the band, or other-
wise placed in electric communication with the earth,
the electricity repelled by C (Fig. 88), will escape, and
the pith balls at B will fall together. The electricity at the
other end will be held by the mutual attraction between it
and its opposite kind at C. The line of communication
with the ground being broken and the conductor being
removed from the vicinity of C, the former will be found
charged with electricity opposite in kind to that of (7.
226 ELECTRIC CHARGES. 161
A body may be thus charged by induction with
no loss to the inducing body.
225. Polarization Precedes Attraction. Wforn
an electrified glass rod is brought near an insulated
uncharged pith ball (electric pendulum), the pith ball is
polarized, as shown in the figure.
As the of the ball is nearer the
-f- of the glass than is the + of the
ball, the attraction is greater than
the repulsion.
If the pith ball be suspended, not p IG g9.
by a silk thread but by some good
conductor, the attraction will be more marked, for the
+ of the ball will escape to the earth through the sup-
port and, thus, the repelling influence will be removed.
226. Provisional Theory of Electricity. While
the real nature of electricity remains unknown, the fol-
lowing theory will be found convenient for classifying
results already attained and suggesting directions for
further inquiry. But we must not let it influence our
judgment as to what is the true and full explanation of
electrical phenomena, which explanation may be found
hereafter :
We may assume that a neutral or unelectri-
fied body contains equal and equally distributed
quantities of positive and of negative electricity.
We may assume these electricities to be un-
limited in amount.
We shall then conceive that a positively elec-
NATURAL PHILOSOPHY.
227
trified body has an excess of 4- electricity and
that a negatively electrified body has an excess
f electricity.
fn this light, we shall see that communicating
4- electricity to a body is equivalent to removing
an equal amount of electricity from it, and
conversely.
227. The Electrophorus. This simple instrument
consists generally of a shal-
low tinned pan filled with
resin, oil Aivhich rests a mov-
able metallic cover with a
glass or other insulating
handle. The resinous plate
may be replaced by a piece
of vulcanized India-rubber.
The metal surface and the
resinous surface touch at
only a few points; they are
PI practically separated by a
Fio. 90. thin layer of air.
(a.) The resinous plate may be prepared by melting together
equal quantities of rosin and Venice turpentine and then adding a
like quantity of shellac. The substances should be heated gradu-
ally and stirred together so as to prevent the forming of bubbles.
Take care that the mixture does not take fire in course of prepara-
tion. The Venice turpentine is desirable but not necessary. For
a handle, a stout wire may be soldered to the centre of the disc
and covered with rubber tubing; or a piece of sealing wax, of
convenient size, may be fastened to the disc for the purpose. A
till better plan is to make the cover of wood, a little less in
ELECTRIC CHARGES.
163
diameter than the resinous plate and with its edges carefully
rounded off. For a handle, a glass rod or tube may be tightly
thrust or cemented into a hole in the middle of the cover. Paste
tin toil all over the cover and smooth down all rough edges of the
foil with the finger nail or a paper folder. The wire support for a
pith ball or paper electroscope may be thrust into'the wood of the
cover, care being taken that it touches the tin -foil.
(b.) The plate is rubbed or struck with flannel or catskin, and
thus negatively electrified. The cover is then placed upon the
resin and thus polarized by induction. Touch tlie cover with the
fineer, as shown in Fie:. 90 ; the free electricity escapes and
the leaves fall. The cover is now charged positively. The charged
cover will give a spark to the knuckle or other unelectrified body
presented to it. (fig. 91.)
228. The Elec-
trophorus charged
by Induction. The
cover may be thus
charged and dis-
charged an indefinite
number of times, in
favorable weather,
without a second
electrifying of the
resinous plate. This
could not happen if
the electricity of the
cover were drawn
from the plate. More-
over, JT v^ ch'irsre ^
the cover were drawn
lie 91
_64 NATURAL PHILOSOPHY. 22Q
from the plate, it would be , and not 4* . The cover
is charged by induction and not by conduction.
229. Whence this Energy ? At every discharge
of the electrophorus, it gives a definite amount of elec-
tricity, capable of doing a definite amount of work. As
this is obtained not by the expenditure of any part of
the original charge, we are led to seek for the source of
this apparently unlimited supply of energy.
"As a matter of fact, it is a little harder work to lift
the cover when it is charged with the + electricity than
if it were not charged for, when charged, there is the
force of the electric attraction to be overcome as well as
ihe force of gravity. Slightly harder work is done at
the expense of the muscular energies of the operator and
this is the real origin of the energy stored up in the
separate charges."
230. A Charge Resides on the Surface. Many
experiments have been made showing that when a
conductor is electrified, the electricity passes to
the surface and escapes if the body be not insu-
lated.
Experiment 92. Place a carrot horizontally upon an insulating
support. Into one end of the carrot stick a sewing-needle. Bring
the electrified glass rod near the point of the needle without
touching it. The electricity of the carrot quietly escapes front
the point to the rod and the carrot is charged with the + elec,
tricity that remains.
231. Density. Experiments show that when a
spherical conductor is charged, the electricity is evenlt
distributed over jtfce. SJK;&@, ggpvided no other electrified
232 ELECTRIC CHARGES. 165
body be near. The electric density is the same at ever^
point.
Experiments on an elongated cylinder, like the prime
conductor of the electric machine, show that the density
is greater at the ends. On an egg-shaped conductor,
like that shown in Fig. 92, the density is greatest at the
smaller end.
In general, the electric density is very great at
any pointed part of a charged conductor.
FIG. 92.
This density at a point may become so great that the
electricity will escape rapidly and quietly, the air par-
ticles rapidly carrying off the charge by convection.
This explains the action of points, which plays so im-
portant a part in the action of electric machines.
232. Electric Machines. Machines have been
made for developing larger supplies of electricity more
easily than can be done with a rod of glass or sealing-
wax or with the electrophorus. Each of them consist;
of one part for producing the electricity and anoth'
part for collecting it.
166
NATURAL PHILOSOPHY.
233. The Plate Electric Machine. This instru-
ment is represented in Fig. 93. It consists of an insu-
lator (or electric), a rubber, a negative and a positive
or prime conductor. The electric is a glass (or ebonite)
plate, A. generally one, two or three feet in diameter.
FIG. 93.
This plate has an axis, B, and handle, C, and is sup-
ported upon two upright columns. The rubber, D, 13
made of two cushions of silk or leather, covered with
amalgam (see 197, a). They press upon the sides of the
plate and are supported from the negative conductor,
with which they are in electric connection.
The negative conductor, N, is supported upon an
insulating column and, when only positive electricity is
desired, is placed in electrical connection with the earth
by means of a chain or wire, W. The prime conductor,
P, is insulated, One end of the prime conductor termi-
234 ELECTRIC CHARGES. 167
nates in two arras, F, which extend one on either side ol
the plate. These arms, being studded witli points pro-
jecting toward the plate, are called combs. The teeth of
the combs do not quite touch the plate. A silk bag, S,
is often supported so as to enclose the lower part of the
plate. All parts of the instrument except the teeth of
the combs are carefully rounded and polished, sharp
points and edges being avoided to prevent the escape of
electricity as already explained. This avoiding of points
and edges is to be regarded in all apparatus for use with
electricity of high potential.
(a.) The pupil may make a plate machine without much ex-
pense. A glazier will cut for him a disc of plate glass, possibly
from a fragment on hand. The edges of this disc may be rounded
on a wet grindstone. A hole may be bored in the middle with a
round file kept moistened with a solution of camphor in turpentine.
The conductors, N and P, may be made of wood covered with
gold foil or Dutch leaf and supported on pieces of stout glass tub-
ing. The prime conductor may well have two such supports. The
arms may consist of two stout wires thrust into the end of a prime
conductor, their free ends being provided with knobs of lead or other
metal. The combs may be made by soldering pin points to one
side of each arm. See that the gold foil makes actual contact with
the metal arms. See that all metal parts except the pin points are
polished smooth. The columns that support the plate may be
made of seasoned wood. The part of the handle to which the
hand is applied may be made of glass or insulated by covering it
with rubber tubing.
234. Operation of the Plate Machine. The
plate is turned by the handle. Electric separation is
produced by the friction of the rubbers. The -f elec-
tricity of the rubber and negative conductor passes tc
168 NATURAL PHILOSOPHY. 234
the plate; the electricity of the plate passes to the
rubber and negative conductor. The part of the plate
thus positively charged passes to the combs of the prime
conductor. The + of the plate acts inductively upon
the prime conductor, polarizes it, repels the + and at-
tracts the electricities.
Some of the electricity thus attracted streams from
the points of the combs against the glass, while some of
the + electricity of the glass escapes to the prime con-
ductor. This neutralizes that part of the plate and leaves
the prime conductor positively charged.
The rubber and negative conductor are kept in equi-
librium by means of their connection with the earth.
As the plate revolves, the lower part, passing from N to
P, is positively charged ; the upper part, passing from P
to N, is neutralized. If negative electricity be desired,
the chain or other ground connection is changed from
N to P, and the charge taken from N.
NOTE. Other forms of electric machines are made. One of
the latest of these, known as the Toepler-Holtz, is very compact
and efficient and remarkably free from the limitations of atmos-
pheric conditions. It may be described as a continuously acting
electrophoras ( 227). A very good one may be bought of JAMES
W. QUEEN & Co., of Philadelphia, for $25 or more. One should
be provided for the school in some way if possible. Any electrical
machine should be free from dust and perfectly dry when used.
It should be warmer than the atmosphere of the room, that it may
not condense moisture from the surrounding air. The drier the
atmosphere the better will be the action of the machine.
235- Construction of the Leyden Jar. The
Leyden jar consists of a glass jar, coated within and with-
237
ELECTRIC CHARGES.
169
out for about two-thirds its height with tin-foil, and a
metallic rod, communicating by means of a
small chain with the inner coat and terminat-
ing above in. a knob. The upper part oi the
jar and the cork which closes the mouth of the
jar and supports the rod are generally coated
with sealing-wax or shellac varnish to lessen
the deposition of moisture from the air.
(a.) Select a candy or fruit jar of greenish glass ;
paste tin-foil within and without, as above described, p IG>
using flour paste ; thrust a wire through a dry cork ;
bend the wire so that, when the cork is in its place, the wire shall
touch the tin-foil on the side of the bottle without tearing it ;
solder the upper end of the wire to a smooth button or thrust it
into a lead bullet ; charge your Leyden jar with a few sparks from
the electrophorus and take a shock.
236. Charging the Leyden Jar. To charge the
jar, hold it in the hand, as shown in Fig. 95, and bring
the knob near or into contact with the prime conductor
of an electrical machine which is in action.
FIG. 95.
237. Discharging the Leyden Jar. The jar
might be discharged by touching the knob with the
NATURAL PHILOSOPHY. 237
In this case the experimenter will feel a " shock."
If the charge be intense, the shock will be
painful or even dangerous. It is better
to use a "discharger," one form of which
is represented in Fig. 96. This consists
of two metal arms hinged together, carry-
ing knobs at their free ends and carried by
insulating handles. The outer coat should
FlG - 96 ' be touched first.
(a.) A good discharger may be made by passing a piece of stout
copper wire, about a foot long, through a piece of rubber tubing
and providing a metal knob for each end of the wire. The flexi-
bility of the wire avoids the necessity for a hinged joint.
238. Modes of Discharge. An electrified con-
ductor may be discharged in at least three ways, viz., by
the disruptive discharge, by the conveciive discharge
and by the conductive discharge.
The discharge in any of these ways is accom-
panied by a transformation of energy, sound,
light, heat, chemical action and other phenom-
ena being produced.
Experiment 93. Present a knuckle of the hand or a metal
knob to the prime conductor of an electric machine and " draw
sparks" therefrom.
239. The Disruptive Discharge. A discharge of
electricity taking place suddenly through a non-con-
ductor is called a disruptive discharge, e. g., the sparki
drawn from an electric machine in action.
242 ELECTRIC CHARGES. 173
Experiment 94. At-
tach a pointed wire to the
prime conductor of the
electric machine. The
flame of a candle held
near will be blown away,
as shown in Fig. 97. If
the candle be placed upon
the prime conductor and
a pointed conductor be
held in the hand near
the candle, the flame will
still be blown away. Fl 97 -
240. The Convective Discharge. When electric-
ity of high potential accumulates with so great a density
as to electrify the neighboring particles of air which,
driven by electric repulsion, fly off carrying part of the
charge with them, we have what is called the convective
discharge. Such discharges are best manifested in gases
at low pressure, in tubes exhausted by an air-pump.
241. The Conductive Discharge. The flow of a
continuous current of electricity constitutes the con-
ductive discharge.
When electricity flows through a wire from the prime
conductor of an electric machine to the rubbers or from
the positive pole of a voltaic cell or battery to the nega-
tive, we have a conductive discharge. It will be con-
sidered in the section especially devoted to voltaic
electricity.
242. Lightning. When an electrified cloud floats
over the earth, separated from it by a layer of insulating
172 NATURAL PHILOSOPHY. 242
air, the inductive influence of the cloud renders the
ground beneath oppositely electrified. Then the cloud,
ground and insulating air correspond respectively to
the inner and outer coatings and the insulating glass of
a Leyden jar.
As the charge of a Leyden jar may be made so intense
that the attraction of the separated electricities will
result in their rushing together and thus piercing the
jar, so the charge of a cloud may become sufficiently
intense to overcome the resistance of the air and a light-
ning stroke ensues. Such electric sparks are sometimes
more than a mile in length but the duration is not more
than 0.00001 of a second.
243. Lightning-Rods. The value of lightning-rods
depends upon the tendency of electricity to follow the
best conductor, and upon the effect of pointed conductors
upon electrical density ( 231).
The lightning-rod should be made of a good conduc-
tor; copper is better than iron. It should terminate
above in one or more points, tipped with some substance
that can be corroded or fused only with extreme diffi-
culty. Platinum or iridium is a metal which satisfies
these conditions very well.
The rod should extend above the highest point of the
building in order to offer the electricity the shortest
path to the ground. It is important to have each pro-
jecting part of the building, as chimneys, towers and
gables, protected by a separate rod. All metal work
about the roofs or chimneys should be connected with
the rod.
243 ELECTRIC CHAROES. 173
The rod should afford an unbroken connection ; the
joints, if there be any, should be carefully made.
The rod should terminate below in water or in earth
that is always inoist. It is well to connect the rod with
underground water-pipes when possible or with a large
metal plate. Personal attention should be given to thia
matter when the rod is put up as, being under ground
and out of sight, this part of the rod is not easily inspected
subsequently.
A rod having a blunted tip, a broken joint or
terminating in dry earth is more dangerous
than no rod at all. Lightning-rod insulators
are undesirable.
(a.) The greatest value of a lightning-rod is due to its quiet
work in the prevention of the lightning stroke. Bring the point
of a knife-blade near the conductor of an electric machine in
operation and notice the instant cessation of sparks. The quiet
passage of electricity from the earth neutralizes the charge of the
conductor and restores the electric equilibrium. In the same way,
a lightning-rod tends to restore the electric equilibrium of the
cloud and prevent the dangerous discharge. For this quiet but
very valuable service, few persons ever give the rod any credit.
Every leaf of the forest and every b'.ade of grass is a pointed coa
iuctor acting in the same way. (231.)
174 NATURAL PHILOSOPHY. 243
NOTE. It is neither necessary nor very desirable that all of the
following experiments be performed. Several of them involve
the same principle ; but one school may have one piece of apparatus
and another, another piece. Additional experiments may be found
in The Elements of Natural Philosophy.
Experiment 95. Figure 98 represents the " electric bells."
The metal frame is hung from the prime con
ductor. The right-hand bell is suspended by a
wire ; the other bell is suspended by a silk cord
and connected with the ground by means of a
chain hanging on the floor. Work the machine
slowly ; the clapper vibrates and rings the bells.
Explain.
_ Experiment 96. Place the two ends of a
p " pane of window glass upon two books so that
the pane shall be about two inches above the
surface of a table. Place several pith balls or numerous bits of
tissue paper on the table under the glass. Electrify the glass by
rubbing its upper surface with silk. Notice the lively motions of
the balls or paper bits.
Experiment 97. Electrify a glass rod. Toss a small sheet of
gold leaf into the air. Bring the rod near the leaf. The leaf is
drawn toward the rod and then thrown off. Chase the leaf with
the rod without letting it touch the ground. Explain.
Experiment 98. If a pupil, standing upon an insulating stool
(a board supported by four warm tumblers will answer) and hav-
ing one hand upon the prime conductor of an electric machine in
action, bring a knuckle of the other hand near one end of the
balanced yard-stick Experiment 73, it will follow the knuckle.
Explain.
Experiment 99. Place a few bits of paper upon the cover of
the electro>x>orus When the cover has been touched with the
243
ELECTRIC CBAKGES.
175
finger and lifted by the insulating handle, the paper will be
thrown ofE Explain.
Experiment 100. Electrify a doll's
head covered with long, dry hair. The
hairs will stand out, as shown in Fig.
99, producing an exaggerated appear-
ance of fright.
Experiment 101. Vary Experiment
94 by placing the candle on the prime
conductor and holding the point of a
needle toward it, as shown in Fig. 100.
The Same will be driven away by the
convective discharge.
FIG
Fio. 100.
Experiment 102. Fasten
one end of a long fine wire to
the knob of the electroscope.
Charge the disc of the elec-
trophorus and touch it to the
other end of the wire. Notice
the action of the electroscope.
Try the experiment with a dry
silk thread instead of the wire.
Describe the action of the elec-
troscope. What does this ex-
periment teach about the wire
and the thread?
Experiment 103. Place an "electric whirl" (which
consists of a set of horizontal wire arms radiating from
a pivot-supported centre, the pointed ends being all bent
in the same direction) upon the prime conductor. Work
the machine and the arms will revolve. (See Fig. 101.)
Fio. 101
176 NATURAL PHILOSOPHY. 243
Experiment 104. Place a pupil on the insulating stool. Let
him hold an electroscope, with his finger on the knob. Let a second
pupil strike him on the back with a cat-skin. Notice the leaves
of the electroscope at every stroke.
Experiment 105. Get a smooth board and a sheet of paper.
Heat them both before the fire. Place the paper on the board and
rub it vigorously with a piece of india-rubber. Remove the elec-
trified paper from the board and hold it near the wall. It will fly
to the wall and cling to it for some time.
Experiment 106. Electrify the paper as in the last experi
ment. Remove it from the board and hold it by the edges. Let
a pupil place a pith ball on the paper. Notice and explain the
action of the ball.
Experiment 107. Electrify the paper as in the last experiment.
While it is lying on the board, cut it into narrow strips. Take
hold of all the strips at one end and lift them from the board.
Notice the repulsion of light bodies similarly charged.
Experiment 108. Place a pupil upon an insulating stool (a
board supported by four warm tumblers will answer) and charge
him by giving him twenty or more sparks from the disc of the
electrophorus. Let another pupil, not insulated, bring his knuckle
to any part of the body of the first pupil. Let the pupils describe
the result.
Experiment 109. Cover one knob of the discharger with gun-
cotton sprinkled with powdered rosin. When the Leyden jar is
discharged with this discharger, the cotton and rosin are ignited.
Experiment 110. Let a pupil, standing on an insulating stool,
become charged by holding one hand on the prime conductor
when the machine is in operation. If he then bring his knuckle
to a metal burner from which a jet of gas is issuing, a spark will
nass between the knuckle and the burner, igniting the gas. An
Argand or Bunsen burner answers well for this experiment. The
experiment may be modified by using, instead of the knuckle, an
244 ELECTRIC CHARGdS. 17?
Jcicle.held in the hand. The gas burner may be replaced by a
pupil (not insulated) holding a spoonful of ether or chloroform
which readily gives off an easily combustible vapor.
244. Relation of Electricity to Energy. The
work necessarily performed in operating an electric
machine is not all expended in overcoming inertia and
friction. Much of it is employed in producing electric
separation. It matters not whether this separation be
the separation of two fluids or of something else.
Wliatever be the nature of the realities sepa-
rated, mechanical kinetic energy is employed in
the separation and converted into the potential
variety ( 100).
An electrified pith ball or a charged Leyden jar is
simply an electrostatical reservoir of potential energy.
In the discharging of such a body, the passage of the
current is accompanied by a loss of potential energy.
What becomes of this energy ? This leads us to look
for effects due to it, to work done by it.
Many illustrations of work thus done have been fur-
nished in the experiments just described. In every case
of electric attraction or repulsion, we have an evident
reconversion of this potential energy into mechanical
kinetic energy. We shall soon see that the sound, heat
and light accompanying electric discharges are forms of
energy due to the conversion of the potential energy of
electric separation.
We shall see other effects, more or less powerful, when
we come to study voltaic and other forms of current
electricity
178 NATURAL PHILOSOPHY. 245
245. Recapitulation. To be amplified by the pupil
for review.
f KINDS AND NAMES.
ELECTROSTATIC LAWS.
ELECTROSCOPES.
( CONDUCTORS.
INSULATORS.
L RKSISTANCB.
3
-
POTENTIAL AND E. M. F.
CAPACITY.
BY CONTACT.
ELECTRIFICATION. { BY INDUCTION. | "
Electrophoru*
DISTRIBUTION OF CHARGE.
DENSITY.
LEYDEN JAR.
O
DisRUPTive.
DISCHARGE \ CONVECTIVE.
CONDUCTIVB.
( LIGHTNING.
THUNDER-STORMS \
\ LIGHTNING-ROD*,
EXPERIMENTS.
RELATION TO ENERGY
245 EXERCISES. 279
EXERCISES.
1. Why do we regard the two electric charges produced simul-
taneously by rubbing together two bodies as being of opposite
kinds?
2. Quickly pass a rubber comb through, the hair and determine
whether the electricity of the comb is positive or negative.
3. Twist some tissue paper into a loose roll about six inches
long. Stick a pin through the middle of the roll into a vertical
3upport. Present an electrified rod to one end of the roll and thus
cause the paper to turn about the pin as an axis. Give this piece
of scientific apparatus an appropriate name.
4. (a.) Prepare two wire stirrups, A and B, like those shown
in Fig. 79, and suspend them by threads. Electrify two glass rods
by rubbing them with silk and j-lace them in the stirrups. Bring
A near B. Notice the repulsion. (6.) Repeat the experiment with
two sticks of sealing-wax that have been electrified by rubbing
with flannel. Notice the repulsion, (c.) Place an electrified glass
rod in A and an electrified stick of sealing-wax in B. Notice the
attraction. Give the law illustrated by these experiments.
5. Why is it desirable that a glass rod used for electrification
be warmer than the atmosphere of the room where it is used ?
6. Electrify one insulated egg-shell conductor ( 222, 6). Bring
it near a second conductor but not into contact with it. Touch the
second egg-shell with the finger, (a.) Experimentally, determine
whether the second egg-shell is electrified or not. (6.) If you find
that it is, what word explains the method of charging? (e.) If
the second egg-shell is charged, will its potential and the potential
of the first be of the same or of opposite signs 1
180 NATURAL PHILOSOPHY. 246
SECTION III.
VOLTAIC AND THERMO-ELECTRICITY.
246, Chemical Action. All chemical changes
( 11, a,) are accompanied by electric separation. The
chemical action between liquids and metals gives results
the most satisfactory. Electricity thus developed is
called voltaic or galvanic electricity.
247. Current Electricity. The principal classes of
electric currents are as follows :
( 1.) Currents produced by chemical action, i. e.,
voltaic currents.
(2.) Currents produced by heat, i. e., thermo-
electric currents.
(3.) Currents produced by other electric cur-
rents or by magnets, i. e., induced cur-
rents.
(a.) We have seen that, when a body having an electrical charge
is properly connected with another of lower potential, there is a
transfer of electricity from the former to the latter. This implies
that there is an electric current. But this current is only mo-
mentary and of little importance in comparison with the currents
that we are about to consider.
(J>.) Current electricity may differ from static electricity in quau
tity, electromotive force, etc., but not in its nature.
249
VOLTAIC ELECTRICITY.
181
248. The Voltaic Current.
copper and one of zinc are placed
in dilute sulphuric acid or in a
battery solution like the one
already used, the two strips being
connected above the acid by a wire
conductor, a current of electricity
is produced. The apparatus here
described is called a voltaic or
galvanic element or cell.
When a strip of
FIG. 102.
(a.) For voltaic purposes, the sul-
phuric acid should be diluted by slowly
pouring the acid into ten or twelve times its bulk of soft water.
Do not pour the water into the acid.
249. Direction of the Current. The metal most
vigorously acted upon by the liquid constitutes the gen-
erating or positive plate ; the other, the collecting or
negative plate.
When the wires from the two plates are in contact, it
is said that the circuit is closed; when the plates are not
thus in electric connection, it is said that the circuit is
broken.
When the circuit is broken, the ends of the wires are
called poles or electrodes. The negative pole is at-
tached to the positive plate and vice versa. Strips of
platinum are often fastened to the ends of the wires;
these platinum strips then constitute the electrodes.
In the liquid, the current is from the + to the
plate. In the wire, the current is from the
182 NATURAL PHILOSOPHY. 2$Q
+ to the electrode. In each case, the current
passes from -f to .
The direction of the current is indicated by the arrows
in Fig. 102.
250. Internal Resistance. We may imagine that
the two plates of a voltaic cell are connected by a liquid
prism. The greater the distance between the plates, the
longer this prism and the greater its resistance. The
larger the plates, the larger the prism and the less its
resistance.
Gases are poor conductors. Hence, the hydrogen
bubbles that often adhere to the negative plate increase
the internal resistance of the cell by lessening the effec-
tive surface of the plate. ( 272.)
251. The Ampere. The strength of current or its
rate of flow will depend upon electromotive force and
resistance, increasing with the former and decreasing
with the latter.
T/ie unit of current is called an ampere.
(a.) At any given instant, the current is the same at every part
of the circuit.
252. Ohm's Law. The following important for-
mula is known as Ohm's law :
r Us = Amperes, or 2=0.
Ohms~ R
(a.) If we have a difference of potential that secures an E. M. F.
of 18 volls and if the total resistance of the circuit be 3 ohms, the
strength of the current will be 6 amperes. 18 + 3 = 6.
2 55 VOLTAIC ELECTRICITY. 183
253. The Coulomb. The unit of quantity is
called the coulomb. It is the quantity of elec-
tricity given by a one ampere current in one
.econa.
(d.) A 10 ampere current will give 30 coulombs in 3 seconds.
254. Amalgamating the Zinc. Ordinary com-
mercial zinc is far from being pure. The chemically pure
metal is expensive. When impure zinc is used, small
closed circuits are formed between the particles of foreign
matter and the particles of zinc. This local action, which
takes place even when the circuit of the cell or battery
is broken, rapidly destroys the zinc plate and contributes
nothing to the general current. This waste is pre-
vented by frequently amalgamating the zinc. This ia
done by cleaning the plate in dilute acid and then rub-
bing it with mercury. See Elements of Natural Phi-
losophy, 386, a.
255. Polarization. It was stated in 250 that the
accumulation of hydrogen bubbles at the negative plate
increases the internal resistance of the cell. But the
hydrogen affects the current in another way. It acts
like a positive plate (being almost as oxidizable as the
zinc) and sets up an opposing electromotive force which
tends to set a current in the opposite direction.
A cell or battery in this condition is said to be
polarized.
Sometimes, as a result of polarization, the strength of
the current falls off very greatly within a few minutes
after closing the circuit
184
NATURAL PHILOSOPHY.
256
256. Varieties of Voltaic Cells. All voltaic cellg
belong to one of two classes :
(1.) Those using only one liquid.
(2.) Those using two liquids.
All of the earlier batteries were composed of one-fluid
cells. *
257. Smee's Cell. A Smee's cell is represented by
Fig. 103. It consists of a platinized
silver plate placed between two zinc
plates hung in dilate sulphuric acid.
The hydrogen bubbles accumulate at
the points of the rough platinum sur-
face and are more quickly carried up
to the surface of the liquid and thus
gotten rid of. The cell has an electro-
motive force of about 0.65 volts.
258. Potassium Di-chromate
Cell. The potassium di-chromate cell
has a zinc plate hung between two carbon plates.
lution of potassium di-chromate in dilute sulphuric acid
is the liquid used. The hydrogen is given an opportunity
for chemical union as fast as it is liberated.
The E. M. F. of this cell is great to start with (from
1.8 to 2.3 volts) but it falls very quickly when the external
resistance is small. It quickly recovers and may be used
with advantage where powerful currents of short duration
are wanted. It is the only single fluid cell that is free
from polarization.
259 VOLTAIC ELECTRICITY. 185
(a.) The bottle form of this cell, represented in Fig. 104, is the
most convenient for the laboratory or lecture
table. By means of the sliding rod, the
zinc plate may be raised out of the solution
when not in use. Thus adjusted, the cell
may remain for months without any action,
if desired, and be ready at a moment's notice.
(&.) One of the best proportions for the
solution is as follows : One gallon of water,
one pound of di-chromate of potash, and
from a half pint to a pint of sulphuric acid,
according to the energy of action desired.
A small quantity of nitric acid added to the
solution increases the constancy of the bat-
tery.
259. The Leclanche Cell.- FlG " 104
This cell, shown in Fig. 105, contains a zinc plate or
rod and a porous earthenware cup filled with carbon and
peroxide of manganese. This cup replaces the other
metal plate. The liquid used is a solution of ammonium
chloride (sal ammoniac) in water.
This cell is tolerably constant if it be not used to pro-
duce very strong currents, but its great merib is that it
is very permanent. It will keep in good condition for
months with very little attention, furnishing a current
for a short time whenever wanted. It is much used for
working telephones, electric bells (see H in Fig. 105)
and clocks, railway signals, etc.
The manganese oxide prevents polarization by destroy-
ing the hydrogen bubbles. If the cell be used continuously
for some time, the power of the cell weakens owing to
the accumulation of hydrogen, but if left to itself it
186
NATURAL PHILOSOPHY.
259
gradually recovers as the hydrogen is oxidized. Some-
times the manganese oxide is applied to the face of the
carbon and the porous cup dispensed with. This cell
has an E. M. F. of about 1.5 volts. It should be left on
open circuit when not in use.
FIG. 105-
261
VOLTAIC ELECTRICITY.
187
FIG. 106.
260. Daniell's Cell. This cell consists of a copper
plate immersed in a saturated solution of copper sulphate
(blue vitriol) and a zinc plate immersed in dilute sul-
phuric acid or a solution of zinc sulphate (white vitriol).
The two liquids are separated ;
usually one liquid is contained in
a porous cup placed in the other
liquid. Large crystals of copper
sulphate are placed on a perforated
shelf in the solution of copper sul-
phate to keep the latter saturated.
Such a cell will furnish a nearly
constant current, with an E. M. F.
of 1.079 volts and keep in order for
a long time. // should be kepi on
closed circuit when not in use.
The outer cell is sometimes made of copper and serves
as the copper plate, as is shown in Fig. 106. The
hydrogen passes through the
porous cell and acts upon the
solution of copper sulphate.
Copper, instead of hydrogen, is
deposited upon the copper plate.
Polarization is thus avoided.
261. The Gravity Cell.
This is a modification of the
Daniell's cell, no porous cup
being used. The copper plate
is placed at the bottom of the
cell and the zinc plate near the
FIG. 107.
188 NATURAL PHILOSOPHY. 26l
top. Crystals of copper sulphate are piled upon the cop-
per plate and covered with a saturated solution of copper
sulphate. Water or, preferably, a weak solution of zinc
sulphate rests upon the blue solution below and covers
the zinc plate. The two solutions are of different specific
gravities and remain clearly separated if the cell be kept
on closed circuit when not in use. (Fig. 107.)
This cell is very largely used in working telegraph
lines. It is sometimes called the Callaud cell.
262. Grove's Cell. The outer vessel of a Grove's
ceil contains dilute sulphuric acid. In this is placed a
hollow cylinder of zinc. Within the zinc cylinder is
placed a porous cup containing strong nitric acid. The
negative plate is a strip of platinum placed in the nitric
acid. The hydrogen passes through the porous cup and
reduces the nitric acid to nitrogen peroxide, which
escapes as brownish-red fumes. These nitrogen fumes
are disagreeable and injurious; it is well, therefore, to
place the battery in a ventilating chamber or outside the
experimenting room.
The E. M. F. of the Grove cell, under favorable con-
ditions, is nearly two volts, while its internal resistance
is small, being about one-fifth that of a Daniell's cell.
It is much used for working induction coils ( 306),
for generating the electric light, etc. It is, however,
troublesome to fit up and should have its liquids renewed
every day that it is used. Fig. 109 represents a, Grove's
battery with cells joined in series.
263. Bunsen's Cell. Bunsen's cell (Fig. 108) differs
26 4
VOLTAIC ELECTRICITY.
189
from Grove's in the use of carbon instead of expensive
platinum for the negative plate, thus reducing the cost.
The plates are made larger
than for Grove's battery.
Its E. M. F. is about the
same as that of the Grove
cell but its internal resist-
ance is greater. Fig. 110
represents a battery of Bun-
sen's cells joined in multiple
FIG. 108.
264. A Voltaic Bat-
tery. A number of vol-
taic elements connected
in such a manner that the current has the same
direction in all, constitutes a voltaic battery.
The usual method is to connect the positive plate of
one clement with the negative plate of the next, as
shown in Fig. 109. When thus connected, they are said
to be coupled "tandem" or "in series." Sometimes all
of the positive plates are connected by a wire and all of
the negative plates by another wire. The cells are then
said to be joined "parallel," "abreast" or "in multiple
arc." (See Fig. 110.)
(a.) When two or more cella are joined together, the points of
contact should be as large as is convenient and kept perfectly clean
The connecting wire should be of good size and, for the sake ol
pliability, a part of it may well be given a spiral form by winding
it upon a pencil or other small rod.
100 NAtURAL PHILOSOPHY 265
265. Batteries of High Internal Resistance.
Each kind of galvanic cell has an internal resistance, aa
explained in 250. A battery of cells joined in series
is called a "battery of high internal resistance." (Fig.
109.) This method of joining the cells increases the
length of the liquid conductor through which the cur-
FIG. 109.
(a.) In a battery of cells joined in series, the E. M. F. and the
internal resistance are those of a single cell multiplied by the
number of cells.
266. Batteries of Low Internal Resistance.
A battery of cells joined parallel is called a "battery of
low internal resistance." (Fig. 110.) This method of
joining the cells does not increase the length of the liquid
conductor traversed by the current but is equivalent to
increasing its diameter or area of cross section.
For a circuit of great external resistance, a battery
of high internal resistance is needed. For a circuit of
267 VOLTAIC ELECTRICITY. 191
small external resistance, large cells, or several cells joined
parallel are preferable.
(a.) In a battery of cells joined parallel, the E. M. F. is that of
a single cell but the internal resistance is that of a single cell
divided by the number of cells.
FIG. 110.
(6.) A battery of high internal resistance was formerly called
an intensity battery, while a battery of low internal resistance
was called a quantity battery.
267. The Best Arrangement of Cells. The best
method of coupling cells depends on the work to be
done by the battery. The maximum effect is at-
tained when the internal resistance of the bat-
tery is equal to the resistance of the external
circuit. For example, suppose that in a given battery
of eight cells :
( 1.) Each cell has an E. M. F. of two volts.
(2.) Each cell has the very high internal resistance of
eight ohms.
(3.) The battery is to work through a wire that has a
resistance of sixteen ohms.
192 NATURAL PHILOSOPHY. 267
(.) First, couple the cells parallel. The E. M. F. of the battery
is that of a single cell, 2 volts. The internal resistance is 8 ohms
-5-8 = 1 ohm. Adding the external resistance, we have a total
resistance of 17 ohms. (See 252.)
This arrangement gives us a current of 0.1176+ amperes.
(&.) Next, couple the cells in series. The E. M. F. of the battery
is 8 times 2 volts, or 16 volts. The internal resistance is 8 times 8
ohms or 64 ohms. Adding the external resistance, we have a total
resistance of 80 ohms.
C - E - 16 -02
= =
This arrangement gives us a current of 0.2 amperes.
(e.) Finally, join the ceils in two rows of four cells each in series
and join the rows parallel. The E. M. F. of the battery will be 4
times 2 volts or 8 volts. The internal resistance will be 4 times
8 ohms or 32 ohms for each row, but only half that, or 16 ohms,
for the whole battery. Adding the external resistance, we have
a total resistance of 32 ohms.
C = 1 = 16^16 ^
This arrangement, in which the internal and the external resist-
ances are equal, gives us a current of 0.25 amperes, the greatest
possible under the given conditions.
(d.) A similar application of Ohm's law shows that when the
external resistance is large, there is little gain from joining cells
parallel and that when the external resistance is very small, there it
little gain in joining cells in series.
Experiment Ml. From the poles of a potassium di-chromate
battery, lead two stout copper wires and connect their free ends by
268 VOLTAIC ELECTKTCirr. 195
two or three inches of very fine iron icire. Coil the iron wire
around a lead pencil and thrust a small quantity of gun-cotton into
the loop thus formed. Plunge the zinc plate of the battery into
the liquid and the iron wire will be heated enough to explode the
gun-cotton ; it may be heated to redness or even fusion.
The resistance of iron wire is about seven times as great as that
of a similar copper wire ; in other words, its conducting power is
only about one-seventh as great. The decrease in the size of tha
wire also adds to its resistance.
268. Thermal Effects of the Electric Current.
Whenever an electric current flows through a con-
ductor, part of the electricity is changed into heat.
Electric energy is changed into heat energy.
T7ie amount of electricity thus changed inh>
heat u-iU depend upon the amount of resistance-
offered by the conductor.
In the last experiment, the stout copper wires wero
good conductors, offered but little resistance and con
verted but little of the electrical energy into heat energy.
The change of material from copper to iron increased
that resistance. This increased resistance was again in-
creased by reducing the size of the conductor. For this
double reason, the fine wire offered so much resistance
that a considerable of the current energy was trans-
formed into heat.
Resistance in an electric circuit always pro^
duces heat at the expense of the electric current
Thus, electricity is often used in firing mines in milt
tary operations and in blasting. All known metals have
been melted in this way.
194 NATURAL PHILOSOPHY. 269
269. Luminous Effects of the Electric Current.
When an electric circuit is closed or broken, there is a
spark at the point of contact, due to the heating of a part
of the conductor to incandescence. We have seen lumi-
nous effects produced by winding the wire from one plate
of a voltaic cell around one end of a file and drawing the
other electrode along the side of the file, thus rapidly
closing and breaking the circuit.
If the iron wire used in the last experiment was heated
sufficiently, it also gave a luminous effect and illustrated
the fundamental principle of the incandescent electric
lamp.
(a.) The most important luminous effects of electricity will be
considered in connection with dynamo electric machines ( 311).
It will be noticed that all of these are secondary thermal effects.
270. Physiological Effects of the Electric Cur-
rent. An electric current may produce muscular con-
vulsions in a recently killed animal. Experiments with
the Leyden jar and the induction coil ( 30G) show that
similar effects may be produced upon the living animal.
Electricity is largely used as an agent for the cure of
disease; experiments of this kind may do injury and
would better be left to the educated physician. The
discharge of a large battery may be fatal and a number
of persons have lost their lives within the last few years
by coming, accidentally or otherwise, into the circuit of
a dynamo-electric machine.
271. Chemical Effects of the Electric Current.
Many chemical compounds in solution may be decom-
272
T8ERMO- ELECTRICITY.
195
posed by forcing the current to traverse the solution.
Substances which are thus decomposed are called elec-
troll/tea ; the process is called electrolysis ; the com-
pound is said to be electrolyzed.
The electrolysis of acidulated water is easily accom-
plished with a current from two Grove's or Bunsen's
cells. (See Chemistry, Experiment 12.) The water is
decomposed into oxygen and hydrogen.
The apparatus, shown in Fig. Ill, may be called a
water-voltameter.
^
FIG. Ill
272. Ions. The products of electrolysis, like the
oxygen and hydrogen, are called ions; the one that
goes to the + electrode (or anode) is called the aniort ;
the one that goes to the electrode (kathode or cathode)
is culled the kathioii or cathioti.
196 NATURAL PHILOSOPHY. 272
Experiment 112. From the + pole of a voltaic battery or
dynamo-electric machine, suspend a plate of copper ; from the
pole, suspend a silver coin. Place the copper and silver electrodes
in a strong solution of copper sulphate (blue vitriol). When the
circuit is closed, the salt of copper is electrolyzed, the copper from
the salt being deposited upon the silver coin and the sulphuric
acid going to the copper or + electrode. The silver is thus
electro-plated. (Fig. 112.)
The countless applications of this process of depositing a me-
tallic coat on a body prepared for its reception, constitute the
important art of electro-metallurgy.
FIG. 112.
273. The E. M. F. of Polarization. The pro-
ducts of electrolysis have a tendency to reunite by virtue
of their chemical affinity. (Chemistry, 8.) For ex-
ample, the electrolysis of zinc sulphate gives zinc and
sulphuric acid. But we now well know that the chemical
action of these two substances has an electromotive force
of its own. This E. M. F. of the ions acts in opposition
to that of the electrolyzing current. In some cases, it
rises higher than the E. M. F. of the original current and
reverses the direction of the current.
274 THERMO-ELECTRICITY. 197
The oxygen and hydrogen, yielded by the electrolysis
of water ( 271), tend to reunite and set up an opposing
E. M. F. of about 1.45 volts. Thus we see that it re-
quires a battery or cell with an E. M. F. of more than
1.45 volts to decompose water.
TJiis electromotive force of the ions is called
the E. M. F. of Polarization.
It may be observed by putting a galvanometer in the
place of the battery of the water-voltameter (Fig. 111).
The polarization in a voltaic cell acts in the same way.
(a.) There is no opposing E. M. F. of polarization when the
kathion and the anode are of the same metal. For example, the
feeblest current will deposit copper from a solution of copper sul-
phate, when ike anode is a copper plate.
274. Secondary Batteries. When a voltameter of
an electro-plating bath is supplying a current of elec-
tricity, as mentioned in the last paragraph, it constitutes
a secondary battery. As the ions do not reunite when
the circuit is open, the energy of the decomposing cur^
rent may be stored up as energy of chemical affinity.
Wlien ft current is again wanted, the circuit
may be closed and the energy of chemical affinity
at once appears as energy of electric current.
Secondary batteries are, consequently, often called
storage batteries.
(a.) The Faure battery consists of two plates of sheet lead coated
with red lead (lead oxide). These plates are separated by a layer
of paper or cloth, rolled up in a loose coil like a roll of carpet and
immersed in dilute sulphuric acid.
198 NATURAL PHILOSOPHY. 274
(6.) When a current from a dynamo-electric machine or a vol-
taic battery is sent through such a cell, chemical action is pro-
duced. Oxygen acts on the coating of the anode plate and converts
it into a higher oxide of lead. Hydrogen unites with the coating
of the kathode plate and reduces it to metallic lead. When these
changes have gone as far as possible, the battery is said to be
" charged." The charged plates will remain in this condition for
days if the circuit be left open.
(e.) By closing the circuit, the plates will, at any time, furnish
a current until they are changed to their original chemical condi-
tion. As the lead plates and the acid are not rapidly destroyed,
the battery may be charged and discharged many times.
(d.) Many serious defects in the Faure battery have been ob-
viated in the Brush battery, which is the only one yet used to any
considerable extent in this country. A Brush dynamo-electric
machine ( 311) is operated in the daytime for charging the bat-
teries. At night, the same dynamo may be used for operating arc
electric lights ( 313), while the charged secondary battery is
furnishing the current for the incandescent electric lighta The
E. M. F. of each Brush cell is about two volts. For electric light-
ing, they are generally prepared in batteries of twenty or more
cells.
275. Magnetic Effects of the Electric Current.
Any conductor is rendered magnetic by passing a cur-
rent of electricity through it. We have already seen
that a bar of soft iron may be temporarily magnetized
by the influence of the voltaic current. It may be
further shown by the action of the bar and helix.
(a.) The bar may be a straight piece of stout iron wire ; the helix
may be made by winding cotton covered copper wire upon a piece
of glass tubing large enough to admit the wire and not quite as
long as the iron,
276 THERMO-ELECTEICITT. 199
(6.) A good bclix, convenient for many purposes, may be mada
upon an ordinary wooden spool. With a sharp knife, make the
shank of the spol as thin as possible and then wind the spool
full of insulated copper wire about as large as ordinary broom or
stove-pipe wire. The iron bar must be small enough to pass easily
through the hole in the spool and long enough to project a little
way beyond each end.
(c.) Either of these helices may be placed in the circuit of a cell
and held in a vertical position, when it will act as a " sucking "
magnet. The movable iron core will be held in mid-air " without
any visible means of support."
(d.) The " helix and ring armature," is shown in Fig. 113.
The armature is of soft iron divided into two semi-
circles with brass handles. When the helix is
placed in a closed circuit, the semicircles resist a
considerable force tending to draw them apart ;
when the circuit is broken they fall asunder of
their own weight The iron ring may be made
without handles by any blacksmith. Stout cords
will answer for handles. The helix may be made
by winding insulated wire upon a pasteboard cylin-
der an inch or an inch and a half long. There
should be four or five layers of the wire which may
be tied together with strings passing through the hole in the helix.
(e.) Such temporary magnets as these are called electro-mag-
nets. The subject of electro-magnets will be further considered
in 298-300.
276. The Electric Telegraph. The electric tele-
graph consists essentially of an electro-magnet and a
" key " placed in the circuit of a battery. The key is an
instrument by which the circuit may be easily broken or
closed at will. The armature, A, of the magnet, M, is
2CO
NATURAL PHILOSOPHY.
2 7 6
FIG. 114.
supported by a spring, S, which lifts it when the circuit
is broken. When the
circuit is closed, the
armature is drawn
down by the attrac-
tion of the magnet.
Thus the armature
may be made to vi-
brate up and down
at the will of the
person at the key.
The armature may
-ct upon one arm of a lever, the other end of which,
being provided with a style or pencil, P, may be pressed
against a paper ribbon, R, drawn along by clock-work.
Thus the pencil may be made to record, upon the
moving paper, a series of dots and lines at the pleasure
of the operator at the key perhaps hundreds of miles
away. When the two stations are several miles apart,
one of the wires is dispensed with, the circuit being
completed by connecting each station with the earth.
The inventor of the practical electric telegraph was an
American, S. F. B. Morse. The system of signals devised
by him is given in the Elements of Natural Philosophy,
445.
To prevent confusion, a small space is left between
successive letters, a longer one between words, and a still
longer one between sentences. Telegraph operators soon
oecome so familiar with this alphabet that they under-
stand a message from the mere clicks of the lever and
277
HERMO-EL ECTR1CITY.
201
do not use any recording apparatus. Such an operatol
is said to "read by sound"; his instrument is called a
"sounder."
The same principle of communicating signals by
making and breaking an electric
circuit is used in fire and burglar
alarms, hotel-annunciators, etc.
277. The Galvanometer.
We have already seen that the vol-
taic current has a marked effect in
turning the magnetic needle from
its north and south position, tend-
ing to place the needle at right
angles to the direction of the cur-
rent.
The galvanometer is a very delicate instru-
ment for detecting the presence of an electric
current and determining its direction and
strength.
The magnetic needle is very light and suspended so
as to turn easily. The wire conductor is insulated and
coiled many times about the needle ; the effect is thus
multiplied. A glass cover protects the apparatus from
dust and disturbance by air currents. The instru-
ment is largely used. One form is represented in
Fig. 115.
If the instrument shows the presence and direction of
the current without measuring its strength, it is a gal'
vanoscope rather than a galvanometer.
FIG. 115.
202
NATURAL PHILOSOPHY.
277
Experiment 113. Connect an iron and a German silver wire to
the binding posts of a delicate galvanometer. Twist the free ends
of the wires together and heat the junction in the flame of an
alcohol lamp. The deflection of the galvanometer-needle will
show that an electric current is traversing the circuit. Cool
the junction with a piece of ice. The galvanometer will show
that a second current is flowing in the opposite direction.
278. Thermo-Electricity. // a circuit be made
of two metals and one of the
junctions be heated or chilled,
a current of electricity is pro-
duced.
A thermo-electric pair may be
made by soldering together a bar
of antimony, A, and one of bis-
muth, B, and joining their free ends by a wire. Several
such pairs may be joined to
form a thermo-electric series,
as shown in Fig. 116. Sev-
eral such series may be joined
to form a thermo-electric pile,
the bars being separated by
strips of varnished paper and
compactly set in a metal
frame so that only the sold-
ered ends are open to view.
The free end of the antimony
bar, representing the -f elec-
trode, and the free end of the
bismuth bar, representing the
electrode, are connected with binding screws, which
FIG.
278 THERMO-ELECTRICITY, 203
may be connected with a sensitive, short coil galvano-
meter (Fig. 115). The thermo-electric pile, with the
addition of conical reflectors, is shown in Fig. 117.
A change of temperature at either exposed face of the
pile produces a feeble current of electricity which is
manifested by the movement of the needle of the gal-
vanometer. The instrument is much used in scientific
work for detecting differences in temperature, being
much more sensitive than the mercury thermometer.
204
NATURAL PHILOSOPHY.
279
279. Recapitulation. To be ampli6ed by the pupil
for review.
(Smee's.
Potassium
Di-chromat
Leclanche.
Ctll. H
f Daniell's.
Two Liquids, -j g^ve'! 8 '
[ Bunsen's.
VOLTAIC.
( Tandem.
Joined \ Abreast.
\ Best Method
Battery. .
High Internal Resistance.
Low Internal Resistance.
Current..
Direction.
Strength.
CHEMICAL
ACTION.
SIGN OF .
Plate.
Pole.
Electrode.
Anode.
KathodQ
RESISTANCE
External.
Internal.
QUANTITY.
ELECTRICITY
LOCAL ACTION .
POLARIZATION . .
THERMAL
LUMINOUS.
i Cause
\ Remedy.
( Cause.
I Remedy.
\ . M.F.
..Relation to Resistance'
CURRENT
EFFECTS.. .
PHYSIOLGICAL.
CHEMICAL -
Electro-Metallurgy.
E. M. F. of Polariza-
tion. ( Fauro's.
Secondary Batteries.. \ Uses^ 8 '
/ Advantages
MAGNETIC.
( Electro-Magnets.
1 Electric Telegraph.
( Galvanometer.
THERMO-ELECTRJCITY.
279 EXERCISES. 205
EXERCISES.
1. Does dilute acid or a battery solution act upon zinc mow
vigorously than it does upon copper, or otherwise ?
2. State three ways in which the internal resistance of a voltaic
cell may be diminished. State two ways in which the strength of
current of a voltaic cell or battery may be increased.
3. What is " local action " and how may it be prevented?
4. If the re.-istance of 18.12 yards of No. 30 copper wire be 3.03
ohms, what length of the same wire is there in a coil, the resist-
ance of which is 22.65 ohms?
5. Given a battery of five Daniell's cells coupled in series.
Each cell has an E. M. F. of 1.1 volts and an internal resistance of
2.2 ohms. The wire of the circuit has a resistance of 44 ohms.
What is the strength of the current? Ans. -fa ampere.
6. It is found that one Daniell's cell, however large, will not
decompose acidulated water. It is also found that two Daniell's
cells, however small, is sufficient for continuous electrolysis.
Explain.
7. If the E. M. F. of a Daniell's cell be 1.08 volts and that of
a Grove's cell 1.73 volts, the internal resistance of the former being
five times as great as that of the latter and the external circuit
being a stout, short copper wire, the resistance of which is so small
that it may be neglected, show that the Grove's cell will give
about eight times as strong a current as the Daniell's.
8. Twenty-four similar cells are arrange:! in four batteries of
six cells, each coupled in series. These batteries are joined abreast.
(a.) How will the E. M. F. of this battery compare with that of a
single cell? (6.) How will Us internal resistance compare with,
that of a single cell ?
9. Given a battery of 100 Grove cells, each having an E. M. F.
of 2 volts and an internal resistance of 0.25 ohms. The wire of
the circuit has a resistance ^f 1000 ohms. Determine the strength
206 NATURAL PHILOSOPHY. 279
of the current (a.) when the cells are joined in series. (6.) When
the cells are joined abreast. Ans. (a.) .195 + amperes.
10. Given the same battery as in the last exercise, the external
circuit now being a short wire of only 0.001 ohni resistance. De-
termine the strength of the current (a.) when the cells are joined
abreast, (b.) When the cells are joined in series.
Ans. (a.) 571.4 amperes; (6.) 7.99 amperes.
11. Given a single cell like those mentioned in the last two
exercises. Join its poles with a short, stout copper wire, which
has a resistance of 0.001 ohm. Determine the current that it will
give and see how it compares with the current of 50 such cells
joined in series, as mentioned in the last exercise.
12. Short circuit the cell mentioned in the last exercise, i. e.,
make the circuit with a wire of so little resistance that it may be
dropped out of the account. Determine the strength of current.
Will joining any number of cells joined in series increase this
effect?
MAGNETISM. 207
SECTION IV.
MAGNETISM.
280. Natural Magnets. One of the most valuable
iron ores is called magnetite (Fe 3 4 ). Occasional speci-
mens of magnetite will attract filings and other small
pieces of iron. Such a specimen is called a, load-
stone. It is a natural magnet.
281. Artificial Magnets. Artificial magnets are
either temporary or permanent. A temporary magnet
is usually made of soft iron and is called an electro-
.nagnet. A permanent magnet is usually made of steel.
Artificial magnets have all the properties of natural mag-
nets and are more powerful and
convenient. They are, there-
fore, preferable for general use.
The most common forms are
. , , , FIG. 118.
the straight or bar magnet and
the horseshoe magnet. The first of these is a straight
bar of iron or steel ; the second is shaped like a letter U,
the ends being thus brought near together, as shown in
Fig. 118.
A piece of iron placed across the two poles of a horse-
shoe magnet is called an armature. We have already
learned how to make artificial magnets.
282. Retentivity. It is more difficult to get the
magnetism into steel than into iron. It is also more
difficult to get the magnetism out of steel than out of
208 NATURAL PHTLOSOPHY. 282
iron. This power of resisting magnetization or demag-
netization is called coercive force or retentivity.
The harder the steel, the greater its retentivity. Soft
wrought iron has but little retentivity.
Experiment 1 14. Roll a bar magnet in iron filings. Withdraw
the magnet; the filings cling to the ends of the bar but not
to the middle.
283. Magnetic Poles. Magnetic attraction is not
evenly distributed throughout the bar.
It is greatest at or near the ends. These points
of greatest attraction are called the poles of thfi
magnet.
284 MAGNETISM. 209
It is impossible, by any known means, to develop one
magnetic pole without simultaneously developing anothei
pole of opposite sign. The middle of the magnet doet;
not attract iron and is called the equator or neutral
point.
Experiment 115. Bring either end of a bar magnet near the
end of a piece of iron, A B ; the iron
is attracted. Bring the same end of
the magnet near the middle of the
iron ; the iron is attracted. Bring tin:
the same end of the magnet near the
other end of the iron ; the iron is at-
tracted. Repeat the experiments with ~ ~~ ~~
the other end of the magnet : in each
case the iron is attracted.
284. Attraction between a Magnet and Iron.
Either pole, of a magnet mill attract ordinary
Experiment 116. Freely suspend threa bar magnets, A, B and
C, at some distance from each other. This may be done by placing-
each magnet in a stout paper stirrup supported by a cord or upon
a board or cork floating on water. See Fig. 120. When they
have come to rest, each will lie in a north and south line.
Magnets for this experiment may be made by magnetizing
( 300) three stout knitting-needles. If there is any electric light,
apparatus in your neighborhood in charge of a good natured man,
he will probably magnetize the needles for you.
Each needle may be suspended by means of a triangular piece
of stiff writing-paper. Pass the needle through the paper near
the lower corners ; at the other corner affix by wax the end of a
horse-hair. The poles may be indicated by little bits of red and of
white paper, fastened by means of wax to the ends of the needlea
Mark the north-seeking poles, and the south-seeking poles, +.
210 XATtTttAL PHILOSOPHY. 28$
285. Characteristics of Magnets. Magnets are
chiefly characterized by the property of attract-
ing iron and by a tendency to assume a particu-
lar direction of position when freely suspended.
Experiment 117. (a.) Take magnet A of Experiment 116 from
its support, and bring its + end near the end of B or 0. Notice
the attraction.
(6.) Bring the + end of A near the + end of B or C. Notice
the repulsion.
(c.) Bring the end of A near the end of B or C. Notice
the repulsion.
(d.) Bring the end of A near the + end of B or C. Notice
the attraction.
(e.) From experiment (a) we learned that the ends of B and C
were each attracted by the + end of A. Bring the end of B
near the end of 0. Notice that they now repel.
(/.) From experiment (b) we learned that the + ends of B and
C were each repelled by the + end of A. Bring the + end of B
near the + end of C. Notice that they now repel.
(gr.) In similar manner show that the + end of B will attract
the end of 0; that the end of B will attract the + end of C.
Record the results of your experiments in tabular form, thus :
(a.) + attracts .
(d.) attracts +.
etc.
(6.) + repels +.
(c.) repels .
etc.
Experiment I! 8. Var? <ue last experiment by pasting a paper
image of a man at the + .ue of each of the magnetized needles
and a paper image of a wr/nan at the end of each. Notice that
the men are unfriendly and will not approacli each other ; that the
women turn from each ot> *, but that the man and the wornajj
are attracted toward each o" er.
286 MAGNETISM. 211
Experiment 119. Magnetize a number of fine sewing-needles
by drawing the + end of a bar magnet three or four times from
the eye to the point of each. Cut several small corks into slices
about an eighth of an inch thick. Through each cork disc, ouob
FIG. 121.
a needle up to its eye and place them in a round dish of water.
These little magnets have their like poles presented to each othef
and they mutually repel. Bring the bar magnet, with its + end
downwards, over the needles; they will be driven to the sides.
Similarly, bring the end over them ; they will be attracted
toward the centre.
286. Laws of Magnets. (1.) Every magnet
has two similar poles; like poles repel each
other ; unlike poles attract each other.
(2.) Magnetic force, like other forms of attrac-
tion and repulsion, varies inversely as the square
of the distance.
Experiment 120. Dip one of the magnetized knitting-needles
Into iron filings, as in Experiment 114. Notice that filings cling
to the ends, near the paper discs but that none cling to the
middle. Now break the needle in the middle and dip each piece
212 NATURAL PHILOSOPHY. 28P.
into iron filings. Notice that the unmarked ends, which were a*
the middle of the unbroken magnet, now attract iron filings as
well as do the marked ends. Poles have been developed in
parts of the needle that previously showed no magnetic at-
traction.
287. Effect of Breaking a Magnet. If a mag-
net be broken, each piece becomes a magnet with two
poles and an equator of its own. These pieces may lie
repeatedly subdivided and each fragment will be a per-
fect magnet.
It is probable that every molecule has its jtoJrs
or is polarized and that, could one be isolated,
it would be a perfect magnet.
288. Magnetized, Magnetic and Diamagnetic
Substances. A magnetized body is one that can bo
made to repel a pole of a freely suspended magnet.
Substances that are attracted by a magnet are called
magnetic ; e. g., iron, steel and nickel.
Substances that are repelled by a magnet are called
diamagnetio ; e. g., bismuth, antimony and arsenic.
Of these, iron is by far the most magnetic, while bis-
muth is the most diamagnetic.
Experiment 121. Wrap a bar magnet in a piece of cloth
With it, attract and repel the poles of a suspended magnet.
2QO MAGNETISM. 213
Experiment 122. Repeat the last experiment, holding a slata
or sheet of zinc between the two magnets.
Experiment 123. Put one piece of the broken magnet into a
bottle ; cork the bottle tightly. With it, attract and repel the
poles of a suspended magnet.
289. Magnetic Screens. Nothing but a mag-
netic body can cut off the action of a magnet.
Experiment 124. Place a piece of card-board or rough draw
ing paper over a good bar magnet. Sprinkle iron filings upon the
card-board and tap it lightly. The iron particles will move and
arrange themselves in well defined curved lines. See Fig. 123.
290. Magnetic Field. A magnet seems to be sur-
rounded by an atmosphere of magnetic influence called
the magnetic field. The magnetic curves, shown in
the above experiments, show the direction of the lines
FIG. 123.
of magnetic force. If a small magnetic needle be
suspended over the card-board, its length will tend to lie
in the direction of the lines of magnetic force as mapped
out by the iron filings.
214 NATURAL PHILOSOPHY. 2QI
The " magnetic curves," formed in the last experiment
are very interesting and instructive. The filings in any
one of th.ese curves are temporary magnets with adjoin-
ing poles opposite and therefore attracting. By using
two bar magnets placed side by side, first, with like pules
near each other, and, secondly, with unlike poles near
each other, their combined effect on the iron filings may
be easily observed.
Experiment 125. Rub one end of a steel pen against the end
of a magnet. Dip the pen into iron filings and notice that the
newly made magnet has a pole at each end. Determine the sign
of each of these poles, as indicated in Experiment 116.
291. Magnetization by Contact..^ bar of iron
or steel may be magnetized by rubbing it against
a magnet.
Pure or soft iron is easily magnetized but quickly
ioses its magnetism when the magnetizing influence is
removed. Hardened steel is magnetized with more diffi-
culty but retains its magnetism after the removal of the
magnetizing influence.
Experiment 126. Move the point of an unmagnetized steel
pen to and fro very near one end of a magnet but without touch-
ing it to the magnet. Dip the pen into iron filings and determine
FIG. 124.
whether or not it has been magnetized. If it has, determine the
sign of each pole, as in the last experiment and notice whether
2Q2 MAGNETISM. 215
the point of the pen is of the same polarity as the end of the
magnet near which it was moved.
Experiment 127. Bring a short bar of soft iron, /, very near a
strong bar magnet, M, end to end, as shown in the figure. Sprinkle
iron filings over the end of the iron bar and they will cling as they
would to a magnet. The iron bar is a magnet, while it remains
in this position.
292. Magnetic Induction. If the end of a bar of
soft iron be brought near one of the poles of a strong
magnet, the iron becomes, for the time 'being, a
magnet. ' The poles of the temporary magnet will be
opposite to those of the permanent magnet, t. <?., if the
+ or positive pole of the magnet be presented to the
iron bar, it will develop a or negative pole in the
nearest end of the iron bar and a -j- pole at the further
end. Bring the iron bar nearer the magnet and this
effect will be increased.
Actual contact is not necessary, but when the iron and
the magnet touch, the magnetizing force is the greatest.
If a steel bar be used instead of an iron bar, it will be
permanently instead of temporarily magnetized.
The iron or the steel is induced to become a
magnet by the influence of the magnet used. It
is said to be magnetized by induction.
Experiment 128. Bring a soft iron ring to the end of a mag-
net. It will be supported. Bring a second ring into contact with
the first ring and it will be supported. In this way quite a num-
ber of rings may be supported, each ring being magnetized by
the bar or ring magnet above it. Of course, the attractive force
216
NATURAL PHILOSOPHY.
2 9 2
is continually weakening
from the first to the last
ring.
Now support the upper
ring upon your finger and
remove the magnet. Each
ring ceases to be a magnet
and the chain is broken
into its separate links.
Experiment 129. Vary
PIG. 125. the last experiment by
using, instead of the
rings,
1. Soft iron nails.
2. Steel sewing-needles.
t3ee if there is any difference in the results.
Experiment 130. Suspend an iron key from the positive end
of a bar magnet. A second bar magnet of about the same power,
with its poles opposite, is moved along the first magnet. When
the end of the second magnet comes over the key, the key
drops.
FIG. 126.
The first magnet tends to induce a pole at the upper end of
the key. The second magnet tends to induce a + pole at the same
point. Hence the effect of each magnet neutralizes that of the
2p4 MAGNETISM. 217
Experiment 13!. Magnetize a piece of watch spring about sii
Inches long (easily obtainable at the watch repairer's) by drawing
it several times between the thumb and the end of a magnet.
Dip it into iron filings Lift it carefully with its load. Bring the
poles of the spring magnet together, bending the magnet into a
ring. The magnet drops its load.
293. Induction Precedes Attraction. We now
see why a magnet attracts ordinary iron; it first magnet'
izes it and then attracts it. The attrac- *
tion between unlike poles is greater than
the repulsion between like poles because
of the smaller distance between them.
Compare 225.
Experiment 132. Test a common fire-poker
for magnetism by bringing a small magnetic
aeedle near its ends and seeing whether the poker
-epels either pole of the compass needle or whether
the two ends of the poker attract different poles
uf the needle.
Experiment 133. If the poker is not slightly magnetic, place
it with its upper end sloping toward the south BO as to make an
angle of a little less than half a right angle. In other words,
place it in the position assumed by the dipping needle. ( 295.)
While the poker is in this position, strike it a few blows with a
wooden block or mallet. Test it again for magnetism.
294. The Earth is a Magnet. The earth acts
like a huge magnet in determining the direction of com-
pass and dipping needles. Its inductive influence, as
shown in the last experiment, strengthens the belief
JO
218
NATURAL PHILOSOPHY.
295
that it has such action. In short, many facts seem to
teach that the earth is a great magnet with mag-
netic poles near its geographical poles.
Experiment 134. By means of a fine wire fork, gently lay one
of the magnetized sewing-needles of Experiment 119 on the surface
of water. It will float without any cork or similar support and
will assume a north and south position. It may be considered the
needle of a small compass.
295. Magnetic Needles. A small bar magnet
suspended in such a manner as to allow it to
assume its chosen position is a magnetic needle.
(a.) If it be free to move in a hori-
zontal plane, it is a horizontal needle ;
6.g., the mariner's or the surveyor's com-
pass (Fig. 128). It will come to rest
pointing nearly north and south. If the
magnet be free to move in a vertical
plane it constitutes a vertical or dipping
needle (Fig. 130).
FIG. 138.
(6.) Make a horizontal needle of a
piece of watch spring about six inches long and straightened by
drawing it between thumb and finger. Heat the needle to red-
ness in a flame and bend it double. Bend the ends back into
a line with each other, as shown in Fig. 129. Magnetize each end
separately and oppositely. Wind a waxed
thread around the short bend at the mid-
dle to form a socket and balance the
needle upon the point of a sewing-needle
thrust into a cork for support. A little
filing, clipping with shears or loading with
wax may be necessary to make it balance.
Fig. 129.
MAGNETISM.
219
(c.) Make a dipping needle by thrusting a knitting-n<jedl
through a cork so that the cork shall be at the middle of the needle.
Thrust through the cork, at right angles to the knitting-needle,
half a knitting needle, or a sewing-needle, for an axis. Support the
ends of the axis upon the edges of two glass goblets or other con-
venient objects (see Fig. 130). Push the knitting-needle through
the cork so that it will balance upon the axis like a scale beam.
Magnetize the knitting-needle and notice the dip.
(d.) A magnetized
sewing-needle, sus-
pended near its middle
(at its centre of gravity)
by a fine thread or hair
or an untwisted fibre
will serve as a dipping
needle. It should first
be suspended so as to
hang horizontal and
magnetized afterward.
A simple form of dip-
ping needle is repre-
sented in Fig. 180.
Experiment 135.
Measure the angle that
your dipping needle
makes with the surface
of quiet water. The
angle in question is
indicated by the dotted arc of Fig. 130.
FIG. 130.
296. Inclination or Dip. The angle that a
dipping needle makes with a horizontal line is
called its inclination or dip.
220 NATURAL PHILOSOPHY. 2Q7
At the magnetic poles, the inclination is 90 ; at the
magnetic equator, there is no inclination. The inclina-
tion at any given place is not greatly different from the
latitude of that place.
Experiment 136. Set two stakes so that a string joining them
will point toward the North Star. The string will run north and
south or nearly enough so for our purpose. Place a long magnet
suspended as a needle under or over the string. Looking down-
ward at the magnet and the string, it will probably be found that
the needle and the string do not point in the same direction.
The North Star may be easily found any evening in the direc-
tion indicated by " The Pointers" of the well known constellation,
" The Great Dipper." " The Pointers " are the two stars marked
by the Greek letters a and ft in the diagram below.
NORTH
STAR
GREAT
* r
*
297. Declination or Variation. The magnetic
needle, at most places, does not lie in an exact north and
south line,
MAGNETISM. 221
Tlie angle which the needle makes irith the
geographical meridian is its declination or va-
riation.
Experiment 137. Send a current of electricity from the small
cell, mentioned in Experiment 84 through its wire. Pour half a
teaspoonf ul of iron filings upon a sheet of paper and bring the wire
conductor of the cell into contact with the filings. Notice that
the filings cling to the wire as though it were a magnet. Break
the circuit and notice that the filings fall from the wire.
298. Electro-Magnets. From the last experiment,
we see that while the wire conductor was carrying an
electric current it had the properties of a magnet. "We
have already seen that under similar circumstances, the
conductor deflects a magnetic needle as if it were itself
a magnet. In fact, such a conductor is a temporary
magnet. The magnetic effect is
much increased if a considerable
length of the conductor be made of
cotton covered (insulated) wire and
wound into a coil, as shown in
Fig. 131. Such a coil is a magnet
with a + pole at one end and
a pole at the other. It has an
easily perceptible magnetic field.
If a soft iron rod or core be intro-
r IG. lol.
duced into the coil, it enters the
magnetic field of the coil or helix and becomes a
magnet
NATURAL PHILOSOPHY.
S209
This combination of coil and core constitutes an
electro-magnet and is more powerfully magnetic than
the coil alone.
An electro-magnet is a bar of iron surrounded
by a coil of insulated wire carrying a current
of electricity.
It may be made more powerful than any permanent
magnet but loses its power as soon as the current ceases
to flow through its coil. The fact that the magnetism
of this apparatus is under control adapts it to many im-
portant uses, such as electric bells and telegraphic instru-
ments.
299. Forms of Electro-Magnets. The bar ol
275, a, and the ring of Fig. 113, with their helices,
are electro-magnets. The electro-magnet more often
has the horse-shoe form shown
in Fig. 132, so that the attrac-
tion of both poles may act upon
the same body at the same time.
The middle of the bent bar is
bare, the direction of the wind-
ings on the ends 1 being such
that, were the bar straightened,
the current would move in the
same direction round every
part.
More frequently, the two he-
lices, A and B, have separate cores which are joined
by a third straight piece into which the ends of the
tores are screwed. An armature is often placed across
FIG.
300 MAGNETISM. 223
the two poles of the magnet, as shown in the figure.
Electro-magnets have been made capable of supporting
several tons.
(a.) When the circuit is broken and the current thus inter-
rupted, the iron is generally not wholly demagnetized. The small
magnetism remaining is called residual magnetism. The residual
magnetism seems to vary with the hardness and impurity of the
iron. The cores of electro-magnets for some purposes are made of
the softest and purest iron attainable.
300. Making Permanent Magnets. A steel bar
may be permanently magnetized by drawing it, from
its centre, in one direction over one pole of a powerful
magnet and then, from its centre, in the opposite di-
rection over the other pole, and repeating the process
a few times. (Fig. 133.)
Fio. 133.
A bar of steel placed within a helix through which a
strong current is passing, will be permanently magnet-
ized.
224 NATURAL PHILOSOPHY. 30!
A steel bar may be magnetized by striking it on end
with a wooden mallet while it is held in the direction
assumed by the dipping needle.
301. Relation of Magnetism to Energy. A
magnet is a reservoir of potential energy. This energy
is due to the e^pnraiture, at some time, of a definite
amount of energy of some kind. By virtue of its poten-
tial energy, it can u'o a definite amount of work and no
mo-re. For instance, it may attract a certain amount of
iron. When thus fuLv loaded, the magnet has done its
full work and can do no more. When the iron is torn
from the magnet, more energy is expended and the
magnet thus endowed agai.n with potential energy. A
magnet lias not an inexhaustible supply of energy, as
some have supposed.
302
KECA PITULA TION.
302. Recapitulation. To be amplified by the pupti
for review.
MAGNETS.
NATURAL.
Permanent .
Temporary or
Electro-Magnets.
MOLECULAR.
POLES.
CHARACTERISTICS.
LAWS.
RELATION TO
RETENTIVITY.
MAGNETIC SCREENS.
f BY CONTACT.
I Magnetic. )
( Diamagnetic .... f
Forma.
How Mad*.
Definition.
Advantages.
Forms.
Residual Magnetic
Substances.
MAGNETIZATION.
TERRESTRIAL..
f Magnetic Curvn.
Bv INDUCTION 4 Lines of Force.
\ Precedes Attr*cti#
POLBS.
Compass.
Dipping.
DECLINATION.
DIP.
MAGNETIC NEEDLES.
L RELATION TO ENERGI
NATURAL PHILOSOPffT. 302
EXERCISES.
t State the first law of magnets and tell why you believe it ia
be true.
2. When you break a bar magnet have you two parts of a
magnet?
8. What is meant by retentivity ? What other name has the
same thing ?
4. Give good reasons for believing that the earth is a magnet
5. Mention three magnetic substances.
6. How can you obtain a magnet with a single pole ?
7. A dozen steel sewing-needles are hung in a bunch by threads
passed through their eyes. How will they behave when hung
over the pole of a strong magnet ?
8. Devise an experiment to show how to cut off the influence
of a magnet from a piece of iron that is not far distant.
9. If one should carry a dipping needle from the north magnetv- 1
pole of the earth to the south magnetic pole, how would the needle
change its position during the journey ?
10. How can I magnetize an iron bar without using a current
of electricity or a steel or iron magnet ?
11. What is the difference between magnets and magnetized
matter?
12. Six sewing needles are magnetized and thrust vertically
through six little floats of cork. They are placed in a vessel of
water, with the + pole of each needle imgnet pointing upward.
What will be the effect of holding th* - pole of a larger magnet
over them 1
304 INDUCED ELECTRICITY. 22?
SECTION V.
, INDUCED ELECTRICITY.
1 303. Induced Currents. From our study of frio
tional electricity and magnetism, we are familiar with
the term induction, by which we understand the influ-
ence that an electrified body exerts upon a neighboring
unelectrified body or that a magnetized body exerts upon
a neighboring magnetic but unmagnetized body. In
1831, Faraday discovered an analogous class of phe-
nomena which we are now about to consider.
An induced current is a current produced in
a conductor by the influence of a neighboring
current or magnet.
A current used to produce such an effect is called an
inducing current.
304. Inductive Effect of Closing or Breaking
a Circuit. In Fig. 134, B represents a double coil
made as follows: On a hollow cylinder of wood or card-
board are wound several layers of stout copper wire, insu-
lated by being covered with silk or cotton. The two
ends of this wire, which constitutes the primary coil,
are seen dipping into the cups, g g\
Upon this coil and carefully insulated from it, ia
wound a much greater length of finer, insulated copper
ivire. The two ends of this wire, which constitutes the
secondary coil, are seen connecting with a delicate, long
coil galvanometer, 0. Remember that there is no elec-
228
NATURAL PHILOSOPHY.
304
trical connection between the two coils. Wires from the
two plates of a voltaic cell, P, dip into mercury in the
cupsgg', thus closing an inducing circuit through the
primary coil of B.
While this circuit is closed, the galvanometer is at rest,
showing that no current is passing through the secondary
coil. By lifting one of the wires from one of the cups,
the inducing current is interrupted. At this instant the
FIG. 134.
galvanometer needle is deflected as by a sudden impulse,
which immediately passes away. This movement of the
galvanometer needle shows the existence of a momentary
induced current in the secondary coil. If the wire just
removed from the cup be replaced and the inducing cur-
rent thus re-established, the galvanometer needle will be
momentarily turned in the direction opposite to that in
which it was previously turned.
When a current begins to flow through th&
primary coil, it induces a current in the sec-
ondary coil.
306 INDUCED ELECTRICITY. 229
W7ien it ceases to flow through the primary
coil, a current flowing in the opposite direction
is induced in the secondary coil.
Both induced currents are merely momentary
in duration.
305. The Extra Current. When a circuit is made
or broken, each convolution of a coil placed in the cir-
cuit acts inductively upon the other convolutions of the
coil as if they were portions of two unconnected circuits.
This action is called the induction of a cur-
rent upon itself; the current thus produced is
called the extra current.
(a.) When the circuit is made, the extra current is inverse or
opposite in direction to the primary current and acts against it.
The extra current at the breaking of the circuit is direct and addt
its effect to that of the primary current.
(6.) Hence, a spark is seen on breaking but not on making con-
tact. Increasing the number of coils or convolutions in the circuit
will increase the brilliancy of the spark. If the coil has an iron
core (electro-magnet) the effect is especially marked.
306. Ruhmkorff's Coil. The induction coil is
a contrivance for producing induced currents in
i secondary coil by closing and opening, in
rapid succession, the circuit of a current in the
primary coil.
The essential parts are described in 304. In the
complete instrument, the axis of the coils is a bundle of
soft iron wires. The primary circuit is rapidly brokeij
230
NATURAL PHILOSOPHY.
307
and closed by an automatic interrupter, shown at the
Jeft hand end of the coil in Fig. 135.
FIG. 135.
The primary coil is placed in the circuit of a voltaic
battery. The current induced in the secondary coil is
vi high potential.
307. Currents Induced
by Change of Distance.
The primary coil may be made
movable.
When the primary coil,
bearing a current, is
brought near or thrust
into the secondary coil, a
current is induced in the
latter.
When the coils are sepa-
rata! , (i current flowing in
the opposite direction is in-
duced in- the, secondary coil.
FIG. 136.
308 INDUCED ELECTRICITY. 231
Tfie induced currents flow while a change of
distance is varying the inductive effect of the
primary current.
Removing the primary coil to an infinite distance
would be equivalent to breaking its circuit, as in 304.
308, Magneto - Electric Currents. We have
already noticed that there is an intimate relation be-
FIG. 137.
tween electric and magnetic action. We have seen that
an electric current may develop magnetism and have,
perhaps, wondered if magnetism may be made to develop
an electric current Faraday found that electricity may
be thus produced ; the results of this discovery have al-
ready become of incalculable commercial importance. If,
instead of the primary coil bearing the inducing current,
a bar magnet be used, as shown in Fig. 137, the effects
produced will be like those stated in the last paragraph.
232
NATURAL PHILOSOPHY.
309
When the magnet is thrust into the interior
)f the coil, an induced current will flow while
the motion of the magnet continues.
Wlien the magnet is stationary, the current
ceases to flow and, the needle of the galvano-
meter gradually comes to rest.
When the magnet is withdrawn, an induced
current flows in the opposite direction.
Of course, it makes no difference whether the magnet
be moved toward the coil or the coil be moved toward
the magnet. The more rapid the motion, the stronger
will be the induced currents.
309. The Inductive Action of a Temporary
Magnet. If within the coil a soft iron bar (or still
better, a bundle of straight, soft, iron wires) be placed,
FIG. 13&
310 INDUCED ELECTRICITY. 233
as shown in Fig. 138, the induced current may be more
effectively produced by bringing one end of a permanent
magnet near the end of the soft iron. In this case the
induced currents are due to the varying magnetism of
the soft iron, this magnetism being due, in turn, to the
inductive influence of the permanent magnet ( 292).
When the intensity of the magnetism of a bar
of iron is increased or diminished, currents are
induced in the neighboring coil.
Similar effects may be produced by moving one pole
of the magnet across the face of the coil from end to
end.
310. The Wheel Armature. Imagine the soft
iron bar in the helix of Fig. 138, to be grooved and sev-
eral times as long as the helix through which it passes.
Imagine the ends of this bar to be brought together so
as to form a complete iron ring carrying one helix. If
the number of helices upon the ring be increased to
eight or more, we shall have the wheel armature shown
(partly wound) in Fig. 139.
If the pole of a magnet be passed around the face of
this wheel, it will pass eight coils of wire and induce a
current of electricity as it approaches each coil and an
opposite current as it leaves each coil, thus inducing
sixteen currents for each revolution. Of course, it
makes no difference whether the magnet be permanent
or temporary, whether the pole of the magnet mores by.
f /ic coil or the coil pauses by the pole of the magnet.
Then, if the magnet be fixed and the wheel turns upon,
234 NATURAL PHILOSOPHY. 310
its axis in such a way as to carry its coils across the ends
of the magnets, we shall be inducing sixteen currents of
electricity for each revolution of the wheel. This is
what happens in the operation of a dynamo-electric
machine.
When a closed circuit conductor moves in a
magnetic field so as to cut across the lines of
magnetic force ( 290), an induced current of
FIG. 189.
electricity flows through the conductor in one
direction while the conductor is approaching the
point of greatest magnetic intensity and in the
opposite direction while the conductor is moving
away from such point of maximum intensity.
The varying magnetic intensity of the iron core ol
each moving coil increases this effect, as explained \
3H INDUCED ELECTRICITY. 235
309. Of course, the number of coils on the armature
may be more or less than eight, or the armature may be
of a form almost wholly different from that just de-
scribed but, in every case, the principle of its action is
as above stated
311. Dynamo-Electric Machines. In the Brush
dynamo-electric machine, represented in Fig. 140, a
shaft runs through the machine from end to end, carry-
ing a pulley, P, at one end, a commutator, c, at the other
and a wheel armature, R, at the middle. The armature,
R, carries eight or more helices of insulated wire, H H.
As the shaft is turned by the belt acting upon P, R and
c are turned with it. As R turns around, it carries the
eight coils, H H, rapidly across the poles of the four
powerful field magnets, MM.
As each coil passes each pole, it necessarily
traverses the magnetic field and cuts across the
lines of magnetic force; consequently, currents
are induced in the coil.
These currents are carried on insulated wires to the
commutator rings, cc, where they are united in such a
way as all to flow in the same direction, forming a con-
tinuous current. The electricity is taken from cc. by
the four or more copper plates, i i, technically called
"brushes," then carried down the flexible copper strips,
s s, then passed through all the insulated wire of the
electro-magnets, M M, and, finally, to the + binding
posts.
Thence the current passes by a wire to the external
NATURAL PHILOSOPHY.
circuit, e. g., to an arc lamp (Fig. 142) and from this to
a second lamp, and so on through all of the lamps of
the circuit and from the last lamp back to the bind-
ing post of the dynamo-electric machine, thus making
the circuit complete. Sixty or more arc lamps in series
may be worked by one of these machines.
FIG. 140.
Dynamo-electric machines are being rapidly introduced
for purposes of electric lighting, electro-plating, motive
power, telegraphy, etc. They are made in various forms,
but the principle underlying the action of them all is
the same as that stated in the last paragraph. After
mastering the action of one dynamo -electric machine,
tlie pupil will have little trouble in understanding the
action of any other that he may have a chance to exam-
ine, Pynamo-eJectric machines are often called "dyna-
3^2 INDUCED ELECTRICITY. 23 1 }
mos." A small dynamo, with hand power, suitable fot
school tise, may be had for $30 or more.
(a) If permanent magnets are used instead of electro-magnets,
ihe machine is called a magneto-electric instead of a dynamo-elec-
tric machine.
(6.) If instead of expending mechanical energy to turn the shaft
of the dynamo and thus produce an electric current, we pass a
strong current of electricity through the dynamo, the shaft of
the dynnmo will be turned in the opposite direction and may
be made to drive ordinary machinery as an electric motor. In the
former case, we convert mechanical
THE S-WAN ELECTRIC LAMP. energy into electric energy ; in the
latter case, we convert electric en-
ergy into mechanical energy.
312. Incandescent Elec-
tric Lamps. When a con-
ductor is heated to incandes-
cence by the passage of a cur-
rent, we have an illustration of
the fundamental principle of
incandescent electric lighting.
To prevent the fusion of the
conductor, a carbon filament,
about the size of a horse-hair,
is used earbon never having
been melted.
To prevent the combustion
of the carbon filament, it is en-
closed in a glass globe contain-
ing either a high vacuum or
FIG. 141. only some inert gas, incapable
NATURAL PHILOSOPHY.
313
of acting chemically upon the carbon at even the high
temperature to which it is to be subjected.
The filament is carbonized in different ways and given
different shapes by different inventors. Fig. 141 repre-
sents the Swan incandescent lamp and is half the
actual size.
BRUSH ELECTRIC LAMP.
313. The Vol-
taic Arc. The
most brilliant lumi-
nous effect of current
electricity is the arc
lamp. (Fig. 142.)
This consists essen-
tially of two pointed
bars of hard carbon,
generally copper
coated (Experiment
112), placed end to
end in the circuit of
a very powerful cur-
rent. If the ends of
the carbons be sep.i-
rated a short distance
while the current is
passing, the carbon
points become incan-
descent and the cur-
rent will not be inter-
rupted.
Fict. 143.
314 tNbUCED ELECTRICITY. 239
Wlien the carbons are thus separated, their
tips glow ivith a brilliancy which exceeds that
of any other light under human control, while
the temperature of the intervening arc is un-
equaled by any other source of artificial heat.
The mechanism shown in the upper part of Fig. 142,
is for the purpose of automatically separating the car-
bons and " feeding " them together as they are burned
away at their tips and for the purpose of cutting the lamp
out of the circuit in case of any irregularity or accident.
Such lamps of from one to two thousand candle power
and requiring an expenditure, at the dynamo, of about
one horse-power per lamp, are now quite common.
Lamps of a hundred thousand candle power have been
made. The current may be furnished by a battery of
forty or more Grove's cells but, for economical reasons,
it is almost universally supplied by a dynamo-electric
machine.
314. The Telephonic Current. An electric cur-
rent may be induced in a coil of insulated wire surround
ing a bar magnet by the
approach and withdrawal
of a disc of soft iron. The
disc a (Fig. 143), is mag-
netized by the inductive
influence of the magnet
m ( 292). The disc, thus
magnetized, reacts upon the magnet, m, and changes the
distribution of magnetism therein. By varying the dis-
S40 JfAtrtTRAL pmLOsopffr. 315
fcance between a and m, the successive changes in the
distribution of the magnetism of m induce to-aud-fro
currents in the surrounding coil ( 309). When a ap-
proaches m, a current flows in one direction ; when it
recedes, the current flows in the opposite direction.
315. The Telephonic Circuit. If the wire sur-
rounding the magnet mentioned in the last paragraph
be continued to a distance and then wound around a
second bar magnet, as shown in Fig. 144, the currents
LIHf
FIG. 144.
induced at M would affect the magnetism of the bar at
M' ( 298) or the intensity of its attraction for the neigh-
boring disc a'.
A vibratory motion in the disc a would induce electric
currents at M\ these currents, when transmitted to M' ,
perhaps several miles distant, would affect the magnetism
of the bar there.
When the current generated at M flows in such a
direction as to reinforce the magnet at M', the latter
attracts a' more strongly than it did before. When the
current flows in the opposite direction, it weakens the
magnetism of M ', which then attracts a' less. The disc,
RECA PITULA TWtf.
241
therefore, flies back. Thus, the vibrations of a' are like
those of a.
(a.) We have here the principle of the telephone, so far as elec-
tric action is involved. Further consideration of this instrument
must be deferred until we have learned more concerning sound.
(See 335.)
316. Recapitulation. To be amplified by the pupil
for review.
CLOSING.. I t Primary... \
} PRIMARY CIRCUIT, i Secondary . L Coils.
BREAKING J [ Ruhmkorff J
<
HO
2t
if
CHANGE OF DISTANCE OF PRIMARY CIRCUIT.
( Telephones.
MAGNETS.
PERMANENT.
TEMPORARY.
Magneto- Electric Machinet.
Wheel Armature.
Dynamo - Electric
Machines which
art largely used
for
Electro- Plating.
Incandescent
Electric Lignt
ing.
Arc Electric
Lighting.
Charging Stor
age Batten**
NATURAL PHILOSOPHY. 316
EXERCISES.
1. A manufacturer has surplus power at his mill. How can he
utilize this power to illuminate his residence, two miles distant?
2. The ends of a coil of fine insulated wire are connected with
the terminals of a long coil galvanometer. A steel bar magnet is
slowly pushed into the hollow of the coil and then suddenly jerked
out. What actions will be observed in the needle of the galvano-
meter ?
3. Experience showed that the actual cost of 71 arc electric
lights of 2000 nominal candle power each, was as follows :
Consumption of carbons per hour $0.89
Power used for dynamo-electric machine. . .65
Interest on cost of machines 30
Attendance, oil, wear and tear, etc. 86
Total cost per hour $2.20
These electric lamps displaced 578 gas burners. Ignoring all
considerations except that of dollars and cents, reckoning the con-
sumption of gas at six cubic feet per hour for each burner and the
cost of gas at $2 per 1000 cubic feet, (a.) which light is the
cheaper? (6.) What is the difference in cost per hour? (c.) What
is the difference in cost per year, the lights being burned 3000
bourn per year ?
316 REVJEW QUESTIONS. 243
REVIEW QUESTIONS.
1. (a..) Describe the barometer, (It.) Describe the lifting-pump
2. What class of lever is represented by a common wheel
barrow ?
3. What simple machine is represented by a
carpenter's chisel ?
4. What three elements of work measure are
involved in the term " horse-power " ?
5. From a bottle, cork and glass tubing, con-
struct the apparatus shown in Fig. 145. Make all
joints air-tight. Place a in water and suck at 6 until
a jet is formed at j. Explain the action of the
apparatus.
6. (a.) What art, the essentials of a good light-
ning rod ? (6.) What is an anion ?
7. A circular copper dish is joined to the zinc
plate of a small battery. Acidulated water is
poured into the dish. A wire from the carbon
plate of the battery dips into the middle of the
liquid. A few scraps of cork are thrown in to
render visible any motion of the liquid. The
pole of a strong bar magnet is held above the dish. -pio 145
What is the effect?
8. What phenomenon resulted from the greatest difference of
electric potential that you ever knew anything about ?
9. What property of matter is illustrated by the fact that when
a stone is thrown into water, it will displace its own bulk of water ?
10. What is the difference between a liquid and a gas ? Whicb
Is a fluid ?
11. Describe the three classes of levers.
12. The E. M. F. of a dynamo-electric machine furnishing cur-
rent for 16 arc electric lamps, is 839 volts. The lamps are placed
in series, each one having a resistance of 4.56 ohms. The internal
resistance of the dynamo is 10.54 ohms. The line wire has a re
sistance of 0.4 ohms. What is the strength of the current ?
244
NAfTJRAL PHILOSOPHY.
13. When I rub together two bodies, which is developed sooner,
f or electricity ?
14. How does the shape of a conductor affect electric density ?
15. (a.) What is Ohm's law ? (6.) What is an ohm?
16. A ball has been freely falling for five seconds, (a.) What
to its velocity? (6.) How far has it fallen?
17. How can you fire gunpowder
with a voltaic current ?
18. Find the current (in amperes)
given by six Grove cells, joined as
shown in Fig. 146, assuming each cell
to have an E. M. F. of 2 volts and
an internal resistance of 0.30 ohms, the
external resistance being 10 ohms.
19. A strip of paper that has been
rubbed with india-rubber is brought
near a glass rod that has been rubbed
with silk. What happens ?
20. Was the paper strip mentioned
in the last question charged with + or
electricity?
21. If you rub with flannel a stick of sealing-wax held in the
hand, it becomes electrified. If similarly you rub a rod of brass it
4oes not become electrified. Explain the difference.
22. A falling body was known to move 337.68 ft. in a single
second. How long had it been falling at the end of the obser-
vation ?
23. A piston, the area of which is 6 sq. in., is inserted in the
side of a vessel of water at an average distance of 3 ft. 6 in. be-
low the surface. Find the force that must be exerted on the
piston to keep it from being thrust out by the water.
FIG. 146.
CHAPTER
SOUND.
SECTION I.
NATURE, REFRACTION AND REFLECTION OF
SOUND.
317. Definition .of Sound. Sound is the mode
of motion that is capable of affecting the audi-
tory nerve.
(a.) The word sound is used in two different senses. It is often
used to designate a sensation caused by waves of air beating upon
the organ of hearing; it is also used to designate these aerial
waves themselves. If every living creature were deaf, there could
be no sound in the former sense, while in the latter sense the
sound would exist but would be unheard. The definition above
considers sound in the latter or physical sense only.
318. Undulations. In beginning the study of
acoustics, it is very important to acquire a clear idea of
the nature of undulatory motion. When a person sees
waves approaching the shore of a lake or ocean, there
arises the idea of an onward movement of great masses
of water. But, if the observer give his attention to a
piece of wood floating upon the water, he will notice
that it merely rises arid falls without approaching thq
246 NATURAL PHILOSOPHY. 318
shore- He may thus be enabled to correct his erroneous
idea of the onward motion of the water.
Again, he may stand beside a field of ripening grain
and, as the breezes blow, he will see a series of waves
pass before him. But, if he observe carefully and re-
flect, he will see clearly that there is no movement of
matter from one side of the field to the other; the
grain-ladened stalks merely bow and raise their heads.
Most persons are familiar with similar wave movements
in ropes, chains and carpets.
Each particle of matter has a motion, but that
motion is vibratory, not progressive. The onward,
movement is that of the wave but not of the part-
icles which compose the wave.
It is a familiar fact that a wave may transmit energy.
(a.) The motion of the wave must be clearly distinguished from
the motion of particles which constitute the wave. The wave
may travel to a great distance : the journey of the individual par-
ticle is very limited.
Experiment 138. Suspend a pith ball by a thread so that it
shall hang lightly against one prong of a tuning fork. When the
fork is sounded, the pith ball will be thrown off by the vibra'
tions of the prongs.
Experiment 139. Place the two ends of a common friction
match on convenient supports, as on two fingers of the left hand ot
the upper edges of a partly opened book standing on end ou a
319 CAUSE OF SOUND. 247
Strike a olow with a common tuning fork to set its prongs ill
vibration, bring: the fork near the ear and notice the sound;
quickly bring the broad face of one prong beneath the middle of
the match ; the match will be thrown upward by the sudden
blow.
Experiment 140. In similar manner, sound a fork and with
the ends of the prongs, quickly touch the surface of a glassful of
water. The vibrating prongs of the sounding fork will throw
:wo showers of spray from the water.
319. Cause of Sound. All sound may be traced
to the vibrations of some material body.
The particles of a sounding body strike the adjacent
particles of air, these pass the motion thus received to
the air particles next beyond, and these to those still
oeyond.
Experiment 141. Provide a tube four or five metres (or yards)
Jong, and about ten centimeters (four inches) in diameter. A few
lengths of common spout from the tinner's will answer. Furnuh
it with a funnel shaped piece, having an opening about 2^ ci.i.
(one inch) in diameter. Place the tube on a table with a candle
flame opposite the opening at B. With a book, strike a sha>p
blow upon the table opposite the opening at A. The flame will
be blown out. Something went from A to B. Did it go through
the tube?
Experiment 142, iet us ask this question of Nature, speakir g
248 NATURAL PHILOSOPHY. 320
<rith her, as we must, in the Language of Experiment. Close the
opening at .4 and repeat the experiment : the flame is not put out.
Remove the tube and repeat the blow ; the flame is not put out.
The answer has come. The tube is necessary ; whatever blew
out the candle, did go through the tube.
Experiment 143. Was this something a wind or a wave ? W
must ask our question in the same language as before ; we must
make an experiment. Dissolve as much potassium nitrate (salt-
peter), as you can in half a cupful of hot water. Soak a piece of
unsized paper in this liquid and dry it. This " touch-paper " burns
with much smoke but no flame. Burn the paper in the tube near
A, filling that end of the tube with smoke. Repeat the experiment
as before. No smoke issues at B. Another answer has come ;
it was not a wind that passed through the tube.
Experiment 144. Replace the tin tube by about the same
length of rubber tube, that has an internal diameter of from ten
to fifteen mm. (| inch). Thrust the neck of a tin or glass funnel
into the end of the tube at A. Get a few inches of glass tubing
that will fit snugly into the rubber tubing. Heat the middle of
the glass in a flame until it softens. Slowly draw the ends
asunder until the softened part is reduced to a diameter of about
two mm. Break the tube at this narrow neck and push the large
end of one piece into the rubber tube at B. Place a small flame
opposite the small opening of the glass tube. Strike a blow in front
of the funnel at A and notice that a puff or pulse of air blows the
flame. Make a loose loop in the rubber tube and repeat the ex-
periment. Clap the hands at A and notice the series of puffs at B.
While an assistant is clapping his hands at A, pinch the rubber
tube so as to prevent the motion from passing through it. Notice
that the puffs at /> cease while the tube is thus pinched and
reappear as soon as the tube is released.
320. Propagation of Sound. Sound is ordinarily
propagated through the air. The first layer of air is
by the vibrating body, The particles of this
321 WAVE LENGTH. 249
layer give their motion to the particles of the next layer,
and so on until the particles of the last layer strike upon
the drum of the ear.
(a.) See Elements of Natural Philosophy, % 484, a. If the teacher
or pupil has a copy of Alfred M. Mayer's little book on " Sound," 1
he may well read the explanation of the propagation of sound
given on p. 89. He will also do well to make and use Crova's
Disk, as described in Experiment 58 of that book. If necessary, the
pupils should "club together" and buy the book /or the class.
321. Wave Length. In such a series of similar
waves, measuring in the direction in which the waves
are traveling, the distance from any vibrating par-
ticle to the next particle that is in the same
relative position or " phase " is called a wave
length. In the case of water waves, for example, the
horizontal distance from one crest to the next crest
would be a wave length. The wave length may be
found by dividing the velocity by the number of
vibrations.
(a). Every one knows how to produce a series of waves in g
rope as shown in Fig. 148, each curved line of which we may
imagine to be an instantaneous photograph of a rope thus shaken,
The distance ub or cd, is a half wave length,
250 NATURAL PHILOSOPHY. $2$
222. Amplitude. Amplitude means the distance
between the extreme positions of the vibrating
particle, or the length of its journey. Referring to
Fig. 148, the distance de or cf is the amplitude of the
Experiment 145. Hold one end of a straight spring, as a hick-
ory etick, in a vise, pull the free end to one sido and let it go.
Elasticity will return it to ita
position of rest, kinetic energy
will carry it beyond and so on, a
vibratory motion toeing thus pro-
duced. (Fig. 149.) When the
spring is long, the vibrations
may be seen. By lowering the
spring in the vise, the vibrating
part is shortened, the vibrations
reduced in amplitude and in-
creased in rapidity. As the
spring is shortened, the vibra-
tions become invisible but audi-
ble, showing that a sufficiently
rapid vibratory motion may pro-
duce a sound.
FIG. 149.
323. Sound Waves. The layers of air are crowded
more closely together by each outward vibration of the
sounding body ; a condensation of the air is thus pro-
duced. As the sonorous body vibrates in the opposite
direction, the nearest layer of air particles follow it; a
rarefaction of the air is thus produced.
A sound wave, therefore, consists of two part&
tt> condensation and a rarefaction/.
323 SOUND WAVES. 251
--u-v.-tuv.--' : - .- ..-' ':/.::'' - :'.;.//. .
Fw. 150.
The motion of any air particle is backward and for-
ward in the line of propagation, and not " up and down "
across that line, as in the case of water waves.
Experiment 146. Provide a wooden rod about half an inch
square (or 1 inch by } inch) and five or six feet long. Place one
end of this rod (preferably made of light, dry pine) against the
panel of a door, hold the rod horizontal and place the handle of a
vibrating tuning-fork against the other end. Notice the sound
given out by the panel.
Experiment 147. With the help of an assistant, vary the last
experiment by placing one end of the rod against the ear or be-
tween the teeth instead of against the panel. See if the sound of
the fork at the same distance is perceptible without the help of
the rod.
Experiment 148. Make a "string telephone" as follows:
melt the bottoms from two small, round, tin boxes about an inch
or an inch and a half in diameter. Ground spices are often sold in
boxes of the kind desired. The bottom may be removed by set-
ting it for a moment on a hot stove. Over one end of each tubo
thus provided, firmly tie a piece of well-soaked bladder, which
NATURAL PHILOSOPHY.
$324
may be had of a butcher or apothecary. When the bladder is dry,
pass one end of a long, fine string through the middle of the head
of each tin box and tie a knot in each end of the string to prevent
it from drawing back. The knot should be on the inaide of the
box. Let one pupil hold the open end of one box to his ear, the
other box being held by another pupil at such a distance that the
FIG. 151.
string shall be drawn tight. If this pupil bring the foot of a
vibrating tuning-fork to the tin, the vibration will travel along
the string and the sound be heard by the first pupil. If the string
be a hundred feet long or more and tightly stretched, conversa-
tion may be carried on through the apparatus. It is desirable
that no solid touch the string between the tin boxes.
324. Sound Media. Any elastic substance may
become the medium for the transmission of sound,
bat such a medium is necessary. Sound cannot
be transmitted through a vacuum.
327 VELOCITY OF SOUND. 263
(a.) Soldiers and Indians sometimes detect the approach of the
enemy at a great distance by putting their ears to the ground.
325. Velocity of Sound in Air. The transmission
of sound is not instantaneous. The blow of a hammer i.
often seen several seconds before the sound is heard ;
steam escaping from the whistle of a distant locomotive
becomes visible before the shrill scream is audible ; the
lightning precedes the thunder.
17z,<? velocity of sound in air at the freezing tern*
perature is about 332rti., or 1090 ft. per second.
The freezing temperature is 32 degrees by Fahren-
heit's thermometer or zero by the centigrade thermom-
eter. (360.)
(a.) The velocity above given is more than 600 miles an hcur.
A wind of 75 miles an hour is a terrible hurricane. Fortunately,
we are here dealing with wave mot.on and not with wind motion.
326. Effect of Temperature upon Velocity.
TJiere is an added velocity of about 1.12 feet
for every Fahrenheit degree, or of 2 feet (GO
centimeters) for every centigrade degree of in-
crease of temperature.
Thus, the velocity of sound in air at a temperature of
59 F. or 15 C. is about 1120 ft.
327. Continuous Sound. A sound may be momen.
tary or continuous. A momentary sound consists of a
single pulse produced by a single and sudden blow. 4
continuous sound consists of a rapid succession of pulses.
254 NATURAL PHILOSOPHY* $ 32J
The ear is so constructed that its vibrations disappeai
rery rapidly but the disappearance is not instantaneous.
// the motion imparted to the auditory nerve by
each individual pulse continue until the arrival
of its successor, the sound will be continuous.
(a.) Momentary sounds may be produced by pounding with a
hammer, stamping with the foot, clapping the hands or draw-
ing a stick slowly along the pickets of a fence. Continuous
eounds may be produced by sawing boards or filing saws. They
are more or less familiar in the rattling of wheels over a stony
pavement, the roar of waves or the crackling of a large fire.
328. Noise and Music. The sensation produced
by a series of blows coming at irregular intervals, is
unpleasant and the sound is called a noise. But
when the air waves come with sufficient rapidity to
render the sound continuous and with perfect regularity,
the sensation is pleasant and the sound is said to be
musical.
To secure this pleasing smoothness of music, the
sounding body must vibrate with the unerring
regularity of the pendulum, but impart nuiclt
sharper and quicker shocks to the air. Every musi-
cal sound has a well-defined period and tea re
length.
329. Elements of Musical Sounds. Musical
sounds or tones have three elements intensity or loud-
ness, pitch and quality.
332 INTENSITY OF SOUND. 255
330. Intensity and Amplitude. Loudness of
sound depends upon the amplitude of vibration.
TJie greater the amplitude, the louder the sound.
(a.) If the middle of a tightly-stretched cord or wire, as a guitar
string, be drawn aside from its position of rest and then set free,
it will vibrate to and fro across its place of rest, striking the air
and sending sound waves to the ear. If the middle of the string be
drawn aside to a greater distance and then set free, the swing to
and fro will be increased, harder blows will be struck upon the
air and the air particles will move forward and backward through
a greater distance. In other words, the amplitude of vibration
has been increased and we say that the sound is louder. (See
Mayer's " Sound," Experiment 93.)
Experiment i49. Whisper into one end of a length (50 ft.) ol
garden hose. A person listening with his ear at the other end
of the hose can distinctly hear what is said although the sound
be inaudible to a person holding the middle of the hose.
331. Acoustic Tubes. -If the sound ware be not
allowed to expand as a spherical shell, the energy of
the wave cannot be diffused. In acoustic tubes (Fig.
152), this diffusion is prevented ; the waves are propa-
gated in only one direction. In this way, sound may
be transmitted to great distances without considerable-
loss of intensity.
Experiment 150. Draw the finger nail across the teeth of a
comb, slowly the first time and rapidly the second time. Notice
the difference in the sounds.
332. Pitch. The second element of a musical sound
is pitch, the quality that makes the difference betweeu
a low tone and a high tone.
256
NATURAL PHILOSOPHY.
27&e pitch of a sound depends upon the rapidity
6f vibration of the sounding body* The more
rapid the vibrations, the higher the tone.
(a.) That pitch depends upon rapidity of vibration, may be
shown satisfactorily by means of Savart's
wheel, shown in Fig. 153. This consists
of a heavy, metal ratchet-wheel, supported
on a frame and pedestal The wheel may
be set in rapid revolution by a cord wound
around the axis. By holding a card agains<
the teeth, when in rapid motion, a shriU
tone will be produced, gradually falling ij
pitch as the speed is lessened. (See Mayer*8
FIG. 153. " Sound" Experiments 77-80.1
334
REFLECTION OF SOUNt).
25?
333. Reflection of Sound. When a sound ray strikes
an obstacle, it is reflected in obedience to the principle
given in 57.
(a.) " The great dome of St. Paul's Cathedral in London is so
constructed that two persons at opposite points of the internal
gallery, placed in the drum of the dome, can talk together in a
mere whisper. The sound is transmitted from one to the other
by successive reflections along the course of the dome."
A similar phenomenon is observable in the dome of the Capitol
at Washington. The "guides" about the building will point out
for you the proper position in the gallery of the dome and also cer-
tain places on the floor of Statuary Hall (formerly the Hall of
Representatives), where remarkable acoustic phenomena may be
noticed.
334. Recapitulation. To be amplified by the pupil
for review.
SOUND.
DEFINITION.
CAUSE.
WAVES
MEDIA.
VELOCITY
MOMENTARY.
CONTINUOUS...
REFLECTION...
MODE -JF PROPAGATION.
LENGI ...
AMPLITUDE.
( Condensation.
PARTS <
( Rarefaction.
AT FREEZING TEMPERATURE.
AT OTHER TEMPERATURES.
NOISY.
Loud.
MUSICAL..
( Cause.
( Acoustic Tubes
Pitch Cause.
Quality.
I WHIS
ISPERING GALLERIES
NATURAL PHILOSOPHY. 334
EXERCISES.
1. Make a pencil sketch of an " up-and-down " wave having
a length of two inches and an amplitude of half an inch.
2. Water is just beginning" to freeze in the open air. What U
the velocity of sound ?
3. If a tuning fork vibrates 256 times a second and its sound
travels 1280 feet in a second, what is the wave length ?
4. The thermometer records a freezing temperature. In a sec-
ond, 218 sound waves pass a given point. What is the length of
each wave? Ans. 5 feet.
5. What is the rate of vibrations of a body that produces sound
waves just a meter long when it is just freezing cold ?
6. What is the velocity of sound in air at a temperature of
30 G? Am. 1130 feet.
7. When sound has a velocity of 1126 feet per second, what is
the temperature? An*. It is 18 C.
8. Give the definition anf* correct pronunciation of the word
SECTION II.
THE TELEPHONE COMPOSITION AND ANALYSIS OF
SOUNDS.
335. The Telephone. This instrument is repre-
sented in section by Fig. 154. A is a permanent bar
magnet, around one end of which is wound a coil, E, of
fine copper wire carefully insulated. The ends of this
coiled wire are attached to the larger wires, CC y which
FIG. 154.
communicate with the binding posts, DD. In front oi
the magnet and coil is the soft iron diaphragm, E> which
corresponds to the disc, a, of Fig 144.
In front of the diaphragm is a wooden mouth-piece
with a hole, about the size of a dime, at the middle of
the diaphragm and opposite the end of the magnet.
The outer case is made of wood or of hard rubber. The
external appearance of the complete instrument is repre-
sented by Fig. 155. The binding posts of one instru-
300
NATURAL PHILOSOPHY.
337
toent being connected by wires with the binding posta
rf another at a distance, conversation may be carried on
between them. Carefully review
314 and 315.
336. Action of the Tele-
phone. When the mouth-piece is
brought before the lips of a person
who is talking, air waves beat upon
the diaphragm and cause it to vi-
brate. Eacli vibration of the dia-
phragm induces an electric current
in the wire of B. These curreu ts are
transmitted to the coil of the con-
nected telephone, at a distance of,
perhaps, several miles, and there
produce vibrations exactly like the
original vibrations produced by the
voice of the speaker. These vibra-
tions of the second diaphragm send out new air waves.
The two sets of air waves being alike, the resulting sen-
sations produced in the hearers are alike. Not only dif-
ferent words but also different voices may be recognized.
Remember that an electric current, and not sound waves,
passes along the line wire. The arrangement being the
same at both stations, the apparatus works in either di-
rection. No battery is necessary with this arrangement
337. The Transmitter. In practice, a transmitter,
shown at C in Fig. 156 is generally used. The vibrations
of the diaphragm of C, when acted upon by sound waves,
. 155.
337
THE TELEPHONE.
produce a varying pressure upon a carbon button placed in
the circuit of a galvanic battery, D. This varying pnr >
ure results in a varying resistance to the passage of the
Fio. 156.
current through the button and, consequently, in varia
tions in the current itself. This varying current, passing
through the primary circuit of a small induction coil ir.
the box, C, induces a current in the secondary IMOUII
thereof. This current, thus induced, flows over the L .e
phone wires and, at the other station, passes thro igii a
telephone like that shown at B 3 which is held close If
the ear of the listener.
262 NATURAL PHILOSOPHY. 337
The message is transmitted by C at one station and
received by B, of a similar instrument at the other station.
At each station is placed an electric bell, A, which may
be rung from the other station, for the purpose of at-
tracting attention. When the stations are a considerable
distance, apart, one binding post of each instrument
may be connected with the earth, as in the case of the
telegraph. See Fig. 15G.
(a.) In most of our cities, the telephones are connected by wire
with a central station, called a telephone exchange. The " Ex-
change" may thus be connected with the houses of hundreds of
patrons in all parts of the city or even in different cities. Upon re-
quest by telephone, the attendant at the central station connects
the line from any instrument with that running to any other in-
strument. Thus, each subscriber may communicate directly with
any other subscriber to the exchange.
Experiment 151. The effect of repeated impulses, each feeble
but acting at the right instant, may be forcibly illustrated as follows:
Support a heavy weight, as a bucket of coal, by a long string or
wire. To the handle of a bucket, fasten a fine cotton thread. By
repeated pulls upon the thread, each pull after the first one being
given just as the pendulum is beginning to swing toward yo*
from the effect of the previous pull, the weight may be made to
swing through a large arc, while a single pull out of time will
snap the thread. A little practice will enable you to perform the
experiment neatly.
Experiment 152. Vary the last experiment by setting the
pendulum in motion by well-timed puffs of air from the mouth or
from a hand bellows.
The same principle is illustrated in the action of the spring
board, familiar to most boys, who know that the desired effect
san be secured only by "keeping time." Soldiers are often or
337
SYMPATHETIC VIBRATIONS.
263
deb
dered to " break step " in crossing a bridge, lest the accumulated
energy of many footfalls in unison break the bridge.
Experiment 153. Suspend several pendulums from a frame as
shown in Fig. 157. Make two of equal length so that they will vi-
brate at the same rate. Be sure that tliey will thus vibrate. The
other pendulums are to be of different lengths. Set a in vibration.
The swinging of a will produce slight vibrations in the frame
which will, in turn, transmit them to the other
pendulums. As the successive impulses thus
imparted by a keep time with the vibrations
of b, this energy accumulates in b, which is
soon set in perceptible vibration.
As these impulses do not keep time with the
vibrations of the other pendulums, there can
be no such accumulation of energy in them, for
many of the impulses will act in opposition to
the motions produced by previous impulses and
lend to destroy them.
Experiment 154. Place two mounted tun-
ing forks (Fig. 159) several feet apart. The
forks must be exactly in unison. Sound one j
fork by rapidly separating its prongs with a
wooden rod or by drawing a resined bass-viol
bow across their ends. After a few seconds,
stop the vibrations of this fork with the fin-
gers ; it will be found that the other fork has
been put into sympathetic vibration and is giving forth a souno.
Weight one of the prongs of the second fork with wax ; an at-
'.empt to repeat the experiment will fail.
Experiment 155. Tune, to unison, two strings upon the same
eonometer (Fig. 158). Upon one string, place two or three paper
riders. With a violin bow, set the other string in vibration. The
sympathetic vibrations thus produced will be shown by the
264
NATURAL PHILOSOPHY.
338
dismounting of the riders, whether the vibrations be audible or
not
Change the tension of one of the strings, thus destroying the
unison. Repeat the experiment and notice that the sympathetic
vibrations are not produced.
338. Sympathetic Vibrations. The string of a
violin may be made to vibrate audibly by sounding near
it a tuning-fork of tbe same tone. By prolonging a
vocal tone near a piano, one of the wires seems to take
up the note and give it back of its own accord. If
the tone be changed, another wire will give it back.
Thus the vibrations of the strings may produce sonorous
waves and the waves, in turn, may produce vibrations
in another string.
The string absorbs only the particular kind of
vibration that it is capable of producing.
PIG. 158.
(a.) The sonometer box may be made by any carpenter. It is
about fifty-nine inches long, 4f inches wide and 4f inches deep.
The ends are made of inch oak boards, the sides of ^ inch oak
boards and the top of inch pine board. The top should be glued
338 SYMPATHETIC VIBRATIONS. 265
on ; no bottom is needed ; the box may sit directly on the table
Three or four one-inch holes may wel) be bored in each side-piece
The two bridges, shown at A and B (Fig. 158) should be of very
hard wood and glued to the cover just 47f inchec (120 centimeters)
apart, measured from centre to ceutre. The strings may be such
as are used on bass-viols ; they should be alike. Two similar
pieces of piano-forte wire (large size) may be used. The strings
may be stretched by weights as shown in the figure or by two piano
string pegs turned with a wrench or a piano tuner's key. The
familiar screw arrangement of the bass-viol may be used for the
purpose. If piano wires are used for strings, the ends must be
annealed by heating them red hot and cooling them slowly, BO
that they may remain fixed when wound around their fasten
ings. Lines should be drawn across the top of the box, exactly
dividing the distance between the middle of the bridges (at which
points the strings are supported) into halves, thirds and quarters.
Provide a block of wood, about two inches wide, 4 inches long
and just thick enough to slip between the strings and the top of
the box.
(6.) When the two strings are in unison, they will vibrate at
exactly the same rate. The second and subsequent pulses sent out
by the first string strike the second string, already vibrating from
the effect of the first pulse, in the same phase of vibration, and
thus each adds its effect to that of all its predecessors. If the
strings be not in unison, they will vibrate at different rates and but
few of the successive pulses can strike the second string in the
same phase of vibration ; the greater number will strike it at the
wrong instant.
Experiment 156. Strike a tuning-fork held in the hand.
Notice that the sound heard is feeble. Strike the fork again and
place the end of the handle upon a table. The loudness of the
sound heard is remarkably increased. Repeat Experiment 146.
Experiment 157. Strike the fork and hold it near the eat
NATURAL PHILOSOPHY.
339
counting the number of seconds that you can hear it: Strike the
fork again with equal force, place the end of the handle on the
table and count the number of seconds that you can hear it.
339. Sounding-boards. In the case of the sonom-
eter, piano, violin, guitar, etc., the sound is due more
to the vibrations of the resonant bodies that carry the
strings than to the vibrations of the strings themselves.
The strings are too thin to impart enough motion to the
air to be sensible at any considerable distance ; but as
they vibrate, their tremors are carried by the bridges to
the material of the sounding apparatus with which they
are connected.
These larger surfaces throw larger
masses of air into vibration and thus
greatly intensify their sound. It
necessarily follows that the energy
of the vibrating body is sooner ex-
hausted ; the sounds are of shorter
* duration.
FIG. 159. (<*) For class or lecture experiments,
tuning forks should be mounted as shown
in Fig. 159.
Experiment 158. Support horizontally, between two fixed sup-
ports, a soft cotton rope a few yards in length. With a stick,
Strike the rope near one end a blow from below and a crest will be
FIG. 160.
34O COINCIDENT WA VES. 267
formed as shown in Fig. 160. Vary the tension of the rope, if
necessary, until the crest is easily seen. Notice that the crest, c,
travels from A to B where it is reflected back to A as a trough,*.
By striking the rope from above, a trough may be started which
will be reflected as a crest. See Mayer's "Sound," Experiment
57, in which tho wire spring shows waves of condensation and
rarefaction. Tie a piece of string to one turn of the wire and no-
tice the motion of the string as the wave passes.
Experiment 159. Start from A a trough. At the moment of its
reflection as a crest at B, start a crest at A as shown in Fig. 161.
The two crests will meet near the middle of the rope. The crest
at the point and moment of meeting results from two forces acting
FIG. 161.
in the same direction consequently, it will be grea^r than either
of the component crests.
340. Coincident Waves. In the case of water
waves, when crest coincides with crest, the water reaches
a greater height. So with sound waves, when, conden-
sation coincides with condensation, this part of
the wave will be more condensed; when rarefac-
tion coincides with rarefaction, this part of the
wave will be more r art fled.
This increased difference of density in the two parts
of the wave means increased loudness of sound, because
there is an increased amplitude of vibration for the par-
ticles constituting the wave.
268 NATURAL PHILOSOPHY. 341
341. Reinforcement of Sound. This increased
intensity may result from the blending of two or more
series of similar waves in like phases, or from the union
of direct or reflected waves in like phases.
Under such circumstances, one set of waves is
said to reinforce the other. The pJienomenon is
spoken of as the reinforcement of sound.
Experiment 160. Hold a sounding tuning-fork over the mouth
of a glass jar, 18 or 20
inches deep ; a feeble
sound is heard. Care-
fully pour in water until
the liquid reaches such
a level that the sound
suddenly becomes much
louder. The water has
shortened the air col-
umn until it is able to
vibrate in unison with
the fork. The length
of the air column is one-
fourth the length of the
wave produced by the
fork.
342. Resonance.
Resonance is a
variety of the re-
inforcement of
sound due to
sympathetic vibrations. The resonant effects of
solids were shown in 339. The resonance of an ail
g 343 RESONANCE. 269
column was shown in the last experiment. The loudness
of sound of wind instruments, like the pipes of a church
organ, is largely due to the resonance of the air enclosed
in them.
343. Helmholtz's Resonators. Helmholtz, the
German physicist, constructed a series of resonators, each
one of which resounds powerfully to a single tone of
certain pitch or wave length. They are metallic vessels,
nearly spherical, hav-
ing a large opening,
as at A in Fig. 163,
for the admission of
the sound waves. The
funnel-shaped projec-
tion at B has a small
opening and is inserted
in the outer ear of the
observer. Fm
Experiment 161. Using the rope as described in Experiment
158, start a crest at A. At the moment of its reflection at B as a
trough, start a second crest at A. The trough and crest will meet
near tlie middle of the rope. The rope at this time and place will
be urged upward by the crest and downward by the trough. The
FIG. 164.
resultan* effect of these opposing forces will, of course, be equal
to their difference. If crest and trough exert equal forces, the
difference will be zero. Consequently the motion of the rope at
270 NATURAL PHILOSOPHY. 345
the meeting of crest and trough will be little or nothing. Thu
one wave motion may be made to destroy the effect of
another wave motion.
Experiment 162. Hold a vibrating tuning-fork near the eat
and slowly turn it between the fingers. During a single complete
rotation, four positions of full sourd and four positions of
perfect silence will be found. When a side of the fork is parallel
FIG. 165.
to the ear, the sound is plainly audible , when a comer of the
prong is turned toward the ear, the waves from one prong com-
pletely destroy the waves started by the other.
Experiment 163. Over a resonant jar, as shown in Pig. 164,
slowly turn a vibrating tuning-fork. In four positions of the fork
we have loud, resonant tones ; in four other positions we have
complete interference. While the fork is in one of these positions
344 INTERFERENCE OF SOUND. 271
of interference, place a pastoboard tube around one of the vibrating
prongs, Fig. 167; a resonant tone is instantly heard; the cause
of the interference has been removed. (See Mayer's "Sound,'
Experiments 60 to 67.)
344. Interference of Sound. If, while a tuning,
fork is vibrating, a second fork be set in vibration, the
waves from the second must traverse the air set in mo-
tion by the former. If the waves from the two forks be
of equal length, as will be the case when the two forks
have the same pitch, and the forks be any number of
whole wave lengths apart, the two sets of waves will
unite in like phases (Fig. 1G6), condensation with con-
densation, etc., and a reinforcement of sound will ensue.
PIG. 166.
But if the second fork be placed an odd number of
half wave lengths behind the other, the two series of
waves will meet in opposite phases ; where the first fork
requires a condensation, the second will require a rarefac-
tion. The two sets of waves will interfere, the one with
the other. If the waves be of equal intensity, the air
particles, thus acted upon, will remain at rest; this means
silence. In Fig. 167, an attempt is made to represent
this effect to the eye, the uniformity of tint indicating
the absence of condensations and rarefactions.
272 NATURAL PffTLOSOPffT. 345
FlG. 167.
By adding sound to sound, both may be de-
stroyed. This is the leading, characteristic prop-
erty of wave rnotion. The phenomenon here de-
scribed is called interference of sound.
(a.) The sound of a vibrating tuning-fork held in the hand
is almost inaudible. The feebleness results largely from inter-
ference. As the prougs always vibrate in opposite directions at
Ihe same time, one demands a rarefaction where the other de-
mands a condensation. By covering one vi orating prong with a
pasteboard tube, the sound is more easily heard.
Experiment 164. Use the two forks mentioned in Experiment
154, one of them being loaded with wax. Set both forks in vibra-
tion and notice the palpitating effect.
Experiment 165. In a quiet room, strike simultaneously one
of the lower white keys of a piano and the adjoining black key.
The palpitating effect will be heard.
345. Beats. If two tuning-forks, A and B, vibrating
respectively 255 and 256 times a second, be set in
vibration at the same time, their first waves will meet in
like phases and the result will be an intensity of sound
greater than that of either. After half a second, B
having gained half a vibration upon A, the waves will
meet in opposite phases and the sound will bo weakened
347 VIBRATIONS OF STRINGS. 273
DF destroyed. At the end of the second, we shall have
another reinforcement ; at the middle of the next second,
another interference.
This peculiar palpitating effect is due to a suc-
cession of reinforcements and interferences, and is
called a beat.
The number of beats per second equals the differ-
ence of the two numbers of vibrations. (See Mayer's
" Sound," Experiments 71 and 72.
346. Vibrations of Strings. The laws of musical
tones are best studied with the help of stringed instru-
ments. The Laws of the Vibrations of Strings is treated
in 455 of the Elements of Natural Philosophy.
Experiment 166. Give a fixed support to one end of a rubber
tube about two yards long and hold the other end in the hand.
Move the hand up and down so as to send a series of waves along
the tube. By varying the tension of the tube and the rate of vi-
bration of the hand, you will soon be able to produce an appear-
ance of gauzy spindles much like those shown in Figs. 148 and
170. This gauzy appearance is due to an optical effect known as
" the persistence of vision." The motion of the hand produces a
regular succession of equal crests and troughs, which are reflected
and neutralize each other as explained in Experiment 161.
347. Fundamental Tones and Overtones. A
string may vibrate transversely as a whole, or as inde-
pendent segments. Such segments will be aliquot parts
of the whole string and separated from each other by
points of no motion called nodes or nodal points. (See
a and b, Fig. 148.) The part of the string between tw
adjacent nodes is called a ventral segment.
2?4 NATURAL PHILOSOPHY. 347
The tone produced by the vibrations of ili6
whole length of a string is called its fundamental
tone. The tones produced by the vibrations of
the segments of a string are called its overtones or
harmonics.
348. Fundamental Tones. When a string vibrates
so as to produce its fundamental tone, its extreme po-
sitions may be represented
by the continuous and the
M8- dotted lines of Fig. 1C8.
This effect is obtained by leaving the string free and
bowing it near one of its ends.
If a number of little strips of paper, doubled in the
middle, be placed like riders upon the string and the
string bowed as just described, all of the riders will be
thrown up and most of them off. This shows that the
whole string vibrates as one string ; that there is no part
of it between the fixed ends that is not in vibration.
349. The First Overtone. If the string of the
sonometer be touched exactly at its middle with a feather,
a higher tone is pro-
duced when che string is ^--- IZx^ HI =
howod This tone is an ^ ^ ;
octave higher than the
fundamental. The string now vibrates in such a way
that the point touched remains at rest; it is a node.
The tone is due to the vibrations of the tuo
Halves of the string, which thus give the octave
instead of the fundamental. The existence of the
350 orsnTONES. 275
node and segments will continue for some time after the
feather is removed. If riders be placed at C, JVand D,
the one at N will remain at rest while those at and
D will probably be dismounted.
(a.) Instead of the sonometer string, a piece of rubber tubing
filled with sand and hung in a vertical position with both ends
fastened may be used. Hold the middle of the tube to form a
node and pluck at the middle of the lower half.
FIG. 170.
350. Higher Overtones. In like manner, if the
vibrating string be touched at exactly one-third (see
L'ig. J 70), one-fourth or one-fifth of its length from one
end, it will divide into three, four or live segments, with
276
NATURAL PHILOSOPHY.
350
vil: rations three, four or five times as rapid as the fun-
damental vibrations.
351. Quality or Timbre. As a sounding body vi-
brates as a whole and in segments at the same timCj
the fundamental and the harmonics blend. The result-
ant effect of this blending of fundamentals and
harmonics constitutes what we call the quality
or timbre of the sound.
We recognize the voice of a friend not by its loudness
not by its pitch, but by its quality. When a piano and
violin sound the same tone, we easily distinguish the
Bouivd N f one from that of the other because, while the
fundamentals are alike, the harmonics are different.
Experiment 167. Take your seat before the key-board of a
piacd. Press and hold down the key of "middle C," marked 1 in
Fi^ 171 which represents part of the key -board. This will lift the
FIG. 171.
damper from the corresponding piano wire and leave it free to vi-
brato. Strongly strike the key of 0", an octave below. Held this
key down for a few seconds and then remove the finger The
damper will fall upon the vibrating wire and bring it to rest. Whoa
the sound of 0' has died away, a sound of higher pitch is heard.
The tone corresponds to the wire of 1, which wire is now vibrating
These vibrations are sympathetic with those that produced the
first overtones of the wire that was struck. These vibrations in
35 2 COMPOUND TOXES. 277
the wire of 1 prove the presence of the first overtone In the
vibrating wire of C'. (See 338.)
In similar manner, successively raise the dampers from the wires
of 2, 3, 4, 5, 6 and 7, striking C" each time. These wires will ac
cumulate the energy of the waves that correspond to the respec
tive overtones of the wire of G* and give forth each its proper tone
Thus we analyze the sound of the wire of C' and prove that
at least seven overtones are blended with its fundamental.
Some of these tones of higher pitch, thus produced by VIM aliens
sympathetic with the vi'srcUions of the segments of the wire o)' 7 ., r-
feebler than others. This shows that the quality of a f on vie
pends upon the relative intensities as well as the numbei Hut.
overtones that blend with the fundamental.
352. Simple and Compound Tones. The weP.
trained ear can detect several sounds of different pitch
when a single key of a piano is struck. In other wwiiia.
the sound of a vibrating piano wire is a compound sound.
The sound of a timing fork is a fairly good example of a
simple sound. Simple sounds all have the same quality,
differing only in loudness and pitch.
(a.) A series of Helmholtz's resonators enables the student of
acoustics to analyze any compound sound. Each component tone
may be reproduced by a tuning fork of appropriate pitch. By
sounding simultaneously the necessary number of forks, each of
proper pitch and with appropriate relative intensity, Hehnholtz
showed that the sounds of musical instruments, including evei the
most wonderful one of all (the human voice), may be produced
synthetically.
NOTE. The author takes this last opportunity to advise the
pupil to procure and study Mayer's " Sound." See Chapter 5VH
278
NATURAL PHILOSOPHY.
353
353. Recapitulation. To be amplified by the pupil
for review.
1
SYMPATHETIC
VIBRATIONS.
ANALYSIS OF
f CONSTRUCTION.
Resonance.
SONOMETER
MOUNTED TUNING-FORKS. . .
SOUNDING BOARDS
AIR COLUMNS.
HELMHOLTZ'S RESONATORS..
REINFORCEMENT
f Characteristic of wave motion
INTERFERENCE... \
[Seats.
FUNDAMENTAL TONES.
OVERTONES.
353 EXERCISES. 279
EXERCISES.
1. Two forks vibrate 256 and 264 times a second respectively.
(.) Which yields the longer soundwaves? Which produces
tones of the higher pitch ?
2. What is the length of waves produced by a fork vibrating
280 times a second, when a centigrade thermometer shows a tem-
perature of 15 degrees ? Ana. 4 feet.
3. A resonant air column (Fig. 162) is one foot long. What is
the wave length of the tone to which it will respond most satis-
factorily ? Ana. 4 feet.
4. One tuning-fork vibrates 256 times a second and makes two
beats per second with a second fork. What is the rate of vibra-
tion of the second fork ?
5. Is the motion of a sound wave vibratory or progressive?
6. What is the difference between a longitudinal and a traaa
verse vibration ? Illustrate each.
7. What is the velocity of sound in a vacuum?
8. What determines the pitch of a musical tone*
NATURAL PHILOSOPHY.
353
REVIEW QUESTIONS.
1. (a.) How may one vary the pitch of a violin string? (6.)
Describe the electrical condition of a polarized body.
2. When a door is swung open or shut by pressing with the
hand near the hinges, what class of levers is illustrated ?
3. Fig. 172 represents apparatus made of glass tubing and a
bottle. There are openings at a, b and c. Tell how the apparatus
a may be used to transfer a liquid that is dangerous to
handle, e. g. sulphuric acid.
4. Describe the waves produced by throwing a stone
into quiet water.
5. Describe the waves produced by striking a bell
in the air.
6. What is the difference between kinetic and po-
tential energy.
7. Define physical change and give an illustration
thereof.
8. Describe the barometet
9. What is always the divisor in problems in spe-
cific gravity?
10. WhatisaKathion?
11. What is meant by the length of a sound wave ?
On a certain dav at the freezin & temperature, I
saw the flash of a gun. Ten seconds later, I heard the
report, (a.) What was the distance of the gun? (b.) Why do I
state the temperature ?
FIG 172
'
FIG. 173.
13. Fig. 173 represents an electric current flowing through an
insulated wire wound around a piece of soft iron. What is the
name of this combination?
CHAPTEB UI*
HEAT.
SECTION I.
TEMPERATURE, THERMOMETERS, EXPANSION.
Experiment 168. Rub a brass button on the floor or carpet
it soon becomes uncomfortably warm.
Experiment 169. Place a nail or coin on an anvil or stone and
hammer it vigorously ; it soon becomes too hot to handle. In
this way, blacksmiths sometimes heat iron rods to redness.
Experiment 170. Hold a piece of iron or steel against a dry
grindstone in rapid revolution ; the shower of sparks noticeable ia
due to the fact that the small particles of metal torn off by the
grindstone are heated to incandescence.
354. Heat Produced by Mechanical Energy.
Tlie arresting of mechanical motion transforms
visible energy into heat. (See 99.)
355. A Day Dream. As our young philosopher notices tha
quickly falling blows of the blacksmith's hammer on the nail and
anvil, he begins to think very intently. He knows that the descend/
282 NATURAL PHILOSOPHY. 355
ing hammer nas energy for it is able to do work. He understands
that this is so because the hammer has weight and motion. After
the last blow has been struck and the hammer lies at rest on the
anvil, he begins to wonder what has become of all the energy that
he knows was in the hammer when it was in motion a moment
ago. He knows that the visible, kinetic energy has disappeared,
for the hammer has no motion. He cannot see that it has been
transformed into potential energy. Yet he remembers that he has
been told that energy, like matter, is indestructible. Here is a prob-
lem ; shall he give it up? No, for he is our " young philosopher."
The puzzling problem perplexes him so that be falls into a day-
dream. His " scientific imagination " begins to work. He again
sees the falling hammer; he hears and, with quickened percep-
tion, almost feels the shock. The motion of the hammer, as a ham-
mer, ceases ; the energy of the hammer, as a hammer, is destroyed.
But his mind's eye sees the myriad molecules of the hammer, nail
and anvil, each for itself, pick up a portion of the motion lost by
the hammer as though the shock had given to each a shiver.
He sees that, as these molecules now have weight and motion,
they necessarily have kinetic energy. He sees that, at least,
part of these molecular motions were produced by the mass motion
of the hammer and that the total quantity of energy thus gained
oy the molecules must be just equal to the quantity of energy lost
by the hammer.
He then awakens ; he sees the hammer, nail and anvil but he
cannot see the molecular motions that were so vivid in his day-
dream. He places his hand upon the iron masses and finds that
they were heated by the blows. Like Archimedes, he shouts
"Eureka! I cannot see these molecular motions but I can feel
them. The visible energy of the moving mass would have been
jevealed to my hand as a crushing blow ; the invisible energy of
the moving molecules is revealed to my hand as heat."
Experiment 171. Pass a bent glass tube through the air-tight
cork of a flask half full of water, and let it dip beneath the sur-
the water. Heat the flask. The heat will raise some
358
TEMPERATURE.
283
FIG. 174.
of the water to the end of the tube where it may be caught as
shown in Fig. 174.
356. Mechanical En-
ergy Produced by Heat.
Previous experiments
showed us that mechanical
energy can produce heat.
The last experiment shows
that heat can perform me-
chanical work: it lifted
water against the force of
gravity There certainly
seems to be a close con nee
tion between heat and the
ordinary forms of energy.
357. What is Heat? Heat is a form of en-
ergy. It consists of the ceaseless, vibratory motions
of the molecules of matter or results from such
motions and gives rise to the sensations of warmth
and cold.
358. What is Temperature ? The temperature
of a body is its condition considered with refer-
ence to its ability to communicate heat to other
bodies.
Temperature is a very different thing from quantity
jf heat. A cup of water dipped from a lake will have
the same temperature as the lake, but the water in the
lake will have incomparably more heat than the water
ua the cup.
NATURAL PHILOSOPHY.
359
359. Thermometers. An instrument for meas-
uring temperature is called a thermometer. The
mercury thermometer is the most common.
360. Thermometric Scales. There are
two scales used in this country, the centigrade
and Fahrenheit's. For these scales, the fixed
points are marked as follows :
Centigrade. Fahrenheit.
Freezing point, 32
Boiling point, 100 212
The tube between these two points is divided
into 100 equal parts for the centigrade scale
and into 180 for Fahrenheit's. Hence a change
of temperature of 5 C. is equal to a change
of 9 F., or an interval of one centigrade degree
is equal to an interval of -| of a Fahrenheit degree.
Fiu. 175.
Experiment 172. Provide a
metal ball which, at ordinary
temperatures, will easily pass
through a certain ring ; heat the
ball and it will no longer pass
through the ring. If the ball be
cooled by plunging it into cold
water, it will again pass through
the ring. (Fig. 176.)
361 Expansion. In-
crease of volume is the
first visible effect of heat
upon bodies.
FIG. 17ft,
364 H&PAiratoX. 285
Whatever raises the temperature of a body increases
:he energy with which the molecules of that body swing
to and fro. Molecules thus vibrating must push each
other further apart, and thus cause the body which they
constitute to expand.
Experiment 173. Saw a piece from one side of a large iron
link and force the ends of the opened link slightly together so
that, the small piece may be pressed hard enough to hold it in
place. When the opposite side of the link is heated, it will expand
and the piece will fall out of its place.
362. Expansion of Solids. The energy of ex-
pansion and contraction of solids, whan heating and cool-
ing, is remarkable. This expansion of metals by heat
is utilized by coopers in setting hoops, by wheelwrights
in setting tires, by builders in straightening bulging walls,
fe
363. Anomalous Expansion of Water. Water
presents a remarkable exception to the general rule. If
icatcr at 0. be heated, it will contract until it
readies 4 C., its temperature of greatest density.
Heated tiborc fit is point, it expands.
364. Results of this Exception. This property
of water is of great importance. Were it otherwise, the
ice would sink and destroy everything living in the
water. The entire body of water would soon becoma
a solid mass which the heat of summer could not wholly
meh, for, ns we shall soon see. wafer has little power to
carry heat downward.
286 KAtVRAL PHILOSOPHY. 365
Experiment 174. Partly fill a bladder with cold air, tie up
the opening and place the bladder near the fire. The air will ex-
pand and fill the bladder.
Experiment 175. Heat a closed flask having a delivery tube
terminating under water.
Some of the expanded air
will be forced to escape, and
may be seen bubbling
through the water. By
" collecting over water " the
air thus driven out, it may
be accurately measured.
(Fig. 177.)
Experiment 176. Cut
a flat spiral with two or
three turns from a piece of
stiff writing paper about
three inches in diameter
thrust a pin through the
centre of the paper, take
the pin by the point and
FIG. 177. bold the spiral over a hot
stove or lighted lamp. As-
cending currents of heated air will cause the spiral to turn alxmt
the pin as an axis.
365. Expansion of Gases. The ascension of "fire
balloons" and the draft of chimneys are due to the ex-
pansion of gases by heat. When the air in the chimney
of a stove or lamp is heated, it is rendered lighter than
the same bulk of surrounding air and, therefore, rises.
The cooler air conies in to take its place.
367
R EGA PITULA TION.
287
366. Absolute Zero of Temperature. Tlie tern
perature at which the molecular motions constitut-
ing heat wholly cease is called the absolute zero.
It has never been reached, and has been only approxi.
mately determined.
(a.) Temperature, when reckoned from the absolute zero, ia
called absolute temperature. Absolute temperatures are obtained
by adding 460 to the reading of a Fahrenheit thermometer,
or 273 to the reading of a centigrade thermometer.
367. Recapitulation. To be amplified by the pupil
for review.
DEFINITION.
PRODUCED BY FRICTION.
INVISIBLE MOLECULAR ENERGY.
DEFINITION.
TEMPERATURE.
EXPANSION.,..
THERMOMETERS
ABSOLUTE.
CAUSE.
Souus.
LIQUIDS.
(Centigrade. 1
L
I Fahrenheit. I
NATURAL PHILOSOPHY. 367
EXERCISES.
1. If a centigrade thermometer records 15, what will be th
reading of a Fahrenheit thermometer by its side ?
2. Express a temperature of 59 F. in degrees centigrade.
8. What is the absolute temperature of 15 C. ? What does
your answer mean ?
4. How can you easily show that mechanical energy may be
converted into heat energy ?
5. Why does not Lake Erie freeze solid to the bottom ?
6. Show how the draft of a chimney depends upon
SECTION II.
LIQUEFACTION AND VAPORIZATION.
Experiment 177. Melt some ice in any convenient dish
When it is partly melted, stir the
liquid part with a chemical thermometer
and notice carefully the temperature at
which ice melts. Frequently deter-
mine the temperature until the ice is
all melted and notice that the tem-
perature is constant until the last
solid particle disappears.
p IG
Experiment 178. Repeat the ex-
periment with tallow, beeswax and
Bulphur in succession. It may be ob-
served, in each case, that the hotter
the fire, the more rapidly the solid will
melt but that there will be no rise of
temperature from the time the solid
begins to melt until it is all melted.
Heat the sulphur somewhat above the
melting poi it and allow it to cool. Notice the temperature when
it begins to solidify and while it is solidifying
368. Laws of Fusion. It has been found by ex-
periment that the following statements are true :
(1.) Evefy solid begins to melt at a certain tern*
perature wliicli, for a given substance, is invariable
if the pressure be constant. When cooling, the sub-
stance will solidify at the temperature of fusion.
13
290 NATURAL PHILOSOPHY. 368
(2.) TJie temperature of the solid or liquid re-
mains at the melting point from the moment
that fusion or solidification begins until it is
complete.
369. Vaporization. If, after liquefaction, further
additions of heat be made, a point will be reached at
which the liquid will pass into the aeriform condition.
This change of form is called vaporization. Vaporization
is of two kinds evaporation and ebullition.
370. Evaporation. Evaporation signifies the
quiet formation of vapor at the surface of a liquid.
371. Ebullition. Ebullition or boiling, signifies
the rapid formation of vapor bubbles in the mass
of a liquid. (See Elements of Natural Philosophy,
501.)
Experiment 179. Tn a beaker half full of water, place a ther-
mometer and a test tube half filled with ether. (Fig. 179.) Heat
the water. When the thermometer shows a temper-
ature of about 00 C., the ether will begin to boil.
The water will not boil until the temperature rises
to 100 0. The temperature will not rise beyond
this point.
Experiment 180. Half fill a Florence flask with
water. Boil the water until the steam drives the
Jj'ia. 179. air from the upper part of the flask. Cork tightly,
remove the lamp and invert the flask. (Fig. 180.)
The exclusion of the air may be made more certain by immers-
ing the corked neck of the tiask in water that has been
boiled.
372
BOILING.
291
When the lamp was removed, the temperature was not abov
100 ( '. By the time that the flask is inverted and the boiling ha3
ceased, the temperature will have fallen below 100 C. When the
boiling stops, pour cold water upon the flask ; directly the boil
ing begins again.
The cold water poured upon the flask lowers the temperature oi
the water in the flask still further, but it also condenses some or
the steam in the flask 01 reduces its tension ( 179). This reduc-
tion of the tension lessens the work necessary to boiling. There
being enough heat iu the water to do this lessened amount of
work, the water again boiteand increases the pressure until the
boiling point is raised above
the temperature of the
water.
The flask may be drenched
and the water made to boil
a dozen times in succession
with a single heating. The
experiment may be made
more striking by plunging
the whole flask under cool
water.
While water boils at 212
P. under a pressure of one
atmosphere, it must be
heated to about 2oO F. to
boil under a pressure of
two atmospheres or to more
than 350 F., under a pres-
sure of ten atmosj v hfn\s. rio.
372. Laws of Ebullition.
(1.) Even/ li(/ni<J ln'giiis to boil at a certain
temperature, which, for it given substance, is in-
292 NATURAL PHILOSOPHY. 372
variable if the pressure be constant. Wlien cooling, .
ttie substance will liquefy at the boiling point.
(2.) TJie temperature of the liquid,, or vapor,
remains, at the boiling point from the moment
that it begins to boil or liquefy.
(3.) An increase of pressure raises the boiling
point; a decrease of pressure lowers the boiling
point.
373. Concerning Steam. A given mass of wa-
ter in the aeriform condition occupies nearly
1700 times as much space under a pressure of
one atmosphere as it does in the liquid condi-
tion. In other words, a cubic inch of water will yield
pearly a cubic foot of steam.
Steam is invisible. What is commonly called steam
is not true steam, but little globules of water condensed
by the cold air and suspended in it. By carefully
noticing the steam issuing from the spout of a tea-kettle,
it will be observed that for about an inch from the spout
there is nothing visible. The steam there has not had
opportunity for condensation. The water particles vis-
ible beyond this space passed through it as invisible
steam. The steam in the flask of Fig. 180 is invisible.
Experiment 181. Boil water colored with ink in the flask, a.
/Pig. 181.) The steam passes through the delivery tube into c
which is placed in a vessel of iced water. The steam that con-
denses in the delivery tube and the smaller flask will be found to
be colorless water. The ink has been left In a.
374
DISTILLATION.
374. Distillation. Distil-
lation is a process of sepa-
rating a liquid from a
solid which it holds in
solution, or of separating a
mixture of two liquids hav-
ing different boiling points.
The process depends upon the
fact that different substances
are vaporized at different tem-
peratures.
The apparatus, called a
still, is made in many forms,
but consists essentially of two parts the retort for pro
ducing vaporization, and a condenser for changing the v
FIG. 181.
Fw. 183.
294
NATURAL PHILOSOPHY.
374
por back to the liquid form. Fig. 182 represents one form
of the apparatus. It consists of a retort, db, the neck
of which is connected with a spiral tube, dd, called the
worm. The worm is placed in a vessel containing water.
This vessel is continually fed with cold water carried to
the bottom by the tube 7i. As the water is warmed by
the worm it rises and overflows at i.
(a. ) Suppose that water is to be separated from the salt it holds in
solution. The brine is placed in a retort and heated a little above
212 F. At this temperature, the water is vaporized while the salt
is not. The steam is driven from the retort through the worm,
where it is rapidly condensed and passes into a vessel, g, prepared
to receive it. The salt remains in the retort, a.
FIG. 183.
(&.) Fig. 183 represents a simpler form of apparatus for this
purpose. The retort is a Florence flask, F t the delivery tube of
which passes through a " water-jacket,'' J. The method of supply
tng this condenser with cold water is evident from the figure.
(0.) Suppose that alcohol is to be separated from water. The
375
RECAPITULATION.
29.7
solution is placed in the retort and heated about 90 C., which ig
above the boiling point of alcohol but below that of water. The
alcohol will pass over in a state of vapor and be condensed, while
the water, etc., remain behind.
375. Recapitulation. To be amplified by the pupil
for review.
CHANGE OF
PHYSICAL
QQUDITION.
LIQUEFACTION. LAWS.
EVAPORATION.
VAPORIZATION.
BOILING.
Definition.
Laws.
Effect of Pressure
Steam.
Di*till*ti<m.
NATURAL PHILOSOPHY. 375
EXERCISES.
1. Sulphur begins to melt at 115 C. At which temperature
will melted sulphur begin to solidify?
2. What is the difference between evaporation and ebullition 1
3. How can water be heated above the ordinary boiling point t
4. On the top of high mountains, water boils at so low a tem-
perature, owing to diminished atmospheric pressure, that it does
not become hot enough to cook many kinds of food. Can you sug
geet a remedy for this difficulty ?
5. What does steam look like?
6. What fact underlies all processes of distillation ?
7. How may sea water be rendered tit lor drinking?
8. Ice and snow are being melted in a kettle over a hot fire.
How warm can they be made before they are wholly melted ?
9. A cubic foot of water is vaporized, (a.) What is the volume
of the steam under a pressure of one atmosphere ? (6.) Under g
pressure of two atmospheres? (See Exercise 7, page 137.)
SECTION I I I.
LATENT AND SPECIFIC HEAT.
376. Thermal Units. A thermal or heat unit
is the amount of heat necessary to warm a weight
unit of water one degree above the freezing point.
The weight unit generally used is the gram or pound ;
any other weight unit may be used with equal propriety.
The degree may be centigrade or Fahrenheit.
(a.) We have three units in common use. They are the amounts
of heat necessary to warm
(1.) A kilogram of water from C. to 1 C. (A calorie.)
(2.) A gram of water from C. to 1 C. (A lesser calorie./
(3.) A pound of water from 32 F. to 33 D F.
Experiment 182. Take a block of ice at a temperature of
-10 C. (14 F.) and warm it. A thermometer placed in it rises to
C. The ice begins to melt, but the mercury no longer rises.
Heat is still applied but the thermometer remains stationary
until the last particle of ice has been liquefied. Then the tem-
oerature begins to rise and continues to do so until the ther-
mometer marks 100 C. The water then begins to boil, and
the temperature a second time becomes fixed. See Element*
of Natural Philosophy, 580-582.
377. Definition of Latent Heat. The latent
heat of a substance is the quantity of heat that
must be communicated to a body to change its
condition without changing its temperature.
298 NATURAL PHILOSOPHY. 377
It may be made to reappear as sensible heat during the
opposite changes, after any interval of time.
378. Latent Heat of Fusion. When ice or any
other solid is melted by the direct application of heat,
much of the heat is rendered latent. We may represent
the process of liquefaction of ice as follows :
Water at C. = ice at C + latent heat of water.
Experiment 183. Mix two weights of pulverized ammonium
nitrate and one weight of pulverized ammonium chloride (sal am-
moniac) and dissolve the mixture in three weights of cold water,
stirring the substances together with a small test tube containing
a little cold water. Determine the temperature of the dissolved
mixture with a chemical thermometer and notice the condition
of the water in the test tube.
379. Latent Heat of Solution. During the pro-
cess of solution, as well as during fusion, heat is ren-
dered latent. Hence, the solution of a solid in-
volves a diminution of temperature.
(a.) A cup of coffee is cooled by sweetening it with sugar and a
plate of soup is cooled by flavoring it with salt.
380. Freezing Mixtures. The latent heat of so-
lution lies at the foundation of the action of
freezing mixtures. For example, when ice is melted
by salt and the water thus formed dissolves the salt itself,
the double liquefaction requires a deal of heat which is
often furnished by the cream in the freezer.
(a.) The freezing mixture roost commonly used consists of one
38 1 LATENT HEAT. 299
weight of salt and two weights of snow or pounded ice. The,
mixture assumes a tmperature of 18 C., which furnished the
zero adopted by Fahrenheit.
(6.) Persons who sleep in cold chambers sometimes notice that
as soon as they touch a pitcher of water that has been standing in
the room over night, the water quickly freezes. If a particle of
ice be dropped into the water, the same result follows. We may
say that, in this condition, liquids have a tendency to become
solid and are restrained only by the difficulty of making a be-
gLnniag.
Experiment 184. Surround with a freezing mixture, a small
glass vessel containing water and a mercury thermometer. The
temperature of the water may be reduced to 10 C. or 12 C.
without freezing the water. A slight movement of the thermome-
ter in the water starts the freezing and the temperature quickly
rises to C.
Experiment 185. Dissolve two weights of Glauber's salt in one
weight of hot water, cover the solution with a thin layer of oil
and al!ow to cool, in perfect quiet, to the temperature of the room.
By plunging a thermometer into the still liquid substance, solidi-
fication (crystallization) is started and the temperature rapidly
rises. Dr. Arnott found that this experiment was successful after
keeping the solution in the liquid condition for five years.
Experiment 186. To three weights of quicklime, add one weight
of water. The water will be completely solidified in the
slaking of the lime with remarkable manifestations of heat.
Carts containing quicklime have been set on fire by exposure to
heavy rains
381. Solidification. Solidification signifies the pas-
sage from the liquid to the solid condition. During
solidification, tJtere is an increase of temperature.
300 NATURAL PHILOSOPHY. 381
The heat energy, being no longer employed in doing
the work of maintaining liquidity, is. reconverted into
sensible heat.
Experiment 187. Pour a teaspoonful of sulphuric ether into
the palm of the hand, being sure that there is no flame near to
ignite the inflammable vapor of the ether. As the liquid evapo-
rates, notice that the hand is furnishing the heat needed to
perform the work of vaporization.
Experiment 188. Wet a block of wood and placea watch crystal
upon it. A film of water may be seen under the central part of the
glass. Half fill the crystal with sulphuric ether and rapidly
evaporate it by blowing over its surface a stream of air from a
small bellows. So much heat is rendered latent in the vaporiza-
tion that the watch crystal is firmly frozen to the wooden
block.
Experiment 189.
Nearly fill the porous
cup of a Grove or Bun-
sen cell ( 262, 263)
with water at the tem-
perature of the room.
As the water evapo-
rates at the surface of
the porous cell, the
water in the cell be-
comes cooler, heat
having been with-
drawn for the work of
vaporization. After
the lapse of 15 or 20
FIG. 184. minutes, use a chemi-
383 LATENT HEAT. 301
cal thermometer to determine the temperature of the water. In
this way, water is often cooled in tropical regions.
Experiment 190. In a vessel of sulphuric ether, place a test
tube containing water. Force a current of air through the ether.
(Fig. 184.) Rapid evaporation is thus produced and, in a few
minutes, the water is frozen.
382. Latent Heat of Vaporization. The vapor-
ization of a liquid is accompanied by the disappearance
of a large quantity of heat and, frequently, by a dimi-
nution of temperature. There is a change of sensible
into latent heat; of kinetic into potential energy. We
may represent the vaporization of water as follows:
Steam at 100 C. = water at 100 C. + latent
heat of steam.
383. Condensation of Gases. Gases may be con-
densed by union with some liquid or solid, by cold or by
pressure. It has been recently shown that any known
gas may be liquefied by cold and pressure. In any case,
the condensation of a gas renders sensible a large
amount of heat.
(a.) When a gas that has been condensed is allowed to expand
under such circumstances that ft must do work (e.g. forcing bach
the surrounding air), its temperature is lowered. Icicles have
l>een formed by allowing condensed air charged with watery vapor
to escape suddenly from the confining vessel. The rapid expan-
sion of carbon dioxide (" carl onic acid gas") when a bottle of beet
or champagne is opened, often produces a fog in the neck of the
bottle. (See C/temintry, % 197.)
302 NATURAL PHILOSOPHY. 384
(6.) Carbon dioxide may be liquefied by great pressure. When
this liquid escapes through a small orifice into the air, evapora-
tion is so rapid that some of the dioxide is frozen in the form of a
fine snow. This carbon dioxide snow dissolves in sulphuric ether
and with it forms one of the most intense freezing mixtures kmown.
By aiding the evaporation of this mixture with an air pump,
Faraday obtained a temperature of 110" C.
Experiment 191. Mix a pound of ice-cold water (0 C.) with
a pound of water at 80 C. Note the temperature of the mixture.
We have two pounds of water at 40 C. The heat lost by the
hot water in cooling 40 degrees was equal to the heat gained by
an equal weight of water in warming 40 degrees in fact it was
identical with it.
Experiment 192. Place a pound of melting ice (0 C.) in a
pound of water at 80 C. When the ice is all melted, note the
temperature of the water. We have two pounds of water at
C.
384. The Latent Heat of Water In Experi-
ments 192 and 193, the heat which warmed one pound
of water from to 80 C. was used to melt a like
weight of ice. Hence, by definition, the latent heat
of water is 80 C. (or 144 F.).
TJie amount of heat required to melt a quan-
tity of ice without changing its temperature is
eighty times as great as the heat required to
warm the same quantity of water one centigrade
degree.
(a.) Because of this great latent heat of water, the processes of
melting ice and freezing water are necessarily slow.
386 LATENT HEAT. 303
385. The Latent Heat of Steam. The amount
of heat necessary to evaporate one gram of water would
suffice to raise the temperature of 537 grams of water
1 C. Hence, the latent heat of steam is 537 C. (or
967 F.).
TJie amount of heat required to evaporate a
quantity of water without changing its tempera-
ture is 537 times as great as the heat required
to warm the same quantity of water one centi-
grade degree.
(a.) When a gram of steam is condensed, 537 heat units (centi-
grade-gram-water) are liberated. In this, we see an explanation
of the familiar fact that scalding by steam is so painfully severe.
(6.) Were it not for the latent heat of steam, when water
reached its boiling point it would instantly flash into steam with
tremendous explosion.
386. Problems and Solutions. (1.) How many grams of ice at
C. can be melted by one gram of steam at 100 C. ? One gram
of steam at 100 C., in condensing to water at the same tempera-
ture, parts with all its latent beat, or 537 heat units. The gram of
water thus formed can give out 100 more heat units. Hence, the
whole number of beat units given out by the steam in changing
to water at C., the temperature at which it could no longer
melt ice, is 537 + 100=637.
Since it requires 80 heat units to melt one gram of ice, 637 heat
units will melt as many grams as 80 is contained times in
637, which is 7.96. Therefore, the steam will melt 7.96 grams of
ice.
(2.) How many pounds of steam at 100 C. will just melt 100
pounds of ice at C. ? To melt 100 pounds of ice, (80 x 100=),
8,000 heat units will be required. Each i>ound of steam can fur-
304 NATURAL PHILOSOPHY. 386
nish 63? heat units for the work required. 8,000 -H 637 = 12.55.
the number of pounds of steam.
(3.) What weight of steam at 100 C. would be required to
raise 500 grams of water from D C. to 10 3 C. ? Each gram o*
water will require 10 heat units ; 500 grams of water will require
5000 heat units. Each gram of steam can furnish (537 + 90 ) 627
heat units for the work required. 5000 -*- 627 = 7.97, the number
of grams of steam.
Experiment 193. Mix any convenient quantity of ice-cold
water (0 C.) with the same quantity of water at a temperature
of 30 C. Ascertain the temperature of the mixture; jt will be
about 15 C. If you used a kilogram of water in each case, the
15000 heat units lost by the warm water and the 15000 heat units
gained by the cold water were identical.
387. Equality of Loss and Gain. A substance
loses as much heat in cooling a given number
of degrees as would be gained by a like quantity
when warmed an equal number of degrees.
Experiment 194. Suspend a vessel containing 3 kilograms of
mercury in boiling water until you are sure that it has the tem-
perature of the water (100 C.) Quickly pour the mercury into one
kilogram (or liter) of ice cold water, C. The temperature of
the mixture will be about 9 C.
The kilogram of water, in being warmed 9 C., gained 9000
heat units ; consequently, the 3 kilograms of mercury in cooling
91 C., must have lost 9000 heat units, for our experiment has
neither created nor destroyed any heat and, if it was neatly per-
formed, but little was lost.
For each degree of change of temperature, the kilogram of water
gained 1000 heat units ; for each degree of change of temperature
9000
one kilogram of mercury lost ^ -= = 33 heat units. In other
yi x o
words, it takes about 30 times as much heat to warm a giveq
39 SPECIFIC HEAT. 305
yeight of water one degree as it does thus to warm the same
weight of mercury.
388. Definition of Specific Heat. The specific
heat of a body is the ratio between the quantity
of heat required to warm it one degree and the
quantity of heat required to warm an equal weight
of water one degree.
(.) It is very important to bear in mind that specific heat, like
specific gravity, is a ratio ; nothing more nor less. The 8|>ecific
heat of water, the standard, is unity. The specific heat of iron is
0.1138; of copper, 0.0902; of tin, 0.0563 ; of lead, 0.0314 ; of bis-
muth, 0.0308 ; of ice, 0.5 and of steam, 0.48. These ratios will be
the same for any given substance, whatever the thermal unit or
ihermometric scale adopted.
389. Heated Balls Melting Wax. The difference
between bodies in respect to specific heat may be roughly
illustrated as follows : small balls of equal weight, mad
severally of iron, copper, tin, lead and bismuth are heated
to a temperature of 180 or 200 C. by immersing them
in hot oil until they all acquire the temperature of the
qil. They are then placed on a cake of beeswax about
306
NATURAL PHILOSOPHY.
389
half an inch thick. The iron and copper will melt their
way through the wax, the tin will nearly do so, while the
lead and bismuth sink not more than half way through
the wax. This shows that the iron or copper had more
heat than an equal weight of lead or bismuth at the same
temperature.
390. Specific Heat of Water. Water in its
liquid form, has a higher specific heat than any
other substance except hydrogen. For this reason,
the ocean and lakes are cooled and heated more slowly
than the land and atmosphere. They thus modify sud-
den changes of temperature, and give rise to land and
sea breezes and to the well known fact that the climate of
the sea-coast is warmer in winter and cooler in summer
than that of inland places of the same latitude.
391. Recapitulation. To be amplified by the pupil
for review.
UNITS.
HEAT.
ENERGY..
KINETIC=SENSIBLE HEAT.
Convertibility on
' Critical occasions. 1
I. POTENTIAL = LATENT HEAT
Definition.
Of Fusion.
Of Solution.
Of Water.
Of Steam.
Freezing Mixture*
Solidification.
DEFINITION.
SPECIFIC. I ILLUSTRATION.
I OF WATER.
8 39* EXERCISES. 307
EXERCISES.
1. Why does sprinkling the floor of a room on a warm day add
to the con: fort of the occupants of the room ?
2. Does a dog's lolling his tongue on a summer day render him
less warm ? Explain.
3. Why do we often bathe a fevered brow with water or with a
mixture of alcohol and water ?
4. How many heat units are required to melt three pounds of
ice ? Am. 432 units (pound-Fahrenheit).
5. How much heat is required to raise three pounds of ice-cold
water to the boiling temperature ? Ann. 540 units.
6. How much heat is required to vaporize 3 grams of boiling
water? Aw. 1611 lesser calories.
7. How much heat is needed to melt and evaporate 8 pounds
of ice ? AM. 3873 units (pound-Fahrenheit).
8. How much heat is needed to change a kilogram of ice at
-10 C. to water at 15 C. ? (Take note of the specific heat of ice.)
Ans. 100 calories, or 100,000 lesser calories.
SECTION IV.
MODES OF DIFFUSING HEAT.
Experiment 195. Place one end of an iron poker into the firfe
The other end soon becomes too warm to handle.
392. Conduction. TJie process by which heat is
transferred from, the hotter to the cold v parts of
a body, passing from one molecule to tlie next
nioteviUe, is called conduction of heat. The propa-
gation is very gradual and as rapid through a crooked
as through a straight bar.
Experiment 196. Instead of the iron poker of Experiment 195,
use a glass rod or wooden stick. The end of the rod may be
melted or the end of the stick burned without rendering the
other end uncomfortably warm.
Experiment 197. Thrust a silver and a German-silver spoon
into the same vessel of hot water; the handle of the former will
become much hotter than that of the latter.
Experiment 198. Place a bar of iron and one of copper end to
end so as to be heated equally by the flame of the lamp. Fasten
small balls (or nails) by wax to the under surfaces of the bars ?t
393 MODES OF DIFFUSING HEAT. 309
equal distances apart. More balls may be melted from the cop-
per than from the iron.
393. Difference in Conductivity. These experi-
ments show that some substances are good conductors
of heat and that some are not.
(a.) Relative thermal conductivity of some metals :
Silver 100 Lead 9
Copper 74 Platinum 8
Brass 24 GernuJi silver 6
Iron 12 Bismuth 2
The above-named metals arrange themselves in the same order
with reference to the conduction of electricity, silver being the
best and bismuth the poorest. This relation suggests a similarity
of nature between these two forms of energy.
Experiment 199. Pass the tube of an air thermometer or of
an inverted mercury thermometer through a cork in the neck of a
funnel. Cover the thermometer bulb to
the depth of about half an inch with
water. Upon the water, pour a little sul-
phuric ether and ignite it. The heat of
the flame will be intense enough to boil
a small quantity of water held over it, but
the thermometer below will be scarcely
affected.
Experiment 200. Fasten a piece of
ice at the bottom of a glass test or igni-
tion tube and cover it to the depth of
several inches with water. Hold the tube
obliquely and apply the flame of a lamp be-
low the upper part of the water. The water there may be made
to boil without melting the ice beiow. Instead of using ice and
water, pack the tube full of moist snow if you can get it.
310 NATURAL PHILOSOPHY. 394
394. Conductivity of Fluids. Liquids and aeri-
form bodies are poor conductors of heat. The sui>
face of a liquid may be intensely heated without sensibly
affecting the temperature an inch below. The conduc-
tivity of iron is about 80 times that of water ; that of
copper is about 500 times that of water, 'f he conduc-
tivity of gases is probably less than that of liquids.
Experiment 201. Drop a small quantity of cochineal or oak
sawdust into a glass vessel containing
water. Heat the water by a lamp
placed below. Notice the currents
indicated by the motion of the solid
particles.
395. Convection. Fluids
(with the exception of mercury
which is a metal) being poor
conductors, they cannot be
heated as solids generally are.
Water, e.g., must be heated
from beloiv ; the heated mole-
cules expand and rise while the
FIG 188 cooler ones descend to take their
place at the source of heat. TJiis
method of diffusing heat, by actual motion of
heated fluid masses, is called convection.
396. Luminiferous Ether. In the case ot kinetic,
mechanical energy, the rapid motion of bodies, e. g., a
vibrating guitar string, is partly carried off by the air in
lb. shnpe ut SQUUCJ.,
397 MODES OF DIFFUSING HEAT. 311
There is sufficient reason for believing that thert
is a medium pervading all space which carrier
off part of the invisible motions of molecules, just
as the air carries off a portion of the motion of
moving masses. This medium is called the -lu~
jniniferous ether.
It is supposed to occupy intermolecular as well as inter-
planetary space, and to pass as freely between the particles
of ordinary matter as the winds do between the trees of
the forest.
Experiment 202. Take a white-hot poker into a dark room.
It emits heat and a white light. The light gradually becomes
reddish and less bright, and finally fades from view as a dull red
glow. Long after it has ceased to be visible, the poker continues
to give heat to surrounding objects, as may be shown by holding
the hand or face near it on any side, above or below. There has
been a continuous change from the emission of white light and
much heat to that of no light and less heat.
397. Radiant Heat. The molecules of a heated
body are in a state of active vibration. The motion of
these vibrating molecules is communicated to the ether
and transmitted by it, as waves, with wonderful velocity.
Thus, when you hold your hand before a fire, the
warmth that you feel is due to the striking of these
ether-waves upon your skin; they throw the nerves into
motion just as sound-waves excite the auditory nerve
and the consciousness corresponding to this motion ia
what we call warmth.
Heat thus propagated by the ether, instead of by
ordinary forms of matter, is called radiant heat.
312 NATURAL PHILOSOPHY. 397
(a.) From the last experiment, we naturally conclude that
radiant heat and light are identical. Thorough experimental in-
vestigation has confirmed this conclusion. When the energy of
the ether waves produces a cer!ain effect upon the retina of the eye,
we call it light ; when it produces another effect upon the nerves
of touch, we call it radiant heat. " Thus radiant heat is brought
under the undulatory theory of light, which in its turn becomes an-
nexed as a magnificent outlying province of the kinetic theory of
teat." ( 420.)
398. Radiation of Heat. The transfej-rence of
heat from one body to another at a distance ir-
respective of the temperature of the intervening
medium, is called the radiation of heat.
399. Incident Rays. When radiant heat falls upon
a surface, it may be transmitted, reflected or absorbed.
If transmitted, it may be refracted. Eock-salt crystal
transmits nearly all, reflects yery little, and absorbs
nardly any. Polished silver reflects nearly all, absorbs
a little, and transmits none. Lampblack absorbs nearly
all, reflects very little, and transmits none.
Experiment 203. Hold a pane of glass between the face and a
hot stove. Notice that the glass shields the face from the
heat of the stove. Next, hold the glass between the face and the
sun. Notice that the glass does not shield the face from the
heat of the sun.
400. Diathermancy. -Bodies that transmit ra-
diant heat freely are called diathermanous ;
those that do not, are called athermanous. These
terms are to heat, what transparent and opaque are
to light. Bock salt Is the most diathermous substance
known.
402 MODES OF KTFFUSING HEAT. 313
401. Obscure and Luminous Heat. Heat that
is radiated from a non-luminous source, as from a ball
heated below redness, is called obscure heat ; while part
of that radiated from a luminous source, as from the sun
or from a ball heated to incandescence, is called lumi*
nous heat. Heat from a luminous source is generally
composed of both luminous and obscure rays.
Glass, water or a solution of alum allows luminous heat
rays to pass but absorbs nearly all of the heat rays from
a vessel filled with boiling water. In other words, these
substances are diathermanous for luminous rays but
athermanous for obscure rays.
A solution of iodine in carbon di-sulphide transmits
obscure rays but absorbs luminous rays. By means of
these substances, luminous and obscure rays may be
sifted or separated from each other.
Experiment 204. Hold a spectacle-glass or a larger lens
(g 450) from an opera glass in the sunlight, perpendicular to the
sun's rays. A point may easily be found below the lens where it
is unusually warm. At this point, called the focus, hold the tip
of a common friction match or a hit of gun cotton that has been
blackened with lamp black or soot. The concentrated rays of
the sun will set fire to the easily combustible substance. GTIH
cotton may be ignited at the focus of a lens made of ice.
Experiment 205. Stand beside an open fire and let your as-
sistant stand in front of the fire. Let him use a piece of bright
tin with which to throw back the light of 'the fire into your face.
Notice that heat as well as light is thrown back, your face feel
ing warmer.
402. Refraction and Reflection of Heat. Heat
rays may be bent from a straight line on entering and
314 NATURAL PHILOSOPHY. 402
leaving a body, as shown in Experiment 204. This
bending of the ray is called refraction.
The refraction of obscure rays cannot be shown by a
glass lens, since glass is athermanous for such rays. A
rock-salt lens and a thermopile may be used for such an
experiment. (See 278.)
Kadiant heat may be reflected like light. (See 430.)
403. Change of Radiant into Sensible Heat
Of all the rays falling upon any substance, only those
that are absorbed are of effect in heating the body upon
which they fall. The motion of the ether waves may be
changed into vibrations of , Dlecules of ordinary matter,
and thus produce sensible heat but the same energy can-
not exist in waves of ether and in ordinary molecular
vibrations at the same time. Lamp-black is a remark-
ably good absorbent.
Experiment 206. Provide two small tin cans of the same size
and shape. You can get them for the asking. In the cover oi
each, make a hole through which you may pass the bulb of a
chemical thermometer. Blacken the outside of one can with
paint or candle soot. Fill both cans with hot water from the same
vessel and, consequently, of the same temperature. At the end oi
half an hour, pass the bulb of the thermometer through the holes
in the covers and ascertain the temperature of the water in each
jan. It will be found that the blackened can has radiated its
heat (or cooled) more rapidly than the other.
404. Relations of Absorption, Reflection and
Radiation.-Cr00$ absorbents are good radiators and
poor reflectors, and vice versa. The powers of ab-
sorption and radiation go hand in hand. ( 338.)
405 RECAPITULATION. 315
The radiating power of a body depends largely upon
the nature of its surface ; smoothing and polishing the
surface increases reflecting power and diminishes absorb-
ing and radiating powers; roughening and tarnishing
the surf^e increase the absorbing and radiating powers
and diminish the reflecting power.
405. Recapitulation. To be amplified by the pupil
for review.
DIFFUSION OF HEAT.
316 NATURAL PHILOSOPHY. 405
EXERCISES.
1. Why is a moist, cold atmosphere more severe than a dry
atmosphere of the same temperature ?
2. Why does fanning one's self on a warm day increase one's
personal comfort?
3. Why is it that, in a cold room, some things seem colder than
others ?
4 (a.) Is a good conductor or a non-conductor better for keeping
a warm body warm ? (6.) For keeping a cold body cool ?
5. How is heat transmitted through a crooked iron rod ?
6. Is a good absorbent of heat better for radiation or for reflec
tion of heat ?
SECTION V.
THERMODYNAMICS.
406. Definition of Thermodynamics. Thermo-
dynamics is the branch of science that considers
the connection between heat and mechanical
work. It has especial reference to the numerical rela-
tion between the quantity of heat used and the quantity
of work done.
407. Correlation of Heat and Mechanical En-
ergy. We know that heat is not a form of matter be-
cause it can be created in any desired quantity. We
must continually remember that it is a form of energy.
When heat is produced, some other kind of energy must
be transformed. Conversely, when heat disappears,
some other form of energy appears.
408. Heat from Percussion. A small iron rod
placed upon an anvil may be heated to redness by re-
peated blows of a hammer. (Experiment 169.) The
energy of the moving mass is broken up, so to speak, and
distributed among the molecules, producing the form of
molecular motion that we call heat The same trans-
formation was illustrated in the kindling of a fire by the
" flint and steel " of a century ago. The bullet is heated
by its blow against the target and the water is wanner
at the foot of Niagara than in the rapids above.
318 NATURAL PHILOSOPHY. 41 1
409. Heat from Friction. Common matches are
ignited and cold hands warmed by the heat developed
by friction. It is said that some savages kindle fires by
skillfully rubbing together well-chosen pieces of wood.
A railway train is really stopped by the conversion of its
motion into heat by means of the brakes. Examples of
this change are matters of every-day experience. (Ex-
periment 168.)
410. Heat from Chemical Action. When coal is
burned, the carbon and oxygen particles rush together
with tremendous violence, energy of position being con-
verted into energy of motion.
The molecular motions produced by this clash-
ing of particles constitute heat and have a me-
chanical value.
411. Heating Powers. If a given weight of carbon
be burned, the heat of the combustion would warm about
8000 times that weight of water 1 C. In like manner,
the combustion of a gram of hydrogen would yield more
than 34000 heat units (gram-centigrade).
(a.) The following table shows the heating powers of several
substances when burned in oxygen :
Hydrogen 34,462 I Carbon
Petroleum 12,300 Alcohol (C 8 H S O).
(6 ) The heating powers mentioned above may be adapted to
Fahrenheit degrees by multiplying them respectively by . As
they stand, the numbers represent the number of times its own
weight of water could be wanned 1 C. by burning the substance
in oxygen.
414 THERMODYNAMICS. 3l9
412. First Law of Thermodynamics. When
heat is transformed into mechanical energy or
mechanical energy into heat, the quantity of
heat equals the quantity of inechanical energy,
4 I 3- Joule's Equivalent. It is a matter of great
importance to determine the numerical relation between
heat and mechanical energy; to find the equivalent of a
heat unit in units of work. This equivalent was first as-
certained by Dr. Joule, of Manchester, England. His
experiments were equal in number and variety to the
importance of the subject. He showed that the mechan-
ical value of the heat required to warm a given weight of
water
I" C., would lift the water J424 meters against gravity.
\ 1390 feet
1 F., would lift the water 772
Any weight unit may be used without changing the
above values which should be remembered.
Eeferring to the first iinit mentioned in 376, we say
that the mechanical value of a heat unit is 424 gram-
meters or 1390 foot-pounds.
Referring to the second unit there mentioned, we say
that the mechanical value of the heat unit is 772 foot-
pounds.
414. The Steam-Engine. The steam-engine is a
machine for utilizing the tension of steam. Its essential
parts are a boiler for the generation of steam and a
cylinder for the application of the tension to a piston.
320 NATURAL PHILOSOPHY. 4*5
415. Double-Acting Engine. In a double-acting
steam-engine, the steam is admitted to the cylinder
alternately above and below the piston. This alternate
admission of the steam is accomplished by means of a
sliding-valve. The sliding-valve is placed in a steam-
chest, 8, which is fastened to the side of the cylinder, G.
FIG. 189.
(?,.) In the figure, the steam-chest is represented as being placed
at a distance from the cylinder; this is merely for the purpose of
making plain the communicating passages to and from the chest.
Steam from the boiler enters at M, passes through A to the cylin-
der, where it pushes down the piston as indicated by the arrows
in Fig. 189. The steam below the piston escapes by B and F.
(&.) As the piston nears the opening of B in the cylinder, tho
sliding-valve is raised, by means of the rod, R, to the position
indicated in Pig. ISO. Steam now enters the cylinder by B aud
416 THERMODYNAMICS. 321
pushes up the piston. The steam above the piston escapes by A
and N. As the piston neare the opening of A in the cylinder, the
sliding- valve is pushed down by R and the process is thus repeated.
FIG. 190.
(c.) The piston-rod and the sliding-valve rod work through
s: earn-tight packing-boxes. The piston is connected with a crank
on the shaft of the engine, so that the to-and-fro motion of the
piston produces a rotary motion of the shaft. Smoothness of
motion is secured by attaching a heavy fly-wheel to the shaft of
the engine. A Uttle reflection will show that the fly-wheel also
acts as an accumulator of energy.
416. Non-Condensing Engines. When the steam
is forced out at N (Fig. 190), it has to overcome an at-
mospheric pressure of 15 pounds to the square inch.
Such an engine is known as a non-condensing engine.
It may be recognized by the escape of steam in puffs.
PHiLOSOPBT. 416
It is generally a high-pressure engine. The railway
locomotive is a high-pressure, non-condensing engine.
417. Condensing Engines. The steam may be
conducted from the exhaust pipe, N (Fig. 190). to a cham-
ber called a condenser. Steam from the cylinder and a
spray of cold water being admitted at the same time, a
vacuum is formed and the loss of energy due to atmos-
pheric pressure is avoided. Such an engine is known as
a condensing, or low-pressure engine. Steamboat en-
gines are generally condensing engines.
(a.) Low-pressure engines are always condensing engines. A
low-pressure engine will do more work with a given amount o*
fuel than a high-pressure, non-condensing engine will, is less liable
to explosion and causes less wear and tear to the machinery. But
it must be larger, more complicated, more costly and less easily
portable.
418. Heat and Work of Steam-Engines. More
heat is carried to the cylinder of a steam-engine than ia
carried from it.
Tlie piston does work at every stroke and this
work comes from the heat that disappears. Every
stroke of the piston transforms heat energy into
mechanical energy.
Careful experiments show that the heat destroyed and
the work performed are in strict agreement with Joule's
equivalent. With a given supply of fuel, the engine wih
give out less heat when it is made to work haid than
when it runs without doing much work.
419
RECAPITULA TtON.
419. Recapitulation. To be amplified by the pupil
for review.
f DEFINITION.
MECHANICAL POWER CHANGED TO HEAT BY
(PERCUSSION,
FRICTION.
ATOMIC ATTRACTION.
JOULE'S EQUIVALENT.
STEAM-ENGINES
A SOURCE OK HEAT.
IEATING POWERS OF SEVERAL SUBSTANCES
ESSBNTIAI PASTB.
DOUBLE ACTING.
CONDENSING.
NON-CONI
R3LATION BKTWKBN HEAT AMD W OWt
824 NATURAL PHILOSOPHY. 419
EXERCISES.
1. If it were possible to convert heat into mechanical power
without lobs, how far would the heat giren out by 1 gram of water
In cooling 1 C. lift another gram of water against the force o?
gravity ? Give your answer in meters and in feet.
2. How higli would it lift 2 grams of water?
Ans. 212 m., or 695 feet.
3. How high would it lift 2 grams of lead ?
4. How high would the heat thus given out by a pound of
water lift a pound weight ?
5. How high would the heat thus given out by 10 pounds of
water lift a 5 pound weight ? Ans. 848 m., or 2780 feet.
6. How high would the heat thus given out by 5 pounds of
water lift a 10 pound weight?
7. If a pound of water falls 772 feet, how much will it be heated
by the s'opping of its motion ?
8. If a 2 pound weight falls 772 feet, how much heat will be
developed by the percussion when it strikes the ground ?
9. How many heat units (gram-centigrade) must be transformed
In order to raise 1000 grams to a height of 424 meters ?
10. How many, thus to raise 2 kilograms? Ans. 2000.
11. How many grams of water can be heated from the freezing
to the boiling temperature by the heat produced by burning 1
gram of hydrogen ?
12. How many weights of water may be thus heated by burn-
ing 1 weight of pure coal (carbon)? Ans. 80.8.
13. If a good steam-engine utilizes only about 10 per cent, of the
heat energy of its fuel, how many foot pounds of work can it do
with a ton of coal that is assumed to be pure carbon ?
Ans. 2000 x 8080 x 1390 x T V
14 Would it do more work or less with the burning of a like
weight of petroleum ?
15. Can heat be destroyed? Can energy?
419 REVIEW QUESTIONS. 325
REVIEW QUESTIONS.
1. What three things determine the rapidity of vibration of a
string ?
2. (a.) Will sound waves pass through water? (b.) Through a
vacuum ?
8. What is in the upper part of a thermometer tube ?
4. Upon what does the loudness of sound depend ?
5. Why are spaces left between the ends of the rails in laying
railway tracks?
6. Why is a gallon of alcohol worth more in cold than in warm
weather ?
7. Will a siphon work in a vacuum ? Wliy ?
8. Can you pump any water out of an air-tight cistern full of
water? Explain.
9. Can you pump any water out of an air-tight cistern half full
of water? Explain.
10. Construct a simple piece of apparatus to illustrate your
answers to the last two questions.
11. Which seems colder, a wiudy or a still day, the temperature
being the same 1 Why ?
12. If there were no water on the earth, would the difference*
in temperature between day and night and between summer and
winter be greater or less than they now are? Why ?
13. Does heat always expand water?
14. How can you heat
water above 212 F. ?
15. Do all solids and gasea
expand equally ?
16. Show that the appa.
ratus represented in Fig. 191
is a lever and tell of what
PlO. 191. class. Indicate the positions
for P f W and F.
17. How does perspiration increase one's personal comfort iu
warm weather?
326
NATURAL PHILOSOPHY.
419
18. If a pound of water at 100 C. be added to a pound of watei
at C., what will be the resulting temperature?
19. Fig. 192 represents a " dropping bottle." (a.) Why does ail
bubble up from the lower end of tube a
when liquid drops from the lower end of c t
(6.) Why does the liquid cease to drop when
the finger closes the tube at a ?
20. The " return ball " is a common toy
made by fastening a wooden ball to the end
of an elastic cord. When thrown from the
hand, the cord being held by the free end,
the bail returns and is caught in the hand.
Does the motion of the ball resemble water
waves or sound waves the more closely?
21. A centigrade thermometer records 33. What will be
the reading of a Fahrenheit thermometer under similar circum-
FIG. 192.
22. Show haw the apparatus represented in Fig. 193 acts to
obviate the trouble arising from
the " boiling away " of the water
in the basin.
23. Describe the suction
pump.
24 What is the cause of dif-
ference of pitch ?
25. What is meant by the
temperature of the maximum
density of water?
26. What is meant by the ab-
solute zero of temperature ?
27. What is the ordinary me-
dium for the transmission (a.) of
sound ? (6.) Of radiant heat ?
28. Give the rule for finding
the downward pressure of
FIG. 193. liquids caused by gravity.
419
REVIEW QUESTIONS.
327
29. What are ions?
30. A bullet is thrown downward
with a velocity of 20 meters per
second. What will be its velocity
at the end of 4 seconds ?
31. Fill with water a cup sus-
pended by three cords. Turn the
cup round and rouud, ti^us twisting
together the cords. Leave the cup
free to untwist the cords and water
will fly from the edges of the cup as
shown in Fig. 194. What physical
law is thereby illustrated ?
32. Note the temperature by the
school-room thermometer. What
is the velocity of sound now and
here?
33. Define chemical change and
give an illustration thereof.
FIG. 194
CHAPTEB IX.
LIGHT.
SECTION I.
THE NATURE, VELOCITY AND INTENSITY OF LIGHT.
420. What is Light ? Light is the mode of
motion that is capable of affecting the optic nerve.
It is physically identical with radiant heat, the
only difference, in any case, being one of wave
length. (See 397.)
(a.) We have seen that the vibrations of air particles in a sound
wave are to and fro in the line of propagation. In the case of ra-
diant heat and light, the ether particles vibrate to and fro across the
line of propagation. Vibrations in a sound wave are longitudinal;
those of a heat or light wave are transversal.
421. Luminous and Non-Luminous Bodies.
Bodies that emit light of their own generating, as the
sun or a candle, are called luminous. Bodies that merely
diffuse the light that they receive from other bodies are
said to be non-luminous or illuminated. Trees and
plants are non-luminous.
422. Transparent, Translucent and Opaque
Bodies. Transparent bodies allow objects to be seen
distinctly through them, e. q., air, glass and water.
424
MATURE OF LIGHT.
329
Translucent bodies transmit light but do not allow
bodies to be seen distinctly through them, e. g., ground
glass and oiled paper.
Opaque bodies cut off the light entirely and prevent
objects from being seen through them at all.
423. Luminous Rays. A single line of light is
called a ray. Tlie ray may, without considerable
error, be deemed the path of the wave.
424. Luminous Beams and Pencils. A collec-
tion of parallel rays constitutes a beam; a cone of rays
constitutes a pencil. The pencil may be converging or
diverging.
FIG. 195.
Experiment 207. Provide two or three perforated screens and
arrange them as shown in Fig. 195, so that the holes and a candle
game shall be in the saine straight line. When the eye is placed
330 NATURAL PXILOSOPHT. 425
in this line behind the screens, light passes from the flame to the
eye ; the flame is visible. A slight displacement upward, down-
ward or sidewise of the eye, the flame or any screen, cuts off the
light and renders the flame invisible.
Make the screen as follows: Prepare a piece of wood, Ix2|
x 18 inches, taking care that the edges are square. Saw it into
six pieces, each three inches long. Prepare three pieces of wood,
3 x 4 x inches. Place three postal cards one over the other on a
board, and pierce them with a fine awl or stout needle, ^ inch from
the end and 1 inch from either side of the card. With a sharp
knife pare off the rough edges of the holes, and pass the needle
through each hole to make the edges smooth and even. Over the
^ x 3 inch surface of one of the blocks, place the unperforated end
of one of the postal cards and over this place one of the 3x4 inch
pieces, so that their lower edges shall be even. Tack them in
this position. Make thus two similar screens. The three screens,
with a bit of candle three inches long, piaced upon one of the
remaining blocks, furnishes the material for the experiment
above. Save the screens and three blocks for future use. (See
Pig. 200.)
425. Rectilinear Motion of Light. A medium
is homogeneous when it has uniform composition and
density. In a homogeneous medium, light travels
in straight lines.
(a) The familiar experiment of "taking sight" depends upon
this fact, for we see objects by the light which they send to ifie eye.
We cannot see around a corner or through a crooked tube.
Experiment 208. Place a lighted candle about a yard from a
white screen iu a darkened room. (The wall of the room may
answer for the screen.) Pierce a large pin-hole in a card and
hold it between the flame and the screen. An inverted image of
the flame will be found upon the screen,
426
NATURE OF LIGHT.
331
Experiment 209. Cover one end of a tube, 10 or 12 cm. long,
with tinfoil ; the other end with oiled paper. Prick a pin-hole in
the tinfoil and turn it toward a candle flame. An inverted
image may be seen upon the oiled paper. The size of the
image will depend upon the distance of the flame from the
pin-hole.
This apparatus rudely represents the eye ( 473), the pin-hole
corresponding to the pupil and the oiled paper to the retina. Al-
most any housekeeper will give you an empty tin can. Place it
upon a hot stove just long enough to melt off one end, thrust a
stout nail through the centre of the other end, cover the nail-hole
with tinfoil and you will have the greater part of the apparatus.
426. Inverted Images. If light from a highly-
illuminated body be admitted to a darkened room through
a small hole in the shutter and there received upon a
white screen, it will form an inverted image of the ob-
ject upon the screen. Every visible point of the illumi-
nated object sends a ray of light to the screen. Each
ray brings the color of the point which sends it and
yrints the color upon the screen. As the rays are
NATURAL PHILOSOPHY.
425
straight lines, they cross at the aperture; hence, the in-
version of the image. The image will be distorted un-
less the screen be perpendicular to the rays. The dark-
ened room constitutes a camera obscura. The image of
the school playground at recess is very interesting and
easily produced.
427. Velocity of Light. Light moves with a
velocity of about 186,000 miles per second.
(a.) It would require more than 17 years for a rannon-ball to
pass over the distance between the sun and the earth ; light makes
the journey in 8 min. 18 sec. For the swiftest bird to pass around
the earth would require three weeks of continual flight ; light
goes as far in less than one seventh of a second. For terrestrial
distances, the passage of light is practically instantaneous (% 325).
36'
FIG. 197.
Experiment 210. Let a candle or lamp at L, (Fig. 197) be the
source of light ; 8, a cardboard screen with a small perforation at
the level of the flame ; A, a screen one inch square and a foot from
S; B, a screen two inches square, two feet from 8; C. a screen
three inches square, three feet from S. It will easily be seen thai
A will cut off all the light from B and C. If A be removed, ti,e
quantity of light which it received, no more and no less, will fall
upon B. If now B be removed, the quantity of light which pre
riously illuminated A and B wjlj fall upon Q
428
NATURti OF LIGHT,
333
We thus see the same quantity of light successively illuminating
one, four and nine square inches. * One square inch at B will re-
-eive one-fourth, and one square inch at C will receive one-ninth as
much light as one square inch at A. The light being spread over
* greater surface is correspondingly diminished in intensity.
428. Effect of Distance upon Intensity. The
intensity of light received bij an illuminated body
caries inversely as the square of its distance from
the source of light.
FIG. 198.
(a.) This principle is applied in photometry or the measurement
of light. A simple photometer is represented in Fig. 198. S is a
screen of white paper or cardboard ; R is a small rod placed up-
right a few inches from S (a cheap j en and pen-holder or a lead
penc 1 held by a bit of wax on the table will answer). The flame
of the candle and that of the lamp should be at the same level ; the
flat lamp-wick should stand diagonally to the screen.
Place the candle about 20 inches from 8 and move L about until
the two shadows upon S just touch and are of equal darkness.
The candle and the lamp are now throwing equal amounts of light
upon H.
PHILOSOPHY. 429
If the distance from 8 to L be twice that from 8 to C, then S is
four times as powerful a light as'C; if the distance be three times
as far, L is nine times as powerful. If C be 20 inches from <S and
L be 65 inches, then L is ~ times as powerful as C.
(b.) Another method is to use a screen of paper with a grease
spot in its centre. If this spot be viewed from the side toward the
light, it will appear darker than the rest of the screen ; if viewed
from the other side, it will appear brighter. Place the two lights
to be compared on opposite sides of the screen so that they shall
be in the same straight line with the centre of the grease spot.
Move one light toward or from the screen until the grease spot
is invisible ; the two sides of the screen are then equally illumi-
nated.
Find the distance of each light from the screen ; square it ; di-
vide the greater square by the less : the quotient will be the ratio
between the intensities of the two lights.
NOTE- In either of these methods of photometry (light meas-
uring), there should be no light falling on the screen except whaf
comes from the two lights being compared.
429. Recapitulation. To be amplified by the pupil
for review.
LIGHT
DEFINITION.
TRANSPARENCY OF
LUMINOUS ....
BODIES.
f BODY.
1RAY.
BEAM.
RIGHT LINE MOTIOI
INVERTED IMAGES.
VELOCITY.
INTENSITY.
PENCIL.
I,
429 EXERCISES. 335
EXERCISES.
1. Do waves of light resemble sound waves more or less than
they do water waves ? Explain.
2. How does light differ from radiant heat?
3. What is a sunbeam ?
4. Explain the formation of inverted images.
5. If the sun were blotted out of existence, how long would it
be before the earth would be wrapped in darkness ?
6. If the moon were twice as far from the earth as it is, what
would be the effect upon the brilliancy of moonlight ?
7. An electric lamp 100 ft. north of me and one 200 feet south
of me illuminate opposite sides of a sheet of paper in my hand
and render invisible a grease spot on the paper.
(a.) Which lamp is giving the more light to the paper ?
(6.) How do the luminous powers of the lamps compare 1
8. What is the difference between a luminous and an illumi-
nated body ?
9. An opaque screen, 3 inches square, is held 12 inches in front
of one eye ; the other eye is shut ; the screen is parallel with a
wall 100 feet distant. What area on the wall may be concealed
by the screen ?
10. A " standard " candle (burning 120 grains of sperm per
hour) is 2 feet from a wall, a lamp is 6 feet from the wall. They
cast shadows of equal intensity on the wall. What is the " candle
power " of the lamp ?
SECTION 1 1 .
REFLECTION OF LIGHT.
Note. The hx-li..sut, or poi-te-iumiere, is composed of one or
more mirrors, by means of which a beam of light may be thrown
in any desired direction. The instrument may be had of appara-
tus manufacturers at prices ranging from $12 upward. Direc-
tions for making one may be found in Mayer & Barnard's little
book on "Light."
43O. Reflection. If a sunbeam enter a darkened
room by a hole in the shutter, as at A, and fall upon s>
PIQ. 199.
polished plane surface, as at B, it will be continued in
a different direction, as toward C. AB is called the
incident beam and BC, the reflected beam. The
43^ REFLECTION OF LIGHT. 337
incident and the reflected beams are in the same modium,
the air.
A change in the direction of light without a,
cJtange in its medium is called reflection of
light.
Experiment 211. Place two of thesceens and the three
extra blocks mentioned in Experiment 297 in position, as shown
in Fig. 200. At the middle of the middle block, place a bit of
win-low glass, painted on the underside with black varnish. Ou
the blocks that carry the screens, place bits of glass, n and o, of
the same thickness as the black mirror on the middle block.
FIG. 200.
Place a candle flame near the hole in one of the screens, as
shown in the figure. Light from the candle will pass through A,
be reflected at m and pass through B. Place the eye in such a
position that the spot of light in the mirror may be seen through
Ji. Mark the exact spot in the mirror with a needle held in place
by a bit of wax.
Phice a piece of stiff writing paper upright upon m and n. mark
the position of B and of m, and draw on the paper a straight line
joining these two points. The angle between this line and the
lower edge of the paper coincides with the angle Bmn. Reverse
*}ie paper, placing it upon m and o. It will be found that the same
angle coincides with Amo. Amo and Bmn being thus equal,
the angle of incidence equals the angle of reflection. ( 57.)
15
338 frATfrSAL ffflLOSOPHf. 43!
Experiment 212. Get a piece of looking glass, about an inch
square. On the back of this little mirror, at the edges and in a
triangular position, place three bits of soft wax each the size of a
pea. Place the mirror on the wrist with one of the wax supports
on- the pulse.
Let this mirror replace the mirror shown in Fig. 199, hold the
arm steady and watch the motions of the sun spot on the wall or
ceiling. They are like the beatings of the pulse which they
make visible to the whole class.
The reflected beam moves through an angle twice as great as
does the mirror. If the pupil trying the experiment laughs, or
becomes excited in any way, the change in the movement of the
pulse becomes evident to all in the room.
431. Law of Reflection. The reflection of light
from a polished surf ace obeys the now familiar law : The
angle of incidence is equal to the angle of reflec-
tion.
Experiment 213. Let a beam of light fall upon a sheet of
drawing paper in a darkened room ; it will be scattered and illu-
minate the room. Let it fall upon a mirror ; nearly all of it will
be reflected in a definite direction and intensely illuminate only a
part of the room.
Experiment 214. Place, side by side upon a board, a piece of
black cloth (not glossy), a piece of drawing paper and a piece of
looking-glass. Allow a beam of sunlight to fall upon the cloth and
notice the absorption. Let it fall upon therpaper, and notice the
diffusion of the light and its effects. Let it fall upon the look-
ing-glass, and notice the regular reflection and its effects. Move
the board so that the cloth, paper and glass shall pass through
the beam in quick succession and notice the effects.
Experiment 215. In the darkened room, place a tumbler of
water upon a table ; with a hand mirr.,r, reflect me sunbeam down
433 SEfLECTlOtf OP LIGffT. $3$
into the water ; the tumbler will be visible. Stir a teaspoonful of
milk into the water and again reflect the sunbeam into the liquid ;
the whole room will be illuminated by the diffused light, the
tumbler of milky water acting like a luminous body.
432. Diffused Light. Light falling upon an opaque
body is generally divided into three parts : the first is
regularly reflected in obedience to the law above given ;
the second is irregularly reflected or diffused ; the third
is absorbed.
The irregular reflection is due to the fact that the
bodies are not perfectly smooth, but present little pro-
tuberances that scatter the light in all directions and
thus render them visible from any position. (Fig. 239. )
Light regularly reflected gives an image of the body
from which it came before reflection ; light irregularly
reflected gives an image of the body that diffuses it. A
perfect mirror would be invisible.
Luminous bodies are visible on account of the
light that they emit; non-luminous bodies are
visible on account of the light that they diffuse.
433- Apparent Direction of Bodies. Every point
of a visible object sends a cone of rays to the eye. The
pupil of the eye is the base of the cone. TJie point al-
ways appears at the place where these rays seem
to intersect (i. e., at the real or apparent apex of the
cone).
If the rays pass in straight lines from the point to the
eye, the apparent position of the point is its real poi-
340 NATUItAL PHILOSOPHY. 433
tion. If these rays be bent by reflection or in any other
manner, the point will appear to be in the direc-
tion of the rays as they enter the eye. No matter
how devious the path of the rays in coming from the
point to the eye, this rule holds good.
Experiment 216. Hold a printed page before a common mirror.
Notice that each printed line is reversed ; each letter is turned side
for side. "Look at yourself " in the mirror. The right hand of
the image is opposite your left hand just as it would be if the image
were another person facing you. This reversing effect is called
lateral inversion.
Experiment 217. Place a jar of water 10 or 15 cm. back of a
pane of glass placed upright on a table in a dark room. Hold a
lighted candle at the same distance in front of the glass. The jar
will be seen by light transmitted through the glass. An imago
of the candle will be formed by light reflected by the glass. The
image of the candle will be seen in the jar, giving the ap-
pearance of a candle burning in water. The same effect may
be produced in the evening by partly raising a window and holding
the jar on the outside and the candle ou the inside.
434. Plane Mirrors ; Virtual Images. If an ob-
ject be placed before a mirror, an image of it appears be-
hind the mirror. This is called a virtual image. All
virtual images are optical illusions and are to be clearly
distinguished from the real images to be studied soon.
Each point of the image will seem to be as far
behind the mirror as the corresponding point
of the object is in front of the mirror, llence, images
seen in still, clear water are inverted.
435
REFLECTION OF LIGHT.
341
(a.) In Fig. 201, rays of
light from A are reflected
at B and B' and enter the
eye fit 7). These r;i ys HUH ply
1 1 />/'(!> 1o converge at A',
\vhidi is, therefore, called
the rirtiiiil image of A.
435- Concave
Mirrors. A spheri-
cal, concave mirror
may be considered as
a small part of a spher-
ical shell with its inner surface highly polished. Let MN
(Fig. 202) represent the section of such a concave, spheri-
cal mirror and C, the centre of the corresponding sphere.
C is called the centre of curvature ; A is the centre ot
FIG. 201.
FIG. 202.
the mirror. A straight line drawn from A through C,
as A CX, is called the principal axis of the mirror. A
straight line drawn from any other point of the mirror
through C, as JCd, is called a secondary axis. The
point, F, midway between A and C, is called the princi-
pal focus. The distance, AF, is the focal distance of the
342 NATURAL PHILOSOPHY. 435
mirror; the focal distance is, therefore, one-half the
radius of curvature. The angle, MCN, is called the
aperture of the mirror.
(.) It should be borne in mind that radii drawn from G to points
in the mirror, as / and J, are perpendicular to the mirror at these
points. Thus, the angles of incidence aud of reflection for any ray
may be easily determined.
436. Effect of Concave Mirrors. The tendency
of a concave mirror is to make incident rays
converge more or diverge less.
437. The Principal Focus. A focus is the point
toward which rays converge. All incident rays parallel
to the principal axis of a concave mirror will, after re-
flection, converge at the principal focus. The princi-
pal focus is the focus of rays parallel to the prin-
cipal axis.
The rays will be practically parallel when their source
is at a very great distance, e.g., the sun's rays. Solar
rays coming to the earth do not diverge a thousandth of
in inch in a thousand miles.
438. Conjugate Foci. Rays diverging from a lumi-
nous point in front of a concave, spherical mirror and at
a distance from the mirror greater than its focal distance,
will converge, after reflection, at another point. Kays
diverging from B will form a focus at b. (Fig. 203.)
Kays diverging from b would form a focus at B. Two
such points a.re called conjugate foci.
439 REFLECTION OF LIGHT. 343
Conjugate foci are two points so related that
each forms the ima&e of the other.
FIG. 203.
Experiment 218. In a dark room, hold a candle between the
eye and the concave side of a bright silver spoon held a little ways
in front of the face. Notice that the inverted image of the
flame is in front of the spoon.
Place the spoon between the flame and your face but so as to
allow the face to be illuminated by the candle. Notice the image
of the observer.
439. Projection of Real Images by Concave
Mirrors. The real image formed by a concave mirror
may be rendered visible by projecting it upon a screen.
In a darkened room, let a candle flame be placed in front
of a concave mirror, at a distance from it greater than
the focal distance and less than the radius of curvature
of the mirror. Incline the mirror so that the flame
shall not be on the principal axis. Place a paper screen
at the conjugate focus of any point in the luminous ob-
ject. The proper position for the screen may easily be
found by trial. Shield the screen from the direct, rays
of the flame by a card painted black. The inverted
image may be seen by a large class, (Fig. 204.)
344
XA Tl'L'A L PHIL OSOPHY.
439
FIG. 204
If the image fall between the mirror and the candle, as
it will if the candle be at a distance from the mirror
greater than the radius of curvature, the screen should
be quite small. The image of any powerfully illuminated
object may thus be produced, as shown in Fig. 205.
In both of these cases, the reflected rays actually con-
verge on the screen ; these images are, therefore, called
real images to distinguish them from virtual images.
Experiment 219. Hold the convex side of a bright silver
spoon toward you and bring the spoon and a candle into the
positions described in Experiment 218. Notice that the erect
image of the flame is back of the spoon.
Place the spoon between the flame and your face but so as to
allow the face to be illuminated by the candle. Notice the image
of the observer,
441
REFLECTION OF LIGHT.
345
FIG. 20.').
440. Convex Mirrors. In convex mirrors, the
images are virtual, erect and smaller than their objects.
441. Recapitulation. To be amplified by the pupil
for review.
DEFINITION. f Plane.
MIRRORS. -I Co
LAW.
<?/ Curvature.
j Principal.
\ Secondary.
REGULAR..
IRREGULAR.
f Definition.
Foci .... -I Virtual.
j Principal.
I- Conjugate.
( Real.
I Virtual.
IMAGES..-! ,,
I Erect -
(. Inverted.
APPARENT DIRECTION OF OBJECTS.
346 NATURAL PHILOSOPHY, 44!
EXERCISES.
1. Given three points, A, B and C, not in a straight line. Show,
by a diagram, how you would place a plane mirror at C so that
light proceeding from A shall be reflected to B.
2. If you hold a sheet of paper with a greased spot on it be-
tween you and the light, the spot will look lighter than the rest
of the sheet. Why is this ?
3. If you hold the sheet in front of you when you are turned
away from the light, the spot will look darker than the rest of the
sheet. Why is this?
4. (a.) Is the image formed by a convex mirror real or virtual ?
(b.) Is it erect or inverted? (c.) Is it larger or smaller than the
object ?
5. What is the difference between real and virtual fofc or
images ?
6. Why are the images seen in a pond of water invert**! *
SECTION III.
REFRACTION OF LIGHT.
Experiment 220. Procure a clear glass bottle with flat sHes.
about 4 inches (10 cm.) hroad. On one side, paste a piece of pawr,
in which a circular hole has been cut. On this clear space, draw
two ink-marks at right angles to each other, as shown in Fig. 206.
FIG. 206.
Fill the bottle with clear water up to the level of the horizonta\
ink-mark. Hold it so that a sunbeam coming through a hole in
the shutter of a darkened room may pass through the clear sides
of the bottle above the water and notice that the beam passes
348 NATURAL PHILOSOPHY. 441
through the bottle in a straight line. Raise the bottle so that the
beam shall pass through the water and notice that the beam is
still straight.
In a card, cut a slit about 5 cm. long and 1 mm. wide. Place
the card against the bottle as shown in Fig. 206. With a mirror,
reflect the beam through this slit at S, so that it shall fall upon
the surface of the water at i, the int3rsection of the two ink marks
Notice that the reflected beam is straight until it reaches the
water but that it is bent as it enters the liquid.
Experiment 221. Put a small coin into a tin cup and place the
cup so that its edge just keeps you from seeing the coin. A ray of
light coming from the coin toward you must pass above the eye
and thus be lost to sight. Pour water into the cup and the coin
will become visible. The rays are bent down as they leave the
water and some of them enter the eye.
FIG. 207.
Experiment 222. Notice that an oar or other stick half im-
mersed in water seems bent at the water's surface, while rivers and
ponds whose bottoms are visible are generally deeper than they
**an to be. (Fig. 207.)
443 REFRACTION OF LlOttT. 349
Experiment 223. Place a bright spoon in a tumbler of water
held at the level of the eye. Bring the bowl of the spoon to the
side of the glass nearest you. Notice the appearance of the spoon.
Move the spoon from you to the opposite side of the water in the
tumbler. Notice the changed appearance of the spoon.
442. Refraction. As a general thing, when a lumi-
nous beam falls upon a substance, some of the rays are
turned back or reflected. Other rays enter the substance,
being rapidly absorbed when the substance is opaque or
freely transmitted when the substance is transparent
\Vo have now to consider those rays that enter a trans-
parent substance. Under some circumstances, such rays
are bent.
Tliis bending of a luminous ray when it passes
from one medium to another is called, refraction
of light.
443. Refraction Explained. Let us consider the
passage of a ray of light through a glass prism, ABC.
The velocity of light is less
in glass than in air and ihc
direction in it-Inch a ini re-
moves is perpendicular to the
wave front.
A wave approaches the side of
the prism, A B. When at a, the lower end of the wave
front first strikes the glass and enters it This end of
the wave moves more slowly th:in does the other, which
is still in the air, and is continually retarded until the
350 NATURAL PHILOSOPHY. 443
whole wave has entered the glass. The wave front thus
assumes the position shown at c.
The path of the wave being perpendicular to
the front of the wave, this change of front causes
a change in the direction of the ray which is
thus refracted toward a perpendicular to the
side, AB.
The wave now moves forward in a straight line until
the top of the wave front strikes AC, the surface of the
prism, as shown at m. The upper end of the wave front
emerging first into the air gains upon the other end of
the front, which is still moving more slowly in the glass.
When the lower end emerges from the glass, the wave
has the position shown at n.
This second change of front involves another
change in the direction of the ray which is now
refracted from the perpendicular.
(a.) Imagine a military company to be marching in " column of
platoons," about 20 men abreast. Imagine them to be obliged to
march in a direction perpendicular to the platoon front. The line
of march lies through a triangular morass (ABC of Fig. 208). At
a, the soldiers on the right of the first platoon enter the morass,
and find that they cannot move as rapidly as they had previously
done. The soldiers on the left of the platoon, maintaining their
previous length and time of step, gain on the right of the platoon
until they too enter the morass. But this gain has changed the
alignment of the platoon to the i>osition represented at c. Conse-
quently, the line of march was bent or refracted at the point where
the platoon passed from hard to soft ground. The velocity being
lessened, the line was refracted toward tlie perpendicular.
444 JtEFRACTIOy OP LtGHT. 351
In similar manner, at m, the left of the platoon first emerges
from the morass and again gains upon the right flank. This
changes the platoon front to the position represented at n and re-
fracts the line of advance from the perpendicular at the point
where the platoon emerges from the morass and is able to increase-
its velocity.
444. Laws of Refraction of Light. (1.) When
light passes perpendicularly froin one medium
to another it is not refracted.
(2.) When light passes obliquely from a rarer
to a denser medium it is refracted toward the
perpendicular, or toward a line drawn, at the point of
incidence, perpendicular to the refracting surface.
(3.) Wlien light passes obliquely from a denser
to a rarer medium, it is refracted from the
perpendicular.
Experiment 224. Place the bottle shown in Fig. 206 upon
several books resting upon a table and invert the card so that a
beam of light reflected obliquely upward from a mirror on the
table may enter through the slit near the bottom of the bottle,
taking a direction through the water similar to the line IA of
Fig. 212. Notice that the sunbeam is turned downward at the
upper surface of the water.
Experiment 225. Look into an aquarium in a direction similar
to that represented by the line IA of Fig. 212, and you may often
see images of the fish or turtles near the surface of the water.
Experiment 226. Place a strip of printed paper in a test tube ;
hold it obliquely in a tumbler of water and look downward at the
printing, which will be plainly visible. Change the tube gradually
toward a vertical position, and soon the part of the tube in the
water takes a silvered appearance and the printing becomes
invisible. The disappearance of the reading is due to "total re
NATURAL PHILOSOPHY.
445
flection." By dissolving a small bit of potassium dichromate in
the water, the tube will have a golden instead of a silver-like ap-
pearance. Fill the test tube with water and notice that the read-
ing is visible, the total reflection at the surface of the air in the
tube being destroyed.
Experiment 227. Place a bright spoon in a tumbler of water
with the handle leaning from you. Hold
the tumbler considerably above the level of
the eye. Notice that you see not only the
lower part of the spoon in the water but
also an image of the shank of the spoon
above the upper surface of the water as
represented in Fig. 209. The free liquid
surface glistens and reflects as does a
mirror.
445. Total Reflection. When
a ray of light attempts to pass from
a denser into a rarer medium, there
are conditions under which the angle
of refraction cannot be greater than
FIG 209. the angle of incidence.
Under such circumstances, the ray cannot pas*
out from, the denser medium but will be wholly
reflected at the point of incidence.
Fig. 210 represents luminous
rays emitted from A, under water,
and seeking a passage into air.
Passing from the perpendicular,
the angle of refraction increases
more rapidly than th'e angle of
incidence until one ray is found
FIG. 210,
446
REFRACTION Of LIGHT.
353
that emerges and grazes the surface of the water. Rays
beyond this eaunot emerge at all.
(a.) Fig. 211 represents a glass vessel partly filled with water.
Mirrors are placed at m and n. In this
way, a ray may be reflected at m, n and
o, and refracted at i.
446. The Critical Angle.
Imagine a spherical flask (Fig, 212)
half filled with water. A ray of
light from L will be refracted at A FIG. ail.
in the direction of R. If the angle
of incidence, CAL, be gradually increased, the angle of
refraction will be gradually increased until it becomes 90,
when the ray will graze the surface of the water, AM,
If the source of light be still further removed from C,
as to I, the ray will be reflected to r.
For cM m-edia, there is an incident angle of
this kind, called the crit-
ical or limiting angle, be-
yond which total internal
reflection will take the
place of refraction.
(a.) The reflection is called
"total" because all of the inci
dent light is reflected, which is
never the case in ordinary reflec-
tion. Hence, a surface at which
total reflection takes place con-
stitutes the most perfect mirror possible.
PIG. 212.
354 NATURAL PHILOSOPHY. 447
447. Three Kinds of Refractors. When a ray
of light passes through a refracting medium, three cases
may arise :
(1.) When the refractor is bounded by planes, the re-
fracting surfaces being parallel. The refractor is then
called a plate.
(2.) When the refractor is bounded by planes, the re-
fracting surfaces being not parallel. The refractor is
then called a prism.
(3.) When the refrac-
tor is bounded by two
surfaces of which at
least one is curved.
The refractor is
then called a lens.
448. Plates.
^^ When a ray passes
through a plate, the
refractions at the two surfaces are equal and contrary in
direction. The direction of the ray after passing through
the plate is parallel to its direction before entering.
Objoctsseen obliquely through such plates appear slightly
displaced from their true position. An object at 8 would
appear to be at S' (Fig. 213).
449. Prisms. A prism produces two simultaneous
effects upon light passing through it ; a change of di-
rection and decomposition. The second of thes. effects
will be considered under the head of dispersion ( 4G3).
450
ItEFRACTTON Of LTGffiT.
355
(a.) Let mno represent the section of a prism. A ray of ligh*.
from L being refracted at a and 6 enters the eye in the direction
FIG. 214
be. The object being seen in the direction of the ray as it enters
the eye ( 433), appears to be at r.
(6.) An object seen through a prism seems to be moved in the
direction of the edge that separates the refracting surfaces.
(c,) The refracted rays are bent toward the side that separates
the refracting surfaces, or toward the thickest part of the prism.
450. Lenses. The curved surfaces of lenses are
generally spherical. With respect to their shape, lenses
are of six kinds.
FIG. 215.
356 NATURAL PHILOSOPHY. 450
(1.) Double-convex, "1
(2.) Plano-convex, ' Thl< * er at the middle tha "
;"' ' at the edges,
(d.) Concavo-convex, or meniscus, J
The double-convex may be taken as the type of these.
(4.) Double concave, |
(5.) Plano-concave, I Thinner at the middle than
(6.) Convex-concave, or diverging | at the edges,
meniscus,
The double -con cave may be taken as the type of these.
(a.) The effect of convex lenses may be considered as produced
by two prisms with their bases in contact ; that of concave lenses,
by two prisms with their edges in contact.
(6.) Tlie effects of lenses may be illustrated with spectacles or
eye-glasses. The common magnifying: glasses used in botanical
study and for other purposes will answer. The larger lenses
which may easily be removed from an opera-glass or a magic lan-
tern mny be made to furnish the apparatus needed for our present
purposes. See description of "The Water Lens" and "The
Fountain of Fire," in the little book of " Light " mentioned at the
top of page 336.
451. Centre of Curvature ; Principal Axis ;
Optical Centre. A double convex lens, ab, (Fig. 216),
may be described as the part common to two spheres whicl;
PIG. 216.
452 REFRACTION OF LIGHT. 357
intersect each other. The centres of these spheres, c and
C, are the centres of curvature of the lens. The straight
line, XY, passing through the centres of curvature is
the principal axis of the lens. In every lens there is
a point, o, on the principal axis called the optical centre.
It is generally at equal distances from the two faces of
the lens. Any straight line, other than the principal
axis, passing through the optical centre is a secondary
axis, as II K.
Experiment 228. Hold one of the large lenses of an opera
glass in the sun's rays. Notice the converging pencil formed
by the rays (after passing through the lens) as they pass through
air made dusty by striking together two blackboard erasers. T1i'>
focus and its distance from the lens maybe seen. Measure ti.Y
distance. Hold a similar lens by the other, face to face. Noli j
that the rays after passing through both lenses converge r..- .
quickly, lessening the distance of the focus from the lens.
452. Principal Focus.-/// incident rays pnr'
li'l to tJte principal axis of a convex lens if/- 1
(tj'trr tiro refractions, converge at a point P-<('!I //
the principal focus.
This point may lie on either side of the lens, ncconl
ing to the direction in which light moves ; it is a rc;;l
focus. Its distance from the lens is called thefwal dis-
tance-
FIG. 217.
358
NATURAL PHILOSOPHY.
452
(a.) The position of the principal focus of a lens is easily deter-
mined. Hold the lens facing the sun. The parallel solar rays in.
cident upon the lens will converge at the principal focus, F (Fig.
217). Find this point by moving a sheet of paper back and forth
behind the lens until the sunny spot formed upon the paper is
as bright and small as you can make it. Owing to the identity
between heat rays and luminous rays, a convex lens is also a
" burning glass."
(6.) Kays diverging from/, a point at twice the principal focal
distance from the lens, will converge at f at twice the focal dis-
tance on the other side of the lens. This may be shown by expert
menting with a lens and candle-flame until the flame and its image
upon a movable screen are at equal distances from the lens.
(c.) In such experiments, it is well to fit neatly the lens into a
pasteboard or other opaque screen, to cut off from the screen all
luminous rays other than those passing through the lens.
(d.) The foci situated at twice the principal focal distance are
called secondary foci. They are conjugate foci.
Experiment 229. Hold a magnifying glass so as to see an ob-
ject distinctly. Now move the object from the lens. The eye
must be placed closer to the lens to secure distinct vision.
453. Conjugate Foci of Convex Lenses. Kays
. 21$.
453
REFRACTION OF LIGHT.
359
diverging from a luminous point in the principal axis at
a small distance beyond the principal focus on either
side of the lens will form a focus on the principal axis
beyond the other principal focus. Thus, rays from L
will converge at I (Fig. 218) ; conversely, rays from I will
converge at L. The luminous point and the focus of
its rays will lie in the same primary or secondary axis.
Two points thus related to each other are called
conjugate foci ; the line joining them always
passes through, the optical centre.
(a.) When the luminous point is at the focal distance, the re-
fracted rays will be parallel (Fig. 219 [;] ) and no focus will be
formed.
FIG. 219.
(6.) When the luminous point, L, is at less than the focal dis-
tance, the refracted rays will still diverge as if from a point, I, on the
same side of the lens, more distant than the principal focus. (Fig.
219 [2 ] ). This focus will be virtual.
(c.) Conversely, converging rays falling upon a convex lens will
form a focus nearer the lens than the principal focus.
360 NATURAL PHILOSOPHY. 45/ 1 .
454. Images Formed by Convex Lenses.
The analogies between the convex lens and the concave
mirror cannot have escaped the notice of the thoughtful
pupil. Others will appear.
If secondary axes be nearly parallel to the principal
axis, well-defined foci may be formed upon them, as well
as upon the principal axis. A number of these foci
may determine the position of an image formed by a
lens.
455. Diminished Real Image.// the Abject.
A B, be more than twice the focal distance from
the convex lens, its image will be real, smaller
than the object and inverted. (Fig. 220.)
FIG. 220.
Kays proceeding from A are focused at a upon the
secondary axis, A0a\ similarly with rays from B and
from all intermediate points. Hence the image, ab.
456. Magnified Real Image.--// the object be
further from the lensfwn the principal focus, but
457
REFRACTION OF LIGHT.
361
at a distance less than twice the focal distance,
the image will be real, magnified and inverted.
(Fig. 221.)
FIG. 221.
457. Virtual Image. If the object, AB, be placed
nearer the lens than the principal focus, the
FIG. 232.
image will be virtual, magnified and erect.
from .1 appear to come from a; rays from B appear to
come from b.
Experiment 230. Repeat Experiment 228, using the eyt*
glasses or small leases of an opera glass. Notice th.at the re-
fracted rays form a diverging pencil.
362 NATURAL PHILOSOPHY. 458
458. Conjugate Foci of Concave Lens. Rays
from a luminous point at any distance whatever will
be made more divergent by passing through a concave
lens. Rays parallel to the principal axis will diverge
after refraction as if they proceeded from the principal
focus. In any case, the focus will be virtual and nearer
the lens than the luminous point. In Fig. 223, the vir-
tual image of L is at I, from which point the refracted
rays appear to proceed.
459. Image formed by Concave Lenses.
Images formed by a concave lens are virtual,
tmaller than the object and erect.
460
RECA P ITU LA TION.
363
460. Recapitulation. To be amplified by tbe pupil
for veview.
REFRACTION OF LIGHT.
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564 NATURAL PHILOSOPHY. 460
EXERCISES.
1. (a.) What is refraction of light? (6.) State the laws govern-
ing the same, (c.) Give an illustrative diagram.
2. (a.) Name and illustrate by diagram the different classes of
lenses. (6.) Explain, with diagram, the action of the burning-
glass.
3. Explain total reflection.
4. Show, with diagram, how the secondary axes of a lens mark
the limits of the image.
5. Using a convex lens, what must be the position of an object
in order that its image shall be real, magnified and inverted ?
6. Show, by a diagram, the course of a ray of light passing
through the centre of a glass sphere.
7. Show, by a diagram, the course of a ray of light passing
through a glass sphere at one side of the centre.
8. Draw the section of a prism and draw lines showing the path
of a ray of light through it.
9. A cathetal prism (one whose section is an isosceles, right-
angled triangle) may be used as a mirror. Draw a diagram show-
ing how this may be done.
10. Draw diagrams showing the direction of rays of light before
and after refraction by a double convex lens, the rays starting from
a luminous point (a.) at the principal focus. (6.) On the principal
axis between the principal and secondary foci, (c.) On the prin-
cipal axis beyond the secondary focus, (d.) On the principal axis,
lietween the lens and the principal focus.
SECTION IV.
CHROMATICS. SPECTRA.
461. Other Results of Refraction. Most lumi-
nous objects emit light of several kinds blended together.
We must learn to sift these varieties one from the other
and to deal with any one kind by itself.
F'TG. 224.
Experiment 231. Admit a sunbeam through a very small
opening in the shutter of a darkened room The opening may be
prepared by cutting a slit an inch (25 mm.) long and % of an
inch (1 mm.} wide in a card. See that the edges of the slit are
smooth. Tack the slit over a larger opening in the shutter. If
we look at the aperture from E (Fig. 224), we shall see the pun
beyond. The path of the beam from S to E is made visible by the
floating dust.
If a prism be placed in the path of the beam , as shown in the
figure, the sides of the slit and edges of the prism Wing horizontal,
the beam will be refracted upward. If the refracted beam be
366 NATURAL PHILOSOPHY. 462
caught upon a screen, it will appear as a band of differently
colored light, passing very gradually from red at the bottom,
through orange, yellow, green, blue and indigo to violet at the
upper end of the beautifully colored band. By placing the slit in
a vertical position and standing the prism on its end so that its
edges will be parallel with the sides of the slit, the spectrum m:iy
be projected as a horizontal band.
The rays thus separated by the prism are identical with each
other and with radiant heat except in the matter of wave length
or (what amounts to the same thing) rapidity of vibration.
462. The Solar Spectrum. The colored land
produced in Experiment 231 is called the solar
-pectruin.
The initials of the names of these different colors f, r -
the meaningless word, VIBGYOR, which may aid the ]
pil in remembering these prismatic colors in their pro , r
order.
463. Dispersion. By looking at Fig. 224, it will l.o
seen that the red rays have been refracted the least and
the violet the most of all the luminous rays. Tfiis sejxt-
ration of the differently colored rays by the prism
is called the dispersion of light ; it depends upon the
fact that rays of different colors arc refracted in different
degrees.
Experiment 232. Let the rays that have been dispersed by a
prism fall upon a convex lens as shown in Fig. 225. They will
be refracted to a focus and recombined to form white light. A
toncave mirror may be used to reflect the rays to a focus instead
of using the lens as above described.
463
CUE OMA TICS SPECTRA .
367
PIG. 225.
Experiment 233. Make a "Newton's disc" of cardboard
painted with the prismatic colors in proper proportion as indicated
by Fig. 226. It is better to divide the surface given to each color
into smaller sectors arranged alternately as shown in Fig. 227. You
FIG. 226.
FIG. 227.
may paste sectors of properly colored paper upon the card-board
instead of painting them. Cause this disc to revolve rapidly by
means of the whirling table or by fastening it to a large top.
Notice that the colors are blended and that the disc appears
grayish white.
Experiment 234. Hold a second prism near one that is used to
produce a solar spectrum, the position of the second being inverted
with reference to the first. If the dispersing prism be held as
shown in Fig. 224 or 225, the second should be held with the
refracting edge uppermost, the facing surfaces being parallel.
36$ NATURAL PfflLdSdP&f. 464
The dispersed rays emerging from the first prism will pass
through the second. The rays separated by the first will be
again blended by the second and appear as white light.
Experiment 235. Hold a hand mirror near the dispersing prism
so as to reflect the refracted rays to a distant wall or ceiling. Give
to the mirror a rapid, angular motion so that the spectrum is made
to move to and fro very quickly in the direction of its length.
The spectrum changes to a band of white light with a colored
spot at each end. The effect is due to what is known as the
"Persistence of Vision," familiarly illustrated by the experi-
ment of producing a ring of light by whirling a firebrand around
a circle.
464. The Composition of White Light. We
have now shown, by both the processes of analysis and
synthesis, that white light is composed of the seven
prismatic colors. We have decomposed white light
into its seven constituents and recombined these constit-
uents into white light.
Experiment 236. Paint three narrow strips of cardboard, one
vermilion red, one emerald green, and the other aniline violet.
Be sure that the coats are thick enough thoroughly to hide the
cardboard. When dry, hold the red strip in the red of the solar
spectrum ; it appears red. Move it slowly through the orange and
yellow ; it grows gradually darker. In the green and colors be-
yond, it appears black. Repeat the experiment with the other
two strips and carefully notice the effects.
Experiment 237. Make a loosely wound ball of candle wick ;
soak it in a strong solution of common salt in water ; squeeze most
of the brine out of the ball ; place the ball in a plate and pour al-
cohol over it. Take it into a dark room and ignite it. Examine
objects of different colors, as strips of ribbon or cloth, by this yellow
light. Only yellow objects will have their usual appearance
4^7 CHROMATICS SPECTRA. $6$
465. Color of Bodies. The color of a body is its
property of reflecting or transmitting to the eye rays of
& particular kind, the other rays being generally absorbed.
(a.) Properly speaking, color is not a property of matter, but of
.iglit. A ribbon is called red, but the redness belongs to the light,
not to the ribbon. There would be more propriety in saying that the
ribbon has ail the other colors of the rainbow, because it keeps the
others and reflects the red. If the red ribbon be placed in the green
or blue of the spectrum, it will appear black because it receives no
red rays to reflect.
466. The Rainbow. The rainbow is due to re-
fraction, reflection and dispersion of sunlight by water-
drops. The necessary conditions are :
(1.) A shower during sunshine.
(2.) That the observer shall stand with his back to
the sun, between the falling drops and the sun. (See
Elements of Natural Philosophy, 706-710.)
467. The Luminous Spectrum. The color of
light depends only upon wave length or rapidity
of vibration. Color is to optics what pitch is to acous-
tics.
"Red light has a wave length o.f ST JoiF nicn 5 violet light
lias a wave length of -g-r&w inch. The wave lengths
that correspond to the other colors are intermediate be-
tween these in value.
The shorter the wave, the more it is refracted (See Fig.
224). Of course, the wave length depends upon the
rapidity of vibration of the molecule that produced the
wave in the ether.
370 NATURAL PHILOSOPHY. 468
468. Other Properties of the Sunbeam. We
have decomposed a sunbeam, and thus produced the
seven prismatic colors. But we must go still further.
Beyond the limits of the visible spectrum, in, both
directions, there are rays that do not excite the op-
tic nerve, the existence of which, however, may be easily
proved. These visible and invisible rays differ only in
respect to wave length. Some of these waves are so short
and fall so rapidly that the optic nerve cannot respond
to their vibrations. OtLers are so long and fall so slowly
that the optic nerve cannot respond to their vibrations
either. In either case, the rays are incapable of exciting
vision.
469. Actinic Spectrum. The ac'inic or chemi-
cal effects of sunlight are familiar to all. The sensitive
paper of the photographer will remain unchanged in the
dark ; it will be quickly blackened in the light.
If a piece of paper, freshly washed in a solution of sul-
phate of quinine, be held successively in the different
parts of the visible spectrum, it will be affected least in
the red and most in the violet.
In this way actinic effects may be found at a
point beyond tJie violet^ wholly outside the visible
spectrum. We thus detect ultra-violet rays con-
stituting an actinic spectrum.
A quartz prism is desirable for this experiment, as
glass quenches most of the actinic rays.
470. Thermal Spectrum. If a very delicate ther-
471
RECA PITVLA TIOX.
371
mometer or thermopile be successively placed in various
parts of the spectrum, it will be found that the tempera-
ture is scarcely affected in the violet, but that there is a
continual increase in temperature as the thermometer
is moved toward the other end of the spectrum, it being
quite marked in the red.
A very marked rise of temperature takes place
beyond the red, wholly outside the visible spec-
trum. We thus detect ultra-red rays constitut-
ing a heat spectrum.
A rock-salt prism is desirable for this experiment, as
glass absorbs most of the ultra-red rays.
Radiant heat differs from light only in wave length.
All luminous rays have heating power ; heat rays are
luminous if their vibrations come at such a rate that the
organ of sight can absorb their energy and respond with
sympathetic vibrations ( 338).
471. Recapitulation. To be amplified by the pupil
for review.
C By Prisms.
(_ By Water Drops.
f Luminous.
Actinic.
COMPLEXITY OF
SUNBEAM....
COLOR OF BODIES.
ANALYSIS.
Spectra
Thermal.
By Lenses.
By Mirrors.
By Prisms.
By Persistence of Vision.
NATURAL PHILOSOPHY. 471
EXERCISES.
1. What is the difference between the " diffusion of light " and
" the dispersion of light " ?
2. Why does a red body appear red ?
3. Name the colors of the solar spectrum in their proper order,
beginning with the one of least wave length.
4. To what quality of sound does color correspond ?
5. Why does the moon look green when viewed through a
piece of green glass ?
6. You hold in your hand a red rose. You carry it into a room
that is wholly dark. Has the rose then any color? Explain your
answer.
7. Imagine a line 186,000 miles long extending from your eye
toward the sun. Radiant energy (heat or light) will traverse such si
line in a second ( 427). Suppose that light from the sun is com-
ing toward you. (a.) After the first wave reaches the far end of
this line, how long will it take the same wave to reach you V (b.)
If the light is red, how many such waves can lie on this line r.t
once ? (c.) How many such waves will enter your eye in a second ?
8. You see a red ribbon, (a.) Does the ribbon reflect any light ?
(It.) How do you know ? (c.) If it does, what is the color of the re
fleeted light ? (d.) Some of the sun's rays enter the ribbon ; what
is their effect ?
9. " Converging " and " diverging " lenses are sometimes men-
tioned. What are they?
10. You see a rainbow. State the threefold work done upon a
sunbeam by each raindrop that contributes to the success of the
exhibition.
SECTION V.
A FEW OPTICAL INSTRUMENTS.
472. Photographer's Camera. The photogra-
pher's camera is nearly the same as the camera-obscura
described in 426. Instead of the darkened room we
have a darkened box ; instead of the simple hole in the
shutter we have a convex lens, placed in a tube at A.
(Fig. 228.)
(a.) A ground-glass plate is placed in the frame at E, which is
adjusted so that a well-
defined, inverted image of
the object in front of A is
projected upon the glass
plate. This adjustment
or "focusing" is com-
pleted by moving the lens
and it 3 tube by the toothed
wheel at D until the ob-
ject in front of A and the
plate at E are at the con-
nigate foci of the lens at
A. When the " focus-
ing" is satisfactory, A is covered with a cloth, the ground-glass
plate is replaced by a chemically-prepared sensitive plate, the cloth
removed and the image projected on the plate. The light works
certain chemical changes where it falls upon this plate and thus a
more lasting image is produced. The many other processes in-
volved in photography cannot be considered here.
473- The Human Eye. This most admirable of
all optical instruments is a nearly spherical ball, capable
FIG. 228.
374
NATURAL PHILOSOPHY.
473
of being turned considerably in its socket. The outer
coat, S, is called the white of the eye. Its transparent
part in front, C, is called the cornea. The cornea is
more convex than the rest of the eyeball. The cornea
fits into the coat, S, as a watch crystal does into its case.
Behind the coat, S, is a dark, opaque coat, N. Behind
the cornea -is a curtain, /, called the iris. It is colored
and opaque ; the circular window in its centre is called
the pupil. The color of
the iris constitutes the color
of the eye. Back of the
pupil is the crystalline
lens, L, built of concentric
shells (layer on layer, as in
an onion) of varying den-
sity. Its shape is shown in
the figure. This lens di-
vides the eye into two
chambers. The anterior
chamber contains a limpid liquid called the aqueous
humor ; the posterior chamber contains a transparent
jelly, F. called the vitreous humor.
The cornea, aqueous humor, crystalline lens and
vitreous humor are refracting media. Back of the
membrane, H, which incloses the vitreous humor, is the
retina, R, an expansion of the optic nerve.
The eye, optically considered, is simply art ar-
rangement for projecting inverted, real images of
visible objects upon a screen, li, made of nerve
filaments. If the images thus formed be well defined
and sufficiently luminous, the vision is distinct.
FIG.
475 OPTICAL INSTRUMENTS. 375
(a.) Objects are not seen distinctly unless the image falls di-
rectly upon the retina. If the image of a distant object falls in
front of the retina, the person is said to be near-sighted. Near-
sightedness may be relieved by the use of concave glasses.
If the image of a near object tends to fall back of the retina,
the person is said to be far-sighted. Far-sightedness may be re-
lieved by the use of convex glasses.
(b.) The eye of the common house-fly is said to have 4,000
lenses ; that of some kinds of beetles is said to have 25,000, each
with its own cornea and retina. " If each of these lenses forms a
separate picture of each object, what an awful army of cruel giants
must the beetle behold when he is captured by a schoolboy I"
474. Magnifying Glasses. A magnifying glass, or
simple microscope, is a con-
vex lens, generally double-
convex. The object is
placed between the lens and
its principal focus. The
image is virtual, erect and
magnified.
475. Compound Mi-
croscope. The compound
microscope consists of two
or more convex lenses placed
iu a tube. One of these, o,
called the object-glass or
objective, is of short focus.
The object, ab, being placed
slightly beyond the principal
focus, a real image, cd, mag-
nified and inverted, is formed
within the tube ( 456), Fio. 380.
376
NATURAL PHILOSOPHY.
475
The other lens, E, called the eye-glass, is so placed
that the image formed by the objective lies between the
eye-glass and its focus. AB (Fig. 230) is a magnified,
virtual image of the real image, formed by the eye-glass
and seen by the observer.
476. Galilean Telescope ; Opera Glass. In
the telescope attributed to Galileo, the objective, 0, is
a double convex and the eye-piece, C } is a double con-
cave lens. The concave lens intercepts the rays before
they have reached "the focuc of the objective.
Fio. 231.
The rays from A, converging after refraction by 0,
are rendered diverging by C '; they seem to diverge from
a. In like manner, the image of B is formed at b.
The image, ab, is erect and very near. An opera glass
consists of two Galilean telescopes placed side by side.
477. Astronomical Telescope ; Refractor. As-
tronomical telescopes are of two kinds refractors and
reflectors. Fig. 232 represents the arrangement of the
lenses and the direction of the rays in the refracting
telescope. The object glass is of a large diameter that
it may collect many rays for the better illumination qi
the image.
478
OPTICAL INSTRUMENTS.
377
FIG. 232.
The inverted, real image, ab, formed by the objective,
0, is magnified by the eye-piece, as in the case of the
compound microscope. The visible image, cd, is a vir-
tual image of ab, which is the real image of AB. The
lenses are enclosed in a tube.
(a.) The largest refracting telescope (1884) is at the Pulkowa
Observatory. Its object glass is 30 inches in diameter. A 36
inch object glass is making for the telescope of the Lick Obser-
vatory in California
478. Reflecting Telescopes. A reflecting tele-
scope consists of a tube closed at one end by a concav.e
FIG. 233.
mirror, M, so placed that the image formed by it maj
be magnified by a convex lens used as an eye-piece.
The rays from the mirror are reflected at mn, andareaJJ
78 NATURAL PHILOSOPHY. 478
image formed at db. This image is magnified by the
glasses of the eye-piece and a virtual image formed &ted.
The Earl of Rosse built a telescope with a mirror six
feet in diameter and having a focal distance of fifty-four
feet.
Experiment 238. Reflect a horizontal beam of sunlight into
a darkened room. In its path, place a piece of smoked glass
on which you have traced the representation of an arrow, AS,
(Fig. 234) or written your autograph. Be sure that every stroke of
Fro. 234.
the pencil has cut through the lamp black and exposed the glass
beneath. Place a convex lens beyond the pane of glass, as at L,
so that rays that pass through the transparent tracings may be
refracted by it as shown in the figure. It is evident that an image
will be formed at the foci of the lens. If a screen, SS, be held at
the positions of these foci, a and 6, the image will appear clearly
cut and bright. If the screen be held nearer the lens or further
from it, as at ff or 8", the picture will be blurred.
479. Magic Lantern. In the magic lantern (Fig.
235), a lamp is placed at the common focus of a convex
lens (called the "condenser") in front of it and of a con-
cave mirror behind it. The light is thus concentrated
upon ab, a transparent picture, called the "slide." A
of lenses, m, is placed at a little more than its
479
OPTICAL INSTRUMENTS.
379
FIG. 235.
focal distance beyond the slide. A real, inverted, mag-
nified image of the picture is thus projected upon the
screen, 8. The tube carrying in is adjustable, so that
the foci may be made to fall upon the screen and thus
render the image distinct. By inverting the slide, the
image is produced right side up.
(a.) Directions for making a simple magic lantern may be found
on page 84 of Mayer and Barnard's little book on Light. Fig. 23B
FIG. 336.
NATURAL PBILOSOPHT.
/" 1
if
represents a very compact and efficient lantern, known as Marcy's
Sciopticon and furnished by James W. Queen & Co., of Philadelphia.
Experiment 239. Close the left eye and hold the right hand so
that the forefinger shall hide the other three fingers. Without
changing the position of the hand, open the left and close the right
eye. The hidden fingers become
visible in part.
Experiment 240. Place a die on
the table directly in front of you.
Looking at it with only the left eye,
three faces are visible, as shown at
A, Fig. 337. Looking at it with
only the right eye, it appears us shown at B.
480. Stereoscopic Effects. Jrom the last two ex-
periments, we see that when we look aS a solid, the
images upon the retinas of the two eyeo are dif-
ferent. If, in any way, we combine two drawings, so a?
to produce images upon the retinas of the two eyes like
those produced by the solid object, we
obtain the idea of solidity.
481. The Stereoscope. To
blend these two pictures is the office
of the stereoscope. Its action will be
readily understood from Fig. 238.
The diaphragm, D, prevents either
eye from seeing both pictures at the
same time. Kays of light from B are
refracted by the half-lens, E', so that
they seem to come from C. In
the same way, rays from A are re-
Fio. m fracted by E so that they also seem
o A.o2 JlECA FiTULA TlOffl* 381
to come from C. The two slightly different pictures,
thus seeming to be in the same place at the same time,
are successfully blended and the picture "stands out"
or has the appearance of solidity. If the two pictures
of a stereoscopic view were exactly alike, this impression
of solidity would not be produced.
482. Recapitulation. To be amplified by the pupil
for review.
OBSCURA.
CAMERA...
. HUMAN EYE.
( OBSCURA.
PHOTOGRAPHER'S.
I
SIMPLE.
MICROSCOPES.
COMPOUND.
{ REFLECTORS.
REFRACTORS .
MAGIC LANTERN.
STEREOSCOPE.
Galilean.
Opera Glats.
Astronomical.
CONCLUSION.
ENERGY.
483. Varieties of Energy. Like matter, energy
is indestructible. We have already seen that energy may
be visible or invisible (i. e., mechanical or molecular),
kinetic or potential. We have at our control at least
eight varieties of energy.
(a.) Mechanical energy of position (visible, potential).
(6.) Mechanical energy of motion (visible, kinetic).
(e.) Latent heat (molecular, potential).
(d.) Sensible heat (molecular, kinetic).
(e. ) Chemical separation (molecular or atomic ; potential).
(/.) Electric charges (probably molecular, potential).
(g. ) Electric currents (probably molecular, kinetic).
(h.) Radiant energy, thermal, luminous or actinic (molecuiar,
kinetic).
484. Conservation of Energy. The doctrine that,
considering the universe as a whole, the sum of all the
forms of energy is a constant quantity, is known as the
Conservation of Energy.
a+b+c+d+e +f+g + h=& constant quantity.
This does not mean that the value of a is invariable ;
we have seen it changed to other varieties as b or d. We
have seen heat changed to electricity and vice versa, and
either or both changed to mechanical energy. It does
not mean that the sum of these eight variable quantities
in the earth is constant, for we have seen that energy
486
ENERGY.
383
may pass from sun to earth, from star to star. But it
does mean that the sum of all these energies in
all the worlds that constitute Uie universe is a
quantity fixed, invariable.
485. Correlation of Energy. The expression Cor-
relation of Energy refers to the convertibility of one
form of energy into another. Our ideas ought, by this
time, to be clear in regard to this convertibility. One
important feature remains to be noticed. Radiant
energy can be converted into other forms, or other
forms into radiant energy only through the in-
termediate state of absorbed heat.
486. Recapitulation. To be amplified by the pupil
for review.
VISIBLE OR MECHANICAL.
OfPotition
Potential.
Of Motion
Kinetic.
e.g., Hanging Apple,
Head of Water.
e.g., Falling Apple,
Flowing Water.
INVISIBLE OR
MOLECULAR.
{Of Position, e. g.. Latent Heat.
Potential.
Of Motion, e.g.. Sensible Heat.
Kinetic:
Of Motion,
ELECTRICITY .
Of Position, e. g., Charged Leyden
Potential. jar, Battery with cir-
cuit broken.
Of Motion, e. g., Leyden jar dis-
Kinttic. charging ; Battery with
circuit closed.
GENERAL
1. (a.) Define science, matter, mass, molecule and atom. (6.)
Define physics.
2. (a.) What are physical properties of matter? (6.) Define
and illustrate two properties of matter.
3. Define solid, liquid and gas ?
4. (a.) Give Newton's laws of motion. (6.) Give the law of re-
flected motion.
5. How may we measure distances by sound ?
6. Upon what does the pitch of a sound depend ?
7. Why do the rays of the evening sun come to us in curved
lines ?
8. How does distance effect the intensity of light ?
9. What is an angle of reflection ?
10. (a.) What is the general effect of a concave mirror ? (5.) Of
a concave lens ?
11. Fig. 339 represents reflection of luminous rays from a
rough surface. Draw a
similar figure illustrat-
ing reflection by a plane
mirror.
12. Is light always re-
fracted in passing from
one medium to another ?
13. How does a straight
stick appear when partly
immersed in water ?
Why?
14. Under what cir-
cumstances will the object
and the image be on the same side of a convex lens ?
15. Why will an iron gate that opens easily in winter often
ttick in summer ?
486 GENERAL REVIEW. 385
16. What is meant by intermolecular spaces and how do they
compare with molecular diameters?
17. How are centigrade and Fahrenheit thermometers graded ?
18. Why are inland cities subject to greater extremes of tem-
perature than seaside resorts ?
19. Can rays of light cross one another without interfering with
the effect of the rays ?
20. The two sides of the greased paper photometer ( 438,6.) are
equally illuminated by a candle flame on one side, distant 18 inches,
and by a lamp flame on the other side, distant 6 feet. How does
the illuminating power of the lamp compare with that of the
candle ?
21. Are all the luminous rays of the same color?
22. Which is the more bulky, a pound of ice or a pound of water ?
23. How can you make water boil at a lower temperature than
212 F. ?
24. What is an anode? A node?
25. Does the vaporization of water change the size of the mole-
cules or the distances between them ?
26. How do you change centigrade into Fahrenheit readings ?
27. What piece or pieces of physical apparatus have you made
since you began studying this book ?
28. What is meant by ineitia ?
29. How far will a freely falling body move in 10 seconds ?
30. Which is the more general term, fluid or liquid?
31. Give a general law of machines.
32. How can energy be destroyed.
33. Define force and energy.
34. State Archimedes' principle.
35. Define specific gravity.
36. Explain gaseous tension.
37. Describe and explain the experiment with the Magdeburg
hemispheres.
38. What is a dynamo ?
39. Explain the action of intermittent springs.
40. Give Ohm's law.
386
NATURAL PHILOSOPHY.
486
41. What is the difference between noise and music ?
42. Why does not Hudson Bay freeze solid to the bottom?
43. What is an ampere ?
44. Explain interference of sound.
45. What is meant by E. M. F. ?
46. What is meant by the " mechanical equivalent of heat " ?
47. How should the cells of a voltaic battery be joined to give
the best effect ?
48. On what optical fact does the success of a sharpshooter
(Fig. 340) depend ?
FIG. 240.
49. Do you think that you know all about Natural Philosophy*
APPENDIX A.
Mathematical Formulas.
7r=3.14159.
D=diameter.
R radius.
Circumference of circle=7r D.
Area of a circle=7r R*.
Surface of a sphere=4 IT R*=7r D*.
Volume of a sphere f n W=$ n D.
APPENDIX B.
International or Metric Measures. This sys-
tem bids fair to come into general use in this country.
For this reason, as well as for its greater convenience, an
acquaintance with it is now desirable and may soon be
necessary. It has been already legalized by act of Con-
gress. The meter is defined as the forty-millionth
of the earth's meridian which passes through Pans.
It is equal to 39.37 inches. Like the Arabic system
of notation and the table of U. S. Money, its divisions and
multiples vary in a tenfold ratio.
8 NATURAL PHILOSOP&Y.
Metric Measures of Length. Ratio = 10.
(Millimeter (mm.)= .001 m.= 0.03937 in.
T " \Centimeter (cm.) = Loi w.= 0.3937 "
[Decimeter (dm.) = .1 w.= 3.937
UNIT. Meter (m.) 1. m.= 39.37 "
fDekameter (Dm.)= 10. m. =393.7
g MTOTI-! Hectometer (JZ TO.) = 100. m. =328 ft. 1 in.
1 PLES. I Kilometer (Km.) = 1000. i.= 0.62137 mi.
lMyriameter(J/>.)= 10000. m.= 65137 "
3 -ZVote. The table may be read : 10 millimeters make
g one centimeter ; 10 centimeters make one decimeter, etc.
| (Fig. 241.) The denominations most used in practice are
v printed in italics. The system of nomenclature is very
M simple. The Latin prefixes, mitti-, centi- and deci-, sig-
2 nifying respectively ^Vs* -j-^, and -j^, and already fa-
miliar in the mill, cent and dime of U. S. Money, are
s used for the divisions, while the Greek prefixes deka-,
" hecko-, kilo- and myria-, signifying respectively 10, 100,
^ 1000 and 10000, are used for the multiples of the unit.
Each name is accented on the first syllable. It may be
| noticed that the meter corresponds somewhat closely to
S the yard, which it will replace. Kilometers will be used
o instead of miles.
- 1 inch= 25.4000 TOOT. 0.0254 m. or about 2| cm.
lfoot= 30.4800 cm. =0.3048 m. " 30 "
1 yard= 0.9144m. " {? of a meter.
1 mile=1609.0000 m. =1.6090 Km. " 1 T % Km.
Metric Measures of Surface. Ratio =io 2
FIG. 241. =100.
f Square millimeter (sq. 7i.)=0.000001 sq. m.
DIVISIONS. -| Square centimeter (q. cm.) =0.0001 "
[ Square decimeter (sq. dm.) =0.01
UNIT. Square meter (sq. m.) =1. "
etc. , etc.
APPENDIX. 389
Note. The table may be read: 100 sq. mm.=l sq. em. ; 100
iq. cm. 1 sq. dm. , etc. The reason for the change of ratio from
10 to 100 may be clearly shown by representing 1 sq. dm., and
dividing it into sq. cm. by lines, which shall divide each side of
the sq. dm. into 10 equal parts or centimeters.
Metric Measures of Volume. Ratio =io 3 =
1000.
f Cubic millimeter (cu. mm.)=0.000000001 cu. m,
DIVISIONS. \ Cubic centimeter (cu. cm.) =0.000001 "
[Cubic decimeter (cu. dm.) =0.001 "
UNIT. Cubic meter (cu. m.) =1.308 cu. yds.
etc. etc.
Metric Measures of Capacity. Ratio =10.
For many purposes, such as the measurement of articles
usually sold by dry and liquid measures, a smaller unit
than the cubic meter is desirable. For such purposes
the cubic decimeter has been selected as the standard,
and when thus used is called a liter (pronouncecj
leeter).
fMilliliter(mZ.) = 1 cu.cm. =0.061022 cu. in.
DIVISIONS. S Centiliter (cl.) = 10 " =0.338 fld. oz.
1 Deciliter (<B.) =100 " =0.845 gill.
UNIT. Liter (I.) =1000 " =1.0567 liquid qt.
f Dekaliter (Dl.) = 10 CM, dm. =9.08 dry qt.
MULTIPLES. ] Hektoliter (#.)= 100 cu. dm. =2 bu. 3.35 pk.
[Kiloliter (El.) = 1 cu. m. =264.17 gal.
Fortunately, there is but one liter. This is intermediate, in
value, between the dry and the liquid quarts which it will replace.
The dekaliter does not differ very much from the peck and might
be substituted for it without much confusion.
390 NATURAL PfffLOSOPHT.
1 U. 8. liquid quart = 0.946 J. or about 1 liter.
1 U. S. dry " = 1.101 1. " 1 "
1U. S. gallon = 3.785 1. " 8 r 8 "
1 U. 8. bushel =35.240 1. " T * T of a hektoliter.
Metric Measures of Weight. Ratio = 10.
(" Milligram (mg.) = 0.0154 grains avoirdupois.
DIVISIONS. \ Centigram (eg.) = 0.1543 " "
{ Decigram (dg,) = 1.5432
UNITS. Gram (g.) = 15:483 "
f Dekagram (Dg.) = 0.3527 oz. "
I Hektogram (Eg.) 3.5274 "
MULTIPLES, j Kilogram (Kg.) = 2.2046 Ibs.
(.Myriagram (Mg.) = 22.046 "
A gram is the weight of one cubic centimeter
of pure water, at its temperature of greatest density
(4 C. or 39.2 F.). A 5-cent nickel coin weighs five
grams. Fortunately, there is but one gram or one kilo-
gram.
1 avoirdupois ounce = 28.35 g. or a little less than 30 grams.
1 Troy or apothecaries ounce=31.10 g. or a little more than 30 grams.
1 avoirdupois pound 453. 59 g. or about T S T of a kilogram.
The best way for the pupil to become familiar with these
weights and measures is to use them. He will quickly discover
that very many computations with these units consist only of in-
telligent shiftings of the decimal point.
APPENDIX. 391
EXERCISES.
1. How much water, by weight, will a liter flask contain ?
2. If sulphuric acid is 1.8 times as heavy as water, what weight
of the acid will a liter flask contain ?
3. If alcohol is 0.8 times as heavy as water, how much will 1360
CM. cm. of alcohol weigh ?
4. What part of a liter of water is 250 g. of water?
5. What is the weight of a cu. dm. of water?
6. What is the weight of a cU. of water?
NUMBERS REFER TO PARAGRAPHS, UNLESS OTHERWISE INDICATED^
A.
Absolute temperature, 366.
" zero, 366.
Absorption of heat, 403, 404.
Acid for batteries, 248*.
Acoustic tubes, 331.
Actinic spectrum, 469.
Adhesion, 29, 30.
Aeriform bodies, 40, 41, 180.
Air, 180, 181.
" chamber, 193.
" pump, 187.
Amalgamating battery zincs, 354.
Ampere, 351.
Amplitude of wave, 322, 330.
Analysis of solar light, 464.
Angle, Critical, 446.
" of incidence, 57.
" of reflection, 57.
Anion, 272.
Anode, 272.
Aperture of mirror, 435.
Apparent direction of bodies, 433.
Appendix, p. 387.
Aqueous humor, 473.
Archimedes' principle, 162.
Armatures, 281, 310.
Artesian well, Ex. 14, p. 139.
Artificial magnets, 205, 281.
Astronomical telescopes, 477, 478.
Athermanous, 400.
Atmospheric electricity, 242.
Atmospheric pressure, 183, 185.
Atom defined, 3.
Atomic attraction, 7.
- motion, 8.
Attraction, Electric, 198, 335.
" Forms of, 7.
" Magnetic, 284, 86.
Axis of lens, 451.
44 mirror, 435.
B.
Balance, 113-114.
Barometer, 183.
Base, 72.
Batteries, Brush, 274^.
44 Bunsen, Fig. no.
44 Electric, 264-367.
" Faure, 2740.
44 Grove, Fig. 109.
44 Secondary, 374.
44 Storage, 274.
Battery, see Cell or Element
" zincs, 254.
Beam of light, 424.
Beats, 345.
Bellows, Hydrostatic, 149.
Blake transmitter, 337.
Blue vitriol, 260.
Boiling point, 360, 372.
Breast wheel, 176.
Brittleness, 29, 33.
Broken magnets, 287.
Brush battery, 274^; Ex. 33, p. 144
41 lamp, 313.
Bunsen cell or battery, 363.
Buoyancy, 162, 163.
Burning glass, 4520.
c.
Callaud cell or battery, 361.
394
INDEX.
Calorific powers, 410.
Camera obscura, 426.
" Photographer's, 472.
Candle, Standard, Ex. 10, p. 335.
Capstan, 121.
Carbon dioxide snow, 383^.
Carbonic acid snow, 383^.
Cathetal prism, Ex. 9, p. 364.
Cathion, 272.
Cathode, 272.
Cause of sound, 319.
Cells, Galvanic or Voltaic, 456.
Centigrade thermometer, 360.
Centimeter, App. B.
Centre of buoyancy, 163^.
" curvature, 435, 451.
* gravity, 65, 66.
M oscillation, Ezp. 38.
Centrifugal force, 52.
Changes of physical condition, 42.
Physical and chemical, n.
Characteristic properties of matter,
15, 28, 29.
Charging by contact, 221.
" induction, 224.
Chemical action develops electricity,
246.
Chemical action produces heat, 410.
" changes, n.
" effects of the electric cur-
rent, 271, 274.
Chemistry, 10.
Chromatics, 461.
Circuit, Electric, 249.
Clocks, 90.
Clouds, Electrified, 242.
Coercive force, 282.
Cohesion, 29, 30.
Coincident waves, 340.
Colors, 465, 467.
Combustion produces heat, 410.
Communicating vessels, 147, 161.
Commutator, 311.
Compass, 204.
Composition of solar light, 464.
Compound lever, 116.
" machines, 140.
* masses, 5.
molecules, 33, 4*.
Compound tones, 353,
Compressibility, 17, 25.
Concave lens, 450, 458, 459.
" mirror, 435-439.
Concavo-convex lens, 450.
Condensation of gases, 383.
Condenser, Electric, 235, 236, 237.
" for gas, 188.
lens, 479.
" of still, 374.
Conditions of matter, 37.
Conduction of electricity, 216, 220, 250.
heat, 392.
Conductive discharge, 238, 241.
Conductivity, Electric, 216, 220, 250.
" Thermal, of gases, 394.
liquids, 394.
solids, 392-
Conductors of electricity, 216, 220, 250.
" heat, 393.
Conjugate foci, 438, 453.
Conservation of energy, 484.
Constant force, 77.
Constitution of matter, 9.
Continuity of matter, 6.
Continuous sounds, 327.
Convection of heat, 395.
Convective discharge, 238, 240.
Convertibility of energy, 100, 268.
Convex-concave lens, 450.
Convex lenses, 450-457.
" mirrors, 440.
Copper plating, Exp. 112.
Copper sulphate cell or battery, 261.
Cornea, 473.
Correlation of energy, 485.
Coulomb, 253.
Critical angle, 446.
Crystalline lens, 473.
Current electricity, 200, 201, 206, 247,
33-
Curvature, Centre of, 435, 451.
Curves, Magnetic, 290.
Daniell's cell or battery, 260.
Day dream, 355.
Decimeter, App. B.
TNDEX.
395
Declination, Magnetic, 297.
Electric density, 231.
Dekameter, App. B.
" discharges, 238-241.
Density, Electric, 231.
" energy, 209.
Diamagnetic substances, 288.
" experiments, pp. 174-176.
Diathermancy, 400.
" fluids, 244-
Diffusion ot heat, 392-404.
" force, 209.
light, 432.
" induction, 222, 304.
Dip, Magnetic, 296.
" insulators, 216.
Dipper, The Great, Exp. 136.
" lamps, 312, 313.
Dipping needle, 295 a. and d.
" light, 312, 313.
Direction, Line of, 71.
machines, 232-234, 310, 311.
" of bodies, Apparent, 433.
" manifestations, 210.
Discharger, Electric, 237.
" motor, 31 1.
Discharges, Electric, 238-241.
" pendulum, 199.
Dispersion of light, 463.
" polarization, 222.
Disruptive discharge, 238, 239.
" pojes, 249.
Distillation, 374.
" potential, 218.
Distribution of Electricity, 230, 231.
" repulsion, 199.
Diverging meniscus, 450.
" resistance, 220, 268.
Divisibility, 17, 23.
" spark, 242.
Divisions of matter, 2.
" telegraph, 276, 298.
Double concave lens, 450.
" tension, 217.
" convex lens, 450.
" whirl, Exp. 103.
" weighing, 114.
Electricity, 196-315.
Downward liquid pressure, 152.
" Atmospheric, 243.
Dropping bottle, Fig. 192.
" Distribution of, 230, 231.
Ductility, 29, 35.
" Dynamic, 247.
Duration of electric spark, 242.
Frictional, 199, 209-244.
Dynamo-electric machines, 310, 311.
" Galvanic, 201, 246-273.
Dynamos, 311.
" Induced, 303-315.
Dynamics, 48.
" Relation of, to energy, 229,
244.
E.
Static, 199, 209-244.
" Theory of, 226.
Earl of Rosse, 478.
" Thermo-, 206, 247, 278.
Ebullition, 37 t, 372.
" Voltaic, 201, 246-273.
Edison, Ex. 22, p. 244.
Electrodes 249.
Effect of concave mirrors, 436.
Electrolysis, 271.
Elasticity, 17, 27, 55.
Electrolytes, 271.
Electric attraction, 198, 225.
Electro-magnets, 275^, 298.
bells, Exp. 95, Fig. 105.
Electro-motive force, 219.
" charges, 209-244.
Electrophorus, 227-229.
circuit, 249.
Electro plating, Exit. 112.
" condenser, 233-237.
Electroscopes, 215, Ex. 3, p. 179.
" conduction, 221.
Electrostatics, Law 01, 214.
" conductors, 216.
Elementary masses, 5.
" current, 200, 201, 206, 303.
" molecules, 30., +t.
" " Effects of, 268-271.
Elements, 5.
" Extra, 305.
E. M. F., 219.
INDEX.
Energy, U., 93-103, 144. 229-244, 301,
Fundamental tone, 347, 348.
407, 418, 483-485-
Fusion, Latent heat of, 378.
Engines, Steam, 414-418.
" Laws of, 368.
Equilibrium, 67-70, 160, 161.
Escapement of clocks, 90.
G.
Ether, Luminiferous, 396.
Evaporation, 369, 370.
Exchange, Telephone, 3373.
Galilean telescope, 476.
Galvanic battery, 264-267.
" cell 248 256.
Expansibility, 17, 26.
Expansion, 8c, 361-365.
Extension, 17, 18.
Extra current, 305.
" electricity, 201, 246-273.
" element, 248, 256.
Galvanometer, 277.
Eye, 473.
" glass, 475.
Galvanoscope, 277.
Gases, 41.
F.
" Condensation of, 383.
Fahrenheit's thermometer, 360.
Falling bodies, 75-82.
" Expansion of, 365.
" Thermal conductivity of, 394.
False balance, 113.
44 Type of, 1 80.
Farsightedness, 473*.
Field, Magnetic, 290.
Floating bodies, 163.
Flow of liquids, 171.
" Volume of, Exp. 7, p. 137.
Geissler tubes, ooo.
Graduation of thermometers, 360.
Gram, App. B.
" rivers, 172.
Fluids, 44-
Thermal conductivity of, 394.
Fly wheel, 415^.
Focal distance, 435, 452.
Foci, Conjugate, 438, 4524?., 453.
" Secondary, t,ytd.
Focus defined, 437.
" of lens, 452-459.
" of mirror, 435-440.
Gravitation, 59.
" Laws of, 60.
Gravity, 20, 61, 77.
" cell or battery, 261.
" Centre of, 65.
" Increment of, 81.
" Specific, 165-169.
Great Dipper, Exp. 136.
Grove cell or battery, 263.
Foot-pound, 95.
u
Force, 47.
n.
" Centrifugal, 52.
Hand glass, Exp. 6r.
" Constant, 77.
Hardness, 29, 31.
" pump, 191-193-
Harmonics, 347.
Forms of attraction, 7.
Head of liquids, 171.
" motion, 8.
Heat, Absorption of, 403, 404.
Formulas, Mathematical, App. A.
" Conduction of, 392.
Fountain of fire, 450^.
" Convection of, 395.
" in vacuo, Exp. 65.
" defined, 357.
Freely falling bodies, 78.
" Diffusion of, 392-404.
Freezing mixtures, 380.
" from chemical action, 410.
Friction, 142-144.
" " combustion, 410.
" produces heat, 409.
" " electric current, 268.
Frictional electricity, 199, 209-244.
" " friction, 409.
Fulcrum, 109.
" " mechanical energy, 354.
INDEX.
397
Heat from percussion, 408.
Insulators, 216.
" Latent, 377-386.
Intensity of light, 428.
" Luminous, 401.
" sound, 330.
" Obscure, 401.
Interference of sound, 344.
" Radiant, 397, 403, 420.
Intermolecular spaces, 6, 8r.
" Radiation of, 398, 404.
Internal reflection of light, 445.
" Reflection of, 402, 404.
" resistance, Electric, 250.
" Refraction of, 402.
International measures, App. B.
" related to energy, 407.
Inverted image- , 426.
" Sensible, 377, 43-
Invisible spectrum, 468.
" Specific, 387-390.
Ions, 272.
" unit, 376.
Ins, 473.
Heating powers, 411.
Hectometer, App. B.
J.
Heliostat, p. 336.
Helmholtz's resonator, 343.
Joule's equivalent, 413.
Hiero's fountain, Ex. 4, p. 137.
Holtz electric machine, note, p. 168.
K.
Homogeneous medium, 425.
Horse power, 97.
Kathion, 272.
Horizontal needle, 2953.
Hydrogen, Specific heat of, 390.
Hydrokinetics, 171-177.
Hydrostatic bellows, 149.
" press, 150.
Kathode, 272.
Kilogram, App, B.
Kilogrammeter, 95*.
Kilometer, App. B.
Kinetic energy, 99.
1
" theory of gases, 179.
Images formed by lenses, 454-459.
L.
" mirrors, 439, 440.
" Inverted, 426.
Latent heat, 377-386.
" Projection of, 439.
Lateral liquid pressure, 157.
" Real, 439.
" inversion, Exp. 216.
" Virtual, 434,
Lamps, Arc, 313.
Impenetrability, 17, 19,
" Incandescence, 312.
Incidence, Angle of, 57.
Law of boiling, 372.
Inclination, Magnetic, 296.
" ebullition, 372.
Inclined plane, 133, 134.
" electrostatics, 214.
Increment, of gravity, 81.
" falling bodies, 79, 82.
" velocity, 81.
" fusion, 368.
Indestructibility of energy, 103.
gravitation, 60.
" " matter, 17, 21.
inclined plane, 134.
Induced currents, 303-315.
inertia, 51.
" electricity, 206, 247, 303.
lever, in, 116.
Induction coil, 306.
luminous intensity, 428.
" Electric, 32a, 304.
machines, 108.
Magnetic, 390.
magnets, 286.
Inertia, 17-22.
melting, 368.
" Laws of, 51.
motion, 50-54.
Initial velocity of falling bodies, 80.
pendulum, 86-88.
INDEX.
Law of pulley, 131.
Luminous pencil, 424.
' reflected motion, 57.
ray, 423.
" reflection of light, 431.
" spectrum, 467.
" refraction of light, 444.
Luminiferous ether, 396.
" screw, 139.
" thermodynamics, 413.
M.
" weight, 63.
" wheel and axle, 120.
Machines, 105, 140.
" Ohm' s, 252.
Machines cannot create energy, 106.
Leaning towers, 733.
" Compound 140.
Leclanche cell or battery, 259.
defined, 105.
Lens, 447, 450-459, 473-
" Electric, 232-234, 310, 311.
Lever, 109-116.
" Laws of, 108.
" Classes of, no.
" Simple, 105-139.
" Compound, 116.
" Uses of, 107.
" Laws of, in.
Magic lantern, 479.
Leyden jar, 235-237.
Magdeburg hemispheres, Exp. 66.
Lifting pump, 189, 190.
Magnetic attraction, 284, 286, 293.
Light, Analysis of, 464.
" curves, 290.
" defined, 420.
" declination, 297.
" Diffused, 432.
dip, 296.
" Dispersion of, 463.
" effects of electric current
" Electric, 312, 313.
275-277.
" Intensity of, 428.
" equator, 283 ; field, 290.
" Rectilinear motion of, 425.
" force, Lines of, 290.
" Reflection of, 430-440.
" inclination, 296.
" Refraction of, 442-459.
" induction, 292.
' Synthesis of, 464.
* needles, 295.
" Total reflection of, 445.
" neutral point, 283.
" Velocity of, 427.
" poles, 283.
Lightning, 242.
*' screens, 289.
" rods, 243.
" substances, 288.
Line of direction, 71.
" variation, 297.
Liquid, 39.
Magnetism, 196-315.
" pressure, 146-158.
" related to energy, 301.
Liquids, Equilibrium of, 160.
" Residual, 299*.
" in communicating vessels,
Magnetization, 291, 300.
161.
Magnetized substances, 288.
" Thermal conductivity of, 394.
Magneto-electric currents, 308-315.
Liter, App. B.
" " machines, 311*.
Loadstone, 205, 280.
Magnets, 102-206, 280-300.
Local action, 254.
" Artificial, 281.
Locomotive, 416.
" Broken, 287.
Lodestone, 205, 280.
Electro-, 275*., 298.
Loudness of sound, 330.
" How made, 291, 300.
Luminous beam, 424.
" Laws of, 286.
body, 42..
" Making, 291, 300.
" effect of electricity, 242, 269.
" Molecular, 287.
14 heat, 401.
" Natural, 280.
399
Magnets, Neutral point of, 383.
Motions of molecules, 8.
" Planetary, 294.
Motor, Electric, 311*.
" Sucking, ajy.
Music, 328, 329.
Magnifying glasses, 474.
Malleability, 29, 34.
N.
Mariner's compass, 295*1.
Mass denned, 5.
Natural magnets, 205, 280.
" attraction, 7.
" philosophy, defined, 13.
" motion, 8.
Nature of electricity, 209.
Mathematical formulas, App. A.
Nearsigatedness, 473*1.
Matter defined, i.
Needle, Magnetic, 295.
" Conditions of, 37.
Neutral equilibrium, 70.
" Constitution of, 9.
" point of magnet, 283.
" Continuity of, 6.
Newton's disc, Exp. 233.
" Divisions of, 2.
" laws of motion, 50-54.
" Properties oif, 12.
Nodal points ; nodes, 347.
" Radiant, 43.
Noise, 328.
" Ultra gaseous form of, 43.
North star, Exp. 136.
" Measure of work, 94-97.
Measures, Metric or international,
App. B.
0.
Mechanical effects of electricity, 198.
Ohm defined, 220.
" energy from heat, 356.
Ohm's law, 252.
' equivalent of heat, 413.
Object glass ; objective, 475.
" motion, 8.
Obscure heat, 401.
Mechanics, 48*1.
Opaque bodies, 422.
Megohm, 220.
Opera glass, 476.
Meniscus, 450.
Optical centre, 451.
Meter, metric measures, App. B.
" instruments, 472-481.
Microscope, Simple, 474.
Optic nerve, 473.
Compound, 475.
Optics, 420-481.
Millimeter, App. B.
Oscilliation, Centre of, Exp. 38.
Mirror, Concave, 435.
" of pendulum, 85.
" Convex, 440.
Overshot wheel, 175.
" Plane, 434.
Overtones, 347-351-
Molecular attraction, 7.
" magnets, 287.
u motion. 8.
P.
Molecule defined, 4.
Pascal's experiment, 148, 184.,
Molecules, Motions of, 8.
" principle, 148.
Momentum, 49.
Pendulum, 84-90.
Motion defined, 46.
Pencil of light, 424.
" Forms of, 8.
Percussion produces heat, 408.
" Newton's laws of, 50-54.
Permanent magnets, 203.
" of pendulum, 85.
Philosophy, Natural, defined, 13.
" Reflected, 56.
Photographer's camera, 472.
Law of, 57.
Photometer, 4280. and b.
Motions of atoms, 8.
Photometry, 4280.
" masses, 8.
Physical changes, n.
400
INDEX.
Physical properties, 15.
" science, 10.
Physics, deflned, 13,
Physiological effects of electric cur-
rent, 270.
Pinion, 122
Pipette, Exp. 54.
Pitch ot sound, 332.
Plane, Inclined, 133, 134.
Plano-concave lens, 450.
" convex lens, 450.
Plate electric machine, 233, 234.
Plates, Refracting, 447, 448.
Plating, Electro, Exp. 112.
Pneumatics, 178.
Pointed conductors of electricity, 231.
Polarization, Electric, 222, 225, 255.
" Electromotive force of,
273.
Poles, Electric, 249.
" Magnetic, 283.
Porosity, 17, 24.
Porte lumiere, p. 336.
Position, Energy of, 99.
Potassium dichromate cell or battery,
258.
Potential, Electric, 218.
" energy, 99.
Press, Hydrostatic, tyt.
Pressure, Atmospheric, 182, 185.
" of liquids, Downward, 15*.
" " Sidewise, 157.
" Upward, 155.
" transmitted by liquids, 146.
Primary coil, 304.
Prince Rupert drop, Exp. 44.
Principal axis of lens, 451.
" ' mitror, 435.
" focus. 435, 437, 452.
Prism, 447, 449, 450*1.
" Cathetal, Ex. o, p. 364.
Projection of images, 439.
Propagation of sound, 320, 331.
Properties of matter, 12, 15.
Pulley, 126-132.
Pumps, 187-193.
PupU of eye, 473.
Q.
Quality of sound, 329, 351.
Radiant heat, 397, 403, 420.
" matter, 43.
Radiation of heat, 398, 404.
Ray of heat, 399.
" light, 423.
Rainbow, 466.
Reaction, 54, 55.
Real images, 439, 455, 456.
Rectilinear motion of light, 425.
Reflected motion, 56, 57.
Reflector telescope, 478.
Reflection, Angle of, 57.
of heat, 402, 404,
light, 430-440.
sound, 333.
" Total, internal, 445.
Refraction explained, 443.
" of heat, 402.
light, 442.
Refractors, 447.
Refractor telescope, 477.
Reinforcement of sound, 341.
Repulsion, Electric, 199.
Residual magnetism, 299*1.
Resinous electricity, 213.
Resistance, Electric, 220, 250, 268.
Resonance-, 342.
Resonators, Helmholtz's, 343.
Retentivity of magnets, 282.
Retina, 473.
Retort of still, 374.
RuhmkorfTs coil, 306.
s.
Savart's wheel, 332.
Sciopticon, 479.
Screw, 138, 139.
Secondary axis of lens, 451.
" " mirror, 435.
" battery, 274.
" coil, 304.
foci, 452^.
Second's pendulum, 89.
Segments of strings, 347.
Sensible heat, 377, 403.
INDEX.
401
Simple machines, 105-139.
Stereoscope, 481.
" tones, 352.
Stereoscopic effects, 480.
Siphon, 194.
Storage battery, 274.
Slide, Magic lantern, 479.
Straight line motion of light, 425.
Sliding valve, 415.
Strings, Vibrations of, 346.
Smee's cell or battery, 257.
Sulphate of copper cell or battery, 261.
Solar spectrum, 462.
Surveyor's compass, 295*1.
Solidification, 381.
Swan lamp, 312, Ex. 23, p. 244.
Solids, 38.
Sympathetic vibrations, 338, 342.
Solids, Expansion of, 362.
Synthesis of white light, 464.
" Thermal conductivity of, 393.
Syphon, 194.
Solution, Latent heat of, 379.
So ometer, 23801.
T.
So nd beats, 345.
Cause of, 319.
Telegraph, Electric, 276.
Continuous, 327.
Telephone, Electric, 335.
defined, 317.
" " Action of, 336,
Intensity of, 330.
" exchange, 3373.
Interference of, 344.
" String, Exp. 148.
Loudness of, 330.
Telephonic circuit, 315.
media, 324.
" current, 314.
Musical, 328, 329.
" transmitter, 337.
Propagation of, 320, 331.
Telescopes, 476-478.
Quality of, 329, 351.
Temperature, 358, 366.
Reflection of, 333.
Temporary magnets, 203.
Reinforcement of, 341.
Tenacity, 29, 32.
Transmission of, 324.
Tension, Electric, 217.
Velocity of, 325, 3*6.
" of gases, 179.
waves, 318.
Terrestrial magnetism, 294.
So nding boards, 339.
Theory of electricity, 226.
Spark, Electric, 242.
Thermal effects of the electric current,
Speaking tubes, 331.
268.
Specific gravity, 165-169.
Thermal spectrum, 470,
" heat, 387-39.
" uuits, 376.
Spectrum, Solar, 462.
Thermodynamics, 406-418.
" Actinic, 469.
Thermo-electricity, 206, 247, 278.
Invisible, 468-.
" electric pile, 278.
" Luminous, 467.
Thermometers, 359.
" Thermal, 470.
Thermometric scales, 360.
Spherical mirror, 435, 440.
Timbre, 329, 351.
Stable equilibrium, 68.
Time pieces, 90.
Stability, 73.
Toepler-Holtz electric machine, note.
Standard candle, Ex. 10, p. 335.
p. 168.
Static electricity, 199, 209-244.
Tones and overtones, 347.
" law, note, p. 72.
" Simple and compound, 352.
Steam, 373 .
Torricelli's experiment, 183.
" engine, 414-418.
Total reflection of light, 445.
" Latent heat of, 385.
Towers, Leaning, 73/1.
Stereopticon, 479.
Translucent bodies, 423.
402
INDEX.
Transmission of pressure by liquids,
146.
Transmission of sound, 324.
Transmitter of telephone, 337.
Transparent bodies, 422.
Tubes, Acoustic, 331.
Tuning forks, Mounted, 339*.
Turbine wheel, 174.
Types of energy, 99.
u.
Ultra-gaseous form of matter, 43.
" red rays, 470.
" violet rays, 469.
Undershot wheels, 177.
Undulations, 318, 420*1.
Undulatory theory of light and heat,
397*.
Unit of heat, 376.
" work, ys , 97,
" Thermal, 376.
Universal properties of matter, 13. 16.
Unstable equilibrium, 69.
Upward liquid pressure, 155.
Uses of machines, 107.
V.
Vaporization, 369.
" Latent heat of, 382.
Vapors, 41.
Variation, Magnetic, 297.
Varieties of energy, 483.
Velocity, Increment of, 81.
" of falling bodies, 75, 80.
of light, 427.
of sound, 325. 326.
" related to energy, 98.
Ventral segments, 347.
Vertical needle, 29501.
Vibrations of pendulum, 85-89.
" : of strings, 346.
" Sympathetic, 338, 342.
Virtual images, 434, 457, 459.
Vision, Persistence of, Exp. 235=
Vitreous electricity, 213.
" humor, 473.
Volt, 219.
Voltaic arc, 313.
Voltaic battery, 264-267.
" cell, 248, 256.
" current, 248.
" element, 248, 256.
" electricity, 201, 246-473.
Voltameter, 271.
Volume of gases, Ex. 7, p. 137.
w.
Water, Expansion of, 363, 364.
" jacket, 374^.
Latent heat of, 384.
" lens, 4506.
" power, 173.
" Specific heat of, 390.
" voltameter, 271.
" wheels, 174-177.
Wave, Amplitude of, 322.
." length, 321.
" motion, 318.
Waves, Coincident, 340.
Wedje, 135-137:
Weighing, 113, ,14.
Weight, 17, 20, 62.
' of air, 181.
" Law of, 63.
Wheel and axle, 118.
" work, 122-125.
Whispering galleries, 333*.
White of the eye, 473.
" vitriol, 261.
Windlass, 121.
Work, 92, 94-96-
Worm of still, 374.
z.
Zero of temperature, 366.
Zincs, Amalgamating battery, 354.
This book is DUE on the last date stamped below
JWR 7 1934
OCT
c '
.OCT 1
JiPB W*
Form L-9-15m-7,'32
A 000 941 341