Book II -- Physics
The Science That Explains Heat, Light,
Sound, Electricity, Magnetism
City, Magnetism… . . . .. .. . . . . . . . . . . . . . . . . . . . . .
.. .. . . . . . . . . . . . . . .. . . . . . . 73
Solids and Liquids . . . . . . . . . . . . .. .. . . . . . . . . . . .
. . .. . . . . . . . . . . . . . . . . . .73
Solids, Liquids, and Gases are made Up of Millions of Small Particles .
. . . . 74
Heat makes Solids, etc., expand . . .. .. . . . . . . . . . . . .. . .
. . . . . . . . . . . . . . . . . 74
Most Gases are Invisible . . . . . . . . . . .. . . . . . . . . . . ..
.. . . . . . . . . . . . . . . . . . . 77
The Diving Bell . . . . . . . . . . . . . . . . . . . . .. . . . . . .
. . . . .. . . . . . . . . . . . . . . . . . 78
The Earth's Atmosphere . . . . . . . . . . .. .. . . . . . . . . . . ..
. . . . . . . . . . . . . . . . . . .78
Balloons . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . .
. . . . . .. . . . . . . . . . . . . . . . . . . 80
Air is Heavy . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .
. . . . .. . . . . . . . . . . . . . . . . . . 81
Reservoirs, Fountains, and the Water Supply of Cities . . . . . . . . .
. . . . . . . . . . 81
The Barometer . . . . . . . . . . . . . .. .. . . . . . . . . . .
. . . . . .. . . . . . . . . . . . . .. . . . . . . . . 83
The Air presses about Fifteen Pounds On Every Square Inch . . . . . . .
. . . . . . . .84
How to measure the Heights of Mountains . . . . . . . . . . . . . . .
.. .. . . . . . . . . . . . . . 85
The Barometer is a Weatherglass . . . . .. . . . . . .. .. . . . . . .
. . . . . . . . . . . . . . . . .86
United States Weather Bureau Predictions . ... .. . . . . . . . . . . .
. . . . . . . . . . . . . 88
Thermometers . . . . . . . . . . . . . . . . . . . . .. . . . . .. .. .
. . . . . . . . . . . . . . . . . . . . . .88
Steam . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . .
.. .. . . . . . . . . . . . . . . . . . . . . . . 90
The Steam Engine . . . . . . . . . . . .. .. . . . . . . . .. . . . . .
. . . . . . . . . . . . . . . . . . . 91
The Locomotive . . . . . .. . . . . . . . . .. .. . . . . . . . .. . .
. . . . . . . . . . . . . . . . . . . .91
The Steamship .. .. . . . . . . . . . . . . .. .. . . . . . . . . . . .
. .. . . .. . . . . . . . . . . . . . 96
Light . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . ..
.. . . . . . . . . . . . . . . . . . . . . . . . . 96
The Sun's Rays travel in Straight Lines . . .. .. .. . . . . . . . . .
. . . . . . . . . . . . . . . .96
Shadows . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .
.. . . . . . . . . . . . . . . . . . . . . . 101
Eclipses of the Sun and Moon . . . . . . .. . . . . .. .. . . . . . . .
. . . . . . . . . . . . . . . 102
Reflection of Light . . . . . . . . . . . . . . . .. . . . . . . . . ..
. . . . . . . . . . . . . . . . . . . . 104
Refraction of Light . . . . . . . . . . . . .. .. . . . . . . . . . ..
. . . . . . . . . . . . . . . . . . . . 105
Prism; the Spectrum . . . . . . . . . . .. . . . . . . . . . .. .. . .
. . . . . . . . . . . . . . . . . . . 105
Lenses . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . .
. . .. . . . . . . . . . . . . . . . . . . . . 106
Spectacles . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . .
.. .. . . . . . . . . . . . . . . . . . . . 107
Sound . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .
. . . . .. . . . . . . . . . . . . . . . . . . . . .110
Velocity of Sound and Light . . . .. .. . . . . . . . . . . . . . .. .
. . . . . . . . . . . . . . . . .. 110
Sound is a Vibration . . . . . . . . . . . .. . . . . . . . . . . . . .
.. . . . . . . . . . . . . . . . . . . . 112
Musical Insturments (Bells, Pianos, Violins, Organs, Drums) . .
.. . . . 113
Reflection of Sound . . . . . . . . . .. . . . . . . . . . . . . . . .
. . .. . . . . . . . . . . . . . . . . . .116
Echoes . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . .
. . . . . . .. . . . . . . . . . . . . . . . . . 116
Musical Notes . . . . . . . . . . . . .. .. . . . . . . . . . . .
. . . . . .. . . . . . . . . . . . . . . . . . . 116
The Phonograph . . . . . . . . . . .. .. . . . . . . . . . . . .
. . . .. .. . . . . . . . . . . . . . . . . . .117
Electricity . .. . . . . . . . . . . . . . . .. .. . . . . . . . . . .
. . . . . . .. . . . . . . . . . . . . . . .. . . .119
Apparatus needed . . . . . . . . . . . . . .. .. . . . . . . . . . . .
. . . . . . .. . . . . . . . . . . . . . 119
Experiments . . . . . . . . . . . . . . .. .. . . . . . . . . . . .
. .. . . . . .. .. . . . . . . . . . . . . .. . . 120
Benjamin Franklin's Kite . . . . . . . . .. . . . . . . . . . . . . . .
. .. .. . . . . . . . . . . . . . .123
Experiments . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .
. . . . . . .. . . . . . . . . . . . . . . . 123
Electric Batteries . . . . . . . . . . . . . . . .. . . . . . . . . . .
. . .. .. . . . . . . . . . . . . . . . . .124
The Telegraph . . . . . . . . . . . . . . .. . . . . . . . . . . . . .
.. . . . . . . . . . . . . . . . . . . . . .125
Telegraphic Alphabet . . . . . . . . . . . . . . . . . . .. . . .
. . . . . . . . . . . .. . . . . . . 127
Velocity of Electricity. . . . . . . . . . . . . . . . . . . .. . . . .
. . . . . . . . . . . . . .. . . .128
Magnetism . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . .
. . . . . . . . . . . . . . . . . . . .128
Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . .. . .
. . . . . . . . . . . . . . . . .129
Magnets . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . ..
. . . . . . . . . . . . . . . . . 129
Natural Magnets (Lodestones) . . . . . . . . . . . .. . . . . . .
. . . . . . . . .. . . . 133
Electro-Magnets . . . . . . . . . . . . . . . . . . . . . . .. . . . .
. . . . . . . . . . . . . . . 133
Telegraph Instruments . . . . . . . . . . . . . . . . . .. . . .
. . . . . . . . . . . .. . . . . . . .133
Electric Bells . . . . . . . . . . . . . . . . . . . . . . . . . .. . .
. . . . . . . . . . . . . . . . . . . . 134
The Telephone . . . . . . . . . . .. . . . . . . . . . . . . . .. . . .
. . . . . . . . . . . . . .. . . . .137
The Mariner's Compass . . . . . . .. . . . . . . . . . .. . . . . . . .
. . . . . . . . . . . . . . . 138
The Electric Light . . . . . . . . . . . . . . . . . . . . . .. .
. . . . . . . . . . . . . .. . . . . . . .140
The Dynamo . . . . . . . . . . . . . . . . . . . . . . . . . .. .
. . . . . . . . . . . . . .. . . . . . . .142
Electric Railways . . . . . . . . . . . . . . . . . . . . .. . .
. . . . . . . . . . . . .. . . . .. . . . 143
Appendix . . . .. . . . . . . . . . . . . . . . . . . . . .. . .
. . . . . . . . . . . . .. . . . . . . .144 – 147
--------------------------------------------
Solids and Liquids.--"What is the difference between a solid
and a liquid?" said Tom one hot afternoon when the children were all
together on the porch, fanning themselves.
Mary. You can pick up a solid
with your fingers, and you cannot pick up a liquid--that's one
difference.
Agnes. You mean you can pick
up a piece of ice, and you cannot pick it up when it has melted
into water?
Mary. Of course you can't.
Fred. Oh, yes, you can--and
with that Fred took a lump of sugar and put it in a teaspoon partly
filled with water. The sugar took up the water, and Fred picked
up the sugar and left the spoon quite empty, saying: "Look at that!
I've picked up a liquid in my fingers. It's magic."
Agnes. That is just foolish,
Fred.
Fred. I know it--but it is
magic. You said I couldn't do it.
Tom. It isn't fair, Fred; you
can pick up a sponge with water in it, but you cannot pick up the
water without the sponge, nor the water without the sugar, either.
pg 74
Fred. All right. I was just
playing. It is a kind of
magic, though.
Mary. Well, wasn't I right? A
solid is a thing you can pick up in your fingers, and a liquid is
something you can't pick up.
Tom. The real magic of it is
that a piece of ice and a spoonful of water are just the same thing.
The same thing is different at different times; sometimes it is ice,
and sometimes water. I wonder why. Let us ask Jack.
Fig 54 A solid, a piece of iron for instance, is made
up of thousands and thousands of little particles, each one like every
other one, all crowded together like the lower part of the picture.
When you heat a solid the little particles are forced farther apart, so
that by and by they look like the upper part of the picture. The solid
will get larger if you heat it.
Jack. I think Mary's
definition is a pretty good one--a solid is a thing you can pick up
with your fingers. You can change a solid into a liquid, if you want
to, by heating it. You can change a piece of ice into water by letting
it melt. A little heat will do it.
Fred. How does heat do it,
Jack?
Solids, Liquids, and Gases are made up
of Millions of Small Particles.--Jack. Well, you have to begin far
off if you wish to understand. The scientific men have proved that all
solids--and all liquids too--are made up of little particles
crowded close together. When you heat a solid the particles are
forced farther and farther apart.
Heat makes Solids, etc., expand.--"A
piece of solid iron gets larger when you heat it. When a
blacksmith wants to fit a new tire on a wheel, he first heats it
and puts it on; then the tire, as it gets cold again, shrinks tightly
on the wheel and stays where it is put."
pg 75
"Every little particle of the tire has been forced apart by the heat,
and by and by the whole tire, which was in the first place smaller than
the wheel, grows large enough to slip over the rim. Then
the blacksmith slips it on and lets it cool. As it cools, it shrinks
and fits the rim tightly. The heat has loosened the particles you might
say."
Agnes. How do you know the
particles are farther apart when the iron is hot?
Jack. There are just so many
particles in the cold iron to begin with, Agnes--say ten millions of
them, if you please. And you haven't put any more particles in the
tire, you know;
Fig. 55 The right-hand picture shows a wheel ready to
be fitted
with a tire; the middle picture shows the tire heated in a fire. When
the tire has expanded--grown large enough--the blacksmith
fits it on the wheel and lets it shrink tight by cooling.
you have simply heated it. But the tire really has grown bigger--the
proof is that it will slip over the rim of the wheel. The same ten
million little particles of the cold iron fill a larger space than when
they are cold; so they must have been forced farther apart somehow, you
see. And the heat did i --nothing else could have done it.
Mary. Suppose you had heated
the iron more and more, Jack. What then?
pg 76
Jack. If you had put the iron
in the furnace and kept on heating it, you would have had a hot solid
at first; then it would have become pasty, almost like a dough, and by
and by it would become a liquid--it would flow like water. You
cannot try the experiment with iron, but you have often seen the boys
try it with lead when they are molding bullets for their guns. Lead
melts more quickly than iron.
Agnes. Yes, they put the lead
in a ladle and melt it, and then pour it into molds and let it
cool.
Jack. Iron is made up of one
kind of small particles, and lead is made up of another kind of
particles; and it takes less heat to separate the lead particles. But
heat does the same thing always. It separates the particles farther,
the more heat you apply. First you have a solid, and then a
liquid; and if you heat the liquid enough, you have a gas--iron
gas, lead gas.
Tom. If you were to go on
heating iron gas for a week, what would you get? something different
than gas?
Jack. No; you would get very
hot iron gas and nothing more. You can have matter in only three
forms--solid, liquid, gas--but you can turn one form into the
other by heat or by cold. Take ice, for instance.
Fred. Ice is solid. If you
make it colder and colder, it is nothing but ice--the north polar
regions are ice and nothing else.
Agnes. And if you heat ice, it
becomes water.
Mary. And if you heat water in
a teakettle, it becomes steam.
Tom. And if you heat steam,
Jack, more and more, is it always steam?
Jack. It is never anything
else. It is simply very hot steam. The boiler of a
locomotive or of a steamship makes steam and
pg 77
nothing else. Solid, liquid, gas--that is all you can get. If
you cool a gas like steam enough, you will get a liquid; and if
you cool a liquid enough, you will get a solid.
Most Gases are Invisible.--Mary. I have seen solids and
liquids; but I'm not sure I have ever seen gases.
Fred. Well, you have smelled
them, anyway, from leaky gas fixtures in the house, or when the match
went out before you could light the gas at the burner.
Mary. Oh, yes; and of course,
there is gas inside of the little toy balloons. But I have never seen
gas.
Jack. Most gases cannot be
seen. They are invisible. Air is invisible, but it is all around
us. If any one asked you to prove that air was really all around us,
Mary, how would you prove it?
Mary. Why, I should say that
the wind was nothing but moving air.
Fig. 56 A glass bowl partly filled with water, a cork,
and a glass
tumbler are needed to prove that the tumbler was filled with air. This
experiment should be tried in the classroom.
Jack. That is a good idea, my
dear. But suppose the wind wasn't blowing? Then what?
Mary. I should wait till it
did.
Jack. You could make an
experiment to prove it this way. Take an empty tumbler and hold
it upside down. We call it empty, but it is really full of air--of
invisible air. Then take a glass bowl half full of water and
float a cork in it. Now gently press the tumbler down over the
cork (see the picture) and see what you will see. If there is nothing
in the tumbler
pg 78
--if the tumbler were really empty--then the water would fill
it full; but you can see that the water rises only for a certain
distance, and no higher. There is air in the tumbler still.
Tom. I can prove that there is
air in the tumbler now. Tip the tumbler sideways a little while it is
in the bowl and the air will come out in bubbles.
Fred. What becomes of the air
in the bubbles when they rise to the surface?
Tom. Why, it just mixes with
the other air all around us.
Fig. 57. Bubbles of
Air Escaping
From the Mouth of a Goldfish in a Globe
The Diving Bell.--Agnes. The diving bell that men use
at the bottom of rivers is like the tumbler, isn't it?
The Earth's Atmosphere.--Jack. The air is all around us
everywhere, for wherever you go you find air to breathe.
Agnes. Not on the tops of
mountains, Jack.
Jack. There is not so much air
at the tops of mountains as there is below, Agnes, but there is air.
Men have climbed the very highest mountains, and as they went up, they
found less and less air. Birds fly nearly as high. The condor--a bird
like an eagle--flies high in the Andes of South America,
and balloons carrying men have gone five miles high. Balloons without
men have gone as high as ten miles, but the air is so thin at that
height that men could not breathe.
pg 79
Fig. 58 The diving bell is lowered by a chain from a
ship, and
air is pumped into it by the pipe marked T in the picture. The
whole diving bell is underwater, but the water rises no higher than its
floor. The pressure of the air keeps it out. The diver goes down to the
bottom of the harbor and fastens ropes to whatever he wants to hoist to
the surface. He has a waterproof and air-proof suit of clothes, and air
is pumped down for him to breathe (through the small pipe in the
picture). The foundations for the piers of bridges can be laid by
men working in this way.
pg 80
Mary. The air gets thinner and
thinner as you go up. Where does it stop then?
Jack. We do not know exactly;
but there is some air--a very little--as high as sixty miles,
because shooting stars begin to burn as high as that. They burn when
they first meet the air, about sixty miles above the ground. (1)
Fig. 59 The Hima'laya Mountains are about five miles,
and Mont Blanc, in the Alps, is about three miles above the level of
the sea.
A balloon carrying men has gone up five miles and very light
balloons filled with gas have gone nearly ten miles above sea level.
Balloons.--Tom. The balloon
floats in the air because it is lighter than the air, just as a
chip floats on the water.
(1) See Book I (Astronomy), Meteors,
page 45.
pg 81
Mary. If the balloon is
lighter than the air, then the air itself must be heavy; for a balloon
weighs a good deal, especially when it is carrying men.
Air is Heavy.--Jack. Yes, the air is heavy, and
there is a simple way to prove it.
Reservoirs, Fountains, and the Water
Supplies of Cities.--"If you have a reservoir full of water
and a fountain joined to the reservoir by a pipe, the fountain will
play as soon as the
Fig. 60 All these glass vessels are joined together by
the brass tube
at the bottom. If you fill the large jar at the left hand side with
water, all the other tubes will at once fill up to the same level.
The air presses on the water in the large jar and forces it up into
the other tubes and makes the little fountain play.
water is turned on, and the fountain will play as high (1) as the water
in the reservoir. That is because the air above the reservoir is heavy
and presses down on the water in it and forces it up in the
fountain. (See Fig. 61) Now here is an
(1) Or nearly as high; the friction of the water in
the pipe
makes some difference.
pg 82
Fig. 61. Reservoirs, Fountains, and the Water Supplies
of Cities--The picture shows a lake which is the source from which the
water is
obtained. A dotted line across the picture marks the level of the upper
surface of the lake. An aquaduct (water pipe) takes the water
from the lake, carries it under the hill, under a pond, up another
hill, where there is a fountain, and delivers the water to the city
reservoir. From the city reservoir pipes conduct the water all over the
city--to
public fountains, to the upper stories of houses, and so forth. Notice
that all the fountains send their jets to about the same height.
pg 83
experiment that we can try ourselves with this bit of glass tube bent
into the shape of a U.
"You can see that the air must be heavy; it must press down with
weight because it makes the fountain play (Fig. 61) and keeps the water
standing high (Fig. 62).
The Barometer.--"Instead of
using water in the U-shaped tubes, you can use any liquid--milk,
kerosene oil, quicksilver. It is convenient to use quicksilver because
it is heavy and because it is so clean. (2)
"You need a hollow glass tube about thirty-two inches long, closed at
one end, a lot of quicksilver, and a flat basin of glass or china. Hold
the long tube with its open end upwards. It is full of air.
Make a paper funnel and pour quicksilver from a pitcher into the
open tube, slowly and carefully, until you have quite filled it. As the
quicksilver goes in it will drive out the air, and finally you
will have a tube with no air in it--nothing but quicksilver. You must
handle it carefully because it is quite heavy. Now put your
finger over the open end of the long tube and turn
Fig. 63 How to Make a
Barometer--See the description in the
text.
(1) The barometer is an instrument to measure the
weight--the pressure--of the air. The name comes from two Greek words,
one
meaning "weight" and the other "to measure."
(2) Quicksilver is a poison if taken by the mouth; it
is
perfectly safe to handle it unless there are cuts on the hands
and fingers. If it touches a gold ring, however, it will cover the gold
with a thin layer of quicksilver that will not wear off for some
time.
pg 84
the tube swiftly upside down. (See the left-hand picture, Fig. 63.) If
you should now take your finger away, all the quicksilver would fall
out. There would be nothing to support it. But dip the open end of the
long tube in the basin of quicksilver, take your finger away, and see
what happens. The quicksilver will fall in the tube a little
distance--a few inches--and then it will stop. The air is pressing on
the quicksilver in the basin and is pressing some of it up into the
tube. There is no air above the quicksilver in the tube; nothing
is pressing downward except the weight of the quicksilver in the tube
itself. The weight of the quicksilver in the tube pressing downward
just balances the pressure of the air on the quicksilver in the basin.
The two pressures just balance each other."
The Air Presses about Fifteen Pounds
on Every Square Inch.--Tom.
The height of the column of quicksilver in the tube is about thirty
inches.
Fig. 64. A Quicksilver Barometer Ready for Use.--The
long glass tube
has a scale of inches at the upper end; 28, 29, 30, 31 inches are
marked, as well as the tenths of inches. The basin of quicksilver is at
the bottom.
Jack. Yes, and if the tube
were an inch square, the column of quicksilver in it (thirty inches
high and an inch square) would weigh fifteen pounds. The air pressure
on the basin keeps that column standing. It keeps a weight of fifteen
pounds standing.
pg 85
Fred. That is what is meant by
saying the air presses fifteen pounds on every square inch of your
body, isn't it?
Jack. It presses fifteen
square inches on every square inch of the whole world; on your body, on
the ground, and on the water--everywhere. It presses down, and it
presses up too.
Tom. How does it press up? I
see that it presses down.
Fred. The air must press up,
or the balloon would not rise.
Jack. That is one proof, and
here is another that you can try for yourself. (See Fig. 65.)
Fig. 65. Fill (or partly fill) a tumbler with water and
press a stout
piece of writing paper over the top closely. Put the palm of your hand
over the paper and hold it on tightly. Now quickly turn the tumbler
upside down and take away your hand from the paper. (See the
picture.) The pressure of the air from below is so much greater
than the weight of the water, and of the small amount of air in the
tumbler, that the paper will hold the water up. (This experiment should
be tried in the school room.)
How to measure the Heights of Mountains.--"Now
I want you to say what would happen if I had taken the barometer to the
top of the mountain."
Mary. The air is lighter at
the top of the mountain than it is here, and does not press down so
much.
Tom. So the quicksilver in the
tube would not stand so high; it would not have so much air pressure to
balance.
Jack. Bravo! that is just
right. At the level of the ocean all the air--the whole atmosphere--is
above you, and it presses fifteen pounds on every square inch of the
ground. The quicksilver stands thirty inches high. When you go up about
1000 feet the air above you presses less because there is less of it;
you have left 1000 feet of it below you, and the column of quicksilver
is about twenty-nine inches high; if you
pg 86
go up 2000 feet, there is less air above you and the column is about
twenty-eight inches high, and so on.
Tom. So you could measure the
height of a mountain by noticing the height of the column of
quicksilver in the tube? On high mountains the column would be short.
Jack. That is right, and that
is the way the height of mountains are really measured. A barometer
measures the weight of the air above you. The higher you go the less
air above you and the less pressure on the basin of the barometer.
The Barometer is a Weatherglass.--Fred. Sometimes we see in the
newspapers a notice of a storm. The Weather Bureau says there is an
area of low barometer coming.
Jack. It happens that where
storms are the air weighs less and the barometer is low--the column of
quicksilver is short. In fine weather the air is heavy and presses down
more; so the barometer is high and the column of quicksilver is long.
If you watch the quicksilver from day to day, you will find this is the
case, generally; when fine weather is coming or is here the barometer
is high; when storms are coming or are here the barometer is low. So
the barometer is a kind of
Fig. 66. An Aneroid Barometer (A Barometer without
Quicksilver)--Aneroid is a Greek word that means "without any liquid."
Inside the
outer metal case is a tightly sealed box containing no air. On this box
the outside air presses, sometimes more, sometimes less. The little box
changes its shape under this pressure, and things are so arranged that
changes of pressure make a needle pointer move around a dial. This form
of barometer is very convenient for travelers and seamen.
pg 87
pg 88
weatherglass. It tells you beforehand what type of weather is coming.
(1)
Fred. And it tells ships at
sea when to look out for storms.
United States Weather Bureau
Predictions.--Jack.
Every day at a hundred places in the United States, in Cuba, and so
forth, the observers of the Weather Bureau notice how their barometers
are standing and telegraph to the central Weather Bureau at Washington.
There they make a weather map of the whole country several times daily.
If a storm is traveling eastwards, it will show on the map by an area
of low barometer, as they call it. The barometer in the country round
Omaha will be low on Monday, for instance; by Tuesday the storm has
traveled to Buffalo; so the Weather Bureau tells New York to look out
for a storm on Wednesday.
Agnes. Well, I never knew
before how that was done.
Thermometers.(2)--Fred. A thermometer has quicksilver
in it, too, but it is closed at both ends.
Tom. A thermometer is to
measure how hot the air is. It is different from a barometer; that
measures how heavy the air is.
Jack. Yes, a thermometer
measures how hot the quicksilver in it happens to be by the height of
the quicksilver in the glass tube. If the quicksilver column is long,
then the temperature
(1) Barometers often have words engraved opposite
points of their
scales; as: 30 1/2 inches, set fair (meaning that the weather will be
fair for some time); 30 inches, fair; 29 1/2 inches, change (meaning
expect a change soon); 29, rain; 28 1/2, much rain; 28, stormy. The
weather is foretold by a change in the barometer rather than by the
actual height of the quicksilver. If the quicksilver is rising, then
the weather is changing towards fair; if it is falling, then the
weather is changing towards stormy.
(2) The word thermometer is from
two Greek words, and it means "an instrument to measure
heat--temperature."
pg 89
is high; if the column is short, then the temperature is low. The
higher the temperature the longer is the quicksilver column.
Mary. It is like the iron tire
of the cart wheel. (See Figure 55.) The hotter the fire, the longer the
tire is.
Agnes. By just putting my hand
on a thermometer I can make the quicksilver mount up in the tube.
Tom. Jack, why do they make
the scale this way? 32 degrees is marked freezing, and 212 degrees is
marked boiling.
Jack. A German named
Fahrenheit (1) invented the thermometer we use about 200 years ago.
(See the right-hand picture in Fig. 68.) He put his thermometer into
melting ice and made a mark on the tube just where the quicksilver
stood; and then into boiling water and made a mark on the tube where
the quicksilver stood. It is too complicated to tell you why he named
the first mark 32 degrees and the second 212 degrees, but anyhow he did
so. The distance between his two fixed marks is 180 equal
parts--degrees. His thermometer was used in England; the pilgrims
brought it over to America; and we use it today. But there is another
scale of degrees--the centigrade (2) (see the left hand picture in Fig.
68)--which was invented in France
Fig. 68. The Glass Tubes of Two Thermometers--The tubes
are closed at
both ends are entirely empty of air, and are partly filled with
quicksilver.
(1) Pronounced --FAR en-hit. (long sound marked over
'i')
(2) Pronounced --SEN ti-grayd.
pg 90
about a hundred years ago, that is nearly everywhere in Europe. On the
centigrade thermometer the freezing point is marked zero (0 degrees),
and the boiling point of water one hundred (100 degrees); and the scale
between 0 and 100 is divided into equal parts. Zero of Fahrenheit's
scale is 17 8/10 degrees below the zero of centigrade scale. (See Fig.
68.)
Mary. Was Centigrade a man?
Tom. Of course not; don't you
see that centi means "one
hundred," and grade means
"degree"?
Mary. Why certainly; I thought
he might be a Frenchman though.
Jack. If you put one of our
thermometers into melting ice, it will always mark 32 degrees; if you
put it in boiling water, it will always mark 212 degrees; and if you
put the bulb of it in your mouth, it will always mark 98 degrees--that
is, unless you have a fever.
Fred. The doctor always takes
my temperature with a thermometer when I am ill.
Tom. And if your temperature
goes as high as 104 degrees, he looks very serious. The sign of being
well is to have a temperature of 98 degrees, they say.
Steam.--Jack. If you took a teakettle and
boiled the water in it, the temperature of the boiling water would be
212 degrees. Inside the kettle there is water at the bottom, and
above the water there is steam. If we had a glass kettle, you could
look through the sides and you would see nothing at all above the
water. True steam is invisible;
but there is steam there all the while. How do we know?
Fred. I thought steam was
visible. What is that cloud coming out of the nozzle of the kettle?
pg 91
Jack. That is water; cooled
steam; water in small drops like fog or clouds. The real invisible
steam is inside, trying to lift the lid and escape. If we fastened the
lid down and closed the nozzle, we should have a little steam engine.
Fig. 69. A Teakettle
with Boiling Water In It--It gives out clouds of
what we call steam. The clouds are really not
steam, but steam cooled back into water. If you hold an alcohol lamp
under the cloud, the hot flame will turn it back into steam and you
will se no cloud over the flame, because true steam is invisible. It is
there though, as you can tell by holding a cold spoon over the
invisible spot. The invisible steam will turn into visible water
(like fog or cloud) and gather in drops on the spoon. (This experiment
should be tried in the schoolroom.)
The Steam Engine.--Then Jack
explained the working of the steam engine to the children in this way.
(See Fig. 70).
F is the firebox; B is the boiler with the water in the bottom of it; S
is the steam pipe that carries the live steam over to the valve chest
VC. There are two pipes in the valve chest, pipe M and pipe N, and both
pipes open from the valve chest and run to the cylinder C. But things
are so arranged that both pipes M
and N cannot be open at the same time. If N is open, M is shut
(as in the picture). If M is open, N must be shut.
The picture is drawn with the pipe N open. The live steam rushes into
the pipe N, fills it, and rushes into the right-hand end of the
cylinder C and presses against the piston
pg 92
head P. (The piston head is a large iron disk that fills up the whole
of the diameter of the cylinder.) The pressure of the live steam moves
the piston head P (to the left in the picture) to the other end of the
cylinder C and pushes the piston rod R against the crank G on the crank
shaft A, and turns it. You must imagine now that the piston head P is
at the
Fig. 70 Drawing of
part of a steam engine.
left-hand end of the cylinder C and that the live steam fills the whole
of the cylinder. Find the letter R' in the picture. R' is fastened to
the slide valve V at one end and to the crank shaft at H, and things
are so arranged that now the rod R' closes
the pipe N by which the steam came in and at the same time opens the pipe M. The live steam is
filling the valve chest VC all
pg 93
this time. It cannot get into the pipe N (which is now closed), and so
it rushes into the pipe M (which is now open) and presses against the
left-hand side of the piston head P (which is now at the left-hand end
of the cylinder C). The piston is now pushed back by the steam to where
it started from (just as in the picture), and the crank shaft A is
turned still more. When the piston gets back to where it started the
pipe M is closed and the pipe N is opened again (by the rod R') and the
piston head is moved to the left again, then to the right, then to the
left, and so on as long as the engine is running. Every the piston head
P travels the length of the cylinder C the crank G makes half a turn,
and in this way the crank shaft GA keeps on turning. W' is a little
pulley wheel fastened to the crank shaft; and if you put a leather belt
on this pulley wheel, you can carry the power of the engine wherever
you like. You can carry it as far as the belt goes and drive any other
machine--a lathe, a saw, a drill. W is a heavy fly wheel fastened to
the shaft A to keep the motion steady.
All this description of how an engine works is perfectly easy to
understand if you take one thing at a time and pay attention; but it is
rather long, and you had better read it over again carefully with a pin
in your hand to point with. The live steam starts from the boiler B
(put your pointer there); fills the valve chest VC (put your pointer
there); rushes through the pipe N (point at it); presses against the
piston head P (point at it); drives the piston head to the left-hand
end of the cylinder (point at it); moves the stiff piston rod R so as
to turn the crank G (point at R and G). At this time the pipe N is
closed and the pipe M is open (point at N and M); the live steam is all
the while filling the valve chest VC (point there) and cannot escape
through the pipe N (which is closed) and
pg 94
now rushes through the pipe M (which is now open), and so on. You must
go through the whole description again and again till you understand it.
The Locomotive.--Figs. 70 and
71 show just how steam from a boiler can be made to turn a crank shaft
(G in both pictures) round and round. Suppose we put wheels on this
crank shaft and make the engine into a locomotive.
Fig. 71. A Stationary
Steam Engine--C, cylinder; P, piston; R, piston
rod. The reader should trace the
course of the steam (which enters through the pipe S) throughout a
complete motion of the piston.
In Fig. 72 you should put your pointer on the sleepers, the rail, the
front wheels P, the front driving wheel, the fire box A, the fuel grate
R, the boiler G, the valve chest C, the cylinder B (there is a separate
picture of the cylinder above the main picture), the smokestack E, the
cowcatcher, the headlight, the bell, the sand box M (the sand box is
used to hold sand to sprinkle on the rails when they are wet and
slippery, so that the driving wheels may not slide on the track), the
whistle O, and the cab. The to-and-fro motion of the piston in the
cylinder
pg 95
Fig 72 A Locomotive
Engine Running on a Railway Track
pg 96
moves the driving wheels round; the engine moves forward as they move
round, and the train follows the engine.
All steam engines work very much like the ones shown in the pictures.
You can see them at work in factories, on ships, in locomotives, in
automobiles--everywhere. With a machine like this you can take a little
water and a little coal and turn them into a power that will drive a
locomotive sixty miles an hour, or a great ship twenty-five miles an
hour from New York to Liverpool.
Fig. 73. An Ocean
Steamship
Light.--Jack. The very first thing to know
about light is that it travels in straight lines. You cannot see round
a corner, you know, though you can hear round a corner.
The room was darkened, and the sun's rays were let in through a very
small hole in a card and made an oval spot on the floor. Tom took a
newspaper, crumpled it up, and set it on fire in
pg 97
a coal hod, so that the room was partly filled with smoke and dust.
This made it easy to trace each little sunbeam along its whole course,
as the picture shows.
Fred. That spot on the floor
looks like a picture of the sun.
Jack. It is a picture--an
image--of the sun. It is oval because the sunlight falls slanting on
the floor. But take this
Fig. 74 The sun's rays
travel in straight lines.
(This experiment
should be tried in the schoolroom.)
large sheet of white pasteboard and hold it perpendicular to the sun's
rays and you will get a round image. You can get a picture of the
landscape outside by letting light in through a small hole in the same
way. (See Fig. 75.)
Mary. Well, I'm sure I don't
understand how you can get a picture of out-of-doors by just letting
light through a hole.
pg 98
Jack. It is the easiest thing
in the world to understand when it is explained; but it is not so easy
to understand it when you see it the first time, as you have,
Mary--when it is sprung on you, as the boys say.
Fred. Well, Jack, how is it?
Explain it to us, now that you have sprung it on us.
Jack. It all comes from light
traveling in straight lines. Let us begin with a simple case, and
explain the harder one afterwards.
Fig. 75. A Picture of
the landscape Formed Inside a Dark Room (Camera
obscure) by Light that passes through a very Small Hole--This
experiment can be tried in the school room. The room should be quite
dark. The hole should be pierced in a sheet of cardboard or, better, a
neat hole should be drilled in a sheet of tin.
pg 99
Fig. 76 The light of a candle travels in straight
lines. Until you have
the candle (C) and the two holes (A and B) in the same straight line
you cannot see the flame. (This experiment should be tried in the
schoolroom.)
Fig. 77 Some of the rays from the points of the candle
flame are drawn
in this picture. They fall on a screen (ab) and make it bright. From
every point of the flame there are such rays. And there are many more
than are drawn in the picture.
pg 100
The light of the candle (Fig. 76) travels in straight lines. So does
the light from every brilliant thing. Every point of the sun and every
part of a candle flame is always sending out rays of light, and the
rays go off in every possible direction.
If you take a pincushion shaped like a ball and stick it full of pins
so that the pins stand out all over it everywhere, that might
Fig. 78 In a
dark room a candle shining through a pin hole will
form its own image on a screen.
serve as a model of brilliant point of a candle flame. Every such point
sends out rays of light in every direction--up, down, sidewise. You
"see" by the rays that happen to come your way.
Rays of light from the candle flame go out in every possible direction.
How do you know that, Tom?
pg 101
Tom. Because you can see the
candle flame no matter what part of the room you are in. If you see it,
you must get rays from it.
Jack. Exactly; now most of the
rays from the flame light up the card and the table and the walls of
the room; a few of them--only a few--get through the hole in the card
(Fig. 78). Some ray from the top of the flame gets to the hole, goes
through it, and goes on till it meets the screen of while pasteboard.
There it stops, and there you have an image of the top of the flame.
Some ray from the candle wick gets through the hole and goes on to meet
the screen and, when it meets it, forms an image of the wick. Some rays
from each of the other parts of the flame get through the hole and make
images, so that finally an image of the whole flame is shown on the
screen. The image of the flame is built up of hundreds of little
separate images, you see. (1)
Fig. 79 The image of
candle shining through a pin hole is formed upside
down on a screen, and this drawing shows why.
Agnes. The image of the candle
on the screen is upside down, and so was the picture of the landscape
(Figs. 75 and 78).
Jack. You can see why it was
so from this drawing, Agnes.
Shadows.--"The shadow of any
square or cube is bounded by straight lines (Fig. 78), and this is
another proof that
(1) The reader should lay a straight edge (the edge of
a card will do)
on Fig. 78. He will see that the wick, the hole, and the image of the
wick are in one straight line. Again, the top of the flame, the hole,
and the image of the top of the flame are in one straight line, and so
on.
pg 102
light travels in straight lines. When the point of light is really a
point, or when it is only a small spot (as in the electric street lamp)
the edges of the shadow are sharp; but when the light comes from a
large body like the sun the true shadow (the umbra) is bordered by a
less dark shadow (the penumbra).
If you hold a piece of cardboard in
front of a lighted candle in a dark room, you can see the shadow of the
cardboard on the wall. The shadow is made up of two parts--the dark
center (the umbra) and a less
dark part (the penumbra).
Move the cardboard till it is quite near the wall and you will see the
umbra get dark and sharp and
the penumbra almost
disappear." (1)
Fig. 80 A point of light at A lights half of a globe at
B, and B casts
a shadow. The electric street lamp casts a shadow with sharp
edges as in the picture, because the light of an electric street lamp
comes from a very small spot--a point of light.
Eclipses of the Sun and Moon.--Eclipses
of the sun and moon can be explained by Fig. 81. The globe of the lamp
stands for the sun, the ball B for the earth, the ball C for the moon.
Suppose you were on the earth (B) inside the shadow of the moon. (Take
a pin and point out the place.) The sun would be hidden from you
if you were there; the sun would be eclipsed to you. An eclipse of the sun occurs for any place
on the earth when that place is in the moon's shadow. (See Fig.
51.)
The moon revolves around the earth, you know. Take the little ball C
and suspend it on that side of the ball B which is farthest from the
lamp. It will be in the shadow of the ball B. When the moon is in the
shadow of the earth no light from
(1) This experiment should be tried in the darkened
schoolroom. When
the appearances are thoroughly understood a second candle should be
lighted and the shadows of the two made to overlap.
pg 103
Fig. 81 A schoolroom experiment on shadows. The room
must be dark and
the lamp should have a ground-glass globe. The ball B may be an orange
fastened to the pincushion by a knitting needle. The little ball C (a
small ball of twine) can be suspended by a string so as to cast a
shadow on the globe B. Notice that the ball C has two shadows, a dark
central shadow (the umbra)
and a less dark shadow around it (the
penumbra). The large brilliant
globe of the lamp makes two shadows to
C. (By a little thinking you can see why.)
Fig. 82 A beam of light enters a dark room through a
hole in the wall
(A) and falls on a mirror at B. It is reflected from the mirror upwards
to form a spot on the ceiling at C. by putting a pencil vertically at
B, in the line BD, you will see that the ray of light AB and the ray of
light BC make the same angles with the pencil BD. That is, the angles
ABD and CBD are always equal to each other, no matter where the mirror
may be.
pg 104
the sun can reach it, and it is eclipsed. An eclipse of the moon occurs whenever the
moon is in the shadow of the earth.
Reflection of Light.--Jack. Light that falls on any
surface is reflected from it. That is the way we see the surface. The
sunlight falls on the ground and is reflected up to our eyes, else we
should not see the ground. A feather that is floating in the air
reflects light to us, else we should not see it. The moon floating in
the sky reflects the sunlight to us, else we should not see it.
Fred. The sun sends us its own
light though. We do not see it by reflected light.
Jack. The sun, the stars, an
electric lamp, a candle, have light of their own. They send us light
directly. The moon, the planets, distant mountains and clouds, near-by
houses and rocks and fields, reflect sunlight to us. If you could shut
off the sunlight, you would not see them.
Fig. 83 A ray of sunlight enters a dark room through a
hole in the
wall, and it falls on water contained in a box with glass sides (a box
with one glass side will do). The ray is beat (refracted) as soon as it
enters the water.
Tom. The sunlight is shut off at night (at least it
is shut off from everything on the earth) and you do not see the
mountains and the houses then. (1)
(1) The reasons why you see the moon and the planets
at night are
explained in Book I (Astronomy), page 33.
pg 105
Refraction of Light.--Jack. Light always travels in
straight lines; but when a ray of light that has been traveling along
one straight line in the air enters something different from air--water
or glass, for instance--it is bent (refracted) into another line. This
second line is straight, too; but it is not the same line as the first
one.
Water will bend a ray of light, and so will glass. You know what a
prism is? A glass pendant to a chandelier is a prism, for instance.
If you let sunlight pass through a prism and then fall on a sheet of
paper, you will get a beautiful spectrum of all the colors of the
rainbow. If a plate of glass or a metal mirror is ruled with fine
parallel lines equally distant, say 1000 or 10,000 to an inch, you can
get a beautiful spectrum by laying it out in the sunshine. The colors
of mother-of-pearl are made in this way. The inside of the oyster shell
is made up of very fine parallel ridges, and the light reflected from
them is scattered into a spectrum of colors. You can prove that it is
the ridges that make the colors by taking an impression of the inside
of the mother-of-pearl shell in wax. The wax gives just the same
colors. The scattering of sunlight by raindrops in somewhat the same
way has to do with forming the rainbow.
pg 106
Lenses.--"Pieces of glass or
certain shapes are called lenses.
We use them to make magnifying glasses, spectacles, microscopes,
telescopes. You children had better get some old spectacle glasses and
try experiments with them. (See Figs. 45, 88-90)
Fig. 85 A glass prism is mounted, for convenience, on a
stand; but the experiment can be tried by a prism held in the hand. The
candle flame seen through the prism seems to be in a different place
from the real candle flame, because the rays of light sent out by the
flame are bent by the prism and when they come to the eye they seem to
come from a place where the real candle is not.
pg 107
"Two (or more) lenses used together make a telescope, you know. (1)
Convex lenses concentrate the light that falls on them (Fig. 89), and
concave lenses disperse the light that falls on them. Persons who are
nearsighted use concave lenses in their
spectacles, and persons who are farsighted use convex lenses."
Fig. 86 A beam of sunlight (white light) is separated
by a prism into rays of violet, indigo, blue, green, yellow, orange,
and red, and most of the heat in the beam falls near the red end of the
spectrum. The heat rays are invisible.
Fig. 87. Glass Lenses
of Different Shapes The Three to
the left of the
middle of the picture are convex lenses; the other
three are concave lenses.
(1) See Book I (Astronomy), page 58.
pg 108
Fig. 88 A convex lens
in a dark room will make a sharp
image of a
candle flame on the wall if the lens is at the right
distance.
(The distance to the wall must be different for different
lenses and can be found by trial.)
Fig. 89 A convex lens
concentrates light falling on it
to a focus (at F in the picture.)
Fig. 90 A concave lens
disperses light falling on it.
(The light
comes from F in the picture and is dispersed by the lens.)
pg 109
Fig. 91. A Powerful Microscope The object to be
examined is placed on the stand S and looked at through the long tube.
Light can be thrown on the object by the lens N or by the mirror M. The
right-hand picture shows the way the lenses are arranged in the tube.
The eye is placed near H, and there is one lens there, another at n,
and three others at O (an enlarged picture of these three is given at
L). Such a microscope as this can be arranged so as to magnify about
2000 times--to make things seem 2000 times larger.
pg 110
Velocity of Sound and Light.--The
children were sitting on the porch one afternoon when they heard the
church bell in the village ringing.
Agnes. Listen to the bell! how
plainly you can hear it, and yet it is nearly three miles away.
Mary. Two--three--four. It is
four o'clock. The hammer has just this moment struck the bell.
Fred. You mean the hammer
struck the bell a moment ago, and we have heard it this minute.
Mary. Why do you say that,
Fred?
Fred. You know that you do not
hear the sound of a blow when the blow is struck--not till afterwards.
Haven't you ever seen a gun fired by a man a mile away from you and
then waited to hear the sound?
Mary. Why do you have to wait?
Fred. Why, you know the light
of the flash comes to you instantly--the very minute the gun is fired;
and it takes time for the sound to travel. Let us ask Jack to tell us
how fast sound travels; he is sure to know.
Jack. Light travels almost
infinitely fast; (1) but sound moves much slower--about 1100 feet in a
second. It takes sound nearly five seconds to go a mile.
Mary. Do you mean, Jack, that
we didn't hear the village clock strike till fifteen seconds after it
had really struck?
Jack. Yes; the hammer struck
the bell first and set it vibrating; then the air round the bell began
to vibrate, and the sound began to travel off in every
direction--north, east,
(1) The velocity of light is 186,330 miles in a second
of time. Light
travels from the sun to the earth in 500 seconds, a little more than
eight minutes.
pg 111
south, west. If you had been in the village, you would have heard the
bell the moment it was struck; if you had been a mile away, you would
have heard it five seconds late; and as we are three miles away, we all
heard it about fifteen seconds later.
Fig. 92. A Church
Bell--It is rung by the rope that
you
see on the left-hand side of the picture.
Tom. It is something like
throwing a stone into a pond of water. Little waves travel in every
direction from the place where the stone went into the pond.
pg 112
Jack. Yes; and you remember
that the waves get smaller and smaller the farther they go. Sound is
like that. The vibrations of the air are powerful near the sounding
bell, but they get weaker and weaker as you go away from it.
Tom. So sound is a vibration
is it, Jack?
Jack. There would be no sound
unless there were some vibration in the first place. But there wouldn't
be any sound
Fig. 93 A wave of sound if it were visible, as it is
not, would look something like the picture. Such waves go out from a
sounding bell in every direction. When they come to your ear you hear
the bell, but not before. Sound waves travel about 1100 feet in a
second--a mile in about five seconds.
unless there were some person to hear it. If there were a mechanical
piano playing at the north pole, by machinery, there would be vibration
of the strings--and of the air, too; but unless there were some one to
hear it there would be no sound, only vibration.
Tom. Well, usually there are
persons to hear in our part of the world. Are all the sounds we hear
caused by vibrations?
