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.


Fig. 62 The U-shaped tube is partly filled with water as in the right-hand picture. Air fills the rest of both branches of the tube. Now tip the tube so that one branch of the tube shall be completely filled with water--water on one side, air on the other. Then cover the water side with your finger, as in the left-hand picture. You will see that the water will still stand at different heights on the two sides. There is air pressure on one side of the tube and no air pressure on the other (your finger keeps the air out). The air pressure keeps the water standing high. If you take your finger away and let the air in, the water on both sides of the tube will stand at the same level on both sides. (This experiment should be tried in the classroom.)

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


Fig. 67.--A map made by a Weather Bureau one November morning. An area of low barometer was near Omaha and it was moving towards Canada (in the direction of the curved arrow). Wherever there are little dots observations have been taken and telegraphed to Washington. The arrows through the dots fly with the wind's motion at each place. Where the dots are black it is raining; where they are square it is snowing; where they are circles with white centers it was cloudy. The full lines ( ____ ) join all places where the barometer was at the same height, as 30 4/10, 30 2/10, 30, 29 8/10 inches. The dotted lines (..........) join all places where the thermometer stood the same, as 70 degrees, 60 degrees, 50 degrees, 40 degrees, 30 degrees, 20 degrees, 10 degrees, 0 degrees. There was zero weather near the Rocky Mountains, while it was warm and cloudy east of the Alleghenies. Northwest of the area of low barometer there was snow; southeast of it there was rain.

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?



Previous Page | Next