If I should ask you why things in the air fall to the ground, you would probably say it is because it is downward, and every thing must come down that is not held up in some way. But what is down, and what is up? This I will explain to you.
How it is known that the earth is round.
The earth, as perhaps you know, is as round as an orange, and people can travel around it just as you can pass your finger around over the orange. This, indeed, was one of the ways in which it was found to be round. Another proof of its being round is this: As you see a ship go out to sea, if you watch it for a long time, after a while the body of the ship will go out of sight, and you will see nothing but the sails, and then the sails will gradually go out of sight also. What does this prove? Why, that the water is not flat, as it appears to be to us, but that it makes a part of the rounded surface of the earth. This figure will make this plain to you. The eye that is represented sees the whole ship atb; but when it gets as far asa, the eye can see only the streamer at the top of the mast.
The reason that we do not see that the earth is round is thatwe are so small and the earth is so large. We see that a globe is round, but it probably seems flat to any little fly that lights upon it, just as the earth does to us.
What is up and what is down.
You can see, then, that as the earth is round, what is down to people on the other side of the earth is up to us. If a boy there throw up a ball at the same time that you throw up one here, the two balls fall toward each other when they come to the ground.
Figure illustrating this.
What we call down, then, is simply toward the ground, or, rather, it is toward the middle of the earth, for we say down in a well or down in the ground. Indeed, if any thing could keep on in the same line in which it falls, it would go right to the centre of the earth. If the ball which you throw up and the ball thrown up by a boy on the other side of the earth should keep on in the ground in the same direction that they fall, they would meet exactly at the earth’s centre. This is represented in this figure. The circle represents the round earth. The lines drawn from the two falling balls to the middle of the circle show how they would come together at the centre of the earth if they could keep on, instead of being stopped when they reach the ground. And all the things that are falling any where on the earth are going toward the same point.
Now why is this? What is it that makes things in the air come to the ground when they are not held up? They do not come down of themselves. They are drawn down. The earthattracts or draws them. How it does this we do not know. We can not see how it is done, but the earth does it as really as if we could see it put up a hand and pull things down.
Attraction not a thing that we can see.
There are other kinds of attraction that operate in a way that we can not see nor understand. There is the attraction of magnetism. If, as I have told you in the last chapter, you bring a magnet toward a piece of iron or steel, for example a needle, when you get it quite near, all at once the needle will go to the magnet and stick to it. You can not see any thing between the magnet and the needle to draw the needle to it. You only know that the needle is drawn or attracted, but you do not know how this is done.
It is just so with this attraction which the earth has for all things, drawing them to it. You can not see any thing any more than you can in the case of the magnet and the needle, but the attraction is as real as if you could see it. You can see what it does, as you can see what is done by the attraction of magnetism.
This attraction is called the attraction of gravitation. It is stronger with some things than it is with others. When any thing is drawn strongly to the earth, we say that it is very heavy; but when a thing is not strongly attracted, we say that it is light. When you take hold of a stone to raise it up, you have this attraction of the earth acting against you. This is pulling the stone down while your muscles are trying to raise it. If the stone is very large, the earth attracts it so strongly that the force of your muscles can not overcome the attraction. If the earth would only stop pulling upon the stone, you could raise it easily enough.
Attraction the cause of weight.
You see, then, what weight is. It is the pressure made by athing as the earth draws or attracts it to itself. The stronger this attraction is, the greater is the pressure—that is, the weight. If you lay a foot-ball upon your foot, you scarcely feel the pressure of it; but if you lay a stone of the same size upon your foot, it presses very hard. The reason is, that the stone is drawn toward the earth much more strongly than the foot-ball. The foot-ball is drawn lightly, and so presses a little; but the stone is drawn much, and so presses a great deal. Your foot, being between the stone and the earth, is pressed by the stone as the earth draws it to itself. It is just as you would be pressed if you were between me and some one that I was drawing toward me.
The reason that the stone is attracted more strongly, or has more weight, than the foot-ball is, that there is more substance to it—that is, the particles in it are closer together. So lead or iron is heavier than wood, because the wood is much more porous: you can see pores and spaces in it, while you can not in the lead and iron. You remember what I told you about the hot-air balloon. This has not as much weight as it would have if it were full of cold air. The reason is, that the particles of cold air are closer together than the particles of hot air; for, you know, heat swells air—that is, it puts its particles farther apart.
If you drop a bag of feathers, it falls to the ground because the earth attracts it. If, now, you drop a stone upon this bag, it sinks down in the midst of it, because the earth attracts it much more strongly than it does the loose feathers. It is for the same reason that a stone sinks in water. The earth attracts the stone more than it does the water.
Why light things rise in the air and in the water.
Wood will not sink in water as the stone does, for it is not drawn down to the earth as hard as the water is; but wood will fall through air to the ground, because the wood is attracted by the earth more strongly than the air is. If you put a block of wood down in the water, and then let it go, it rises to the surface. Why is this? It is because the water, being more strongly drawn down by the earth than the wood, pushes the wood up out of the way. It is for the same reason that the balloon filled with hot air or with light gas rises. It is not attracted to the earth as much as the cool air around it is, and so it is pushed up out of the way.
Every thing, you see, then, is attracted by the earth. The air itself is kept close to the earth by this attraction. It makes a sea, as we may say, all around the earth about forty-five miles deep. Beyond that there is no air except around some of the other worlds that we see far off in the sky. Now the air would fly off and spread every where among the stars if the earth did not attract it and thus keep it around itself. The air moves about freely like the water, but it can not fly away from the earth any more than the water can. The earth keeps both its air and water all to itself by attraction.
Every thing tries to get as close to the earth as possible.
Every thing gets as close to the earth as it can, because every thing is attracted by the earth. There is nothing that is of itself disposed to go up, but every thing, even the air, is pressing down, the heaviest always getting the lowest if it can, and there is sometimes a sort of strife as to which shall be lowest. When a stone is put upon a heap of feathers, the earth pulls upon it so much harder than it does on the feathers that the stone presses to getthrough them to the earth; but as it can not thrust them out of the way, it crushes them down in the struggle to get below them. The struggle is a different one with the stone in water. The water clings to the earth, but it is easily pushed away by the stone as it tries to get below the water. Even in the going up of a balloon you can see the same struggle. It would stay down if it could. It goes up, as I have before told you, simply because the cold air about it, being more strongly attracted by the earth than the balloon is, tries to get below the balloon. If the cold air could be taken away, the balloon would stay down, for the same reason that a block of wood would remain in the bottom of a bowl if there were no water in it. The block, attracted by the earth, will stay as near the earth as it can. The water pushes it up because it is attracted by the earth more than the block is, and for the same reason the air pushes up the balloon.
Questions.—What is the common idea about the falling of things to the ground? What is one of the proofs that the earth is round? What is another proof? Why can not we see that the earth is round? What is meant by down and up? Tell what is represented by the figure. What is it that makes things fall to the ground? Give the comparison about the attraction of magnetism. What is said about the earth’s attracting some things more strongly than others? What is weight? Explain by telling about the foot-ball and the stone. Why is the stone attracted more strongly than the foot-ball? Why are lead and iron heavier than wood? Why is a hot-air balloon lighter than the air around it? Tell about the feathers and the stone. Why will not wood sink in water as stone does? Give the comparison between the block of wood and the balloon. What is said about the earth’s attracting the air? Is there any thing that does not press down? Which always gets the lowest if it can? Tell about the stone put on the feathers and dropped in the water. Give the comparison between the balloon and the block of wood in a bowl.
Questions.—What is the common idea about the falling of things to the ground? What is one of the proofs that the earth is round? What is another proof? Why can not we see that the earth is round? What is meant by down and up? Tell what is represented by the figure. What is it that makes things fall to the ground? Give the comparison about the attraction of magnetism. What is said about the earth’s attracting some things more strongly than others? What is weight? Explain by telling about the foot-ball and the stone. Why is the stone attracted more strongly than the foot-ball? Why are lead and iron heavier than wood? Why is a hot-air balloon lighter than the air around it? Tell about the feathers and the stone. Why will not wood sink in water as stone does? Give the comparison between the block of wood and the balloon. What is said about the earth’s attracting the air? Is there any thing that does not press down? Which always gets the lowest if it can? Tell about the stone put on the feathers and dropped in the water. Give the comparison between the balloon and the block of wood in a bowl.