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 get through 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.
CHAPTER XXXIII.
THE MOTION OF THE EARTH.
Why a ball thrown up comes down.
When a boy throws a ball up into the air, he thinks that it comes down of itself. He thinks that it comes down merely because the force with which he sent it up is spent or lost; but this is not so. It is pulled down. The earth pulls it down. The earth is pulling upon it all the time as it goes up, and gradually overcomes the force with which he threw it up.
There is another thing that helps to overcome the force by which the ball is sent up. It is the resistance of the air. As the ball goes up, it has to spend a part of its force in pushing the air away to make a path for itself.
These two things—the pulling of the earth and the resistance of the air—gradually stop the going up of the ball. If there was no air, and if the earth would let the ball go, instead of drawing upon it, it would not come down. It would fly off out of sight; and more than that, it would never stop till something stopped it. It could never stop of itself.
This, perhaps, seems strange to you; but look at it. A ball, you know, has no power. It lies still if you do not touch it. It can not move itself, and, for the same reason, it can not stop itself. Once set it agoing, and it would move on forever if it was not stopped by something.