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 a thing 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.