You see some boys playing foot-ball. What is it that they are kicking about? It is an India-rubber ball, you will say. But is this all? Is there not something else besides the India-rubber? Suppose that you prick a hole in the ball. It is good for nothing now; but the India-rubber is all there. What makes it good for nothing? It is because the air escapes from the hole. The ball is of no use unless you can keep it full of that thing that we call air; and in playing with it, you kick about air locked up in the India-rubber.
Life-preservers.
You have heard of life-preservers, and perhaps you have seen them. They are India-rubber bags that you can fill with air by blowing into them. They are made of such a shape that they can be tied around the body. When used in this way, a life-preserver will keep one from sinking in water. But why? It is the air in it that does this. The air is as really a thing as the water is, but it is a lighter thing, and therefore a thing full of air will float on the water. If you kick a foot-ball into the water, it will float, because it is full of that light thing—air. But if you should prick a hole in it, and press out the air, and then throw it into the water, it would sink. So, too, the life-preserver would do no good if you tie it around you without blowing it up. It is the air that you blow into it that buoys you up in the water.
Boats.
Why does a boat float on the water? It is not because the boat itself is lighter than the water is. It is commonly heavier, because there is so much iron about it. The reason that it floats is that it is full of air. Even a boat made entirely of iron will float for the same reason. But if there should be a leak, so that the boat can be filled with water, it will sink. So, too, it will sink if you put too much weight in it.
Life boats.
How life-boats are made.
You have heard of life-boats. These are made in such a way that they will not sink, even if they are filled with water. How do you think that they are made to be so much lighter than other boats? It is not because they are built of different materials. They are made of wood, and are fastened together in every part with iron. Sometimes they are made entirely of iron. But they are built in a different way from common boats. They are made double, and in such a way that there are chambers of air between the two parts. These chambers are air-tight. If they were not they would do no good. If there were any opening into these chambers, the water would go in and force out the air. The boat would no longer be a life-boat. It would be of no more use than a life-preserver with no air in it, or with water instead of air.
We can feel air, but can not see it.
You can not see air, although it is a thing; but you can sometimes feel it. You can not feel it while it is still, as you can such things as a table or water. You can only feel it when it is in motion. When the wind blows upon you, it is air in motion that you feel. When there is a gust of wind, as we say, the air comes against you just as a wave of water does. When you fan yourself, you make the air strike upon your face, and you feel it as you feel any thing else that strikes you, as water or a stick.