Elasticity of the air.
It is this springiness of the air, called its elasticity, that makes the foot-ball bound so. If the ball were filled with water instead of air, it would not bound at all, because the water has no elasticity.
Operation of the air-gun explained.
I have told you that the more the air is pressed the greater is its springiness. In what is called the air-gun, a great deal of air is crowded into a very small space—much more than there is in a pop-gun; and a bullet can therefore be fired from it with force enough to go through a board. It is done in this way: The pressed air is shut up tight, and all at once it is let into the barrel of the gun where the bullet is. It throws the bullet out just in the same way that the gas of the powder does in a common gun. This air-gun is only a curiosity. It will never come into use, for it is quite a tedious operation to load it with pressed air. The common gun, you know, is very easily loaded with powder, and the gas which it turns into does the work even better than the pressed air in the air-gun.
Questions.—What makes the cork fly out of the pop-gun? Explain just how the pop-gun operates. Tell how the quill pop-gun is made. Why do we have the potato in both ends of the quill? What is said about the springiness of the air? How would the pop-gun work if it were filled with water? Why is this? Give the experiment with the pop-gun showing how springy the air is. How is it when you try the same experiment with the gun filled with water? Give the comparison between your pop-gun and a cannon. What is said about the foot-ball? Tell about the air-gun. Why is this not in common use?
CHAPTER XI.
BALLOONS AND BUBBLES.
What makes the balloon go up?
What is it that makes a balloon go up in the air? It is because it is so light, you will say; but what it is made of is not as light as air is. It will not, you know, fly off into the air before it is filled. It is what it is filled with, then, that makes it so light.
Its car.