I will mention only a few of the experiments that may be tried with the air-pump. If you put an India-rubber bag, or a foot-ball, with but a little air in it, under the glass jar, when you begin to pump this will begin to swell, as represented here; and if you pump for some time, it will swell very much. The reason is this. As you take away the air from around the ball, the air in the ball expands. If you then turn the screw that lets the air into the jar, the ball will become small again, because it is pressed upon by the air that is let in.
Bubbles.
So, too, if some soap-bubbles be put under the jar, when you pump out the air they will swell; that is, the air shut up in the bubbles will expand, because the pressure of the air around them is lessened.
Shriveled apples.
It is amusing to see a shriveled apple under the jar of the air-pump. After pumping a little it will swell out, and appear like a plump, fresh apple; but let in the air again, and the apple becomes shriveled as before. This is owing to the air that is in the apple, for there is air in every thing. There is air in our bodies; and if the air all about us could be lessened very much, just as it is in the jar of the air-pump, we should swell up like a puff-ball. It is the pressure of the air all around us that keeps us just of the size we are.
The degree of pressure of the air.
The air does more in pressing than you think for. As you move about in it, it does not seem to press upon you at all; but it really presses upon you very hard. It presses on you with the weight of about fifteen pounds upon every square inch—that is, a space of this size. It would take many such spaces to cover over your hand. The air really presses upon your hand, as you hold it out flat, with more than the weight of a hundred pounds. You can hardly believe this, and you will want to know how it is that you do not feel this weight or pressure of the air. I will tell you.
