How this is borne.
Hold out your hand flat in the air. You know that there is air underneath your hand as well as over it. And this air underneath presses up just as much as that above presses down. Now this is the reason that you do not feel the pressure. If the air underneath your hand could be taken away, you would feel the pressure of that which is above. You would not only feel it, but you could not bear it. This we can prove by the air-pump. Take the jar off from the plate, and then put upon it a small glass vessel, open at both ends, such as you see here. Place your hand over it tightly as represented, and then let some one work the pump. Your hand will be pressed down into the cup so hard after a little pumping that you will be glad enough to have the pump stopped and the air let in.
Observe what is done to your hand by the pumping. Some of the air is taken away from beneath your hand—that is all; and, this being done, you feel now the pressure of the air above it, because there is no pressure below to balance it.
You can show the same in another way with this glass cup. Tie a piece of bladder or India-rubber over one end of it, and then place this over the hole in the plate of the air-pump. As you pump out the air, the India-rubber will be pressed down into the cup by the air above, as represented here.
How the boy’s sucker operates.
The pressure of the air is very well shown by the sucker, as it is called, with which boys sometimes amuse themselves. This sucker is a round piece of leather, with a string fastened to the middle of it. The leather is moistened, and then pressed evenly upon the smooth surface of a stone, and now the stone can be raised, as you see here, by the string, even if it be a pretty large one. But how is it that the leather sticks so fast to the stone? It is by the pressure of the air upon it. When you pull on the string, you raise the middle of the leather a little from the stone, and this makes a little space there in which there is no air. But all the leather around by its edge is pressed very tight upon the stone by the air outside; and it is because no air can get between the leather and the stone that the leather holds on to it so well. If the leather is not pressed down exactly even, or if there be some unevenness in the stone where the leather is put upon it, the air will get in between the leather and the stone, and the sucker will not operate.
Suckers in the feet of flies.