"And that is not a good way to find out any thing, Uncle Philip?"
"No, boys; though some persons much older than you are, did nothing but guess about this very thing, and guessed very far from the truth too. Some thought that the fly had a sponge in its foot, and squeezed a sort of glue out of it which made it stick fast; others said that the glass or wall was so rough that the fly's feet would catch hold of the little points upon it; but both were wrong."
"How does it hold on, Uncle Philip?"
"Did you ever see what the boys call a sucker, made of a piece of soft sole leather? That will show you how the fly's foot sticks fast. This leather is cut round, and has a string through the centre; the boys wet it, and then put it upon a board or something smooth, and stamp on it, and try to raise it up from the board by the string; and it requires some strength to pull it up: sometimes they put it on a small smooth stone, and then lift up the stone by it. The reason why the leather sticks so fast is because the air is pressing on it upon the outside, and there is very little or no air between it and the board, to press the other way."
"Why, Uncle Philip, is the air heavy?"
"Oh yes, boys, when there is so much of it as there is above the earth, it presses down very heavily. Now the fly's foot is like the sucker; when he puts it down he has a contrivance to drive out the air from under it, so that there will be little or none between it and the wall; and then the outer air presses upon it, and holds it fast."
"But, Uncle Philip, how does he get it up again?"
"Why, boys, by another contrivance, he can let air in under his foot again, and then he can easily move it; for we do not feel the weight of air when it presses upon both sides of us. The reason why you stand up straight is because the air is pressing all around you; if it were on one side of you only, it would press you down on the other side. Here is a picture of the fly's foot, as it appears through the microscope. You will see it has three suckers with the edges all like saws; these are to make it stick the closer. This picture, boys, is sixty-four hundred times as large as the fly's foot is."
