"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." [Illustration: Fly's foot magnified.] "But, Uncle Philip, there is one thing yet hard to understand." "What is it?" "Why, the fly walks on the wall over our heads; now the air cannot press down upon his feet there." "Very true, boys: it cannot press _down_, but it can and does press _up_ against his feet; for the air presses up and down and sidewise all alike." "Ah, now it is plain enough, and we are much obliged to you, Uncle Philip, for telling us what we wished to know." "You are quite welcome, my dear boys, to all that I can teach you: if it makes you to be wiser and better men when you grow up, I shall be very thankful to God that I have been able to do you any good." "Good morning, Uncle Philip." "Good day, boys; I shall expect to see you all in church to-morrow." "We shall be there, Uncle Philip." CONVERSATION XV. _Uncle Philip tells the Boys how Hats are made; and then talks to them about Animals that can make Felt like the Hatter._ "Boys, do you remember my telling you of a remarkable bird, called the tailor-bird, which sews very neatly?" "Oh yes, Uncle Philip; it is not easy to forget such an excellent little workman; but why do you ask--have you any thing more to tell us about that bird?"