178 CONVERSATIONS ON 



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 re- 

 quires 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 con- 

 trivance 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 



