200 



THE ATMOSPHERE. 



Since the piston is sustained in the position represented in fig. 6, it follows 

 that the elasticity of the air beneath it is equivalent to the weight of the piston, 

 A B ; and, therefore, that the air included in the cylinder acquires double its 

 original elasticity when it is compressed into half its original bulk. 



Let the piston be now loaded with three times its original weight, or 48 

 ounces ; it will be observed to descend into the cylinder, and further to com- 

 press the air, until its distance from the bottom is reduced to 4 inches. At 

 that distance it will rest, being balanced by the increased elasticity of the air : 

 this air is now compressed into one third of its original bulk, and it has three 

 times its original elastic force. 



In the same manner, in whatever proportion the weight of the piston be 

 augmented, in the same proportion will the distance from the bottom at which 

 it will rest in equilibrium be diminished, and, consequently, the elastic force of 

 the air is increased in the same proportion as the space into which it is com- 

 pressed is diminished. 



Let us, again, suppose the piston to be loaded with sixteen ounces, and to 

 be balanced, as in fig. 5, by the resistance of the air at 12 inches from the bot- 

 tom of the cylinder. But let us also suppose the cylinder continued upward 

 to a height exceeding 24 inches ; let the weight upon the piston be now re- 

 duced to eight ounces. Since the elasticity of the air beneath the piston was 

 capable of supporting sixteen ounces, it will now prevail against the dimin- 

 ished pressure of eight ounces. The piston will continue to rise in the cylin- 

 der until the elasticity of the air is so far diminished by expansion that it is 

 capable of supporting no more than eight ounces ; the piston will then remain 

 in equilibrium. If the height of the piston above the bottom be now measured, 

 it will be found to be 24 inches, that is, double its former height ; the air has, 

 therefore, expanded to double its former dimensions, and is reduced to half its 

 former elasticity. 



In like manner it may be shown that if the weight upon the piston were re- 

 duced to four ounces, or a fourth of its original amount, the piston Avould rise to four 

 times its original height, or 48 inches, before it would be capable of balancing 

 the reduced elasticity of the air. Thus, by expanding to four times its primi- 

 tive dimensions, the elasticity of the air is reduced to one fourth of its primitive 

 amount. 



By like experiments, it is easy to see how the general law may be estab- 

 lished. In whatever proportion the weight of the piston may be increased or 

 diminished, in the same proportion exactly will the space filled by the air 

 which balances it be diminished or increased. 



The preceding illustration has been selected with a view rather to make the ) 

 property itself intelligible, than as a practical experimental proof of it. The { 

 use of pistons moveable in cylinders is attended with inconvenience in cases ) 

 of this kind, arising from the effects of friction, and the difficulties of making \ 

 due allowance for them. There is, however, another method of bringing the ) 

 law to the test of experiment, which is not less direct, and is more satisfactory. { 



Let A B C D, fig. 7, be a glass tube curved at one end, B C, and having ( 

 the short leg, C D, furnished with a stop-cock at its extremity ; let the leg B A ^ 

 be more than 60 inches in length. The stop-cock D being opened so as to ? 

 allow a free communication with the air, and the mouth A of the longer leg 

 being also open, let as much mercury be poured into the tube as will till the 

 curved part B C, and rise to a small height in each leg. By the principles of 

 hydrostatics, the surfaces of the mercury E and F will stand at the same level. 

 Let the stop-cock D be now closed, the levels E F will still remain undis- 

 turbed. When the stop-cock D was opened, the surface F sustained a pres- 

 sure equal to the weight of a column of air continued from F upward as far as 



