ELASTICITY OF AIR. 



57 



Let A, B, fig. 12, be a cylinder furnished with a piston P, which moves air- 

 tight in it. Let C be a tube proceeding from the bottom, and furnished with a 

 stop-cock. Let us suppose this tube to communicate with the receiver or 

 vessel R, in which it is intended to condense the air. Let another tube, D, 

 proceed from the cylinder, also furnished with a stop-cock. Let the piston be 

 now drawn to the top of the cylinder, both stop-cocks being open. The re- 

 ceiver R, being in free communication with the atmosphere, will contain air 

 of the same density and pressure as the external atmosphere. Let the stop-cock 

 D be now closed, and let the piston be pressed to the bottom of the cylinder ; 

 the air confined in the cylinder below the piston will thus be forced through 

 the tube C into the vessel R, while the piston is pressed against the bottom B. 

 Let the stop-cock C be closed so as to prevent the escape of the air from the 



Fig. 12. 



vessel R, and let the stop-cock D be opened, so as to allow a free communica- 

 tion between the cylinder A, B, and the external atmosphere. Let the piston 

 be again drawn to the top of the cylinder. The cylinder will then be filled 

 with atmospheric air of the same density as the external atmosphere. Let the 

 stop-cock D be closed, and C opened, and let the piston be once more forced 

 to the bottom of the cylinder ; the contents of the cylinder will be thus again 

 discharged and forced into the receiver R. Let the stop-cock C be again 

 closed, and let the process be repeated. It is evident that at each stroke of 

 the piston a volume of atmospheric air will be forced into the receiver equal to 

 the dimensions of the cylinder A, B, and there is no limit to the degree of con- 

 densation, except that which depends on the strength of the receiver R, and 

 the cylinder and tubes, and on the power by which the piston is urged. 



After each stroke of the piston, the density of the air in R is increased by 

 the admission of as much atmospheric air as fills the cylinder A, B, and there- 

 fore the density, as the process advances, receives equal increments at each 

 stroke of the piston. Let us suppose that the receiver R has ten times the 

 capacity of the cylinder A, B, and let us suppose that the elastic pressure of 

 the air in R, at the commencement of the operation is expressed by the number 

 10. After the first stroke this pressure will be expressed by the number 11, 



