18 



the bottom of its barrel, H is depressed below O by a cleat fixed to 

 the rack which moves P, and thus a free communication is es- 

 tablished between the receiver and the passage E. As before, 

 suppose P at the top of B ; H is also at the top of G ; on depress- 

 ing P, while the space below it discharges its air at C, that above 

 it is left exhausted, for the valve I cannot be opened by the rarified 

 air. At last the cleat already mentioned forces down L, and the 

 air in the receiver expands into the barrel : on raising the piston, 

 H rises to its place and shuts off the communication ; and the air 

 in the upper part continues to be condensed by the ascent of the 

 piston till it passes F, when a free passage is opened between the 

 two portions of the barrel, and the air passes from the upper to the 

 lower, from which it is discharged at the descent of P. 



The valves C, D, and I are leather ; and the piston H is in my 

 opinion preferable to any valve both for the ease with which it can 

 be made and the security of its action : if any part may seem ob- 

 jectionable, it is the narrow passage E F, but the trials wiiich I' 

 ma?le of Mr. Kiernan's pump, in which the piston passed a small 

 aperture, shiw that no inconvenience can result from it. 



If a still higher exhaustion be required, screwing an exhausted 

 flask on C we have 



in which the second term is altogether insensible, and this mode of 

 proceeding secures the valve C from leakage, thus enabling the 

 machine to exert its full power. 



To exhibit the comparative values of the results obtained above^ 



* 1 

 suppose V =0. 2, A = 28. 8, and ^=3^0 which is easily attainable 



