IW Description of an Air -Pump. 



A, when it is stopped by the shoulder X In its progress,, 

 a vacuum is formed in the barrel, until it arrives at Ay 

 and opens a communication between the barrel and re- 

 ceiver, when the air, by its elastic force, rushes into the 

 barrel and fills it. All the back space between the collar 

 O and the piston 3f, both in its advance and recess, is to 

 be considered as making a part of the capacity of the 

 rsceiver. But to speak more accurately, it is only the 

 space OA which is a real addition to the capacity of the 

 receiver ; the space AAI, between the duct and the pis- 

 ton, whilst it moves forward towards JB, being only a 

 temporary dilatation of the capacity and the small vacuity 

 between the collar and the piston, after it has passed the 

 duct^ in moving backward, being a temporary contrac- 

 tion of it : In both cases it is restored to its original ex- 

 tent, on the return of the piston. Having attained to its 

 limit in moving back, the piston now proceeds forward, 

 and after it has passed by the duct A, intercepts the 

 communication between the receiver and barrel, and 

 drives before it the air contained in the baiTcl, together 

 with the short piston jY, until it is stopped by the shoul- 

 der X, at the end of the bar HS^ at the instant that it ar- 

 rives against the middle of the duct £, at which the air 

 is all forced out. The two pistons now form one air- 

 tight stopper, that completely closes the orifice £, and 

 prevents any communication of the receiver and barrel 

 with the external air. Now the piston PFM, is drawn 

 back towards T, and the short piston JVH, by the force 

 of the springs Z), ii", F, follows in close contact with it, 

 and thus continues to interpose a barrier between the 

 barrel and external air, until it is stopped by the meeting 

 of the bolthead H w ith the slide G, after having passed 

 the duct B, and having arrived at the situation JV, where 

 the description began. 



When the pump is intended to exhaust, the receiver 

 must be placed over the duct A, leaving the duct B open 

 to the external air ; but when it is designed to condense, 

 nothing more is necessary than to shift the situation of 

 the receiver on the plate, placing it over the duct B, and 

 leaving the duct A open to the external aii\ 



