[ 341 ] 



The a£lion of this pump in exhaufting is as follows : The 

 pjfton being at the bottom, and the key of the cock turned to 

 its limit on that fide by which the communication is open be- 

 tween the barrel and rec', and confequently that with the cir- 

 culating pipe clofed, the piflon being then raifed to the top of 

 the barrel, difcharges the air in it through the valve, and that 

 in the rec' ru(hes into the barrel ; the key being now turned 

 to its limit on the other fide, the communication with the rec' 

 is clofed, and that with the circulating pipe opened ; while it 

 is kept fo, the pifton is depreffed, and drives the air (which had 

 pafled from the rec' into the barrel, and was left there on 

 turning the key, and fo clofing the communication between it 

 and the rec'j into the circulating pipe, and through it into the 

 top of the barrel : the pifton being down to the bottom, and 

 the key again turned to open the paffage to the rec' and to 

 clofe that with the circulating pipe ; if the pifton be again 

 raifed, the air above it will, as before, be thrown out by the 

 valve, which air is no other than what in the former ftroke, 

 had come out of the rec' ; for the valve at the top clofing, as 

 foon as the pifton began to defcend, excluded all entrance of the 

 external air into the barrel. As the pifton rifes, more air will 

 pafs from the rec' into the barrel under it, which on turning 

 the key, and depreffing the pifton, is by the circulating pipe 

 conveyed to the top of the barrel, to be pumped out through 

 the valve on raifing the pifton as before. Thus the ftrokes be- 

 ing. 



