r 



_ [ 343 J 



barrel with the circulating pipe opened by the key of the cock, 

 and the pifton raifed to the top ; the external air then prefTcs 

 through this pipe into the bottom of the barrel, which it fills : 

 on opening the r^c'' pipe by turning the key (which flouts the 

 other pipe) and depreffing the pifton, the air under it is forced 

 into the rec''. The cock being again turned while the pifton 

 is kept at the bottom, and this again elevated, the external 

 air fills the barrel by the circulating pipe as before, and this 

 on turning the key and depreffing the pifton, as in the for- 

 mer ftroke, is forced into the rec"; and the condenfation is in 

 this manner encreafed by a repetition of the ftrokes ; the de- 

 gree to which it is carried being eafily known, even without 

 a gage, by knowing the proportion between the capacity of the 

 barrel and rec'', which the operator Ihould, whether he is ex- 

 haufting or condenfing, be always certainly informed of, as 

 otherwife he cannot tell how many ftrokes are requifite to 

 produce a given degree of rarefadlion or condenfation ; in which 

 latter operation the full of the barrel of air of the natural 

 denfity, is thrown into the rec"' at every ftroke ; a dangerous 

 operation with a glafs rec"", efpecially when large, unlefs it is 

 clofely grafped in a cage of metal rods. 



To let air fuddenly into the exhaufted rec"', there is a pin 

 fattened in the end of the reC du6t (at N, fig. i .) with cement, 

 by pulling out or loofening which, the air is admitted.. 



I HAVE 



