[ 352 1 



a part of the natural air contained in the circulating-pipe,^ as 

 the cavities under the plfton are of the content of the whole 

 cyl'i i. e. as above fhewn, to about the 3530th part. Now the 

 crofs fedlion of the channel of the circulating pipe, which is 

 fquare, is only tV^^ °f ^^ inch, and its length 2 1 inches ; its 

 contents are therefore rVo'ot^ P^i'ts of a cubic inch ; of the 

 air in which, only the 3530th part is contained in the cavi- 

 ties under the pifton ; i. e. nearly the 16809th part of one 

 cubic inch, or the 712700th part of 42,+ inches (the content 

 of the cylr) which N°. will therefore exprefs the meafure of 

 air left under the pifton, and confequently the rarefa€lion in 

 the cyl"". 



This will be eafily conceived, if it be confidered, that when 

 the pifton has been raifed to the top, the valve muft at that 

 inftant be open, difcharging the laft of the air pumped out 

 of the rec'' ; the circulating pipe as far as the key, will there- 

 fore be filled with common air; but when by turning the key 

 of the cock, this pipe is opened into the rarefied air or vacuum 

 under the pifton, the parts of the cyl'', both over and under 

 the pifton, are one vacuum conne(5led by this pipe ; and then 

 the valve, not prefled underneath, muft fliut, and no air enters 

 above the defcending pifton, but that which expands itfelf 

 out of the circulating pipe ; fo that there is no air in the 

 ■whole cyK, over or under the pifton, but what had been con- 

 tained 



