[ m ] 



ftroke, beings the fame, its quantity mufl at length be diredlly 

 as the recr ; and the quantity remaining after each ftroke, as 

 the capacity of the rec' to that of the cyl"; confequently the 

 rarefa(5lion the reciprocal of this. 



Hence appears the mifchief of leaving any air under the 

 depreffed pifton ; for the whole of it muji pafs into tb: rec 

 at every ftroke ; and if the rec,- be fmall, the greater will be 

 its denfity there; but if the rec' be large, there muft be more 

 ftrokes to exhauft it, and fo more additions to the * refiduai 

 air in it ; alfo lefs of it can be pumped out again : however, 

 though no air were left under the pifion, yet if air intruded 

 from the machine's being leaky, its power in extracting fuch 

 furreptitious air, would not at all correfpond with the fame 

 in extra<fting the primitive air in the rec' ; for with refpecSl 

 to this latter, the greateft rarefadlion in the cyl'' itfelf is given, 

 Vol. VI. 3 B and. 



graduated ; firft, either by a continual bifedlion of the divifions towards the top 

 >^ (which will be always one extremity of the interval bifefted) for fuch bife£lion, as 

 it halves each length of the fegment, muft double the rarefadtion correfponding to 

 the lower extremity of the portion bifecled, thefe portions being inverfely as their 

 correfponding rarefa£lions ; and fo the leaft rarefaftion (of looo) would be conti- 

 nually doubled ; or, fecondly, according to the above-mentioned analogy, by dividing 

 the produil of the leaft rarefaflion, multiplied by the length of the fegment (viz. 

 in the above cafe of 985 by 141 ; or of 1000, by 138,88, which is the fame) by 

 any greater Mi's expreffing fuch rarefadlion as one would chufe to marlc on the 

 fcale ; for inftance, by 1500, 2500, &c. : then the quotients will be tlie lengths - 

 from the top, at which thcfe rarefadlions Ihould be marked. The lower or wider.- 

 fegment of the ftem is to be feparately graduated in the very fame manner. 



