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■and fo likcwife mufl be ultimately that in the reCr, let its capacity- 

 be what it may. In this machine, the air under the pifton 

 being conveyed away by the circulating-pipe, before the com- 

 munication of the cyl' with the rec' is opened, it cannot get 

 into the rec', and the rarefaftion in the cyl"^ being as it were 

 indefinitely great, equal as above-mentioned to iy6ooo, that 

 in the rcc' may be indefinitely encreafed within this limit ; 

 whereas the intruded and unextradled air under the pifton 

 (were there any) would get into the rec', and the part of 

 it which could be pumped out, would depend on the pro- 

 portion between the cyl' and rec", jufl as in,. the cafe of air 

 admitted through leaks in the machine. 



It is not therefore here pretended, that the rarefadlion could 

 be carried to an unlimited degree, even on the principles of 

 the pump's mechanifm • and yet perhaps it may be prefumed 

 that, even on thefe, its conflrudion is fuperior even to that of 

 Mr. Cuthbertfon, the only one which pretends to a power of 

 indefinite exhauftion : and as it is only from a belief of 

 this, that the machine here defcribed is publifhed, fo I may 

 be permitted to obferve, (having had no opportunity of afcer- 

 talning by trial the excellence of his, though I believe it is 

 excellent ; nor of feeing any defcription of it, but that in the 

 Encyclopedia Britannica, juft publifhed here) that I conceive 

 its perfeclion mull depend on the fuppofed circumftance of 



no 



