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in the rec'' would indefinitely approximate to a vacuum ; and 

 of confequence, 



2dly, Th,\t the degree of rarefadlion in the reC can nev-er 

 exceed fuch rarefadlion in the cyl' : from both which it fol- 

 lows, 



3dly, That the moft perfcd pump, c: p^ is that in which 

 the moft perfed vacuum is made under the elevated pifton, 

 in the cyl' unconnedled with any thing elfe. 



The firft pofition is in fadl felf-evident, if it be admitted 

 that the elafticity of the air is as its denlity (and I have reafon 

 to believe it has elaftic force to expand itfelf 30,000 times;) 

 for thus, whilever there remains any air, it will have power 

 to diffufe itfelf out of the rec^ into the cyl' (if there be no 

 valve to raife, nor any thing to oppofe its paflage) fo long as 

 there is lefs air in the latter than in the former. 



The fecond pofition (or confequence rather) is no lefs evi- 

 dent ; to fuppofe the contrary, is to fuppofe that a lefs elaftic 

 force could overcome a greater, or that the rarer air in the rec" 

 could rufh into the denfer in the cyl', which is fo manifeftly 

 abfurd, that it is furprizing the firft idea of an air-pump 

 which would ftrike any one, fliould not be that which firft 

 occurred to me, when I thought of making one ; that no air 



X X X fhould 



