[ 34S ] 



-'iliould be lefc under the piftoa when put down; yet this was 

 certainly not adverted to by Mr. Boyle, nor (which indeed *I 

 admire) by Doiflor Hooke, who affil-Led in contriving for him 

 the firfl air-puttip; for Mr. Boyle complains that he could not, 

 tifing rcC'S of any fize, make the mere" fink in his barom"- 

 gage to the level of that in the ciftern, and he attributes this to 

 ;air cafually intruded, when he might have feen that he left 

 it behind in his pump at every ftroke ; for it is manifefl from 

 the defcripcion and figure given of it, that a confidcrable fpace 

 was left between the bottom of the cyl' and the cock ; in 

 which air of the natural denfity would reft, and for ever limit the 

 degree of exhauftion ; now had this been corredled, his air-pump 

 would exhauft as perfedlly as any fince made, as will appear 

 hereafter. All who have not attended to this circumfl:ance ', 

 I fuppofe, have imagined, that the air left under the piflion 

 could after pafling into the rec be extracted from it again ; 

 whereas on the contrary, if we fuppofe a perfe(5l vacuum in 

 the rec% and an imperfedl one in the cyl', from air left be- 

 hind at every ftroke and difFufed in it, this air would at each 

 ftroke get into the rec', juft for the fame reafon that in op- 

 pofite circumftances, it paflTes out of the rec"^ into the cyl'', 

 viz. that it is rarer in the latter : to expedl the contrary, is 

 I to expect it would miftake the way it fhould go. 



As 



M T. Smeaton was too fagacious not to attend to-it. 



