KEW mercurial AIR-^UMP. 189 



pendicular glafs tube M N and the glafs fphere P» This fphere Air pump oper- 

 is conne6led with an iron cock Q R, having two oppofite per- cut7 i/a wood* 

 forations, one going horizontally to R, where a fmall outfideen barrel, &c« 

 valve permits a communication with the glafs ball SY; the 

 other, placed perpendicularly, opens a pafiage from the com- 

 mon receiver VT to the fphere. The inftrument being thus 

 conftruded, the box HEFG, is filled with mercury to the dot- 

 ted line a a, or about fix inches, and the pifton being alter- 

 nately preffed down or raifed as ufual by the crank work, the 

 effedl of the machine is as follows: 



The cock Q R being turned as in the figure, the communi- 

 cation with the receiver is fliut, and that with the external air, 

 through a fmall hole S in the glafs fphere S Y, being open, the 

 pifton in defcending prefTes upon the farface a a, and forces the 

 mercury out of its place, it confequently rifes in the fmall fpace 

 between the piflon and the wooden tube, a a to bb, and at the 

 fame time in the glafs tube N P to a correfponding height N. 

 The prefTure being continued, the mercury flill rifes fo as to fill 

 the whole tube, the fphere P, and laft of all makes its way 

 through the cock Q R and the valve R. forcing every particle 

 of the air contained in M N P out before it. When a drop of 

 mercury appears in Y, the piflon is moved the contrary way, 

 and by its afcenfion the prefTure on the mercury is diminifhed, 

 which finks and evacuates the fphere P, leaving a place for the 

 air from the receiver which is admitted by turning the cock 

 Q R, in the other diredlion *. P being filled with air, the * 



cock is turned again, the piflon again defcends, and expels the 

 air, by raifing the mercury a fecond time, and in this manner 

 the exhauflion may be continued at pleafure ; The raechanifm 

 and efFed of this inflrument being, as I prefume, rendered 

 clear by the preceding defcription, I fhall only add a few re- 

 marks. 



The necefTary force to prefs down the piflon, being a little 

 more than the whole weight of a column of mercury of the 

 bafis D C, and the height of the barometer, is confiderable in 

 the beginning of the operation, but diminifhes as in other air- 

 pumps, when the air in the receiver becomes more rarefied ; 

 when the external atmofphere afls more powerfully in fupport 



• The alternate turnings of the cock may eafily be performed 

 by a fimple appendage conne^ed with the motion of the piflon. 



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