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be more apt to imbibe the air into its pores when they were 



agitated together by the carriage. If, on this account, it be 



thought beft to prevent fuch agitation, it may be done more 



eafily than by any of the contrivances I have met witli ; for, 



when the gage-hole is flopped, invert the inftrument, unfcrew 



the bottom of the box, and put in a piece of cork that may 



fill the fpace which was occupied by the air, and the cover 



being fcrewed on again will keep all tight. The cork having 



a thread put through it will be eafily removed, and it ought to 



go into the box fo eafily as to let the air pafs out by its fides. 



I have not met with any experiments made to fhew what 



quantity of air mercury will abforb after being well purged 



of air. An experiment for this purpofe may be conveniently 



and accurately made in the following manner : As foon as 



a portable barometer is filled with mercury, well purged of air, 



let it be hung up along with a thermometer in a cool place, 



where the temperature of the air is not like to vary ; and, 



when the mercury has attained the temperature of the place, 



fiiut the box of the barometer and mark the height at 



which the mercury then ftands. On this occafion not more 



than y'- of an inch in length fhould be left for the air in the 



box. When the barometer has remained in this fituation for 



fome time (during which the mercury in it fhould be now and 



then agitated), if it has imbibed any proportion of the included 



air, fuppofe y'-, the air will then have loft Vo of '^s elafticity, 



and confcquently the column of mercury fuftained will lofe 



-•- of its height, or will have defcended in the tube about three 



Inches. Thus the defcent of the mercury will fhew accurately 



the proportion of the air that has been abforbed. 



As 



