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will meafure the preiTure of the air on the furface of the mercury 

 in the veffel ; let A be the altitude of the mercury in the veffel 

 above the upper orifice of the pipe, and P the length of the pipe ; 

 then the whole force preffing downwards the plate of mercury 

 which is immediately in the upper orifice of the pipe, will be 

 = D + A; and the whole force preffing the fame plate upwards 

 will be D — P ; and the difference between thefe forces will be the 

 abfolute force preffing the fame plate of mercury downwards ; 

 while D is greater than P,. this abfolute force will confequently be 

 equal to A + P ; when D = P, D — P vanilhes, and the force 

 preffing the plate downwards is = D + A = P + A; hence therefore 

 no variation in the time of the efflux will be perceived, while the 

 altitude of the mercury in the gage is equal to or lefs than the 

 difference between the length of the pipe and the flandard altitude. 

 When D is lefs than P, the force upwards is alfo nothing ; and 

 therefore, as before, the whole force preffing the plate down- 

 wards is = D+ A; and A being given, it decreafes according as 

 D decreafes ; and when D vanifhes, that is, when the receiver is 

 abfolutely exhaufted, the force becomes equal to A, and the time 

 of the efflux will be the fame, as if the pipe had not been in- 

 ferted in the bottom of the veffel. To try the truth of thefe 

 things by experiment, I inferted a tube 7,8 inches long in a cy- 

 lindrical veffel, and clofing the orifice of the pipe, I filled the 

 veffel with mercury to the height of 6 inches ; then placing the 

 apparatus under the; receiver of an air-pump, when the barometer 

 was at 30 inches, and the gage at 28,5, the time of the efflux 



was 



