216 Prof. Regnault on the Determination of 



y the barometric pressure, also unknown. 



V the capacity at 0° of the tube No. 1 of the barometric 

 system. 



V the capacity at 0° of the tube No. 1 of the thermometric 

 system. 



The mercury being brought to the level a of the tube a b under 

 the pressure of the atmosphere H, a thermometer placed at the 

 side of a b indicating the temperature t, the tube No. 1 of the 

 barometric system is fitted on the manometer. This tube is 

 surrounded by melting ice; mercury is allowed to flow out by the 

 stopcock to expand conveniently the air of the space v ; it is 

 thus seen whether the junction is tight; lastly, the stopcock of 

 the tube No. 1 is opened. By pouring mercury into the mano- 

 meter the level is brought to a, and the difference h of the two 

 mercurial columns is measured. The elastic force of the air in 

 the apparatus is H — h. 



The weight of this air is expressed by 



Tr H-A 1 H-A 



760 ^ 1 + ut 760 



But this air is the volume v of air under the pressure H, and at 

 the temperature t t which was in the manometer at the time of 



1 TT 



observation, and which is expressed by v- -• =t^l. Further, 



1 J \-\-ut 760 



of the volume V of air under the pressure y which filled the tube 

 No. 1 at the time of closing the stopcock ; the weight of this 



air is V^r. We have thus 

 760 



V. y+ ^H = V(H-7,) + »-LH-/,, 

 whence 



w=H— A — ^ -A. 



This equation will give the unknown barometric pressure y ; but 



v 



■=■ must be known. It is determined directly in the apparatus 



just as it is arranged. Mercury is allowed to flow out of the 

 manometer until its level is at C ; the difference of the two mer- 

 curial columns is then A', the elastic force of the air is H — h\ 

 and we ought to have 



T7 H-7* 1 H-A uH-A' , 1 H-A' 



V ■ A- v = V 1- v' ■ • . 



760 + 1 + at 760 760 * l + at 760' 



