360 Hydrostatics. 



be taken by means of a thermometer suspended in the air, 

 while T, T', are supposed to be indicated by a thermometer at- 



tached to the barometer. We take n = -- 32. Moreover, 



the coefficient T | 7 or 0,00208, representing the elastic force, re- 

 quires to be increased somewhat for the purpose of taking ac- 

 count, as far as can be done, of the quantity of water in a state 

 of vapour which is at all times mixed with the air in a greater 

 or less quantity. Indeed, under the ordinary pressure of the 

 atmosphere, the density of aqueous vapour is to that of air, as 

 10 to 14; consequently, the atmosphere is so much the lighter 

 according as it is composed in a greater degree of this vapour. 

 Now it contains so much the more vapour according as its tem- 

 perature is more raised, whereby, when the air is dilated by 

 heat, its weight must be diminished in a higher ratio than that 

 of its augmentation of bulk. We increase the coefficient 0,00208 

 therefore to 0,00223f or T i , which has been found by actual 

 trial to give the most correct results. We have, accordingly, 



e n = 0,00223 



' 32\ 



We now substitute in equation (n.) for e n the above value, 

 and for log. - the value found in equation (in.), and we shall ob- 

 tain 



log. - + 2 log. (1 + *"\= 

 g W ' " + *' 



Whence 



x 



* - * 



a (I + 0,00223(I^ _ 32)) R +fc 



- 32 )) ^ 



jThis is the value adopted by Laplace, Poisson,Biot and others from 

 the very extensive and careful researches of M. Ramond. That of 

 Sir George Shuckburgh is 0,00238, which is generally employed by 

 English writers upon this subject. The mean of the two is 0,00235. 



