W. J. M. Rankine, Esq., on the Elasticity of Vapours. 29 



The /orw? of the equation has been given by theory ; but 

 three constants, represented by a, /S, and y, have to be de- 

 termined for each fluid by experiment. 



The inverse formula, for finding the temperature fi'om the 

 pressure, is of course 



f2.) 



,-=y^ 



Log. P ^ 



y 47^ 



It is obvious that for the determination of the three con- 

 stants, it is sufficient to know accurately the pi*essures cor- 

 responding to three temperatures ; and that the calculation 

 will be facilitated if the reciprocals of those temperatures, as 

 measured from the absolute zero, are in arithmetical pro- 

 gression. 



In order to calculate the values of the three constants, for 

 the vapour of water, the following data have been taken from 

 M Eegnault's experiments : — 



These data give the following results for the vapour of 

 water, the pressures being expressed in millimetres of mer- 



