26 Sir J. W. Lubbock on the Heat of Vapours. 



In this expression a, y, and E are constants, 6 and p are corre- 

 sponding temperatures and pressures. 



1— <y 1 



If -=fi, and if - + = t, t being the absolute tempera- 



iy a. 



ture, the following equation is easily obtained : 



if p' be taken equal to unity at the boiling-point of the fluid, t 1 

 is the absolute temperature at that boiling-point, 



and if 



(l-E)r' 



E 



= F, 



lo gP=-0 lo 8 E + £ lo S\ l + r 



a- 



This formula resembles that given by Mr. Rankine in the last 

 Number of this Journal, inasmuch as it expresses the logarithm 

 of the pressure in a series proceeding according to the negative 

 powers of t. 



In my treatise above referred to, the following numerical 

 values are given for water : 



£ = 1-17602, /3='0134; 

 but having gone over the work again, I find the slightly different 

 values 



£=1-18028, £=-01372; 



t=448° + number of degrees of Fahrenheit, t' = 660, and the 

 pressure being reckoned in inches of mercury, 



1 W =OT»106 + [l-8626459]log{l- B^ffl}, 



or the pressure being expressed in millimetres of mercury, and 

 the temperatures in Centigrade degrees, 



log^=8-12766+ [1-8626459] log j 1 - L 1 ' 74 ** 2322 ] "^. 



From the same three observations of M. Regnault that Mr. 

 Rankine used in his paper in the Edinburgh Philosophical 

 Journal, vol. xlvii. p. 31, to determine the constants in his for- 

 mula, I found 



