MOTIVE POWER OF HEAT. 169 



latent heat of a pound of saturated steam, by the 

 equation 



p 1 + .00366 X 100 . 

 ~760 1 + .00366 X t 



where p denotes the pressure in millimetres, and 1 

 the latent heat of a pound of saturated steam; the 

 values of A, being calculated by the empirical for- 

 mula f 



A = (606.5 -f 0.3050 -(t + .00002** + 0.0000003*'), 

 given by Regnault as representing, between the 



* It appears that the vol. of 1 kilog. must be 1.69076 ac- 

 cording to the data here assumed. 



The density of saturated steam at 100 is taken as ~^ 

 of that of water at its maximum. Rankine takes it as T ^. 



f The part of this expression in the first vinculum (see 

 Regnault, end of ninth memoir) is what is known as " the 

 total heat " of a pound of steam, or the amount of heat 

 necessary to convert a pound of water at into a pound 

 of saturated steam at t ; which, according to " Watt's 

 law," thus approximately verified, would be constant. 

 The second part, which would consist of the single term 

 t, if the specific heat of water were constant for all tem- 

 peratures, is the number of thermic units necessary to raise 

 the temperature of a pound of water from to t, and 

 expresses empirically the results of Regnault's experi- 

 ments on the specific heat of water (see end of the tenth 

 memoir), described in the work already referred to. 



