#1 Mr. W, J. M. Rankine on the Mechanical Theory of Heat. 



The mechanical value of one Cimtigrade degree in atmospheric 

 air, as deduced from N, is 



238-66 English feet = 72-74 metres. 



The mechanical value of one Centigrade degree in liquid 

 water, as determined by Mr. Joule from experiments on friction, 

 being 



1389-6 English feet = 423*54 metres, 



it follows that the real specific heat of unity of weight of atmo^ 

 spheric air is 



1389-6 ""^'^'^ 

 and the apparent specific heat under constant pressure 

 0-1717x1-4=0-2404; 



according to De la Roche and Berard it is 0-2669. 



I was at first disposed to ascribe this difierence to some un- 

 known loss of power in Mr. Joule's apparatus ; but now tlia^; I 

 am better acquainted with his experiments^ I am inclined rather 

 to believe that the error lies chiefly in the experiments of De la 

 Roche and Berard. 



The apparent specific heat of vapour maintained at its maxi- 

 mum pressui'e is 



Real specific heat x /l 4- N (^1 -^]^) T , 



and is a negative quantity, as M. Clausius has concluded. 



The third section applies the principles of the first to the 

 latent and total heat of evaporation, and it is shown — 

 • TJiat the total heat of evaporation, where the vapour is sensibly 

 a perfect gas, increases at a sensibly uniform rate with the tempe- 

 rature ; and that the coefficient of its increase with tempei'ature is 

 sensibly equal to the apparent specific heat of the vapour as a gas 

 under constant pressure. 



The value of this coefficient for steam, as determined by Reg- 

 nault, is 



Specific heat of liquid water x 0-305. 



This then is also the apparent specific heat of steam as a per- 

 manent gas under constant pressure. The real specific heat of 

 steam is 



=0-194, and the ratio H-N = l-57. 



These values differ slightly from those given in my original 

 paper, being calculated from Joule's equivalent, instead of the 

 experiments of De la Roche and Berard on atmospheric air. 



The fourth section applies the principles of the second and 

 third to the theory of the steam-engine, and shows the modifica- 



