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LXVIII. Mechanical Theory of Heat. — Specific Heat of Air. 

 By W. J. Macquorn Rankine, C.E., F.R.S.E. ^c * 



THE results of the recent experimental researches of M. Re- 

 gnault on the specific heat of air at constant pressure 

 {Comptes Rendus, April 18^ 1853) agree so closely with those de- 

 duced by me theoretically, about three years since, from the 

 principle of the convertibility of heat with mechanical power, 

 that a tabular comparison of them may prove interesting, espe- 

 cially as they have an important bearing on the theory of heat 

 in general. The annexed table contains also the mean result of 

 Mr. Joule's experiments, as given in the Philosophical Transac- 

 tions for 1852, page 74. 



The theoretical calculation may be found in the Transactions 

 of the Royal Society of Edinburgh, vol. xx. p. 192. The ratio 

 of the specific heat of air at constant pressure to that at constant 

 volume, as determined from the velocity of sound, is there taken 

 as approximately equal to 1*4 ; but as the true value of this ratio 

 probably lies between 1-4 and 1*41, I have added the results of a 

 computation in which the latter value is used. 



The following are the numerical data and formulae employed : — 

 Height of the modulus of elasticity of air at 0° Centigrade, 



A =26214 feet. 



Temperature of melting ice, as measured in Centigrade degrees 

 from the absolute zero of a perfect gas-thermometer, 



To=274-6. 



Mechanical value of the specific heat of liquid water (Joule's 

 equivalent), 



Kw= 1389*6 feet of fall per Centigrade degree. 



Ratio of the specific heat of air at constant pressure, to that at 

 constant volume, 



1-|-N = 1*4 in the original calculation, 



1*41 in the second calculation, 



being the limits between which the true value is probably con- 

 tained. 



Mechanical values of the specific heat of air : — 



at constant volume, 

 at constant pressure, 



* Communicated by the Author. 



