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XXXIX. On a Method of Calculating the Constant y, expressing 

 the Relation of the Specific Seat at Constant Pressure to the 

 Specific Heat at Constant Volume. By W. Moon*. 



LET B^ equal the specific heat of air at constant pressure, 

 and H the specific heat at constant volume; then, when 

 a volume of air is compressed through dv, the adiabatic pres- 



■J JT TT 



sure of the air will be 1 + - dv — ^ — times the isothermal 



v Jti 



pressure; and if this value is equated with the pressure cal- 

 culated from the work required to produce the compression, 



TT 



then the value of J may be ascertained without experi- 

 mentally knowing the value of H. 



The pressure may be thus calculated from the work required 

 to produce the compression: — 



If the temperature of a volume of air is increased from t 

 to t ± (the volume being constant), then the pressure of the 



air will be increased from 1 to := — } nnnnne * and therefore 



1 + ^OOoboo -. 



if, while the volume of air is compressed from 1 to -, the tem- 

 perature of the air varies from t to t x , then the pressure of the 

 air will be 



760 1Q33796 V l + ^003665 h S™ es > 



where h = barometric height, and 1033*796 the pressure upon 

 a square centimetre at 760 millim. 



The heat given off in the gas is equal to the work performed 

 against the gas in compressing it divided by Joule's equi- 

 valent. Therefore, when the air is compressed through dv, the 

 heat given off 



42355 



To obtain from this expression the increase of temperature of 

 the air, it must be divided by the specific heat at constant 

 volume and the weight of a cubic centimetre of air, 



dv 



calories. 



^"^mss)> 



42355 / degrees 



"TT ' h 1 i Centigrade. 



^•2375x-001293187^ TT ^ Mgr5 l 



* Communicated by the Author. 



