Section III, 1919 [loi] Trans. R.S.C. 



The Velocity of Sound and the Ratio of the Specific Heats for Air 



By Dr. T. C. Hebb 



Presented by Dr. D. McIntosh 

 (Abstract, Read May Meeting, 1919.) 



The paper contains an account of a new determination of the 

 velocitN' of sound by the method previously used by the author.^ This 

 was considered desirable as the author's first determination did not 

 seem to give a ratio of the specific heats equal to that obtained by the 

 Lummer-Pringsheim method. This was shown by Moody,- who ob- 

 tained 1 • 4003 for V using the author's value of V, whereas the Lummer- 

 Pringsheim method gives a value in the neighbourhood of 1-4030. 

 He, however, failed to correct for the fact that air is not an ideal gas. 

 But even with that correction the value is still too low by the velocit>' 

 of sound method. This new determination supports the old value — 

 the new and the old values being 331-26 and 331-29 meters per 

 second respectively. The explantion of the discrepanc>' betvs'een the 

 two values of y as obtained by the two methods lies in the method of 

 obtaining the velocit\' of sound for dr\- air at 0°C. from observation 

 taken in moist air at t°C. The formula for conversion used was 



V =V '1 



m y, 



/o 



where \'° is the velocit>- at O^C, \^ is the velocity at the time of 

 the experiment, J^ is the density of air at the time of the experiment 

 and.'"o is the densit\- of dr\' air at 0°C. and pressure equal to that at 

 the time of the experiment. This equation assumes that ;■' for moist 

 air is the same as ;' for dn.- air. To correct the formula the right hand 

 side of the equation should be multiplied by 



/~ 



Where ;■'„ is the ratio of the specific heats for dr>- air at 0°C. and /'m 

 is the ratio of the specific heats for air at the time of this experiment. 

 This ratio is small and is calculated. When the correction is ap- 

 plied to the two values of the velocity- of sound there results the values 

 331-44 and 331-38 meters per second. From these two values, y is 



1 Phys. Rev., Vol. XX, No. 2. Feb., 1905. 

 »Phys. Rev., Vol. XXXIV, No. 4, p. 275, 1912. 



