114 Mr. W. J. M. Rankine on the Mechanical Action of Heat. 



temperature measured from that point, m the specific gravity of 

 mercury, A that of the gas at the temperature of melting ice, 

 and pressure corresponding to a column of mercury of the 

 height h. It follows that the ratio 7 is given by the formula 



«^A 

 7=1 -I- N nearly = — rr^ — ^^^^.. . . (23) 



Calculations have been made to determine the ratio 7 from 

 the velocity of sound ; but as many of them involve erroneous 

 values of the coefficient of elasticity E, the expei'iments have to 

 be reduced anew. 



The following calculation is founded on an experiment quoted 

 by Poisson on the velocity of sound in atmospheric air, the 

 values of E, m, and A being taken from the experiments of 

 M. Regnault. 



ff = 340'89 metres per second. 



^=9'»^-80896. h = Qi^-7Q. T = 15°-9 Centigrade. 



E =0-003665; ? =10513. 



' A 



Consequently, for atmospheric air, 



7 = 1-401. 



The results of a reduction, according to correct data, of the 

 experiments of Dulong upon the velocity of sound in atmospheric 

 air, oxygen, and hydrogen, are as follows : — 



Atmospheric air .... 7=1-410 

 Oxygen . . . ♦. . . . 1-426 

 Hydrogen 1-426 



Thus it appears, that for the simple substances, oxygen and 

 hydrogen, the ratio N is the same, while for atmospheric air it 

 is somewhat smaller*. 



* The following are some additional determinations of the value of y for 

 atmospheric air, founded upon experiments on the velocity of sound : — 



T. «. y. 



observers. Centigrade. Metres per second. 



Bravais and Martins : mean of several "I 



experiments at temperatures varying I P, S'-f'^''^7 r40955 



from 5° to 11° Centigrade, reduced f 



to 0° {Comptes Rendus, xix.) J 



Moll and Van Beek : reduced to . . 332-25 1-40853 



Siampfer and Myrhack: reduced 10 0°}^ q 3;32-96 1-41456 



(not corrected for moisture) . . / 

 Academic des Sciences, 17 38 (not cor- \ g.j 337-10 1"418 



rected for moistiu-e) .... / 



A variation of one metre per second in the velocity of sound at U° corre- 

 sponds to a variation of -0085 in the value of y. See also Phil. Mag. 

 for June 1853. 



