the Velocity of Sound. 97 



tated. Thus Poisson, in his " Memoir on the Theory of Sound/' 

 says that the difference between calculation and observation may 

 be attributed solely to development of heat, " if no objection can 

 be raised against the analysis by which the velocity of sound was 

 deduced." Then after assuming that air in vibration undergoes 

 changes of temperature proportional to the changes of density, 

 and finding that if a compression equal to T y-g be accompanied 

 by an increase of 1 degree of temperature the velocity of sound 

 will be accounted for on that assumption, he admits that such 

 an increment of temperature in aerial waves " cannot be verified 

 by any direct experiment " [Journal de VEcole Poly technique, 

 vol. vii. p. 326 and p. 363) . Dulong, in the memoir already cited, 

 writes thus (p. 128) : " Poisson arrived at the same theorem 

 [that of Laplace relative to the ratio of the specific heats] by a 

 calculation more direct and wholly unencumbered by the impro- 

 bable hypotheses which the author of the Mecanique Celeste had 

 adopted respecting the mode of existence of heat in elastic fluids." 

 On turning to the memoirs of Poisson here referred to [Ann. de 

 Chim. et de Phys. vol. xxiii. p. 337, and Conn, des Temps, 1826, 

 p. 257), it will be found that he dispenses with Laplace's a priori 

 considerations by making the assumption just mentioned, which 

 is accordingly the foundation of his theory. In a note in the 

 Conn, des Temps (p. 269) he adds, for the sake of further ex- 

 planation, that "the actual temperature of a slice of fluid in 

 movement subsists only for an instant, and only extends through 

 an infinitely small thickness ; but that does not prevent our con- 

 ceiving of that temperature and its measure." A physical cir- 

 cumstance which is only " conceivable " is, I submit, not a vera 

 causa. In short, the objection I have constantly made to La- 

 place's correction is, that there are absolutely no experimental 

 data for calculating the effect of the development of heat in 

 aerial waves, and that experiments on air in confined spaces do 

 not furnish such data. From not taking this distinction into 

 account, Prof. Le Conte has not correctly represented my views 

 where he says that I contend that "experiment proves the deve- 

 lopment of heat by condensation only when the fluid is confined 

 within narrow limits." I allow that heat is developed by con- 

 densation, whether the fluid be confined or not, but deny that 

 the effect of the developed heat in changing the temperature is 

 the same under the two circumstances. 



It must, however, be conceded that full weight is to be given 

 to the evidence adduced by Prof. Le Conte of the close agree- 

 ment of the theoretical velocity of sound with observation after 

 the application of the Laplacian correction. If the assumed 

 modus operandi were made matter of demonstration, the evidence 

 would be irresistible. Till this be done, the accordance of my 



Phil. Mag. S. 4. Vol, 27. No. 180. Feb. 1864. II 



