12 Prof. J. Le Conte on the Discrepancy between the Computed 



incuts of Dr. Olinthus Gregory prove that sounds produced by 

 bells, muskets, and cannon are propagated with the same speed. 

 Indeed, a candid and impartial examination of the evidence which 

 has been adduced in relation to this point will, I think, warrant 

 the assertion that Mr. Earnshaw has failed to produce a single 

 unexceptionable fact, or a single satisfactory observation, in verifica- 

 tion of his theoretical deductions. 



Resuming the more immediate consideration of Laplace's ex- 

 planation of the excess of the observed velocity of sound over 

 that calculated by Newton, it should be borne in mind that the 

 development of heat and cold by sudden condensation and rare- 

 faction, is not a mere hypothetical cause evoked to account for 

 the phenomena, but is a well-known physical fact, proved by 

 direct experiment. Moreover, inasmuch as Laplace's formula is 

 a rigorous deduction from this physical fact, " the only way/' 

 to use the language of Professor Stokes, " of escaping from the 

 conclusion that the velocity of sound is really increased by the 

 cause assigned, is to suppose that the heat produced by con- 

 densation passes away so rapidly by radiation, that the result is 

 the same as though condensation and rarefaction were incapable 

 of changing the temperature of air/' Indeed, Professor Challis* 

 has been driven to the adoption of such an assumption : he asks, 

 " May it not be that the developed heat (whether positive or 

 negative) is carried off too quickly by radiation to affect the tem- 

 perature of the fluid?" Again, the same mathematician con- 

 tends that experiment only proves that heat is developed by 

 condensation " when the fluid is confined within narrow limits," 

 and that it is an unsupported hypothesis to assume " that there 

 is increase of temperature in fluid of unlimited extent" Let us 

 scrutinize this view more narrowly. 



In the first place, it will not escape attention that, inasmuch 

 as during the propagation of elastic pulses the condensations 

 and rarefactions are necessarily momentary, it is prima facie 

 physically improbable that the heat developed should escape by 

 radiation. The loss of heat by radiation requires time; so that 

 the condensation of the aerial molecules in the transmission of 

 sound-waves is performed precisely under the circumstances most 

 favourable to the free and full production of the whole effect in 

 question. It is obvious, that the condensations being so mo- 

 mentary as to afford no time for the escape of heat by radiation, 

 an unlimited mass of air is really better adapted to give this cause 

 its full influence, than any experiments on the compression of 

 gases in close vessels. Furthermore, in the second place, Pro- 

 fessor Stokes f has submitted this view to an elaborate analytical 



* Phil. Mag. S. 3. vol. xxxii. p. 283 (1848) ; S. 4. vol. i. p. 40/(1851). 

 f Ibid. S. 4. vol. i. pp. 306-317 (1851). 



