Prof. Potter's Rephj to tfie Observatidns of Mr. Raiikine. M7 



capable of traversing gi-eat lengths of air before any considerable 

 portion of it is absorbed. This is especially the case with heat 

 of such high refrangibility as to place it within the limits of the 

 visible spectrum ; whereas heat of low refrangibility, such as that 

 which would emanate from shghtly heated air, is absorbed more 

 rapidly. Should the distance to which radiant heat can proceed 

 in air before a given fraction of it, such as one-half, is absorbed, 

 not be extremely great compared with the length of a wave of 

 sound, it may be seen after a little reflection that the general 

 conclusions arrived at will be unchanged, though the nimierical 

 details would be somewhat altered. I have not met v^ath any 

 experiments relating to the absorption of non-luminous heat by 

 air which could be made a foundation for numerical calculatiotiv' 

 March 11, 1851. ' ^' "" ']'>' 



XXXIX. A Reply to the Observations of Mr. Rankine in the last 

 Number of the Magazine. By Professor Potter, A.M.* 



IN the last Number of the Magazine, Mi*. Rankine sayahe 

 thinks I have misunderstood " the theoiy of Laplace and 

 Poisson as to the propagation of sound in gases : " I assure him 

 I have never so misunderstood that theoiy as to think it to be a 

 solution of the problem, but have always considered it as begging 

 the question. It does not appear as if Poisson looked upon it 

 at all in the light of a strict solution, when he had first in 1807 1, 

 put Laplace's views into a tangible mathematical form ; for he 

 says, "En admettant ce resultat, qu'on ne pent verifier par 

 ancune experience directe, on fera disparaitre la difference que 

 Newton a remarquee, Ic premier, entre la vitesse du son donnee 

 par calcul, et cclle qui resulte de F observation." The amended 

 calculations have, however, always been far from close approxi- 

 mations to the trae velocity. 



In showing the point of failure in the solutions, I shall refer 

 to the simplest and most improved form, as given in Poisson's 

 Traite de Mecanique, edition 1833, vol. ii. p. 695. He there 

 puts ymh equal to the pressure in the gas before disturbance, ff 

 being the force of gravity, m the density of mercury, and h the 

 height of the barometric colunm. In the state of motion, and 

 neglecting chang6 of temperatui-e, the pressure (») will be re- 

 presented by '""'^ ''^ '. ''I'"'- "c ^0 nozJfioxldjjq ') 



>■,■ Mllyllll;}! 1 ' '.■■,<'<'■ 1.' ' ' ■ ' .11 ' lu^! 



where s represents the wjiideiisatimi j)Q8ltiyj5^m:j^^l^gjt|Yy^|,j.^I|g 



="' '■'■:■■• ' ! I" ii;il' ,li)/ii<'!'j ybasila 8bw eo tbaaoqqi/e svb/I I 

 * Communicated by the Author.* 

 t Sec Journal de I Beol.e Polytechtiifjue^ Cahier XIV, pp. 363, 364. 



