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>c 



10-1 Trof. Potter on the adequacy 0/ Laplace's suggestion 



clement of the medium can be kept at an elevated temperature 

 for the o^jth or -\jth of a second, during which it is within the 

 limits of a sound-wave, without some heat being lost by radia- 

 tion in that comparatively long time. If some heat is lost, then 

 the finite-interval theory will give a velocity of sound which will 

 be in excess of experiment, because that theory essentially sup- 

 poses that no heat is lost. The velocity to which my investiga- 

 tions lead is therefore the superior limiting value of sound-velo- 

 city, and in practice must be reduced by an allowance made for 

 the loss of heat — that is, for the loss of absolute molecular force 

 which occurs during the motion. I look upon 1130 feet, the velo- 

 city as determined by my theory, as the extreme possible value of 

 ordinary sound-velocity. It is probable the experimental value 

 will be something less than this theoretical value, just in pro- 

 portion as it is probable there is a loss of heat by radiatiorvKAnd 

 upon this point I may just remark that Dr. Le Conte thinks 

 that "during the propagation of elastic impulses the condensa- 

 tions and rarefactions are necessarily momentary." I am of a 

 contrary opinion. They are not momentary ; for they last during 

 the whole time that it takes a wave to pass a given element of 

 the medium. In the case of long waves this may be the g^th 

 or even the y^h of a second, which is surely much larger than 

 a moment, and quite long enough to allow heat to escape by 

 radiation. It is certainly not the moment ariness of the conden- 

 sations which prevents the escape of heat by radiation. <v Nothing 

 that Dr. Le Conte has written has changed my opinion of the 

 state of the problem of sound. We want careful experiments on 

 the comparative velocities .of sounds due to different exciting 

 causes, such as musical instruments, blows with a hammer on a 

 hard substance, explosions, discharges of electricity, cannon, 

 thunder, &c. We know but little yet of the properties of differ- 

 ent types of sound. 



Sheffield, January 16, 1864. 



XX. A Reply to the Observations of Prof. Le Conte, M.D., 

 respecting the adequacy of Laplace's suggestion for a correc- 

 tion in the Theory of Sound. By Professor Potter, A.M.* 



IN Dr. Le Conte's paper published in the Philosophical Ma- 

 gazine for January 1864, he says it appears to him that the 

 obscurity which marks many of the discussions relating to the 

 problem of sound arises from two distinct causes, " namely, first, 

 from a misconception of the physical theory of Laplace and that of 

 Poisson, which is substantially the same ; and secondly, from the 

 difficulties and obscurities which invest the mathematical theory of 

 * Communicated by the Author. 



