106 0/?Laplace's suggestion for a correction in the Theory of Sound. 



interpretation, capable of being tested by observation or experiments 

 Experiments show that the temperature of air escaping from a 

 greater to a less pressure does not change suddenly, but occupies 

 a short interval of time, like the change of temperature in the 

 high-pressure steam-jet* at different distances from the aperture. 

 This want of simultaneousness of the change of temperature is 

 of itself fatal to Poisson's solution. Again, it was found by 

 myselff that near the aperture in a jet of air the change of tem- 

 perature varies as the cube of the rarefaction, and not as the first 

 power, which is required in Poisson's solution, which conse- 

 quently fails also on a second account. Dr. Le Conte's concep- 

 tion of Laplace's correction in the theory of sound, whether on 

 the original suggestion, or in the discussion of Poisson, has thus 

 no experimental confirmation, which he rightly states to be 

 necessary. 



The methods of finding the heat and cold developed in con- 

 densations and rarefactions by the air- thermometer, after the 

 method of Clement and Desormes, which was used also by Gay- 

 Lussac and Welter at the wish of Laplace, I have shown to be 

 radically faulty J from the beginning, since the experiment is a 

 dynamical one before it becomes a thermometrical one, and the 

 dynamical results have all along been taken for thermometrical 

 ones. The values of the ratio (k) of the two specific heats of 

 gases so determined are consequently nugatory, and this part of 

 the discussion in Dr. Le Conte's paper falls to the ground. ^C 



With respect to my own method of investigating the funda- 

 mental differential equations for the motions of fluids in the 

 Philosophical Magazine for March 1851, " On the Considera- 

 tion of their Atomic Constitution," I believe it is sufficient to 

 insist that the same atomic theory which is the basis of modern 

 chemistry must be considered in other cases of physical problems 

 which are affected by it; and hydrodynamical problems un- 

 questionably are so affected. 



With respect to my theoretical value for the velocity of sound 

 from atomic considerations, which is 1122-2 feet per second, or 

 32-2 feet per second more than 1090 feet, the received velocity 

 at the freezing temperature, I find reasons for believing it to be 

 the correct result when sound passes over still water, from the 

 experience of the artillery officers, and is therefore the normal 

 velocity. The distance of an object which is to be fired at, is 

 often found by noticing the interval of time between seeing the 

 flash and hearing the report of a gun at the object; and it is 

 found that the gun, the shot of which is required to strike the 

 object, must be elevated rather more when the sound has tra- 



* Phil. Mag. for January 1862. + Ibid. September 1853. 



% Ibid. October 1862. 



