SECT, xvj.] VELOCITY OF SOUND. 149 



distance, would arrive at the ear in different times. A rapid 

 succession of notes would in this case produce confusion and 

 discord. But, as the rapidity with which sound is transmitted 

 depends upon the elasticity of the medium through which it 

 tas to pass, whatever tends to increase the elasticity of the 

 air must also accelerate the motion of sound. On that accoun t 

 its velocity is greater in warm than in cold weather, supposing 

 the pressure of the atmosphere constant. In dry air, at the 

 freezing temperature, sound travels at the rate of 1090 feet in 

 a, second, and for any higher temperature one foot must be 

 added for every degree of the thermometer above 32 : hence 

 at 62 Q of Fahrenheit its speed in a second is 1120 feet, or 

 765 miles an hour, which is about three-fourths of the diurnal 

 velocity of the earth's equator. Since all the phenomena of 

 the transmission of sound are simple consequences of the phy- 

 sical properties of the air, they have been predicted and com-, 

 puted rigorously by the laws of mechanics. It was found, 

 however, that the velocity of sound, determined by observa- 

 tion, exceeded what it ought to have been theoretically by 

 J73 feet, or about one-sixth of the whole amount. La Place 

 suggested that this discrepancy might arise from the increased 

 elasticity of the air in consequence of a development of latent 

 heat (N. 173) during the undulations of sound, and calcula- 

 tion confirmed the accuracy of his views. The aerial mole- 

 cules being suddenly compressed give out their latent heat ; 

 and, as air is too bad a conductor to carry it rapidly off, it, 

 occasions a momentary and local rise of temperature which, 

 increasing the elasticity of the air without at the same 

 time increasing its inertia, causes the movement to be pro-, 

 pagated more rapidly. Analysis gives the true velocity 

 of sound in terms of the elevation of temperature that a 

 mass of air is capable of communicating to itself, by the 

 disengagement of its own latent heat when suddenly COHH 

 pressed in a given ratio. This change of temperature? 

 however could not be obtained directly by any experiments 

 which had been made at that epoch ; but by inverting the 



