128 VELOCITY OP SOUND SECT. XVI 



sounded together at a little 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 has 

 to pass, whatever tends to increase the elasticity of the 

 air must also accelerate the motion of sound. On that 

 account its velocity is greater in warm than in cold 

 weather, supposing the pressure of the atmosphere con- 

 stant. 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 of 

 Fahrenheit its speed in a second is 1120 feet, or 765 

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

 nal velocity of the earth's equator. Since all the phe- 

 nomena of the transmission of sound are simple conse- 

 quences of the physical properties of the air, they have 

 been predicted and computed rigorously by the laws of 

 mechanics. It was found, however, that the velocity of 

 sound determined by observation, exceeded what it ought 

 to have been theoretically by 173 feet, or about one-sixth 

 of the whole amount. La Place suggested that this dis- 

 crepancy 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 calculation 

 confirmed the accuracy of his views. The ae'rial mole- 

 cules being suddenly compressed give out their latent 

 heat ; and as air is too bad a conductor to carry it rap- 

 idly off, it occasions a momentary and local rise of tem- 

 perature which, increasing the elasticity of the air 

 without at the same time increasing its inertia, causes 

 the movement to be propagated more rapidly. Analysis 

 gives the true velocity of sound in terms of the elevation 

 of temperature that a mass of air is capable of commu- 

 nicating to itself, by the disengagement of its own latent 

 heat when suddenly compressed 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 problem and assuming 

 the velocity of sound as given by experiment, it was 

 computed that the temperature of a mass of air is raised 



