134 VELOCITY OF SOUND. SECT. XVI. 



nature, extent, and intensity of the primitive disturbance. Con- 

 sequently sounds of every quality and pitch travel with equal 

 speed. The smallest difference in their velocity is incompatible 

 either with harmony or melody, for notes of different pitches and 

 intensities, 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 diurnal velocity of the earth's equator. Since all the 

 phenomena of the transmission of sound are simple consequences 

 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 obser- 

 vation, exceeded what it ought to have been theoretically by 

 173 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 or 

 absorbed heat (N. 178) during the undulations of sound, and 

 calculation confirmed the accuracy of his views. The aerial 

 molecules being suddenly compressed give out their absorbed 

 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 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 

 communicating to itself, by the disengagement of its own absorbed 

 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 



