SMCT. XVI. INTENSITY OF SOUND. ]31 



these effects. We estimate that intensity by the im- 

 petus of these fluid molecules on our organs, which is 

 consequently as the square of the velocity, and not by 

 their inertia, which is as the simple velocity. Were 

 the latter the case there would be no sound, because the 

 inertia of the receding waves of air would destroy .the 

 equal and opposite inertia of those advancing ; whence 

 it may be concluded that the intensity of sound dimin- 

 ishes inversely as the square of the distance from its 

 origin. In a tube, however, the force of sound does 

 not decay as in open air, unless perhaps by friction 

 against the sides. M. Biot found from a number of 

 highly interesting experiments made on the pipes of the 

 aqueducts in Paris, that a continual conversation could 

 be carried on in the lowest possible whisper, through 

 a cylindrical tube about 3120 feet long, the time of 

 transmission through that space being 2-79 seconds. In 

 most cases sound diverges in all directions so as to oc- 

 cupy at any one time a spherical surface ; but Dr. Young 

 has shown that there are exceptions, as for example 

 when a flat surface vibrates only in one direction. The 

 sound is then most intense when the ear is at right an- 

 gles to the surface, whereas it is scarcely audible in a 

 direction precisely perpendicular to its edge. In this 

 case it is impossible that the whole of the surrounding 

 air can be affected in the same manner, since the particles 

 behind the sounding surface must be moving toward it, 

 whenever the particles Before it are retreating. Hence 

 in one half of the surrounding sphere of air its motions 

 are retrogade, while in the other half they are direct ; 

 consequently at the edges where these two portions 

 meet, the motions of the air will neither be retrograde 

 nor direct, and therefore it must be at rest. 



It appears from theory as well as daily experience, 

 that sound is capable of reflection from surfaces (N. 174) 

 according to the same laws as light. Indeed any one 

 who has obs'erved the reflection of the waves from a 

 wall on the side of a river after the passage of a steam- 

 boat, will have a perfect idea of the reflection of sound 

 and of light. As every substance in nature is more or 

 less elastic, it may be agitated according to its own law 

 by the impulse of a mass of undulating air ; and recip- 



