534 Prof. O. Reynolds on the 



by an inclination of the front backwards, and by an upward motion of 

 the wave. A similar effect would be produced in a direction opposite to 

 that of the wind, only as the top of the wave would then be moving 

 faster than the bottom, the waves would incline forwards and move 

 downwards. In this way the effect of the wind is to lift the waves 

 which proceeded to windward, and to bring those down which move 

 with it. 



Thus the effect of wind is not to destroy the sound, but to raise the 

 ends of the wave, which would otherwise move along the ground, to such 

 a height that they pass over our heads. 



When the ends of the waves are raised from the ground they will 

 tend to diverge down to it, and throw off secondary waves, or, as I shall 

 call them, diverging waves, so as to reconstitute the gap that is thus 

 made. These secondary waves will be heard as a continuation of the 

 sound, more or less faint, after the primary waves are altogether above 

 our heads. [This phenomenon of divergence presents many difficulties, 

 and has only as yet been dealt with for particular cases. It may, how- 

 ever, be assumed, from what is known respecting it, that in the case of 

 sound being lifted up from the ground by refraction, or, what is nearly 

 the same thing, passing directly over the crest of a hill so that the 

 ground falls away from the rays of sound, diverging waves would be 

 thrown off very rapidly at first and for a considerable distance, depending 

 on the wave-length of the sound ; but as the sound proceeds further the 

 diverging rays would gradually become fainter and more nearly parallel 

 to the direct rays, until at a sufficient distance they would practically 

 cease to exist, or, at any rate, be no greater than those which cause the 

 diffraction-bands in a pencil of light*. The divergence would introduce 

 bands of diffraction or interference within the direct or geometrical path 

 of the sound, as in the case of light. These effects would also be com- 

 plicated by the reflection of the diverging waves from the ground, which, 

 crossing the others at a small angle, would also cause bands of inter- 

 ference. The results of all these causes would be very complicated, but 

 their general effect would be to cause a rapid weakening of the sound 

 at the ground from the point at which it was first lifted ; and as the 

 sound became weaker it would be crossed by bands of still fainter sound, 

 after which the diverging rays, as well as the direct rays, would be lifted, 

 and at the ground nothing would be heard. September 1874.] 



If we leave out of consideration the divergence, then we may form 

 some idea as to the path which the bottom of the sound, or the rays of 

 sound (considered as the rays of light), would follow. If the variation 

 in the speed of the wind were uniform from the surface upwards, then 



* Taking sound of 1 foot wave-length, and comparing it with light whose wave- 

 length is the 50,000th part of an inch, then the divergence of the sound at a mile from 

 the point at which it left the ground would be comparatively the same as that of the 

 light at T V of an inch from the aperture at which the pencil was formed. 



