EESEAECHES IN SOUND. 



515 



In these, Figure 1 represents the effect of a favorable -^ind in depress- 

 ing- the waves of sound, S being the signal-station and O the point of 

 observation. The wind blowing from W to E, as the spheroidal faces 

 of the sonorous waves become more pressed forward by the greater ve- 

 locitj^ of the wind above, assuming it to be retarded at the surface by 

 friction, and the direction of the acoustic beam being constantly normal 

 to the wave-surfaces, the lines of direction of the sound will gradually 

 be bent downward and reach the ear of the observer with an accumu- 

 lated effect at the point O. 



Fio-. 2, 



Figure 2 represents the ordinary effect of an opposing wind blowing 

 from E to W against the sound ; the wave-faces being more resisted 

 above than below, assuming as before a retardation at the surface, the 

 sound-beams are curved upward, and the lowest ray that would reach, 

 in still air, the distant observer at O, is gradually so tilted np that it 

 passes above the ear of the listener, leaving him in an acoustic shadow. 



Fiff. 3. 



mmsy;m. 



Figure 3 represents the disturbing effect of two winds, the lower in 

 opposition to the sound at the surface, and the upper with it. In this 

 case the principal effect will be a depression of the sound-beam, similar 

 to that shown in Figure 1, but more strongly marked, as the difference 

 of motion will be greater as we ascend. Attending this action, says 

 Mr. Taylor, there will probably be some lagging of the lower stratum 

 by reason of the surface- friction, the tendency of which will be to dis- 

 tort the lower part of the sound-waves, giviug them a reverse or serpen- 

 tine curvature. In this case the upper ray of sound would only have a 

 single curvature, similar to that shown in Figure 1, while the lower rays 

 would be represented by the lower line S O, rendering the sound less 

 audible at an intermediate xioint, ^, than at the more distant station O. 

 This hypothetical case of compound refraction offers a j)lausible ex- 

 planation of the paradox of a nearer sound being diminished in power 

 by the wind which increases the effect of a more distant one. 

 S. Mis. 59 33 



