102 W. B. Taylor— Recent Researches in Sound. 



above by the freer contrary wind (assuming as before a surface 

 retardation), the sound-beams are curved upward, and the low- 

 est ray that can reach the distance of the observer at <?, is that 

 which touchinof the surface of the sea is gradually so tilted up- 



2. — ^Adverse Wind, 

 ward that it passes above the ear of the listener, leaving bin 

 practically in an acoustic shadow ; very much as an observe 

 on the deck of a vessel when losing the sight of the bull c 

 another vessel ten miles off, by reason of the interposed i 

 vexity of the ocean, stands in the optical shadow 

 In both cases if the conditions favor, the boundary < 

 shadow may be re-crossed by ascending from the deck 1 

 }-head, and the sisrht or the sound-beam thus regainet 



of the eartii. 



3. — Compound Wind. 



Fig. 3 represents the disturbing effect of a lower contrary 

 wind with an opposite wind above. In this case the principal 

 result will be a depression of the sound- beam as in fig. 1, but 

 more strongly marked, as the differences of motion as we ascend 

 will be more rapid. Attending this action, however, tbere will 

 probably be some lagging of the lower stratum of the adverse 

 wind by reason of the surface friction ; the tendency of which 

 will be to slightly distort the lower sound radiations, by giving 

 them a reverse or serpentine curvature. The upper rays of 

 sound would probably have only a single declining curvature, 

 similar to that shown in tig. 1. 



One result of this condition of the locus of the normals (to use 

 a mathematical phrase) would be to make the sound less audi- 

 ble (or possibly sometimes inaudible) at a point (as at t) midw;iy 

 between the two stations. This hypothetical case of compoun<l 

 refraction would appear to offer a plausible explanation, not 

 only of the paradox of a nearer trumpet-sound being dimin- 

 ished in power by the wind which increased the effect of a 

 more distant whistle, but also of the puzzling "belt" of inaudi- 

 bility previously noticed, Duane [D], and Henry [b]. 



