512 



EESEARCHES IN SOUND. 



ially in one locality; in wliicli the sound is heard against a northeast, 

 snow-storm more distinctly than when the wind is in an opposite direc- 

 tion. This anomaly was referred to the action of an upper current in 

 an opposite direction to that at .the earth, such a current being known 

 to exist in the case of northeast storms on our coast. But in what man- 

 ner the action of the wind increased or diminished the audibility of 

 sound was a problem not solved. It could not be due, as might be 

 thought at first sight, to the acceleration of the sonorous impulse by the 

 addition of the velocity of the wind to that of sound, on the one hand, 

 nor to the retardation of the latter by the motion of the wind, on the 

 other. The inadequacy of this explanation must be evident when we 

 reflect that sound moves at the rate of 750 miles an hour, and therefore 

 a wind of 7^ miles an hour would only increase its velocity one per cent. ,• 

 whereas the actual increase in audibility produced by a wind of this 

 intensity is in some instances several hundred per cent. 



In this state of our knowledge, a suggestion of Professor Stokes, of 

 Cambridge, England, which offered a plausible exi^lanation of the action 

 of the wind, became known to us, and was immediately adopted as a 

 working hypothesis to direct investigations. 



This suggestion, the importance of which appears to have escaped 

 general recognition, is founded on the fact that the several strata into 

 which a current of air may be divided do not move with the same veloc- 

 ity. The lower stratum is retarded by friction against the earth and by 

 the various obstacles it meets with, the one immediately above by friction 

 against the lower, and so on ; hence the velocity increases from the ground 

 upward — a conclusion established by abundant observation. ISTow, in 

 I)erfectly still air, a sounding instrument, such as a bell, produces a series 

 of concentric weaves perfectly spherical ; but in air in motion the differ- 

 ence of velocity above and below disturbs the spherical form of the 

 sound-wave, giving it somewhat the character of an oblique ellipsoid, 

 by tending to flatten it above — to the windward, and to increase its con- 

 vexity above — to the leward ; and since the direction of the sound is per- 

 jiendicular to the sound-wave, against the wind it will be thrown upward 

 above the head of the observer, and in the opposite direction downward 

 toward the earth. A similar effect will be produced, but with some varia- 

 tions and perhaps greater intensity, by a wind above, opposite to that at 

 the surface of the earth. 



These propositions will be rendered plain by the following illustrations 

 (Figures 1, 2, and 3), for which I am indebted to an article in the Ameri- 

 can Journal of Science, by William B. Taylor. 



Fiff. 1. 



