448 WRITINGS OF JOSEPH HENKY. [1875 



one hand, nor to the retardation of the latter by the motion 

 of the wind, on the other. The inadequacy of this explana- 

 tion 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 

 explanation of the action of the wind, became known to me, 

 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 velocity. The lower stratum is 

 retarded by friction against the earth and by the various 

 obstacles it meets with, the one immediately above by fric- 

 tion against the lower, and so on ; hence the velocity increases 

 from the ground upward ; a conclusion established by abun- 

 dant observation. Now in perfectly still air, a sounding 

 instrument — such as a bell, produces a series of concentric 

 waves perfectly spherical ; but in air in motion the difference 

 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 wind- 

 ward, and to increase its convexity above to the leeward ; and 

 since the direction of the sound is perpendicular to the 

 sound-wave, when moving 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 variations 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 just published in the American Journal of Sci- 



