1874] WRITINGS OF JOSEPH HENRY. 419 



powerful fog-signals. The effect of the wind is usually at- 

 tributed (without due consideration) to the motion of the 

 body of air between the hearer and the sounding instrument • 

 in the case of the wind coming toward him, it is supposed that 

 the velocity of the sound is re-enforced by the motion of the 

 air, and when in the opposite direction that it is retarded in 

 an equal degree. A little reflection however will show that 

 this cannot be the cause of the phenomena in question, since 

 the velocit}'- of sound is so vastly greater than that of any 

 ordinary wind that the latter can only impede the progress 

 of the former by a very small percentage of the whole. 



Professor Stokes, of Cambridge University, England, has 

 offered a very ingenious hypothetical explanation of the 

 effect of wind on sound, which we think has an important 

 practical bearing — especially in directing the line of re- 

 search, and subsequent application of principles. 



His explanation rests upon the fact that during the pass- 

 age of a wind between the observer and the sounding instru- 

 ment, its velocity will be impeded at the surface of the earth 

 on account of friction and other obstacles; and the velocity 

 of the stratum immediately above will be somewhat reduced 

 by that below, and so on, the retardation being gradually les- 

 sened as we ascend through the strata. From this it follows 

 that the sound wave will be deformed, and the direction of 

 its normal changed. Suppose for example that the wind is 

 blowing directly from the observer. In this case the retard- 

 ation of the sound wave will be greater above than below, 

 and the upper part of the wave-front will be thrown back- 

 ward, so that the axis of the phonic ray will be deflected 

 upward, and over the head of the observer. If on the 

 other hand a deep river of wind (so to speak) is blowing 

 directly toward the observer, the upper part of the front of 

 the wave will be inclined down and toward him, concen- 

 trating the sound along the surface of the earth. 



The science of acoustics in regard to the phenomena of 

 sound as exhibited in limited spaces has been developed 

 with signal success. The laws of its production, propaga- 

 tion, reflection, and refraction have been determined with 



