492 EESEARCHES IN SOUND, 



of the breakers at a distance of about a mile in no case apjieared to be 

 coincident with the variations of an aneroid barometer or a thermome- 

 ter, bnt in every instance it was affected by the direction of the wind. 



The variation in the distinctness of the sound of a distant instrument 

 as depending on the direction of the wind is so marked that we are war- 

 ranted in considering it the principal cause of the inefficiency in certain 

 cases of the most powerful fog-signals. The effect of the wind is usu- 

 ally attributed, without due consideration, to the motion of the body of 

 air between the hearer and the sounding instrument ; in the case of its 

 coming towards him it is supposed that the velocity of the sound is re- 

 inforced by the motion of the air, and when in the opx)osite direction 

 that it is retarded in an equal degree. A little reflection, however, will 

 show that this cannot be the cause of the i^henomeuon in question, since 

 the velocity of sound is so vastly greater than that of any ordinary 

 wind that the latter can only imjiede 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 explana- 

 tion of wind on sound, which we think has an important practical bear- 

 ing, especially in directing the line of research and subsequent ai^plica- 

 tion of principles. 



His explanation rests upon the fact that during the passage of a wind 

 between the observer and the sounding instrument the velocity of this 

 will be more retarded at the surface of the earth on account of friction 

 and other obstacles, and that the velocity of the stratum immediately 

 above will be retarded by that below, and so on, the obstruction being 

 lessened 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 ob- 

 server. In this case the retardation of the sound wave will be greater 

 above than below, and the ujiper part of the wave-front will be thrown 

 backwards so that the axis of the phonic ray will be deflected upwards, 

 and over the head of the observer. If, on the other hand, a deep river 

 of wind (so to speak) is blowing directly towards the observer, the upper 

 part of the front of the wave will be inclined down and towards him, 

 concentrating the sound along the surface of the earth. 



The science of acoustics in regard to the phenomena of sound as ex- 

 hibited in limited spaces has been developed with signal success. The 

 laws of its production, propagation, reflection, and refraction have been 

 determined with much ftrecision, so that we are enabled in most cases 

 to explain, predict, and control the phenomena exhibited under given 

 conditions. But in case of loud sounds and those which are propagated 

 to a great distance, such as are to be employed as fog-signals, consider- 

 able obscurity still exists. As an illustration of this I may mention the 

 frequent occurrence of apparently abnormal phenomena. General War- 

 ren informs me that at the battle of Seven Pines, in June, 1862, near 

 Eichmond, — General Johnston, of the Confederate army, was within three 



