C. GENERAL PHYSICS. 185 



and cold air, he sees a true and sufficient cause of the phe- 

 nomena described by Tyndall in the deadening of the sound, 

 caused by a deformation of its wave-front in consequence 

 of the different velocities existing in the various directions 

 of the sonorous impulses. 



In the two following abstracts it will be seen that both 

 Professor Henry, who has had a long experience with such 

 phenomena on our own coast, and Professor Reynolds in En- 

 gland, do not agree with Professor Tyndall, but attribute 

 the apparent impermeability of the air to sound observed by 

 Tyndall to the tilting of the sonorous wave-front, caused by 

 the difference of the velocities with which the air moves at 

 the ground, or on the surface of the sea, and at an elevation. 



EFFECT OF WIND ON SOUND-WAVES. 



Professor Henry presented a series of observations on this 

 subject at the meeting of the National Academy pf Sciences, 

 in New York and Washington, but since then he had made 

 many additional observations, and Professor Tyndall had 

 also been investigating* the same matter. It is in accordance 



CD CD 



with the most extended observations, and with the experi- 

 ence of all who have given attention to the subject of the 

 propagation of sound, that the audibility of it is much influ- 

 enced by the wind, and that as a general rule, to which there 

 are some exceptions, sound is heard more distinctly when prop- 

 agated in the direction of the wind than when in opposition 

 to it. There are some well-authenticated cases, however, in 

 which sound has been heard at a Greater distance against the 



CD CD 



wind than with it. The effect of the wind on sound is by no 

 means a simple phenomenon of ready explanation. At first 

 sight it might appear that the sound w r as carried onward 

 with the wind when the two were coincident in direction, 

 and that it was retarded wdien in an opposite direction; but 

 this explanation is not sufficient to account for the phenome- 

 non when we consider the fact that sound moves at the rate 

 of upward of seven hundred miles an hour, while a wind of 

 seven miles an hour is sufficient to give a penetrating power 

 to a given sound of double the intensity, whereas from the 

 foregoing consideration it should have an effect of only one 

 per cent. The only explanation which has been offered for 

 the phenomena is that in a river of air of considerable depth 



