40 BULLETIN OF THE 



deck of a vessel, might sometimes be regained by ascending to the 

 mast-head ; that is, sound is sometimes more readily conveyed by 

 the upper current of the air than by the lower. 



This fact, with other corroborative facts, did not, he says, reveal 

 its full significance to him until he was able to interpret it by the 

 aid of the hypothesis of Prof. Stokes, ( Transactions of the British 

 Scientific Association for 1867, Vol. 24,) according to which there 

 is — when the wind blows — a difference of velocities between the 

 upper and the lower strata of the atmosphere, resulting from the 

 retardation of the lower stratum by friction with the ground. This 

 unequal movement of the atmosphere disturbs the spherical form 

 of the sound waves, and tends to make them somewhat of the form 

 of an ellipsoid, the section of which by a vertical diametral plane, 

 parallel to the direction of the wind, is an ellipse, meeting the 

 ground at an obtuse angle on the side towards which the wind is 

 blowing, and at an acute angle on the opposite side. But as sound 

 moves in a direction perpendicular to the front of the sound waves, 

 it follows that sounds moving with a favorable wind tend to be tilted 

 downwards toward the ground ; and sounds moving against an 

 opposing wind tend to be tilted upward until, finally, they pass 

 above the head of a listener standing on the ground. 



The eifect of different elevations on the audibility of the same 

 sound has been brought within the sphere of scientific experiment. 

 In some experiments made by Prof. Reynolds in 1874, on " a flat 

 meadow," by the aid of an electrical bell, placed one foot from the 

 ground, it was found that elevation affected the range of sound 

 against the wind " in a much more marked manner than at right 

 angles." He adds : " Over the grass no sound could be heard with, 

 the head on the ground at twenty yards from the bell, and at thirty 

 yards it was lost with the head three feet from the ground, and its 

 full intensity was lost when standing erect at thirty yards. At 

 seventy yards, when standing erect, the sound was lost at long in- 

 tervals, and was only faintly heard even then ; but it became con- 

 tinuous again when the ear was raised nine feet from the ground, 

 and it reached its full intensity at an elevation of twelve feet."* 



In some experiments made by Prof. Henry, in 1875, he found 

 that while sound moving at right angles to the wind could not be 

 heard as far as sound moving with the wind, yet it was equally true 



* London, Ed., and Dub. Ph. Mag. for 1875, Vol. 50. 



