210 



THE EEFRACTION OF SOUND. 



deflection presented the only satisfactory explanation of the familiar 

 fact of observation that sound is usually heard many times farther in 

 the direction of a wind than in a direction opposed to its action. His 

 explanation of this phenomenon is as follows : 



"If we imagine the whole massof airin the neighbourhood of the source 

 of disturbance divided into horizontal strata, these strata do not all 

 move with the same velocity. The lower strata are retarded by friction 

 against the earth, and by various obstacles they meet with ; the upper 

 by friction against the lower, and so on. Hence the velocity increases 

 from the ground upward, conformably with observation. This difference 

 of velocity disturbs the spherical form of the sound-wave, tending to 

 make it somewhat of the form of an ellipsoid, the section of which, by a 

 vertical diametrical plane parallel to the direction of the wind, is an 

 ellipse meeting the ground at an obtuse angle on the side toward which 

 the wind is blowing, and an acute angle on the opposite side. Now sound 

 tends to propagate itself in a direction perpendicular to the sound-wave ; 

 and if a portion of the wave is intercepted by an obstacle of large size, 

 the space behind is left in a sort of sound-shadow, and the only sound 

 there heard is what diverges from the general wave after passing 

 the obstacle. Hence, near the earth, in a direction contrary to 

 the wind, the sound continually tends to be propagated upward, 

 and consequently there is a continual tendency for an observer in 

 that direction to be left in a sort of sound-shadow. Hence, at a 

 sufficient distance, the sound ought to be very much enfeebled ; but 

 near the source of disturbance this cause has not yet had time to oper- 

 ate, and therefore the wind produces no sensible effect, except what 

 arises from the augmentation in the radius of the sound-wave, and this 

 is too small to be perceptible. In the contrary direction — that is, in the 

 direction toward which the wind is blowing — the sound tends to propa- 

 gate itself downward, and to be reflected from the surface of the earth ; 

 and both the direct and reflected waves contribute to the effect per- 

 ceived."— (JKep- ^rit. Assoc, 1857, xxvii, p. 23 of Abstracts.) 



:^^a& ' >. 



Fig. 4. — Vental relractiou. 

 This action may be illustrated by Fig. 4, in which the larger arrow 



