NATURAL PHILOSOPHY. 249 



the ratio of the velocity of sound to the sum of the velocities of sound and 

 of the wind, and the intensity would be diminished in the inverse duplicate 

 ratio. But the effect is much too great to be attributable to this cause. It 

 would be a strong wind whose velocity was a twenty-fourth part of that 

 of sound ; yet cveh in this case the intensity would be diminished by only 

 about a twelfth part. The first volume of the "Annalcs de Chimie," ( 181G), 

 contains a paper by M. Delaroche, giving the results of some experiments 

 made on this subject. It appeared from the experiments, First, that at 

 small distances the wind has hardly any perceptible effect, the sound being 

 propagated almost equally well in a direction contrary to the wind and in 

 the direction of the wind ; Secondly, that the disparity between the intensity 

 of the sound propagated in these two directions becomes proportionally 

 greater and greater as the distance increases ; Thirdly, that sound is propa- 

 gated rather better in a direction perpendicular to the wind than even in the 

 direction of the wind. The explanation offered by the author of the present 

 communication is as follows. If we imagine the whole mass of air in the 

 neighborhood 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 the various obstacles they meet 

 with ; the upper by friction against the lower, and so on. Hence the velo- 

 city increases from the ground upwards, 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 diametral plane parallel to the direction of the Avind is 

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

 the wind is blowing, and an acute angle on the opposite side. Xow, 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 con- 

 tinually tends to be propagated upwards, and consequently there is a con- 

 tinual 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 operate, 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 towards which the wind is blowing, the sound tends to propa- 

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

 and both the direct and reflected waves contribute to the effect perceived. 

 The two waves assist each other so much the better as the angle between 

 them is less, and this angle vanishes in a direction perpendicular to the wind. 

 Hence, in the latter direction the sound ought to be propagated a little bet- 

 ter than even in the direction of the wind, which agrees with the experiments 

 of M. Delaroche. Thus the effect is referred to two known causes, the 

 increased velocity of the air in ascending, and the diffraction of sound. 



