52 BPaXISH ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE. 



mass of air being supposed to be carried uniformly along, the time ■which the 

 sound would take to reach the observer, and consequently the radius of the sound- 

 wave, would be increased by the wind in the ratio of the velocity of sound to the 

 sum of the velocities of sound and of the wind, and the intensity would be dimin- 

 ished in the inverse duplicate ratio. But the effect is much too great to be attrib- 

 utable to this cause. It would be a strong wind whose velocity was a twenty- 

 fourth part of that of sound ; yet even in this case the intensity would be 

 diminished by only about a twelfth part. The first volume of the " Annales de 

 Chimie," (1816), 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 propagated 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 neighbourhood of the source of dis- 

 turbance 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 

 Tarious obstacles they meet with ; the upper by friction against the lower, and 

 so on. Hence the velocity 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 wind is aa 

 •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. Now, sound tends to propa- 

 gate 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 Bort 

 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 upwards, 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 operate, and therefore the wind produces no sensible effect, except what 

 arises fi'om 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 propagate 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 better 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. 



