respecting Sound and Light. 119 



impervious to sound. Indeed, as M. Lambert has very truly 

 asserted, the whole theory of the speaking trumpet, supported 

 as it is by practical experience, would fall to the ground, if it 

 were demonstrable that sound spreads equally in every direc- 

 tion. In windy weather it may often be observed, that the 

 sound of a distant bell varies almost instantaneously in its 

 strength, so as to appear at least twice as remote at one time as 

 at another ; an observation which has also occurred to another 

 gentleman, who is uncommonly accurate in examining the 

 phaenomena of nature. Now, if sound diverged equally in all 

 directions, the variation produced by the wind could never 

 exceed one-tenth of the apparent distance : but, on the suppo- 

 sition of a motion nearly rectilinear, it may easily happen that a 

 slight change in the direction of the wind, may convey the 

 sound, either directly or after reflection, in very different de- 

 grees of strength, to the same spot. From the experiments on 

 the motion of a current of air, already related, it would be 

 expected that a sound, admitted at a considerable distance from 

 its origin through an aperture, would proceed, with an almost 

 imperceptible increase of divergence, in the same direction; for, 

 the actual velocity of the particles of air, in the strongest sound, 

 is incomparably less than that of the slowest of the currents in 

 the experiments related, where the beginning of the conical 

 divergence took place at the greatest distance. Dr. Matthew 

 Young has objected, not without reason, to M. Hube, that the 

 existence of a condensation will cause, a divergence in sound: 

 but a much greater degree of condensation must have existed 

 in the currents described than in any sound. There is indeed 

 one difference between a stream of air and a sound ; that, in 

 sound, the motions of different particles of air are not synchro- 



