Experiments and Inquiries refpeSimg Sound and Light, 83 



place where his hearer ftands : and I am aflured by a very refpe6lable Member of the 

 Royal Society, that the report of a cannon appears many times louder to a perfon towards 

 whom it is fired, than to one placed in a contrary direction. It muft have occurred to 

 every one's obfervation, that a found fuch as that of a mill, or a fall of water, has ap- 

 peared much louder after turning a corner, when the houfe or other obftacle no longer 

 intervened ; and it has been already remarked by Euler, on this head, that we are not 

 acquainted with any fubftance perfe£Hy impervious to found. Indeed, as M. Lambert has 

 very truly aflerted, the whole theory of the fpeaking trumpet, fupported as it is by prac- 

 tical experience, would fall to the ground, if it were demonftrable that found fpreads 

 equally in every direction. In windy weather it may often be obferved, that the found 

 of a diftant bell varies almoft inftantaneoufly in its ftrength, fo as to appear at leaft twice 

 as remote at one time as at another ; an obfervation which has alfo occurred to another 

 gentleman, who is uncommonly accurate in examining the phscnomena of nature. Now, 

 if found diverged equally in all diredlions, the variation produced by the wind could never 

 exceed one-tenth of the apparent diftance : but, on the fuppofition of a motion nearly 

 re£lillinear, it may cafily happen that a flight change in the diredlion of the wind, may 

 convey the found, either diredtly or after refledtion, in very different degrees of ftrength^ 

 to the fame fpot. From the experiments on the motion of a current of air, already 

 related, it would be expe£tcd that a found, admitted at a confidcrable diftance from its 

 origin through an aperture, would proceed, with an almoft imperceptible increafe of 

 divergence, in the fame direftion ; for, the a£lual velocity of the particles of air, in the 

 ftrongeft found, is Incomparably lefs than that of the floweft of the currents in the ex- 

 periments related, where the beginning of the conical divergence took place at the greateft 

 diftance. Dr. Matthew Young has obje£led, not without reafon, to M. Hube, that the 

 exiftence of a condenfation will caufe a divergence in found : but a much greater degree of 

 condenfation muft have exifted in the currents defcribed than in any found. There Is 

 indeed one difference between a ftream of air and a found ; that, in found, the motions of 

 different particles -of air are not fynchronous : but It is not demonftrable that this circum- 

 ftance would affeft the divergency of the motion, except at the inftant of its commence- 

 ment, and perhaps not even then in a material degree ; for. In general, the motion Is 

 communicated with a very gradual increafe of intenfity. The fubje£l, however, defervcs a 

 more particular Inveftigation ; and, in order to obtain a more folid foundation for the 

 argument, It is propofed, as foon as circumftances permit, to inftltute a courfe of experi- 

 ments for afcertaining, as accurately as pofhble, the different ftrength of a found once 

 proje£led In a given dire£lIon, at different diftances from the axis of its motion. 



VII. Of the Decay of Sound. 



Various opinions have been entertained refpefting the decay of found. M. de la Grange 

 has publifhed a calculation, by which its force is (hown to decay nearly in the fimple ratio of 

 the diftances i and M. Daniel Bernouilli's equations for the founds of conical pipes lead to 



M 2 a fimllar 



