8l Experiment/ and Inquiries refpeSllng Sound and Li^htt " ^ 



V. Of fonorous Cavities. 



M. de la Grange has alfo demonftrated, that all Impreflions arc tefle£led by an obftacle 

 terminating an claftic fluid, with the fame velocity with which they arrived at that obftacle. 

 When the walls of a paflage, or of an unfurnifhed room, are fmooth and perfeftly parallel, 

 any exploGon, or a ftamping with the foot, communicates an impreffion to the air, which 

 is refle£ted from one wall to the other, and from the fecond again towards the ear, nearly 

 in the fame dire£lion with the primitive impulfe : this takes place as frequently in a fecond, 

 as double the breadth of the paflage is contained in 1130 feet; and the ear receives a 

 perception of a mufical found, thus determined in its pitch by the breadth, of the paflage. 

 On making the experiment, the refult will be found accurately to agree with this ex- 

 planation. If the found is predetermined, and the frequency of vibrations fuch as that 

 each pulfe, when doubly reflefted, may coincide with the fubfequent pulfe proceeding 

 direftly from the founding body, the intenfity of the found will be much increafed by the 

 refleflion ; and alfo, in a lefs degree, if the refle£led pulfe coincides with the next but 

 one, the next but two, or more, of the dire£t pulfes. The appropriate notes of a room 

 may readily be difcovcred by finging the fcale in it ; and they will be found to depend on 

 the proportion of its length or breadth to 1 130 feet. The found of the ftopped diapafoa 

 pipes of an organ is produced in a manner fomewhat fimilar to the note from an explofion ir» 

 a pafl'age ; and that of its reed pipes to the refonance of the voice in a room : the length of 

 the pipe in one cafe determining the found, in the other, increafing its ftrengfh. The 

 frequency of the vibrations does not at all immediately depend on the diameter of the 

 pipe. It muft be confefled, that much remains to be done in explaining the precife 

 manner in which the vibration of the air in an organ pipe is generated. M. Daniel 

 Bernouilli has folved feveral difficult problems relating to the fubjedlj yet fomc of bis 

 aflumptlons are not only gratuitous, but contrary to matter of fad. 



Vl. Of the Divergence of Sound. 



It has been generally aflerted, chiefly on the authority of Newton, that if any found be 

 admitted through an aperture into a chamber, it will diverge from that aperture equally in 

 all direflions. The chief arguments in favour of this opinion are deduced from confider- 

 ing the phxnomena of the preflure of fluids, and the motion of waves excited in a poo! of 

 water. But the inference feems to be too haftily drawn : there is a very material difFerenoc 

 between impulfe and preflTure ; and> in the cafe of waves of water, the moving force at 

 each point is the power of gravity, which, ading primarily in a perpendicular dirediion, 

 is only fecondarily converted into a- horizontal fwrce, in the diredlion of the progrefs of the 

 waves, being at each ftep difpofed to fpread equally in. every direction: but the impulfe 

 tranfmitted by an elaftic fluid, a£ls primarily in the dire£lion of its progrefs. It is well 

 knovoj that if a perfon calls to another with a fpeaking trumpet,, he points it towards the 



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