76 Mr Ch. Martins on the Intensity of Sound 



Xy the intensity in air of the density d (air of the station) 



at the distance R : we have, in virtue of the first law, 



i '. X : : \ : d ; 



and from the second, 



J : a: : : R2 . ^2 . 



whence, 



Vd 



To calculate the distance for the limit of audibility in air 

 of density 1, we have then to divide the distance observed 

 by the square root of the density of the air in which the 

 experiment was made ; we obtain the density d by the fol- 

 lowing formula, in which H represents the height of the 

 barometer, and t the temperature of the air in degrees cen- 

 tigrade. 



H 



d 



\^ ^ 3000 ) 



760 ( 1 + 



All the distances for the limit of audibility reduced to what 

 they would have been in air at density 1, will be found in 

 the following table : — 



If we analyse these observations, we observe that there 

 are causes which favour the hearing of sound on high moun- 

 tains, that more than compensate the rarefaction of the air. 

 "We see, in fact, that we have heard a sound at the greatest 



* T. 



p. 706. 



