in the liarified Air of High Mountains. 73 



tion of the branches of the diapason was 5™"^ 6 and 8 mille- 

 nietres when the wooden cylinder employed to put them in 

 motion was placed between them. With a sound having 

 always the same intensity in air of equal density, it is evident 

 that the variable distance at which it ceases to be percepti- 

 ble in mediums of different densities, should give us a mea- 

 sure of the variations of this intensity. 



The agitation of the air complicates these experiments. Its 

 influence has been successively studied by M. de Holdat at 

 Nancy,* and by M. de la Roche at Paris. t They found that 

 the limit of distance for hearing is increased or diminished 

 for a person having the wind from or to the origin of the 

 sound. But both agree in affirming that the sound is heard 

 at the greatest possible distance in air at rest ; the noise 

 from the wind, come from what part it may, interfering with 

 the perception of the sound. Our experiments having always 

 been made during calm weather, or during a slight intermit- 

 tent breeze which permitted us to choose the intervals of re- 

 pose, we shall not occupy ourselves with this complication. 

 We had, besides, two diapasons that we sounded alternately : 

 if, then, the wind had favoured the hearing for one, it would 

 have hindered it for the other ; now we have never observed 

 this circumstance. At the limiting distance, the sound 

 ceased to be perceived by both listeners at the same time. 



Our first trial took place the 22d June 1844, between one 

 and two o'clock afternoon, upon a desert plain in front of the 

 village of Saint-Cheron (Seine-et-Oise.) M. Lepileur and I 

 successively increased our distance till it attained 254 metres ; 

 at this distance I no longer heard the diapason of M. Lepileur, 

 and in six experiments he heard mine only once. The day 

 was calm, the sky overcast, a very faint breeze blew from 

 the south, that is to say almost perpendicularly to the line 

 which joined the observers. The silence was imperfect and 

 disturbed by the cries of birds and the buzzing of insects ; 



* Recherches sur la propagation du son dans I'air agit6par le vent {JowmaX 

 de Physique, t. Ixxix. p. 285 ; 1814.) 



t >Sur Tinfluence que le vent exerce dans la propagation du son, sous le 

 rapport de son intensity {Annates de Chimieet dePhynque, t. i. p. 176; 1816.) 



