74 Mr Ch. Martins on the Intensity of Sound 



the temperature of the air was 24° cent., the barometer was 



744mm.3^ 



The same experiment having been repeated at eleven 

 o'clock in the evening in the same place, it was only at a 

 distance of 379 metres that the sound of the diapason ceased 

 to be heard by each of us. This difference of 125 metres in 

 the distance at which the sound ceased to be perceptible in 

 the day and in the night, is in accordance with the conse- 

 quences that Zanotti* deduced from the researches of 

 Hauksbee ; it confirms also the results obtained by De la 

 Roche, near Paris, and the observations of M. de Humboldt 

 upon the banks of the Orinoco,t where the cataracts were 

 heard much better during the night than during the day, 

 although the buzzing of insects was more intense, and the 

 cries of wild animals were louder than during the day. M. 

 Lepileur and I were equally surprised not to find in the 

 night a more complete silence than in the middle of the day. 

 The hum of the insects, the fall of the small branches from 

 the trees, the baying of the dogs in the distance disturbed 

 our experiment, nevertheless the sounds of the diapason 

 were heard at a distance greater by 125 metres that at mid- 

 day. The air was calm, the sky overcast, the thermometer 

 at 17° cent., and the barometer at 744«i"i-7. 



The third experiment was made by M. Bravais and I, the 

 1st October 1844, between eleven o'clock forenoon and noon, 

 upon the western ridge of the Faulhorn in Switzerland, at a 

 mean height of 2620 metres above the sea. The sky was 

 clear, the air at 7°*2 cent. ; the tension of the vapour of water 

 5mm. 52 ; a weak breeze blew from WSW., that is to say 

 in the direction of the two observers ; the barometer stood at 

 55gmm.5, The distance at which the diapason ceased to be 

 heard was 550 metres ; on approaching each other 15 metres, 

 we heard the diapason anew. The silence was complete. 

 M. Bravais and I made the fourth experiment on the grand 

 plateau of Mont Blanc, on the grand circus of snow, sheltered 

 on the north side, situated 900 metres below the summit, 

 and 3910 metres above the sea. At the time of the experi- 



* Commentarii hononienses, t. i. p. 179 ; 1748. 

 t Tableaux de la Nature, t. i. p. 248. 



