68 



is greater in the same radius. The intensity is 

 the same in dry air, and in air mingled with 

 vapours ; but it is feebler in carbonic acid gas, 

 than in mixtures of azot and oxygen. From these 

 facts, which are all we know with any certainty, 

 it is difficult to explain a phenomenon observed 

 near every cascade in Europe, and which, long 

 before our arrival in the village of Atures, had 

 struck the missionary and the Indians. The 

 nocturnal temperature of the atmosphere is 3° 

 less than the temperature of the day ; at the 

 same time the apparent humidity augments at 

 night, and the mist that covers the cataracts 

 becomes thicker. We have just seen, that the 

 hygroscopic state of the air has no influence on 

 the propagation of the sound, and that the cool- 

 ing of the air diminishes it's swiftness. 



It may be thought, that, even in places not 

 inhabited by man, the hum of insects, the song 

 of birds, the rustling of leaves agitated by the 

 feeblest winds, occasion during the day a con- 

 fused noise, which we perceive the less because 

 it is uniform, and constantly strikes the ear. 

 Now this noise, however slightly perceptible it 

 may be, may diminish the intensity of a louder 

 noise ; and this diminution may cease, if during 

 the calm of the night the song of birds, the hum 

 of insects, and the action of the wind upon the 

 leaves, be interrupted. But this reasoning, 

 even admitting it's justness, can scarcely be ap- 



