264 KOCTUBffAL PEOPAOATION OF SOUNDS. 



but this is untrue. When the noise is heard in the plain 

 that surrounds the mission, at the distance of more than a 

 league, you seem to be near a coast skirted by reefs and 

 breakers. The noise is three times as loud by night as by 

 day, and gives an inexpressible charm to these solitary 

 scenes. What can be the cause of this increased intensity 

 of sound, in a desert where nothing seems to interrupt 

 the silence of nature ? The velocity of the propagation of 

 sound, far from augmenting, decreases with the lowering of 

 the temperature. The intensity diminishes in air agitated 

 by a wind which is contrary to the direction of the sound ; 

 it diminishes also by dilatation of the air, and is weaker in 

 the higher than in the lower regions of the atmosphere, 

 where the number of particles of air in motion 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 azote 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. 



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, occasicn during the 

 day a confused 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 i Qsects, and the action of the wind upon the leaves be 

 interrupted. But this reasoning, even admitting its just- 

 ness, can scarcely be applied to the forests of the Orinoco, 

 where the air is constantly filled by an innumerable quantity 

 of mosquitos, where the hum of insects is much louder by 

 night than by day, and where the breeze, if ever it be felt, 

 blows only after sunset. 



I rather think that the presence of the sun acts upon the 

 propagation and intensity of sound by the obstacles met in 

 currents of air of different density, and by the partial un- 

 dulations of the atmosphere arising from the unequal heating 



