264 
NOCTURNAL PROPAGATION OP SOUNDS. 
but tMs 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. Prom 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, occasion 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 hu® 
of insects, and the action of the wind upon the leaves 
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. 
1 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 
