--_Memoirs of the Duchess of Abrantes._

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SOUNDS DURING THE NIGHT.


The great audibility of sounds during the night is a phenomenon of
considerable interest, and one which had been observed even by the
ancients. In crowded cities or in their vicinity, the effect was
generally ascribed to the rest of animated beings, while in localities
where such an explanation was inapplicable, it was supposed to arise
from a favourable direction of the prevailing wind. Baron Humboldt
was particularly struck with this phenomenon when he first heard the
rushing of the great cataracts of the Orinoco in the plain which
surrounds the mission of the Apures. These sounds he regarded as
three times louder during the night than during the day. Some authors
ascribed this fact to the cessation of the humming of insects, the
singing of birds, and the action of the wind on the leaves of the
trees, but M. Humboldt justly maintains that this cannot be the cause
of it on the Orinoco, where the buzz of insects is much louder in the
night than in the day, and where the breeze never rises till after
sunset. Hence he was led to ascribe the phenomenon to the perfect
transparency and uniform density of the air, which can exist only at
night after the heat of the ground has been uniformly diffused through
the atmosphere. When the rays of the sun have been beating on the
ground during the day, currents of hot air of different temperatures,
and consequently of different densities, are constantly ascending from