Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 2 - By Alexander Von Humboldt And Aime Bonpland.
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Oscillations, That At
First Seem Partial, React On The Whole Liquid Mass Contained In The
Trunk As Well As In Its Numerous Ramifications.
Some of the Missionaries in their writings have alleged that the
inhabitants of Atures and Maypures have been struck with deafness by
the noise of the Great Cataracts, 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.
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