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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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.
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 hum of
insects, and the action of the wind upon the leaves be interrupted.
But this reasoning, even admitting its justness, 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 undulations of the atmosphere
arising from the unequal heating of different parts of the soil. In
calm air, whether dry or mingled with vesicular vapours equally
distributed, sound-waves are propagated without difficulty. But when
the air is crossed in every direction by small currents of hotter air,
the sonorous undulation is divided into two undulations where the
density of the medium changes abruptly; partial echoes are formed that
weaken the sound, because one of the streams comes back upon itself;
and those divisions of undulations take place of which M. Poisson has
developed the theory with great sagacity.* (* Annales de Chimie tome 7
page 293.) It is not therefore the movement of the particles of air
from below to above in the ascending current, or the small oblique
currents that we consider as opposing by a shock the propagation of
the sonorous undulations. A shock given to the surface of a liquid
will form circles around the centre of percussion, even when the
liquid is agitated. Several kinds of undulations may cross each other
in water, as in air, without being disturbed in their propagation:
little movements may, as it were, ride over each other, and the real
cause of the less intensity of sound during the day appears to be the
interpretation of homogeneity in the elastic medium. During the day
there is a sudden interruption of density wherever small streamlets of
air of a high temperature rise over parts of the soil unequally
heated. The sonorous undulations are divided, as the rays of light are
refracted and form the mirage wherever strata of air of unequal
density are contiguous. The propagation of sound is altered when a
stratum of hydrogen gas is made to rise in a tube closed at one end
above a stratum of atmospheric air; and M. Biot has well explained, by
the interposition of bubbles of carbonic acid gas, why a glass filled
with champagne is not sonorous so long as that gas is evolved, and
passing through the strata of the liquid.
In support of these ideas, I might almost rest on the authority of an
ancient philosopher, whom the moderns do not esteem in proportion to
his merits, though the most distinguished zoologists have long
rendered ample justice to the sagacity of his observations. "Why,"
says Aristotle in his curious book of Problems, "why is sound better
heard during the night? Because there is more calmness on account of
the absence of caloric (of the hottest).* (* I have placed in a
parenthesis, a literal version of the term employed by Aristotle, to
express in reality what we now term the matter of heat. Theodore of
Gaza, in his Latin translation, expresses in the shape of a doubt what
Aristotle positively asserts. I may here remark, that, notwithstanding
the imperfect state of science among the ancients, the works of the
Stagirite contain more ingenious observations than those of many later
philosophers. It is in vain we look in Aristoxenes (De Musica), in
Theophylactus Simocatta (De Quaestionibus physicis), or in the 5th
Book of the Quest. Nat. of Seneca, for an explanation of the nocturnal
augmentation of sound.) This absence renders every thing calmer, for
the sun is the principle of all movement." Aristotle had no doubt a
vague presentiment of the cause of the phenomenon; but he attributes
to the motion of the atmosphere, and the shock of the particles of
air, that which seems to be rather owing to abrupt changes of density
in the contiguous strata of air.
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