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Antiqua
Antiqua; Auszeichnung: Kursivierung.
The inhabitants of Atures and Maypures, what-
ever the missionaries may have asserted in their works,
are not more struck with deafness by the noise of the
great cataracts, than the Catadupes of the Nile. When
this 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 in-
tensity 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 con-
trary 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
From these facts, which are all we know with any cer-
tainty, 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. The nocturnal tempera-
ture of the atmosphere is 3° less than the temperature
of the day; at the same time the apparent humidity
augments at night, and the mist that covers the cata-
racts becomes thicker. We have just seen, that the
hygroscopic state of the air has no influence on the
propagation of the sound, and that the cooling of the
air diminishes its swiftness.
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, occa-
sion, during the day, a confused noise, which we per-
ceive 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
Oroonoko, where the air is constantly filled by an in-
numerable quantity of moschettoes, 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 the
sound by
the obstacles which they find in the currents of air
of
different density, and the partial undulations of the
atmosphere
caused by the unequal heating of different
parts of the soil. In calm air,
whether it be dry, or
mingled with vesicular vapours equally
distributed,sonorous
undulation is 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 Mr. Poisson has
recently
developed the theory with great sagacity.
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 oppos-
ing by a shock
the propagation of the sonorous undu-
lations. A shock, given to the surface
of a liquid,
will form circles around the center 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 ride over each other, and the
real cause
of the less intensity of sound during the day appears to
be
the interruption of homogeneity in the elastic medium.
During the
day, there is a sudden interruption of den-
sity wherever small streamlets
of air of a high tempe-
rature rise over parts of the soil unequally heated.
The
sonorous undulations are divided, as the rays of light
are
refracted, and form the mirage (looming,)
wherever
strata of air of unequal density are contiguous.
The
propagation of sound is altered, when a stratum of hy-
drogen gas is
made to rise in a tube closed at one end
above a stratum of atmospheric air;
and Mr. Biot has
well explained, by the interposition of bubbles of
car-
bonic acid gas, why a glass filled with Champagne
wine is little
sonorous so long as the gas is evolved,
and continues to pass through the
strata of the liquid.
Humboldt’s Personal Narrative.