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Zitierempfehlung
Alexander von Humboldt: „Nocturnal increase of sounds“, in: ders., Sämtliche Schriften digital, herausgegeben von Oliver Lubrich und Thomas Nehrlich, Universität Bern 2021. URL: <https://humboldt.unibe.ch/text/1820-Sur_l_accroissement-08-neu> [abgerufen am 26.04.2024].
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| Titel | Nocturnal increase of sounds | ||||
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| Jahr | 1821 | ||||
| Ort | London | ||||
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Nachweis in: The Morning Post 15780 (12. Oktober 1821), [o. S.].
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| Sprache | Englisch | ||||
| Typografischer Befund | Antiqua; Spaltensatz; Auszeichnung: Kursivierung, Kapitälchen; Fußnoten mit Asterisken. | ||||
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Humboldt endeavours to account for the increaseof sounds during the
night, from observing that thepresence of the sun affects the propagation
and inten-sity of sound by the obstacles opposed to its trans-mission
by currents of air of different densities andpartial undulation, the result
of the unequal heatingof various parts of the earth’s surface. In
air at rest,whether it be dry or mixed with elastic vapoursequally
distributed through it, the sonorous undula-tion is propagated without
difficulty; but when thisair is crossed in every direction by small
currents of awarmer temperature, the sonorous undulation dividesinto
two waves at the spot where there is the mostsudden change in the density
of the medium; thusproducing partial echoes, which weaken the body
ofsound, because one of the sonorous waves is reflectedback upon
itself. The theory of these partitions ofsonorous waves has been explained
by M. Poisson.*It is not, therefore, the motion of the passage of
theparticles of air from below upwards, nor the smalloblique currents
of this fluid that we consider asopposing, by impulse, the propagation of
the so-norous waves. A stroke or impulse impressed
onthe surface of the liquid will form circles around theimpinging
centre, even when the liquid is in agitation.Several kinds of waves may
cross in air, as well as inwater, without interfering with each other; but
thetrue cause of the less intensity of sound in the day-time, appears
to be the want of homogenity in theelastic medium. There is at this time a
suddenchange of density throughout, produced by smallcurrents of air,
of a high tempetature, rising fromportions of the earth’s surface
that are unequallyheated. The sonorous waves are then divided in
thesame manner as luminous rays are refracted, andfrom a mirage of sound wherever strata of air of un-equal
density are contiguous. A distinction must bekept between the intensity of sound or of light andthe direction of the sonorous or luminous wave.When
these waves are propelled across strata ofdifferent densities, two
simultaneous effects willbe produced—there will be a change in
thedirection of the wave, and extinction of lightor sound. The
reflection that accompanies eachrefraction weakens the intensity of light;
theseparation of the sonorous ware causes partialechoes, and that
portion which returns on itself be-comes insensible to our ear, in weak
noises, at thespot where the density or the medium suddenlychanges. In
the mirage with double images, thatwhich has
undergone refraction contiguous to theearth is always weaker than the
direct image. Strataof fluids, of very different density, may so
alternate,that the primitive direction of the luminous or the
so-norous ray will remain the same, but the intensity ofthe ray will
be not the less weakened on that account.During the night the surface of
the earth cools, theparts covered with grass, or with sand, take the
sametemperature: the atmosphere is no longer crossedby currents of hot
air, rising obliquely or verticallyin every direction. The medium being now
becomemore homogeneous, the sonorous wave passes withless difficulty,
and the intensity of sound increases,as the separations of the sonorous
waves and echoesbecomes less frequent.
NOCTURNAL INCREASE OF SOUNDS.
* Ann. de Chimie, t. 7.