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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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?
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