The Sun.
London, England, 1821-10-10 00:00:00

LITERARY AND SCIENTIFIC INTELLIGENCE.
LFROM 'VARIOUS SOURCES.]

No. XXI. •
_NOCTURNAL INCREASII- o F S9UND3.—llumboldt
endeavours to account for the increase of sounds
during the night, from observing that the presence
of the sun affects the propagation-and intensity of
sound by the obstacles opposed to its transmis-
sion by. currents of air of different: densities and
partial undulation, , the result of the unequal
heating of various parts of the earth's surface.
In air at rest, whether it be dry or mixed with
elastic vapours equally distributed through it, the
sonorwis undulation is propagated without diffi-
culty ; but when this air is crossed in every di.7
.rection by small currents warmer tempera-
: ture, 'the sonorous unduLgtioh divides into two
waves at the spot where there is the most sud-
den change in the density of the medium; thus
.producing partiatechoes, which Weaken the hoc's,
of sound, because one of the sonorous waves
is reflected back upon itself. The theory 'of
these- partitions of sonorous waves has. been ex-
plained by M. •Poisson.* It is not, therefore,
the motion of the passage of the particles
of air from below • upwards, nor the small
oblique currents of this fluid that we consider as
opposing, by impulse, the propagation of the so-
norous waves. - A stroke or impulse impressed on
the surface of the liquid will form circles around
the impinging centre, even when the liquid is in
agitation. Several kinds of waves may cross in
air, as
,well as_ in water, without interfering . with
each other ; but the true cause of . the less inten-
sity of sound in the day-time, appears to be the
want of homogeneity in the elastic medium.—
There is at this time a sudden change of density
throughout, produced by small currents of air, of
a high temperature, rising from portions of the
earth's surface that are unequally heated. The
sonorous waves are then divided in the same man-
ner as luminous rays are refracted, and from a mi-
rage of sound wherever strata of air of unequal
density are contiguous. A distinction must be
kept between the intensity of sound or of light,
and the direction of the sonorous or luminous wave.
When these waves are propelled across strata of
different densities,,two simultaneous effects will
be produced--there will be a change in the
direction of the wave, and extinction of light
or sound. The reflection that accompanies each
refraction weakens the intensity of light; the
separation of . the sonorous wave causes partial
.echoes, and that portion which returns on itself
becomes insensible to our ear, in weak noises, at
the spot where the density or the medium sud-
denly changes. In the mirage with double
images, that which has undergone refraction con-
tiguous to the earth is always weaker than the
direct image. Strata of fluids, of very different
density, may so alternate, that the primitive di-
rection of the. luminous or the sonorous ray will
remain the same, but the intensity of the ray
will be not the less weakened on-that account.
During the night the surface of the earth cools,
the parts covered with grass, or with sand, take
the same temperature : the atmosphere is no
longer crossed by currents of hot air, rising
obliquely or vertically in every direction. The
medium being now become more homogeneous,
the sonorous wave passes with less difficulty, and
the intensity of sound increases, as the separa-
tions of the sonorous waves and echoes become
less frequent.
• FoluviATION OF MISTS-IN PARTICULAR SITUA-
TIONS.—Sir Humphrey Davy has transmitted the
following remarks on this subject to the Royal So-
ciety:—ssoon as the Sun has disappeared from any
part of theglobe the surface begins to lose heat by
radiation, and in greater proportion as the sky is
clearer: but the land and water are cooled by
this operation in a very different manner; the
impression of cooling on the land is limited to the
surface, and very slowly transmitted to the in-
terior ; whereas, in water above 409 Fahren-
heit, as soon as the upper, stratum is cooled,
whether by radiation or evaporation, it sinks in
the mass. of fluid and its place is supplied by
warmer waters from below ; and till the tempe-
rature of the whole mass is reduced nearly to
40° Fahrenheit, 'the surface cannot be the coolest
part. It follows, therefore, that wherever water
exists in considerable masses, and has a tempera-
ture nearly equal to that of the land, or only a
few degrees below it, and above 45° Fahrenheit
at sun set, its surface during. the night, in calm
and clear weather, will be warmer than that of
the contiguous land, and the air above the land
will necessarily be cooler than that above the
water; and when they both contain their due
proportion of aqueous vapour, and the situation
of the ground is such as to permit the cold air
froM the land to mix with the warmer air above
the water, mist or fog will be the result, which
will be so much greater in quantity as the land
surrounding or inclosing the water is higher, the
water deeper, and the temperature Of the water,
which will coincide with the.quantity or strength
of vapour in the air above it, greater.
ORGANIC REMAINS.—The quarries of marble
whence the .bloCks are taken for the construction
of the Plymouth Break-water, are situated at
Oreston, on the eastern shore of catwater. They
consist,of one vast mass of compact close-grained
marble;, seams of clay, however, are interposed
through the rock, in which there are also large
cavities, some empty, and others partially' filled
with clay. In one of these caverns in the solid
rock, fifteen feet wide, forty-five feet long, and
twelve feet deep, nearly filled with compact
clay, were found imbedded fossil with
be-
longing to. the Rhinoceros, and portions of the
skeletons of three difft;rent animals, all of them

** Arm. de CLimie, L. 7.