Nocturnal increase of Sounds.

— Humboldt accounts for the increase of sounds during the night, by observing, that the presence of the sun affects the propagation and intensity of sound by the obstacles opposed to its transmission 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 sonorous undulation is propagated without difficulty.

But when this air is crossed in every direction by small currents of a warmer temperature, the sonorous undulation divides into two waves, at the spot where there is the most sudden change in the density of the medium; thus producing partial echoes, which weaken the body of sound, because one of the sonorous waves is reflected back upon itself.

The theory of these partitions of sonorous waves has been explained 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 sonorous 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 intensity of sound in the day-time appears to be the want of homogeniety 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 manner as luminous rays are refracted, and form a mirage 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 producedthere 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 of the medium suddenly changes.

In the mirage with double images, that which has undergone refraction contiguous to the earth is always weaker than the direct image.

Strata of fluids, of very different density, may so alternate, that the primitive direction 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 separations of the sonorous waves and echoes become less frequent.

Ann. de Chimie, t. 7.