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.Ann. de Chimie, t. 7.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.