Nocturnal Increase of Sounds.

—Hum-boldt endeavours to account for the in- crease of sounds during the night, fromobserving that the presence of the sunaffects the propagation and intensity ofsound by the obstacles opposed to itstransmission by currents of air of diffe-rent densities and partial undulation—theresult of the unequal heating of variousparts of the earth’s surface. In air at rest,whether it be dry, or mixed with elasticvapours equally distributed through it,the sonorous undulation is propagatedwithout difficulty. But when this air iscrossed in every direction by small cur-rents of a warmer temperature, the sono-rous undulation divides into two waves, atthe spot where there is the most suddenchange in the density of the medium;thus producing partial echoes, whichweaken the body of sound, because oneof the sonorous waves is reflected backupon itself. The theory of these parti-tions of sonorous waves has been explain-ed by M. Poisson.Ann. de Chimie, t. 7. It is not, therefore,the motion of the passage of the particlesof air from below upwards, nor the smalloblique currents of this fluid that we con-sider as opposing, by impulse, the pro-pagation of the sonorous waves. A stroke or impulse impressed on the surface ofthe liquid will form circles around theimpinging centre, even when the liquidis in agitation. Several kinds of wavesmay cross in air, as well as in water,without interfering with each other; butthe true cause of the less intensity ofsound in the day-time appears to be thewant of homogeniety in the elastic me-dium. There is at this time a suddenchange of density throughout, produced bysmall currents of air, of a high tempera-ture, rising from portions of the earth’ssurface that are unequally heated. Thesonorous waves are then divided in thesame manner as luminous rays are re-fracted, and form a mirage of soundwherever strata of air of unequal densityare contiguous. A distinction must bekept between the intensity of sound or oflight, and the direetion of the sonorousor luminous wave. When these wavesare propelled across strata of differentdensities two simultaneous effects will beproduced—there will be a change in thedirection of the wave, and extinction oflight or sound. The reflexion that ac-companies each refraction weakens theintensity of light; the separation of thesonorous wave causes partial echoes, andthat portion which returns on itself be-comes insensible to our ear, in weak noises, at the spot where the density ofthe medium suddenly changes. In the mirage with double images, that which hasundergone refraction contiguous to theearth is always weaker than the directimage. Strata of fluids, of very diffe-rent density, may so alternate, that theprimitive direction of the luminous orthe sonorous ray will remain the same,but the intensity of the ray will be notthe less weakened on that account. Du-ring the night the surface of the earthcools; the parts covered with grass, orwith sand take the same temperature:the atmosphere is no longer crossed bycurrents of hot air, rising obliquely orvertically in every direction. The me-dium being now become more homoge-neous, the sonorous wave passes withless difficulty, and the intensity of soundincreases, as the separations of the so-norous waves and echoes become lessfrequent.