The following passage from Dr. Prout's Treatise is so pertinent, that we beg leave to transcribe it: "Over the greater portion of the earth, the air which, during the day at least, is warmed by contact with the earth's surface, and thus becomes lighter, has a constant tendency to rise into the higher atmosphere. Now if this air were saturated with vapour, of course, whenever by rising it became mixed with colder air, its vapour would be more or less condensed, and a cloud would be formed. Hence, if we lived in such an atmosphere, we should be always enveloped in a mist, through which the sun would not be visible. But, by the benevolent arrangement we enjoy, this consequence is so entirely prevented, that, unless under peculiar circumstances, and always for beneficial purposes, the air at the earth's surface is hardly ever saturated with moisture. The air which has been warmed by contact with the earth can, therefore, rise from the surface without any condensation of its moisture within the limits of its point of saturation. Thus, at the equator, before the air reaches the temperature of 61°, the presumed point of its saturation, it must ascend to the height of 6,000 or 7,000 feet. At this height its vapour will be condensed, and a cloud will be formed, which may either be precipitated on the spot from which its constituent vapour had risen, or may be transported by the currents of the atmosphere, similarly to refresh a distant country, or may again be dissolved in the air; while, under all these contingencies, the whole of the lower portion of the atmosphere is exempt from mist, and continues perfectly transparent. These operations are unceasing; moreover, the very clouds, by giving out their latent heat, and shielding the earth's surface from the direct influence of the sun, have a still further effect, and have a constant tendency to modify their own formation and existence."