RAIN. The ancients appear to have been very imperfectly ac quainted with the constitution of the atmosphere; and Descartes was probably the first who, iu attempting to refer meteorological phenomena to their causes, approached near the hypotheses now generally received ; for he ascribes the formation of clouds, snow, rain, and hail to varia tions of temperature in the upper regions of the air. /le supposes that when the coldness of any portion of those regions becomes Intense, the subtle matter disseminated among the particles of vapour becoming too weak to keep those particles at a distance from one another, the latter must rush together, and either form small spicular filaments or spherical drops of ice. The superficies of these filaments or drops being supposed to be considerable when compared with their volumes, lie conceives that the resistance of the air may be great enough to pre vent them from descending by their weight, and that thus a great assemblage of them may remain suspended in the form of a cloud above the earth. The filaments becoming by au accession of heat partially liquefied, it may happen that many of them will adhere together, and thus form flakes of snow, m bleb, at length acquiring sufficient weight to overcome the resistance of the air, descend to the ground. In order to explain the origin of rain and hail, he supposes that the flakes, on arriving near the surface of the earth, may pass through a warmer region than that in which they were formed, and there dissolving, they assume the figure of spherical or spheroidal drops of water. Again, if in the descent the latter should meet a current of cold air, they become globules of ice. (` Dleteora,' cap. v. vi.) The absence of observation in all this, however, is evinced by the theory of the formation of snow, to which it is evident that Descartes never applied even such a moderate magnifier as he might readily have obtained, or he could not have failed to see the true nature of its aggregation.
The diffusion of electricity through the earth and atmosphere has led some meteorologists to believe that the variations in its quantity or intensity in particular regions may be the cause of the formation of snow, rain, and hail. The electrical particles, being endowed with a
great repulsive power, are supposed to keep in general the particles of vapour asunder ; and when, from any cause, some given volume of air is deprived of its natural quantity of electricity, these particles unite by their mutual attractions, and thus form drops of rain or ice. From the showers which accompany a thunder-storm, there is no doubt that electricity co-operates in some measure in the production of rain ; and it may be remarked in support of the above hypothesis that rain is most abundant among mountains, their elevated summits being favour able for receiving and discharging electricity ; while in some regions where thunder is little known there is also little rain. But the idea that repulsive electrical particles keep asunder the particles of aqueous vapour is itself purely hypothetical.
The theory first proposed by Dr. Hutton of Edinburgh (' Trans. Roy. Soc. Edin., 1784), is that which appears to correspond most satis factorily to the observed phenomena of the atmosphere ; and accord ingly it has been adopted by nearly every distinguished meteorologist since that time. This theory will be briefly described.
The atmosphere surrounding the earth is known to consist of air and aqueous gas or vapour, both of which are elastic ; and, according to the experiments of M. Gay-Lussac, the elasticity of the vapour is equal to that of the air at an equal temperature, both when the vapour exists alone, and when it is diffused through the air ; hence it is inferred that in the atmosphere the vapour and air are in mechanical mixture only, and also that the particles of the former have the power of moving freely in the intervals between those of the latter. The atmosphere is supplied with humidity by evaporation from the waters of the earth, and its power to hold the water in suspension depends on its temperature, an increase of the latter augmenting that power, and a decrease diminishing it : but in the theory of Hutton, the diminution of the power takes place in a higher ratio than the diminu tion of the temperature.