in the supposition that the detached and ascending vesicles of wa. ter are impregnated with highly rarefied air, instead of with the subtle ether of Des Cartes. But the hypothesis now generally admitted is that of solution first of all advanced by the Abbé Nollet, in his Leçons de Physique Experimentale. Water and air, it is contended, have a mutual power of dissolving each other; and air is not more frequently ex tricated from the former than water is from the latter. The lower part of the atmosphere being pressed, then, by the weight of the incumbent column on the surface of the water, and perpetually rubbing against it, attracts and dissolves those particles with which it is in contact and separates them from the rest of the water. The aqueous particles thus detached, and absorbed by the lower column of air, are, next, still more forcibly attracted by the superior in consequence both of its being drier, and possessing ampler pores to receive the dissolved vapour. When the aqueous particles attain a certain degree of elevation, the coldness of the atmosphere condenses them, and they coalesce into particles of a larger diameter, and gradually produce the phenomenon of a CLOUD. When the particles, of which such cloud consists, are more closely com. pacted, either by the mutual attraction of cohesion, or the external pressure of the wind against it, they run into drops sufficiently ponderous to descend in the form of RAIN. If the cloud become frozen by any current of cold air, before its particles are formed into drops, small fragments of them being condensed, and conse quently increased in weight, will detach themselves from the general mass, and fall down in thin flakes of snow. If its particles have coalesced into drops prior to its being frozen, these drops will then descend in the form of HAIL-stones: And when the lower air is replete with aqueous vapour dissolved in its pores, and a sudden current of cold wind brushes through it, producing the natural frigidity of the superior atmosphere, a MIST or FOG, which is only a kind of inferior cloud, is immediately created; and as suddenly again dispersed on the return of the natural warmth of the air, which then re-dissolves, to invisible minuteness, the vapoury particles. In like manner DEW.drops may be regarded as a species of inferior rain, the cold attaching the dissolved vapours of the lower atmosphere, being either more intense than in the case of fogs, or continued for a greater length of time. VOL. IV. [GooD's Trans. of Lucretius. Note on Book vi. 467. Rain never begins to fall while the air is transparent: the invisible vapours first pass their maximum, and are changed into vesi. cular vapours; clouds are formed, and these clouds gradually dissolved in rain. Clouds, however, are not formed in all parts of the horizon at once; the formation begins in one particular spot, while the rest of the air remains clear as before: this cloud rapidly increases till it overspreads the whole horizon, and then the rain begins. It is remarkable, that though the greatest quantity of vapours exist in the lower strata of the atmosphere, clouds never begin to form there, but always at some considerable height. It is remark. able, too, that the part of the atmosphere at which they form, has not arrived at the point of extreme moisture, nor near that point, even a moment before their formation. They are not formed, then, because a greater quantity of vapour had got into the atmo. sphere than could remain there without passing its maximum. It is still more remarkable, that when clouds are formed, the temperature of the spot in which they are formed is not always lowered, though this may sometimes be the case. On the contrary, the heat of the clouds themselves is sometimes greater than that of the surrounding air*. Neither, then, is the formation of clouds owing to the capacity of air for combining with moisture being lessened by cold; so far from that, we often see clouds, which had remained in the atmosphere during the heat of the day, disappear in the night, after the heat of the air was diminished. The formation of clouds and rain, then, cannot be accounted for by the principles with which we are acquainted. It is neither owing to the saturation of the atmosphere, nor the diminution of heat, nor the mixture of airs of different temperatures, as Dr. Hut. ton supposed; for clouds are often formed without any wind at all either above or below them; and even if this mixture constantly took place, the precipitation, instead of accounting for rain, would be almost imperceptible. It is a very remarkable fact, that evaporation often goes on for a month together in hot weather without any rain. This sometimes happens in this country; it happens every year in the torrid zone. Thus at Calcutta, during January 1785, it never rained at allt: the mean of the thermometer for the whole month was 66 degrees; * De Luc sur la Meteorol. ii. 100, + Asiatic Researches, vol. ii. Appendix. there was no high wind, and indeed during great part of the month little wind at all. The quantity of water evaporated during such a drought must be very great; yet the moisture of the air, instead of being increased, is constantly diminishing, and at last disappears almost entirely; for the dew, which is at first copious, diminishes every night and if Dr. Watson's experiment formerly mentioned be attended to *, it will not be objected that the quantity of evaporation is also very much diminished. Of the very dry state to which the atmosphere is reduced during long droughts, the violent thunder. storms with which they often conclude is a proof, and a very de. cisive one. Now what becomes of all this moisture? It is not accumulated in the atmosphere above the country from which it was evaporated, otherwise the whole atmosphere would in a much less period than a month be perfectly saturated with moisture. If it be carried up daily through the different strata of the atmosphere, and wafted to other regions by superior currents of air, how is it possible to account for the different electrical state of the clouds situated between different strata, which often produces the most violent thunder-storms? Are not vapours conductors of the electric fluid? and would they not have daily restored the equilibrium of the whole atmosphere through which they passed? Had they tra versed the atmosphere in this manner, there would have been no negative and positive clouds, and consequently no thunder-storms. They could not have remained in the lower strata of the atmo sphere, and been daily carried off by winds to other countries; for there are often no winds at all during several days to perform this office; nor in that case would the dews diminish, nor could their presence fail to be indicated by the hygrometer. It is impossible for us to account for this remarkable fact upon any principle with which we are acquainted. The water can nei ther remain in the atmosphere, nor pass through it in a state of vapour. It must therefore assume some other form; but what form is, or how it assumes it, we know not. There are, then, two steps of the process between evaporation and rain, of which at present we are completely ignorant: 1. What * See p. 134. becomes of the vapour after it enters into the atmosphere. 2. What makes it lay aside the new form which it must have assumed, and return again to its state of vapour, and fall down in rain. And till these two steps be discovered by experiments and observations, it will be impossible for us to give a rational or a useful theory of rain. Dr. Pratt of Exeter has endeavoured to prove, in a very ingenious treatise, that water is decomposed during its evaporation, and converted into oxygen and hydrogen gas; but the absence of any perceptible quantity of this last gas in the atmosphere, even when rain is actually forming, cannot be accounted for, unless we suppose that the products of the decomposition are different. Girtanner's theory, that azote is composed of hydrogen and oxygen, would remove every difficulty; but unfortunately that theory is not only destitute of proof, but militates against the known properties of water, azote, and hydrogen. We must therefore be cautious in drawing any conclusion till future discoveries have removed the ob. scurity in which the phænomena of rain are at present involved. The mean annual quantity of rain is greatest at the equator, and decreases gradually as we approach the poles. Thus at Granada, Antilles, 12° N. lat. it is 126 inches On the contrary, the number of rainy days is smallest at the equa. tor, and increases in proportion to the distance from it. From north latitude 12° to 43° the mean number of rainy days is 78; from 43° to 46° the mean number is 103; from 46° to 50° it is 134; from 51° to 60° 161 1. * Cotte, Jour. de Phys. Oct. 1791, p. 246. The number of rainy days is often greater in winter than in summer; but the quantity of rain is greater in summer than in winter. At Petersburgh the number of rainy or snowy days during winter is 84, and the quantity which falls is only about five inches ; during summer the number of rainy days is nearly the same, but the quantity which falls is about 11 inches . More rain falls in mountainous countries than in plains. Among the Andes it is said to rain almost perpetually, while in Egypt it hardly ever rains at all. If a rain-guage be placed on the ground, and another at some height perpendicularly above it, more rain will be collected into the lower than into the higher; a proof that the quantity of rain increases as it descends, owing perhaps to the drops attracting vapour during their passage through the lower strata of the atmosphere, where the greatest quantity resides. This, however, is not always the case, as Mr. Copland of Dumfries discovered in the course of his experiments ‡. He observed, also, that when the quantity of rain collected in the lower guage was greatest, the rain commonly continued for some time; and that the greatest quantity was collected in the higher guage only either at the end of great rains, or during rains which did not last long. These observations are important, and may, if followed out, give us new knowledge of the causes of rain. They seem to show, that during rain the atmosphere is somehow or other brought into a state which induces it to part with its moisture; and that the rain continues as long as this state continues. Were a sufficient num. ber of observations made on this subject in different places, and were the atmosphere carefully analysed during dry weather, during rain, and immediately after rain, we might soon perhaps discover the true theory of rain. Rain falls in all seasons of the year, at all times of the day, and during the night as well as the day; though, according to M. Toaldo, a greater quantity falls during the day than the night. The causé of rain, then, whatever it may be, must be something which ope. rates at all times and seasons. Rain falls also during the conținuance of every wind, but oftenest when the wind blows from the south. Fails of rain often happen likewise during perfect calms. * Id. ibid. |
