RAIN. 



the rain is impetuous. Hence it appears that the vapours 

 are condcnfcd when the atmofpherc is warm as well as when 

 it is cold. 



Others only allow the cold a part in the aftioii, and afcribe 

 a (liare of it to the winds, alleging, that a wind blowing 

 againft a cloud will drive its vcficuhc upon one another, by 

 which means fever;il of them, coalefcing as before, will be 

 enabled to defcend ; and the cffeft will be Hill more con- 

 fiderable, if two oppofite winds blow together toward the 

 fame place. To which they add, that clouds already formed, 

 happening to be augmented by frcfli acceflions of vapourj 

 continually afcending, may on this account be enabled to 

 defcend. 



Againft the above ftated hypothcfis of Derham, it has 

 been objeiSled that the veficulse of vapour, if they be fuch, 

 are filled, nst with air, but tire or heat in a latent ftate, and till 

 they part with this heat, the vapour cannot be condenfed. 

 Cold is not always fufficient to produce this efTeft, for in 

 the moft feverefrofts the air is ferene, and parts with little 

 or none of its vapours, for a confiderable time. It is alfo 

 alleged, thit the winds have no confiderable agency, fince 

 blowing upon vapour is fo far from condenfing it, that it 

 unites more clofely with the air ; and wind is known very 

 much to promote evaporation. Sec Vapour. 



Yet the grand caufe, according to Rohault, ftill remains : 

 that author conceives it to be tlie heat of the air, which 

 after continuing for lome time near the earth, is at length car- 

 ried up on high by a wind, and there thawing the fnowy 

 villi, or flocks of the half-frozen velicul^, reduces them 

 into drops ; which coalefcing, defcend, and have their 

 diflolution perfeAed in their progrefs through the lower and 

 warmer ftages of the atmofphere Others, as Dr. Clarke, 

 tie. afcribe this defcent of the clouds rather to an altera- 

 tion of the atmofphere than of the veficulx ; and fuppofe 

 it to arife from a diminution of the fpring, or elaltic force 

 cf the air. 



This elafticity, which, as they fay, depends chiefly or 

 wholly on the dry terrene exhalations, being weakened, the 

 atmofphere finks under its burden, and the clouds fall, on 

 the common principle of precipitation. 



Now, the little veficulac, by any or all thefe means, being 

 once upon the defcent, will perfift therein, notwithltanding 

 the increafe of refiftance they every moment meet with in 

 their progrefs through ftill denfer and denfer parts of the 

 atmofphere. For, as they all tend toward the fame point, 

 viz. the centre of the earth, the farther they fall, the more 

 coalitions will they make ; and the more coalitions, the 

 more matter will there be under the fame furface ; the 

 furface only increafing as the fquares ; but the folidity as 

 the cubes ; and the more matter under the fame furface, 

 the lefs friftion or refiftance there will be to the fame 

 matter. 



Thus if the cold, the wind, &c. happen to aft early 

 enough to precipitate the afcending veficula:, before they 

 are arrived at any confiderable height, the coahtions being 

 few in fo fliort a defcent, the drops will be proportionably 

 fmall : and thus is formed what we call de^u. See Dew. 



If the vapours prove more copious, and rife a little higher, 

 we have a mi/l or fog ; which fee refpeftively. 



A little iiigher ftill, and they produce a fmall rain, kc. 

 If 1 hey neither meet with cold, nor wind enough to con- 

 denfe or diflipate them, they form a heavy, thick, dark flcy ; 

 which lafts fometimes feveral weeks. Tliis hypothefis ot 

 the diminution of the atmofphere's elafticity, requires a 

 more fatisfaftory exphcation of the caufe of this diminution. 

 By afcribing it to terrene exhalations, we only folve one 

 difficulty by introducing another ; for we are totally un- 



acquainted with the nature and operation of thefe cxhala- 

 tibns. But let the caufe be what may be fuppofcd, if 

 it aft equally, and at once, upon all the vapour in the air, 

 all that vapour muft be at once precipitated ; and the con- 

 fequcncc muft be, that, inftead of gentle fliowers continuing 

 for a confiderable time, wo muft have copious watcr-fpouts, 

 lafting only for a few minutes or feconds, which, inftead of 

 refrediiiig the earth, would drown and lay it wafte. 



Thofc who admit either of the above hypothcfoj, account 

 for many of the phenomena of the weather ; e. gr. why a 

 cold is always a wet fumnier, and a warm a dry one ; be- 

 caufe the principle of precipitation obtains in the one cafe, 

 and is wanting in the other. Why we have ordinarily moft 

 rain about the equinoxes ; becaufe the vapours arife more 

 plentifully than ordinary in the fpring, as the earth becomei 

 loofened from the brumal conitipations ; and becaufe, as 

 the fun recedes from us in autumn, the cold increafing, the 

 vapours that had lingered above during the fummer heats, 

 are now difpatchcd down, &c. Why a fettled, thick, clofe 

 Iky fcarcely ever rains till it have been firft clear ; becaufe 

 the equally diff'ufed vapours muft firft be condenfed, and 

 congregated into feparate clouds, to lay the foundation of 

 rain ; by which means the reft of the face of heaven is left 

 open, and pervious to the rays of the fun, &c. 



For other phenomena of rain, as they relate to the 

 vveather-glafs, fee Barometer, and Weather. 



It cannot be doubted, that there is a conneftion between 

 the defcent of the barometer and the fall of rain ; but no 

 latisfaftory reafon has yet been afligned for the circumftance ; 

 nor is it poflible to foretell with certainty that rain will follow 

 any changes in the height of the barometer that have been 

 obferved. The immediate dependence of rain, or of any 

 other atmofpherical phenomena, on the influence of the 

 moon, appears to be rendered highly improbable, not only 

 by mathematical calculations of the effefts of the moon's 

 attraftion, but alfo by the irregularity of the obfervation* 

 which have been adduced in favour of a fuch a conneftion. 

 (See Injluence of the MooN.) But however uncertain the 

 ultimate caufes of rain may be in general, their effefts in 

 fome places are fufficicntly conftant, to be attributed 

 to permanent local circumilances, and in pattjcular to the 

 periodical recurrence of fimilar winds. 



For a more particular account of other theories, for 

 explaining the afcent of vapours, the formation of clouds, 

 and the fall of rain, fee Vapoui:. 



Befides the caufes of rain, mentioned above, Defaguliers 

 thinks it owing to the lofs of eleftricity in the vapours of 

 which they were formed. 



Other writers, in the progrefs of this part of philofophical 

 fcience, have confidercd rain as an eleftrical phenomenon : 

 or at leaft they have fuppofed, that the powers of eleftri- 

 city may concur with other caufes in producing it. Sir- 

 nior Beccaria, whofe obfervations on the general ftate of 

 eleftricity in the atmofphere have been more extenfive and 

 accurate than thofe of any other perfon, reckons rain, hail, 

 and fnow, among the efiefts of a moderate eleftricity in 

 the atmofphere. Clouds that bring rain, he thought, were 

 produced in the fame manner as thunder clouds, only by 

 a moderate eleftricity. He defcnbes them at large, and 

 the refemblance which all their phenomena bear to thofe of 

 thunder-clouds is very rtriking. He notes feveral circum- 

 ftances attending rain without lightning, which render it 

 probable, that it is produced by the fame caufe as when it 

 is accompanied with lightning. Light has been feen among 

 the clouds by night in rainy weather ; and even by day- 

 rainy clouds are fometimes feen to have a brightnefs evi- 

 dently independent of the fun. The uniformity with 

 Y y 2 which 



