5? CQiiJiderations on Dr. Huiion's Theory of Rain. 



two oppofite currents of air are formed in the fummer hemi- 

 fphere, one moving along the furfyce of the earth from the 

 pole to the equator, the other flowing above. Thefe oppo- 

 llte currents, while feparate, might pafs each other without 

 producing rain; but when fufficient portions are mixed, not 

 only clouds but fliowers will be produced, lince the fudden 

 formation of a mean degree of heat, in the mixture of two 

 portions of different teniperatures, muft condcnfe a quantity 

 of vapour fufficient to form rain. 



" Without this law of condenfation of evaporation nei- 

 ther rain nor dew could take place any where in the fummer 

 hemifphere, perhips not even in tropical latitudes : there 

 would be evaporation r.nd a general tendency to laturate the 

 atmofphere with water, but the mixture of dilVerent portions 

 of air would only temper t'le faturaiion without producing a 

 condenfation of vapour in the mean degrees of heat. At 

 niffht, from the influence of the cold, the atmofphere 

 would become gradually clouded ; this cloudinefs would in- 

 creale to a general diflillalion of condenfed vapour, which 

 would be uniformly continued until the returning fummer 

 Ihould change the ftate of condenfation to evaporation ; and 

 inllead of the beautiful return of feafons, tempered with va- 

 rious degrees of heat and refrefliing (liowers, fix months 

 rain and fix months drought would follow each other in an 

 invariable fucceflfiou." 



Such is the hypothefis which Dr. Hutton is willing we 

 fliould confider as the theory of rain, and which he proceeds 

 to applv to natural appearances; fceming to experience no 

 great dit'Iiculty in foh ing by it every problem that occurs, as 

 the foHliiial, equinoftial, and other periodical rains, the 

 comparative ftate of different climates and of land and fea, 

 as to its frequency and (juantity, 8cc. But, if we except 

 alliort remark on hail and thunder (lorms (which feemsonly 

 to give occafion to a confeffion of our imjierfeift knowledge 

 of atmofpherical elc(ilricity), in no infl:ance does the doilor 

 avail himfelf of the aid of any other principle than the mix- 

 ture of currents of air in travcrfing the irregular furface of 

 the globe, and confequcnt precipitation of water, to account 

 for the whole train of phcenomena conftituting the fubjc6l of 

 meteorology. 



The fimpliclty of this theory, the impofing air of a dia- 

 gram at its introduftion, the ingenious and dcfcriptive man- 

 ner of the application, all contribute to render it fufiicicntly 

 plaufiblc. It is rather extraordinary that the objc6lions urged 

 an-ainll it by De Luc * fliould have extended no further tharx 



• Idccs fur la MOttorologie, tome ii. p. 67- 



it 



