VAPOUR. 



weight of air. Mr. Eeles alio .endeavours to fiiew, that the 

 afcent and defcent of vapour, attended by this fire, are the 

 caufe of all our winds, and that they furnifh a fatisfaftory 

 folution of the general phenomena of the weather and 

 barometer. (Phil. Tranf. vol. xlix. p. 124, &c.) Dr. 

 Darwin, in 1757, pubhihed remarks on the theory of 

 Mr. Eeles, with a view of confuting it ; and attempted 

 to account for the afcent of vapours, by confidering the 

 power of expanfion which the conftituent parts of fome 

 bodies acquire bv heat, and alio that fome bodies have a 

 greater affinity to heat, ;. e. acquire it fooner, and retain it 

 longer than others. On thefe principles, he thinks, it is very 

 intelligible how water, whofe parts appear from the Eolipile 

 to be capable of immeafurable expanfion, Ihoiild by heat alone 

 become fpecifically lighter than the common atmofphere. 

 A fmall degree of heat is fufficient to detach or raife the 

 vapour of water from the mafs to which it belongs ; and 

 the rays of the fun communicate heat only to thofe 

 bodies by which they are refrafted, reflefted, or obilrufted, 

 whence, by their impulfe, a motion or vibration' is caufed in 

 the parts of fuch bodies. Hence he infers, that the fphe- 

 ricles of vapour will, by refracting the folar rays, acquire a 

 conftant heat, though the furrounding atmofphere remain 

 cold. If it be alked, how clouds are fupported in the 

 abfence of the fun ? it mud be remembered, that large 

 maffes of vapour muft for a confiderable time retain much 

 of tlie heat they have acquired in the day ; at the fame time 

 reflefting, how fmall a quantity of heat was neceffary to 

 raife them, and that doubtlefs even a lefs will be fufficient 

 to fupport them ; as from the diminifhed prelTure of the 

 atmofphere at a given height, a lefs power may be able to 

 continue them in their prefent ftate of rarefaction ; and 

 lailly, that clouds of particular ihapcs will be fuftained or 

 elevated by the motion they acquire from A^nnds. Phil. 

 Tranf. vol.i. p. 246. 



For the effect of vapour in the formation of fprings, &c. 

 fee Sprisg and River. 



The quantity of vapour raifed from the fea by the 

 warmth of the fun, is far greater than one would imagine. 

 Dr. Halley has attempted to eftimate it. The refult of 

 liis eftimate is contained in the following articles. 



1. That water falted to about the fame degree as falt- 

 water, and expofed to a heat equal to that of a fummer's 

 day, did, from a circular furface of about eight inches 

 diameter, evaporate at the rate of fix ounces in twenty-foiu- 

 hours. Whence, by a calculus, he finds that the thick- 

 nefs of the pellicle or Hcin of water, evaporated in two 

 hours, was the fifty-third part of an inch ; but, for a round 

 number, he fuppofes it only a fixtieth part ; and argues 

 thence, that if water as warm as the air in fummcr eva- 

 porates the thicknefs of one-fixtieth part of an inch in two 

 hours, from its whole furface ; in twelve hours it will 

 evaporate the tenth of an inch ; which quantity, he ob- 

 ferves, will be found abundantly fufficient to furnifh all the 

 rains, fprings, dews, &c. 



In cfFeft, on this principle, every ten fquare inches of 

 the furface of the water yield in vapour p;r diem a cubic 

 inch of water ; and each fqiiare foot half a wine pint ; 

 every fpacc of four feet fquare, a gallon ; a mile fquare^ 

 6914 tons ; and a fquare degree of fixty-nine Enghfh miles 

 J™1 ."^^^Porate 33 milhons of tons a day ; and die whole 

 Mediterranean, computed to contain 160 fquare degrees, at 

 leaft 5280 millions of tons. Phil. Tranf. N= 189, or Abr 

 vol. II. p. 108, &c. See River. 



2. A furface of eight fquare inches, evaporated purely 

 by the natural warmth of the weather, without either wind 

 or fun, in the eourfe of a whole year, 16,292 grains of water. 



or fixy-four cubic inches ; confequently the depth of water 

 thus evaporated in one vear amounts to eight inches. But 

 this being too little to anfwer the experiments of the French, 

 who found that it rained nineteen inches of water in one 

 year at Paris ; or thofe of Mr. Townley, who found the 

 annual quantity of rain in Lancafhire above forty inches ; 

 he concludes that the fun and wind contribute more to 

 evaporation than any internal heat or agitation of the 

 water. 



It has fince been difcovered, that there was a fource of 

 error in Mr. Townley's experiments, with which the world 

 was not at that time acquainted : his rain-gauge was fixed 

 ten yards above the furface of the earth ; but Dr. Heberden 

 has found, that a rain-gauge fixed below the top of a houfe, 

 received above a fifth part more rain than another of the 

 fame fize above the top of the fame houfe ; and that 

 there fell upon Weilminfler Abbey not much above one- 

 half of that which fell in the fame fpace below the tops of 

 the houfes ; and by feveral experiments made by Dr. Dob- 

 fon of Liverpool, it appears, that the quantity of rain re- 

 ceived in a veffel placed on the ground exceeded that re- 

 ceived by another of the fame dimenfions eighteen yards 

 higher more than one-third, and lefs than one-half. Phil. 

 Tranf. vol. lix. art. 47. and vol. Ixvii. art. 13. p. 256. See 

 Rain. 



With regard to the caufe of this difference, it may be 

 obferved, that as in chemical precipitations a greater por- 

 tion of the precipitating fubftance will be received on the 

 real bottom of a veffel containing the folution than on a 

 fuppofed falfe bottom placed any where above it, and that 

 in proportion to its height above the real bottom ; fo 3 

 greater quantity of water, confidering rain as a precipitation 

 of water before diffolved in air, ought, on parting with its 

 former folvent, to fall on the furface of the earth than on 

 an imaginary horizontal plane of the fame dimenfions above 

 it ; and though the cafes are not exaftly parallel, yet the 

 drops of rain in their defcent mufl be fomewhat increafed 

 either in number or fize ; partly by fucceffively impinging 

 on the aqueous particles contained in the air through which 

 they pafs, and by attrafting others in virtue of their beiilg 

 poffeffed of a different electricity ; and partly by the fpon- 

 taneous feparation and precipitation of that moifture, which 

 is known to be contained in confiderable quantities in the 

 air at all times, and the appearance of which, dripping down 

 the walls of our houfes, &c. is one of the popular figns 

 of approaching rain. (Monthly Review, vol. xhv. p. 322.) 

 Dr. Dobfon flates the annual evaporation at Liverpool, 

 taking the medium of four \ears, at 36.78 inches. Dr. 

 Halley fixes the annual evaporation of London at 48 

 inches. See Phil. Tranf. vol. Ixvii. parti, p. 252. 



3. The effeft of the wind is very confiderable, on a 

 double account ; for the fame obfervations (hew a very odd 

 quality in the vapours of water, •viz. that of adhering and 

 hanging to the furface that exhaled them, which they 

 clothe, as it were, with a fleece of vaporous air ; which 

 once inveiling the vapour, it thenceforward rifes in much 

 lefs quantity. Whence, the quantity of water loft in 

 twenty-four hours, when the air was very ftill from wind, 

 was very fmall, in proportion to what went away when 

 there was a ftrong gale of wind abroad to diffipate the 

 fleece, and make room for the emiflion of vapour ; and 

 this, even though the experiment was made in a place as 

 clofe from the wind as could be contrived. 



Add, that this fleece of water hanging on the furface of 

 waters in ftill weather, is the occafion of very ftrange appear- 

 ances, by the refraftion of the vapour's differing from and 

 exceeding that of common air ; whence every thing appears 



raifed, 



