DEW. 



nantly found it to be lefs than that of the air, any where 

 between one inch and nine feet above the ground, the latter 

 being the greateft height at which he ever mai-ked he 

 heat of the atmofphere in thefe experiments. At the 

 height of four feet above the ground and in calm and clear 

 mcrhts. ho freouentlv found the grafs feven, eight, or nine 



iiiehts, he frequently - c , - , . . 



decrees colder than the air at that height ; feveral times it 

 wl .o» and 11°, and once ,2°, coder than the air. In 

 lome few obfcrvations, the greater coldneia of gra.s than that 

 of the air began to appear, in clear and calm weather, m 

 places (heltered from the aftornoon fun, and yet open to a 

 conf.derable portion of the fl.y, foon after the heat of the 

 atmofphere had dechned. A fimilar coldnefs continued 

 upon rra<"s in Hill and ferenc mormngs, for lome time alter 

 the rilino- of the fan, in places fhadcd from its uireft light, 

 but othe?wife open to the llcy. In cloudy mghts, par- 

 tlcularly with wind, the grafs was never much colder than 

 the air. Sometimes the temperatures were the fame ; at 

 other times that of the grafs was higher, even when it had 

 been wetted by preceding rain, and when it muft have cooled 

 by evaporation. When the night changed from clear to 

 cloudy, though without change as to calmnefs, a confiderable 

 alteration in'the temperature of the grafs always enfued, 

 and this change occurred fometimes very fuddenly. On 

 one night, the grafs_, after having been 1 2° colder than the 

 air, became 2° colder, the temperature of the air at both 

 obfervations being the fame. On a fecond night, the grafs 

 became 9° warmer in an hour and a half. On a third night, 

 in lefs than forty-five minutes the temperature of the grafs 

 rofe 15°, while that of the neighbouring air incrcafed 3^"^. 

 On a fourth night, the temperature of the grafs at half-paft 

 9 o'clock was 32° ; in twenty minutes afterwards it was 

 found to be 39°, the Hey in the mean time having become 

 cloudy. At the end of twenty minutes more, the fky 

 being clear, the temperature of the grafs was again 32°. 

 On the connexion of fog or mift with cold. Dr. Wells 

 made feveral experiments, which we cannot recite. He 

 obferves, that he has always found on dewy nights the tem- 

 perature of the earth half an inch or an inch beneath its 

 furface much warmer than the grafs upon it ; and the earth 

 at this depth was alfo almoft conftantly warmer on dewy 

 nights than the air. Metals, fays our author, furnifh 

 proofs of the conneftion cf dew with cold in the fnbftanc?s 

 on which it forms fuperior to that of the neighbouring 

 atmofphere. Upon the whole, our author's experiments, 

 which we cannot minutely detail, Ihew, that when bodies 

 which had been equally expofed to the night air were 

 examined at the fame time, thofe v/hich were mod dewed 

 were the coldeft. In the profecution of experiments with 

 other fubflances befides grafs, he found that thofe of the 

 filamentous downy kind were the moft produftive of cold. 

 Thus, wool of moderate fincnefs, very fine raw filk, very 

 fine unfpun cotton, fine flax, and- fwan-down, were not only 

 more fleadily cold, upon clear and calm nights, than grafs, 

 but alfo gave rife to a greater degree of cold than was 

 almofl at any time obferved upon it even in its beft flatc. 

 Wool produced the leaft cold, and we have found before, 

 that it attrafted kfs d^w than fi!k, cotton, and ilax. Fre!n, 

 unbroken flraw, and fhreds of white paper, were found to be 

 a little more productive of cold than wool. The next clafs 

 confifted of bodies in the ftate of powder ; thefe were, clean 

 river fand, glafs, chalk, charcoal, lamp-black, and a brown 

 calx of ir*i. Chalk produced the leafl, and the three lafl 

 fubflances produced the greateft cold. Solid f-^bflances, 

 expofing to the (ley a furface of at leaft twenty-five inches 

 fquare, formed a third clafs, on which our author made 

 experiments. The fubftances of this dcfcription fubjcftedto 



trial were, glafs, brick, cork, oak-wood, and wax ; all of whicli 

 were found inferior to the filamentous fubftances. His prin- 

 cipal experiments, however, of this kind were made on fnow. 

 The next fubjeft to which Dr. Wells diredls his atten- 

 tion is the theory of dew. Accordirjg to Ariftotle among 

 the ancients, and many philofophical writers among the 

 moderns, dew is a fpecies of rain, formed in the lower 

 atmofphere, in confequence of its moifture being condenfed 

 by the cold of the night into miniite drops. But opinions 

 of this kind have been found erroneous, by the confideration 

 of a faft firft noticed by Gerften in 1733 ! '""^* that bodies 

 a little elevated in the air often become moift with dew, 

 while fimilar bodies, lying on the ground, remain dry, 

 though neceffarily, from their pofition, as liable to be 

 wetted, by whatever falls from the heavens, as the former. 

 It was foon afterwards obferved by Mufchenbroeck, that 

 metals will be free from dew, while other bodies attraft it 

 copioufly : hence Dufay concluded, that dew is an eleAri- 

 cal phenomenon, fince it leaves untouched the bodies which 

 conduft eleftricity, whilft it appears upon thofe which 

 cannot tranfmit that influence. Againft this hypothefis 

 feveral objeftioas have been urged, however plaufibly it has 

 been fupported. It has been alleged that charcoal, which 

 next to the metals is the beft folid conductor of eleftricity, 

 attrafts dew very powerfully ; and dew, as we have feen 

 above, frequently forms upon metals themfelves. It hai 

 aJfo been urged againft this hypothefis, that dew forms in 

 different parts of the night, in quantities difproportioned to 

 the degrees of eleftricity found in the atmofphere at the 

 fame time. Thus, it is commonly more copious in the 

 morning than in the evening, though the air is obferved to 

 be, in the latter feafon, more highly eleftrical than in the 

 former. But another argument applies alike to all the 

 theories which have hitherto been made public on the caufe 

 of dew ; and this is, that none "of them include the impor- 

 tant faft, that its produftion is attended with cold. Mr. 

 Wilfon and Mr. Six have indeed maintained, that the form- 

 ation of this fluid is the caufe of the cold that accompanies 

 it. Dr. Wells once held the fame opinion ; but finding 

 that bodies v/ould fometimes become colder than the air 

 without being dewed, and that when dew was formed, its 

 quantity, and the degree of cold which appeared with it, 

 were very far from being always in the fame proportion to 

 each other, he firft doubted its truth, and at length became 

 convinced that it was erroneous ; and by farther inquiry he 

 was led to conclude, that dew is the produftion of a pre- 

 ceding cold in the fubftances upon v/hich it appears ; and 

 that it has precifely the fame immediate caufe as the prefence 

 of moifture upon the outfide of a glafs or metallic veflel, when 

 a liquid confiderably colder than the air has been poured 

 into it fhortly before. This faft is applied by our author, to 

 the explanation of feveral atmofpherical appearances. 



" I. The variety in the quantities of dew, which were 

 found by me upon bodies of the fame kind, exnofed to the 

 air during the fame time of the night, but in different fitua- 

 tions, is now feen to have been occafioned by the diverfity 

 of temperature, which exifted among them. 



" II. Agreeably to the opinion of Mr. Wilfon and Mr. 

 Six, the cold connected with dew ought always to be pro- 

 portional to the quantity of that fluid ; but this is contra- 

 difted by experience. On the other hand, if it be granted, 

 that dew is water precipitated from the atmofphere, by the 

 cold of the body on which it appears, the fame degree of 

 cold, in the precipitating body, may be attended with 

 much, with little, or with no dew, according to the exifting 

 ftate of the air in regard to moifture ; all of which circum- 

 ftances are found actually to take place. 



" III. The 



