1) E W 



6. The length of the branches of the trees, the tendernefs 

 of thcii twigs, and the pliancy of the footftalks of their 

 leaves, will caufe in the leaves an almoft perpetual motion, 

 even in ftatcs of air that may be denominated calm. I have 

 hence frequently heard, during the ftillncfs of night, a 

 ruftling noife in the trees, which formed one of the bounda- 

 ries of the ordinary place of my obfervations, while the air 

 below fcemed without motion. 



«' Nearly in the fame manner is to be explained, why 

 flirubs and bufhcs alfo receive dew more readily than lofty 



trees. , n ■ 



" Bright metals, expofed to a clear (ky in a calm night, 

 will be Icfs dewed on their upper fui face than othsr folid 

 bodies ; fince of all bodies they will, in fuch a fituation, lofe 

 the fmalleft quantity of heat by radiation to the heavens, at 

 the fame Lme that they are capable of receiving, by conduc- 

 tion, at lead as much heat as any others from the atmofphere, 

 and more than any others from the v/armer folid fubftances, 

 which they happen to touch. 



" If the expofed pieces of metal be not very fmall, another 

 reafon will contribute fomewhat to their being later and lefs 

 dewed than other folid fubftances. For, in confequence of 

 their great conducing power, dew cannot form upon them, 

 unlefs their whole mafs be fufficiently cold to condenfe the 

 watery vapour of the atmofphere ; while the fame fluid will 

 appear on a bad conduftor of heat, though the parts a very 

 little beneath the furface are warmer than the air." 



It appears from the difcoveries of profefTor Leflie, that 

 the metals differ in their capacities of radiating heat ; and 

 hence will arife a difference among themfelves with regard 

 to their attradlion of dew. Gold, filver, copper, and tin, 

 as we have already faid, refift the formation of dew more 

 ftrongly, than other fubftances of the fame clafs ; but thefe 

 metals, according to Mr. Leflie, radiate heat the moft 

 fparingly. On the other hand, lead, iron, and fteel, which, 

 according to the fame author, radiate heat more copioufly 

 than the former metals, were found by Dr. Wells to acquire 

 dew more readily. The fame obfervations may probably be 

 applicable to platina and zinc. In the article Dew, we have 

 already taken fome notice of the opinion of thofe who main- 

 tain, that it rifes from the earth at night. The firft trace 

 of this opinion, according to Dr. Wells, occurs in the 

 Hift. Acad. Sci. for 1687. Gerften advanced it anew in 

 1733. It was embraced by Mufchenbroeck and Dufay ; 

 though the former foon admitted that dew fometimes falls. 

 Mr. Webfter of New England has adopted the fame opinion. 

 We refer thofe who wifh to fee the arguments for and 

 againft this opinion fully ftated to WeRs's EfTay. 



Agreeably to another opinion on the origin of dew, that 

 which is found upon growing vegetables, is faid to be the 

 condenfed vapour of the plants on which it appears. " But 

 this feems," fays our author, " to be erroneous for feveral rea- 

 fons. I. Dew forms as copioufly upon dead as upon living 

 vegetable fubftances. 2. The tranfpired hum.our of plants 

 will be carried away by the air which paffes over them, 

 when they are not fufficiently cold to condenfe the watery 

 vapour contained in it ; unlefs, which is almoft never the 

 cafe if mift does not already exift, the general mafs of the 

 atmofphere be incapable of receiving moifture in a pellucid 

 form. Accordingly, on cloudy nights, when the air, con- 

 lequently, can never be cooled more than a little below the 

 point of repletion with moifture, by bodies in contaft with 

 It, dew IS never obferved upon any plants, that are elevated 

 a iew feet above the ground. 3. If a plant has become, by 

 radiating us heat to the heavens, fo cold, as to be enabled to 

 bring the air m contaft with it below the point of repletion 

 with moifture, that which form* upon it, from. its own 



D I A 



tranfpiration, will not then, ir.deed, evaporate. But othcj- 

 moifture will, at the fame time, be communicated to it by 

 the atmofphere ; and v/hen the difference in the copioufnefs 

 of thefe tv/o fources is confidered, it may, I think, be fafely 

 concluded, that almoft the whole of the dew, which will 

 afterwards form on the plant, muft be derived from tiie air ; 

 more efpecially when the coldnefs of ?. clear night, and the 

 general inactivity of plants in the abfence of light, both 

 leffening their tranfpiration, are taken into account. 



" An experiment, however, has been appealed to in proof, 

 that tlie dew of plants aftually does originate from fluid 

 tranfpired by them ; that, namely, in which a plant, fhut up 

 in an air-tight cafe, becomes covered with moifture. But 

 this experiment, if attentively examined, will be found to 

 have little weight. Firft, the iiiclofed plant being exempt 

 from the cold, which its own radiation would have produced 

 in its natural fituation, on a dewy night, will tranfpire a 

 greater quantity of fluid, than a fimilar plant expofed at the 

 fame time to the open air. Again, the fmall quantity of 

 air, contained in the cafe, muft foon be replete with moifture, 

 after which, the whole of what is further emitted by the 

 plant will neceffarily affume the form of a fluid, whatever 

 may be the condition of the external atmo^here ; whereas, 

 during even the cleareft night, only a part of the fmallet 

 quantity of moifture, emitted by the expofed plant, will be 

 condenfed on its furface. In the laft place, notwithftand- 

 ing the circumftances which favour the appearance of 

 moifture upon inclofed plants from their own tranfpiration, 

 ftill the quantity obferved on them is faid to be, for I have 

 made no experiment myfelf refpefting tliis matter, much kfs 

 confiderable, than what is feen upon plants of the fame 

 kind, expofed to the air for the fame time, during a calm 

 and ferene night." For feveral appearances connedfed with 

 dew, we are under a neceflity of referring to the third part 

 of Dr. Wells's Eflay. 



DEWAN, or DuAN, a term which has various fignifi- 

 cations in India. It denotes a place of affembly, a native 

 minifter of the revenue department, and chief juflice of civil 

 caufes within his jurifdlclion, and receiver general of a pro- 

 vince. The term is alfo ufed to defignate the principal 

 revenue fervant under an European colleftor, and even of a 

 Zemind?.r. By this title, the Eaft India company are receivers 

 of the revenues of Bengal, under a grant from the GreaL 

 Mogul. Accordingly Dewannu denotes the office or jurif- 

 diftion of a Dewan, 



DEWEYSBURG, in Geography, a town of Caledonia 

 county, in Vermont, having 200 inhabitants. 



DEZPHOUL, a town of Perfia, in Kuziftan, or Chufif- 

 tan, 28 miles W. of Shufter, containing nearly as many inha- 

 bitants as that city, and fituated on the eaftern bank of the 

 Abzal, on a beautiful and fpacious plain. Its only ornament 

 is an elegant bridge of twenty-two arches, erefted by com- 

 mand of Sapor, notorious for deftroying as well as famous 

 for building cities. The bridge is 450 paces in length, 

 20 in breadth, and about 40 in height. The piers are 

 made of large ftones, and the arches and upper parts of 

 burnt brick. 



DlAL.ff'ori of a Clock, col. 2, 1. 24, for Plate XXIII. 

 ,: Plate XVIII. 



DIALLAGE. See Mineralogy, Addenda. 



DIAMOND, col. 4, 1. 43, add— With a fmall portion 

 of oxygen, as fir Humphrey Davy has lately difcovered. 



Diamond. See Diamond, and Mineralogy, Addenda. 



DIANA, col. 3, 1. 20 from the bottom, for 27 r. 30. 



DIANELLA, in Bot.iny, a poetical name, in honour of 

 the fylvan goddefs Diana, to whom Commerfon its author 

 thought fo beautiful an inhabitant of the woods peculiarly 



appropriate^ 



