D E W 



leaves both fmooth, and nearly equal in lengfth. — Obferved 

 by Mr. Brown, in the tropical part of New Holland. 



9. D. artftata. Awned Devauxia. — Receptacle naked. 

 Styles fix or feven, combined at the bafe. Sheath fmooth, 

 with longifh awns. Stalks two-edged. — Found by Mr. 

 Brown on the fouthern coaft of Nev/ Holland. 



Mr. Rudge, very commendably anxious to retain, if pof- 

 lible, Labillardiere's original generic name, has, by taking 

 xEvl^ov for a pr'tckh, as it undoubtedly means a point, or Jharp 

 Jpiie, made Centrolepis to tY.^r:e{s a prickly fcale, alluding to 

 the hifpid_yZ>i?a/,^; of fome fpecies. But tliis is not appofite, 

 the fcales, or glumes of the fioiuer, to which the original 

 name applied, being, in no fenfe, pointed, or prickly ; nor 

 do we fee that this name can be forced into any appropriate 

 meaning, xhe fcales being acknowledged on all hands not to 

 be central. 



DEVIZES, col. 2, 1. penult, r. 696, and 3750. 



DEVONSHIRE, col. 6, 1. 11 and 12, r. 62,318, and 

 383.308. 



DEW. To this article we (hall fubjoin fome appro- 

 priate remarks, felefted from an " Effay on Dew, &c." by 

 the ingenious Dr. Wells, lately (w'z. 18 19) republifhed in 

 an edition of his works, to which is prefixed a Memoir 

 of liis life written by himfelf. To this author it occurred 

 in 1784, that the formation of dew is attended with the 

 produftion of cold. The fame opinion was announced 

 ill 1788 by Mr. Wilfon of Glafgow (vol. i. Edinb. Tranf.), 

 and alfo by Mr. Six of Canterbury (Phil. Tranf. for 1788, 

 ajid in a pofthumous worJc printed at Canterbury m 1 794. ) 

 All thefe writers at firft concurred in regarding the cold 

 wliich accompanies dew as an effeft of the formation of that 

 fluid. Dr. Wells, however, upon mature confideration of 

 the fubjeft, was led to fufpeft that this opinion was erro- 

 neous ; and his fufpicion was afterwards juftified by a 

 variety of obfervations and experiments. Although Dr. 

 Wells agrees, in general, with Ariftotle and other writers, 

 and maintains that dew appears only on calm and ferene 

 nights, yet this opinion is not univerfally true ; for he fre- 

 quently found a fmall quantity of dew on grafs, both on 

 windy nights, if the Iky was clear or nearly fo, and on 

 cloudy nights, if there was no vnnd ; but he never perceived 

 dew on nights both cloudy and windy. Dew, he fays, pro- 

 bably begins, in this country, to appear upon grafs, (haded 

 from the fun, during clear and calm weather, foon after the 

 heat of the atmofphere has declined ; and it continues to 

 form, in (haded places, after fun-rife ; the interval between 

 fun-rife and its ceafing to form being confiderably (horter 

 than that between its firft appearance in the afternoon and 

 fun-fet. If the weather be favourable, however, more dew 

 forms a little before, and in (haded places a little after fun- 

 rife, than at any other time ; whereas Mufchenbroeck 

 afterts, that dew does not form after the fun has rifen. Our 

 author alfo maintains, in oppofition to the opinion of M. 

 Prieur, that dew, after it has once commenced, continues 

 during the whole night, if the weather remain ftill and 

 ferene. During nights that are equally clear and calm, 

 whether they be longer or (horter, dew often appears in very 

 unequal quantities ; the quantity of moifture in the atmo- 

 fphere' ferving to increafe the produftion of dew ; and, 

 accordingly, in equally clear and calm nights, dew is more 

 abundant foon after rain than during a long continuance of 

 dry weather : it is alfo more abundant in Europe, and in 

 fome parts of Afia and Africa, with foutherly and wefterly 

 winds, than with thofe which blow from the north and the 

 eaft. Another circumftance upon which the quantity of 

 dew depends, is the diminution of the weight of the atmo- 

 fphere ; for though the falling of the mercury in the baro- 

 VoL. XXXIX. 



DEW 



meter is commonly attended with wind or clouds, both 

 unfavourable to the produftion of dew, yet the p-reateft 

 dew obferved by our author occurred while the barometer 

 was finking. M. De Luc alfo has obferved, that rain may 

 be foretold when dew is uncommonly abundant in relation 

 to the climate and feafon. We have already obferved, 

 that dew is commonly more plentiful in fpring and autumn 

 than in fummer, a faft which our author alfo has noticed ; 

 it is always very copious on clear and calm nights which 

 are followed by mifty or foggy mornings ; and on a clear 

 morning which fucceeds a cloudy night. Heat of the 

 atmofphere, if other circumftances are favourable, occafioiis 

 a great formation of dew ; and upon the fuppofition of 

 the fame clearnefs and flillnefs of the atmofphere," more dew 

 is formed between midnight and fun -rife than between 

 fun-fet and midnight ; the cold of the atmofphere being 

 greater in the latter than in the former part of the night. 

 Our author's experiments ferve to iView, that various differ- 

 ences with regard to fituation, mechanical (late, and real 

 nature of bodies, have a very confiderable influence upon 

 the produftion of dew. As to fituation he obferves, that 

 whatever diminifhes the view of the (ley, as feen from the 

 expofed body, diminifties the quantity of dew that is pro- 

 duced ; thence the quantity is greater when the expofnre 

 to the (ley is more complete. There are other circum- 

 ftances, regarding fituation, which ferve to augment or 

 diminifh the quantity of dew that is produced, when the fub- 

 ftances that are ufed for indicating it are the fame. 



It is obferved farther, that when other circumftances are 

 fimilar, a difference in the mechanical ftate of bodies has an 

 effeft with regard to the quantity of dew which they 

 attraft ; and hence it is, that fine raw filk, fine unwrougbt 

 cotton, and flax, were found to attraft more dew than the 

 wool employed by our author in his experiments ; the fibres 

 of wool being thicker than thofe of the other fubftances 

 juft mentioned. Bright metals alfo attraft dew much lefs 

 powerfully than other bodies : this faft was obferved by 

 Mufchenbroeck and Dufay ; but they erroneoufly afferted, 

 that dew never appears on the upper furface of bright 

 metals. There are others, and our author in particular, who 

 have known dew to be formed on gold, filver, copper, tin, 

 platina, iron, ileel, zinc, and lead. This inaptitude of 

 metals to attraft dew is communicated to bodies of a very 

 different nature, which touch or are near to them. Wool, 

 fays our author, laid upon 3 metal, will acquire much lefs 

 dew than an equal quantity laid upon grafs in the imme- 

 diate vicinity. It is maintained, that the upper furfaces of 

 metals are moft readily and moft copioufly dewed on thofe 

 nights and in thofe parts of the night, during which other 

 fubftances are the moft readily and the moft copioufly 

 dewed. All metals, our author remarks, do not refift the 

 formation of dew with the fame force. " I faw," he fays, 

 " for example, platina one night diftinftly dewed, while 

 gold, filver, copper, and tinj though fimilarly fituated, were 

 entirely dry ; and I have alfo feveral times foen thefe four 

 metals free from dew, while iron, fteel, zinc, and lead, were 

 covered with it." 



Our author proceeds to inveftigate and afcertain the 

 degree of cold connefted with the formation of dew. 

 Mr. Wilfon, he thinks, is the firft philofopher who ever 

 fufpefted the exiftence of fuch a conjunftion ; though dew 

 is often fpokcn of as cold by our popular writers. Hero- 

 dotus mentions it as poffeffing this quality ; Cicero and 

 Virgil apply to it the epithet of " gehdus ;" Milton that 

 of " chill;" and Colhns that of " cold." With thermo- 

 meters adapted to the purpofe, he has, in ferene and ftill 

 nights, examined the temperature of dewed grafs, and con- 

 3 N rtantly 



