DEW. 



" III. The formation of dew, indeed, not only does not 

 produce cold, but like every other precipitation of water 

 from the atmofphere, produces heat. 



" IV. In very calm nights, a portion of air, which comes 

 in contaft with cold grEfs, will not, when the furfacc is 

 level, immediately quit it, more efpecially, as this air has 

 become fpecifically heavier than the higher, from a diminu- 

 tion of its heat, but will proceed horizontally, and be 

 applied fucceflively to different parts of the fam.e furface. 

 The air, therefore, which makes this progrcfs, muR; at 

 length have no moifture to be precipitated, unlefs the cold 

 of the grafs which it touches faould incrcafe. Hence in 

 great meafure is to be explained, ♦hy, o;i fuch nights as 

 have been juft mentioned, more dew was acquired by fub- 

 ftances placed on the raifed board, than by others of the 

 f dme kind on the grafs, though it began to form ir.uch 

 fooner in the latter than in the former fituation, thofe on the 

 raifed board having received aii», which had previoudy 

 depofited lefs of its moifture. 



. " A reafon is now alfo afforded, why a flight agitation of 

 the atmofphere, when very pregnant with moifture, fnould 

 increafe the quantity of dew ; fince frefh parcels of air will 

 hence be more frequently brought into contaft with the 

 cold furface of the earth, than if the atmofphere were 

 entirely calm. 



" V. Dew, in agreement with the immediate caufe which 

 has been affigned by me for its produftion, can never be 

 formed, in temperate climates, upon the naked parts of a 

 living and healthy human body, during the night ; fince 

 their heat is never lefs in this feafon, in fuch climates, than 

 that of the atmofphere. I have, in faft, never perceived 

 dew on any naked part of my own body at night, though 

 my attention was much occupied, for three years, with 

 every thing relative to this fluid, and though I had been, 

 during that period, much expofed to the night air. On 

 the other hand, in very hot countries, the uncovered parts 

 of a human body may fometimes, from being confiderably 

 colder than the air, condenfe the watery vapour of the 

 atmofphere, and hence be covered with a real dew, even in 

 ,the day-time. 



" VI. Hygrometers formed of animal or vegetable fub- 

 ftances, when expofed to a clear fky at night, will become 

 colder than the atmofphere ; and hence, by attrafting dew, 

 or, according to an obfervation of Sauffure, by merely cool- 

 ing the air contiguous to them, mark a degree of moifture, 

 beyond v/hat the atmofphere ailually contains. This ferves 

 to explain an obfervation made by M. De Luc, that in ferene 

 and calm weather, the humidity of the air, as determined by 

 aa hygiometer, increafes about, and after fun-fet, with a 

 greater rapidity, than can be attributed to a diminution of 

 the general heat of the atmofphere." 



Having eftablifted the fad\, that bodies become colder 

 than the neighbouring air, before they are dewed, and 

 apphed this faft to the explication of many atmofpherical 

 appearances, we fhall now proceed with the author to 

 complete the inveftigation of his theory with refpeft to the 

 caufe of dew ; and hence he avails himfelf of the difcoveries 

 on heat and its radiation, that have been made by profeffor 

 Lellie, Dr. Herfchel, and count Rumford. (See Heat.) 

 " The experience of moft perfons," fays Dr. Wells, 

 " refpefting the communication of heat among bodies in 

 the open air, is confined to what happens during the 

 day ; at which time, thofe that are fituated near to one 

 another are always found to poflefs the fame temperature, 

 unlefs fome very evident reafon for the contrary fhould 

 «xift. To many, therefore, it may appear incredible, 

 that a perfeftly dry body, placed in contaft, on all fides, 



with other bodies of the fame temperature with itfelf, (hall 

 afterwards, without undergoing any chemical change, 

 become much colder than they are, and (hall remain fo 

 for many hours ; yet thcfe circumftances are found to 

 occur in fubllances attractive of dew, when laid on the 

 furface of the earth, in a- ftill and ferene night, and are in 

 perfeA agreement with the doftrine of heat, now univer- 

 fally admitted to be juft. 



" To rci.der this more eafy of apprehenfion, let a fmall 

 body which radiates heat freely, and pofTelfes a temperature, 

 in common with the atmofphere, higher than 32°, be placed, 

 while the air is clear and ftill, on a flow conduftor cf heat 

 lying on the furface of a large open plain, and let a firma- 

 ment of ice be fuppofed to cxifi at any height in ths atmo- 

 fphere ; the confequence muft be, that the fmall body will, 

 from its fituation, quickljt become colder than the neigh- 

 bouring air. For, while it radiates its own heat upwards, 

 it cannot receive a fufficient quantity from the ice to ccm- 

 penfate this lofs ; little alfo can be conveyed to it from the 

 earth, as a bad conduftor is interpofed between them ; and 

 there is no folid, or fluid except the air, to communicate it 

 laterally either by radiation or conduftion. This fmall 

 body, therefore, unlefs it (hall receive from the air, nearly 

 as m.uch heat as it has emitted, which, confidering the little 

 that can be communicated from one part of the atmofphere 

 to another, in its prefent calm ftate, muft be regarded as 

 impoffible, will become colder than the air, and condenfe 

 the watery vapour of the contiguous parts of it, if they 

 fhould contain a fufficient quantity to admit of this effeft. 

 But events fimilar to thefe occur, when dew appears in an 

 open and level grafs field, during a ftill and ferene night. 

 The upper parts cf the grafs radiate their heat into regions 

 of empty fpace, which confequently fend back no heat in 

 return ; its lower parts, from the fmallnefs of their cou- 

 dudting power, tranfmit little of the earth's heat to the 

 upper parts, which at the fame time receiving only a fmall 

 quantity from the atmofphere, and none from any other 

 lateral body, muft remain colder than the air, and condenfe 

 into dew its watery vapour, if ihia be fufficiently abundant, 

 in refpedl to the decreafed temperature of the grafs. 



" This fubjeil may be further iUuftrated by a reference to 

 what happens in the experiment, wh'ch has been ufed to 

 prove the refleftion of cold. 



" In the fimpleft form of this experiment, a fmall body, 

 the bulb of a thermometer, pofFefTrng the temperature of 

 the atmofphere, is placed before a larger cold body, ren- 

 dered equal in effeft to one ftill larger, by means of a con- 

 cave metallic mirror. In this fituation, the fmall body 

 radiates heat to the larger, without receiving an equivalent 

 from it, and, in confequence, becomes colder than the air, 

 through which its heat is fent, notwithftanding that it is 

 continually gaining fome heat, both from the air which fur- 

 rounds it, and from the walls and contents of the apartment, 

 in which the experiment is made. Dew, therefore, would 

 as readily form upon the thermometer in this experimert, 

 as it would upon one fufpended in the open air at night, 

 under a clear fky, provided that the-two inftruments were 

 equ.-J!y colder than the atmofphere, and that this was in 

 both cafes equally near to being replete with moifture. 



" Regarding now as eftablilhed, that bodies fituated on or 

 near to the furface of the earth become, under certain eir- 

 cumftances, colder than the neighbouring air, by radiating 

 more heat to the heavens, than they receive in everj- way, 

 I fhall in the firft place offer a few remarks on the extend 

 and ufe cf this occurrence, and /hall afterwards apply the 

 knowledge of it to the explanation of feveral more of the 

 appearances Uefcribtd in the former part cf this Effay, and 

 3 N 2 of 



