DEW. 



of foine others, which have not hitherto been mentioned 



by me. 



" Radiation of lieat by the earth to the heavens mult exilt 

 at all times ; but, if the fun be at fome height above the 

 horizon, the degree of which li hitherto undetermined, and 

 probably varies according to feafon, and feveral other cir- 

 cumftances, the heat emitted by it to the earth will over- 

 balance, even in places fhaded from its dired beams, that 

 whicli the earth radiates upwards. 



" In a calm and ferene night, however, when confequently 

 little impediment exifts to the efcape, by radiation, of the 

 eartli's heat to the heavens, and when ho heat can be radiated 

 by the fun to the place of obfcrvation, an immenfe degree 

 of cold would occur on the ground, if the following cir- 

 cumftances did not combine to lefFen it. I. The incapacity 

 of all bodies to prevent, entirely, the palTmg of heat, by 

 eonduftion, from the earth to fubftances placed upon them. 



2. The heat radiated to thefe fubftances by lateral objefts. 



3. The heat communicated to the fame fubilances by the 

 air. 4. The heat which is evolved, during the condenfation 

 of the watery vapour of the atmofphere into dew. 



" The extent of the effeft of all thefe checks upon the pro- 

 duftion of cold, by the nightly radiation of heat from bodies 

 on the furface of the earth, cannot, in the prefent ftate of 

 our knowledge, be properly eftimated ; but fafts Ihew that, 

 Hotwithftanding their operation, the cold originating in this 

 fource muft be often very confiderable. 



" I fhall add, with the greateft difBdence, a few words upon 

 a final caufe of the radiation of heat from the earth at night, 

 and upon fome of the circumftances which modify its aftion, 

 though fully confcious of the danger of error, which is 

 always incurred in the attempt to appreciate the works of 

 our Creator. 



" The heat which is radiated by the fun to the earth, if 

 fuffered to accumulate, would quickly deftroy the prefent 

 conftitution of our globe. This evil is prevented by the 

 radiation of heat by the earth to the heavens, during the 

 night, when it receives from them little or no heat in return. 

 But, through the wife economy of means, which is witnefied 

 in all the operations of Nature, the prevention of this evil is 

 made the fource of great pofitive good. For the furface of 

 the earth, having thus become colder than the neighbouring 

 air, condenfes a part of the watery vapour of the atmo- 

 fphere into dew, the utility of which is too manifeft to 

 require my fpeaking of it. I may remark, however, that 

 this fluid appears chiefly where it is moft wanted, on herbage, 

 and low plants, avoiding, in great meafure, rocks, bare 

 earth, and confiderable mafles of water. Its produ Aion too, 

 by another wife arrangement, tends to prevent the injury, that 

 might arife from its own caufe ; fince the precipitation of 

 water, upon the tender parts of plants, muft leffen the cold 

 in them, which occaiions it. I {hall obferve in the laft 

 place, that the appearance of dew is not confined to any one 

 part of the night, but occurs during its whole courfe, from 

 means the moft fimple and efficacious. For after one part 

 of tJae air has depofited its moifture, on the colder furface 

 of the earth, it is removed, in confequence of that agitation 

 in the atmofphere which exifts during its ftilleft ftates, and 

 gives pjace to another having its quantity of water undimi- 

 nifhed ; and, again, as the night proceeds, a portion of air, 

 which had before depofited all the moifture, which circum- 

 ftances at that time permitted, is rendered fit, by the gene- 

 ral increafe of the cold of the atmofphere, to give out a 

 frefh parcel, when it comes anew into contaft with the 

 ground. 



" The firft faft, which I fhall here attempt to explain, 

 is the prevention, either wholly or in part, of cold, from 



5 



radiation, in fubftances on the ground, by the interpofitioii 

 of any folid body between them and the flcy. This evi- '^ 

 dently appears to arife in the following manner. The 

 lower body 'radiates its heat upwards, as if no other inter- 

 vened between it and the fl<;y ; but the lofs, which it hence 

 fuffers, is more or lefs compenfated by what is radiated to 

 it, from the body above, the under furface of which pof- 

 fefles always the fame, or very nearly the fame temperature 

 as the air. 



" No direft experiments can be made to afcertain the 

 manner, in which clouds prevent, or occafion to be fmall, 

 the appearance of a cold at night, upon the furftce of the 

 earth, greater than that of the atmofphere ; but it may, 1 

 think, be firmly concluded, from what has been faid in tbe 

 preceding article, that they produce this effeft, almofl 

 entirely, by radiating heat to the earth, in return for that 

 which they intercept in its progrefs from the earth towards 

 the heavens. For although., upon the fky becoming fud- 

 denly cloudy during a calm night, a naked thermometer, 

 fufpended in the air, commonly rifes two or three degrees, 

 little of this rife is to be attributed to the heat evolved by 

 the condenfation of watery vapour in the atmofphere, as 

 was fuppofed by Mr. Wilfon ; fince, in confequence of the 

 ceafmg of that part of the cold indicated by the thermo- 

 meter, which was owing to its own radiation to a clear flcy, 

 the temperature of the atmofphere may feem to increafe 2'^, 

 or more, notwithftanding that it has received no real addi- 

 tion. Befides, the heat which is extricated by the con- 

 denfation of vapour, during the formation of a cloud, muft 

 foon be diffipated ; whereas the effedl: of greatly leflening, 

 or preventing altogether, the appearance of a fuperior cold 

 on the earth to that of the air, will be produced by a^cloudy 

 Iky, during the whole of a long night. 



" Denfe clouds, near the earth, muft poflefs the fame heat 

 as the lower atmofphere, and will therefore fend to the 

 earth as much, or nearly as much, heat as they receive from 

 it by radiation. But fimilarly denfe clouds, if very high, 

 though they equally intercept the communication of the 

 earth with the fl<y, yet being, from their elevated fituation, 

 colder than the earth, will radiate to it lefs heat than they 

 receive from it, and may, confequently, admit of bodies on 

 its furface becoming feveral degrees colder than the air. 

 In the firft part of this Eflay, an example was given of a 

 body on the ground becoming at night 5° colder than the 

 air, though the whole flcy was thickly covered with high 

 clouds. 



" Iflands, and parts of continents clofe to the fea, being, 

 by their fituation, fubjeft to a cloudy (ky, will, from the 

 fmallcr quantity of heat loft by them through radiation to 

 the heavens at night, in addition to the reafons commonly 

 afligned, be lefs cold in winter, than countries confiderahly 

 diftant from any ocean. 



" Fogs, like clouds, will arreft heat, which is radiated 

 upwards by the earth, and, if they be very denfe, and of 

 confiderable perpendicular extent, may remit to it as much 

 as they receive. Accordingly, Mr. Wilfon found no differ- 

 ence at night, in very foggy weather, between the tempera- 

 ture of the furface of fnow, and that of the air. 



" In mifts and low fogs it was found by profelfor Leflie, 

 that the diminution of the fun's heat is fmall, when com- 

 pared with what occurs, when the flcy is obfcured by a 

 denfe body of clouds ; and it will, I prefume, be readily 

 granted, that the fame ftate of the atmofphere, which allows 

 the heat of the fuu to pafs copiouOy, will alfo give a ready 

 tranfit to heat radiated by the earth." 



From previous reafoning, the author concludes, therefore, 

 that *' fogs do not in«ny inilance furnifh a real exception to 



the 



