DEW. 



the general rule, that whatever exifts in the atmofphere, 

 capable of ftoppiiig or impeding the paffage of radiant heat, 

 will prevent or leflen the appearance at night of a cold on 

 the furface of the earth, greater than that of the neighbour- 

 ing air. 



" It follows alfo, from what has been faid in this article, 

 that the water depofited upon the earth, during a fog at 

 night, may fometimes be derived from two different fources, 

 one of which is a precipitation of moifture from a confider- 

 able part of the atmofphere, in confequence of its general 

 cold ; the other, a real formation of dew, from the con- 

 denfation, by means of the fuperficial cold of the ground, 

 of the moifture of that portion of the air, which comes in 

 contaft with it. In fuch a ftate of things, all bodies will 

 become moift, but thofe efpecially which moft readily 

 attract dew in clear weather. I have had no opportunity, 

 however, of trying this conclufion by the teft of obferva- 

 tion, fince it occurred to me. 



" When bodies become cold from radiation, the degree 

 of effeft obferved muft depend, not only on their radiating 

 power, but in part alfo on the greater or lefs eafe, with 

 which they can derive heat, by conduclion, from warmer 

 fubftances in contaft with them. Tlius grafs, on a clear 

 and ftill night, was conftantly colder, fometimes very much 

 colder, than the gravel-walk, though a fmall quantity of 

 fand, placed upon grafs, was always nearly as cold as this 

 fubftance. In this cale, the difference in temperature be- 

 tween the gravel-walk and fand, evidently depended on the 

 different quantities of heat which they received from the 

 parts beneath. A like reafon is to be given for dew appear- 

 ing in greater quantity on (havings of wood, than on the 

 fame fubftance in a more denfe and compaft form ; and for 

 filamentous and downy fubftances becoming colder than all 

 others, even than laiiip-black, which is placed by Mr. Leflie, 

 at the head of the beft folid radiators of heat. For the 

 lamp-black expofed by me, being about two lines in depth, 

 poflefTed, in confequence, a fund of internal heat, which 

 would more readily pafs to its cold furface, than the heat of 

 the lower parts of the downy fubftances would to their 

 upper furface. 



" Bodies, expofed in a clear night to the (ky, muft 

 radiate as much heat to it during the prevalence of wind, 

 as they would do if the air were altogether ftill. But in 

 the former cafe, little or no cold will be obferved upon 

 them above that of the atmofphere, as the frequent applica- 

 tion of warm air muft quickly return a heat equal, or nearly 

 fo, to that which they had loft by radiation A flight 

 agitation of the air is fufficient to produce fome effeft of 

 this kind ; though, as has already been faid, fuch an agita- 

 tion, when the air is very pregnant with moifture, will 

 render greater the quantity of dew, one requifite for a con- 

 fiderable produdlion of this fluid being more increafed by 

 it, than another is diminiftied." 



Theophraftus remarks, and the remark has been con- 

 firmed by other writers, that " the hurtful effefts of cold 

 occur chiefly in hollow places. If this be reftrifted to 

 what happens on ferene and calm nights, and it does not, I 

 believe, hold true in any other circumftances, two reafons 

 from diff"erent fources are to be afligned for it. The firft 

 is, that the air being ftiller in fuch a fituation, than in any 

 other, the cold, from radiation, in the bodies which it con- 

 tains, will be lefs diminiftied by renewed apphcations of 

 warmer air ; the fecond, that from the longer continuance 

 of the fame air in contaft with the ground, in deprefled 

 places than in others, lefs dew will be depofited, and there- 

 fore lefs heat extricated during its formation. It will be 

 fecn in the laft part of this Effay, that, in the Eaft Indies, 



depreflions in the earth are artificially made, for the purpofe 

 of increafing the cold, which appears in ferene nights. On 

 this fubjeft, however, it is to be obferved, that if the 

 depreffed or hollow places be deep, in proportion to their 

 horizontal extent, a contrary effedt mult follow ; as a cafe 

 will occur more or lefs fimilar to that which exifted in fome 

 experiments formerly related by me, in which a fmall por- 

 tion of grafs was furrounded by a hollow cylinder. 



" An obfervation clofcly connefted with the preceding, 

 namely, that in clear and (till nights, frofts are lofs fevere 

 upon hills, than in neighbouring plains, has excited more 

 attention, chiefly from its contradifting what is commonly 

 regarded an eftabliihed faft, that the cold of the atmo- 

 fphere always increafes with the diftance from the earth. 

 This inferior cold of hills is evidently a circumftance of the 

 fame kind, with that afcertair.ed by Mr. Piftet and Mr. Six, 

 refpefting the increafing warmth, in clear and cUm nights 

 at all feafons of the year, of the different ftrata of the atmo- 

 fphere, in proportion as thefe are more elevated above the 

 earth. As the greater cold of the lower air is the lefs com- 

 plicated faft, I fhall attempt to explain it in the firft places. 

 Mr. Piftet, indeed, furnifties an explanation himfelf, by 

 afcribing it to the evaporation of moifture from the ground. 

 But to ftiew that this is not juft, it need only be mentioned, 

 that the appearance never occurs in any conilderable degree-, 

 except upon fuch nights as are attended with fome dew, 

 and that its great degrees are commonly attended with a 

 copious formation of that fluid ; fince it cannot be thought, 

 that the fame ftratum of air will depofit moifture on the 

 ground, from an infufficiency of heat, at the very time it is 

 receiving moifture from the ground, in the ftate of pellucid 

 vapour, as this prefuppofes, that it is not yet replete with 

 water." 



Ariftotle and Plutarch, and of late Mr. Jefferfon, (Notes 

 on Vi^g^nia,) have obferved that dew is much lefs copious 

 on hills than it is upon plains. In order to account for 

 this fad, we may allow, at firft, that the furface of the 

 ground is in both fituations equally colder than the air con- 

 tiguous to it ; yet, " as tlie produftion of dew muft be in 

 proportion to the whole depreifion of the temperature of the 

 air which fumifhes it, below what its heat had been in the 

 preceding day, and as one part of this depreffion, the gene, 

 ral cooling ol the atmofphere, is much more confiderable on 

 the plain than on the hill, moifture muft necelFarily be depo- 

 fited more copioufly in the former than in the latter place. 

 If the greater agitation of the atmofphere, and the lefs 

 quantity of moifture, during clear weather, in its higher 

 region than in the lower, be added, it may readily be 

 inferred, that dew fhall fometimes be altogether wanting on 

 a hill, though abundant on a plain at its loot, agreeably to 

 what has been actually obferved by Mr. Jefferfon. 



" The leaves of trees often remain dry throughout the 

 night, while thofe of grafs are covered with dew. As this 

 is a fimilai- faft to the fmallnefs of dew on hills, I (hall in 

 accounting for it do little more, than enumerate the circum- 

 ftances on which it depends. 



" I. The atmofphere is feveral degrees warmer near the 

 upper parts of trees ou dewy nights, than dole to the 

 ground. 2. The air in the higher fituation is more agitated, 

 than that in the lower. 3. The air at a little diftance from 

 the ground, from being nearer to one of its fources of 

 moifture, will on a calm evening contain more ol it, than 

 that which furrounds the leaves of elevaleii ; ■ ees. 4. Only 

 the leaves of the very tops of trees are fii.i) .- \pofed to the 

 fky. 5. The declenfion of the leaves from an horizontal 

 pofition will occafion the air, which has been cooled by them, 

 to Hide quickly away, and be fucceeded by warmer parcels. 



6. The 



