CAUSES OF DEW.] 



METEOROLOGY. 



1139 



perfectly well understood, no atmospheric phenomenon, 

 before the happy researches of Dr. Wells, was more im- 

 perfectly explained and involved in greater mystery. 

 The formation of dew is immediately referable to the 

 function of radiation, concerning which it will be proper 

 to make a short explanation, in addition to that which 

 has been already stated. 



The general indication only has been made that a 

 heated body for example, a cannon-ball if suspended 

 in si ace, darts off heat cognisable under the condition of 

 rays. It remains now to be stated that the function of 

 radiation is determined as to its extent by the surface of 

 bodies : rough metallic surfaces radiate more than those 

 which are smooth ; glass surfaces radiate more than 

 metallic surfaces ; plants radiate more than the earth ; 

 Lfass and leaves more than bushes and trees ; loose 

 gravelly land more than hard soil. 



To demonstrate the efl'ect of surface on radiation, 

 many instructive experiments may be performed by 

 means of the differential thermometer and a cubical 

 canister of tin plate. If such a canister be taken, and 

 one side of it scratched, another polished smooth, 

 another painted white, and the fourth black a mixture 

 of lamp-black and size being used by preference for the 

 latter purpose : if the canister be now filled with hot 

 water, ainl held between the two bulbs of a differential 

 thermometer, as represented in the accompanying dia- 

 gram (Fig. 40), each side of the canister will represent 

 and indicate a different amount of 

 radiating influence, aa shown by 

 the complementary disturbance of 

 the two mercurial columns. It 

 will be found that the polished 

 aide has the minimum, and the 

 blai -kcued side the maximum, 

 radiating effect. It will soon be 

 perceived that these deductions 

 concerning the property of radia- 

 tion are intimately connected with 

 the philosophy of dew.* 



I'ntil the experiments of Dr. 

 Wells, which will be soon ad- 

 verted to, the most erroneous 

 notions prevailed concerning the 

 theory nf dew. According to some it fell from the sky; 



ling t.i others it rose from the ground ; both which 



are altogether untenable. 



It is a sufficient answer to the proposition that dew 

 falls from the sky, to say that dew never occurs when 

 night* aro cloudy ; and it is a sufficient answer to the 

 statement that it rises from the ground, to remark, that 

 a slight screen thrown on the ground, or elevated 

 atxive the ground, is incompatible with the formation 



The theory of dew is easily explained by 

 a consideration of the laws of radiant heat. 

 The starting-point of the investigation is the 

 atmosphere. Now the atmosphere always 

 .,i.t:iing moisture, as we have already ex- 

 plained ; and the amount of this moisture 

 will, w /.,;< /xiriVxu, be correlative with the 

 nf atmospheric heat at the time. If, 

 fli'-n, the atmosphere be in:,' rai-e.i to its 

 ]M>int of saturation for i.ny given 

 of temperature, that temperature 

 should by any chance fall, the result will 

 necessarily be a deposition of moisture. Let 

 us now apply these principles to the coinii- 

 ti. IM of a heat-radiating surface of the earth and a 

 clou.led sky. In this case no dew occurs, nor, according 

 to theory, should any occur, inasmuch s the clouds 



nn the functions of a second radiatin,' sin 

 earth radiates heat owing to the clouds ; but the 



clouds, in their turn, radiate heat back !i','ain to the 

 earth ; whence it follows that the earth practically 

 not lose heat, and its temperature not falling below the 

 temperature of the circumambient atmosphere, no atrno- 



Tin- rtnrivnt mnj rrfer to the article " lUcluUiun." in the Section on 

 Utat, in chu volume.- Kit. 



spheric moisture can be deposited ; in other words, no 

 dew can occur. 



For these facts we are indebted to the late Dr. Wells. 

 They are demonstrated by thousands of natural condi- 

 tions, and bear the test of any properly-devised experi- 

 ment. 



The following diagram (Fig. 41) is intended to show- 

 Fig. 41. 



the manner in which a screen will prevent the occurrence 

 of dew. Two plates of glass are represented as supported 

 over an expanse of grass. Underneath the glass plate 

 not the slightest dew will be round, though the grass 

 around will be dewed heavily. 



A very pretty illustration of the conditions which regu- 

 late the formation of dew, will frequently be supplied by a 

 sheep lying down on the grass, on a clear, tranquil, cloud- 

 less ni",ht, when, to use a popular but incorrect expres- 

 sion, dew is falling ; it will be found that the upper part 

 or .upect of the wool of a sheep, is completely drenched 

 with dew, although the under part or aspect of the 

 animal is dry, as represented in the accompanying dia- 

 gram (Fig. 42). 



fig. 11 



The explanation of this phenomenon will be so obvious 

 that no further remark concerning it is necessary. 



Consideration of the laws of radiant heat will render 

 manifest the reason wherefore some surfaces are more 

 bedewed than others. The amount of dew will depend, 

 caeterit jmri'in.i, on two circumstances firstly, on the 

 kind of Kin-face ; and secondly, on its angle of inclination. 

 k'-Vn nee has already been made to the comparative 

 facility wherewith certain bodies found in nature, favour 

 the deposition of dew upon them ; and the most casual oil- 

 server canuot fail to bo struek with the difference. In all 



cas^s, the bodies which radiate heat are the most favour- 

 able to tint deposition of dew upon them. Few, if any, 

 urriii',', are so solicitous of dew an 



spiders' webs ; and no object presents tho phenomena of 

 dew under a guise so beautiful. Not (infrequently, a thin 

 filament of cobweb, so small that it would be invisible 

 to the naked eye, presents itsolf to tho vision on a 

 dewy norning as if it wore strung with little pearls 

 (1-X'i. !:;, 44). 



That angular inclination of a body should influence, 

 and be intimately connected with, the function of dew 



