254 On Dft Wells's Essay on Dew. [April, 



Considering the purpose to which Dr. Y. has applied his quota- 

 tion from Mr. P., I shall venture to examine this a little closely. 



1. What has hitherto been called by me Mr. P.'s explanation 

 of the effect of clouds, at night, upon the temperature of the sur- 

 face of the earth and of the lower atmosphere, is in reality only a 

 conjecture; for, 1st, It is denominated by himself an " Essai d'Ex- 

 plication : " 2dly, It is supported by no experiment : 3dly, It takes 

 as established, that air can radiate heat, whereas he says in p. 24 of 

 his last work, " On peut siipposer, que les molecules de I'air 

 rayonneiit" 



2. Mr. P. mentions, that clouds send back to the earth a little 

 more heat than transparent air can do ; which is equivalent to 

 affirming, that clouds alter the temperature of bodies upon the 

 surface of the earth at night only a little. Nothing can show more 

 strongly Mr. P.'s want of practical knowledge upon this subject ; 

 since 1 have given in my Essay, p. 32, an instance of the tempera- 

 ture of grass having risen 15° in less than 45 minutes, on the sky 

 becoming cloudy. 



3. It is remarked by Mr. P. that the operation of clouds, in 

 keeping the surface of the earth warm at nigiit, is exactly similar, 

 " exactement comparable," to that of cloathing on the human 

 body ; and Dr. Y. assents to this observation. Now it appears to 

 me, that very dissimilar things are here confounded together. 

 Cloaths keep us warm by being bad conductors of heat; but clouds 

 warm the earth by radiating heat to it. Hence, the eft'ect of clouds 

 is immediate, but cloaths require to be applied some time, before 

 they completely answer their purpose. Cloathing must touch the 

 skin to produce its full effect, whereas clouds produce theirs at the 

 greatest distance. The thicker our cloathing is, the warmer it 

 renders our bodies ; but, if a cloud be sufficiently dense to prevent 

 the heat, that is radiated into it, from passing through its inter- 

 stices, the earth is kept as warm by it, as far as the principle of 

 radiation is concerned, as it would be by one many times deeper. 

 This is shown by a piece of cambric preventing the occurrence of 

 cold on the earth's surface, from radiation, as effectually as a thick 

 blanket. 



4. Annexed to the disquisition of Mr. P., are two meteorolo- 

 gical facts communicated to him by Mr. Pictet. One is, that upon 

 a night in January, 17775 a thermometer, suspended in the open 

 air, rose nearly 3^" of Fahr. in the space of an hour, the weather 

 having in the mean time become cloudy. This was observed by 

 himself; but it does not appear, that he afterwards made any 

 similar observations. The second is given on the authority of 

 husbandmen, who arc said constantly to find that, though other 

 circumstances are favoiirable for the production of dew, none, or 

 almost none, appears, if the sky be cloudy; and that hoar frosts, 

 which are so frequently injurious in spring and autumn, do not 

 occur in those seasons, during cloudy weather. These facts, if 



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