ON DEW, &c. 207 



Islands, and parts of continents close to the 

 sea, being, by their situation, subject to a cloudy 

 sky, will, from the smaller quantity of heat lost 

 by them through radiation to the heavens at 

 night, in addition to the reasons commonly 

 assigned, be less cold in winter, than countries 

 considerably distant from any ocean. 



III. Fogs, like clouds, will arrest heat, which 

 is radiated upwards by the earth, and, if they 

 be very dense, and of considerable perpendi- 

 cular extent, may remit to it as much as they 

 receive. Accordingly, Mr. Wilson found no 



atmosphere, and of the surface of the earth ; but he seems 

 not to have known, that their effect on the temperature of 

 the latter is much greater than that which they produce upon 

 the air. My explanation of this influence of clouds, on the 

 temperature of the surface of the earth, during the night, is 

 a direct consequence from the facts, which I had observed 

 respecting the prevention of cold on the ground from radia- 

 tion, by the interposition of solid bodies between it and the 

 heavens, and occurred to me in 1812. Mr. Prevost's work, 

 indeed, was published in IS 9; but I did not see it before 

 the summer of 1813 ; when it was lent to me by his relation 

 Dr. Marcet of London, who at the same time said, that he 

 believed there was no other copy of it in Great Britain, ex- 

 cept one, which had been sent by himself to Edinburgh. 



Note to second edition.'] I did not know, until after the 

 first edition of this Essay was printed, that Mr. Prevost had 

 published his opinion on the effect of clouds in preventing 

 the occurrence of cold at night in the atmosphere, and upon 

 the surface of the earth, as early as 1792, in a work entitled 

 6 Recherches sur la Chaleur.' 



