1815.] On Dr. IFelh's Essaij on Dew. 255 



considered in themselves, are certainly very important, but they ace 

 not new, though they appear to have been thougjit so by Dr. 

 Young, both by their being brought in support of his position, that 

 I had been anticipated in several of the opinions contained in my 

 Essay, which I had regarded as original, and by the use, which he 

 has made of the Italick character, in presenting Mr. Pictet's words 

 to his readers. With respect to the first fact, I referred in p. 78 of 

 my Essay to Mr. Wilson's being acquainted with it, and as his 

 observations concerning it appeared in 17S6 iu the Edin. Phil. 

 Trans, his claim to the discovery must, by the common rule in such 

 cases, be held superior to that of Mr. P., whose communication 

 upon it was not published till \']\)2. Besides, Mr. W. formed his 

 conclusion from several observations, whereas Mr. P. has given 

 only a single one. But the same fact, though in forms somewiiat 

 different, was known to two authors anterior to both Mr. W. and 

 Mr. P. For Lord Bacon says, Nat. Hist. § 366, " Star-liks 

 nights, yea, and bright moonshine nights, are colder than cloudv 

 nights ; " and Professor A. Wilson, of Glasgow, has mentioned, 

 Phil. Trans. 1771) t^^^t on a winter night, during which the 

 atmosphere was several times misty and clear alternately, he observed 

 a thermometer, suspended in the air, always to rise from a half to 

 a whole degree, whenever the 'former state began, and to fall as 

 much, as soon as the weather became serene. In regard to the 

 facts related by Mr. P. on the authority of husbandmen, I shall 

 only say, that Aristotle expressly mentions, that neither dew nor 

 hoar frost occurs except the night be clear, and that the same 

 observation has been made by many other authors prior to Mr. P. 



5. Mr. Prevost says at the end of his disquisition, tliat all the 

 facts mentioned by Mr. Pictet may be accounted for on the prin- 

 ciples previously established by him ; but, as I have proved, I 

 think, that he did not know that the surface of the earth ever 

 becomes colder than the atmosphere, by radiation, any explanation 

 he could offer of those facts must necessarily be imperfect. 



Having now, 1 think, shown, that nothing is contained in the 

 extract given by Dr. Y. from Mr. Prevost that required being taken 

 notice of in my Essay, besides what he has said on the effect of 

 clouds on the temperature of the earth and air at night, I shall 

 mention, in justice to this philosopher, that the Essai d'Explica- 

 tion, just discussed, has risen considerably in my estimation, since 

 I knew, that it had been given to the world so long ago as in 1 792 ; 

 for as I had before supposed, that he had been assisted in forming it 

 by the works of Count Runiford and Mr. Leslie on heat, which 

 were published in 1804. His opinion, therefore, on the effect of 

 clouds in keeping the earth, and lower atmosphere, warm at night, 

 appears to me now not only to be entirely original, but to exhibit a 

 degree of knowledge on the subject of radiant heat, which I did not 

 conceive had been possessed by any person at the time of its puljli- 

 cation. 



IJI. I menlioned in a note in the I05th page of my Essay, th^t 



