1815.] On Dr. IFells's Essay on Deiv. 253 



leur" of Mr. Prevost, published in 1732. This work was, indeed, 

 unknown to me when I composed my Essay ; but Dr. Y. 1 presume 

 was ignorant, when he wrote his Criticism, that the very passages, 

 which he has cited, are contained in another and much later work 

 on heat by Mr. P. unaccompanied with any intimation, that they 

 were copied from a preceding publication. Mr. P.'s later work, 

 which was printed in 1809, was first seen by me in IS 12, two 

 years before my Essay came out. What appeared to me the most 

 worthy of attention in the passages cited by Dr. Y. was spoken of 

 in one of mv notes, p. 7''^- I mentioned there, very distinctly, that 

 Mr. P. had already accounted for the effect of clouds, in diminish- 

 ing the cold of the air at night, by making this to depend upon 

 their preventing the escape of its heat by radiation to the heavens, 

 but only impliedly, that he had accounted, in this way also, for a 

 similar effect produced by them upon the temperature of bodies on 

 the surface of the earth, as 1 said only, that he did not seem to have 

 known, that they have a much greater effect upon the temperature 

 of such bodies, than upon that of the air. My full meaning was, 

 that Mr. P. did not seem to know, that the degree of cold, which 

 is prevented by clouds from appearing on the surface of the earth, 

 is much greater, than that which they prevent from appearing in 

 the air ; or in other words, that he was ignorant, that, bodies on 

 tlie surface of the earth become much colder than the air in a clear 

 night, this being one of the principal facts, on which my theory of 

 dew is built. That I had no desire to conceal any thing which Mr. 

 P. had said upon this subject is shown, by my referring to the latest 

 work, in whicii he has mentioned the effect of clouds upon the 

 Iieat of the earth and atmosphere at night, and by my referring, 

 likewise, in three different parts of my Essay, (p. 68, 74, and 

 118,) to Count Kumford as supposing, that the earth is cooled by 

 radiation at night ; since it cannot bethought, that, althoug'i un- 

 acquainted with both of these authors, I should withhold the know- 

 ledge of the possessions of one, and yet repeatedly speak of similar 

 possessions of the other. 



I have said in the preceding paragraph, that Mr. P. did not seem 

 to know, that the earth ever becomes colder, at its surface, than 

 the air by radiation. My reasons are, 1st, That he has not men- 

 tioned this fact : 2dly, That he has said what is equivalent to a 

 denial of it in his late work, Du Calorique Kayonnant, p. 2-19 : 

 and 8d!y, That it is ajjparently in opposition to an observation of his 

 friend Mr. Pictct, from whom he seems to have derived all the 

 fads which he has related in this disquisition on the effect of clouds ; 

 for that philosopher found, that, a'tliough, in clear and calm 

 niglrfs, the heat of the air decreased from the height of 75 feet 

 above the ground to within four lines of it, yet a thermometer, 

 lying iip'iii the ground, and iiaving its bulb slightly covered with 

 carih, " preci-cment enterree," was higher than all those which 

 were suspended in the air above it. — Putet sur le Feu, p. 180. 



