1815.] On Dr. IFelh's Essay on Dew. 25S 



that this source of cold cannot be resorted to for the removal of the 

 present ditlicnlty, since, 1 think, 1 have shown^ that l)v. Y. was 

 ignorant of the necessity of a clear sky for the success of any of 

 ]\lr. P.'s experiments, and unless the sky be clear, the outer surface 

 of the window-glass will not be rendered colder than the atmosphere 

 by radiation. 



To conclude the consideration of Dr. Y.'s claim to having justly 

 accounted for the facts related by Mr. P., I siiall remark, that, 

 although his explanation could be made to apply to the whole of 

 them, and all tlieir attending circumstances, still, as it was never 

 verified by experiments, that would admit the existence of no cause 

 of the appearances observed, but that which is assigned by him, it 

 can strictly be regarded in no other light than in that of a con- 

 jecture.* The conjecture, however, was, I believe, original ; 

 it was most happy too, since, if admitted to be just, it com- 

 pletely accounted for several important circumstances in Mr. 

 P.'s experiments. If then its learned and ingenious author had 

 establislied its truth by facts clearly seen by himself, and had after- 

 wards pursued the subject of dew tiirough its various ramifications, 

 by means of the clue which would have thus been obtained, he 

 must soon have acquired a knowledge of the theory, which has 

 lately been submitted by myself to the consideration of the learned, 

 and which he, as a member of that body, has pronounced to be 

 just. But, I must, on the other hand be permitted to say, that, if 

 Dr. Y., forgetting that Newton became a glass-grinder in the 

 service of science, will neglect to employ, for the increase of 

 natural knowledge, the slow and laborious method of observation 

 and experiment, and will frequently exhibit his speculations in a 

 manner, unsuited to the capacities of ordinary men, he ought not 

 to think it .strange, that opinions, advanced by him on difficult 

 points of philosophy, are not, agreeably to liis own remark at the- 

 end of the Criticism, received as truths beyond doubt, and are often 

 not jindcPbtood. I am. Sir, 



Your most obedient humble servant, 



London, Feb. 15, 1815. WlLLI.VM ChaHLKS WkLLS. 



• In p. 475 of the second volume of Dr. Y.'s work on NHdiral Philosopliy is 

 Ihf followino passajfo : — " Must of tlic facts [related by Mr. P ] may pcikapi be 

 explained by Mr. Leslie's dii-coverics." Hence 1 naUir.iUy loiicluded, (liat the 

 siiliior liiinself at 111- lime of tii^ publi'Iiin!; that work, regarded his evpl:iiiatli)ii, 

 not onlj as a conjecture, liut as a conjecture inapplicable to the whole of the 

 facts iibsi-ivcd by .\|r. P. I have very lately learned, however, that i)r. Y.'ifiist 

 Voinme Mas printed aftt r ihr secoud. What is said, tlierefoie, b\ liiiii, re-pecliug 

 thecau-e of those fact's, in the paisa^e quoted in bis Criticism from tlie /i/v< voliune, 

 must be held to !>uyrr'<rde what h cited by luein the beginning of this note from tbr. 

 -und. 



11 2 



