1815.] On Dr. Wells's Essmj on Dew. 2oJ 



leQins to cool. Sometimes, however, in warmer weather, the dew 

 begins to descend in the evening." For, what connexion with the 

 doctrine of radiation has this theory of the dew, which is deposited 

 by the atmosphere, and in what does it differ from that which was 

 invented by Aristotle ? In like manner it may be said, that if Dr. 

 Y. had entertained a just notion of the cause of dew, he would not 

 have conjectured, that the remarkable cold observed by Mr. Wilson 

 on snow, in conjunction with hoar frost, a substance admitted by 

 Dr. Y. to be frozen dew, might arise from the contact of air occa- 

 sioning the ^ow to melt more readily ; ii. 395 ; nor have sup- 

 posed, that, when the surface of a metal remains dry, in the 

 neighbourhood of a piece of glass which is covered witli moisture, 

 this arises from the different attractions for moisture in those bodies 

 wi)ile of the same temperature; i. 70S ; since both of these facts 

 must now, I think, if he considers them fully, appear to him to 

 owe their explanation to the doctrine of radiation ot heat. Having 

 offered this argument in the first place, as being of easy compre- 

 hension, I pass to others of a different kind, which I am afraid I 

 shall not be able, for various reasons, to render very perspicuous. 

 To facilitate reference, I shall place at the bottom of this p;ige all 

 that he has transcribed in his Criticism from his Lectures concern- 

 ing Mr. P.'s experiments.* 



The account given by Dr. Y. of such of Mr. P.'s experiments as 

 he means to consider, is, to my apprehension, involved in much 

 obscurity, part of which would have been prevented by his men- 

 tioning, that the experiments were made upon a glazed window- 

 frame, pertaining to a room the air in which was warmer than that 

 without. But, notwithstanding that these circumstances were 

 known to me, I mistook one of iiis facts on the first reading of his 

 statement, and was not undeceived, until I consulted Mr. P. In 

 Mr. P.'s apparatus a thin rectangular plate of metal is attached to 

 the inner or outer side, or the warm or cold side, (to use Dr. V.'s 

 expression,) as occasion may require, of a pane of glass of the 

 window, in such a manner, as to be surrounded by a margin of bare 

 glass. When, therefore, Dr. Y. spoke of moisture being deposited 

 on the glass near ihe metal, I naturally concluded, as I remembered 

 the construction just described, that the portion of glass meant was, 

 the naked margin on the .••ame side with the metal. But on looking 

 into Mr. P. 1 found, that Dr. Y.'s meaning must be, that the 

 moisture was deposited on the glassy surface immediately opposite to 



• " It ha« hoen ob-«'rvfd, ll.ata piece of metal, placed on Rla^s, usually protects 

 alio ibe uppoiiitr side of ihc glass from the deposition of dew ; and Mr. Benedict 

 I'rtviist lia> kIiuwi), that, In general, HJienevcr the metal is placed on tlic waruitr 

 »l'le of the gliiMi, tlie tiiiiMidily is deposited more copiously, either on itself, or on 

 the |flai« ne.-ir it, [as in the ca^e of the sliultfr'] ; that \vlien it is on the colder 

 «Wr, it neither receive* the humidity, nor permits its deposition on the glass ; but 

 • b»i the addition of a ^etond piece of glass <iver the melal destroys the cllect, and 

 a second piece of metal reHorej it. It appeaf.-, that, from its properties with 

 rc»pcc! 10 rniliant heat, the mr.tullic turfact prodiiees these cfl'octs by preventiiig 

 »«i(/v comiiiuiiiialiun eilliir of htat, or of luUt to the g'ass." 



Vol. V. N° IV. K 



