38 Harvey on the Deposition of Dew. 



contact with them, may be referred the cause of the phenomena 

 above described. The glass, for example, with a very superior pro- 

 pellent energy, had a portion of its surface touching that of the 

 polished tin, which radiates heat but imperfectly. The resistance 

 which this feeble radiation offers to the formation of dew, being 

 communicated to the vitreous surface equally in all directions, but 

 confined to within certain limits, will explain the cause of the ab- 

 sence of moisture in the bottoms of the glasses, and also why their 

 dry portions were of a circular form. The uniform breadth of the 

 dry zone, in the crystal surrounded with the ferrule, may be referred 

 to the same cause. 



The gradual increase also of the particles of the dewy zones, 

 from the borders of the lucid portions of the crystals, affords a 

 beautiful proof of the general influence of the metal, and of the 

 gradual steps by which that influence is diminished. In the 

 glass surrounded with the ferrule, the deposition of dew was 

 checked, both by the metal on which it rested, and by the ferrule 

 itself. This double influence necessarily occasioned each border 

 of the zone of moisture, to be formed of the smallest parti- 

 cles, and the middle portion of the same, of the largest; that 

 being the part of the surface where the joint effect of the metals 

 was the least. The other glass, being subject only to one 

 of the influences here alluded to, necessarily had its minutest 

 particles confined to the inner edge of its zone; and, conse- 

 quently, those of the largest size, on the extreme border of the 

 crystal ; that being the part farthest removed from the influence 

 of the tin. 



In particular conditions of the atmosphere, and after a metallic 

 surface has been, for some time, exposed to its influence, it will 

 approach very nearly to a state favourable to the formation of 

 dew ; or, as sometimes happens, moisture will be deposited on it*. 

 During the night when these experiments were performed, the latter 



* The difficulty with which polished inetals are dewed has been long known. 

 But there are some circumstances connected with the deposition of dew on 

 their surfaces, at particular elevations above the ground, which merit the fur- 

 ther attention of philosophers. 



