ON DEW, &e. 229 



small, another reason will contribute somewhat 

 to their being later and less dewed than other 

 solid substances. For, in consequence of their 

 great conducting power, dew cannot form upon 

 them, unless their whole mass be sufficiently 

 cold to condense the watery vapour of the 

 atmosphere ; while the same fluid will appear 

 on a bad conductor of heat, though the parts a 

 very little beneath the surface are wanner than 

 the air *. 



From the same ready passage of heat from 

 one part of a metal to another, a metallic plate 

 suspended, horizontally, in the air several feet 

 above the ground, will be found dewed on its 

 lower side, if the upper has become so ; while 

 the lower surface of other bodies, more attrac- 

 tive of dew, but worse conductors of heat, are 

 without dew in a similar situation. 



A metal placed at night in the air, near to 

 the ground, is, for the most part, sufficiently 

 cold to condense, on its underside, the vapour 

 which arises from the warmer earth ; though 



* I hence think it probable, that dew will sometimes form 

 on the bulb of a thermometer, before the mercury in it is 

 cooled below the temperature of the air. It seems certain 

 to me, also, that dew may appear upon substances, which, 

 from the thinness of the layer of matter their cold is con- 

 fined to, will produce little or no sensible effect upon a ther- 

 mometer that is applied to them. 



