ON DEW, &c. 233 



surface had been metallic. In this experiment, 

 the outer edge of the metallic surface, from 

 being nearest to the colder glass, will be the 

 first and the most dewed, while the parts of the 

 uncovered glass, which are contiguous to the 

 warmer metal, will be the last and the least 

 dewed, of their respective substances. 



A piece of glass, covered on one side with a 

 metal, being placed on grass, with this side 

 down, its upper surface attracts dew as readily 

 as if no metal were attached to it ; since the 

 metal, in this situation, has no power to lessen 

 the radiation of heat from the upper surface of 

 the glass. I conclude, however, from general 

 principles, for I have not made the trial, that if 

 the same piece of glass, having its metallic side 

 still undermost, were raised in the air a little 

 above the grass, it would be more readily dewed 

 on its upper surface, than if it had been without 

 a metallic coating on the lower, as this coating 

 must resist the introduction of heat radiated by 

 the warmer grass, and thus preserve nearly un- 

 diminished the cold acquired, from radiation 

 of heat to the sky, by the bare upper surface. 



The preceding remarks apply to the whole 

 class of metals ; but the discoveries of Mr. 

 Leslie, respecting the difference in the capa- 

 cities of these bodies to radiate heat, furnish 

 an explanation of a diversity among themselves, 

 in regard to attraction for dew, which was 



