230 ESSAY 



its upper surface may be dry, from possessing 

 the same, or almost the same temperature, as 

 the atmosphere near to it. 



As the temperature of metals is never much 

 below that of the neighbouring air, a slight 

 diminution of their cold from radiation will 

 often occasion them to evaporate the dew, 

 which they had previously acquired, though 

 other substances, which had been more cooled 

 by radiation, are still attracting dew. For a 

 like reason, a metal, which has been purposely 

 wetted, will often become dry at night, while 

 other substances are becoming moist. 



A substance highly attractive of dew, such 

 as wool, if laid upon a metal, will derive heat 

 from it, and will therefore acquire less dew, 

 than an equal portion of the same substance 

 laid upon grass. 



A large metallic plate will be less readily 

 dewed while lying on grass, than if it were 

 placed in the air, though only a few inches 

 above the grass ; because, in the former situa- 

 tion, it receives freely, by means of its great con- 

 ducting power, heat from the earth ; whereas, 

 when placed in the air, it powerfully resists by 

 another property, possessed in a great degree 

 by bright metals, the entrance of heat radiated 

 towards it by the grass beneath. Besides ; the 

 grass under the metal possesses now less heat, 



