to 



On Climate, fyc, 



body be in a liquid or aeriform state, although the process may 

 go on freely, as in water, the cold produced by it will not accu- 

 mulate upon the surface, but will be dispersed by known laws 

 throughout the mass ; and if a solid body be a good radiator 

 but a bad conductor of heat, the ftigorific effect will be con- 

 densed upon the face which is exposed. So upon the surface 

 of the earth absolute stillness of the atmosphere is necessary 

 for the accumulation of cold upon the radiating body ; for if 

 the air be in motion, it disperses and equalizes the effect, 

 with a rapidity proportioned to its velocity. 



It is upon these principles that Dr. Wells has satisfac- 

 torily explained all the phenomena connected with dew or 

 hoar frost. This deposition of moisture is owing to the cold 

 produced in bodies by radiation, which condenses the at 

 mospheric vapour upon their surfaces. It takes place upon 

 vegetables, but not upon the naked soil. The fibres of short 

 grass are particularly favourable to its formation. It is not 

 produced either in cloudy or in windy weather, or in situa- 

 tions which are not perfectly open to the sky. It is never 

 formed upon the good conducting surfaces of metals, but is 

 rapidly deposited upon the badly conducting surfaces of 

 filamentous bodies, such as cotton, wool, &c. 



In remarking that dew is never formed upon metals, it is 

 necessary to distinguish a secondary effect which often causes 

 a deposition of moisture upon every kind of surface indis- 

 criminately. The cold which is produced upon the surface 

 of the radiating body, is communicated by slow degrees to 

 the surrounding atmosphere, and if the effect be great and of 

 sufficient continuance, moisture is not only deposited upon 

 the solid body, but is precipatedin the air itself; from which 



