THEORY OF DEW. 357 



from which it would appear, that the caloric 

 evolved from aqueous vapour during its con- 

 densation into clouds, warms the atmosphere 

 and prevents the surface of the earth from being 

 cooled to the dew point. Admitting that a por- 

 tion of the caloric radiated from the surface of 

 the earth, is reflected back, instead of being 

 absorbed by clouds, it is insufficient to produce 

 so decided a change of temperature. During 

 clear and serene nights, when no caloric is given 

 out to the atmosphere by the condensation of 

 vapour, the earth is cooled down by radiation, 

 because it receives nothing to compensate for 

 the loss of What is given off; so that the trans- 

 parent aqueous vapour of the lower strata of the 

 atmosphere, is condensed into dew. There is 

 no dew on windy nights, for the same reason 

 that evaporation is accelerated by wind : or that 

 a renewal of water and agitation promote the 

 solution of salts. 



It was before stated, that caloric is radiated 

 more copiously from rough surfaces than from 

 such as are smooth. In accordance with that 

 general fact, we find that dew is formed more 

 copiously on swansdown, wool, cotton, raw silk, 

 flax, tine shavings of wood, &c. than upon 

 compact pieces of the same materials. It 

 forms more copiously on rough metals than on 

 polished ones ; doubtless because the former pre- 

 sent a more extended radiating surface. See 

 Theory of Radiation. Book II. Chap. I. 



