CHAP. 1. 10. OF CLOUDS. 39 



it less capable, agreeable to M. Howard's theory, 

 of containing so much aqueous gas in solution, 

 which thus contributes to effect its own con- 

 densation into clouds: by which process there 

 is produced again an increase of heat; and by 

 this means the degree of temperature is in some 

 measure restored. 



" The theory of dew seems to be simply this: during the 

 heat of the day a great quantity of vapour is thrown into the 

 atmosphere from the surface of the earth and waters. When 

 the evening returns, if the vapour has not been carried off in 

 part by currents, it will often happen that more remains 

 diffused in the general atmosphere than the temperature of 

 the night will permit to subsist under the full pressure of the 

 aqueous atmosphere. A decomposition of the latter then 

 commences, and is continued until the general temperature 

 and aqueous pressure arrive at an equilibrium, or until the 

 returning sun puts an end to the process. The caloric of the 

 decomposed vapour goes to maintain the general temperature ; 

 while the water is separated in drops, which, minute as they 

 are, arrive successively at the earth in the space of a few hours. 

 That the ordinary production of dew is by a real descent of 

 water from the atmosphere, and not by decomposition of 

 vapour on surfaces previously cooled (as in the experiment 

 already mentioned) any one may readily be convinced by 

 observing in what abundance it is collected by substances 

 which are wholly unfit to carry off the requisite quantity of 

 caloric for the latter effect." Phil, Mag. Sep. 1803. 



