CHAP. 1. 4. OF CLOUDS. 13 



fogs. It must be remembered, however, that 

 all fogs are not strati: some appear to be of the 

 modification of cirrostratus. Of the latter kind 

 are generally the wet mists, which moisten every 

 thing on which they alight. 



In speaking of the cumulus, I have repre- 

 sented the manner in which elastic vapour may 

 rise into the air, on the accession of diurnal 

 temperature. As the sun sinks, the heat also is 

 diminished, and the lower atmosphere becomes 

 cooler than that above. The air, no longer ca- 

 pable of containing so much vapour in solution 

 as when it was warmer in the day, may deposit 

 it in minute particles of water, which may fall 

 in the form of mist or stratus. In the evening, 

 too, the under atmosphere being as cold, or per- 

 haps colder, than the upper, the vapour plane 

 is not preserved, and cumuli by degrees may 

 sink down in dew. Under these circumstances, 

 they appear often to evaporate.* This vesper- 

 tine subsidence of the cumulus is a circumstance 

 which induces me to believe that its diurnal 

 existence, as an aggregate, is not merely the 

 result of the attraction of aggeration. Its sub- 



* For further observations respecting the nocturnal eva- 

 poration of clouds, the decomposition and recomposition of the 

 air, etc. I refer the reader to the next Chapter. 



