i'heir Prodiidiofi, Suspension, and Destruction. 353 



This nocturnal evaporation is usually most powerful in 

 the autunui, about the time that the temperature of the 

 nights undergoes a considerable and sometimes pretty sudden 

 depression attended with a calm. 



In this state of things the vapour arising from the heated 

 earth is condensed in the act of diffusing itself (if we may 

 be allowed the expression) : the cold particles of water thus 

 formed, in descending meet the ascendmg stream of vapour, 

 and condense a portion on their surfaces : if they touch the 

 earth they are again evaporated, which is not necessarily the 

 case if they alight on the herbage. In this way an aggregate 

 pf visible drops is sooner or later formed ; and as from the 

 temperature thus communicated to the air next the earth 

 the vapour has still further and further to rise in order to be 

 condensed, the cloud will be propagated upward ia propor- 

 tion. 



Hence the stratus most usually makes its appearance in 

 the evening succeeding a clear warm day, and in that qui- 

 escent state of the atmosphere which attends a succession 

 of them. Hence also the frequency of it during the pe- 

 netration of the autumnal rains into the earth; while in 

 spring, when the latter is acquiring temperature together 

 with the atmosphere, it is rarely seen. 



Of t/te Formation of the Cumulus. 



When the sun's rays traverse a clear space of atmosphere, 

 it is well known that they connnunicate no sensible increase 

 of temperature thereto. It is by the contact, and what may 

 be termed the radiation, of opakc substances exposed to the 

 light, that caloric is thrown into the atmosphere. 



This eft'ect is first produced on the air adjacent to the 

 earth's surface, and proceeds upward, more or less rapidly, 

 according to the season and other attendant circumstances. 

 In the morning, therefore, evaporation usually prevails 

 again ; and the vapour which continues to be thrown into 

 air, now increasing in temperature, is no longer condensed : 

 on the contrary, it exerts its elastic force on that which the 

 nocturnal temperature had not been able to decompose, and 

 which consequently remained universally diffused. The 

 latter, in rising through the atmosphere to give place to the 

 Eiqjply from below, must necessarily change its climate, quit 

 the lower air of equal temperature, and arrive among more 

 elevated and colder air, the pressure from above still conti- 

 nuing unabated. The consequence is a partial dccompo- 



• A plentiful dc-.v may often be fcuiid on the grass after a stratus. 



Vol. X\T. No. Cl. Z sitioa, 



