454 PHILOSOPHY OF STORMS. 



no account of how invisible vapor begins originally to con- 

 dense into visible vapor, or cloud. If you say that con- 

 densation of invisible vapor begins in consequence of an 

 upward vortex of air, the query then is, what originates 

 this upward vortex of air ? l According to my notions on 

 the subject, the formation of clouds is chiefly occasioned by 

 whatever reduces the temperature of the atmosphere ; and 

 this may be produced in a variety of ways, such as, 1st. 

 By the reduction in the temperature which takes place dur- 

 ing the transitions from day to night, and from summer to 

 winter. 8 2d. By the transportation of air by means of 

 aerial currents, from a warm towards a comparatively cold 

 latitude or locality. 3 3d. By the elevation of atmospheric 

 currents in surmounting hills and elevated lands. 4 And 

 also, as I hypothetically conceive, (though I am very doubt- 

 ful of the truth of this hypothesis,) by the slow ascension of 



1 The up-moving column of air may take its rise either from acquiring 

 more heat or more vapor than surrounding portions of the atmosphere ; for it 

 is known, that vapor is only five-eighths the specific gravity of air. 



2 This cannot be the cause ; for we have in this climate at all seasons nu- 

 merous instances of entirely clear nights succeeding days with many clouds. 

 No phenomenon is more common than for clouds to begin to appear at eight 

 or nine, A. JVL, and increase till the hottest part of the day, and then grad- 

 ually disappear after sunset. And in Jamaica this occurs every day in the 

 dry seasons, almost always producing rain in the interior of the island about 

 one or two o'clock. 



3 If cloud is produced in the first way, it ought to be in contact with the 

 ground ; for it can only be chilled by contact. This undoubtedly sometimes 

 occurs, and fog is the result. But I have constantly observed, that when a 

 very warm, and even damp atmosphere begins to blow from the south, after 

 a very cold spell of weather, when the ground and stones of our pavement are 

 so cold as to condense upon them a portion of vapor from the air, none of 

 those clouds called cumuli are ever formed 5 the reason I suppose to be, that 

 no up-moving columns can then be formed. Neither is cloud formed by 

 the change from summer to winter; for there is more rain in the spring than 

 in the autumn. 



4 This is undoubtedly one cause of clouds ; but unless the cloud is speci- 

 fically lighter than the surrounding air, how are these currents surmounting 

 hills produced ? 



