FORMATION OF CLOUDS. 191 



ence gradually extends upward, till all the vapour 

 in the sky, up to a very considerable height, is 

 much colder than before. As the heat diminishes, 

 the tendency of the particles of water to each 

 other, which has been suspended, but not in the 

 least destroyed by the heat, returns to its action, 

 and the particles approach each other, and form a 

 cloud That cloud gathers vapour from all the 

 space surrounding it, not only while it is barely 

 visible, but after masses of cloud have been formed. 

 Everybody who has looked at the sky must have 

 seen the clouds " congregating," even when there 

 was no wind but wind of their own making ; and 

 must have observed that, true to the law of that 

 attraction which is the real cause of their forma- 

 tion, the little clouds always move towards and 

 unite with the larger one. 



If the wind blows from a dry quarter, in the 

 higher part of the air, the cloud is often swept 

 away as fast as it forms ; and if it be blown to a 

 place where there is no such action on the surface 

 as that which produced the cloud, it may be again 

 dissolved by the air. But mornings, when the 

 disappearance of hoar frost denotes rain, are ge- 

 nerally calm ; and in those cases, there usually 

 is rain. Indeed, a moderate surface wind, one of 

 those " unhearty " winds, which we call " raw," 

 and which hiss in the crevices like scotched snakes, 

 rather brings on than retards the rain ; as wind al- 

 ways increases evaporation even those winds 

 that we call " moist," dry more than air at the 

 same degree of saturation with moisture, but at 

 rest. 



When the heating cause is local and confined, 

 the result is not rain but fog. In the evening, 



