188 FORMATION OF CLOUDS. 



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 formation, 

 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 generally calm ; and in those 

 cases there usually is rain. Indeed, a moderate sur- 

 face 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 always 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, the land, 

 especially where it is bare and dry, cools much 

 sooner than the water; and as it is the change of 

 temperature, and not the absolute temperature that 

 produces the change of evaporation, vapour then 

 gathers over the pools and marshes, and the courses 

 of the rivers ; and among bare hills with deep valleys, 

 and lakes and rivers, the fog is often seen white and 

 dense, in the hollows, as if some white fluid had 

 been poured into them. 



City fogs, such as the fog of London, which is at 

 times very annoying, and always very offensive, are 

 owing to a similar cause ; only in the case of these, 

 that cause is in the city. In the early morning, 

 when the production of fog has been lessened by the 



