III.] RAIN AND DEW. 41 



from a clear and cloudless sky. By local refrigeration, after 

 sunset, the vapour, invisibly diffused through the atmosphere, 

 is condensed, at once, into excessively fine drops of liquid 

 water, forming the rain called serein. But such phenomena 

 are rare ; and, as a rule, we may fairly expect the formation 

 of rain to be preceded by that of cloud. 



Many opinions have been advanced to explain the 

 precise condition in which water exists in a cloud. At 

 one time it was commonly supposed that a cloud is made 

 up of a vast number of minute vesicles, or little watery 

 bladders, which remain suspended in the air by reason of 

 their small size and hollow structure. It appears probable, 

 however, that the water is merely condensed in a very 

 finely-divided state, its extremely minute particles remaining 

 suspended in the surrounding moist air, as fine dust would 

 do. Such particles have, indeed, been expressively called 

 by Prof. Tyndall "water-dust." It is supposed that, in the 

 upper regions of the atmosphere, the watery cloud-drops are 

 frequently frozen into ice — a supposition strongly supported 

 by the optical characters of certain clouds, which appear to 

 be explicable only by the presence of a crystalline structure. 



When a current of warm air, laden with moisture, rises 

 from the surface of the earth, and reaches the higher and 

 colder regions, the uppermost portion of the ascending 

 current deposits its moisture in visible form, and thus 

 produces a cloud, supported at the top of an invisible 

 column. If the temperature fall, or the course of the 

 current be arrested, the cloud descends, and regaining 

 the lower and warmer regions, returns to its original state 

 of invisible vapour, and thus becomes dissipated. Observe 

 the clouds of steam which issue from the chimney of a 

 locomotive engine, and you will see that as they float 

 away in the air they gradually disappear. They are, in 



