110 EVAPORATION, 8cc. [Lesson xix. 



will diffuse themselves, rising gradually higher and 

 higher, and so leave the lowest air not so much satu- 

 rated but that it will still be able to dissolve and take 

 up fresh particles of water; which process is greatly 

 promoted by the motion of the wind. When the 

 vapours are thus raised and carried by the winds into 

 the higher and colder regions of the atmosphere, 

 some of them will coalesce into small particles, 

 which slightly attracting each other, and being 

 intermixed with air, will form clouds; and these 

 clouds will float at different heights (varying from 

 less than half a mile to somewhat more than two 

 miles) according to the quantity of vapour borne 

 up, and the degree of heat in the upper part of the 

 atmosphere: and thus, clouds are generally higher 

 in summer than in winter. 



When the clouds are much increased by a con- 

 tinual addition of vapours, and their particles are 

 driven close together, by winds, cold, and other 

 causes, then will they run into drops heavy enough 

 to fall down in Rain : 



The clouds consign their treasures to the fields, 

 And, softly shaking on the dimpled pool 

 Prelusive drops, let all the moisture flow 

 In large effusion o'er the fresLen'd world. 



THOMSON. 



In the winter, when the air is disposed for freez- 

 ing, it is not unlikely that the atoms of vapour 

 which are near the earth are congealed, whereby 

 they become more opaque, and form those ob- 

 scure fogs which constitute a hoar frost. At this 



time 



