THE EEASON WHY. 



"With clouds hecovereth the light, and commandeth it not to shine by the 

 cloud that cometh betwixt." JOB xxxvi. 



376. Why, if they are invisible ivJien they rise, do they 

 become visible when they have ascended ? 



Because the vapours become coolecL in passing through the air, 

 and form a denser body. 



377. TP hy, when they are condensed, do they not follow the 

 course of gravitation, and descend ? 



Because the vapours form into minute vesicles, which we may 

 call vapour bubbles, and these, being warmed by the sun, are 

 specifically lighter than the air. 



Because, also, the lower parts of clouds do partially descend, but 

 again becoming more rarefied by meeting with a warmer atmo- 

 sphere, they again ascend, and are thus poised upon the air. 



Because, also, there is always a degree of atmospheric motion 

 upward, caused by the convection of heat from the earth's sur- 

 face. And, although there must also be downward movements of 

 the air to supply the place of that which has ascended, still the 

 heat of the ascending air, combined with its upward movement, 

 expands and floats the vapour of the clouds. 



378. At what height do clouds usually fly ? 



They fly ac every degree of altitude; but clouds of specific 

 character are said to fly at given altitudes, or to occupy certain 

 ranges of altitude. We will give their probable altitudes when 

 speaking of the specific clouds. 



Fig. 5. CIRRUS, OK CURL CLOUD. 



379. How many descriptions of clouds are there ? 

 There are seven. 



