CLOUDS AND CLOUD FORMS 



67 



do not fall, why they do not rest upon the 

 earth instead of in the air. There are several 

 reasons for their not doing so, and all of these 

 reasons taken together may account for the ap- 

 parent defiance of the law of gravity. 



Thistle-down will speedily find an abiding- 

 place on the ground if there be no wind, but a 

 gentle breeze will carry it drifting for miles, 

 now high, now low, always soaring, sinking, 

 floating. Something of this effect is produced 

 upon the clouds by the winds and the moving 

 currents of air. They are always forming and 

 changing and being kept in motion by the 

 winds. The travelling capacity of the different 

 cloud flocks is, as we shall see hereafter, much 

 greater than is generally supposed. 



Another and perhaps more potent cause of 

 certain clouds being kept above us lies in the 

 warm currents of air that are continually rising 

 from the earth and buoying them up, very 

 much as the heated air from a stove or lamp- 

 chimney may buoy up a feather. We can see 

 this illustrated in the formation of the clouds 

 that sometimes hang about a mountain's top. 

 The warm currents of air in the valley seek to 

 rise up the side of the mountain because it is a 

 natural conductor protecting them in measure 



