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 
Effect of the 
winds. 
Effect of 
the air- 
currents, 
