70 



NATURE FOR ITS OWN SAKE 



Cloud 

 changes. 



Reconden- 



sation. 



of its heat, forms into tiny, cold-water globules 

 which have a tendency to sink down toward the 

 earth. If the globules are large and heavy 

 enough, owing to sudden condensation, they do 

 fall to the earth in the shape of rain ; if they 

 are small, as is usually the case, they no 

 sooner sink down into the warmer air from 

 whence they came, than they are evaporated 

 and carried up to the top of the cloud, to be 

 once more condensed into mist. The "re- 

 newal " of the cloud then means that the water- 

 globules are continually falling down only to be 

 evaporated and sent up again for recondensa- 

 tion. The cloud is always losing at the bottom, 

 and its flat base shows the line where evapora- 

 tion takes place ; but it is continually adding to 

 itself on the top. The tendency of the cloud at 

 the top is to form above itself drifts of higher 

 clouds, but this is held in check by the loss of 

 moisture, the dryness of the upper air, and the 

 dissipating action of the sun's rays from above ; 

 the tendency of the cloud at the bottom is to 

 sink down, but this is held in check by the 

 continual evaporation as the water-globules fall 

 into the warmer, lower air. The cloud then, 

 though in reality always changing, is apparent- 

 ly stationary and without change. The ascend- 



