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 eyapora- 
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- 
