INFLUENCE ON PRECIPITATION-GENERAL RESULT. 179 
their craggy slopes, the Sahara and other great African and 
Asiatic deserts, and all such other portions of the solid surface 
as are permanently unfit for the growth of wood, we shall find 
that probably not one tenth of the total superficies of our 
planet was ever, at any one time in the present geological 
epoch, covered with forests. Besides this, the distribution of 
forest land, of desert, and of water, is such as to reduce the 
possible influence of the former to a low expression; for the 
forests are, in large proportion, situated in cold or temperate 
climates, where the action of the sun is comparatively feeble 
both in elevating temperature and in promoting evaporation ; 
while, in the torrid zone, the desert and the sea—the latter of 
which always presents an evaporable surface—enormously pre¬ 
ponderate. It is, upon the whole, not probable that so small 
an extent of forest, so situated, could produce an appreciable 
influence on the general climate of the globe, though it might 
appreciably affect the local action of all climatic elements. 
The total annual amount of solar heat absorbed and radiated 
by the earth, and the sum of terrestrial evaporation and atmos¬ 
pheric precipitation must be supposed constant; but the distri¬ 
bution of heat and of humidity is exposed to disturbance in 
both time and place, by a multitude of local causes, among 
which the presence or absence of the forest is doubtless one. 
So far as we are able to sum up the general results, it would 
appear that, in countries in the temperate zone still chiefly 
covered with wood, the summers would be cooler, moister, 
shorter, the winters milder, drier, longer, than in the same 
regions after the removal of the forest. The slender historical 
evidence we possess seems to point to the same conclusion, 
though there is some conflict of testimony and of opinion on 
this point, and some apparently well-established exceptions to 
particular branches of what appears to be the general law. 
One of these occurs both in climates where the cold of 
winter is severe enough to freeze the ground to a considerable 
depth, as in Sweden and the Northern States of the American 
Union, and in milder zones, where the face of the earth is 
exposed to cold mountain winds, as in some parts of Italy and 
