INFLUENCE OF PRECIPITATION.—GENERAL RESULTS. 199 
the present geological period, 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 woods to a low 
expression ; for the forests are, in large proportion, situated in 
cold or temperate climates, where the action of the sun is com- 
paratively 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—enormous- 
ly preponderate. Itis, upon the whole, not probable that so small 
an extent of forest, so situated, could produce a sensible influ- 
ence on the general climate of the globe, though it might appre- 
ciably affect the /ocal 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 atmospheric 
precipitation, must be supposed constant ; but the distribution 
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 results, it would appear 
that, in countries in the temperate zone still chiefly covered 
rith wood, the summers would be cooler, moister, shorter, the 
winters milder, drier, longer, than in the same regions after the 
removal of the forest, and that the condensation and precipita- 
tion of atmospheric moisture would be, if not greater in total 
quantity, more frequent and less violent in discharge. 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. 
Among the many causes which, as we have seen, tend to 
influence the general result, the mechanical action of the forest, 
if not more important, is certainly more obvious and direct than 
the immediate effects of its organic processes. The felling of 
the woods involves the sacrifice of a valuable protection against 
the violence of chilling winds and the loss of the shelter afforded 
to the ground by the thick coating of leaves which the forest 
sheds upon it and by the snow which the woods prevent from 
