152 METEOROLOGICAL INFLUENCE OF THE FOREST. 
ually extend themselves over soils where now scarcely any 
green thing but the bitter colocynth and the poisonous fox- 
glove is ever seen. 
General Meteorological Influence of the forest. 
The physico-geographical influence of forests may be divided 
into two great classes, each having an important influence on 
vegetable and on animal life in all their manifestations, as 
well as on every branch of rural economy and productive 
industry, and, therefore, on all the material interests of 
man. The first respects the meteorology of the countries ex- 
posed to the action of these influences ; the second, their super- 
ficial geography, or, in other words, the configuration, con- 
sistence, and clothing of their surface. 
For reasons assigned in the first chapter, and for others that 
will appear hereafter, the meteorological or climatic branch of 
the subject is the most obscure, and the conclusions of physicists 
respecting it are, in a great degree, inferential only, not founded 
on experiment or direct observation. They are, as might be 
expected, somewhat discordant, though one general result is 
almost universally accepted, and seems indeed too well sup- 
ported to admit of serious question, and it may be considered 
as established that forests tend to mitigate, at least within 
their own precincts, extremes of temperature, humidity, and 
drought. By what precise agencies the meteorological effects 
of the forest are produced we cannot say, becanse elements of 
totally unknown value enter into its action, and because the 
relative intensity of better understood causes cannot be meas- 
ured or compared. I shall not occupy much space in discuss- 
ing questions which at present admit of no solution, but I 
propose to notice all the known forces whose concurrent or 
conflicting energies contribute to the general result, and to 
point out, in some detail, the value of those influences whose 
mode of action has been ascertained. 
