I 
EFFECTS OF HTJMAN ACTION ASCERTAINABLE. 25 
approach to precision, yet even here we have no present means 
of knowing how much of the water absorbed by the earth is 
restored to the atmosphere by evaporation, and how much 
carried off by infiltration or other modes of underground 
discharge. When, therefore, we attempt to use the phe¬ 
nomena observed on a few square or cubic yards of earth, as a 
basis of reasoning upon the meteorology of a province, it is 
evident that our data must be insufficient to warrant positive 
general conclusions. In discussing the climatology of whole 
countries, or even of comparatively small local divisions, we 
may safely say that none can tell what percentage of the 
water they receive from the atmosphere is evaporated ; what 
absorbed by the ground and conveyed off* by subterranean 
conduits; what carried down to the sea by superficial chan¬ 
nels ; what drawn from the earth or the air by a given extent 
of forest, of short pasture vegetation, or of tall meadow-grass ; 
what given out again by surfaces so covered, or by bare 
ground of various textures and composition, under different 
conditions of atmospheric temperature, pressure, and humid¬ 
ity ; or what is the amount of evaporation from water, ice, or 
snow, under the varying exposures to which, in actual nature, 
they are constantly subjected. If, then, we are so ignorant of 
all these climatic phenomena in the best-known regions inhab¬ 
ited by man, it is evident that we can rely little upon theo¬ 
retical deductions applied to the former more natural state of 
the same regions—less still to such as are adopted with respect 
to distant, strange, and primitive countries. 
Mechanical Effects produced by Man on the Surface of the 
Earth more easily ascertainable. 
In investigating the mechanical effects of human action on 
superficial geography, we are treading on safer ground, and 
dealing with much less subtile phenomena, less intractable 
elements. Great physical changes can, in some cases, be posi¬ 
tively shown, in some almost certainly inferred, to have been 
produced by the operations of rural industry, and by the labors 
