evaporation. Precise actual measurement of these quantities upon even a
single acre of ground is impossible; and in all cabinet experiments on
the subject, the conditions of the surface observed are so different
from those which occur in nature, that we cannot safely reason from one
case to the other. In nature, the inclination and exposure of the
ground, the degree of freedom or obstruction of the flow of water over
the surface, the composition and density of the soil, the presence or
absence of perforations by worms and small burrowing quadrupeds--upon
which the permeability of the ground by water and its power of absorbing
and retaining or transmitting moisture depend--its temperature, the
dryness or saturation of the subsoil, vary at comparatively short
distances; and though the precipitation upon very small geographical
basins and the superficial flow from them may be estimated with an
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 phenomena 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 channels; 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 humidity; or what is the amount of evaporation from water,