INFLUENCE OF PRECIPITATION.—GENERAL RESULTS. 901 
There are, in the constitution and action of the forest, many 
forces, organic and inorganic, which unquestionably tend pow- 
erfully to produce meteorological effects, and it may, therefore, 
be assumed as certain that they must and do produce such 
effects, wnless they compensate and balance each other, and 
herein lies the difficulty of solving the question. To some of 
these elements late observations give a new importance. For 
example, the exhalation of aqueous vapor by plants is now be- 
lieved to be-much greater, and the absorption of aqueous vapor 
by them much less, than was formerly supposed, and Tyndall’s 
views on the relations of vapor to atmospheric heat give im- 
mense value to this factor in the problem. In like manner the 
low temperature of the surface of snow and the comparatively 
high temperature of its lower strata, and its consequent action 
on the soil beneath, and the great condensation of moisture by 
snow, are facts which seem to show that the forest, by protect- 
ing great surfaces of snow from melting, must inevitably exer- 
cise a great climatic influence. If to these influences we add 
the mechanical action of the woods in obstructing currents of 
wind, and diminishing the evaporation and refrigeration which 
such currents produce, we have an accumulation of forces 
which must manifest great climatic effects, unless—which is 
not proved and cannot be presumed—they neutralize each 
other. These are points hitherto little considered in the discus- 
sion, and it seems difficult to deny that as a question of argu- 
ment, the probabilities are strongly in favor of the meteorologi- 
eal influence of the woods. The ez¢dence, indeed, is not satis- 
factory, or, to speak more accurately, it is non-existent, for there 
really is next to no trustworthy proof on the subject, but it 
appears to me a case where the burden of proof must be taken 
wind being more variable, snow is less permanent, and perhaps the same 
remark may be applicable to. the ice of the rivers. These effects seem to 
result necessarily from the greater quantity of heat accumulated in the earth 
in summer since the ground has been cleared of wood and exposed to the rays 
of the sun, and to the greater depth of frost in the earth in winter by the 
xposure of its uncovered surface to the cold atmosphere.”— Collection of 
Papers by NoAu WEBSTER, p. 162, 
