INFLUENCE OF PRECIPITATION. 193 
When, therefore, man destroys these natural harmonizers of 
climatic discords, he sacrifices an important conservative power, 
though it is far from certain that he has thereby affected the 
mean, however much he may have exaggerated the extremes of 
atmospheric temperature and humidity, or, in other words, may 
have increased the range and lengthened the scale of thermo- 
metric and hygrometric variation. 
Special Influence of Woods on Precipitation. 
With the question of the action of forests upon temperature 
and upon atmospheric humidity is intimately connected that 
of their influence upon precipitation, which they may affect 
by increasing or diminishing the warmth of the air and by 
absorbing or exhaling uncombined gas and aqueous vapor. 
The forest being a natural arrangement, the presumption is: 
that it exercises a conservative action, or at least a compen- 
sating one, and consequently that its destruction must tend to 
produce pluviometrical disturbances as well as thermometrical 
variations. And this is the opinion of perhaps the greatest 
number of observers. Indeed, it is almost impossible to sup- 
pose that, under certain conditions of time and place, the 
quantity and the periods of rain should not depend, more or 
less, upon the presence or absence of forests; and without in- 
sisting that the removal of the forest has diminished the sum- 
total of snow and rain, we may well admit that it has lessened. 
the quantity which annually falls within particular limits. 
Various theoretical considerations make this probable, the 
most obvious argument, perhaps, being that drawn from the 
generally admitted fact, that the summer and even the mean 
temperature of the forest is below that of the open country in 
the same latitude. If the air in a wood is cooler than that 
around it, it must reduce the temperature of the atmospheric 
stratum immediately above it, and, of course, whenever a satu- 
tween the humidity exhaled by the trees and that absorbed and condensed 
invisibly by the earth, 
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