INFLUENCE ON PRECIPITATION. 189 



humidity, and it is liiglily probable that, in analogy with most 

 other works and workings of nature, they, at certain or uncertain 

 periods, restore the equiUbrium which, whether as hfeless masses 

 or as hving organisms, they may have temporarily disturbed.* 



"When, therefore, man destroys these natural harraonizers 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 Injhience of Woods on Precvpitation. 



"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 in- 

 creasing or diminishing the warmth of the air and by absorbing 

 or exhahng imcombined gas and aqueous vapor. The forest be- 

 ing a natural arrangement, the presumption is that it exercises a 

 conservative action, or at least a compensating one, and conse- 

 quently that its destruction must tend to produce pluviometrical 

 disturbances as well as thermometrical variations. And this is 



* There is one fact which I have nowhere seen noticed, but which seems to 

 me to have an important bearing on the question whether forests tend to main- 

 tain an equilibrium between the various causes of hygroscopic action, and con- 

 sequently to keep the air within their precincts in an approximately constant 

 condition so far as tliis meteorological element is concerned. I refer to the 

 comparative rarity of fog or visible vapor in thick woods in full leaf, even 

 when the air of the neighboring open grounds is so heavily charged with con- 

 densed vapor as completely to obscure the sun. The temperature of the atmos- 

 phere in the forest is not subject to so sudden and extreme variations as that of 

 cleared ground, but at the same time it is far from constant, and so large a sup- 

 ply of vapor as is poured out by the foliage of the trees could not fail to be 

 frequently condensed into fog, by the same causes as in the case of the adja- 

 cent meadows which are sometimes covered with a dense mist while the forest 

 air remains clear, were there not some potent counteracting influence always in 

 action. This influence, I believe, is to be found partly in the equalization of 

 the temperature of the forest, and partly in the balance between the humid- 

 ity exhaled by the trees and that absorbed and condensed invisibly by the 

 earth. 



