174 TOTAL INFLUEl^CE OF THE FOREST ON TEMPERATURE. 



them as to make possible the growing of many plants in or near 

 tlie wood wliicTi could not otherwise be reared in that climate. 



The evaporation of the juices of trees and other plants is doubt- 

 less their most important thermoscopic function, and as recent ob- 

 servations lead to the conclusion that the quantity of moisture ex- 

 haled by vegetables has been hitherto underrated, we must ascribe 

 to this element a higher value than has been usually assigned to 

 it as a meteorological influence. 



The exlialation and evaporation of the juices of trees, by what- 

 ever process effected, take up atmospheric heat and produce a 

 proportional refrigeration. This effect is not less real, though to 

 common observation less sensible, in the forest than in meadow 

 or pasture land, and it can not be doubted that the local tempera- 

 ture is considerably affected by it. But the evaporation that cools 

 the air, diffuses through it, at the same time, a medium which 

 powerfully resists the escape of heat from the earth by radiation. 

 Visible vapors, fogs and clouds, it is well known, prevent frosts 

 by obstructing radiation, or rather by reflecting back again the 

 heat radiated by the earth, just as any mechanical screen would 

 do. On the other hand, fogs and clouds intercept the rays of the 

 sun also, and hinder its heat from reaching the earth. The invis- 

 ible vapors given out by leaves impede the passage of heat reflect- 

 ed and radiated by the earth and by all terrestrial objects, but op- 

 pose much less resistance to the transmission of direct solar heat ; 

 and indeed the beams of the sun seem more scorching when re- 

 ceived through clear air charged with uncondensed moisture than 

 after passing through a dry atmosphere. Hence the reduction of 

 temperature by the evaporation of moisture from vegetation, 

 though sensible, is less than it would be if water in the gaseous 

 state were as impervious to heat given out by the sun as to that 

 emitted by terrestrial objects. 



Total Tnjluence of the Forest on Temperature. 



It has not yet been found practicable to measure, sum up, and 

 equate the total influence of the forest, its processes and its prod- 

 ucts, dead and hving, upon temperature, and investigators differ 

 much in their conclusions on this subject. It seems probable 

 that in every particular case the result is, if not determined, at 



