180 INFLUENCE OF FOREST ON HUMIDITY. 
The hygroscopicity of humus or vegetable earth is much 
ereater than that of any mineral soil, and consequently forest 
ground, where humus abounds, absorbs the moisture of the 
atmosphere more rapidly and in larger proportion than common 
earth. The condensation of vapor by absorption develops 
heat, and consequently elevates the temperature of the soil 
which absorbs it, together with that of air in contact with the 
surface. Von Babo found the temperature of sandy ground 
thus raised from 68° to 80° F., that of soil rich in humus from 
68° to 88°. 
The question of the influence of the woods on temperature 
does not, in the present state of our knowledge, admit of precise 
solution, and, unhappily, the primitive forests are disappear- 
ing so rapidly before the axe of the woodman, that we shall 
never be able to estimate with accuracy the climatological 
action of the natural wood, though all the physical functions of 
artificial plantations will, doubtless, one day be approximately 
known. 
But the value of trees as a mechanical screen to the soil they 
cover, and often to ground far to the leeward of them, is most 
abundantly established, and this agency alone is important 
enough to justify extensive plantation in all countries which 
do not enjoy this indispensable protection. 
Influence of Forests as Inorganic on the Humidity of the 
Air and the Earth. 
The most important hygroscopic as well as thermoscopic in- 
fluence of the forest is, no doubt, that which it exercises on the 
humidity of the air and the earth, and this climatic action it 
exerts partly as dead, partly as living matter. By its interposi- 
tion as acurtain between the sky and the ground it both checks 
evaporation from the earth, and mechanically intercepts a certain 
proportion of the dew and the lighter showers, which would 
otherwise moisten the surface of the soil, and restores it to the 
