HTFLUETTOE OF FORESTS ON HUMIDITY. 177 



The liygroscopicitj of Immus or vegetable earth is much greater 

 than that of any mineral soil, and consequently forest ground, 

 where humus abounds, absorbs the moisture of the atmosphere 

 more raj)idly and in larger proportion than common earth. The 

 condensation of vapor by absorption develops heat, and conse- 

 quently elevates the temperature of the soil which absorbs it, to- 

 gether with that of air in contact with the surface. Yon 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 disappearing 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 aU the physical functions of artificial plan- 

 tations win, doubtless, one day be approximately known. 



But the value of trees as a mechanical screen to the soU 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 aU countries which do not enjoy 

 this indispensable protection. 



Influence of Forests, as Inorganic^ ontJie Humidity of the Air 



and the Earth. 



The most important hygroscopic as well as thermoscopic influ- 

 ence of the forest is, no doubt, that which it exercises on the hu- 



with humus, with arable and with garden earth, are the soils which least con- 

 duct heat. It is for this reason that sandy ground, in summer, maintains a 

 high temperature even during the night. "We may hence conclude that when 

 a sandy soil is stripped of wood, the local temperature wUl be raised. After 

 the sands, follow successively argillaceous, arable and garden ground, then 

 humus, which occupies the lowest rank. 



"The retentive power of humus is but half as great as that of calcareous 

 sand. "We will add that the power of retaining heat is proportional to the 

 density. It has also a relation to the magnitude of the particles. It is for 

 this reason that ground covered with silicious pebbles cools more slowly than 

 silicious sand, and that pebbly soils are best suited to the cultivation of the 

 vine, because they advance the ripening of the grape more rapidly than chalky 

 and clayey earths, which cool quickly. Hence we see that in examining the 

 calorific eilects of clearing forests, it is important to take into account the 

 properties of the soil laid bare." — Becquerel, Des CUmats et des Sols boiaes, 

 p. 137. 



