iOO INFLUENCE OF FOEEST ON HUMIDITY OF SOIL. 



perature, the Immiditj, the texture and consistence, tlie configu- 

 ration and distribution of the mould or arable soil, and very often 

 of the mineral strata below, and on the permanence and regularity 

 of springs and greater superficial watercourses, are much less dis- 

 putable as well as more easily estimated and more important, than 

 its possible value as a cause of strictly climatic equilibrium or dis- 

 turbance. 



The action of the forest on the earth is cliiefly mechanical, but 

 the organic process of absorption of moisture by its roots affects 

 the quantity of water contained in the vegetable mould and in the 

 mineral strata near the surface, and, consequently, the consistency 

 of the soil. In treating of the effects of trees on the moisture of 

 the atmosphere, I have said that the forest, by interposing a 

 canopy between the sky and the ground, and by covering the sur- 

 face with a thick mantle of fallen leaves, at once obstructed in- 

 sulation and prevented the radiation of heat from the earth. 

 These influences go far to balance each other ; but familiar obser- 

 vation shows that, in summer, the forest-soil is not raised to so 

 high a temperature as are open grounds exposed to irradiation. For 

 this reason, and in consequence of the mechanical resistance op- 

 posed by the bed of dead leaves to the escape of moisture, we 

 should expect that, except after recent rains, the superficial strata 

 of woodland-soil would be more humid than that of cleared land. 

 This agrees with experience. The soil of the natural forest is al- 

 ways moist, except in the extremest droughts, and it is exceed- 

 ingly rare that a primitive wood suffers from want of humidity. 

 How far this accimaulation of water affects the condition of 

 neighboring grounds by lateral infiltration, we do not know, but 

 we shall see, in a subsequent chapter, that water is conveyed to 

 great distances by this process, and we may hence infer that the 

 influence in question is an important one. 



It is undoubtedly true that loose soils, stripped of vegetation 

 and broken up by the plough or other processes of cultivation, 

 may, until again carpeted by grasses or other plants, absorb more 

 rain and snow-water than when they were covered by a natural 

 growth ; but it is also true that the evaporation from such soils is 

 augmented in a still greater proportion. Rain scarcely penetrates 

 beneath the sod of grass-ground, but runs off over the surface ; 

 and after the heaviest showers a ploughed field will often be dried 



