216 ACTION OF FORESTS ON THE SUEF ACE-FLOW OF EAIN WATER. 



Herr Wex has expressed his views on this point in a passage I have 

 quoted, and Marsh in his treatise on the " Earth as Modified by 

 Human Action," states in regard to a forest : — 



" By its interposition, as a curtain between the sky and the ground, 

 it both checks the evaporation from the earth, and mechanically 

 intercepts a certain portion of the dew and lighter showers, which 

 would otherwise moisten the surface of the soil, and restores it to the 

 atmosphere by exhalation. While in heavier rains the large drops 

 which fall upon the leaves and branches are broken into smaller ones, 

 and, consequently, strike the ground with less mechanical force, or are, 

 perhaps, even dispersed into vapour without reaching it. 



" The vegetable mould, resulting from the decomposition of leaves 

 and of wood, seems as a perpetual mulch to forest soil by carpeting 

 the ground with a spongy covering which obstructs the evaporation 

 from the mineral earth below, drinks up the rains and melting snows 

 that would otherwise flow rapidly over the surface, and perhaps be 

 conveyed to the distant sea, and then slowly give out by evaporation 

 infiltration, and percolation, the moisture thus imbibed. The roots, 

 too, penetrate far below the superficial soil, conduct water along 

 their surface to the lower depths to which they reach, and then by 

 partially draining the superior strata, remove a certain quantity of 

 moisture out of the reach of evaporation." 



The meteorological effects produced thus by forests resolve them- 

 selves into the prolongation and consequent increase of the evapora- 

 tion of water falling in the forms of rain, snow, and hail, effected in 

 two distinct operations, first the absorption and retention of a large 

 portion of the rainfall, and second the retardation of the flow of the 

 remainder towards the great reservoir and source of all, in accord- 

 ance with the observation of the Hebrew preacher, " All the rivers 

 run into the sea ; yet the sea is not full : for unto the place from 

 whence the rivers come thither they return again." 



Mr Marsh speaks of the ever-renewed and increasing vegetable 

 mould as a perpetual mulch, and in reference to the humidity of 

 forest soil he cites the following passage from Etudes sur P Economic 

 Forestiere, by Jules Clav6 :— " Why go so far for the proof of a 

 phenomenon which is repeated every day under our own eyes, and of 

 which every Parisian may convince himself without venturing beyond 

 the Bois de Bologne, or the Forest of Meudon ? Let him after a few 

 rainy days pass along the Chevreuse road, which is bordered on the 

 right by the wood, and on the left by cultivated fields. The fall of 

 water, and the continuance of the rain, have been the same on both 



