63 



rock or compact clay. It contributes, only in a very irregular manner, to the feeding of 

 rivers and streams, as it delivers considerable volumes at certain times, and becomes no- 

 thing as soon as the rain ceases. But, on the contrary, when the soil is permeable, it 

 absorbs all the water that falls, and does not deliver it again at the surface until some 

 days after the rain, if completely absorbed. It is then that the action of forests begins 

 to be felt. But if, in fact, the soil is uncovered, the liquid volume descends with a velo- 

 city proportionate to the slope, and brings with it the materials of every kind that 

 obstruct its course, at the same time increasing its volume and destructive power. If 

 these form torrents of limited ravage when the rainfall is local, they become fearful inun- 

 dations when it is more general in extent. But, on the contrary, if the soil is covered 

 with woods the flow is more gentle. Being arrested at every point, broken by the trees, 

 their branches, and the mosses which it encounters on the way, the water arrives at the 

 bottom of the valley much slower, without erosions, and without bringing with it any 

 foreign substances. The forest, therefore, in hindering the delivery of the water, lessens 

 the chances of engorgement." 



" Evaporation. — We know that evaporation is going on at all temperatures, with 

 greater or less rapidity, whenever the surrounding air is not already saturated with moisture. 

 All other things being equal, it is greater when the ground is cleared than when covered 

 with forests, because the latter arrest the action of the winds and prevent the masses of 

 air, when saturated, from being renewed, and keeps the temperature lower by shielding 

 the surface from the sun's heat. In lesseningjthe amount of water evaporated, it by so 

 much increases the quantity that is absorbed. It is, moreover, needless to insist upon a 

 ■fact which everybody knows — for no one can be ignorant of the fact — that the soil in a 

 forest after a rain remains wet much longer than where the surface has been cleared. 



" Evaporation can only take place when, at a given temperature, the air is not satu- 

 rated with moisture. But the rains themselves prove that there is an excess of satura- 

 tion in the air at the time, and therefore there can be no evaporation when it rains. 

 They can, therefore, have no very serious influence upon inundations properly so-called, 

 and in this regard cleared lands present no advantage over others." 



" Absorption. — A part of the water which falls is absorbed by the soil. Some of this 

 is used by the vegetation, and serves to carry into the tissues of plants their soluble 

 mineral elements, and is then returned in a certain degree to the atmosphere by the exha- 

 lations of the leaves. Another portion filters slowly into the soU till it meets an imper- 

 vious stratum, and then flows along this bed, following its undulations, till it appears at 

 the surface in the form of springs, unless it is drawn down into the depths of the earth's 

 crust. It is this part alone, which is absorbed by the earth, that feeds the springs and 

 furnishes the aliment of rivers. Every cause which tends to increase, to its detriment, 

 the evaporation or pure loss of water, or to augment the superficial flow, has to this ex- 

 tent an influence upon the regulation of the water flow, and in this regard forests exer- 

 cise a most important influence. All soils are not equally permeable. Some, as in the 

 oolitic formation, absorb nearly all the rain that falls upon their surface. Others, like 

 the primary rocks and liasic soils, allow rain to penetrate only so far as they are covered 

 with vegetable mould. It is implied, therefore, that these vegetable beds should be pre- 

 served at the highest points, since they tend to increase the subterranean contingent of a 

 part of the water, which, without its presence, would flow ofl" upon its surface. But 

 forests serve marvellously the functions of fixing the soil upon the steepest slopes. There 

 will be no need of conviction upon this point to one who shall pass over the Alps or Pyrenees, 

 where every peasant knows that to consolidate the banks of the brooks that cross his fields, 

 and to prevent the gullying of the slopes of the roads, he has only to plant a few trees. 

 Who does not, moreover, know the cohesive power of grass turf in fostering the roots of 

 plants ? The forests are turf upon a large scale, in which the blades of herbage are re- 

 placed by trees, of which the roots strike two or three yards into the soil. They can, 

 therefore, oppose an invincible resistance to this washing away of the soil. According to 

 M. Brougniart, the roots of trees contribute to augment the permeability of certain soils 

 by offering a kind of vertical drainage." 



