64 



" Clay Soils. — Nor is this all. When the soil is carried away, it confines a certain 

 proportion of clay, which, when moistened to a depth which, according to M. Becquerel, 

 does not exceed six times the depth of the sheet of falling water, it forms a natural cup, 

 its pores being obstructed mechanically by the rains which harden them. It is then im- 

 permeable, and free to deliver, by superficial flow, all the liquid that has not been absorbed. 

 But when, on the contrary, the surface is covered with forests, the dome of foliage breaks 

 the force of the rains, which only reach the soil in a state of minute division, and this 

 impervious condition cannot then take place to hinder effective absorption. Finally, by 

 the humus which they produce, forests increase the absorbent qualities of different soils, 

 and consequently the amount of liquids with which they may be charged. This absorbent 

 quality is about twenty-five per cent, in weight in sandy soils, and varies from fifty to 

 ninety per cent, for argillaceous soils, and in humus it rises to one hundred and ninety 

 per cent. 



" We must admit," says M. Hun, " that the sheet of water produced by the heaviest 

 rains scarcely exceeds 3.9 inches in depth. But the bed of soil in a well-stocked forest 

 comprises a layer of humus over a great part of the surface of more than double this 

 depth. In speaking of forests I do not refer to the thin and ruined woods to which this 

 name has been improperly applied ; but to the timber lands like the forests belonging to 

 the state, and to all the communal forests in the eastern departments, where the soil has 

 a capacity for absorption greater than the volume of water yielded by the heaviest show- 

 ers. From this we may explain the fact that after a deluging rain, the water-courses 

 issuing from a well-stocked forest, show only a moderate increase in their volume, and 

 that they keep this up for quite a time, their transparency being scarcely affected." 



" General Conclusions : — Thus, to resume our subject, forests hinder the superficial 

 flow, or delay its progress ; they hinder evaporation, and in a rain of given amount they 

 tend to increase the portion that is absorbed by the soil, and to diminish the surplus flow, 

 which is lost without profit. 



" The data of the problem being stated, it is easy to adduce the conclusions. If we 

 assume that the mean annual number of rainy days is 120 and of dry days 244, it follows 

 that, in order that the rivers shall always keep at a constant level, the time required for 

 the flow of their waters should be nearly three times greater than that in which they feU 

 as rain. It would be necessary, therefore, that they should be stored in a reservoir of 

 which the outlet should only be one-third as great as the inlet, thus allowing the waters 

 to escape in a time three times as long as that in which they are received. If the flow 

 takes place more rapidly, the reservoir will be dry for a season, after having flowed in ex- 

 cessive abundance, which might cause either a local or a general inundation. If, on the 

 contrary, the flow is not so fast, it will not discharge in a proper time all the liquid mass, 

 and there will be an engorgement producing marshes, and finally inundations. Thus, an 

 undue excess of rapidity or of slowness in the discharge of rainwater will cause, as we 

 shall hereafter see, either from an absence from an extreme abundance of forests, the same 

 results. 



"Forests retard the flow of waters: — Forests, by favouring absorption, allow only the 

 minimum of waters to be liberated. Moreover, in prolonging the discharge of the liquid 

 absorbed, they extend the time required for its flowing off, and serve like a reservoir, of 

 which the springs are the outlets, and thus insure the regular feeding of the water-courses. 

 Denuded soil, on the contrary, allows a part of this water to escape both by evaporation 

 and by superficial flow, retaining only imperfectly what it absorbs, and allows the sun's 

 rays to pump up the moisture from the lower beds. For these reasons the springs become 

 dry in summer and the rivers engorged in winter. 



" Examples near at home : — But why should we seek so far away for the proofs of 

 phenomena that are renewed daily under our eyes, and of which any Parisian may con- 

 vince himself without venturing beyond the Bois de Boulogne or the forest of Meudon ? 

 Let him walk out, after some days of rain, along the Chevrence road, bordered on the 

 right by the forest of Meudon, and on the left by cultivated fields. The amount of rain 



