INFLUENCE OF THE FOEEST ON FLOODS. 231 



The importance of the mechanical resistance of the wood to 

 the flow of water over tJie surface has, however, been exaggera- 

 ted bj some writers. Rain-water is generally absorbed by the 

 forest-soil as fast as it falls, and it is only in extreme cases tliat it 

 gathers itseK into a superficial sheet or current overflowing the 

 ground. There is, nevertheless, besides the absorbent power of 

 the soil, a very considerable mechanical resistance to the trans- 

 mission of water heneath the surface through and along the su- 

 perior strata of the ground. This resistance is exerted by the 

 roots, which both convey the water along their surface down- 

 wards, and oppose a closely wattled barrier to its descent along 

 the slope of the permeable strata which have absorbed it.* 



Rivers fed by springs and shaded by woods are comparatively 

 uniform in volume, in temperature, and in chemical composition.f 



a wooded belt for a quarter of the height of the slope, which was from 1,300 

 to 1,800 feet above the brook at its foot. 



In the first section, comprising six-sevenths of the whole surface, the rains 

 had not produced a single ravine ; in the second, occupying about a tenth of 

 the ground, were three ravines, increasing in width from the summit to the 

 valley beneath, where they had, all together, a cross-section of 600 square feet ; 

 in the third section, of about the same extent as the second, four ravines had 

 been formed, widening from the crest of the slope to the belt of wood, where 

 they gradually narrowed and finally disappeared. 



For important observations to the same purpose, see March and, Les Tor- 

 rents des Alpes, in Revue des Eaux et Forets for September, 1871. 



* In a valuable report on a bill for compelling the sale of waste communal 

 lands, now pending in the Parliament of Italy, Senator Torelli, an eminent 

 man of science, calculates that four-fifths of the precipitation in the forest are 

 absorbed by the soil, or detained by the obstructions of the surface, only one- 

 fifth being delivered to the rivers rapidly enough to create danger of floods, 

 while in open grounds, in heavy rains, the proportions are reversed. Suppos- 

 ing a rainfall of four inches, an area measuring 100,000 acres, or a little more 

 than four American townships, would receive 53,777,777 cubic yards of water. 

 Of this quantity it would retain, or rather detain, if wooded, 41,000,000 yards ; 

 if bare, only 11,000,000. The difference of discharge from wooded and un- 

 wooded soils is perhaps exaggerated in Col. Torelli's report, but there is no 

 doubt that in very many cases it is great enough to prevent, or to cause, de- 

 structive inundations. 



f Dumont gives an interesting extract from the Misopogon of the Emperor 

 Julian, showing that, in the fourth century, the Seine — the level of which now 

 varies to the extent of thirty feet between extreme high and extreme low water 

 mark — was almost wliolly exempt from inundations, and flowed with a uni- 

 form current through the whole year. " Ego ohm eram in hibemis apud ca- 



