INFLUENCE OF THE FOREST ON FLOODS. 2935 
water soaked through the sod and burst it up by hydrostatic 
pressure.* 
The importance of the mechanical resistance of the wood to 
the flow of water over the surface has, however, been exagge- 
rated by some writers. Rain-water is generally absorbed by 
the forest-soil as fast as it falls, and it is only in extreme cases 
that it gathers itself into a superficial sheet or current overflow- 
ing the ground. There is, nevertheless, besides the absorbent 
power of the soil, a very considerable mechanical resistance to 
the transmission of water beneath the surface through and 
along the superior strata of the ground. This resistance is 
exerted by the roots, which both convey the water along their 
surface downwards, and oppose a closely wattled barrier to its 
descent along the slope of the permeable strata which have 
absorbed it.+ 
* Die Hochwasser in 1868 im Biindnerischen Rheingebiet, pp. 12, 68. 
Observations of Forster, cited by Cézanne from the Annales Horestiéres for 
1859, p. 358, are not less important than those adduced in the text. The 
field of these observations was a slope of 45° divided into three sections, one 
luxuriantly wooded from summit to base with oak and beech, one completely 
cleared through its whole extent, and one cleared in its upper portion, but re- 
taining 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 Marcuanp, Les Tor- 
rents des Alpes, in Revue des Haux et Foréts 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, im heavy rains, the proportions are reversed. Suppos- 
ing a rain-fall of four inches, an area measuring 100,000 acres, or a little more 
