220 ACTION OF FOBEBTS ON THE FLOW OF BIVEES. 



this and is carried off, feeding springs and fountains, and streams, the 

 saddest thing that can be said is that it is lost, thougti this can only 

 be said with a show of truth of that portion of it whicii is lost in the 

 sea ; but of what is carried off by rtiissellemfnt comparatively little is 

 utilized ; a much greater proportion of it finds its way to the sea, and 

 this in rushing thither often carries with it devastation and destruc- 

 tion. 



According to the statements I have cited from Surell, evaporation, 

 infiltration, and ruissellement, are equal in their sum to the quantity 

 of the rainfall, and if after evaporation has taken up its part the in- 

 filtration be complete, the ruissellement will be nil. 



To the negative good which might thus be done, or damage which 

 might thus be prevented, and the effect of trees in doing these, by 

 equalizing to some extent the flow of rivers, would I next draw 

 attention. 



In connection with a statement made in the preceding section, 

 relative to the effect of the destruction of forests upon the Durance, 

 reference was made to the important circumstance that while the 

 river is confined to a current little more than thirty feet in width, 

 the bed in some places exceeds a m'ile and a quarter in breadth, and 

 so far back as 1789 it was computed that it had covered an area of 

 not less than 130,000 acres with gravel and pebbles; and it was 

 intimated that thus was brought before us another of the effects of 

 forests, or rather of the destruction of forests, which would be sub- 

 sequently brought under discussion. 



Mr Marsh, writing on this subject, says : " The traveller who visits 

 the depth of an Alpine ravine, observes the length and width of the 

 gorge, and the great height and apparent solidity of the precipitous 

 walls which bound it, and calculates the mass of rock required to fill 

 the vacancy, can hardly believe that the humble brooklet which 

 purls at his feet has been the principal agent in accomplishino- this 

 tremendous erosion. Closer observations will often teach him that 

 the seemingly unbroken rock which overhangs the valley is full of 

 cracks and fissures, and really in such a state of disintegration that 

 every frost must bring down tons of it. If he compute the area of 

 the. basin, which finds here its only discharge, he will perceive that 

 a sudden thaw of the winter's deposit of snow, or one of those terrible 

 discharges of rain so common in the Alps, must send forth a deluge 

 mighty enough to sweep down the laigpst masses of gravel and of rock. 

 The simple measurement of the cubical contents of the semicircular 



