23'6 ACTION OF FORESTS ON THE FLOW OF RIVERS. 



junction between plain and mountain, and carry back a long reach 

 of the Adriatic coast many miles to the west. 



" It is, indeed, not to be supposed that all the degradation of the 

 mountains is due to the destruction of the forests — that the flanks of 

 every Alpine valley in Central Europe below the snow-line were once 

 covered with earth and green with woods, but there are not many 

 particular cases in which we can, with certainty, or even with strong 

 probability, affirm the contrary." 



There is thus brought under consideration a secondary effect of 

 trees in arresting the flow and escape of the rainfall, and so to some 

 extent equalizing the flow of rivers. How this is affected next 

 demands our attention. 



As the evaporation, to which attention has been called, may be 

 considered as only a continuation of evaporation, begun so soon as 

 the rain-drop was formed in the atmosphere, so may the infiltration 

 and ruisseUement be considered as only a continuation of the descent 

 by which, under the influence of gravitation, it fell ; but it is effected 

 under different conditions, and it presents different phenomena. 



So soon as it falls part is absorbed by the hydroscopicity of the 

 soil, more, it may be, escapes by infiltration through the soil, and the 

 remainder flows over the surface to a lower level. 



It may have been observed that on the footpath, or the rock, it 

 accumulates in pools, but not on the grass or turf of herbage, or field 

 of corn ; and on the bare ground, it may be seen flowing off in runnels ; 

 but on the grassy turf the phenomenon is somewhat different. Even 

 on the declivity of a knoll, or on the declivity of a mountain side, 

 the grass arrests, divides and subdivides, and so retains the super- 

 ficial flow of the superabundant rainfall. In accordance with this is 

 the action of forests thereon. 



The establishment of this fact has followed the study of the natural 

 history of Alpine torrents. In some of these the whole rainfall in a 

 mountain basin, rushing off impetuously to the sea, and undermining 

 the banks of its channel, carried off the detritus, and buried there- 

 with, it may be, fertile lands, and villages and towns beyond. The 

 destructive effects thus produced in France called attention to the 

 subject nearly a century ago, and in 1793 there was published by M. 

 Fabre, a civil engineer, Essai sur la theorie des Torrents et des Riviers, 

 in which he alleges that the destruction of forests in the mountains, 

 and the uprooting of their stumps, had been the primary and the 

 secbMary causes of the formation of these torrents, and had thus been 



