236 ACTION OF POEEBTS ON THE FLOW OF RIVERS. 



junictiou 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 ih arrestirig the flow and escape of the rainfall, and so to some 

 extent equalizing the flow of rivers; How this is afieoted 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 ndssellement be Considered as only a continuation of the descent 

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

 imder different iSonditions, and St presents different pheuoi^nena. 



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

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

 reniaihder flows over the surface to a lower level. 



It may have been observted that on the footpath, or the Took, 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 ; 

 b'ut 6il the gi'assy turf the phenomenon is somewhat different. Even 

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

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

 ficial flow of the superabunda'ikt 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 

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

 Fabre, a civil engineer, Essai sur la tkeorie des Torrents et des Riders, 

 in which h6 alleges that the destruction of forests in the mountains, 

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

 secondary causes of thfe formation of these torrents, and had thus been 



