srATEilENI BV 11. HERICOUBT. 253 



which characterised the flood of October 1846, in the upper basin of 

 the Loire, I will admit with him that if we could have held back 

 1 75,000,000 cubic metres of water, the inundation which proved so 

 sad a calamity to France would not have proved so painful an 

 event. The upper basin of the Loire, as far as Roanne, comprises an 

 area of 640,000 hectares (158,080,000 acres), of which at least a 

 third, say 213,000 hectares (52,693,000 acres), might be profitably 

 re-forested. The inundation was caused by a rain which lasted sixty 

 hours, and poured upon the soil a sheet of water 153 millimetres (about 

 6 inches) in depth. This portion of the basin of the Loire, therefore, 

 received 979,200,000 cubic metres of water. On the hypothesis of 

 M. Valles, 244,800,000 cubic metres were absorbed. There accord- 

 ingly remained for superficial flow, 734,400,000 cubic metres. 



'•But let us suppose that, in 1S46, the 213,000 hectares above- 

 mentioned to have been covered with massive woods, and then let us 

 calculate what would have happened.' These 213,000 metres would 

 have received,as theirshare, 290,000,000 cubic metres. The absorbent 

 quaUties of the soil are increased 40 per cent, by reforesting, and this 

 operation would have withdrawn 130,116,000 cubic metres from the 

 superficial flow, which would have reduced the amount upon the re- 

 timbered portions to 195,174,000 cubic metres. But this liquid 

 mass would have been hindered in its course down the valley, as we 

 have above explained, by the passive resistances of every kind which 

 the forest presents, and a half, at least, would not have arrived until 

 after the other half, which had fallen in other portions of the basin, 

 had passed ofi'. We may therefore conclude that the superficial 

 flow would not have exceeded 500,000,000 cubic metres, and that the 

 calamities occasioned by the inundation of 1846 would have been 

 completely prevented by reforesting.*' 



As rain is the precipitation from the air of a quantity of moisture 

 which it can no longer retain, so rivers are composed of the draining 

 off' of a quantity of the rainfall which the ground upon which it has 

 fallen can no longer retain ; and — while the flow and escape of the 

 rainfall is arrested by much of it being absorbed and retained by the 

 decaying debris of leaves and twigs lying on the surface of the 

 ground in a forest, and by the humus in the soil, and by being 

 carried to a great depth along the channels of the roots — a secondary 

 efiiect of this is to regulate, and, in doing so, to some extent to 

 equalise over a considerable time the continuous flow and delivery of 

 the streams and rivers by which it is drained. 



