DBPAHTMENT OF l'aRI)ECH!E. 277 



square miles of surface, or more than one thousand times as much as that 

 of the former. 



" The average annual precipitation in the basin of the Ardfeche is not 

 greater than in many other parts of Europe, but excessive quantities of rain 

 frequently fall in that valley in the autumn. On the 9th of October 1827, 

 there fell at Joyeuse, on the BeaUme, no less than thirty-one inches between 

 three o'clock in the morning and midnight. Such facts as this explain the 

 extraordinary suddenness and violence of the floods of the Ardfeche, and the 

 basins of many other tributaries of the Ehone exhibit meteorological pheno- 

 mena not less remarkable. The Ehone, therefore, is naturally subject to 

 great and sudden inundations, and the same remark may be applied to 

 most of the principal rivers of France, because the geographical character 

 of all of them is approximately the same. 



" The volume of water in the floods of most great rivers is determined by 

 the degree in which the inundations of the difierent tributaries are coinci- 

 dent in time. Were all the afiluents of the Lower Rhone to pour their highest 

 annual floods into its channel at once, as the smaller tributaries of the 

 Upper Ehone sometimes do- — were a dozen Niles to empty themselves into 

 its bed at the same moment — its waters would rise to a height and rush with 

 an impetus that would sweep into the Mediterranean the entire population 

 of its banks, and all the works that man has erected upon the plains which 

 border it. But such a coincidence can never happen. The tributaries of 

 this river run in very different directions, and some of them are swollen 

 principally by the melting of the snows about their sources, others almost 

 exclusively by heavy rains. When a damp south-east wind blows up the 

 valley of the Ardfeche, its moisture is condensed, and precipitated in a 

 deluge' upon the mountains which embosom the head-waters of that stream, 

 thus producing a flood; while a neighbouring basin, the axis of which lies 

 transversely or obliquely to that of the Ardiche, is not at all aflfected. 



" It is easy to see that the damage occasioned by such floods as I have 

 described must be almost incalculable, and it is by no means confined to 

 the effects produced by overflow and the mechanical force of the superficial 

 currents. In treating of the devastations of torrents, I have hitherto con- 

 fined myself principally to the erosion of surface and the transportation of 

 mineral matter to lower grounds by them. The general action of torrents, 

 as thus far shown, tends to the Ultimate elevation of their beds by the 

 deposit of the earth, gravel, and stone conveyed by them ; but until they 

 have thus raised their outlets so as sensibly to diminish the inclination of 

 their channels — and sometimes when extraordinary floods give the torrents 

 momentum enough to sweep away the accumulations which they have 

 themselves heaped up — the swift flow of their currents, aided by the 

 abrasion of the rolling rocks and gravel, scoops their beds constantly deeper, 

 and they consequently not only undermine their banks, but frequently sap 

 the most solid foundations which the art of man can build for the support 

 of bridges and hydraulic structures, 



"In the inundation of 1857, the Ardfeche destroyed a stone bridge near 

 La Beaume which had been built about eighty years before. The resistance 

 of the piers, which were erected on piles, the channel at that point being of 

 gravel, produced an eddying current that washed away the bed of the river 

 above them, and the foundation, thus deprived of lateral support, yielded 

 to the weight of the bridge, and the piles and piers fell up-stream. 



" By a curious law of compensation, the stream which, at floods, scoops 



