476 BASEL'S OF EECEPTION. 



I have noticed in tlie next preceding chapter the recent legis- 

 lation of France upon the preservation and restoration of the for- 

 ests, with reference to their utility in subduing torrents and 

 lessening the frequency and diminishing the violence of river 

 inundations. The provisions of these laws are preventive rather 

 than remedial, but most beneficial effects have abeady been expe- 

 rienced from the measures adopted in pursuance of them, though 

 sufficient time has not yet elapsed for the complete execution of 

 the greater operations of the system. 



JBasins of Reception, 



Destructive inundations of large rivers are seldom, if ever, 

 produced by precipitation within the limits of the principal val- 

 ley, but almost uniformly by sudden thaws or excessive rains on 

 the mountain ranges where the tributaries take their rise. It is 

 therefore plain that any measures which shall check the flow of 

 surface-waters into the channels of the affluents, or which shall 

 retard the dehvery of such waters into the principal stream by 

 its tributaries, will diminish in the same proportion the dangers 

 and the evils of inundation by great rivers. The retention of 

 the surface-waters upon or in the soil can hardly be accomphshed 

 except by the methods already mentioned — replanting of forests 

 and furrowing or terracing. The current of mountain streams 

 3an be checked by various methods, among which the most 

 familiar and obvious is the erection of barriers or dams across 

 their channels, at points convenient for forming reservoii's large 



tions, but of foreseeing them, and thus in some measure averting their evil con- 

 sequences. On all considerable streams, observations are now regularly made, 

 in rainy weather, at what may be called critical points along their course, 

 where the rising of the water above its normal level, and the rate of its incre- 

 ment, have been found by experience to furnish trustworthy indications as to 

 the time and measure of floods at important towns lower down the river. 

 Notice can be given, long beforehand, as to the time when the flood-wave 

 will reach such points. Thus, for example, it being known that the flood- 

 wave of the Tiber propagates itself from Orte — sixty miles above Rome, at 

 the confluence of that river and the Nera — to Rome in from sixteen to seven- 

 teen hours, notice of an impending inundation can be communicated to the 

 Capital in time for the removal of exposed objects along the banks, for secur- 

 ing the entrances to cellars, and for other precautionary measures. 



