BASINS OF RECEPTION. 493 
siderable measure to mitigate, the evils they are intended to 
guard against. 
The last argument is more easily reduced to a numerical 
question than the others. The mean and extreme annual pre- 
cipitation of all the basins where the construction of such 
works would be seriously proposed is already approximately 
known by meteorological tables, and the quantity of water, 
delivered by the greatest floods which have occurred within 
the memory: of man, may be roughly estimated from their 
visible traces. From these elements, or from meteorological 
records, the capacity of the necessary reservoirs can be cal- 
culated. Let us take the case of the Ardéche. In the inun- 
dation of 1857, that river poured into the Rhone 1,305,000,000 
cubic yards of water in three days. If we suppose that half 
this quantity might have been suffered to flow down its chan- 
nel without inconvenience, we shall have about 650,000,000 
cubic yards to provide for by reservoirs. The Ardeche and 
its principal affluent, the Chassezac, have, together, about 
twelve considerable tributaries rising near the crest of the 
mountains which bound the basin. If reservoirs of equal 
capacity were constructed upon all of them, each reservoir 
must be able to contain 54,000,000 cubic yards, or, in other 
words, must be equal to a lake 3,000 yards long, 1,000 yards 
wide, and 18 yards deep, and besides, in order to render any 
effectual service, the reservoirs must all have been empty at 
the commencement of the rains which produced the inun- 
dation. 
Thus far I have supposed the swelling of the waters to be 
uniform throughout the whole basin; but such was by no 
means the fact in the inundation of 1857, for the rise of the 
Chassezac, which is as large as the Ardéche proper, did not 
exceed the limits of ordinary floods, and the dangerous excess 
came solely from the headwaters of the latter stream. Hence 
reservoirs of double the capacity I have supposed would have 
been necessary upon the tributaries of that river, to prevent 
the injurious effects of the inundation. It is evident that the 
