67 



or plateaux is, in some degree, a conquest over the dominion of the floods, and a reduction 

 of the ravages that they may commit. 



"Forests in Excess : — But, carried to too great an extent, this operation will work 

 precisely against the end which we desire to obtain. If the forests cover too great an 

 extent of country, we may fear that the springs or subterranean water-courses may not 

 be able to deliver all the rain that falls in a given time before other rains fall, which will 

 cover the country with stagnant water. This was the condition of Gaul at the time when 

 it was covered with forests, and such is still the condition of certain parts of America, 

 which are wooded in this excessive degree. By this means we explain the apparent con- 

 tradictions of which the partisans of reforesting are accused. 



" Reforesting wliere Needed: — It will be necessary, before coming to the desirable 

 conclusion as to where the true proportion lies, and which cannot now be known with 

 precision, that we should be able to show for each river- basin how much of a reservoir a 

 forest should furnish that shall discharge, freely and with regularity, the rains that it re- 

 ceives only at intervals. However the case may be, it is evident that the reforesting 

 should be carried on upon the mountainous parts of the different basins. It is there, 

 practically, that the humid winds condense the vapours that they contain, on account of 

 the lower temperature which there prevails, and from thence comes the superficial flow of 

 waters, the absorption of which we wish to increase, and make to appear in the springs, 

 whose number and volume we would regulate. It is from thence, in short, the torrents 

 begin, which become the forerunners of the inundations, which it is our wish, if possible, 

 to control. 



" Certain Changes Beyond our Control : — It may be asked as to whether, these in- 

 vestigations being ended, ' shall we always be able to guard against these inundations t ' 

 Probably not ; for it is not in the power of man to prevent atmospheric perturbation, and 

 we have never yet found the remedies against the return at times of the warm and humid 

 currents of air from the Atlantic, to which the diluvial rains are due which cause these 

 damages. But at least, if we do not by reforesting entirely allay these evils, we may, 

 peradventure, considerably reduce their magnitude, and enhance the efficacy of other 

 means of defence which have until now been held as quite illusory. 



" Dikes and other Structures : — At the present time most of the works constructed 

 for the preventing of these evils, in fact, only increase them. It is held by a great num- 

 ber of engineers, that transverse dikes, in order to be of service, should be built in the 

 lower parts of the valleys and near the mouths of affluents ; but the first result of this 

 would be to cause inundations in these parts which are usually fertile and well-cultivated, 

 and where, if they had not been built, they might not have been felt. We might have to 

 pay damages for the property injured, and the sums, although considerable, would not 

 always be compensated for by the advantages claimed. This system, moreover, amounts 

 only to transferring the evil to another place, without escaping it, and it is at best but a 

 secondary, not a radical, remedy. As for longitudinal dikes, not only are they frequently 

 unable to withstand, in time of flood, the power of the waters, but they tend to erode the 

 river bed, and to create obstacles which stop the materials carried down. Rivers, there- 

 fore, become for the country which they traverse a permanent source of danger, for, by a 

 moderate flood the plains are often overflowed. Reforesting quite removes this peril, and 

 by hindering the erosion of torrents they check the wearing out of the channels of the 

 rivers and the obstructions at their mouths from accumulations of sand and gravel. They 

 also tend to favour the construction of longitudinal dikes, at points where their utility is 

 recognized. 



" Denuding Power of Rains : — In the torrid zone the degradation of land is generally 

 very rapid, but the waste is by no means proportioned to the superior quantity of rain, or 

 the suddenness of its fall, the transporting power of water being counteracted by a greater 

 luxuriance of vegetation. A geologist, who is no stranger to tropical countries, observes 



