INUNDATIONS AND TORRENTS. 
383 
for many months in succession, it is drawn from rivers at the 
seasons when their proportion of salts is greatest, and it either 
sinks into the superficial soil, carrying with it the saline sub¬ 
stances it holds in solution, or is evaporated from the surface, 
leaving them upon it. Hence irrigation must impart to the 
soil more salts than natural inundation. The sterilized 
grounds in Egypt and Nubia lying above the reach of the 
floods, as I have said, we may suppose them to have been first 
cultivated in that remote antiquity when the Nile valley re¬ 
ceived its earliest inhabitants. They must have been artifi¬ 
cially irrigated from the beginning; they may have been 
under cultivation many centuries before the soil at a lower 
level was invaded by man, and hence it is natural that they 
should be more strongly impregnated with saline matter than 
fields which are exposed every year, for some weeks, to the 
action of running water so nearly pure that it would be more 
likely to dissolve salts than to deposit them. 
INUNDATIONS AND TORRENTS. 
In pointing out in a former chapter the evils which have 
resulted from the too extensive destruction of the forests, I 
dwelt at some length on the increased violence of river inun¬ 
dations, and especially on the devastations of torrents, in coun¬ 
tries improvidently deprived of their woods, and I spoke of 
the replanting of the forests as the only effectual method of 
preventing the frequent recurrence of disastrous floods. There 
are many regions where, from the loss of the superficial soil, 
from financial considerations, and from other causes, the res¬ 
toration of the woods is not, under present circumstances, to 
be hoped for. Even where that measure is feasible, and in 
actual process of execution, a great number of years must 
elapse before the action of the destructive causes in question 
can be arrested or perhaps even sensibly mitigated by it. Be¬ 
sides this, leaving out of view the objections urged by Bel- 
grand and his followers to the generally received opinions 
concerning the beneficial influence of the forest as respects 
