230 
EFFECTS OF INUNDATIONS. 
ice in proportion to its length. Consequently, the widening 
of a fissure to the extent of one inch might give an additional 
drainage equal to a square foot of open tubing. 
The observations and reasonings of Belgrand and Valles, 
though their conclusions have not been accepted by many, are 
very important in one point of view. These w'riters insist 
much on the necessity of taking into account, in estimating 
the relations between precipitation and evaporation, the ab¬ 
straction of water from the surface and surface currents, by 
absorption and infiltration—an element unquestionably of 
great value, but hitherto much neglected by meteorological 
inquirers, who have very often reasoned as if the surface earth 
were either impermeable to water, or already saturated with 
it; whereas, in fact, it is a sponge, always imbibing humidity 
and always giving it off, not by evaporation only, but by infil¬ 
tration and percolation. 
The destructive effects of inundations considered simply as 
a mechanical power by which life is endangered, crops de¬ 
stroyed, and the artificial constructions of man overthrown, 
are very terrible. Thus far, however, the flood is a temporary 
and by no means an irreparable evil, for if its ravages end here, 
the prolific powers of nature and the industry of man soon 
restore what had been lost, and the face of the earth no longer 
shows traces of the deluge that had overwhelmed it. Inunda¬ 
tions have even their compensations. The structures they 
destroy are replaced by better and more secure erections, and 
if they sweep off a crop of corn, they not unfrequently leave 
behind them, as they subside, a fertilizing deposit which en¬ 
riches the exhausted field for a succession of seasons.* If, 
* The productiveness of Egypt has been attributed too exclusively to the 
fertilizing effects of the slime deposited by the inundations of the Nile; 
for in that climate a liberal supply of water would produce good crops on 
almost any ordinary sand, while, without water, the richest soil would 
yield nothing. The sediment deposited annually is but a very small frac¬ 
tion of an inch in thickness. It is alleged that in quantity it would be 
hardly sufficient for a good top dressing, and that in quality it is not chem¬ 
ically distinguishable from the soil inches or feet below the surface. But 
