INUNDATIONS IN FRANCE. 243 
the soil follow them downwards, but their swelling growth 
powerfully tends to enlarge, not to obstruct, the crevices of 
rock into which they enter; and as the fissures in rocks are 
longitudinal, not mere circular orifices, every line of additional 
width gained by the growth of roots within them increases the 
area of the crevice 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 cbservations and reasonings of Belgrand and Valles, 
though their conclusions have not been accepted by many, are 
very important in one point of view. These writers insist much 
on the necessity of taking into account, in estimating the rela- 
tions between precipitation and evaporation, the abstraction of 
water from the surface and surface-currents, by absorption and in- 
filtration—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 infiltration and percolation. 
The remarkable historical notices of inundations in France 
in the Middle Ages collected by Champion® are considered by 
many as furnishing proof, that when that country was much 
more generally covered with wood than it now is, destructive 
inundations of the French rivers were not less frequent than 
they are in modern days. But this evidence is subject to this 
among other objections: we know, it is true, that the forests 
of certain departments of France were anciently much more 
extensive than at the present day; but we know also that in 
many portions of that country the soil has been bared of its 
forests, and then, in consequence of the depopulation of great 
provinces, left to reclothe itself spontaneously with trees, many 
* Les Inondations en France depuis le Vie siécle jusqwa nos jours. 6 vols. 
8vo. Paris, 1858-64. See a very able review of this learned and important 
work by Prof. Messedaglia, read before the Academy of Agriculture at Verona 
in 1864, 
