238 DTUNDATIOIirS IN FEANCE. 



there is no doubt tliat they, independently of their action as ab- 

 sorbents, mecbanically promote it. N"ot only does the water of 

 vtlie soil follow them downwards, but their swelling growth power- 

 fully 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 observations and reasonings of Belgrand and Yalles, 

 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 account, in estimating the relations 

 between precipitation and evaporation, of the abstraction of water 

 from the surface and surface-currents, by absorption and infiltra- 

 tion — 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, al- 

 ways imbibing humidity and always giving it off, not by evapo- 

 ration 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 gen- 

 erally 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 objec- 

 tions : we know, it is true, that the forests of certain departments 

 of France were anciently much more extensive than at the pres- 

 ent 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 spon- 

 taneously with trees, many times during the historic period ; and 

 our acquaintance with the forest topography of ancient Gaul 



* Les Inondations en Prance depuis le Vie sihlejusqu'd 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 Ve* 

 rona in 1864. 



