STATEMENT BY M. OOSTA DE JSAbTELlOA. 245 



been almost exclusively the permeability or impermeability of the 

 soil, and the proportion borne by the water absorbed to that which 

 flows off. This is certainly an important question, and no difficulty 

 is found in showing that forests diminish to an enormous extent the 

 amount of water which flows away ; but the service which they 

 render is perhaps greater still in regulating, as they do, the flow, and 

 in securing the delivery of only water of perfect fluidity, 



" The study of torrents has shown that the evil done consists not 

 so much in the greater or less volume of water discharged as in the 

 disturbances or perturbations of the flow connected with this. The 

 principal causes of these are sudden changes or variations in the 

 delivery and in the degree of fluidity of the flood. And if it be 

 shown that the forests have, in relation to both of these, a regulating 

 power superior to that of any other force operating on the torrent, 

 it will be proved that they are the most potent means of extinguish- 

 ing torrents. 



" If we could expose, by a vertical section, a wooded slope, it would 

 show in the upper portion a layer of varymg thickness, but most 

 frequently of from 30 to 40 centimetres (12 or 15 inches) of humus 

 in which the fibrous rootlets ai-e so developed that the whole has the 

 appearance of a woolly material. This layer is at once a sponge and 

 a filter. The large roots of the trees penetrate more or less into the 

 subjacent rock. 



" When the rain falls on ground covered with wood a considerable 

 portion of the water is restored to the atmosphere by evaporation • 

 another portion is absorbed by the immense expansion of foliage and 

 boughs. If the rain be prolonged the water comes at length to the 

 ground, which again is capable of absorbing an immense quantity. 

 A flow from this is slow to establish itself; it is necessary, first that 

 the saturation of the sponge-like layer be complete ; and when this 

 is effected — when the water has been able to make a passage for it- 

 self by an infinite number of imperceptible channels — the flow, like 

 that of a charged syphon, maintains a certain uniformity of flow and 

 this it continues for a long time after the rain has ceased. 



" So much is this the case that opponents have alleged that forests 

 are more hurtful than beneficial, as they tend to prolong floods. The 

 flood is prolonged, it is true, but the delivery is regulated — diminished 

 at the commencement and increased at the close ; the total quantity 

 of water drained away takes a longer time to flow ; it flows durino- the 

 whole of that longer time ; and, what is of more importance, it flows 

 vmiformly and equally, with no sudden variations, and thereby much 



