TOHBBNTS, ETC., BY COSTA DB BASTELICA. 119 



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 extinguishing torrents. 



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

 in the upper portion a layer of varying thickness, but most frequently of 

 from 30 to 40 centimfetres (12 or 15 inches) of humus, in which the fibrous 

 rootlets are 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 itself 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 during the whole of that 

 longer timej and, what is of more importance, it flows uniformly and 

 equally, with no sudden variations, and thereby much evil is avoided ; and, 

 what is of more importance still, the forest acts at the same time as a filter, 

 delivers no water but what is of perfect fluidity, scarcely even discoloured 

 by the washing away of organic matter, and unable to wash away the earth 

 of the subsoil protected against erosion by its thick covering of humus. 



" When, on the contrary, the rain falls on a soil stript of vegetation, it 

 tends to cut this up into ravines, and it does so if the tenacity and resist- 

 ance of the ground be not sufficient to withstand it ; and the flood is subject 

 to great variations in its current, carrying off' here and there the earth and 

 other debris of the soil. 



" Forests have, then, a double action ; on the one hand they consolidate 

 the soil, on the other hand they reduce and regulate the flow of the current 

 — acting at once both on the delivery and on the perturbation, — in other 

 words, on the primary cause and on the secondary causes of the overflowing 

 of water-courses. 



" It has been tried to subject to experiment and observation the meteoro- 

 logical and hydrological influences of forests. And doubtless studies so 

 interesting are by no means lost to science. They cannot be too much 

 encouraged ; but it should be borne in mind that they can have compara' 

 tively little value in this question, seeing that they cannot take cognizance 

 of this modulating and regulating action. 



" In regard to any flood which we may wish to make the subject of study, 

 it would avail comparatively little to know what quantity of rain falls 

 annually in the basin drained by it. What is necessary to be known is — 

 In what way did the flow of the flood operate during the duration of 



