TORRENTS, ETC., BY 008TA DB BASTELICA, 113 



" Experience shows that this perturbation, produced by second causes, 

 exercises on water-courses a much more powerful action than that proceeding 

 from simple variations in the quantity of water delivered. 



" In the great torrents of the Alps, which bear along at the time of great 

 floods enormous masses of Diaterial, from the grain of sand to the largest 

 blocks of rock, and which, moreover, are extremely muddy, the perturbation 

 is such that the laws of hydraulics would appear at times to be entirely 

 reversed, and to produce efiects diametrically opposite to what are produced 

 in a normal condition. For example, — the bed, instead of being concave, 

 is convex ; the current, instead of following such depressions in the soil as 

 ofier the most rapid declivity, tends to raise itself, and to follow the pro- 

 minent points in the ground. The surface of the water itself is convex; the 

 most extraordinary dynamical effects are produced ; and the water-course 

 — a prey to a veritable revolutionary state of things — ^becomes the picture 

 of the maddest instability. 



" We have there, I repeat, an extreme case of the torrential phenomenon, 

 and one the study of which is pre-eminently adapted to reveal to us the 

 laws by which it is regulated ; for, though less remarkable, this perturbation 

 is nevertheless perceptible in the currents of ordinary streams which bear 

 away solid matter when in flood. This formidable phenomenon betrays 

 itself by certain indications. The surface of the stream tends to assume a 

 convex form ; it is furrowed with currents which change their position with 

 great mobility a,nd varying rapidity. The principal current, instead of 

 establishing itself in the deepest part of the bed, tends, on the contrary, to 

 follow the line of the highest parts of this, and to invade the banks of 

 gravel, if there be such there. In rebound from the normal state the 

 greatest rapidity of flow is along by the banks, and this is one cause of the 

 erosion of these. 



" It is evident that these are effects which cannot be other than the 

 product of secondary disturbing causes, since it is physically impossible that 

 variations in the quantity of water passing along could be the producing 

 cause of any such instability. 



" A trained eye, morever, can judge at once, by the appearance alone of a 

 water-course, what is the degree of torrentiality to which it is subject. 



" First, when the banks are, through a stretch of some length, covered 

 with verdure to the water edge — or when the willows allow with impunity 

 their branches to be borne along by the current — it is a certain sign of great 

 stability and tranquilityof flow. If, on the contrary, the banks are despoiled 

 of vegetation, and show traces of erosion — and further, if there are to be 

 seen here and there banks of gravel — this is symptomatic of the first stage 

 of torrentiality. 



" These indications become more and more pronounced, according to the 

 special regime of each water-course ; and when, as in the Durance, the tor- 

 rential phenomenon attains a great degree of intensity, the water may be 

 seen straggling over immense plains of pebbles, and dividing into many 

 branches, which change their position on the smallest increase of flood. 



" The condition and appearance of the islands formed by these branches 

 present also a certain characterestic of greater or less stability in the regime 

 of a water-course. When these islands are covered with old trees, and 

 better still, if people have made up their mind to dwell on them, although 

 there be occasionally great floods, it is a sign of great stability. If, on the 

 contrary, these deposits are devoid, or despoiled, of vegetation, or have not 



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