TORHEITCS, ETC., BY COSTA DE BlSTELICA. 12? 



serve to illustrate his subject, he prepares for the discussion of the possi- 

 bility of preventing inundations by a discussion of the torrential phenomena 

 in great rivers. And considering, as he did, that attention had been given 

 too exclusively to variations in the delivery of water-courses — no previous 

 writer, so far as was known to him, having even admitted the ideaof per- 

 turbations in the flow giving rise to a confusion of the so-called hydraulic 

 laws, of something like a revolution breaking out in a water-course and 

 producing an instability which mocked all provisions and precautionary 

 measures alike — to these perturbations and their phenomena he gives special 

 attention ; and he again brings under consideration what it is which consti- 

 tutes torrentiality, with a view to showing that it is seen in rivers as well 

 as in what are designated torrents. 



According to M. Costa, his definition of a torrent embodies the idea of its 

 bearing along earthy matter in suspension ; and he states that it does so both 

 in a mass and in what is known in France as triage, dropping some and 

 carrying on others of the materials in question ; in the former case all the 

 rocks, pebbles, and lesser fragments are carried along in something like 

 their relative positions, as would be the case in a viscid mass or in a glacier ; 

 in the latter the weightier materials are dropped first, and this going on more 

 or less continuously, the matters in a state of extreme comminution are 

 carried furthest. The difference in mode of transport appears in connection 

 with difference in the velocity of the flow. When this is so great as to bear 

 the whole along in a mass, the stones, whatever their size, do not come into 

 collision, and if any were withdrawn they would be found to be as little 

 rounded as are the stones falling from a glacier and forming a moraine. 



But when the velocity is being impaired, as this goes on the stones begin 

 to roll, suspended in the water, and they may come into collision one with 

 another ; and the heavier linking, these are for a time rolled along the 

 bottom and subjected to collision and friction. At length they rest, and 

 where they rest the collision of others following and proceeding further 

 subjects them to continued abrasion; and what happens thus to the heavier 

 masses happens there or further in advance, in succession as the velocity 

 is reduced, to others of lesser weight. 



In view of these phenomena he makes valuable suggestions in regard to 

 the structure of barrages. He suggests that by submerged barrages of little 

 height, if these be properly disposed, the velocity of a torrent might be so 

 reduced as to secure a deposit over a great extent of ground of the impal- 

 pable mud borne down by a torrent. He states that much of the mud thus 

 can-ied along would be infertile, but that much fertile vegetable mould is 

 thus buried in the sea ; and he proposes that in certain circumstances in 

 which this may be practicable and desirable this should be so secured ; he 

 points out places of great extent in regard to which he proposes that this 

 should be done ; and he cites what has been eff'ected in the High Alps in 

 evidence of the practicability and advantages of the measure. 



M. Costa looks, in the light of these observations, at the geological 

 phenomena which led M. Cezanne to conclude that the so-called glacial or 

 drift period was succeeded by what he considered a torrential era ; the 

 different appearance of stones found in some heaps from that presented by 

 these in others— these being in the one angular and in the other rounded, 

 the former like the stones forming moraines deposited by glaciers, the 

 others like those found in lits de dejection, and attributable to attrition, 



