H6 LITERATURE ON T0RBENT3. 



its Causes, among whioli the practice of floatation is particularly noticed. 

 The amount of damage done to the commune of Campo, on the Rovana, (a 

 tributary of the Maggia, in the canton of Tisino) in great part from the 

 eflfect of floatation, is most striking {Rapport I., pp. 7-13). The force of the 

 torrent Rovana has been augmented to such a degree by baring the soil, 

 and by suddenly opening the dams near its sources, that in the course of 

 four years it excavated below the village a new channel one hundred feet 

 deeper than its ancient bed, and of course undermined the left bank, which 

 was composed of comparatively loose materials, for a long, distance. 

 Deprived of its original support, the steeply inclined soil of the commune to 

 the extent of twenty-five hundred acres, including the village of Campo, 

 began to slide downwards in a body. The movement stiU continues (1875). 

 Many of the houses have been carried ofi', some overthrown, and the walls 

 of most of the remainder dangerously cracked. Unless costly measures of 

 protection are soon adopted, the whole of this vast moving mass will be 

 washed by the Rovana into the Maggia, and by that river into Lake 

 Maggiore. So insecure is the soil considered at Campo, that as I was lately 

 told on the spot, meadow and pasture grounds, which if safe would be worth 

 100 dollars (£20) per acre, cannot now be sold for 10 dollars (£2)." 



In the first part of his work, M. Costa treats of the phenomena of 

 transport of solid materials by running water, and the laws regulating these 

 in difierent states of the current — from that of a tranquil flow and the 

 first movement of sand and stones through acceleration of the flow, through 

 various degrees of speed, to the deposit of these in consequence of a diminu- 

 tion of this — and having shown that these phenomena include two modes 

 of transport — one appropriately characterized as triage, or selection, bearing 

 onwards lighter or smaller material, while heavier or larger is left, or only 

 rolled along, or dropped, and another in which the whole appears to be 

 borne along en masse, water and stones and mud commingled, but keeping 

 their relative position while being borne onward — a section is devoted 

 to the discussion of the laws of viscosity, of which this is a form, and of 

 density, as this is efiected by immersion in a fluid. 



An opportunity will afterwards present itself for stating somewhat in 

 detail the phenomena he has observed in connection with the transport of 

 solid materials in both of the modes described. 



Proceeding, in the second part of his work, to treat of the torrents, he 

 calls attention to two different typical forms of torrential floods — the 

 comparatively limpid floods of the Vosges and the Pyrenees, and the floods 

 of the High Alps, loaded with earth and stones, which they are sweepmg 

 along ; and looking upon the former as virtually extinct torrents, to employ 

 the phrase introduced by Surell and now consecrated by use, he confines 

 his remarks to the latter class of torrents, and discusses in connection with 

 them what he considers the essential parts of these — the basin and the 

 deposit. These are treated of at length, and more especially so the 

 geometrical form of the deposit and the laws regulating its increase ; also 

 certain remarkable incidental phenomena connected with torrents, and the 

 phenomena attending extinction of torrents. 



One of the remakable phenomena of which he treats is the bounding of 

 stones before the mountain wave, of which mention has been made. 



In regard to this he writes, — " Some of the eff'ects of torrents have 

 appeared so extraordinary that, the law of torrentiality not having been 



