ETUDES BY SCIPION GEAS. 81 



two torrents, uniting their floods, will pour out the mass of water in a 

 regular sheet, 32 centimetres, or 12 inches, in fall." 



Of this locality it is mentioned elsewhere by M. Cezanne, that in 1157, 

 after a storm of rain, two torrents of the Oisans, which look directly across 

 from one bank to the other of the Romanche, the Vandaine and the Infernay, 

 raised a barrier across the principal valley ; a lake formed itself immediately 

 behind this dam, which was known under the name of the Lake Saint 

 Lauranf, because the storm had burst on the day of St Laurant. This lake 

 stood for sixty-eight years, but in the night between the 14th and 15th 

 September 1219 the barrier gave way, the waters laid waste the lower parts 

 of the valley, and two towns, Vozille and Grenoble, were almost entirely 

 destroyed. Since the thirteenth century onwards there has often been a 

 threatening of the formation again of this barrier, but in despite of this 

 there has sprung up, in the dried basin of the Lake Laurant, the Bourg 

 d'Oisans, which M. Gras proposed thus to protect. 



M. Cfeanne states in detail objections to which the measure was deemed 

 by him to be open ; and referring to two practical applications which had 

 been made of the system proposed by M. Gras — one on the Roise, near 

 Grenoble, the other on the Riou-Bourdoux, in the Lower Alps — ^he cites 

 observations made by Professor Culmann, who visited the former some three 

 years after the publication of the memoire, and reported of it thus : — " At the 

 time of our visit (October 1860), we found that a strong debdde had just 

 passed over a barrage, and that a great mass of rubbish had been stopped 

 behind the upper barrier. The little wooden bridge a little above it had 

 evidently had too weak a channel, and it was carried away, and the barrier 

 itself could not resist more. . . . 



" It is clear that the work had maintained its resistance until the deposit 

 above it had attained the top of its slope, and that so soon as blocks of even 

 small size began to roll over the inclined plane the links of the binding 

 chains, formed of iron bars '02 m6tre, or four-fifths of an inch, in thickness, 

 yielded to the shock and opened." 



" Beyond this," says M. Cezanne, " M. Cuhnann criticises the mode of 

 constructing rather than the theories of these barriers, but he does not 

 appear to attribute to them other effect than to determine a deposit in the 

 same way as does every other kind of barrier." 



il. C6zanne visited La Roise in 1869, and he says, — " In point of fact, the 

 bed of La Roise presents to a visitor the ordinary appearance of the bed of 

 a torrent. The repaired barriers are surmounted by deposits, and the old 

 state of things appears to be exactly reproduced at a higher level. 



" According to M. Marechal, Inginieur des Fonts et Chaussees, the experi- 

 ment tried on the Riou-Bourdoux has not been more successful ; the barrier 

 has perished through defective or vicious construction." 



M. C6zanne states, in concluding, that notwithstanding failures, which 

 have followed a practical application of it, which have been made, in some 

 of which the failure was attributable to unsatisfactory workmanship, 

 engineers who have to do with torrents, but who have not had much 

 personal experience in connection with torrential phenomena, will read with 

 much profit the memoir by M. Gras ; they wUl find a great many facts 

 carefully noted, and will learn how to make observations themselves. And 

 others who have written upon the subject go, I may mention, far beyond 

 this in their commendations of the measure proposed by M. Gras. 



In regard to triage, selection or successive deposit of materials of different 



H 



