■MBMOIEE BY PHILIPPI BRETOif. 83 



constructed entirely of rockwork, and those constructed of dry stones, never 

 cost much less, and they sometimes cost more, than those built with Roman 

 cement, and these have a great advantage over the others in their greater 

 cohesion. As soon as a breach occurs at any height in a larrage of rocks 

 or dry stones, the violent current, passing through the breach, begins at 

 once to enlarge it, and it soon effects a great destruction. In the hydraulic 

 masonry any opening can only enlarge itself slowly, and the flood will have 

 exhausted itself before the destruction has become serious. 



" In saying what I have done I am only extending to barriers retaining 

 gravel the practical rule adopted in the department of Isfere for longitudinal 

 dykes. M. Picol, and the engineers under his orders, have often remarked 

 that a dry- stone dyke is rent from the bottom to the top when a small 

 breach has been made in the foundation. Wishing to make these observa- 

 tions complete by comparison, they made the experiment of building with 

 stones set in good hydraulic mortar. The experiment was not long in 

 revealing — ^first, that the dykes so constructed did not cost much more than 

 did those bmlt of dry stones, as they could build with smaller material, and 

 they did not require to give the same thickness to the wall ; and then, 

 what is of primary importance, that a wall built with good mortar can 

 sustain a considerable destruction at its base without being instantly rent 

 to the top, for the part above sustains itself in the condition of an arch or 

 vault ; and thus time is afforded for assistance." 



After having discussed in detail the different questions which are 

 connected with barrages, M. Breton thus meets an objection which is often 

 brought up : — " I have frequently heard educated and intelligent men 

 object against the system of retaining gravel by larrages the danger of a 

 rupture in the works. When these works shall have amassed a great mass 

 of gravel behind them, if a rupture should occur, that entire mass, so 

 retained above its level, would, it is said, suddenly begin to move, and 

 would produce a frightful catastrophe below. And as a proof in support of 

 this fear they adduce the effects attributed to the sudden emptying of the 

 Lake St Laurent, which, escaping from the plain of the Oisans, laid waste 

 the vaUeys of the Romanche, and of the Drac, as far as to Grenoble. They 

 might adduce, in like manner, the lamentable disasters produced in a single 

 night by the rupture of the reservoir at Sheffield ! But they forget that in 

 these two cases, as in all others which may be cited in which the rupture 

 of a reservoir has caused a sudden catastrophe at a lower level, the state- 

 ment refersjto a reservoir of water, and not to a reservoir of sand, and earth, 

 and gravel. 



" It is thus that I have no dread of this objection, if the work be judged of 

 only by builders accustomed to see the movements of water, and of sand 

 and gravel, and know the difference between them ; never will an engineer 

 bring himself to believe that gravel wiU flow as does water." 



Numerous cases illustrative of the effects of the rupture of a barrage are 

 then given. But M. Breton, while writing thus, is not unmindful of the 

 importance of the boisement or gazonnemervt of the basin drained by the 

 torrent. He admits distinctly that it is vegetation which has the power to 

 extinguish torrents ; he only proposes barrages as a temporary expedient 

 against torrents which cannot be prevented, as are sometimes those connect 

 ted with glaciers, or as temporary appliances where, through the strength 

 of prejudice or legal difficulties, the forest treatment must be for a consider" 

 able time postponed. 



