44 b:^sum^ op surbll's study of 



immense prisms, in accordance with laws similar to those regulating land- 

 shoots (pousee des terres). So that it is- in the abundance of certain kinds of 

 grounds, and in the composition of the soil itself, that we find the secret of 

 the principal power of these torrents. 

 " And this is the evil to be met." 



With these destructive effects of the torrents are conjoined the devastat- 

 ing effects of the deposit of debris covering up fertile soil with barren sand, 

 and gravel, and stones, — and, in some places, overwhelming not only 

 cultivated ground, but houses and property not less necessary for the 

 maintenance of the life of man, his wealth, and his comfort. M. Surell 

 brings under consideration the several defensive appliances which had been 

 employed in the bed of the torrent to prevent those destructive effects, and 

 describes the respective merits of these. 



The first of these brought under consideration is a wall built along the 

 base of banks in danger of being undermined; and the impotency and 

 inefiB.ciency of such a defence is exposed. The second consists of stone 

 erections or wears raised across the bed of the torrent, to create an artificial 

 fall diagonal to the torrent's course, diverting it away from the ground 

 which it is desired to protect ; such erections, it is stated, operate beneficially, 

 and do so in two different ways,— they retain the bed of the torrent, and 

 they diminish the velocity of the torrent for some distance above them. 

 The first action prevents the sweeping away of the ground, the second 

 deadens the violence of the current, thus not only preventing erosion, but 

 destroying the cause of erosion. And details of their structure, of the 

 extent of some, and of beneficial results which have followed the erection of 

 them, are given. Eeferences are also made to fascinages, structures of 

 fascines, or bundles of bushes, and to pallissades clayonnJees, or stockades of 

 wicker work, which are successfully employed elsewhere — but not there. 



In another chapter are discussed the defences employed in the valleys. 

 Amongst the mountains, as has been intimated, the evil against which 

 protection is sought is the erosion, and subsidence, and destruction of the 

 ground ; in the valleys the evils to be guarded against are those resulting 

 from the deposit of the debris of the mountain in places where it does harm. 

 Of the magnitude of these evils illustrations are given ; and the defences 

 ( employed are classified under two heads, — epis, blocks or piles, and longitu- 

 dinal dams. The effects of a single epi, and of a line of these placed 

 diagonally across a portion of the stream, are described, as are also the 

 structure and effect of dams, and the structure and effect of a third defence 

 consisting of a combination of the two. A chapter is then devoted to the 

 more full discussion of endiguements, the designation given to embank- 

 ments designed for the defence of one bank of a river, the designation 

 encaissement, or enclosing banks, being applied to structures designed for the 

 simultaneous defence or protection of both banks. 



In regard to these effects, it is stated that, whenever in the bed of a 

 water-course a resisting obstacle to the flow of the water presents itself — 

 be it the projection of a rook, be it the bluff side of a mound or hillock, or 

 be it an artificial obstruction — two effects manifest themselves. (1) The 

 current is directed towards the obstacle and maintains this flow ; (2) The 

 current is thence reflected and directed against the opposite bank. The 

 hurtful consequence of this reaction is constant, and it is So serious that it 



