360 WOOD TRANSPORT BY WATER. 



thick facing of rough heavy stones is piled on the side of the 

 dam, away from the reservoir. The impermeability of the 

 dam by water is specially influenced by the nature of the 

 ground on which it rests, and for its site, a place is therefore 

 chosen where there is solid rock, or a clay bed ; if this is 

 some depth down, it may be necessary to have artificial clay 

 foundations. 



(b) Wooden sluices. Wooden sluices have a framework 

 of wood strengthened by means of earth or stones, usually 

 the latter, in which case the wooden framework is lined with 

 clay and filled with stones. Fig. 231 shows the ground-plan 

 of such a sluice, there being three rows of partitions to be 

 filled with stones. On the side away from the reservoir, these 

 partitions are only half as high as the other two rows, and 

 are planked over (c, c). A roof is usually placed over the 

 sluice, and it is crowned by a planked bridge. Buttresses 

 (a a a a) of somewhat similar construction to the rest of the 

 sluice are added to strengthen the structure. They may, 

 however, consist only of coarse, dry, stone masonry ; b is the 

 channel for the passage of the water in the direction m n, 

 and is closed by a sluice-gate. 



Fig. 232 shows another weaker kind of wooden sluice in 

 the Black Forest, at Absdach, on the river Wolf. It consists 

 of piles boarded over, and strengthened, away from the reser- 

 voir, by large blocks of stone between which an opening is left 

 for the sluice-gate. 



(c) Masonry-dams. These are built very strongly, chiefly, 

 or entirely, of large hewn stones. As a rule, however, they 

 are only faced with hewn stones, the interior being filled with 

 rammed broken stones, or with gravel or rough stones 

 imbedded in clay ; buttresses are then required. 



In order to increase their strength, they are frequently 

 made in a regular curved shape, the convex side of which is 

 opposed to the water-pressure, but in that case it must rest on 

 either side on firm rocks, and then resists the pressure of the 

 water like a great vat. 



Fig. 233 shows the plan and elevation of a masonry-dam 

 at Herrenwies, in the Black Forest, with two sluice-gates (b b) ; 

 a a are smaller gates which are opened first to relieve the 





