FLOATING. 



363 



reservoir, to make it watertight, and another vertical layer of 

 clay or loam a', in the middle of the dam will prevent rats 

 from perforating it. In order to strengthen the work, a 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 



Fig. 204. 



Sluice at Absdacli (Black Forest). 



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. 



(/3) 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. 203, 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 ovei- the sluice, and it is crowned by 

 a planked bridge. Buttresses {a a a a) of somewhat similar con- 



