358 



WOOD TRANSPORT BY WATER. 



iii. Dams and Reservoirs. 



Whenever lakes and ponds are not available, the water of 

 the floating-stream itself may be dammed-up, and thus a 

 stronger head of water obtained. This is secured by means 

 of a dam furnished with a sluice-gate, that is erected 

 transversely across the valley in which the stream runs so as 

 to maintain the level of the water behind it. A reservoir is 

 thus formed, the water in which may be made available for 

 floatage when required, by opening the sluice-gates. 



There is much variety in the mode of construction of dams, 

 and according to the material used for them, they are made 

 of earth, wood or masonry. The chief point is to make the 

 dams and sluices watertight ; cemented masonry-dams are 

 best in this respect, but earth-dams are superior to wooden 

 sluices. 



(a) Earth-dams. Earth-dams are formed of heaps of earth 

 at the ordinary angle of repose for the material used, as 



Fig. 230. Earth-dam. 



shown in Fig. 230, which gives the section of such a dam. A 

 facing (a) of clay or loam is added to the dam on the side 

 near the reservoir, to make it watertight, and another vertical 

 layer of clay or loam (a') in the middle of the dam will prevent 

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



