FLOATING. 



371 



pose, therefore, a small overflow channel is generally provided at 

 the top of the dam, unless there is a special gate constructed 

 for this purpose. It may also be necessary to completely drain 

 the reservoir of water, in case of repairs, or to free it from sand, 

 gravel, &c. ; for this purpose a third opening may be necessary 

 lower down than the principal gates. It is usual to admit 



Fig. 212. 



a little water into the floating-channel so as to set the logs 

 slowly in motion, before the chief rush of the water comes. 

 This can be done at pleasure by means of sluice-valves, but 

 where there are sluice-gates, a special opening must be made in 

 the large gate for this purpose, unless the floating-channel is 

 provided with a small quantity of water by a side-channel, open- 

 ing with its own sluice-gate. The size of the principal sluice- 

 gate depends on whether it serves only for the passage of water, 

 or for the wood as well, and in the latter case it must evidently 

 be 4 or 5 meters broad (fig. 204). 



(t) Dimensions of Dams. — Dams vary much in size ; there are 

 some dams which maintain reservoirs capable of submerging a 

 whole valley below them ; these are 450 feet long, and over 

 65 feet in breadth, and in the construction a considerable amouui. 



B B 2 



