368 WOOD TRANSPORT BY WATER. 



can be closed by a conical plug (n) which is raised by means 

 of a vertical bar and screw (b) ; (p) is a plank bridge for 

 giving access to b. The chamber in which this plug plays is 

 covered with a fine grating to exclude rubbish. This kind of 

 valve weakens the dam much less than any other form of 

 opening for the water, and the water can be allowed to pass 

 through the channel as gradually as one could wish ; it is 

 however very liable to become filled with silt and mud, that 

 are removed with difficulty. 



All sluice-gates must allow for an overflow of excessive water 

 from the reservoir and also for passing a small quantity of 

 water into the floating-channel before the principal sluice- 

 gates are opened. The principal rush of water, which is 

 required for floating, passes through the sluice-gates, of which 

 there may be several in the case of large dams, but when once 

 the reservoir is full of water, any more water coming in must 

 be allowed to escape, otherwise the top of the dam would be 

 injured. For this purpose, therefore, a small 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, etc. ; for this purpose a third 

 opening may be necessary lower down than the principal 

 gates. It is usual to admit 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, opening 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. 232). 



(i) 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 their construc- 

 tion a considerable amount of capital is invested, whilst 



