370 



W ATER-TK A NSPO RT. 



Fig. 211. 



being lifted, is cftrricd down by tlic water ; the other logs are 

 similarly lifted out of the way, 



]^alks of wood one above the other may also be suspended 

 horizontally as is usual in the Black Forest, by chains before the 

 opening, as shown in tig. 205. They are raised one after the other 

 by hooked-poles. Fig. 212, shows 

 the so-called plug-valve which is 

 much used, especially in Austrian 

 Silesia. The valve tits vertically 

 into a channel (a) excavated under 

 the dam and projecting 4 or 5 

 yards into the reservoir where it 

 is strongly closed, the open end of 

 the channel leading down-stream. 

 The end under the reservoir is 

 open at (m) and can be closed by 

 a conical plug (») which is raised 

 by means of a vertical bar and 

 screw {b) ; {p] is a plank bridge 

 for giving access to (/'). The 

 chamber in which this plug plays 

 is covered with a fine grating to 

 exclude rul)liisli. 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 chan- 

 nel, as gradually as one could wish; 

 it is however very liable to become 

 tilled with silt and mud dilllcult 

 to remove. 

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

 from the reservoir and also for i)assing 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 w^hen 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 pur- 



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