FLOATING. 



363 



channel, which conveys the rush of water at some distance 

 from the dam into the natural bed of the stream. This pro- 

 tects the lower side of the dam from heing undermined by the 

 water, and is specially important in the case of wooden sluices 

 and earth-dams, as in Fig. 231, in b n. The sluice-gates are 

 closed by various contrivances, and they may be distinguished, 

 according as they open with a rush, like ordinary sluice- 

 gates, or are raised gradually, as in the case of vertically 

 opening valves. 



(f) Sluice-gates opening in the ordinary way. This is 



i.'_ r . 2ir>. Sluice-.<:at<>. 



effected by means of hinges, but the gates are closed by 

 various contrivances. The usual method of closing them is 

 shown in Fig. 235. A is the gate revolving on hinges at a. 

 B is a revolving elliptical cylinder of wood, which is kept 

 closed by means of a peg (b), a lever placed between b and 

 the wall of the dam and the pressure of the water in the 

 reservoir, until the lever (m) is withdrawn ; the pressure on 

 B then causes it to revolve on its axis through an angle 

 of 90 and present its smaller diameter to A, so that the 

 latter can open, b entering a recess in the wall made to 

 receive it. 



Another mode of opening a sluice-gate is shown in Fig. 236: 



