366 



WOOD TRANSPORT BY WATER. 



opening the latter, the violent rush of water would damage the 

 banks below ; such gates can therefore be used only where the 

 watercourse below has steep rocky banks. They have also 

 the disadvantage, that the sudden rush of water may not be 

 able to carry downstream all the wood which is lying on the 



bed of the watercourse, so that 

 much of its effect is lost. In 

 the Tyrol, self-opening sluice- 

 gates are used, which open when 

 the reservoir is full. 



(g) Sluice - valves. Sluice - 

 valves are used in well-con- 

 structed floating-channels and 

 wherever the banks need pro- 

 tection against the downward 

 rush of water, so that the 

 amount of water passing through 

 the passage in the dam may be 

 regulated at will. These valves 

 are opened by means of a lever 

 fitting into cogs, a ratchet pre- 

 venting the descent of the valve 

 (Fig. 238). In the Absdach 

 sluice, the so-called ladder sluice- 

 gates are adopted, the construc- 

 tion of which may be seen in 

 Fig. 232. In order to avoid the 

 use of heavy valves, two smaller 

 ones side by side may be used, 

 or several, each of which works 

 in its own groove and may be 

 raised by a revolving axis by 

 means of rollers and chains, or winches. 



The mechanism for raising these heavy valves with a small 

 expenditure of strength should be of a very simple nature. 

 Fig. 239 gives a simple combination of cog-wheels and endless 

 screw for the purpose. This mode of raising valves is in 

 general use for the tanks which will be described lower 

 down. 



Fig. 239. Sluice-valve. 



