FLOATING. 



36.9 



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. 211 gives a simple combination of cog-wheels and endless 



Fig. 210. 



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

 general use for the tanks which will be described lower down. 



(6) Sluice-gates made of logs. — The roughest method adopted 

 for closing sluices is to place a number of round logs, split in half, 

 vertically alongside one another, with their ends resting against two 

 strong beams above and below. The crevices between them are then 

 stopped with moss and a pile of earth is often made behind them. 

 When it is desired to release the water, a hook attached to a 

 rope is passed through an iron ring in the central log, which, on 



VOL. V. B B 



