282 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



The whole system with the cement lining presents a perfectly 

 smooth conduit to the passage of flowing water. 



The main feeders or large pipes are built in behind the wing 

 walls, bedded in masses of concrete. 



The intakes from the Langton Dock are commanded by 

 double clough-paddles of large dimensions, actuated by both 

 hydraulic and hand-power machinery. 



Water for sluicing purposes can as a rule be drawn off 

 without interfering with the working of the docks ; in fact, as 

 regards the Langton Dock, the running off of water from it is 

 a necessity of the mode of working, seeing that the water in it 

 must each tide be lowered to suit the level of the next tide two 

 hours before high water, when the gates have to be opened for 

 the docking of ships. 



Sweeping is carried on daily, and has proved ampty sufficient 

 to prevent the accumulation of silt in the immediate vicinity of 

 the entrances, and for a limited distance outside the foot of the 

 walls. Sluicing is, however, most effective during springs, 

 owing to the lower level of the low- water in the basin and 

 fairway. 



For dealing with those portions of the basin and fairway 

 beyond the reach of the wall sluices, pipes are laid under the 

 bottom of the part of the basin to be kept clear, which, to render 

 the retention of silt more difficult, is covered throughout with a 

 concrete floor. 



The pipes are laid in trenches excavated in the clay to a 

 depth of 18 feet below the low-water level, bedded on, and 

 surrounded with, concrete. The main pipes, 8 feet in diameter, 

 are connected with the adjoining dock, and are controlled by 

 paddles worked by hydraulic and hand power. The pipes are 

 provided with short branches, having splayed openings or 

 nozzles leading upwards, terminating about 9 inches below 

 the finished concrete surface of the floor, and so spaced as to 

 admit of the sluicing water commanding the entire area of this 

 part of the basin. 



Upon these openings are laid heavily framed discs of green- 

 heart, connected to the pipe by four strong loose links of 2-inch 

 iron. When in a quiescent state, these discs rest upon the 

 openings of the upcast pipes, but when the paddles in the dock 

 are lifted they are forced up by the flowing water to the extent 

 of the range allowed by the links, thereby causing annular jets 



