264 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



on to the driving shaft at the head of the chamber, the slack 

 of the chains being carried by the chain-rollers. The driving- 

 shaft is worked by a worm-and-spur wheel wrought off a capstan 

 on the quay. 



Arrangements are made for keeping the caisson berth and 

 chamber clear of mud by placing the outlets of the filling and 

 emptying culverts of the dock in the end walls of the berth 

 and chamber. 



The chamber can be converted into a graving dock for 

 repairing and painting the caisson by placing a temporary 

 dam in a groove at the entrance to the chamber, and the water 

 can be pumped out by a connection with the dock pumps. The 

 caisson can also be floated out of its berth like an ordinary 

 caisson, and placed against an outer meeting-face, and so enable 

 repairs and renewals of any work about the dock entrance and 

 chamber to be effected without the aid of a coffer-dam. When 

 it is desired to do this, the caisson is disconnected from the 

 hauling machinery ; the adjusting valves are closed at low- 

 water, and as the tide rises the caisson floats and is hauled 

 back about 15 inches, so that one end clears the side of the 

 dock entrance ; it is then pulled round, and sunk against the 

 outer face in the position shown by dotted lines (Fig. 268). 



Garvel Graving Dock Caisson. 1 In this design the rollers 

 are attached to the caisson, and run upon plate-rails laid on the 

 floor of the chamber and berth, instead of the rollers being laid 

 on the floor and the keels secured to the bottom of the caisson. 



The lowering bridge is mounted on the caisson, the roadway 

 being at the same level as the quay when the caisson is in 

 position closing the entrance. When the caisson is drawn into 

 the chamber, the bridge and handrails are lowered sufficiently 

 to allow them to pass under the covering of the chamber. 

 The bridge roadway is carried by a series of levers or parallel 

 bars, the upper ends of which are fixed to axles working in 

 plummer-blocks secured to the bottom flanges of the girders 

 of the bridge, and the lower ends to axles fixed in a similar 

 manner to the cross-girders of the caisson (Figs. 272, 273). 

 The outside bars are secured to the ends of the axles, and are 

 continued to 4 feet above the roadway to serve as railing 

 standards, the whole forming a parallel motion like an ordinary 

 pair of parallel rulers. 



1 MP.LC.K, vol. Ixv. p. H4. 



