266 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



To counterbalance the weight of the bridge, the ends on 

 two pairs of levers are extended downwards, and boxes con- 

 taining ballast attached to them. 



The raising and lowering of the bridge platform is effected 

 by rollers fixed at each end which work against curved plates 

 in the abutment, and the curved girder or lowering plate across 

 the entrance to the recess. In hauling the caisson into the 

 chamber, in order to open the dock, these rollers one of which 

 is shown in Fig. 272 abut against the convex lowering plate, 

 causing the platform and the handrails to fall down auto- 

 matically into the position shown by the full lines at the end 

 of the elevation. To close the dock, the caisson is driven across 

 the entrance, and the rollers on the other end of the platform 

 come against a concave raising plate fixed in the opposite side 

 wall of the entrance, causing the bridge platform to rise up 

 to the level of the quay. When the bridge is "up," it is so 

 locked between the abutments that it cannot fall down until 

 the caisson is drawn back. The machinery for working the 

 caisson consists of an oscillating three-cylinder hydraulic engine 

 driving the shaft with chain-wheels and chains. The dock 

 can be opened or closed at any state of the tide in three 

 minutes. 



To make the caisson scour out the berth and the chamber, 

 the ends are close plated, so that there is no opening through 

 the body. Thus, when the caisson is drawn back into the 

 chamber, the whole of the water displaced by the advancing 

 caisson rushes under it and past its sides, carrying along with 

 it any mud which may have been deposited in the chamber 

 or berth. 



