252 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



sufficient capacity to float the caisson with all ballast and 

 machinery and with four tons of water in the tank, when the 

 water, line is level with the top of the chamber. In the centre 

 of the lower deck of the air-chamber is placed the well for the 

 drainage pump, and at each end is a water-tight trunk connect- 

 ing the upper and lower water-chambers. 



In the lower water-chamber is another deck, formed of 

 diagonal bracing and supporting the pump -well in the bottom 

 of the air-chamber before referred to. The timber keel and 

 stems consist of the best seasoned English oak, bedded on the 

 iron with canvas coated with red lead so as to form a water- 

 tight joint. The outer surfaces are planed perfectly true to 

 meet the masonry faces of the groove. 



The method of working a ship caisson will differ somewhat 

 according to the details of construction, but generally it is as 

 follows : Supposing it to be afloat, it is towed or hauled into 

 the entrance. One of the stems is then placed in the groove or 

 stop constructed in the masonry, the caisson is then swung 

 round until the other stem is opposite the groove on the other 

 side of the entrance ; then, having adjusted the caisson so that 

 it has an equal bearing on either side against the masonry, 

 water is let into the tank until the caisson sinks into place. To 

 raise the caisson, the valve in the water-tank is opened to allow 

 as much water as necessary to escape, when the caisson will rise ; 

 then bringing one of the stems well into the groove, the caisson 

 may be swung round and out of the entrance. 



Iron Floating Box Caisson. 1 Figs. 249, 250 show the elevation, 

 plan, and cross-section of the caisson at Messrs. Green's new 

 graving dock, Blackwall, designed by Messrs. Kinipple and 

 Morris, M.I.C.E. It is a box-caisson 67 feet 3 inches in length, 

 10 feet in width, and 28 feet 9 inches in depth, having the outer 

 corners rounded. 



The caisson has one meeting-face of teak wood, which is 

 protected from injury while the caisson is being shifted in and 

 out of the berth by horizontal and vertical fenders projecting 

 beyond the face. The outer plating is of wrought-iron plates 

 ^ inch in thickness up to the water-tight deck, and above that 

 level f inch and if inch in thickness. The plating runs hori- 

 zontally in alternate inside and outside strakes, lap-jointed and 

 single riveted. The vertical butt-joints have cover-plates double 

 > M.P.I.C.E., vol. Ixv. p. 339. 



