CAISSONS. 253 



riveted. The plating of the bottom of the caisson is J inch in 

 thickness, and is stiffened by the vertical floor-plates, % inch in 

 thickness, riveted to the side and bottom angle-irons. The 

 spaces between the floor-plates are filled with cast-iron 

 kentledge and Portland-cement concrete as ballast, to give 

 stability to the caisson when floating. The angle-irons forming 

 the side framing are 3 inches by 3 inches by |- inch, and placed 

 18 inches apart ; the cross-beams of angle-iron up to the level of 

 high water, are 4 inches by 4 inches by inch, and above that 

 level are 3 inches by 3 inches by f inch, fastened with gussets 

 to the side angle-irons. The uprights in the centre of the 

 caisson are 4 inches by 4 inches by f inch, and 18 inches apart ; 

 the angle-irons under the water-tight deck are 5 inches by 3 

 inches by f inch, widened at the ends and secured to the side 

 frames ; the deck-beams are 3 inches by 3 inches by f inch, 

 secured by gussets. Horizontal plate-stringers 15 inches in 

 width and J inch in thickness, below the second tier from the 

 top, and above that f inch and J inch in thickness, are riveted 

 to the cross-beams and secured by angle-irons to the outside 

 plating and frames. The upper portion of the caisson is divided 

 by water-tight bulkheads into three compartments, each pro- 

 vided with regulating valves for admitting the water in order 

 to sink the caisson into its berth in the dock entrance. At 

 each end of the upper portion a water-tight bulkhead forms a 

 ventilator and manhole access to the air-chamber below the 

 water-tight deck. The bulkheads are of plating f inch thick, 

 stiffened with angle-irons riveted to the frames and beams, and 

 secured to the water-tight deck by angle-irons, thus forming 

 efficient cross-bracing to the caisson. The meeting-face of the 

 caisson is of teak 14 inches in width by 7 inches in thickness, 

 scarfed at the junctions, bedded in red lead, and bolted to 6-inch by 

 C-inch by J-inch angle-irons, riveted to the caisson, and following 

 the curve of the masonry invert and the batter of the stop-quoins. 



The seams between the angle-irons and teak face are caulked 

 with oakum and pitch. The fenders are of rock elm, 10 inches 

 by 10 inches, secured by wrought-iron clips or brackets to the 

 caisson. 



The planking to the roadway deck is English oak, 3 inches 

 thick, secured to the deck-beams and made water-tight by 

 caulking. Manholes are provided for access to the three upper 

 compartments and the ventilators. 



