CAISSONS. 



257 



Immediately below the bottom deck, which is water-tight, there 

 are two sluices, each 4 feet by 2 feet, for lowering or raising the 

 water level on the two sides. Cast-iron kentledge is packed 

 as ballast in the bottom of the caisson. The caisson contains 



r \Lo7L-er Jl-'afer 



SCALE 



FIG. 255. 



FIG. 25G. 



about 328 tons of wrought-iron and steel, 22 tons of cast-iron, 

 354 tons of iron ballast, and 335 cubic feet of timber. 



Kowloon Dock Caisson. 1 This caisson (Figs. 257, 258, 259) is 

 of the ship form, built entirely of iron. It is divided vertically 



FIG. 257. 



FIG. 259. 



into four compartments by four water-tight decks. Under the 

 first or lower deck the permanent ballast is placed, and between 

 the first and second decks a water-tight tank is formed in the 

 centre of the caisson, having an air-chamber at each end. This 



1 Engineering, vol. xliv. p. 146. 



S 



