BLACKBALL DOCK. 179 



generally slope. A speci'al feature in this dock is that a 

 portion of the invert in the centre has been omitted, and 

 castings (Fig. 164) substituted to take the thrust of the 

 arch, and forming a series of shallow wells down the centre 

 of the dock. The castings take the place of the lower 

 keel-blocks ; the upper blocks being removed, and the vessel 

 grounded upon the castings, advantage can be taken of the 

 full depth of water in the dock whilst the wells or spaces 

 between the castings admit of free passage under the vessel's 

 keel. 



The work was carried out in brickwork faced with Staf- 

 fordshire blue bricks, set in 3-to-l cement mortar, and backed 

 with concrete composed of 1 part Portland cement to 10 

 parts of ballast and sufficient sand to fill the interstices. Stone 

 was only used in the caisson groove to form the water-tight 

 faces. 



A caisson was adopted for closing the entrance in preference 

 to gates, in order that the water might be retained in the dock 

 for sluicing the outside approach. 



Messrs. Green's Dock, Blackwall. 1 This dock (Figs. 165 to 

 167), constructed from the plans of Messrs. Kinipple and Morris, 

 is arranged to take in two .vessels when required, by placing 

 two diagonal lines of keel-blocks. 



The dock increases in width from the entrance towards the 

 head, and at a point about 136 feet from the caisson stop 

 or meeting-face of the invert there is an offset or break in 

 the east-side wall for stairs and timber slide. From this point, 

 the side walls are continued to the head in regular curves 

 of 1152 feet radius. The head of the dock is circular to a 

 radius of 25 feet at coping level. 



The following are the principal dimensions : 



Ft. in. 



Clear length on centre line from caisson stop to coping at head -110 



Width of entrance at coping level G5 



at coping level at centre timber slide and steps 74 



at centre, clear of steps 84 



Depth of water on sill nt high water of ordinary spring tides 23 



over bottom of dock at ordinary spring tides 25 



Height of coping above high-water level, Trinity standard 5 G 



Depth from coping to floor of caisson berth 29 6 



from coping to surface of apron 30 G 



Batter of walls 1 in 24 



1 Engineering, xxvi. p. 110. 



