260 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



means of small slide-valves for the purpose of adjusting the 

 water ballast. 



To give the necessary displacement for any required flotation 

 level, the water ballast is admitted to the four central com- 

 partments from which it flows to the others by two 9-inch 

 cast-iron inlet-pipes A A (Fig. 262), or is discharged through the 

 outlet pipe B (Fig. 262), each of which is fitted with a brass- 

 faced valve worked from the deck. 



A small steam-pump is fixed on the water-tight deck, 

 for the purpose of discharging the water ballast, in case of 

 the depth of water on the sill being reduced while the vessel is 

 afloat. 



For filling the dock, 24-inch cast-iron pipes, fitted with sluice- 

 valves worked from the deck, are fixed in the lower part of the 

 caisson. 



In addition to the water ballast, there are about 120 tons 

 of iron ballast, which, with the weight of the caisson itself, 

 including the cast-iron pipes, pumps, etc., amounting to 152 tons, 

 is equal to the displacement at the lowest water level at which 

 the caisson requires to be floated. 



Esquimalt Dock Caisson (Figs. 264 to 267). 1 This caisson, 

 constructed from the designs of Messrs. Kinipple and Morris, 

 is of somewhat more than ordinary interest, on account of the 

 arrangements for facilitating removal from the stops with but 

 slight lifting. 



The sides of the caisson are vertical, but are bevelled hori- 

 zontally, the inner face being of less width than the outer ; this 

 was done in order that, by moving the caisson a short distance 

 back into the chamber, and allowing it to rise by the removal 

 of ballast sufficiently only to clear the stop, the caisson may 

 be more readily floated out when necessary. Further, by con- 

 structing the caisson in this way, the usual battering sides are 

 dispensed with, and a saving of about 10 feet in the width of 

 the entrance is effected. 



The height from the under side of the keel plates to the 

 coping level is 34 feet 10 inches. The length on the inner or 

 dock side is 67 feet 2 inches, and on the outer or harbour side 

 71 feet 2 inches. 



The caisson is divided into two parts ; the lower being the 

 air-chamber, of sufficient capacity to reduce the weight on the 



1 Engineering, vol. xlvi. p. 87. 



