258 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



tank has also a longitudinal bulkhead in the centre, but 

 it is not water-tight, as it only saves as a break to the water 

 in case of the vessel taking a list. Between the second and 

 third decks is what is termed the tidal chamber, the water 

 being allowed free access to the whole with the exception of 

 two water-tight trunks for the ladders; between the third 

 and fourth decks another tank is formed in the centre of the 

 caisson of a capacity of 60 tons. No pumps are fitted, the 

 caisson being lowered and raised by the use of the tanks, 

 the upper tank being filled from a hydrant on the dock side 

 through a hose. 



The floating draught of the caisson is 20 feet from the 

 top of the sill ; this leaves the second or tidal dock 2 feet above 

 the water line. 



To sink the caisson, the lower tank valves are opened and 

 the tank filled. As this tank contains 150 tons, the tidal 

 deck will then be 3 feet below the water level, therefore the 

 valves of the tidal chamber being opened the water will flow 

 in and the caisson gradually sink until it touches the bottom, 

 when the water in the tidal chamber will be level with the 

 water inside and outside of the dock. The valves of the lower 

 tank on each side of the caisson are then closed, also those 

 of the tidal chamber on the side next the dock, and the 

 pumping out of the dock proceeded with, at the same time the 

 upper tank of 60-tons' capacity is filled. 



To raise the caisson, previous to filling the dock, the lower 

 tank is emptied and the valves closed ; the water is then let 

 into the dock. As the valves of the tidal chamber on the side 

 next the sea have always been open, the water in this compart- 

 ment will be at the general level 'also. All the tidal valves 

 on the dock side are then opened ; also the valves of the 

 upper tank, allowing the water to flow from it into the tidal 

 chamber; the caisson then gradually rises until it reaches its 

 flotation draught, viz. 20 feet. 



From the floating draught -line of the caisson to the tidal deck 

 the displacement is 60 tons. This weight being in the upper 

 tank, no matter what the state of the tide may be on the tidal 

 deck, the caisson will be in a state of equilibrium when the water 

 in the dock is level with the water outside. This is proved 

 by the fact that in the tidal chamber weight and displacement 

 are practically neutralized ; therefore, when the tank is empty, 



