At this immersion the walls have a freeboard of 3 feet 6 inches, which in urgent cases might be safely reduced by a foot or more in order to increase the depth of water over the blocks. Its lifting power up to pontoon-deck level is 15,500 tons, but by utilising the “pound” formed by the bulwark surrounding the pontoon decks, additional lifting power up to 17,500 tons can be gained. The dock, without its machinery, weighs 6500 tons. When called upon to perform its maximum lift the dock is sunk until the summit of its walls is but 2 feet 6 inches above sea-level. Water is admitted into the three pontoons and the two side walls, and from them removed by eight 16-inch centrifugal pumps at a rate sufficient to lift an ironclad of 15,000 tons in three and a half hours. In order that the dock may not tilt as it rises, the whole is divided into fifty-six divisions, each of which is separately connected with the pumps. By turning off cocks, water can be left in any desired divisions, and the dock forced to incline in any direction for purposes of cleaning and repairs. When undergoing its official tests the Bermuda dock lifted H.M.S. _Sans Pareil_ over 11,000 tons, and after its arrival at Bermuda it received and raised completely out of the water H.M.S. _Dominion_, when that vessel was badly damaged through stranding and was so down in the water as to displace nearly 17,000 tons. It is specially important that a structure of this kind should be self-docking, that is, able to lift any part of itself clear of the water. To expose the bottom of one side the dock is first lowered to a depth of 20 to 21 feet, the water inside the wall compartments being brought to the same level as that of the water outside. The dock is then raised by emptying the pontoons, and when these are exhausted the water is released from the side to be exposed until the outer corner is two feet or more clear. The pontoons are lifted in turn by withdrawing the pins of one, and allowing it to float, while the rest of the dock sinks. The pontoon is then made fast to the walls at its floating level, and the dock emptied, so exposing the whole of the bottom of the raised pontoon. The two end sections can be treated simultaneously, and floated if required on to the central portion, but the latter must be moved only when the other pontoons are in position. Electric lights and hauling machinery are distributed over the dock. A crane capable of lifting five tons runs along each wall from end to end.