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DOCKYARDS
are frequently used, and are of two types—floating and sliding. The former, when the entrance is opened, is floated out of its position, while the latter is drawn endways into a chamber constructed at the side of the entrance, being moved over ways with plane sliding surfaces, or on rollers fixed either to the masonry or to the bottom of the caisson. The advantages of caissons are that they save room by shortening the length of the entrance, they dispense with hollow quoins, they facilitate maintenance and repairs, and, by being capable of carrying a road or railway along their top, they do away with the cost and inconvenience of a swing bridge across the entrance. The deck forming the roadway on top of the sliding caisson can be so arranged that it falls automatically as the caisson is drawn into the chamber, and rises again when
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it is moved out. Caissons are raised and lowered by the aid of air and water chambers, the water being ejected or admitted by pumps. Sliding caissons are constructed of rectangular section, and are drawn backwards and forwards by hauling gear fixed at the inner end of the caisson chamber and worked by steam, compressed air, or hydraulic power, the weight of the caisson on the sliding ways or rollers being adjusted by the amount of water admitted to the water chambers. Floating caissons are in shape somewhat similar to the hull of a ship, with a width at the top varying according to the nature of the road or footway required along it. Floating and pontoon docks are constructed in various forms, but their function is in every case to lift a vessel out of the water and support it by means of their buoyancy. They can only be used in completely sheltered positions. Circumstances, such as a bad foundation, which might render the construction of a solid dock almost if not quite impracticable, would favour the adoption of a floating dock, but each case must be judged on its own merits. Reference may be made to Proc. Inst. C. E.—0. Colson, Notes on Docks and Dock Construction.—I*. F. Vernon-Harcourt, Harbours and Docks.—William Shield, Principles and Practice of Harbour Construction. (w. E.)
DOCKYARDS. 1. British. Recent Structural Developments. — The extensions of H.M. dockyards at Portsmouth and Chatham, described in the ninth edition article on Dockyards, marked the most important stage in the history of dockyard construction which had been reached up to the date of their completion and prior to the year 1880. It is proposed to describe briefly in the present article the extent to which H.M. dockyards have developed since that date, with special reference to the great works which were sanctioned by the Naval Works Acts of 1895 and subsequent years. The works proposed under these Acts are classified under three heads, viz., (a) the enclosure and defence of harbours against torpedo attack; (b) adapting naval ports to the present needs of the fleet; (c) naval barracks and hospitals. Under the first heading are included the defensive harbours at Portland, Dover, and Gibraltar; under headUOder I
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are included the deepening of harbours legislation, and approaches, the dockyard extensions at Gibraltar, Keyham, Simons Bay, and HongKong, with sundry other items. Under heading (c) are included the naval barracks at Chatham, Portsmouth, and Keyham; the naval hospitals at Chatham, Haslar, and Haulbowline ; the colleges at Keyham and Dartmouth, with other items. The total estimated cost of these works, as stated in the Act of 1899, amounts to upwards of twentythree and a half millions sterling, and they will form, when completed, the most important additions yet made at any one period to the dockyard and harbour works required to meet the necessities of the fleet. These works will now be briefly described under the headings of their various localities, together with the principal additions to other dockyards carried out between 1880 and 1901. (a) Enclosure and Defence of Harbours against Torpedo Attach. Portland.—The existing harbour of refuge at this station was commenced in 1847 under the direction of the Admiralty, and completed about fifteen years later in the form and extent in which it remained up to the date of
the commencement of the additional works now being carried out. The original design consisted of the partial enclosure of an extensive area of water bounded on the south and west by the island of Portland and the adjacent coast, and sheltered thereby from winds from those quarters, and on the east by a breakwater of large dimensions starting from the north-east corner of Portland island. This breakwater consisted of a rubble mound of stone quarried by convict labour at the summit of the island, lowered by a wire-rope incline to the sea-level, and deposited by means of staging in its position in the mound. The breakwater consisted of two portions, an inner arm terminating in a masonry head and fort, and an outer or detached breakwater terminating in a circular fort at its northern extremity, an- entrance for shipping being left between the two. The enclosure thus formed afforded a magnificent sheltered roadstead of great extent, with a depth of water of not less than thirty feet over a large proportion of the area. The developments of naval strategy and the necessity of greater protection from torpedo attack have, however, led to additional works. It will have been observed from the foregoing remarks that the harbour, while protected on the west, south, and east sides, was still open on the north to Weymouth bay and the Channel. It is therefore to the further and complete enclosure of the roadstead on this northern side that the additional works have been directed (Fig. 1). The distance from the circular fort at the extremity of the eastern breakwater above described to the Bincleaves rocks on the western shore of the harbour is about two miles. This opening is now closed by two additional breakwaters, one portion being detached and about 4465 feet in length, the other starting from the Bincleaves shore and about 4642 feet in length. Each breakwater consists of a rubble mound of stone, quarried as before on the summit of the island, lowered by wire-rope incline, shipped into specially designed hopper barges, towed out and deposited on the site of the breakwaters, which have thus been brought up from the sea bottom to above the water-line in depths of water ranging from 30 to 50 feet. Each section of breakwater
