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"26 MARINE REVIEW. [August 8, (5 tons per square inch). In all the principal elements the strain has been kept below this figure, the main transverse girders that support the ship working only 6,627 kilos (419 tons). The walls are of great longitudinal strength, it having been a provision of the contract that the dock should be of sufficient stiffness to support a vessel of 10,000 tons when the same only bore over 80 meters of the length of the dock. Although the use of the gates does away with the necessity of this excessive stiffness, still excess of longitudinal rigidity is by no means a bad fault. Another point to which special attention has been given, warranted by the size and magnitude of the structure, is the question of its stability. A curve of the meta-centric heights and co-efficients of transverse stability Si i Pees oh ers 1 at a | pen ne {_.__442 ou . 1 ' : Nt We ee, 1 qe eo TOS, Oe ooo oo TRANSVERSE SECTION--AS ORDINARY FLOATING DOCK LIFTING MERCHANT VESSEL. from the time when the ship first touches the dock until she is completely lifted is appended. It will be seen that this curve varies considerably in its value, attaining a minimum when the tops of the broad altars are just emerging. At this point the dock has a meta-centric height of 1.186 M. (3 ft. 1034 in.), this being a little more than that of the largest ironclad that can use the dock. This state, however, only lasts for a moment, as immediately the altar emerges it instantly rises to a height of 5.114. M. (16 ft. 936 in.), and when the pound is empty or the deck of the pontoon above water, that is to say the condition of the dock when the ship is lifted, END ELEVATION--AS CAISSON DOCK LIFTING IRONCLAD. it attains the large value of 15.292 (5,082 in.). Expressing these figures in co-efficients of stability, we obtain the enormous value of 22,796 meters, which is equivalent to saying that it would require an external effort of 19,355 meter tons (63,502 foot tons) to lay the dock over 5° from the hori- zontal. When it is remembered that the largest vessel which uses the dock would only require an effort of 851 meter tons to produce the same heel, and this vessel moreover has to face heavy seas whilst the dock lies always at anchor in the calmest water, it will be evident that the conditions of stability are such as to afford the most complete security in every imaginable contingency. METHOD OF WORKING THE DOCK. _ This floating graving dock is worked in exactly the sante way as any ordinary floating dock, and therefore but the barest description is neces- Saty, itis lowered in the first place by admitting water into the pontoons, and walls being kept in a horizontal position by regulating the amounts Bar ORO 888 =. admitted into the various compartments by means of the valves, until there is sufficient depth of water over the keel blocks to enable a ship to be floated in over them. When in position in a fore-and-aft sense, the self-centering shores are screwed out and these push the ship between them until its keel comes true over the center line of the dock. Water is then pumped out of the compartments of the dock, which rises and begins to lift the ship. As this goes on further ordinary timber shores are in- serted so as to keep the vessel steady, the dock itself being kept on a level keel by manipulating the valves and pumping more or less water either from one side or the other. When a considerable portion of the weight of the vessel has been taken by the keel blocks, the bilge shores are hauled in by means of their chains, thus affording a secure berth for the vessel. In the case of an ordinary ship the dock is merely pumped out until the deck is brought above water, when the ship is lifted, but when an iron clad is on the dock the caissons are floated into position over their slots whilst the dock is rising. When the dock has risen suffi- ciently to catch these in their slots, further pumping from the inner com- partments is stopped and the pound valves are opened and the water removed therefrom, when the bottom of the ship will be left dry, and bye somes Swen en =e \ t owing to the much greater area of the pound compared with that of the ship, the dock will have risen to a considerable extent, so that the tops of the caissons come well above the water line. In lowering a vessel the exact converse takes place. The pound is allowed to fill until inside and outside water levels are the same. Water is then admitted into the interior of the dock, which then sinks, leaving the caissons floating. The lowering is continued until the ship floats again, when shores, etc., are removed and she is hauled out. The dock is either then ready to receive another vessel-or can be pumped out until the pontoon deck comes above the water, and left until again required. METHOD OF SELF-DOCKING. The system adopted is a modification of that applied to a dock re- cently designed by the same engineers for the Mexican government. In order to follow the arrangement it should be first imagined that it is re- quired to lift the pontoons in order to get at their bottoms. There are five pontoons, but for the purposes of repair either two or three alternate ones are dealt with together, as by this arrangement the dock always remains in equilibrium. At the same time if required two contiguous pontoons could be dealt with, the weight balanced by water. In the first place the dock is pumped out so as to bring it to its lightest draught line te-- St ability Curves ---- 4 ghaho metres. 8, 554 metres S165 749 mes neas' 70.789 mts 4907 mx __ Top of Gangway passage ray mes 3769 make F749 my 2789 met- 6130 me, Top of Altar 5.769 mes Pecke_of Pantoon. Seeaae tore eet (Fig. 1). Men then descend into the spaces between the walls and the pontoons, and remove the bolts that connect the pontoons that are to be docked so that they are left floating free on the surface of the water. The dock is then lowered, leaving these pontoons floating behind, until the decks of the remaining attached pontoons come about 3 ft. below the bottoms of the floating pontoons. In this position a fresh set of angles will be found riveted to the walls and the free pontoons are attached to these 'by the same fish plates and bolts that held them in their former position. When attached (Fig. 2) the dock is pumped out, and as it rises it brings with it all the pontoons, but those which are attached in the upper position will have their bottoms brought above the water level as shown in Fig. 3. Rafts can then be floated under them and every part got at for painting or repairing. When the painting or repairs are fin- ished, the dock is again lowered to such a depth that the repaired pon- toons are in a position to float again. They are then detached and left free, whilst the dock is raised again until it resumes its primitive posi- tion, when the pontoons are again made fast to the walls. The remaining untouched pontoons are then dealt with in their turn in an exactly similar