FLOATING-DOCKS. So iong as ships remained of a small size, no difficulty was found in effecting repairs on their hulls by the simple method of laying them on any convenient beach or sandbank at high-water. The receding tide would leave them high and dry for a few hours at a time; and by actively working at the repairs during low water, they could generally be without any special contrivance for taking the vessel out of the water. Even now this plan is not.unfrequently resorted to, the part of the beach on which it is to be carried out beimg laid with parallel rows of timber beams, called collectively a gridiron. The•rise and fall of the tides is in many localities insufficient for the purpose of leaving the-hull dry at low-water, and the larger the ship the greater the risk of "beaching." Numerous plans have been adopted for getting at the bottoms of large vessels. of heeling over ships was at one time very extensively used. A lighter of sufficient size and weight was provided with very powerful " heaving-down tackle," consisting of strong ropes passing through very large blocks. These tackles were made fast to the masts of the vessel, previously secured by extra stays, on the upper side, and then, by working the tackles, the hull of the ves sel is heeled over on its side. Of course, this plan necessitates the removal of the whole of the cargo. By hauling the vessel over first on one side, and then on the other, the whole of the hull can be got at without difficulty. Of course, still water is required to carry out this method with safety. When there is no rise and fall of tide, the lighter is not required, as the tackle can be made fast to the quay or pier. It was while being careened over that the Royal George went down at Spithead in 1782, with 600 men on board. Graving or dry docks (q.v.) offer a very easy plan for repairing slips; but they are always very costly to construct, and in many, localities sufficiently firm foundations are not attainable. Another common method of getting at the bottom of ships is by bringing them on to sloping ways, called slips, carried out from the yard a under water, and then hauling them right up on to the shore end of the ways by means of suitable tackle, generally worked by hydraulic power. During the operation, the vessel rests upon a suitable carriage. Mr. Morton, of Leith, in 1818, invented a carriage for this purpose, which has contributed much to render this method of hauling up vessels easy.
Floating-docks have been in use for many years. Until of late years, they were built of timber, in the form of a large box with a flap-door falling 'down on strong hinges at one end. They are moored in still and shallow water, with a depth just suf ficient to allow the vessel to float into them as they rest on the bottom. The flap-door is then raised up, and the water pumped out. These timber docks are incapable of being used iu deep water, in consequence of their want of stability. If the vessel bein docked happened to be so light that the dock began to float before the water was all pumped out of the dock, it was very apt to heel over, and thus cause the water to rush to one side, endangering both ship and dock. A considerable number of wooden float
imr-docks, of a size sufficient to dock large vessels, have been built in the United States d'America. Some of these American docks have been built in sections—that is, a number of short docks are joined together to make a structure long enough to fake in a long ship; but those wooden erections have little strength or durability.
It was not until the introduction of iron as the material for constructing them, that floating-docks were made capable of working in deep water, and able to take in the largest class of ships.
Mr. It W. Thomson, c.E., of Edinburgh, designed in 1859 a great iron floating-dock for the port of Sourabaya, Java. Contrary to the method which had always before been adopted, Mr. Thomson determined to make every separate piece of the Sourabaya dock from drawings, and to dispense altogether with the costly operation of building up in this country. Some idea may be formed of the skill and care required for the proper fulfillment of this undertaking, when it is stated that there were upwailds of 75,000 separate plates, ribs, And every panelled with numerous holes, and ready in every respect to be riveted into their places without any further preparation. It was absolutely necessary that every one of the two millions of holes that were to be punched in all these plates and pieces of iron should be accurately in its tight place. 'Mr. Thomson succeeded in carrying out his system so completely, that there were only about 450 separate forms to be made for all the 75,000 different pieces. By systematizing the work in this manner, it became possible to spend sufficient time and care on the making of drawings and templates for each of the separate 450 forms which composed the whole dock, to insure almost mathematical accuracy in the form of each piece, and in the positions of every hole in it. Another advantage of this method is the immense saving of labor in erecting the dock. Under the old plan of shaping each piece of iron s.o that it would fit only into one special place, it had to be searched for amid thousands of pieces similar to, and yet not capable of being substi tuted for it. The mere turning over of the innumerable plates and angle-irons in search for individual pieces becomes a source of great expense. tinder Mr. Thomson's system, however, when the material for the docks is discharged from the ships, each of the 450 classes of pieces is piled up by itself, and the workmen have nothing more to do than to take the piece on the top of the pile, perfectly sure that it will fit accurately any of the hundred possible positions to which its class belongs. All the iron for the Sourabaya dock was used just as it came from the rolling-mills. The plates were all flat and rectangular, and the angle and T-iron all straight. The structure was so designed that no bending or heating .0f the pieces was required. It can easily be imagined that a dock so carefully planned would be cheaply made.