DOCK, an enclosure for the accommodation of shipping, and of which there are three kinds—viz., wet or floating; tidal, which may with more propriety be called harbors or basins; and dry or graving. Wet docks are for the purpose of maintaining a level nearly uniform with that of laigh-water, so as to keep vessels always afloat, and to save them from rubbing up and down the quays with the rise and fall of the tide, and being sometimes too high and at other times too low for convenience in shipping or dis charging cargoes.
Wet docks are generally surrounded by quay or wharf walls of masonry or brickwork, but where they arc wanted chiefly for laying up vessels in, and not for loading or unload ing, their margin is, sometimes only a natural sloping beach. They are of most impor tance in places where there is a great rise and fall of tide, such as at Bristol or Liver pool, where they are almost indispensable; while, again, in the Clyde, where the tides are small, they have not long been erected, and are on a much more limited scale. Wet docks are generally entered by means of what is called a lock (see LOCK), having two gates, in one leaf, or more frequently in two folding-leaves each, which enables vessels to enter or depart for a considerable time before and after high-water; but frequently, for the sake of economy both in space and in cost, they. have only one gate, so that vessels can only enter or depart at or very near high-water, unless the water in the D. be run down considerably below that level.
The water in wet docks is sometimes kept, by means of pumping, permanently at as high a level as that of the highest tides, when a supply of pure water can be procured, to prevent the silting caused by the admission of any considerable body of turbid water by the gates, but that involves the necessity of locking up or down always except at the highest tides. The tendency to silt up by deposits of fine mud is of common occur rence, and dredging, or some other plan, must be resorted to for the purpose of keeping the D. reasonably clear. In almost all cases, wet docks require to be occasionally emptied for the purpose of cleaning.
Dock-gates are generally opened and shut by means of chains worked by hand, either by winches or capstans; but of late years they have in some cases been moved by hydraulic machinery, as at Great Grimsby, the Victoria docks in the Thames, and Albert dock, Leith.
Tidal docks require no particular description; they are merely basins surrounded by quay walls, and having open entrances permitting the free flow and ebb of the tide, as at Greenock and Troon, and they have the advantage of requiring no opening or shutting of gates. With small tides, they answer very well, and they are sometimes made deep enough to keep vessels afloat at low-water; but with tides of considerable range they are attended with the disadvantage of large vessels grounding at low-water, and from the large volume of water, generally more or less turbid, which enters at every tide, they are much more liable to silt up than wet docks are. For ridding them of muddy
deposits, the plan is sometimes resorted to of letting out a reserve of water with a sud den gush from an inclosure at the inner end, at the time the tide has receded. This is called scouring. Such is the process pursued at Boulogne and elsewhere.
The quays of wet and of tidal docks must have mooring-ports or rings. for making vessels fast to. They are generally provided with sheds to keep goods dry, with cranes (see HYDRAULIC CRANE) for shipping or unloading heavy articles, and with staiths or drops in the case of coal-shipping ports, and now they very frequently have rails laid along them.
Dry docks are used for the purpose of laying vessels dry for examination or repairs. They may have their entrance either from a wet D. or from a tidal harbor; but the former is by much the better arrangement, as it admits of vessels being docked or taken out at any time of tide, and it keeps a more equal pressure on the gates, thereby making them less liable to leak. They require to be built of good water-tight masonry. The entrance has generally a pair of folding-gates pointing outwards, to exclude the water; but sometimes it is closed by means of a caisson—viz., a vessel shaped some thing like the hull of a small ship, and having a keel and two stems, which fit into a groove in the masonry. The caisson is sunk into the groove by admitting water into its interior, and is floated out again by pumping out the water. When the tides are very large, the bottom of the D. may sometimes be placed above low-water, so that it may be run dry without pumping; but generally the bottom of a dry D. for the reception of any but very small vessels is below that level, in which case a steam-engine and pumps, with a wall and water-channels leading to it, are required for emptying the dock. The floor is tietirly level, and the keel of the vessel to be docked rests on wooden blocks fastened down to prevent them floating, and of such a height as to admit of the ship wrights getting under the vessel's bottom. Side-shores are put in, to keep the vessel in an upright position, and blocks are fitted in under the bilges as soon as possible after the water has been got out of the dock. The sides generally consist of stone steps called altars, for the purpose of fixing the lower ends of the shores, and also for the con venience of supporting the workmen's scaffolds. Dry docks are frequently made long enough to hold three or four vessels of considerable size at one time, in which case they are placed, not in the center line of the D., but obliquely across, so as to give more available length.