DOCK, an inclosure for the accommo dation of shipping. Docks may be divided into two principal classes, viz., wet docks and dry docks (or graving docks).
Wet docks are used almost exclusively for purposes of marine commerce. Where the range of tide is more than 10 to 12 feet, docked vessels are kept alongside of the quay or dock at as nearly a uniform elevation as practicable by means of in closing requisite water areas and pre venting by suitable means the outflow of water during ebb tides. Such docks are the side walls of the lock chamber, or under the floor of the latter, through which water may flow from the dock into the lock chamber till the elevation of water in the latter is the same as that in the dock. When the lock is filled with water the gates between it and the dock are opened. After the ships have been admitted the gates are closed and the water is allowed to flow from the lock through sluices like those already described leading into the lower water of the basin or outer har bor. When the lock water in which the ship is floating has fallen nearly to the level of the water of the tidal basin or harbor, the gates at the exit end of the lock are opened, enabling the ship or ships to pass freely outward. The simple reversal of the latter operation enables a ship to enter the wet dock at any frequently approached through what is called a tidal basin, or sometimes a half tide basin, the latter expression indicat ing the fact that ships may freely enter or leave such basins during the upper half of the tidal range.
In order that ships may enter or leave wet docks at any stage of tide, it is necessary that the entrances be fitted with locks. A lock in a wet dock is a long narrow chamber with its sides constructed of masonry or timber, with what are called gates at each end. Each of these gates usually consists of two parts or leaves, each precisely like the other. These leaves or half gates swing about vertical axes, and then close against each other, forming an obtuse angle like the two sides of a short and broad letter "A," pointing inward to the dock where the highest water is to be found. Such gates are called miter
gates; the pressure of water against them tends to hold them shut till the height of water is equalized on both sides. Sluices fitted with proper valves are made, either in the lock gates, in stage of the aide. The comparatively small range of the tides on the coast of the United States makes it unneces sary to construct closed docks in Ameri can ports; hence American docks, which correspond to the wet docks in Great Britain or other foreign countries, are simply open oblong spaces of water be tween substantially constructed timber or masonry piers.
Tidal-basin or half-tide docks are in closures between wet docks and the open harbor. The gates or lock through which vessels enter or leave them are kept open during the upper half of the tidal range, so that ships may freely enter or leave during that period of time. The gates are closed, however, when the tide has half ebbed; from that time to the next mid-tide ships must enter and leave the tidal basin, if at all, in precisely the same manner as that of entering or leaving the wet dock. The tidal basins are useful for the pur pose of receiving ships prior to entering the wet dock, or on coming from the lat ter, having received their cargo. 2 • Tidal docks are basins surrounded by quay walls, and having open entrances permitting the free flow and ebb of the tide; they have the advantage of re quiring no opening or shutting of gates. With small tides they answer very well; they are sometimes made deep enough to keep vessels afloat at low water. They are much more liable to silt up than wet docks.
Dry docks are inclosures with one end movable. Like wet docks, they are built of both masonry and timber, the former material being the more substantial.
Dry docks of the floating type are built of both steel and timber, the for mer material being better adapted to structures of large capacity and now generally used.