170 DOCK the water is lowered several inches in the dock in a few minutes, thus hastening the shoring and producing an immediate pressure on the gates, so as to effectually prevent the admission of water and fix them steadily. A complete command of the level at the moment the gates are closed, or when a ship, especially a large one, is about to touch the blocks and requires the placing of shores, is important ; and the above method gives a more perfect control of the operation for the first foot than could be obtained by the best regulated pumps and machinery for driving them. There are also naval dry docks at the navy yards of Bos- ton, Portsmouth, Norfolk, Pensacola, and San Francisco. The Boston dry dock, situated at Charlestown, is of granite, 256 ft. long and 86 ft. wide, with a depth at mean high tide of 25 ft. The length of the dock may be increased 53 ft., like the one at Brooklyn, by leaving open the turning gates and using the caisson alone. It was commenced in 1827 and finished in 1834. The dry dock at Norfolk, Va., built at the same time, is also of granite. Before its completion the ship of the line Delaware was docked in it in 1833, on the anniversary of the battle of Bunker Hill (June 17), being the first national ship ever docked in a United States dry dock. Its dimensions and construc- tion are like those of the Boston dock. There are 11 dry docks at the Portsmouth navy yard, England ; among them two (double locks), each 644 ft. long, with gates 80 and 88 ft. in width and a depth of water at highest tides of 25 and 27 ft. ; and another 426 ft. long, with gates 70 ft. wide, and a depth of 25 ft. 9 in. at high tide. At Devonport there are five dry docks, one 437 ft. long, with an entrance 73 ft. wide, and a depth of water at high tide of 31 ft. There are 18 dry docks at Liverpool, some of them over 500 ft. long, the Canada graving dock having an entrance width of 100 ft. and a maximum depth of water of 26 ft. At Birkenhead there are six dry docks, two of them 750 ft. long at the bottom, with gates 85 and 50 ft. wide, and a depth of 29 ft. 9 in. at high water. There are numerous other magni- ficent structures in other parts of the kingdom. The number of floating basins in the English government dockyards is 11, comprising a total area of 35 acres, and having a lineal quay space of 13,500 ft. The dry docks at Se- bastopol, which were mainly destroyed after the capture of the city, Sept. 8, 1855, were situated considerably above the level of the Black sea, and the expense of pumping was entirely avoided ; ships were raised into a gen- eral dock basin by a series of three locks, each of which had a rise of 10 ft. The water for supplying the basin, for filling the docks, and for working the locks, was brought by a canal
2 m. long, which taps a mountain stream. Ves-
sels, after being elevated into the general basin, were floated into one of these dry docks, and, the gates being closed, the water was let off into the sea through a subterranean culvert A floating dry dock, called a balance dock, and which was patented by John S. Gilbert of New York, consists of a pontoon divided into compart- ments, which may be so filled with either air or water as to preserve a balance of position, and by its buoyancy to be capable of raising ves- sels. It may be built of timber and planking, or of wrought iron and planking. Those which are used in New York have the framework en- tirely of wood, and one of them has been in use for nearly 30 years. The pontoons may be from 8 to 12 ft. in depth, and 100 or more in breadth by 350 or more in length. They are strongly girded and trussed, and have a strong bulkhead running through the middle for the whole length, upon which the keel of the ves- sel is supported by keel blocks. At either side the dock rises into walled chambers, which may be also filled with water or air, and upon the deck of which are placed steam engines for the purpose of pumping the water from the in- terior. The ends of the dock are left open, so that when the vessel is raised the water readi- ly flows from the dock. Fig. 5 represents the larger of two docks owned by the New York balance dock company. It is 325 ft. long, 100 ft. wide, and 30 ft. from deck of pontoons to FIG. 5. Balance Dock. deck of side walls, or 40 ft. in all, the pontoons being 10 ft. deep. It has 8 gates on either side for admitting, and 8 others for discharging water, which is pumped out by steam engines, one upon either wall, of 40 horse power each. The pumps are 14 in number, 7 on either side, of 36 in. diameter and 35 in. stroke, capable of exhausting with sufficient rapidity to raise a vessel of 3,000 tons in an hour and a half. Its total lifting power is estimated at 8,000 tons. In docking a vessel on the balance dock, the pumps are first set in motion by the steam engines on the deck above, and the discharge opening being closed by a gate for that purpose, the water rises in the chamber above the pumps until it is full to the deck of the dock. It is next allowed to flow into the upper cham- ber of the dock until its weight, acting as bal- last, sinks the dock to the required depth. When the ship is floated into the dock, this bal- last is drawn off by means of valves, causing the dock to rise by its own specific gravity until it touches the bottom of the ship, after
