The use of the graving D. is frequently •superseded by that of Morton's patent slip. See SLIP. Graving-docks of large dimensions are very expensive works, and the difficulty of making them water-tight is very great. In many cases, therefore, recourse is had to a pontoon or floating-dock, sometimes termed a "camel." See FLOATING Docx. The use of the floating D., together with the appliogtioli of hydraulic pres sure for the raising of ships, is to be seen at the "Thames graving-docks," where there are two rows of cast-iron columns, 5 ft. diameter, and 16 in each row. The rows are 60 ft. apart, and the practical working length is 350 feet. Each column incloses a hydraulic press of 10 in. diameter, with a length of stroke of 25 feet. There are cross-heads on the top of each ram, from the ends of which cross-girders extend across the D. to the corresponding column on the opposite side, which girders form a large wrought-iron gridiron or platform, which is raised or lowered at pleasure with the vessel upon it. When a vessel is to be repaired, an open pontoon is selected to suit its dimensions, which is sunk in position to the bottom of the D., and resting on the iron gridiron. After the vessel is floated over the pontoon, the whole is raised by the hydraulic presses, and the pontoon being emptied of water, sustains the weight of the vessel to be repaired, and is then floated away into some convenient part of the dockyard.
Liverpool has 20 graving-docks, many of them being 600 ft., and some even 750 ft. in length. No docks in the world are on so splendid a scale as those of London, Liver pool, and Birkenhead, which are of immense area, covering hundreds of acres. Sur rounded with substantial stone quays, provided with gates, placed under a proper police, and otherwise managed in a costly manner, these, as well as nearly all other docks in Great Britain, require to be supported by rates levied from the vessels resorting to them; and for levying these rates, powers are taken in the acts of parliament author izing the construction of the respective docks. Sometimes the D. dues or rates are
imposed on vessels in bulk according to tonnage, and in other instances, the rates are so much per ton, according to the nature or value of the goods. See M'Culloch's Commer cial Dictionary. Generally, the dues are complained of as being a heavy burden on com merce; but so enormous is the cost of constructing docks, that the joint-stock compa nies by which they are for the most part owned, do not often realize good returns for their investments. The most remarkable circumstance connected with English docks, is the rapid extension of the dock-system on the Mersey at Liverpool. The original old D. contained an area of 3 acres 1200 yards, and 557 lineal yards of quay space. The total quay space is now above 20 miles.
DOCK (ante). Docks in the United States are of not so much importance to cora merce as in England, the rise and fall of the tide being far less. There is no difficulty In lading and unlading ships at the wharves of any of the Atlantic or gulf ports, and hence little need of inclosed docks. There are, however, some very fine docks for the more convenient handling of merchandise, especially of grain, sugar, cotton, etc., and for the care of valuable goods. The Atlantic docks in Brooklyn are a specimen. For the repair of vessels, the lifting dock is most in use, and generally hydraulic power is used. The graving-dock in the government navy-yard at Brooklyn is one of the finest in the world, though built on a most difficult foundation. The floor is an arch, upside down; the masdn work is granite-faced, and the facing stones weigh as much as three tons. The gates are of iron, and with apparatus for opening and closing them, weigh about 400,000 lbs. The floating caisson is of iron, and weighs over 420,000 lbs. besides ballast. There is pumping power sufficient to empty the dock in two hours. The principal measurements are: length and breadth of main chambers, at bottom, 286 by 30 ft.; at top, 307 by 98. The caisson added makes the extreme length 359 feet. This dock cost over $2,000,000.