DOCK 387 DOCK Tidal docks are basins surrounded by quay walls, and having open entrances permitting the free flow and ebb of the tide I 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. The ordinary dry dock, whether built of timber or of masonry, is preferably located where a foundation of very firm enter it At one end of this floor is placed a sill, against which and the cor- responding surfaces on the two vertical sides of the entrance the caisson rests in order to make the closed chamber. The sides and one end of the dry dock are formed of substantially built steps called altars, the approximate outline of which is more or less nearly parallel to the sides of the ships as they lie in the dock. Floating dry docks are composed of one or more large pontoons, so con- structed and arranged as to carry along each side pumps and other appliances on suitable stiff frames. When the pon- toons are filled with water they sink, and when water is pumped out of them they rise to a height corresponding to the amount of water taken out. In 1919 considerable progress was made in the construction of dry docks of FLOATING DRY DOCK material is available. The top surface is a few feet only above high water; the bottom is placed at such depths as will accommodate the ships of greatest draught which are to use them. The movable end of the dock, if small, may be formed of a pair of gates like those of wet docks; but for structures of the dimensions requisite to accommodate large ships it is usually formed by a caisson, i. e., a floating vessel usually of steel, and deep enough to close the opening to the dock, and wide and stiff enough to sustain the water pressure at its full height on the outside, with no water on the other or dock side. The interior of the dry dock consists of a floor constructed with sufficiently strong foundations to sustain the weight of the heaviest ship which is to the navy and merchant marine. In Nor- folk, a navy yard dry dock measuring 1,011 feet long, 144 feet wide, and 40 feet deep, built at a cost of $4,500,000, was opened April, 1919. There was at the same time under construction in Boston a dry dock, 1,106 feet long and 149 feet wide. This dock was pur- chased by the Government and was used to provide accommodations for the larg- est naval and commercial ships. A dock of practically the same size as the Nor- folk dock was constructed at the Phila- delphia Navy Yard, and in 1920 the construction of a similar dock at the Charleston Navy Yard v/as begun. A dock of over 1,000 feet long and 150 feet wide was also constructed in San Fran- cisco. In August, 1919, the great dry dock at
