Engineering Wonders of the World
Volume I

DOCKS. 185 (Photo, R. Banks.) NEW DOCK AT MANCHESTER. FILLING DOCK WITH WATER OVER TEMPORARY WALL; varies with circumstances. The sill is gener- ally about two feet above the level of the gat,e floor, and the stones composing it are, as a rule, of granite. The gate floor is, of course, flat, as are the aprons at each end of the lock entrance. Otherwise, the construction is very much like ordinary dock work. We may note that the floor of a dry dock is the reverse of an invert, in that it slopes down towards each side from the centre line of the dock. Again, whereas the sides of a lock-chamber are ver- tical, the sides of a dry dock are stepped. But the foundation and construction of both call for equal care. Structurally, the gateway is very like that of a lock entrance, although there are some small features yard there are three such docks, the largest 465 feet long over the blocks, 68 feet wide at the entrance, and 19 feet deep over the sill. At San Francisco there is one of reinforced concrete, 1,050 feet long over all. In the case of one other American dock the ground was frozen artificially to make excavation possible. This is an expensive process, however, and an unpopular one with engineers. In a few docks have been hewn out of instances dry solid rock. A small sample of this method of construction is in Barbados ; larger examples are at Rio de Janeiro and at Nagasaki. The process looks expensive, even where labour is cheap, but the stone quarried incidentally has a considerable off-setting value. It remains to say something about dock gates. As regards material, development here has kept in line with shipbuilding. Docks of early origin have gates of timber— greenheart for choice, although ^ates- greenheart is heavier than water. To them succeeded iron gates composed of outer and inner skins of plates, braced horizontally and peculiar to each. Dry docks are built of masonry, bricks, or timber— in this country usually of the first-mentioned of these mate- rials. Timber docks are to be found in most parts of the world, especially in the United States. They are much less liable than docks of stone to damage through frost, and are usually built in clay. The floors are, as a rule, of con- crete. In the Boston navy BLASTING AWAY TEMPORARY WALL AT ENTRANCE TO THE NEW DOCK. (Photo, R. Banks, Manchester.)