Page:The American Cyclopædia (1879) Volume VI.djvu/174

 166 DOCK DOCK (Gr. doxt, Dutch dole, Ger. Dock, a re- ceptacle), an artificial enclosure in connection with a harbor or river, used for the reception of vessels, and provided with gates for keeping in or shutting out the tide. There are two kinds of docks in general, wet and dry docks, the former so called because they retain their water for the purpose of keeping vessels afloat. In the United States wharves are popularly but erroneously called docks. Enclosed basins not closed by gates, in consequence of there being no necessity for it because of small tides, are called docks with better reason, as the At- lantic dock at Brooklyn, which will be de- scribed in this article, although according to the accepted definitions it should strictly be classed as a basin. The late English engineer Rankine calls a reservoir surrounded by quay walls, and having a single gate without a lock, a basin, restricting the term wet dock to a reservoir which is entered through a lock. Either kind of structure, however, is usually called a wet dock. A dry dock is one from which the water may be shut or pumped out so as to become dry, leaving a vessel in a posi- tion to be inspected or repaired. Floating docks and screw docks are varieties of dry docks by means of which vessels are raised out of the water by the buoyancy of pontoons or the application of screw power. Vessels are also drawn out upon inclined railways which are called slips ; but these cannot prop- erly be called docks, as they are not recepta- cles in the sense in which the word is under- stood. WET DOCKS are important structures in harbors where there is considerable rise and fall of tide, serving not only to keep ships afloat, but to maintain a convenient level. They are of comparatively recent date. The Mediterranean, upon which most of the com- merce of the world till within a few centuries was conducted, has so little rise and fall of tide, and the vessels used by the ancients were of so small a size, that the necessity of main- taining a level equal to that of high tide did not exist. Liverpool was the first city to embark to any extent in their construction, and for a long time her docks were unrivalled in size and magnificence. In the harbor of New York, at Philadelphia, and at Baltimore and other ports in Chesapeake bay, the rise and fall of the tides are so inconsiderable as to render such docks as those upon the Mersey and the Thames not only unnecessary but inconvenient. The Atlantic dock, therefore, has no gates. The maintaining of a level of 4 or 5 ft. above low water would doubtless often be an advantage, but the expense necessary to secure it would be greatly disproportionate. At Liverpool, however, where the difference between high where it is 18 ft, the case is different. Liver- I ] owes all her great commercial prosperity to the number and extent of her docks, which cover an area of over 200 acres. Without them the Mersey would never have afforded much more than an indifferent harbor for fishing and other small vessels. In the Clyde, where the tides are small, they have not been used, al- though the idea of providing them has been entertained. Wet docks are constructed with a wall of masonry, or of piling with concrete and tamped clay filling, and with a clay or concrete bottom. The enclosed area may vary from four or five acres, as at Leith, to 70 or more, as the Victoria dock at London. The higher the level of the water in the dock is kept above the low or mean tide of the harbor, the stronger and more impervious the walls require to be made. When the area is not too great the water is sometimes maintain- ed at the highest tide level by pumps, mainly to avoid the necessity of admitting too much sedimentary matter with the entrance of the tide when the water in the harbor is very tur- bid. In planning a dock, among other things to be considered is the proportion of the sur- face of water to the length of the quay walls, which should be as small as is consistent with convenience in manoeuvring the vessels. The docks of Great Britain are usually entered through locks having two gates similar to those on canals. The entrance locks to the docks of London vary in width from 40 to 80 ft., and in length from 100 to over 326, depending upon the class of vessels which it is intended to ac- commodate. The Victoria docks comprise a tidal basin of 16 acres at the entrance from the Thames, and a main dock of 74 acres. The earthy strata which occupied the site of the dock consisted of a top soil one foot deep, a Inyer of clay about 5| ft. thick, then one of peat from 5 to 12 ft., and beneath this a bed of gravel, lying upon the London clay. The dock and basin were excavated to a depth of 26 ft. be- low high-water mark, and its bottom puddled with clay to a depth of 2 ft., leaving the fin- ished surface 24 ft. below Trinity high-water .mark. The entrance from the river into the basin is by a lock having two pairs of wrought- iron gates, revolving in hollow quoins, the walls of the lock being constructed of cast-iron pi- ling, T-shaped in section, backed with hydraulic concrete. The gates are what are called cylin- drical in form ; that is, they are portions of a cylinder, with the convexity turned toward the basin. The lock chamber is 80 ft. wide at the bottom and 326^ ft. long, including the upper and lower gate platforms upon which the gates are supported while turning upon a circular roller path. On the site of the lock the surface of the London clay was 37 ft. be- low high-water mark, and to this depth the excavation was carried at this point, and the foundations of the gate platforms were laid. Between the platforms the bottom of the lock was filled with clay puddle to a level of 28 ft. below high-water mark. The upper gate plat- form is 25 ft. below that mark, while the lower one is 28 ft., or at the same depth as the bottom of the lock ; so that, the mean fall of tide being 18 ft., there will be 10 ft. depth of
 * md low tide is about 15 ft., and at London,