DOCKS.] HARBOURS 469 ckage. The late Dr Rankiue in bis Manual of Civil Engineering gives the following table for the expenditure of water due to the passage of vessels through the lock : Let L denote alockful of water, that is, the volume contained in the lock chamber between the upper and lower water levels, and B the volume displaced by a ship, then the quantities of water discharged from the dock are shown in the table. The sign - prefixed to a quantity of water denotes that it is displaced from the lock into the dock. Single Lock. Lock Found. Water Discharged. Lock Left. Empty. L-B Empty. Do do Full. -B Empty. Empty or full. L + IS Full. 2n ships descending and ascend- J Descending full, ascend- 1 n L and j l v (rt+l)L ) Descending empty, ns- Train of n ships descending Do. do Empty. Full. ) ( n L - n B f(-l)L ) l Empty. Empty or full. 1 -B ) nL-HiB Full. Two trains of n ships the s descending, the ~2d ascending]" Full. (2 n - 1)L Full. From these calculations it appears that ships ascending and descend ing alternately cause less expenditure of water than equal numbers of ships in train. The level of the surface of the water in a dock will of course sink in proportion to the number of times the lock has to be filled during each tide, and may also be affected to a small extent by leakage. In order to provide for this depression of level, the old dock of Grimsby, and also the Bute Docks, Cardiff, are supplied by land streams, while at Penarth Sir John Hawkshaw was unable to get a supply of pure water, and the dock at that port is supplied from the tide water of the Bristol Channel, which holds in suspen sion a large amount of alluvial matter. In the old London docks the level is kept up by pumping. If the masonry of the walls has been properly constructed, and if the gates and lock are kept in proper repair, and especially if the basin be of large area, the depression of the water level due to leakage, will, as appears from the following table, be comparatively trifling : Name of Dock. Area in Acres. Leakage. Authority. Ardrossan Belfast 4 3:1 44 16 64 4 18 in. per tide. At first 1 to 2 in., now 6 in. springs, and 4 in. neaps. 3 in. in. in 24 hours. H in. 12 in. per tide. Mr Moffat, C.E. MrSalmond, C.E. Sir Brunlees, C.E. Do. Mr Boyd, C.E. Whiteliaven Avonmouth. ... King s Lynn... Silloth SVorks The principal works required in the construction of docks >f con- are first, cofferdams for excluding the tide, and pumps for keeping the basin dry while the works are going on ; second, excavation of the basin and lock pit ; third, piling for the foundations of quay walls and lock, where the bottom is soft ; fourth, the building of the quay walls, lock, and outer tide basin ; fifth, entrance gates, gangway, and dolphins in the channel outside of the dock for facilitat ing the entry and departure of vessels. As to the first of these works, namely, the cofferdam and pumping apparatus, reference, is made to the articles on these subjects in this work ; and as to the second and third, there is nothing requiring special notice, as no peculiarity attaches to the mode of carrying on excavation or piling for docks more than for any other engineering undertaking where these are needed. It is only necessary to point out that when the bottom is soft a large amount of piling is required, more especially at the lock pit, where rows of sheeting piles should be driven across the lock, at least at the entrance, and in the lines of the clap or point ing sills against which the gates shut. The fourth class of works, namely, quays and lock, are in all cases most important features in a dock. Dif ferent rules have been given by engineers for the propor- Proper tions of quay walls. Mr A. Giles recommends that they tions should be made strong enough to resist the pressure of a head of water equal to their own height ; and Minard recommends their thickness at bottom to be yg-ths of their height. The recent use of Portland cement concrete in the con- Quay struction of harbours has led to its introduction at dock walls < works. In the small dock erected in 1875 at Berwick-on- coacrc Tweed, Messrs Stevenson had the dock walls and gate chamber constructed entirely of Portland cement concrete, excepting a small portion at and above the high water level, against which the vessels might rub ; and Mr llendel at the new Victoria Docks, London, which are the longest in the world, is making the whole masonry of cement concrete. Graving docks are basins sometimes 600 feet long and GraTin 60 or 70 feet wide, fitted with gates, from which the tide- docks, water which floats the vessel into the dock is pumped out, so as to let the carpenters get access to the ship s bottom. The sides of graving docks consist of a series of steps oi masonry, called "altars," against which small timber props, generally of Gulf of Bothnia timber, are placed, for support ing the vessel s sides as she ceases to be water-borne. Her keel is supported on blocks, generally of hard wood, bub of late years they have in some places been made of cast iron. The sides, in order to save pumping, are in some places made of a curved form, so as to suit the shape o the vessel s sides. The advantages are, however, more than counterbalanced by the undue contraction of the space allotted for the carpenters, who are unable to move about easily on the sloping surface of the masonry. Of all the different kinds of masonry which enter into the construction of marine works, there is none which requires greater accuracy of workmanship, or more easeful circumspection, than the graving dock. Leakage in a wet dock, provided it does not originate at a place where- it is liable to increase through time, and is of no greater extent than to depress the surface of the water a few inches, can not be regarded as a serious evil. But in a graving dock, where the requirements are different, there should be no leakage. A very little water, accumulating on the platform of a dry dock, interferes to a serious extent with the comfort and convenience of the carpenters. Although it may occasion considerable additional expense, there ought to be in all cases, especially where the soil is full of springs, an ample underground storage provided by a system of drains for receiving the leakage, which can then be pumped out periodically, as required, without ever allowing the water to rise above the platform. Mr. G. B. Rennie s docks are stated to be the first of the Mr G. kind that have been made of iron. They consist of floating caissons for holding the vessel to be repaired. They are sunk by allowing them to fill with water, and are raised by pump ing. The caissons are made with water-tight compartments, and they are carried up as high as the vessel s bulwarks, excepting that through which the vessel enters and leaves. Among several advantages that have been claimed for this kind of dock may be mentioned its independence of the rise and fall of the tide, the power of applying breast shores as in an ordinary graving dock, and the stiffness produced by the side walls. As the upper parts of the side walls or altars are always full of air, this dock may be used in deep water, and is therefore independent of the nature of the bottom. Mr Edwin Clark s hydraulic lift, at the Victoria Docks, Mr E. London, consists of a pontoon which is filled with water and Clark sunk between two rows of iron columns. After a vessel hj dra has been floated and steadied upon the pontoon, the whole is raised by twelve hydraulic force-pumps of 2 feeb diameter,
acting on the pontoon by means of chains. After the