Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/496

Rh 468 Mr Ure, in a report on Glasgow harbour, gives the fol- work lowing table showing the amount of work per yard per r - vard quay. annum : Tonnajre Port. per Acre of Water. per Lineal Yard of Remarks. Quay. Glasgow, including steamers. Glasgow, excluding steamers. Southampton Tidal Harbour. 30,670 20,361 20,000 440 293 350 Fully worked. Could do more. Liverpool Docks 21,200 185 Fully worked. Hull Docks.. . . . 19 000 143 During 9 months. St Catherine s Docks 20,500 142 Fully worked. inery m- A great increase in the amount of trade, with the same ong s size of dock and length of quay, may be effected by Sir ^ U &quot; William Armstrong s patent hydraulic machinery. This principle of applying hydraulic pressure for opening gates, bridges, and sluices, or to the capstans for hauling vessels out and into dock and waggons on and off the coal tips, and for turning them on the turntables, the discharging of ballast and loading of coals, and the shipment and dis charging of general cargoes, has now been successfully adopted at many harbours. Its use is, however, only warrantable where there is a great amount of traffic, and especially at places where the spring tides remain only a short time at the same level. At the Victoria Docks, London, the late Mr Bidder mentions that 41 craft and 17 ships, or 11,711 tons, came in at one tide. In one month the number of craft entering the harbour was 1229 ; craft leaving, 1288 ; ships entering, 250 ; ships leaving, 258 ; or an aggregate of 2517 craft and 508 ships during the month. The gates, which are 80 feet span, are opened in less than 1J minutes. At Sunderland the accumulators are equivalent to a head of 600 feet, and the engine is 30 horse-power. A wrought-iron bridge 16 feet wide, and including counterweights equal to nearly 200 tons, is raised vertically 18 inches, and drawn back in about 2^ minutes. At Swansea the accumulators are equivalent to an effective pressure of 750 Ib per square inch, and there are three high-pressure engines of 80, 30, and 12 horse-power respectively. The time employed in either opening or closing the gates is about 2^ minutes, which is the shortest period consistent with safety. The wrought- ii-on swing-bridge can be opened or shut in 1^ minutes. The ballast cranes, which are distributed round the dock, can each discharge from 350 to 400 tons in the day. The quantity of coal that can be shipped is about 1000 tons per day, and the effective quantity of water required for the port is 21,050 cubic feet per week. The large new bridge at Leith, designed by Messrs Kendel & Robertson, and worked by the Armstrong apparatus, is 120 feet span, length of girder 214 feet, weight moved 750 tons, and is opened in 1J minutes. The saving of time effected by this method is very great, for at Liverpool, according to Mr A. Giles, gates of 70 feet require 20 minutes, and six men on each side to open them. The proportion of water area to length of quays will obviously depend on the form of the docks. The following table, deduced from information kindly supplied by Mr ui fand Lyster, C.E., gives examples on this head at Liverpool: Total Water Area and Quay Space at Liverpool and Birkcnlwad. !ter to Water Area. Lineal Quayage. Lineal Yards of Quay per Acre. Liverpool Docks Acres. Yards. 252 1601 Miles. Yards. 17 1472 Yards. 124-4 Liverpool Basins Liverpool Docks & Basins Birkenhead Docks 16 4441 269 1202 159 4535 1 1088 19 800 9 110 168-3 127-2 99 7 ,, Docks and Basins 164 2538 9 779 101-0 In order to utilize the area of a dock to the best advantage, a certain amount of land space dependent on the kind of trade should be provided around it; for without this space, which is needed for giving room for railway sidings, ware houses, and the like, the traffic would be cramped. As an illustration of the large extent of railway which is required in a coal-port, we may refer to Cardiff, whereMrM Connochie says, &quot; The extent of sidings provided and maintained by [DOCKS. the Bute trustees, in connexion with the docks, amounts to 16 miles in length, the whole of which is at times fully occupied.&quot; The proportion of the area of outer tide basin to that of Propoi the dock to which it forms the access varies necessarily t 011 0&amp;lt; with the requirements of the trade, as is shown in the ^^ 1 following table : Dock. Outer Basin. Ratio. Bute Dock (East) Bute Dock (West) Penarth Dock Acres. 42-33 16-86 17-11 Acres. 2-15 1-40 2-68 19-69 : 1 12-04 : 1 6-39 : 1 Tyne Dock 50- 9 50 5-27 : 1 Liverpool generally 107- 13&quot; 8-23 : 1 The level of the bottom of a dock is always kept lower Depth than that of the sill of the entrance gates, in order to dock, admit of a certain amount of deposit taking place without injuriously affecting the depth of water in which the vessels lie, or interfering with the working of the gates. There should never be less than 1 foot of greater depth over the bottom of the dock than over the entrance sill. The commercial advantage of a great depth of water over Com- the sill has been well shown by Mr George Robertson in me y cii his report on the Albert Dock at Leith. By increasing a j es the depth to the extent of 2 feet he found there would be p r0 po: 396 tides in the year giving a depth of 23 feet over the tional sill, as against only 102 tides in the year when that depth j Ulj ? could be had if the sill were only 2 feet high. But besides the advantage of availing ourselves of a greater number of tides, Mr T. Stevenson found, from an examination of the draughts and tonnage of a large number of sailing vessels, that the commercial advantages in a navigable river, dock, or harbour are proportional to the cubes of the (Jej)ths. The relation subsisting between the depth of a dock or channel and the tonnage of sailing vessels which can use it is given by the formula #= 1 13c? 3, or d= ^/7 7 t, from which the following table has been calculated : Feet. Tons. Feet. Tons. Feet. Tons. 6 16 12 225 19 892 6 28 13 286 20 1040 7 44-5 14 357 21 1201 8 66-5 15 439 22 1384 9 95 16 532 23 1581 10 130 17 639 24 1797 11 173 18 758 25 2031 In fixing the minimum width that will admit vessels of Widt the size which are expected to trade at the harbour, the f en - engineer must take care to preserve a proper relation tranc between the area of the dock and the width of the entrance; for if this relation be neglected, a current may be gener ated through the entrance when the tide begins to ebb, which would be incompatible with the safe working of the gates. A very small current is sufficient to act injuriously in this way, especially if the gates be of great width. Whenever the internal area of the proposed basin is con siderable, it will be a safe precaution to calculate the velocity of the outgoing current, at different periods of time, after the tide has begun to ebb ; and for this pur pose observations of the fall of the tide must be carefully made and compared with the proposed transverse sectional area of the passage through the lock chamber. If the velocity thus obtained be thought too great for the safe working of gates of the size required, the only alternative is to provide an additional lock. But a double outlet will at any rate be in all probability needed, or will at least prove a great convenience in all cases where a large internal basin is required.