Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/503

DOCKS.] HARBOURS 471 Mr Kingsbury adds half of this compressive strain to the strain representing the compression due to the transverse pressure, and deducts it from the same amount for the extension ; and these results being divided as before by 4 and 5 respectively, being the allowances per square inch of metal for compression and extension, give the areas for each. He, however, expresses a doubt whether the whole of the compressive strain from the other gate may not perhaps come upon the compressed section. The sections close to the posts he takes as requiring to resist the compression due w tan to transmitted pressure only, or= j For cylindrical gates Mr Kingsbury gives the formula, whenp = pressure per unit of surface, and r = radius of curvature, py -T- sectional area of metal in square inches. It must be remembered in designing gates which are exposed to the waves that they should be mads strongsr than these formula require. And the gates for graving or dry docks sustain a greater pressure with the same rise of tides than those of wet docks, and should therefore be made correspondingly stronger. sons. Caissons are sometimes employed instead of gates, and are the invention of the late General Bentham. They are constructed of malleable iron plates and are sometimes made in the form of a ship s hull, and sometimes have vertical sidss. The caisson fits into checks made for its reception in the side walls of tbe entrance to the dock. They require to be very carefully ballasted, and to be most accurately fitted to the masonry on the sides and bottom, and even when they are faced with timber which has been finely dressed and fitted, coal cinders are often needed to make the joints water tight. At high water they are floated into the grooves, and are then scuttled by admitting the water into the interior chamber. When a vessel has to leave the dock, the water is again pumped out, and the caisson is then floated out of its place and taken into the river or harbour, so as to be out of the way of the vessel which is leaving. General Bentham says caissons are cheaper than gates, occupy less room, are more easily repaired, and the same caisson may be used for different places at different times, while they answer for roadways, and require less labour for opening. The caisson at Keysham was designed by Mr Scamp, deputy- director of the Admiralty works, and is 80 feet wide and 43 feet deep, with an air-chamber at the bottom. When raised a few inches above the bottom, the caisson is drawn back into a recess or chamber in the side walls. The total weight is 290 tons, and the deflexion was f inch near the bottom, wlien the pressure of the whole surface was 2000 tons. The time for opening and closing the entrance at Keysham is 10 minutes and 8 minutes respectively. The cost was about 10,000. Mr M Connochie says that the iron caisson at Cardiff is 60 feet long, 26 feet greatest width, and 30| feet deep, with a total weight of 152 tons. er The outer gangway at entrance to docks is generally a r^yay structure of open timber work, erected in the line which .^ will best suit the entrance of vessels when under the influ- a . ence of currents. This gangway is sometimes useful for ce. a vessel discharging her pnssengers or completing her cargo while the gates are shut. Mr Redman remarks that " the practice in the port of London is to dock a ship upon the flood just before the high water, and to undock her at ab- it the same period of tide. . . . The directions as apparently the most desirable are an angle of about 45 pointing up th& streams for graving docks, an angle of about 60 "in a similar direction for wet docks, and a right angle for biiilding slips." It is of importance in discharging coal into ships that tlls - the fall should be as small as possible. At Bramlcy Moor Dock, Liverpool, the "VVigan coal is discharged by railway 18 feet above the quay. At Sunderland Mr Meik finds that 20 feet above "the quays, as at the south dock, is too small for the shoots, and prefers 36 feet, At Penarth the height is 22, and they are able to lift 180 tons per hour. At Greenock, where steam cranes are used, they load about 500 tons a day. At Middlesboro they can discharge 150 tons per hour, but 106 tons are as much as can be trimmed on board. At Cardiff, where there is no hydraulic machinery, they load 100 tons an hour. The distance between the staiths is 180 feet. At the Tyne Docks, Mr Harrison states that on one occasion, when a vessel had very long hatchways admitting of several waggons being simultaneously discharged, 420 tons of coal have been shipped in 55 minutes, a feat which shows the admirable mechanism employed. At Swansea, with hydraulic machinery, they can discharge from Ballast 350 to 400 tons a day, and those at Penarth load at the rate of 60 cranes, tons an hour. The Law of Harbours and Ports. The power of appointing suitable places to be ports or havens in England is one of the ancient prerogatives of the crown, founded, as Blackstone thinks, partly upon the rights of the king as conservator of the kingdom, partly upon his interest in the marine revenues. In England it hath always been holden that the sovereign is lord of the whole shore, and particularly is guardian of the ports and havens which are the inlets and gates of the realm, and therefore, so early as the reign of King John, we find ships seized by the king s officers for putting in at a place that was not a legal port." The right of the king to make a harbour, and to grant the dues arising from the use thereof, in any suitable place, although the ownership of the soil may be in another, is subject to any vested rights of harbour already existing ; and in the recent case of Foreman v. the Free Fishers of Whitstable, it was said that the right of the public freely to navigate the high seas is a paramount right to which the right of the crown in the soil is necessarily subservient, and therefore, as the right of navigation must include the right of anchorage, the crown or its grantee could not impose a toll on this use of its soil. To justify such a toll some facts must be shown proving that the soil was originally within the precincts of a port or harbour, or that some service or aid to navigation was rendered, in respect of which the alleged right of toll was granted. The crown having once granted a harbour could not afterwards narrow its limits, but by 16 and 17 Viet, c. 107 the crown is enabled to ascertain the limits of all ports, and to assign quays thereon for the enclosure, load ing, and landing of merchandize, so that frauds on the revenue may be prevented. The Act 10 and 11 Viet c. 27 consolidates the clauses usually inserted in private acts authorizing the construc tion of harbours; 24 and 25 Viet. c. 45 facilitates the for mation and improvement of piers and harbours in Great Britain and Ireland ; and 25 and 26 Viet. c. 69 and 28 and 29 Viet. c. 100 transfer from the Admiralty to the Board of Trade certain powers and duties relative to har bours. Besides these general Acts, there are many special Acts relating to particular harbours. Reference may be made to Brit. Assoc. Reports, 1848 (J. S. Russell), and 1850 (Scoresby); Smeaton s Reports ; J. S. Russell, Researches on Hydrodynamics," Trans. Roy. Soc. Edin., vol. xiv. , 1837; Thomas Stevenson, "Account of Experiments on Force of Waves of Atlantic and German Oceans," Trans. Roy. Soc. Edin., vol. xvi., 1845; T. Stevenson, " On Reduction of Height of AVaves after passing into Harbours," Edin. New Phil. Journ., 1852; J. Bremner, Treatise on Planning and Constructing Harbours, Wick, 1845 ; Sir J. Rennie, Theory, Formation, and Construction of British and Foreign Harbours, Loud., 1854; Belidor s Archi tecture Hydrauliquc, Paris; Capt. Washington, Royal Tidal Harbour Com. Reps., Lond., 1845-46; the article on "Tides and Waves," in the Encyclopaedia Metropolitans,, by G. B. Airy, Astronomer Royal ; Minutes of Proceedings of Institution of Civil Engineers; A. Cialdi s writings on Waves and Harbours; Report of the Committees on Harbours of Refuge, 1857-58 ; Report of the Royal Commission on Harbours of Refuge, 1858-59; C. J. Minard, Cours de construction dcs outrages hydrauliques des ports de mcr, Liege, 1852; Thomas Stevenson, Design and Construction of Harbours (2d ed.), Edin., 1874; Professor Mitchell s American Reports. The gates figured in Plate XI. are those at Great Grimsby and Victoria Dock, London; and the caisson, Plate XII., is that in

use at Cardiff Docks. (T. ST.)