Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/495

Rh DOCKS.] HARBOURS 467 Height of Waves in which Gates can le Worked. Height of Width of Waves in Name. Entrance which Gates Authority. in Feet. are worked in Feet. 40 2 Mr Moffat, C.E. 36 2 Sunderland Jforth 50 3 Mr Meik, C.E. Do South 60 24 Do. Silloth 60 2 Mr Boyd, C.E. It might at first sight be expected that the strain would increase as the squares of the breadth of the gate, but, owing to the manner in which the forces are distributed, this does not seem to be the case. Where b = the width of opening of the entrance in feet, and h = the height of the waves at the place, the formula fiO 6 = may perhaps be regarded as safe; or, where h has not n/ been observed, and D is length of fetch in miles, 60 Should the calculation by the formula give higher waves than are compatible with safety or convenience, the proper course will be to provide an outer harbour or covering break water, so as to reduce the height of the waves. For this purpose the formula given at page 462 is applicable. In cases where the course now recommended cannot be adopted, from motives of economy or from other causes, recourse may be had to protecting booms such as are described at page 463. If the traffic be considerable and the interrup tion caused by the shipping and unshipping of the booms be regarded a formidable objection, the gates may in some cases be protected by fastening together the tops of the mitre posts by chains. Another mode of securing gates against the sea was adopted at Ramsgate by Mr Ramsay, and is stated by Sir John Coode to be perfectly satisfactory. It consisted of the following arrangement : &quot; One end of a stout beam or stay of greenheart timber was con nected, by means of a massive iron movable joint, to the fore part of each gate, near the middle posts on the inner or basin side, and at about the level of high water of equinoctial spring tides ; the opposite end of this beam passed into an opening in the masonry of the side walls of the entrance. The inner end of this beam was supported by a small bogie truck running upon a pair of rails laid in a horizontal plane. On the vertical face of the beam nearest the gate there was a strong toothed rack working into a pinion at the end of a train of gearing, the first motion of which was a screw-and- worm wheel. By means of this gearing the power of the men was communicated to the gate through the beam or stay just described.&quot; After the engineer has succeeded in designing a dock which is sufficient in so far as the sea is concerned, it may after all not prove safe and convenient if it be exposed fully to the force of the wind, which, acting on the rigging and hulls of the shipping, causes vessels to break their moorings or produces a grinding action of the vessels against the quays. At Sunderland south dock and at the docks in the Tyne, for example, the gales of October 1863 occasioned very considerable damage, from vessels breaking adrift from their moorings and coming into collision with other vessels, ipacity The number of vessels that can be accommodated in each idhar a re ^ a nar ^ our mav ^ e termed its &quot; available capacity.&quot; JUTS &quot; ^^ s must obviously vary with the sizes of the craft which T trade, frequent the port and with the ratio of sheltered to un sheltered acreage ; or in other words, with the exposure, depth, and reductive power. It will therefore be highest for a dock with gates, less for a tide-basin into which the waves have access, and least of all for an anchorage break water or roadstead. But it also depends on the form of the basin and on its depth. The following table contains the greatest number of vessels that can be accommodated at one time per acre for several docks : Name. Tonnage of Vessels. Number of Vessels per Acre. Authority. Ardrossan 100 to 1000 15 Mr Moffat, C.E. Avonmouth. 2000 075 Mr Brunlees C E Kind s Lynn 1500 aver. 6 Do. Silloth 50 to 800 7 50 Mr Boyd, C.E. If represents the number of vessels per acre, and t their average tonnage, and a is a coefficient which may be taken at from 3 to 4 according to the tonnage inversely, the formula ?z = (-a t may perhaps be convenient for showing, though necessarily in a very approximate way, the relative capacity for mixed shipping of proposed docks of symmetrical propoitions. But the capacity ill very largely depend on whether both large and small vessels fre quent the port, or only vessels of one size. The capacity for tonnage per acre is shown in the follow ing table : Name Depth of Water Tonnage per Acre at one Tonnage shipped and unshipped per Authority. in Feet. Acre per annum. Ardrossau 184 3400 90,000 Mr Moffat. Belfast 22 3074 40,307 Mr Salmond. King s Lvnn 25 9000 22,650 Mr Brunlees. Silloth 22 6000 40,718 Mr Boyd. The general form of a dock will in most cases be depend- Form ent on local peculiarities. Where the nature of the ground dock. and other circumstances admit of it, the width of the basin should, in order to provide for the largest amount of traffic, be greater near the entrance than at a distance from it. The diagram (fig. 8) may perhaps be regarded as an ap proach to the form of maximum capacity which should be kept in view as a general guide in making a design. But in most situations the form will, as we have said, be mainly if not entirely dependent on the outline of the land and depth of water, on the position of adjoining streets and buildings, and on the geological formation and nature of the subsoil. The serrated outline of the jetties shown in the diagram affords a larger amount of accommodation for shipping than when the quays are continuously straight. Sir John Hawkshaw was, we believe, the first to adopt these indented jetties, which are specially suited for the coal trade ; but the radiating form shown in the diagram has not, so far as we know, been adopted. Not only are numerous cross sections of the ground required for fixing on the best site for a dock, but borings of the subsoil are essential, as, upon a careful comparison of these, the position of the entrance must be determined. It is always of the greatest importance to place the foundations of the lock or entrance gates at a place where the bottom is rock, or, where that is not to be found, on the hardest and most impervious part of the subsoil, or, failing that, on a place where the bottom- though soft, is of uniform consistency.