A Treatise On The Principles And Practice Of Dock Engineering

CRADLE. 473 gradually becoming flatter as the summit is reached. The permanent way generally consists of three or four main lines of rails, arranged in pairs close together, the rails being of a shallow type, with 3 to 6 inclies flat bearing surface. Between the centre pair is a strong cast-iron rack to receive the pawls of the cradle. The rails are spiked to longitudinal sleepers which, in their turn, are carried by cross sleepers laid upon or bedded in the prepared foundation. For the immersed portion of the way, it has been found convenient to construct short lengths of a timber platform upon which the rails are laid, and to float these out successively into position between guide piles, D (fig. 460). The platform has then been lowered into position by means of a winch, the necessary weight for effecting this being supplied by the ballast.* Great care is requisite in laying the rails to see that there are no inaccuracies in the joints. In order to ensure an even bearing, it is advis- able to bed the rails upon a layer of tarred felt. The Cradle is a framework of timber or iron, usually consisting of three main longitudinals, of which the centre one, carrying the keel blocks, is much stronger than the other two. All three longitudinals are connected by transverse pieces of iron or wood. The latter also serve to carry the sliding bilge blocks. The whole structure is mounted over numerous cast- iron rollers, working in carriages of the same metal. Pawls are attached to the centre longitudinal of the cradle, at intervals of about 20 feet, and these engage in the rack in the permanent way and prevent any back slip. It will generally be found useful to provide short supplementary lengths of cradle to attach to the main one, in case a very large vessel has to be accommodated. A wrought-iron plough, for the removal of silt accumula- tions upon the rails, is a serviceable adjunct to each longitudinal. With the object of utilising a slipway to its füllest extent, various contrivances have been adopted for releasing the cradle from its first load, in order that it may return for a second. One method of achieving this result is that of pivotiug the cross pieces to the side longitudinals, so that they may be swung round to rest upon the latter. After the vessel has been drawn up to its assigned position, it is wedged up on fresh blocks placed upon the ways between the longitudinals, the cross pieces are swung round, the bilge blocks and keel blocks released, and the cradle is available for a second journey. Another method (Thompson and Ooopers) is to employ two cradles with ways constructed at different inclinations. When the vessel has reached a certain point, it is transferred from the first cradle to the second by means of fresh bilge blocks on the latter. The cradles move simultaneously, and the steeper slope of the second causes it to gradually raise the vessel off its previous bearings. In this case also the cross pieces are pivotted for removal. Hauling Machinery. —The subject of hauling machinery will be more ap- propriately considered under the head of Working Equipment in Chap. xii. * Min. Proc. Inst. C.E., vol. Ixxii., p. 168.