Considerable attention has been given for some years past to the subject of floating pontoon docks by Mr. Robert Turnbull, naval architect, of South Shields, Eng., who has devised the ingenious arrangement which forms the subject of the annexed illustration. The end aimed at and now achieved by Mr. Turnbull was so to construct floating docks or pontoons that they may rise and fall in a berth, and be swung round at one end upon a center post or cylinder--nautically known as a dolphin--projecting from the ground at a slight distance from the berth. The cylinder is in deep water, and, when the pontoon is swung and sunk to the desired depth by letting in the necessary amount of water, a vessel can be floated in and then secured. The pontoon, with the vessel on it, is then raised by pumping out the contained water until she is a little above the level of the berth. The whole is then swung round over the berth, the vessel then being high and dry to enable repairs or other operations to be conducted. For this purpose, one end of the pontoon is so formed as to enable it to fit around the cylinder, and to be held to it as to a center or fulcrum, about which the pontoon can be swung. The pontoon is of special construction, and has air-chambers at the sides placed near the center, so as to balance it. It also has chambers at the ends, which are divided horizontally in order that the operation of submerging within a berth or in shallow water may be conducted without risk, the upper chambers being afterwards supplied with water to sink the pontoon to the full depth before a vessel is hauled in. When the ship is in place, the pontoon with her is then lifted above the level of the berth in which it has to be placed, and then swung round into the berth. In some cases, the pontoon is provided with a cradle, so that, when in berth, the vessel on the cradle can be hauled up a slip with rails arranged as a continuation of the cradle-rails of the pontoon, which can be then furnished with another cradle, and another vessel lifted.

It is this latter arrangement which forms the subject of our illustration, the vessel represented being of the following dimensions: Length between perpendiculars, 350 feet; breadth, moulded, 40 feet; depth, moulded, 32 feet; tons, B. M., 2,600; tons net, 2,000. At A, in fig. 1, is shown in dotted lines a portion of the vessel and pontoon, the ship having just been hauled in and centered over the keel blocks. At B, is shown the pontoon with the ship raised and swung round on to a low level quay. Going a step further in the operation, we see at C, the vessel hauled on to the slipways on the high-level quay. In this case the cylinder is arranged so that the vessel may be delivered on to the rails or slips, which are arranged radially, taking the cylinder as the center. There may be any number of slips so arranged, and one pontoon may be made available for several cylinders at the deep water parts of neighboring repairing or building yards, in which case the recessed portion of the pontoon, when arranged around the cylinder, has stays or retaining bars fitted to prevent it leaving the cylinder when the swinging is taking place, such as might happen in a tideway.

Fig. 1. IMPROVED FLOATING PONTOON DRY DOCK.

The arrangements for delivering vessels on radial slips is seen in plan at fig. 2, where A represents the river or deep water; B is the pontoon with the vessel; C being the cylinder or turning center; D is the low-level quay on to which the pontoon carrying the ship is first swung; E is the high-level quay with the slip-ways; F is an engine running on rails around the radial slips for drawing the vessels with the cradle off the pontoon, and hauling them up on to the high-level quay; and G shows the repairing shops, stores, and sheds. A pontoon attached to a cylinder may be fitted with an ordinary wet dock; and then the pontoon, before or after the vessel is upon it, can be slewed round to suit the slips up which the vessel has to be moved, supposing the slips are arranged radially. In this case, the pivot end of the pontoon would be a fixture, so to speak, to the cylinder.

The pontoon may also be made available for lifting heavy weights, by fitting a pair of compound levers or other apparatus at one end, the lifting power being in the pontoon itself. In some cases, in order to lengthen the pontoon, twenty-five or fifty foot lengths are added at the after end. When not thus engaged, those lengths form short pontoons suitable for small vessels.--Iron.