DEPOSITING DOCK. 301 



side, but are perfectly free at the other. Each pontoon is divided 

 into several compartments by means of internal bulkheads, some 

 of which are permanently closed so that it is impossible to sink 

 the dock. Each compartment is fitted with independent pipe 

 connections with the pumps which are placed in wells close to 

 the bottom of the vertical side, and worked by suitable engines 

 placed on an engine-deck. 



On the other side of the vertical member is attached a broad, 

 shallow pontoon, the chief function of which is to keep the dock 

 horizontal and to ensure its stability during the operations of 

 raising or lowering a vessel. It forms also a very convenient 

 working platform and store for spare blocks. This pontoon 

 is also divided into several water-tight compartments, and is 

 ballasted so as to always be immersed to half its depth. It is 

 attached to the vertical side in such a way that it always floats 

 on the surface, whatever may be the position of the dock. 



Access to the pontoons forming the bottom is gained by 

 means of gangways passing through the vertical side, and to 

 the upper deck by self-adjustable ladders. 



The special feature of this form of dock, and from which it 

 takes its name, is that it is capable of depositing a vessel on 

 staging provided for the purpose in any convenient position. 

 One lifting dock may therefore be made to serve a large number 

 of vessels, its usefulness, in a suitable position, being limited only 

 by the length of staging provided. 



The staging is formed of piers constructed of parallel rows 

 of vertical piles of iron or wood, capped by horizontal timbers 

 and platforms to carry the keel- and bilge-blocks. These piers, 

 which are erected at right angles to the shore line, are generally 

 from 4< to 5 feet wide, and spaced from 12 to 15 feet apart in the 

 clear. 



In docking a ship, the pontoons and vertical side are lowered 

 to the necessary depth by admitting water, The vessel is thsn 

 brought over the keel-blocks and centred by means of movable 

 side shores, which are placed a little above the water-line and 

 controlled from the upper deck. Sufficient water is then pumped 

 out to cause the vessel to take a bearing on the keel-blocks. 

 The bilge-blocks are then drawn in by means of chains worked 

 from the upper deck. Pumping is then continued until the 

 vessel is raised clear of the water. 



To deposit a vessel, the dock is brought up to the timber 



