304 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



design of the double-power dock of which Fig. 300 is a side 

 elevation, showing a heavy vessel docked. 



The sides marked A are arranged to slide up and down 

 between the fixed corners, B. The part marked C is the pon- 

 toon. When the sides are secured in the usual position, the 

 appearance of the dock is similar to an ordinary floating dock, 

 and it is lowered in the usual way to receive a vessel. 



The vessel having been secured by shores or sliding bilge- 

 blocks, it is raised as far as possible by pumping out the pontoon ; 

 the sides are then lowered one at a time by allowing water to 

 enter them, and are secured in their new position, in which they 

 have at least 5 feet freeboard. They are then pumped out, thus 

 adding their lifting power to that of the pontoon ; the vessel is 

 thereby raised (as shown in Fig. 300), the buoyancy of every 

 part of the dock except the four fixed corners having assisted in 



:; . 



FIG. 300. 



the operation of lifting. These corners, which have a freeboard 

 of 5 feet, are divided into water-tight compartments. 



The ends of the pontoon are pointed, so that the lifting 

 power may be exerted under the heaviest part of a vessel. 



On the pontoon are built strong upright frames, every 6 feet 

 apart ; these are used for supporting the shores to the vessel, and 

 also as guides to the sliding sides. 



At the first glance, it may perhaps appear that lowering one 

 of the sides while a vessel is on the dock would dangerously 

 affect the stability; but that this quality is always ample 

 will be evident when it is remembered that the actual stability 

 is due to the fact of the vessel and the other secured side of the 

 dock being practically in rigid connection, and really forming 

 one structure, with a breadth about twice that of the vessel. 



Any movement towards either side is immediately resisted 

 by immersing the vessel to a greater depth, and by raising the 

 fixed side out of the water to a corresponding extent, or vice 

 versa, the centre of gravity lying between the secured side and 



