The Evolution of Floating and Other Dry Docks. 67 



built on vertical supports. These vertical supports acted as 

 guides for a float, and the process of docking of vessel was : — 

 First fill the Pontoon with water, raise the floats by means of 

 screws which depressed and submerged the pontoon, haul the 

 vessel between the floats, screw down the floats until the keel 

 of vessel rested on pontoon, then shore and pump out the 

 pontoon, floating the vessel and dock to a convenient berth, 

 where the vessel if so desired could be hauled with its launch- 

 ways to a prepared track, thus introducing the principle of 

 the more modern depositing dock. Each section was detachable 

 and could be docked for repairs or painting, on the remaining 

 sections. 



The introduction of iron and the use of Hydraulic jacks 

 instead of screws for raising and lowering the floats, have made 

 this dock a great favourite for despatch and convenience. 



Balance Docks. 



In 1848 the American Government had constructed of wood 

 a dock of the " Balance " type, which consists of a bottom 

 pontoon with cellular wall sides, and derives its name from the 

 method of submerging. Being of soft wood it will not sink 

 when water-logged ; to overcome this there is an upper deck 

 in the side walls which can be filled with water, thus loading 

 the structure conveniently to submerge it to a suitable draft. 



When the vessel is berthed the water is allowed to drain off 

 the deck and the cellular structure is pumped out, lifting itself 

 and the vessel. The dock is sometimes fitted with gates and 

 then is capable of raising much weightier vessels. One of the 

 main features of the modern and most perfect type of floating 

 dock. 



Rennie's Dock. 



The father of the modern metal floating dry dock was Mr. C. 

 B. Rennie, who designed and had constructed in 1859 a dock 

 for Carthagena, to lift a dead weight of about 6,500 tons, an 

 achievement to be proud of in those early days of iron structures. 



