The Evolution of Floating and Other Dry Docks. 69 



the dock is being hauled about to deposit its load on the grid- 

 iron, and the large area required for operation somewhat 

 prevents the more general adoption of this type, but with 

 plenty available room and a brisk shipping business this is 

 about the cheapest and most convenient form of dry dock. A 

 large rise and fall of tide, however, somewhat curtails its use. 



The Off Shore Dock. 



The Off Shore Dock is of recent date and has been designed 

 to meet special circumstances and localities. It somewhat 

 resembles in form the depositing dock, except that it merely 

 rises and falls with its load and the tide. Its principal advantage 

 is that a vessel can be hauled broadside on, a merit worthy of 

 consideration with firms or public bodies who have limited 

 water frontage, or where there is a narrow water-way with 

 strong currents or tides. Its principal objection is that it is not 

 self-contained but must have shore attachments, involving 

 additional expense in construction and upkeep, and preventing 

 its removal to another site without preparation. 



Generally, this outline description covers the field in dry 

 dock architecture ; there are numerous modifications, but the 

 general principles are embodied in these remarks or explained 

 during the course of reading when referring to the illustration, 

 and I shall therefore only briefly refer^to the necessary pumping 

 arrangements associated with dry docks. 



In most of the smaller graving docks the emptying of the 

 docks follows the falling of the tide as a matter of course, where 

 there is a sufficient rise and fall of tide to be of service, but in 

 the larger and more modern type a powerful pumping in- 

 stallation is provided to more quickly empty the dock and so 

 permit access to the vessel's bottom. These pumps are now 

 almost universally of the centrifugal type, which for small 

 lifts are found to be very effective, convenient, and easily 

 maintained. Usually the pumps are connected direct to high 

 speed steam engines, but in a recent floating dry dock electric 

 motors have been utilised and coupled direct to the pumps 



