320 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



Another plan for shortening the slipway, applicable in situa- 

 tions where there is sufficient range of tide, is to enclose the 

 upper part of the slip within water-tight walls provided with 

 gates, which, being shut at low water, exclude the tide. 1 



Mode of Working 1 . The method of working a slip will not 

 greatly vary in the different cases; therefore, the following 

 description of the operations of hauling up and launching, given 

 by Messrs. Lightfoot and Thompson, may be taken as generally 

 applicable. " The size 2 and general description of the vessel 

 to be taken on having been ascertained, the bilge-blocks on the 

 cradle are trimmed to fit the ship as nearly as possible, and the 

 guide-rods and other fittings looked to and put in proper 

 position. 



"The cradle is then run down into the water by its own 

 weight, assisted if necessary by a down-hauling chain worked 

 by an independent apparatus at the top, arranged also for quickly 

 drawing up the empty cradle after launching a vessel. The 

 extreme end of the cradle may project 30 to 50 feet over the 

 end of the rails, the main ribs being made strong enough for 

 this purpose. The vessel is then floated into position as accu- 

 rately as possible, being guided by hawsers manipulated from 

 a jetty or the shore, and also by the guides fixed at the front 

 of the cradle, which are drawn up into a vertical position by 

 ropes, afterwards secured to the vessel. The hauling up then 

 commences, the ship all the time sitting on the keel-blocks 

 placed on the centre rib, and being guided at the stern by the 

 after-guides. At the proper time, the sliding bilge-blocks are 

 drawn in by ropes previously taken up to the jetties, or on 

 board the vessel ; the operation thus proceeds until finally the 

 ship is drawn out of the water, safely seated on the cradle, <in<l 

 supported uniformly over the whole length of keel, as well as 

 by the bilge-blocks on each side. 



" In launching, the reverse process takes place. The vessel 

 is lowered by the machinery to within a convenient distance 

 of the water; the cradle is then disconnected from the links, 

 and, the holding pawls being raised, is left supported entirely 

 on one special single pawl or dagger, at the top. The dagger 

 is then knocked away by a blow from a hammer, and the crall i 

 with its burden runs down the way until it reaches the water, 

 and the vessel floats off. The empty cradle is drawn up by tin 



1 M.PJ.C.E., vol. Ixxii. p. 138. ' JIM., p HO. 



