295 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



within the ends and sides of the pontoon. This arrangement 

 permits of an all-round internal examination and free com- 

 munication between the two engine-rooms, when the pontoon 

 is sunk. The outside shell-plates, up to and a little above the 

 tank top, are -^ inch thick. They and the tank top-plates arc 

 double riveted in seams and butts throughout. 



Two centrifugal pumps, engines, and boilers, are placed in 

 the forward end. The pumps are placed, and draw from the 

 main cross-drain M (Fig. 291), which forms part of the structure, 

 and is in communication with the various tanks adjoining. 

 There is a sluice- valve, S, in the cross-drain in the longitudinal 

 centre line, so that usually the port pump only pumps from 



.^j^.^ 



\iO O O O1 / 



.'-. L ul 



MIDSHIP SECTION SECTION THROUGH SILL & TOWER 



Fio. 292. 



the port tanks, and the starboard pump from the starboard 

 tanks. 



Before sinking the dock, the distributing valves shown on 

 Fig. 291 are opened, and remain open until the dock is 

 completely raised again. In fact, it would be better not to 

 have these distributing valves, but to draw direct from the 

 end of each tank, and thus each longitudinal set of tanks 

 would be entirely independent of its neighbours. The water 

 coming in or going out is entirely controlled by the admission 

 or exclusion of air from the various tanks ; that is, if in lifting 

 the dock it was considered necessary to keep the water in 

 any particular tank, the air-pipe for that tank would be kept 

 shut, and this gives the necessary control over the water. The 

 pumps are run full speed during the lifting of a ship, and the 

 man in charge is on the dock or ship and controls the air-pipes, 

 as above described. There are two donkey pumps and a range 



