CAISSONS. 



249 



basin entrances, Portsmouth Dockyard. When closing the 

 passage, the caisson is raised off the bottom by the removal of 

 the water ballast to an extent sufficient to give it a lifting 

 power in excess of any load it may have to bear, the caisson 

 being kept down to the wharf level by strong cross-girders fixed 

 to the masonry on each side of the entrance. When it is re- 

 quired to open the entrance, water is let into the ballast-tanks, 

 and the caisson sinks to the bottom of the berth ; it is then 

 drawn over the sliding ways under the fixed deck of the 

 chamber. The pipes shown on the section are connected with 

 the compressed-air s} T stem, and are used for stirring up and 

 blowing out any mud that may collect in the caisson berth and 

 chamber. 



When the conditions are such that it is not imperative that 

 the whole of the deck of the caisson should be continuous with 



+'' Caisson Chamber. 



Caisson 



FIG. 244. 



the coping level, as in the case when only light traffic has to be 

 accommodated, the main portion of the deck may be fixed at 

 such a level as to clear the underside of the chamber cover, 

 while a portion at each end is hinged and capable of being lifted 

 to the coping level, thus forming a convenient ramp for the 

 passage of light traffic. This arrangement is shown by Fig. 

 244. 



Sliding caissons have the advantage over floating caissons in 

 so far that the latter require space in which to stow them when 

 removed from the entrance. This becomes an important matter 

 where the water area is at all restricted ; this difficulty may, 

 however, be met by the construction of a recess in the wharf 

 adjoining the entrance sufficiently deep that the caisson, when 

 housed, may not project beyond the face of the wall 



Floating caissons take a longer time to handle. As a rule. 



