202 NOTES ON DOCKS AND DOCK CONSTRUCTION. 



Means of communication between the compartments was 

 provided by specially arranged openings through the transverse 

 girders. 



It was a matter of the first importance to ensure a perfect 

 connection between the upper mass of masonry which formed 

 the floor proper of the dock and the filling of the working 

 chamber. 



An ordinary floor, or ceiling, of the working chamber of iron 

 plates would have constituted a permanent separation between 

 the two masses, arid would have established water-ways, which, 

 considering the nature of the work, it was imperative should be 

 avoided. Iron, as a roof or ceiling of the working chamber, was 



Temporary 

 Wall bounding 



fart of the 

 Basin retained 



(-13018) 



FIG. 189. 



therefore replaced by brickwork arches resting upon the lower 

 flanges of the girders. These arches, covered with a rendering 

 of cement, were as impervious to the compressed air as a ceiling 

 of iron plates, and after the working chamber was filled up, their 

 soffits were in direct contact with the concrete filling (Fig. 188). 



Nine air locks, of which four were for materials only, were 

 provided for the entry into the working chamber and the 

 removal of debris. The sinking was rendered difficult by meet- 

 ing with a part of the old circular head of the dock, which 

 necessitated blasting within the working chamber. 



The cutting edge of the caisson reached its proper level of 

 42 feet 8 inches below datum in 113 days. 



