LEGHORN DOCK. 203 



During the time the sinking was in progress, the masonry 

 immediately above the ceiling of the working chamber of the 

 caisson, and forming the floor of the clock, was proceeded with 

 to within about 6 feet 7 inches of the end abutting upon the 

 old work. Upon this floor a second temporary transverse wall 

 was erected, parallel to the previous one constructed across the 

 head of the old dock, arid, like it, capable of resisting the full 

 head of water (Fig. 189). 



At the same time, the rough masonry of the sides of the 

 extension was proceeded with. The debris arising from the 

 sinking was deposited in the interior of the dock to give weight 

 to overcome the friction, and to counteract the upward pressure. 



Between the ends of the caisson and the second transverse 

 wall there was a narrow width of invert, upon which a mass 

 of dry masonry was placed, for the purpose of equalizing the 

 weighting of that part of the caisson. 



When the caisson had reached its proper depth, the working 

 chamber was filled in, an operation which occupied twenty- 

 seven days. The debris was then removed from the floor, the 

 locks were removed, and the wells through the floor filled in, 

 and the face lining of the sides and invert completed. 



The only part then remaining to be done to complete the 

 work was to connect the old and the new masonry within the 

 space between the two temporary transverse walls, which 

 enclosed a space about 9 feet 10 inches wide (Fig. 189). For 

 this purpose, rails were laid on the tops of the walls, and upon 

 which was placed a travelling-stage and crab. 



A caisson, 24 feet 7 inches long by 5 feet 11 inches wide, was 

 suspended to the travelling- stage by six chains ; by the aid of 

 this caisson, or bell, the old masonry and rubbish found between 

 the two walls was removed down to the top of the ceiling beams, 

 that is, to 8 feet below the floor. By successive removals of the 

 caisson, the two floors were connected by filling in the gap with 

 concrete 'aid in thin layers. 



In order to complete the enclosure of which the concrete 

 formed the bottom and the temporary walls the two sides, sheet- 

 piling was driven at the ends, that is, in line with the back of 

 the side walls. The enclosure was then pumped out, and the 

 masonry of the sides completed. The two temporary walls were 

 then demolished, and the paving of the floor made good. 



Foundation of the Pump-house. The extension of the dock 



