498 Sir Frederick Bramicell [Feb. 22,. 



length is raised to a few feet above high water, the water is pumped 

 out, the bed of the river is exposed, the earth is excavated dowu to the 

 required depth, and the concrete put in, and the brickwork of the tunnel 

 including its covering arch is completed. The cofferdam is then pro- 

 longed for another length, which is similarly treated, leaving, it 

 will be seen, the front end of the original cofferdam as a mere 

 partition in the middle of a longer cofferdam pumped dry on both 

 sides of this partition. Under these circumstances it is easy to 

 make the partition tight against the executed work. Then the upper 

 part of the sides, and the hinder end of the first length of dam, can be 

 removed, and with the requisite new parts, be formed into another 

 length, in front of the portion being executed, and thus step by step the 

 work can be carried across the river with absolute certainty. The 

 remark may be made, supposing bad ground is met with, why won't 

 the whole structure sink '? Bad ground means in this case quicksand 

 full of water, and under this condition of things there would be no pres- 

 sui'e, forcing the tunnel to sink, greater than that wliich the bad ground 

 had already borne ; for, as seen in the case of the Thames Tunnel, after 

 allowing for the openings, the tunnel as a whole is lighter than the 

 earth, which it replaces. With respect to the nature of the cofferdam, 

 it may be urged by those acquainted with these subjects, that the 

 depth is too great for piling, but piling is not the only mode of making 

 a cofferdam. 



Sir Joseph Bazalgette, for that portion of the Victoria Embank- 

 ment between Westminster and some one hundred yards to the east 

 of Waterloo Bridge, employed a succession of oval iron caissons, 

 which were sunk in a continuous line and were united by grooves 

 and tongues. These caissons have the advantage of being sinkable, 

 to any needed depth, by internal excavation, and of forming by their 

 lower parts an admirable protection to the comjileted work, while 

 the upper parts are readily unbolted by the aid of a diver, and are 

 used over and over again, in the successive stages of the cofferdam. 



There is another mode, however, by which, without any cofferdam 

 ordinarily so called, cut-and-cover work can be executed, and that 

 is by employing an enormous diviug-bell, and using compressed air. 

 Some years ago Mr. Fowler and Mr. Baker proposed an application 

 of this system for the Humber Tunnel. A sketch of the apparatus 

 is shown upon tliis diagram. 



An open bottom vessel some 160 feet long, by 42 feet wide, by 

 12 feet deep, immediately below, and made in one with an air-tight 

 vessel of exactly equal size, forming a float. On the top of this last- 

 named vessel there was a framework 45 feet high, carrying a working 

 deck 160 feet long by 55 feet wide, on which was to be placed the 

 whole of the machinery. The framework supported a certain number 

 of screw piles on each side. The apparatus was to be used in the 

 following manner : — It was to be towed over the place where the 

 tunnel was to be made, and then, the machinery being set to work, the 

 screw piles were to be di-iven into the earth on each side of the vessel ; 



