Compressed Air Work And Diving 1909

84 COAIPRESSED AIR WORK. wide respectively. The last ring was recessed to take the tail, which overlappecl it for a length of 2 ft. 9 in. The tail was 7 ft. 3 in. long exclusive of the overlap, and was built up of three j-in. plates rivetecl together. The external diameter of the tail was 30 ft. 7^ in., and the internal diameter 30 ft. 3 in., so that there was a space all round of ij- in. between the cast-iron lining of the tunnel and the shielcl where it overlappecl. The cast-steel portion was 30 ft. 8 in. external diameter. This front portion was diviclecl into sixteen compart- ments by three horizontal and three vertical divisions. Each of the eight top compartments was fittecl with face rams, which took the place of tunnel “guns.” The two central top compartments were fittecl with sliding plates, so that the face of the shield was always kept closed 4 ft. 6 in. clown from the crown. About 5 ft. from the cutting edge a curtain plate was fixed in each compartment, and about 3 ft. 6 in. further back a cliaphragm rising from the floor of each compart- ment, and slightly overlapping the curtain plate. A kind of water trap was thus formed. The excavatecl material was passeel through openings 18 in. square in the bottom half of the water trap. The rams for “ shoving ” the shielcl, forty in number, were fixed to the flanges forming the joint between the 3 ft. 6 in. and 4 ft. 9 in. wide cast-steel rings. They were constructecl for a working pressure of 3 tons to the square inch, and at this pressure the total force shoving- the shielcl forward was about 6,000 tons. At the back were two hydraulic erectors workeel by rack and pinion. The term water trap, as useel above, is perhaps not quite a correct one, because, these traps being at three different levels, a balance could not be obtained between