Compressed Air Work And Diving 1909

82 COMPRESSED AIR WORK. I he distance from the centre of Shaft No. i to the centre of Shaft No. 2 was 927 ft. From the centre of Shaft No. 2, and under the river to the centre of Shaft No. 3, the distance was 1,571 ft. 6 in., and from the centre of Shaft No. 3 to the centre of Shaft No. 4 the distance was 1,190 ft. 6 in. There was also a length of cut-and-cover at each end, as well as the open approaches. I he shield usecl for driving the section under the river was starteel from Shaft No. 3, which was on the north side of the river, and the caisson for sinking- the shaft is shown in Pigs. 14A and 14B, and is similar to those usecl for the other three shafts. It was circular in section, and had two shells. The external diameter was 60 ft., and the internal diameter 50 ft., and the five- foot space between was filled with Portland cement concrete. The working chamber was formed by an air-tight floor 12 ft. 8 in. above the level of the cuttino- edo-e, and this was supportecl 011 three girders, 8 ft. 6 in. cleep, with three others at right angles to them, or six girders in all, each 8 ft. 6 in. cleep. Access to the working chamber was obtained by means of two shafts 8 ft. 9 in. in diameter, placed 12 ft. apart, centre to centre, at one side of the caisson (Fig. 14B). These were connected together at the top by a large dome- shapecl air-lock, from which projected horizontally out- wards two circular chambers 5 ft. 9 in. in diameter, and with cloors opening inwarcls towards the caisson and therefore kept closecl by the pressure of the air only so that they were quite safe and no interlocking device was necessary. 1 hey had a length inside, clear from all obstructions, of 16 ft. 6 in. and were made this length in orcler to take two waggons at once when these