Compressed Air Work And Diving 1909

92 COMPRESSED AIR WORK. shown in Fig. 15A, and in general principle is very similar to that of the Blackwall Tunnel shields. One or two novel features were however introduced. The tunnels were driven mostly through rock, as to the lower part and sand in the upper, but in one place, for over 1,100 ft. in each tunnel, the rock dipped and quicksand only was met with, so both soft ground and rock had to be provicled for when getting out the design. The shields were 18 ft. in length and 23 ft. 6| in. outsicle diameter. The ta.il was built up of three f-in. plates, leaving a space of 1 in. between the shield and the cast-iron tunnel lining. There was a double bulkhead which alloweel a difference in pressure at the front and back of the shield if necessary, and a means of communication between front and back was given by locks for men and material. There were locks for men on each floor level and material shoots to each compartment. The compartments on each floor level were in communication with each other. There were two horizontal and two vertical divisions which stiffened the shield and divided the front into nine compartments. The front part of the shield was entirely shut off from the back clown to the level of the bottom of the safety screen. This screen, a device of Mr Moir’s, extendeel 9 ft. clown from the crown of the tunnel, and converted the whole of the working section behind for the top 9 ft. into a huge diving bell on the principle alreacly alluclecl to. This screen is perhaps one of the most distinetive features of these shields, and is an arrangement in every way aclmirable, and which no shield should be without when there is any danger of the tunnel becoming flooded. An emergency lock was provicled in the top of the bulkheads, with the bottom of the door level about on a level with the bottom of the screen. It might be mentioned again here that should the air