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 tipper, but in one place, for over 1,100 ft. in each tunnel, the rock clippecl and quicksancl only was met with, so both soft ground and rock had to be provided for when getting out the design. The shields were 18 ft. in length and 23 ft. 6| in. outside diameter. I he ta.il was built up of three plates, leaving a space of 1 in. between the shielcl and the cast-iron tunnel lining. There was a double bulkhead which allowed a difference in pressure at the front and back of the shielcl if necessary, and a means of communication between front and back was given by locks for men and material. 1 here were locks for men on each floor level and material shoots to each compartment. 1 he compartments on each floor level were in communication with each other. Diere were two horizontal and two vertica.1 divisions which stiffeneel the shielcl and divided the front into nine compartments. The front part of the shielcl was entirely shiit 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 converteel the whole of the working section behind for the top 9 ft. into a huge diving- bell on the principle already alluded to. This screen is perhaps one of the most distinetive features of these shields, and is an arrangement in every way admirable, and which no shielcl shoLikl be without when there is any danger of the tunnel becoming flooded. An emergency lock was provided 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 shonld the air