Compressed Air Work And Diving 1909

94 COMPRESSED AIR WORK. tended half-way clown the shields. Two types of hoocl were usecl—some of the shields had one type and some the other. One type was fixed, and the other was made up of segments which could be pushed forward as the excavation proceeded, and which, when the “ shove ” was made, would slide back again. T. he object of the hoocl is to enable the excavation to be taken out with a sloping face instead of a vertical one. They were boltecl to the cutting edge, and could be detached at any time if it became desirable to force the shield through soft ground instead of excavating in front of it. They were found very useful, particularly the sliding ones, and saved a good deal of timbering. The sliding floors were another novel feature. They were made to slide on the top of the permanent floors, and were workecl by hydraulic rams underneath the floors, and therefore on the ceiling of the compartment below, from which they were workecl. These were of great use in rock, and were kept pushed out against the face and thus prevented any pieces that got loose from falling down. They also interceptecl any small piece which did become detached, and thus made it safe for the workers in the invert of the tunnel in front of the cutting edge. As, however, the surface of the rock was naturally rough, spaces would be left through which pieces might fali. To guard against this, ordinary llat-bottomed rails were fixed to slide on the top of the sliding floors. These were pushed forward singly in such a way that they took the shape of the rock, and as they were placecl close together, all spaces through which pieces of rock might fall were closed up. When the shove was made, valves in the rams were opened, which allowed the water to escape, and