Compressed Air Work And Diving 1909

70 COMFRESSED air worx. overflow. It woulcl thus present a horizontal face to the air pressure, and if this pressure were equal to the head of water at the centre of the tunnel, no water coulcl enter. At this particular tunnel, however, the pressure was kept very much lower than that due to the head of water, and to meet this difficulty the trap was fitted with a lid, which could be closecl down if any sign was noticed of the water coming in. It is difficult to keep the pressure sufficiently high to make a water trap quite efficient, but to have the front part of the shield closecl half-way clown is thoroughly souncl, as even if the air pressure is not high enough to balance the water, the latter as it enters will compress the air as it rises, and, provicled the tunnel is 011 the level, there will always be enouo-h for the workers to breathe, and to enable them to escape through the emergency lock. The tunnel will, in faet, from the bottom of the closed-in portion of the shield back to the bulkhead, be converted into a huge diving bell. If the tunnel has a gradient, it may be necessary to introduce hanging screens, with an emer- gency lock in each, at frequent intervals, more particu- larly if the gradient is rising towards the face. A water trap will, however, be best in a tunnel of small diameter in which there is not sufficient room for an emergency lock in the tipper part of the bulkhead, since the water, when it overflows the lower diaphragm, will stop the escape of air, and the compression of air will start at once instead of only when the lower edge of the screen is touched. But that ampie security is given by a hanging screen, or half closed-in shield, in a tunnel of large diameter, will be seen by a glance at Table XIII. The emergency lock should be always fixed as high up as possible for this reason, and at its level there should be a gangway supported on light brackets fixed to the