Engineering Wonders of the World
Volume I

THE TUBE RAILWAYS OF LONDON. 305 CONVEYOR USED FOR DELIVERING INTO SKIPS THE SPOIL CUT AWAY BY THE ROTARY DIGGER. {Photo, F. Milner.) calculated in a rough-and-ready manner by allowing half a pound per square inch for every foot of the “ head ” or column of water. Thus, to exclude 30 feet of water, an air pressure of about 15 lbs. per square inch must be maintained. We will assume that a person is about to enter the lock, in which the air stands at the same high pressure as in the part of the tun- nel beyond the wall. He sig- Passing- nals for j00r through . . J _ _ „„ be shut, and then turns a Air-lock. valve by which the air inside the lock escapes till it attains atmospheric pressure. Then he enters, closes the valve and door carefully, and turns a cock admitting compressed air from the tunnel. At this period most novices experience a curious sensation of pressure in the ears, followed by pain and bleeding at the nose and ears, which renders a retreat advisable for the time. Discomfort is reduced by sucking a sweet to cause free salivation, and by swallowing fre- quently. When the pressure inside the air-lock has risen to that in the tunnel, the inner door is pushed open easily, while the outer door is (1,408) kept tightly closed by the full pressure that is acting upon it. To come out from the workings through a lock, the procedure is reversed, the air inside being allowed to escape gradually until the outer door can be opened. Any reader who has had experience of river-locks will under- stand that the principle, on which they work is similar to the compressed air-lock in all respects, the level of the water in the one case corresponding to the air pressure in the other. Before the passenger can enter or leave a lock, the air or water conditions must be the same inside the lock and on the side from which or to which he wishes to go. Tunnelling and shaft-sinking with the help of compressed air differ somewhat. For sink- ing a shaft it generally suffices to provide as a maximum air pressure that corresponding to the maximum head of water to be dealt with, because, the base of the shaft being horizontal, the water pressure will be the same over the whole area of the base of the shaft—assuming the simple case in which the strata pierced by the shaft are uniform in character. Fig. 10.—THE ASSISTED SHIELD METHOD OF TUN- NELLING THROUGH WATER-LOGGED GROUND. The solid black lines indicate shield and tunnel lining in section.