Engineering Wonders of the World
Volume I

THE TUBE RAILWAYS OF LONDON. 233 zero marks are fixed to the tunnel lining, it can be seen at a glance, when the shield is actually advancing, whether the advance is even. If the readings on the rods do not tally, the ganger in charge of the shield cor- rects the error by manipulation of the rams. The same principle, somewhat modified, is and that of the nearer wall 4 yards, then the divisions on rod a must be to the divi- sions on rod b as 5 is to 4 ; and so long as the marks on the rods keep even relatively to the zero marks on the tunnel, the shield must be following the correct curve. Useful as the guide rods are, they do not STATION SHIELD ON WATERLOO AND CITY RAILWAY. (Photo, The Woodburytype Photographic Printing Co.') used to drive this case the Steering Round a Curve. a shield round a curve. In spaces between the division marks on the rods are respec- tively proportioned to the radii of curvature of the two sides of the tunnel. The conven- tional diagram (Fig. 4) will make my meaning clear. Suppose the radius of the outer wall of the tunnel to be 5 yards, entirely obviate the need for an independent means of checking the position of the shield. But before going further we must pay some attention to Fig. 5. tt is a line tan- gential to the centre line of the curve—that is, at right angles to the radius & , Offsets of the curve—at o. Know- ing the radius of the curve, we can find successive points P1, p2, p3, p4, etc., on the