Engineering Wonders of the World
Volume I

234 ENGINEERING WONDERS OF THE WORLD. Fig. 4.—DIAGRAM TO SHOW HOW A SHIELD IS GUIDED ROUND A CURVE BY MEANS OF PROPORTIONALLY GRADUATED RODS. curve to the right of o by making “ off- sets,” s1, s2, s3, s4, etc., of calculated length, at right angles to the tangent, at points b1, b2, B’, B4, etc. The engineer has to calculate Fig. 5.—DIAGRAM ILLUSTRATING HOW THE POSITION OF A SHIELD IS CHECKED BY “ OFFSETS ” S1, S2, S3, ETC., FROM A TANGENT LINE. the respective lengths of these offsets and make a table of them for the ganger in charge ; also to provide plumb-lines at points a, a1 on the tangent for sighting the line. When the shield has been driven, say, to the posi- tion shown in Fig. 5, the ganger measures the distance from 0 to the centre of the shield, marked on a piece of wood called a “ fiddle.” Looking at his table of offsets, he finds that the offset from the tangent to the centre should be of such and such a length. He then gets the wires at a, a1 in line, and an assistant makes a mark on the “ fiddle,” and measures from that mark to the centre of the shield. If the measurement equals the theoretical offset, the shield is travelling cor- rectly ; if not, the difference indicates how far the shield is out of line, and informs the ganger what must be done to put it right again. It is obvious from Fig. 5 that a tangent line must eventually meet the side of the tunnel, and that offsetting from it cannot be con- tinued indefinitely. As soon, therefore, as the need arises, Setting1 the engineer sets out another Tangents tangent from which offsetting may be continued. In Fig 6, ao b represents the first tangent, and s the last offset made Fig. 6.—ILLUSTRATING THE METHOD OE OBTAINING A NEW TANGENT LINE. from it. The engineer has to line out a new tangent at n. At n is built a stage for the theodolite—one already exists at o, the old tangent point. When the distance from o to n has been measured with a tape, the point N can be definitely fixed, as calculations show what the size of the angle a o n must be to allow both o and n, the ends of a chord of a circle of known radius, to be in the circum- ference. The theodolite is moved to n, and an angle, ond, is turned off equal to bon, the complementary angle to AON. Points, d, d, are fixed with plumb-lines, and the line joining them gives, when produced, the new tangent. Offsetting is then continued from the new zero point, n, and further tangents are established in the same way as required. So far lateral guidance only has been con- sidered. To keep the shield on the level or on a gradient, its movements are checked by a plumb-line hanging from a support fixed to the top of Vert*cal the shield. A two-foot rule held square to the shield near the bob shows whether the shield is vertical, or to what