=Lining.=--The tunnels were lined with cast-iron circular rings of the segmental bolted type. In some special cases, cast steel was used instead of cast iron. The rings were made 30 ins. long, with an internal diameter of 21 ft. 2 ins. and an external one of 23 ft. The rings were composed of nine equal segments of 77¹⁄₂ ins. external circumferential length each, except the two segments adjoining the key which were equal to the other segments with the difference, that one end joint was not radial but formed so as to make an opening 12.25 ins. wide at the outside and 12.60 ins. at the inside, which was closed by the key segment. Each segment had six bolts in the circumferential joint, the key had one, so that there were 67 bolts in one circumferential joint. Each of the twelve longitudinal or radial joints had five bolts, in all 127 bolts per ring. The circumferential flanges of each plate were strengthened by two transverse webs or feathers on each flange. Each segment was provided with a 1¹⁄₂ in. grout hole closed with a screw plug. In order to pass around curves, whether horizontal or vertical, or to correct deviation from the line or grade, tapering was used; by this is meant the placing of rings in the tunnels which were wider than the standard rings, either at one side (horizontal tapers or liners), or at the top (depressors), or at the bottom (elevators). Tapers ¹⁄₂, ³⁄₄ or even 1 in. were used. The taper rings were made by casting a ring with one circumferential flange much thicker than usual and then machining it off to the taper.