The first plan adopted in putting in the masonry was to remove all the timbering; but owing to the large number of falls and slides this was abandoned, and the plan followed was to leave in the three roof segments of the timbering with the overlying cord-wood packing and débris. In carrying on the work the first step was to remove the side timbers. This was done by supporting the roof timbers, as shown in Fig. 159; that is, the first and fourth arch rib of an 8-ft. section containing four arch ribs were supported by temporary posts. The intermediate arch ribs were supported against the downward pressure by 6 × 6 in. timbers, extending from the side ribs near the tops of the temporary posts to the opposite sides of the intermediate roof segments, as shown in the longitudinal section, Fig. 160. To resist the pressure from the sides, 4 × 6 in. braces were placed across the tunnel from near the center of the intermediate segments to the upper ends of the hip segments, as shown in the cross-section, Fig. 159. The hip segments were then sawed off below the notch, and the side timbering removed and the masonry built. The stone was conveyed into the tunnel on flat cars, and laid by means of small derricks located on the cars. Two derricks were used, one for each side wall, and the work on both walls was carried on simultaneously. The arch was built upon a centering, the ribs of which were 5¹⁄₂ ins. less in diameter than the distance between the side walls, so as to permit the use of 2³⁄₄ ins. lagging. Each center had three ribs, made in 1-in. or 2-in. board segments, 10 ins. thick and 14 ins. deep. These ribs were mounted on frames, which followed the opposite walls, and were 4 ft. apart, making the total length of the center out to out about 9 ft. The frames, upon which the ribs were supported, are shown in Fig. 161. As will be seen, they were mounted on dollys to enable the center to be moved from one section to another. Jacks were used to raise and lower the center into its proper position. [Illustration: ~Cross Section.~ ~Longitudinal Section.~ FIG. 161.--Relining Timber-Lined Tunnel, Great Northern Ry.]