=Excavation.=--The two parallel tunnels were driven exclusively from the ends reached by shafts; thus the tunnels were attacked at four parts. It was in these tunnels that a comparative test was made of the different methods of driving tunnels through rock. The contractor applied the heading and drift method at the southern ends of the tunnels, the eastern tunnel being driven by means of a drift while in the western tunnel the usual heading method was followed. This latter method is illustrated in the chapter following and the eastern tunnel at 33rd Street, excavated by means of a drift, is here considered. Fig. 58 shows the sequence of cuts adopted for this tunnel. It was begun by a bottom drift, about 10 ft. high, 8 ft. wide and 7 ft. deep, which was located at one side of the axis of the tunnel, as indicated in the figure. This drift was immediately widened by removing the portions marked 2. About 50 ft. in the rear the part marked 3 was taken away, thus clearing the entire lower portion of the tunnel. Section 4, about 50 ft. to the rear of section 3, was then broken down and removed. The methods of drilling and blasting were as follows: In taking out the original drift, a wedge-shaped center cut was made and then enlarged to the full size of the drift by drilling parallel holes. The succeeding sections, 2 and 3, were removed by driving parallel holes, while the top section, 4, was taken away by a center cut and parallel holes. The drills were mounted on columns, two drills to a column, and the holes were usually drilled about 7 ft. deep, starting with a diameter of 2³⁄₄ in. and ending with a diameter of 1³⁄₄ in. They were blasted with 40% dynamite in light charges, only a few holes being fired at a time, usually not more than three or four. [Illustration: FIG. 59.--Traveling Platform for the Excavation of the Upper Side of the Murray Hill Tunnel.] To remove section 4, a traveling platform 10¹⁄₂ ft. long and 25 ft. wide was used. This platform, as shown in Fig. 59, consisted of two longitudinal beams mounted on four double flanged wheels which were running on tracks laid 23 ft. apart. Resting on top of these beams were four 12 in. × 12 in. uprights braced in every direction against the framework of the platform. This frame was built of 12 in. × 12 in. beams laid longitudinally, the transverse beams being 12 in. × 14 ins. The platform proper was made of 3 in. planks, and was set 9 ft. above the tunnel floor. The columns supporting the drills for the excavation of the upper section 4, were set up above the platform which was then reinforced by other vertical props, as indicated by the dotted lines in the figure. These props, however, were placed so as to leave a clearance beneath the platform for the cars to carry away the débris from the front. During the blasting the platform was moved back so that the blasted rock fell to the floor of the tunnel, whence it was loaded into boxes on the cars.