On the concrete bed of the tunnel the segments of the metal lining were placed and surrounded by concrete, as required by the plans and specifications (Fig. 145). The contractors had planned to unbolt the roof from its holdings, to remove by means of dredgers the earth which had been heaped on it, and thus set the roof afloat, after which it was to be towed within the two working platforms already erected on the Bronx shore. But Mr. McBean devised a simpler and more economic, but at the same time more dangerous, way of constructing this second section of the tunnel. He thought that the upper half of the tunnel proper could be used instead of the timber roof, thereby reducing the capacity of the working chamber, and limiting the use of compressed air. In this way he dispensed with the removal of timber, and also of the earth heaped on the roof. In building this Bronx section, a channel was dredged along the line of the tunnel to a depth of 5 ft. from the foundation-bed of the proposed tunnel. The working platforms were constructed on both sides of this channel, quite similar to those erected on the other half of the tunnel; and between them pile-bents were sunk, capped with 12-in. by 12-in. beams. Over the cap-pieces rangers were placed longitudinally, which also rested on the sides of the wooden working caisson, Fig. 146. The sheeting-piles were cut off at level, but much lower down than in the first half of the tunnel. The roof was built on floats made of 12-in. by 12-in. timber laid transversely 4 ft. apart and supporting a floor of 3-in. by 12-in. planks rendered water-tight. The sides of the floats were made by verticals, 4 ins. by 6 ins., and planks, 3 ins. by 12 ins., carefully caulked. A temporary floor was built on the base of the float, consisting of transverse beams, 16 ins. by 16 ins., placed 8 ft. apart. A center piece, 10 ins. by 16 ins., was laid so as to correspond with the axis of the tunnel; and on each side of it, other parallel beams, 16 ins. by 16 ins., corresponding to each center of the circular metal lining of the tunnel; the beams, longitudinal and transversal, were strongly bolted together. The temporary floor was completed by boarding the spaces left between the various beams. [Illustration: FIG. 146.--Showing Sides of the Caisson and Supports for the Roof.]