The longitudinal sides of the caisson were constructed by sinking two rows of sheeting piles, each row being close to a service platform. The sheeting piles were made up of yellow-pine timbers 12 ins. by 12 ins.; three piles bolted together formed a section 3 ft. wide. Each section was grooved and tongued, so as to be firmly connected with the adjacent sections to be sunk. The lower ends of the piles were cut wedge-shaped, with a sharp edge to offer a small resistance while penetrating the soil. The sheeting-piles were then cut off under water, which operation was successfully carried out by means of a circular saw operated by a pile-driving machine. The sheeting was also extended between two platforms to make a bulkhead, and in this way the four sides of the caisson were built up. Particular attention was always given to the alignment of the sheeting piles, which was obtained by guiding the piles with the timbers placed longitudinally, one below the water-line in connection with the frames located between the pile-bents, and the second along the inner edge of the service platform, as shown in Fig. 143. [Illustration: FIG. 143.--Showing Sheeting-Piles for the Sides of the Caisson and Trussed Beam for the Roof.] The caisson was completed by placing a roof covering the sides. This roof was 40 ins. thick, made up of three layers of 12-in. beams placed transversely to the axis of the caisson, while between the beams planks 2 ins. thick were placed lengthwise and bolted together, so as to make a firm, solid structure. The roof was built ashore, in sections each varying from 39 ft. to 130 ft. long. The edges of the roof fitted the sides of the caisson perfectly; and when each section was towed to the proper spot, it was sunk and made secure. Under the roof were placed six longitudinal beams, 12 ins. by 14 ins., called “rangers,” resting on the cap-pieces of the pile-bents that were laid across the space of the proposed tunnel; while the extreme rangers were used for the purpose of fitting above the sheeting-piles of the caisson, in order to make the latter water-tight. The two extreme rangers were provided with =T=-irons, the flat side being laid on the sheeting-piles, while the web penetrated the ranger by reason of the weight of the load resting on the roof, for the purpose of sinking it to the required point. Earth was next heaped on the roof, and in this way a large working-chamber was prepared, as shown in Fig. 144. [Illustration: FIG. 144.--Showing the Caisson with the Working-Chamber.]