The remaining sections of the tunnel roof were built in the same way, until the last abutted against the part of the work constructed within the caisson under the high wooden roof on the Manhattan side of the river. The following method was adopted for the purpose of connecting the few parts of the tunnel which had been differently constructed. The diaphragm at the end of the last section of the tunnel roof was constructed so as to abut against the last circumferential flanges of the iron lining without leaving a projecting ring. It was continued above the metal and concrete lining of the roof in a rectangular form, and of the same height and width as the wooden bulkhead of the working-chamber on the Manhattan side of the river. The diaphragm was made of riveted plates and angles, with an opening 20 ins. by 30 ins., bolted so as to be removable at will. The diaphragm was of the same height as the roof and was connected with a roof-plate to the rangers supporting the thick wooden roof. Other steel plates, placed vertically, were riveted to the diaphragm and bolted to the caisson. All this work was carried on by divers. The wooden bulkhead was cut to the springing-line of the arch; and between the two parts of the tunnel, built by different methods, a bulkhead was placed, made of steel plates 14 ins. long, which prevented the entrance of water into the working-chamber. [Illustration: FIG. 148.--Showing the Tunnel Completed by Building the Lower Half within the Caisson.] When the different sections were joined together, and all the openings closed and made water-tight, cement-grout was poured on the roof, and earth was heaped up to a height of 5 ft. The 300 ft. of the roof, resting on sheeting-piles and provided with diaphragms at the extreme ends, formed a water-tight working-chamber, or caisson, communicating with the exterior by means of the shafts and air-locks. The lower portion of the tunnel was built under air-pressure. The pile-bents were first cut off at the plane of the tunnel sub-grade, after which the foundation-bed of concrete was laid. The lower segments of the iron lining were then placed in position, and the structure made continuous by building up the lateral walls, consisting of concrete (Fig. 148). No accidents occurred while building the second part of the tunnel. The Harlem River tunnel was completed in contract time, although the opening of the subway was delayed by difficulties encountered in tunneling through rock in the borough of the Bronx. The writer endeavored to obtain information regarding the expense per linear foot, but all his efforts were rewarded with a general assurance that it proved to be the cheapest method. SINKING AND JOINING TOGETHER SECTIONS OF TUNNELS BUILT ON LAND. THE SEINE. THE DETROIT RIVER TUNNELS.