After the caissons had been sunk to the required place and in continuation of one another, a space of nearly 5 ft. was left between them. The construction of the tunnel within the bank of earth separating the two caissons was as follows: A cofferdam was built around this space. It was formed by two diaphragms closing the ends of the tunnel, and by two longitudinal walls sunk as temporary caissons, one on each side of the tunnel and inclosing their ends. This cofferdam was covered with a metal working-chamber whose lower edges rested on top of the four walls of the cofferdam. The joints were made tight by means of rubber or packed clay. The water in the cofferdam was then pumped out, the earth excavated, and the masonry built in continuation of the two ends of the tunnel sections. The submerged sections of the tunnel which were allowed to remain full of water to render them more stable and to save effort in pumping them, were now made dry; the diaphragms were removed from the ends of the caisson tunnels and the work made continuous. Fig. 149 shows the cross-section of the caissons. At the Pont Mirabeau crossing of the Seine, a slightly different method was used, described in “Eng. News,” May 18, 1911. The caissons were sunk to the required line and grade with an intervening longitudinal space of 15³⁄₄ ins. between two adjoining caissons. At each end of this space, which was filled with the river marl, was sunk against the edges of the caissons a hollow cylinder 20 ins. outside diameter. The interior of these cylinders was excavated and filled with concrete, thus forming a continuous wall on both sides of the two adjoining caissons. The earth from the intervening space was then removed and concrete deposited from bottom opening tremies up to the level of the top of the caisson. After nearly one month the tunnel was entered from the shaft and an opening the shape and size of the tunnel section cut through the diaphragms of the 15³⁄₄-in. wall and the concrete tunnel lining made continuous between the two sections. Fig. 150 shows the method of joining the caissons. =The Detroit River Tunnel.=[15]--With some modifications which permitted dispensing with compressed air, the tunnel under the Detroit River was built for the Michigan Central Railroad, connecting Detroit with Windsor, Canada. The tunnel is 6625 ft. long; of this, however, only 2625 ft. are under the river, while the approach on the American side is 2000 ft. long and that on the Canadian side, 4000 ft. The tunnel consists of two parallel circular tubes 23 ft. in diameter, built up of ³⁄₈-in. steel plate. They are placed 26 ft. apart, center to center, and are connected by diaphragms at 12-foot intervals. [15] Condensed from a paper by B. H. Ryder.