In the year 1896, Mr. Erastus Wyman secured a patent for building subaqueous tunnels close to the river, by sinking and joining together small sections of tunnels previously built on land. Each section would have been provided with a long vertical tube for the air-lock when compressed air was to be admitted to expel the water and permit the construction of the lining within the sunken shell. Thus each section of the tunnel would have acted as a pneumatic caisson; being, however, an improvement on Professor Winkler’s suggestion inasmuch as the caisson was a portion of the tunnel itself, instead of a simple inclosure for facilitating the construction of the shield. Mr. Wyman proposed to use this method in the construction of a tunnel between South Brooklyn and Stapleton, Staten Island; a charter was granted him but the tunnel was never built. =The Tunnel under the Seine River.=--The caisson method of building tunnels under water was used at Paris, France, in the construction of the Metropolitan Railroad under the Seine River. The caissons designed by Mr. L. Chagnaud were for a double track line. They were sunk, ends to ends, and formed a portion of the tunnel lining which was enveloped by a framework of metal embedded in concrete. Built-up frames carried a shell of steel plating on the sides, from toes to springing lines, and on the sides and roof of the working-chamber. A temporary plate diaphragm closed the open ends. This construction formed a vessel capable of floating with a very light draft. The method of sinking the caissons was as follows: The caisson was erected on the river bank and when completed it was launched and towed into position between pile stagings which served the double purpose of guiding the descent at the beginning of the sinking and of forming a working platform. The caisson when launched and, consequently, before the cast-iron lining had been put in place within it, weighed 280 metric tons; but, beyond some difficulty in taking it under the bridges in the way, the towing was accomplished without serious trouble. [Illustration: FIG. 149.--Transversal Section of the Caissons for the Tunnel under the Seine River.] Previous to placing the caisson in position between the stagings, the portion of the river bed it was to rest upon had been leveled by dredging. Once in position, the first work was the erecting of the cast-iron lining segments within the framework. Work was then begun by filling the annular space between the lining and the shell with concrete; this additional weight gradually sunk the caisson to the river bottom. The working shafts, made up of steel cylinders, were placed as the sinking progressed to this point. [Illustration: ~Section A-B.~ ~Section C-D.~ ~Plan at Joint.~ FIG. 150.--Showing the Joining of the Caissons at the Pont Mirabeau Tunnel under the Seine River.]