Fig. 2.—The Old Railroad Ferry.
The tunnel could not have been built without this shield. The credit of its invention appears to be due to Mr. Alfred E. Beach, of New York, who designed it in 1868 for use in the construction of the tunnel under Broadway. It was subsequently used in Buffalo, Chicago, at the Hudson River Tunnel, and other places. The use of the shield in tunneling was first introduced by Sir Mark I. Brunei in 1825, and it was afterward employed by Mr. Greathead, in the Thames Tunnel and other works; but the St. Clair shield differs in some important respects from any before employed. It is a cylinder of iron, twenty-one feet and six inches in diameter and sixteen feet long, built of steel one inch thick, and with a sharp cutting edge in front. It is divided into twelve compartments by two horizontal and three vertical stays. It weighs fifty tons, and was built on the spot, the material having been prepared in the workshops at Hamilton. Against the rear end of the shield were ranged twenty-four hydraulic rams, eight inches in diameter and having a stroke of twenty-four inches. These forced the cutting edge forward into the clay, which was then excavated within the shield. By means of a Worthington pump, a pressure of five thousand pounds per square inch, or three thousand tons in all, could be exerted. The greatest pressure used was seventeen hundred pounds per square inch, or a thousand and sixty tons in all. The pressure could be exerted on any or all of the rams so as to preserve the true direction of the shield. The keeping of this direction was one of the interesting engineering feats of the work. It was done by means of a specially made London transit, set on masonry, a series of disks and cross-wires indicating the slightest deviation.
