The remarkable success of the London Tower tunnel encouraged Barlow to form in 1871 a company to tunnel the Thames between Southwark and the City, and Greathead, in 1876, to project a tunnel under the same waterway known as the North and South Woolwich Subway. Barlow’s concession was abrogated by Parliament in 1873, without any work having been done. Greathead progressed far enough with his enterprise to construct a shield and a large amount of the iron lining when the contractors abandoned the work. From the brief description of his shield given by Greathead to the London Society of Civil Engineers, it contained several important differences from the shield built by him for the London Tower tunnel, as is shown by Fig. 127. The changes which deserve particular notice are the great extension of the shield behind the diaphragm, the curved form of the diaphragm, and the use of hydraulic jacks. Greathead had also designed for this work a special crane to be used in erecting the cast-iron segments of the lining. [Illustration: FIG. 129.--Shield for City and South London Railway.] While these works had been progressing in England, Mr. Beach, an American, received a patent in the United States for a tunnel shield of the construction shown by Fig. 128, which was first tried practically in constructing a short length of tunnel under Broadway for the nearly forgotten Broadway Pneumatic Underground Railway. This shield, as is indicated by the illustration, consisted of a cylinder of wood with an iron-cutting-edge and an iron tail-ring. Extending transversely across the shield at the front end were a number of horizontal iron plates or shelves with cutting-edges, as shown clearly by the drawing. The shield was moved ahead by means of a number of hydraulic jacks supplied with power by a hand pump attached to the shield. By means of suitable valves all or any lesser number of these jacks could be operated, and by thus regulating the action of the motive power the direction of the shield could be altered at will. Work was abandoned on the Broadway tunnel in 1870. In 1871-2 Beach’s shield was used in building a short circular tunnel 8 ft. in diameter in Cincinnati, and a little later it was introduced into the Cleveland water-works tunnel 8 ft. in diameter. In this latter work, which was through a very treacherous soil, the shield gave a great deal of trouble, and was finally so flattened by the pressures that it was abandoned. The obviously defective features of this shield were its want of vertical bracing and the lack of any means of closing the front in soft soil. [Illustration: FIG. 130.--Shield for St. Clair River Tunnel.] [Illustration: ~Longitudinal Section.~ ~Cross Section.~ FIG. 131.--Shield for Blackwall Tunnel.]