The shield used was Barlow’s idea put into practical shape by Greathead. It consisted of an iron cylinder, or, more properly, a frustum of a cone whose circumferential sides were very slightly inclined to the axis, the idea being that the friction would be less if the front end of the shield were slightly larger than the rear end. The shell of the cone was made of ¹⁄₂-in. plates. The thinned plate cutting-edge of Barlow’s shield was replaced by Greathead with a circular ring of cast iron. Greathead also altered the construction of the diaphragm by arranging the angle stiffeners so that they ran horizontally and vertically, and by fastening the diaphragm plates to an interior cast-iron ring connected to the shell plates. This was a decided structural improvement, but it was accompanied with another modification which was quite as decided a retrogression from Barlow’s design. Greathead made the diaphragm opening rectangular and to extend very nearly from the top to the bottom of the shield, thus abandoning the element of safety provided by Barlow in case of an irruption of water. Fortunately the material penetrated by the shield for the Tower tunnel was so compact that no trouble was had from water; but the dangerous character of the construction was some years afterwards disastrously proven in driving the Yarra River tunnel at Melbourne, Australia. To drive his shield Greathead employed six 2¹⁄₂-in. screw-jacks capable of developing a total force of 60 tons. The tails of the jack bore against the completed lining, which consisted of cast-iron rings 18 ins. wide and ⁷⁄₈ in. thick, each ring being made up of a crown piece and three segments. The different segments and rings were provided with double (exterior and interior) flanges, by means of which they were bolted together. The soil behind the lining was filled with liquid cement injected through small holes by means of a hand pump. [Illustration: FIG. 127.--Shield Suggested by Greathead for the Proposed North and South Woolwich Subway.] [Illustration: FIG. 128.--Beach’s Shield Used on Broadway Pneumatic Railway Tunnel.]