Two general types of construction are employed for the cutting-edge. The first type consists of a cast-iron or cast-steel ring, beveled to form a chisel-like cutting-edge and bolted to the ends of the forward shell plates. This construction was first employed in the shield for the London Tower tunnel, and has since been used on the City and South London, Waterloo and City, and the Clichy tunnels. The second construction consists in bracing the forward shell plates by means of right triangular brackets, whose perpendicular sides are riveted respectively to the shell plates and the diaphragm, and whose inclined sides slant backward and downward from the front edge, and carry a conical ring of plating. The shields for the St. Clair River, East River, and Blackwall tunnels show forms of this type of cutting-edge construction. A modification of the second type of construction, which consists in omitting the conical plating, was employed on some of the shields for the Clichy tunnel. This modification is generally considered to be allowable only in materials which have little stability, and which crumble down before the advance of the cutting-edge. Where the material is of a sticky or compact nature, into which the shield in advancing must actually cut, the beveled plating is necessary to insure a clean cutting action without wedging or jamming of the material. =Cellular Division.=--It is necessary in shields of large diameter to brace the shell horizontally and vertically against distortion. This bracing also serves to form stagings for the workmen, and to divide the shield into cells. The following table shows the arrangement of the vertical and transverse bracing in several representative tunnel shields. +------------------+----------+-------+-------+-------+ | NAME OF TUNNEL. | DIAMETER.| HORI- |PLATES,| VERT. | | | |ZONTAL.| DIST. |BRACES.| | | | | APART.| | +------------------+----+-----+-------+-------+-------+ | |Ft. | In. | No. | Ft. | No. | |Hudson River |19 |11 | 2 | 6.54 | 2 | |Clichy |19.4| 0 | 2 | 6.54 | None | |St. Clair River |21 | 6 | 2 | 6.98 | 3 | |Waterloo (Station)|24 |10¹⁄₂| 2 | 7.12 | None | |Blackwall |27 | 8 | 2 | 6.0 | 3 | |East River |11 | ³⁄₄| None | ... | 1 | +------------------+----+-----+-------+-------+-------+