With the foregoing brief review of the early development of the shield system of tunneling, we have arrived at a point where the methods of modern practice can be studied intelligently. In the pages which follow we shall first illustrate fully the construction of a number of shields of typical and special construction, and follow these illustrations with a general discussion of present practice in the various details of shield construction. [Illustration: ~Transverse Section.~ ~Longitudinal Section.~ FIG. 132.--Elliptical Shield for Clichy Sewer Tunnel, Paris.] [Illustration: ~Longitudinal Section.~ ~Cross Section.~ FIG. 133.--Semi-elliptical Shield for Clichy Sewer Tunnel.] Mr. Raynald Légouez, in his excellent book upon the shield system of tunneling, considers that tunnel shields may be divided into three classes structurally, according to the character of the material which they are designed to penetrate. In the first class he places shields designed to work in a stiff and comparatively stable soil, like the well-known London clay; in the second class are placed those constructed to work in soft clays and silts; and in the third class those intended for soils of an unstable granular nature. This classification will, in a general way, be kept by the writer. As a representative shield of the first class, the one designed for the City and South London Railway is illustrated in Fig. 129. The shields for the London Tower tunnel, the Waterloo and City Railway, the Glasgow District Subway, the Siphons of Clichy and Concorde in Paris, and the Glasgow Port tunnel, are of the same general design and construction. To represent shields of the second class, the St. Clair River and Blackwall shields are shown in Figs. 130 and 131. The shields for the Mersey River, the Hudson River, and the East River tunnels also belong to this class. To represent shields of the third class, the elliptical and semi-elliptical shields of the Clichy tunnel work in Paris are shown by Figs. 132 and 133. The semi-circular shield of the Boston Subway is illustrated by Fig. 134. [Illustration: ~Half Transverse Section A-B.~ ~Half Rear-End Elevation.~ ~Details of Casting Supporting Ends of Jacks.~ ~Details of Castings under Ends of Girders.~ ~Longitudinal Section C-D.~ FIG. 134.--Roof Shield for Boston Subway.]