EXPANDED STEEL the Williams system small flat bars are used, which are not quite horizontal, but pass alternately over and under the rolled joists which support the slabs. A concrete arch is reinforced in much the same way as a wall, the stresses being somewhat similar. The reinforcing rods are generally laid both longitudinally and circumferentially. In the case of a culvert the circumferential rods are sometimes laid continuously in the form of a spiral as in the Bordenave system. To those wishing to pursue the subject further, the following books among others may be suggested :—Sabin, Cement and Concrete (New York); Taylor and Thompson, Concrete, Plain and Reinforced (London) ; Sutcliffe, Concrete, Nature and Uses (London) ; Marsh and Dunn, Reinforced Concrete (London) ; Twelvetrees, Concrete Steel (London) ; Paul Christophe, Le Beloit arme (Paris) ; Buel and Hill, Reinforced Concrete Construction (London). (F. E. W.-S.)