The semi-elliptic arch, sxiitable for masonry construction, shown in the accompanying illustration is set out in the following manner. First draw the span AB and the rise CD, and draw AX and D X parallel to CD and CA respectively. Divide A X and AC each into three equal parts, make CZ equal CD, and draw lines through the points, as shown, intersecting each other at Y and X. Make the angle D X H equal to the angle X D C, and produce D C to meet N H in H: then H will be the first centre. Join YX and bisect it, cutting X H in M, which will be the second centre from which the curve Y X is struck. The curve A Y is obtained in a similar manner, and the other side of the arch, being symmetrical, is easily found.

Setting Out Semi-elliptic Arch.