The accompanying diagram represents an easy way of striking out an elliptic arch. First draw the span A 13 and the rise C D, then the parallel line AE the same length as the rise. Divide the rise CD into three equal parts, of which two-thirds is the radius at A F G to strike the shoulder of the arch. Then bisect EA, and from the point H obtained draw line to C; then square off centre of F C to intersect at J; then with J as centre and J F as radius describe the crown of arch from F to C.

Striking Out an Elliptic Arch.