A circle-on-circle arch is false construction, and should only be adopted in exceptional cases. When it is a case of necessity, a semicircular arch of 3 ft. span may be turned in a 7 1/2n. stone wall curved to a radius of lit. 2in. A template A B C D (Fig. 1) should be set out on the plan for the base of the centre, and the outside B C being a semicircle the framing may be set out as in Fig. 2, being flat on face like an ordinary centre; allowance is made for the thickness of the laggings, which are 3ft. over all. The back A D will be the same height (18in.), but only 2ft. (3A in. wide, and therefore elliptical, as in Fig. 3. The laggings, when nailed on, will form a Hewing surface upon which the voussoirs or bricks may be laid. The supports for the centre would be the usual ones, the overhang not being sufficient to necessitate any exceptional course being adopted.

Centre for Circle-on-circle Arch. An Easily Made Kitchen Table.