Chap. XXIV.
1463
the corresponding divisions marked with the same letters. The divisions on the frontcentres are determined by prolonging the joints until they meet the vertical planes ofthe projecting angle; the curved arrises which they form with the soffites are, on accountof the property of the sphere, quarters of a circle of which ABODE FGII in theelevation only indicate that the ellipsis formed with the ordinates Bl, C2, D3, K4, is ofa quarter of a circle, of which A K is radius. One of the two faces developed is shown atfig. 2635., with its divisions of joints to serve as moulds. Fig. 2636. shows the section
Fig. 2635.
Fig. 2632.
Fig. 2633.
taken on the common axis, RIK, of the projection in plan and elevation. As this nicheis supposed to be of such dimensions that the voussoirs cannot be made in one singlestone, the profile of the joint which divides the key into two parts, and that of the trom-pillion, is indicated in the part shaded ; the lines formed by the joints of the other voussoirson the concave surface are also indicated in the plan and elevation, and in the part ofthe profile which is not shaded. The upper part of the key, the profile of which infig. 2634. is represented by fig. 2637.: to form this part of the voussoir, begin with the upperhorizontal bed, which is the^ largest face, indicated in fig. 2636. by K, L, x, y> P ; afterhaving applied a mould with the same form to trace its contour, cut the two faces L R D Kand RKEP square with the upper part; apply to each the mould RLKD,taken on the fig. 2633. on the return, in order that the side R K should fall on thearris KR for the two sides. These moulds, with the angular bevel RLD, will formthe surfaces, on which should be applied the moulds of the joints, which will give thecurves and arrises of the soffites: according to these curves, and those of the faces D Kand KE, formed by tracing with the aid of points on K 16 of the profile, the curvedsurface of this soffite may be set out, on which having traced the line 1112 terminate thispart of the voussoir by forming the back section with a bevel, producing the angleK 16 15 on fig. 2634.
Fig. 2632. is another portion of the voussoir, uniting with the level courses; this maybe traced, like the preceding, by commencing with the upper horizontal bed, for whichapply the mould to the plan, make the large joint on the side of the key by meansof bevels taken on the elevation; then the facing, and the part of the joint forming a rebate,which should be square with the upper bed; having prepared the face of the otherjoint, trace with the moulds the straight and curved arrises which are to terminate them;for the soffite, use points taken on the profile, as for the preceding voussoir.
Spheroidal Vaults upon a circular and elliptical Plan. — Fig. 2640 . shows one quarter of aspheroidal vault on a circular plan, the half of which is an elevation or half ellipse.Fig. 2639. is divided into three courses of voussoirs to the key; the first, the section of
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