. Cyclopedia of architecture, carpentry, and building; a general reference work ... n in plan at ^ C andB D. It is easy to find theplan of these ribs because theymust pass from corner to cornerdiagonally. To find the eleva-tion we must use the same prin-ciple that was employed in finding the position of the valley rafterand the shape of the curved hip rafter for an ogee roof, namely,any line drawn in the roof or ceiling surface parallel to the plateor side walls must be horizontal, and all points in it must be atthe same elevation. We start with the assumption that one of the vaults is semicir
. Cyclopedia of architecture, carpentry, and building; a general reference work ... n in plan at ^ C andB D. It is easy to find theplan of these ribs because theymust pass from corner to cornerdiagonally. To find the eleva-tion we must use the same prin-ciple that was employed in finding the position of the valley rafterand the shape of the curved hip rafter for an ogee roof, namely,any line drawn in the roof or ceiling surface parallel to the plateor side walls must be horizontal, and all points in it must be atthe same elevation. We start with the assumption that one of the vaults is semicir-cular, as shown in elevation at A, Fig. 267. Taking any line in thevaulted surface, shown in plan, as the line S P 0,we produce it untilit intersects the plan of the diagonal rib AC at the point 0. Thispoint must be the plan of one point in the line of intersection of thevaulted surfaces. The elevation of the point 0 above the springing line of thevaults is shown by the distance P S, since the line *S P 0 is exactlyhorizontal throughout. This distance is laid off at EF with the. Fig. 267. Diagram Showing Method of Finding Diagonal Rib for Two Intersecting Vaults 201 190 CARPENTRY line G E H representing the horizontal plane which contains thespringing Hnes of the vaults. The point H is the point from whichthe diagonal rib starts. The point F, as we have seen, is anotherpoint in the curve, and we can by a similar process locate as manypoints as we need. This will enable us to draw the complete curveGFH of the line in which the vaults intersect, and to which thediagonal rib must conform. By continuing the line from the point 0 at right angles to itsformer direction and parallel to the wall line, we may obtain thepoint K, which is a plan of one point in the surface of the ellipticalvault. The elevation of this point also above the springing linesmust be the same as for the point S and may be laid off, as shown atK M. By finding other points in a similar way the curve N M R oithe
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