1468
THEORY AND PRACTICE OF ENGINEERING.
Book II.
zontal joints, by setting off their distance from the line O B on the lines 1 5, 2 6, 8 7, and 4 8,of the plan ; thus for the joint P 2 1 of the section set off Pa on the plan from B to g,Yb from 5 to i, Pc from 6 to m, and Pdfrom 7 to o : through the points draw a curve,
which will be the projection of the joints situate on the line P 2 1.
The projection of the plan of these joints and the intersections of those represented bythe lines 15, 2 6, 3 7, 48, furnish an easy method of drawing the foreshortening of the jointson the elevation, by setting off the distances of the intersections from the line B C on theplan on the right lines, which indicate the horizontal joints of this elevation on the verticalB» C The stone in perspective exhibits one cut to suit this arrangement.
Groined Vault over a circular Plan consists of two semicircular vaults, differing in dia-meter, crossing each other; the principal, A FIDE, is comprised between two circularconcentric walls, and traversed by an irregular conical vault, I G QS, whose heights agree.
.v o
Fig. 2658.
Fig. 2659.
CIRCULAR GROINED VAULT.
If we take as a primitive centre the quadrant M S, having forits radius the least width, Q S, it results that the curvature,starting from this line, is formed by different quarter ellipses,whose greater axes increase from Q S to I G, while the lesserremain the same. The only difficulty in this kind of vault is
to form the voussoirs at the arris properly.
After having projected them on the plan, the mould is taken from them, by theassistance of which prisms are cut, having a mixtilinear base; on the faces of this solidmoulds are applied, taken from the centres on the sections. To form the hollow parts re-gularly curved rules are made use of. The development and the voussoirs are alsoindicated.
Constructions for a Circular Staircase round a newel, called Vis Saint Gilles, firstpractised at the priory of that name, situated about four leagues from Nismes, has thereputation of being the most difficult operation that could be performed by a mason; all thesurfaces of the voussoirs are irregular, and the arrises formed of double curves.