Chap. XXIV.

MASONRY.

1453

judicious extradossing, vaults of this kind may be made almost to balance themselves, andto exert but little thrust, certainly not more than can be overcome bv a weight put uponthe external walls, or that deposited by the timber roof, which usually covers the entirebuilding.

Conical Faults have their interior surfacesparts of cones ; the most simple are thoseconstructed on two walls, forming an angle,so that the centre of the face represents thebase of the cone.

All the joints tend to the apex of thecone, diminishing in breadth; but ifcontinued throughout, they would be-come too weak, therefore a single stoneis adopted, called a trompillion , A. To formthe voussoirs we must first set out theirhorizontal projections, their vertical andtheir profile or section, which are shown inthe elevation, plan, and the developmentof the diagram.

To transfer the moulds of the intrados,joints, and beds, with the bevels of theangles formed by the union of the sur-faces to which these moulds are to beapplied ; the workman should take care toselect the greatest face as a base from whichto set out the others; thus in trompes hecommences with the intrados of the vous-soirs, and dresses a face preparatory toapplying the mould.

In the example given the develop-ment is a semi-cone or sector of a circle,whose radius is equal to BC on the plan,and whose arc is equal to the developmentof the semicircle which forms the face ;whence it results that each intrados is re-presented by a small sector or isoscelestriangle, whose base is formed by the arc,or by the corresponding chord of eachvoussoir, and by two radii or sides equal toCB.

If from the entire development, or fromthat of each intrados, we cut off the partanswering to the trompillion, the surpluswill be the entire intrados, or each partof the intrados; thus the intrados of thetrompe is expressed in the development byC, a,fc,c,d, E, n 43 2 10, for one half, theother being perfectly similar.

The moulds for the joints are supposedto be laid flat on the development of theintrados, and attached to the side to whichthey have reference; thus the mould (fig.

2591.) 5,6,7,4, d h is that indicated by theline hd4, shown on the elevation, and gc3 ,

10, 9 8, answers to the line g c 3 : fb 2, 11,has reference to fb 2 on the elevation, andea 1 v to e,a, 1. This result being formedof a right cone, hollow', and of equal thick-ness, the angles of all the joints taken per-pendicularly to the intrados are all equal.

To find these angles, observe that thevoussoirs are supposed to be divided on

a circle, whose radius is perpendicular to the inclination of the inner surface of thecone, and drawn from a centre placed on the axis; by a reference to the figures andletters on the elevation, plan, and development, the whole setting out may be understood,difficult as it may appear.

V ig. 2SS8.

Fig.2589

PLAN.

Fig. 2550.

development

Fig. 2591.

CONICAL VAULTS.