Chap. XXIV.

MASONRY.

1445

jection of that point: as a series of points in succession represent lines, the boundary ofany figure may be arrived at and accurately defined in every position; by droppingperpendicular lines from the sections of an arch, the position of the several voussoirs on theplan are perceived, and their form may be traced according to their several positions, whichvary as tiie stones approach or recede from each other.

Groined Vault , on an hexagonal Plan .—The primitive centre, from whence the rest isderived, is here a semicircle composed of seven voussoirs : after drawing the six diagonalsfrom the opposite angles, and perpendiculars dropped from the arches to intersect withthem, on either of the diagonal lines may be raised others at right angles, upon whichby setting out the heights the curvature of the groin may be traced through. Thecentral stone or key is set out on the plan, and its sides are perpendicularly cut, afterwhich the mould of curvature is applied to mark out on its section the intrados andextrados : B is the central key, C represents the form of the other voussoirs.

Fig. 2549 GROINED ARCHES ON AN HEXAGONAL FLAN.

In vaults of this description the pressures are reduced to six points of support,consequently buttresses or walls of considerable depth are required in the direction of thatpressure, between which a very thin wall may enclose the building; this was the practiceof the freemasons, who did not require their external walls to have one uniform thick-ness throughout, but applied the greater portion to resist the external pressure whereit was most needed, and by this system of construction they greatly economised theirmaterial.

In the plan the position of every stone is laid down by dropping perpendiculars fromthe joints drawn in the section above, after which their dimensions can be obtained, and themould taken, by which they can be accurately cut. To ascertain the form of all the plane

4 z 3