Chap. XXIV.

MASONRY.

1449

N D from p to s, and by raising through this last point a parallel pk , on which is set offthe height C G, taken on the section from s to m : and having drawn the chord km, raiseon its centre a perpendicular cutting the base E F in a point, which is found as p ; it willbe the centre of the arc mk> answering to D N.

To facilitate the drawing of ribs of this kind of vault, the diagram may be referred to,where their various forms are divided into voussoirs. 'The keys being the parts whichrequire the most care should be set

out with reference to all their projec-tions.

Hanging Keys .—The last voussoirof arches of this kind, having theirabutments against the principal key,form together one independent ofthat which constitutes the centre; thishas given the idea of suspendingkeys, as well as those hollowed out,found in many vaults of the middleages, which have excited astonishmentin all who have beheld them.

The filling in between the ribsforms surfaces of double curvature,which would be very difficult toexecute, if not sufficiently small to besupported by plaster or mortar, andto follow the curvature of the arcswithout its being necessary to cutthem expressly.

Gothic Vaults , formed of fourquadrants of an inverted concaveparabolic conoid with a central key,are the perfection of the mason’s art,and we have examples of themof great beauty in buildings con-structed during the fifteenth centuryin England. If a cone of this formbe constructed upon its broad base,with rings of voussoirs, in the samemanner as for adome, their ver-tical joints di-verging to a ver-tical line drawnthrough its cen-tre, it will standin a similar man-ner ; when theconoid is so con-structed, if it bestood upon itsapex, and thebase preventedfrom spreading,the voussoirs willretain their po-sition. We mayimagine such afigure cut intofour parts, eachhaving one flatside against thewall of the build-ing and the open-ing formed by thefour quadrants ofouter voussoirs,which consti-tuted its originalbase, filled in with

Fig. 2563.

BIBS OF GOTHIO VAULT8

lg.

1 12 .

Fig. 2560.

KEYSTONE AND ITS MOULDINGS.