1464

THEORY AND PRACTICE OF ENGINEERING.

Book II

■which 13 expressed by E,rfaAG. Thecurve of the extrados gives at the centreof the key a thickness equal to the

Fig. 2G39.

/

Fig. 2638.

half of what it is where detached fromthe wall. Fig. 2638. shows in perspectiveone of the voussoirs of the second course,developed in a portion of the hollowcylinder, the base of which is taken on theplan, where it is marked by the lettersh,i,k,m , comprehending the projection ofthe lower section : on the straight jointsof this species of prism apply the moulddale taken on the section, fig. 2639., whichmust be turned to trace the other side.

Spheroidal Vault on an oval or ellipticalPlan , the inner surface of which isproduced by the ellipsis or oval ofthe plan,which turns round its greatest axis orgreatest diameter, so that all its verticalsections made in the direction of its length,parallel to the lesser axis, will be semi- lg '

circles spheroids on a circular plan.

The primitive centre for dividing thevoussoirs is the qua-drant of a circle,whose radius A C isequal to half the lesseraxis of the ellipsis onthe plan. The rowsof voussoirs beinghorizontal, the curveof the greater axisbeing more elongatedthan that of the lesser,the intradosses arenot of equal breadth,but increase from thelesser to the greateraxis. The projectionon the plan of thehorizontal joints formsimilar ellipses, thatis to say, their twoaxes preserve thesame proportion, butthey are not equi-distant.

The execution ofthis kind of vault isattended with manydifficulties, and re-quires more opera-tions than that forspherical vaults witha circular base, be-cause, by reason of

2641.

SPHEROID ON AN ELLIPTICAL PLAN.

Fig. 2043.