1466

THEORY AND PRACTICE OF ENGINEERING.

Book II.

as before explained for the spherical vaults, forming first the parts of a cylinder in whichthey are comprised, in order to trace them by means of moulds of the curves taken on thesection and plan ; the voussoirs, where the open parts or lunettes intersect with the other,should be obtained out of portions of cylinders, comprising their greatest height, length, andwidth : besides the moulds of the section and plan we must have others of the intrados andextrados, made of flexible material; this method would suit every variety of conoidalvault, whatever be the curve of its centre, hyperbolic, parabolic, or any other, and evenwhatever the form of base, circular, oval, or elliptic.

Pendentites that support domes are portions either of spherical or spheroidal vaults,resulting from the section of several parts of these vaults by vertical and horizontalplanes : they are made use of in gathering over a square or polygonal figure into thatof a circle or ellipsis. The domes at the intersection of the naves and transepts of a churchare so constructed.

o

Sim

Fig. 2G51.

PENDENTITES,

When the plan upon which the dome is to be placed is square, the feces of the fourgreat arches that are to support it may be considered as four vertical planes cutting aspherical dome into as many segments, and the base of the cupola usually constructed onit, as a fifth horizontal plane which cuts off the upper portion of the sphere, so that fourspherical triangles alone remain; but as the effect of these resting on a point is unpleasantto the eye, various arrangements have been made to overcome this defect. The faces ofthe pendentives are not always portions of spherical vaults, but irregular surfaces, theplan and elevation of which are shown.

Where rows of horizontal voussoirs are introduced, their execution and arrangement willbe found extremely simple; the projection of the rows is shown on the plan by lines ofconcentric circles. To obtain these curves suppose between G and C as many verticalarcs as there are points of each curve; to unite them on the section, so that theyhave a common point B, set off AG, abcd> B C, projections on the plan of these aresfrom O to C, db G, and draw the chords B G, Bd, B b t Be, on the middle of which raiseperpendiculars cutting the horizontal line BI in 1,2, and 3, which will be the centres ofeach of these arcs.