Chap XXIV.

MASONRY.

1457

Trompe Circular, on plan erected on a straight wall. — This may be taken as an example ofvoussoirs sustaining a round tower. The utmost projection which may be given to thepart of the tower it sustains must not be greater than two-thirds the radius of itsouter circumference, andthe centre of the voussoir Efmust have a greater heightthan the projection.

To find the curve ofthe voussoir or vault, theprimitive centre is shownon the elevation, as thebase of a horizontal halfcylinder, cutting anotherof greater diameter placedvertically, and whichserves to represent thetower ; the curve formedby the intersection ofthese two cylinders is thearris of the voussoir.

This curve being sym-metrical and double, ifright lines are drawn,from all the points ofone half to the other,they will indicate thesurface of the voussoirwhich sustains the tower;we shall have the profileof this voussoir by draw-ing from the divisionpoints a , 6 , c, of the primi-tive centre, lines parallelto the vertical on theplan, and other horizontala a, bb, cc, on the sec-tion. The distances areset off at 1 c, 26, 3a, onthe plan at Cl, C2, C"3on the section; throughthese points raise per-pendiculars, which willcut the parallels drawnfrom the elevation in the

Fig. 259?

Fig. 2598.

2600.

Fig. 2602.

Fig 2603.

CIRCULAR ON A STRAIGHT WALL.

points a, 6, c. These points of intersection will indicate the curve of the section of thevoussoir, and more exactly as the division points are multiplied.

To draw the voussoirs which form the trompe, begin by cutting on each its largest face ;then for the second voussoir, which we suppose to form the thickness of the wall, get outthe square mass 1230 in the elevation, and 4567 on the plan. Take the mould mionkp ,expressing the contour which it will form on the surface of the wall, indicated on the planby G H, and the mould 486, B 95, of its projection on the plan; having cut the twostraight surfaces at a right angle, apply the mould mionkp to one of them, and 4 8 6, B 9 5to the others.

After having cut the joints mi and nk , apply the mould kefibe to trace their contour :according to which and that drawn on the upper bed to, the stone is cut to form the convexsurface of the part corresponding to the tower, which should be square with the bed above.The surface mb nc is formed with a curve taken from the section, placed always vertically,as the lines bs, tv, c x indicate: the curved joint mn is hollowed perpendicularly to theinner face of the wall; this voussoir is shown in perspective.

Skewed and inclined Arch in an inclined Wall . — The interior of a round window orbull's-eye presents the surface of an oblique cone with a circular base ; the joints of the in-trados of the voussoirs with which it is formed all tend to the summit of the cone, and thejoints of the face to the axis; from the double obliquity of this cone, the vertical facealone, and the two horizontal beds, E F, G H, give the real size of the moulds andthe joints; the development of the intrados is performed in a similar manner to that ofoblique cones.

To trace the voussoirs, for example, of the bull’s-eye or the key K, we must apply the

5 A