Chap. XXIV

MASONRY.

1451

lengthened out forms the ellipsis on the plan. The voussoirs of the angle, instead of beingcomprised in parallelepipeds with a square base, are contained in a prism with a pentagonalbase ; the key is a portion of a truncated octagonal pyramid, whose bases are formed by anassemblage of triangular portions of curved surfaces, concave below and convex above.

The Arc de Cloitre on an irregular Plan has a less solid form than others, and oughtnever to be used; when adopted, the operation of constructing it is tedious, but notdifficult. The rows of voussoirs should be always parallel to the walls, so that thekey has the same form as the plan; the projections on the plan give for regular vaults thebases of the prisms in which the voussoirs are comprised; the sections perpendicular to eachside, through the centre, give the profile of the moulds at the several joints; the only dif-ference is, that in regular vaults it is sufficient to draw out a single side, the others beingsimilar, while in irregular we must draw each, as they are all different in form, and asthe distance from the centre of the key to the walls is unequal, different curves result,

but they are only prolongations ot that whichhas the least radius, considering it as the pri-mitive centre.

Fig. 2576.

Fig. 2579. ARC DE CLOITKE WITH AN OCTANGULAR PLAN. Fig. 2580. ARC DB CLOITRB ON AN OBLONG PLAN.

Faults of Arc de Cloitre., on an Oblong Plan. — On the section the form of the severalstones L, M, N, O, D is shown, and the plan of their intrados below; K is the secondstone at the angle, in perspective. In this example the re-entering angles correspondto the diagonals of the rectangular plan, which makes the centre over the lesser sideof greater length, when drawn out, than that over the greater, which docs not producea good effect; therefore it is preferable, when an oblong square is to be vaulted, to makethe middle portion semicircular, and the angles 45°, which gives an equal curvature to allthe sides.