540

tical, cycloide. pointed, comp A rampant unequal heig When the is said to be

There are. for example, those less t nished arch

Arches a

than a semic A vault

overtopp

an apartinen lars, plac Hence

and ever not be t

A gro tion of dos of An ar spring In

If a cylinder intersect a sphere of greater diameter than the cylinder, the arch is called a sphero-cylindric arch; but on the other hand, if a sphere pierce a cylinder of greater diameter than the sphere, the arch is called a cylindro-spheric arch,

If a cylinder pierce a cone, so as to make a complete perforation through the cone, two complete arches will be formed, called cono-cylindric arches; but, on the contrary, if a cone pierce a cylinder, so that the concavity made by the cone is a conic surface, the arch is called cylindro-conic arch.

If, in a straight wall, there be a cylindric aperture continuing quite through it, two arches will be formed, called plano-cylindric arches.

Every description of arch is, in a similar manner to the above, denoted by the two preceding words; the former ending in o, signifying the principal vault, or surface cut through; and the latter in ic, signifying the description of the aperture which pierces or intersects the wall or vault.

When groins are introduced merely for use, they may be built either of brick or stone; but, when introduced by way of proportion or decoration, their beauty will depend on the generating figures of the sides, the regularity of the surface, and the acuteness of the angles, which should not be obtunded. In the best buildings, when durability and legance are equally required, they may be constructed of wrought stone; and, when elegance is wanted, at a trifling expense, of plaster, supported by timber ribs.

In stone-cutting, a narrow surface formed by a point or chisel, on the surface of a stone, so as to coincide with a traight edge, is called a draught.

The formation of stone arches has always been considered a most useful and important acquisition to the operative mason; in order, therefore, to remove any difficulties which might arise in the construction of arches of different descriptions, both in straight ana circular walls, we shall here introduce a few examples, which, it is hoped, with careful examination, will greatly facilitate a knowledge of some of the most abstruse parts of the art.

Fig. 551, No. 1. To find the moulds necessary for the construction of a semicircular arch, cutting a straight wall obliquely.

W be teftud of most using spherical domes.
DLA E DEL In the ouers surate to the vertical joints ;
in the same stars to the other joints; and in both

Ja te zde re if the ime.

Lii and are nouits wilth apply on the convex surface to
to bra
war de ius c i, o d, eƒ, &c. being at right angles to
te Chor radi, in so fa, &, and produced until they intersect
endruar er; the dferent intersections are the centres which
gre de criar jeg of the mould, and the straight part gives the hori-

Faas excute the plan of a groined vault.

Lay down the arch, either at the full or half size, on a floor or piece of for, then divide and draw on the plan the number of joints in the semicircular arch, and from the intersections with the diagonals, draw the Transverse joints on the plan, and produce them till they touch the inTracius of the elliptical arch, the curve of which may be found by setting te corresponding distances from the line of the base to the curve; thus * equal to a b. This being accomplished, draw the joints of the ellipti