ARCH is the general name for any solid work, whether of masonry or otherwise, of which the lower part is formed into an arc of a curse supported at the two extremities.
The lower supports are the piers of the arch; the arch is said to spring from the tops of those piers ; the summit of the arch is the crown, and on either side of the crown are the flanks of the arch. The lower line of the arch stones is called the intrados or soffit, the upper, the extrados or buck; the arch stones are Balled voussoirs, and the highest voussoir the key-stone. The width from pier to pier is the span ; and the distance from the crown to the level of the top of the piers is the 'height. The voussoirs are cemented together; and if the cement were sufficiently strong, any form might be given to the arch, or at least any form which would stand if cut out of the solid material. If we suppose the stones unce mented, their friction upon one another would tend to prevent the disturbance of equilib rium, and allow considerable variety of form in arches constructed with stones of the same weight. But if we suppose the stones per fectly smooth, so that each of them is kept from slipping only by the pressure of the ad joining two, then each intrados has one parti cular form of extrados and one only, so long as the manner in which the stones are cut follows one given law.
The applications of the arch, in practice, arc noticed under ABUTMENT; BRIDGE; BUT TRESS; IMPOST; PIER, SZE.
Sir M. I. Brunel introduced, a few years ago, a singular mode of constructing arches. In the construction of stone arches, some kind of centering is necessary, to support the single stones until by mutual pressure they can support each other ; but Brunel devised the following mode in respect to brick arches. The piers of the bridge or arch having been constructed in the usual manner up to the springing, he commenced building a portion of the arch right and left, on both sides, taking care that both arches progressed at an equal rate, so that they might balance each other.
In order to increase the cohesion of the structure, he introduced bands of hoop iron longitudinally between the courses ; and by these means he was enabled to carry on the two semi-arches, until they met those pro duced in a similar way from the opposite piers. A narrow arch, say about four feet in width, having been thus completed, Brunel proposed to extend it to the requisite width by building on each side of it, adding from nine to eighteen inches at a time. To spew that—whatever might be the difficulties in practice—the principle is sound, Sir M. I. Brunel constructed two semi-arches, of bricks laid with mortar prepared with blue lias lime. Several bands cf hoop iron, and several slen der rods of fir, were inserted longitudinally between the courses, extending throughout the whole length of the structure. The radius of curvature of the arch was 177 feet, and although only 4i feet wide at the top, it was extended to the length of 40 feet on each side of the centre pier. One end was after wards extended an additional 20 feet; and as the other end could not be extended in the same way, in consequence of want of space, a weight of 28i tons was suspended from it as a counterpoise. Although the structure fell about three years after its completion, the fall was owing to circumstances independent of the principle on which the arch was con structed.
The origin of the arch in actual construc tions is still unknown ; it Cannot he stated with any degree of certainty, either in what country or at what epoch it was first used. There is reason to think that it was unknown to the Greeks at the time when they produced their most beautiful temples, in the fifth, fourth, and third centuries before the Chris. tian min,. The want of the arch would lead them to contract the intercolumniations, or spaces between the columns, and to the gene ral and frequent adoption of columns as the only mode of supporting a superstructure.