Arch (Lat. are us, a bow), a curved structure supported by its own curve. An arch is distinguished from a vault by its length being much less than its width, as is the case with the arch forming the roof of a door or of a window; but this distinction does not apply to structures built entirely above ground and open on both sides, as the arch of a bridge or a triumphal arch. It was long supposed that domes were unknown to the Egyptians and early Greeks, the first arched monument on record being the cloaca maxima of Rome, built in the age of the Tarquins; but it is now certain that arches were used by the Assyrians and Babylonians long before the foundation of Rome, and also that the Egyptians were acquainted with the principle of the arch, though they did not see fit to make use of it to any great extent. - The earliest arches in Italy were bailt by the Etruscans. The original Etruscan dome was supported by a few pillars, under which stood the augurs; the object was to protect the priest against the sun and rain, and at the same time allow him to study the horizon and be seen by the people.

The Romans scarcely deviated from the semicircle, which is the simplest form of the arch, and in building it did not follow true mechanical principles; so that the great strength of their numerous aqueducts, viaducts, and monuments is to be ascribed to their massiveness and to the good cement employed. It was not till the middle ages that the arch was properly built and widely used. Strong abutments are generally found around the monuments of that period, which consist of a succession of arches built one above the other, from the ground to the top of the monument, the uppermost one being used as an aqueduct for the roof gutters, appearing from below as light as if made of tin plate. The roofs of many of these edifices are formed of large arches as main ribs, which sustain smaller arches abutting on them; they are as slender as possible, and so appropriately shaped and ornamented as to appear much lighter than they are. The wedge-shaped stones of which an arch is composed are called voussoirs; the uppermost is the keystone; the two blocks of masonry on which the arch rests are the abutments; the line from which the arch springs is called impost; the inner curve, in-trados or soffit; the curve outside the voussoirs, extrados; the span is the distance between the piers; the distance of the keystone above the impost is the height of the arch.

The names of the parts of the arch proper are, the springs of the arch, the haunches, and the crown. When the arch has only to support itself, each voussoir sustains the weight of those placed above it, and consequently they must be made larger and larger from the crown to the spring; but when the arch has to support weights, the various modes in which they may be disposed require as many different constructions, and the finding of the resulting force acting on each part is one of the most difficult tasks of the architect. The use of arches in the form of an arc smaller than a semicircle is comparatively recent, and superior for many purposes to older forms. In bridges, for example, it leaves in ordinary times a larger passage for boats, and in times of freshet offers less resistance to the water, and the bridge runs less risk of being carried down. Since the introduction of cast iron in architecture, arches of that metal and of a single piece have been built; in such cases the arch is used only to please the sight, as the solidity of the structure depends entirely on other portions of the work. - A triumphal arch is a monumental structure erected in honor of some celebrated person and his deeds, or to commemorate some great event.

Triumphal arches probably originated with the Romans, and L. Stertinius is the first recorded who erected such a monument. Two were built by him, one about 190 B. C. in the Forum Boarium, and another in the Circus Maximus. A few years later, Scipio Africanus built one on the Clivus Capitolinus, and in 121 Q. Fabius Maximus erected one on the Via Sacra. Of these none remain. Different writers record 21 as having been built in the city of Rome. The most celebrated Roman arches are those of Augustus at Rimini, of Trajan at Beneventum and Ancona, and those of Titus, Drusus, Septimius Severus, and Constantine at Rome. That of Titus is one of the best. It is situated at the foot of the Palatine, and was probably completed after his death and apotheosis, as in the inscription he is called Divus. It commemorates his conquest of Judea. Remains of Roman arches are to be seen in Spain, Greece, and other countries. The custom of raising magnificent triumphal arches began under the first emperors. During the republic arches were decreed to victorious generals, but not to the dead.

When Augustus was emperor, the senate proposed to have one built in honor of Drusus the elder, who died in Germany. Augustus consented, and a marble arch was constructed on the Appian Way. - Paris, of all modern cities, has the most numerous and the most beautiful arches. The Portes St. Denis and St. Martin were erected in 1673-'4; the arc du Carrousel in the years 1806-'9, in honor of the armies of France. The latter is at the W. entrance of the Tuileries; its height is 47 feet, its breadth 55. Its two principal faces have each eight Corinthian columns, surmounted by statues. The most magnificent is the arc de I'Etoile, at the extremity of the avenue des Champs Elysees, built for the purpose of commemorating the victories of Napoleon. (See Paris.) 'The arch at Hyde Park corner, with the equestrian statue of the duke of Wellington, and Cumberland gate, are the only specimens in England.