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AROADIUS ARCH 641 under the Roman power. Thenceforth they have no separate history from that of the em- pire, and of mediaeval and modern Greece. At present Arcadia, comprising the larger part of the ancient division, with the addition of the ancient district of Cynuria and a part of Laconia, forms one of the nomarchies of the kingdom of Greece, bounded E. in part by the gulf of Nauplia ; area, 2,028 sq. m. ; pop. in 1870, 131,740. Capital, Tripolitza. ARCADIl'S, the first of the Byzantine empe- rors, born in Spain, A. D. 383, died in Constan- tinople, May 1, 408. He was the elder son of Theodosius the Great, the last ruler of the whole Roman empire. In 395, a few months before his death, Theodosius divided the empire be- tween his two sons, Arcadius and Honorius, giving to the former the eastern part, extend- ing from the Adriatic to the Tigris and from Scythia to Ethiopia. Arcadius ruled at first under the regency of Rufinus, who was soon assassinated at the instigation of his rival, Sti- licho, the regent of Honorius. Eutropius, a eunuch, afterward became regent, and held the place till 399, when Tribigild, a Gothic chief in Phrygia, revolted and compelled Arcadius to put his favorite to death. Trigibild and his tribe also obtained permission to pass the Bos- porus and settle on the European side; but being Arians, they were massacred or driven out by the people of Constantinople. In re- venge for this the empress Eudoxia, who had now acquired the absolute control over her husband, caused Chrysostom, the great adver- sary of Arianism, to be banished to Comana in Pontus (404). Arcadius was a contemptibly feeble man, but of strict religious orthodoxy. ARCESILAUS, a Greek philosopher, the founder of the Middle Academy, born at Pitane in ^olis about 316 B. C., died about 241. He was originally intended for a rhetorician, but while pursuing his studies at Athens decided to devote himself entirely to philosophy, and succeeded Crates in the chair of the academy of Athens. From the little we know of his opinions, it seems that he was a skeptic, but not in the Pyrrhonic sense of that term ; and his celebrated saying, " that he knew nothing, not even his own ignorance," seems to have been but an utterance of humility. He was also distinguished from the pure Pyrrhonists by his predilection for questions appertaining to practical life, and by the undeviating mod- eration of his tone. ARCH (Lat. arcus, a bow), a curved structure supported by its own curve. An arch is dis- tinguished 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 42 VOL. i. 42 in the age of the Tarquins ; but it is now cer- tain that arches were used by the Assyrians and Babylonians long before the foundation of Rome, and also that the Egyptians were ac- quainted 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 built 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 de- viated 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 em- ployed. 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 monu- ment, 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 vous- soirs, extrados; the span is the distance be- tween 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 it- self, 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 construc- tions, 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 ex- ample, it leaves in ordinary tunes a larger pas- sage 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 struc-