ARCUS (also fornix), an arch. A true arch is formed of a series of wedge-like stones, or of bricks, supporting each other, and all bound firmly together by their mutual pressure. It would seem that the arch, as thus defined, and as used by the Romans, was not known to the Greeks in the early periods of their history. But they made use of a contrivance, even in the heroic age, by which they were enabled to gain all the advantages of our archway in making corridors, or hollow galleries, and which in appearance resembled the pointed arch, such as is now termed Gothic. This was effected by cutting away the superincumbent stones in the manner already described, at an angle of about 45° with the horizon. The mode of construction and appearance of such arches is represented in the annexed drawing of the walls of Tiryns. The gate of Signia (_Segni_) in Latium exhibits a similar example. The principle of the true arch seems to have been known to the Romans from the earliest period; it is used in the _Cloaca Maxima_. It is most probably an Etruscan invention. The use of it constitutes one leading distinction between Greek and Roman architecture, for by its application the Romans were enabled to execute works of far bolder construction than those of the Greeks. The Romans, however, never used any other form of arch than the semicircle. The arcus triumphalis, triumphal arch, was a structure peculiar to the Romans, erected in honour of an individual, or in commemoration of a conquest. Triumphal arches were built across the principal streets of Rome, and, according to the space of their respective localities, consisted of a single archway, or a central one for carriages, and two smaller ones on each side for foot-passengers. Those actually made use of on the occasion of a triumphal entry and procession were merely temporary and hastily erected; and, having served their purpose, were taken down again, and sometimes replaced by others of more durable materials. Stertinius is the first upon record who erected anything of the kind. He built an arch in the Forum Boarium, about B.C. 196, and another in the Circus Maximus, each of which was surmounted by gilt statues. There are twenty-one arches recorded by different writers, as having been erected in the city of Rome, five of which now remain:--1. _Arcus Drusi_, which was erected to the honour of Claudius Drusus on the Appian way. 2. _Arcus Titi_, at the foot of the Palatine, which was erected to the honour of Titus, after his conquest of Judaea; the bas-reliefs of this arch represent the spoils from the temple of Jerusalem carried in triumphal procession. 3. _Arcus Septimii Severi_, which was erected by the senate (A.D. 207) at the end of the Via Sacra, in honour of that emperor and his two sons, Caracalla and Geta, on account of his conquest of the Parthians and Arabians. 4. _Arcus Gallieni_, erected to the honour of Gallienus by a private individual, M. Aurelius Victor. 5. _Arcus Constantini_, which was