The amphitheatres of Rome, so called from being as it were double theatres, formed a complete circle, or rather an ellipse. They, too, like theatres, were built of wood, until the time of Augustus, from which period they gradually increased in size and splendor, until Vespasian and Titus caused the crowning work of the kind--the Colisæum--to be erected for the convenience of the immense crowds of sight-seers flocking to the public games. This most celebrated remain of Roman architectural art covers five acres of ground; the longer diameter being 615 feet, and the shorter 510. Four orders of columns, the first pure Doric, the second Ionic, the third Corinthian, and the fourth, Corinthian pilasters, rise one above the other till they attain the great elevation of 160 feet. These columns have one-fourth of their thickness buried in the face of the wall which they ornament. There are eighty columns in each tier, and from their summits spring arches supporting the tier above. There were, therefore, four tiers of seats for the accommodation of spectators, and this sufficiently explains how the building could contain the immense number of 87,000. The numbers contained by these buildings of antiquity were very large; the Attic theatre held 50,000, and the wooden theatre of Scaurus, before-mentioned, could accommodate 80,000. We have mentioned that the perfect arch was not known to the Greeks; indeed, they have no word for it in their language. But the frequent use of the arch by the Romans, even in walls, where plain masonry would have sufficed, shows it to have been a favorite form of construction, and the triumphal arches built to commemorate victories are among their most splendid remains. Those erected by Drusus, Titus, Septimius Severus, Gallienus, and Constantine, still remain, and with the bas-reliefs commemorative of the events which they were designed to perpetuate still attest the great amount of Roman skill, and the extent of Roman conquests.