The outer encircling wall is pierced with 80 openings, leaving of course an equal number of piers; every opening is arched, and in or against every pier is a column projecting about half its diameter, and supporting an entablature which runs in an unbroken line all round the structure. With the exception of the four central openings, which lie on the diameters of the ellipse, and are each nearly two feet wider than the rest, all the openings are very nearly the same, their width being 14 feet 6 Inches. An exactly similar series of arches, diminished only in proportion to the smaller extent of the ellipse, separates the second corridor from the first ; and auother, bearing the same relation to the second series that the second does to the first, or outer, bounds the second corridor.
The external elevation of the Colosseum (which alone can be noticed, as a detailed description of the building is given in another part of the English Cyclopsedia is composed of three stories of attached or engaged columns with their usual accessories, and a pilastraded ordinance, forming a species of attic, which is pierced with windows —one in every other interspace. Tho lowest ordinance of columns rests on the upper step of the substructions, or on the ground-floor of the structure; it is of what is termed the Doric style. The inter vening arches are semicircular; they spring from moulded imposts, and have moulded archivolts on their outer faces. The second ordi nance is in the Roman Ionic style, having voluted capitals to the columns ; and the third is in the Corinthian or foliated style : these rest upon continued, but recessed, stylobate; their entablatures are unbroken throughout, and the arches in the intereolumniations in both correspond exactly—except in minor details—with those of the lowest or Doric ordinance. The pilasters have foliated capitals also, and are called composite; they rest on deep plinths under which there is a continued and recessed dado superimposing the Corinthian eatable. tnre ; this dodo is pierced with holes or small windows alternating with those of the ordinance above, to give light to the corridor behind the lower and under the upper gallery on the inside. The crowning entablature, which runs its cornice round in one unbroken line, is made bold and effective by deep modilliou blocks or consoles occupying the whole depth of the frieze.
The Amphitheatre Castrensis, a small brick amphitheatre, erected probably in the first century of the Christian era. It originally stood
without the walla of Servius Tullius, but during the reign of Honorius it was employed to form part of the new inclosure, and the arches were filled up. On the inside the form of a semi-ellipse on its greatest axis is all that can be discerned ; but externally the engaged Corinthian columns of the lower order; with their brick capitals, aro well pre served; of the upper order there only remain a pilaster and part. of an arch.
Arclies.—The Arch of Titus, which stands near the ruins of the Temple of Venus and Rome, was erected by Domitian in honour of Titus and his conquest of Jerusalem. It Is of Peutelic marble, and of an elegant design, but with only one arch. On the sides of the piers under the arch, which is highly decorated, are two very fine bas-reliefs, illustrating the victory of Titus over the Jews. In one of them is represented the golden table, the trumpets and horns of silver, and the golden candlestick with its branches. The triumph of Titus is repre sented also on the frieze on the outside of the arch.
Tho Arch of Septimius Severus, erected A.D. 205 by the senate and Roman people, in honour of Septimius Severus and his eons Caramlla and Gets, for their victories over the Parthiene and other oriental nations. This arch is also of Pentelic marble, with archways and transverse archways through the piers of the centre arch. Each front is decorated with four fluted columns, and a series of bas-reliefs. Above the attic was a marble group represeutiug the emperor and his two sons, in a chariot drawn by six horses. The whole of the mould ings and the vaulting are highly enriched with carved ornaments.
The Arch of Constantine, ereoted in commemoration of his great victory over Maxentius, stands near the Meta Sudsas, and fronting tho Colosseum. It is formed with three archways, adorned with four beautiful columns of giallo entice on each side, and enriched with many fine bas-reliefs and statues. Above the attic was a triumphal qusdriga. The roadway passes under the arch.
The Arch of Dolabella. This single arch of travertine was con structed A.D. IO, by the consuls Publius Cornelius Dolnbella and Caius Junius Silithus. It stands near the church of San Giovauni and Paolo, and la thought to have been the entrance to the Campus Martialia, where the Equiria, or equestrian games in honour of Mars, were cele brated, when the Campus Martius was inundated by the Tiber.