184 COMPARATIVE ARCHITECTURE. bishop's throne, surrounded by six rows of seats in the apse, still exist, giving a good idea of the Early Christian arrangements. BAPTISTERIES are another description of building met with in Early Christian architecture. They were originally used only for the sacrament of baptism ; hence the name " Baptistery." The form was derived from the Roman circular temples and tombs, already described (page 136). There was generally one baptistery in each city, as at Ravenna and Florence, and it was as a rule a detached building, usually adjoining the atrium or forecourt. Indeed, until the end of the sixth century of our era the baptistery appears to have been a distinct building ; but after this period the font came to be placed in the vestibule of the church. In adopting the Roman tombs as models for these buildings, the early Christians modified them to some extent, for the internal columns which in Roman examples were generally used in a decorative way were now used to support the walls carrying the domes. To cover a large area with one roof was difficult, but by the addition of an aisle in one story round a moderate-sized circular tomb, the inner walls could be replaced by columns in the lower half, resulting in such a building as these early baptisteries (No. 75 h, j). The Baptistery of Constantine, Rome (No. 75 j, k, l), is octagonal, and the roof is supported by a screen of eight columns two stories in height. The Baptistery, Nocera, between Naples and Salerno, is circular, being 80 feet in diameter, with two rings of columns. This building is domed and covered with a wooden roof, and appears to be the first instance of the use of both, as the Roman architects always allowed the stone vault to show externally, as in the Pantheon. In the case of this building, however, the vault is merely an internal ceiling which is covered with an external wooden roof, and is similar to the practice of Gothic architects, who, in the mediaeval period, covered the stone vaults of their churches with timber roofs (No. 109). S. Stefano Rotondo, Rome (a.d. 470), though not a baptistery, is a good example of a circular plan of similar type (Nos. 75 F, G, H, and 77), being 210 feet in diameter, and with roof supported on two circular rings of columns, all taken from older buildings, the outer range supporting arches, and the inner a horizontal architrave. The two central columns are an addition to support the roof timbers. Tiie Baptistery, Ravenna, founded at the end of the fourth century, is an octagonal structure with two arcades in the interior, one above the other. The dome, constructed of hollow tiles, has
