The dome of the temple of Bacchus is also internally hemispherical, though without coliers. Externally it is now covered %‘ ith a common roof', which may have been the original form ; such a roof is also to be seen over the dome of the temple of Jupiter, in the palace of Diocletian, at Spalatro.
The donic of one of the chambers of the Therime of Catania was 111 feet in diameter. In the Therune of Titus there are two domes, each 84 feet in diameter ; and in the baths of Constantine there was one of 70 rect. There were three domes in the baths of Diocletian, of which two still remain ; one is 73 feet 6 inches in diameter, and the other 62 feet 3 inches. Jaq,ring from those that remain, it would seem that in the Theraue they were lighted from the top, in the same manner as in the dome of the Pantheon. In the neigh bourhood of Puzznoli there is a circular edifice which has a dome built of purnice-stone and volcanic tufh : its diameter is about 96 feet. The temple of Al inerva Dledica at Rome, was on the plan, a pol? gonal dome of ten sides, without any opening at the top. Domes were sometimes constructed on corbels by the ancients. In one of the octagonal rooms of the enclosure round the baths of Car•calla the corbels which supported the dome still remain, and at Catania there is a spherical dome covering a square vestibule.
The dome of Santa Sophia, at Constantinople, built in the reign of Justinian, ranks next to the Pantheon in point of antiquity, and is the most remarkable and the earliest con structed after those of the Romans. Anthemins of Tralles, and Isidorus of Aliletus, were the architects. Anthemins had promised to raise a dome over this edifice, of such magnitude as to eclipse the magnificence of thelkunan Pantheon. With this view, he erected fur pillars at the angles of a square, at about the distance of 115 feet from each other, and nearly of the same altitude. The church was to be of the 11)•in of a cross. and vaulted with stone ; he therefore threw arches over the pillars, and filled up the angular spaces between the arehivaults, till he had gradually shaped them into a complete circle, at the level of the extradoses of the arches. On the ring tints formed, the dome was raised, being the first ever built on pendentives. The pressure of the eastern and western arches was resisted by four walls almost solid, form 'ng transepts, and running longitudinally, two from the north, and two from the south sides of the pillars, to the distance of about 90 feet. The east and west arches were abutted upon by half-domes, resting on cylindrical walls, which, it was supposed, would have been sufficient to resist the pressure of the arches on the north and smith ; but in this the architect was mistaken ; for the superstructure gave way towards the cast, and, at the end of a few months, fell, taking with it the on that side. After the death of Anthemius, the
superintendence of the building devolved on lsidorus, who strengthened the eastern pillars, by filling up certain voids left by his ; lint when the dome was turned upon them, the east end proved still too weak for the support of so great a load, and again gave way, betbre the work was completed. In order to counteract this thrust on the east, Isidorus now built strong pillared buttresses against the eastern wall of a square cloister that ran round the building ; from which lie threw flying buttresses over the void• and then raised the a third time, hut with very little success ; fm• though every precaution was, taken to lessen its weight, by using pumice-stone and other light materials, and by reducing its thickness, the arches were so much fractured. that he was under the necessity of filling lip the large arcades on the north and south sides, with arches of less dimensions, in three stories.
We have mentioned these circumstances, to show that the architects of the ago to which this building is referred, were not so well acquainted with the principles of dome-vaulting, as those of more modern date; for the latter would probably have hooped or chained such a dome immediately over the arches and pendentives, so as to confine its pressure to a per pendicular thrust, or nearly so. Such was the case, in the tiir more Ponderous dome of St. Peter's. at 1Zotne, erected by Michael Angelo ; and such, more recently, was the practice of our countryman, Sir Christopher Wren. in the cupola of St. Paul's. at London. The present dome, however, of Santa. Sophia, was reconstructed by the nephew of Isidorns. It rests on the square fbrined at the intersection of the of the Greek cross ; the diameter being about 111 feet, and the dome 40 feet high, and is supported by corbellings placed in the angles of the square. These corbels are surmounted by a kind of cornice on which rests a circular gallery. The lower part of the dome has a row of adorned with columns on the exterior, and the top is surmounted by a lantern on which is a cross. . The dome of Anthenflus and lsidorns, was not so high ; and was partly destroy ed by an earthquake a few years after its construction. In rebuilding it, the nephew of Isidorus used a very light white brick, made at Rhodes, and much lighter than the common brick.