Page:The Classical Heritage of the Middle Ages.djvu/324

 906 THE CLASSICAL HERITAGE [chap. tress of antique culture as well as of the Grseco-Roman Empire. Besides the basilica form, the Greek circu- lar temple had passed to Rome in pre-Christian times. The Pantheon, built, as it stands, under the Antonines, is the great example of a circular temple crowned with a dome of concrete constructed in the Roman method. This dome is set directly upon a circular drum and therefore needs no pendentives. These are rare and rudimentary in Roman dome construction, for the Romans never set a dome upon a square base, and only in a few secondary instances upon an octa- gon.^ Byzantine domes are not " cast " with concrete, but are constructed out of layers of brick or tiles. Hence they are not held together by the cohesion of the material. They rest either upon a circular base, an octagon, or a square. The first are related to the Pantheon. The second are represented by St. Sergius at Constantinople and St. Vitale at Ravenna, both built in Justinian's time, and employing pendentives. The great church representing the third group stands for the climax of Byzantine architectural achieve- ment. In the latter part of Justinian's reign, the Greek architects of St. Sophia solved most beautifully the problem of setting a dome upon a square, by the use of pendentives in the form of spherical triangles resting upon arches.* 1 An instance is afforded by one of the smaller halls in the Baths of Caracalla. 2 This means of adjusting a dome to a square base is quite differ- ent and far more beautiful than the ancient (originally Persian) mode of conical vaults — "trompes" — rising from the corners of the square and joining with the sphere of the main dome (Choisy, L'histoire, etc., I, 125, II, 8). An interesting modification of the