212 COMPARATIVE ARCHITECTURE. B, Walls. — These were often constructed of brick. Internally, all the oriental love of magnificence was developed, marble casing and mosaic being applied to the walls ; hence a flat treatment and absence of mouldings prevailed. Externally the buildings were left comparatively plain, although the fa9ade was sometimes relieved by alternate rows of stone and brick, in various colors. c. Openings. — Doors and windows are semicircular headed (No. 8g G, h), but segmental and horse-shoe arched openings are sometimes seen. The windows are small and grouped together (Nos. 80 a and 87). The universal employment of mosaic in Byzantine churches, and the consequent exclusion of painted glass, rendered the use of such large windows as the Gothic architects employed quite inadmissible, and in the bright climate very much smaller open- ings sufficed to admit the necessary light. Tracery was, in con- sequence, practically non-existent as a northern architect would understand it. The churches depend largely for light on the ring of windows at the base of the dome, or in the " drum," or circular base on which the dome is sometimes raised (No. 86), and on openings grouped in the gable ends (No. 80 a). Such windows, grouped in tiers within the semicircular arch beneath the dome, are a great feature in the style. Portions of the windows are occasionally filled with thin slabs of translucent marble (No. 89 g). D. Roofs.— The method of roofing these buildings was by a series of domes formed in brick, stone, or concrete, with fre- quently no further external covering. In S. Sophia the vaults are covered with sheets of lead, a quarter of an inch thick, fastened to wood laths, resting on the vaults without any wood roofing (No. 80 b). Hollow earthenware was used in order to reduce the thrust on the supporting walls (No. 83 d). The Byzantines introduced the dome placed over a square or octagonal plan by means of pendentives (No. 79 j), a type not found in Roman architecture. In early examples the pendentives were part of one sphere. A good idea of this type is obtained by halving an orange, cutting off four slices, each at right angles to the last, to represent the four arches, and then scooping out the interior ; the portion above the crown of these semicircles is the dome, and the inter- vening triangles are the pendentives. Such domes are rare, however, perhaps the only example in Europe being that over the tomb of Galla Placidia (No. 73 h, j, k), already described (page 187). In the later type the dome is not part of the same sphere as the pendentives, but rises independently from their summits (Nos. 80 b, hi c). The early domes were very Hat; in later times they were raised on a drum or cylinder.
