Bk. IX. ^H. III. CHURCHES WITH DOMES. 437 was that twice the necessary amount of materials was consumed its construction. The second, that the mode of lighting by a hole in the roof, which also admitted the rain and the snow, was most objectionable before the invention of glass. The third, that a simply circular })lan is always unmeaning and inconvenient. A fourth, that a circular building can hardly, by any contrivance, be made to fit on to any other buildings or apartments. In the Minerva Medica (Woodcut No. 227) great efforts were made, though not quite successfully, to remedy these defects. The building would not fit on to any others, and, though an imj^rovement on the design of the Pantheon, was still far from perfect. The first step the Byzantines made was to enclose the circle in a square, as a (Woodcut No. 874), and then to insert a great niche in 874. Diagram of Byzautiue Arrangement. 875. Diagram of Byzautiue Pendentives. each of the angles. By this means, the thickness of the outer walls was very considerably reduced, and the whole square was practically utilized. A second step was to cut away as much as j^ossible of the outer wall, leaving only what was requisite to support the dome, and enclosing the whole in an octagon, as at b, or a square, as at c. When tills was done, it is evident that a church of any required dimensions could be constructed without serious effort, and great variety of per- spective obtained without affectation. The octagonal arrangement in the last woodcut was that adopted at St. Vitale at Ravenna ; the square, that which produced the church of SS. Sergius and Bacchus at Constantinople. So vong as the octagonal arrangement was adhered to, no difiiculty of construction occurred ; the difference between the circle and octagon, represented by the shaded parts at a in the dia- gram (Woodcut No. 875), is so small, that it is easily got over in construction, but such a polygon has many of the architectural defects of the circle, and the triumph of the Byzantine architects was complete, when, by the introduction of pendentives — represented by the shaded parts at b (Woodcut No. 875), they were enabled to ])lace the circular dome on a square apartment.
