299. Plan, Kalat Sema’n. Scale 100 ft. to 1 in.
Churches with Domes.
Whether the dome of the Pantheon at Rome (p. [320]) was erected in the time of the Antonines, or before the time of Augustus, as was formerly supposed, it is evident that the Romans had conquered the difficulties of domic construction long before the transference of the seat of power to Byzantium; the Pantheon being, up to this hour, the largest (single) dome ever constructed by the hand of man. Simple and grand as it undoubtedly is, it had several glaring defects in its design which the Byzantines set themselves to remedy. The first was that twice the necessary amount of materials was consumed in 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 plan 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. [229]) great efforts were made, but not quite successfully, to remedy these defects. The building would not fit on to any others, and, though an improvement on the design of the Pantheon, was still far from perfect.
300. Diagram of Byzantine Arrangement.
301. Diagram of Byzantine Pendentives.
The first step the Byzantines made was to carry the dome on arches resting on eight piers enclosing an octagon A (Woodcut No. [300]); this enabled them to obtain increased space, to provide nave, choir, and transepts, and by throwing out niches on the diagonal lines, virtually to obtain a square hall in the centre. The difference between the octagon and circle is so slight, that by corbelling out above the extrados of the arches, a circular base for the dome was easily obtained B. The next step was to carry the dome on arches resting on four piers, and their triumph was complete when by the introduction of pendentives—represented by the shaded parts at D (Woodcut No. [301]), they were enabled to place the circular dome on a square compartment. The pendentives and dome thus projected formed part of a sphere, the radius of which was the half-diagonal of the square compartment. Constructively it would probably have been easier to roof the space by an intersecting vault; and even if of 100 or 150 ft. span it would without difficulty have been effected. The difference between the intersecting vault and the dome (as shown in Woodcuts [302] and [303]; the former the tomb of Galla Placidia, built 450 A.D., the latter the chapel of St. Peter Crysologus attached to the archiepiscopal palace of about the same date, and both in Ravenna) is perhaps the most striking contrast the history of architecture affords between mechanical and ornamental construction. Both are capable of being ornamented to the same extent and in the same manner; but the difference of form rendered the dome a beautiful object in itself wholly irrespective of ornament, whereas the same cannot always be said of the intersecting barrel vault. Altogether, the effect would have been architecturally so infinitely inferior, that we cannot but feel grateful to the Byzantines that they persevered, in spite of all mechanical temptations, till they reached the wonderful perfection of the dome of Sta. Sophia.
