Page:EB1911 - Volume 27.djvu/984

The widest hall vaulted by the Romans was that of the throne room in the palace of Diocletian on the Palatine Hill, and this had the enormous span of 100 ft., its thrust being counteracted by other halls on, either Iside with buttresses outside. In provincial towns and in other parts of the Roman Empire, where the material pozzolana was not procurable, the Romans had to trust to their mortar as a cementing medium, but this, though excellent of its kind, was not of sufficient cohesive strength to allow of the erection of vaults of more than about 40 ft. span, which were generally built in rubble masonry. There still, exist in Asia Minor and Syria some vaulted halls, generally attached to thermae, which are. carried on walls of great thickness. There were many varieties of the Roman vault, whether continuous or intersected, such as those employed over the corridors on the Colosseum and the theatre of Marcellus, but in these Cases the springing of the vault was above the summit of the Arches of the main front, so that there was no intersection; on the other hand, over the corridors were either elliptical or semicircular, or over the staircases rising vaults, all of which were more difficult to construct; there were also numerous solutions of vault over circular halls, of which that of the Pantheon was the most important example, having a diameter of 142 ft., and over the hemicycles,: which were sometimes of great size; that known as Canopus in Hadrian’s villa at Tivoli had a diameter of 75 ft., and was vaulted over with a series of ribs, between which were alternating rampant flat and semicircular webs and cells; in the same villa and in Rome were octagonal halls with various other combinations of vault. Another type of vault not yet referred to is that of the Tabularium arcade where the cloister vault was employed. Fig. 3 compared with fig. 2 will show the difference; in the former the angles of intersection are inset and in the latter they are groins with projecting angles at the base, which die away at the summit.

The vault of the basilica, commenced by Diocletian and completed by Constantine, Was the last great work carried out by the Romans, and two centuries pass before the next important development is found in the church of Sta Sophia at Constantinople. It is probable that the realization of the great advance in the science of vaulting shown in .this church owed something to the eastern tradition of dome Vaulting seen in the Assyrian domes, which are known to us only by the representations in the bas-relief from Nimrud (fig. 1), because in the great water cisterns in Constantinople, known as the Yeri Batan Serai (the underground palace) and Bin bir-derek (cistern with a thousand and one columns), both built by Constantine, we find the intersecting groin vaults of the Romans already replaced by small cupolas or domes. These domes, however, are of small dimensions when compared with that projected and carried out! by Justinian in Sta Sophia. Previous to this the greatest dome was that of the Pantheon at Rome, but this was carried on an immense wall 20 ft. thick, and with the exception of small niches or recesses in the thickness of the wall could not be extended* so that Justinian apparently instructed his architect to provide an immense hemi cycle or apse at the eastern end, a similar apse at the western end, and great arches on either side, the walls under which would be pierced with windows.

Fig. 3.

Fig, 4.

Fig. 5.—AA, pendentive.

The diagram (fig. 4) shows the outlines of the solution of the problem. If a hemispherical dome is eut by four vertical planes, the intersection gives four semicircular arches ; if cut in addition by a horizontal plane tangent to the, top of these arches, it describes a circle; that portion of the : sphere which is below this circle and between the arches, forming a spherical spandril, is the pendentive (fig. 5), and its radius is equal to the diagonal of the square on which the four arches rest. Having obtained a circle for the base of the dome, it is not necessary that the upper portion of the dome should spring from the same level as the arches, or that its domical surface should be a continuation of that of the pendentive. The first and second dome of Sta Sophia apparently fell down, so that Justinian determined to raise it, possibly to give greater lightness to the structure, but mainly in order to obtain increased light for the interior of the church. This was effected by piercing it with forty windows—the effect of which was of an extraordinary nature, as the light streaming through these windows gave to the dome the appearance of being suspended in the air. The pendentive which carried the dome rested on four great arches, the thrust of those crossing the church being counteracted by immense buttresses which traversed the aisles, arid the other two partly by smaller arches in the apse, the thrust being carried to the outer walls, and to a certain extent by the side walls which were built under the arches. From the description giy#h by Procopius we gather that theicentring employed for the great arches consisted of a wall erected to support them during their erection. The construction of the peridenrives is not known, but it is surmised that to the top of the pendentives they were built in horizontal courses of brick, projecting one oyer the other, the projecting angles being cut off afterwards and covered with stucco in which the mosaics were embedded ; this, was the method employed in the erection of the Perigordian domes to which we shall return; these, however, were of less diameter than those of Sta Sophia, being only about 40 to 60 ft. instead of 107 ft. The apotheosis of Byzantine architecture, in fact, was reached in Sta Sophia, for although it formed the model on which all subsequent; Byzantine churches were based, so far as their plan was concerned, no domes approaching the former in dimensions were even attempted. The principal difference in some later examples is that which took place in the form of the pendentive on which the dome was carried. Instead of the spherical spandril of Sta Sophia, large niches were formed in the angles, 3s: in the .mosque of Damascus, which was built by Byzantine workmen for the Sherif al Walid in A.D. 705; these gave, an -octagonal base on; which the hemispherical dome rested (fig". 6); for again, as in the Sassanian palaces of Serbistan and Firuzabad of, the 4th and 5th century of our era, when a series of concentric arch rings, projecting one in front of the other, were built, giving also an octagonal base; each of these pendentives is known as a squinch.

Fig. 6.—BB, niche or squinch pendentive.

There is one other remarkable vault, also built by Justinian, in the church of S. Sergius and Bacchus in Constantinople. The central area of this church was octagonal on plan, and the dome is divided into sixteen compartments; of these eight consist of broad flat bands rising from the centre of each of the walls, and the alternate eight are concave cells over the angles of the octagon, which externally and internally give to the roof the appearance of an umbrella.

Although the dome constitutes the principal characteristic of the Byzantine church, throughout Asia Minor are numerous examples in which the naves are vaulted with the semicircular barrel vault, and this is the type of vault found throughout the south of France in the 11th and 12th centuries, the only change being the occasional substitution of the pointed barrel vault, adopted not only on account of its exerting a less thrust, but because, as pointed out by Fergusson (vol. ii. p. 46), the roofing tiles were laid directly on the vault and a less amount of filling in at the top was required. The continuous thrust of the barrel vault in these cases was met either by semicircular or pointed barrel vaults on the aisles; which had only half the span- of the naVe; of this there is an interesting example hi the chapel of St John in the Tower of London—and sometimes by half-barrel vaults. The great thickness of the walls, however, required in such constructions would seem to have led to another solution