From the importance attached to wood ties or girdles built into the small domes of Mount Athos, we may be certain that some system of chaining was applied to the great dome of S. Sophia. Choisy gives an example of the former, and also a dome constructed by interlocking semicircular bricks, “two courses of which make a circlet absolutely inextensible.” See B in Fig. 45. The dome of S. Vitale at Ravenna is built of layers of earthenware pots or tapering tubes, the end of one fitting into the next and rising in a continuous spiral course, round and round from the bottom to the crown of the dome. The question of dome construction without centring is of the greatest interest, and much might doubtless be gathered of the traditional methods still followed in modern Greece, Egypt, Persia, and S. Italy. Our Fig. 43 represents modern domes in Persia, the upper diagram being an ordinary type of exterior from a photograph of Koum. The dome beneath, Fig. 44, is from a sketch made in a Persian caravanserai by Mr. Wm. Simpson,[344] who describes it as built of burnt brick, square below, round above. “As I was told that centring was never used in Persia I presume this one was constructed without it.” This beautiful form may be considered as four conical squinches penetrating a hemisphere as at A, or as a gradual transition from square to round, B. Ancient Persian domes of substantially the same form, in which a hemisphere penetrates a pyramid, are shown by Dieulafoy.[345] [Illustration: FIGS. 43 and 44.--Modern Domes built without Centring.] _Chainage and Walling._--In the East the frequency of severe earthquakes necessitated a manner of construction which should resist disruption. The massive walls of stone of the Classic period are cramped together with metal. The stone Byzantine church at Ezra has a course of interlocking stones forming a chain around the octagon beneath the dome (Fig. 45 a). At S. Sophia the continuous courses of stone some feet above the floor, mentioned by Salzenberg, are almost certainly converted into a chain by cramps; and the stone course at the springing of the great arches probably has the same function. In brickwork lateral cohesion was usually obtained by a system of continuous wood ties, which is described by Choisy as built into the wall at every five or six feet of height. According to the Greek architect, M. Kouppas, ties of bond timbers were used in this way in the construction of the cisterns, “laid not only along the outside walls but also in parallel rows beneath the lines of pillars and arches;” other rows of timber were built in either as ties or struts in continuous lines at the springing of the vaults.