Ctesiphon on the Tigris, so that there was no lack of opportunity for the study of Oriental architecture on the very spot where it was born. They could judge of and admire the beauty it certainly possessed when the great buildings of Mesopotamia were still clothed in all the richness of their decoration. The genius of the Greeks had come nigh to exhausting the forms and combinations of the classic style; it was tired of continuous labour in a narrow circle and sighed for fresh worlds to conquer. We can easily understand then, how it would welcome a system which seemed to afford the novelty it sought, which seemed to promise the elements of a new departure that might be developed in many, as yet unknown, directions. If we put ourselves at this point of view we shall see that Isidore and Anthemius, the architects of St. Sophia, were the disciples and perpetuators of the forgotten masters who raised so many millions of bricks into the air at the bidding of Sargon and Nebuchadnezzar.[208] Whatever may be thought of this hypothesis, there seems to be little doubt that the Assyrians knew how to pass from the barrel vault to the hemispherical, and even to the elliptical, cupola. As soon as they had discovered the principle of the vault and found out easy and expeditious methods of setting it up, all the rest followed as a matter of course. Their materials lent themselves as kindly to the construction of a dome as to that of a segmental vault, and promised equal stability in either case. As to their method of passing from the square substructure to the dome we know nothing for certain, but we may guess that the system employed by the Sassanids (see Fig. 54) was a survival from it. It is unlikely that timber centerings were used to sustain the vaults during construction. Timber was rare and bad in Chaldaea and men would have to learn to do without it. M. Choisy has shown--as we have already mentioned--that the Byzantine architects built cupolas of wide span without scaffolding of any kind, each circular course being maintained in place until it was complete by the mere adherence of the mortar.[209]