All the chambers were vaulted with circular arches (Fig. 60). The arches consist of numerous ring courses, separated from each other by level courses (Fig. 61), exactly as in the eastern door of the Citadel. [Illustration: FIG. 60.—Arches of the Vaulted Building. ] We must here observe the difference that exists between arches, underground vaulting, and outstanding vaulting. The wall in which the arch is placed provides it with the necessary abutments; there are no difficulties to encounter in its construction, and we meet with it in the earliest times, at Nippur and Fara as early as the invention of writing. In Fara there is an underground canal which consists of actual arches placed close together; in Babylon and Assur there are underground vaults which certainly date back to the year 1000. Such vaultings are easily constructed, for the earth in which they are buried affords the necessary abutments. But the case is very different when the vaulting has to be carried from one free standing wall to another. Then the building has to be so constructed that the thrust of the vaulting is counterbalanced by the walls themselves. This distinct advance appears to have been first attempted, or at any rate planned, in Mesopotamia by Nebuchadnezzar. Certainly, no house vaulting older than ours on the Southern Citadel has been found in Mesopotamia, roofing as it does a huge connected complex of chambers. The vaultings asserted by Place to be over the chambers at Khorsabad are, without exception, absolute inventions. Sargon was only acquainted with the arch in the wall, which, as we have already seen, is not a noteworthy achievement, and with the sloping courses employed in forming the arched roofing of a canal. Those Assyrian-Babylonian palaces were entirely roofed with wooden beams, like the cedars of Lebanon of our Southern Citadel. It is possible that the throne-room of the principal court was vaulted, but that is not certain. The vaulted building shows clear signs of tentative and inexperienced work in the arrangement of the vaulting. It consists merely of simple barrel-vaults, and there is, of course, no cross vaulting, cupola, or any arrangement of the kind. The thrust of the central chambers is on the north against the strong Citadel wall, and on the south against the outer row of chambers vaulted in the other direction (Fig. 62). [Illustration: FIG. 61.—Abutments of arches of the Vaulted Building. ]