the kings are unceasingly obliged, as they relate in their inscriptions, to repair or rebuild walls which crumble under the dissolving action of water from the sky. The unusual thickness of the walls, the long, narrow form of all the chambers, are also justified by the employment of the vault as the essential element of the Assyrian buildings. V. Place unearthed at Khorsabad a great doorway surmounted by a semicircular arch. The sides of the doorway, as well as the arch itself, are of brick; there are three rows of voussoirs one above the other, forming as it were three concentric door-frames half-fitting into one another. All the voussoirs, which have issued from a single mould, have a slightly trapezoidal shape, like the stone voussoirs of our most carefully built edifices. The height of the doorway, under the keystone, is 19 ft. 8 in., and the breadth 11 ft. At other points, Place recognised that the enormous accumulation of materials which filled up the halls could only come from the falling in of the clay vaults. Some blocks still, at the time of the excavations, formed an arch, sometimes several yards in diameter, solid enough to serve as shelter for the shepherds of the neighbourhood; they were, on the concave side, covered with carefully laid stucco, or with paintings in fresco--a circumstance which proves positively that these blocks are sections of crumbled vaults. [Illustration: Fig. 39.--Vaulted and domed houses (after Layard).] The square chambers were surmounted by a dome; there are in the palace of Sargon two of these rooms as much as 44½ ft. square. In a bas-relief discovered at Kouyunjik (fig. 39) a group of houses figures, among which some are surmounted by hemispherical cupolas, others by elongated domes in the form of sugar-loaves. The houses of Babylon were vaulted, as Strabo tells us. The Mesopotamian palaces of the Achæmenid, Parthian, or Sassanian epoch, the halls of which are surmounted by domes which scarcely yield in boldness to those of St. Sophia, evidently only handed on the Assyro-Chaldæan tradition which is also represented before our eyes by the modern houses of Mosoul, Bagdad and southern Persia. The technical methods of contemporary masons also do not fail to make known to us what steps their ancestors of the time of Sargon or Nebuchadnezzar took to supply the want of wood, and in consequence to do without a previous arched framework: travellers tell us that they have observed the commonest workmen of the country erecting their hemispherical or elliptical cupolas by layers in rings, laid one above the other, and narrowing in proportion to their nearness to the keystone; it is the same principle as that of the corbelled vaulting. [Illustration: Fig. 40.--Vaulted drain (after Layard).]