The place where it has been possible to observe the employment of the vault in the architecture of the Assyrian palaces, is in the very bowels of the basements of these edifices. A vast corridor, surmounted by a semicircular vaulting, was discovered by the English explorers, in the flanks of the mound of Nimroud; the lower courses are of enormous slabs of stone, all the rest is brick. In the scientific system of drains which carried off the sewage of the palace of Sargon, Place distinguished every kind of vaulting: the pointed or ogival vault, the semicircular vault, the flat-arched vault, the shallow vault, the elliptical vault. Never, at any point in their history, did the Egyptians [Illustration: Fig. 41.--Vaulted drain at Khorsabad (after Place).] or the Romans push the application of the vault to an equal degree of perfection. In most of the halls of Sargon’s palace a slab pierced with a hole was remarked in the middle of the bricks which form the pavement: this was the orifice of a vertical conduit opening into a vaulted drain concealed in the terrace. One of these drains had an ogival vaulting, the description of which we will borrow from MM. Perrot and Chipiez.[29] “The bricks composing it are trapezoidal in shape, two of their sides being slightly rounded--the one concave, the other convex. The radius of this curve varies with each brick, being governed by its destined place in the vault. These bricks go therefore in pairs, and as there are four courses of bricks on each side of the vault, four separate and different moulds would be required, besides a fifth, of which we shall presently have to speak. The four narrow sides of these bricks differ sensibly from one another. The two curved faces, being at different distances from the centre, are of unequal lengths; while, as the lower oblique edge is some inches below the upper in the curve, these two edges have different directions. In their disinclination to use stone voussoirs the Assyrian builders here found themselves compelled to mould bricks of very complicated form, and the way in which they accomplished their task speaks volumes for their skill.” The two upper voussoirs meeting and touching one another by one of their corners, the triangular space left empty between their edges was filled up either by wedge-shaped bricks or by mortar. The drain which we have just given as an example is 4 ft. 7 in. high under the keystone and 3 ft. 8 in. broad; the explorers were able to follow it to a length of 216 feet. To facilitate its construction the architect conceived the ingenious idea of building it upon an inclined plane--that is to say, that all the rows of voussoirs, instead of being perpendicular, lean considerably backwards, and are supported one upon the other; this system, which did not at all affect the solidity of the vaulting, allowed the builders to do without circular wooden frames. [Illustration: Fig. 42.--Vaulted drain at Khorsabad. Slope of the bricks (after Place).]