The most remarkable thing about this drain is the construction of the vault. The bricks composing it are trapezoidal in shape, two of their edges 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, for a brick of which we shall presently have to speak. The four narrow sides of these bricks differ sensibly one from 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. [Illustration: FIG. 92.--Drain at Khorsabad, with pointed arch. Section in perspective.] If we cast a glance at our Fig. 92 the first thing that strikes us is the absence of a keystone to the vault. The two rows of voussoirs that are in full view thrust against each other only by a single sharp edge; there is no keystone between them. In the row immediately behind, however, there is a stone (imperfectly seen in our illustration) that seems to play the part of a key. Thus we find that only at each alternate vertical course was the arch of burnt and moulded brick complete. The openings left at the summits of the other courses must have been filled in in some way, and, in fact, the line of voids which ran along the top of the extrados was filled in with brick earth, beaten tight and forming the best of keys. So that the vault was completed and consolidated by the same material as that used to make its channel impervious to water.[289]