ne slabs overpassing the
inside width of the channel by several inches. By this means the internal
joints were reduced to a minimum, and a further precaution was taken by
placing the slabs in a bath of asphalte, which was also used to coat the
oblique channels and the foot of the vertical pipe. The low perpendicular
walls upon which the vault was to be placed were built upon the outer edge
of these wide slabs. They were of four-inch bricks, carefully laid.

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]

This