ts. It
has two channels, one above the other, which are similar neither in slope
nor section. Moreover this double sewer is abruptly interrupted in the
middle of the artificial mound through which it runs. Must we believe that
it was never finished or used? We shall not attempt to answer this
question, but shall content ourselves with pointing to the similarities
between this tunnel and the last described. The same large stone slabs upon
a layer of bitumen, the same inclination of the body of the vault, the same
bricks formed in different moulds according to their place in the vault,
are found in each.

Our Fig. 93 shows the two channels and their position one above the other.
The pavement of the terrace, which consists of a double bed of large
bricks, rests upon the extrados of the upper channel. This vault is
semicircular; it has three voussoirs on each side, which, with the key,
make seven in each vertical course. But in consequence either of an error
in measurement or of a mistake in calculating the shrinking of the bricks,
there was a gap between the third voussoir on the right and the key. This
gap was filled in by the insertion of a stone cut into the shape of a
wedge. But for this fault--which, however, had no appreciable effect upon
its solidity--the vault would be perfect.[290] The narrow triangular
opening of the lower channel may be seen below it.

The semicircular vault gradually and insensibly changes into an elliptical
one. The side walls become lower, at each yard their height is diminished
by the thickness of a brick, and finally they disappear about the middle
of the total length. At the point shown in our Fig. 94 the arch has lost
its supports and rests directly upon the pavement of the channel. Its
ellipse is composed of eight voussoirs, four on each side, and a key with a
small wedge-shaped stone voussoir on each side of it. Between the two
points shown in our Figs. 93 and 94 the upper and lower sewers have become
one, the vaulted roof of the first and the paved floor of the second being
continued in a single tunnel. At the point where this tunnel comes to a
sudden end it is closed by a wall, through which two small openings are
pierced to serve as outlets for the sewer within (Fig. 94).

[Illustration: FIG. 93.--Sewer at Khorsabad, with semicircular vault;
compiled from Place.]

At different points on the Khorsabad mound, M. Place found other sewers,
some with depressed, some with basket-handle