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 vault has another strange singularity which at first is very surprising. The whole structure has a sensible inclination in the direction of its length, suggesting that some accident had happened to it in course of erection. Such an explanation must be rejected, however, because at the moment of discovery the whole arrangement was uninjured, and, moreover, the filling of clay must have rendered any movement of the kind impossible. M. Place's explanation seems the best. He thinks the slope was given merely to facilitate the work of the bricklayers. The first course of voussoirs would be sloped in this fashion, and would rest upon some mass of crude brick in the centre of the building. The bricks of the second course would lean against it, and their weight would be brought in to add cohesion and solidity to the whole structure instead of being entirely occupied in adding to the perpendicular thrust, while the ease with which they could be placed without an internal support would be much increased. Assisted by this simple expedient, two bricklayers with their labourers could build the vault at a very rapid rate. We may believe that the notion of building in this way would never have occurred to the Assyrian architects but for their habit of dispensing with timber centres.
This slope had an effect upon the arrangement of the bricks which should be noticed. In all other vaults, such as those of the city gates, the units are laid upon their longest sides, and a vertical section shows their shortest diameters. Here, on the other hand, the bricks stand on their edges, and their largest surfaces are in contact, on each side, with the next vertical course. If the full benefit of the natural cohesion between one brick and another was to be obtained, this method of laying them was absolutely necessary.
Internally, the drain we have been studying was four feet eight inches high from the floor to the crown of the vault. Its width was three feet nine inches, and its general slope very slight. It may be followed for a total length of about 220 feet, after which falls of earth have carried away the arch and the whole northern part of the esplanade, so that no trace of the mouth by which it opened on the plain can be traced.
The other sewer described by M. Place may be more summarily dismissed. In spite of their drawings and minute descriptions, explorers have not yet succeeded in explaining the eccentricities of construction it presents. 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]).
Fig. 93.—Sewer at Khorsabad, with semicircular vault; compiled from Place.