But in regard to the study of what may be called the arch-principle, the subterranean channels which formed part of the system of drainage employed by the Assyrians are of greater importance. These aqueducts are found in all the palaces, both at Nimrûd and Kouyunjik, but Khorsabad furnished the best preserved examples, and therefore afforded the most valuable material for the careful examination of this architectural contrivance. At Khorsabad, Place discovered several arched drains of different shapes, some of them being round, others elliptical, while others again were pointed, but apparently in every case the stones or bricks were set at an angle, so that each course had the support of the course preceding it, and thus the pressure on the centre of the arch was reduced to the minimum. In the case of the pointed arched aqueducts found by Place, the arches in question are no true keystone arches, indeed they have no keystones of any kind as will be seen in Figs. 19, 20; this arched drain measures four feet eight inches from the ground to the centre of the vault, its width is about three feet nine inches, while its original length is unfortunately not known, though Place succeeded in tracing it for some two hundred and twenty feet. The floor was made of large slabs of limestone set in asphalt, while the ends of the stone-slabs extended beyond the walls of the vault on either side. The rounded type of arch is seen in Fig. 21; it is semicircular in shape and is formed of three voussoirs on each side, which together with the key, thus make seven in all, but owing to some miscalculation, the keystone appears to have been too small, in consequence of which there was a gap between it and the top voussoir on the right, which was filled in by means of a stone wedge which can be seen in the figure. Its width and height vary at different points, in some places it is said to be wide enough for two men to walk abreast in it.[86] The floor was composed of slabs of limestone which were laid in asphalt just like the floor of the pointed arch described above. An elliptical-shaped arch, also found at Khorsabad, is illustrated in Fig. 22: it is formed of eight voussoirs and a keystone, the gap on either side of which is filled in by means of two stone wedges. The failure to make the keystones sufficiently large, and the consequent necessity for these supplementary wedges may be due to the architects not having allowed for the shrinkage of the bricks. [Illustration: FIG. 19. (Place, _Nineveh_, Pl. 38.)] [Illustration: FIG. 20. (Place, _Nineveh_, Pl. 38.)] [Illustration: FIG. 21. (Place, _Nineveh_, Pl. 39.)] [Illustration: FIG. 22. (Place, _Nineveh_, Pl. 39.)]