In regard to the arched structures at Nimrûd (Calah) Layard says he found a vaulted room and more than one arch. He tells us that “the arch was constructed upon the well-known principle of vaulted roofs, the bricks being placed sideways, one against the other, and having been probably sustained by a framework until the vault was completed.” Knowledge of the principle of dome-shaped roofs in Assyria as well as in the mother-country, is evidenced both by the discovery of rooms whose dimensions would have rendered any other mode of roofing impossible, and also by the representations on Assyrian bas-reliefs, as we have already seen.[87] [Illustration: FIG. 23. (After Andrae.) FIG. 24. (After Taylor.)] The arch-principle is further embodied in some of the Babylonian and Assyrian graves, and as there is no other opportunity of discussing the burial-places of the Babylonians and Assyrians in this volume, it may be permissible to give here a brief and general description of one or two of the best preserved of these burial-vaults. At Muḳeyyer (Ur) Taylor found a number of arched vaults (cf. Fig. 24) which in most cases measured about 5 feet in height, and 3 feet 7 inches in breadth, while they were about 7 feet long at the bottom and 5 feet long at the top. The arch is formed by successive layers of overlapping bricks. It is interesting to compare the burial vaults discovered by Andrae at Ḳalat Sherḳat (Ashur), one of the best preserved of which is seen in Fig. 23.[88] This vault was discovered about 16 feet below the level of the floor of a Parthian door in the neighbourhood, and over 13 feet below a later Assyrian pavement. At the time of its construction the vault would appear to have been about 9 feet beneath the surface of the soil.