It is difficult to say how the Assyrians set about building these arches of crude brick, but long practice enabled their architects to use that unsatisfactory material with a skill of which we had no suspicion before the exhumation of Nineveh. Thanks to its natural qualities and to the experienced knowledge with which it was prepared, their clay was tough and plastic to a degree that astonished the modern explorers on more than one occasion. The arched galleries cut during the excavations--sometimes segmental, sometimes pointed, and often of a considerable height and width--could never have stood in any other kind of earth without strong and numerous supports. And yet M. Place tells us that these very galleries, exactly in the condition in which the mattock left them, "provided lodging for the labourers engaged and their families, and ever since they have served as a refuge for the inhabitants of the neighbouring villages. Workmen and peasants have taken shelter under vaults similar to those of the ancient Assyrians. Sometimes we cut through the accidental accumulations of centuries, where the clay, far from having been carefully put in place, had rather lost many of its original qualities. Even there, however, the roof of our galleries remained suspended without any signs of instability, as if to bear witness that the Assyrian architect knew what he was about when he trusted so much to the virtues of a fictile material."[202] We may refer those who are specially interested in constructive methods to M. Place's account of the curious fashion in which the workmen of Mossoul will build a pointed vault without the help of any of those wooden centerings in use in Europe. In our day, certainly, the masons of Mossoul use stone and mortar, but their example none the less proves that similar results may once have been obtained in different materials.[203] A vault launched into mid-air without any centering, and bearing the workmen who were building it on its unfinished flanks, was a phenomenon calculated to astonish an architect. Taking everything into consideration the clay vaults of Khorsabad are no more surprising than these domes of modern Mossoul.[204] We cannot say for certain that the Assyrian builders made use of domes in addition to the barrel vaults, but all the probabilities are in favour of such an hypothesis. A dome is a peculiar kind of vault used for the covering of square, circular, or polygonal spaces. As for circular and polygonal rooms, none have been found in Assyria, but a few square ones have been disinterred. On the principal facade of Sargon's palace there are two of a fair size, some forty-eight feet each way. Thomas did not believe that a barrel vault was used in these apartments; the span would have been too great. He sought therefore for some method that would be at once well adapted to the special conditions and in harmony with the general system. This he found in the hemispherical dome.