All doubts on the subject were taken away, however, by the discovery of the bas-relief (Fig. 43) reproduced on page 145, in which we find a group of buildings roofed, some with spherical vaults, some with elliptical domes approaching a cone in outline. This proves that the Mesopotamian architects were acquainted with different kinds of domes, just as they were with varieties of the barrel vault. It has been guessed that this bas-relief, which is unique in its way, merely represents the brick-kilns used in the construction of the palace of Sennacherib. To this objection there is more than one answer. The Assyrian sculptures we possess represent but a small part of the whole, and each fresh discovery introduces us to forms previously unknown. Moreover, had the sculptor wished to represent the kilns in which the bricks for the palace were burnt, he would have shown the flames coming out at the top. In reliefs of burning towns he never leaves out the flames, and in this case, where they would have served to mark the activity with which the building operations were pushed on, he would certainly not have omitted them. Again, is not the building on the left of the picture obviously a flat-roofed house? If that be so we must believe, before we accept the kiln theory, that the sculptor made a strange departure from the real proportions of the respective buildings. The doorways, too, in the relief are exactly like those of an ordinary house, while they bear no resemblance to the low and narrow openings which have been used at all times for kilns. Why then should we refuse to admit that there were vaults in Nineveh, when Strabo tells us expressly that "all the houses of Babylon were vaulted."[205] Thomas invokes the immemorial custom of the East to support the evidence of this curious relief:--the great church of St. Sophia, the Byzantine churches and the Turkish mosques, all of which had no other roof but a cupola. In all of these he sees nothing but late examples of a characteristic method of construction which had been invented and perfected many centuries before at Babylon and Nineveh. From the monuments with which those two great cities were adorned nothing but the foundations and parts of the walls have come down to our day; but the buildings of a later epoch, of the periods when Seleucia and Ctesiphon enjoyed the heritage of Babylon, have been more fortunate. In the ruins which are acknowledged to be those of the palaces built by the Parthian and Sassanid monarchs, the upper structures are still in existence, and in a more or less well preserved condition. In these the dome arrangement is universal. Sometimes, as at Firouz-Abad (Fig. 52), we find the segment of a sphere; elsewhere, as at Sarbistan (Fig. 53), the cupola is ovoid. Our section of the latter building will give an idea of the internal arrangements of these structures, and will show how the architect contrived to suspend a circular dome over a square apartment.[206]