The three domes of Firûzâbâd, the earliest of the Sasanian palaces, have a diameter of 13·30 metres; the larger of the two domes at Sarvistân is about 12 metres across, the dome in the smaller palace at Qaṣr-i-Shîrîn covered a chamber 16·15 metres square.[129] If the audience chamber in the larger palace at Qaṣr-i-Shîrîn was domed, as I suspect, it covered an area about 16 metres square. Under this dome, at each angle, at a distance of 2·90 metres from the walls stands a corner pier 1·40 metres square, terminating on the two inner sides in an engaged column 1 metre in length. The distance between the piers is thus about 16 metres, that is to say that the dome would have been no larger in diameter than that which covered the principal chamber in the neighbouring palace. The walls there are 3·90 metres thick, whereas the side walls of the chamber in the palace of Khusrau are never more than 2 metres thick, but in the one case the wall was the only support, whereas in the other the thrust would have been taken first by the angle supports and by them transferred to the outer wall. Moreover, the walls themselves were buttressed by vaulted rooms. The piers are buried about 1 metre in the ruins with which the hall is filled (the ruin heaps lie deepest along the walls and reach almost to the height of a doorway arch which remains in place on the south side); the best preserved of the four piers projects less than 1 metre out of the present surface; that is to say that its whole height is at present under 2 metres. It is conceivable that the piers may at no time have been carried very much higher. Like the columns under the small dome at Sarvistân, they may have been bound into the wall at that level by arches carrying a barrel vault, which would in this instance have had a span of 5·20 metres, and the dome placed upon the square substructure thus formed would reproduce the Sarvistân dome in magnified proportions.[130] It is clear that Ukhaiḍir shows a retrogression in the art of dome building, both in point of span and in point of distribution of thrusts, nor is the fact surprising. The desert ḥîrah of an early Mohammadan prince need not be expected to rival in architectural achievement the summer palace of the Sasanian king of kings, situated upon one of the high roads of his empire.