great vaults of the palace at Hatra.[102] Throughout the city, so far as our knowledge goes, the vault is systematically used, and for the first time it is constructed of dressed stone, not of brick. For it must be borne in mind that the expansion in Asia of the Roman Imperial stone and mortar vaulted architecture encountered a similar expansion of brick vaulted architecture in which both material and structure point to an ancient oriental tradition and an independent Asiatic origin.[103] If Hatra is the oldest example of the systematic use of the vault in a monumental building, the very presence there of a method so fully developed postulates a long evolution. That this evolution was oriental is suggested by the fact that the forms which the vault assumes at Hatra can be traced back, almost without exception, to Asiatic brickwork, while the systematic employment of the vault is foreshadowed in hollow substructures which date from the Hellenistic era, and even from earlier times.[104] In Babylon such substructures, several stories high, roofed with stone slabs, would seem to have been devised before Alexander’s conquest, while Strabo’s description, which probably applies to a Hellenistic reconstruction, mentions terraces in which the vaults rested on cube-shaped piers, vaults and piers being built of burnt brick with a mortar of asphalt. Moreover, Strabo notes that in Seleucia, the capital of the Hellenistic kingdom on the Tigris, all the houses were vaulted on account of the want of timber.[105] That these vaults were of brick goes without saying; stone was even more difficult to obtain at Seleucia than wood. In this connexion the possibility that Nebuchadnezzar’s great hall at Babylon may have been covered with a vault should not be overlooked. The vaults of Hatra fall into five groups. 1. A primitive vault, composed of oversailing horizontal courses of stone is found in the small chambers of tombs (_Hatra_, ii, Figs. 93, 111, 155). Sometimes the walls incline smoothly inwards from base to summit until the space between them is narrowed sufficiently to admit of the imposition of a covering slab (_Hatra_, ii, Figs. 99, 118, 120, 155. In Fig. 155 the slope begins in the fourth course above the base). The vault built of oversailing horizontal courses was an obvious expedient for the roofing of narrow spaces, and it is, as might have been expected, widely distributed.[106] There is one instance at Hatra of a dome constructed in the same manner. It covers a rectangular chamber, 1·50 × 1·70 metres, and it is the solitary known example of an attempt on the part of Parthian builders to solve the problem of a circular vault over a rectangular substructure (_Hatra_, ii, Fig. 93).