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).
2. The true vault oversailing the wall occurs in numerous tomb chambers (Hatra, ii, Figs. 100, 105, 125, 130, 144, 145, 149, 152, 163), as well as in most of the smaller rooms of the inner palace (Hatra, ii, Figs. 225, 226, 237, and Plate 8) ([Plate 74], Fig. 2). It is a form which originated in brick building. It is found in Assyrian brick tombs,[107] but never, so far as my knowledge goes, in any dressed stone vaults save in those of Hatra. It appears at Ctesiphon in the side vaults,[108] and in the rough stonework of Qaṣr-i-Shîrîn ([Plate 52], Fig. 2, and Plate 68, Fig. 1). It is constant at Ukhaiḍir and in early Mohammadan architecture,[109] and it is used invariably in the brick vaulted constructions of Mesopotamia at the present day. It is perhaps the triumphant survival of the old brick vault of horizontal oversailing courses, represented by Mughair, and it bears, at Hatra and elsewhere, another indubitable mark of its brick origin in the horizontal or almost horizontal joints of its lower courses.[110]
3. The vault springing flush with the walls is used in tombs (Hatra, ii, Figs. 103, 118, 128, 139, 159), in the southern and in the northern lîwâns of the main palace and in the two lîwâns which were added at the northern end (Hatra, ii, Plate 8), in the western annex, the so-called temple (Hatra, ii, Plate 9), and in building B (Hatra, ii, Fig. 183). The moulded cornice, which usually divides this vault from the walls below, is absent in most of the tombs. The high stilt formed by the horizontal lower courses, which is especially remarkable in the larger of these vaults, differentiates them from western Hellenistic vaulting and connects them more closely with brick forms. In one of the smaller palaces there is a striking example of the survival of brick building methods (Hatra, ii, Fig. 74). The stone vault is composed, almost to its whole height, of horizontal courses, and only the very top of the arch is filled in with radiating voussoirs. Nor is the elliptical vault, which is the form naturally assumed by oriental uncentered brickwork[111] wanting at Hatra (Hatra, ii, Figs. 108 and 162, Fig. 162 being a primitive example, where the vault is carried down to the floor of the chamber).
4. One room on the upper floor of the palace shows a fuller comprehension of the thrust and buttressing of the vault (room No. 12, Hatra, ii, Plate 10 and Fig. 226). The space to be covered is diminished by placing two arched niches on either side, a system which points the way to the breaking up of the wall into buttressing piers. This principle was carried out yet further by Sasanian builders. In the palace of Sarvistân the lower portion of the piers was detached from the body of the wall and further lightened by being divided into two small columns,[112] while angle piers terminating in a single detached column bore the dome of a chamber situated at the back of the palace ([Plate 74], Fig. 1). The advance in structural knowledge thus gained was carried little further in these regions; indeed it is curious to observe that Ukhaiḍir exhibits a movement in the opposite direction. Although in rooms 33 and 40 the vaults are set upon columns which stand absolutely free, the vault of the great hall rests upon arched niches whereof the piers are connected with the wall, and the principle of the detached column is recalled only by the engaged columns which form part of the pier. The arcade on free standing columns with a vaulted corridor behind it is of frequent occurrence, but the fact that in all the palace only one, and that one the shortest, of these arcades remains standing (No. 20) shows that the skill of the builders was at fault. Again, in the church of Mâr Ṭahmâsgerd at Kerkûk the engaged columns are present, as in the great hall of Ukhaiḍir, but in the same manner they are structurally one with the piers behind them[113] ([Plate 75], Fig. 1); and in the churches of northern Mesopotamia, where deep niches under the vault are a constant feature, the engaged pier of Hatra returns in all its primitive simplicity.[114] Whether the data afforded by extant monuments in Mesopotamia and Persia are conclusive would be hard to determine. The setting of arch, vault, and dome on free standing supports would seem to have been a conception deeply rooted in Hellenistic art, but for actual examples we can adduce only the evidence of relief architecture or the disposition of rock-cut tombs and temples. The blind order under the vault of the men’s caldarium near the forum at Pompeii,[115] the rock-cut dome on engaged columns of the Hellenistic tomb of Akeldama at Jerusalem[116] exhibit a motive to which the architecture of a later age was to give fully developed plastic execution. Yet more explicit are the indications afforded by the rock-cut monuments of Egypt and of India. At Memphis one of the graves of the Persian period shows a vaulted nave resting on piers,[117] and the rock-cut temples of Hellenistic India, with their long vaulted naves resting on columns,[118] point to similar achievements in the Seleucid architecture of Mesopotamia from which they are derived. The existence of an underlying desire to solve statical problems which were of the highest importance to the spatial interior is attested by the sporadic survival of such buildings as the Praetorium at Musmiyyeh and a room in the Golden House of Nero,[119] where the four-sided and the round dome were placed respectively on piers and on columns; but the final mastery was reserved for early Christian builders of the Hellenistic coast-lands, or developed in the same age in Rome out of methods which were specifically Roman, such as the intersecting barrel vault and construction in concrete. In Rome also the original impulse may have come from the East.[120]
5. In three of the upper rooms in the palace (Nos. 13, 15, and 16, Hatra, ii, Figs. 227 and 228, and Plate 10) the roof is formed by means of transverse arches (respectively five, three, and one in number) carrying stone slabs which cover the space between them. This type of roof was universally employed in Syria from Nabataean times until the Mohammadan invasion.[121] It was a simple and a satisfactory method of roofing in stone in a country where centering beams, sufficiently massive to sustain a stone vault, were difficult to obtain. I know no other Mesopotamian example of it in stone, but it was copied in Sasanian brickwork, where the stone slab was replaced by a brick vault running at right angles to the main axis.[122] In this form it finds a place at Ukhaiḍir in room 32, and it continued to be used by Mohammadan builders in the Middle Ages, the most renowned example being that of Khân Orthma, at Baghdâd.[123]
The absence of the dome at Hatra is significant. The small square chambers of the palace were well suited to dome construction, yet nothing but the barrel vault is present. Moreover, it is the barrel vault in its simplest expression; not even an intersection is attempted. In the vaulted passage surrounding the central chamber of the western annex, the ‘temple’, one end of the vault terminates on each of the four sides against a transverse arch, whereby the insuperable difficulty of intersection was avoided[124] ([Plate 75], Fig. 2). Hellenistic builders had attacked the problem as early as the second century B.C. in Asia Minor,[125] and yet more boldly in Rome.[126] I know no single example of the intersection of barrel vaults in Sasanian buildings; even at Ukhaiḍir the system is sparingly used, and never without careful abutment. Where two barrel vaults meet at right angles, they are either joined together diagonally, without intersection, as in the chemin de ronde, or they terminate against transverse arches, and not infrequently in the rectangular space thus formed, a semi-dome takes the place of the intersecting vault, as in the mosque and in the upper gallery No. 134. The rock-cut temples of India exhibit a similar termination of the barrel vault in a semi-dome.[127] The dome, though it is at Ukhaiḍir of frequent occurrence, the chambers of the chemin de ronde in all the round towers being domed as well as the two chambers north and south of the great hall, Nos. 4 and 27, is never placed over a span wider than 3·10 metres. The square rooms, Nos. 30 and 141, behind the two lîwâns 29 and 140, where, on the analogy of the Sasanian palaces (see below, pp. [74], [76] and [78]) a dome might be expected, are covered in one case by a barrel vault, and in the other case by a groined vault. There was no question here of a dome on free standing columns; where the opportunity occurred, in rooms 33 and 40, it was set aside in favour of parallel barrel vaults. The domed chambers in the towers have a circular ground-plan, and when the problem presented by the rectangular substructure arose, it was met in a fashion which is applicable only to very small edifices. The dome in No. 4, and all the calottes over rectangular niches, are set over the angles upon horizontal brackets of masonry. On the octagon, or half-octagon, thus formed, a circle or segment of a circle of small diameter could be placed without any difficulty. It was an expedient which had been adopted by early dome builders both in Syria and Asia Minor,[128] but it was inadequate when the space to be covered assumed larger dimensions and, before the date of Ukhaiḍir, Byzantine and Sasanian architects had elaborated solutions of the problem. In the West the great dome of Santa Sofia had already been placed securely upon stone pendentives; in Persia the use of the arched angle niche, or squinch, had enabled Sasanian builders to throw their domes over a span of 16 metres. 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.
Firûzâbâd affords the earliest extant example of the dome in Persia. In Babylonia and Assyria no dome is standing which can be dated earlier than Ukhaiḍir. Possibly the Lakhmid ḥîrahs would have provided us with other instances, but the tentative nature of dome building at Ukhaiḍir throws doubt upon the proficiency of Lakhmid construction in this respect.[131] In the Babylonian cultural sphere the dome does not seem to have played an important part in monumental building until a late period, and in my opinion too much significance has been attached to the celebrated relief exhibiting domed buildings which Layard found at Quyundjik.[132] We have here a representation of village architecture, and it is natural to suppose that the domes were of small dimensions. They are to be found to this day in the village architecture of northern Syria and northern Mesopotamia, indeed no other form of roof exists; and they take the shapes depicted upon the relief. They are built of sun-dried brick held together by a mortar of clay. The high ovoid domes which appear upon the relief and in modern villages are built of oversailing rings, like the solitary dome at Hatra. I imagine that the summit of the round domes is constructed over a light centering, but I have not actually seen them in process of being built. The difficulties presented by these methods are practically nil, owing to the light and malleable material and the smallness of the span. The translation of this primitive dome into larger diameters was a very different matter, and there is no evidence for the belief that this step was taken in Mesopotamia in an early age.
The Sasanian conquerors came out of lands on which Hellenism had made an impression less deep than on Mesopotamia, lands where Rome had never penetrated; and they came of a stock more tenacious of its own traditions and less eclectic than the Parthians. To a large extent they re-orientalized the territories which they occupied. No doubt there was less for them to copy, for in the interval of some 300 years during which the Parthians were predominant, Seleucid monuments must have disappeared, and the blurred Arsacid copy of Greek or Roman models had taken their place. The Sasanians created an art of their own, less dependent than that of Parthia on Western forms, and more potent to influence those who came into contact with it, not excluding the Byzantines. In the earliest of their palaces, so strongly marked is the reversion to Achaemenid types that Dieulafoy relegated it unhesitatingly to the earlier Persian period. In its general characteristics the plan of Firûzâbâd differs little from that of an Achaemenid khilâni palace ([Plate 73], Fig. 2). The lîwân has deepened, and the employment of the vault has enabled the builder to dispense, as at Hatra, with the columns that sustained its roof. The greater depth of the lîwân, combined with a desire to keep the vaulting span within moderate bounds, have led to the breaking up of the tower room on either side into two narrow chambers. In order to counteract more effectually the thrust of the main vault (13·30 metres wide) the side chambers are placed at right angles to the lîwân, a principle which was not adopted at Hatra, but which rules at Ctesiphon, and at Ukhaiḍir. The towers themselves have disappeared, and though their place remains in the plan, in the elevation it is probable that the façade presented an unbroken line. The audience hall of the khilâni palace is reduced to a domed chamber, and the clumsy construction of the dome makes it evident that the builder would not have ventured to stretch its diameter further. Finally, round the posterior courtyard are grouped, besides the living-rooms, two smaller lîwâns, placed, like those in the Ukhaiḍir courts, so that they may serve respectively for winter and for summer.
The resemblances in detail between the Achaemenid palaces and Firûzâbâd are no less striking. The high fluted gorge and narrow torus of stone which cover the doorways and niches of the one are repeated in the plaster-work of the other. The plain fillets which surround the openings at Persepolis reappear at Firûzâbâd, but in the latter case all the openings are arched, and the moulded archivolt is set within the rectangle formed by the fillets. The ṭâqchah niches, which, so far as my knowledge goes, are found for the first time in the palace of Darius, are present also at Firûzâbâd,[133] and henceforth assume a permanent place in Persian architecture, from which they were borrowed by Mohammadan builders.
The building material at Firûzâbâd is undressed stone, very roughly coursed and set in a bed of mortar. In the domes the stones are cut thinner, more carefully coursed and provided at intervals with a bonding course; in the vaults the thin slabs are laid vertically, parallel with the main axis of the chamber. Exactly the same principles are observed at Ukhaiḍir. Nor do the resemblances end here. Tubes are not absent from the vaulting system,[134] and most of the archways are set back from the jambs to facilitate the placing of centering.[135] The arches are semicircular as at Qaṣr-i-Shîrîn. In the vault of the big lîwân centering would seem to have been used, for it is set back from the face of the walls, doubtless in order to leave a convenient ledge for the centering beams. The vaults and domes here and in all other Sasanian buildings have the ovoid shape common to Ukhaiḍir and to subsequent Mohammadan work in Mesopotamia. It is the old Mesopotamian vault contour. The exterior walls of Firûzâbâd are broken into a continuous series of recessed and arched blind niches divided by engaged columns carrying an entablature of modest proportions.[136] The appearance of this decoration is to my eyes so entirely un-Hellenistic that I have difficulty in connecting it with any classical influence, and in point of fact an arched niche from one of the reliefs from Quyundjik, in the British Museum ([Fig. 11]), is nearer akin to it than such