result is in plan a double column, similar to the double columns which carry the arcades of every early Christian church in central Anatolia.[355] I saw one of these double columns in a graveyard at Raqqah, where it is used as a tomb-stone. They are foreshadowed in the Nabataean façade at Si’ in the Ḥaurân.[356] [Illustration: FIG. 31. Hatra, façade of palace reconstructed. (From _Hatra_, by kind permission of the Deutsche Orient-Gesellschaft.)] The triple-arched façade must have been popular in the early Abbâsid period. It is found in the Bait al-Khalîfah at Sâmarrâ, where it is as pronounced as it was at Hatra. It was present in the two main façades of the audience chambers at Balkuwârâ.[357] But the single arched motive was to play an equally important part in Mohammadan architecture, a part of which an early (perhaps the earliest) indication is to be seen at Ukhaiḍir. On the north wall of the great hall the central feature is the great arch with its shallow calotte. Within this frame is set the smaller arched opening of the door. Here, as Fergusson has observed,[358] is the ‘perfectly satisfactory solution of a problem which has exercised the ingenuity of architects of all ages’. It has always been manifest ‘that to give a large building a door at all in proportion to its dimensions was, to say the least of it, very inconvenient. Men are only six feet high and they do not want portals through which elephants might march. It was left, however, for the Saracenic architects completely to get over the difficulty. They placed their portals--one or three, or five, of moderate dimensions--at the back of a semi-dome. This last feature thus became the porch or portico, and its dimensions became those of the portal, wholly irrespective of the size of the opening. No one, for instance, looking at this gateway (south gate of Akbar’s mosque at Fatehpur Sîkrî) can mistake that it is a doorway, and that only, and no one thinks of the size of the openings that are provided at its base. The semi-dome is the modulus of the design, and its scale that by which the imagination measures its magnificence’. The same principle rules over two of the smaller doorways of Ukhaiḍir, the doors at the outer ends of the corridor 5-6.