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540 about the middle of the fifth century, that is half-way between the reigns of Constantine and Justinian.

In the East from a very early time ordinary building works were for the most part done with sun-dried mud bricks. In hot, dry countries this forms a fairly good material. Besides this use of crude bricks there had come down a still simpler way of building by aggregations of clay. The mud, even when subdivided into crude bricks, adhered so thoroughly when put together in a mass with liquid mud in the joints, that a type of structure was developed which was homogeneous; the roofs and floors being of the same materials as the walls, and continuous with them. The chambers, large or small, were cells in a mass-material. Such a method of building was common to the valleys of the Nile and the great rivers of Western Asia. Burnt bricks were in turn developed from mud bricks by an extension of the method found so successful in making pottery. Such bricks were often used for special purposes in combination with the crude bricks from an early time. The building forms made use of in typical Byzantine architecture largely depended on the use of brick, which may be regarded as the bringing together of small units well cemented so as to form continuous walls and vaults. Burnt bricks were usually set in so much mortar, the bricks being thin and the joints thick, that the whole became a sort of built concrete. The mortar in a wall, in fact, must frequently have been much more in quantity than the bricks.

Arising doubtless out of primitive ways of forming mud roofs, it became customary later to construct vaults of mud bricks, and then of burnt bricks, by leaning the courses against an end wall so that the vault was gradually drawn forward from the end of a given chamber in inclined layers. Each layer was thus supported by the part already done and no centring was required. Domes came to be erected in a somewhat similar way. A rod or a cord being attached to the centre so as to be readily turned in any direction, a dome was reared on its circular base, a course at a time, the curvature being determined by the length of the rod or cord. About 1670 Dr Covel described this method of procedure, and it is still practised in the East, although skilled dome builders are now but few.

If a dome is not set over a circle, but over an octagon or a square, a troublesome question arises in regard to the angles. Where the chamber is small, and especially in the case of the octagonal form, the work can easily be jutted out in the angles so as roughly to conform to the circular base required for the dome. When, however, a square area is large, some regular solution becomes necessary. The angles of the square may be cut off by diagonal arches so as to form an octagon. If such arches are so built as to continue back into the angles forming little vaults, on a triangular base, they are called squinches. In such cases as these the base of the dome is governed by the width across the chamber, but it is possible to plan a dome on the diagonal dimensions of the area to be