[106] Another explanation has been given of the employment of the vault in subterranean work. Mariette believed the arch to be symbolic, to signify the canopy of heaven, the heaven of Amen. One objection to this is the fact that the vault was not universal in tombs; some of those at Beni-Hassan have flat ceilings, others have coves. §5.--_Compact Construction._ [Illustration: FIG. 80.--Granaries, from a bas-relief.] The methods employed in what we may call _compact construction_ permit the use, in considerable quantities, of moulded clay mixed with chopped straw. This material was used in buildings which were homogeneous; it was poured into a mould formed by planks, which was raised as the work progressed and the mixture dried. But the material had little strength, and was far inferior to those modern concretes which have the density and durability of the hardest stone. The Egyptians do not seem to have been acquainted with concrete proper, and unburnt bricks did not differ essentially from pisé. Such bricks, when placed one upon another after being imperfectly dried, combined, under the influence of the weather and their own weight, into one homogeneous mass so that the separate courses became undistinguishable. This latter fact has been frequently noticed in Assyria, by those who had to cut through the thickness of walls in the process of excavation. [Illustration: FIG. 81.--Modern pigeon house, Thebes.] If voids have to be covered in pisé, one of those self-supporting curves which we have described under the name of vaults, must be made use of, and the vault must be constructed over a centring of wood. But we have no evidence that the Egyptians could carry the art of construction to this point in pisé. On the contrary, we have good reason to believe that they generally made use of this material for the quiescent body of the edifice alone, and that voids were mostly covered with stone or wood. In a word, the Egyptians did not carry the use of artificial material far enough to form a complete system based upon it. They made great use of it, but only in a strictly limited fashion. It is only found in certain well-defined parts of buildings, which were never of any very great interest from an artistic point of view (Fig. 80). It deserved to be mentioned, if only for the frequency of its use in Egypt, in the private architecture of both ancient and modern times (Fig. 81), but it need not detain us longer. § 6.--_Construction by Assemblage._