[Illustration: FIG. 73.--Egyptian construction, epitomized by Ch. Chipiez.] Consequently, if the proportions of the vertical and horizontal elements of a building, that is to say, its sections, have been skilfully determined, there is in the building itself no latent cause of disruption; its equilibrium is perfect, and can only be destroyed by external physical causes, by long exposure to the weather, by earthquakes, or by the hand of man.[104] [104] The vertical support and the architrave form the two vital elements of an Egyptian building, which is therefore enabled to dispense with those buttresses and other lateral supports which are necessary to give stability to the edifices of many other nations. We see then that the first impression caused by the external lines of the architectural monuments of Egypt is confirmed and explained by further study. They are built, as said the Pharaohs themselves, "for eternity." Stability, in a word, in its highest and most simple form is the distinguishing characteristic, the true originality, of Egyptian architecture. This character is most strongly marked in stone buildings, but it is by no means absent from those built of materials created by human industry. Works in brick form the transition between the construction that we have described and that which we call compact. A stone roof is not often found, and the termination is generally a terrace in which wood is the chief element. In some cases the secondary parts of such edifices, and sometimes the whole of them, are covered in by brick vaults, and maintained by walls of a sufficient thickness. Although the use of monoliths for roofing purposes was general in Egypt, it must not be thought that the architects of that country were ignorant of the art of covering voids with materials of small size, that is to say, of building vaults. There are numerous examples of Egyptian vaults, some of them of great antiquity, and, moreover, the Egyptian builders constructed their vaults after a method of their own. In spite of the facilities which they afforded, they played, however, but a secondary _rôle_ in the development of art. They were never used in the buildings to which greater importance was attached; they are introduced chiefly in out-of-the-way corners of the building, and in the substructures of great monumental combinations. This method of construction, being confined within such narrow limits, never resulted in Egypt in an architectural system;[105] neither did it give birth to any of those accessory forms which spring from its use. [105] We may here remark that the modest dwellings of the Egyptian fellah are often covered by vaults of pisé, that is to say, of compressed and kneaded clay. None of the ancient monuments of Egypt possess such vaults, which are of much less durability than those of stone or brick. We are, however, disposed to believe that they were used in antique times.