These theoretical chapters will be illustrated in the same fashion as the others, except that the illustrations will partake of the generalized and abstract character of the text which they accompany. In most cases they will be simple diagrams composed for the express purpose of illustrating the definitions or descriptions to which they belong. They will each refer to some essential element in the national architecture, to some element which is not peculiar to any one edifice more than another, but is to be found in all those which have similar aims and are constructed of the same materials. Such elements are above and outside such accidental variations as may be found in details of plan or ornament; they form part of the substantial inner constitution of the arts of Egypt and Chaldæa, and make their originality indisputable. § 2.--_General Principles of Form._ The external forms of Egyptian edifices are _pyramidoid_; in other words, the outward surfaces of their walls affect the form of a _trapezium_. Thus if we prolong these surfaces vertically we find that they unite at last in a point, in the case of square buildings (Fig. 58), and in a ridge in those which are oblong in plan (Fig. 59).[92] A square building will sometimes end in a ridge, or _aréte_, when the principal _façade_ and the corresponding one in its rear are vertical, the other two being inclined. [92] _Lois générales de l'Inclinaison des Colonnes dans la Construction des Temples grecs de l'Antiquité_, dedicated to his Majesty, Otho I., by CHARLES VILLEROI, engineer. Athens, 1842, 8vo. Horizontal lines predominate over inclined or vertical lines, and buildings, therefore, tend to develop in length and depth rather than in height. To this general rule, however, the pylons afford exceptions. [Illustration: FIG. 58.--Square building.] [Illustration: FIG. 59.--Rectangular and oblong building.] The terminations of their edifices were also horizontal. There was no necessity for sloping roofs, as, away from the immediate proximity of the sea, it hardly ever rains in Egypt. Moreover, the natural conformation of the country had its influence upon the creations of its inhabitants. The unforeseen and sudden variations, the contrasts of hill and plain, which we find in a mountainous country like Greece, are here unknown. Lower Egypt is a verdant plain, intersected by canals, and stretching from the sea to the desert; in Upper and Middle Egypt the lazy river is accompanied throughout its journey from south to north by two long chains of hills, the Arab chain and the Libyan, whose summits form an almost unbroken line. Between these aspects of nature and the works of man which they enframe, there is a striking general sympathy.[93]