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.
Egyptian vaults may be divided into two great categories, according to the method of their construction.
1. Off-set vaults. These vaults are composed of courses off-set one from another, and with their faces hollowed to the segment of a circle. ([Fig. 74].)
Fig. 74.—Element of
an off-set arch.
Fig. 75.—Arrangement of the courses
in an off-set arch.
If the face of those stones which, in the form of inverted steps, are turned to the void which has to be covered, be cut into the line of a continuous curve, the superficial appearance of a segmental arch or barrel vault will be obtained; but this appearance will be no more than superficial, the vault will be in fact a false one, because, in such a construction, all the stones which enframe the void and offer to the eye the form of a vault, are really laid horizontally one upon another, and their lateral joints are vertical. ([Fig. 76].) When the units of such vaults are properly proportioned they are stable in themselves, and they have no lateral thrust.