7. The _wall_ may be considered the lateral continuation of the column, answering the purposes both of support and inclosure. It is constructed of various materials, but chiefly of brick, stone, and marble, with a suitable proportion of mortar or cement. Walls are also made of wood, by first erecting a frame of timber and then covering it with boards; but these are more perishable materials, which require to be defended from the decomposing influence of the atmosphere, by paint or some other substance. 8. The _lintel_ is a beam extending in a right line from one column or wall to another over a vacant space. The _floor_ is a lateral continuation or connexion of beams, by means of a covering of planks. The strength of the lintel, and, in fact, every other elementary part of a building used as a support, can be mathematically determined by the skilful architect. 9. The _arch_ answers the same purpose as the lintel, although it far exceeds it in strength. It is composed of several pieces of a wedge-like form, and the joints formed by the contact of flat surfaces point to a common centre. While the workmen are constructing the arch, the materials are supported by a _centring_ of the shape of its internal surface. The upper stone of an arch is called the _key-stone_. The supports of an arch are called _abutments_; and a continuation of arches, an _arcade_. 10. The _vault_ is the lateral continuation of an arch, and bears the same relation to it that a wall bears to a column. The construction of a simple vault is the same with that of an arch, and it distributes its pressure equally along the walls or abutments. A complex or groined vault is made by the intersection of two of the common kind. The groined vault is much used in Gothic architecture. 11. The _dome_, or _cupola_, is a hemispherical or convex covering to a building or a part of it. When built of stone it is a very strong kind of structure, even more so than the arch, since the tendency of the parts to fall is counteracted by those above and below, as well as by those on each side. During the erection of the cupola, no centring is required, as in the case of the arch. 12. The _roof_ is the most common and cheap covering to buildings. It is sometimes flat, but most commonly oblique, in shape. A roof consisting of two oblique sides meeting at the top, is denominated a _pent_ roof; that with four oblique sides, a _hipped_ roof; and that with two sides, having each two inclinations of different obliquities, a _curb_ or _mansard_ roof. In modern times, roofs are constructed of wood, or of wood covered with some incombustible material, such as tiles, slate, and sheets of lead, tin, or copper. The elementary parts of buildings, as just described, are more or less applicable in almost every kind of architecture.