In making the foundation for a house, the ground should be excavated, so as to secure a solid bed of earth or rock not liable to be affected by the weather. A continuous bed of the best cement concrete should then be laid, not only under the walls, but covering the entire site of the house, and extending on every side at least 6 inches beyond the footings of the wall; and for footings it should never be less than 18 inches thick. The concrete serves two purposes: it, to a large extent, cuts off the entrance of the ground-air through the basement floor into the house; and prevents the entrance of damp into the house from below. To further ensure dryness where the floor is below the level of the adjacent ground, a _dry area_ is frequently provided, that is, a closed chamber lined with stone or cement below the ground level of the house, and surrounding the underground part of its four walls, or a hollow wall is built below the ground-level, as shown in Fig. 38. Neither a dry area nor a hollow wall constitutes the best arrangement, as the cavity is usually inaccessible, and rather aids than hinders the entry of the ground air into the house. The best plan is to provide a solid wall, impervious to both moisture and air. A vertical layer of roofing slates is sometimes used for this purpose; or, still better, a narrow cavity about 1∕2 to 3∕4-inch wide is provided in the body of the wall, and this is run full with molten asphalte. The =Walls= of the house must be provided with a “=_damp-proof course_=” carried through their whole thickness, slightly above the highest point at which the ground is touched. It may be formed by (1) sheet lead, which possesses the disadvantage of being costly; (2) two layers of ordinary roofing slate, set in cement, with broken joints,” _i.e._ the joints of the upper layer over the centre of the slates below them; (3) a layer of good asphalte, about ¾-inch thick; (4) perforated glazed stoneware slabs; or (5) two or three courses of hard blue Staffordshire bricks, laid without mortar. The use of asphalte is an excellent plan, and is now commonly adopted in good buildings. [Illustration: FIG. 38. DOUBLE DAMP-PROOF COURSE AND HOLLOW WALL. _a_, _a_—Damp-proof courses. _b_—Level of neighbouring ground. _c_—Floor-boards. _d_—Floor joist. _f_—Vertical space in wall. _g_—Concrete under foundation and over site of house.] This suffices when there is no basement. If there is a basement an open area around the house is desirable. Where for a given wall this cannot be secured, the vertical damp-proof course already described must be made to extend from the foundations well above the ground level. The open area, however, should be insisted on whenever practicable. The damp-proof course protects the walls from damp proceeding from the soil around or beneath the house.