It is necessary also that the walls above the level of the ground should, as far as possible, be kept free from damp. =_Damp walls_=, not due to ascent of moisture, =_may be caused by_= (1) Rain falling on window-sills which do not project beyond the walls, and consequently do not throw the water clear of them. This is remedied by constructing the window-sills so as to project beyond the walls, and “throating” them to prevent rain from running along the bottom of the sill. The throat is shown at _a_, Fig. 39. (2) Rain falling on cornices and other projecting portions of the wall itself. The evil from this source may be diminished by sloping the top of the projection, downwards from the face of the wall. (3) Parapet walls, gables, etc., not being properly covered with coping. All such walls should be topped with a projecting slab of stone, or with a damp-proof course under the top course of bricks, which should be laid on edge. (4) Overflow from defective roof-gutters or rain-water pipes. In this case, either clearing out, repairing, or renewing is required. (5) Rain beating against the walls. This as a rule produces no great harm, if the walls are well constructed. Most of the water runs off as it falls on the surface. It is advisable, however, to protect a much exposed wall by a coating of Portland cement, or in extreme cases with slate. Various impervious paints have also been employed. [Illustration: FIG. 39. SECTION THROUGH WINDOW. Showing stone sill “weathered” at _i_, and “throated” at _a_. _b_—Wall. _c_—Inside plaster of room. _d_—Window-board. _e_—Oak sill. _f_—Beading and _g_—bottom rail of window-sash. _h_—Window. _j_—Iron tongue let into slot in _i_ and _e_ to prevent rain driving in.] If it is not proposed to coat exposed surfaces of brickwork, the wall may be formed of two parallel walls, two inches apart, and tied together by a sufficient number of bonding-ties of iron or glazed stoneware, or some other non-absorbent material. This arrangement is shown in Fig. 40. An excellent plan is to fill in the narrow space between two such walls, as the building proceeds, with asphalte or slab slate, thus forming a _vertical damp course_, in the same way as below the ground level. The evils arising from damp can be avoided in every new house by proper methods of construction. In an old house, however, they are much more difficult to remove. The dampness is indicated on entering, by a peculiar mouldy smell, and by the discolouration and destruction of wall-papers, and dry rotting of floor timbers. In such a case a damp course may, with care and patience, be inserted in the wall, and the soil under the basement may be covered with concrete, and a dry-area excavated around the basement. Free ventilation under the floor-boards of the lower floors also helps in keeping the house dry. [Illustration: FIG. 40. SHOWING HOLLOW WALL AND BONDING-TIES OF GLAZED STONEWARE.