The hygienic necessities of =Floors= are that they shall be impervious to moisture and to dust. On the =ground floor= the ordinary arrangement is to provide a joisted and boarded floor raised about a foot above the ground. Dry rot is one of the dangers in connection with such =boarded floors= on the ground floor. The chief causes which tend to induce rotting, are damp walls, lack of ventilation, contact with mortar, damp earth, or vegetable mould, and worst of all, alternations of damp and dryness, or wet along with heat. In order to avoid these dangers in connection with boarded floors, the ends of all timbers resting on walls should have a clear air-space around them, and communicate with the external air by means of perforated bricks. The larger timbers, girders, etc., should rest on stone templates, and the smaller joists on hoop-iron bonds. In all cases, the timber used should be well seasoned, and properly ventilated. The ends of oak posts, which are to be driven into the ground, should be charred, if the timber is old, or steeped in a solution of chloride of zinc. The ends of the joists should be trimmed, so as not to come too near to chimney flues. The best plan for flooring is to place an impervious flooring resting on the solid ground. This is more secure against rot than the boarded floor, and affords no space for dirt and vermin to lodge. Such an impervious floor may be formed of concrete over a layer of asphalte, as in the well-known terrazzo flooring. This is very suitable for corridors, pantries, etc. For living-rooms =wood-block flooring= is placed over the cement, molten pitch connecting the two. The blocks are 2 to 3 inches thick. If the wood is soft, as deal, it must be kept clean by washing; if hard, as oak or teak, it can be wax-polished. _Parquetry_ consists of small pieces of hard woods carefully fixed and polished. For =upper floors= the ordinary flooring is of floor-boards supported on wood joists, beneath which are wood laths and plaster. The floor-boards should be thoroughly seasoned, otherwise they will shrink, and the joints be filled with dirt. This dirt may accumulate for years between the floor and the ceiling of the room below, vitiating the air and helping to increase the stuffiness characteristic of dirty houses. Various plans are adopted for uniting the edges of floor-boards and preventing dust from dropping between the boards. [Illustration: FIG. 41. TONGUED FLOOR.] The one most commonly employed is the _ploughed and tongued floor_ (Fig. 41). In this, both edges of the floor are grooved so as to receive strips or tongues of iron or wood, an equal half of each strip being in the groove of each of two boards when they are in place. A less expensive method than the above is to _splay_ the ends of the boards so that they slightly overlap each other. This is not so efficient as the above, but is much better than simply placing the boards side to side as is commonly done.