The method of constructing a furnace suitable for a small public bath is, however, shown at Fig. 6. The excavations for stokery and heating chamber being completed, and the position of the furnace determined a solid foundation of concrete must be prepared, upon which the brickwork to support the stove must be laid. At the same time, the foundations for walls of furnace chamber, stokery, coke store, and the side walls for the horizontal cold-air conducting flues will be prepared. These latter must then be built in half-brick with glazed interior face, and the furnace inclosed in similar work, as shown in perspective sketch. The flues must be covered with York stone slabs 3 in. thick, up to within three inches or so of the convolutions of the stove, at which distance the side walls of the furnace must be erected, the back one similarly, and the front one round the four projecting doors, which are, respectively, the ash-pit door, the fire door, and two doors for cleansing the horizontal smoke-box and interior of convolutions. The furnace walls must be continued up to a few inches above the bend of iron smoke flue, and then--if, as shown, the furnace be small--covered with a 4-in. York slab in one piece. If the furnace be large, a flat brick arch must form the covering, as at Fig. 8, where this arch supports the flooring of the laconicum. The openings for the admission of the heated air into the conduit leading into the hot rooms may be either directly above, as shown in the last-named illustration, or in the side, as in Fig. 6, with inclined flues. As a rule, it is more economical, in heating on the principle now under consideration, to place the furnace below the level of the hot rooms; but if desirable to place both on one level, the back wall of the furnace chamber becomes the party wall of the laconicum, and it must be stopped short of the ceiling, and the air debouched over it. In cheap baths the interior face of furnace chamber may be of stock brickwork; but best glazed work should be adopted in good ones. All hot and cold-air ducts should be similarly lined with glazed ware. In first-class work the floors of horizontal and inclined flues should be of white glazed tiles set in cement. Manholes must be provided for cleaning when necessary. Every portion of furnace chamber, flues, shafts, and conduits for hot and cold air must be "get-at-able" either by means of manholes or by long brushes. Air-tight doors must be indicated on the plans wherever this necessity demands them. The iron smoke-pipe from furnace must be conducted to the smoke flue, and the connection between furnace chamber and flue hermetically sealed. The walls for a small furnace chamber need not be more than 4-1/2 in. thick. Large furnaces require walls one-brick thick. [Illustration: FIG. 7. An Air Filter.]