It is a well-known fact that hot air is lighter than cold. Consequently, when a room is artificially heated, the highest temperature will be found just under the roof or ceiling, and articles placed in that part of a heated room will dry much faster than those near the floor. If the drying-room is heated by an ordinary stove, articles placed on a fairly low level will only dry very slowly, because the hot air flowing from the stove tends to ascend. In order, therefore, to utilise the entire space of the drying-room, it is necessary to place the heating apparatus in such a position that the whole of the room will be warmed as uniformly as possible. The stove should therefore be situated in a chamber underneath the drying-room proper. [Illustration: FIG. 20.] Because air that is already saturated with moisture cannot take up any further quantity, care must be taken to remove the damp air continuously from the drying-room, and to replace it by dry air. This may be effected by suitably designed ventilation, on the lines shown in Fig. 20, which represents a drying-room arranged in such a way as to provide for all the above-mentioned contingencies, and ensure continuous drying. The heating apparatus is located in the cellar, and consists preferably of a slow-combustion stove comprising a cast-iron cylinder, with an air inlet _A_ (with sliding regulator _T_), for the air of combustion, and a shoot _F_ at the top, through which the stove is fed with fuel--preferably coke, on account of its great heating power. The stove is surrounded by an iron or brick shell _M_, having two flues _R_ and _R1_ leading to the chambers I and II, where they terminate in register cowls _K_, which can be adjusted, by turning the handles _h_, so that when the slots _o_ in _K_ coincide with corresponding slots in the end of the pipe, the maximum amount of hot air from the stove is delivered into the drying-chambers; and, by suitably adjusting the cowls and the draught through the fire-door _T_, it is possible to regulate the temperature of the chambers to within one degree of the thermometer scale. When only one of the drying-chambers is required to be heated, the register in the other is closed, and the whole of the hot air is delivered to the first one. With this arrangement none of the heat is wasted, and the contents of one chamber can be dried while those of the other are being removed and replaced. The moisture-laden air from the drying-chambers can be led direct into the stove chimney. When coke is used, the flue gases consist almost entirely of carbon dioxide. If the vent pipes are led from the top of the drying-chambers into the chimney, the hot gases ascending the latter induce a strong draught in the chambers and carry off the moist air into the open. These pipes, also, are fitted with registers, which, when suitably adjusted, assist in the maintenance of a uniform drying temperature.