The ordinary method of heating by the use of radiators is known as the _direct_ method. The air is heated by coming directly into contact with the radiators and distributed through the room by convection. If the arrangement is such that the air is brought from outdoors and heated by the radiator before entering the room, it is called the _indirect_ method of heating. Such an arrangement is illustrated in Fig. 29. The radiator is located beneath the floor, in a passage that takes the air from outdoors and after being heated, enters the room through a register located in the wall. Fig. 30 is still another arrangement known as the _direct-indirect_ method of heating. The radiator is placed in position, as for direct heating, but the air supply is taken from outdoors. The radiator base is enclosed and a double damper _T_ regulates the amount of air that comes from the outside. When the inside damper is closed and the outside damper is open, as is shown in the drawing, the air comes from outdoors and is heated as it passes through the radiator on its way to the room. If the dampers are reversed, the air circulates through the radiator as in the case of direct radiation. [Illustration: FIG. 27.--Two-column radiator suspended from the wall by brackets.] [Illustration: FIG. 28.--Dining-room radiator containing a warming oven.] In the use of the _direct_ or the _direct-indirect_ method of heating the principal object to be attained is that of ventilation, but quite generally the passages are so arranged that the air may be taken from outdoors or, if desired, the air of the house may be sent through the radiators to be reheated. In extremely cold and windy weather it is sometimes difficult to keep the house at the desired temperature when all of the air supply comes from the outside. Under such conditions the outside air is used only occasionally. In mild weather it is common to use the outdoor air most of the time. The cost of heating, when these methods are used, is higher than by direct radiation, because the air is being constantly changed in temperature from that of the outside to 70°. [Illustration: FIG. 29.--Ventilation by the indirect method of heating.] [Illustration: FIG. 30.--Ventilation by the direct-indirect method of heating.]