Another method, where steam or hot-water radiators are placed in the room, is to connect the outer air, either through the lower part of the window or through the wall of the room just below the window opening, with a space back of the radiator, so that the cold air entering will pass around and through the radiator and so be warmed as it enters. [Illustration: FIG. 15.--Ventilating device.] The picture (Fig. 15, after Jacobs) shows the arrangement of the radiators in one of the buildings of the University of Pennsylvania. A is the opening in the wall below the window; _D_ is a valve which regulates the amount of air entering through the opening; _R_ is the radiator; _B_ is a tin-lined box which surrounds the radiator; _T_ is a door in front of the box, which when raised allows the air of the room to be heated and to circulate through the radiator. By adjusting the two valves _D_ and _T_, air of any desired temperature can usually be obtained. Figure 16 (after Billings) shows an English device intended for the same purpose. The valve _D_ in this case operates to admit air, either through the radiator or to the space between the radiator and the wall, in order to vary the temperature of the entering air. The valve _T_ may be open or closed, and its position, together with that of the valve _F_, determines the proportion of the room air which is reheated. [Illustration: FIG. 16.--Ventilating device.] The writer remembers one schoolhouse where these methods were used successfully, the radiators being placed directly in front of the window and inclosed at the back, sides, and top, except for an opening to the outer air through the wall, properly controlled by a damper. In the writer's own office the radiators are by the side of the window and are boxed in, the connection being made with the outside air through a wooden box entering under the radiator. This is an admirable method, provided the radiator has sufficient surface to warm the fresh air admitted. Another excellent arrangement is to provide a narrow screen similar to that used for protection against flies, but with the screening material of muslin cloth instead of wire cloth. This muslin will break up the current of air so completely that no draft is felt by persons sitting even close to the open window. _Position of inlet._ The inlet for fresh air, if connecting directly with the outside air, should not be at the top of the room, since then the inlet would not serve to admit air, but rather to allow the warm air of the room to escape, and a burning match would inevitably show a draft outward instead of inward.