Neither is it desirable to have the fresh-air inlet near the floor of the room unless the entering air is warm, because cold air admitted will flow across the floor and remain there, not disturbing the warm upper layers. The effect then is not to improve the ventilation, but only to chill the feet of persons sitting in the room. The position of the window lends itself, therefore, to admission of fresh air, since it is neither at the top nor at the bottom of the room, but at the level most suitable for such admission. _Foul-air outlet._ Very few houses have any provision for the outlet of spent air, and if ventilation is thought of at all, the only idea usually is to provide, in part at least, for the admission of air and to make no adequate arrangement for its egress. Whenever a stove or fire-place is in use, the mere burning of fuel requires the consumption of air, and in cases where apparently no air is admitted to the room, insensible ventilation is at work bringing into the room, through the walls and through cracks around the doors and windows, the necessary air for combustion. [Illustration: FIG. 17.--Ventilation by means of coal stove.] It may be proved by the laws of physics that a coal stove burning freely in a room causes adequate ventilation; and that only where the dampers of the stove are closed, so that not merely is the supply of fresh air diminished, but also the products of combustion are thrown out into the room, is there danger from lack of ventilation. The stovepipe in this case furnishes the necessary outlet for the impure air, and the following suggestion has been made in order to utilize this outlet, even when the fire is not burning freely or when the damper in the stovepipe is closed. If the stovepipe from a stove is carried horizontally, as it usually is, an elbow must be provided to raise the pipe to the stove hole in the chimney. Then providing a T connection at the point marked _A_ in Fig. 17 (after Billings), the lower part of the _T_ may be carried to within a foot of the floor with a damper at the points _B_ and _C_. When the fire is burning freely, the damper at _C_ is closed, and ventilation is secured through the stove, the damper at _B_ being open. When the damper at _B_ is closed and the fire checked, then the damper at _C_ may be opened and the impure air drawn up the chimney from the level of the floor. This, it is said, is an effective arrangement for drawing off the polluted air of a room. [Illustration: FIG. 18.--Coal-stove ventilation.]