Another simple method of admitting fresh air to a room consists in leaving an aperture in the external wall, at a level between the ceiling of one apartment and the floor of the room immediately above, then to convey the fresh air through a channel from the external wall to the centre of the ceiling of the apartment below, where the air can be admitted by an opening, and dispersed by having a flat board or disc to impinge against, suspended 4 in. or 6 in. below the opening of the ceiling, and so scattered over the room. The cold air, however, thus admitted, plunges on the heads of the occupants of the room and mixes with the hot air which has risen near the ceiling. A top window-sash lowered a little to admit fresh air has the same disagreeable effect, the cold air being drawn towards the floor by the chimney draught, and leaving the hot air to stagnate near the ceiling. In any siphon system placed vertically the current of air will enter by the short arm, and take its exit by the long arm, and thus the chimney flue acts as the long arm of a siphon, drawing the fresh air from the nearest opening. Fresh air may be introduced through perforations made in the woodwork of the bottom rail of the door to the room, or through apertures in the outer wall, admitting the fresh air to spaces behind the skirting board, and making the latter perforated. The only objection to this plan is the liability for vermin to lodge between the skirting board and the wall. This may be prevented by covering the outside apertures with perforated zinc, but such covering also helps to keep out the full supply of fresh air. Butler recommends, while admitting the cold air through side walls near the floor level, and allowing the foul air to escape at the ceiling, that the fire draught should be maintained quite independent of the air inlet to the room, the requisite amount of air for combustion being supplied by a separate pipe led through the hearthstone with its face towards the fire, the latter acting as a pump, which is sure to procure its own allowance from the nearest source; thus the draught which would otherwise be felt by the fire drawing its supply from the inlet across the room is considerably reduced. The foul air may enter the ceiling in the centre, and be conducted by an air-flue either to the outside or to the chimney. The chimney is the best extractor, as its heated condition greatly favours the ventilating power.