In a very large room, it is better to have several medium-sized valves, than a few larger ones, the air being thus more completely diffused. If there are two valves, they should not be opposite one another, as the air may then simply pass from one to the other, without becoming diffused through the room. If there is only one valve, it may occasionally serve as an outlet when the wind is to leeward. By means of this form of valve, the air is projected upwards in a diverging current towards the ceiling. The valve should be placed above the level of one’s head, but not too near the ceiling; as in the latter case, the current of air is driven hard against the ceiling, and falls thence with considerable force towards the floor. A combination of Sheringham’s inlet and Boyle’s mica outlet into the chimney at the opposite side of the room ensures efficient ventilation in a dining-room. Better than the outlet into the chimney is an opening into a special flue alongside the chimney-flue, if this be available. (4) =Ellison’s Inlet= consists of a brick pierced with conical holes, the apex of the cone being towards the external air. By this means any great draught is avoided, and the air is distributed over a considerable area. In order that this may prove an efficient means of ventilation, a considerable number of bricks are required. The =Floor= of a room is always the source of considerable currents of air, even when well carpeted. Air mounts up through the crevices of the wood-work, being aspirated into the room when its temperature is higher than that of the rooms below. In the case of rooms on the ground floor, air is often drawn from the subjacent soil, or through dust-bins, etc. Theoretically, in all measures of ventilation, the floor would be the best point for the entry of cold air. This, however, is intolerable when the incoming air is cold, and the floor must therefore be abandoned as a means of ventilation, apart from heating apparatus. The floor may be used as a means of entry of fresh air in a modified manner, by directing the air entering at the floor-level for some distance up a tube at the side of the wall. This apparatus is known as =Tobin’s tube=. It consists of a rectangular or cylindrical tube from 4 to 6 feet high, which communicates at the lowest point with the external air by means of a perforated brick or grating. The air enters the room in an upward direction, and is consequently sent towards the ceiling, where it becomes mixed with warmer air, before diffusing itself throughout the room. But when the incoming air is very cold, it may fall more rapidly, causing cold draughts on the heads of those in the room.