(1) =A Simple Grating=, may be inserted; but this is apt to become blocked with dirt, and does not allow a large amount of air to enter. Louvred openings in the walls are objectionable, except for very large rooms.
(2) =Sheringham’s Valve= is the most convenient means of ventilating through the wall. An opening in the external wall is made by a ventilating brick or grating; into the wall is fixed an iron box, which has in front of it an iron valve hinged along its lower edge, so that it can open towards the room. On the sides of the valve cheeks are attached, which fit into the box when the valve is shut. A heavy piece of iron pressing against the valve from within the box, tends to keep it constantly open. By means of a string and pulley, the valve can be opened or closed at will, or fixed in any intermediate position.
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.