(3) In =Boyle’s Valve= these objections are partially obviated. It consists of an iron frame, across which lie iron rods; and from these are suspended thin talc plates, only capable of moving in the direction of the chimney (Fig. 13). Even this apparatus is rather noisy when there is a strong wind. [Illustration: View from room. View from chimney. FIG. 13. BOYLE’S MICA FLAP VENTILATOR.] Neither of these plans answers so well as a second flue alongside the chimney flue, communicating with each room near its ceiling; but the latter can only be arranged for when the house is built, while the valves may be inserted at any time. =The Ceiling= may be utilised for removing foul air; and thus serve to diminish the draught which is often produced by the currents of air towards the chimney, when this forms the only means of outlet. In large rooms (1) a sunlight gas-burner forms an important means of ventilation. It causes a strong up-current from every part of the room. If there is a fire in the room, the burner is apt to become an inlet for air, or the chimney to smoke, according to the relative strength of the two currents. (2) Benham’s and other forms of =Ventilating Gas Burners= serve the same purpose. In each of them the products of combustion are conveyed by special ducts above the ceiling to the outer air. (3) =McKinnell’s Ventilator= is useful in single-storied buildings, like certain barracks. It consists of two tubes encircling one another, the inner forming an outlet tube, because the casing of the outer tube maintains the temperature of the air in it. It is made higher than the outer tube, and is protected by a hood. The outer tube forms the inlet for fresh air. The entering air is thrown up towards the ceiling and then to the walls by a flange placed at the bottom of the inner tube. The air after traversing the room, and becoming heated, passes upwards to the inner tube. When doors and windows are open, both tubes become outlets; if there is a fire in the room, they may both become inlets; but this may be prevented by closing the outlet tube. [Illustration: FIG. 14. MCKINNELL’S ROOF VENTILATOR.] (4) Various other means have been devised for carrying foul air from the ceiling through channels between the ceiling and the floor of the room above. All share the disadvantage that the channels become dirty and are difficult or impossible of access for cleaning.