(5) Various cowls connected by metal tubes with the ceilings of rooms have been placed on roofs, and their aspirating effect used in ventilating these rooms. When a room is furnished with a chimney such cowls are most undesirable. In large rooms without a fire-place they are helpful, but much more confidence can be placed in cross-ventilation by hinged windows. It is doubtful if any of the advertised fixed cowls produce materially greater aspiration of air from rooms than a simple open tube of the same size. It is desirable that the tube should be protected at its upper end against the entry of rain, and that a grating should be provided to prevent birds building their nests in the tube. In the preceding plans of ventilation, the ceiling serves almost entirely as an outlet for impure air. In the following plan, it is used as an inlet for pure air. (6) In =Sylvester’s Method of Ventilation=, the perflating force of the wind is employed to produce an abundant entry of fresh air. A cowl is placed, always turning towards the wind; the air received is conducted to the basement, where it is warmed by a stove or hot-water pipes, and then passed through tubes into the upper rooms. From these it is carried by tubes above the roof, these tubes being covered with cowls turning from the wind, so that in this way the aspirating power of the wind is likewise used. Ships are often ventilated in a somewhat similar manner. The tube to which a windward cowl is attached above, ought to be bent at right angles, so as to lessen the velocity of the entering air. By covering other air-shafts with movable cowls, turning from the wind, the aspirating action of the wind is brought into action to aid the escape of foul air. =The Walls= of a room, unless covered with an impervious material, are constantly traversed by gentle currents of air, which play an important part in the ventilation of rooms. Special apertures may be made to furnish a freer supply, and these may be in various forms. (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. [Illustration: FIG 15. SHERINGHAM’S VENTILATOR.]