The opening of windows and doors is possible only in warm weather; and as ventilation becomes a problem only in temperate and cold weather, the opening of windows and doors cannot very well be utilized without causing colds, etc. Various methods have therefore been proposed for using windows for the purposes of ventilation without producing forcible currents of air. The part of the window best fitted for the introduction of air is the space between the two sashes, where they meet. The ingress of air is made possible whenever the lower sash is raised or the upper one is lowered. In order to prevent cold air from without entering through the openings thus made, it has been proposed by Hinkes Bird to fit a block of wood in the lower opening; or else, as in Dr. Keen's arrangement, a piece of paper or cloth is used to cover the space left by the lifting or lowering of either or both sashes. Louvers or inclined panes or parts of these may also be used. Parts or entire window panes are sometimes wholly removed and replaced by tubes or perforated pieces of zinc, so that air may come in through the apertures. Again, apertures for inlets and outlets may be made directly in the walls of the rooms. These openings are filled in with porous bricks or with specially made bricks (like Ellison's conical bricks), or boxes provided with several openings. A very useful apparatus of this kind is the so-called Sheringham valve, which consists of an iron box fitted into the wall, the front of the box facing the room having an iron valve hinged along its lower edge, and so constructed that it can be opened or be closed at will to let a current of air pass upward. Another very good apparatus of this kind is the Tobin ventilator, consisting of horizontal tubes let through the walls, the outer ends open to the air, but the inner ends projecting into the room, where they are joined by vertical tubes carried up five feet or more from the floor, thus allowing the outside air to enter upwardly into the room. This plan is also adapted for filtering and cleaning the incoming air by placing cloth or other material across the lumen of the horizontal tubes to intercept dust, etc. McKinnell's ventilator is also a useful method of ventilation, especially of underground rooms. [Illustration: FIG. 5. HINKES BIRD WINDOW. (TAYLOR.)] [Illustration: FIG. 6. ELLISON'S AIR INLETS. (KNIGHT.)] [Illustration: FIG. 7. SHERINGHAM VALVE. (TAYLOR.)] [Illustration: FIG. 8. THE TOBIN VENTILATOR. (KNIGHT.)] [Illustration: FIG. 9. McKINNELL'S VENTILATOR. (TAYLOR.)] To assist the action of winds over the tops of shafts and chimneys, various cowls have been devised. These cowls are arranged so as to help aspirate the air from the tubes and chimneys, and prevent a down draught.