=Outlets=, under ordinary circumstances, are best placed near the ceiling. They should be enclosed as far as possible within walls, so as to prevent the outgoing air being cooled; and should have smooth walls, reducing friction to a minimum. Where artificial warmth increases the temperature of the air, the discharge of outlets is much more certain and constant. The chimney with an open fire forms one of the best outlets. Gas, again, may be made to heat an outlet tube, which carries off the products of combustion. Two forms of ventilation are usually described—natural and artificial. The former term is used to describe any plan not requiring heating apparatus or the motive power of steam, or gas, or electricity, while the latter implies the use of some such motive power or source of heat. Obviously, however, there is no sharp line of demarcation between the two. A lighted fire is strictly an artificial plan of ventilation, but inasmuch as no apparatus intended for ventilating purposes is required, it is hardly a means of artificial ventilation. =Natural Ventilation.=—The most important means of natural ventilation are the window and the chimney; but openings in outer walls and over the door may form valuable adjuncts. =The Window= is perhaps the most important agent in purifying a room—both the light and air it admits being essential for health. The window is invaluable (1) =for flushing the room= with fresh air at intervals. Where possible, opposite windows should be opened, or window and door. Cross-ventilation by opposite windows open at the top forms one of the best means of natural ventilation, in large rooms, such as school-rooms. This can, as a rule, be borne without discomfort, while the room is occupied, unless the wind is very high. (2) =The Upper Segment= of a window may be made to work inwards on a hinge, and turned so that the current of air may be upwards. Where this plan is adopted, triangular pieces of glass should be placed at the two sides to prevent cold air from falling directly down at the sides of the opening. (3) =A Block of Wood=, two or three inches wide, may be inserted at the bottom of the window sash at =A= (Fig. 12), and then the window pulled down on this. The consequence is that air is admitted between the two sashes at =B=, its current being necessarily directed upwards (Fig. 12). This plan answers admirably in admitting pure air; but it possesses a disadvantage common to all the plans in which external air much colder than the internal is admitted into a room. The current of cold air passes upwards for some distance, but may then fall down on the heads of those occupying the room. [Illustration: FIG. 12. WINDOW VENTILATION.] (4) The top sash of the window may be opened, and some =zinc gauze= fastened across the open part. This is practically the same as the last arrangement, except that the air is admitted through the apertures of the zinc, and the amount admitted is greatly diminished (page 136).