Secondly, and not less important, the windows should fit properly, and be perfectly hung and balanced, so that the sash will come down at a finger's touch, stay exactly where it is put, and go up again like a feather, instead of having to be pried loose, wrested open, held in place with a stick, and shoved up, or down again, only with a struggle. [Illustration: A WELL-AIRED CLASSROOM The windows to the left of the pupils cannot, of course, be shown in the picture, but it can be seen that the lighting of the room is chiefly from that side. Notice that the windows are both down from the top and up from the bottom.] There should be, if possible, windows on two sides of every room, or, if not, a large transom opening into a hall which has plenty of windows in it. With this equipment and a good supply of heat, any room can be properly ventilated and kept so. But it _will not ventilate itself_. Ventilation, like the colors of the great painter Turner, must be "mixed with brains"; and those brains must be in the room itself, not down in the basement. In the schoolroom, each teacher and pupil should regard the ventilation of the room as the most important single factor in the success of their work. The teacher has a sensitive thermometer and guide in, first, her own feelings and, second, the looks and attention of her pupils. There should be vacant seats or chairs in every room so that those too near the window in winter can move out of the strong current of cold air. [Illustration: A HEALTHFUL ARRANGEMENT OF WINDOWS AND SHADES The windows face in more than one direction. The shades are hung in the middle, not only regulating the light in the room, but allowing free passage of air at the top.] Windows should reach well up toward the ceiling and be opened _at the top_, because the foul air given off from the lungs at the temperature of the body is warmer than the air of the room and consequently rises toward the ceiling. It is just as important in ventilation to _let the foul air out_ as to let the fresh air in. In fact, one is impossible without the other. Air, though you can neither see it, nor grasp it, nor weigh it, is just as solid as granite when it comes to filling or emptying a room. Not a foot, not an inch of it can be forced into a room anywhere, until a corresponding foot or inch is let out of it somewhere. Therefore, never open a window at the bottom until you have opened it at the top. If you do, the cold fresh air will pour in onto the floor, while the hot foul air will rise and bank up against the ceiling in a layer that gets thicker and thicker, and comes further and further down, until you may be actually sitting with your head and shoulders in a layer of warm foul air, and your body and feet in a pool of cool pure air. Then you will wonder why your head is so hot, and your feet so cold!