[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!
Currents and Circulation of Air. In fact, this tendency of hot air to rise, and of cold air to sink, or rush in and take its place, which is the mainspring of nature's outdoor system of ventilation, is one of our greatest difficulties when we wall in a tiny section of the universe and call it a room. The difficulty is, of course, greatest in winter time, when the only pure air there is--that out of doors--is usually cold. This is one of the few points at which our instincts seem to fail us. For when it comes to a choice between being warm or well ventilated, we are sadly prone to choose the former every time. Still we would much rather be warm _and_ well ventilated than hot and stuffy, and this is what we should aim for.
The main problem is the cost of the necessary fuel, as it naturally takes more to heat a current of air which is kept moving through the room, no matter how slowly, than it does a room full of air which is boxed in, as it were, and kept from moving on after it has been warmed. The extra fuel, however, means the difference between comfort and stuffiness, between health and disease. Fortunately, the very same cold which makes a room harder to heat makes it easier to ventilate. When air is warmed, it expands and makes a "low pressure," which sucks the surrounding cooler air into it, as in the making of winds; so that the warmer the air inside the room, or the colder the air outside of it, which is practically the same thing, the more eagerly and swiftly will the outdoor air rush into it. So keen is this draft, so high this pressure, that some loosely-built houses and rooms, with only a few people in them, will in very cold weather be almost sufficiently ventilated through the natural cracks and leaks without opening a window or a door at all. And what is of great practical importance, an opening of an inch or two at the top of a window will admit as much fresh air on a cold day as an opening of a foot and a half in spring or summer, so swiftly does cold air pour in.