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.

Bearing this in mind, and also that it is always best to ventilate through as many openings as possible, both to keep drafts of cold air from becoming too intense, and to give as many openings for the escape of the foul air as possible, there will be little difficulty in keeping any room which has proper window arrangements well ventilated in winter. An opening of an inch at the top of each of three windows is better than a three-inch opening at the top of one. But you must use your brains about it, watching the direction of the wind, and frequently changing the position of the window sashes to match the changes of heat in the room, or of cold outside.

No arrangement of windows, however perfect, is likely to remain satisfactory for more than an hour at a time, except in warm weather. This watchfulness and attention takes time, but it is time well spent. "Eternal vigilance" is the price of good ventilation, as well as of liberty; and you will get far more work done in the course of a morning by interrupting it occasionally to go and raise or lower a window, than you will by sitting still and slaving in a stuffy, ill-smelling room.

Plenty of Heat Needed. Any method of heating—open fireplace, stove, hot air, furnace, hot water, or steam—which will keep a room with the windows open comfortably warm in cold weather is satisfactory and healthful. The worst fault, from a sanitary point of view, that a heating system can have is that it does not give enough warmth, so that you are compelled to keep the windows shut. Too little heat is often as dangerous as too much; for you will insist on keeping warm, no matter what it may cost you in the future, and a cold room usually means hermetically sealed windows. Remember that coal is cheaper than colds, to say nothing of consumption and pneumonia.

A HEALTHFUL BEDROOM

Windows on two sides; shades rolling from the middle; draperies few and washable; no carpet, but rugs by the bedside.

Ventilating the Bedroom. The same principles that apply to ventilating a living-room or day-room apply to ventilating a bedroom. Here you can almost disregard drafts, except in the very coldest weather, and, by putting on plenty of covering, sleep three hundred days out of the year with your windows wide open and your room within ten degrees of the temperature outdoors. You need not be afraid of catching cold. On the contrary, by sleeping in a room like this you will escape three out of four colds that you usually catch. Sleeping with the windows wide open is the method we now use to cure consumption, and it is equally good to prevent it.

No bedroom window ought to be closed at the top, except when necessary to keep rain or snow from driving in. Close the windows for a short time before going to bed, and again before rising in the morning, to warm up the room to undress and dress in; or have a small inside dressing-room, with your bed out on a screened balcony or porch. But sleep at least three hundred nights of the year with the free air of heaven blowing across your face. You will soon feel that you cannot sleep without it. In winter, have a light-weight warm comforter and enough warm, but light, blankets on your bed, and leave the heat on in the room, if necessary—but open the windows.