Therefore, regular provision must be made for the ventilation of rooms regardless of the feelings of their inmates: at least an inch of window open at the top, day and night, renders a room tolerably safe, because it allows of the escape of the vitiated air, which, being light, ascends, leaving room for the influx of colder, fresher air by cracks and crannies in doors and floors. An open chimney is a useful, though not a sufficient, ventilator; it is needless to say that the stopping-up of chimneys in sleeping rooms is suicidal. It is particularly important to accustom children to sleep with an inch or two, or more, of open window all through the year — as much more as you like in the summer.