It goes without saying that persons unaccustomed to ventilation, and consequently over-sensitive to drafts, should avoid over-exposure while they are in process of changing their habits. But after even a few days of enjoyment of air in motion, with cautious exposure to it, the likelihood of cold is greatly diminished; and persons who continue to make friends with moving air soon become almost immune to colds. The popular idea that colds are derived from drafts is greatly exaggerated. A cold of any kind is usually a catarrhal disease of germ origin, to which a lowered vital resistance is a predisposing cause. The germs are almost always present in the nose and throat. It is exposure to a draft plus the presence of germs and a lowered resistance of the body which produces the usual cold. Army men have often noted that as long as they are on the march and sleep outdoors, they seldom or never have colds, but they develop them as soon as they get indoors again. See SUPPLEMENTARY NOTES, "Avoiding Colds." Of course, one must always use common sense and never grow foolhardy. It is never advisable that a person in a perspiration should sit in a strong draft. [Sidenote: Windows] The best ventilation is usually to be had through the windows. We advise keeping windows open almost always in summer; and often open in winter. One should have a cross-current of air whenever practicable; that is, an entrance for fresh air and an exit for used air at opposite sides of the room. Where there can not be such a cross-current, some circulation can be secured by having a window open both top and bottom. [Sidenote: Window-boards] In winter, ventilation is best secured by means of a window-board. This is a board the edge of which rests on the edge of the window-sill, the ends being attached firmly to the window-frame. It affords a vertical surface three or four inches high and situated three or four inches in front of the window, so as to deflect the cold air upward when the window is slightly opened. The air will then reach the breathing-zone, instead of flowing on to the floor and chilling the feet, which is the usual consequence of opening a window in winter. It seems tragic to think that for lack of some such simple device, which anyone can make or buy, there is now an almost complete absence of winter ventilation in most houses. [Sidenote: Air-fans] Air should never be allowed to become stagnant. When there is no natural movement in the air, it should be put in motion by artificial means. This important method of practising air-hygiene is becoming quite generally available through the introduction of electric currents into dwellings and other buildings and the use of electric fans. Even a hand fan is of distinct hygienic value. [Sidenote: Heating Systems]