The object sought in ventilating a room is to keep up the quality of the air by constant addition of fresh air, and in order to bring about a uniform purification of the entire atmosphere the entering air must be mixed with that already in the enclosure. If the discomforts of drafts are to be avoided, this mixing process must be brought about by admitting the cold air at the upper part of the room. [Illustration: FIG. 163.--A chimney flue used as a ventilator.] Warm air rises to the top of the room because it is lighter than the colder air beneath it. The coldest air is always lowest in point of elevation and unless there is some means to stir up the entire volume this condition will always remain the same. When the easiest means of air for entering and leaving are near the floor, the cold entering air and that which goes out will always be in the lower part of the room, even when the supply is amply large. If no opportunity is given for the fresh air to mix with that already in the room, a poor average quality will result. In the process of ventilation, the entering air should be admitted at, or directed toward, the highest part of the room, so that the pure cold air may have a chance to mix with that which is warmest. Air is not a good conductor of heat, and in mixing warm and cold air the cold particles will tend to float downward and take up heat from the warmer air with which it comes into contact, and thus produces a more uniform temperature. [Illustration: FIG. 164.--Method of admitting cold air into rooms so as to produce the best condition of ventilation.] The condition most to be desired is that of admitting cold air at a point where it will most readily mingle with the warm air from the source of heat. The reduction in temperature that must take place from this mixture will produce a gravitational circulation. Unfortunately this is not always possible to attain in an old building, but in the construction of a new building air ducts placed to admit air at points near the ceiling and located with reference to the supply of heat will bring about the best effect of ventilation. The air which enters a room should, therefore, be near the top or so directed that the entering shaft will carry it upward. The air which is taken out of the room should leave from a point near the floor. In so doing it will tend to produce a more uniform quality and a more even distributor of the heat. In order that the most desirable quality of atmosphere may be attained, there should be a constant supply of pure air entering and an equal amount discharging from the house. In the better-constructed dwelling such a condition is often provided through a ventilating flue that is a part of the chimney. This flue is arranged with registers placed to take air from the parts of the house requiring the greatest amount of air. Such an arrangement is shown in the picture in Fig. 163.