Fig. 164 shows the method of Fig. 163 combined with a direct means of admitting fresh air from the inside. The fresh air ducts should be provided with dampers to control the effect of extreme cold and wind. =Quantity of Air Discharged by a Flue.=--Any change of temperature of air produces a change equal to 1/491 part of its volume, for each degree variation. If a cubic foot of air is raised in temperature 1°F., its volume is 1/491 part larger than the original volume, and its buoyancy in the surrounding air is increased correspondingly. Air that has a temperature higher than that surrounding it will tend to rise because it is lighter. The air rising from a hot-air register or from a heated surface are illustrations of this condition. Since the change of volume--or what is the same thing, its tendency to rise--increases 1/491 for each degree difference in temperature, the upward velocity of highly heated air will be very great. In warm air that fills a chimney flue or a room, the same tendency exists, the warmest air rises to the highest point and if the air can escape, as in the case of a chimney, a draft will result. The draft of a chimney, in quiet air, is due to the difference in temperature between the air inside the flue above that outside the house. A chimney that does not “draw” and causes a stove to “smoke,” will often produce sufficient draft after the flue has been warmed. The upward movement of the warmer air in the flue produces a constantly increasing velocity, until it reaches the top of the chimney. This is an accelerated velocity that may be calculated by use of the formula given in physics, to express the velocity of accelerated motion. The well-known formula _V_ = √(2_gh_) may be modified to express the conditions existing in a flue and permit of the calculation of the quantity of air discharged.