It has been stated that the method of calculating volumes of air that will pass through a flue is based on the formula used to express the velocity of accelerated motion. The fundamental formula must be changed to suit the conditions produced when air is heated and made buoyant by expansion. As has been stated, the change in temperature of air 1°F. causes an increase or decrease 1/491 part of its volume for each degree change. Any portion of air, warmer than that which surrounds it, tends to rise because of its lighter weight; the tendency to rise increases with the difference in temperature. The draft of a flue is caused by this condition of difference in temperature between the air inside the flue and the outside atmosphere. In order that this general condition may be expressed in the simplest form let: _T_ = the temperature inside the flue in degrees F. _t_ = the temperature outside the flue in degrees F. _H_ = the height of the flue in feet. The quantity (_T_-_t_)/491 expresses the difference in temperature in degrees, divided by the change of volume for each degree. This gives the constant upward tendency of the air in passing through the flue. If this quantity is placed in the formula _V_ = √(2_gh_), so as to exert its influence through the height of flue _H_, the condition may be expressed: _V_ = √(2_g_((_T_-_t_)/491)_H_) The factor _g_, representing the acceleration of gravity, is constant and equal to 32 feet per second. The quantity 2_g_ may be removed from under the radical and the formula becomes: _V_ = 8√(((_T_-_t_)/491)_H_) The formula may now be used to express the volume of discharge of air from a flue. Suppose such a flue contains an area of 1 square foot in cross-section and that it is desired to estimate the air discharged from the flue per hour. The value of _g_ is given in feet per second, and in order to make the formula express the volume of air discharged in cubic feet per hour, it must be multiplied by the number of seconds in an hour. Volume discharged in cubic feet per hour = 60 × 60 × 8√(((_T_-_t_)/491)_H_) = 28,800√(((_T_-_t_)/491)_H_) This formula applies to conditions such as will permit uniform movement of the air in a straight flue, uninfluenced by irregular, odd-shaped passages and rough surfaces. Moreover, it is supposed that the air may enter the house as rapidly as it escapes. The theoretical discharge will, in most instances, be less than the calculated amount, because the air cannot enter the house as fast as it may be discharged by the flue. It is a common custom to consider the theoretical flue only 50 per cent. efficient. As applied to the formula, the constant 28,800 when reduced 50 per cent. will become 14,400, and will be so used in the calculations as follows.