The object of this report is to give a practical definition of the longitudinal stability in flight, or apparent stability, and to show the relation between the apparent stability and the static or basic stability (stability at constant speed and various angles of attack) which can be measured in the wind tunnel or calculated. 1. The basic stability is characteristic of the airplane design and is not altered by changes in the setting of the stabilizer. 2. The apparent stability of an airplane is greatly affected by the setting of the stabilizer with reference to the wing. 3. Basic stability is represented by the curve of elevator angles measured in flight at various speeds; also by the curve of stick forces at one mile per hour and at various angles of attack (stick forces in flight divided by V squared). 4. Apparent stability is represented by the curve of stick forced measured in flight at various airspeeds and plotted against these speeds. 5. Apparent stability can always be obtained by a proper setting of the stabilizer, even if the airplane is basically unstable. In such a case the stick force will be tail heavy at all speeds and must be balanced (if too large) by a force applied on the stick or on the elevator. 6. An excellent representation of the basic stability can be obtained by calculating the pitching moment of the airplane at one mile per hour elevator free.