A machine possesses statical stability if, when its attitude is disturbed, moments tending to restore it to this attitude are set up by the action of the air on the machine; e. g., if an aircraft, after an initial disturbance, oscillates with swings of constantly increasing amplitude, it is statically stable but not dynamically stable. STABILIZER: A fixed horizontal, or nearly horizontal, tail surface, used to steady the longitudinal motion and to damp oscillations in pitch. _Mechanical._—A mechanical device to steady the motion of an aircraft. STAGGER: The amount of advance of the entering edge of the upper plane of a biplane over that of the lower, expressed as percentage of gap; it is considered positive when the upper surface is forward and is measured from the entering edge of the upper plane along its chord to the point of intersection of this chord with a line drawn perpendicular to the chord of the lower plane at its entering edge, all lines being drawn in a plane parallel to the plane of symmetry. (_In directions for rigging_).—The horizontal distance between the entering edge of the upper plane and that of the lower when the airplane is in the standard position; i. e., when the arbitrary line of reference in the airplane is horizontal. (This line is usually the axis of the propeller shaft.) STALLING: A term describing the condition of an airplane which from any cause has lost the relative speed necessary for control. STATOSCOPE: An instrument to detect the existence of a small rate of ascent or descent, principally used in ballooning. STAY: A wire, rope, or the like, used as a tie piece to hold parts together, or to contribute stiffness. For example, the stays of the wing and body trussing. STEP: A break in the form of the bottom of a float. STREAM-LINE FLOW: The condition of continuous flow of a fluid, as distinguished from eddying flow. STREAM-LINE SHAPE: A shape intended to avoid eddying and to preserve stream-line flow. STRUT: A compression member of a truss frame. For instance, the vertical members of the wing truss of a biplane. SUSPENSION BAND: The band around a balloon to which are attached the basket and the main bridle suspensions. SUSPENSION BAR: The bar used for the concentration of basket suspension ropes in captive balloons. SWEEP BACK: The horizontal angle between the lateral axis of an airplane and the entering edge of the main planes. TAIL: The rear portion of an aircraft, to which are usually attached rudders, elevators, stabilizers, and fins. TAIL CUPS: The steadying device attached at the rear of certain types of elongated captive balloons. TANDEM: An airplane whose sets of planes are placed one in front of the other. TRACTOR: See Airplane. TRAILING EDGE: The rearmost edge of an aerofoil or propeller blade. TRIPLANE: A form of airplane whose main supporting surface is divided into three parts, superimposed.