_Of attack_ (or _of incidence_) _of an aerofoil_.—The acute angle between the direction of the relative wind and the chord of an aerofoil; i. e., the angle between the chord of an aerofoil and its motion relative to the air. (This definition may be extended to any body having an axis.) _Critical._—The angle of attack at which the lift-curve has its first maximum; sometimes referred to as the “burble point.” _Gliding._—The angle the flight path makes with the horizontal when descending in still air under the influence of gravity alone; i. e., without power from the engine. ANGLE OF INCIDENCE (_in directions for rigging_): In the process of rigging an airplane some arbitrary definite line in the airplane is kept horizontal; the angle of incidence of a wing, or of any aerofoil, is the angle between its chord and this horizontal line, which usually is the line of the upper longitudinals of the fuselage or nacelle. APPENDIX: The hose at the bottom of a balloon used for inflation. In the case of a spherical balloon it also serves for equalization of pressure. ASPECT RATIO: The ratio of span to chord of an aerofoil. ATTITUDE: The attitude of an aircraft is determined by the inclination of its axes to the “frame of reference”; e. g., the earth, or the relative wind. AVIATOR: The operator or pilot of heavier-than-air craft. This term is applied regardless of the sex of the operator. AXES OF AN AIRCRAFT: Three fixed lines of reference; usually centroidal and mutually rectangular. The principal longitudinal axis in the plane of symmetry, usually parallel to the axis of the propeller, is called the _longitudinal_ axis; the axis perpendicular to this in the plane of symmetry is called the _normal_ axis; and the third axis, perpendicular to the other two, is called the _lateral_ axis. In mathematical discussions the first of these axes, drawn from front to rear, is called the X axis; the second, drawn upward, the Z axis; and the third, running from right to left, the Y axis. BALANCING FLAPS: See Aileron. BALLONET: A small balloon within the interior of a balloon or airship for the purpose of controlling the ascent or descent and for maintaining pressure on the outer envelope so as to prevent deformation. The ballonet is kept inflated with air at the required pressure, under the control of valves by a blower or by the action of the wind caught in an air-scoop. BALLOON: A form of aircraft comprising a gas bag, rigging and a basket. The support in the air results from the buoyancy of the air displaced by the gas bag, the form of which is maintained by the pressure of a contained gas lighter than air. _Barrage._—A small spherical captive balloon, raised as a protection against attacks by airplanes. _Captive._—A balloon restrained from free flight by means of a cable attaching it to the earth.