_Air-speed_--The speed of aircraft as related to the air in which they are moving; as distinguished from land-speed (which see). _Alighting Gear_--Devices on the under side of the aeroplane to take up the jar of landing after flight, and at the same time to check the forward motion at that moment. _Angle of Entry_--The angle made by the tangent to the curve of the aeroplane surface at its forward edge, with the direction, or line, of travel. _Angle of Incidence_--The angle made by the chord of the arc of a curved “plane,” or by the line of a flat plane, with the line of travel. [Illustration] _Angle of Trail_--The angle made by the tangent to the rear edge of a curved plane with the line of travel. _Apteroid_--A form resembling the “short and broad” type of the wings of certain birds--as distinguished from the pterygoid (which see). _Arc_--Any part of a circle, or other curved line. _Arch_--The curve formed by bending the wings downward at the tips, leaving them higher at the centre of the machine. _Aspect_--The view of the top of an aeroplane as it appears when looked down upon from above. _Aspiration_--The (hitherto) unexplained tendency of a curved surface--convex side upward--to rise and advance when a stream of air blows against its forward edge and across the top. _Attitude_--The position of a plane as related to the line of its travel; usually expressed by the angle of incidence. _Automatic Stability_--That stability which is preserved by self-acting, or self-adjusting, devices which are not under the control of the operator, nor a fixed part of the machine, as are the adjusting planes. _Aviation_--Flying by means of power-propelled machines which are not buoyed up in the air, as with gas bags. _Aviator_--The operator, driver, or pilot of an aeroplane. B _Balance_--Equilibrium maintained by the controlling mechanism, or by the automatic action of balancing-surfaces--as distinguished from the equilibrium preserved by stabilizing surfaces. _Balancing Plane_--The surface which is employed either intentionally, or automatically, to restore a disturbed balance. _Biplane_--The type of aeroplane which has two main supporting surfaces or planes, placed one above the other. _Body_--The central structure of an aeroplane, containing the machinery and the passenger space--as distinguished from the wings, or planes, and the tail. _Brace_--A construction member of the framing of aircraft which resists a compression strain in a diagonal direction--as distinguished from a “stay,” or “diagonal,” which supports a pulling strain; also from a strut which supports a compression strain in a vertical direction. C