_Camber_--The distance from the chord of the curve of a surface to the highest point of that curve, measured at right angles to the chord. _Caster_, or _Castor_, _Wheel_--A wheel mounted on an upright pivoted shaft placed forward of its axle, so that it swivels automatically to assume the line of travel of an aeroplane when landing: used in the alighting gear. To be distinguished from a fixed wheel, which does not swivel. _Cell_--A structure with enclosing sides--similar to a box without top or bottom stood upon one side. The vertical walls of the cell give lateral stability, and its horizontal walls fore-and-aft stability. [Illustration: The first Santos-Dumont biplane, constructed of cells.] _Centre of Gravity_--That point of a body where its weight centres. If this point is supported, the body rests in exact balance. _Centre of Lift_--The one point at which the lifting forces of the flying planes might be concentrated, and produce the same effect. _Centre of Resistance_--The one point at which the forces opposing the flight of an air-craft might be concentrated, and produce the same result. _Centre of Thrust_--The one point at which the forces generated by the revolving propellers might be concentrated, and produce the same effect. _Chassis_--The under-structure or “running-gear” of an aeroplane. _Chord_--The straight line between the two ends of an arc of a circle or other curved line. _Compound Control_--A mechanical system by which several distinct controls are operated through different manipulations of the same lever or steering-wheel. _Compression Side_--That side of a plane or propeller blade against which the air is compressed--the under surface of a flying plane, and the rear surface of a revolving propeller. _Curtain_--The vertical surface of a cell--the wall which stands upright. D _Deck_--A main aeroplane surface. The term is used generally in describing biplanes; as the upper deck, and the lower deck; or with aeroplanes of many decks. _Demountable_--A type of construction which permits a machine to be easily taken apart for transportation. _Derrick_--A tower-shaped structure in which a weight is raised and allowed to fall to give starting impetus to an aeroplane. _Dihedral_--That form of construction in which the wings of an aeroplane start with an upward incline at their junction with the body of the machine, instead of stretching out on a level. _Dirigible_--The condition of being directable, or steerable: applied generally to the balloons fitted with propelling power, or airships. _Double Rudder_--A rudder composed of two intersecting planes, one vertical and the other horizontal, thus enabling the operator to steer in any direction with the one rudder. [Illustration]