DRIFT METER: An instrument for the measurement of the angular deviation of an aircraft from a set course, due to cross winds. DRIP CLOTH: A curtain around the equator of a balloon, which prevents rain from dripping into the basket. DROOP: A permanent warp of an aerofoil such that the angle of attack increases toward the wing tips. (The opposite of “wash out.”) ELEVATOR: A movable auxiliary surface, usually attached to the tail, the function of which is to control the longitudinal attitude of an aircraft by rotating it about its lateral axis. EMPENNAGE: The tail surfaces of an aircraft. Sometimes the word is limited to the fixed stabilizing portion of the tail—usually comprising the tail plane and vertical fin, to which are attached the elevator and rudders. ENTERING EDGE: The foremost edge of an aerofoil or propeller blade. ENVELOPE: The outer covering of a rigid airship; or, in the case of a balloon or a nonrigid airship, the gas bag which contains the gas. EQUATOR: The largest horizontal circle of a spherical balloon. FINS: Small fixed aerofoils attached to different parts of aircraft, in order to promote stability; for example, tail fins, skid fins, etc. Fins are often adjustable. They may be either horizontal or vertical. FLIGHT PATH: The path of the center of gravity of an aircraft with reference to the earth. FLOAT: That portion of the landing gear of an aircraft which provides buoyancy when it is resting on the surface of the water. FUSELAGE: The elongated structure to which are attached the landing gear, wings and tail. A fuselage is rarely used with pushers; and in general it is designed to hold the passengers. GAP: The shortest distance between the planes of the chords of the upper and lower planes of a biplane, measured along a line perpendicular to the chord of the lower plane at its entering edge. GAS BAG: See Envelope. GLIDE: To fly without engine power. GLIDER: A form of aircraft similar to an airplane, but without any power plant. When utilized in variable winds it makes use of the soaring principles of flight and is sometimes called a soaring machine. GLIDING ANGLE: See Angle, gliding. GORE: One of the segments of fabric composing the envelope. GROUND CLOTH: Canvas placed on the ground to protect a balloon. GUIDE ROPE: The long trailing rope attached to a spherical balloon or airship, to serve as a brake and as a variable ballast. GUY: A rope, chain, wire or rod attached to an object to guide or steady it, such as guys to wing, tail, or landing gear. HANGAR: A shed for housing airships or airplanes. HELICOPTER: A form of aircraft whose support in the air is derived from the vertical thrust of propellers. HORN: A short arm fastened to a movable part of an airplane, serving as a lever arm, e. g., aileron horn, rudder horn, elevator horn.