_Turnbuckle_--A device with a nut at each end, of contrary pitch, so as to take a right-hand screw at one end, and a left-hand screw at the other; used for drawing together, or toward each other the open ends of a stay, or tie. U _Uniform Pitch_--That varying pitch in a propeller blade which causes each point in the blade to move forward in its own circle the same distance in one revolution. _Up-wind_--In a direction opposite to the current of the wind; against the wind; in the teeth of the wind. V _Vertical Rudder_--A rudder for steering toward right or left; so called because its surface occupies normally a vertical position. W _Wake_--The stream of disturbed air left in the rear of a moving aircraft, due mainly to the slip of the propeller. _Wash_--The air-currents flowing out diagonally from the sides of a moving aeroplane. _Wing Bar_--The larger construction members of a wing, running from the body outward to the tips. The ribs are attached to the wing bars, usually at right angles. _Wing Plan_--The outline of the wing or main plane surface as viewed from above. _Wing Section_--The outline of the wing structure of an aeroplane as it would appear if cut by a plane passing through it parallel to the longitudinal centre of the machine. _Wing Skid_--A small skid, or runner, placed under the tip of the wings of an aeroplane, to prevent damage in case of violent contact with the ground. _Wing Tip_--The extreme outer end of a wing or main plane. _Wing Warping_--A controlling device for restoring disturbed lateral balance by the forcible pulling down or pulling up of the tips of the wings, or of the outer ends of the main surface of the aeroplane. _Wing Wheel_--A small wheel placed under the outer end of a wing or main plane to prevent contact with the ground. An improvement on the wing skid. THE END [Transcriber’s Note: Inconsistent spelling and hyphenation are as in the original.] End of the Project Gutenberg EBook of How it Flies or, Conquest of the Air, by Richard Ferris