An essential part of the equipment of either the aerial photographer or the designer of aerial photographic apparatus is a working knowledge of the principles and construction of the airplane, and considerable actual experience in the air. Conditions and requirements in the flying plane are far different from those of the shop bench or photographic studio. As a preliminary to undertaking any work on airplane instruments a good text-book on the principles of flight should be studied. Such general ideas as are necessary for understanding the purely photographic problems are, however, outlined in the next paragraphs. [Illustration: FIG. 1.—The elements of the plane.] =Construction of the Airplane.=—The modern airplane (Fig. 1) consists of one or more _planes_, much longer across than in the direction of flight (_aspect ratio_). These are inclined slightly upward toward the direction of travel, and their rapid motion through the air, due to the pull of the _propeller_ driven by the _motor_, causes them to rise from the earth, carrying the _fuselage_ or body of the airplane. In the fuselage are carried the pilot, observer, and any other load. Wheels below the fuselage forming the _under-carriage_ or _landing gear_ serve to support the body when running along the ground in taking off or landing. The pilot, sitting in one of the _cockpits_, has in front of him the _controls_, by means of which the motion of the plane is guided (Figs. 2 and 3). These consist of the engine controls—the _contacts_ for the ignition, the _throttle_, the oil and gasoline supply, air pressure, etc., and the steering controls—the _rudder bar_, the _stick_ and the _stabilizer control_. The rudder bar, operated by the feet, controls both the _rudder_ of the plane, which turns the plane to right or left in the air, and the _tail skid_, for steering on the ground. The _stick_ is a vertical column in front of the pilot which, when pushed forward or back, depresses or raises the _elevator_ and makes the machine dive or climb. Thrown to either side it operates the _ailerons_ or wing tips, which cause the plane to roll about its fore and aft axis. The stabilizer control is usually a wheel at the side of the cockpit, whose turning varies the angle of incidence of the small stabilizing plane in front of the elevator, to correct the balance of the plane under different conditions of loading. [Illustration: FIG. 2.—Forward cockpit of DeHaviland 4, showing instrument board.] [Illustration: FIG. 3.—Rear cockpit of DeHaviland 4, showing rear “stick” and rudder bar.]