The _fuselage_ consists usually of a light hollow framework of spruce or ash, divided into a series of bays or compartments by upright members, connecting the _longerons_, which are the four corner members, running fore and aft, of the plane. Diagonally across the sides and faces of these bays are stretched taut piano wires, and the whole structure is covered with canvas or linen fabric. Cross-wires and fabric are omitted from the top of one or more bays to permit their being used as cockpits for pilot and observer. In later designs of planes the wire and fabric construction has been superseded by ply-wood veneer, thereby strengthening the fuselage so that many of the diagonal bracing wires on the inside are dispensed with. This greatly increases the accessibility of the spaces in which cameras and other apparatus must be carried. [Illustration: FIG. 4.—Biplane in flight.] The fuselage differs greatly in cross-section shape and in roominess according to the type of engine. In the majority of English and American planes, with their vertical cylinder or V type engines, the fuselage is narrow and rectangular in cross-section. In many French planes, radial or rotary engines are used and the fuselage is correspondingly almost circular, and so is much more spacious than the English and American planes of similar power. The shape and size of the plane body has an important bearing on the question of camera installation. [Illustration: FIG. 5.—A single-seater.] =Types of Planes.=—The most common type of plane is the _biplane_ (Fig. 4), with its two planes, connected by struts and wires, set not directly over each other, but _staggered_, usually with the upper plane leading. _Monoplanes_ were in favor in the early days of aviation, and _triplanes_ have been used to some extent. According to the position of the propeller planes are classified as _tractors_ or _pushers_, tractors being at present the more common form. Planes are further classified as _single-seaters_ (Fig. 5), _two-seaters_, and _three-seaters_. These motor and passenger methods of classification are now proving inadequate with the rapid development of planes carrying two, three, and even more motors, divided between pusher and tractor operation, and carrying increasingly large numbers of passengers. Aside from structure, planes may be further classified according to their uses, as _scout_, _combat_, _reconnaissance_, _bombing_, etc. Planes equipped with floats or pontoons for alighting on the water are called _seaplanes_ (Fig. 182), and those in which the fuselage is boat-shaped, to permit of floating directly on the water, are _flying boats_ (Fig. 183). =The Plane in the Air.=—The first flight of the photographic observer or of the instrument expert who is to work upon airplane instruments is very profitably made as a “joy ride,” to familiarize him with conditions in the air. His experience will be somewhat as follows: