Due to its stretching, there is a pretty definite limitation to the feasible length of the Bowden wire. This length is about four feet. Where according to English practice the pilot makes the exposure, a considerably longer wire and sheath are called for. In this case the effective length of the release is increased by giving the pilot a second releasing lever, connected to the first by a rigid rod (Fig. 69). The releasing lever, wire, and all mechanical parts of the Bowden release should be made much stronger than would be indicated by bench tests of the camera. In the air it is impossible to decide either by sound or by delicacy of touch whether the mechanism has acted, so that the observer is apt to pull much harder than necessary and to strain or break the release if it is weak. The Bowden wire is used in the American service only for shutter release. In the English service it has been used for plate changing with the L camera. [Illustration: FIG. 69.—Bowden wire release in rear cockpit, with rod connected to similar release for the pilot.] [Illustration: FIG. 70.—Bowden wire release with stop watch attached, for use in timing for overlaps.] =Sights.=—In airplane photography the need for a finder or sight is fully as great as in everyday work. A new condition, however, prevails, for except with hand-held cameras, and even to some extent with them, the operation of pointing the camera involves pointing the whole vehicle that carries the camera. The pointing of airplane cameras is therefore akin to the sighting of great guns. While the observer may perform the actual operation of taking the picture, the responsibility for covering the objective rests with the pilot. Teamwork counts equally with tools. Airplane camera sights may accordingly be divided into two classes: sights attached to the camera, for use principally with hand-held apparatus, and sights attached to the plane, for the use of pilot, of observer, or of both. =Sights for Hand-held Cameras.=—The simplest form of sight attached directly to the camera is modeled on the _gun sight_, consisting of a forward point or bead and a rear V. This sight of course serves merely to place the objective in the center of the plate and gives no indication of the size of field covered. Another simple sight of rather better type is the _tube sight_—a metal tube of approximately one inch diameter and three inches length, carrying at each end pairs of wires crossed at right angles. The camera is in alignment when the front and back cross wires both exactly match on the object to be photographed. The best way to mount the cross-wires is with one pair turned through 45 degrees with respect to the other, so that it is at once apparent which is the front and which the rear pair (Figs. 31 and 39).