Figures 90 and 91 show a bell crank mounting furnished with a rotating turret. This was designed to facilitate the changing of magazines in the English B M camera, which is swung around through 90 degrees from the exposing position to bring the magazine near the observer. The camera shown in the mounting is the American hand-operated model (type M), in which there is the same necessity for turning in order to manipulate the bag magazine easily. The camera is shown in both exposing and plate changing positions. An important detail of these mounts is a _safety catch_, which must be fastened before the plane lands, in order to prevent the shocks of landing from producing oscillations sufficient to throw the camera out of the mount. =Center of Gravity Rubber Pad Supports.=—Given a camera whose center of gravity does not change during operation, a simple and entirely adequate anti-vibration support is furnished by a ring of sponge rubber in the plane of the center of gravity. But if provision has to be made for oblique views or for adjusting the camera to the vertical, something more elaborate is necessary. [Illustration: FIG. 90.—U. S. model deRam camera on anti-vibration mounting adjustable for the angle of incidence of the plane.] [Illustration: FIG. 91.—U. S. deRam camera and mount installed in photographic DeHaviland 4 (Fig. 100). Viewed from above the observer's cockpit.] [Illustration: FIG. 92.—U. S. type “K” film camera on universal mounting, vertical position.] [Illustration: FIG. 93.—U. S. type “K” film camera on universal mounting, oblique position.] Mountings for the American deRam and for the Air Service film camera, embodying the results of complete study of the anti-vibration problem, are shown in Figs. 90, 92 and 93. Trusses carrying the cameras on pivots rest on four pads of sponge rubber which are mounted on frames correctly spaced ready for attachment to the cross-pieces of the airplane camera supports. In the deRam (Fig. 90) the pivots, attached to the camera body, permit it to be leveled fore and aft, to compensate for the inclined position of the fuselage assumed at high altitudes or in some conditions of loading. This will sometimes amount to as much as 11 or 12 degrees, which is very serious, since one degree causes (with an angular field of 20 degrees) about one per cent. difference of scale at the two sides of the plate. The film camera mounting carries the camera in a conical ring, and is pivoted not only for vertical adjustment, but for the taking of obliques as well (Fig. 93). These mounts transmit practically no vibration. [Illustration: FIG. 94.—Tests on two types of camera mount: (_a_) Support at bottom of camera; (_b_) support above center of gravity.]