The operation of the film driving mechanism is comparatively simple. It consists of a train of gears, driven by a flexible revolving shaft attached to some separate source of power capable of speed variation. The action of the gears is to move the film, set the shutter and then expose it; in the earlier cameras with the film continuously moving. In the first cameras constructed the space between the pictures varies as the film rolls up, due to the increasing diameter of the roll. In later cameras the film roller is disengaged from the gears just before the shutter is tripped, so that the film stands still during the exposure, and is then re-engaged at a new point on a ratchet wheel governed by the diameter of the receiving roll, whereby the pictures are equally spaced. In all the cameras, punch marks made at the time of exposure enable the limits of the picture to be detected in the dark room by touch. [Illustration: FIG. 63.—Film winding and exposing mechanism in German film camera.] Variable speed is arranged for in any one of several ways. For peace-time uses a turbine attached to the side of the plane is simple and positive, and, provided it is made of sufficient size—which is not the case with the one shown in the Figure—will give adequate speed regulation upon varying the aperture through which the air enters. The Venturi tube may be carried upon the same mount, or a small rotary pump can be attached on the same shaft. Where the high wind resistance of the turbine is an objection the camera is driven electrically, by a motor acting through the intermediary of a variable speed control described in the next chapter (Fig. 68). The camera weighs complete about forty pounds, and the film rolls about four pounds. The latter can be changed in the air without great difficulty provided the camera is mounted accessibly and so that the top may be opened. CHAPTER XII MOTIVE POWER FOR AERIAL CAMERAS As long as circumstances permit, hand operation still remains the most reliable and satisfactory method of driving a camera. It is always available, can be applied to just the amount desired, and at the time and place needed. For instance, in a magazine of the Gaumont type (Fig. 40), what is needed is the periodic application of a very considerable force rather quickly, and while this can be done quite simply by hand, no mechanism has even been attempted to go through this same operation automatically. Instead, the fundamental design of automatic magazines has been made along other lines calculated to utilize smaller forces more steadily applied.