The wind propeller does not lend itself at all well to speed variation. It cannot be partially covered from the air stream, as can the turbine, because of the resulting strain on its mount. A possible form of variable speed propeller, one which, however, has not yet been practically developed, is a propeller with controllable variable pitch. If this could be made mechanically sound it would be well-suited for camera operation. That such a propeller could be worked out is indicated by the good performance of a constant speed propeller developed for radio generators and used on the French deRam camera (Fig. 54). Parenthetically, it maybe questioned whether a constant speed propeller is really desirable with an airplane camera. What is required is not exposures at a definite time interval—although most of the data are in that form—but exposures at definite intervals with respect to the motion of the plane, which practically means with reference to its air speed. Rather than build a camera calculated to give exposures at intervals of so many seconds when it is attached to a constant speed propeller, we would do better to use a propeller which responds to the speed of the plane, in conjunction with some form of tachometer to show the rate at which exposures are being made. This in turn should be coördinated with the indications of a proper camera-field indicating sight. One solution of the problem of speed control with a propeller of practically fixed speed, is to use a governor and slip clutch as in the English Type F film camera (Fig. 57). Here the propeller shaft and the camera driving axle are connected by two friction discs. That on the camera mechanism is forced against the other by a spiral spring, whose tension is controlled by a ball governor. If the camera speed becomes too high the governor reduces the tension on the spiral spring and the discs slip over each other. The point where this slipping occurs is determined by the position of the governor as a whole, and this is controlled by a lever on top of the camera. Another speed control device, perhaps more positive but certainly more complicated and wasteful of power, consists of a large flat disc, driven by the propeller or electric motor, and from which the camera is driven by a shaft from a smaller friction disc which may be pressed against any point from the center to the periphery of the larger disc. The speed range attainable in this way is limited only by the size of the large disc. An application of this idea is shown in the speed control (Fig. 68), designed for the American Type K camera when operated on an electric motor or on a simple propeller. The same idea is utilized in the Duchatellier film camera, in connection with the constant speed propeller already described. [Illustration: FIG. 68.—Friction disc speed control.]