=Control of Camera Speed.=—In the semi-automatic camera the only control required on the speed of the operating motor is at the upper and lower limits. It must not go so fast as to anticipate the completion of any steps in the cycle of camera operation, such as the fall of plates or pawls into position, which would jam the camera. On the other hand, it must not be so slow that pictures cannot be obtained with the requisite overlap for maps or stereoscopic views. In the American deRam camera the cycle of operations cannot safely be put through in less than four seconds, a short enough interval for most purposes. It is also highly desirable in the semi-automatic camera to have the motive power capable of stopping completely. This saves wear and tear on both motor and camera mechanism. In the automatic camera an extreme range of speed is called for by the several problems of mapping, oblique photography, and the making of stereoscopic views. For mapping alone, the shortest likely interval may be taken as that required for work at approximately 1000 meters altitude, for a plane speed of 150 kilometers per hour, which demands an interval of six seconds with a ten inch lens on a 4 × 5 inch plate. For vertical stereos at the same altitude and speed this interval is divided by three, and low oblique stereos need even quicker operation. Hence a range of from 1 to 30 pictures per minute should be provided for. This requirement is difficult to meet with any simple mechanism. From the standpoint of simplicity in speed regulation the wind turbine of adequate vane surface has much to recommend it. It is only necessary to present more or less of its vane area to the wind in order to secure a considerable range of speed. The method of doing this by a shutter interposed in front is uneconomical, but it is probable that the design can be so altered that more or less of the turbine is exposed beyond the side of the plane, possibly by varying the angle, to secure the same result without introducing useless head resistance. A serious practical objection to the turbine lies in the large vane surface necessary to give adequate power combined with proper speed variation. In the automatic film camera (Type K) this area should be as much as 40 to 50 square inches.