On the whole it is eminently desirable from the standpoint of power operation that the automatic camera should embody its own means for altering the interval between exposures, so that all the external attachment needed is a single connection to a source of power either of constant speed, as an electric motor, or of speed proportional to that of the plane, as with a simple wind propeller. This makes the camera largely independent of the nature of the power supply, whereas a camera designed for a special variable speed device is of little use on a plane where this is not available. =Transmission of Power to the Camera=—It has already been pointed out that the ease of transmission of electrical energy makes it particularly convenient for use in a plane. All other sources of power, except clock-work incorporated in the camera, require flexible shafting, so that the question of bearings and connections becomes a serious one, especially when the shaft runs continuously for long periods at very high speeds. The shafting found most suitable is the spirally wound form commonly known as dental shafting. This must be encased in a smoothly fitting sheath, flexible enough to permit of easy bends. The ends of the shaft should be equipped with square or rectangular pins to fit into corresponding slots in the motor and camera shafts. The ends of the shaft casing may be fitted either to attach by bayonet joints or by smoothly fitting screw collars. At the point of attachment to the camera it is desirable to have some form of junction adjustable as to the direction from which the shaft may be connected, so that it need be bent as little as possible. A right angle bevel gear offers one means of doing this. Bearings, such as those of the propeller, should be of the ball variety, while heavy lubrication, such as vaseline, should be freely used, both in the bearings and in the shaft casing. An important feature of any power drive system should be a safety device, so that the power will race in case of any jam or stoppage in the camera. This will often prevent serious damage through the breakage of some relatively weak part of the camera mechanism on which the whole force of the driving apparatus is suddenly thrown. The “L” camera propeller is fitted with a spring friction clutch with the idea that if the camera refuses to operate the propeller will slip instead of wrenching the shaft to pieces. CHAPTER XIII CAMERA AUXILIARIES