Spring motors have the very real advantage that by their use the camera can be made entirely self contained. The simplest application of the spring motor would be to the semi-automatic camera, where no close regulation of speed is required. In such a camera the operation of exposing the shutter would release the spring, which would then change the plate or film and re-set the shutter, repeating this operation as long as the spring retained sufficient tension. Small film hand-cameras of this type, using self-setting between-the-lens shutters, have been designed, though not for aerial work. The possibilities of using springs as motive power in semi-automatic cameras have not apparently been seriously considered. When a spring motor is used for automatic camera operation it at once becomes necessary to add to the motor an elaborate clock mechanism for controlling and regulating its speed of action. Springs are much better fitted for giving power by quick release of their tension than by slow release, and the necessary clock mechanisms for their regulation become very heavy, as well as complicated and delicate, when they are made large enough to do any real work. For their repair they require the services of clock makers rather than the usual more available kind of mechanic. Coming next to electric motors, we meet with a source of power of very great flexibility both in its derivation and in its application. If a source of electric current is already provided for heating and lighting as it is on the fully equipped military plane, and if it has sufficient capacity to handle the camera, its use is rather clearly indicated, irrespective of how efficiently or by what method it is produced. Especially is this the case, from the standpoint of economy and simplicity, if a propeller-driven generator is the source of current, and the alternative power drive is an additional propeller for the camera. If, on the other hand, the camera must have its own source of electric power, the advantages and disadvantages must be closely scrutinized. In this case either a generator must be provided, or resort be made to storage batteries, or a combination of the two. Ruling out a special propeller-driven generator, we are left with either the generator driven from the engine or the storage battery. Inasmuch as storage batteries are practically indispensable with generators, in order to maintain the voltage constant at all speeds, it is on the whole advisable to rely upon batteries alone. An advantage of their use is that the power plant is entirely within the plane: All projections such as propellers are avoided. Another merit is that the power is drawn upon only as needed. Against storage batteries is their weight, the need for frequent charging, and their loss of efficiency at low temperatures—a loss so serious with those of the Edison form as to preclude their use.