In order to apply this relation to the study of spring motors for driving cameras, data are necessary on the power delivery per pound weight of such mechanisms. Such data are not easily accessible, largely because clock-work has not generally been seriously considered as a motive power for large apparatus. To arrive at an approximate figure we may take the fact that in an 8 × 10 inch film camera designed by one of the manufacturers who have utilized clock-work, the motor weighed 30 pounds. This is equivalent to six pounds head resistance. Now the type K, 18 × 24 centimeter film camera is operated, even with the addition of a friction drive speed control, by means of the L camera propeller. As shown in Fig. 66, at 100 miles per hour the head resistance of this propeller is still less than three pounds. Consequently, it appears that from the efficiency standpoint the clock mechanism is quite outclassed by the wind propeller. Coming next to the _electric motors_, the L camera and the K are both operated satisfactorily with a 1/20 horse power motor, weighing 6 pounds. For the deRam a ⅒ horse power motor has been adopted. Taking up efficiency considerations, we have, if the current is supplied by a generator from the engine, a transformation factor of 70 to 80 per cent. from mechanical to electrical energy and a similar factor in using a motor for the camera. When batteries are employed the matter of weight _versus_ head resistance again arises. The batteries found most satisfactory for operating the K and deRam cameras are of the six-cell 12 volt lead type. Their capacity is 40 ampère hours at three ampères or 36 at five ampères—more than is necessary for a single reconnaissance, but a practical figure when economy of charging and replacement are considered. The weight of this unit is 27 pounds. To this must be added the weight of the motor—6 lbs.—making a total of 33 pounds, equivalent to a head resistance of nearly 7 pounds. This is more than twice the propeller head resistance invoked to do the same work. These considerations of efficiency have been gone into because they are usual in studying any engineering problem and because of the insistent demand from the plane designer that every ounce of weight and head resistance be saved. Actually, as already stated, the load imposed by any method of power drive is trivial in comparison with the whole load of the plane. There is, however, an important reservation to be made, which applies against clock-work and batteries: This is, that while the equivalent head resistance of any camera motive power carried as dead weight is small, its effect on _balance_ may not be so. While the use of a propeller need not disturb the plane's balance, the weight of the camera alone, without any driving apparatus, is already seriously objected to on this score. The merely mechanical superiority of the propeller as a source of motive power is on the whole rather marked.