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FILM MACHINES 147 arranged to exactly counterbalance the difference in the mass of the cam drum, and counteract any deflecting strain on the bearings. The cylinder is driven by means of a fixed ring, h, cut with internal teeth, and three planet wheels on the spider g, gearing both with the ring h and driving the central pinion e on the axis of the cylinder a. The examples given will sufficiently indicate the in- genuity which has been spent on this type of feed. It is not much used for projectors, but is very extensively used for feeding the film in perforating and printing machines. The action of the pins serve in the latter to feed the positive film along with the negative film, both films being thereby fed at exactly the same rate. The motion of the film during a feed by the claws can be represented by curves similar to those shown in Fig. n8A. In the case of the claw-feed the claws always enter and leave the perforations when stationary, and the curve will always start from X and finish at X'. The quicker shift movements are represented by steep curves, and the consequent strain on the film is correspondingly great. The slower shift movements are represented by flatter curves, and the consequent strain on the film is correspondingly less. 2 (d [i]). Film moved intermittently by the Pressure of a Revolving Eccentric. The original so-called dog-motion, invented by Demeny, is shown in Fig. 82. The amount of film fed forward can be adjusted by adjustably mounting the eccentric roller R along a slot in the revolving disc D (Fig. 147). If the film passes between two gripping rollers, such as A, B, which continually rotate, and thereby continually put tension on the film, it is necessary to have pressure pads, P, at the gate to hold the film stationary when the dog is not acting on the film. It is more usual, however, to use a sprocket- roller in lieu of the rollers A, B, for the purpose of taking