The shaft coming in at the upper side of this photo comes from the 'second' gear outside the mill - the one driven by the wheel. It connects with the smaller gear which sends the rotational power to the millstone assembly. The smaller gear, and tbe main body of the larger one, are iron, but the teeth of the large gear are wooden. The reason for the wooden teeth is that iron-on-iron in ineveitably produce sparks on occasion - a very bad thing in an environment where the air is loaded with ultra-fine particles of flammable flour.