Here's the gear assembly that drives the millstones. The shaft coming in at the lower left turns the lower, conical, gear (with wooden teeth) which in turn drives another conical gear out of sight below, with iron teeth. (it doesn't touch the large horizontal gear). The gear below translates the rotation to the horizontal plane, and drives the central large gear of the photo (with wooden teeth). Engaged with it, behind, is the drive gear for one of the sets of millstones. Closer to us, on the right side of the photo and raised out of engagement, is the drive gear for the other set of stones. The drive gears are raised and lowered by cranks, above.