This circular contraption encloses the corn-grinding stones, drive by (and directly above) the engaged gear in the previous photo. Dried corn from the hopper at the top is fed into the metal funnel in the center of the wooden cylinder, which covers the stones themselves. To the left, uncovered, you can see the wheat-grinding stones, the upper encased in a band of iron, the lower projecting just an inch or so above the platform. The wheat stone is driven by the disengaged gear in the previous photo, and so wasn't spinning.