This photo shows a couple brilliant devices. The unmilled grain is pouring from a bin on our right into the millstones below. The vertical shaft rotates with the millstones. The three ridges on the lower part of the shaft push the pouring spout and cause it to knock against the shaft, a tapping that helps keep the grain moving. The cam at the top will ring a bell once per rotation (off screen to the left), but only once the rope on the right has gone slack, which will happen after the cloth it's tied to is no longer buried by grain. So when the hopper runs low, a bell starts to ring.