I'm in Frankfurt for a couple of days hoping to finalize some new projects and generally having a good look round. On the way across I stopped in the small town to Xanten where lives this delightful working windmill.

Outside the windmill were posted various bits of information but unfortunately the windmill itself was not open to the public on a Monday.

This picture of a mechanism was posted on the notice board outside. The top wheel obviously connects to the sails of the windmill. The drive is then turned through 90 degrees and driven down to the wheel a the bottom where the speed of rotation is increased through gearing to drive the mill wheels at the bottom of the picture.
The unanswered question is, what it the part with the question mark for? 
A modest prize for anyone who can work it out in the comments section below. I'll be calling in to Xanten on my way back to the ferry so I'll find out and let you know.