I have posted about watermills and windmills in my region of The Netherlands, but I almost forgot about one ancient source of power that was also use extensively for driving mills: horses.
On one of my walks I saw this relatively modern (around 1910, I think) horse mill. In local dialect, this type of horse mill is known as a geubel, and just across the border in Germany as a Göpel.
Olympus Stylus 1s, 33mm, ISO200, f5.6, 1/60
Basically, it is a set of gears with a connection (bottom, right-hand side in the photo) for driving early mechanised farming equipment. Horses pulled the beams around and there it went. These days, we locals also use groups of tourists as a power source, they seem to like it and we get to grin.
These horse mills were used for driving farming equipment like threshing and heckling machines. These old pictures from Meyers Konversationslexikon give you some idea:
In my region, this was not the only way horses walking in circles were used; the local peat came out of the ground so wet, the water had to be squeezed out before it could be cut and left in the sun to dry. For this, a kluundel was used, a cobbled open space near the peat bog:
Olympus XZ-1, reposted photo of a kluundel in the Aamsveen
A horse would walk in circles all day to squeeze the water out, much like a horse driving a horse mill, but less tiring. I've seen demonstrations of both in action, and the horses didn't seem to mind going round and round again, but maybe they were so dizzy they were beyond caring.