Horizontal Water-Wheel
Remains of a primitive horizontal water wheel in a culvert beneath a ruined mill.
The horizontal water wheel was invented by the Greeks and surpassed by the Romans, who introduced the very much more efficient vertical wheel. In England the horizontal wheel was already extinct by the Middle-Ages, but in Spain it remained in use until the 1950s.
Horizontal waterwheels are particularly suited to very shallow, fast flowing streams. In England shallow streams and rivers were dammed (by the construction of the mill) and a big millpond was created. This allowed the collection of a large head of water with which to turn a vertical overshot wheel. Clearly, the creation of the pond required that the mill-owner be sufficiently affluent to buy a sizeable piece of land. Having invested his money he hoped to do very nicely from the profits of this fairly lucrative business.
The Spanish horizontal mills ("rodeznos", or "rodetes") generally belonged to peasant farmers. They were not millers, as such - they probably milled only their their own produce - and they would not have had the money to buy land for the creation of a pond.
(Grazalema, Andalusia, Spain. 2007)