the romans used both fixed and floating water wheels and introduced water power to other countries of the roman empire. so-called 'greek mills' used water wheels with a vertically mounted shaft. a "roman mill" features a horizontally-mounted shaft. greek style mills are the older and simpler of the two designs, but only operate well with high water velocities and with small diameter millstones. roman style mills are more complicated as they require gears transmit the power from a shaft with a horizontal axis to one with a vertical axis. an example of a roman era watermill would be the early 4th century site at barbegal in southern france, where 16 overshot waterwheels were used to power an enormous flour mill.