Over 5,000 years ago, the ancient Egyptians used wind to sail ships on the Nile River. While the proliferation of water mills was in full swing, windmills appeared to harness more inanimate energy by employing wind sails. The wind wheel of Heron of Alexandria marks one of the first known instances in history of wind powering a machine . The first practical windmills were the vertical axle windmills invented in eastern Persia, as recorded by the Persian geographer Estakhri in the 9th century, see Hassan and Hill . Prototypes of windmills were probably known in Persia (present day Iran) as early as the 7th century AD with their sails mounted on a vertical axis, (see Figure 14.2). Towards the end of the 12th century, windmills with sails mounted on a horizontal axis appeared in Europe; the first of this kind probably appeared in Normandy, England. These are post mills, where the sails and machinery are mounted on a stout post and the entire apparatus has to be rotated to face the wind.