List me down some key aspects about windmills from given text.
Wind-powered��machines may have been known earlier, but there is no clear evidence of windmills
 before the 9th century. Hero of Alexandria��(Heron) in first-century��Roman Egypt��described what appears
 to be a wind-driven wheel to power a machine. His description of a wind-powered��organ��is not a
 practical windmill but was either an early wind-powered toy or a design concept for a wind-powered
 machine that may or may not have been a working device, as there is ambiguity in the text and issues
 with the design. Another early example of a wind-driven wheel was the��prayer wheel, which is believed
 to have been first used in��Tibet��and��China, though there is uncertainty over the date of its first
 appearance, which could have been either��c.���400, the 7th century, or after the 9th century.
1. Before 9th century there is no evidence of Windmills.
 2. In first century roman Egypt , Heron described what is wind driven wheel to power machine.
 3. As per the description given by him the wind-powered organ was either an early wind-powered
 toy or a design concept for a wind-powered machine that may or may not have been a working
 instrument.
 4. One more early example(either the 7th century, or after the 9th century) of a wind-driven wheel
 was the��prayer wheel, which was first used in��Tibet��and��China.