instruction: List me down some key aspects about windmills from given text.
context: 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.
response: 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.