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.