The origin of the windmill is controversial. In a manuscript on Pneumatics, Heron of Alexandria includes the description of a wheel driven by the wind and driving the piston of an organ. But the authenticity of this passage has not been resolved — it could have been added during the Islamic period. In any case, even if this passage is authentic, the link between this invention and the appearance of the true windmill in central Asia seems to be very weak.
The backdrop of Heron’s apparent reference to a windmill is the ubiquitous presence of water mills on all of the rivers. On the Tigris at Baghdad, on the Seguro at Murcia, on the Ebre at Saragossa, there are even boat-mounted water mills. Mills are the indispensable tools of an agricultural economy. But on the Persian plateau there are few, if any, watercourses. This undoubtedly led to the invention of the windmill at an indeterminate date but perhaps prior to the Arab conquest. In about 660 a Persian had affirmed to the caliph Omar that a windmill could be built. The windmills of Seistan (to the east of Khorassan) were known to Arab geographers from the 9th century on, in particular to the Banu Musa brothers.[363] These mills turn on a vertical axis; their vanes are inside a chamber at the top of a sort of tower, open to the four cardinal directions with shutters that one can close depending on the direction or the force of the wind. The rotation of the axle directly turns a millwheel that is in the chamber just below. These mills are described somewhat later by Chinese authors who give a precise account of them:
“In the western countries called Herat and Samarcand, there are many windmills. Brick walls are built to form a sort of house having openings at its summit, facing the four directions, outside of which screens can be placed to direct the wind. In the chamber below is placed a wooden axle, with sails (literally planks to ride the wind) fixed to it. Whatever the direction of the wind, the axle always turns, and the stronger the wind, the more work can be accomplished.”[364]
