Networks of Wind Energy Enthusiasts and the Development of the “Danish Concept” - Wind Power for the World: The Rise of Modern Wind Energy - page 120

gears and axles for translating the horizontal to vertical motion
for grinding. Initially, the entire structure of the post-mill had to
be rotated into the wind for use.

Figure 5.2 Early image of a post-mill from “Schembartbuch”, 1590-1640.

The technology evolved into the “tower-mill” and other
variations which still used a horizontal-axis but only required
rotation of the sails, wind-shaft and brake-wheel rather than the
entire tower structure. Moving into the 18th and 19th centuries,
various innovations in particular surrounding the control of these
windmills were developed including mechanisms for orienting
the mill with respect to the incident wind direction, braking
mechanisms and regulation of power production. For example, the
“yaw” mechanism for orienting the rotor in and out of the wind was
automated using a fantail on the backside of the tower, oriented
perpendicular to the rotor axis. For braking, governors dependent
on centrifugal forces building up at higher wind speeds would
engage automatic mechanical braking mechanisms or furling of the
rotor blades. Having achieved such advancements, many elements
of modern wind turbines already existed by the 19th century:
upwind orientation, a small number of blades, a geared driveshaft
connecting to a work machine, and automated control mechanisms
as mentioned above.