These picturesque objects are buildings containing machinery, to be
driven by the wind, for grinding corn, sawing wood, and any other
purpose that may be required. They consist of a basement, generally
of stone or brick, and a superstructure surmounted by a sort of dome
capable of being turned round. From this dome projects the shaft of
a wheel, and on this is fastened four fans or sails made of long bars
of wood crossed by shorter ones; these being covered with canvass,
form a surface to catch the wind. These sails are placed obliquely to
the front of the cross, so that when the wind blows upon them right
in front, they are at an angle with it, they are therefore turned
round; for the wind which pushes them from the front, as they are
oblique, tends also to push them on one side; when once in motion,
being heavy, they form a sort of fly-wheel to the machinery. The
dome has several small wheels attached to its lower border, to act
as friction rollers and cause it to be easily turned round (which
is often required), that the sails may be made to face the wind in
whatever direction it may blow; this is sometimes done by ropes
attached to the dome, but is more frequently effected by means of
a small set of sails, shown in the cut, which are placed at right
angles to the large set, so that when the wind acts on the large
sails the small ones are not affected; but should the wind shift,
these small ones begin to move, and they are connected with a toothed
wheel acting upon a band which surrounds the dome; this is therefore
caused to turn round whenever the small sails are turned, and as the
dome turns, it brings with it the large sails until they are in the
right position. These sails are generally fixed not quite upright,
but inclined with their fronts looking a little upwards, which is
found to be the best position to catch the wind.