WINDMILL, a mill which receives its motion by the wind acting on sails, and i which is used for grinding grain, rais ing or pumping water, and other pur poses. When wind is employed as the first mover of machinery, it may be ap plied in two ways : (1) By receiving it on sails which are nearly vertical, and which give motion to an axis nearly horizontal, in which case the machine is called a vertical windmill, or (2) by re ceiving it on vertical sails which move in a horizontal plane, and give motion to a vertical axis, in which case it is called a horizontal windmill. Sometimes the whole mill is made to turn on a strong vertical post, and is then called a post mill; but more commonly the roof or head only revolves, carrying with it the wind wheel and its shaft, this weight being supported on friction rollers.
As it is that the extremity of the wind shaft must always be placed so as to point to the quarter from which the wind blows, a large vane or weather cock is placed on the side which is op posite the sails, thus turning them always to the wind. But in large mills the
motion is regulated by a small supple mentary wind wheel, a pair of sails occupying the place of the vane, and situated at right angles to the principal wind wheel. When the windmill is in its proper position with the shaft parallel to the wind, these supplementary sails do not turn; but when the wind changes they are immediately brought into action, and, by turning a series of wheel work, they gradually bring round the head to its proper position. On account of the inconstant nature of the motion of the wind, it is necessary to make some pro vision for accommodating the resistance of the sails to the degree of violence with which the wind blows. This is done by clothing and unclothing the sails; that is, by covering with canvas or thin boards a greater or smaller portion of the frame of the sails according to the force of the wind.