WINDMILL. A motor which utilizes the energy of the wind for pumping water, driving mills, or doing other work. The wind acts on a set of sails or slats attached to an axis so as to form a revolving wheel. A crank or gear ing transmits the power from the axis to the pump or other media nism to lie driven. :Most wimhnills are mounted on towers at a consider able elevation above the ground, or else placed on the tops of buildings, in order to take ad vantage of the unobstructed action of the wind. A comparatively few wind wheels are set on a vertical axis, lent as a rule the axis is either horizontal or slightly inclined therefrom. A deviation from the horizontal is sometimes es sential to permit the sails tee clear flee tower.
The typical Dutch windmill is composed of four long sails at right angles to each other, fixed un an axis inclining about 10° to the horizontal and mounted on an inclosed tower eef masonry or wood. Modifications of this type include six or more sails. The web eef the sails is generally of canvas, hit wood may he sub stituted. lee the best forms of European wind mills each sail is composed of arms or whips 30 feet or more in length, attached at right angles to the sail axle. Transverse lears or rods are secured to the whip :It intervals throughout its length, and on these the sails are stretched. Dv setting the at varying angles critic the plane of revolution of the whiles, a warped surface is produced. This is essential, since the sails re volve in a plane at right angles to the direction of the wind, so if the surface was not curred the only effect of the wind would be to press against a fixed surface. The width of the sails is gen erally greater at the outer than at the inner ex tremity, and the total sail area is smaller in proportion to the area eef the wind zone than is the ease in American mills.
American windmills may he divided into two broad classes: (1) those that revolve in the same direction as the wind and resemble paddle wheels or certain types of water wheels, and (2) those revolving at right angles to the wind. The
members of the second class in this one respect are similar to the Dutch and European mills already described, and may for convenience be called sail wheels. Most of thee windmills in use fall in the second class. The paddle-wheel mills must have one-half of their fans guarded from the action of the wind, for otherwise they will not revolve. since the pressure on the two halves will balance. This protection may be afforded. in the ease of wheels revolving on horizontal axis. by easing in the lower half of the wheel: and for wheels on a vertical axis, by either casing in one-half or by using folding vanes that close on themselves when they cone into the counter-pressure. In either of these sub-classes, it is evident that only a por tion of the total wind area of the wheel is effec tive. The horizontally set paddle-wheel mills are generally placed near the ground and facing the prevailing wind. They are of no use except when the wind blows their way. Sometimes the vanes are made in the form of a warped or screw surface to obviate this defect, whereupon the mill assumes somewhat of the sail-wheel type. In the vertical paddle-wheel mills the wind guard may be made in the form of a semi-cylindrical revolving hood, self-adjusting by means of the changing direction of the wind itself, through the action of the wind on a rudder.
The American sail-wheel windmills vary in design from four or more arms attached to a horizontal axis, with a single plain board nailed to the arm at such an angle as to catch the wind, to an elaborate scrips of fans in one or more annular rings. the fans being composed of narrow slats of wood, also set at an angle with the wind. From this complex type there is a strong tendency for the wheels to revert to a simple form, somewhat like the old European mills, except that in the place of a few curved sails of (1111VZIS and wood there are a number of metal sails with curved surfaces, all connected to make a compact, strong wheel.