WINDMILL, a machine for furnishing power for grinding grain, pumping water, or doing other useful work, operated by the wind. History does not record the date of invention of the windmill; but it is known that it was used in Europe as early as the 12th century AD A common form of European windmill is shown in Fig. 1. The sails, upon which the wind acts in driAing the machine, consist of a light framework upon which canvas is stretched. The surface of the canvas makes an angk (called the angle of weather) with the plane of the windwheel, and this angle is usually about Di° at the inner end of the sail, gradual!. decreasing to about 7. at the outer end The length of the slit is generally about five-sixths the length of the arm, the width of the outer end one-third the length, and the width of the inner end one-fifth the length. For proper action the axis of the windwheel shaft should be parallel to the direction of wind. In most cases it is, however, inclined upward at an angle of about 15° to permit the ends of the sails to clear the tower, and since the direction of the wind is, in general, horizontal, this condition for proper action is not fully realized in windmills of the European type. To allow the windwheel to be turned as. the direction of the wind changes, the upper part of the structure is adapted to rotate about the verticle axis of the tower, being supported upon rollers at A and the auxiliary wheel B is provided to maintain it in its proper relation with the direction of the wind. This auxiliary wheel is so connected, by gearing, with the fixed part of the structure that in rotating it causes the movable part of the structure to turn about its axis. When the main windwheel is properly directed the auxil iary windwheel presents its edges to the wind, and is, therefore, not affected by it; a change in the direction of the wind tends to cause rota tion of the auxiliary wheel which in rotating rings the main windwheel into proper relation with the wind. Before the invention of the auxiliary steering wheel, and to a large extent after its invention, the main windwheel was matic governor. It is to be observed that the sails of the European windmill occupy only a small portion of .the area swept by them. In the American type the sails are much greater in number and occupy, comparatively, a much larger part of the area swept by them. The kept in position by hand. In many cases the whole tower, instead of merely the upper part thereof, is made to pivot about its vertical axis, this latter construction being generally fol lowed in German practice, while that above de scribed is common to machines built by the Dutch. In some very crude windmills of the European type no provision whatever was made for changing the direction of the windwheel. The speed of a windwheel tends to vary with the wind velocity. In windmills of the European type, where speed regulation is desired, it is accomplished by varying either the load or the sail area exposed to the action of the wind. A friction brake is often used to vary the load, while to vary the sail area the canvas forming the sail surface is rolled up or unrolled, or the sail surface is formed from slats, after the fashion of the Venetian blind, and changed by opening or closing the slats. The devices for varying the sail area are actuated either by hand or by a suitable auto greater number of sails makes it possible to re duce the diameter of the windwheel, for a given power, considerably below that necessary in the European windmill; as a result the American windmill is much smaller than that of Europe.
The windwheels of the latter were often as large as 100 feet in diameter; those of Ameri can type are rarely larger than 30 feet in diameter.
American windmills are built in a very large variety of styles, some being constructed prin cipally of wood, others entirely of metal. Each is. however, designed automatically to maintain the windwhcel in its proper relation with the direction of wind, to deliver the power devel oped either by means of a rotating shaft or re ciprocating rod, and automatically to keep its speed below a certain definite and safe limit. A certain number of methods have been de vised for each of these operations. Different combinations of these methods, together with variations in minor details, result in the large variety of styles. Figs. 2, 3 and 4 show sev eral windmills of the American type, these particular forms being selected because among them may be found examples of each of the methods by which the results before mentioned arc accomplished. The windwheel may be held in proper position with regard to the direction of wind in three ways, namely, by means of a rudder or tail, as in Figs. 2 and 3, by means of an auxiliary steering wheel, as in Fig. 4; and by the pressure of the wind upon the wind wheel itself, which is placed on the opposite side of the tower to that from which the wind proceeds, as in Fig. 4. When the windmill is to be used for pumping purposes exclusively, the power is, in general, transmitted from the windmill to the pump by means of a reciprocat ing sertical rod, as in Figs. 2 and 3. Those in which the pump rod makes one complete stroke for each resolution of the windwheel, as in Fig. 4, are termed direct stroke windmills: when the pump rod makes only a fraction of a stroke for each resolution of the windwheel, as in Figs. 2 and 3, the windmill is said to be hack geared. The back granng may be accomplished either by means of a pair of spur gears, as in • Fig. 2, or by means of a rack and pinion, as in Fig. 3. In pumping windmills of recent construction back gearing is very largely em ployed, its object being to avoid the great shocks and violent churning of the water resulurig from the rapid action of direct stroke wind mills, and to reduce the losses in the pump doe to friction and to backlash of the valves. When the windmill is to be used for general power purposes, which may include mg, sawing, grinding, etc., the power is transmitted through a train of gears to a vertical shaft, those of this general type being known as geared, or power windmills. Any torque exerted by the gearing upon the vertical shaft reacts upon the windmill itself, tending to rotate it out of its normal position with gard to the direction of wind lecting friction, the torque at the vertical shaft is equal to that exerted by the windwheel divided by the number of turns which the vertical shaft makes to one turn of the wheel. Hence the effect of the vertical shaft in disturbing the tion of the windwheel becomes smaller in amount as the ratio of gearing between them is increased, and for this reason the ratio of ing is in best practice made about six to one; that is, the vertical shaft makes six revolutions to one tion of the windwheel. This high ratio is also an advantage in that it allows the diameter of the vertical shaft. for a given size of windwheel, to be made considerably kss than that which would be necessary with a smaller ratio.