Page:The New International Encyclopædia 1st ed. v. 20.djvu/655

* WINDMILL. 559 WINDMILL. vaiilngc of the uiiobslructed action of the wind. A c-uiiipanilivoly f(,'W wind wheels are set on a vertical axis, but as a rule the axis is either horizontal or slif;litly inelined therefrom. A deviation from the horizontal is sometimes es- sential to poriiut th(> sails to clear the tower. The typical Dutch windmill is composed of four long sails at right angles to each other, lixed on au axis inclining aliout 10° to the horizontal and mounted on an inclosed tower of masonry or wood. Modifications of this type include six or more sails. The web of the sails is generally of canvas, but wood may be sub- stituted, in the best forms of European w'ind- niills each sail is cosnposed of arms or whips .30 feet or more in lengtli. attaclicd at right angles to the sail axle. Transver.sc bars or rods are secured to the wliip at intervals throughout its length, and on these the sails are stretched. By setting the bars at varying angles with the plane of revolution of the wliips. a warped surface is jiroduced. This is essential, since the sails re- volve in a plane at right angles to the direction of the wind, so if the surfa<'o was not curved the only efTcct of the wind would be to press against a lixed 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 of the wind zone than is the case in American mills. American windmills may be 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 the 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 alTorded, in the case of wheels revolving on a horizontal axis, by casing 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 come 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, wdth a single plain board nailed to the arm at such an angle as to eatch the wind, to an elaborate series 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 canvas and wood there are a number of metal sails with curved surfaces, all connected to make a compact, strong wheel. Whatever the type of mill employed, its higliest eHiciency can be attained onl^' by keeping it con- stantly in the wind ; that is, so that its plane of revolution will be at right angles to the wind in the sail-wlieel mill, and parallel with it in the paddle-wheel type. In the early European mills the wheel and tower were mounted on a pivoted post and turned by hand. Later on the mills were so arranged that only the wheel, its accessories, and the dome or cap of the tower were turned. This was done either V>y hand or by wind power, an auxiliary windmill, with gearing, being employed in the latter case. In the American mill, also, only the upper part of the mill revolves. The turntable to effect this movement is actuated by the direct pressure of the wind on either the wheel itself or a rudder attached to it; or else by means of a secondary wlu-el, which is used to turn the main wheel into the wind. Besides devices to keep the wheel in the wind others must be employed to regulate the wheel under varying conditions of wind velocity. Such regulation is necessary to secure the maximum amount of power when the wind is low, a fairly constant speed, so far as is possible, under all conditions, a reduction of speed in high winds, and the complete stoppage of the windmill during gales. Regulation may be effected by applying a brake to check the speed, or, as is commonly the case, by diminishing the area of sail exposed to the wind. The latter may be effected by chang- ing the angle of the groups of vanes of a sec- tional wheel, by means of centrifugal action ap- plied through lever rods and resisted by means' of weights or springs. The greater the velocity of the wheel the more the vanes will be turned until, if the wind be high enough, they offer it no purchase wdiatever. A second method in- volves turning the wheel partially or wholly out of the wind, by revolving it about the axis of the tower. Counterweights may also be used here, as in the case of both the centrifugal gov- ernor and some of the devices used for holding the wheel in the wind. The object of the coun- terweights is to prevent too sudden action of the regulating devices, which might result in damage to some portion of the mechanism in- volved. The chief use to which windmills are put in America is the pumping of water from wells, but they are also employed, often conjointly, for grinding grain and cutting fodder for stock, turn- ing grindstones and other machinery' for repair- ing and making farm tools, and mnuerous other purposes where a relatively small amount of ir- regular power is needed and where low first cost and a mininnun of expense for maintenance and operation are essential. The power of windmills increases a little- faster than the square of the wind velocity and about 1.2.5 times the square of the diameter of the wind wheel. According to Murphy {see bililiography below) "a good 12-foot ste'd mill should furnish one horsepower in a 20-miIe wind {indicated! and 1.4 horsepower in a 25- niile wind. This is the smallest amount of