When the pressure of the wind upon the windwheel becomes so great as to approach the danger point. it is lessened by reducing the sail area posed to the wind. In the windmill shown in Fig. 2 the windwheel is so placed that a line drawn through its centre parallel to the normal direction of the wind passes to one side of the axis about which the windmill pivots. The tail is in each case supported upon the windmill frame by the hinge joints. C, C. and is held in its normal position with reference to the windwheel by the spring D. pressure upon the windwheel tends, therefore, to stretch the spring, and it does so when it is sufficient to overcome the initial tension of the spring, thus turning the windwheel so that the wind acts obliquely upon it and reducing the exposed sail area. The initial tension of the spring is so determined that it will begin to stretch when the pressure of the wind upon the windwhcel approaches the safe limit. In the windmill shown in Fig. 3 the windwheel is so placed that a line through its centre parallel to the normal direction of the wind intersects the axis about which the windmill pivots. The side vane E is fixed to the windmill frame. upon which the tail is also carried, being ported by the hinge joints, C. C. The tail is held in its normal position with reference to the windwheel by means of the spring F. ure of the wind upon the vane E tends, there turns which a windwheel will make in a given time varies inversely as its diameter. (6) There is nothing gained by having the sail area of a windwheel greater than seven-eighths of the area swept by the sails, and there is little gained h having it more than three-fourths the latter area.
In order that the maximum power may be fore, to stretch the spring F, and the spring is so determined that it will be stretched when the wind pressure upon the vane E approaches the is evident that any change in the relative posi tion of the windmill is changed so that the exposed sail area is reduced, in the same man nrr as with the windmills shown in Figs. 2 and 3 In the windmill shown in Fig. 4 it I, evident that any change in the relative posi t!-ro of the steering wheel and the main wind shed will result in a change in the position .1 the latter relative to the wind. The frame carrying the steering wheel is pivotally mounted upon the frame which carries the main wind wheel, and is held in its normal position with reference to it by the coiled spring H. In the ordinary running position the wind falls per pendicularly upon the side of the tail of the steering wheel, and when the pressure upon it exceeds the safe limit it overcomes the resist ance of the spring and changes the position of the steering wheel so that the wind falls upon its lace; this causes the steering wheel to act, which moves the main windwheel so that the wind falls obliquely upon it, thus reducing the exposed sail area. Other methods of regula tion. involving centrifugal governors or varia tions from the arrangements above described, have been devised, but have not come into ex tended use. The particular mechanical details involved are subject to very wide variation, but the arrangements shown in the figures serve to illustrate the principal types.
Windmills of the usual forms are, in gen eral, mounted upon towers of heights ranging from 10 to 100 or more feet, so that the wind which acts upon them will not be seriously affected by obstructions which may exist in their immediate neighborhood, Provision is al ways made for controlling, or furling the wind mill from a convenient place near the base of the tower, generally by means of a wire, termed the furl-wire. Furling is in general accom
plished by reducing the exposed sail area to such an extent as to prevent the action of the windmill The reduction of the exposed sail area is in each case accomplished in the same mariner as in the process of regulation, except that the force necessary to lift the weight or overcome the tension of the spring is applied through the furl-wire and transmitted by a suit able arrangement of levers or pulleys, instead of Icing supplied by the energy of the wind. Thus in the windmills shown in Figs. 2 and 3 a pull transmitted through the furl-wire is applied to the tail in such manner as to bring it into a posi tion parallel to the plane of the windwheel. In that of Fig. 5 the auxiliary wheel is turned through an angle of 90 degrees by means of a full on the furl-wire. It is to be observed that In each of the windmills heretofore mentioned the windwheel is adapted to rotate about an nits the normal position of which is parallel to the direction of the wind. A number of types tit aindnufb tra‘c been des ised in which the windwheel rotates about an axis, the normal position of which is in a plane perpendicular to the direction of the wind, hut they have not attained a sufficient degree of success to war rant considering them here. If A represents the plane area, in square feet, swept by the sails of the windwheel, V. the velocity of the wind in feet per second, and if the weight in pounds of a cubic foot of the air, then the weight of air available per second for use by the wind wheel is W.e.e AV& and its energy is equal to WV' g .g being a constant representing the accele ration due to gravity. The power in pounds per second brought to the windwheel by ViAa this air is, therefore, L =ai = —. This 2g 2g expression indicates that, other conditions re maining the same, the power of a windwheel varies directly as the cube of wind velocity, and directly as the area swept by the sails, both of which relations have been substantially verified by experiment. In the construction of wind mills considerations of strength require that, in general, the linear dimensions of all of the parts shall bear the same proportional relation to each other, regardless of the size of the windwheel. Other conditions being equal, the weight of a windmill, therefore, varies directly as the cube of the diameter of its wheel, while its power varies directly as the square of the diameter of the wheel. Hence, in increasing the size of windmills, the weight, and the conse quent cost of material, increase more rapidly than the capacity, and it is not, therefore, found practicable to increase their sire beyond a cer tain degree. When a windmill is in operation the air currents which act upon it are so modi fied and broken up, by their contact with the sails and other members, as to render accurate mathematical treatment of the case impossible, in the present state of our knowledge. For this and other reasons, mathematical considera tions of the case are chiefly of theoretic interest and the development of the form of the wind wheel has been due rather to experiment than to theory.