I. WIND WHEELS The wind-wheel was first utilized as a source of motive power by the Germans during the eleventh century. The oldest form of was doubtless that of the " post "-mill, in which the whole structure was carried on a post set in the ground and rotated by a long lever ( 3, fig. S). The post-null was succeeded by the tower- or Holland mill (fig. 7), a structure which consisted of a stationary tower, the wind-shaft and sails being carried on a revolving cap at the top. Tower-mills are largely employed in Holland for drainage purposes. Our present subject, however, is not specifically windmills, but in general, as applied for the obtainment of motive power. Wind-wheels, front their mode of action, are of two classes—(I) those rotating verlically on horizontal axes, and (2) those rotating horizonially on vertical axes.
Vertical Uancs or the ordinary early form of windmill the wheel consists of planes combined with and placed at right angles to a shaft, which is revolved by the impactive force of the wind acting on the planes. These planes are called " vanes " or "sails," a number of which—generally four or five, though exceptionally three, six, or even twelve—are connected with the head of the axle or shaft, which is set horizontally or inclined to the horizon at an angle of from 8° to 15°. Each of these vanes, or " whips,'' as they are sometimes called, usually from 3o to zto feet in length, is composed of a long tapering bar of wood with short cross-pieces or rungs, whose extremities are connected with one another by wooden strips. This frame is covered with sail-eloth or with thin boards. By an inclination of the rungs obliquely to the geomet rical plane of the whips the sails receive an inclination in the direction of the wind blowing parallel to the axle, in consequence of which the wheel is rotated. Without this inclination the wind would impart to the wheel a pressure only in the line of its axis, but with it the impactive force of the wind is divided into a component of a power directed in a line parallel and normal to the surface of the vane; hence, while in the first case the wind exerts an axial pressure only, in the latter it effects a rotation. The
inclination of the axis referred to above is given for a similar reason, it having been found from experience that the direction of the wind is some what downward, generally forming, in open countries, an angle of abdnt i S° with the surface of the ground.
In the earlier wheels the same inclination was given to all the rungs, but in modern constructions it is less for the rungs at the greatest distance from the axle, the lower part forming whit the axis an angle of about 6o°, and the upper of about So°, in consequence of which the covering of the vanes forms an oblique and somewhat winding surface, with their concav ities facing the wind. The inclination of the rungs being the basis of a suitable construction, and their difference of inclination being conducive to a winding surface, which is an improvement, the best form of construe tion results from further bending the surface and making it concave by a curvature of the whip itself.
Figure r (p1. 7o) gives an illustration of the upper portion of a tower mill. A is the tower, which, for containing the machinery driven by the vertical shaft L, may be constructed of wood or of masonry; B is the tower-cap, which is movable in a horizontal plane; CDE is the axle of the wheel; F, F are the whips, composed of two parts, and FG is a cable, which is secured to E at G for the support of the outer ends of each whip. On the axle of the wind-wheel there is secured a cog-wheel, which gears into a like wheel, of smaller diameter, upon a vertical shaft placed cen trally in the mill, so that the vane-wheel and the cap of the tower, when revolving to face the wind from another quarter, may continue in gear therewith, and in all positions may communicate motion to the mill-ma chinery, which is located about the lower end of this shaft.