ropean mills, and the other by the direct pressure of the wind against a side-vane. In centrifugal mills the wind-wheel consists of a number of radial arms firmly secured in a metal hub, with sections between them pivoted so that they can swing into a position in which the ends of the slats only are exposed to the wind. They are held in the plane of the wheel by a counterweight, and thrown out of this position by the action of a ball-governor. This governor may be placed in various positions on the wheel, and act upon the movable section directly or through the medium of connecting rods. In one form of wheel the balls are placed upon the framing so that when the wheel is at rest they hang down upon its face, but as it revolves fly out, and in doing so turn the sectors. The angle at which the wheel-surface is exposed to the wind is thus altered with every variation in its velocity, and the motion of the wheel consequently kept nearly uniform, in a manner similar to that of a steam-engine. When the wind attains a velocity greater than a certain number of miles an hour, the action of the governor keeps the slats in the position in which their ends are alone exposed to the wind. The velocity at which the wheel will completely close can be regulated by the counterweight, which is movable on its arm by means of appropriate connecting rods, from the base of the tower.
This method of regulation has been found to answer very well in practice, but it has several grave objections. The construction is necessarily such that there are a large number of joints on which the wear is very considerable; and with so many movable parts the liability to derangement is greatly increased. The failure of any of the parts during a high wind would endanger the safety of the wheel, and perhaps cause its destruction. The second form of mill, that using the vane-governor, is much simpler in construction, has fewer parts, and is consequently more durable. It is, therefore, to a considerable extent supplanting the older form. The wheel in it is solid—that is, without movable sections—and is turned about a vertical axis in such a way that its angle with the direction of the wind varies with the pressure of the latter. The devices by which this is accomplished vary somewhat in different mills, but the method is essentially the same in all. In one, a small vane, placed back of the wheel, is hinged upon the frame of the large rudder-vane, and when the wheel is at rest hangs vertically downward. It is connected by means of rods with the wheel in such a way that, when the pressure on this exceeds a certain amount, the vane will be raised toward an horizontal position. In so moving it turns the wheel by suitable mechanism toward the rudder-vane, when the pressure of the wind is sufficiently great the small vane is raised to an horizontal position and the wheel swings parallel with the rudder. The whole apparatus, wheel and rudder, then becomes simply a weather-vane, is exposed as little as possible to the wind, and is in the best position to escape injury when this is very high.