following table illustrates the power which is guaranteed by manufacturers, and which has been attained by hundreds of wind-wheels in the United States and elsewhere. "The actual results tabulated are in close agreement which those obtained by theoretical anal ysis of the impulse of wind upon windmill-blades" (Wolff).
Regulation of the accommodate the mill to the variations in the direction of the wind the frame or a portion of the tower carrying the wheel, in the ordinary form of construction, turns around in a hori zontal plane on a fixed vertical axis. If, on account of the strength of the wind, it is necessary to check the rapidity of the wheel, a portion of the vane-covering is removed; this regulation of the wheel is in conformity with rules derived from experience with wind-mills, and varies with each type. In more modern constructions the regulation of the wheel is fre quently attained by altering the angle at which the sails arc presented; this is done by turning the vanes on their whips, by which adjustment, however, the previously mentioned influential inclination of the surfaces of the vanes is changed. Andrew I\Ieikle of Scotland in 1772 invented a plan for automatically adjusting the area of the sails to the force of the wind, and Bywater of Nottingham, England, in 1804 patented a inethod of rolling up the sails for adjusting them in like manner by means of a weighted lever. Sir IV. Cubitt in 18o7 introduced automatic reefing arrangements. The sails were made of thin boards or pivoted slats like those of a Venetian blind, and were held up to the wind by a counteract ing- weight; as the strength of the wind increased the slats were pressed back, by which action they exposed less surface to the wind.
Regulation of the an economical standpoint it is important that both the above changes should be effected by utilizing the force of the wind itself; efforts have consequently been directed toward making these constructions self-acting. It is stated that the first method adopted to present the vanes to the wind was to float the mill and turn it in the water as occasion required. The next was to put the will on a post and turn it on this axis; this was called the "German system." Subse quently the cap or roof of the mill was made to revolve by means of a vane of considerable area attached to the movable framework; this was a Dutch invention of the sixteenth century. Meikle in 175o introduced an
auxiliary windmill or fan, placed at right angles to the principal wheel, for automatically turtling the face to the wind. When the wind shifted, the small fan began to revolve and by an actuating gear rotated the cap of the mill. Our illustrations exhibit various forms of the self-acting setting of the wind-shaft, both on the principle of a large vane of wood, sail-cloth, or sheet-iron, and on that of a small wheel acting on a mech anism which rotates the upper portion of the frame or tower. The self acting change of the surfaces and of the inclination of the vanes is effected by numerous devices, a few of which will be described.
7)7111' s —Figure 6 (pi. 7o) shows the Trull wind-wheel, which turns on a plate placed on the top of a frame (A) and is rotated to face the wind by a large vane set horizontally to the plane of the wheel. The four sails can be turned (as shown in Figure 5 on an enlarged scale) by segments of cog-wheels on the lower ends of the whips. These cog wheels engage with a square rod, toothed on its four sides, which lies in the hollow wheel-shaft and is movable in the axis of the shaft. This rack rod connects with a mechemism which is moved by a ball-governor placed on the vertical shaft, near the foot of the frame. The faster the wheel rotates with an increasing wind or a decreasing resistance of the operating machines, the more rapidly the vertical shaft I, connected with the wheel shaft by cog-wheels, is revolved; and the farther the halls of the governor are centrifugally forced apart, the flatter the vanes are set by means of the rack-mechanism. The rotation of the wheel being thus retarded by the flattening of the vanes, its velocity will remain quite constant with even' change in the force of the wind; and if the inclination of the sails is reg ulated for a minimum velocity of the wind for a degree of effect as advan tageous as possible, the power developed will assume the desired strength in all the variations. The dropping of the lever A' completely flattens the vanes and stops the wheel. The transmission of the motion and power to a mill, or other machine is effected by means of a pulley or a cog wheel usually provided.