Whatever the type of mill employed, its highest efficieney eau be attained only by keeping it con stantly in the wind; that is, so that its plane of revolution will he at right angles to the wind in the sail-wheel 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 by 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 tee it or else by means of a secondary wheel, which is used to turn the main wheel into the wind.
Besides devices to keep the wheel in the wind others must he employed to regulate the wheel under retrying 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 cheek the speed, or, as is commonly the ease, hy diminishing the area of sail exposed to flee wind. '1'lle 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 ter 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 whatever. 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 conn terweig,hts is to prevent too sudden action of the regulating devices, which might result in damage to some portion of the mechanism in vol ved.
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 numerous other
purposes where a relatively small amount of ir regular power is needed and where low first cost and a minimum 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 1.25 times the square of the diameter of the wind wheel. According to Murphy (see bibliography below) "a good 12-foot sted mill should furnish one horsepower in a 20-mile wind (indicated) and 1.4 horsepower in a 25 mile wind. This is the smallest amount of power that will do any considerable amount of useful work. A 16-foot mill will furnish 1.5 horsepower in a 20-mile wind (indicated) and 2.3 horsepower iu a 25-mile wind.
"A 12-root steel mill and a 50-foot steel tower as commonly made weigh about 2000 pounds. A 16-foot steel mill and a 50-foot steel tower weigh about 4250 pounds. The 1G-foot outfit weighs more than twice that of the 12-foot, and its power is only 1.5 that of the latter. In addition, the 12-toot mill will govern more easily and is less likely to he injured in a storm than the I6-foot will. hi most eases, therefore, it is better to use two 12-foot mills than one l6-foot mill." "Steel mills." lie says. "with few large sails, have much more power than the wooden mills with their many small sails." Mills should be on towers 50 to 70 feet high, in order to get them at least 30 feet above the tallest trees and buildings. Mills should start in as light a wind as corresponds to a velocity of four or five miles per hour. For information as to velocity and other phases of the wind, see WIND.
BIBLIOGRAPHY. Consult: Wolff, The WindBibliography. Consult: Wolff, The Wind- mill as a Prime Mover (New York, 1885), a technical treatise; Perry, Experiments with Windmills: Barbour, WcIls and Windmills in Yebraska, which contains a very interesting de scriptive review of home-made windmills; and Murphy. The Windmill: Its Efficiency and Economic Use, also mentioned above (Washing ton, 1). C., 1899 and 1001, being Nos. 20, 20, and 41-42. respectively, of Water Supply caul Irrigation. Papers of the United States Geological Survey).