WINDMILL

In Mechanics, a mill which receives its motion from the impulse of the wind. The general appear ance of the windmill is familiar to every one. The building containing the machinery is usually circular. To the extremity of the principal axis, or wind-shaft, are attached rectangular frames (generally five), on which cloth is usually stretched to form the sails. The surfaces of the sails are not perpendicular to the axis, but inclined to it at a car tain ancle, about 72° at the extremities nearest to the axle, and 83° at the farther extremities; so that their form is in some degree twisted, and different from a plane surface. Suppose the axis to be placed in the direction of the wind: the wind will then strike the sail obliquely, and the force may therefore be resolved into two parts, one of which, acting in the direction perpendicular to the axis, gives a motion of rotation to the sails, and consequently to the wind shaft, from which it is communicated to the machinery, The wind-shaft is inclined to the horizon in an from 8° to 15°, principally with a view to allow room for the action of the wind at the lower part, where it would be weakened if the sails came too nenrly in contact with the building. As the direction of the wind is constantly chancing, soma apparatus is required for bringing the axle and sails into [s. 1352]