WINDMILL is a building containing machinery for grinding corn, pumping water, sawing wood, or for any purpose depending on wheel work, to which motion is communicated by tho impulse of the wind. Windumilla are of two kinds : in one, the wind is made to act upon vanes or sails, generally four, which are disposed so as to revolve by that action on an axis which is nearly horizoutal, in a plane which is nearly vertical ; and in the other, the axis of revolution being precisely vertical, any point on the surface of the vane revolves in a horizontal plane. The former is called a vertical, and the latter a horizontal windmilL The building is generally a wall of timber or brickwork in the form of a frustum of a cone ; and the smaller kind of mill when formed of timber is capable, by means of a lever, of being turned round horizon tally on an axis, in order that the plane in which the radii or arms of the sails revolve may be placed perpendicularly to the direction of the wind, for the purpose of allowing the latter to act upon the sails in the most advantageous manner. In other kinds of mills the conical wall, A E, is terminated above by a wooden dome, c, which is capable of revolving horizontally upon it. A ring, E F, of wood, forming the lower part of the dome, rests upon a ring, c n, of the same material at the top of the wall, and the surfaces iu contact being made very smooth, the former may easily be turned round upon the latter, being pre vented from sliding off by a rim which projects from it, as at s, and descends over the interior circumference of the lower ring. The revo lution is however facilitated by placing between the two rings of wood one of metal, in which are fixed four or six small wheels or rollers, as a 1,, on horizontal axles. The weight of the dome is supported on these rollers, which turn by its motion. Small wheels or rollers, as d, are also fixed on vertical axes in the projecting rim just mentioned ; and as the dome revolves the circumferences of these rollers press against and turn upon the interior faces of the ring which is fixed on the top of the wall.
The dome in turning carries with it the windsails, Si x, and their axle, ; and thus the latter may be made to coincide with the direction of the wind, or the piano in which the radii of the sails turn may be made perpendicular to that direction. The revolution is sometimes accom plished by the force of a man applied to a winch near the ground. An endless rope, as it is called, or one whose ends are spliced together, "asses under a pulley on the axle of the winch, and over one near the top of the mill ; and the latter pulley in revolving gives motion to a wheel and pinion, the last of which works in teeth on the exterior circumference of the ring which forms the lower part of the dome.
But in general the wind itself is made to turn the dome of the mill so that the sails may continue in the proper position with respect to the direction of the wind. For this purpose there is provided a set of small vanes, 1., which are situated at the extremity of a long horizontal arm projecting from the dome in a plane ',ageing through the vertical shaft of the mill, and on the side opposite to the great sails. These vanes turn on a horizontal axis at right angles to that plane, and are act in motion by the pressure of the wind when the latter deviates from the plane of their motion, or from a plane perpendicular to that in which the radii of the great sails revolve. A pinion on the axis gives motion to a wheel, and the axle of this last carries a pinion, m, whose teeth work in those of the wheel n ; the axle of this last carries a pinion, p, whose teeth work in others which are formed on the eite rior circumference of the ring u 11, forming the base of the dome. By this means the dome is made to revolve horizontally, so as always to present the axle q) of the windsails in the direction of the wind. It is to Sir W. Cubitt that we are indebted for this simple but invaluable improvement in the details of windmill machinery. Strangely enough, it has never yet been applied in Holland or Northern Germany.