WINDMILL. Windmills are of two kinds: in one the 'kind is made to act upon vanes or sails, generally four, which are disposed so as to revolve by that action in a plane which is nearly vertical ; and in the other, the axis of revolution being precisely vertical, any point on the surface of a vane revolves in a hori zontal plane. The former is called a Vertical Virmdniill and the latter a Horizontal Wind mill.
The building for a Vertical Windmill is generally a wall of timber or brickwork in the ' form of a frustum of a cone, and terminated above by a wooden dome which is capable of revolving horizontally upon it. A ring of wood, forming the lower part of the dome, rests upon a ring Of the same material at the top of the wall, and the surfaces in contact being made very smooth, the dome may easily be turned round upon the wall ; and is prevented from sliding off by a rim which projects from it, and descends over the inte rior circumference of the lower ring. The dome in turning carries with it the wind sails and their axle; and thus the windsails may be made to coincide with the direction of the wind, or the plane in which the radii of the sails turn may be made perpendicular to that direction. The revolution is sometimes
accomplished by the force of a man applied to a winch near the ground; but in general the wind itself is made to turn the dome or the mill by means of a set of small vanes which are situated at the extremity of a long horizontal arm projecting from the dome in a plane passing through the vertical shaft of the mill, and on the side opposite to the great sails.
A Horizontal Windmill is a great cylindri cal frame of timber, which is made to revolve about a vertical axis, and its convex surface is forMed of boards attached in vertical posi tions to the upper and lower parts of the frame. The whole is inclosed in a fixed cylinder having the same vertical axis as the other : this consists of a screen formed by a number of boards which are disposed so that, in whatever direction the wind may blow, it may enter between them on one side only of a vertical plane passing through the axis, and thus give motion to the interior cylinder. The effective power of the Vertical Windmill is however so much greater than that of the Horizontal Windmill that the latter is now seldom constructed.