This axle is usually inclined about 10 degrees to the horizontal line. It is supported at the inner extremity, P, which is st or near the centre of the base of the dome, on the top of the vertical shaft, s T, of the mill, and near the opposite extremity on a block under a perforation in the dome. The axle passes through this perforation, and the radii, or arms of the sails, are affixed to it on the exterior ; the axle and the sails which it carries revolve with the dome about the lower point of support. A toothed wheel, n, is fixed perpendicularly on the axle, and .revolves with it by the pressure of the wind on the sails; and the teeth or cogs of this wheel drive those of a lantern or pinion, s, on the vertical shaft of the mill. To this shaft, as an axle, the upper mill stone is fixed, so as to revolve with it in a horizontal position; and the corn being placed in a hopper, or funnel, is allowed to run from thence between the stones through a small channel, and through a perforation about the centre of the upper one. The lower millstone is stationary, and the corn being ground, the meal is received in vessels underneath. The principal wheel, n, is furnished with a brake, by which its motion may be checked or stopped at pleasure.
The four radii, or whips, as they are called, of the sails, are let into the axle at right angles to it and to one another, so that a plane passing through them will decline about 10 degrees from a vertical position. Into each of these radii or arms are fixed a number of staves of wood, each five or six feet long, at right angles to it and inclined to the plane passing through the arms, but approaching nearer to coincidence with such plane, as they are more distant from the axle. The ends of these staves are inserted in a rod of wood extending nearly the whole length of the arm ; and thus there is formed a sort of lattice-work on which canvas is spread to receive the action of the wind. In most cases each radius or whip of a windmill sail is about thirty-three feet in length from the axle to its extremity.
The variations in the force of the wind require that the quantity of canvas on the sails should be varied accordingly; and the contraction as well as the expansion of a sail is usually effected by means of ropes fastened to it in three places or more. These ropes may be either drawn tight or relaxed as required ; but for either purpose it is neces sary that the mill should for a time be stopped ; and as the stoppage is attended with great inconvenience, several methods have been devised for rolling and unrolling the sails while in motion. Oue of these, which was invented by Mr. Bywater, consists in the application, on each arm or whip, of a cylinder or roller to which the canvas is attached: this extends the whole length of the arm, and has a toothed wheel at the extremity nearest to the axle ; the teeth of this wheel work in those of two other wheels, and the motion of one or the other of these being stopped, the cylinder xolls up or unrolls the canvas, being made to turn on its axis by the action of the wind on the sail. Several methods have also been proposed for equalising the action of the wind on the sails of a mill, and they consist generally in the employment of a series of valves fixed in the frame work of each sail. These valves
revolve on pivots which are let into the frames ; and as the force of the wind increases, they present, in turning, less of their surfaces to its action, so that the pressure is rendered nearly equable. None of the methods seems however to be in use, probably on account of the great additional expense with which the construction would be attended. Uf late years wooden Lattices working somewhat on the principle of the Venetian blinds, have been used instead of canvas for the sails of wind mills, and their angle towards the wind is regulated by a species of governor fixed on the main shaft ; but there seem to be mechanical difficulties in the way of this system which have hitherto opposed its general adoption. , A horizontal windmill is a great cylindrical frame of timber, which is made to revolve about a vertical .axis, and its convex surface is formed of boards attached in vertical positions to the upper and lower parts of the frame. The plane of each board is oblique to the lines in which the wind impinges on it, the direction in which the latter blows being supposed to be parallel to the horizon ; and 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. The wind thus entering acts upon the oblique surfaces of the boards about the interior cylinder on one side of the axis, while it is, in great degree, prevented by the screen from acting upon the boards on the opposite side; these boards therefore meet with small resistance when, during each revolution, they come up towards the quarter from whence the wind blows. In horizontal mills one board may receive an impulse equal to that which the wind communicates to a sail of equal area in a vertical mill ; but in the latter all the sails a•e acted upon equally at the same time, whereas in the former only one or two can receive the impulse of the wind, and there is always, besides, sonic resistance experienced in returning against the wind. Mr. &neaten estimated that the power of a horizontal mill was only about one-tenth of the power of a vertical mill, the dimensions of the sails or vanes being equal in both ; but it is observed by Sir David Brewster that in this estimate no account is taken of the resolved part of the wind's force which presses the pivot of the axle against its support, and which is lost on the sails of the vertical mill ; and he concludes that the power of the latter is not more than thrco or four times as great as that of a horizontal mill. The effective power of the vertical mill is however so much greater than that of the other kind, that the latter is now seldom constructed.
The effective force of the wind in turning the sails of a mill is investi gated in the article WINDBA11,13.
(See Brewster's edition of Ferguson's Lectures; &neaten, On the Power of Wind and Wider; Borgnia, Traitd Complet de Micanique Leendert van Naturus, Groot Volkonten Moolenboek, &c.)