WINDMILL. In mechanics, a mill which receives its motion from the im pulse of the wind. The general appear ance of the windmill is familiar to every one. The building containing the ma chinery is usually circular. To the ex tremity 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 certain angle, about 72° at the extremities nearest to the axle, and 83° at the farther extremi ties ; so that their form is in some degree twisted, and different from a plane sur face. 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 direc tion perpendicular to the axis, gives a motion of rotation to the sails, and con sequently to the wind-shaft, from which it is communicated to the machinery. The wind-shaft is inclined to the horizon in an angle of 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 nearly in contact with the building.
As the direction of the wind is con stantly changing, some apparatus is re quireI for bringing the axle and sails into their proper position. This is some times effected by supporting the machin ery on a strong vertical axis, the pivot of which moves in a socket firmly fixed in the ground ; so that the whole structure may be turned round by a lever. Bnt it is now usual to construct the with a movable roof, which revolves up on friction rollers ; and the shaft being fixed in the roof is brought round along with it. The roof is brought into the required position by means of a small vane wheel furnished with wind sails, which turns round when the wind strikes on either side of it, and drives a pinion which works into the teeth of a large crown wheel connected with and sur rounding the movable roof.
Of the form and position of the sails.• From the investigations of Parent and Belidor, it appears that the maximum effect of the wind on the sails is pro duced when their inclination to the axis of rotation is about 54f degrees; or when the angle of weather, that is to say, the angle formed by the plane of the sail and the plane of its revolution, is 354 degrees. But this result, being obtained from con sidering the effect of the wind on the sails when at rest, does not agree with that which is found by experiment. In fact, as the velocity of the sail tends to withdraw it from the wind, it is neces sary to counteract the diminution of force by diminishing the angle of weather, or to bring the sail into such a position that the wind strikes its surface more directly: and since the velocity of the different parts of the sail is in proportion to their distance from the axis, it follows that in order to produce the greatest effect every elementary portion of it ought to have a different angle of weather, diminishing from the centre to the extremity of the sail.
From the experiments of Smeaton, it appears that the following positions are the best. Suppose the radius to be divid ed into six equal parts, and call the first part, beginning from the centre, one, the second two, and so on, the extreme part being six :— Angle Angle with the will Plane of Motion, or No. the !LAS. Angle of Weather.
1 18a 2 71 19 3 72 18 4 74 16 5 774 124 6.... .....83 7 As it is necessary that a windmill should face the wind from whatever point it blows, the whole mill is made to turn upon a strong vertical post, and is there fore called a post-mill; but, commonly, the roof or head only revolves, carrying with it the windwheel and its shaft, the weight being supported on friction rol lers.
The following are the maxims of Smea ton with regard to mills:-1. The velo city of windmill sails, whether loaded or unloaded, so as to produce a maximum effect, is nearly as the velocity of the wind, their shape and position being the same. 2. The load at the maximum is nearly, but somewhat less, as the square of the velocity of the wind, the shape and position of the sails being the same. 8. The effects of the same sails at a maxi mum are nearly, but somewhat less, as the cubes of the velocity of the wind. 4. The load of the same sails at the maxi mum is nearly as the squares, and their effects as the cubes, of their number of turns in a given time. 5. When the sails are loaded so as to produce a maxi mum effect at a given velocity, and the velocity of the wind increases, the load continuing the same, then the increase of effect, when the increase of the velocity of the wind is small, will be nearly as the square of those velocities ; when the ve locity of the wind is doubled, the effect is nearly as 10 to VI. When the veloci ties compared are more than double of that where the given load produces a maximum, the effects compared increase nearly in the same ratio of the velocity of the wind. 6. In sails where the posi tions and figures are similar and the ve locity of the wind the same, the number of turns in a given time will be recipro cally as the radius or length of the sail.