WINDMILL. A mill of any kind actuated by the impulse of the wind. They are of two kinds—oertical and horixowtsls Vertical windmills (to which • decided preference has been hitherto given) usually consist of a strong shaft or aids inclining a litde upwards from the horizon, with four long yards or arms fixed to the highest end, perpendicular to the shaft, and erossing each other at right angles. Into these acme are monied several small cross bars, and to them are fastened two, time, or four loag bars, running in a direction parallel with the length of the arms; so that the bars intersect each other, and form a kind of lattice-work, on which the cloth is weed to receive the action of the wind. Them are called the sofa, and are is the shape of a trepezium, usually about nine yards long and two wide. The direction of the wind being *hoar very uncertain and variable, it becomes necessary to provide some contrivance for bringieg the sails into a proper posi tion for receiving its impression. Two methods have been devised for this , one of which is denominated the post-mill, the other, the smock-mill. purpose, is so called from the circumstance of the mill being built round a massive central post, made out of the whole trunk of s stout tree, which is sunk vertically in the ground, and supported in its position by oblique struts or. braces, which extend from a platform on the ground to the middle of the port, leaving 10 or 12 feet of the upper part free from the braces. The part thus left free from obstruction is rounded and made to pass through a circular collar, formed in the flooring of the lower ohamber, and to enter into a socket fixed into the flooring of the upper chamber, and to one of the strongest crom-beams, which must sustain the whole weight of the mill-house; so that by means of pivot, or, fastened on that part of the post which enters into the socket, the whole machinecan turn about horizontally to face the wind. A strong framing, united by joints at the back of the mill-house, descends in a doping direction to the ground, and is there fastened to short posts, when placed in the position required for the sails to be acted upon by the wind. To this fruits
ladder is attashed, which leads into or out of the mill-house. To the boned this frame a rope is fastened and conducted to tackle in the mill-house, by *di the frame can be lifted from the ground, while its position is being changed, io the inanner of a capstan poet, to snit the wind.
The smock-mill does not depend upon a central post for its bat it is generally a strong independent building, the upper portion usually a tower of the form of a truncated cone, constructed of wood, md mounted upon a vertical wall of masonry, containing two or three floors, whom the work of the mill is tower above containing a vertical shaft, by which the motion and force is communicated from the saih to the mill-stones. The head or cap in the upper pert of the mill is provided with a cap, which is contrived so that it may turn itself about as the wind change; for this purpose there is a nearly horizontal framed projection at the back pet of the head, which carries some small sails acting as a vane, there being, coo. centric with the axis, a large grooved ring, around which a circular hoop, provided with anti-friction rollers, traverses.
The velocity of motion of the sails or vanes is very considerable. Mr. Fer guson calculated the motion of the tips of the sails, even when operated upon by a very moderate wind, to be thirty miles per hour.
Horizontal windmills, as their name implies, are such as are worked by their sails revolving in a horizontal plane. All disinterested authors who have writ ten on this subject condemn them, as being very inferior in effect to those d the vertical kind. Smeaton considered their effect to he only one-eighth, but Dr. Brewster shows that they have from ene-third to one-fourth of the effect of the verticaL It is probable, however, that means may be discovered of impact ing them considerably.
To ascertain the best form and position cif windmill-mils, Mr. Sangre tuted a series of experiments, of which the results are liven in the sultiened table :—