941 



WINDMILL. 



WINDOW. 



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 F, is terminated above by a wooden dome, c, which is capable of 



revolving horizontally upon it. A ring, K 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 in 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 K, and 

 descends over the interior circumference of the lower ring. The revo- 

 lution ia 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 b, on hori/ont '.1 axles. The weight of the dome is supported on these 

 rollers, which turn by its motion. Small wheels or rollers, as <l, are 

 also fixed on vertical axes in the projecting rim just mentioned ; and 

 as the dome revolves the circumferences of there 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 \vimU-iils, M N, and their axle, 

 1Q ; and thus the latter 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 accom- 

 plished by the force of a inau applied to a wiuch near the ground. An 

 endless rope, as it is called, or one whose ends are spliced together, 

 j-nifp 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 

 and pinion, the last of which works in teeth on the exterior 

 circumference of the ring which forms the lower part of the dome. 



lint 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 

 email vanes, L, 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. These 

 vanes turn on a horizontal axis at right angles to that plane, and are set 

 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 iu 

 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 

 pinic.n, p, whose teeth work in others which are formed on the exte- 

 r mnference of the ring u n, forming the base of the dome. 

 By this means the dome is made to revolve horizontally, so as 

 always to present the axle (i' it) 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 hag never yet been applied in Holland or Northern 

 ( MTinany. 



This axle is usually inclined about 10 degrees to the horizontal line. 

 It is supported at the inner extremity, F, which is at or near the centre 

 of the base of the dome, on the top of the vertical shaft, 8 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 .-inn* "f tli" H ills, arc affixed to it on the exterior ; the axle and the 

 ails which it cirries revolve with the dome about the lower point of 

 support. A toothed wheel, B, in 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, B, is furnished with a brake, by which its motion 

 may be checked or stopped at pleasure. 



The four radii, or whipi, 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 leugth 

 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 iu 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 lolling and unrolling the sails while in motion. One 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 jolls 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 

 it* 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. 

 Of late years wooden lattices working somewhat on the principle of the 

 Venetian blinds, have been used instead of canvas for the sails of wiud- 

 mill.-i, and their angle to wards the wind is regulated by a species of 

 governor fixed on the main shaft ; but there seem to be mechanical 

 dithculties 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 lo.ver 

 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 

 ; of a screen formed by a number of boards which are disposed 

 so that, in whatever direction the wind may blow, it may enter betu'oon 

 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 a 

 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 are 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, some 

 resistance experienced in returning against the wind. Mr. Smeaton 

 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 three 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 Wixns.ur.s. 



(See Brewster's edition of ferguion'i Lectures; Smeaton, On Ike 

 Power of Winil un<! Water; Borgnis, Trait/! Complet de Mtccmique ; 

 Leendert van Naturus, Groot Volkomm Moolcnboek, &c.) 



WINDOW, Though almost unknown in ancient architecture, at 



