APPLIED MECHANICS. 



[WIND-POWER WINDMILLS. 



tag to the force of the wind, has been suocess- 

 fully adopted. The uili connist of a framework filled 

 in with louvre-boards hinged on pivot-pins nonr one 

 of their edge*, and all connected by levers and rods. 

 with a slnlni.: KM* on the central axis of the windmill 

 111). When the wind blows strongly against the 

 1. 'ii v ro- board's it forces them out of their vertical position, 

 ami paste* freely through the openings between them. 

 The surface of the sails is thus diminished by the pres- 

 sure of the wind itself- To prevent its being too much 

 diminished, the sliding-boss connected with the louvre- 

 boards is pressed upon by a lever loaded by a certain 

 weight sufficient to balance, as far as may be desirable, 

 the pressure tending to force aside the louvres, and thus 

 to keep them, to a certain extent, up to 

 their work. When the load on the mill 

 that is to say, the quantity of work 

 effected by it is varied, the weight may 

 be varied accordingly ; and thus the 

 effective amount of surface in the sails 

 may be adjusted to the average force of 

 the wind, and the work to be done by it. 

 When the wind-force exceeds or falls 

 short of its average, the greater or less 

 inclination of the louvres very nearly 

 compensates for the variation. 



The sails of a windmill should directly 

 face the wind in order to receive its most 

 advantageous action ; but, as the direc- 

 tion of the wind often changes, it is neces- 

 sary to adopt some arrangement for 

 varying that of the mill-shaft accordingly. 

 The summit of the mill-tower, in which 

 the mill-shaft is mounted, is therefore 

 made to revolve, so that at any time the 

 direction of the shaft may be varied, and 

 the sails presented to the wind. In old 

 mills, and indeed in many small mills 

 still existing, this change of direction is 

 effected by hand. A long lever is fixed to the movable 

 cap or summit of the tower, and extends obliquely to 

 Fig. 112. 



its proper position. But in large mills this would re- 

 quire considerable power; and, moreo\. 'it at- 



tention would have to be paid to the changes of the 

 wind. Were a single change neglected, the mill mi-jlit 

 be destroyed ; for as the sails are made and strengthened 

 by tie-rods to receive the wind's pressure on their face, 

 a change of the wind to the opposite direction might 

 throw a great strain on their back, for meeting which 

 no provision is made. A simple mode of making the 

 change of direction self-acting, is to fit the back of the 

 cap with a large vane, which, like that of a weathercock, 

 would cause the sails to be presented to the wind from 

 whatever Quarter it might blow. But when mills are of 

 considerable size, the vane would require to be very Urge 

 and cumbrous. The contrivance generally employed is 

 neat and ingenious. Behind the cap (Fig. 112), on the 

 side opposite that through which the wind-shaft passes, 

 a framing is made to project outwards. On this framing 

 there is mounted a small windmill, on an axis transverse 

 to that of the main arms. The cap rests on rollers fitted 

 in the circular top of the tower, so that it may move 

 freely round ; and a toothed circular rack is also fixed 

 on the summit of the tower. A spindle, fitted with 

 bevil-gearing so that it may be caused to revolve by the 

 revolution of the small mill, conveys motion to a toothed 

 pinion, which gears into the circular rack. When the 

 main mill has its face presented to the wind, the small 

 one stands edgeways to it, and therefore remains at rest; 

 but as soon as the wind veers, it begins to act on one 

 side or the other of the small mill, and thus causes it to 

 revolve. The pinion is thus made to travel along the 

 fixed rack, and turn the cap of the mill round, until the 

 main mill is again brought to face the wind in its new 

 direction. This arrangement is found to be very effective ; 

 and when it is properly applied, the mill requires no 

 attention in respect of the direction of its sails to the 

 wind. 



In estimating the velocity with which the sails of a 



windmill revolve, we have to consider not only the force 



of the wind upon them, but also the resistance to their 



motion occasioned by the work done by the mill A B 



Fig. 113. 



the ground. The miller watches the direction of the 

 wind, and by mnviug this lever, turns the cap round to 



(Fig. 113) may represent the edge of a surface presented 

 obliquely to the wind, and capable of moving in the 

 direction C D at right angles to that of the wind. If 

 the surface be free and unresisted in its motion, and the 

 wind be considered to produce its full effect upon it, the 

 proportion of its velocity to that of the wind would be 

 estimated by that of the line B B' to the line B A' ; for 

 it is clear that while the wind travels over the distance 

 B A' the surface moves to the position dotted, that is, 

 over B B'. But if the motion of the surface be resisted, 

 its velocity in relation to that of the wind is diminished. 

 In the case of windmill sails, we may suppose such a load 

 of work on the mill that the velocity of the sails is not 

 more than half what it would be were there no resist- 

 ance. We may, therefore, assume that the velocity of 

 the sail relatively to the wind would be expressed by the 

 ratio of half the length of the line B B' to the length of 

 A B. Taking the wind as a gentle breeze, the velocity 

 of which, in the table, is about 5 miles per hour, and the 

 inclination of the sail or angle A' B B' half way from the 

 centre 18, we should find the half of B B' to be about 1 J 

 times A' B, or the velocity of the sail 1 J X 5 = 7 miles 

 per hour about C60 feet per minute. If the windmill 

 be about 60 feet in diameter, the diameter of the middle 

 point of tin; arm is 30 feet ; the circumference of the 



