APPLIED MECHANICS. 



[WIXD-POWBR WIXDMIIL8. 



travel long distances in sledge* over ice, have taken 

 advantage of favourable wind to propel their vehicle! 



Tig. lox 





fixed obliquely, or in such lines as, if produced inwards, 

 would touch the circumference of the wind-wheel liy 

 Kg. 107. 



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by means of kites attached to them. Chariots have also 

 been made with sails, so as to be propelled by wind along 

 a road, as ships are propelled at sea. But the uncer- 

 tainty and variations of the wind render such applica- 

 tions of its power matters of curious experiment rather 

 than of practical utility. 



Windmill*. For giving motion to machinery, wind- 

 mills have been and stUl are very extensively used. 

 Engineers of the hut generation devoted great attention 

 to the construction of windmills, and brought them to 

 great perfection. The introduction of steam-power a 

 power economical, manageable, and always to be de- 

 pended on has, in a great measure, superseded that of 

 wind as a mover of machinery. It is true that after the 

 first cost of a windmill, the power is comparatively in- 

 expensive ; but it is so variable in intensity sometimes, 

 when it is not required, exerting great force, and somo- 

 timcs, when it may be most wanted, totally ineffective 

 that it is generally preferable to apply a force, perhaps 

 considerably more expensive in ita production, but con- 

 stant, steady, and completely under control. 



Windmills are of two kinds, horizontal and vertical. 

 The former have been very little used, for it is found in 

 practice that they are by no means so effective as the 

 latter. The mode of constructing a horizontal windmill 

 in like that represented on the plan (Fig. 100), or some 

 Fig. 106. 



modification of the wimo principle of construction. A 

 ftheel, A, ni<mnt>-<l <>n avertiml nxix or shaft, and having 

 at vane* or board* fitted round it* cin-iimfi-ivnco, in 

 in a circular casing, which is fitted with boards 



this arrangement, the wind, from whatever point it may 

 blow, causes the wheel to revolve in the same direction. 

 Part of the breeze passes between the oblique board* "i 

 the casing, and acts on the blades of the wheel ; while 

 part is intercepted by the boards, and either reflected 

 inwards so as to propel the blades in the same direction, 

 or reflected outwards so as not to act upon them in the 

 opposite direction. Sometimes horizontal windmills have 

 been made with a casing partially surrounding the wind- 

 wheel (Fig. 107), and capable of being turned round !>y 

 means of a vane, so as to permit the wind only to act on 

 one side of the wheel, while the other is completely 

 sheltered. 



The vertical windmill, as is well known, consists of an 

 axle or shaft, nearly horizontal, mounted in bearings at 

 the summit of a tower, with four or more blades or sails 

 attached to it. These sails are set at an angle with the 

 axis, so that when the wind blows directly on the face of 

 the mill, ita oblique action on the sails is resolved into 

 two forces one in the direction of the axis, and the 

 other perpendicular to it, the direction in which the sails 

 revolve. Numerous experiments and computations were 

 made to determine the most advantageous angles for 

 setting the sails, and their moat effective forms and pro- 

 portions. If we suppose the radius of a sail divided 

 into six equal parts (Fig. 108), and circles traced through 

 Fig. IDS. 



the points of division, the velocity of each point in 

 revolving, is proportional to the piirt of its circle inter- 

 cepted between two radii, or proportional to its own 

 radius. If, then, we make a series of plans of the wiil 

 at these different part*, we see that as we approach the 

 centre we should increase the obliquity of the sail to ito 



