WIND- POWER WINDMILLS.] 



APPLIED MECHANICS. 



823 



plane of motion, so as to allow for its more slow escape 

 sideways from 'the impulse of the wind. The sails 

 accordingly are not made flat surfaces, inclined equally 

 to the plane of their revolution, but surfaces of varying 

 inclination, somewhat like portions of screw blades, 

 twisting as it were from a certain obliquity at their ex- 

 tremes to a greater obliquity at the centre. The angles 

 found most advantageous in practice are given by the 

 celebrated engineer Smeaton as follow, as well as those 

 used by some other engineers : 



Distance from centre ............ 1 ... 2 ... 3 ... 4 ... 5 ...6 



Inclination to plane of motion 



(Smeaton) ........................ 18 ... \9> ... 18 ... 16> ... 12J ... 7 



Ditto (otherwise) ......... 24 ... 21 ... 18 ... 14 ... > ... > 



In the angles given by Smeaton, an irregularity is ob- 

 served in the first, which should, by theoretical reasoning, 

 be greater than the second ; whereas Smeaton makes it 

 less. The following rule may be adopted as a very near 

 approximation. To find the angle at which the sail 

 should be inclined to the plane of revolution at any dis- 

 tance from the centre : 



Ride. Multiply 18 twice by the distance from the 

 centre ; divide the product twice by the total radius, and 

 subtract the quotient from 23 ; the remainder is the in- 

 clination in degrees. 



Example. In a windmill 60 feet in diameter, required 

 the inclination of the sail 20 feet from the centre. 



M feet is the total radios, and 



18 * ^ * 2 



8, 



30 X <>0 



which, subtracted from 23, gives 15, the angle of that 

 point. 



Were we to divide the radius 30 feet into 6 equal 

 parts, and calculate the angles at each point, we should 

 find them correspond nearly with the means of those 

 given by Smeaton and others, as may be seen by the 

 following table : 



Having determined the proper inclination of the sails 

 at different distances from the centre, it next becomes 

 important to inquire how much of the surface of the 

 whole circle should be filled with sails. Mills are gene- 

 rally made with four strong wooden arms, or radii, fixed 

 firmly in a central socket, and steadied and stiffened by 

 tie-rods, connecting their extremities together, and with 

 a projecting strut on the central boss. The width of 

 each sail at the extreme, should be about half of the 

 radius ; so that in a mill 60 feet diameter, or 30 feet 

 radius, each sail would be 15 feet wide at the extreme. 

 The part of the arm next the centre, for about Jth of the 

 radius, that is, 5 feet in the case supposed, is not fitted 

 with sails, because the surface there is so little effective, 

 Fig. 109. 



as well from its short leverage as from its obstructing 

 the wind reflected from the head of the turret behind it. 

 The width at the inner end should be Jrd of the radius, 

 or 10 feet. The surface of each sail is therefore 312 j 

 square feet, and the total of the four is 312| x 4= 1,250 

 square feet. 



The total area of a circle 60 feet in diameter is some- 

 what above 2,800 square feet, so that not half the sur- 

 face of the circle is clothed with sails. There would be 

 no disadvantage in extending the surface by making the 

 sails broader, or more numerous, until it became ths of 

 the whole surface. Beyond this, additional sail-surface 

 is disadvantageous, for it appears to obstruct the free 

 passage of the currents reflected from the sails, and thus 

 clogs their motions. It is found advantageous to arrange 

 the surface of a sail somewhat in the proportions of the 

 diagram (Fig. 109), which represents the front view of 

 one sail. 



The covering of the surface, so as to catch the impulse 

 of wind, formerly consisted of canvas fixed on a roller 

 at one side of the arm, on which it could be rolled like 

 a window-blind ; or from which it could be unrolled so 

 as to cover the whole sail, which was filled in with 

 wooden framing to support the canvas pressed against it 

 by the wind. Sometimes the canvas, instead of being in 

 one sheet, was subdivided into numerous separate sheets 

 mounted on rollers ; and apparatus was provided so that 

 the canvas might be wound on the rollers or unwound 

 at pleasure, while the mill was in motion. As the wind 

 is exceedingly variable, and as the quantity of work re- 

 quired of the mill also varies to a considerable extent, it 

 was found necessary to provide some apparatus by which 

 the mill might regulate itself, so that its velocity should 

 not be excessive at one time and too small at another. 

 One mode of effecting this object was to apply to the 

 Fig. no. 



x. 



If A be . 

 Then A E . 

 A D or E F 

 AB . . 



30 feet. 



25 



10 



6 , 



machinery of a mill a governor, like that of a steam-engine 

 (which we shall have occasion to describe in detail here- 

 after). This governor consists of two heavy 

 balla suspended from the summit of a vertical 

 revolving spindle by jointed rods (Fig. 110). 

 The spindle being at rest, the balls hang close to 

 it on each side ; but on the spindle being caused 

 to revolve rapidly, the balls, impelled by cen- 

 trifugal force, fly away from the central axis, 

 as marked by the dotted lines. A system of 

 levers and rods connected this apparatus with 

 the sail-rollers, so that when the balls flew out- 

 wards, from increased velocity, the sails were 

 furled ; and when they fell inwards, from dimi- 

 nished speed of revolution, the sails were un- 

 furled. The quantity of surface thus presented 

 to the wind was adjusted to its force, and a 

 tolerably equable velocity of the machinery was at- 

 tained. In some more recent mills; an ingenious 

 contrivance for regulating the surface of sail accord- 



