908 



Wind Power. 



[FEB., 



owner some knowledge of the theory of wind power. With 

 any motor it is always desirable to know how much work it 

 can be expected to do, what power it is exerting at any par- 

 ticular time, and the working conditions under which it 

 suffers least from depreciation. The necessity of having this 

 knowledge has been realised by such firms as the Stover 

 Manufacturing Company, Illinois, U.S.A., to whom the 

 writer is indebted for much technical information concerning 

 the management of windmills. 



The windmill derives its power from the kinetic energy 

 of the moving atmosphere : the greater the velocity of the 

 wind the greater the power transmitted, providing the mill 

 is suitably constructed. 



It should be taken as a general rule that for any given 

 velocity of the wind the speed of the wheel should not change, 

 but the load should be arranged and the governor adjusted 

 to suit any particular wind velocity. The mean average wind 

 velocity varies for different localities ; but a pretty good idea 

 can be formed of the mean velocity of any one centre by 

 taking an average of the velocities which have been deter- 

 mined at two or three places near where the windmill is to 

 be erected. 



The mean average velocity increases with increase in eleva- 

 tion : double the elevation and the mean average velocity is 

 also nearly doubled. This shows the value of high towers. 

 Now the value of wind velocity, or its capacity for doing 

 work, increases in the ratio of the squares of the different 

 velocities. For instance, the value of a 20 mile-per-hour wind 

 compared with a 10 mile-per-hour wind is (20) 2 : (io) 2 or 



42? = 4. That is, if the value of a 10 mile-per-hour wind be 

 100 



regarded as 1, the value of a 20 mile-per-hour wind is four 

 times this value. 



The mean average velocity of the wind should always be 

 taken into account in adjusting the load. A 16 ft. mill in a 

 centre of low mean average wind velocity must be provided 

 with a lighter task than one in a centre of higher average 

 velocity. It has been determined that wind below 8 miles 

 per hour has a lower percentage of working power than where 

 the mean average velocity is over 8 miles per hour. The 



