i9io.] 



Wind Power. 



909 



difference is roughly in the ratio 2 :2j. Now, supposing the 

 mean average velocity for a certain spot is 7 miles per hour, 

 this value multiplied by 2 (=14) gives the comparative wind 

 value for which the mill should be governed. Again, supposing 

 the mean average wind velocity were 15 miles per hour, then 

 i5x2j = 37j- is the value in this case for which the machine 

 should be governed. It is obvious that when the governor 

 begins to act the wheel has reached its maximum speed, for 

 at this point it has begun to be turned out of the wind. The 

 speed of the wheel is transferred to the shaft, so that at this 

 point the shaft must also be revolving at its maximum rate. 

 In most of the modern mills the highest speed of the shaft 

 should not be more than § greater than at its lowest velocity. 

 Therefore, let v - velocity at which mill is to govern and 



= velocity at which mill is to be loaded. Then v — Vi. 

 J 100 



Under these conditions the mill will begin to work when 

 the wind reaches a velocity equal to v 1} and the speed of the 

 machinery will increase until the velocity = v is reached, and 

 the speed will then be at a maximum, because v is the velocity 

 at which the mill is adjusted to govern. 



Knowing the velocity at which the mill should be loaded, 

 it is next necessary to determine what power is exerted at 

 this velocity. In order to arrive at this some standard must 

 be taken as a unit. For this purpose a 15 miles-per-hour wind 

 has been selected, because this is nearly equal to 1 lb. pressure 

 on 1 sq. ft. It is generally allowed that a 12 ft. windmill of 

 best construction, and working in a 15 miles-per-hour wind, 

 will develop very nearly 1. horse power gross. 



The power developed by any mill can be determined 

 approximately from the following formula : — 



A V 8 



Horse power — — . 



1080000 



Where A = area, of sails. 



V = velocity of wind in feet per second. 



The area of the sails varies and must be determined for 

 each make of mill, but as a working formula the sails may 

 be regarded as occupying 65 per cent, of the total wheel area. 



Animal and Wind Power Compared. — H.P. represents 



