WINDMILLS 309 



The above table is given where the wind velocity is 

 such that the mill makes the number of revolutions a 

 minute given ; of course, if the velocity increases, the 

 R.P.M. will increase likewise and consequently the 

 power. 



Smeaton drew from his experiments that the power in- 

 creases as the cube of the wind velocity and as the square 

 of the diameter of the wheel. Murphy did not check this 

 result, but found that the power increases as the squares 

 of the velocity and as about 1.25 of the diameter of the 

 wheel. This latter conclusion is probably the more re- 

 liable, as the instruments which Smeaton used were more 

 crude than those of Murphy. The former determined 

 the velocity of the wind by taking the time which it would 

 take a feather to travel from one point to another as the 

 velocity. The latter used a Thompson anemometer. 



431. Tests of mills. The following tests were made by 

 E. C. Murphy to determine what windmills actually did 

 in the field, also to see whether mills in practice carried 

 cut the rules made by previous experimenters. Perry 

 found by his experiments in a closed room that the power 

 of a wheel increases as the cube of the velocity, while 

 Murphy found that it varied from this. 



It will be noticed from the following table that some 

 steel wheels as well as wooden gave much more power 



