Windmill 535 



where V is the velocity at the hight of the tower, v the ve- 

 locity at 50 feet, and H the hight of the windmill. 



Taking the efficiency of the wind as increasing with the 

 cube of the velocity, the relative efficiency of the same mill 

 at the four hights would be at 25 feet .51, at 50 feet 1, at 

 75 feet 1.73 and at 100 feet 2.74, from which it appears 

 that a mill placed on a 100-foot tower may have more than 

 5 times the efficiency of one placed at 25 feet, and a mill 

 on a 75 foot tower is likely to do three-fourths more work 

 than one on a 50-foot tower. 



674. Observed Amount of Work Done by a Windmill in 

 Pumping Water. We have measured the amount of water 

 which was pumped during one entire year by the 16-foot 

 geared windmill represented on the cover of this book.* 

 This mill was provided with three pumps arranged so as 

 to lift water 12.85 feet whenever there was wind enough 

 to enable it to do any work. When the wind was lightest 

 it was given the pump of smallest capacity, when stronger 

 the one of next size, when still stronger both pumps to- 

 gether, the third pump being used only in the very high- 

 est winds. 



The water was pumped into a large tank holding 141.2 

 cu. ft., so arranged that when full it emptied itself auto- 

 matically in of a second, and at the same time recorded 

 the time of emptying. In connection with this an auto- 

 matic U. S. Weather Bureau anemometer made a continu- 

 ous record of the miles of wind passing through the mill 

 each hour of the day for a whole year and the amount of 

 water pumped during the same intervals. 



The amount of work done by this windmill during 10- 

 day periods for the whole year is computed in acre-inches 

 of water lifted to a hight of 10 feet and expressed in the 

 table below: 



Bulletin 68, Wisconsin Agricultural Experiment Station. 



