534 Farm Mechanics. 



of the sails increases with the distance of their points from 

 the center of the wheel. But the angular velocity must be 

 the same in all parts of the sail, and this causes the wind 

 sail to be forced around away from the wind passing 

 through the wheel with very different velocities, and this 

 difference reduces the relative efficiency so that large wind- 

 mills of like pattern do not increase the available horse- 

 power as much as the size is increased. 



672. Unsteadiness of Wind Velocity. It should be under- 

 stood that the wind rarely blows with anything like uni- 

 form velocity for even a single minute, and an anemometer 

 which gives the total number of miles of wind in an hour 

 furnishes no sufficiently reliable data from which to cal- 

 culate the work which the windmill should be expected to 

 do. It very often happens that a wind which is registered 

 as 10 miles per hour may have been blowing during a con- 

 siderable portion of the time at the rate of 20 miles per 

 hour and these high velocities are very much more effective 

 than the mean 10-mile wind, and this would cause the 

 wheel to show a relatively high efficiency in such a case. 



673. Hight of Towers. The wind velocity near the 

 earth's surface is not only less than at higher elevations 

 at the same time, but near the ground it is very much less 

 uniform, so that for both of these reasons mills should be 

 placed upon as high towers as practicable when the great- 

 est efficiency is desired. If there are obstructions to the 

 wind movement even within 1,000 feet of the windmill 

 the tower should carry it several feet higher than these. 

 Observations indicate that, taking the velocity of the wind 

 at a hight of 50 feet as 1, at 25 feet its velocity would be 

 nearly .8 ; at 75 feet it would be 1.2 and at 100 feet it 

 would be nearly 1.4. These are deduced from Steven- 

 son's formula,* which is 



* Journal of the Scottish Meteorological Society, 1831, 



