Windmill. 



533 



G70. Ability of Wind to Do Work. Tlio woik which 

 wind can do depends upon the amount which passes 

 through a given windmill per minute and the pressure 

 which it exerts. But as the pressure varies with the square 

 of the velocity, and the quantity passing the mill varies di- 

 rectly as the velocity, the theoretic working capacity of the 

 wind must increase as the cubes of the wind velocity. 



Perry regards it approximately correct to state that a 

 12 ft. windmill in a 5 mile wind may develop ^ of a 

 horse-power and the figures in the last line in the table 

 above are his. 



671. Relation of Diameter of Wheel to Its Efficiency. In 

 increasing the horse-power of an engine it is not usually 

 necessary to increase its weight and strength much more 

 than in proportion to the increase of power which is to be 

 developed, but in the case of two wind wheels, having the 

 same type of construction, the one which is to develop 

 double the horse-power must have a strength of resistance 

 practically 8 times as great in order to withstand the high- 

 est wind pressures to which it is liable to be subjected. This 

 is so because doubling the diameter of the wheel not only 

 makes the surface of wind pressure four-fold, but at the 

 same time carries the center of pressure farther from the 

 axis of the wheel, causing it to act upon a longer lever 

 arm. But to increase the strength of resistance of the 

 wheel 8-fold makes it necessary to build it much heavier 

 and this detracts from its relative efficiency. 



Besides this, with wheels of largo diameter there are 

 much greater differences in the wind pressure on the dif- 

 ferent parts of the wind sails because the actual velocity 



