340 AGRICULTURAL ENGINEERING 



be given up. It is easy to see that it would be impossible to 

 reduce the velocity to zero and obtain all of the energy of the 

 wind, because it must flow past the windmill. 



Types of Mills. There are many types of windmills on 

 the market, and they may be classified in several ways: (1) 

 by the material used in construction, (2) by the type of con- 

 struction, and (3) by the use to which they are put. For- 

 merly the wheels were made almost entirely of wood, but 

 steel has now practically displaced wood. It is claimed by 

 good authorities that the steel wheel is more efficient and 

 will operate in a lighter wind than the wooden wheel, owing 

 to the thinness and the shape of the fans. Windmills may 

 be also either direct-stroke mills or geared mills. Direct- 

 stroke mills are used solely for pumping purposes; a stroke is 

 made with each revolution of the wind wheel. In order to 

 produce a mill which will operate in lighter winds, gearing is 

 often used to reduce the number of strokes in proportion to 

 the number of revolutions of the wheel. Most steel mills are 

 now geared in this way. 



Windmills used solely for pumping are called pumping 

 mills, and the power is transmitted from the wind wheel by 

 means of a pumping rod having a reciprocating motion. 

 When a rotating motion is desired a vertical shaft is run from 

 the mill to a point from which the power may be transmitted 

 to a machine by any of the more usual methods. Such a mill 

 delivering its power by a rotating shaft is said to be a power 

 mill. 



Size of Mills. The size of windmills is indicated by the 

 diameter of the wheel. Common sizes used for pumping 

 purposes are 8- and 10-foot wheels. Power mills are often 

 built much larger, with wheels 20 or more feet in diameter. 

 Wheels of large diameter must be made very strong to be 

 able to withstand the wind, and the extra weight thus 



