124 OUR PHYSICAL WORLD 



The mill usually bore four sails. In the modern mill the blades 

 are smaller, more numerous, and made of wood or steel. 



In many parts of the Old World as well as in America the 

 country landscape is dotted with such mills raised into the breeze 

 on towers. They furnish the farmer with power for pumping 

 water, for running his electric plant, his churn, and many other 

 small farm machines. They have been used, too, for power to 

 grind his grain. When not in use the mill is turned with the 

 edge of the wheel into the wind instead of its face. This is 

 easily accomplished with the modern, small, light mill but it 

 was not so easy a task with the old mill with its great expanse of 

 sails. Sometimes the sails were furled as on a ship. Again a great 

 slanting beam was attached at one end to the axle of the mill 

 while the other end rested on the ground or was attached to a 

 wheel on the ground. Then horses or oxen could be attached to 

 this end so the mill could be turned on a pivot into the desired 

 direction. Another scheme was to have the tower or perhaps 

 merely its top rotate on its axis and turn by means of a rack and 

 pinion that could be operated by a great hand crank. 



The early water wheel was a paddle wheel. The blades were 

 wide, usually four in number, and radiated from the hub with 

 their faces set at right angles to the plane of the wheel. Such a 

 mill wheel might be set so its blades dipped one after another 

 into a stream of water that ran under it, the undershot wheel, or 

 the water coming from some source above the wheel was led by 

 a trough or flume so it fell on the tip of the blade, first one, 

 then another, as the wheel was made to revolve by the falling 

 water, the overshot wheel. 



Now, however, it is much more customary to set a wheel like 

 a windmill at the bottom of a vertical pipe through which water 

 is flowing from some height when the wheel is turned by the 

 passing water just as the windmill is turned by the passing air. 

 Such a wheel is known as a turbine. We have seen (p. 112) that 

 water in a vessel exerts a pressure of about 15 pounds per square 

 inch for every 33 feet of height of the water. So that if the 



