[Illustration: FIG. 120.--A mountain stream turns the wheels of the
mill.]
172. Water Wheels. In Figure 120 the water of a small but rapid
mountain stream is made to rotate a large wheel, which in turn
communicates its motion through belts to a distant sawmill or grinder.
In more level regions huge dams are built which hold back the water
and keep it at a higher level than the wheel; from the dam the water
is conveyed in pipes (flumes) to the paddle wheel which it turns.
Cogwheels or belts connect the paddle wheel with the factory
machinery, so that motion of the paddle wheel insures the running of
the machinery.
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