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Two thirds of the surface of the Earth is covered by water. The tides, caused by gravitational interactions between the Earth, Moon and Sun, are one of the most persistent and well-known natural patterns. However, it is water produced by rain over land that is most used as a power source by humans.
Water power has been in use for millenia as well. It typically involves placing
a wheel-like structure or propeller into a moving stream, waterfall or river.
Dams help to even out and intensify the amount of power availalble. In earlier
days, descending water has powered irrigation systems, grain mills and eventually
textile mills. Today hydroelectric dams produce great amounts of electric power.
Where available, wind powers the same types of activities as water power. Wind
pumps are have been used on many farms to provide water for irrigation and household
use.
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