In the 17th century, Christiaan Huygens, inventor of the pendulum clock, noticed a curious phenomenon.  When two identical pendulum clocks are mounted next to each other on a wall, their swings eventually become synchronized, swinging in opposite directions.  He called the effect “an odd kind of sympathy” in his letter to the Royal Society; these days it is often referred to as entrainment.

from Forbes – Tech http://ift.tt/1Dbxd2i
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