Odd Sympathy

Laid up in bed during a brief illness and idly watching two clocks mounted in one case, Huygens noticed something strange: No matter how the pendulums started out, eventually they always ended up swinging in exactly opposite directions. Huygens wondered whether this odd sympathy might solve the longitude problem. Perhaps, he thought, two such clocks could regulate each other. If one got dirty, for instance, and started running slow, the influence of the other clock would lessen this effect. I...