The problem with clocks in the 1700s: the ship’s rocking messed up a clock’s pendulum movement; the salty sea air corroded the metal gears; changes in temperature and humidity made metal clock parts expand & contract. All these things made a clock inaccurate—it was too slow or too fast.
Harrison came up with some innovative ideas to counter-act these problems. The first one was a dumbbell-style of pendulum. Instead of a rod with a weight at the bottom swinging from an axis, Harrison put the axis in the middle of 2 rods with weights at top and bottom—then he connected them with springs so they would keep moving back and forth no matter how the ship bounced around.
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