To encourage a solution, governments established prizes for anyone whose method or device proved successful. The largest reward of ?20,000-- truly a king's ransom-- was offered by Britain's Parliament in 1714. The scientific establishment-- from Galileo to Sir Isaac Newton-- had been certain that a celestial answer would be found and invested untold effort in this pursuit. By contrast, John Harrison imagined and built the unimaginable: a clock that told perfect time at sea, known today as the chronometer. Harrison's trials and tribulations during his forty-year quest to win the prize are the culmination of this remarkable story.