John Harrison's H1

John Harrison's first marine timekeeper, unveiled in London in 1735 and now on display at the Royal Observatory Greenwich. A ship that knows the exact time at home can work out its longitude, and H1 was Harrison's first attempt at a clock that could keep that time on a rolling deck. Instead of a pendulum, it uses two linked balances topped with brass balls, which swing in opposition so the ship's motion cancels out. It sailed to Lisbon in 1736 aboard HMS Centurion and came home on HMS Orford.