John Harrison's H4

More than two decades after H1, Harrison solved the longitude problem with this: a watch about 13 centimeters across, finished in 1759 and shown here from the back, with its engraved, pierced movement. On a trial voyage to Jamaica aboard HMS Deptford, which arrived in January 1762, it was found to be only about five seconds slow, an error in longitude of roughly a nautical mile. It is displayed with H1, H2 and H3 at the Royal Observatory Greenwich.