John Harrison (1693-1776), English clockmaker, sitting in front of two of his inventions. At upper right is the gridiron pendulum, which he invented in 1726. At upper left is the H3 version of Harrison's marine chronometer. Harrison is most famous for his development of his portable H4 chronometer that could work on board a moving ship, a problem that was essential to solve to allow accurate navigation at sea. A portable timepiece is on the table in front of Harrison. This reproduction is based on a 1767 oil painting by Thomas King.