John Harrison's fourth marine timekeeper, H4.

John Harrison's fourth marine timekeeper, H4. © National Maritime Museum, London

Repro ID: D0789_1

Description: Harrison had been working on improving watches as a sideline to his development of the much larger H3. In 1753 a pocket watch was made for Harrison, to his design, by watchmaker John Jefferys. This went so well that Harrison began to realise it pointed to a longitude solution that lay not in H3 but in smaller watches. Work began on H4 in 1755, and, with its very stable, high-frequency balance, it proved the successful design. It is probably the most important timekeeper ever made.

Creator: John Harrison.

Date: 1755-1759