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The Encyclopedia of Horology

Marine chronometer (large watch form) · England

Harrison H4

Verified

A radical departure from Harrison's earlier sea clocks: a large silver pocket-watch-sized instrument rather than a pendulum-like machine.

Technical facts

Escapement
Verge-derived escapement with diamond pallets, remontoire-assisted
Diameter
Approximately 13 cm — large by pocket-watch standards of the day
Case
Silver pair case
Compensation
Bimetallic compensation curb affecting effective balance-spring length

Historical significance

  • A radical departure from Harrison's earlier sea clocks: a large silver pocket-watch-sized instrument rather than a pendulum-like machine.
  • On its 1761–1762 sea trial to Jamaica it kept time accurate enough to determine longitude well within the Board of Longitude's requirements, though Harrison had to fight for full recognition and payment of the prize.
  • Established that a watch-form timekeeper, not a large clock, was the practical solution to the longitude problem, shaping all subsequent marine chronometer design.

Background

John Harrison designed and completed the H4 marine timekeeper in 1759, intending to solve the problem of calculating longitude at sea. Intended to function as a marine timekeeper, the H4 departed from the design of Harrison's earlier sea clocks by resembling an oversized pocket watch. The movement is highly complex, utilizing a coiled steel spring inside a brass mainspring barrel to provide 30 hours of power. During its sea trials, the instrument proved extremely accurate, allowing navigators to determine longitude with high precision.

Written from the encyclopedia entries listed under Sources; each statement was checked against the cited text.

Construction and mechanism

  • The instrument is housed in a silver pair case measuring approximately 13 cm in diameter.
  • Its escapement features diamond pallets with faces set parallel to one another rather than at a traditional angle.
  • The movement includes a fusee with maintaining power to ensure the watch continues running while being wound.

Later history

  • The H4 was used in a sea trial to test its accuracy in conjunction with the Method of Lunar Distances.

Photographs

Harrison H4 chronometer
Harrison H4 chronometer — Colonel Warden at en.wikipedia, CC BY-SA 3.0. Wikimedia Commons file page
Harrison H4 clockwork 2
Harrison H4 clockwork 2 — Photograph by Mike Peel (www.mikepeel.net)., CC BY-SA 4.0. Wikimedia Commons file page
Harrison H4 clockwork 3
Harrison H4 clockwork 3 — Photograph by Mike Peel (www.mikepeel.net)., CC BY-SA 4.0. Wikimedia Commons file page
The Greatest Horologists Of Their Time: John Harrison (1693-1766 ...
The Greatest Horologists Of Their Time: John Harrison (1693-1766 ... — mrwatchmaster.com, Rights unverified — web image search result, review pending. Web page (mrwatchmaster.com)

Photographs are illustrative examples of the type, not necessarily the exact object described. Verify details against the source file page.

Sources & references

  1. John Harrison, Wikipedia (English), retrieved 2026-10-28 (text CC BY-SA 4.0)
  2. Marine chronometer, Wikipedia (English), retrieved 2026-10-28 (text CC BY-SA 4.0)
  3. Longitude rewards, Wikipedia (English), retrieved 2026-10-28 (text CC BY-SA 4.0)
  4. Royal Museums Greenwich, catalogue and gallery notes on Harrison's timekeepers
  5. Sobel, D., Longitude
  6. Gould, R. T., The Marine Chronometer: Its History and Development

Where sources disagree, the disagreement is stated rather than resolved silently.