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The Encyclopedia of Horology

Marine timekeeper (spring-and-balance sea clock) · England

Harrison H1

Verified

John Harrison's first sea clock, built to compete for the longitude prize established by the Board of Longitude's 1714 act.

Technical facts

Escapement
Grasshopper escapement of Harrison's own design
Regulator
Twin balances linked by connecting wires, largely temperature-compensated by bimetallic strips
Materials
Predominantly wood (lignum vitae and oak) with brass fittings
Weight/size
Large — approximately 34 kg; not a portable pocket instrument

Historical significance

  • John Harrison's first sea clock, built to compete for the longitude prize established by the Board of Longitude's 1714 act.
  • Uses paired balances connected by wires and springs to cancel the effect of a ship's motion, entirely without a pendulum.
  • Trialled at sea to Lisbon in 1736 with encouraging results, leading the Board to fund further work (H2, H3, H4).

Background

John Harrison, a Yorkshire carpenter, completed his first sea clock, known as H1, in 1735. The clock was developed as a proposal for the British government's longitude prize, which offered rewards for determining longitude at sea. Harrison demonstrated the machine to the Royal Society, who recommended it to the Board of Longitude for its first sea trial.

In 1736, the clock was trialled on a voyage to Lisbon aboard HMS Centurion and returned on HMS Orford. While the clock lost time on the outward journey, it performed effectively during the return trip. The ship's sailing master noted that the H1 correctly predicted the vessel's landfall, whereas his own calculations had been incorrect.

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

Construction and mechanism

  • The clock utilizes a system of counter-oscillating weighted beams connected by springs that are not affected by gravity or ship motion.

Later history

  • The restored H1 is currently held within the collection at the Royal Observatory in Greenwich.
  • The H1 timepiece remains in working condition today.

Photographs

Replica of John Harrison's first marine timekeeper, by Herbert Davies, Emsworth, Hampshire, completed c. 1962 - Clockmakers' Museum - London, UK - DSC05428
Replica of John Harrison's first marine timekeeper, by Herbert Davies, Emsworth, Hampshire, completed c. 1962 - Clockmakers' Museum - London, UK - DSC05428 — Daderot, CC0. Wikimedia Commons file page

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. Royal Museums Greenwich, catalogue and gallery notes on Harrison's timekeepers
  4. Sobel, D., Longitude

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