Marine timekeeper (large sea clock) · England
Harrison H3
VerifiedHarrison's third sea clock, on which he worked for nearly two decades, introducing the bimetallic strip (a direct ancestor of the bimetallic compensation balance used in later chronometers) and caged roller bearings to reduce friction.
Technical facts
- Escapement
- Grasshopper-derived escapement of Harrison's own design, further refined from H1
- Innovation
- Believed to be the first practical use of a bimetallic strip for temperature compensation
- Bearings
- Caged roller bearings, an early anti-friction device
Historical significance
- Harrison's third sea clock, on which he worked for nearly two decades, introducing the bimetallic strip (a direct ancestor of the bimetallic compensation balance used in later chronometers) and caged roller bearings to reduce friction.
- Never itself fully solved the longitude problem at sea in the way H4 later did, but the mechanical innovations developed while building H3 fed directly into H4's design.
- Illustrates that Harrison's path to H4 was incremental across four increasingly refined machines (H1–H4), not a single isolated invention.
Background
John Harrison developed the Third Sea Clock, known as H3, while waiting for the War of the Austrian Succession to end. The clock features a vertically mounted balance wheel linked to another wheel of the same size, a layout chosen to eliminate issues caused by the ship's motion. Harrison adopted circular balances for this machine to address complications caused by the ship's yawing action during maneuvers.
The construction of H3 was part of Harrison's incremental development process aimed at improving the accuracy of timekeeping at sea. Harrison worked on the machine for several years following his experience with the second sea clock.
Written from the encyclopedia entries listed under Sources; each statement was checked against the cited text.
Later history
- Rupert Gould later performed restorative work on the timekeeper to ensure its survival as a functional mechanism.
- The H3 clock is currently housed and displayed at the Royal Observatory in Greenwich.
Photographs
Photographs are illustrative examples of the type, not necessarily the exact object described. Verify details against the source file page.
Sources & references
- John Harrison, Wikipedia (English), retrieved 2026-10-28 (text CC BY-SA 4.0)
- Royal Museums Greenwich, catalogue and gallery notes on Harrison's timekeepers
- Sobel, D., Longitude
Where sources disagree, the disagreement is stated rather than resolved silently.