Thomas Earnshaw Box Chronometer
date: label: date made
content: ca 1798
name: label: maker
content: Earnshaw, Thomas
notes: label: Description
content: This instrument is a specialized timekeeper for finding longitude at sea. Thomas Earnshaw made this chronometer in England about 1798. It became part of the James Arthur Collection at New York University, and the university donated a portion of the collection, including this chronometer, to the Smithsonian in 1984.
label: Description
content: To find longitude at sea, a chronometer was set to the time of a place of known longitude, like Greenwich, England. That time, carried to a remote location, could be compared to local time. Because one hour of difference in time equals 15 degrees difference in longitude, the difference in time between the chronometer and local time would yield local longitude. The instruments require careful handling to keep precise time. Although the original box for this instrument has not survived, most chronometers are fitted in a wooden box in a gimbal to remain level and compensate for the movement of a ship at sea.
label: Description
content: Thomas Earnshaw (1749-1829) was a pioneer in chronometer development. He is credited with introducing to chronometer design two important features that became standard parts of the timekeeper in the 19th century—the detached detent escapement and, independently of his rival John Arnold, the bimetallic compensation balance. His simplifications permitted others to undertake batch production of chronometers, and his work received an award of £2500 from Britain’s Longitude Board in 1805.
label: Description
content: Mechanism details:
label: Description
content: Escapement: Earnshaw spring detent
label: Description
content: Duration: 1 day
label: Description
content: Power source: Spring drive with chain and fuse
label: Description
content: Balance spring: helical, blued steel
label: Description
content: Balance: Earnshaw, two-arm
label: Description
content: Inscription: “Thos. Earnshaw / Inv. Et Fecit No. 451” on back plate
label: Description
content: Barrel bridge possibly a replacement
label: Description
content: Dial details:
label: Description
content: Engraved and silvered brass
label: Description
content: Indicates hours, minutes, seconds
label: Description
content: Inscription: “Thos. Earnshaw / INVT. ET FACIT /No. 451” on dial
label: Description
content: Blued steel lunette hands (minute hand is a replacement)
label: Description
content: Brass bowl; screwed and milled bezel; convex and chamfered crystal
label: Description
content: No box
label: Description
content: No winding key
label: Description
content: References:
label: Description
content: 1. Mercer, Tony. <i>Chronometer Makers of the World.</i> Essex: NAG Press and Tony Mercer, 1991.
label: Description
content: 2. Thompson, David. <i>Clocks.</i> London: British Museum Press, 2005.
label: Description
content: 3. Whitney, Marvin E. <i>The Ship’s Chronometer.</i> Cincinnati: American Watchmakers Institute Press, 1985.
label: Location
content: Currently not on view
place: label: place made
content: United Kingdom: England, London
setName: label: See more items in
content: Work and Industry: Mechanisms
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content: Time and Navigation
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content: Measuring & Mapping
dataSource: label: Data Source
content: National Museum of American History
identifier: label: ID Number
content: 1984.0416.013
label: accession number
content: 1984.0416
label: catalog number
content: 1984.0416.013
objectType: label: Object Name
content: box chronometer without case
physicalDescription: label: Physical Description
content: brass (overall material)
label: Physical Description
content: steel (overall material)
label: Physical Description
content: glass (overall material)
label: Measurements
content: overall: 2 7/8 in x 3 1/2 in; 7.3025 cm x 8.89 cm