Artificial horizon

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The sea horizon appears to offer an ideal base for measuring the height of heavenly bodies. But this is often obscured, and artificial horizons were used on land by surveyors, in conjunction with a sextant, when the horizon was not visible. This artificial horizon (pictured here in its fitted case) was made around 1870, probably by Negretti and Zambra, scientific instrument makers based in London.

The artificial horizon consists of a cast iron trough, prismatic draught cover and mercury bottle. Engraved on the cover is 'Negretti & Zambra, London'. The fitted case has a trade label for 'John Lilley & Son, 9 London Road, Fenchurch Street, London EC.'

Mercury is poured into the trough. The liquid metal gives a highly reflective flat surface, and to prevent the wind from disturbing it, it is provided with a roof-like triangular lid with slanting sides of plate glass. This simple type of 'quick-silver' horizon was used by landing parties of voyages of exploration, for example on Cook's second voyage; however, the mercury was always susceptible to vibration.

Record details

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• Online ID: 000-100-104-302-C
• Image Rights Holder: National Museums Scotland
• Project: National Museums Scotland
  Project description   View all records in project
• Ref: National Museums Scotland  T.1987.153
• Date: Around 1870
  c. 1870
  
• Material: Inscription: Negretti & Zambra, London
  Horizon, artificial, mercury / case
  
• Dimensions: 140 mm [trough]
  
• What: Horizon, artificial, mercury / case
  
• Subject: 9. NAVIGATION (Departmental Classification)
  
• Who: John Lilley and Son (Seller)
  Negretti and Zambra, London (Maker)
  The Leith Nautical College Collection
  
• Where: England, London
  England, London, Fenchurch Street (9 London Street)
  
• Event:
• Description: Mercury artificial horizon consisting of a cast trough, prismatic draught cover and mercury bottle, made by Negretti and Zambra of London about 1870
  
• References:
  • For a discussion on the development and use of the artificial horizon, see E.G.R. Taylor and M.W. Richey, The Geometrical Seaman: a book of early nautical instruments, London, 1962, pp 79-81. 
  • For dates of Negretti & Zambra, and John Lilley, see Gloria Clifton, Directory of British Scientific Instrument Makers 1550-1851, London, 1995, pp 168 and 198. 
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