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Artificial horizon

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probably made in London

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.

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