Artificial horizon

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 collapsible artificial horizon (pictured here with its fitted box and mercury bottle - now empty) was made in England around 1800. It is unsigned.

The instrument consists of a trough of wood, into which mercury is poured. 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.

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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