The artificial horizon was used to enable observation of the altitude of objects when the true horizon was not visible.   The most common form, dating from about 1790, consists of an oblong trough containing mercury, which provides a level reflecting surface, and two panes of glass set at right angles to the observer's line of sight and held in a protective frame.

The artificial horizon set shown above consists of a cast iron tray, brass mercury bottle with screw on funnel and black oxidized brass triangular wind hood with a pair of right angle oriented optical windows.  It was used on land in conjunction with a sextant to obtain celestial sightings when no sea level horizon was visible.  By viewing the sun or a star directly in the reflection of the mercury surface, one would measure twice the angular altitude of the body above the horizontal (and then divide by two).