Antique artificial horizon

Antique artificial horizon by Hicks London ca.1850 in original mahogany box.

It was devised as a means of navigation when the horizon was obscured by darkness or through the effect of inclement weather.

The principle was based upon the first law of optics, the angle of reflection from a mirror is equal to the angle of incidence, therefore a sextant would be employed to measure the angle between the sun or the stars and the reflection on the mercury contained in the metal trough