Artificial or mercury roof horizon, rectangular black-lacquered cast iron trough with pouring hole and brass-framed roof-shaped cover with clear glass panels. Round iron bottle filled with mercury with stopper and iron funnel, in fitted wooden carrying box, signed and made by Sewill, maker tot he Royal Navy, Liverpool. Only a few copies of Sewill are known. Even the Maritime Museum in Greenwich does not have a copy.
The artificial horizon was used to establish the elevation of the sun or a star when the horizon was not visible. The vertical angle between the celestial body and its reflection in the mercury was mesured. This had to be halved to establish the angle of elevation above the horizon.

Catalog: NM.5-15
Date: ca. 1850
HWL case: 15x22x15 cm
Signed: Sewill, Maker tot he Royal Navy, Liverpool
Origin: England
Condition: excellent, perfect, the strap is broken and incomplete
Mercury is poisonous, please pay attention!