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  Surveying Instrument Collection 


not known


Artificial mercury horizon

Serial Number



167 x 93 x 114 mm


Artificial Horizon


The wooden box, marked 'A7' in felt pen, contains

- (Steel?) tray (150 x 86 x 22 mm) containing a wooden insert (150 x 86 x22 mm) with a depression (90 x 75 x 8 mm) for the mercury horizon and a threaded filling hole.

- Mercury container (wood/baklite, 51 mm diameter, 95 mm high) with thread on top fitting the tray filling hole.

- Brass cover for mercury bath with 2 glass windows at 90 to each other and at 45 to the base (176 x 93 x 113 mm)

For measuring the altitudes of stars or the sun up to about 60, this artificial horizon was employed when it was impossible to see the actual horizon. At sea, the horizon is formed by the junction of sea and sky. On land, the true horizon is often obscured by hills or other topological features. For making accurate observations with a sextant of the altitude of the sun or of a star an artificial horizon was required. In order to obtain the altitude of a celestial body, the angle between the sun or star and its reflection in a plane, level surface was measured and divided by two. The instrument consists of a shallow wooden tray about 150 mm by 75 mm and a turned wooden bottle for the mercury. The wooden tray is set on the ground or on a stand in an approximately level position and filled with the mercury. When the instrument is not in use, the mercury is kept in the bottle shown. The corner of the tray has a small hole and socket so that the mercury may be poured back without any danger of spilling it. A roof with sloping faces of glass plates of uniform thickness (shown on the left) covers the tray during the observations, in order to prevent rippling by wind striking the mercury surface. This artificial horizon gives very good results within its limits, its main disadvantage being that it is rather bulky and heavy.

History & comments

A similar device is shown as item 0103.


in wooden box (195 x 178 x 108 mm). Key to box missing.


some oxidation on brass and steel parts.


The mercury container is seized and cannot be opened.


Catalogued by JMR in July 2010. Photo by JMR.

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