What is a glass artificial horizon?
On Amundsen's polar expedition they had,
Of instruments and apparatus for the sledge journeys we carried two sextants, three artificial horizons, of which two were glass horizons with dark glasses, and one a mercury horizon, and four spirit compasses, made in Christiania.
The South Pole An Account of the Norwegian Antarctic Expedition in the “Fram”, 1910–1912
You can see in this answer the team using the mercury artificial horizon, but what is a glass horizon?
1 answer
I'd suppose, that the glass horizon is the one that uses a well-polished glass as a reflective surface. The quote from the answer you've mentioned:
The basic arrangement needs a horizontal reflective plane, for which Amundsen used a pool of mercury. A precisely weighted mirror could work also, but a pool of mercury is more robust and doesn't go out of calibration.
The device could be seen at the website of Royal Museums Greenwich where it's described as:
A circular brass case on three levelling feet covered with a blackened glass plate serving as mirror.
BTW, other glass artificial horizons exemplars could be found in the mentioned museum collection.
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