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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?

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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.

enter image description here

  • Interesting that they used a black glass, no a mirror. Would the be because its cheaper? – Paul Paulsen Jan 16 at 11:03
  • @PaulPaulsen I doubt that a mirror would be much more expensive. I'd assume that either a black glass could be better in some weather/atmospheric conditions or it would last longer than a mirror would - I mean that amalgam deterioration that could be seen on many old mirrors. But these are just rough assumptions. – Usurer Jan 16 at 11:48
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    It could also be to make the reflection of the sun less blinding. In black glass the suns reflection will be plenty visible, while a proper mirror might actually make it hard to get an accurate reading. – fgysin Jan 16 at 14:40
  • Another question that arises for me: how (by means of measuring) do you get that thing level. I cannot see any indicator to show the inclination. – Benedikt Bauer Jan 17 at 21:47
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    @BenediktBauer As the description of the current item says (see the link to Royal Museums Greenwich) - "The spirit level is missing". So at least we should expect a sort of a measuring device to be used together with the horizon. – Usurer Jan 18 at 10:16

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