C.V. referred me to two books, 'Wrinkles in Practical Navigation', published in 1918, and 'Hints to Travelers', published in 1871 [2,3]. Apparently, the idea of putting a spirit level on the index arm of a sextant originated with Karl Friederich Knorre (1801-1883), Professor of Practical Astronomy at the School of Navigation in Nikolaev, a major Black Sea port city in what is now Ukraine. The purpose of the spirit level was to allow someone measuring the altitude of a celestial body with the aid of an artificial horizon to ensure that the measurement was being made correctly. An artificial horizon is a container holding a pool of reflective liquid, usually mercury, that allows a sextant user to measure the altitude of a celestial body when a proper horizon, such as can be seen on a clear day at sea, is not available [4]. The user first locates the image of the celestial body in the reflective surface of the artificial horizon and then ‘brings down’ into alignment with it the image of celestial body reflected from sextant’s index mirror; dividing the resulting angle by two gives the altitude of the celestial body. Apparently, it was fairly easy to mistakenly ‘bring down’ the wrong celestial body when a similar-looking celestial body was close in the sky to the correct one. The index arm spirit level prevents the user from making this error. Its angle relative to the index mirror is such that the level’s bubble will ‘play’ when two images of the same celestial body are in alignment.
Many thanks to C.V. for solving the puzzle of the index arm spirit level. Congratulations on your lifetime subscription to the GoG blog, C.V.! To all my other readers, there's still hope: A second lifetime subscription is still available to the first person to find a surviving example of a Brandis Astronomical sextant.
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[1] Although C.V. has given me permission to use his full name, in the name of protecting the innocent only his initials will be used
[2] 'Wrinkles in Navigation' can be downloaded at: https://archive.org/stream/wrinklesinpracti00leckrich#page/n7/mode/2up
[3] 'Hints to Travelers' can be downloaded at:
http://galton.org/essays/1870-1879/galton-1871-jrgs-hints-travellers.pdf
[3] More on artificial horizons can be found at: http://www.longcamp.com/horizon.html
