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Frank,

 I have the 1871 ‘Hints to Travellers’ document that you excerpted. I can post the complete pages 16 and 17 from it if anyone is interested.

 The date and translation of your Knorre find is interesting. I had only been able to learn that Knorre’s revelation pre-dated 1872.

 You wrote: “Someone asked about a level attached to the index arm at an odd angle on a sextant c.1900.”

 My 2016 pursuit was prompted by just such a question.  A long-time correspondent of mine closed a blog post on astronomical sextants by jokingly offering a prize to anyone who could identify the purpose of the spirit levels shown mounted on them. The following link includes my response and his challenge post, with images of two astronomical sextants that had appeared in Brandis catalogs.

 https://gardnerghost.blogspot.com/2016/12/

 I have attached the two sextant images from the blog post along with a Stackpole & Brother sextant shown at this Smithsonian link (which wrongly attributes introduction of the index arm spirit level to William Harkness):

 https://amhistory.si.edu/navigation/object.cfm?recordnumber=1060172

Chuck Varney

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