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Apparently double sextants were popular in survey work, as you've already learned. I'm attaching portions of two articles from works on surveying from the late 19th century and also a short description with a very nice engraving from The Book of the Sextant (A.J. Hughes, 1915).

While I'm at it, it's probably worth describing how I found these articles. Dave Walden pointed out to us a few years ago that hathitrust.org has the best online search facility for books. I regularly use the hathitrust advanced search tool here. You can search on an exact phrase in the full body text of all books. For example, search on "double sextant" and select "this exact phrase" from the pull-down menu. Then for year of publication, you probably want to select "Before or during" and then enter some date that reasonably matches the end date of the use of instruments like this, maybe 1925. Note that there is a slight flaw in the coding of this web page. Depending on your browser, if you return to this page to modify your search, you may need to select another date option and then re-select 'before or during' in order to get it to display properly. Hit "Search" and the service will return a long list of documents. You can look at most of these, though not recent publications with copyright limitations (many more, however, than at Google Books). There are download limitations. You can download individual pages as PDFs, and if you learn how, you can combine these into whole books. But if you download more than about 25 pages in a short period of time, you will need to take a break since that is considered a (small) abuse of their system. Once you have opened one of these old books to view, you'll discover one inconvenience of this hathitrust search system: you have to enter your search term again to find the exact location within the document: look for the box labeled "search in the this text" and enter your search term, e.g. 'double sextant', again. Loads of fun and interesting reading out there!

By the way, the "double sextant" is also mentioned briefly in Lecky's Wrinkles, and it's in the context of finding one's position in coastal waters by simultaneous angles from shore positions, as expected. He notes the interesting detail that he considers this instrument an inexpensive item which can be purchased for "only a few pounds". And that's another huge difference from the "reflecting circle" which the double sextant so closely resembles.

Frank Reed
Conanicut Island USA

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