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Henry asks about terminology for the friction arrangement for moving the index arm of a vernier sextant.

In contemporary books, only the tangent screw and the clamp seem to be named, so in attempting a detailed description for my book (The Naked Nautical Sextant), I invented some terminology which I hope is apt. I am attaching a picture which shows the layout for most nineteenth century sextants. There were some variations, but the principles remained the same except for some Brandis sextants, which used an early, and unsatisfactory, eighteenth century arrangement. C Plath and Hughes and Son added some refinements that must have increased the cost for no particular gain.

Bill Morris
Pukenui
New Zealand
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