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    Vernier explanation
    From: Don Seltzer
    Date: 2022 Jun 22, 07:30 -0400
    A poster on the Gunroom forum had the following question.

    “I have recently bought a sextant dating most probably from the 1890s and bearing
    the name of H. G. Blair & Co. of Cardiff, although I suspect that this company
    was only a retailer. It was incomplete (lacking the telescope, the magnifier on
    a swivelling arm over the vernier scale, and one of the smoked-glass shades),
    but I have managed to supply the missing parts after a fashion. Unfortunately
    this instrument can no longer give very precise readings, since the threaded
    shaft of the vernier adjuster is slightly bent so that the vernier scale can
    only move through about a fifth of its original travel before the shaft jams
    solid. The sextants familiar to Jack Aubrey and his officers would have been
    very similar, I believe, although the Ramsden instrument of 1772 shown in the
    illustrated edition of 'Longitude' is much larger (15" radius, as against about
    8" on mine) and has a less complicated frame.

            It is the vernier that puzzles me. As I understand it, the principle of
    this device is that the units on the separate vernier scale are made either
    slightly larger or slightly smaller than the divisions on the main scale; thus
    on my Celestaire cardboard sextant the vernier scale spans one degree in
    5-minute increments, the whole degree being equal to 11 degrees on the main
    scale. Readings are taken by noting the nearest whole degree on the main scale
    to the zero mark on the vernier scale, and then establishing the number of
    minutes by finding a point where the marks on the two scales exactly coincide.
    (Please correct this as required if I am mistaken.) On the Blair instrument, by
    contrast, the vernier is engraved 0 to 10 (are these minutes, or are they tenths
    of a degree?), each unit being subdivided into six, and the 0–10 range spans
    exactly twenty degrees on the main scale; thus each half-unit on the vernier is
    precisely equal to one degree on the main scale and there is no staggering or
    offsetting between the two scales, so that the 'interpolation' method of reading
    which I have tried to describe above cannot apply. Can anyone tell me how this
    instrument would have been read?

            I have been calling the Blair instrument a sextant, but I believe it
    should strictly be designated a quintant, since the main scale extends over 150°
    rather than the more usual 130°. The engraving on the scales (still done by hand,
    I believe) is extraordinarily delicate, each degree on the main scale being ruled
    off in six ten-minute divisions which my tired old eyes can barely resolve

    Can someone help him out and explain how this vernier works?

    Don Seltzer



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