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    Re: Cleaning arc of Vernier sextant.
    From: George Huxtable
    Date: 2006 Oct 28, 22:27 +0100

    
    Robert Eno asked what he described as "the obvious question".
    |
    | George, once you decided to acquire a metal sextant, why didn't you
    just go
    | the whole way and purchase a micrometer drum sextant? Then you would
    not
    | have to concern yourself with stockpiling other people's cigar ashes
    or
    | going through the painful tedium of trying to read a vernier scale
    under
    | poor light conditions.
    
    That's a fair question. Not sure that I really know the answer myself,
    but I will analyse my motives for getting a Vernier instrument as best
    I can.
    
    First, it's not to navigate with. My plastic Ebbco is still good
    enough for measuring up-from-the-horizon observations, from the
    unstable deck of my little boat.
    
    Nor is it to display on the mantlepiece as a decoration.
    
    It's an instrument that has remained pretty well unchanged, since the
    advent of engine-divided arcs in the 1780s. It represented the
    ultimate achievable precision over more than a century. And it wears
    its precision, obviously, on its sleeve. You just have to look at the
    details of the arc and the Vernier to appreciate the precision that
    went into it. In that respect, to my mind, it's a thing of beauty,
    comparable with any art object. Yes, a micrometer instrument is just
    as precise, but that precision is hidden, in the cutting of the rack.
    
    In my boyhood days, my physics teacher kept a Vernier sextant in the
    lab cupboard, and would get it out for us to look at from time to
    time, but we lads were never allowed to touch it, and since then I
    have always coveted one.
    
    Already, I feel I have got better "in-touch" with 19th-century
    navigators, in appreciating the importance of getting the
    illumination-angle right for reading the Vernier, and understanding
    the problems in night-reading with a lantern, candle or oil. That was
    a matter that would not have crossed my mind, before trying it out.
    
    Like many others on the list, I wish, some time, to try measuring
    lunars, to the extent that my old eyes will allow. The limitations
    will not lie in the sextant, I think. It will be a bit of a challenge,
    for me.
    
    I have no illusions about its technical inferiority to the modern
    micrometer instrument, which is certainly easier to read-out, and is
    likely to have wider aperture, bigger mirrors, and a modern prismatic
    telescope.
    
    As for its accuracy, there's a test certificate from the NPL, dating
    from 1920, stating calibration errors to be zero seconds for all the
    quoted angles. I think that implied that all were within the
    test-limit of 10 arc-seconds. With a Vernier instrument, there isn't
    much to go wrong, to derange that calibration.
    
    One interesting feature is that the telescope position can be shifted,
    out from the plane passing through the edge of the silvering on the
    horizon mirror, by what's known as a "rising-piece", adjusted by a
    screw-knob. This allowed the relative brightness of the two images to
    be adjusted, at the expense of allowing a slight wobble in the
    telescope mounting. That gimmick is mentioned by Peter Ifland, in
    "Taking the Stars", as being used "for a period about 1800", but it
    must have been common in English instruments much later than that. To
    judge, that is, by its appearance on the two Vernier instruments for
    sale last week, and on another for sale a couple of months ago,
    labelled as "Lillie & Wilson". I suspect all three were products of
    the same factory, in spite of their various labels.
    
    So, those are my reasons for acquiring something that's a bit of a
    museum-piece (like I am).
    
    George.
    
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
    
    
    
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