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    The first drum sextants.
    From: George Huxtable
    Date: 2005 Jun 14, 23:32 +0100

    Perhaps Alex is away across the North Sea already; if so, he should be
    enjoying a spell of pleasantly settled weather, just now.
    Alex asked, a few days ago, which firm was the first to introduce the drum
    sextant, to supersede the Vernier scale.
    I was surprised to discover that none of my books on the history of
    navigation provided an answer to that simple question.
    Since sending that reply, I've taken a look at "Taking the Stars", that
    lovely book on navigation instruments by list member Peter Ifland. To my
    surprise, that hasn't taken the answer to Alex's question much further.
    Peter's book tells us that in 1909 the British firm Heath, in their
    "Hezzanith" range, introduced the patented "endless tangent screw", which
    allowed fine-adjustment over the whole range of the sextant's scale,
    without meeting any sort of stop. He writes "in the example shown, one
    revolution of the fine-adjustment screw moves the index arm precisely 5
    seconds of arc"; but that must surely be in error. From the look of the
    rack cut on the sextant's arc, its spacing appears to be very similar to
    that of a more modern drum sextant, which advances a whole scale-degree for
    every turn of the screw. 5 vseconds per turn would be absurd.
    But that development in 1909 allowed only fine-adjustment of the sextant,
    not fine-reading. No attempt was made to produce a precisely cut arc which
    corresponded to exact degrees; and there was no attempt made to put any
    scale to show fractional turns of the adjusting knod. Just as before, the
    navigator had to squint through a microscope to get an accurate reading of
    angle, using a Vernier scale against the angular arc.
    Then Ifland moves on to Drum Micrometers. He makes to definite stament
    about which firm was first to introduce them, but shows an illustration of
    a drum micrometer sextant, by Heath, stated to be "late 1920s". An added
    Vernier, against the drum scale, allows the readings to be interpolated to
    10 seconds. So, basically, there we have a fully-developed modern drum sextant.
    There's an interesting note, which reads- "The earliest known application
    of the drum micrometer to navigation instruments is found on a sextant by
    Jesse Ramsden, ca 1787, now in the National Maritime Museum in Greenwich."
    I'm a bit sceptical that the engineering standards of Ramsden's day allowed
    for a drum sextant to be made which could read angles from the drum
    corresponding to the whole arc, to navigational accuracy. I must look it up
    on my next visit to Greenwich. Can anyone tell us more about that sextant,
    how well it lived up to its aims, and why (and if) no successor appeared
    for another 130 years?
    Contact George at george@huxtable.u-net.com ,or by phone +44 1965 820222,
    or from within UK 01865 820222.
    Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
    5HX, UK.

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