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    Re: Sextants,vernier and micrometer
    From: George Huxtable
    Date: 2006 Oct 31, 23:55 -0000

    I hold Henry Halboth's sea-experience in great respect. No doubt, he
    has read Vernier sextants thousands of times more often than I ever
    have, so when he and I differ, even about an essentially trivial
    matter, it's a worry to me. It seems worth while, then, to thrash the
    matter out between us, a bit further, to get to the bottom of the
    difference between us. And it may be no more than that he and I are
    using different words to describe what we are both seeing, when we
    look at the image of an arc and Vernier that Alex has made available,
    at http://www.math.purdue.edu/~eremenko/pic1.jpg   , which is divided
    in a way that's unfamiliar to me, and may also be unfamiliar to Henry.
    Henry wrote-
    |  .... the arc of Alex's sextant  is divided into six spaces,
    | i.e., respectively 10-20-30-40-50 & 60 seconds to make-up 1-minute;
    Here I differ. Each DEGREE on the main arc is indeed divided into six
    fine spaces, each such space corresponding to 10 MINUTES, not 10
    seconds. Next, the Vernier must be used to interpolate between those
    markings, 10 minutes apart.
    And if you look at the Vernier, that is subdivided into 50 parts. It
    is marked 0 to 10, and the space between each such marking has five
    subdivisions, not six. The figures 0 to 10 correspond to
    whole-minutes, in the interpolation between those fine markings on the
    main scale, 10 minutes apart. And the fact that those whole-minutes
    are divided into five on the Vernier allows angles to be measured to
    the nearest fifth of a minute, or 12 arc-seconds. So that's why we
    have here a 12-second Vernier. Reading angles by it to the nearest 0.2
    minutes, rather than to sixths or quarters of a minute as were more
    common, fits in much better with calculations being to decimal parts
    of a minute, rather than to seconds as was earlier practice.
    I wonder if Henry will look at that picture yet again, and whether we
    can agree about how the thing works. I do hope so.
    Henry concluded-
    | I have a several page dissertation on reading-off verniers which I
    | be pleased to scan and send separately to anyone wishing to receive
    and I put my hand up and ask for a copy, please. But I am still on a
    dialup line, not broadband, so (depending on the number of pages) I
    ask that it be scanned at the minimum resolution that can be got away
    with without spoiling it. Or, alternatively, put in the post.
    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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