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    Re: How Worsley Navigated [Was Navigation and Whaling]
    From: Brad Morris
    Date: 2009 Feb 24, 10:49 -0500

    I have examined the book cover and yes, that appears to be the
    appropriate ladder sextant.  Since it is a painting (and clearly not a
    photograph, hehehe, like they brought a camera) then we can permit the
    artist some degree of leniency with regards to the angle of the endless
    tangent angle adjustment knob.  I will check tonight to see if my knob
    is at the same angle.
    In regards to reading the vernier, there is a small sheet of translucent
    glass, mounted perpendicularly to the arc.  Its purpose is to diffuse
    the light over the arc where you are attempting to read it.  You hold
    the light parallel to the arc, not pointed at the arc, rather at the
    back of the translucent glass.  I use a small red LED but I do agree
    with George's point, it is a little tough to read.  Once you get used to
    it, it isn't so bad.  Am I reading it to 10"?  I doubt it, but I do take
    extra-ordinary care to assess the reading.  I use the scintillation of
    the light on the graduations to make the final call and yes, it is
    always a judgment call.  Thanks Bill for the assessment of wear and
    George makes an excellent point about which sextant.  I foolishly
    assumed he picked the best one available.  Maybe it was the only one
    available!  There was discussion about how overloaded the boats were
    when they journeyed from the ice to Elephant Island.  Perhaps they
    ditched the other sextants, retaining the best one?  The logbook does
    not carry down to seconds when the reduction is performed, only to the
    nearest minute of degrees. So clearly, he didn't use the terrific
    accuracy inherent in the sextant.  
    There are two chronometers I have found images of which purport to be
    the ones used by Worsley.  One is the box gimbaled type, while the other
    is a pocket watch type.  I can lend no clarity which one is correct, in
    particular which one was used on the journey.
    The log book has a note in the margin which states "Lat proved to be
    correct within about 2m.  Long ditto but Chron was much slower than I
    had allowed which made us about 20 miles of distance further xxxx than
    obs showed".  Try as I might, I could not read what word he has for
    xxxx, however, if the chronometer was slower than expected; then NavList
    can tell me, was that east or west? The chronometer rate determined at
    Elephant Island was 5 seconds slow per day.  
    I have been contacted by Canterbury Museum.  The missing page of the log
    is on its way!  Honorable folks. The page missing clearly contains 4
    May, 5 May and 6 May, so some more clarity will be had about the
    decision making process Worsley entertained, when it arrives.
    Best Regards
    Navigation List archive: www.fer3.com/arc
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