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    Dip measurement : was Index corr., Octant as dipmeter
    From: George Huxtable
    Date: 2004 Nov 22, 11:17 +0000

    I haven't yet found my photocopy of Blish's paper about what he calls "the
    Navigator's prism", to be attached to a sextant for measuring dip.
    But I have come across an email I sent, 3 years ago, which refers to the
    paper and says something about it, if that's useful.
    Brenda Corbin , the helpful librarian at the US
    Naval Observatory, helped to get me a photocopy from the original, which is
    kept at the Operational Archives Branch of the Naval Historical Centre.
    It's "The Navigator's Prism", by Lieut.-Commander John B Blish, US Navy, in
    Proceedings of the United States Naval Institute, Annapolis, Md., Vol XXIX,
    No.1, March 1903, p.175.
    Until now, all I have had to go on is a mention in Cotter "A History of
    Nautical Astronomy", p.91., and a copy of the US patent specification.
    Blish's paper contains an anecdote of the precise navigation required for
    the cable-laying vessel "Faraday". between Nova Scotia and the Azores. She
    carried nine navigators with nine sextants (itself a recipe for disaster,
    one might think!) and made precise dead-reckoning by paying out a tethered
    measuring wire behind her. And one day all nine navigators found their noon
    sextant readings were three miles out, though the next day they were back
    on track. Blish ascribes this to variation of dip.
    Blish goes on to describe the use of his prism, with a sextant and also on
    its own. And he provides a (very) short table of some measurements,
    scattered in time, made by USS Alert, on the coasts of California and Lower
    California. This shows that on Oct 16 1901 at 1.25 pm, the measured dip "in
    all azimuths" was 7 min 53 s, compared with the expected value from tables
    of 4min 40 s. This was the greatest positive value of dip recorded in the
    table, but a few days later on Oct 21 at noon a dip was observed of -2min
    28 sec, a discrepancy from the "book" value of over 7 min.
    These seem to me to be remakarbly wild fluctuations of dip when compared
    with the Carnegie's records, and perhaps they might put the observing
    techniques used into question. Or perhaps the Californian coast is an abode
    of inconsistent dips, and a bad place for sextant sights.
    Since writing that, I've been informed by an authority on the subject (Andy
    Young, of
    the Astronomy Dept., San Diego State University, San Diego, CA-
    It's the latter.  The California coasts are notorious for large variations
    in refraction ...
    October is our "Santa Ana" season, when the hot wind from the desert blows
    out over the cold ocean.  So these observations make perfectly good sense.
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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