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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.

George.


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