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Here are a few collected responses to mailings from Alex, under various
threadnames-

>(I was looking yesterday whether any dipmeter was ever mentioned
>in any standard Western navigation book accessible to me.
>No.)

Try Charles H Cotter, "A History of Nautical Astronomy" (Hollis & Carter
1968), page 91, about the Blish prism. To save trouble finding it, here's
the relevant paragraph, quoted below.

"To eliminate the effect of refraction on the dip of the sea horizon,
Commander Blish of the United States Navy invented, in the early part of
the 20th century, an attachment for a sextant known as the Blish prism.
This decice has the top and bottom faces bevelled at 45 degrees. It is
fitted to the sextant so that the longer of the front and back surfaces
faces the observer. This face is provided with two polished surfaces, the
lower of which is directly opposite the index mirror, and the higher of
which faces over the observer's head towards that part of the horizon, 180
degrees away from the part the observer is facing.. With the index of the
sextant set to zer on the arc the observer looks directly at the sea
horizon in front of him and sees, at the same time, the back horizon
reflected from the prism. When the fore and back horizons are brough into
line, the sextant reading is twice the angle of dip, assuming that the
sextant is free from index error."

There's a mass of interesting stuff in Cotter, but also many errors. Some
know errors have been uncovered by members of this list, and collected at-

www.huxtable.u-net.com/cotter01.htm

==============

Alex will find a picture of the Gavrisheff dipmeter (and much more besides)
if he can access the article, already referred to, by Captain Henry H
Shufeldt, USNR., "Precision Celestial Navigation Experiments", on page 301
of the Journal of the Institute of Navigation (the British one), vol 15
(1962).

===============

Alex asked, about my Ebbco plastic sextant-

>Is this what you have?
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=2285965198&ssPageNam
e=STRK:MEWA:IT

Can't say, Alex. My old browser crashed when trying to download that
picture. That's not unusual, with my old Mac.

================

and-

>Have you ever tried to measure the dip of the horizon with your periscope?

Yes, but not as part of a thorough campaign, only haphazardly. And very
boring it has been, never deviating by more than 1 minute or so from the
predicted dip.

My modified Ebbco sextant-dipmeter was taken by the "Cook-method"
navigators on a voyage north of Australia in the replica Endeavour, as part
of a (dreadful) programme series made by the BBC in 1991, called "The
Ship".

They didn't try it much, but again, the dip they found corresponded with
prediction. I think that will normally be the case, and significant
deviations will be rare, but when they exist, they can have a big effect,
if Blish's experience off Southern California is typical.

==================

I'm grateful to Alex for pointing me to the Mystic facsimile of Halley's
paper, which explains how to set the index correction of a quadrant in
back-observation mode. This is similar to (but presumably earlier than) the
quote from Norie on the same topic. Both agree that you align the images of
fore and aft horizons, having allowed first for the dip. That must have
assumed a standard "book-value" for the dip, and would have no way of
allowing for any anomalous dip. In Halley's time, the refractive component
of the dip was only imperfectly understood. No wonder the method was
distrusted by navigators.

Alex and I seem to agree that by making two observations, one inverted,
both the dip and the index error can be discovered.

Alex says that back-observations never worked well, and is investigating
further. I can suggest one possible reason why. There was no telescope
fitting for the back-observation, and observing through the peep provided,
an open horizon could be seen ahead of the observer. His picture of this
fore  horizon  wasn't "framed" by the horizon mirror assembly, because for
back observations, the peep and horizon mirror were so close, perhaps just
for compactness. So there's little to constrain the observer's view to be
in the plane of the sextant. Just a thought, though not fully thought-out.

George.

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01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
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