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Murray Buckman

Re: The Tracks.  Forgive me if I don’t repeat your long and informative post.  To me, one seemed to emanate from near Lisbon and the other from near Porto.  If I remember Onedin started by carrying Port wine from Portugal to Liverpool and possibly returning the empty casks.

Re: The chart.  It appears to be a Mercator chart of the North Atlantic with five-degree lat and long spacing. Onedin’s sextant box appears to be to the right of the chart.   

Can you identify the interference lines parallel and NE of track.  Is it a fluke of Frank making a copy of a copy or is there a Perspex ruler lying along track interacting with the cameraman’s lighting; which can be very intense; residual Perspex cooling stress possibly?

Can you account for what looks like the outline of an inverted triangle around 57N 015W?

Re The Isogonals.  I’m pretty sure they’re isogonals and not isobars or lines of constant bearing, because Google Images of similar charts show them with values and annual changes on. I came across this little programme on Wikipedia which shows how the magnetic variation/declination has changed over the years. It might help to work out the date of the chart. https://en.wikipedia.org/wiki/File:Earth_Magnetic_Field_Declination_from_1590_to_1990.gif  It kind of matches the turn of the 19th/20th Century.  DaveP