Dear Expert Friends,
I have yet another question that I am sure that one of you will
be able to answer, or at least put me on the track to finding the
answer.
We all know that Magnetic Declination marked on a compass is an
excellent guide to its date of manufacture. I have the figures for
London, Paris, Rome and New York for the last 400+ years.
However, I have been studying a compass, actually a portable
dial, that was made for 22.5 degrees SOUTH! I measured this from its
gnomon angle, so there is some possibility of error, perhaps 1 degree.
This latitude passes through Australia, South Africa and South America
plus a few islands like Mauritius. I can virtually rule out the first
two as there are no major towns at this latitude. (Perhaps I should
consider Walfish Bay in S Africa, but the community there would have
been really small). However, Rio de Janeiro (or possible Sao Paulo) sit
almost on my line. It is an English made dial, and I know that Rio had
a large British population. The dial date is between 1800 and 1840, and
the magnetic declination indicated seems to be around zero.
Therefore I am seeking for records of declination at around this
period to confirm that Rio is the intended location for the dial. If
not, where else had zero declination in 1820?
One possibility would be to look for old shipping charts, but I
am hoping that someone will have a computer program that will calculate
such things for any point on the Earth for any date? This will be a
most important tool for anyone interested in Portable Dials or
Compasses.
Any help from any source will be welcomed.
Regards,
Mike Cowham.
<[EMAIL PROTECTED]>
Cambridge, UK.
