Secrets of the Selden Map - Map of China: Decoding the Secrets of a Vanished Cartographer - page 156

of the Laud rutter, he copied it out by hand while he was there. He returned to Ch-
inatoservehiscountry,andendedupbeingtorturedtodeathattheageofsixty-six
during the scandalous first year of the Cultural Revolution for the sin of having
travelled abroad and knowing foreigners. I like the fact that Xiang's careful work
on the rutter has now come back to prove something about a map he never had the
chance to see: that the ruler is indeed the scale by which the cartographer drew the
routes on the Selden map. His reputation outlives his tormentors. The vindication
is a small one, but among the people I hang out with, this triumph matters. Xiang
was one of our finest, and this is what we do.
____________________
We get to the third secret of the map's construction via the one we have just ex-
posed. This secret is that the map has a magnetic signature.
Look carefully at the compass routes. You will recall that most segments of
these routes are labelled with their compass bearings, seventy-two of them spaced
around the circumference of the compass at an angle of 5° from one to the next.
The cartographer has done more than mark their bearings. He has gone a step fur-
ther and drawn the route lines so that they depict the actual angles they are sup-
posed to represent. The reference for these angles is the compass rose. If a line is
marked zi (0°), then it has been drawn to conform to how zi appears on the com-
pass rose. Now look more closely at the compass, and you will notice that zi , the
northern point of the compass, does not in fact point to the top of the map. It is
tiltedslightlytotheleftbyabout6°.Iftherosehadbeendrawntolineupalongthe
map'snorth-southaxis, zi shouldpointtruenorth.Itdoesn't-because itshouldn't.
It is common knowledge that the earth's magnetic field does not align perfectly
along its north-south axis, for the magnetic pole is not fixed. It meanders errat-
ically inside the Arctic Circle among the islands of the Canadian north. Actually,
as I write, it is just about to leave the sector of the Arctic Ocean under Canadian
sovereignty and enter Russia's Arctic sector - claims of jurisdiction, incidentally,
that neither country could make were it not for John Selden's argument that it was
reasonable to delineate boundaries on the sea. This means that the rate of magnet-
ic declination - the difference between geographical or 'true' north (the top of the
map) and magnetic north (the zi position on the compass circle) - is always chan-
ging. If the Selden compass points 6° to the left, it is because in China at that time
the magnetic pole must have been 6° to the left of the North Pole. As it turns out,
accordingtothereconstructionofhistoricdeclinationspublishedbytheUSGeolo-