The Compass Rose - Map of China: Decoding the Secrets of a Vanished Cartographer - page 109

What, then, is the compass rose doing on the Selden map? Was it an attempt to im-
itate a conspicuous feature of European maps? It is certainly possible. The Selden
cartographer was operating within a zone in which Europeans had been sailing for
almost a century. He would have seen their ships and he would have seen their
crews, so it is likely that he saw their maps. What cartographer would not want
to examine maps that came from outside his own tradition? Is the compass, then,
evidence that he was simply copying what he saw?
The foot ruler beneath the rose seems to be an even stronger piece of evidence
of borrowing. A distance scale was a strict requirement of European charts. Often
itisdepictedliterallyasaruleroverwhichapairofcalipersordividers('compass'
in the other, now archaic sense of the word) has been opened. Ruler and calipers
were the mapmaker's signature tools, their presence signifying that careful meas-
uring had been done. Together they were the trademark of scientific accuracy. El-
izabethan poets picked up the image of the compass/calipers as a symbol of con-
stancy. This is how Ben Jonson uses it when he addresses John Selden in a poem,
praising his polymath friend as someone who is
Ever at home, yet have all countries seen:
And like a compass, keeping one foot still
Upon your centre, do your circle fill
Of general knowledge; watched men, manners too,
Heard what times past have said, seen what ours do.
If we are right in supposing that the Selden cartographer drew visual inspiration
from European design, this could explain why he extended two rhumb lines 15° to
each side of 180° ( bing and ding ) downwards from the magnetic compass, produ-
cing a triangle that imitates a pair of calipers opened above a ruler.
Stephen Davies, a researcher at the Hong Kong Maritime Museum, has conjec-
tured that the ruler was not purely decorative but was in fact used to determine dis-
tances on the map. He proposes that each of the hundred ticks ( fen or tenths of an
'inch', of which there are ten in a Chinese 'foot') along the ruler's edge marks the
distanceashipcoveredinonewatch,ofwhichtherewereteninatwenty-four-hour
day.Ifeachtenthofaninchmarksatenthofaday,thenaninchequalsthedistance
covered in twenty-four hours. Using my estimated speed of 6¼ knots, a day's sail
amounted to 150 nautical miles. As a Chinese ruler has ten inches, the length of
the ruler on the map should represent 1,500 nautical miles. Davies concedes that