The Compass Rose - Map of China: Decoding the Secrets of a Vanished Cartographer - page 98

It wasn't quite that easy. Deducing a ship's position requires knowing its direc-
tionanditsspeed.Directionwaseasy:youcouldreaditoffthecompass.AChinese
pilot was expected to check his bearing every 20 nautical miles, taking readings at
both the bow and the stern to ensure he was not getting a distorted reading. Speed
was more difficult. It could be calculated by dividing distance by time, but neither
distance nor time could be exactly measured at sea. Time could be approximated
by tracking the passage of the sun across the sky or by burning a fixed amount
of incense for which the burn rate was known. Distance was the bigger challenge
once a ship sailed out of sight of land. For purposes of general calculation, Ming
navigators estimated that a ship could sail four-fifths of a 'stage' - a land distance
of roughly 30 kilometres - in 1¼ geng or 'watches', of which there were ten per
twenty-four-hour day. At that speed a ship would cover 24 kilometres, or roughly
15 nautical miles, in 2 hours and 24 minutes, which calculates as a speed of 6¼
knots(nauticalmilesperhour). * Apilotcouldadjusthisestimateofspeedbydrop-
pingapieceofwood-whatEuropeanmarinerscalleda'speedlog'-fromthebow
andpacingwithittothestern,thenmultiplyingthetimetakenbytheratiobetween
a given distance (one 'stage') and the length of the ship. The result of this calcula-
tion was not terrifically exact, as ships were subject to tide, current and swell, any
of which could confound measuring how far it actually travelled over the ground.
All of this is to say that a compass on its own was not enough to navigate blue
water where the distances were too vast and the markers too infrequent. To fill
the knowledge gap, pilots turned to written records that explained how particu-
lar routes went. These records could be anything from point-form notes to rough
sketches to full-blown sea charts. John Saris called them 'platts' or 'plotts', an ar-
chaic usage that survives today in the expression 'plotting a course'. Saris refers
to them several times in his journal, usually to complain of their inaccuracy. More
intriguing is an account in a letter Saris later received from his kinsman Edmund
SayersdescribingadifficultjourneyfromSiamtoHirado.Whenthejunk'sincom-
petent Chinese pilot became so ill that he couldn't 'creepe out of his cabbin', Say-
ershadtotakeoverwithnoideaoftheship'slocation.Hethendiscoveredthatone
of the cooks on board had what he called an old 'platt'. He was able to read it well
enough to set a bearing that brought the ship to Japan six days later, with barely
five men standing.
Almost no platts survive. What do survive are route guides: the English called
them'rutters';thePortuguese, portolanos ,literally'listsofports';andtheChinese,
zhenjing , 'compass manuals'. These hand-copied documents furnished pilots with