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During theThree Kingdoms Period(220-280), when the sea boats of the Wu State reached the South Sea, some people wrote a book called A Record of the Eyewinkers in the South Sea, which recorded the then mileage-plotting method. To measure the sailing speed and mileage, first, a sailor would throw a wooden plate into the sea at the bow before immediately running to the stern to see whether the wooden plate arrived at the same time.

As the embryonic form of the mileage-plotting instrument, the method remained in use until theMing Dynasty(1368-1644), when a more precise method was developed and when incenses were burned to measure the time length of a certain distance.

The ancient mileage method was quite similar to the structure of the more modern fan-shaped mileage-plotting instrument (used around the early 20th century), where a fan-shaped wooden plate was also used, but was fastened to a rope of the length of the boat before being thrown into the sea. A sand filter (or an hourglass) was used to measure the time duration, just like the incense-burning method in ancient China.

Author: Jessie