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I think it is an illustration of the 16th century "rule to raise a degree",
i.e. a means of calculation of a ship's position based on the assumption
that there are 17 1/2 leagues to a degree:

If you sail north or south you have to sail 17 1/2 leagues to arrive one
degree to the north or south. And so on for every point of the compass
(11,25 degrees).  And there can only be 8 numbers, because if you sail east
or west your latitude doesn' t change at all.

Diagrams of this sort can be found in manuals of navigation of the 16th and
17th century. 

Dr. Wolfgang Köberer
Wolfsgangstr. 92
D-60322 Frankfurt am Main
Tel: + 49 69 95520851
Fax: + 49 69 558400

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