Note to Macbeth, Act 1, Scene 3, line 17: "shipman's card"

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to
Macbeth,
Act 1, Scene 3, line 17.
A "shipman's card" is now known as a "compass card." A basic compass is made by floating a magnetic needle above a compass card.
Compass Card

Image Source:
Chest of Books 		Compass

Image Source:
Compass Card Systems