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compass.jpg - 18934 BytesBy the late sixteenth century, the mariner's compass had evolved into an instrument not very different from the compass of today. The case in which the compass itself is housed was made of wood or ivory in the early days. After that, brass came into use since it does not affect the magnetic needle.

The Pole Star (Polaris) served as the seaman's lodestar (or star that shows the way). Therefore the magnetic stone which was used to magnetize the compass needle was called a lodestone. The magnetic, direction-finding property of the lodestone had been discovered in China as early as the twelfth century.

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By the sixteenth century, the mariner's compass was made with a soft iron wire bent to a lozenge shape and attached to the underside of a circular compass card, which was suspended at the center on an upright needle.

Because the iron wire tended to lose its magnetism over a period of time, it was necessary for each ship to carry a good lodestone to re-magnetize the wire when it weakened.

There was one basic problem encountered in the use of the mariner's compass: The magnetized wire in the compass was drawn by large land masses. This caused the compass to have variations in its readings. The mariners and mathematicians of this early period were concerned about this problem, and a number of corrective measures were tried. However, at the time the Mayflower sailed in 1620, the problem had not been satisfactorily solved.


Note to teachers: A classroom activity for learning about the mariner's magnetic compass may be found here.

Sources
Text:
2003 by Duane Cline, author of The Pilgrims and Plymouth Colony: 1620. Used by permission of the author.

Illustrations:
Mariner's Compass and Exploded View
1990 by Duane Cline. Used by permission of the artist.
 

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