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Dear George,

let me add some historical detail to your post about Halley:

He wasn`t the first one to propose finding longitude by magnetic variation,
of course. Notable precursors were Simon Stevin in his "Haven-Finding Art"
(London 1599; reprinted Amsterdam/New York 1968) and Guillaume de Nautonnier
"M?cometrie de Leymant" (Venes 1603). There is a complete refutation of the
latter`s theory in the second edition of Edward Wright "Certaine errors in
navigation detected and corrected..."(London 1610), the work that laid the
mathematical foundations of the Mercator projection.

More detail on the efforts of finding longitude by variation can be found in
D.W. Waters "Art of Navigation in England in Elizabethan and Early Stuart
Times" (London 1958) and of course in "The Quest for Longitude". A.R.T.
Jonkers "Earth?s Magnetism in the Age of Sail" (Baltimore/London 2003), a
condensation of his dissertation - which is a mine of information concerning
magnetism, compasses and navigation - gives more information on the
background.

Regards,

Wolfgang

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