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Lisa asked-

>Our hand bearing compass is one designed for the southern hemisphere, as
>we're in Australia. I remember being taught that it's important to have
>a southern hemisphere compass in the southern latitudes and a northern
>hemisphere compass in the northern latitudes.
>
>What's the principle involved here, and how did the early navigators (eg
>going from Europe to rounding Cape Horn etc.) compensate?

Good question. It's the dip angle of the magnetic field, which does its
best to pull down one end of the magnet(s), therefore one side of the
compass card. It can be a real problem in compasses such as the
French-designed (by Morin, originally) Opti-compas or Mini-compass (superb
in all other respects), because the pillbox shape doesn't allow for much
tilt of the card.You need to hold it rather level, to view the horizon
across the top. If the card isn't balanced horizontally it can foul on the
housing.

In the days before liquid-damped compasses, you could simply lift the glass
lid and drop a spot of sealing wax under the card, and this could be done
as often as necessary on a voyage to maintain the balance. Some designs had
a small movable weight or you could add a twist of wire under the card.

George.

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contact George Huxtable by email at george@huxtable.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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