THE origins of the magnetic compass are lost in the mists of time. There is a legend that it all began with a shepherd called Magnes who tended his flocks on the slopes of Mount Ida in a region that was then called Asia Minor, and which now forms part of modern Turkey.
Magnes, we are told, noticed that his iron crook and the iron nails of his sandals were inclined to cling to a large black stone; he had discovered what came to be called magnetite, an iron oxide which happens to be naturally magnetic.
The Chinese, some six or seven centuries BC, are believed to have been the first to notice that a piece of magnetite always pointed in the same direction if it was allowed to swing freely. This unique quality had an obvious application as an navigational aid - information which the Chinese passed to the Arabs, and they in turn to the Europeans. By the 13th century the magnetic compass was in common use among western seafarers.
But the "north" indicated by a compass is an elusive concept. The magnetic north pole, unlike its geographical counterpart, does not remain fixed in one spot; it drifts over the years. These days the northern magnetic pole is about 1,000 miles "off position", lying near the coast of Canada; the southern magnetic pole is similarly displaced, being located near the outer edge of the Antarctic ice-cap.
Both lie about 15 degrees of latitude from their respective geographic poles. And to complicate the issue further, they are not direct antipodes; a straight line joining the two does not pass through the geometric centre of the Earth.
Because of this displacement, and for other reasons related to the nature of the Earth's magnetic field, the deviation of the compass needle from true north, called the magnetic declination, varies irregularly as one travels east or west.
Columbus was somewhat taken aback by this phenomenon on his first voyage towards the new world, but he wisely kept his worries to himself; he was afraid that if they knew of this strange occurrence, his crew might become terrified and force him to turn back.
In those olden times, magnets were known as lodestones, because, give or take a vowel or two, they used to "lead" the way. But our ancestors attributed a number of other mysterious and very useful properties to the lodestone.
It was believed, for example, that placed upon the pillow of a guilty wife, the lodestone would make her confess her indiscretions while she slept; the stone could also be used for the treatment of many minor ailments, and even - although we are not told exactly how - as a form of contraceptive.