What was known in Gilbert's TimeThe ancient Greeks knew about lodestones (sometimes spelled loadstones), strange minerals with the power to attract iron. Some were found near the city of Magnesia in Asia Minor (now Turkey), and that city lent its name to all things magnetic.
The early Chinese also knew about lodestones and about iron magnetized by them. Around the year 1000 they discovered that when a lodestone or an iron magnet was placed on a float in a bowl of water, is always pointed south. From this developed the magnetic compass, which quickly spread to the Arabs and from them to Europe. The compass helped ships navigate safely, even out of sight of land, even when clouds covered the stars. Compasses were also built into portable sundials, whose pointers had to face north to give the correct time.
The nature of magnetism and the strange directional properties of the compass were a complete mystery. For instance, no garlic was allowed on board ships, in the mistaken belief that its pungent fumes caused the compass to malfunction. Columbus felt the compass needle was somehow attracted by the pole star, which maintained a fixed position in the northern sky while the rest of the heavens rotated around it.
Two things were noted in those centuries. First, the compass needle did not point exactly north (towards the pole star) but veered off slightly to the east. As Columbus sailed across the Atlantic Ocean, he claimed changes in the direction of the needle. How much of this was the spurious effect of inaccurate observations is still being debated. One letter, written by Columbus in 1498 from Haiti, claimed the compass direction changed from east of true north to west of it, suggesting that the discrepancy might depend on location (as it indeed does).
And second, the force on the needle was not horizontal but slanted downwards into the Earth. If a compass needle was balanced evenly on its pivot before being magnetized, then afterwards its north end would be pulled downwards, and a tip had to be snipped off to restore balance. This was studied by Robert Norman of London, England, who was drawn to investigate this, after snipping off too much of a needle he was preparing, and spoiling it. In 1581 he published his finding in a book, The Newe Attractive.
Norman used a compass needle balanced on a horizontal axis, able to swing in a vertical plane lined up in the north-south direction. That way, the north pointing end of the needle could still point northwards, and it did so, but it was no longer horizontal. Instead it slanted downwards at a steep "dip angle." This demonstrated that the magnetic force was not horizontal, either.