If you magnetize a needle (holding it for awhile over a magnet), and
attach it in the middle to a pivot in such a way that it can move freely
around, and let it loose, it will turn with one end toward midday
(south), and with the other toward midnight (north).
When the magnet was not known, people did not sail far out to sea. When
they went out far into the sea, so that land was not to be seen, they
could tell only by the stars and the sun where they had to sail. But
when it was dark, and the sun or stars could not be seen, they did not
know which way to sail. And a ship was borne by the winds and carried on
rocks and wrecked.
So long as the magnet was not known, they did not sail far from the
shore; but when the magnet was discovered, they made a magnetic needle
on a pivot, so that it should move around freely. By this needle they
could tell in which direction to sail. With the magnetic needle they
began to sail farther away from the shores, and since then they have
discovered many new seas.
On ships there is always a magnetic needle (compass), and there is a
measuring-rope with knots at the stern of a ship. This rope is fixed in
such a way that when it unrolls, they can tell how far the ship has
travelled. And thus, in sailing in a boat, they always know in what spot
it is, whether far from the shore, and in what direction it is sailing.