As for the variation, I believe the star has the quality of all the four quarters, like the needle, which if touched to the east side points to the east, and so of the west, north, and south; wherefore, he that makes a compass covers the loadstone with a cloth, all but its north part, or that which has the power to make the needle point to the north.

" Astragon (the poet says) discovered the loadstone and its use in navigation.