s to the North; in the case of a piece of iron also, whether it has
been excited by a loadstone or not, the southern end moves toward the
North. An oblong piece of iron of three or four digits' length[196], when
skilfully rubbed with a loadstone, quickly turns north and south. Wherefore
mechanicians, taking a piece of iron prepared in this way, balance it on a
pin in a box, and fit it up with the requisites of a sun-dial; or they
prepare the versorium out of two curved pieces of iron with their ends
touching one another, so that the motion may be more constant. In this way
the mariners' versorium is arranged, which is an instrument beneficial,
useful, and auspicious to sailors for indicating, like a good genius,
safety and the right way. But it must be understood on the threshold of
this argument (before we proceed further) that these pointings of the
loadstone or of iron are not perpetually made {116} toward the true poles
of the world, do not always seek those fixed and definite points, or remain
on the line of the true meridian; but usually diverge some distance to the
East or to the West. Sometimes also at certain places on land or sea they
do indicate exactly the true poles. This discrepancy is called the
_Variation_ of the iron or of the loadstone; and since this is brought
about by other causes, and is merely a certain disturbance and perversion
of the true direction, we are directing our attention in this place to the
true direction of the compass and of the magnetick iron (which would be
equally toward the true poles and on the true meridian everywhere on the
earth, unless other obstacles and an untoward pervertency hindered it). Of
its variation and the cause of the perversion we shall treat in the next
book. Those who wrote about the world and about natural philosophy a
century ago, especially those remarkable elementary philosophers, and all
those who trace their knowledge and training to them down to our own times,
those men, I say, who represented the earth as always at rest and, as it
were, a useless weight, placed in the centre of the universe at an equal
distance from the sky on every side, and its nature to be simple, imbued
only with the qualities of dryness and cold, sought diligently for the
causes of all things and of all effects in the heavens, the stars, the
planets, in fire, air, waters and substances of mixed natures. Never indeed
did they recognize that the terrestrial globe had, besides dryness a