BOOK THIRD.
Chap. I.
ON DIRECTION.
n referring to the earlier books it will be found shown that a loadstone has its poles, and that a piece of iron has also poles, and rotation, and a certain verticity; finally, that the loadstone and the iron direct their poles toward the poles of the earth. Now, however, we must make clear the causes of these things and their admirable workings, pointed out indeed before, but not proven. All those who have written before us about these rotations have left us their opinions so briefly, so meagrely, and with such hesitating judgment that they seem hardly likely ever to persuade anyone, or even to be able to satisfy themselves; and all their petty reasons are rejected by the more prudent as useless, uncertain, and absurd, being supported by no proofs or arguments; whence also magnetick science, being all the more neglected and not understood, has been in exile. The true austral pole of a loadstone, not the boreal (as all before us used to think), *if the loadstone is placed in its boat on the surface of water, turns 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, 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 madetoward
