Book III
In this Third Book, we learn of the directive (or versorial) force which is called verticitas—verticity—what it is, how it resides in the loadstone, and how it is acquired when not naturally produced; how iron acquires it and how this verticity is lost or altered; why iron magnetized takes opposite verticity; of magnetizing stones of different shapes; why no other bodies save the magnetic are imbued with verticity by friction with a loadstone and why no body which is not magnetic can impart and awaken that force; of disagreements between pieces of iron on the same pole of a loadstone, and how they may come together and be conjoined; that verticity exists in all smelted iron not excited by the loadstone, as shown by its lying, being placed—or, preferably, by hammering hot iron—in the magnetic meridian; that the magnetized needle turns to conformity with the situation of the earth; of the use of rotary needles and their advantages; how the directive iron rotary needles of sundials and the needles of the mariner’s compass are to be rubbed with loadstone in order to acquire stronger verticity.