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.