Chap. XV. “The magnetic force imparted to iron is more apparent in an iron rod than in an iron sphere or cube, or iron in any other shape.”
Chap. XVI. “Motion is produced by the magnetic force through solid bodies interposed: of the interposition of a plate of iron.”
Chaps. XVII-XXII. Herein are detailed as many as twelve different experiments to prove the increased efficiency of armed loadstones.
Chap. XXV. “Intensifying the loadstone’s forces.” Magnetic bodies can restore soundness (when not totally lost) to magnetic bodies, and can give to some of them powers greater than they originally had; but to those that are by their nature in the highest degree perfect, it is not possible to give further strength.
Chap. XXVIII. “A loadstone does not attract to a fixed point or pole only, but to every part of a terrella, except the equinoctial line.”
Chap. XXIX. “Of differences of forces dependent on quantity or mass.” Four experiments.
Chaps. XXXVIII and XXXIX are the last, and they treat of the attractions of other bodies and of mutually repellant bodies. All electrics attract objects of every kind: they never repel or propel.
In the preceding Chapter XXXV, Gilbert had alluded to the perpetual-motion engine actuated by the attraction of a loadstone, which we have given an account of at Peter Peregrinus, A.D. 1269.
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.