Chap. VII. “Of the potency of the magnetic force, and of its spherical extension.” The magnetic energy is not hindered by any dense or opaque body, but goes out freely and diffuses its force every whither: in the case of the terrella, and in a spherical loadstone, it extends outside the body in a circle, but, in the case of an oblong loadstone, it extends into an area of form determined by the shape of, and is everywhere equidistant from, the stone itself. Chap. XIII. “Of the magnetic axis and poles.” 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