He shows the effects of cutting a spherical loadstone (which he calls a "terrella") through the poles and equator and the direction of attraction at different points. Magnets act at a distance but the force has no permanent presence and is not hindered like light. Materials including gold, silver and diamonds are not affected by magnets, nor can one produce perpetual motion. The Earth's normal magnetism. He proposes (incorrectly) that the angle of the ecliptic and precession of the equinoxes are caused by magnetism. A loadstone cut out of rock and floated in water returns to the same direction. Iron heated to white heat and cooled lying along a meridian also acquires magnetism. But stroking with other materials fails—he proved this with an experiment with 75 diamonds in front of witnesses. The best way to magnetize a compass (magnetized versorium). Declination. The compass does not always point to true north. There is considerable variation. Using the terrella he shows that variations in the height of the surface can lead to differences but insists that variation is a global issue. In the midst of the ocean or continent there is no variation. He shows how to measure variation and the sources of common errors. Magnetic dip. The angle of inclination (dip) of a compass to the horizon differs according to latitude. He shows how to construct a dip instrument. At the equator it is level and increases towards the poles as he has shown earlier with his terrella. Terrestrial rotation. Heraclides and others held that the Earth rotates from west to east and this is supported by Copernicus (the "restorer of astronomy"), but Aristotle said otherwise. "If the rotations of the earth seems headlong and not to be permitted by nature because of its rapidity, then worse than insane, both as regards itself and the whole universe is the motion of the primum mobile." He rejects the idea of a sphere of the fixed stars for which no proof has been offered and leaves aside the question of other movements of the Earth but "infers not with mere probability, but with certainty the diurnal revolution of the earth." He states that "the cause of the diurnal motion are to be found in the magnetic energy and the alliance of bodies" but offers no further guidance. The inclination of the Earth's pole to the ecliptic produces the seasons. He explains the Precession of the equinoxes as the movement of the Earth's axis.