Chap. 8. On the construction of the common mariners' compass, and on the diversity of the compasses of different nations. Chap. 9. Whether the terrestrial longitude can be found from the variation. Chap. 10. Why in various places near the pole the variations are much more ample than in a lower latitude. Chap. 11. Cardan's error when he seeks the distance of the centre of the earth from the centre of the cosmos by the motion of the stone of Hercules; in his book 5, _On Proportions_. Chap. 12. On the finding of the amount of variation: how great is the arc of the Horizon from its arctick to its antarctick intersection of the meridian, to the point respective of the magnetick needle. Chap. 13. The observations of variation by seamen vary, for the most part, and are uncertain: partly from error and inexperience, and the imperfections of the instruments: and partly from the sea being seldom so calm that the shadows or lights can remain quite steady on the instruments. Chap. 14. On the variation under the æquinoctial line, and near it. Chap. 15. The variation of the magnetick needle in the great Æthiopick and American sea, beyond the æquator. Chap. 16. On the variation in Nova Zembla. Chap. 17. Variation in the Pacifick Ocean. Chap. 18. On the variation in the Mediterranean Sea. Chap. 19. The variation in the interior of large Continents. Chap. 20. Variation in the Eastern Ocean. Chap. 21. How the deviation of the versorium is augmented and diminished by reason of the distance of places. _Book 5._ Chap. 1. On Declination. Chap. 2. Diagram of declinations of the magnetick needle, when excited, in the various positions of the sphere, and horizons of the earth, in which there is no variation of the declination. Chap. 3. An indicatory instrument, showing by the virtue of a stone the degrees of declination from the horizon of each several latitude. Chap. 4. Concerning the length of a versorium convenient for declination on a terrella. Chap. 5. That declination does not arise from the attraction of the loadstone, but from a disposing and rotating influence. Chap. 6. On the proportion of declination to latitude, and the cause of it. Chap. 7. Explanation of the diagram of the rotation of a magnetick needle. Chap. 8. Diagram of the rotation of a magnetick needle, indicating magnetical declination in all latitudes, and from the rotation and declination, the latitude itself. Chap. 9. Demonstration of direction, or of variation from the true direction, at the same time with declination, by means of only a single motion in water, due to the disposing and rotating virtue. Chap. 10. On the variation of the declination. Chap. 11. On the essential magnetick activity sphærically effused. Chap. 12. Magnetick force is animate, or imitates life; and in many things surpasses human life, while this is bound up in the organick body. _Book 6._ Chap. 1. On the globe of the earth, the great magnet.