[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.