Book 4.
[Chap. 1]. On Variation.
[Chap. 2]. That the variation is caused by the inæquality of the projecting parts of the earth.
[Chap. 3]. The variation in any one place is constant.
[Chap. 4]. The arc of variation is not changed equally in proportion to the distance of places.
[Chap. 5]. An island in Ocean does not change the variation, as neither do mines of loadstone.
[Chap. 6]. The variation and direction arise from the disponent power of the earth, and from the natural magnetick tendency to rotation, not from attraction, or from coition, or from other occult cause.
[Chap. 7]. Why the variation from that lateral cause is not greater than has hitherto been observed, having been rarely seen to reach two points of the mariners' compass, except near the pole.
[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.