Chapter XLVIII: Book IV: chap. ii. Variation is due to inequality among the

earth’s elevations.

Book V. chap. ii. Illustration of the direction and dip of a
_terrella_ representing the earth relative to the standard
representation of the globe of the earth, at north latitude 50°.

Book V. chap. iii. Instrument for showing by the action of a
loadstone, the degree of dip below the horizon in any latitude.

Book V. chap. vi. Of the ratio of dip to latitude and the cause
thereof.