CHAP. VII.
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.
he earth, by reason of lateral eminences
of the stronger globe, diverts iron and loadstone by some degrees from the true pole, or true meridian. As, for example,
with us English at London it varies eleven degrees and ⅓: in some other places the variation is a little greater, but in
no other region is the end of the iron ever moved aside very much more from the meridian. For as the iron is always
directed by the true verticity of the earth, so the polar nature of the continent land (just as of the whole terrene
globe) acts toward the poles: and even if that mass divert magnetick bodies from the meridian, yet the verticity of those
lands (as also of the whole earth) controls and disposes them so that they do not turn toward the East by any greater
arc. But it is not easy to determine by any general method how great the arc of variation is in all places, and how many
degrees and minutes it subtends on the horizon, since it becomes greater or less from diverse
causes. For both the strength of true verticity of the place and of the elevated regions, as well as their distances from
the given place and from the poles of the world, must be considered and compared; which indeed cannot be done exactly:
nevertheless by our method the variation becomes so known that no grave error will perturb the course at sea. If the
positions of the lands were uniform and straight along meridians, and not defective and rugged, the variations near lands
would be simple; such as appear in the following figure.
This is demonstrated by a long loadstone the poles of which are in the ends A B; let C D be the middle line and the æquinoctial, and let G H and E F (the lines) be for meridians on which versoria are disposed, the variations of which are greater at a greater distance from the æquator. But the inequalities of the maritime parts of the habitable earth, the enormous promontories, the very wide gulfs, the mountainous and more elevated regions, render the variations more unequal, or sudden, or more obscure; and, moreover, less certain and more inconstant in the higher latitude.