On the Magnet, by William Gilbert

Chap. x.

Why in various places near the pole the variations are much more ample than in a lower latitude.

V ariations are often slight, and generally null, when the versorium is at or near the earth's æquator. In a higher Latitude of 60, 70 or 80 deg. there are not seldom very wide variations. The cause of this is to be sought partly from the nature of the earth and partly from the disposition of the versorium. The earth turns magnetick bodies and at the æquator directs them strongly toward the pole: 227at the poles there is no direction, but only a strong coition through the congruent poles. Direction is therefore weaker near the poles, because by reason of its own natural tendency to turn, the versorium dips very much, and is not strongly directed. But since the force of those elevated lands is more vigorous, for the virtue flows from the whole globe, and since also the causes of variation are nearer, therefore the versorium deflects the more from its true direction toward those eminences. It must also be known that the direction of the versorium on its pin along the plane of the Horizon is much stronger at the æquator than anywhere else by reason of the disposition of the versorium; and this direction falls off with an increase of latitude. For on the æquator the versorium is, following its natural property, directed along the plane of the horizon; but in other places it is, contrary to its natural property, compelled into æquilibrium, and remains there, compelled by some external force: because it would, according to its natural property, dip below the horizon in proportion to the latitude, as we shall demonstrate in the book On Declination. Hence the direction falls off and at the pole is itself nothing: and for that reason a feebler direction is easily vanquished by the stronger causes of variation, and near the pole the versorium deflects the more from the meridian. It is demonstrated by means of a terrella: if an iron wire of two digits length be placed on its æquator, it will be strongly and rapidly directed toward the poles along the meridian, but more weakly so in the mid-intervals; while near the poles one may discern a precipitate variation.

227 Page 168, line 29. Page 168, line 33. Directio igitur inualidior est propè polos. Here as in many passages direction means the force which directs. A similar usage prevails with the nouns variation and declination, meaning frequently the force causing variation or declination respectively.

Page 172, line 13. perquirere. The edition of 1633 reads perquirero, in error.

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Last updated Friday, March 14, 2014 at 22:17