Page:On the Magnet - Gilbert (1900 translation of 1600 work).djvu/101



S conceived by astronomers the æquinoctial circle is equidistant from both poles, cutting the world in the middle, measures the motions of their primum mobile or tenth sphere, and is named the zone of the primum mobile. It is called æquinoctial, because when the sun stands in it (which must happen twice in the year) the days are equal to the nights. That circle is also spoken of as æquidialis, wherefore it is called by the Greeks. In like manner it is also properly called Æquator, because it divides the whole frame of the earth between the poles into equal parts. So also an æquator may be rightly assigned to a terrella, by which its power is naturally divided, and by the plane of which permeating through its centre, the whole globe is divided into equal parts both in quantity and strength (as if by a transverse septum) between verticities on both sides imbued with equal vigour.

ERIDIANS have been thought out by the geographer, by means of which he might both distinguish the longitude and measure the latitude of each region. But the magnetick meridians are infinite, running in the same direction also, through fixed and opposite limits on the æquator, and through the poles themselves. On them also the magnetick latitude is measured, and declinations are reckoned from them; and the fixed direction in them tends to the poles, unless it varies from some defect and the magnetick is disturbed from the right way. What is commonly called a magnetick meridian is not really magnetick, nor is it really a meridian, but it is understood to pass through the termini of the variation on the horizon. The variation is a depraved deviation from a meridian, nor is it fixed and constant in various places on any meridian.