CHAP. X.
On the variation of the declination.
irection has been spoken of previously,
and also * variation, which is like a
kind of dragging aside of the direction. Now in declination such irregular motion is also noticed, when the needle dips
beyond the proper point or when sometimes it does not reach its mark. There is therefore a variation of declination,
being the arc of a magnetick meridian between the true and apparent declination. For as, on account of terrestrial
elevations, magnetick bodies are drawn away from the true meridian, so also the needle dips (its rotation being increased
a little) beyond its genuine position. For as variation is a deviation of the direction, so also, owing to the same
cause, there is some error of declination, though often very slight. Sometimes, also, when there is no variation of
direction in the horizontal, there may nevertheless be variation of the declination; namely, either when more vigorous
parts of the earth crop out exactly meridionally, i.e. under the very meridian; or when those parts are less
powerful than nature in general requires; or when the virtue is too much intensified in one part, or weakened in another,
just as one may observe in the vast ocean. And this discrepant nature and varying effect may be easily seen in certain
parts of almost any round loadstone. Inæquality of power is recognized in any part of a terrella by trial of the
demonstration in chap. 2 of this book. But the effect is clearly demonstrated by the instrument for showing declination
in chap. 3 of this book.