a
great circle or magnetick meridian passing through the Zenith or vertex,
and the point of variation on the horizon, and the lowest point of the
heavens, which they call the Nadir. This fact is shown by placing a rather
long magnetick versorium on one side a little way from the vase. This is a
demonstration of a more absolute conformity of a magnetick body with the
earth's body as regards unity; in it is made {204} apparent, in a natural
manner, the direction, with its variation, and the declination. But it must
be understood that as it is a curious and difficult experiment, so it does
not remain long in the middle of the water, but sinks at length to the
bottom, when the cork has imbibed too much moisture.

* * * * *

CHAP. X.

On the variation of the declination.

Direction 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. Inaequality 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.

* * * * *

{205} CHAP. XI.

On the essential magnetick activity sphaerically
_effused._

Dis