The variation in any one place
_is constant_.
[Illustration]
Vnless there should be a great dissolution of a continent and a subsidence
of the land such as there was of the region Atlantis of which Plato and the
ancients tell, the variation will continue perpetually immutable; the arc
of the variation remains the same in the same place or region, whether it
be at sea or on land, as in times past a magnetick body has declined toward
the East or the West. The constancy of the variation and the pointing of
the versorium to a definite point on the horizon in individual regions is
demonstrated by a small versorium placed over a terrella the surface of
which is uneven: for it always deviates from the meridian by an equal arc.
It is also shown by the inclination of a versorium toward a second magnet;
although in reality it is by the turning power of the whole, whether in the
earth or in a terrella. Place upon a plane a versorium whose cusp is
directed toward the north A: place beside it a loadstone, B, at such a
distance that the versorium may turn aside toward B to the point C, and not
beyond. Then move the needle of the versorium as often as you will (the box
and the loadstone not being moved), and it will certainly always return to
the point C. In the same manner, if you {160} placed the stone so that it
may be truly directed toward E, the cusp always reverts to E, and not to
any other point of the compass. Accordingly, from the position of the land
and from the distinctive nature of the highest parts of the earth (certain
terrene and more magnetick eminences of the regions prevailing), the
variation indeed becomes definite in one and the same place, but diverse
and unæqual from a change of place, since the true and polar direction
originating in the whole terrestrial globe is diverted somewhat toward
certain stronger eminences on the broken surface.
* * * * *