, to the point
_respective of the magnetick needle_.

Virtually the true meridian is the chief foundation of the whole matter:
when that is accurately known, it will be easy by a mariners' compass (if
its construction and the mode of attachment of the magnetick iron works are
known) or by some other larger horizontal versorium to exhibit the arc of
variation on the Horizon. By means of a sufficiently large nautical
variation compass (two equal altitudes of the sun being observed before and
after midday), the variation becomes known from the shadow; the altitude of
the sun is observed either by a staff or by a rather large quadrant.

On land the variation is found in another way which is easier, and because
of the larger size of the instrument, more accurate. Let a thick squared
board be made of some suitable wood, the surface of which is two feet in
length and sixteen inches in width: describe upon it some semicircles as in
the following figure, only more in number. In the centre let a brass style
be reared perpendicularly: let there be also a movable pointer reaching
from the centre to the outmost semicircle, and a magnetick versorium in a
cavity covered over with glass: then let the board be exactly adjusted to
the level of the Horizon by the plane instrument with its perpendicular;
and turn the lily of the instrument toward the north, so that the versorium
may rest truly over the middle line of the cavity, which looks toward the
point of variation on on the Horizon. Then at some convenient hour in the
morning (eight or nine for instance) observe the apex of the shadow thrown
by the style when it reaches the nearest semicircle and mark the place of
the apex of this shadow with chalk or ink: then bring round the movable
index to that mark, and observe the degree on the Horizon numbered from the
lily, which the index shows. In the afternoon see when the end of the
shadow shall again reach the periphery of the same semicircle, and,
bringing the index to the apex of the shadow, seek for the degree on the
other side of the lily. From the difference of the degrees becomes known
[Illustration] {172} the variation; the less being taken from the greater,
half the remainder is the arc of variation. The variation is sought by many
other instruments and methods in conjunction with a convenient mariners'
compass; also by a globe, by numbers, and by the ratios of triangles and
sines, when the latitude is known and one observation is