s. From a plank of wood let a smooth and
circular instrument be prepared, at least six digits in diameter, and affix
this to the side of a square pillar, which stands upright on a wooden base.
Divide the periphery of this instrument into 4 quadrants: then each
quadrant into 90 degrees. At the centre of the instrument let there be
placed a brass peg, at the centre of the end of which let there be a small
hollow, well polished. To this wooden instrument let a brass circle or ring
be fixed, about two digits in width, with a thin plate or flat rod of the
same metal, representing the horizon, fixed across it, through the middle
of the circle. In the middle of the horizontal rod let there be another
hollow, which shall be exactly opposite the centre of the instrument, where
the former hollow was made. Afterward let a needle be fashioned out of
steel, as versoria are accustomed to be made. Divide this at right angles
by a thin iron axis (like a cross) through the very middle and centre of
the wire and the cross-piece. Let this dipping-needle be hung (with the
ends of the cross resting in the aforesaid holes) so that it can move
freely and evenly on its axis in the most perfect aequilibrium, so
accurately that it turns away from no one point or degree marked on the
circumference more than from another, but that it can rest quite easily at
any. Let it be fixed upright to the front part of the pillar, whilst at the
edge of the base is a small versorium to show direction. Afterward touch
the iron, suspended by this ingenious method, on both ends with the
opposite ends of a loadstone, according to the scientifick method, but
rather carefully, lest the needle be twisted in any way; for unless you
prepare everything very skilfully and cleverly, you will secure no result.
Then let another brass ring be prepared, a little larger, so as to contain
the former one; and let a glass or a very thin plate of mica be fitted to
one side of it. When this is put over the former ring, the whole space
within remains inclosed, and the versorium is not interfered with by dust
or winds. Dispose the instrument, thus completed, perpendicularly on its
base, and with the small versorium horizontal, in such a way that, while
standing perpendicularly, it may be directed toward the exact magnetical
point respective. Then the end of the needle which looks toward the north
dips below the horizon in northern regions, whilst in southern regions the
end of the needl