e which looks toward the south tends toward the centre of
the earth, in a certain proportion (to be explained afterward) to the
latitude of the district in question, from the aequator on either side. The
needle, however, must be rubbed on {187} a powerful loadstone; otherwise it
does not dip to the true point, or else it goes past it, and does not
always rest in it. A larger instrument may also be used, whose diameter may
be 10 or 12 digits; but in such an instrument more care is needed to
balance the versorium truly. Care must be taken that the needle be of
steel; also that it be straight; likewise that both ends of the cross-piece
be sharp and fixed at right angles to the needle, and that the cross-piece
pass through the centre of the needle. As in other magnetical motions there
is an exact agreement between the earth and the stone, and a correspondence
manifestly apparent to our senses by means of our experiments; so in this
declination there is a clear and evident concordance of the terrestrial
globe with the loadstone. Of this motion, so important and so long unknown
to all men, the following is the sure and true cause. A magnet-stone is
moved and turned round until one of its poles being impelled toward the
north comes to rest toward a definite point of the horizon. [231]This pole,
which settles toward the north (as appears from the preceding rules and
demonstrations), is the southern, not the boreal; though all before us
deemed it to be the boreal, on account of its turning to that point of the
horizon. A wire or versorium touched on this pole of the stone turns to the
south, and is made into a boreal pole, because it was touched by the
southern terminal of the stone. So if the cusp of a versorium be excited in
a similar manner, it will be directed toward the southern pole of the
earth, and will adjust itself also to it; but the cross (the other end)
will be southern, and will turn to the north of the earth (the earth itself
being the cause of its motion); for so direction is produced from the
disposition of the stone or of the excited iron, and from the verticity of
the earth. But declination takes place when a magnetick is turned round
toward the body of the earth, with its southern end toward the north, at
some latitude away from the aequator. For this is certain and constant,
that exactly under the coelestial aequator, or rather over the aequator of
the terrestrial globe, there is no declination of a loadstone or