When a needle is being excited by a loadstone, begin in the middle, and
draw the needle toward its end; at the end let the application be continued
with a very gentle rubbing around the end for some time; that is to say,
for one or two minutes; do not repeat the motion from the middle to the end
(as is frequently done) for in this way the verticity is injured. Some
delay is desirable, for although the power is imparted instantly, and the
iron excited, yet from the vicinity of the loadstone and a suitable delay,
a more steady verticity arises, and one that is more firmly durable in the
iron. Although an armed stone raises a greater weight of iron than an
unarmed one, yet a needle is not more strongly excited by an armed stone
than by an unarmed one. Let there be two iron wires of the same length,
wrought from the same wire; let one be excited by an armed end, the other
by an unarmed end; it is manifest that the same needles have a beginning of
motion or a sensible inclination at equal distances from the same armed and
unarmed loadstone; this is ascertained by measuring with a longish reed.
But objects which are more powerfully excited move more quickly; those
which are less powerfully excited, more feebly, and not unless brought
rather close; the experiment is made on water with equal corks.

* * * * *

{151} [Illustration]

BOOK FOURTH.

_CHAP. I._

ON VARIATION.

Direction has hitherto been spoken of as if in nature there were no
variation; for in the preceding natural history we wished to omit and
neglect this, inasmuch as in a terrestrial globe, perfect and in every
sense complete, there would be none. Since, however, in fact, the earth's
magnetick direction, owing to some fault and slip, deviates from its right
course and from the meridian, we must extract and demonstrate the obscure
and hidden cause of that variance which has troubled and sore racked in
vain the minds of many. Those who before us have written on the magnetick
movements have made no distinction between direction and variation, but
consider the motion of magnetick iron to be uniform and simple. Now true
direction is the motion of the magnetick body to the true meridian and its
continuance therein with its appropriate ends towards the poles. But it
very often happens at sea and on land that the magnetick iron does not
point to the true pole, and that not only a versorium and magnetick pieces
of iron, and the need