h (as they not infrequently are
fixed in buildings and across windows), those rods, I say, by that long
lapse of time acquire verticity and turn round, whether hanging in the air,
or floating (being placed on cork), to the pole toward which they were
pointing, and magnetically attract and repel a balanced iron magnetick; for
the long continued position of the body toward the poles is of much avail.
This fact (although conspicuous by manifest experiments) is confirmed by an
incident related in an Italian letter[213] at the end of a book of Maestro
Filippo Costa, of Mantua, _Sopra le Compositioni degli Antidoti_ written in
Italian, which translated runs thus: "A druggist of Mantua showed me a
piece of iron entirely changed into a magnet, drawing another piece of iron
in such a way that it could be compared with a loadstone. Now this piece of
iron, when it had for a long time held up a brick ornament on the top of
the tower of the church of St. Augustine at Rimini, had been at length bent
by the force of the winds, and remained so for a period of ten years. When
the monks wished to bend it back to its former shape, and had handed it
over to a blacksmith, a surgeon named Maestro Giulio Caesare discovered
that it was like a magnet and attracted iron." This was caused by the
turning of its extremities toward the poles for so long a time. And so what
has been laid down before about change of verticity should be borne in
mind; how in fact the poles of iron spikes are altered, when a loadstone is
placed against them only with its pole and points toward them, even at a
rather long distance. Clearly it is in the same way that that large magnet
also (to wit, the earth itself) affects a piece of iron and changes its
verticity. For, although the iron may not touch the pole of the earth, nor
any magnetick part of the earth, yet verticity is acquired and changed; not
because the poles of the earth and the point itself which is 39deg distant
{142} from our city of London, changes the verticity at a distance of so
many miles; but because the whole magnetick earth, that which projects to a
considerable height, and to which the iron is near, and that which is
situated between us and the pole, and the vigour existing within the orbe
of its magnetick virtue (the nature of the whole conspiring thereto),
produces the verticity. For the magnetick effluence of the earth rules
everywhere within the orbe of its virtue, and transforms bodies; but those