suitable manner of operating. Rich
ores of iron and such as yield a greater proportion of metal are recognized
by means of an iron needle suspended in aequilibrium and magnetically
prepared; and magnetick stones, clays, and earths are distinguished,
whether crude or prepared. An iron needle (the soul of the mariners'
compass), the marvellous director in voyages and finger of God, one might
almost say, indicates the course, and has pointed out the whole way around
the earth (unknown for so many ages). The Spaniards (as also the English)
have frequently circumnavigated (by an immense circuit) the whole globe by
aid of the mariners' compass. Those who travel about through the world or
who sit at home have sun-dials. A magnetick pointer follows and searches
out the veins of ore in mines. By its aid mines are driven in taking
cities; catapults and engines of war are aimed by night; it has been of
service for the topography of places, for marking off the areas and
position of buildings, and for excavating aqueducts for water under ground.
On it depend instruments designed to investigate its own dip and variation.

When iron is to be quickened by the stone, let it be clean and bright,
disfigured by no rust or dirt, and of the best steel[216]. Let the stone
itself be wiped dry, and let it not be damp with any moisture, but let it
be filed gently with some smooth piece of iron. But the hitting of the
stone with a hammer is of no advantage. By these means let their bare
surfaces be joined, and let them be rubbed, so that they may come together
more firmly; not so that the material substance of the stone being joined
to the iron may cleave to it, but they are rubbed gently together with
friction, and (useless parts being rubbed off) they are intimately united;
whence a more notable [Illustration] virtue arises in the iron that is
excited. A is the best way of touching a versorium when the cusp touches
the pole and faces it; B is a moderately good way, when, though facing it,
it is a little way {148} distant from the pole; also in like manner C is
only moderately good on account of the cusp being turned away from the
pole; D, which is farther distant, is hardly so good; F, which is prepared
crosswise along a parallel, is bad; of no virtue and entirely irresponsive
and feeble is the magnetick index L, which is rubbed along the aequator;
oblique and not pointing towards the pole as G, and oblique, not pointing
toward but turned away fr