On Variety of Strength due to Quantity
_or Mass_.
Quite similar in potency are those stones which are of the same mine, and
not corrupted by adjacent ores or veins. Nevertheless that which excels in
size shows greater powers, since it seizes greater weights and has a wider
orbe of virtue. For a loadstone weighing one ounce does not lift a large
nail as does one weighing a pound, nor does it rule so widely, nor extend
its forces; and if from a loadstone of a pound weight a portion is taken
away, something of its power will be seen to go also; for when a portion is
abstracted the virtue is lessened. But if that part is properly applied and
united to it, though it is not fastened * to nor grown into it, yet by the
application it obtains its pristine power and its vigour returns.
Sometimes, however, when a part is taken away, the virtue turns out to be
stronger on account of the * bad shape of the stone, namely, when the
vigour is scattered through inconvenient angles. In various species the
ratio is various, for one stone of a drachm weight draws more than another
of twenty pounds. Since in very many the influence is so effete that it can
hardly be perceived, those weak stones are surpassed by prepared pieces of
clay. But, it may be asked[179], if a stone of the same species and
goodness weighing a drachm would seize upon a drachm of iron, would a stone
of an ounce weight seize on an ounce, a pound on a pound, and so on? And
this is indeed true; for it both strains and remits its strength
proportionately, so that if a loadstone, one drachm of which would attract
one drachm of iron, were in equal proportion applied either to a suitably
large obelisk or to an immense pyramid of iron, it would lift it directly
in such {98} proportion and would draw it towards itself with no greater
effort of its nature or trouble than a loadstone of a drachm weight
embraces a drachm. But in all such experiments as this let the vigour of
the magnets be equal; let there be also a just proportion in all of the
shapes of the stones, and let the shape of the iron to be attracted be the
same, and the goodness of the metal, and let the position of the poles of
the loadstones be most exact. This is also no less true in the case of an
armed loadstone than of an unarmed one. For the sake of experiment, let
there be given a loadstone of eight ounces weight, which when armed lifts
twelve ounces of iron; if you cut off from that loadstone a certain
portion, which when it has been * reduced to the shape of the former whole
one is then only of two ounces, such a loadstone armed lifts a piece of
iron applied to it of three ounces, in proportion to the mass. In this
experiment also the piece of iron of three ounces ought to have the same
shape as the former one of twelve ounces; if that rose up into a cone, it
is necessary that this also in the ratio of its mass should be given a
pyramidal shape proportioned to the former.
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