received
word of the invention of the telescope in the Low Countries, might
easily lead to the re-invention of just such perpetual-motion wheels as
we have already noted. In many ways, once the idea has been suggested it
is natural to associate such a perpetual motion with the incessant
diurnal rotation of the heavens. Without some such stimulus however it
is difficult to explain why this association did not occur earlier, and
why, once it comes there seems to be such a chronological procession
from culture to culture.

We now turn to what is undoubtedly the most curious part of this story,
in which automatically moving astronomical models and perpetual motion
wheels are linked with the earliest texts on magnetism and the magnetic
compass, another subject with a singularly troubled historical origin.
The key text in this is the famous _Epistle on the magnet_, written by
Peter Peregrinus, a Picard, in an army camp at the Siege of Lucera and
dated August 8, 1269.[40] In spite of the precise dating it is certain
that the work was done long before, for it is quoted unmistakably by
Roger Bacon in at least three places, one of which must have been
written before _ca._ 1250.[41]

The _Epistle_ contains two parts; in the first there is a general
account of magnetism and the properties of the loadstone, closing with a
discussion "of the inquiry whence the magnet receives the natural virtue
which it has." Peter attributed this virtue to a sympathy with the
heavens, proposing to prove his point by the construction of a
"terrella," a uniform sphere of loadstone which is to be carefully
balanced and mounted in the manner of an armillary sphere, with its axis
directed along the polar axis of the diurnal rotation. He then
continues:

Now if the stone then move according to the motion of the
heavens, rejoice that you have arrived at a secret marvel.
But if not, let it be ascribed rather to your own want of
skill than to a defect of Nature. But in this position, or
mode of placing, I deem the virtues of this stone to be
properly conserved, and I believe that in other positions
or parts of the sky its virtue is dulled, rather than
preserved. By means of this instrument at all events you
will be relieved from every kind of clock (horologium),
for by it you will be able to know the Ascendant at
whatever hour you will, and all other dispositions of the
heavens which Astrologers seek after.

It should be noted