that the device is to be mounted like an astronomical
instrument and used like one, rather than as a time teller, or as a
simple demonstration of magnetism. In the second part of the _Epistle_
Peter turns to practical instruments, describing for the first time, the
construction of a magnetic compass consisting of a loadstone or iron
needle pivoted with a casing marked with a scale of degrees. The third
chapter of this section, concluding the _Epistle_, then continues with
the description of a perpetual motion wheel, "elaboured with marvellous
ingenuity, in the pursuit of which invention I have seen many people
wandering about, and wearied with manifold toil. For they did not
observe that they could arrive at the mastery of this by means of the
virtue, or power of this stone."

This tells us incidentally, that the perpetual motion device was a
subject of considerable interest at this time.[42] Oddly enough, Peter
does not now develop his idea of the terrella, but proceeds to something
quite new, a device (see fig. 22) in which a bar-magnet loadstone is to
be set towards the end of a pivoted radial arm with a circle fitted on
the inside with iron "gear teeth," the teeth being there not to mesh
with others but to draw the magnet from one to the next, a little bead
providing a counterweight to help the inertia of rotation carry the
magnet from one point of attraction to the next. It is by no means the
sort of device that one would naturally evolve as a means of making
magnetism work perpetually, and I suggest that the toothed wheel is
another instance of some vague idea of protoclocks, perhaps that of Su
Sung, being transmitted from the East.

[Illustration: Figure 22.--MAGNETIC PERPETUAL MOTION WHEEL illustrated
by Peter Peregrinus; from the edition of S. P. Thompson (see footnote
40).]

The work of Peter Peregrinus is cited by Roger Bacon in his _De
secretis_ as well as in the _Opus majus_ and _Opus minus_. In the first
and earliest of these occurs a description, taken from Ptolemy, of the
construction of the (observing) armillary sphere. He says that this
cannot be made to move naturally by any mathematical device, but "a
faithful and magnificent experimentor is straining to make one out of
such material, and by such a device, that it will revolve naturally with
the diurnal heavenly rotation." He continues with the statement that
this possibility is also suggested by the fact that the motions of
comets, of tides, and