of certain planets also follow that of the Sun and
of the heavens. Only in the _Opus minus_, where he repeats reference to
this device, does he finally reveal that it is to be made to work by
means of the loadstone.

The form of Bacon's reference to Peregrinus is strongly reminiscent of
the statement by Robertus Anglicus, already mentioned as an indication
of preoccupation with diurnally rotating wheels, at a date (1271)
remarkably close to that of the _Epistle_ (1269)--so much so that it
could well be thought that the friend to which Peter was writing was
either Robert himself or somebody associated with him, perhaps at the
University of Paris--a natural place to which the itinerant Peter might
communicate his findings.

The fundamental question here, of course, is whether the idea of an
automatic astronomical device was transmitted from Arabic, Indian, or
Chinese sources, or whether it arose quite independently in this case as
a natural concomitant of identifying the poles of the magnet with the
poles of the heavens. We shall now attempt to show that the history of
the magnetic compass might provide a quite independent argument in
favour of the hypothesis that there was a 'stimulus' transmission.

The Magnetic Compass as a Fellow-traveler from China

The elusive history of the magnetic compass has many points in common
with that of the mechanical clock. Just as we have astronomical models
from the earliest times, so we find knowledge of the loadstone and some
of its properties. Then, parallel to the development of protoclocks in
China throughout the middle ages, we have the evidence analyzed by
Needham, showing the use of the magnet as a divinatory device and of the
(nonmagnetic) south-pointing chariot, which has been confusedly allied
to the story. Curiously, and perhaps significantly the Chinese history
comes to a head at just the same time for compasses and clocks, and a
prime authority for the Chinese compass is Shen Kua (1030-1093) who also
appears in connection with the clock of Su Sung, and who wrote about the
mechanized armillary spheres and other models _ca._ 1086.

Another similarity occurs in connection with the history of the compass
in medieval Europe. The treatise of Peter Peregrinus, already discussed,
provides the first complete account of the magnetic compass with a
pivoted needle and a circular scale, and this, as we have seen, may be
connected with protoclocks and perpetual-motion devices