The Epistle contains two parts; in the 

 first there is a general account of mag- 

 netism and the properties of the load- 

 stone, closing with a discussion "of the 

 in(|uii y whence the magnet receives the 

 natural \irtuc 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 load- 

 stone 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 il 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 con- 

 served, 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 e\ents \ou will be relieved from every 

 kind of clock (horologium), for by it you will 

 be able to know the .\scendant at whatever hour you will, 

 and all other dispositions of the heavens which Astrologers 

 seek after. 



It should be noted 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 demon- 

 stration of magnetism. In the second part of the 

 Ejnstle Peter turns to practical instruments, describing 

 for the fust time, the construction of a magnetic com- 

 pass consisting of a loadstone or iron needle pivoted 

 with a casing inarkcd 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 persuit 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 



Figure 22. — Magnetic Perpetual Motion Wheel 

 illustrated by Peter Peregrinus; from the edition of 

 S. P. Thompson (see footnote 40}. 



time.''- 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. 



The work of Peter Peregrinus is cited by Roger 

 Bacon in his De seaetis as well as in the Opus majus 



*' I have wondered whether the medieval interest in perpetual 

 motion could be connected with the use of the "Wheel of 

 Fortune" in churches as a substitute for bell-ringing on Good 

 Friday. Unfortunately I can find no evidence for or against 

 the conicctiirc. 



PAPER 6: CLOCKWORK, PERPETUAL MOTION DEVICES, AND THE COMPASS 



109 



