310 James Elliot on certain 



A Description of certain Mechanical Illustrations of the 

 Planetary Motions, accompanied by Theoretical Investi- 

 gations relating to them, and, in particular, a new Ex- 

 planation of the Stability of Equilibrium of Saturn's Rings. 

 By James Elliot, Teacher of Mathematics, Edinburgh * 



Orreries, as they are called, have been constructed with 

 much elaborate ingenuity, and rendered capable of exhibit- 

 ing the motions of the planets to a surprising degree of accu- 

 racy ; but they are so complicated and cumbrous in their 

 machinery — so constrained in their movements — so totally 

 different from that which they represent, in regard to their 

 moving principles (their toothed wheels, pulleys, and inclined 

 planes being utterly unlike the laws of attraction and iner- 

 tia) — that they are seldom regarded in any other light than 

 as mechanical curiosities, and are rarely used for explaining 

 the subject of astronomy. In them we look in vain for imita- 

 tions of 



" Heaven's easy, artless, unencumbered plan" 



(to borrow a description applied to a higher subject), and 

 long for illustrations more simple, and governed by laws 

 more nearly related to those which govern the planets them- 

 selves. 



On first commencing the study of astronomy myself, and 

 endeavouring to obtain a distinct conception of that motion 

 of the earth which gives rise to the precession of the equi- 

 noxes, it occurred to me that I had seen the same motion in 

 spinning a hoop or a halfpenny. Thence I traced it to the 

 top and the te- to turn. Afterwards, in teaching the subject, 

 it appeared to me that, if I could reduce their untractable 

 movements to some degree of management, I might obtain a 

 useful auxiliary to my explanations. There is so much dif- 

 ficulty in imparting to learners a distinct idea of the motion 

 alluded to, — in making them conceive the possibility of a 

 rotation of the earth about its axis in one direction, and 



* Read before the Royal Scottish Society of Arts, 27th February and 13lh 

 March 1854 ; and the Silver Medal, value Ten Sovereigns, awarded. 



