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the earth's axis, the retrogradation of the moon's nodes, 

 the equilibrium of Saturn's rings, and the peculiar effect of 

 a magnet upon an iron disc in rotation. He had been led 

 to the construction of the first of these instruments so long 

 ago as 1835, from the difficulty he experienced while teach- 

 ing astronomy, in explaining the subject of precession by 

 mere verbal description. He conceived that the motions 

 of the common spinning-top might be converted into an 

 exact imitation of those of the earth, by altering the posi- 

 tion of the centre of gravity in reference to the point of 

 support. In the conception of such an instrument, he 

 was not aware at the time that he had been anticipated 

 by others; but he now found that the records of the 

 Royal Astronomical Society contained a short and ob- 

 scure notice of a similar instrument, which had previously 

 been constructed by Mr. Atkinson. M. Bohnenberger's 

 gyroscope had also been previously known on the Con- 

 tinent, and partially in this country, which, among other 

 purposes, exhibited correctly the precession of the equi- 

 noxes ; and Mr. Higginson, now present, had, it ap- 

 peared, constructed an instrument of the same kind. 

 The effect of a magnet on a rotating iron disc had been 

 shewn to him (Professor Elliot) by a friend, but almost 

 accidentally, and without reference to its theory or astro- 

 mical application. Its use for that purpose was to ex- 

 hibit the effect of one planet in disturbing the plane of 

 another planet's orbit, in eight different positions of the 

 disturbing body (as described in Newton's Principia), in 

 producing a forward or a backward movement of the 

 nodes, or an increase or diminution of the obliquity 

 of the plane of its orbit to the ecliptic. Professor Elliot 

 stated that, although his experiment with the iron ring 

 and magnet was admitted to be an exact imitation of 

 the peculiar motion of Saturn's rings, yet the coinci- 

 dence of the two in regard to principle had been disputed 



