NATURAL miLOSornr. 179 



Some conversation ensued upon the point thus raised, Mr. Hamilton giv- 

 ing it as his opinion that the effect would be as Mr. Higgins supposed. It 

 would be, he observed, an interesting question whether the force thus applied 

 would produce heat. In every instance where action was arrested heat was 

 developed. M. Foucault arrested the motion of the disk by a magnet, and it 

 became hot. He ascribed it to a mechanical action in arresting the speed of 

 the instrument. The test sxiggested would be an experimentum crucis on the 

 theory of an arrested mechanical action always giving rise to an equivalent 

 amount of heat. 



Professor Elliot thanked Professor Hamilton for the more than ample 

 justice he had dealt out to him in noticing his instruments. He begged 

 leave to make some additional statements regarding his own researches on 

 the subject, as well as those of others. Professor Hamilton had correctly 

 described his (Professor Elliot's) instruments for illustrating, both in regard 

 to fact and principle, the precession of the equinoxes, the mutation of 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 disk in rota- 

 tion. He had been led to the construction of the first of these instruments as 

 long ago as 1835, from the difficulty he experienced, while teaching astrono- 

 my, in explaining the subject of precession by mere verbal description. He 

 conceived that the motion of the common spinning-top might be converted 

 into an exact imitation of those of the earth, by altering the position 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 antici- 

 pated by others ; but he now found that the records of the Royal Asrronoml- 

 cal Society contained a short and obscure notice of a similar instrument, 

 which had previously been constructed by Mr. Atkinson. M. Bohnenber- 

 ger's gyroscope had also been previously known on the Continent, and par- 

 tially in this country, which, among other purposes, exhibited correctly the 

 precession of the equinoxes; and a member of the society, now present, had, 

 it appeared, constructed an instrument of the same kind. The effect of a 

 magnet on a rotating iron disk had been shown to him (Professor Elliot) by 

 a friend, but almost accidentally, and without reference to its theory or 

 astronomical application. Its use for that purpose was to exhibit the effect 

 of one planet in disturbing the plane of another planet's orbit, in eight dif- 

 ferent 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 or the plane of its orbit to the ecliptic. Pro- 

 fessor Elliot stated that although his experiment with the iron ring and mag- 

 net were admited to be an exact imitation of the peculiar motion of Saturn's 

 rings, yet the coincidence of the two in regard to principle had been disputed 

 by some of our best mathematicians. The objection advanced to it by Profes- 

 sor Thomson, of Glasgow, that the one was, to a certain extent, a constrained, 

 and the other a perfectly free motion, was certainly a valid objection; but 

 there were some difficulties attending the subject which required to be 

 removed by further experiments. He (Professor Elliot) had shown the 

 greater part of these experiments to the Liverpool Polytechnic Society in 

 the year 1839. Since that time the subject of rotary motion had become a 

 fashionable study among mathematicians, having been taken up succes- 

 sively by Professors Magnus, Wheatstone, Powell, Foucault, Smythe, and 

 Maxwell. Foucault's experiment for showing the stability (perfect or par- 

 tial) of a rotating disk during the earth's rotation was not new in theory, as 



