Rotatory Motion, the Gyroscope, 4'C- 151 



is the effect of the weight of the wheel compounded into the rotatory 

 motion. The wheel is weighing just as much when spinning as when 

 at rest ; but the fact of its weight, or the attraction of gravitation on 

 it, is not to be looked for in the downward direction, as with quiescent 

 bodies, but in a dii-ection at right angles to it, that is, in the horizontal. 

 . In this horizontal motion, then, is the desired explanation of 

 the mystery, which turns out after all to be no paradox, no upsetting 

 of previously ascertained laws of matter." With all due deference to 

 Professor Smyth, I must submit that his proposed explanation leaves 

 the matter as paradoxical as before, — for he says, that the attraction of 

 gravitation is not to be looked for in a downward direction — that is, he 

 annihilates the downward tendency of gravity, and substitutes a hori- 

 zontal motion for it. In his explanation, the Professor does not appear 

 to have fully worked out his own very good rule of observing everything, 

 otherwise he would have perceived that the attraction of gravitation is 

 still to be looked for in a downward direction ; but that it takes effect 

 upon the point of support of the instrument, instead of acting as it 

 would, were the wheel not spinning. 



The report pubhshed of Mr. Elliot's discourse of the 9th April, 1855, 

 shows that he then possessed a much greater knowledge of the subject 

 of rotatory motions, than when he wrote the paper delivered the year 

 before, and to which I referred at the commencement of this paper ; 

 but it does not appear from the report of his later discourse, that he 

 corrected what was erroneous in his earlier paper, which I shall now 

 proceed to discuss, pi'emising that I admire, as much as any one, his 

 exceedingly ingenious mechanical illustrations of some of the planetary 

 motions ; and that the following remarks refer solely to his explanation 

 of the phenomena.* 



As already mentioned, Mr. Elliot's explanation is peculiar to himself. 

 He applies it, in the first instance, to the case of the peg top, the con- 

 tinued conical motion of which, without falling, when spinning in an 

 inclined position, is precisely the same as the precessional motion of the 

 earth and gyroscope. He says, that the fallacy of the popular idea tliat 

 the top's not falling is due to centrifugal force, is well exposed by Dr. 

 Arnott. Centrifugal force, however, as I have shown, has a great deal, 

 — though not everything, — to do with the phenomena ; but the popular 

 idea refers the centrifugal force to the geometric axis of the top, whereas 

 the centrifugal forces actually in operation have reference to another 

 axis. Mr. Elliot next proceeds to show the insufficiency of Dr. Arnott's 



• On reading this paper I was informed that the error of some of Mr. Elliot's explana- 

 tions had been demonstrated when his paper was read at Glasgow. As, however, I have 

 seen no publisliod correction of his explanations, I have considered it useful to publish 

 this portion of m\- paper entire as read. 



