178 ANNUAL OF SCIENTIFIC DISCOVERY. 



Frisi, in 1700. It was discussed in an abstract form by Ponsot in a Memoiro 

 read before the Academy of Sciences at Paris, and by Poisson, in the Journal 

 de I'Ecole Polyteclmique. The same author had treated the subject at great 

 length in his Traitede Me'dianiqne. Professor Airey also had handled it in 

 a very elegant manner in his "Tract on Precession." As the subject was 

 not capable of being discussed without the use of the higher mathematics, it 

 had not been alluded to by any of the popular writers on mechanics. The 

 method of experimentally illustrating compound rotations was accidentally 

 discovered by Fessel, of Cologne. The instrument which he invented was 

 called poly trope, or gyroscope. It was modified in form and greatly im- 

 proved by Pluckcr, Magnus, Wheatstone, and Foucault. M. Foucault, after 

 making the beautiful pendulum experiment, which afforded the first jiositive 

 demonstration of the earth's rotation, considered that the gyroscope might 

 be employed for the same purpose, as it possessed the property of " main- 

 taining the plane of its rotation unchanged." In experimenting with this 

 instrument, Professor Hamilton referred particularly to its alleged properties 

 of " maintaining the plane of rotation unchanged," and the " fixity of the 

 plane of rotation." He remarked upon the loose sense in which these terms 

 had been employed in descriptions of the gyroscope, and pointed out the 

 restricted and qualified sense in which they ought to be used. The plane of 

 rotation was not fixed in an absolute sense. It held a fixed relation to the 

 plane of a great circle of the earth, and participated in the earth's diurnal 

 motion, which was illustrated by a mathematical diagram. The time of its 

 making an apparent revolution round a circle at the pole was twenty-four 

 hours. The time at any other place varied inversely as the sine of the lati- 

 tude. This was also submitted to a mathematical proof, and the point, in 

 common with every other embraced in the lecture, rendered beautifully sim- 

 ple and clear. The gyroscope was then considered as a mechanical com- 

 pass. Since the free axis of the gyroscope tended to become parallel to any 

 other axis about which the disk was constrained to revolve, and since it was 

 constrained to revolve about the axis of the earth, the free axis tended to 

 become parallel to the axis of the earth. Hence, at any given station, the 

 four parts of the horizontal ring indicated the cardinal points of the hori- 

 zon, the extremity of the axis coincided with the celestial pole, and the plane 

 of the disk with the plane of the celestial equator. And since the elevation 

 of the pole above the horizon was equal to the latitude of the place, the ele- 

 vation of the axis above the horizontal plane was equal to the latitude of the 

 station. The gyroscope was therefore theoretically a mechanical compass, 

 but practically it was of no value, since friction on the pivots produced a 

 horizontal motion in one direction or the other, if the ring deviated in the 

 slightest degree from the true horizontal position. For the same reason the 

 gyroscope afforded no satisfactory proof of the earth's diurnal rotation. It 

 would indeed be a valuable discovery if a method could be devised of mak- 

 ing it, with certainty, deviate from the truth equally in opposite directions, 

 and thus, by a mutual destruction of errors, indicate the true north. Mr. 

 Hamilton then described Professor Elliot's apparatus for illustrating the prc- 

 ccssion of the equinox and the mutation of the earth's axis. 



The Rev. H. H. Higgins put the following question: If the gyroscope 

 were revolving in a vertical plane, the axis being horizontal, and force were 

 applied to incline the axis, where did that force go to? Was it expended in 

 producing an increased friction on the pivot? And if so, would the ultimate 

 result be to arrest the motion of the disk? 



