48 



of a diameter perpendicular to the axis of the inner ring; and, 

 having loosened the latter, place it at right angles to the former. 

 On causing the disc to rotate, its axis will retain its original posi- 

 tion ; but if the slightest effort be made to turn the outer ring round 

 the vertical line, the axis of the rotating disc will instantly fly into 

 this position, and the disc will move in the same direction as that of 

 the impressed rotation. 



2. The horizontality of the loose inner ring being restored, if a 

 weight be suspended from either end of the axis of the disc, that axis 

 will, while it preserves its horizontal or any inclined position, re- 

 volve round the vertical line ; the direction of the motion will change 

 if either the weight be applied to the opposite end of the axis or the 

 disc rotate in the opposite direction. If this rotation be arrested, 

 gravity will immediately cause the weighted end of the axis to 

 descend. 



3. Clamp the rings together either in the same plane or at right 

 angles to each other, and fasten a string, in the first case, at the ex- 

 tremity of a diameter coinciding with the axis of the inner ring, and 

 in the latter case at the extremity of a diameter perpendicular thereto. 

 Having set the disc spinning, if a rotation round the vertical line be 

 given to the system the axis of the disc will ascend, carrying with it 

 the disc and rings notwithstanding their weight, and, even when the 

 impressed rotation has ceased to act, will continue to rotate in the 

 same direction until the motion of the disc ceases. 



In this note I have purposely avoided entering into the mathema- 

 tical theory of the phenomena, my intention having been solely to 

 describe the apparatus exhibited and to give an intelligible account of 

 its effects. Those who wish to investigate the subject more pro- 

 foundly, will find the best guide in the Astronomer Royal's essay 

 on Precession and Nutation published in his Mathematical Tracts. 



