Royal Society. 525 



system of rings canying- the disc be removed from the rest of the 

 apparatus, and by unfastening the tightening screw let the inner 

 ring be allowed to move freely within the outer. Having set the disc 

 in rapid rotation, hold the outer ring at the extremities of the dia- 

 meter which is in the plane in which the axis of motion of the disc 

 is free to move, then giving to the outer ring a tendency to rotation 

 round that diameter, it will be observed that, in whatever jjosition 

 the axis is, it will fly to place itself in the fixed axis thus determined, 

 and rotation will take place round it in the same direction. Consi- 

 derable resistance is felt so long as the moveable axis is changing 

 its position, but when once it coincides with the fixed axis the 

 rotation of the external ring round its diameter is effected with 

 facility. A slight alternate motion of the outer ring, tending to 

 give to it rotations in opposite directions, will occasion a continued 

 rotation of the moveable axis. The same result takes place when 

 an endeavour is made to rotate the outer ring round an axis per- 

 pendicular to its plane. In all cases when the axis of the rotating 

 disc is free to move in a plane, and the outer ring is constrained to 

 rotate round a line in this plane, the moveable axis will place itself 

 so as to coincide with that line, and so that the disc shall rotate 

 in the same direction as the ring ; if the fixed axis be in a different 

 plane the moveable axis will assume permanently that position in its 

 plane which approaches nearest to the former. The moveable axis 

 is thus apparently attracted towards the fixed axis if the rotations 

 are in the same direction, and repelled from it if the rotations are 

 in opposite directions. 



In the experiments just described the free and constrained axes of 

 rotation intersect, but in Fessel's apparatus they are distant from each 

 other. In the latter case the rule must be thus modified, that the 

 free axis of rotation tends to place itself parallel to the constrained 

 axis of rotation, or to as near a position thereto as possible. By this 

 princijde all the results manifested are easily explained. The beam 

 being in equilibrium, a motion impressed on it round the vertical axis 

 causes it to ascend or descend, because the axis of the rotating disc 

 tends to place itself parallel to tlie vertical axis of rotation and so that 

 the disc rotates in the impressed direction. When the equilibrium of 

 the beam is destroyed, gravity tends to make it rotate round a horizon- 

 tal axis ; the axis of the disc endeavours to place itself parallel with 

 that axis, but both being unchangeably at right angles to each other, 

 the tendency to place itself there gives rise to a continued rotation. 

 Other results with this apparatus, to which 1 have not yet adverted, 

 are similarly explained. Fix the outer ring horizontally and loosen 

 the inner ring, keeping them both however in the same plane ; then, 

 on moving the beam round the vertical axis, the axis of the rotating 

 disc will immediately fly to place itself parallel thereto, with rotation 

 of the disc in the impressed direction. The rings being placed in the 

 veitical plane, the same result will take place if the beam be moved 

 in a vertical [)lane, i. e. round a horizontal axis. 



The following additionil experiments may be made with the rings 

 detached from the apparatus. The results are necessary conse- 

 quences of wliat has been previously explained : — 



