47 



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 

 principle 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 the 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 I 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 

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

 in a vertical plane, i. e. round a horizontal axis. 



The following additional experiments may be made with the rings 

 detached from the apparatus. The results are necessary conse- 

 quences of what has been previously explained : 



1 . Suspend, by means of a string, the outer ring at the extremity 



