Royal Society. 523 



that the beam has no tendency to displace itself in any direction. 



The Gyroscope. 



Not so, however, if the equilibrium be in any way disturbed ; on 

 moving the weight towards the centre of the beam, thus causing 

 the disc to preponderate, it will be observed that if the disc ro- 

 tates from right to left the beam will move round the vertical 

 axis also from right to left; and if the motion of the disc be 

 reversed the rotation of the beam will be reversed also. On 

 causing the equipoise to preponderate contrary effects will take 

 place. The velocity of the rotation of the beam round the ver- 

 tical axis increases in proportion to the disturbance of the equili- 

 brium. It will also be observed that, notwithstanding the increased 

 or diminished action of gravity on the disc, its axis of rotation 

 always preserves the same inclination to the vertical axis at which 

 it has been originally placed. The effect produced is a seeming 

 paradox. When the equilibrium is disturbed while the disc is at 

 rest, the beam being placed in any other position than the vertical, 

 gravity acts so as to turn it round a horizontal axis ; but when the 

 disc is in motion the usual effect of gravity disappears, and there is 

 substituted for it a continued rotation round a vertical axis, that is, 

 round an axis perpendicular to the plane which contains the axes of 

 the two original rotations, 



A similar composition of forces takes place when the disc is caused 

 to rotate while the equilibrium of the beam is maintained, by im- 

 pressing on the beam a rotation round the vertical axis. When the 

 disc rotates from right to left, the slightest pressure tending to pro- 

 duce rotation round the vertical axis in the same direction, causes 

 the end of the beam carrying the disc to ascend, and a pressure in 

 the opposite direction causes it to descend, that is, the beam is con- 

 strained to move round a horizontal axis perpendicular to the vertical 



