44 



disc, the axis of rotation of which is in a line with the beam ; at the 

 opposite extremity is a shifting weight by means of which the equi- 

 librium of the beam may be established or disturbed at pleasure. 



The Gyroscope. 



If the beam be brought into equilibrium, and the disc be rapidly 

 rotated, by means of a thread unrolled from its axis, it will be seen 

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

 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 



