58 



(1) the only efiect it can produce is a horizontal motion 

 about the vertical axis. 



4. When one end of the axis of the revolving disc is 

 supported by a string, and the other end left without a 

 support, the disc does not fall, but revolves horizontally 

 roinid the string as a centre. The action of gravity tends 

 to turn the disc round a horizontal axis, and we have seen 

 (1) that the rotating disc resists all forces Avhich tend to 

 alter its plane of rotation. When the speed is sufficiently 

 great it resists the force of gravity, therefore it cannot 

 fall ; but it moves horizontally round the string, because 

 the action of gravity tends to turn the disc round a 

 horizontal axis, as in Exp. 3, and its motion in a horizontal 

 direction round the string is owing to the same cause. 



5. When the disc is inclined to one side, a motion fre- 

 quently ensxies round a vertical axis. This is caused by 

 the friction of the pivots on the outer ring. When the 

 axis of the disc is perfectly horizontal, the friction has no 

 tendency to produce this motion. When the motion com- 

 mences, the inclination of the disc continually increases, 

 because the rotation of the disc tends to carry any point 

 of the circumference in the direction of a tangent, and the 

 motion round a vertical axis turns the plane of the disc out 

 of that direction, the inertia of the point assumed acts as 

 described in (2). This increases the inclination, the 

 friction now acts at a greater advantage, thus the motion 

 about the vertical axis is accelerated, which again in- 

 creases the inclination of the disc, bringing it still nearer 

 the horizontal position. At length this position is attained, 

 and now the whole force of friction is employed in pro- 

 ducing a motion roiuid the vertical axis. 



6. When the gyroscope is fastened by a screw or clamj^ 

 in such a manner as to keep the rotating disc always 

 vertical, and placed in the centre of a turning table, it 

 maintains its plane of rotation unchanged, notwithstanding 



