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THE POPULAR SCIENCE MONTHLY 



8 (sweeping of axis of spin about a vertical axis through 0) brings 

 about inertia reactions of the various particles of the spinning wheel 

 which keep the wheel from falling under the pull of gravity; it is only 

 necessary to show that to produce precessional motion there must act 

 upon the gyrostat- wheel an unbalanced torque (the torque due to the 

 pull of gravity upon the overhanging frame and wheel in Fig. 8). 

 Fig. 10 represents a disk spinning in the direction of the curved 

 arrows (in the front view), the spin being represented by the straight 

 arrow 8 in the side view. Imagine the axle of spin to sweep slowly 



Fig. 11. 



Fig. 12. 



around the vertical line CD in the direction of the curved arrows PP. 

 This sweeping of the axle of spin about the line CD constitutes pre- 

 cessional motion, and CD is the axis of precession. Consider the 

 front view of the spinning disk in Fig. 10 ; every particle in the upper 

 half of the disk has a component of its velocity towards the right, and 

 every particle in the lower half of the disk has a component of its 

 velocity towards the left. After a short interval of time the pre- 

 cessional motion moves the edge E of the disk forwards and the edge 

 E' of the disk backwards in the figure, so that the velocity of every 

 particle in the upper half of the disk is turned slightly backwards and 

 the velocity of every particle in the lower half of the disk is turned 

 slightly forwards, that is to say; every particle in the upper half of the 



