MECHANICAL PARADOXES. 



movement, something must be done by the 

 thrower at the moment of despatching it. 



He causes it to revolve in an almost hori- 

 zontal plane that is, on an almost perpen- 

 dicular axis. 



We have seen, in the section on gyrostatic 

 movements, that a revolving body strongly 

 resists attempts to change the plane or axis 

 of its revolution. You may move it back- 

 wards or forwards, sideways, upwards, or down- 

 wards quite easily, without any resistance 

 other than that due to its weight. But if you 

 attempt to give it a twist, so that it shall be 

 spinning in a different plane, it resists this 

 change with an energy acting quite differently 

 from that of gravity. 



In this spin, then, we have a means * of 

 promoting steady motion by checking in the 

 beginning any tendencies towards deviation. 



The influence is much like that of steadying 

 the front wheel of a bicycle. This wheel is so 

 constructed that it has of itself a tendency 

 to run straight, and if the machine be ridden 

 carefully the wheel does run straight. But 

 without such care, whenever the slightest 

 roughness in the road, wind on one side, or 

 unevenness of pressure, disturbs the balance, 

 the tendency of the wheel to set this right 

 again, and to continue too far the movement 

 by which it sets it right, causes an error on 

 the other side, the over-correction of which 



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