MECHANICAL PARADOXES. 



own accord, to go backwards at the same time. 

 And since a spinner supported at one end only 

 is constantly being depressed by gravity, so 

 also it is constantly having its axis turned 

 horizontally round its fixed end, and in that 

 direction in which the bottom of the gyroscope 

 is spinning at the time. 



So in the first experiment in which a spinner 

 was let down from an upper window, unwinding 

 a long string, it was no doubt observed that 

 besides the vertical spin maintained by the 

 unwinding of the string, there was a horizontal 

 movement of the whole axis round its sup- 

 ported end, which the experimenter doubtless 

 attributed to the unwinding of the string. 

 But it will be noticed that if the string be wound, 

 and consequently the gyroscope spun, in the 

 opposite way, the axis will also turn the other 

 way round, always in the same direction as the 

 bottom part of the gyroscope is spinning. 



The direction in which the axis turns can 

 also be reversed by putting the support under 

 its other end. The direction of movement of 

 the bottom part then points the opposite way 

 round the new point of support. 



This turning of the axis is called in mathe- 

 matics a movement of precession ; but having 

 once mentioned this fact we will not admit 

 so technical-looking a word again. 



There is a very remarkable feature about this 

 turning of the axis which must not be passed over. 



34 



