THE CONQUEST OF TIME AND SPACE 



scopes balance each other in such a way as to make the 

 Brennan car lean in while passing about a curve, we 

 must investigate more fully the action of the individual 

 gyroscopes. I have already said that there is another 

 principle involved as supplementary to the principle 

 of the fixed axis; this we must now investigate. 



Perhaps it would be fairer to say that what we have 

 to consider is not a new principle but a complication as 

 to the application of the principle of gyroscopic action 

 already put forward. In any event there is an element- 

 ary fact about the gyroscope that I have not yet stated. 

 It is this: in order that the gyroscope may exercise its 

 fundamental property of holding its axis fixed, it must 

 have that axis so adjusted that it is free to oscillate or 

 wabble. That sounds distinctly paradoxical, but it is 

 a very essential fact. If Mr. Brennan had merely fixed 

 two wheels rigidly in the frame of his car, they would 

 have had no appreciable effect in balancing it. Had 

 nothing more than that been necessary, some one would 

 have invented a gyrocar long ago. But very much 

 more than that was necessary, as we shall see. 



The complication of which I am speaking is illustrated 

 by the action of the simplest top, which likewise owes 

 its stability to its wabble. Your top does not rise merely 

 because it spins, but because it wabbles as it spins — 

 wabbling being the familiar word for what the machin- 

 ist calls "precession." A freely spinning top, if in 

 equilibrium, has no inherency to rise up against gravita- 

 tion, as your top may have led you to suppose. Your 

 top rises because it is not spinning freely in equilibrium, 

 its action being interfered with by the friction of the 



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