176 FKOM NEBULA TO NEBULA 



You cannot have forgotten our former experiment of 

 whirling the ball at the end of the string, and how I tried 

 to make clear the fundamental distinction between the 

 principle of that experiment, embracing as it does a 

 "flinging motion" outward from the center, and the case 

 of the earth and moon, in which such a flinging motion is 

 conspicuous "by its absence. 



That instance has a parallel. The experiment I now 

 refer to is the one purporting to illustrate and explain to 

 advanced pupils the cause of the earth's equatorial pro- 

 tuberance. An elastic hoop is provided and so adjusted 

 on a vertical axis that by means of a crank it can be 

 rotated around its axis at a high velocity. As the crank 

 is turned faster and faster, the hoop is seen to elongate 

 horizontally and its vertical axis to shorten correspond- 

 ingly, making an ellipse out of what was before a true 

 circle. Here the experiment is invariably brought to an 

 abrupt close. The lesson it is intended to teach is that the 

 earth's oblateness is the physical effect of her axial rota- 

 tion. 



Now it is most earnestly to be hoped that no under- 

 graduate can be discovered so preternaturally dense as 

 to require to be shown this silly experiment in order to 

 teach him no more than the a priori knowledge that such 

 ENFORCED rotation must produce just such effects. What 

 he wants to know, what you want to know, what the world 

 wants to know, and what these blind teachers of the blind 

 cannot answer is, What keeps the earth turning on her 

 axis? The experiment of the hoop demonstrates admir- 

 ably that as long as you are content to turn the crank the 

 hoop will not only dutifully rotate, but that it will also 

 remain distended. But it also most emphatically demon- 

 strates that the hoop will infallibly cease doing either of 

 these things the moment you suspend your exertions. 



Suppose that, while the hoop is rotating at its highest 

 speed and consequently expanded to the limit, you were 

 sufficiently deft to insert in it a horizontal brace, you 

 could then shut off the driving force without impairing 

 the deformation attained ; otherwise the natural elasticity 



