104 FKOM NEBULA TO NEBULA 



therefore, urged against the pail, and forced up its sides, till the 

 excess of height, and consequent increase of pressure downwards, 

 just counterbalance its centrifugal force, and a state of equili- 

 brium is attained. The experiment is a very easy and instructive 

 one, and is admirably calculated to show how the form of equili- 

 brium accommodates itself to varying circumstances. If, for ex- 

 ample, we allow the rotation to cease by degrees, as it becomes 

 slower we shall see the concavity of the water regularly diminish ; 

 the elevated outward portion will descend, and the depressed 

 centre rise, while all the time a perfectly smooth surface is main- 

 tained, till the rotation is exhausted, when the water resumes its 

 horizontal state. (The italics are all his.) 



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. I furthermore pointed out 

 the unsavory fact that astronomers affect to believe 

 that the two phenomena are exactly analogous. 



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 



