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keep alive the fiction, invented by Newton himself, that 

 the only drain upon the moon's store of momental energy 

 lies in the possible resistance offered her by the ether, 

 and that by merely positing this last as "imponderable" 

 the whole theory is irrevocably clinched. Their argu- 

 ment runs that, in the case of the ball and string, the 

 flinging motion is necessary not to stretch the string, but 

 only to overcome the atmospheric resistance, and that, 

 in the case of the moon, there being no atmospheric re- 

 sistance whatsoever, no live flinging force at all is re- 

 quired. 



One of my critics, a professor of astronomy in one 

 of the oldest New England colleges and an uncompromis- 

 ing Newtonian, writes me: "The experiment you describe 

 is demonstrably false. As long as the ball is in motion it 

 requires no ' flinging force' to prevent its flying back to 

 the hand, and were friction absent it would never stop in 

 its motion. The answer to your question, ' Whence comes 

 the centrifugal force to keep driving the moon away from 

 the earth' is found in Newton's first law of motion. " Let 

 us test this view by following out its logical deductions 

 and see whether we can conscientiously subscribe to them. 



Suppose, then, we choose an elastic string exactly a 

 yard in length and capable of extension without rupture 

 to double its length; and to one end of it let us attach a 

 solid lead ball one pound in weight. Suppose, again, that 

 I set the ball awhirling round my head until the string is 

 stretched just to its limit. Now according to the New- 

 tonian view, all the muscular energy I thus expend is 

 wholly and exclusively absorbed in buffeting the air, and 

 not so much as one infinitesimal part of it goes toward, 

 or is required for, the tensing of the cord. If this view 

 be really sound, then these propositions, which to my 

 mind are reductios ad absurdum, necessarily follow : 



I. If, while in the midst of the experiment, it were 

 possible for me to step into, and exist, in a vacuum cham- 

 ber, I could there cease my flinging motion altogether, 

 and the ball would nevertheless continue to gyrate about 

 my hand indefinitely. 



