NEWTON'S THEORY OF PLANETARY MOTIONS 85 



she falls by gravity, they furthermore tell us, she doesn't 

 fall this distance after all, because she comes no nearer ! 



In the text quoted from Sir Oliver Lodge, accom- 

 panying Figure 1, he says: "Now consider circular mo- 

 tion in the same way, say a ball whirled round by a 

 string." This habit of drawing a parallel between the 

 case of the moon revolving around the earth and that of 

 a ball being whirled about at the end of a string, with 

 never a word as to the vital difference between the two 

 phenomena, is nothing short of a pious fraud. It is a 

 common occurrence in one's astronomical reading to come 

 upon such expressions as, "the earth whirls the moon 

 around," "the sun whirls his planets around", "the star 

 whirls its companion around," quite as though these 

 things were "most well-attested facts of human knowl- 

 edge". As a matter of plain fact, the difference is the 

 same as between a dead horse and a live one, between an 

 automobile charged with gasoline and one that has run 

 dry. When I whirl the ball around, I supply the poiver 

 that stretches the string and overcomes the resistance of 

 the atmosphere. When I turn to the earth, however, I can 

 perceive nothing to correspond with my part in the 

 scheme. I can readily see the analogy between the ten- 

 sion of the string and gravity, between the ball and the 

 moon, and between the position of the earth with refer- 

 ence to the moon and that of my hand with reference to 

 the ball; but as to the flinging motion of my arm, its 

 counterpart is conspicuous by its absence. 



It is scarcely to be doubted that Newton cast about 

 long and anxiously for the physical foil to the earth's 

 gravity which enables her, from century to century, to 

 maintain her aloofness. Nor is it any less likely that 

 every beginner in the study of astronomy passes through 

 a period of wholesome skepticism more or less pro- 

 longed and harassing before he can bring himself to be- 

 lieve, if ever he really can believe, the fable that "no such 

 foil is necessary." By dint of incessant iteration, how- 

 ever, coupled with their avowed "policy of discouraging 

 discussion," our scientific institutions have managed to 



