MOTIONS 37 



selves, who nevertheless imply there is. Why, then, does 

 the moon keep her distance, and why does she course 

 round and round, instead of yielding to the tension of 

 the connecting cord and flying back against us f It is a 

 continual shock to me to find such men as Flammarion 

 and our own Doctor W. W. Campbell, men of the first 

 order of merit, not to mention an endless procession of 

 lesser experts, talk with straight faces about "the sun 

 whirling the earth around ", and a "star whirling its 

 companion around", with never an attempt then, or at 

 any other time, in the class-room, laboratory, or printed 

 word, to justify the expression, or to apologize for the 

 palpable misrepresentation ! 



Have you, my reader, any adequate idea of the elastic 

 strength of the earth's attraction upon the moon? Do 

 you realize that, unlike a rubber band, gravitational at- 

 traction never rots or wears out, never relaxes, and, 

 moreover, even increases its tension the closer the bodies 

 are brought together? It is not like a strand of twine, 

 or a ship's cable, or a dog's tether, that remains in a state 

 of laxity until called into play only when stretched to full 

 length. No ! it is unrelentingly pulling at the moon with 

 all its might, striving to bring her down. 



Now, it is not a difficult matter to determine by cal- 

 culation the approximate strength of this attraction, once 

 we know the respective masses of the two bodies and the 

 intervening distance, all of which we do. Expressed one 

 way, this attraction is equivalent to the full tensile 

 strength of a solid steel cable 400 miles in diameter (say 

 from Pittsburgh to New York) capable of sustaining a 

 load of 40 tons to the square inch. Expressed in terms of 

 power (supposing a horse able to sustain a ton against 

 gravity), it is equal to that of 240 million, million horses. 

 Allowing 100 square feet for each horse, it would require 

 to stable them a four-story building covering the entire 

 surface of the earth, not excepting the space taken up by 

 the oceans! Visualize all this to your mind's eye, and 

 imagine the horses, or a steam engine equal to them in 

 power, located at the center of the earth and pulling upon 



