NEWTON'S THEORY OF PLANETARY MOTIONS 81 



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, onoe 

 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 

 such a cable, and ask yourself whether it seems to you 

 reasonable that the unsupported moon, wiiliout any influx 

 of energy whatsoever from without, can, unenfeebled, 

 hold her own against it forever, as astronomers declare 

 she does? To the casual observer the moon looks like a 

 silver wafer pasted upon the sky ; in reality it is a globe 

 of earthy matter 2160 miles in diameter (as far as from 

 New York to Santa Fe) with a mass nearly one-eightieth 

 that of the earth itself. So far as we know or have rea- 

 son to believe, it has remained exactly at its present mean 

 height for at least 3000 years, and possibly has been 

 there for as many hundreds of millions all the time 

 pulling against a strain of 240 trillions of untiring 

 horses ! The moon has no pillar to support it, no pulley 

 to suspend it, no atmosphere to buoy it, no breeze to waft 

 it, no screw to propel it, no engine to haul it, yet still it 

 stays up. Why! Whence comes the centrifugal force 

 to keep driving it away from the earth with identically 

 the same energy that the latter perpetually prompts it to 

 fall? Where is the equivalent of that flinging motion of 

 your hand that supplied the centrifugal force to the little 

 ball in Sir Oliver 's experiment ? 



