THE STORY OF NINETEENTH-CENTURY SCIENCE 



earth ; must, in other words, have been thrown off from 

 the then plastic mass of the earth, as a polyp buds out 

 from its parent polyp. At that time the earth was spin- 

 ning about in a day of from two to four hours. 



Now the day has been lengthened to twenty-four 

 hours, and the moon has been thrust out to a distance 

 of a quarter-million miles; but the end is not yet. The 

 same progress of events must continue, till, at some re- 

 mote period in the future, the day has come to equal 

 the month, lunar tidal action has ceased, and one face of 

 the earth looks out always at the moon, with that same 

 fixed stare which even now the moon has been brought 

 to assume towards her parent orb. Should we choose to 

 take even greater liberties with the future, it may be 

 made to appear (though some astronomers dissent from 

 this prediction) that, as solar tidal action still continues, 

 the day must finally exceed the month, and lengthen 

 out little by little towards coincidence with the year; 

 and that the moon meantime must pause in its outward 

 flight, and come swinging back on a descending spiral, 

 until finally, after the lapse of untold a5ons, it ploughs 

 and ricochets along the surface of the earth, and plunges 

 to catastrophic destruction. 



But even though imagination pause far short of this 

 direful culmination, it still is clear that modern calcula- 

 tions, based on inexorable tidal friction, suffice to revo- 

 lutionize the views formerly current as to the stability 

 of the planetary system. The eighteenth-century math- 

 ematician looked upon this system as a vast celestial 

 machine which had been in existence about six thousand 

 years, and which was destined to run on forever. The 

 analyst of to-day computes both the past and the future 

 of this system in millions instead of thousands of years, 



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