A HISTORY OF SCIENCE 



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 aeons, it ploughs and ricochets along the 

 surface of the earth, and plunges to catastrophic de- 

 struction. 



But even though imagination pause far short of this 

 direful culmination, it still is clear that modern calcu- 

 lations, based on inexorable tidal friction, suffice to 

 revolutionize the views formerly current as to the sta- 

 bility of the planetary system. The eighteenth-century 

 mathematician looked upon this system as a vast celes- 

 tial machine which had been in existence about six 

 thousand years, and which was destined to run on for- 

 ever. The analyst of to-day computes both the past 

 and the future of this system in millions instead of 

 thousands of years, yet feels well assured that the solar 

 system offers no contradiction to those laws of growth 



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