THE STABILITY OF THE SOLAR SYSTEM 297 



but gained no audience. Then he sent to the savant a letter 

 on the principles of mechanics. It brought him a post at the 

 military school, and a courtly word from d'Alembert that such 

 a letter would be an introduction anywhere in Europe. There- 

 with his successful way began. 



In taking up the " acceleration of the moon's mean motion," 

 this steady shortening of the month which Halley had disclosed, 

 Laplace was led to a still more important discovery ; that was 

 the changing eccentricity of the earth's orbit. The elliptical 

 form of the earth's path about the sun, he found, was steadily 

 growing less and less. This was the cause of the quickening I 

 of the moon's motion. Laplace calculated it all out, figured 

 that the real amount of quickening was ten seconds in a century 

 instead of twelve, as Halley had estimated and that this 

 was precisely what the theory of attraction required. 



But immediately came the question : How long had this 

 decrease in the eccentricity of the earth's orbit been going on ? 

 Was the eccentricity once immensely greater than it is now ; 

 did the path of the earth once resemble that of a comet ; in 

 times gone by, did it shoot down close to the sun into regions 

 of blazing heat, then whirl about and rush away again into 

 distant spaces of chilling cold, as the comets do ? It was a 

 tremendously interesting problem, for it might give a clue to 

 the whole past history of the earth perhaps a hint as to its 

 future as well. 



The memoir which Laplace at length presented to the French 

 Academy dealing with this question was of profound philosophic 

 significance. His answer was : The variation of eccentricity is 

 not cumulative ; it is, like the precession of the equinoxes, like 

 nutation, a periodic variation, requiring for its completion a 

 long period of time. The latter Laplace estimated at eighty- 

 six thousand years. It was a glowing triumph of the cal- 

 culating faculty, and it was the foundation of a wonderful work. 

 This was that great MJcanique Celeste the Mechanics of the 

 Heavens to which Laplace owes, largely, his fame. Its central 

 idea is the perfect balance revealed in every motion of the 

 planetary system. It seemed to rule out all change as Newton 

 had excluded chance ; it presented the solar machine as a vast 

 engine, subject to slight variations in the motions of its parts, 

 but whirling on without substantial deviations throughout in- 

 finity. We now know that there was in Laplace's calculations 



