24 FROM NEBULA TO NEBULA 



"There remains one lunar irregularity 

 among the multitude of lesser ones, which is of 

 great interest theoretically, and is still a bone 

 of contention among mathematical astrono- 

 mers, namely, the secular acceleration of the 

 moon's mean motion. It was found by Halley, 

 early in the last century, by a comparison of 

 ancient with modern eclipses, that the month 

 is certainly shorter than it was in the days of 

 Ptolemy, and that the shortening has been pro- 

 gressive, apparently going on continuously 

 * * *. In 100 years the moon, according to 

 the results of Laplace, gets in advance of its 

 mean place about 10", and the advance in- 

 creases with the square of the time?'' (Tne 

 italics are his.) 



Now, the remarkable fact is that falling bodies do 

 by rule increase their velocity precisely in this way, 

 that is, with the square of the time !* 



Indeed, what logic would there be in arbitrarily as- 

 suming that the universe, which we see filled with count- 

 less points of attraction, should not have its gravita- 

 tional influence on our system? After all the various 

 attractions are balanced up, there cannot fail to remain 

 a residuum, an excess, a net resultant, in some one defi- 

 nite direction .(not necessarily toward that quarter of the 

 heavens most filled with stars, for proximity and coun- 

 teraction are also to be considered), which residuum I 

 shall call, for convenience, the PRIME RESULTANT. De- 

 fined, this term denotes the net gravitational pull, both 

 as to force and direction, of the whole universe upon the 

 solar system considered as a GRAVITATIONAL UNIT. And 

 by the term GRAVITATIONAL UNIT I mean any body, 

 simple, compound, or systemal, moving freely in space 



*A resisting medium would of course follow a different law in 

 its effect; that is, it would be practically uniform from year to year. 



