140 FROM NEBULA TO NEBULA 



comprehend his treatise), that Kepler's laws and the law 

 of gravitation are in harmony. As the proponent of the 

 proposition that the law of equilibrium is just as deeply 

 rooted in this great law of gravitation as are Kepler's 

 laws, it would seem to devolve upon me to establish, 

 mathematically, the essential identity of all three. I shall 

 therefore make the attempt. 



The reader will remember that I recognize three 

 sorts of gravitational units, namely: (1) a simple body, 

 like the moon; (2) a compound body, like our binary 

 earth-moon system, and (3) a complex body consisting of 

 a multitude of planets and satellites, all balancing them- 

 selves around their common center of gravity. More- 

 over, I define a gravitational unit as a single body, or a 

 congeries of cosmic bodies, seeking its lowest center of 

 gravity. The proposition I am now going to try to prove 

 is, that our solar system is a family of such bodies so in- 

 timately associated with each other, by virtue of their 

 mutual attractions, and so distantly removed from the 

 stars in general, as to behave as a consolidated mass in 

 this : that, while in the act of falling in the direction of 

 the resultant of the stellar attractions, they seek their 

 common systemal center of gravity and revolve around 

 each other according to the 



LAW or THE LEVEE on BALANCE ARM 



It is a fact already well recognized by astronomers, 

 that the moon does not revolve around the center of the 

 earth, but around the center of gravity of their joint 

 mass, and that a similar principle holds good of the 

 planets with respect to the sun. So far, then, the princi- 

 ple of the balance arm has been scientifically accepted. 

 But this knowledge does not dispose of the riddle as to 

 why these bodies rotate at all ; it does not explain the im- 

 pulsion that lay, or lies, behind those tangential or cen- 

 trifugal motions, nor does it point out what keeps them 

 going. My conception is, that the orbital movements of 

 the circulating bodies is due to their act of falling at the 



