78 FROM NEBULA TO NEBULA 



at perigee has always exactly the same momentum. Con- 

 sidering, then, two successive perigees, let M represent 

 the Moon's momentum at the time of the first perigee, 

 and M' that which she has at the time of the second, and 

 we have, 



M=M' 



Again, by general consent the centripetal and centrifugal 

 forces are equal, which fact we may express, for one lun- 

 ation, thus : 



C=C' 



But astronomers inform us that the momentum, M, of 

 the moon supplies the centrifugal force, consequently, on 

 this theory, the momentum of the moon at the second 

 perigee must be M C' ; substituting which in place of M 

 in the first equation, we get, 



M=M C f 



whence, C"=0 



and, since C= C' C =0 



Reductio ad absurdum: There are no central forces: 

 gravitation is a myth (v. p. 17, ante). 



Newtonians are by no means oblivious to the in- 

 security of the ground they here occupy, so they have 

 added, by way of an alternative or makeweight, the fic- 

 tion that celestial motions are "persistent". This idea 

 they have borrowed from the so-called doctrine of con- 

 servation of energy. They have come to realize that 

 Newton was too modest in asking merely for the gift of 

 rectilinear motions by way of capital on which to run his 

 cosmic system, and so they have added, for the sake of 

 expediency, this new notion. In other words, they have 

 gratuitously substituted in the Newtonian vocabulary the 

 word persistent for inertia, an antonym. Now, our ter- 

 restrial experience offers us not a single example of 

 motion not plainly traceable to a definite physical cause ; 

 so, to mark the difference, and as a sort of honorary dis- 

 tinction, they call the one sort "celestial" mechanics and 

 the other, "terrestrial", exactly traversing, as it were, 

 Drummond's Natural Law in the Spiritual World, by 



