ch. v.] PL A NETAB Y E VOL UTION. 36 1 



manner as this our solar nebula must have acquired its 

 definite rotation from west to east. 



Let us next observe the mechanical consequences of this 

 rotation. No matter what may have been the primitive 

 shape of the nebula — and, if we may judge from the analogy 

 of irresolvable nebula? now existing, it may very likely have 

 been as amorphous as any cloud in a summer sky — no 

 matter what its primitive shape, it must at last inevitably 

 assume the form peculiar to rotating bodies in which the 

 particles move freely upon each other. It must become an 

 oblate spheroid, flattened at the poles and bulging at the 

 equator, because at the equator the centrifugal tendency 

 rjonerated by rotation is greatest. Furthermore as the mass 

 contracts, it must rotate faster and faster; for as the total 

 quantity of rotation is unalterable, the velocity must increase 

 as the space traversed diminishes. 



In accordance with these principles of mechanics, as our 

 solar nebula continued to radiate heat and contract, it con- 

 tinued to rotate with ever-increasing velocity, its poles 

 became more and more flattened, and its equatorial zone pro- 

 truded more and more, until at last the centrifugal tendency 

 at the equator became greater than the force of gravity at 

 that place. Then the bulging equatorial zone, no longer able 

 to keep pace with the. rest of the mass in its contraction, 

 was left behind as a detached ring, girdling, at a small but 

 steadily increasing distance, the retreating central mass. 



What must now have been the career of this detached 

 ring ? Unless subjected to absolutely symmetrical forces in 

 all directions — an infinitely improbable supposition — such a 

 ring must forthwith break into a host of fragments of very 

 unequal dimensions. For in order that it should break into 

 tqual-sized fragments, the strains exerted upon it must be 

 disposed with absolute symmetry; and against this supposition 

 also the probabilities are as infinity to one. It would break, 

 much as a dish, breaks when dropped on the floor, into 



