180 FROM NEBULA TO NEBULA 



terior of the earth, also, is known to possess a high tem- 

 perature. 



The several rings thus thrown off (or abandoned, as 

 some insist) in good time gathered into balls of nebulous 

 matter possessing axial rotations, like the parent nebula, 

 and, following precedent, likewise exchanged the sphe- 

 roidal for the discal shape and eventually flung off 

 smaller rings, which later developed into satellites. 



Granting the premises, then, Laplace, up to this 

 stage, had ostensibly accounted for these things: first, 

 the revolution of the planets in the same direction; sec- 

 ond, their revolution in (about) the same plane; third, 

 the near-circularity of their orbits, and, fourth, the solar 

 heat. 



He had still, however, to account for the fact that all 

 the planets rotate, like the earth, on their axes, and in the 

 same direction in which they orbitally revolve. To do 

 this, he gratuitously assumed that the nebula revolved 

 in hydrostatic equilibrium, that is to say, like a rigid 

 solid. His reason for so doing was this : the planets roll 

 on the perimeters of their orbits like wagon wheels along 

 a cartway, that is to say, their distal halves (as viewed 

 from the sun) move fonvard while their inner halves 

 move relatively backward. It was essential, therefore, 

 for the sake of mechanical consistency, to depict the 

 outer edge of the cast-off ring as traveling faster than 

 the inner edge, just as the circumference of a flywheel 

 travels at a faster rate than the inside of its rim. 



As the data stood during Laplace's life, the Hypo- 

 thesis correlated them all quite faithfully, a circumstance 

 which doubtless accounts for the amazing hold it so long 

 retained upon the scientific mind. The first notable 

 departure from it came in the fifth decade of the past 

 century, when heat was identified as a mode of motion. 

 However, this departure was more in the nature of a 

 development or vindication than a contradiction, inas- 

 much as it relieved the theorists from defending the ab- 

 surdity of the nebula's maintaining its temperature in 

 the face of the cold of space. But the day of disaster ir- 

 retrievable was on its way. In 1877, Prof. Asaph Hall, 



