44 FROM NEBULA TO NEBULA 



plain this uniformity of direction by imagining a com- 

 mon origin for the whole system. Their ideas, however, 

 lacked plausibility because there was nothing in evi- 

 dence, so far as then known, to image this unity to the 

 minds of others. It was some years later that Her- 

 schePs discovery of nebulae occurred, and Pierre Simon 

 de Laplace, a French mathematician second only to the 

 great Newton himself, promptly conceived the idea that 

 these might -be the chrysalides of new systems an idea 

 surely lacking neither in romance nor attractiveness. 



Accordingly, he evolved his celebrated "Nebular 

 Hypothesis," which ever since has dominated astronom- 

 ical theory, although now greatly modified in details. 

 A generation or so ago, such was its vogue that the ad- 

 vanced clergy, taking a leaf out of past experience, went 

 so far as to proclaim the Hypothesis a plagiarism of 

 the Mosaic Cosmogony, and delivered multitudes of 

 sermons in the effort to establish the parallelism ! 



Laplace postulated the parent nebula of our solar 

 system to extend from the sun as a center to a point 

 somewhat beyond the orbit of Uranus (Neptune was not 

 discovered till long after Laplace's death) * and assumed 

 it to contain only what matter is embraced in the sys- 

 tem of our own day. In imagination he endowed this 

 nebula with the west- to-east motion of the planets, and, 

 besides, assumed it to be in a complete state of incan- 

 descence. As the nebula condensed, a differentiation 

 of speed developed between the core and the outer rim, 

 he supposed, so that a ring was shed, which he imagined 

 rolled up afterward to form the planet Uranus. The 

 process of condensation of the core continuing, the in- 

 cident of ring-shedding was repeated, and Saturn, the 

 next planet, was this time the result. And so the pro- 

 cess kept on, as he imagined, until all the planets and 



