1907-190S.] NebulcB an d Nebular Hypotheses, 79 



ference. Laplace imagined that this ring, after separating 

 from the parent mass, revolved for a time as a whole, but 

 -eventually broke and collected at one part to form the first of 

 the planets. After shedding the ring, the nebula regained its 

 equilibrium, but further contraction brought about another 

 increase in the speed of gyration, and necessitated the sacrifice 

 of another ring, which formed the second planet. After re- 

 peating this operation eight or nine times, no further crises 

 of instability occurred, and the remaining nebula condensed 

 steadily until it decreased to the present proportions of our 

 sun. 



Since this theory was propounded knowledge has trans- 

 gressed many boundaries, and set at nought much ingenious 

 theorising. And in common with other hypotheses, the cos- 

 mogony of Laplace has been weighed and found wanting. 

 There are many features of the solar system of which it gives 

 •either no account at all or an erroneous one, and advances in 

 mathematical analysis show that the dynamics of the process 

 are fundamentally at fault. If the material of the solar 

 :system were spread out so as to fill the orbit of Neptune, its 

 rotation would be so slow that centuries would be required 

 for the completion of one revolution. Obviously a gyration 

 so leisurely could never produce a state of instability such as 

 was pictured by Laplace. And as the density of the material 

 when so expanded would be only the 2 5 - millionth part 

 of the density of air at sea-level, it is clear that no coherent 

 rings could ever have separated from it. 



The nebular hypothesis has not, then, stood " four-square to 

 all the winds that blew," and the perception of its inadequacies 

 has led to many attempts at its improvement. The latest, 

 rand in many ways the best, of these is a joint production, 

 having been elaborated by Prof. Moulton of Chicago, an 

 eminent mathematician, and by Prof. Chamberlin, an equally 

 eminent geologist. It is called the Planetesimal hypothesis, 

 and supposes that the sun is the nucleus of what was once a 

 spiral nebula, and that the planets represent the masses which 

 were ejected from it when the nebula was formed in the 

 manner which I have already described. These masses have 

 swept up and gathered into themselves practically all the 

 finely divided matter which was ejected along with them, so 



