INTRODUCTION 



1. The orbits are all nearly circular. 



2. They are all nearly in one plane (excepting the cases of 

 some of the little asteroids). 



3. The revolution of all is in the same direction. 



4. There is a curiously regular progression of distance (ex- 

 pressed by Bode's law, which, however, breaks down at Neptune). 



5. There is a roughly regular progression of density t in- 

 creasing both ways from Saturn, the least dense of all the planets 

 in the system. 



As regards the planets themselves, we have : 



6. The plane of the planet's rotation, nearly coinciding with 

 that of the orbit (probably excepting Uranus). 



7. The direction of the rotation the same as that of the 

 orbital revolution (excepting probably Uranus and Neptune). 



8. The plane of orbital revolution of the satellites coinciding 

 nearly with that of the planet's rotation. 



9. The direction of the satellite's revolution also coinciding 

 with that of the planet's rotation. 



10. The largest planets rotate most swiftly. 



In the first decade of the past century Laplace called 

 the world's attention to these remarkable coincidences, 

 and on the theory of probabilities demonstrated the 

 chances of such an arrangement as now exists being 

 fortuitous to be as many billions to one. At the same 

 time he emphasized the utter helplessness of Newtonian 

 theory to cope with these enigmas. All this he did as a 

 prelude to the launching of his celebrated Nebular Hypo- 

 thesis, the germ ideas of which were, (1) to seek for all 

 the bodies in our solar system a common genesis, and 

 hence concerted relations, and (2) to reduce to a modest 

 single assumption of spontaneous motion the host of 

 separate postulates theretofore so lavishly indulged in. 

 All he asked of scientists was to concede him these two 

 things: (1) incandescence for this nebula, and (2) that it 

 possessed of itself, as something "natural" and (( not re- 

 quiring explanation", a rotation, round a fixed axis, as 

 though it were a rigid solid. 



