34 Condition of the Moon's Surface. [January, 



conditions under which the process of cooling began and 

 proceeded. 



Now here I must recall to the attention of the reader the 

 reasoning which I have made use of in my " Other Worlds 

 than Ours." to show that, in ail probability, our solar system 

 owed its origin rather to the gathering of matter together 

 from outer space than to the contraction of a rotating 

 nebulous mass. It is there shown, and I think that the 

 consideration is one which should have weight in such an 

 inquiry, that there is nothing in the nebular hypothesis of 

 Laplace to account in any degree for the peculiarities of 

 detail presented by the solar system. That theory explains 

 the revolution of the members of the solar system in the 

 same direction, their rotation in the same direction, the 

 approach to circularity of the orbits, and their near coin- 

 cidence with the mean plane of the system: but it leaves 

 altogether unexplained the different dimensions of the 

 primary members of the solar system, the apparent absence 

 of law and order in their axial tilt, and the inclination of 

 the orbits of their satellite families. In particular, the 

 remarkable difference which exists between the outer family 

 of planets, — the giant orbs, Jupiter, Saturn, Uranus, and 

 Neptune, — and the inner family of small planets, — Mars, 

 the Earth, Venus, and Mercury, — is left wholly unexplained. 

 Nor can one recognise in the nebular hypothesis any reason 

 whatever for the comparative exuberance of orb-forming 

 activity in the outer family, and particularly in the two 

 planets lying next to the zone of asteroids, and the poverty 

 of material which is exhibited within the minor family of 

 planets. All these circumstances appear to be explained 

 satisfactorily when we regard the solar system as formed by 

 the gathering in from outer space of materials once widely 

 scattered. We can see that in the neighbourhood of the 

 great primary centre there would be indeed a great abun- 

 dance of gathered and gathering matter, but that, owing to 

 the enormous velocities in that neighbourhood, subordinate 

 centres of attraction would there form slowly, and acquire 

 but moderate dimensions. Outside a certain distance there 

 would be less matter, but a far greater freedom of aggrega- 

 tion ; there we should find the giant secondary centres, and 

 we should expect the chief of these to lie inwards, as Jupiter 

 and Saturn, while beyond would be orbs vast indeed, but far 

 inferior to these planets. And we can readily see that the 

 border region between the family of minor planets and the 

 family of major planets would be one where the formation 

 of a planet would be rendered unlikely ; here, therefore, we 



