ADDRESS. 19 
hypothesis. Thus, for example, the great nebula in Andromeda presents 
a grand illustration of what we may take to be a planetary system in 
course of formation. In it we see the central condensation surrounded 
by a more or less ring-like nebulosity, and in one of the rings there appears 
to be a subordinate condensation. 
Nevertheless it is hardly too much to say that every stage in the sup- 
posed process presents to us some difficulty or impossibility. Thus we 
ask whether a mass of gas of almost inconceivable tenuity can really 
rotate all in one piece, and whether it is not more probable that there 
would be a central whirlpool surrounded by more slowly moving parts. 
Again, is there any suflicient reason to suppose that a series of intermit- 
tent efforts would lead to the detachment of distinct rings, and is not a 
continuous outflow of gas from the equator more probable ? 
The ring of Saturn seems to have suggested the theory to Laplace ; but 
to take it as a model leads us straight to a quite fundamental difficulty. 
If a ring of matter ever concentrates under the influence of its mutual 
attraction, it can only do so round the centre of gravity of the whole 
ring. Therefore the matter forming an approximately uniform ring, if it 
concentrates at all, can only fall in on the parent planet and be re- 
absorbed. Some external force other than the mutual attraction of the 
matter forming the ring, and therefore not provided by the theory, seems 
necessary to effect the supposed concentration. The only way of avoiding 
this difficulty is to suppose the ring to be ill-balanced or lop-sided ; in 
this case, provided the want of balance is pronounced enough, concentra- 
tion will take place round a point inside the ring but outside the planet. 
Many writers assume that the present distances of the planets preserve 
the dimensions of the primitive rings ; but the argument that a ring can 
only aggregate about its centre of gravity, which I do not recollect to have 
seen before, shows that such cannot be the case. 
The concentration of an ill-balanced or broken ring on an interior 
point would necessarily generate a planet with direct rotation—that is to 
say, rotating in the same direction as the earth. But several writers, and 
notably Faye, endeavour to show—erroneously as I think—that a retro- 
grade rotation should be normal, and they are therefore driven to make 
various complicated suppositions to explain the observed facts. But I do 
not claim to have removed the difficulty, only to have shifted it ; for the 
satellites of Neptune, and presumably the planet itself, have retrograde 
rotations ; and, lastly, the astonishing discovery has just been made by 
William Pickering of a ninth retrograde satellite of Saturn, while the 
rotations of the eight other satellites, of the ring and of the planet itself, 
are direct. Finally, I express a doubt as to whether the telescope does 
really exactly confirm the hypothesis of Laplace, for I imagine that what 
we see indicates a spiral rather than a ring-like division of nebule.! 
. Professor Chamberlin, of Chicago, has recently proposed a modified form of the 
Nebular Hypothesis, in which he contends that the spiral form is normal. See Year 
Book, No. 3, for 1904, of the Carnegie Institution of Washington, pp. 195-258. 
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