CHAPTER XVII 
CONCLUSION 
385. Now that the detailed discussion of particular problems is ended, we 
may perhaps attempt to summarise our results and tentative conclusions, 
sacrificing logical and chronological order in favour of the arrangement which 
offers the broadest and simplest view of the whole subject. 
The easiest part of the problem of cosmogony is the interpretation of the 
observed shapes of astronomical bodies and formations. Here the effects of 
rotation have proved to be of primary importance. The earth and many of the 
planets have the shape of flattened oranges. The degree of flattening is such 
as would be produced by quite slow rotation about an axis, and there is no 
room for doubt that this is the actual cause of the observed flattening. It is 
possible to trace out theoretically the shapes assumed by astronomical bodies 
having all possible amounts of rotation. Mathematical investigation shews 
that the flattened-orange shape is assumed by all bodies in slow rotation, no 
matter what their internal constitution and arrangement may be, but that 
with more rapid rotation the shape depends on the internal arrangement of the 
body, being especially affected by the extent to which its mass is concentrated 
at or near its centre. 
Two special and quite extreme types of arrangement have been considered 
in detail. In the first the body is supposed to consist of matter which cannot 
be compressed and is of uniform density throughout; to fix our ideas, we 
may think of a mass of water. As the rotation of such a mass increases, the 
orange becomes flatter and flatter but retains the shape of an oblate spheroid 
throughout, until a stage is reached beyond which the flattening cannot go. 
At this stage the body abandons its circular cross-section; it elongates and 
concentrates its mass around one of the diameters in its equatorial plane, 
thus forming an ellipsoid with three unequal axes. This process continues until 
the mass forms a cigar-shaped figure with a length equal to nearly three times 
its shorter diameter. At this point the mass begins to concentrate about two 
distinct points on its longest diameter, a furrow or waist forming near the 
centre which continually deepens until it cuts the body into two distinct 
detached masses. These rotate in orbital motion about one another like the 
earth and moon, except that the two masses are more nearly equal and are 
closer together. 
It is possible for a single rotating mass to assume all these configura 
tions in turn. The concept, first introduced by Laplace, of a mass shrinking 
and at the same time increasing its speed of rotation, because its rotational 
momentum must remain constant, remains of the utmost importance to 
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