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.