The next marked example of the concentric structure is furnished by comets. We have ah'cady shewn that this is one of their most striking features. In this case, we see the very formation of the strata or envelopes. They grow before our eyes, and afford a type of the structure of the more solid bodies of the solar system.
Most of the planets are so remote from us, or so near the sun, that we cannot speak with certainty of their envelopes. As far as our knowledge extends, the moon stands lowest, as she has no shell of any kind surrounding the solid ball. Mars probably comes next. We can at all times see the fixed features of his surface, and nothing like clouds has been detected, though there is evidence for an atmosphere. Next comes the earth, with her far-extending atmosphere and her stratum of clouds. Jupiter ranks next in order; and lastly, Saturn, the most complete example of concentricity of structure.
In the case of the moon, taken as a whole, we do not indeed find a concentric structure; but in a former chapter we shewed that almost the whole surface is covered with volcanic craters, the principal feature of which is concentricity. The cone in the centre — the encircling cavity—the rim, with its successive terraces, all combine to carry out the planetary ideal.
When we descend from the cosmical type of the planetary system to the lowest forms of organisation, we discover the same concentric feature. The section
