We have seen what the telescope has taught us of the structure of comets, so complex and wonderfully mobile, so different in this respect from that of the planets or the sun. On the one hand we see solid or liquid bodies, bearing the most striking analogy to the terrestrial globe, surrounded like it by atmospheres of comparatively small extent, stable in every portion ; these are the planets, the moon, and the satellites of the planets. As regards the sun and the stars–which shine, like the sun, by their own light, and are, like him, as everything leads us to suppose, foci of light and heat to other planetary groups–if these bodies are incandescent gaseous masses, their condensation is so enormous and their physical constitution is comparatively so stable, that the changes of which they are perpetually the theatre have no appreciable effect upon their equilibrium. In comparison with comets they are permanent stars ; while comets seem to be nothing more than clouds–wandering nebulae, to employ the expression of Laplace, who has but reproduced in a more happy form the term so happily applied by Xenophanes and Theon of Alexandria.