globe, but of its visible atmosphere; and if we assign to the body of Saturn a density the same as that of the earth, then its real diameter is reduced by one-half: most probably it is not more than 38,000 miles, while its atmosphere towers to a height of more than 20,000. If the body of Saturn be so small as this, the rings assume an entirely new character-they cease to be a part of the planet, and now rank among the satellites in whose immediate vicinity they now exclusively are. Instead of being distant from the planet only 19,000 miles, they are in reality more than double that distance, while it must be remembered that they are only 32,000 miles distant from the nearest moon, and 62,000 from the second. They, therefore, form an interesting variety in the lunar system, similar to the asteroids among the planets. The smaller specific gravity of Saturn, and consequently the greater proportion of atmosphere to the solid body, would indicate an earlier period in the history of planetformation, when perhaps the crust is red hot, and no deposit of water has yet taken place. It would then correspond to the state of our earth at a still earlier age. May we not also have here a representation of the manner in which our satellite was formed at that distant period, first as a ring revolving round the earth in twenty-nine days, until, its equilibrium being disturbed, it would collapse in the line of its orbit, and form a spherical instead of an annular satellite? URANUS AND NEPTUNE. Uranus and Neptune may be referred to in company, being very similar to one another in constitution, though their distance from the sun and the length of their years respectively are very different. Their size is much less 2833 than Jupiter or even Saturn, but still much greater than the earth. Their specific gravity being no greater than that of water, would lead us to suppose that their atmospheres are, like Jupiter, very great, and that much of what has been said regarding him may also be said of them. At the same time we observe a singular inversion of the law of revolution-not only the axial motion of these planets, but the motion also of their satellites, is in a direction contrary to that which is found in all the others. This indicates a change of polarity of some kind, which takes place in the planetary system after passing the orbit of Saturn. What the nature of that polarity may be we have as yet no means of ascertaining, and under these circumstances any speculations on these planets must be doubly uncertain. It ought not to be overlooked that the growth of Uranus and Neptune, or even Saturn, may not yet be completed. It is quite possible that there may be a large unexhausted supply of meteoric matter still ministering to a slow combustion still going on. There is nothing which has been observed in these planets inconsistent with such a supposition; and although it may not be possible to obtain proof of the fact, if it be a fact, still their probably late formation, compared with that of the earth, requires such a possibility to be kept in view. In one sense, it may be said that, to a very small extent, the growth of our own earth is not yet completed, because every meteorolite that descends adds to its bulk. We have thus a visible illustration of the earth's growth in former ages continued to the present day. The earth, however, has grown cold since the supply of meteorolites has failed, and now it cannot volatilise them when they come, as it once did. |