table specifying the usual astronomical elements of the solar system, we find that four planets resemble each other very closely in the period of axial rotation, and the same four planets are all found to have high densities, thus:—
| Name of Planet. | Period of Axial Rotation. | Density. | |||
|---|---|---|---|---|---|
| Mercury | 24 | hours | 5 | minutes | 7·94 |
| Venus | 23 | " | 21 | " | 5·33 |
| Earth | 23 | " | 56 | " | 5·67 |
| Mars | 24 | " | 37 | " | 5·84 |
A similar table for the other larger planets, is as follows:—
| Jupiter | 9 | hours | 55 | minutes | 1·36 |
|---|---|---|---|---|---|
| Saturn | 10 | " | 29 | " | ·74 |
| Uranus | 9 | " | 30 | " | ·97 |
| Neptune | — | " | — | 1·02 |
It will be observed that in neither group is the equality of the rotational period or the density more than rudely approximate; nevertheless the difference of the numbers in the first and second group is so very well marked, the periods of the first being at least double and the densities four or five times those of the second, that the coincidence cannot be attributed to accident. The reader will also notice that the first group consists of the planets nearest to the sun; that with the exception of the earth none of them possess satellites; and that they are all comparatively small. The second group are furthest from the sun, and all of them possess several satellites, and are comparatively great. Therefore, with but slight exceptions, the following correlations hold true:—
| Interior planets. | Long period. | Small size. | High Density. | No satellites. |
|---|---|---|---|---|
| Exterior " | Short " | Great " | Low " | Many " |
These coincidences point with much probability to a difference in the origin of the two groups, but no further explanation of the matter is yet possible.
The classification of comets according to their periods by Mr. Hind and Mr. A. S. Davies, tends to establish the conclusion that distinct groups of comets have been brought into the solar system by the attractive powers of Jupiter, Uranus, or other planets.8 The classification of nebulæ as commenced by the two Herschels, and continued by Lord Rosse, Mr. Huggins, and others, will probably lead at some future time to the discovery of important empirical laws concerning the constitution of the universe. The minute examination and classification of meteorites, as carried on by Mr. Sorby and others, seems likely to afford us an insight into the