Fig. 67.—Prof. Keeler's Method of Measuring the Rotation of Saturn's Ring.
Finally, let us consider the case of the rings, consisting of innumerable particles moving round the planet in accordance with Kepler's third law. The actual velocities of these particles would be per second:—
| At outer edge of ring | 10·69 miles. |
| At middle of ring | 11·68 miles. |
| At inner edge of ring | 13·01 miles. |
| Rotation speed at surface of planet | 6·38 miles. |
The shifting of the lines of the spectrum should be in accordance with these velocities, and it is easy to see that the lines ought to lie as in the fourth figure. When Professor Keeler came to examine the photographed spectra, he found the lines of the three spectra tilted precisely in this manner, showing that the outer edge of the ring was travelling round the planet with a smaller linear velocity than the inner one, as it ought to do if the sources of light (or, rather, the reflectors of sunlight) were independent particles free to move according to Kepler's third law, and as it ought not to do if the ring, or rings, were rigid, in which case the outer edge would have the greatest linear speed, as it had to travel through the greatest distance. Here, at last, was the proof of the meteoritic composition of Saturn's ring. Professor Keeler's beautiful discovery has since been verified by repeated observations at the Allegheny, Lick, Paris, and Pulkova Observatories; the actual velocities resulting from the observed displacements of the lines have been measured and found to agree well (within the limits of the errors of observation) with the calculated velocities, so that this brilliant confirmation of the mathematical deductions of Clerk Maxwell is raised beyond the possibility of doubt.
The spectrum of Saturn is so faint that only the strongest lines of the solar spectrum can be seen in it, but the atmosphere of the planet seems to exert a considerable amount of general absorption in the blue and violet parts of the spectrum, which is especially strong near the equatorial belt, while a strong band in the red testifies to the density of the atmosphere. This band is not seen in the spectrum of the rings, around which there can therefore be no atmosphere.
As Saturn's ring is itself unique, we cannot find elsewhere any very pertinent illustration of a structure so remarkable as that now claimed for the ring. Yet the solar system does show some analogous phenomena. There is, for instance, one on a very grand scale surrounding the sun himself. We allude to the multitude of minor planets, all confined within a certain region of the system. Imagine these planets to be vastly increased in number, and those orbits which are much inclined to the rest flattened down and otherwise adjusted, and we should have a ring surrounding the sun, thus producing an arrangement not dissimilar from that now attributed to Saturn.
It is tempting to linger still longer over this beautiful system, to speculate on the appearance which the ring would present to an inhabitant of Saturn, to conjecture whether it is to be regarded as a permanent feature of our system in the same way as we attribute permanence to our moon or to the satellites of Jupiter. Looked at from every point of view, the question is full of interest, and it provides occupation abundant for the labours of every type of astronomer. If he be furnished with a good telescope, then has he ample duties to fulfil in the task of surveying, of sketching, and of measuring. If he be one of those useful astronomers who devote their energies not to actual telescopic work, but to forming calculations based on the observations of others, then the beautiful system of Saturn provides copious material. He has to foretell the different phases of the ring, to announce to astronomers when each feature can be best seen, and at what hour each element can be best determined. He has also to predict the times of the movements of Saturn's satellites, and the other phenomena of a system more elaborate than that of Jupiter.
Lastly, if the astronomer be one of that class—perhaps, from some points of view, the highest class of all—who employ the most profound researches of the human intellect to unravel the dynamical problems of astronomy, he, too, finds in Saturn problems which test to the utmost, even if they do not utterly transcend, the loftiest flights of analysis. He discovers in Saturn's ring an object so utterly unlike anything else, that new mathematical weapons have to be forged for the encounter. He finds in the system so many extraordinary features, and such delicacy of adjustment, that he is constrained to admit that if he did not actually see Saturn's rings before him, he would not have thought that such a system was possible. The mathematician's labours on this wondrous system are at present only in their infancy. Not alone are the researches of so abstruse a character as to demand the highest genius for this branch of science, but even yet the materials for the inquiry have not been accumulated. In a discussion of this character, observation must precede calculation. The scanty observations hitherto obtained, however they may illustrate the beauty of the system, are still utterly insufficient to form the basis of that great mathematical theory of Saturn which must eventually be written.
But Saturn possesses an interest for a far more numerous class of persons than those who are specially devoted to astronomy. It is of interest, it must be of interest, to every cultivated person who has the slightest love for nature. A lover of the picturesque cannot behold Saturn in a telescope without feelings of the liveliest emotion; while, if his reading and reflection have previously rendered him aware of the colossal magnitude of the object at which he is looking, he will be constrained to admit that no more remarkable spectacle is presented in the whole of nature.
We have pondered so long over the fascinations of Saturn's ring that we can only give a very brief account of that system of satellites by which the planet is attended. We have already had occasion to allude more than once to these bodies; it only remains now to enumerate a few further particulars.