Sir J. Herschel estimated the thickness of the rings as 250 miles, but Bond thought it far less—about 40 miles. There are great obstacles in the way of ascertaining the exact proportions of a structure so distant and offering such an extremely slender form to our view.
_Further Observations required._—The globe and rings of Saturn offer an encouraging prospect for additional discoveries. Though the more prominent details have already been descried, there remain other features, probably of more delicate outline and intermittent visibility, which will be glimpsed in future years. Small instruments will scarcely be competent to deal efficiently with this object: observers who can command at least a moderate grasp of light may, however, enter upon the work with every assurance of interesting results. In this, as in other sections of observational astronomy, the student will realize that in oft-repeated observation and comparison of records and drawings he acquires a familiarity with the appearance of the object which will enable him to discern more and more of its configuration, until ultimately he feels confident he has progressed as far as the utmost capacity of his instrument will permit. It is in the sedulous application of his powers that the observer will find the key to success. Partial devotion to a subject offers a prospect far less encouraging; for observations of a disconnected character are seldom valuable.
Changes are unquestionably occurring both in connection with the ball and rings of Saturn[39]. Some of the discrepancies between the observations published from time to time are only to be explained on this assumption. It should therefore be the aim of observers to obtain further evidence of such variations, and this may be best accomplished by assiduously watching the lineaments of the planet during the most favourable periods of each opposition. The collection of a number of reliable materials through a series of years would undoubtedly possess weight in removing some of the anomalies of past observation, and afford us a more thorough knowledge of the delicate markings.
The rotation-period of Saturn is probably not much different from that given by the atmospheric markings seen by Herschel and Hall. But additional determinations are very desirable for many reasons. The spots which are so plentiful on Mars and Jupiter have furnished observers with a valid and concise means of ascertaining the rate of axial motion of those planets. Saturn, however, has far more sparingly provided the data for such an investigation; for if we disregard Schröter’s uncertain figures, we have but two values for the rotation-period. These were fortunately effected by observers of exceptional ability, and the periods may be accepted without reservation; but other independent determinations are much required. By multiplying results of this nature, we have a prolific source of comparison; and comparisons, apart from being interesting, are of importance in denoting erratic results and indicating those entitled to credence. Moreover, a reliable mean value may be sometimes deduced from multiple records; hence it becomes advisable to secure as many as possible.
