The period of rotation was, till within the last few years, thought to be definitely and satisfactorily ascertained. Sir William Herschel fixed it, from his observations, at ten hours sixteen minutes. Professor Asaph Hall, from observations of a white spot near the equator, reduced this period to ten hours fourteen minutes twenty-four seconds. Stanley Williams and Denning, in 1891, reached results differing only by about two seconds from that of Hall; but the former, discussing observations of 1893, arrived at the conclusion that there were variations of rotation presented in different latitudes and longitudes of the planet's surface, the longest period being ten hours fifteen minutes, and the shortest ten hours twelve minutes forty-five seconds. Subsequently Keeler obtained, by spectroscopic methods, a result exactly agreeing with that of Hall. It appeared, therefore, that fairly satisfactory agreement had been reached on a mean period of ten hours fourteen minutes twenty-four seconds. In 1903, however, a number of bright spots appeared in a middle north latitude which, when observed by Barnard, Comas Solà, Denning, and other observers, gave a period remarkably longer than that deduced from spots in lower latitudes--namely, about ten hours thirty-eight minutes. Accordingly, it follows that the surface of Saturn's equatorial regions rotates much more rapidly than that of the regions further north--a state of affairs which presents an obvious likeness to that prevailing on Jupiter. But in the case of Saturn the equatorial current must move relatively to the rest of the surface at the enormous rate of from 800 to 900 miles an hour, a speed between three and four times greater than that of the corresponding current on Jupiter! The resemblance between the two great planets is thus very marked indeed. Great size, coupled with small density; very rapid rotation, with its accompaniment of large polar compression; and, even more markedly in the case of the more distant planet than in that of Jupiter, a variety of rotation periods for different markings, which indicates that these features have been thrown up from different strata of the planet's substance--such points of likeness are too significant to be ignored. It is not at all likely that Saturn has any solidity to speak of, any more than Jupiter; the probabilities all point in the direction of a comparatively small nucleus of somewhat greater solidity than the rest, surrounded by an immense condensation shell, where the products of various eruptions are represented.