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Rh duration"; and in the following year he confirmed his suspicion, and announced the period of rotation as 10 hours 16 minutes.

Five satellites of Saturn were known when Herschel commenced his study of the planet. His discovery of two exceedingly faint inner satellites, rendered fainter still by their proximity to the bright disc and ring of Saturn, was the result of long and continued investigation. He had entertained "strong suspicions" of the existence of a sixth satellite for a considerable time; and on 19th August, 1787, using the 20-foot reflector as a "front view," he noticed an object which he marked down as possibly a sixth moon. The possibility was rendered a certainty when on 28th August, 1789, he turned the new 40-foot on Saturn. Six satellites were manifestly visible, sharing in the planet's motion. On 17th September, he detected another satellite, still fainter, and closer to the ring, revolving in less than one terrestrial day. These moons were named Enceladus and Mimas. Two years later he announced that the fifth satellite Japetus performs its rotation, like the moons of Jupiter and our moon, in a period coincident with its revolution, These conclusions led him to an important generalisation, that "a certain uniform plan is carried on among the secondaries of our solar system; and we may conjecture that probably most of the moons of all the planets are governed by the same law".

The ring-system fascinated Herschel. In 1789 he suspected its division into two, and this suspicion was confirmed in 1791. "Its division into two very unequal parts can admit of no doubt." He appears to have seen the inner dusky or "crape" ring, but he did not recognise it as a portion of the system. In common with Laplace and contemporary astronomers, he believed the rings to be solid structures. In August, 1815, Herschel made his last observations of the planet, with both the 40-foot and the 20-foot telescopes.