Account of the Discovery of a Sixth and Seventh Satellite of the Planet Saturn; with Remarks on the Construction of its Ring, its Atmosphere, its Rotation on an Axis, and its spheroidical Figure

1790. London, Peter Elmsly, 1790. 4to. In recent marbled paper wrappers. Extracted from "Philosophical Transactions". Including title-page of volume. Title-page with professional repair to lower margin, not affecting text. Leaves reinforced in margin. Very light brownspotting through out and light offsetting from folding plate as usual. (2), iii-vi, (2), (1)-20 pp. + three folded plates. First edition of Herschel's important paper on his observations of the rings of Saturn. "Saturn exercised a special fascination for Herschel, and between 1789 and 1808 he devoted seven papers and part of an eighth to the planet, its ring, and its satellites.On 19 August 1787 Herschel suspected he had found a sixth and previously unknown satellite, but he was not able to confirm this until 28 August 1789, when his forty-foot telescope came into commission. A few days later he found a seventh satellite. For some months he carefully tracked the satellites, establishing for Mimas and Enceladus periods within seconds of the modern values, and giving evidence to show that Iapetus rotates in its period of revolution.He also made careful observations of the rings, which he believed to be solid. As the earth happened to be in the plane of the ring structure at the time, he compared the thickness of the ring when seen edge-on with the diameter of Jupiter's satellites; and although his estimate exceeds modern values, his method showed that the thickness did not exceed a few hundred miles." (DSB)