Rotation of ring, Herschel found that it revolves in its own plane in the rings. JQ hours 32 min. 15 4 sec. ; and Laplace arrived at the same result from theory. It is worthy of remark, that this is the period in which a satellite, having for its orbit the mean circumference of the ring, would complete its
revolution according to the third law of Kepler.
Amongst the most remarkable discoveries of recent times with regard to the rings of Saturn is that of the inner dusky or semi-transparent ring, sufficiently obvious to any observer capable of using well a moderately good telescope, but which, previously to the year 1 850, was only once men tioned by any astronomer. Dr Galle of Berlin saw this ring with the Berlin refractor in 1838, but the attention of the scientific world was not generally drawn to it. The account of Galle s observations is accompanied by drawings exhibiting the trace of the dusky ring where it crosses the body of the planet. In the year 1850 the ring was recog nised almost simultaneously by two observers, namely, by Prof. Bond of the Cambridge Observatory, U.S., and by Mr Dawes. Since that time there has been no difficulty in seeing this appendage, though it requires a practised eye and a good telescope.
At the time of the discovery of the dusky ring, Mr Dawes also satisfactorily established the fact of the divi sion of the exterior ring near its outer extremity; and sub sequently he observed a series of discontinuous grada tions of colour or intensity of brightness in a portion of the inner bright ring. He observes that " the exterior por tion of the inner bright ring to about one-fourth of its whole breadth was very bright, but, interior to this, the shading-off did not appear, as under ordinary circumstances, to become deeper towards the inner edge without any dis tinct or sudden gradations of shade ; on the contrary, it was clearly seen to be arranged in a series of narrow con centric bands, each of which was darker than the next exterior one. Four such were distinctly made out : they looked like steps leading down to the black chasm between the ring and the ball. The impression I received was, that they were separate rings, but too close together for the divisions to be seen in black lines." This curious pheno menon was confirmed afterwards by Professor Bond.
Captain Jacobs, at the Madras Observatory, discovered that the dusky ring is semi-transparent, the body of the planet being visible through it. M. Otto Struve shows with tolerable certainty that the inner or dusky ring is not a modern appendage to the planet, but that, at the begin ning of the 18th century, the dark line thrown by it across the planet was known by the name of the equatorial belt. He also finds reason to believe, from a comparison of the measures of Huyghens, Cassini, Bradley, Herschel, W. Struve, Encke. Galle, and himself, that the inner edge of the interior brig/t ring is gradually approaching u>e body of the planet, while at the same time the total breadth of the two bright rings is constantly increasing.
The opinion now generally entertained respecting the Saturnian ring-system is, that it is composed of multitudes of minute satellites, probably intermixed with vapour, travelling independently around the planet. On no other supposition, indeed, can the permanence of the ring-system be explained.
From observations of some obscure belts, and a very conspicuous spot on the surface of Saturn, Sir W. Herschel concluded that his rotation is performed in 10 hours 16 min., on an axis perpendicular to the belts and to the plane of the ring ; so that the planes of the planet s equator and ring coincide. According to the same astronomer, the ratio of the equatorial and polar diameters of Saturn is 2281 to 2061, or nearly 11 to 10. He also observed that the globe of Saturn appeared to be flattened at the equator as well as at the poles. The compression he thought to extend to a great distance over the surface of the planet, and the greatest diameter to be that of the parallel of 43 of latitude, where, consequently, the curvature of the meridians is also the greatest. The disk of Saturn, there fore, resembled a square of which the four corners have been rounded off. According to recent measures the shape of the planet has been found to be that of an exact spheroid of revolution ; but it does not follow that this is always the case. Applying to Saturn reasoning similar to that already employed in the case of Jupiter, we see that the atmospheric envelope may be so deep that the cloud-layers which the astronomer really measures may lie at different levels, under the varying conditions to which the planet is subject.
Saturn is surrounded by a system of eight satellites the most extended as well as most numerous subordinate system within the sun s domain. The span of the orbit of the outermost satellite amounts to nearly 4 millions of miles. This satellite is probably larger than our moon, while Titan, the 6th satellite, is nearly as large as the planet Mercury. The elements of the satellites and the names of their discoverers are indicated in the table at p. 783. Their motions are less interesting to observers than those of Jupiter s satellites, because, owing to their considerable inclination to the plane of Saturn s orbit, they are seldom occulted by the planet, or transit across his disk.
Chapter XVI.—The Planets Uranus and Neptune.
Uranus is the seventh primary planet in order of distance Uranus. from the sun, and, with the exception of Neptune, the remotest. His mean distance from the sun exceeds the earth s more than 19 times, and amounts to 1,753,869,000 miles. But the eccentricity of the orbit is considerable, amounting to 0466 ; consequently, the variation of the planet s distance is also great, his greatest distance amount ing to 1,835,561,000 miles, his least to 1,672,177,000 miles. Subtracting roughly from his greatest, mean, and least distance the earth s distance, we obtain the opposition distances 1,744,000,000 miles, 1,662,500,000 miles, and 1,581,000,000 miles respectively; and, therefore, it is manifest that the planet is seen under much more favour able conditions in some oppositions than in others, especi ally if it is remembered that when at one of his nearer oppositions, Uranus, being also nearer to the sun, is more strongly illuminated, a point of great importance in the case of a planet so far from the sun. The sidereal revolu tion of Uranus is performed in 30,686-8208 days, or 84 years 6 days. His mean diameter is about 33,000 miles, and, as seen from the earth, Uranus in opposition subtends an angle of less than 4 // . The apparent diameter of the sun seen from Uranns is less than T Vth of the diar, ^.ter of the sun as we see him, or is but about ! ; and the ap parent surface of the sun is but about the 367th part of the apparent surface of the sun we see. Accordingly, the light and heat received by Uranus are less in the same proportion. This refers, of course, only to the light and heat received per unit of surface. The whole globe of Uranus receives about -^ th part of the heat which is received by the whole globe of the earth. It has been asserted that Uranus turns upon an axis in about 9 J hours, but the evidence on which this statement rests is slight and insufficient.
13, 1781, while he was examining the small stars in the of neighbourhood of 77 Geminorum. Being struck with the enormous magnitude of a star in this region, he suspected it to be a comet. He examined it with higher power, and found the disk enlarged, which would not be the case with
a star. He therefore announced the discovery of a comet.