In accordance with the suggestion of Sir John Herschel, the names of the Titanian divinities are now commonly applied to the satellites. In order of distance from the primary, they rank as follows:

The first, second, and third satellites are much nearer the planet than the terrestrial moon is to the earth; the fourth is also slightly nearer; but the eighth, the most remote, is upwards of two millions of miles from the primary, or ten times the distance of our moon from us. Nothing is known with exactness respecting their magnitude, except that the sixth, Titan, is the largest, and is perhaps but little inferior to the planet Mars. Their periods of revolution range from one to seventynine days.

The wonderful orb of Saturn has a diameter of 70,000 miles, and a volume which is more than eight hundred times the size of the earth. Yet the planet is carried round upon its axis in about ten hours and a half, moving with a velocity at the equator which is more than twenty times greater than the earth's velocity of rotation at the same part of its surface. The dimensions of the whole system are in round numbers as follows:

Yet so vast is the distance to which this mighty system is removed from us, that the disk of our moon is more than sufficient to hide it altogether from our sight.

The annular appendages of the planet have a very inconsiderable thickness, perhaps not amounting to more than from one to two hundred miles. Such indeed is their thinness, that when the minutest of the satellites, which can only be reached by telescopes of extraordinary power, appears on the edge, it projects on the opposite sides, above and below. Herschel once saw his two little moons in this position, as beads moving along a line of light, "like pearls strung on a silver thread." It is owing to this extreme thinness that the rings are not discernible by ordinary instruments, when the earth being in their plane, only the edge is presented to us. This occurs twice in every revolution of the planet round the sun, at opposite points of its orbit, and at intervals apart from each other by about fifteen years. The last instance was in the year 1862, and the next will be in 1877. In the course of the periodical revolution, the rings are seen under very varying aspects. Alternately we behold one side and then the other; and oval forms of different ellipticity are presented. These are the phases of Saturn, unknown to the shepherd-astronomers of Chaldea, the star-gazers of Egypt, Greece, and Rome, the astrologers of the middle ages, Copernicus and Tycho Brahe, who were only acquainted with one of the most surprising objects in the solar universe, as a dull nebulous star, slowly moving in the heavens.

Placed in the system at nearly twice the distance of Jupiter, or 872 millions of miles from the sun, Saturn has an enormous orbital path to describe, and occupies a period of 29 terrestrial years in accomplishing his

appointed revolution round the central globe. In consequence of the immense distance, the sun, as seen

from the planet, will dwindle down to an orb of oneninth the apparent diameter exhibited to us; and will afford little more than one-eightieth of the light and heat supplied to the carth. But as some compensation, the solar glory returns twice in a terrestrial day to the meridian of any place on the Saturnian globe, owing to the velocity of rotation, while a plentiful supply of reflected light is furnished by means of the satellites and rings. The former by their number, and the latter by their extent, will splendidly adorn the nightsky of the planet.

CHAPTER XI.

URANUS-NEPTUNE.

Aspect of Uranus-Diameter, Distance, and Period-Proportion of Solar Light and Heat-Satellites of the Planet-Their Peculiarity-Discovery of Uranus-Previous Observations-Sir W. Herschel - His Arrival in England - Residence at Bath-Patronized by George III.-Settlement at Slough The Forty-Feet Reflector-Space-Penetrating Power of Telescopes-Description of the Giant Reflector-Death of Herschel-His Sister -Neptune-Unexplained Perturbations of Uranus-Inference from them -Researches of Adams and Leverrier-Discovery of Neptune-A Satellite Discovered-Cincinnati Observatory-Old Observation of NeptuneMagnitude and Distance of the Planet-Details of the Solar SystemIts Great Scale.

THE far remote orb of Uranus, recently regarded as defining the outer boundary of the system, is scarcely to be distinguished by the naked eye, though seventyfive times larger than the earth. But under favourable circumstances, as when the sky is very clear, and the moon absent, a practised gazer, knowing where to look, may detect him, appearing commonly as a star of the sixth magnitude, shining with a subdued light. Viewed with high magnifying powers, a distinct and well-defined disk is exhibited, which presents a uniform greyish tint, without the slightest symptom of spots, belts, or rings. The planet has a diameter of 34,000 miles, and is at the mean distance of 1754 millions of miles from the sun. His period of revolution comprises eighty-four of our years. Hence, as his place in the system was discovered in the year 1781, little more than a single revolution since that date has been accomplished. But slow as is the apparent pace of

Uranus, the actual rate amounts to fifteen thousand miles an hour, from which the immense length of the orbital march may be inferred. Owing to the enormous distance of the planet, the proportion of light and heat received from the sun is 360 times less than that enjoyed by the earth; and the diameter of the solar disk appears nineteen times less than it does to us. But though thus apparently reduced to a speck, the illuminating power of the sun to that remote world will be equal to the light afforded by three hundred of our full moons, while we know certainly, with reference to the temperature of the earth's surface, that it does not depend solely upon the direct rays of the sun, but on the condition of its atmosphere, and the internal heat of its mass. Nothing is known of the physical constitution of Uranus, nor is it likely that this will be illustrated, on account of the vast intervening distance. Though stated by some to rotate upon an axis in nine and a half hours, the fact of the rotation itself is by others considered to be doubtful.

Uranus is attended by several satellites. Although the number has been set down as six, yet it is believed that only four have ever been observed. The most powerful instruments of modern days, used by the most skilful observers, have failed to indicate the presence of more. The known satellites are named according to the order of their distances from the primary, Ariel, Umbriel, Titania, and Oberon. The first two are the nearest to Uranus, and were discovered in 1847. Titania and Oberon are the only two out of the six supposed to have been discovered by Sir W. Herschel which have ever been identified by another observer; and though easily seen under favourable circumstances, it has only been since the year 1828 that his observation of these two has been verified. In that year, his son, Sir