that of the Earth; but as its bulk, or volume, is nearly 1,250 times greater, it follows that its density can only be one-quarter that of the Earth. Jupiter is passing through space at the rate of 28,743 miles an hour, and is also performing the equatorial revolution on its own axis at the rate of 27,726 miles an hour. As seen from the Earth, Jupiter does not present any sensible phase in ordinary telescopes, owing to its great distance from the Sun; but when observed through a powerful telescope the right or left edge of its disk shows occasionally considerable signs of want of illumination.

When viewed with the naked eye, Jupiter shares with Venus that universal attention which is always given to the evening and morning stars; but sometimes Jupiter shines with even greater splendour than Venus, especially when it is due south at midnight in the winter months. At these times, Jupiter passes the meridian at an altitude equal to that of the Sun in summer, while the light of Venus is frequently partially eclipsed by the twilight, or by the hazy nature of the atmosphere near the horizon. But when Venus is at its greatest brilliancy, the greater intensity of its reflected light makes it invariably the brighter planet of the two, although its diameter is much smaller than that of Jupiter. It is, however, as a telescopic object that Jupiter has become so popular and interesting to the astronomer, for by the application to the eye of a very ordinary telescope, the four attendant satellites or moons, and the distinctive lineaments of light and shade on its surface, become plainly visible. The motions of the satellites around Jupiter are very soon perceptible, as they are continually changing their positions with respect to the body of the planet. Sometimes they are seen to disappear into the shadow of Jupiter, and thus become totally eclipsed, similarly to our own Moon; at other times they are observed to pass behind the planet, and then reappear on the opposite side; and again at other times they may be noticed on the disk of the planet. This last appearance is a very interesting phenomenon, as not only is the image of the satellite projected on the disk, but its shadow also is generally seen at the same time as a small round black spot. On some occasions, Jupiter is seen apparently without satellites, which at these times are either obscured in the shadow, or projected on the disk of the planet, but this occurrence is very rare. The last phenomenon of this kind took place on August 21st, 1867, when, notwithstanding the general unfavourable state of the weather, some very interesting observations were made. The most curious was the appearance of the fourth satellite on the disk of Jupiter as a dark object, nearly as black as its shadow. From this observation, it has been inferred that the reflective power of this satellite, which is the most distant from Jupiter, must be greatly inferior to the other three, and that it is also of less intrinsic brightness than the body of the planet. The telescope also reveals to us that the surface of Jupiter is partially covered with brownish-grey streaks parallel with the equator. Two of these are very conspicuous, one north, the other south of the equator. They extend completely around the ball of the planet, for no great deviation in their form can be observed on opposite sides. These streaks, or belts, resemble in some measure the lines of stratus cloud often seen on

JUPITER.

calm evenings near the horizon about the time of sunset. Between the two principal belts, a more brilliant ground marks the equatorial region of the planet. Towards the poles, a continuation of parallel belts of different intensities can be observed. The illumination of the disk near the poles is evidently more feeble than near the equator. Occasionally dark round spots have been seen on the principal belts, which have afforded a good means for the determination of the time of rotation of the planet. Some excellent drawings of Jupiter have been made by Mr. De La Rue, Sir John Herschel, M. Mädler, and others.

The satellites of Jupiter were discovered by Galileo on January 7, 1610. He first imagined them to be fixed stars, in no way connected with the planet. By a mere accident he happened to examine Jupiter on the succeeding day, when he was surprised to find that the arrangement of the stars was quite different, three of them being now west of the planet, whereas on the preceding day two were on the east side, and one only on the west. On January 13, he saw four stars, when he came to the conclusion that they were small planets revolving around Jupiter in the same manner as the ordinary planets revolve around the Sun, forming, in fact, a miniature of the solar system. In the same way as they were first seen by Galileo through one of the newly-invented telescopes, so they can now be easily discerned through any ordinary instrument fitted with an astronomical eye-piece. The satellites are generally known by simple numbers in the order of distance from their primary, but they have been sometimes called Io, Europa, Ganymede, and Callisto. The periods of their revolution around Jupiter are:

The following diagram exhibits the apparent path of Jupiter from April 15th, 1868, to the end of February, 1869, including the period of retrograde motion in the Autumn of 1868:

SATURN.

Saturn is the sixth large planet from the Sun, and is one of the most beautiful celestial objects which come under the notice of the star-gazer. Saturn is a stupendous globe, about seven hundred times greater in volume than the Earth, surrounded by a series of rings of solid matter, and accompanied in its course around the Sun by eight satellites. All this complicated system moves with a common motion so exact that no part interferes with another in their orbital revolution, which is performed in rather less than thirty years. The mean distance of Saturn from the Sun is more than nine times that of the Earth, and nearly double the distance of Jupiter. The equatorial diameter of the ball is 74,000 miles, while its polar diameter is about 66,000 miles; its form is therefore sensibly elliptical, and is something the shape of a well-flattened orange. Streaks of light and shade have been observed on the ball, very similar to the better known belts of Jupiter, affording some kind of evidence of the existence of currents of air analogous to our trade winds. This supposition would lead us to infer that the surface of Saturn is surrounded by an atmosphere subject to all its attendant meteorological phenomena. Saturn revolves on its axis in 10h 29m 16s, consequently a Saturnian day is less than half a terrestrial day.

But the system of rings which encircle the central ball of Saturn is by far the most interesting appendage of this magnificent planet. Before the invention of the telescope, the existence of the rings was unknown. Even the veteran Galileo was unable to view them satisfactorily, owing to the low penetrating power of the telescopes of his time. According to M. Arago, this low-defining power of Galileo's telescopes was a source of great perplexity to him. In a letter to the Grand Duke of Tuscany, he announced that the planet was "three-bodied," remarking that when he observed Saturn with a telescope magnifying thirty times, the central object appeared the greatest, the two others being attached, one on the east, and the other on the west side of the principal ball. At a later period Galileo observed Saturn at the time of one of its equinoxes, when the plane of the ring passes through the Sun, which then illuminates only the edge of it. On this occasion Saturn appeared to the illustrious astronomer as a perfectly round object; he was therefore led to conclude that his previous observations were nothing more than optical illusions. Galileo died in 1642, and it was not till Huygens observed the rings, in 1659, that a true explanation of the cause of the phenomenon was given. The gradual improvement in the construction of telescopes soon, however, supplied the means for observing the form and dimensions of the two bright rings as we see them at the present day, excepting only that the superior definition given by modern telescopes has enabled us to make further discoveries of great importance. For instance, the exterior ring, or that farthest from the planet, is seen separated from the intermediate ring by an empty space, showing that they are quite independent of each other. Between these and the ball of the planet, a dusky, or semi-transparent ring has been observed

since 1850.

SATURN.

At these times of

The tint, or colour of the rings, is varied, the intermediate ring being the most brilliant of the three. The two exterior rings are opaque, and cast a very decided shadow on the disk of Saturn; but the dusky ring is evidently of a gaseous nature, as a part of the luminous disk can be seen through it. The thickness of the rings is very minute, so much so indeed, that when the edge is precisely directed to the Sun, it is nearly invisible even in telescopes of great power. It is assumed from this that the thickness cannot be much greater than 200 miles. its equinox, Saturn has the same globular, or rather spheroidal, appearance as the other large planets; and advantage is always taken of the invisibility of the rings to obtain accurate measures of the form of the central ball. The phenomenon of the disappearance of the rings of Saturn takes place at intervals of about fifteen years. The mass, or weight of Saturn, is about ninety times greater than that of the Earth. The density is, however, only about the eighth part of the Earth's, or, in more

familiar words, a cubic foot of the material of Saturn is eight times lighter than a cubic foot of the material of which the Earth is composed. Saturn moves in its orbit around the Sun at the rate of 21,220 miles per hour, and also turns on its axis at the equator 22,216 miles in the same time. It is impossible for any one to view this magnificent object through a large telescope without admiration, especially when the planet is in such a position with respect to the Sun and Earth as to exhibit the fullest extent of the rings. Many observers have, on such occasions, made elaborate drawings, showing all the minute peculiarities of the planet-those by Mr. Warren De La Rue and Mr. Dawes being almost perfect delineations of the disk and rings. Of the eight moons of Saturn, five were discovered between 1655 and 1684, two by Sir William Herschel in 1789, and one by Messrs. Lassell and Bond, in 1848. The satellites of Saturn are distinguished by the names of Titan, Japetus, Rhea, Dione, Tethys, Enceladus, Mimas, and Hyperion. The three last are very faint objects, visible only occasionally in our

largest telescopes. It has been remarked by Sir John Herschel, that at the time Enceladus and Mimas were discovered by his father, "they were seen to thread, like beads, the almost infinitely thin fibre of light to which the ring, then seen edgeways, was reduced, and for a short time to advance off it at either end, speedily to return, and hastening to their habitual concealment behind the body."

URANUS.

Uranus is the next planet in order of distance from the Sun. Like Jupiter and Saturn, it is the centre of a system, having at least four satellites, while it has probably more, but too small to be detected in our telescopes. The discovery of this large planet by Sir William Herschel in 1781, March 13th, was to some extent accidental. On the evening of that day the illustrious astronomer was examining the small stars in the neighbourhood of H. Geminorum, when he noticed one which seemed much lighter than the rest. The magnitude of this object was so much greater than any which he expected to find there, that his suspicions were aroused; and after watching it for some time he concluded that the stranger was a comet. Being engaged at the time in a series of observations on the parallax of the fixed stars, he had near him several eyepieces of high magnifying power, with which he examined the new object. "I put on," he remarks, "two powers, 460 and 932, and found the diameter of the comet increased in proportion to the power, as it ought to be, on a supposition of its not being a fixed star, while the diameter of the stars to which I compared it was not increased in the same ratio. Moreover, the comet being magnified much beyond what its light would admit of, appeared hazy and ill-defined with these great powers, while the stars presented that lustre and distinctness which, from many thousand observations, I knew they would retain."

Observations of the supposed comet were made, not only by Herschel, but by all the principal astronomers of Europe. Owing, however, to its slow motion in the heavens, several months elapsed before its planetary nature was even conjectured, when it was provisionally placed in the solar system exterior to Saturn. It was not until Laplace communicated to the Academy of Sciences in January, 1783, elliptic elements of the planet, that all doubt was removed. Herschel, as the discoverer, gave the new planet the name of Georgium Sidus, but the French astronomers resolved to know it only by the name of Herschel. Professor Bode proposed Uranus, which was at first unpopular, but this name has been gradually superseding the other two, until, by universal consent, it is now adopted by the astronomers of all countries.

As a telescopic object, Uranus appears with a small round disk shining with a pale yellowish light. It is accompanied in its journey round the Sun by several satellites, but owing to its immense distance from us they are only seen, even in the best of telescopes, with the greatest difficulty. Herschel has recorded observations of six, but some of them have never been confirmed by later astronomers. In fact, we