times bright spots, sometimes dark ones, are seen, which have enabled us to determine the period of the planet's rotation, which, as we have seen, is very rapid—so rapid that on the equator an observer would be carried round at the rate of 467 miles a minute, instead of 17 as on the Earth. It is remarkable that spots near the equator travel faster than those remote from it, just as they do on the Sun, differences of as much as 7 minutes having been observed in the periods of rotation derived from differently situated spots. An enormous red spot, for instance, 30,000 miles long by nearly 7,000 wide, which appeared in 1878, and is still faintly visible, took about 5 minutes longer to complete a rotation than a brilliant white spot conspicuous at the same time. This sun-like mode of rotation, and other facts, favour the opinion that the internal temperature of Jupiter is still very high; and the same conclusion probably holds good for Saturn, Uranus, and Neptune.

265. Although all astronomers do not agree that the surface of the planet is never seen, there are many strong reasons why it should not be seen. In the first place, Mars and the Earth, whose atmospheres are nearly alike, have nearly the same densities (Art. 145), while in the case of Jupiter and Saturn-the belts of which latter planet, as far as we can observe them, resemble Jupiter's —the density, as calculated on the idea that what we see is all planet, is only about one-fifth that of the Earth; and as the density of the Earth is 5 times that of water, it follows that the densities of the two planets in question are not far off that of water.

266. Now, if we suppose that the apparent volume of Jupiter (and similarly of Saturn) is made up of a large shell of cloudy atmosphere and a kernel of planet, there is no reason why the density of the real Jupiter (and of the real Saturn) should vary very much from that of the Earth or Mars, and this would save us from the waterplanet hypothesis. Moreover, a large shell of cloudy

atmosphere is precisely what our own planet was most probably enveloped in, in one of the early stages of its history (Art. 208).

267. In addition to the changing features of Jupiter itself, the telescope reveals to us four moons, which as they course along rapidly in their orbits, and as these orbits lie nearly in the plane of the planet's orbit, lend a

great additional interest to the picture. In the various positions in their orbits the satellites sometimes appear at a great distance from the primary; sometimes they come between us and the planet, appearing now as bright and now as dark spots on its surface. At other times they pass between the planet and the Sun, throwing their shadows on the planet's disk, and causing, in fact, eclipses of the Sun. They also enter into the shadow cast by the planet, and are therefore eclipsed themselves; and sometimes they pass behind the planet and are said to be occulted. Of these appearances we shall have more to say by and by (Lesson XXXVI.).

268. Referring to the sizes of these moons and their distances from the planet, in Table III. of the Appendix, it may be here added that, like our Moon, they rotate on their axes in the same time as they revolve round Jupiter. This has been inferred from the fact that their light varies, and that they are always brightest and dullest in the same positions with regard to Jupiter and the Sun.

In a

269. In Plate X. the black spot is the shadow of a satellite, and the satellite itself is seen to the left; the passage of either a satellite or shadow is called a transit. solar eclipse, could we observe it from Venus, we should see a similar spot sweeping over the Earth's surface.

270. We now come to Saturn; and here again, as in

the case of Jupiter, we come upon another departure from Mars and the Earth. The planet itself, which is belted like Jupiter, is surrounded not only by eight moons, but by a series of rings, one of which, the inner one, is transparent! The belts

FIG. 26.-Saturn and his Moons (general view).

that as the equator of Saturn, unlike that of Jupiter, is greatly inclined to the ecliptic, transits, eclipses, and occultations of the satellites, the orbits of which for the most part lie in the plane of Saturn's equator, happen but rarely. 271. It is to the rings that most of the interest of this planet attaches. We may well imagine how sorely puzzled the earlier observers, with their very imperfect telescopes, were, by these strange appendages. The planet at first was supposed to resemble a vase; hence the name Ansa, or handles, given to the rings in certain positions of the planet. It was next supposed to consist of three bodies, the largest one in the middle. The true nature of the rings was discovered by Huyghens in 1655, who announced it in this curious form :

ааааааа CCCCC d eeeee g h iiiiiii 1111 mm nnnnnnnnn 0000 pp q rr S ttttt uuuuu,"

which letters, transposed, read :

"annulo cingitur, tenui plano, nusquam cohaerente. ad eclipticam inclinato."

There is nothing more encouraging in the history of astronomy than the way in which eye and mind have bridged over the tremendous gap which separates us from this planet. By degrees the fact that the appearance was due to a ring was determined; then a separation was noticed dividing the ring into two; further observations suggested to the French astronomers Du Séjour and Lalande that the number of rings should be multiplied many-fold; the extreme thinness of the ring came out next, when Sir William Herschel observed the satellites "like pearls strung on a silver thread." The making out of the transparent ring by Dawes and Bond followed in 1850; then the transparent ring was discovered to be divided as the whole system had once been thought to be; last of all comes evidence that the smaller divisions in the various rings are subject to change, and that the ringsystem itself is probably increasing in breadth, and approaching the planet.

LESSON XXI.—The other Planets compared with the Earth (continued). Dimensions of Saturn and his Rings. Probable Nature of the Rings. Effects produced by the Rings on the Planet. Uranus. Neptune : its Discovery.

The lower figure of Plate X. will give an idea of the appearance presented by the planet and its strange and beautiful appendage. It will be shown in the sequel (Chap. IV.) how we see, sometimes one surface, and sometimes another, of the ring, and how at other times the edge of it is alone visible.

272. The ring-system is situated in the plane of the planet's equator, and its dimensions are as follow:

so that the breadths of the three principal rings, and of

the entire system, are as follow:

and the distance between the two bright rings is 1,680 miles.

In spite of this enormous breadth, the thickness of the rings is not supposed to exceed 100 miles.

273. Of what, then, are these rings composed? There is great reason for believing that they are neither solid nor liquid; and the idea now generally accepted is that they are composed of myriads of satellites, or little bodies, moving independently, each in its own orbit, round the planet; giving rise to the appearance of a bright ring when they are closely packed together, and a very dim one when they are most scattered. In this way we may account for the varying brightness of the different parts, and for the haziness on both sides of the ring near the planet (shown in Fig. 26), which is supposed to be due to the bodies being drawn out of the ring by the attraction of the planet.

274. Although Saturn appears to resemble Jupiter in its atmospheric conditions, unlike that planet, and like our own Earth, its year, owing to the great inclination of its