They are supposed to be clouds floating about in the atmosphere of the planet; or rather, perhaps, the darker body of the planet appearing through the atmosphere. Their parallelism to Jupiter's equator may arise from currents of air, somewhat analogous to our tradewinds, setting either east or west, but with much greater constancy and regularity.

104. The distinguishing feature of the planet Jupiter is his being accompanied by four moons, which revolve round him in periods of time varying from 1 day 18 hours to 16 days.

As the planet casts a shadow behind it, these moons are constantly suffering eclipses, and not only do they disappear by immersion into the shadow, but by passing behind the planet, and even when between us and Jupiter they are invisible, except with a superior telescope. In the latter case, the shadow of the satellite may be seen, under favourable circumstances and with a good glass, projected on the planet as a dark round spot. The times of these very interesting occurrences are given every month in the Nautical Almanac.

The moons of Jupiter form, with the planet as a

<

central body, a planetary system in miniature; subject to all the laws and obedient to the same forces as those which regulate the connexion of the sun and planets, and which will be explained under the head of Physical Astronomy. Their orbits are ellipses slightly eccentric, having the planet in one of the foci; they describe equal areas in equal times (§§ 201-203), and the cubes of their mean distances are in the proportion of the squares of their periodic times.

The third and fourth are thought to be about the size of Mercury; the first and second about as large as our moon. From the proportion which has been discovered by comparing the periodic times of the first three satellites, a most singular and beautiful result has been obtained. The mean sidereal revolution of the first is about half the time of the second; which latter is about half that of the revolution of the third. Again, the mean longitude of the first, minus three times that of the second, plus twice that of the third, is always equal to 180°: hence it results, that when the first satellite is eclipsed, the other two will always dispense their light; and vice versâ.

105. Jupiter's equator is but very slightly inclined to his orbit, and the paths of his satellites are nearly in the plane of his equator. All the satellites except the fourth eclipse the sun at every revolution; while, from the bulk of their primary, they are themselves eclipsed at every revolution by entering his shadow. In the course of a Jovian year there will occur no less than 4500 eclipses of the moons, and about the same number of eclipses of the sun. When Jupiter is about 90° from the sun, or comes to the meridian at or

near six o'clock, his shadow is projected laterally, in a direction opposite to that of the sun in this case the immersions of the satellites take place at a considerable distance from the body of the planet; and in the case of the third and fourth satellites they often vanish and re-appear on the same side of the planet. The orbit of the fourth is so much inclined, that he escapes, at regular periods, being eclipsed at all. The mean duration of an eclipse

By accurate micrometrical measurements of the distances of the satellites, compared with their periodic times, the Astronomer Royal has determined the mass of Jupiter, as compared with that of the sun, to be as 1 to 1046.77. The density of Jupiter is not more than one-quarter of that of the earth; so that the force of gravity on his surface is not so great as we should conclude, from taking into consideration only his immense volume.

106. The beautiful discovery of the velocity of light arose from the observation of the eclipses of Jupiter's satellites. When Jupiter is in opposition (viz. at x, fig. 16, Plate III.), it was discovered by a Danish astronomer, Romer, in 1675, after a series of observations extending through many years, that the eclipses happened much sooner than the calculated time; on the contrary, when the earth was at its greatest distance from Jupiter, the eclipses were always later than he expected. The explanation given by him, and con

firmed by the subsequent discovery of the aberration of light, was, that the time in the latter instance was lost by the last ray sent out by the satellite before its immersion into the shadow of Jupiter, having to pass through a distance greater than in the former case by the whole diameter of the earth's orbit, before the eclipse could be announced to the inhabitants of the earth. The time occupied by a ray of light in traversing this space of 190,000,000 miles has been found to be 16 minutes 26 seconds; so that, since the time of this discovery, when the eclipses of Jupiter's satellites are calculated, allowance is made for the distance of the planet from the earth, before the occurrence is tabulated. These eclipses are so nicely calculated, that the instant of ingress or egress of a satellite is known to a second; and hence they are made use of to determine the longitude of any place on the globe, as will be shewn more at large under the head of Practical Astronomy.

107. The planet Saturn exhibits a phenomenon which never would have entered into the mind of man to conceive, had not the telescope revealed it to us: it is surrounded by a ring, or rather by several concentric rings, somewhat similar to the horizon around a globe, but at a greater comparative distance. That these rings are opaque is proved by the shadow cast by them on the body of the planet; at the same time the shadow of the planet on the rings may be distinctly seen. The attention of astronomers has lately been directed to this planet in consequence of its favourable situation for observation, while most powerful telescopes have assisted their investigations. Mr. Lassell of Liverpool has

discovered an eighth satellite; Mr. Bond of Cambridge, Massachusetts, and the Rev. W. R. Dawes, independently of each other, have remarked a dark ring between the well-known ring and the body of the planet. It is, however, transparent; for the portion which crosses the planet is of a lighter shade than the rest, as shewn in the drawing. What was originally considered as the ring of Saturn is now found to consist of certainly two, and probably several, distinct rings, though the divisions in the outermost ring are made out only occasionally and with difficulty. These rings are found to revolve round the planet in a period equivalent to that of a satellite at the mean distance of the rings from the planet. Through this revolution they are able to maintain themselves in stable equilibrium, and there will consequently be no fear of their falling upon the planet, from which they are preserved by centrifugal force, in the same manner as the moon is kept at a distance from the earth. Were the rings stationary, the slightest disturbance from attraction of a body beyond would cause them to fall upon the planet, and not recover their position.

108. Saturn also exhibits belts like Jupiter; and thus we may suppose him also to have an atmospheric envelope, like the other members of the planetary system.

109. The figure of Saturn is the flattest of all the planets at the poles; for, in addition to the centrifugal force, which is very considerable, seeing that he revolves in 10 hours 19 minutes 17 seconds, the attraction of the ring over his equator has aided the accumulation of matter in that region: hence, from these two causes arises the remarkable difference in the polar and equa