so rapidly as to communicate to any body on their surfaces a very powerful tendency to fly off, which is, however, counterbalanced by the effect of gravity. But if Jupiter's rotation were only four times faster than it is, the centrifugal force would be so great that all the inhabitants would be sent flying off through the air or rather along with it, for it would go too. When the impulse with which they started was lost, they would of course fall back to the ground, but only to be shot off again at once; and in this state of perpetual oscillation, bouncing up and down like an india-rubber ball, they would spend all their lives, unless they took some means of anchoring themselves to the surface of their planet.

The class of phenomena which we have been last considering depend all of them upon the positions and movements of the planets, and are hence common, with various. modifications, to the whole of them. But besides these there are connected with all of

them special points of individual interest, arising from circumstances peculiar to them

selves alone, and over these we must cast a rapid glance before we proceed in our excursion to visit a new set of worlds.

Of the first of the planets, Mercury, we know but little. From the closeness of his proximity to the sun he can never be seen with the naked eye, except occasionally for a few moments close to the horizon, immediately after sunset or before sunrise; and even these hurried glimpses cannot be got except at considerable intervals and under very favourable circumstances. Hence, though his existence seems to have been known from a very early period, he was comparatively seldom seen before the invention of the telescope. Copernicus lamented upon his death-bed that he had never been able to catch a glimpse of Mercury at all; the mists from the marshes of the Vistula too obstinately fringed the morning and evening horizon round the Observatory of Thorn. A distinguished French

* It is calculated that Mercury, Venus, and the Earth will, from a similar reason, never be visible at all from the surface of Uranus.

astronomer of the same period only saw him twice. The telescope when turned upon him shews us little but a small round disc, which exhibits phases, like the moon, according to its relative positions with regard to the sun and to the earth. Recent observations have revealed enormously lofty mountains upon his surface, eight times as high in proportion to their planet as the Himalayas are to our own globe. The proximity of Mercury to the sun, the eccentricity of his orbit, and the fact that he is unattended by any satellites, rendered the determination of his mass and other elements a matter of much difficulty, and great discrepancies exist between the earlier estimates of them. Fortunately his small size, and the consequent insignificance of the perturbations he produces in the other planets, diminished the importance of having an accurate knowledge of him. Any similar uncertainty about one of the larger planets would have interposed most serious obstacles to the progress of science, and would, for example, have rendered the discovery of Neptune impossible.

over

It is at present uncertain whether there are any planets within the orbit of Mercury. If there are, their light must be so powered by that of the sun as to render them visible only when he is under eclipse, or when they are passing across his disc, in which case they would appear as small black spots. Astronomers have occasionally fancied that they detected planets under the latter circumstances, but they have never felt certain that what they saw were not merely some of the ordinary spots on the sun. A French astronomer, M. Lescarbault, felt pretty confident on one occasion that he had found a real planet, to which he gave the name of Vulcan; but twenty years have passed away, and the discovery has never been confirmed. It was hoped that at the recent total eclipse Vulcan might have been seen near the edge of the moon's disc when the sun's light was cut off; but if he really exists, he lost the glorious chance then offered him of proving the fact, by perversely hiding behind the sun, or between it and the moon.

With Venus we are all familiar. It is the most brilliant of all the planetary or stellar orbs; and the "Star of the Evening, Beautiful Star," has been sung by poets of every age and clime, from Homer to the Christy Minstrels. Like Mercury, and for the same reason, Venus is seldom seen except about sunrise or sunset; but as her elongation from the sun, though limited, is much greater than that of Mercury, she is very frequently visible. Sometimes even, though at rare intervals, she is sufficiently near us to be seen when the sun is above the horizon; and the sight of the little planet, shining softly out in fearless companionship with the dazzling orb of day, is described as singularly striking and beautiful. Varro relates a tradition that Venus shone thus at noonday, a most auspicious portent, upon Æneas' voyage from Troy to Italy. And on the occasion of one of the first Napoleon's triumphal entries into Paris after a successful campaign, Venus joined in the pageant of the procession; exciting the intensest enthusiasm among the populace,