Uranus to be represented by a very large wooden hoop floating on a sheet of water; then, if a small wooden hoop were so weighted as to float almost upright, with one half out of the water, the position of that hoop would represent the position of the path of one of the planet's satellites. It will be seen at once that if we suppose a body to travel round (and upon) the former hoop in a certain direction, then a body travelling round the latter hoop could scarcely be said to travel in the same direction, whether it circled one way or the other. Or to employ another illustration - if a watch be laid face upward on a table, we should correctly say that its hands move from east through south to west; but, if it be held nearly upright and the face rather upwards, we should scarcely say that the hands moved from east through south to west; nor if the face were tilted a little further forward, so as to be inclined rather downwards, should we say that the hands move from east through north to west.

The great slope or tilt of the paths is undoubtedly a more singular feature than the direction of motion. Implying as it does that the planet's globe is similarly tilted, it suggests the strangest conceptions as to the seasonal changes of the planet. It seems impossible to suppose that the inhabitants of Uranus, if there are any, can depend on the sun for their supply of heat. The vast distance of Uranus from the sun, although reducing the heat-supply to much less than the three-hundredth part of that which we receive, is yet an insignificant circumstance by comparison with the axial tilt. One can understand at least the possibility that some peculiarity in the atmosphere of the planet might serve to remedy the effects of the former circumstance; precisely as our English climate is tempered by the abundant moisture with which the air is ordinarily laden. But while we can conceive that the minute and almost starlike sun of the Uranian skies may supply much more heat than its mere dimensions would lead us to expect, it is difficult indeed to understand how the absence of that sun for years from the Uranian sky can be adequately compensated. Yet in Uranian latitudes corresponding to the latitude of London the sun remains below the horizon for about twenty-three of our years in succession. Such is the Arcticl night of regions in Uranus occupying a position corresponding to that of places in our temperate zone.

But the most important result of the discovery of the satellites has been the determination of the mass or weight of the planet, whence also the mean density of its substance has been ascertained. It has been thus discovered that, like Jupiter and Saturn, Uranus is constructed of much lighter materials than the earth. Our earth would outweigh almost exactly six times a globe as large as the earth but no denser than Uranus. It is to be noticed that in this respect the outer planets resemble the sun, whose density is but about one-fourth that of the earth. It seems impossible that the apparent size of any one of the outer planets can truly indicate the dimensions of its real globe. An atmosphere of enormous extent must needs surround, it would seem, the liquid or solid nucleus which probably exists within the orb we see.

1 It has been remarked that there is some incongruity in the name Arctic planets which I have assigned in my 'Other Worlds' to Uranus and Neptune, when considered with reference to the theory I have enunciated that these planets still retain an enormous amount of inherent heat. Many seem to imagine that the term arctic implies cold. I have, of course, only used the name as indicating the distance of Uranus and Neptune from the sun.

In the case of Jupiter or Saturn, the telescope has told us much which bears on this point; and as I have indicated in my 'Other Worlds,' and elsewhere, there is an overwhelming mass of evidence in favour of the theory that those orbs are still instinct with their primeval fires. But in the case of Uranus, it might well be deemed hopeless to pursue such inquiries, otherwise than by considering the analogy of the two larger planets. Direct evidence tending to show that the atmosphere of Uranus is in a condition wholly differing from that of our own atmosphere, cannot possibly be obtained by means of any telescopes yet constructed by men. Some astronomers assert that they have seen faint traces of belts across the disc of Uranus; but the traces must be very faint indeed, since the best telescopes of our day fail to show any marks whatever upon the planet's face. Even if such belts can be seen, their changes of appearance cannot be studied systematically.

It is, however, on this very subject-the condition of the planet's atmosphere - that the discovery I have now to describe throws light.

Faint as is the light of Uranus, yet, when a telescope of sufficient size is employed, the spectrum of the planet is seen as a faint rainbow-tinted streak. The peculiarities of this streak, if discernible, are the means whereby the spectroscopist is to ascertain what is the condition of the planet's atmosphere. Now, Father Secchi, studying Uranus with the fine eight-inch telescope of the Roman Observatory, was able to detect certain peculiarities in its spectrum, though it would now appear that (owing probably to the faintness of the light) he was deceived as to their exact nature. He says : 'The yellow part of the spectrum is wanting altogether. In the green and the blue there are two bands, very wide and very dark.' But he was unable to say what is the nature of the atmosphere of the planet, or to show how these peculiarities might be accounted for.

Recently, however, the Royal Society placed in the hands of Dr. Huggins a telescope much more powerful than either the Roman telescope or the instrument with which Dr. Huggins had made his celebrated observations on sun and planets, stars and star-cloudlets. It is fifteen inches in aperture, and has a light-gathering power fully three times as great as that possessed by either of the instruments just mentioned.