"Undoubtedly," said Cortlandt. "Jupiter was condensed while in the solar-system nebula, and began its individual existence and its evolutionary career long before Mercury was formed. The matter now in Ganymede, however, doubtless remained part of the Jupiter-system nebula till after Mercury's creation, and, being part of so great a mass, did not cool very rapidly. I should say that this satellite has about the same relation to Jupiter that Jupiter has to the sun, and is therefore younger in point of time as well as of development than the most distant Callisto, and older, at all events in years, than Europa and Io, both of which are nearer. This supposition is corroborated by the fact that Europa, the smallest of these four, is also the densest, having a specific gravity of 2.14, its smallness having enabled it to overtake Ganymede in development, notwithstanding the latter's start. In the face of the evidence before us we must believe this, or else that, perhaps, as in the case of the asteroid Hilda, something like a collision has rejuvenated it. This might account for its size, and for the Nautical Almanac's statement that there is a 'small and variable' inclination to its orbit, while Io and Europa revolve exactly in the plane of Jupiter's equator."
They had about as long a journey before them as they had already made in going from the earth to Jupiter. The great planet soon appeared as a huge crescent, since it was between them and the sun; its moons became as fifth- and sixth-magnitude stars, and in the evening of the next day Jupiter's disk became invisible to the unaided eye. Since there were no way stations, in the shape of planets or asteroids, between Jupiter and Saturn, they kept the maximum repulsion on Jupiter as long as possible, and moved at tremendous speed. Saturn was somewhat in advance of Jupiter in its orbit, so that their course from the earth had been along two sides of a triangle with an obtuse angle between. During the next four terrestrial days they sighted several small comets, but spent most of their time writing out their Jovian experiences. During the sixth day Saturn's rings, although not as much tilted as they would be later in the planet's season, presented a most superb sight, while they spun in the sun's rays. Soon after this the eight moons became visible, and, while slightly reducing the Callisto's speed, they crossed the orbits of Iapetus, Hyperion, and Titan, when they knew they were but seven hundred and fifty thousand miles from Saturn.
"I am anxious to ascertain," said Cortlandt, "whether the composition of yonder rings is similar to that of the comet through which we passed. I am sure they shine with more than reflected light."
"We have been in the habit," said Ayrault, "of associating heat with light, but it is obvious there is something far more subtle about cometary light and that of Saturn's rings, both of which seem to have their birth in the intense cold of interplanetary space."
Passing close to Mimas, Saturn's nearest moon, they supplemented its attraction, after swinging by, by their own strong pull, bringing their speed down to dead slow as they entered the outside ring. At distances often of half a mile they found meteoric masses, sometimes lumps the size of a house, often no larger than apples, while small particles like grains of sand moved between them. There were two motions. The ring revolved about Saturn, and the particles vibrated among themselves, evidently kept apart by a mutual repulsion, which seemed both to increase and decrease faster than gravitation; for on approaching one another they were more strongly repelled than attracted, but when they separated the repulsion decreased faster than the attraction, so that after a time divergence ceased, and they remained at fixed distances.
The Callisto soon became imbued with motion also, but nothing ever struck it. When any large mass came unusually near, both it and their car emitted light, and they rapidly separated. The sunlight was not as strong here as it had been when they entered the comet, and as they penetrated farther they were better able to observe the omnipresent luminosity. They were somewhat puzzled by the approach of certain light-centres, which seemed to contain nothing but this concentrated brightness. Occasionally one of these centres would glow very brightly near them, and simultaneously recede. At such times the Callisto also glowed, and itself recoiled slightly. At first the travellers could not account for this, but finally they concluded that the centres must be meteoric masses consisting entirely of gases, possessing weight though invisible.
"We have again to face," said Cortlandt, "that singular law that till recently we did not suppose existed on earth. All kinds of suppositions have been advanced in explanation of these rings. Some writers have their thickness, looked at from the thin edge, as four hundred miles, some one hundred, and some but forty. One astronomer of the nineteenth century, a man of considerable eminence, was convinced that they consisted of sheets of liquid. Now, it should be obvious that no liquid could maintain itself here for a minute, for it would either fall upon the planet as a crushing hail, or, if dependent for its shape on its own tenacity, it would break if formed of the toughest steel, on account of the tremendous weight. Any number of theories have been advanced by any number of men, but in weight we have the rub. No one has ever shown how these innumerable fragments maintain themselves at a height of but a few thousand miles above Saturn, withstanding the giant's gravitation-pull. Their rate of revolution, though rapid, does not seem fast enough to sustain them. Neither have I ever seen it explained why the small fragments do not fall upon the large ones, though many astronomers have pictured the composition of these rings as we find they exist. Nor do we know why the molecules of a gas are driven farther apart by heat, while their activity is also increased, though if this activity were revolution about one another to develop the centrifugal, it would not need to be as strong then as when they are cold and nearer together. There may be explanations, but I have found none in any of the literature I have read. It seems to me that all this leads to but one conclusion, viz.: apergy is the constant and visible companion of gravitation, on these great planets Jupiter and Saturn, perhaps on account of some peculiar influence they possess, and also in comets, in the case of large masses, while on earth it appears naturally only among molecules--those of gases and every other substance."
"I should go a step further," said Bearwarden, "and say our earth has the peculiarity, since it does not possess the influence necessary to generate naturally a great or even considerable development of apergy. The electricity of thunderstorms, northern lights, and other forces seems to be produced freely, but as regards apergy our planet's natural productiveness appears to be small."
The omnipresent luminosity continued, but the glow was scarcely bright enough to be perceived from the earth.
"I believe, however," said Bearwarden, referring to this, "that whenever a satellite passes near these fragments, preferably when it enters the planet's shadow, since that will remove its own light, it will create such activity among them as to make the luminosity visible to the large telescopes or gelatine plates on earth."