pidly. We know that the earth's days were formerly
but half or a quarter as long as now, having lasted but six or eight
hours. The explanation of the elongation is simple: the earth rotates
in about twenty-four hours, while the moon encircles it but once in
nearly twenty-eight days, so that our satellite is continually drawing
the oceans backward against its motion. These tidal brakes acting
through the friction of the water on the bottom, its unequal pressure,
and the impact of the waves on the shore, are continually retarding its
rotation, so that the day is a fraction of a second longer now than it
was in the time of Caesar. This same action is of course taking place
in Jupiter and the great planets, in this case there being five moons
at work. Our moon, we know, rotates on its axis but once while it
revolves about the earth, this being no doubt due to its own
comparative smallness and the great attraction of the earth, which must
have produced tremendous tides before the lunar oceans disappeared from
its surface."

In crossing the orbits of the satellites, they passed near Ganymede,
Jupiter's largest moon.

"This," said Cortlandt, "was discovered by Galileo in 1610. It is
three thousand four hundred and eighty miles in diameter, while our
moon is but two thousand one hundred and sixty, revolves at a distance
of six hundred and seventy-eight thousand three hundred miles from
Jupiter, completes its revolution in seven days and four hours, and has
a specific gravity of 1.87."

In passing, they observed that Ganymede possessed an atmosphere, and
continents and oceans of large area.

"Here," said Bearwarden, "we have a body with a diameter about five
hundred miles greater than the planet Mercury. Its size, light
specific gravity, atmosphere, and oceans seem to indicate that it is
less advanced than that planet, yet you think Jupiter has had a longer
separate existence than the planets nearer the sun?"

"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