s have our guns ready, and, as
old Deepwaters would say, keep our weather eye open."

The stream flowed off in a southeasterly direction, so that by
following it they went towards the volcanoes.

"It is hard to realize," said the professor, "that those mountains must
be several hundred miles away, for the reason that they are almost
entirely above the horizon. This apparent flatness and wide range of
vision is of course the result of Jupiter's vast size. With
sufficiently keen sight, or aided by a good glass, there is no reason
why one should not see at least five hundred miles, with but a slight
elevation."

"It is surprising," said Ayrault, "that in what is evidently Jupiter's
Carboniferous period the atmosphere should be so clear. Our idea has
been that at that time on earth the air was heavy and dense."

"So it was, and doubtless is here," replied Cortlandt; "but you must
remember that both those qualities would be given it by carbonic-acid
gas, which is entirely invisible and transparent. No gas that would be
likely to remain in the air would interfere with sight; water vapour is
the only thing that could; and though the crust of this planet, even
near the surface, is still hot, the sun being so distant, the vapour
would not be, raised much. By avoiding low places near hot springs, we
shall doubtless have very nearly as clear an atmosphere as on earth.
What does surprise me is the ease with which we breathe. I can account
for it only by supposing that, the Carboniferous period being already
well advanced, most of the carbonic acid is already locked up in the
forests or in Jupiter's coal-beds."

"How," asked Bearwarden, "do you account for the 'great red spot' that
appeared here in 1878, lasted several years, and then gradually faded?
It was taken as unmistakable evidence that Jupiter's atmosphere was
filled with impenetrable banks of cloud. In fact, you remember many of
the old books said we had probably never seen the surface."

"That has puzzled me very much," replied Cortlandt, "but I never
believed the explanation then given was correct. The Carboniferous
period is essentially one of great forest growth; so there would be
nothing out of the way in supposing the spot, notwithstanding its
length of twenty-seven thousand miles and its breadth of eight thousand
miles, to have been forest. It occurred in what would correspond to
the temperate region on earth. Now, though the axis of this planet is