practically straight, the winds of course change their direction, and
so the temperature does vary from day to day. What is more probable
than that, owing perhaps to a prolonged norther or cold spell, a long
strip of forest lying near the frost line was brought a few degrees
below it, so that the leaves changed their colours as they do on earth?
It would, it seems to me, be enough to give the surface a distinct
colour; and the fact that the spot's greatest length was east and west,
or along the lines of latitude, so that the whole of that region might
have been exposed to the same conditions of temperature, strengthens
this hypothesis. The strongest objection is, that the spot is said to
have moved; but the motion--five seconds--was so slight that it might
easily have been an error in observation, or the first area affected by
the cold may have been enlarged on one side. It seems to me that the
stability the spot DID have would make the cloud theory impossible on
earth, and much more so here, with the far more rapid rotation and more
violent winds. It may also have been a cloud of smoke from a volcano
in eruption, such as we saw on our arrival, though it is doubtful
whether in that case it would have remained nearly stationary while
going through its greatest intensity and fading, which would look as
though the turned leaves had fallen off and been gradually replaced by
new ones; and, in addition to this, the spot since it was first noticed
has never entirely disappeared, which might mean a volcanic region
constantly emitting smoke, or that the surface, doubtless from some
covering whose colour can change, is normally of a different shade from
the surrounding region. In any case, we have as yet seen nothing that
would indicate a permanently clouded atmosphere."

Though they had walked a considerable distance, the water was not much
cooled; and though the stream's descent was so slight that on earth its
current would have been very slow, here it rushed along like a mountain
torrent, the reason, of course, being that a given amount of water on
Jupiter would depress a spring balance 2.55 times as much as on the
earth.

"It is strange," said Ayrault, "that, notwithstanding its great speed,
the water remains so hot; you would think its motion would cool it."

"So it does," answered the professor. "It of course cools considerably
more in a given period--as, for instance, one minute--than if it were
moving more slowly,