observing the bow in the south belt. The suggestion of a
resemblance to the flowing of a stream past the foot of an elevated
promontory, or mountain, is strengthened by the fact, which was
observed early in the history of the spot, that markings involved in the
south belt have a quicker rate of rotation about the planet's axis than
that of the red spot, so that such markings, first seen in the rear of
the red spot, gradually overtake and pass it, and eventually leave it
behind, as boats in a river drift past a rock lying in the midst of the
current.

This leads us to another significant fact concerning the peculiar
condition of Jupiter's surface. Not only does the south belt move
perceptibly faster than the red spot, but, generally speaking, the
various markings on the surface of the planet move at different rates
according as they are nearer to or farther from the equator. Between the
equator and latitude 30 deg. or 40 deg. there is a difference of six
minutes in the rotation period--i.e., the equatorial parts turn round
the axis so much faster than the parts north and south of them, that in
one rotation they gain six minutes of time. In other words, the clouds
over Jupiter's equator flow past those in the middle latitudes with a
relative velocity of 270 miles per hour. But there are no sharp lines
of separation between the different velocities; on the contrary, the
swiftness of rotation gradually diminishes from the equator toward the
poles, as it manifestly could not do if the visible surface of Jupiter
were solid.

In this respect Jupiter resembles the sun, whose surface also has
different rates of rotation diminishing from the equator. Measured by
the motion of spots on or near the equator, Jupiter's rotation period is
about nine hours fifty minutes; measured by the motion of spots in the
middle latitudes, it is about nine hours fifty-six minutes. The red spot
completes a rotation in a little less than nine hours and fifty-six
minutes, but its period can not be positively given for the singular
reason that it is variable. The variation amounts to only a few seconds
in the course of several years, but it is nevertheless certain. The
phenomenon of variable motion is not, however, peculiar to the red spot.
Mr. W.F. Denning, who has studied Jupiter for a quarter of a century,
says:

"It is well known that in different latitudes of Jupiter there are
currents, forming the belts and zones, moving at various rates of sp