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 speed.
In many instances the velocity changes from year to year. And it is a
singular circumstance that in the same current a uniform motion is not
maintained in all parts of the circumference. Certain spots move faster
than others, so that if we would obtain a fair value for the rotation
period of any current it is not sufficient to derive it from one marking