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 alone; we must follow a number of objects distributed in different longitudes along the current and deduce a mean from the whole."[10]

[Footnote 10:  The Observatory, No. 286, December, 1899.]

Nor is this all.  Observation indicates that if we could look at a vertical section of Jupiter’s atmosphere we should behold an equally remarkable contrast and conflict of motions.  There is evidence that some of the visible spots, or clouds, lie at a greater elevation than others, and it has been observed that the deeper ones move more rapidly.  This fact has led some observers to conclude that the deep-lying spots may be a part of the actual surface of the planet.  But if we could think that there is any solid nucleus, or core, in the body of Jupiter, it would seem, on account of the slight mean density of the planet, that it can not lie so near the visible surface, but must be at a depth of thousands, perhaps tens of thousands, of miles.  Since the telescope is unable to penetrate the cloudy envelope we can only guess at the actual constitution of the interior of Jupiter’s globe.  In a spirit of mere speculative curiosity it has been suggested that deep under the clouds of the great planet there may be a comparatively small solid globe, even a habitable world, closed round by a firmament all its own, whose vault, raised 30,000 or 40,000 miles above the surface of the imprisoned planet, appears only an unbroken dome, too distant to reveal its real nature to watchers below, except, perhaps, under telescopic scrutiny; enclosing, as in a shell, a transparent atmosphere, and deriving its illumination partly from the sunlight that may filter through, but mainly from some luminous source within.