The earth turns on its axis once in 24 hours but no such definite time can be assigned to Jupiter, which, like the sun, seems to have different rotation periods in different latitudes--9h. 50m. in the equatorial belt and 9h. 56m. in the dark belts and higher latitudes. There is some indication that the larger part of the visible surface rotates in 9h. 55.6m., while a broad stream along the equator flows eastward some 270 miles per hour, and thus comes back to the center of the planet, as seen from the earth, five or six minutes earlier than the parts which do not share in this motion. Judged by terrestrial standards, 270 miles per hour is a great velocity, but Jupiter is constructed on a colossal scale, and, too, we have to compare this movement, not to a current flowing in the ocean, but to a wind blowing in the upper regions of the earth's atmosphere. The visible surface of Jupiter is only the top of a cloud formation, and contains nothing solid or permanent, if indeed there is anything solid even at the core of the planet. The great red spot during the first dozen years of its existence, instead of remaining fixed relative to the surrounding formations, drifted two thirds of the way around the planet, and having come to a standstill about 1891, it is now slowly retracing its path. 139. PHYSICAL CONDITION.--For a better understanding of the physical condition of Jupiter, we have now to consider some independent lines of evidence which agree in pointing to the conclusion that Jupiter, although classed with the earth as a planet, is in its essential character much more like the sun. _Appearance._--The formations which we see in Fig. 87 look like clouds. They gather and disappear, and the only element of permanence about them is their tendency to group themselves along zones of latitude. If we measure the light reflected from the planet we find that its albedo is very high, like that of snow or our own cumulus clouds, and it is of course greater from the light parts of the disk than from the darker bands. The spectroscope shows that the sunlight reflected from these darker belts is like that reflected from the lighter parts, save that a larger portion of the blue and violet rays has been absorbed out of it, thus producing the ruddy tint of the belts, as sunset colors are produced on the earth, and showing that here the light has penetrated farther into the planet's atmosphere before being thrown back by reflection from lower-lying cloud surfaces. The dark bands are therefore to be regarded as rifts in the clouds, reaching down to some considerable distance and indicating an atmosphere of great depth. The great red spot, 28,000 miles long, and obviously thrusting back the white clouds on every side of it, year after year, can hardly be a mere patch on the face of the planet, but indicates some considerable depth of atmosphere.