In the first place, they are of singular permanence for clouds. The fleeting forms we know as such assume in the Jovian air a stability worthy of Jove himself. In their general outlines, they remain the same for years at a time. “Constant as cloud” would be the proper poetic simile there. But while remaining true to themselves, they prove to be in slow, unequal shift with one another. Thus Jupiter’s official day differs according to the watch of the particular belt that times it. Spots in different latitudes drift round lazily in appearance, swiftly in fact, those near the equator as a rule the fastest. Nor is there any hard and fast latitudinal law; it is a go-as-they-please race in which one belt passes its neighbor at a rate sometimes of four hundred miles an hour. The mean day is 9ʰ 55ᵐ long. [Illustration: JUPITER AND ITS “GREAT RED SPOT”—A DRAWING BY DR. LOWELL, APRIL 12, 7ʰ 0ᵐ-5ᵐ, 1907.] [Illustration: JUPITER AND ITS “GREAT RED SPOT”—A DRAWING BY DR. LOWELL, APRIL 12, 7ʰ 28ᵐ-42ᵐ, 1907.] A side-light is cast upon the Jovian state of things by the “great red spot,” which has been more or less visible for thirty years, and which takes five minutes longer than the equatorial band to travel round. Its tint bespoke interest in what might be its atmospheric horizon. Yet it betrayed no sign of being either depressed or exalted with regard to the rest of the surface. “In 1891,” as Miss Clerke puts it, “an opportunity was offered of determining its altitude relative to a small dark spot on the same parallel, by which, after months of pursuit, it was finally overtaken. An occultation appeared to be the only alternative from a transit; yet neither occurred. The dark spot chose a third. It coasted round the obstacle in its way, and got damaged beyond recognition in the process.” It thus astutely refused to testify. [Illustration: SUN SPOTS—AFTER BOND.] Now, this exclusiveness on the part of the “great red spot” really offers us an insight to its character. Clearly it was no void, but occupied space with more than ordinary persistency. As it was neither above nor below the dark spot and shattered that spot on approach, which its former surroundings had not done, its force must have been due to motion. This can be explained by its being formed of a vast uprush of heated vapor from the interior. In short, it was a sort of baby elephant of a volcano, or geyser, occurring as befits its youth in fluid, not solid, conditions, but fairly permanent, nevertheless—a bit of kindergarten Jovian geology. This estimate of it is concurred in by Dr. Slipher’s spectrogram of the dark and light belts respectively. For in the spectrum of the dark one we see the distinctive Jovian bands intensified as if the light had traversed a greater depth of Jovian air. Its color, a cherry red, abets the conclusion—that in such places we look down into the fiery, chaotic turmoil so incessantly going on. [Illustration: PHOTOGRAPH OF A SUN SPOT—AFTER THE LATE M. JANSSEN.]