Page:The evolution of worlds - Lowell.djvu/134

104 long ago, is that the planet's disk is noticeably banded by dark belts. Two characteristics of these belts are important. One is that they exhibit a regular secular progression with the lapse of years, the south tropical belt being broader and more salient for many years in succession, and then gradually fading out while the northern one increases in prominence. It has been suspected that the rhythm of their change is connected with that of sun spots. The second is that the belts do not preserve in their several features the same relation in longitude toward one another. They all rotate, but at different speeds. There could be no better proof that Jupiter is no solid, but a seething mass of heavy vapors boiling like a caldron. Tempered by distance we can form but a faint idea of the turmoil there going on. Further indication of it is furnished by its glow. For all the dark belts are a beautiful cherry red, a tint extending even to the darkish hoods over the planet's caps. This hue comes out well in good seeing, and best, as with all planetary markings, in twilight, not at night, because the excessive brightness of the disk is then taken off, preventing the colors from being swamped.

This brings us to the planet's albedo, which Müller at Potsdam has found to be 75 per cent. Now the interest attaching to this determination is twofold, that it bespeaks cloud and that it seems to imply