Page:The evolution of worlds - Lowell.djvu/199

Rh them due to Jovian trade-winds, the planet's swift rotation making up for deficiency of sun; why, does not appear.

Modern study of the planet shows that the bright longitudinal layers between the dark belts are unquestionably belts of cloud. Their behavior indicates this, and their intrinsic brightness bears it out. For they are of almost exactly that albedo. Whether they are the kind of cloud with which we are familiar, clouds of water-vapor, we are not yet sure. But whatever their constitution, their conduct is quite other than is exhibited by our own.

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 9$h$ 55$m$ long.