Page:Popular Science Monthly Volume 16.djvu/775

Rh are sufficient for changes of enormous magnitude. The first question that arises is, What do the dark bands or spots mean? Are they portions of the solid body of the planet, which have some fixity of shape, in any degree analogous to that of our mountain-chains or great continents? Or are they cloudy matter of less light-reflecting power than the bright and dense atmosphere by which the planet appears to be surrounded? Or are they merely more transparent parts of that atmosphere, through which no lower objects happen to reflect light enough to be visible? If the bright parts of the Jovian disk are light-reflecting clouds, and the dark belts the body of the planet, we should suppose it would be common to see a notched appearance of the edges; but this is not so. "Ordinarily," as Captain Noble says, "the belts fade perceptibly as they approach the actual edge of the disk; but," he adds, "I have seen the belts right up to it." The softening of the belts, as the planet's rotation brings them to the edges of the disk, probably arises from the dark parts being considerably below the boundary of the Jovian atmosphere, and thus seen through a greater thickness of it when near the edges. When the dark belts touch the edge without noticeable softening they must be higher up, and less likely to be any part of the solid body, if Jupiter has anything that can be so called. The great spot of this season has never been seen close to the edge. A very moderate magnification is sufficient to show that as the planet rotates it comes into view decidedly at some distance from the luminous margin, and disappears at a similar distance from the opposite margin.

Telescopes, under the most favorable conditions, and of the greatest power, only reveal very large features of the planet. It is impossible to see anything like details of structure, and this makes the identification of objects seen at different times more or less uncertain. If we had glimpses of great mountain-chains in Jupiter, it would be something like seeing the Andes or the Himalayas all in a lump, from some skyey perch, so far off as to prevent the separate peaks and valleys from being noticed. Jupiter is about five and one fifth times as far from the sun as we are—our mean distance, according to the last reckoning, being 92,620,000 miles. With the moon only 240,000 miles off, and very frequently bearing a much higher magnification than can be applied without confusion to Jupiter, telescopes bring no object near. A magnification of 1,000 linear—only usable under very favorable circumstances—makes lunar objects as big, but not as distinct, as a naked-eye vision of them would do if it could approach within 240 miles. With the enormously greater distance of Jupiter it must be evident how impossible it is for anything but huge masses to be seen.

Jupiter's atmosphere is much larger in proportion to any solid matter he may contain than that of our earth to its solid matter. It is also much denser, and from its greater distance only gets about one twenty-fifth as much solar influence as reaches us. For these and