WRAT IS JUPITER DOING? 741 



utes 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 ap- 

 pears 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 

 ai-e light-reflecting clouds, and the dark belts the body of the jDlanet, 

 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 consid- 

 erably 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 de- 

 cidedly at some distance fi-ora the luminous margin, and disappears at 

 a similar distance from the opposite margin. 



Telescopes, under the most favorable conditions, and of the great- 

 est power, only reveal very large features of the planet. It is impos- 

 sible to see anything like details of structure, and this makes the iden- 

 tification 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 val- 

 leys 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 favor- 

 able 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 mat- 

 ter 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 



