16 The Saturnian System. (January, 
piece of evidence on this point the significance of which is 
unmistakable. Scarcely ever, if ever, is any feature stable, 
even for a few successive rotations. It is certain that we 
scarcely ever, if ever, see the real surface of Jupiter or 
Saturn. I doubt myself whether those features whose suc- 
cessive returns have been observed for the determination of 
the rotation-rates of these planets are other than atmo- 
spheric phenomena. They appear to resemble rather the 
solar spots, whose motion may indeed suffice for the deter- 
mination of the general rotation-rate, but cannot possibly be 
regarded as the motion of some obje¢t on the real surface of 
the solar orb (if he have any). However this be, it is 
certain that if any part of the real surface of Jupiter has 
ever been seen, the whole of that surface is usually con- 
cealed from view. Even the dark zones are not zones of 
his real surface, though they unquestionably underlie the 
bright cloud zones. Moreover, signs of violent action 
-such as I have suggested have been by no means wanting ; 
since not only do the features of the Jovian belts change 
rapidly in shape, but special details are sometimes seen,—as 
round white spots lasting but for a short period, &c.,—which 
can be readily explained as due to an uprush of vapour, 
and, in my opinion, can no otherwise be so satisfactorily 
accounted for. 
But so soon as we admit the probability that the atmo- 
spheres of Jupiter and Saturn are very deep, we are led toa 
line of reasoning which seems demonstrative of the fact 
that the condition of these planets is utterly unlike that of 
the earth. A deep atmosphere, subjected to the strong 
gravitating energy of either planet, would acquire at ordi- 
nary temperatures a density altogether incompatible with 
the existence of any resemblance to our own atmosphere in 
any part of its extent. In fact, if we only assume that the 
atmosphere of Jupiter has a depth of 50 miles below 
the cloud layers, we deduce at the surface of Jupiter 
a density incompatible with the gaseous state, a density 
exceeding many times that of our heaviest metals! It is 
difficult to suppose that that atmosphere, the changes in 
which are so manifest at our great distance from the giant 
planets, has a less depth than this would imply; and yet 
the assumption that it has such a depth leads dire¢tly to the 
conclusion that (if at ordinary temperature) it increases in 
density till liquefaction is attained, and that if Jupiter has 
any liquid surface at all, such surface is due to the pressure 
of the Jovian atmosphere producing liquefaction. There 
would, however, be more probably continuity of change 
