n, Jupiter is brightest near
the centre. But the most perplexing part of his results was that Jupiter
actually seemed to give out more light than he received. Bond, however,
rightly considered his data too uncertain for the support of so bold an
assumption as that of original luminosity, and, even if the presence of
native light were proved, thought that it might emanate from auroral
clouds of the terrestrial kind. The conception of a sun-like planet was
still a remote, and seemed an extravagant one.

Only since it was adopted and enforced by Zoellner in 1865,[1043] can it
be regarded as permanently acquired to science. The rapid changes in the
cloud-belts both of Jupiter and Saturn, he remarked, attest a high
internal temperature. For we know that all atmospheric movements on the
earth are sun-heat transformed into motion. But sun-heat at the distance
of Jupiter possesses but 1/27, at that of Saturn 1/100 of its force
here. The large amount of energy, then, obviously exerted in those
remote firmaments must have some other source, to be found nowhere else
than in their own active and all-pervading fires, not yet banked in with
a thick solid crust.

The same acute investigator dwelt, in 1871,[1044] on the similarity
between the modes of rotation of the great planets and of the sun,
applying the same principles of explanation to each case. The fact of
this similarity is undoubted. Cassini[1045] and Schroeter both noticed
that markings on Jupiter travelled quicker the nearer they were to his
equator; and Cassini even hinted at their possible assimilation to
sun-spots.[1046] It is now well ascertained that, as a rule (not without
exceptions), equatorial spots give a period some 5-1/2 minutes shorter
than those in latitudes of about 30 deg. But, as Mr. Denning has pointed
out,[1047] no single period will satisfy the observations either of
different markings at the same epoch, or of the same markings at
different epochs. Accelerations and retardations, depending upon
processes of growth or change, take place in very much the same kind of
way as in solar maculae, inevitably suggesting similarity of origin.

The interesting query as to Jupiter's surface incandescence has been
studied since Bond's time with the aid of all the appliances furnished
to physical inquirers by modern inventiveness, yet without bringing to
it a categorical reply. Zoellner in 1865, Mueller in 1893, estimated his
albedo at 0.62 and 0.75 respectively, t