OBSERVATIONS ON JUPITER IN 1870-71. 
281 
or 3. Such a direction may result from the different values of 
rotation at different heights in the atmosphere, combined with 
the expansion and ascent, or condensation and sinking, of the 
aerial strata in contact respectively with the warmer globe or 
cooler expanse of space : an idea supported by the analogy of 
our own trade-winds. But if we adopt this solution, it will 
lead us somewhat further. We shall find it expedient to 
assume a more regular distribution of heat over the surface 
of that globe than exists upon our own, and some consequent 
difference in its origin ; for notwithstanding perspective fore- 
shortening, belts are occasionally traceable at great distances 
from the equator. A drawing made on Dec. 22 shows a 
defined belt in a South latitude of probably 60° or 65°. Now 
since the situation of the terrestrial axis at either equinox is 
equivalent to the average position of the axis of Jupiter as 
far as solar radiation is concerned, we may suppose that if 
the warmth of Jupiter were derived wholly from the sun, the 
temperature of his equator would bear a similar proportion 
to the temperature of lat. 60° or 65° that the temperature of 
the terrestrial equator does to the mean equinoctial tempe- 
rature of the parallel of Iceland ; and that consequently the 
warming of the lower stratum of the atmosphere by contact 
with the globe would in all probability be too much reduced 
to admit of the formation of definite zones and belts ; espe- 
cially when we add to this the diminution to half its value 
of the velocity of rotation in that latitude. Hence, then, it 
would appear not improbable that a considerable portion of 
the heat of Jupiter may be of an unborrowed character. 
This idea, which has been advocated on other grounds, seems 
favoured by another circumstance. Were his temperature due 
merely to solar radiation, the currents ascending from the 
hotter equatoreal regions would be observed to deviate to some 
extent in an oblique direction, their lateral diffusion being 
unrepressed by equivalent expansion in remoter latitudes ; the 
phenomena, however, afford but very equivocal instances of 
any such tendency, the oblique arrangements which are occa- 
sionally visible being too irregular in aspect or inconsistent in 
direction to be referred with safety to this origin. It must be 
admitted that the foundation of such an attempt at explana- 
tion is very insecure ; still, till some more probable solution 
may be devised, it seems to present the least amount of diffi- 
culty and inconsistency. And as we believe that our own globe 
possesses a certain amount of internal temperature, independent 
of that derived from solar radiation, there is an antecedent 
probability in extending the analogy to globes of greater 
dimensions, such as Jupiter, and we may add Saturn, whose 
similarly belted surface no doubt indicates a similar constitu- 
