1874.] The Saturnian System. 15 
circumstance that there are sometimes so many of them. 
Polar and equatorial air-currents such as exist in our own 
air would naturally explain, perhaps, an equatorial cloud- 
zone and sub-tropical trade zones on either side of it; but 
they can afford no explanation of the existence, even though 
occasional only, of three or four well-marked zones on either 
side of the equatorial one. 
_It appears to me a natural inference from these considera- 
tions that the difference of rotational velocity to which the 
cloud zones of Jupiter are certainly due, does not result 
from polar and equatorial currents carrying cloud-matter to 
regions where the movement of rotation is greater or less, 
but from upward and downward motions within the Jovian and 
Saturnian atmospheres. Or else, (though the assumption is 
not incompatible with the hypothesis of upward and down- 
ward currents), we may assume that some cause resembling 
that which occasions the solar spot zones is at work to 
produce cloud-zones on Saturn and Jupiter. We cannot 
proceed much farther in this direction until we have more 
definite information. But it suffices for our purpose to 
notice that we have in upward and downward currents, 
combined probably with some resemblance in condition to 
the sun, a reasonable explanation of the general features of 
the cloud-zones of the giant planets. 
Only, it is necessary to notice that such views require the 
atmospheric envelopes of these planets to be exceedingly 
deep, and the upward and downward motions taking place 
within them exceedingly rapid. The upward motions 
would, in fact, appear, according to this view, to resemble 
rather the uprush of vapours during volcanic eruptions than 
any ordinary forms of vertical currents. The downward 
motions need not necessarily be so rapid; they might, and 
probably would, be merely the quiet return of the vapourous 
masses which had been propelled upwards, by which they 
resumed the level due to their density. 
It appears to me that the results of telescopic scrutiny 
correspond well with these requirements. It is impossible 
to observe the belts of Jupiter with adequate telescopic 
power without being led to the conviction that the atmo- 
sphere in which these belts exist is exceedingly deep. It is 
impossible to convey by any description, and not easy to 
indicate even by pictorial illustration, the force of the 
telescopic evidence. It may be remarked, however, that 
the shapes and changes of shape of the cloud masses show 
that they have been generated in a deep atmosphere, and 
therein exposed to modifying influences. There is also one 
