i877*J Movements of Jupiter's Cloud-Masses. 193 
motions of downrush carrying them from regions of swifter 
to regions of slower rotational motion. This view seems 
certainly encouraged by what we find when we come to 
study more closely the aspedt of the Jovian belts. The 
white spots — some small, some large — which are seen to form 
from time to time along the chief belts present precisely the 
appearance which we should expedt to find in masses of 
vapour flung from far down below the visible cloud-surface 
of Jupiter, breaking their way through the cloud-layers, and 
becoming visible as they condense into the form of visible 
vapour in the cooler upper regions of the planet’s atmo- 
sphere. Then again, the singular regularity with which 
in certain cases the great rounded white clouds are set side 
by side, like rows of eggs upon a string, is much more 
readily explicable as due to a regular succession of up- 
rushes of vapour, from the same region below, than as due 
to the simultaneous uprush of several masses of vapour 
from regions set at uniform distances along a belt of Jupiter’s 
surface. The latter supposition is indeed artificial and im- 
probable in the highest degree, and in several distindt 
respedts. It is unlikely that several uprushes should occur 
simultaneously, unlikely that regions whence uprush took 
place should be set at equal distances from each other, un- 
likely that they should lie along the same latitude parallel. 
On the other hand, the occurrence of uprush after uprush 
from the same region of disturbance, at nearly uniform in- 
tervals of time, is not at all improbable. The rhythmical 
succession of explosions is a phenomenon, indeed, altogether 
likely to occur under certain not improbable conditions, — 
as, for instance, when each explosion affords an excess of 
relief, if one may so speak, and is therefore followed by a 
readtionary process, in its turn bringing on a fresh explosion. 
Now, a rhythmical succession of explosions from the same 
deep-rooted region of disturbance would produce at the 
upper level, where we see the expelled vapour-masses (after 
condensation), a series of rounded clouds lying side by side. 
For each cloud-mass — after its expulsion from a region of 
slow absolute rotational motion, to a region of swifter 
motion — would lag behind with reference to the direction of 
rotational motion. The earlier it was formed the farther 
back it would lie. Thus each new cloud-mass would lie 
somewhat in advance of the one expelled next before it ; 
and if the explosions occurred regularly, and with a sufficient 
interval between each and the next to allow each expelled 
cloud-mass to lag by its own full length before the next one 
appeared, there would be seen precisely such a series of 
VOL. vii. (n.s.) o 
