i877-] 
Movements of Jupiter's Cloud-Masses. 
197 
and was not perceptibly less dark at egress than in mid- 
transit. ” 
It appears to me demonstrated by the evidence thus far 
noted, that in a semi-transparent atmosphere of enormous 
depth, surrounding Jupiter, there float vast cloud-masses, 
sometimes in layers, at others in irregular heaps, at others 
having well-rounded forms. These cloud-masses undergo 
sometimes remarkable changes of shape, often forming or 
disappearing in a very short time, and thus indicating the 
inferior activity of the forces at work below them, — in other 
words, the intense heat of Jupiter’s real globe. As to the 
actual depth of the semi-transparent atmosphere in which 
these cloud-layers and cloud-masses float, it would be diffi- 
cult to express an opinion. We do not know how many 
cloud-layers there are, how thick any cloud-layer may be, 
how great may be the depth of the vast rounded masses of 
cloud whose upper surface (that is, the surface remotest 
from Jupiter’s true surface) we can alone see under favourable 
conditions. But we can indicate a minimum than which 
the atmosphere’s depth is probably not less ; and from all 
the observations which I have examined as bearing on this 
point, I should be disposed to assign for that minimum at 
least 6000 miles. I am strongly of opinion that in reality 
the depth of the Jovian atmosphere is still greater. I can- 
not doubt that Jupiter has a solid or liquid nucleus, though 
this nucleus — glowing, as it must be, with a most intense 
heat — may be greatly expanded ; yet I should conceive that, 
with the enormous attractive power residing in it, containing 
as it must nearly the whole mass of the planet, its mean 
density cannot be less than that of the earth. Now a globe 
of the mass of Jupiter, but of the same mean density as our 
earth, would have one-fourth of Jupiter’s volume — the mean 
density of Jupiter, as at present judged, being equal to one- 
fourth that of the earth. The diameter therefore of such a 
globe would be less than the present diameter of Jupiter in 
the same ratio that the cube root of unity is less than the 
cube root of 4, or as 1 is less than i'5874. Say, roughly, 
(remembering that the atmosphere of Jupiter must have a 
considerable mass) the diameter of Jupiter’s nucleus would, 
on the assumptions made, be equal to about five-eighths of 
his observed diameter, or about 53,000 miles. This is less 
than his observed diameter by about 22,000 miles, so that 
the radius of his nucleus would be less than his observed 
radius by about 11,000 miles, which therefore would be the 
probable depth of his atmosphere. 
But we have still to consider the velocities with which 
