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upper part of the cloud-layer. This is about the 6,000th part 
of the diameter of Jupiter; and if any student of astronomy 
can believe that that wonderfully complex and changeful cloud 
envelope which surrounds Jupiter has a thickness of less 
than the 6,000th part of the planet’s diameter, I would 
recommend as a corrective the careful study of the planet for 
an hour or two with a powerful telescope, combined with the 
consideration that the thickness of a spider’s web across the 
telescopic field of view would suffice to hide a breadth of twenty 
miles on Jupiter’s disc. 
But we are not by any means limited to the reasoning here 
indicated, convincing as that reasoning should be to all who 
have studied the aspect of Jupiter with adequate telescopic 
power. We have in Jupiter’s mean density an argument of 
irresistible force against the only view which enables us even 
hypothetically to escape from the conclusions just indicated. 
Let it be granted, for the sake of argument, that Jupiter’s cloud 
layer is less than fourteen miles in depth, so that we are freed 
for the moment from the inference that at the lower part of 
the atmosphere there is either an intense heat or else a density 
and pressure incompatible with the gaseous condition. We 
cannot, in this case, strike off more than twenty-eight miles 
from the planet’s apparent diameter to obtain the real diameter 
of his solid globe — solid, at least, if we are to maintain the 
theory of his resemblance to our earth. This leaves his real 
diameter appreciably the same as his apparent diameter, and 
as a result we have the mean density of his solid globe equal 
to a fourth of the earth’s mean density, precisely as when we 
leave his atmosphere out of the question. Now I apprehend 
that the time has long since passed when we can seriously pro- 
ceed at this stage to say, as it was the fashion to say in text- 
books of astronomy, “ therefore the substance of which Jupiter 
is composed must be of less specific gravity than oak and other 
heavy woods.” We know that Brewster gravely reasoned that 
the solid materials of Jupiter might be of the nature of pumice- 
stone, so that with oceans resembling ours a certain latitude 
was allowed for increase of density in Jupiter’s interior. But 
in the presence of the teachings of spectroscopic analysis, few 
would now care to maintain, as probable, so preposterous a 
theory as this. Everything that has hitherto been learned 
respecting the constitution of the heavenly bodies, renders it 
quite unlikely that the elementary constitution of Jupiter differs 
from that of our earth. Again, it was formerly customary to 
speak of the possibility that Jupiter and Saturn might be 
hollow globes, mere shells, composed of materials as heavy as 
terrestrial elements. But whatever opinion we may form as to 
the possibility that a great intensity of heat may vaporise a 
