1875.] The Atmospheres of the Planets. 453 
Moreover, as on the larger planets the depth of the atmo- 
sphere is to be measured, these values become rigorous. 
Keeping for the present to the group of the three greater 
planets, Jupiter, Saturn, and Uranus, supposing the density 
of the upper limit of the cloud-bearing strata of the atmo- 
spheres of these planets to be known, it would be easy to 
compute at what depth beneath this the superincumbent 
pressure would be so increased as to crush the gases forming 
the atmosphere into so dense a condition that they would 
become, to all intents, liquids; though probably long before 
this could be accomplished the atmosphere would have come 
to an end through the condensation of its constituents. 
The measured diameter of Jupiter and, as far as can be 
ascertained, of Saturn and Uranus, isth at of the upper 
limit of the cloud-bearing strata of their atmospheres; and, 
consequently, at this point the density cannot be of extreme 
slightness, for masses of condensed vapours or clouds can 
only exist and remain suspended in a strata of comparative 
density. In fixing the limiting density, where clouds may be 
considered as possibly existing, at as low as one-thousandth 
of the surface density of the earth’s atmosphere, an extreme 
value is probably taken; for not only is it questionable 
whether any mass of brilliantly illuminated condensed vapour 
could remain in suspension in air of this rarity, but it is 
doubtful whether condensation must not always ensue before 
reaching so low. Assuming, therefore, this limit, it remains 
to consider the maximum depths of the atmospheric enve- 
lopes of the three great planets ; supposing the lowest limit 
marked by the point where the gaseous constituents would 
be crushed by the superincumbent pressure into a consistency 
which would class them amongst liquids, not gases, 
even were they not actually liquefied long before reaching 
this point. Supposing that the temperature of these three 
planets is, like our own, not materially different from + 100° 
to —100° C. the atmosphere may be considered as possess- 
ing uniform temperature of o° C. 
On Jupiter at a depth of only 19 miles the density would 
be already over ten times as great as our own; at 28 miles 
it would be denser than water, and at 33 miles as dense and 
heavy as mercury; though long ere this the immense pressure 
would have, through liquefaction, put an end to the atmo- 
sphere and the laws of gaseous compression. On Saturn the 
Same points would be reached at a depth of 38, 57, and 68 
miles; and on Uranus at 41, 66, and 77 miles below the visible 
cloud surface. These comparatively trifling depths under 
the supposed conditions are the maximum that the cloud- 
