824 Stonry— Of Atmospheres upon Planets and Satellites. 
13 and 14, we find for the minimum velocity of escape from each of these planets, 
if not rotating, 
v = 34:92 km. / sec., on Saturn, 
= 21°61 km. /sec., on Uranus, 
= 22°60 km. /sec., on Neptune ; 
whence 
v =v — u = 24°508, on Saturn, 
17°299, on Uranus, 
= 18-002, on Neptune, 
is the least velocity which enables a missile to escape when helped by the 
rotation. 
By dividing these last numbers by 9°27, we find the velocity of mean square 
of the gas which can escape as freely as does helium from the Earth, and then by 
Clausius’s formula, we can calculate p,, its density, which is 
pi = 0°37 of the density of hydrogen on Saturn, 
= 0°74 of the density of hydrogen on Uranus, 
= 0°68 of the density of hydrogen on Neptune. 
On the other hand, by dividing the values for v’ by 18, we learn what is the 
velocity of mean square of the gas which would be detained as firmly as water 
is held by Venus; and then, if we calculate p,; by Clausius’s formula, we find 
p2: = 1°39 times the density of hydrogen on Saturn, 
= 2°78 times the density of hydrogen on Uranus, 
= 2°57 times the density of hydrogen on Neptune. 
Now hydrogen, with a density of 1, stands in each case between p, and p;, and 
we are, therefore, left uninformed whether hydrogen is or is not allowed to 
escape. There is, perhaps, some ground for conjecturing that it cannot escape 
from Saturn, and that it can escape from Uranus and Neptune. But this must 
remain doubtful. Helium, with its density of 2, being more than the value of p, 
upon Saturn, is certainly imprisoned by that planet, but we have no satisfactory 
information as to what is its fate upon Uranus or Neptune. 
Thus the information we gain with reference to these three planets amounts to 
this—that we have no definite information as regards hydrogen; that Saturn is 
able to detain helium, but that we do not know whether the other two planets can 
or cannot; that all other gases known to chemists would be more firmly imprisoned 
by any one of these planets than they are by the Earth; and that, if there be 
gases lighter than hydrogen, it is certain that Saturn cannot detain any of which 
the density falls as low as one-third of that of hydrogen, Neptune cannot hold any 
