322 Sronry—Of Atmospheres upon Planets and Satellites. 
its way under the nitrogen, mixing only slowly with the nitrogen; and, as 
a result, only a very small proportion of the whole stock would be at any one 
time found elsewhere in the atmosphere than near the ground. It is suggested 
that the fogs, the snows, the frosts, and the evaporation of such a constituent of 
the atmosphere may account for the peculiar and varying appearances upon Mars, 
which, though recorded in our maps as if they were definite, are in reality 
very imperfectly seen from our distant Earth. In fact Mars, when nearest the 
Earth, which unfortunately seldom happens, is still 140 times farther off than 
the Moon. Fogs over the low-lying plains which on Mars correspond to the bed 
of our ocean, with mountain chains projecting through the fog, and a border 
of frost along either flank of these ranges, would perhaps account for some of 
the appearances which have been glimpsed; and extensive displacements of the 
vapour, consequent upon its distillation towards the two poles alternately, would 
perhaps account for the rest. 
Cuaprer X.—Of Jupiter. 
In the case of the planet Jupiter, we have the following data :— 
8848 
P (the periodic time of his rotation) = 35,728 seconds, 
r (Jupiter’s equatorial radius) = 378 = 70170 km., 
+ (m being Jupiter’s mass, and J the mass of the Earth) = 311-9. 
Using these data we find— 
u (the velocity at his equator, owing to the rotation) 
= 12:337 km. / sec. 
v (the least velocity which would carry a missile away, if Jupiter were not 
ous) = 59:570 km./see. 
v' =v —u(the least velocity which enables a missile to escape when helped 
by the rotation) — 47-933 km. / sec. 
pi (the density of gas which would escape from Jupiter, at a temperature 
of — 66° C., with as much ease as helium does from the Earth) 
= 0:099 of the density of hydrogen. 
p» (the density of a gas which would be imprisoned by Jupiter as effectually as 
creeneeie 8 Ik) aio) = 0°373 of the density of hydrogen. 
