Srongey— Of Atmospheres upon Planets and Satellites. 321 
By following the same steps as in the case of Mercury, we find successively 
v = 5042 m./sec. 
for the least velocity which would carry a missile away from Mars, if Mars 
were not rotating, and 
v =v —u = 4803 m./sec. 
for the relative velocity which is sufficient in consequence of the rotation. 
From this, and equations 16 and 17, we find 
p = 9°57 
as the density of a gas which would escape from Mars at a temperature of 
— 66° C., with the same facility as helium from the Earth. 
We must here make some allowance for the probability that the highest 
temperature at which a gas has an opportunity of escaping from Mars may 
be lower than the corresponding temperature on the Earth. And we must, on 
the other hand, remember that the molecules of helium are almost certainly 
not quite the heaviest molecules that can rid themselves of the Earth. Taking 
both considerations into account, 7 is legitimate to infer that water, in which p = 9, 
cannot remain on Mars. 
As to what happens to gases with densities of 14 and 16, we cannot speak 
with confidence. They may perhaps be imprisoned. And the conspicuous 
polar snows of Mars make it im a considerable degree probable that carbon 
dioxide, of which p = 22, is abundantly present. 
It appears here to be worth reviewing the state of things that must prevail if 
the atmosphere of Mars consist mainly of nitrogen and carbon dioxide. Without 
water, there can be no vegetation upon Mars, at least not such vegetation as 
we know; and, in the absence of vegetation, it is not likely that there is much 
free oxygen. Under these circumstances, the analogy of the Earth suggests that 
the atmosphere of Mars consists mainly of nitrogen, argon, and carbon dioxide. 
Carbon dioxide, the most condensible gas of such an atmosphere, would 
behave very differently from the way in which water behaves on the Earth. 
Water in the state of vapour is so much lighter than the other constituents 
of our atmosphere that it hastens upwards through the atmosphere ; and, accord- 
ingly, its condensation into cloud, whether of droplets of water or spicules of 
ice, takes place usually at very sensible elevations. There would be no such 
hurry to rise; there would, on the contrary, be great sluggishness in the 
diffusing upwards of carbon dioxide through an atmosphere of nitrogen. When 
brought to the ground in the form of snow or frost (for there would probably 
be no rain), and when subsequently evaporated, the carbon dioxide gas would 
crawl along the surface, descending into valleys, occupying plains and pushing 
TRANS. ROY. DUBL, SOC., N.S. VOL. VI., PART XIII. 3D 
