the planet uninhabitable base their opinion largely upon the assumed
absence of sufficient air to support life. It was long ago recognized
that, other things being equal, a planet of small mass must possess a
less dense atmosphere than one of large mass. Assuming that each planet
originally drew from a common stock, and that the amount and density of
its atmosphere is measured by its force of gravity, it can be shown that
Mars should have an atmosphere less than one fifth as dense as the
earth's.

Dr. Johnstone Stoney has attacked the problem of planetary atmospheres
in another way. Knowing the force of gravity on a planet, it is easy to
calculate the velocity with which a body, or a particle, would have to
start radially from the planet in order to escape from its gravitational
control. For the earth this critical velocity is about seven miles per
second; for Mars about three miles per second. Estimating the velocity
of the molecules of the various atmospheric gases, according to the
kinetic theory, Dr. Stoney finds that some of the smaller planets, and
the moon, are gravitationally incapable of retaining all of these gases
in the form of an atmosphere. Among the atmospheric constituents that,
according to this view, Mars would be unable permanently to retain is
water vapor. Indeed, he supposes that even the earth is slowly losing
its water by evaporation into space, and on Mars, owing to the slight
force of gravity there, this process would go on much more rapidly, so
that, in this way, we have a means of accounting for the apparent drying
up of that planet, while we may be led to anticipate that at some time
in the remote future the earth also will begin to suffer from lack of
water, and that eventually the chasms of the sea will yawn empty and
desolate under a cloudless sky.

But it is not certain that the original supply of atmospheric elements