n has sapiently been made that, mathematical
facts being invariable, the eternal equality of two plus two with four
might serve as a basis of understanding, and that a statement of that
truth sent by electric taps across the ocean of ether would be a
convincing assurance that the inhabitants of the planet from which the
message came at least enjoyed the advantages of a common-school
education.

But, while speculation upon this subject rests on unverified, and at
present unverifiable, assumptions, of course everybody would rejoice if
such a thing were possible, for consider what zest and charm would be
added to human life if messages, even of the simplest description, could
be sent to and received from intelligent beings inhabiting other
planets! It is because of this hold that it possesses upon the
imagination, and the pleasing pictures that it conjures up, that the
idea of interplanetary communication, once broached, has become so
popular a topic, even though everybody sees that it should not be taken
too seriously.

The subject of the atmosphere of Mars can not be dismissed without
further consideration than we have yet given it, because those who think
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
w